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Title:
STACKED HIGH CONTRAST GRATINGS AND METHODS OF MAKING AND USING THEREOF
Document Type and Number:
WIPO Patent Application WO/2021/150304
Kind Code:
A1
Abstract:
Stacked high contrast gratings (S-HCGs) containing grating layers containing high refractive index III-nitride bars and low refractive index air and/or porous III-nitride bars are described. Further disclosed are methods for preparing and using such S-HCGS, for example, as bottom mirrors in vertical cavity surface emitting lasers (VCSELs).

Inventors:
HAN JUNG (US)
ELAFANDY RAMI (US)
MI CHENZIYI (US)
Application Number:
PCT/US2020/061794
Publication Date:
July 29, 2021
Filing Date:
November 23, 2020
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
UNIV YALE (US)
International Classes:
G02B5/18
Domestic Patent References:
WO2011046244A12011-04-21
Foreign References:
US20180101016A12018-04-12
US20020085610A12002-07-04
Other References:
T.-C. LU ET AL., APPLIED PHYSICS LETTERS, vol. 97, no. 7, 2010, pages 071114
Y. HIGUCHI ET AL., APPLIED PHYSICS EXPRESS, vol. 1, no. 12, 2008, pages 121102
K. IKEYAMA ET AL., APPLIED PHYSICS EXPRESS, vol. 5, no. 10, 2012, pages 092104
S. M. MISHKAT-UL-MASABIH ET AL., APPLIED PHYSICS EXPRESS, vol. 12, no. 3, 2019, pages 036504
G. COSENDEY ET AL., APPLIED PHYSICS LETTERS, vol. 101, no. 15, 2012, pages 151113
S. IZUMI ET AL., APPLIED PHYSICS EXPRESS, vol. 8, no. 6, 2015, pages 062702
M. KURAMOTO ET AL., APPLIED PHYSICS LETTERS, vol. 112, no. 11, 2018, pages 111104
T.-C. LU ET AL., APPLIED PHYSICS LETTERS, vol. 92, no. 14, 2008, pages 141102
T.-C. CHANG ET AL., ACS PHOTONICS, vol. 7, no. 4, 2020, pages 861 - 866
YOUTSEY ET AL., APPL. PHYS. LETT., vol. 71, 1997, pages 2151 - 2153
M. MOHARAM ET AL., JOSA, vol. 12, no. 5, 1995, pages 1068 - 1076
Attorney, Agent or Firm:
SHYNTUM, Yvonne Y. et al. (US)
Download PDF:
Claims:
We claim:

1. A stacked high contrast grating (S-HCG), wherein the S-HCG comprising:

(a) a substrate layer formed of a Ill-nitride or is alternatively a substrate formed of sapphire, silicon, or silicon carbide;

(b) a first grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the first grating layer is above the substrate layer or the substrate;

(c) a separation layer formed of a Ill-nitride on top of the first grating layer;

(d) a second grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the second grating layer is on top of the separation layer; and

(e) a capping layer formed of a Ill-nitride on top of the second grating layer; wherein the thicknesses of the first and the second grating layers each independently range in between about 50 to 500 nm; wherein the thickness of the separation layer ranges in between about

50 to 500 nm; wherein the total thickness of the HCG ranges from between about 600 nm to about 2,000 nm; wherein the substrate layer, the separation layer, and the capping layer are non-porous or substantially non-porous and each independently have an index of refraction greater than 1, more preferably greater than 2; wherein the Ill-nitride bars of the first and the second grating layers are non-porous or substantially non-porous and each independently have an index of refraction greater than 1, more preferably greater than 2; wherein the Ill-nitride bars of the first and the second grating and the air bars and/or porous Ill-nitride bars of the first and the second grating layers have a refractive index ratio (An) in the range of about 2 to about 3, preferably about 2.5.

2. A stacked high contrast grating (S-HCG), wherein the S-HCG comprising: (a) a substrate layer formed of a Ill-nitride or is alternatively a substrate formed of sapphire, silicon, or silicon carbide;

(b) a first grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the first grating layer is above the substrate layer or the substrate;

(c) a separation layer formed of a Ill-nitride on top of the first grating layer;

(d) a second grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the second grating layer is on top of the separation layer;

(e) a second separation layer formed of a Ill-nitride on top of the second grating layer;

(f) a third grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the third grating layer is on top of the second separation layer; and

(g) a capping layer formed of a Ill-nitride on top of the third grating layer; wherein the thicknesses of the first, the second, and the third grating layers each independently range in between about 50 to 500 nm; wherein the thickness of the first and the second separation layers each independently range in between about 50 to 500 nm; wherein the total thickness of the HCG ranges from between about 600 nm to about 2,000 nm; wherein the substrate layer, the first and the second separation layers, and the capping layer are non-porous or substantially non-porous and each independently have an index of refraction greater than 1, more preferably greater than 2; wherein the Ill-nitride bars of the first, the second, and the third grating layers are non-porous or substantially non-porous and each independently have an index of refraction greater than 1, more preferably greater than 2; wherein the Ill-nitride bars of the first, the second, and the third grating layers and the air bars and/or porous Ill-nitride bars of the first, the second, and the third grating layers have a refractive index ratio (Δη) in the range of about 2 to about 3, preferably about 2.5.

3. A stacked high contrast grating (S-HCG), wherein the S-HCG comprising:

(a) a substrate layer formed of a Ill-nitride or is alternatively a substrate formed of sapphire, silicon, or silicon carbide;

(b) a first grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the first grating layer is above the substrate layer or the substrate;

(c) a separation layer formed of a Ill-nitride on top of the first grating layer;

(d) a second grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the second grating layer is on top of the separation layer;

(e) a second separation layer formed of a Ill-nitride on top of the second grating layer;

(f) a third grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the third grating layer is on top of the second separation layer;

(g) a third separation layer formed of a Ill-nitride on top of the third grating layer;

(h) a fourth grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the fourth grating layer is on top of the third separation layer; and

(i) a capping layer formed of a Ill-nitride on top of the fourth grating layer; wherein the thicknesses of the first, the second, the third, and the fourth grating layers each independently range in between about 50 to 500 nm; wherein the thickness of the first, the second, and the third separation layers each independently range in between about 50 to 500 nm; wherein the total thickness of the HCG ranges from between about 600 nm to about 2,000 nm; wherein the substrate layer, the first, the second, and the third separation layers, and the capping layer are non-porous or substantially non- porous and each independently have an index of refraction greater than 1, more preferably greater than 2; wherein the Ill-nitride bars of the first, the second, the third, and the fourth grating layers are non-porous or substantially non-porous and each independently have an index of refraction greater than 1, more preferably greater than 2; wherein the Ill-nitride bars of the first, the second, the third, and the fourth grating layers and the air bars and/or porous Ill-nitride bars of the first, the second, the third, and the fourth grating layers have a refractive index ratio (Δη) in the range of about 2 to about 3, preferably about 2.5.

4. A stacked high contrast grating (S-HCG), wherein the S-HCG comprising:

(a) a substrate layer formed of a Ill-nitride or is alternatively a substrate formed of sapphire, silicon, or silicon carbide;

(b) a first grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the first grating layer is above the substrate layer or the substrate;

(c) a separation layer formed of a Ill-nitride on top of the first grating layer;

(d) a second grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the second grating layer is on top of the separation layer;

(e) a second separation layer formed of a Ill-nitride on top of the second grating layer;

(f) a third grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the third grating layer is on top of the second separation layer;

(g) a third separation layer formed of a Ill-nitride on top of the third grating layer; (h) a fourth grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the fourth grating layer is on top of the third separation layer;

(i) a fourth separation layer formed of a Ill-nitride on top of the fourth grating layer;

(j) a fifth grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the fifth grating layer is on top of the fourth separation layer; and

(k) a capping layer formed of a Ill-nitride on top of the fifth grating layer; wherein the thicknesses of the first, the second, the third, the fourth, and the fifth grating layers each independently range in between about 50 to

500 nm; wherein the thickness of the first, the second, the third, and the fourth separation layers each independently range in between about 50 to 500 nm; wherein the total thickness of the HCG ranges from between about 600 nm to about 2,000 nm; wherein the substrate layer, the first, the second, the third, and the fourth separation layers, and the capping layer are non-porous or substantially non-porous and each independently have an index of refraction greater than 1, more preferably greater than 2; wherein the Ill-nitride bars of the first, the second, the third, the fourth, and the fifth grating layers are non-porous or substantially non- porous and each independently have an index of refraction greater than 1, more preferably greater than 2; wherein the Ill-nitride bars of the first, the second, the third, the fourth, and the fifth grating layers and the air bars and/or porous Ill-nitride bars of the first, the second, the third, the fourth, and the fifth grating layers have a refractive index ratio (Δη) in the range of about 2 to about 3, preferably about 2.5.

5. The 5-HCG of any one of claims 1-4, wherein the Ill-nitride of the substrate layer; the capping layer, the separation layers, and the Ill-nitride bars of the grating layers is selected from aluminum nitrides, gallium nitrides, indium nitrides, and alloys thereof.

6. The S-HCG of any one of claims 1 -5, wherein the Ill-nitride bars of the grating layers are non-porous.

7. The S-HCG of any one of claims 1-5, wherein the grating layers comprise porous Ill-nitride bars having a porosity of at least about 50%, 60%, 70%, 80%, or 90%.

8. The S-HCG of any one of claims 1-7, wherein the air bars of the grating layers have a refractive index of about 1.

9. The S-HCG of any one of claims 1 -8, wherein the air bars of the grating layers each independently have a length of about 50 nm to 800 ran.

10. The S-HCG of any one of claims 1-9, wherein the ill-nitride bars of the grating layers each independently have a length of about 50 nm to 800 nm.

11. The S-HCG of any one of any one of claims 1-10, wherein the III- nitride of the substrate layer, the capping layer, the separation layers, and the III- nitride bars of the grating layers each independently maintains a carrier (electron) concentration of at least above about 5xl018 cm-3 and electrical mobilities of at least about 50, 60, 70, 80, 90, or 95 cm2/V s.

12. The S-HCG of any one of claims 1-11, wherein the III- nitride of the substrate layer, the capping layer, the separation layers, and the III~nitride bars of the grating layers each independently have a thermal conductivity in a range of between about 1 to 25, 2 to 20, 2 to 15, or 2 to 10 W/m-K.

13. The S-HCG of any one of claims 1-11, wherein the III- nitride of the substrate layer, the capping layer, the separation layers, and the Til-nitride bars of the grating layers each independently have a thermal conductivity of at least about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 20, 30, 40, or 50

W/m-K.

14. The S-HCG of any one of claims 1-13, w herein the Ill-nitride of the substrate layer, the capping layer, the separation layers, and the 111-nitride bars of the grating layers are the same ill-nitride.

15. The S-HCG of any one of claims 1-14, wherein the Ill-nitride of the substrate layer; the capping layer, and the separation layers is a bulk Ill- nitride that is not doped with any n-type dopant(s).

16. The S-HCG of any one of claims 1-15, wherein the refractive index contrast ratio (Δη) is at least about 2.1, 2.2, 2.3, 2.4, or 2.5.

17. The S-HCG of any one of claims 1-15, wherein the refractive index contrast ratio (Δη) is at about 2.5.

18. A method of making the HCG of any one of claims 1-17, the method comprising the steps of:

(a) providing a substrate or growing a substrate layer, which is a HI- nitride layer;

(b) growing at least a first grating layer of an n-type doped Ill-nitride above the substrate layer;

(c) growing an undoped Ill-nitride first separation layer over the at least first grating layer;

(d) growing at least a second grating layer of an n-type doped III- nitride over the first separation layer;

(e) optionally growing an undoped Ill-nitride second separation layer over the second grating layer;

(f) optionally growing a third grating layer of an n-type doped III- nitride over the second separation layer, if present;

(g) optionally growing an undoped Ill-nitride third separation layer over the third grating layer, if present;

(h) optionally growing a fourth grating layer of an n-type doped III- nitride over the third separation layer, if present;

(i) optionally growing an undoped Ill-nitride fourth separation layer over the fourth grating layer, if present;

(j) optionally growing a fifth grating layer of an n-type doped III- nitride over the fourth separation layer, if present;

(k) growing a capping layer over the top most grating layer present, wherein the capping layer is an undoped Ill-nitride layer;

(l) forming or depositing an ion implant mask layer over the capping layer; (m) implanting ions, through ion implant mask layer, into the n-type doped grating layers to form ion implanted domains or regions therein having reduced electrical conductivity, as compared to non-ion implanted regions or domains in the n-type doped grating layers;

(n) optionally removing the ion implant mask layer;

(o) optionally patterning or etching at least the n-type doped III- nitride grating layers to expose sidewalls of the n-type doped Ill-nitride grating layers which were not ion implanted; and

(p) electrochemically (EC) etching away the non-ion implanted regions or domains in the n-type doped Ill-nitride grating layers in the presence of an electrolyte and under an applied bias voltage to completely etch away or substantially etch away the non-ion implanted regions or domains of the grating layers thereby forming air bars and/or porous III- nitride bars in the n-type doped III- nitride grating layers; wherein following step (p), the grating layers comprise alternating Ill-nitride bars and the air bars and/or porous Ill-nitride bars.

19. The method of claim 18, wherein the III -nitride of the substrate layer, the capping layer, the separation layers, and the n-type doped Ill-nitride grating layers is selected from aluminum nitrides, gallium nitrides, indium nitrides, and alloys thereof.

20. The method of claims 18 or 19, wherein the Ill-nitride of the substrate layer, the capping layer, the separation layers, and the n-type doped III- nitride grating layers are epitaxially or homoepitaxially grown by metal organic chemical vapor deposition (MOCVD).

21. The method of any one of claims 18-20, wherein the thicknesses of the n-type doped Ill-nitride grating layers, as present, each independently range in between about 50 to 500 nm.

22. The method of any one of claims 18-21, wherein the thickness of the separation layers, as present, each independently range in between about 50 to 500 nm.

23. The method of any one of claims 18-22, wherein the total thickness of the HCG formed by the method ranges from between about 600 nm to about 2,000 nm.

24. The method of any one of claims 18-23, wherein the substrate layer, the separation layers, as present, and the capping layer are non-porous or substantially non-porous and each independently have an index of refraction greater than 1, more preferably greater than 2.

25. The method of any one of claims 18-24, wherein following step (p) the Ill-nitride bars in the grating layers are non-porous or substantially non- porous and each independently have an index of refraction greater than 1, more preferably greater than 2;

26. The method of any one of claims 18-25, wherein following step (p) the Ill-nitride bars in the grating layers and the air bars and/or porous III- nitride bars formed in the grating layers have a refractive index ratio (Δη) in the range of about 2 to about 3, preferably about 2.5.

27. The method of any one of claims 18-26, wherein the n-type doped III- nitride grating layers are doped with an n-type dopant selected from a Ge dopant, Si dopant, or combination thereof.

28. The method of claim 27, wherein the n-type dopant is obtained from a dopant source selected from silane (SiH4), germane (GeH4), isobutylgermane (IBGe), and combinations thereof.

29. The method of any one of claims 18-28, wherein the n-type doped III- nitride grating layers are doped with an n-type dopant in a range of between about 0.5 x 1020 cm-3 to 10 x 1020 cm 3; and/or in a range of between about 1 x 1019 cm-3 to less than 1 x 1020 cm 3 or in a range of between about 0.5 x 1019 cm-3 to 10 x 1019 cm 3; and/or in a range of between about 0.5 x 1018 cm-3 to 10 x 1018 cm-3.

30. The method of any one of claims 18-29, wherein the ion implant mask layer is made from photoresist materials, dielectric materials, metals, or combinations thereof.

31. The method of any one of claims 18-30, wherein the ion implant mask layer is formed by a photoresist method, hard mask-etching method, or hard mask- lift off method.

32. The method of any one of claims 18-31, wherein the ion implant mask layer has a thickness in a range of between about 1 pm to about 10μm.

33. The method of any one of claims 18-32, wherein the ion implanting of step (m) forms ion implanted domains or regions comprising ions of aluminum, gold, nitrogen, hydrogen, helium, carbon, oxygen, titanium, iron, or combinations thereof.

34. The method of any one of claims 18-33, wherein the ions implanted during ion implanting step (m) have an ion implant dosage ranging from between about 1012 to 1016 ions/cm-3.

35. The method of any one of claims 18-34, wherein step (m) is carried out at energies from less than about 10 keV to about 1 MeV.

36. The method of any one of claims 18-35, wherein the ion implant mask layer is removed by physically and/or chemically etching the ion implant mask layer.

37. The method of any one of claims 18-36, wherein optional step (o) is performed using inductively coupled plasma reactive-ion etching (ICP-RIE).

38. The method of any one of claims 18-37, wherein optional step (o) forms a trench or via to expose the sidewalls of the n-type doped Ill-nitride grating layers.

39. The method of any one of claims 18-38, wherein the III- nitride bars in the grating layers following step (p) are noil-porous.

40. The method of any one of claims 18-38, wherein the grating layers comprise porous 01-nitride bars having a porosity of at least about 50%,

60%, 70%, 80%, or 90%.

41. The method of any one of claims 18-40, wherein the air bars in the grating layers following step (p) have a refractive index of about 1.

42. The method of any one of claims 18-41, wherein the air bars in the grating layers following step (p) each independently have a length of about 50 nm to 800 nm.

43. The HCG of any one of claims 18-42, wherein the Ill-nitride bars in the grating layers following step (p) each independently have a length of about 50 nm to 800 nm.

44. The method of any one of claims 18-43, wherein the Ill-nitride of the substrate layer, the capping layer, the separation layers, and the n-type doped Ill-nitride grating layers are formed from the same III-nitride.

45. The method of claim 26, wherein the refractive index contrast ratio

(An) is at least about 2.1, 2.2, 2.3, 2.4, or 2.5.

46. The method of claim 26, wherein the refractive index contrast ratio (Δη) is at about 2.5.

47. The method of any one of claims 18-46, wherein the electrolyte of step (p) is selected from hydrofluoric acid, nitric acid, or organic acids or salts thereof.

48. The method of any one of claims 18-47, wherein the applied bias voltage of step (p) is in a range of between about 0.1 to 10 V, 1.0 to 5V, or 1.0 to 2.5V and is applied for at least about 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 10, hours, 15 hours, 20 hours, or 24 hours .

49. The method of any one of claims 18-48, wherein step (p) is carried out at room temperature or at a temperature in the range of about 10 °C to about 50 °C.

50. A device comprising the HCG of any one of claims 1-17.

51. The device of claim 50, wherein the device is selected from light- emitting diodes, field-effect transistors, laser, laser diodes, and biomedical devices.

52. The device of claim 51 , wherein the laser is a vertical cavity surface emitting laser (VCSEL) and the HCG is a bottom mirror in the vertical cavity surface emitting laser (VCSEL).

53. The device of claim 52, wherein the HCG bottom mirror has a stopband at or around 440 nm with a peak reflectance of at least about 99%,

99.1%, 99.2%, 99.3%, 99.4%, 99.5%, 99.6%, 99.7%, 99.8%, or 99.9%.

54. The device of claim 53, wherein the stopband has a wavelength width of between about 10 to about 85 nm or between about 10 to about 80 nm.

55. The device of claim 53, wherein the stopband has a wavelength width of about 10 nm, 35 nm, or 80 nm.

56. The device of any one of claims 52-55, wherein the vertical cavity surface-emitting laser (VCSEL) emits in the blue wavelength range and/or the green wavelength range.

Description:
STACKED HIGH CONTRAST GRATINGS AND

METHODS OF MAKING AND USING THEREOF

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. Provisional Application No. 62/964,724 filed January' 23, 2020, and U.S, Provisional Application No. 63/078,479 filed September 15, 2020, which are hereby incorporated by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED

RESEARCH

This invention was made with Government support under Grant No. ECCS-i 709149 awarded by the National Science Foundation. The Government has certain rights in the invention.

FIELD OF THE INVENTION

This invention is in the field of high contrast gratings (HCGs), which are structures containing multilayer gratings, or more specifically stacked HCGs, which are formed from, for example, epitaxially grown Ill-nitrides, and which can be used in electronic applications, such as photonic devices.

BACKGROUND OF THE INVENTION

Over the last three decades, research and development of lll-nitrides and their alloys have ushered a new era of optoelectronic light emitting devices. Most notably, gallium nitride (GaN) based light emitting diodes (LEDs) and edge emitting laser diodes (EELDs), which covered UV/blue/green spectrum and achieved high conversion efficiencies and high output powers, were employed in numerous lighting, display and data storage applications.

More recently, there has been a growing interest in GaN based vertical-cavity surface-emitting lasers (VCSELs) due to their unique properties: low threshold currents, single mode operation, circular beam Higuchi, et al., Applied Physics Express 1 (12), 121102 (2008); K. Ikeyama, et al., Applied Physics Express 9 (10), 102101 (2016); C. Holder, et al., Applied Physics Express 5 (9), 092104 (2012); S. M. Mishkat-Ul-Masabih, et al., Applied Physics Express 12 (3), 036504 (2019); G. Cosendey, et al., Applied Physics Letters 101 (15), 151113 (2012); S. Izumi, et al., Applied Physics Express 8 (6), 062702 (2015); and M. Kuramoto, et al., Applied Physics Letters 112 (11), 111104 (2018)).

Similarly, GaN based blue VCSELs are expected to find novel applications in virtual and augmented reality, low power sensing and communication, and portable projection devices, to name a few.

Since the first demonstration of an electrically injected VCSEL in 2008 (T.-C. Lu, et al., Applied Physics Letters 92 (14), 141102 (2008)), several device architectures have been developed and optimized over the years by several academic and industrial based research groups. However, more than a decade later of research and development, GaN VCSELs have yet reached technological viability.

It is accepted that the main bottleneck in the GaN VCSEL technology is the preparation of the bottom mirror. Bottom epitaxial distributed Bragg reflector (DBR), which is the current design standard, suffers from the inherent limitations in Ill-nitride material system of having a large lattice mismatch yet with a relatively modest contrast in refractive indices. These limitations cause the DBR to have a challenging growth procedure and a low yield, which both hinder their commercialization.

To date, high contrast grating (HCG) mirrors have been incorporated into GaN-based VCSEL as the top mirror (T.-C. Chang, et al., ACS

Photonics 7 (4), 861-866 (2020)). Such membrane-based HCGs, which act as top mirrors, have a few advantages over the conventional epitaxial DBR mirrors, such as 1) a potentially wider stopband width; 2) a thinner equivalent mirror thickness and shorter cavity length; 3) a reduced complexity in heteroepitaxy; and 4) an inherent polarization anisotropy in mirror reflectivity, leading to mode and frequency stabilization. Although this kind of HCG mirrors can potentially resolve the issues with DBR mirrors they are not, however, mechanically stable and there is no known for example, for VCSELs. Accordingly, current implementations and designs of HCGs can still cannot be easily incorporated as the bottom mirror in a GaN-based VCSEL structure.

Therefore, there is a need for high contrast grating bottom mirrors, which can replace the distributed Bragg reflectors (DBRs) bottom mirrors currently used.

There also is a need for addressing the issues identified with current high contrast gratings (HCGs), as discussed above.

Therefore, it is an object of the invention to provide alternatives to

DBR mirrors.

It is also an object of the invention to provide high contrast grating structures (HCGs) which address and overcome the issues known to-date,

It is yet another object of the invention to provide methods for preparing such high contrast gratings (HCGs).

It is still a further object of the invention to provide methods of using the high contrast gratings (HCGs) described.

SUMMARY OF THE INVENTION

Stacked high contrast gratings (S-HCGs) which are epitaxially grown, polarization selective, and are capable of a wide stopband width at high reflectances are described herein. The S-HCGs are multilayer structures containing at least two grating layers. Methods of manufacturing and using such S-HCGs are also described.

In one non-limiting example, a stacked HCG has at least two grating layers where the S-HCG contains:

(a) a substrate layer formed of a Ill-nitride or is alternatively a substrate formed of sapphire, silicon, or silicon carbide;

(b) a first grating layer having alternating Ill-nitride bars and air bars and/or Ill-nitride porous bars wherein the first grating layer is above the substrate layer or the substrate;

(c) a separation layer formed of a Ill-nitride on top of the first grating layer; (d) a second grating layer containing alternating Ill-nitride bars and air bars and/or Ill-nitride porous bars wherein the second grating layer is on top of the separation layer; and

(e) a capping layer formed of a Ill-nitride on top of the second grating layer.

In other instances, stacked HCGs may contain three, four, or up to five grating layers, as described in detail below.

For stacked HCGs described, the first grating layer is above the substrate or the substrate layer. In instances where the substrate is sapphire, silicon, or silicon carbide, a preferably undoped Ill-nitride layer is present between the substrate and the first grating layer. This layer, when present, can be defined according to the separation and/or capping layers discussed elsewhere.

For stacked HCGs described, the thicknesses of any one of the first, the second, the third, the fourth, or the fifth grating layers, which may be present, may each independently range in between about 50 and 500 nm (and subranges therein). The thicknesses of the first, the second, the third, and the fourth separation layers, which may be present, may each independently range in between about 50 to 500 nm (and subranges therein). In preferred instances, the total thickness of a stacked HCG may range from between about 600 nm to about 2,000 nm.

For stacked HCGs, the substrate layer, the first, the second, the third, the fourth separation layers, and the capping layer are preferably non-porous or substantially non-porous Ill-nitride bulk layers. Each of the aforementioned layers can independently have an index of refraction greater than 1 and more preferably greater than 2. In some instances, the index of refraction is about 2 to 2.5. Further, for stacked HCGs, the substrate layer, the first, the second, the third, the fourth separation layers, and the capping layer may be undoped non-porous or substantially non-porous Ill-nitride bulk layers or they may be low doped non-porous or substantially non- porous Ill-nitride layers (where “low doped” refers to a doping concentration of less than about 1 x 10 18 cm 3 ). More typically, these layers are undoped.

For stacked HCGs, the Ill-nitride bars of the first, the second, the porous or substantially non-porous (where “substantially non-porous” refers to a porosity of less than 5%, 4%, 3%, 2%, or 1% in the Ill-nitride bars). Each of the Ill-nitride bars in each layer may independently have an index of refraction greater than 1, more preferably greater than 2. In some instances, the index of refraction is about 2 to 2.5.

For stacked HCGs, the Ill-nitride bars of the first, the second, the third, the fourth, and the fifth grating layers, as present, and the air bars and/or porous Ill-nitride bars of the first, the second, the third, the fourth, and the fifth grating layers, as present, may have a refractive index ratio (Δη) in the range of about 2 to about 3, preferably about 2.5. In some instances, the refractive index contrast ratio (Δη) is at least about 2.1, 2.2, 2.3, 2.4, or 2.5. In still other instances, the refractive index contrast ratio (Δη) is at about

2.5.

For stacked HCGs, the Ill-nitride of the substrate layer, the capping layer, the separation layers, and the Ill-nitride bars of the grating layers can be selected, without limitation, from aluminum nitrides, gallium nitrides, indium nitrides, and alloys thereof. In still some other instances, the III- nitrides for the S-HCGs can be substituted for non-nitride materials, which are described herein, and include, for example, gallium phosphides, gallium arsenides, silicon, and indium phosphides. Such non-nitride materials can be porosified and/or etched away according to the electrochemical etching techniques described herein. In some instances, the Ill-nitride of the substrate layer, the capping layer, the separation layers, and the 111-nitride bars of the grating layers are formed from the same Ill-nitride or non-nitride material (as specified above). Typically, the Ill-nitride of the substrate layer, the capping layer, and the separation layers is a bulk III-nitride that has not been doped with any dopant(s).

The S-HCG structures can have advantageous properties, such as: (1) avoiding the usage of planar airgap in a buried form, thus greatly enhancing fabrication feasibility and structural robustness; (2) utilizing established fabrication methods to ease the epitaxial difficulty; (3) providing broad stopband with relatively small epitaxial thickness; (4) competitive electrical and thermal conductivity due to its compact stmcture; and (5) providing polarization control for VCSELs due to anisotropy for TE- and TM-polarized light.

The S-HCGs can be formed, for example, by a method that includes the steps of:

(a) providing a substrate or growing a substrate layer, which is a HI- nitride layer;

(b) growing at least a first grating layer of an n-type doped TTT-nitride above the substrate layer;

(c) growing an undoped Ill-nitride first separation layer over the at least first grating layer;

(d) growing at least a second grating layer of an n-type doped III- nitride over the first separation layer;

(e) optionally growing an undoped Ill-nitride second separation layer over the second grating layer;

(f) optionally growing a third grating layer of an n-type doped III- nitride over the second separation layer, if present;

(g) optionally growing an undoped Ill-nitride third separation layer over the third grating layer, if present;

(h) optionally growing a fourth grating layer of an n-type doped HI- nitride over the third separation layer, if present;

(i) optionally growing an undoped Ill-nitride fourth separation layer over the fourth grating layer, if present;

(j) optionally growing a fifth grating layer of an n-type doped III- nitride over the fourth separation layer, if present;

(k) growing a capping layer over the top most grating layer present, wherein the capping layer is an undoped Ill-nitride layer;

(l) forming or depositing an ion implant mask layer over the capping layer;

(m) implanting ions, through ion implant mask layer, into the n-type doped grating layers to form ion implanted domains or regions therein having reduced electrical conductivity, as compared to non-ion implanted regions or domains in the n-type doped grating layers;

(n) optionally removing the ion implant mask layer; (o) optionally patterning or etching at least the n-type doped III- nitride grating layers to expose sidewalls of the n-type doped Ill-nitride grating layers which were not ion implanted; and

(p) electrochemically (EC) etching away the non-ion implanted regions or domains in the n-type doped Ill-nitride grating layers in the presence of an electrolyte and under an applied bias voltage to completely etch away or substantially etch away the non-ion implanted regions or domains of the grating layers thereby forming air and/or porous Ill-nitride bars in the n-type doped Ill-nitride grating layers; wherein following step (p), the grating layers comprise alternating Ill-nitride bars and air and/or porous Ill-nitride bars.

The Ill-nitride of the substrate layer. , the capping layer, the separation layers, and the n-type doped Ill-nitride grating layers is selected from aluminum nitrides, gallium nitrides, indium nitrides, and alloys thereof. Such undoped or doped Ill-nitride, such as GaN, layers can be epitaxially or homoepitaxially grown according to art known methods, such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). For example, undoped Ill-nitride layer (i.e., first grating layer) and second undoped Ill-nitride layer (i.e., first separation layer) can be epitaxially or homoepitaxially grown according to art known methods. In still some other instances, the Ill-nitrides described herein may be substituted with non-nitride materials and include, for example, gallium phosphides, gallium arsenides, silicon, and indium phosphides. Such non-nitride materials can be porosified and/or etched away according to the electrochemical etching techniques described herein. For example, nonnitride materials specified may be doped and ion implanted using the same methods described herein for Ill-nitride materials where an EC step may be applied to etch away or selectively porosify such n-type doped non-nitride materials (see above), just as in the case of the Ill-nitrides described herein.

The stacked high contrast gratings can be used in various applications including electronic, photonic, and optoelectronic applications. The spatial control in etching selected regions or domains to form the grating layers in an S-HCG can be used to control the optical, electrical, thermal or in laser diodes, such as vertical-cavity surface-emitting lasers (VCSELs) where they can serve as the bottom mirrors. As discussed earlier, it is not to- date possible or practical to use or form current HCG stmctures as bottom mirrors in VCSELs. The S-HCGs can address these issues. Such S-HCGs can be prepared according to the methods and these can be incorporated into different devices, such as VCSELs, using art known techniques. Further, as a bottom mirror with a broad stopband width, the S-HCG structures may be used in applications in many optoelectronic devices. For example, the S- HCGs may also be utilized in light-emitting diodes (LEDs).

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a non-limiting representation of a stacked high contrast grating (HCG) structure, where Gl, G2, and G3 denote three grating layers from top to bottom having a given thickness and SI and S2 represent high-index separation layers having a given thickness. The grating layers contain alternating high-index III- nitride bars and low-index medium (i.e., air or porous Ill-nitride) bars. A high-index capping layer is shown on top of Gl and a substrate layer of a given index is shown below G3. Λ represents the length of the period, a represents the length of the low-index bar.

Figure 2 shows a non-limiting exemplary fabrication process for a stacked high contrast grating (HCG) stmcture, where in the process is shown in a flow chart manner read in a clockwise order. In (a) an epitaxially grown stmcture with alternating doped “n+ GaN” and undoped “U-GaN” layers is formed followed by formation of a “mask” layer having nano-scale patterning in (b), such as by nano-imprint or e-beam lithography. In (c) ion implantation is applied through the mask to create vertical regions of reduced conductivity (light grey regions (c)) and the mask may be removed in (d). After the ion implantation, trenches will be formed through dry etching which exposes the sidewalls of the structures, as shown in (e) and (f). Electrochemical etching is performed in (g) to selectively etch away only the n+ GaN regions that were not ion implanted in the doped n+ GaN layers. The final HCG stmcture is shown in (h). Figure 3A shows a schematic representation of a process to selectively implant ions into a doped Ill-nitride layer where a stmcture is first grown having a 50 nm thick “n+ GaN” Ge doped GaN layer sandwiched between two undoped “U-GaN” layers, as shown in top of Figure 3 A. Then using photolithography H+ ions are implanted within the doped “n+ GaN” layer, as shown in middle of Figure 3A. Finally, trenches are formed to expose the sidewalls of the “n+GaN” layer, as shown in bottom of Figure 3A. Figure 3B shows an optical micrograph of an ion implanted sample after porosification in a doped Ill-nitride layer 100 surrounded by trenches or vias 180, continuous and discontinuous ion implanted regions or domains 110, etched and porosified regions or domains 120, and showing etching fronts 125.

Figures 4A - 4C show simulated reflectance spectra of three S-HCG stmctures, such as in Figure 1. In Figure 4A, an S-HCG stmcture can provide >99.5% reflectance for 81 nm stopband width when the thickness of grating layers is: 375 nm, 225 nm, 500 nm; the thickness of separation layers is: 300 nm, 375 nm; the period is: 300 nm; and the filling factor (FF) is 0.22. In Figure 4B, an s-HCG stmcture can provide >99.5% reflectance for a 36 nm stopband width when the thickness of grating layers is: 228 nm, 132 nm, 228 nm; the thickness of separation layers is: 238 nm, 336 nm; the period is: 252 nm; and the filling factor (FF) is 0.35. In Figure 4C, an S-HCG stmcture can provide >99.5% reflectance for a 10 nm stopband width when the thickness of grating layers is: 120 nm, 249 nm, 150 nm; the thickness of separation layers is: 82 nm, 87 nm; the period is: 208 nm; and the filling factor (FF) is 0.61.

DETAILED DESCRIPTION OF THE INVENTION

Stacked high contrast gratings (S-HCGs) which are epitaxially grown, polarization selective, and are capable of a wide stopband width at high reflectances are described herein. The S-HCGs are multilayer structures containing at least two grating layers. Methods of manufacturing and using such S-HCGs are also described. For example, such S-HCGs are useful as an alternative to distributed Bragg reflector bottom mirrors for high- performance GaN blue VCSELs.

I. Definitions

“Porosity,” as used herein refers to the volumetric ratio of air present in a porosified medium, such as a III- nitride layer(s), which is expressed as a percentage.

“Refractive Index” or “Index of Refraction,” are used interchangeably and refer to the ratio of the velocity of light in a vacuum to its velocity in a specified medium, such as a layer of a Ill-nitride, according to the formula n = c/v, where c is the speed of light in vacuum and v is the phase velocity of light in the medium.

“Refractive Index Contrast,” as used herein refers to the relative difference in refractive index between two mediums having different indices of refraction and which are in contact and form an interface.

“Bulk Ill-nitride,” as used herein refers to a pristine Ill-nitride material.

Numerical ranges disclosed in the present application include, but are not limited to, ranges of temperatures, ranges of times, ranges of bias voltages, ranges of porosities, ranges of thermal conductivities, ranges of integers, and ranges of thicknesses, amongst others. The disclosed ranges, disclose individually each possible number that such a range could reasonably encompass, as well as any sub-ranges and combinations of subranges encompassed therein. For example, disclosure of a time range is intended to disclose individually every possible time value that such a range could encompass, consistent with the disclosure herein.

Use of the term "about" is intended to describe values either above or below the stated value, which the term “about” modifies, in a range of above or below the stated value in a range of approx. +1- 5%. When the term "about" is used before a range of numbers (i.e., about 1-5) or before a series of numbers (i.e., about 1, 2, 3, 4, etc.) it is intended to modify both ends of the range of numbers and/or each of the numbers recited in the entire series, unless specified otherwise.

II. Stacked High Contrast Gratings (S-HCGs)

Stacked high contrast gratings (S-HCGs) are described herein. Typically, the HCGs contain at least two grating layers but may contain up to five grating layers, as described in detail below.

In one instance a non- limiting exemplary stacked HCG having two grating layers may contain:

(a) a substrate layer formed of a Ill-nitride or is alternatively a substrate formed of sapphire, silicon, or silicon carbide;

(b) a first grating layer having alternating Ill-nitride bars and air bars and/or Ill-nitride porous bars wherein the first grating layer is above the substrate layer or the substrate;

(c) a separation layer formed of a Ill-nitride on top of the first grating layer;

(d) a second grating layer containing alternating Ill-nitride bars and air bars and/or Ill-nitride porous bars wherein the second grating layer is on top of the separation layer; and

(e) a capping layer formed of a Ill-nitride on top of the second grating layer.

In another instance, a non- limiting exemplary stacked HCG having three grating layers may contain:

(a’) a substrate layer formed of a III- nitride or is alternatively a substrate formed of sapphire, silicon, or silicon carbide;

(b’) a first grating layer containing alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the first grating layer is above the substrate layer or the substrate;

(c’) a separation layer formed of a Ill-nitride on top of the first grating layer; (d’) a second grating layer comprising alternating III- nitride bars and air bars and/or porous Ill-nitride bars wherein the second grating layer is on top of the separation layer;

(e’) a second separation layer formed of a Ill-nitride on top of the second grating layer;

(f’) a third grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the third grating layer is on top of the second separation layer; and

(g’) a capping layer formed of a III- nitride on top of the third grating layer.

In still another instance, a non-limiting exemplary stacked HCG having four grating layers may contain:

(a”) a substrate layer formed of a Ill-nitride or is alternatively a substrate formed of sapphire, silicon, or silicon carbide;

(b”) a first grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the first grating layer is above the substrate layer or the substrate;

(c”) a separation layer formed of a III- nitride on top of the first grating layer;

(d”) a second grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the second grating layer is on top of the separation layer;

(e”) a second separation layer formed of a Ill-nitride on top of the second grating layer;

(f”) a third grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the third grating layer is on top of the second separation layer;

(g’ ’) a third separation layer formed of a Ill-nitride on top of the third grating layer;

(h”) a fourth grating layer comprising alternating III- nitride bars and air bars and/or porous Ill-nitride bars wherein the fourth grating layer is on top of the third separation layer; and

(i”) a capping layer formed of a III- nitride on top of the fourth In still another instance, a non-limiting exemplary stacked HCG having five grating layers may contain:

(a’”) a substrate layer formed of a III- nitride or is alternatively a substrate formed of sapphire, silicon, or silicon carbide;

(b’”) a first grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the first grating layer is above the substrate layer or the substrate;

(c’”) a separation layer formed of a Ill-nitride on top of the first grating layer;

(d’”) a second grating layer comprising alternating III- nitride bars and air bars and/or porous Ill-nitride bars wherein the second grating layer is on top of the separation layer;

(e’”) a second separation layer formed of a Ill-nitride on top of the second grating layer;

(f””) a third grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the third grating layer is on top of the second separation layer;

(g’”) a third separation layer formed of a Ill-nitride on top of the third grating layer;

(h’”) a fourth grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the fourth grating layer is on top of the third separation layer;

(i’”) a fourth separation layer formed of a Ill-nitride on top of the fourth grating layer;

(j’”) a fifth grating layer comprising alternating Ill-nitride bars and air bars and/or porous Ill-nitride bars wherein the fifth grating layer is on top of the fourth separation layer; and

(k’”) a capping layer formed of a Ill-nitride on top of the fifth grating layer.

A non-limiting exemplary stacked HCG having three grating layers is shown in Figure 1, where Gl, G2, and G3 denote three grating layers from top to bottom having a given thickness and SI and S2 represent high-index separation layers having a given thickness. The grating layers contain porous Ill-nitride) bars. A high-index capping layer is shown on top of G1 and a substrate layer of a given index is shown below G3. Λ represents the length of the period, a represents the length of the low-index medium (i.e., air or porous Ill-nitride) bars. For each S-HCG, the length of the period and the length of the low-index medium (i.e., air or porous Ill-nitride) bars for each grating layer present is typically the same. The length of the period Λ can be any suitable length based on desired properties of the S-HCG. In some other instances, the length of the period Λ, is based on the target wavelength of the application, such as a length based on use in UV of less than 400 nm or use in IR of greater than 800 nm. The length of the period may also be in a range between about 100 nm (for UV) to more than 900 nm (for IR). In still some instances, the length of the period Λ may be between about 100 to 400 nm or 200 to 300 nm. In some instances, the low-index medium bars (i.e., air or porous Ill-nitride bars) of the grating layers each independently have a length of about SOnm to 800 nm within a given layer.

In some instances, TIX-nitride bars of the grating layers each independently have a length of about 50nm to 800 nm. Filling factor (FF) may be calculated for the S-HCGs based on the equation (Λ-α)/Λ. In some instances, the filling factor (FF) of the S-HCGs ranges from about 0.1 to 0.9. In some other instances, the filling factor (FF) of the S-HCGs is about 0.1, 0.2, 0.3, 0.4,

0.5, 0.6, 0.7, 0.8, or 0.9. Typically, each grating layer present in an S-HCG has the same filling factor (FF).

For stacked HCGs, the first grating layer is above the substrate or the substrate layer. In instances where the substrate is sapphire, silicon, or silicon carbide, a preferably undoped Ill-nitride layer is present between the substrate and the first grating layer. This layer, when present, can be defined according to the separation and/or capping layers discussed herein. In some instances, where the first grating layer is directly above a substrate layer, there may optionally be a suitable substrate formed of sapphire, silicon, or silicon carbide below the substrate layer.

For stacked HCGs, the thicknesses of any one of the first, the second, the third, the fourth, or the fifth grating layers, which may be present, may each independently range in between about 50 to 500 nm (and subranges therein). The thicknesses of the first, the second, the third, and the fourth separation layers, which may be present, may each independently range in between about 50 to 500 nm (and subranges therein). The thicknesses of the substrate or substrate layer, as may be present, may each independently have any suitable size but may range from about 50 to 500 nm (and subranges therein). In preferred instances, the total thickness of a stacked HCG may range from between about 600 nm to about 2,000 nm. The dimensions and/or shape of the above layers or substrate may be of any suitable shape/dimension required for an application.

For stacked HCGs, the substrate layer, the first, the second, the third, the fourth separation layers, and the capping layer are preferably non-porous or substantially non-porous Ill-nitride bulk layers. Each of the aforementioned layers can independently have an index of refraction greater than 1 and more preferably greater than 2. In some instances, the index of refraction is about 2 to 2.5. Further, for stacked HCGs, the substrate layer, the first, the second, the third, the fourth separation layers, and the capping layer may be undoped non-porous or substantially non-porous Ill-nitride bulk layers or they may be low doped non-porous or substantially non- porous Ill-nitride layers (where “low doped” refers to a doping concentration of less than about 1 x 10 18 cm 3 ). More typically, these layers are undoped.

For stacked HCGs, the Ill-nitride bars of the first, the second, the third, the fourth, and the fifth grating layers, as present, are typically non- porous or substantially non-porous (where “substantially non-porous” refers to a porosity of less than 5%, 4%, 3%, 2%, or 1% in the Ill-nitride bars). Each of the Ill-nitride bars in each layer may independently have an index of refraction greater than 1, more preferably greater than 2. In some instances, the index of refraction is about 2 to 2.5.

For stacked HCGs, the Ill-nitride bars of the first, the second, the third, the fourth, and the fifth grating layers, as present, and the air bars and/or porous Ill-nitride bars of the first, the second, the third, the fourth, and the fifth grating layers, as present, may have a refractive index ratio (Δη) in the range of about 2 to about 3, preferably about 2.5. In some instances, the refractive index contrast ratio (Δη) is at least about 2.1, 2.2, 2.3, 2.4, or 2.5. In still other instances, the refractive index contrast ratio (Δη) is at about

2.5.

In some instances, alternating Ill-nitride bars and air bars are present in the grating layers of the S-HCGs which are formed by a spatial control technique combining conductivity selective electrochemical (EC) etching based on selective ion implantation into selected regions or domains of doped Ill-nitride layers, as described herein. Spatial control of ion implantation allows for non-ion implanted regions or domains in the doped Ill-nitride layer to be completely EC etched away or substantially etched away (where “substantially etched away” refers to etching greater than 95%, 96%, 97%, 98%, or 99%) leaving a void where Ill-nitride material existed. The void represents the low-index medium (i.e., air bar) as shown, for example, in Figure 1. The air bars of the grating layers typically have a refractive index of about 1. The EC etching method used to form the S- HCGs also allows for control and the optimization of the local electrical, thermal, optical properties the Ill-nitride domains and regions contained therein.

In some other instances, alternating Ill-nitride bars and porous III- nitride bars are present in the grating layers of the S-HCGs which are formed by a spatial control technique combining conductivity selective electrochemical (EC) etching based on selective ion implantation into selected regions or domains of doped Ill-nitride layers, as described herein. Spatial control of ion implantation allows for non-ion implanted regions or domains in the doped Ill-nitride layer to be controllably EC porosified to afford porous Ill-nitride bars. The EC etching method used to form the S- HCGs also allows for control and the optimization of the local electrical, thermal, optical properties the porous Ill-nitride domains and regions contained therein.

In still some other instances, it is contemplated that alternating III- nitride bars and a combination of both air bars and/or porous Ill-nitride bars may be present in the same and/or different grating layers of the S-HCGs which are formed by a spatial control technique combining conductivity selective electrochemical (EC) etching based on selective ion implantation into selected regions or domains of doped Ill-nitride layers, as described herein.

In instances of porous Ill-nitride bars being present in the S-HCG grating layers, porosification should be preferably high and between about 50% and 90%, or greater. In some instances, the porosity is at least about

50%, 60%, 70%, 80%, or 90% or greater. The porous Ill-nitride bars, when present, should have a refractive index of less than about 1.6 or about 1.0,

1.1, 1.2, 1.3, 1.4, 1.5, or 1.6 Incorporation of a low index material, such as air, into Ill-nitride by porosification has the effect of lowering the refractive index, as compared to the bulk Ill-nitride.

In instances of porous Ill-nitride bars being present in the S-HCG grating layers, different degrees of porosities and pore morphologies may be obtained by changing the type and concentration of electrolyte (either salt or acid), doping concentration, and applied bias voltage, as discussed in greater detail below. For example, columnar vertically or laterally aligned pores, or dendritic pore morphologies can be achieved due to the pore growth mechanisms operating during EC etching. The porosification produces porous Ill-nitride(s) bars within the given grating layers present. Such porous Ill-nitride bars are preferably nanoporous but may be further defined as being micro-, meso-, or macro-porous, or any combination thereof. Without limitation, columnar vertically or laterally aligned pores may be formed in the porous Ill-nitride bars during the EC etching process of a suitable length. The vertically or laterally aligned pores may be further categorized as microporous (d < 2 nm), mesoporous (2 nm < d < 50 nm), or macroporous (d > 50 nm); where d is the average pore diameter. The morphology of the formed pores may be classified as circular, semicircular, ellipsoidal, or a combination thereof. The pores may have an average size of between about 5 to 100 nm, 5 to 75 nm, 5 to 50 nm, or 5 to 25 nm. In some instances the average pore size is about 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 nm or greater. In some instances, based on the original doping concentration, the etchant used, and the applied voltage during the electrochemical porosification process, the average size of the pores can range from between less than about 20 nm to greater than 50 nm. The spacing between adjacent increases as a function of a lower applied bias and a lower doping concentration. The spacing between pores can range from between about 1 to 50 nm, 5 to 50 nm, 5 to 40 nm, 5 to 30 nm, 5 to 25 nm, 5 to 20 nm, 5 to 15 nm, or 5 to 10 nm.

For stacked HCGs, the III -nitride of the substrate layer, the capping layer, the separation layers, and the [[[-nitride bars of the grating layers can be selected, without limitation, from aluminum nitrides, gallium nitrides, indium nitrides, and alloys thereof. In still some other instances, the III- nitrides described for the S-HCGs can be substituted for non-nitride materials, and include, for example, gallium phosphides, gallium arsenides, silicon, and indium phosphides. Such non-nitride materials can be porosified and/or etched away according to the electrochemical etching techniques described herein. In some instances, the Ill-nitride of the substrate layer, the capping layer, the separation layers, and the III- nitride bars of the grating layers are formed from the same Ill-nitride or non-nitride material (as specified above). Typically, the Ill-nitride of the substrate layer, the capping layer, and the separation layers is a bulk III-nitride that has not been doped with any dopant(s).

The layers of Ill-nitrides, such as GaN, can be grown epitaxially or homoepitaxially according to art known methods. In some instances, the III- nitride layer can be grown, for example, on a suitable substrate (i.e., c -plane of a sapphire substrate, a silicon substrate, or a silicon carbide substrate) by metal organic chemical vapor deposition (MOCVD).

Controlled EC etching to remove non-ion implanted regions or domains within the layers of doped Ill-nitride requires controlling and localizing where the EC etching occurs. To allow for etching Ill-nitrides need to be doped with an n-type dopant. Exemplary dopants include, but are not limited to n-type Ge and Si dopants. Such dopant sources can include, for example, silane (SiH 4 ), germane (GeELi), and isobutylgermane (IBGe) which can be doped into layer(s) of Ill-nitrides during their formation/deposition. In some instances the Ill-nitrides which are porosified are aluminum free or substantially free of aluminum (where “substantially free” means less than about 5%, 4%, 3%, 2%, 1%, 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, some other instances, the Ill-nitrides which are porosified contain aluminum and can be classified as aluminum rich Ill-nitrides (such as containing greater than 10%, 20%, 30%, 40%, or 50%, or greater aluminum by weight in the doped III- nitride layer).

The n-type doping concentration within a doped Ill-nitride can be uniform across the entirety of a Ill-nitride layer or the doping concentration may form a gradient (i.e., a Ill-nitride layer having a graded dopant concentration across an axis of the layer, such width). The doping concentration is considered high at doping concentration levels of at least about 1 x 10 20 cm -3 or higher; or in the range of between about 0.5 x 10 20 cm -3 to 10 x 10 20 cm -3 . The doping concentration is considered to be moderate at doping concentration levels of greater than about 1 x 10 18 cm -3 to less than 1 x 10 20 cm -3 , 2 x 10 18 cm -3 to less than 1 x 10 20 cm -3 , 3 x 10 18 cm -3 to less than 1 x 10 20 cm -3 , 4 x 10 18 cm -3 to less than 1 x 10 20 cm -3 , or 5 x 10 18 cm -3 to less than 1 x 10 20 cm -3 . In some instances, the moderately doped concentration level is in the range of 1 x 10 19 cm -3 to less than 1 x 10 20 cm -3 or in the range of about 0.5 x 10 19 cm -3 to 10 x 10 19 cm -3 . The doping concentration is considered to be low at doping concentration levels of less than about 1 x 10 18 cm -3 or in the range of between about 0.5 x 10 18 cm -3 to 10 x 10 18 cm -3 . In certain instances, the n-type doping concentration within a doped Ill-nitride layer should be greater than about 1 x 10 19 cm -3 to permit formation of highly porous Ill-nitride bars or to permit complete etching of non-ion implanted portions of the doped Ill-nitrides in the grating layers.

Further, selective and controlled etching away of selected regions or domains of a doped Ill-nitride layer relies on the introduction of one or more ion implants into pre-defined and selected regions or domains of doped layer(s) of Ill-nitrides forming the grating layers of the S-HCGs during fabrication. Ion implantation allows for a reduction in the electrical conductivity of the ion implanted regions or domains sufficient and prevents their etching under EC etching conditions. The non-ion implanted regions or domains are etched away (or controllably porosified) under EC etching conditions. This is due to a reduction in electrical conductivity caused by introducing ion implants into doped regions or domains of Ill-nitride layer(s) to the electrical conductivity prior to ion implantation. By contrast, non-ion implanted regions or domains of a layer of a doped Ill-nitride layer retain at least the same electrical conductivity, as compared to the bulk pristine III- nitride, or greater than about 99%, 95%, 90% of the electrical conductivity.

By way of an ion implant mask layer it is possible to create microscale patterns which permit selective ion implantation of only certain doped regions or domains within the doped Ill-nitride layer(s) of the S-HCGs during fabrication, were masked areas are not exposed to ion implants. One or more ion implant mask layers can be used which have any size, area, or shape may be formed on the capping layer during fabrication to controllably select which domains and regions of the doped Ill-nitride layer(s) are exposed to the ion implantation.

Ion implant masks can be formed using several methods. Without limitation, some examples include:

(1) Photoresist method: A layer of photoresist is spin coated onto the capping layer. Then using photolithography, electron beam lithography or stamping techniques, the photoresist mask layer can be pattered into the desired shape. Based on the source of ions and ion energies used, the mask layer can vary from less than about 1 pm to greater than 10 pm, as needed.

(2) Hard mask, etching method: A hard mask layer of a material is deposited on the capping layer. This material can be dielectric (such as, for example, silicon dioxide or silicon nitride) or a metal, such as titanium, aluminum, etc. Based on the material, different deposition techniques can be used such as, for example, thermal evaporators, electron beam evaporators, sputters, spin coating, chemical vapor deposition, atomic layer depositions, etc. Then a layer of photoresist can be spin coated and patterned on top of the deposited hard mask layer of material. Then the hard mask layer of material can be etched away, either chemically or physically, in order to transfer the patterns from the photoresist to the underlying layer of the material.

(3) Hard mask, liftoff method: Similar to the above technique (hard mask, etching method) but carried out in reverse order. First, a layer of photoresist is spin coated and then patterned. Then, a hard mask layer is deposited on top of the photoresist using one of the techniques in (2). The photoresist is then etched away causing the lifting off of regions within the hard mask layer thus transferring the pattern to the hard mask layer.

As noted above, the ion implants are made only in the exposed areas of the doped Ill-nitride layer(s), which are not masked. The ion implant ions can come from various ion sources. Many ion species and sources can be used, such as aluminum, gold, nitrogen, hydrogen, helium, carbon, oxygen, titanium, iron, to modify the electrical conductivity of the implanted regions. In some instances, the ion may be chosen based on higher atomic mass. For example, aluminum ions may be selected due to their greater atomic mass compared to hydrogen ions. The selected energy depends on the depth required of the ion implant. These energies can range from less than about 10 keV to greater than about 1 MeV to control implant at depths in a range from less than about 10 nm up to greater than about 2 pm, about 10 nm up to greater than about 1.5 pm, about 10 nm up to greater than about 1 pm, about 10 nm to about 600 nm, about 10 nm to about 500 nm, about 10 nm to about

250 nm, about 10 nm to about 100 nm, or any suitable sub-range or individual depth value within those ranges disclosed here. The ion dosage can also be used to control the number of implanted ions and therefore the modification in the electrical conductivity. Typical ion implant dosages can range from, but not are limited to about 10 12 to 10 16 cm 3 , based on the ion species and desired depth. In some instances it is possible to use an implant dosage of 3 x 10 16 cm 3 , for example, when using protons for deep implants. The energy of the ion implant source exposure can be used to control the depth of the ion implants made into doped Ill-nitride grating layer(s) of the S-HCGs during fabrication.

EC etching leaves the ion implanted regions or domains of the layer or layers intact with little (i.e., less than 5%, 4%, 3%, 2%, 1%) to no etching or porosification. EC etching is used to completely or substantially etches away (where “substantially etches away” refers to etching greater than 95%, 96%, 97%, 98%, or 99%) leaving a void where Ill-nitride material existed.

The void represents the low-index medium (i.e., air or porous Ill-nitride bar) as shown, for example, in Figure 1. Thus, doped regions or domains of the layer or layers of Ill-nitrides, which were not ion implanted are selectively In some cases, it may be necessary to pattern or etch the layer or layers of Ill-nitrides to expose sidewalls of non-ion implanted (doped) regions or domains of the Ill-nitride grating layer(s) during fabrication and prior to applying the EC etching methods. The porosity of a given region or domain within Ill-nitride layer(s) following electrochemical (EC) can be measured. For example, the porosity of a region or domain of Ill-nitride layer(s) which has been porosified can be determined where the Ill-nitride layer(s) can be weighed, such as on a micro-balance, before and after porosification and the weight difference (loss) in the EC etched porosified over the original (before) weight can be expressed as a percentage to denote the degree of porosity. In some other instances, porosity may also be measured/estimated by imaging processing software, such as Image! , where scanning electron microscopy (SEM) images of the porosified Ill-nitride is used.

As noted above, the regions or domains within a doped Ill-nitride grating layer, following the introduction of ion implants, which are etched away (or controllably porosified) by EC etching are those which are non-ion implanted (i.e., protected from the ion implants by the ion implant mask layer(s)). The one or more regions or domains which are non-ion implanted and which are etched away (or controllably porosified) during the EC etching process are typically bar shaped for the S-HCGs and are based on the ion implant mask layer parameters (i.e., size, area, shape) and the ion implant exposure parameters (i.e., ion source and ion energy) which control the depth of the ion implant.

The dimensions of layer(s) of Ill-nitrides in the S-HCGs above, whether doped or undoped can be of any size, area, or shape suitable for an application. In some instances, the area is in the range of between about 0.1 to 100 cm 2 , 0.1 to 90 cm 2 , 0.1 to 80 cm 2 , 0.1 to 70 cm 2 , 0.1 to 60 cm 2 , 0.1 to 50 cm 2 , 0.1 to 40 cm 2 , 0.1 to 30 cm 2 , 0.1 to 20 cm 2 , 0.1 to 10 cm 2 , 0.1 to 5 cm 2 , or 0.1 to 1 cm 2 .

The S-HCG structures can have advantageous properties, such as: (1) avoiding the usage of planar airgap in a buried form, thus greatly enhancing fabrication feasibility and structural robustness; (2) utilizing established stopband with relatively small epitaxial thickness; (4) competitive electrical and thermal conductivity due to its compact stmcture; and (5) providing polarization control for VCSELs due to anisotropy for TE- and TM-polarized light.

It was also found that the S-HCG structures can provide great flexibility by changing their stmctural parameters. With different total thickness, the stmcture can provide different stopband width, which can meet different kinds of device operational requirements. The effects of such variations are in prophetic examples based on simulations below. a. Optical Properties of Ill-Nitrides Selective incorporation of a low index material, such as air, into selected regions or domains of Ill-nitride grating layer of an S-HCG by EC etching away of those regions or domains has the effect of lowering the refractive index, as compared to the bulk Ill-nitride. Thus, it is possible to tune the refractive index of the porosified regions or domains selectively.

The refractive index (n) of the Ill-nitride grating layer(s) of the S-HCGs are dependent on the degree of porosity (i.e., amount of air in the porous III- nitride), where the refractive index of air is about 1 (at STP) and the refractive index of a bulk (non-porous) Ill-nitride is about 2.6. In some cases, the Ill-nitride grating layer(s) of the S-HCGs can have Ill-nitride bars therein which have a refractive index of about 1 to 2.6 or about 1 to 1.6: or about 1, 1.1., 1.2, 1.3, 1.4, 1.5, or 1.6 , by selectively incorporating porosity into the porosified III-nitride bars using the ion implantation and EC etching methods. Typically, the ill-nitride bars are not porosified to a great extent, b. Electrical Properties of III -Nitrides Selective incorporation of air (or porosity) into selected regions or domains of a Ill-nitride grating layer of an S-HCG by EC etching away of those regions or domains can affect the electrical properties, as compared to the bulk (non-porous) equivalent bulk Ill-nitride. For electrically injected devices, especially those requiring high current densities, good electrical transport is essential for high device performance.

In some instances, selectively incorporating porosity into the Ill- nitride bars using the ion implantation and EC etching methods can be used process has occurred. With limited introduction of porosity into the III- nitride bars it is possible to retain a carrier (electron) concentration of above about 5xl0 18 cm -3 and electrical mobilities of at least about 50, 60, 70, 80, 90, 95 cm 2 /V s, or greater, compared to the bulk (non-porous) equivalent bulk Ill-nitride.

C. Thermal Properties of Ill-Nitrides

For electrically injected devices, especially those requiring high current densities, efficient heat dissipation is essential for high device performance. For example, poor thermal conductivity (~1 W/m-K) can impede the commercialization of electrically injected devices.

It is believed that the inclusion of etched away (or controllably porosified) regions or domains within Ill-nitride grating layers of an S-HCG stmcture whereby the grating layers have alternating Ill-nitride bars and air and/or porous Ill-nitride bars, provides an advantage of improved thermal conduction properties, where the grating layers can function at much lower operation temperatures, as compared to an equivalent device which does not contain such grating layers. The improved thermal conductance may also benefit the threshold, power, and efficiency of such heat generating devices.

The thermal conductivity of the grating layers can be tuned based on the introduction of the air and/or porous Ill-nitride bars as well as by controlled introduction of some porosity in the Ill-nitride bars therein. In some instances, the thermal conductivity of the grating layer(s) are in the range of between about 1 to 25, 2 to 20, 2 to 15, or 2 to 10 W/m-K. In still some other instances, the average thermal conductivity is at least about 1, 2,

3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 W/m-K.

III. Methods of Preparing Stacked High Contrast Gratings

Unlike the photoelectrochemical (PEC) methods previously used, the conductivity selective electrochemical (EC) etching methods rely on electrically injected holes, rather than photogenerated holes, to oxidize predetermined domains and regions within bulk layer(s) of doped Ill-nitrides, such as GaN. The methods do not require exposure to ultraviolet (UV) illumination. The etching behavior of the doped Ill-nitride is well controlled during the EC etching process by the incorporation of ion implants, which reduce electrical conductivity and prevent porosification during EC etching process. Such a process allows selected domains and regions to be etched away to form low-index medium (i.e., air or porous III- nitride) bars within a doped Ill-nitride layer with good spatial control.

In one non- limiting example of a method of forming a stacked high contrast grating (S-HCG), the method includes the steps of:

(a) providing a substrate or growing a substrate layer, which is a HI- nitride layer;

(b) growing at least a first grating layer of an n-type doped TTT-nitride above the substrate layer;

(c) growing an undoped Ill-nitride first separation layer over the at least first grating layer;

(d) growing at least a second grating layer of an n-type doped HI- nitride over the first separation layer;

(e) optionally growing an undoped Ill-nitride second separation layer over the second grating layer;

(f) optionally growing a third grating layer of an n-type doped III- nitride over the second separation layer, if present;

(g) optionally growing an undoped Ill-nitride third separation layer over the third grating layer, if present;

(h) optionally growing a fourth grating layer of an n-type doped III- nitride over the third separation layer, if present;

(i) optionally growing an undoped Ill-nitride fourth separation layer over the fourth grating layer, if present;

(j) optionally growing a fifth grating layer of an n-type doped III- nitride over the fourth separation layer, if present;

(k) growing a capping layer over the top most grating layer present, wherein the capping layer is an undoped Ill-nitride layer;

(l) forming or depositing an ion implant mask layer over the capping layer;

(m) implanting ions, through ion implant mask layer, into the n-type doped grating layers to form ion implanted domains or regions therein having reduced electrical conductivity, as compared to non-ion implanted regions or domains in the n-type doped grating layers;

(n) optionally removing the ion implant mask layer;

(o) optionally patterning or etching at least the n-type doped III- nitride grating layers to expose sidewalls of the n-type doped Ill-nitride grating layers which were not ion implanted; and

(p) electrochemically (EC) etching away the non-ion implanted regions or domains in the n-type doped Ill-nitride grating layers in the presence of an electrolyte and under an applied bias voltage to completely etch away or substantially etch away the non-ion implanted regions or domains of the grating layers thereby forming air and/or porous Ill-nitride bars in the n-type doped Ill-nitride grating layers; wherein following step (p), the grating layers comprise alternating Ill-nitride bars and the air and/or porous Ill-nitride bars.

Typically, the low-index medium bars are air bars that have a refractive index of about I. In instances where porous Ill- nitride bars are present these may have a refractive index in the range of between about 1 to 1.6, depending on the degree of porosity imparted during step (p). The air and/or porous Ill-nitride bars in the grating layers following step (p) each independently have a length of about 50 nm to 800 nm. The III- nitride bars in the grating layers following step (p), which are high-index preferably with a refractive index of greater than 2, each independently have a length of about 50 nm. to 800 nm. These lengths can be controlled by way of the ion masking layer parameters, as discussed below. Thus, the refractive index contrast ratio (Δη) between the high-index non-porous or low porous III- nitride bars and low-index air bars and/or porous Ill-nitride bars is at least about 2.1, 2.2, 2.3, 2.4, or 2.5. Preferably, the refractive index contrast ratio (Δη) between the low-index and high-index materials in the S-HCG is at about 2.5.

Figure 2 provides a visual example of an exemplary fabrication process for a stacked high contrast grating (HCG) stmcture consistent with the methods described above. The process is shown in a flow chart manner read in a clockwise order. In (a) an epitaxially grown structure with followed by formation of a “mask” layer having nano-scale pattering in (b), such as by nano-imprint or e-beam lithography. In (c) ion implantation is applied through the mask to create vertical regions of reduced conductivity (light grey regions (c)) and the mask may be removed in (d). After the ion implantation, trenches will be formed through dry etching which exposes the sidewalls of the stmctures, as shown in (e) and (f). Electrochemical etching is performed in (g) to selectively etch away only the n+ GaN regions that were not ion implanted in the doped n+ GaN layers. The final HCG structure is shown in (h).

For the method described above, the first grating layer is above the substrate or the substrate layer. In instances where the substrate provided is a sapphire, silicon, or silicon carbide substrate, a preferably undoped Ill-nitride layer is also present (i.e., grown) in between the substrate and the first grating layer. This layer, when present, can be defined and fabricated according to the separation and/or capping layers discussed elsewhere. In some instances, where the first grating layer is directly above a substrate layer, there may optionally be a substrate made of sapphire, silicon, or silicon carbide which is provided below the substrate layer.

The ill-nitride of the substrate layer, the capping layer, the separation layers, and the n-type doped Ill-nitride grating layers is selected from aluminum nitrides, gallium nitrides, indium nitrides, and alloys thereof. Such undoped or doped Ill-nitride, such as GaN, layers can be epitaxially or homoepitaxially grown according to art known methods, such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). For example, undoped Ill-nitride layer (i.e., first grating layer) and second undoped Ill-nitride layer (i.e., first separation layer) can be epitaxially or homoepitaxially grown according to art known methods. In still some other instances, the Ill-nitrides may be substituted with non-nitride materials and include, for example, gallium phosphides, gallium arsenides, silicon, and indium phosphides. Such non-nitride materials can be porosified and/or etched away according to the electrochemical etching techniques. For example, non-nitride materials specified may be doped and ion implanted using the same methods for Ill-nitride materials where an EC step may be applied to etch away or selectively porosify such n-type doped non-nitride materials (see above), just as in the case of the Ill-nitrides.

In the methods, the thicknesses of the n-type doped Ill-nitride grating layers, as present, may each independently range in between about 50 to 500 nm (and subranges therein). The thickness of the separation layers, as present, and the capping layer are each independently range in between about 50 to 500 nm (and subranges therein). The thicknesses of the substrate or substrate layer, as may be present, may each independently have any suitable size but may range from about 50 to 500 nm (and subranges therein). The dimensions and/or shape of the above layers or substrate may be of any suitable shape/dimension required for an application. Lastly, the total thickness of the HCGs formed by the method preferably ranges from between about 600 nm to about 2,000 nm.

Etching forms low-index medium (i.e., air or porous III- nitride) bars within the grating layers present in the S-HCG being formed by the methods proceeds by EC etching that requires that the bulk Ill-nitride of the grating layer(s) be doped with an n-type dopant. Accordingly, doped Ill-nitride grating layers, as present, are formed during deposition/formation.

Exemplary dopants can include, but are not limited to n-type Ge and Si dopants. Such dopant sources can include, for example, silane (SiH4), germane (GeH4), and isobutylgermane (IBGe). The doping concentration can be uniform across the entirety of a doped Ill-nitride layer or the doping concentration may form a gradient (i.e., a graded dopant concentration across an axis of the layer, such width). The doping concentration is considered high at doping concentration levels of at least about 1 x 10 20 cm 3 or higher; or in the range of between about 0.5 x 10 20 cm -3 to 10 x 10 20 cm 3 . The doping concentration is considered to be moderate at doping concentration levels of greater than about 1 x 10 18 cm 3 to less than 1 x 10 20 cm 3 , 2 x 10 18 cm -3 to less than 1 x 10 20 cm 3 , 3 x 10 18 cm -3 to less than 1 x 10 20 cm 3 , 4 x 10 18 cm -3 to less than 1 x 10 20 cm -3 , or 5 x 10 18 cm -3 to less than 1 x 10 20 cm-

3 . In some instances, the moderately doped concentration level is in the range of 1 x 10 19 cm -3 to less than 1 x 10 20 cm -3 or in the range of about 0.5 x 10 19 cm -3 to 10 x 10 19 cm -3 . The doping concentration is considered to be low at between about 0.5 x 10 18 cm -3 to 10 x 10 18 cm 3 . Porosification and etching by electrochemical (EC) etching is limited to domains or regions of III- nitride which are doped at moderate to high concentrations.

Introducing ion implant mask layer(s) over one or more areas of the final capping layer formed protects masked areas from ion implants. The one or more regions or domains which are non-ion implanted can be etched away (or controllably porosified) by the EC etching process based on the ion implant mask layer parameters (i.e., size, area, shape) and the ion implant exposure parameters (i.e., ion source and energy) which control the depth of the ion implant. The ion implant mask layer can be made, without limitation, using known surface masking techniques to form a masked region on the undoped layer prior to ion implantation.

Ion implant mask layer(s) can be formed using several methods. Without limitation, some examples include:

(1) Photoresist method: A layer of photoresist is spin coated onto a layer. Then using photolithography, electron beam lithography or stamping techniques, the photoresist mask layer can be pattered into the desired shape. Based on the source of ions and ion energies used, the mask layer can vary from less than about 1 pm to greater than 10 pm, as needed.

(2) Hard mask, etching method: A hard mask layer of a material is deposited. This material can be dielectric (such as, for example, silicon dioxide or silicon nitride) or a metal, such as titanium, aluminum, etc. Based on the material, different deposition techniques can be used such as, for example, thermal evaporators, electron beam evaporators, sputters, spin coating, chemical vapor deposition, atomic layer depositions, etc. Then a layer of photoresist can be spin coated and patterned on top of the deposited hard mask layer of material. Then the hard mask layer of material can be etched away, either chemically or physically, in order to transfer the patterns from the photoresist to the underlying layer of the material.

(3) Hard mask, liftoff method: Similar to the above technique (hard mask, etching method) but carried out in reverse order. First, a layer of photoresist is spin coated and then patterned. Then, a hard mask layer is deposited on top of the photoresist using one of the techniques previously of regions within the hard mask layer thus transferring the pattern to the hard mask layer.

The ion implant mask layer can have any suitable pattern, size, shape, area, or depth. In some instances, the area of the ion implant mask layer covers a range of between about 0.1 to 100 cm 2 , 0.1 to 90 cm 2 , 0.1 to 80 cm 2 , 0.1 to 70 cm 2 , 0.1 to 60 cm 2 , 0.1 to 50 cm 2 , 0.1 to 40 cm 2 , 0.1 to 30 cm 2 , 0.1 to 20 cm 2 , 0.1 to 10 cm 2 , 0.1 to 5 cm 2 , or 0.1 to 1 cm 2 . In some instances, the depth or thickness of the ion implant mask layer(s) can range from between about 1 pm to greater than 10 pm, and sub-ranges within. Following the ion implantation step, the ion implant mask layer is typically removed prior to performing EC etching.

The methods here rely on introduction of one or more ion implants into pre-defined and selected regions or domains within the doped Ill-nitride grating layers, prior to EC etching, allowing for a reduction in the electrical conductivity of the ion implanted regions sufficient to prevent their porosification and etching under EC etching conditions. The reduction in electrical conductivity caused by introducing ion implants into doped regions or domains of the Ill-nitride grating layer(s) can be by at least 10, 20, 30, 40, 50, 60, 70, 80, or 90%, as compared to the electrical conductivity prior to ion implantation. The non-ion implanted doped regions or domains in the at least one layer of a doped Ill-nitride retain at least the same electrical conductivity, as compared to the bulk pristine Ill-nitride, or greater than about 99%, 95%, 90% of the electrical conductivity. The use of an ion implant mask layer also allows for the creation of micro-scale patterns, which permit selective etching of only doped regions or domains of the grating layers of Ill-nitride, which were masked and not exposed to ion implants. Preferably these doped regions or domains of the grating layers of Ill-nitride are completely etched away (or controllably porosified), as discussed above, to leave alternating Ill-nitride bars (which were ion implanted) and low-index (i.e., air or porous III- nitride) bars where the nonion implanted Ill-nitride was etched away (or controllably porosified). In some instances, the remaining Ill-nitride bars, following EC etching, may also demonstrate some limited degree of porosification, such as at the edges The ion implants are made by exposing the doped Ill-nitride grating layers through the ion masking layer opening(s) to ions from various ion sources. Various ion species and sources can be used, such as aluminum, gold, nitrogen, hydrogen, helium, carbon, oxygen, titanium, and iron, to modify the electrical conductivity of the implanted regions. In some instances, the ion may be chosen based on higher atomic mass. For example, aluminum ions may be selected due to their greater atomic mass compared to hydrogen ions. The selected energy depends on the depth required of the ion implant. These energies can range from less than about 10 keV to greater than about 1 MeV to control implant at depths in a range from less than about 10 nm up to greater than about 1 pm, about 10 nm to about 600 nm, about 10 nm to about 500 nm, about 10 nm to about 250 nm, about 10 nm to about 100 nm, or any suitable sub-range or individual depth value within those ranges disclosed here. The ion dosage can also be used to control the number of implanted ions and therefore the modification in the electrical conductivity. Typical ion implant dosages can range from, but not are limited to about 10 12 to 10 16 cm 3 , based on the ion species and desired depth. The ion implant step of the methods can be repeated one or more times, as needed, and with one type or multiple types of ion sources to introduce multiple implants and types of ion implants, if necessary. The energy of the ion implant source exposure can be used to control the depth of the ion implants made into doped Ill-nitride grating layers present in the stacked

HCG.

During step (p), the EC etching leaves the ion implanted regions or domains of the doped grating layers completely intact or with little (less than 5%, 4%, 3%, 2%, 1%, or less) to no porosification. The EC etching etches away or porosifies only the doped regions or domains of the Ill-nitride layers, which were not ion implanted. Although the grating layers typically require etching away all the doped Ill-nitride material that was not ion implanted, in some instances it may be useful to control the degree of porosity of doped regions or domains within Ill-nitride grating layer(s), which were not ion implanted. This can be controlled as a function of two parameters: the doping (carrier) concentration and the anodization or applied bias voltage. In some instances, the porosity is at least about 50%, 60%, 70%, 80%, 90% or greater.

Etching during the electrochemical (EC) etching process of step (p) can be controlled based on concentration of electrolyte, doping concentration, and applied bias voltage (as discussed below). The applied bias voltage is typically a positive voltage in the range of about 0.1 to 10 V, 1.0 to 5V, or 1.0 to 2.5V. In some instances, based on the original doping concentration and the type of etchant used, the applied bias ranges from less than about IV to at least about 10V, or greater. The electric field direction during the EC etching process can control the direction of the etching direction and thereby control the direction of the pores etched into the nonion implanted doped regions or domains of a Ill-nitride layer. For example, during step (p) of the methods, the EC etching direction is determined by the electric field direction. Depending on the Ill-nitride/electrolyte interface, the EC etching can be controlled to be in a vertical etching or lateral etching direction. The rate of vertical or lateral etching during can be about 0.1 pm/min, 0.2 pm/min, 0.3 pm/min, 0.4 pm/min, 0.5 pm/min, 0.6 pm/min, 0.7 pm/min, 0.8 pm/min, 0.9 pm/min, 1 pm/min, 2 pm/min, 3 pm/min, 4 pm/min, 5 pm/min, 6 pm/min, 7 pm/min, 8 pm/min, 9 pm/min, or 10 pm/min.

The EC etching of step (p) can be carried out under an applied bias voltage from about 1 min to 24 hours, 1 min to 12 hours, 1 min to 6 hours, 1 min to 4 hours, 1 min to 2 hours, 1 min to 1 hour, or 1 min to 30 minutes. In some instances, the EC etching of step (p) is carried out under an applied bias voltage for at least about 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 10, hours, 15 hours, 20 hours, 24 hours, or greater. The EC etching of step (p) can be carried out under an applied bias voltage at room temperature or at a temperature in the range of about 10 °C to about 50 °C. The EC etching of step (p) can be carried out under an applied bias voltage under ambient conditions or optionally under an inert atmosphere (such as of nitrogen or argon).

In the methods, the EC etching carried out in step (p) can be carried (either salt or acid). Exemplary high conductivity electrolytes can include, but are not limited to hydrofluoric acid (HF), nitric acid (HNO 3 ), and organic acids and their salts (such as oxalic acid). The concentration of the electrolyte solutions are typically in the range of between 0.1 to 1M.

The optional patterning or etching of step (o) can be performed, for example, by lithographically patterning one or more openings known as “via trenches,” into the S-HCG structure prior to EC etching. This can expose sidewalls, which enable the EC process of step (p). In one example, as shown in Figure 2(e) and 2(f), a silicon dioxide layer may be deposited onto a defined area of the capping layer following step (d) which allows etching or removal, such as by inductively coupled plasma dry etching, of S-HCG stmcture not protected by the silicon dioxide layer. Suitable etching techniques, such as inductively coupled plasma reactive-ion etching (ICP- RIE), can be used to etch down Ill-nitride layer(s) to expose the doped layer(s) sidewalls.

It is believed that the conductivity selective electrochemical (EC) etching process of step (p), which selectively etches away doped regions or domains of doped Ill-nitride that were not ion implanted, is believed to proceed by an anodic etching reaction which involves four steps:

(1) charge carrier transport in the space-charge region;

(2) oxidation of the doped Ill-nitride surface;

(3) dissolution of oxides formed; and

(4) transport of products.

The Ill-nitride/electrolyte interface is understood to behave as a Schottky diode. Under a positive bias, holes (h + ) are generated near the surface of the doped Ill-nitride by tunneling or impact ionization and the holes are swept by electric field onto the Ill-nitride surface for subsequent oxidation reaction. As an example, the oxidation of GaN generates Ga 3+ ions and nitrogen gas (Youtsey, et al. Appl. Phys. Lett. 71, 2151-2153 (1997)).

Near the cathode, hydrogen gas is formed by hydrogen ion reduction reaction.

The reduction reaction completes the charge transfer circle of the electrochemical (EC) process.

IV. Methods of Using Stacked High Contrast Gratings (S-HCGs)

The stacked high contrast gratings can be used in various applications including electronic, photonic, and optoelectronic applications. The spatial control in etching selected regions or domains to form the grating layers in an S-HCG can be used to control the optical, electrical, thermal or mechanical properties of Ill-nitrides. In particular, the S-HCGs can be useful in laser diodes, such as vertical-cavity surface-emitting lasers (VCSELs) where they can serve as the bottom mirrors. As discussed earlier, it is not to- date possible or practical to use or form current HCG stmctures as bottom mirrors in VCSELs. The S-HCGs described here can address these issues. Such S-HCGs can be prepared according to the methods described herein and these can be incorporated into different devices, such as VCSELs, using art known techniques.

In certain instances, the S-HCGs act as a bottom mirror in a vertical cavity surface emitting laser (VCSEL). The S-HCG bottom mirror can have a stopband at or around 440 nm with a peak reflectance of at least about

99%, 99.1%, 99.2%, 99.3%, 99.4%, 99.5%, 99.6%, 99.7%, 99.8%, or 99.9%.

In some instances, the stopband has a wavelength width of between about 10 to about 85 nm or between about 10 to about 80 nm. In other instances, the stopband has a wavelength width of about 10 nm, 35 nm, or 80 nm. Such properties can be tailored by tuning the properties of the S-HCG, such as the number of grating layer, the thicknesses of the grating layers, the thicknesses of the separation layers, the period, the filling factor (FF), etc. The S-HCG bottom mirrors are particularly suited for vertical cavity surface-emitting lasers (VCSELs) that emit in the ultraviolet (i.e., <400 nm), violet (i.e., about 400-420 nm), blue, green, or red wavelength ranges. Further, emission may also be at the near infrared and infrared wavelengths.

The S-HCGs may be used as mirrors which provide the option of a conductive mirror to support vertical current injection vital in attaining high performance VCSELs with excellent optical and electrical performance as compared to previously reported VCSELs. VCSELs have many advantages compared to more commonly used edge emitting laser diodes (EELDs)such as superior beam quality, compact form factor, low operating power, cost- effective wafer-level testing, higher yield and lower cost in manufacturing. VCSEFs, in general, are expected to find important applications in various fields including information processing, micro-display, pico-projection, laser headlamps, high-resolution printing, biophotonics, spectroscopic probing, and atomic clocks.

Further, as a bottom mirror with a broad stopband width, the S-HCG stmctures may be used in applications in many optoelectronic devices. For example, the S-HCGs may also be utilized in light-emitting diodes (LEDs).

Example

To validate the use of the combination of vertical modulation of conductivity (through epitaxial growth) and lateral ion implants to permit formation of the stacked HCG structures, confinement of the electrochemical etching to the specific regions inside a layer was carried out, which would permit formation of such grating layers and stmctures thereof through EC etching, as detailed below.

Accordingly, a structure was grown having a 50 nm thick “n+ GaN” Ge doped GaN layer was sandwiched between two undoped “U-GaN” layers, as shown in top of Figure 3 A. Then, using photolithography and H+ ions were implanted within the doped “n+ GaN” layer, as shown in middle of Figure 3 A. Finally, trenches were formed to expose the sidewalls of the “n+GaN” layer, as shown in bottom of Figure 3 A. After fabrication, the sample was electro-chemically (EC) etched with nitric acid according to the methods. Figure 3B shows an optical micrograph of an ion-implanted sample after porosification in a doped Ill-nitride layer 100 surrounded by trenches or vias 180, continuous and discontinuous ion implanted regions or domains 110, etched and porosified regions or domains 120, and showing etching fronts 125.

This demonstrated that EC etching can be laterally controlled through ion-implants. This example demonstrates that EC etching in a buried layer can be laterally controlled through ion-implants, thus showing that it is possible to build a 3D grating stmcture, such as in the HCGs.

Prophetic Examples

Rigorous coupled wave analysis (RCWA) simulation was utilized to model reflectance spectmm (M. Moharam, et al., JOS A a 12 (5), 1068-1076 (1995)). Simulated reflectance spectmm of an exemplary modeled 3 -stacked grating, as shown in Figure 1, are provided in Figures 4A-4C.

For such 3 -stacked gratings, structural parameters (period, filling factor and thickness of the grating and separation layers) can be optimized. From the simulated reflectance spectrum shown at Figure 4, the reflectance for the TE mode (polarization direction parallel with the grating bars) is simulated to be close to unity near 440 nm while that for TM mode (perpendicular with the grating bars) is below 0.8. Therefore, the S-HCG stmctures are believed to be able to control the polarization direction of the laser.

Tunability in S-HCG structures

The optical performance of the S-HCG structures can be further optimized by RCWA simulation. By varying the structural parameters of a simulated S-HCG, as in Figure 1, it will be possible to provide a broad stopband near 440nm.

Based on simulations of different total thicknesses, the stmcture can provide different stopband widths, which can meet different kinds of requirements. Below it is provided three theoretically modeled designs with different total thicknesses of the S-HCG stmctures.

As shown in Figure 4A, an S-HCG with a total thickness of -1.8 pm can provide a very broad stopband for TE mode near 440 nm. The RCWA simulation result shows the stmcture can provide >99.5% reflectance for 81 nm stopband width when the thickness of grating layers is: 375 nm, 225 nm, 500 nm; the thickness of separation layers is: 300 nm, 375 nm; the period is: 300 nm; and the filling factor (FF) is 0.22. In this simulated reflectance spectrum, one can see the anisotropy for TE- and TM -polarized light. For the TE-polarized light (TE mode curve), a reflectance stop band of greater 80 nm can be achieved, which is much greater than the epitaxial DBRs used in known GaN-based material systems.

As shown in Figure 4B, for an S-HCG with a total thickness of ~1.2 pm RCWA simulation shows the stmcture can still provide a 36 nm stopband with at >99.5% reflectance when the thickness of grating layers is: 228 nm, 132 nm, 228 nm; the thickness of separation layers is: 238 nm, 336 nm; the period is: 252 nm; and the filling factor (FF) is 0.35. By reducing total thickness, the thermal and electrical performance of an S-HCG used as a bottom mirror, for example in a VCSEL, can be enhanced. Compared to a 40 pair AlInN/GaN DBR, the simulated S-HCG structure can provide a doubled stopband width while the total thickness is only about 40% of a 40 pair AlInN/GaN DBR.

As shown in Figure 4C, the thickness of an S-HCG stmcture could be further reduced to 800 nm, which is an extremely thin structure and would provide excellent thermal conductivity allowing for higher driving current and high output power. According to the RCWA simulation, such a stmcture can provide a stopband of 10 nm with a reflectance >99.5% when the thickness of grating layers is: 120 nm, 249 nm, 150 nm; the thickness of separation layers is: 82 nm, 87 nm; the period is: 208 nm; and the filling factor (FF) is 0.61.

Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of skill in the art to which the disclosed invention belongs. Publications cited herein and the materials for which they are cited are specifically incorporated by reference.

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention. Such equivalents are intended to be encompassed by the following claims.