CROSS-REFERENCE TO RELATED U.S. PATENT APPLICATION

[0001] The present application claims priority to, and the benefit of, U.S. Patent Application Serial No. 63/356,445, entitled “REDUCTION OF SURFACE OXIDATION IN MOLECULAR BEAM EPITAXY SOURCES” by Jonathan McCandless, which was filed on June 28, 2022, the entirety of which is incorporated herein by reference.

FIELD OF THE DISCLOSURE

[0002] The present disclosure relates generally to molecular beam epitaxy (MBE) and, more particularly, to technologies for reducing surface oxidation in MBE sources.

BACKGROUND

[0003] Molecular beam epitaxy (MBE), such as plasma-assisted MBE, is a thin-film deposition technique for epitaxial growth or deposition of crystal films to form various devices, including semiconductor devices such as metal-oxide semiconductor field-effect transitions (MOSFETs). Molecular beam epitaxy is performed in a tightly controlled environment. For example, the air quality, temperature, and pressure of an MBE system are monitored and precisely controlled to produce the desired results.

[0004] A typical MBE system includes a growth chamber in which a substrate on which the epitaxial film is to be deposited is placed. During use, the substrate is often heated and the growth chamber is brought to an ultra-high vacuum (UHV). A typical MBE system also includes multiple effusion cells in which a source materials are placed. Depending on the desired film to be despotized, different source materials may be used in different effusion cells. The effusion cells may vary in design, but typically include a conical crucible located in a housing, which is heated to cause a beam of molecules to exit the effusion cell. Oftentimes, the molecular beam from the effusion cells are directed toward the substrate. The ejected molecules slowly build up on the substrate to form very thin layers.

[0005] Monoclinic beta-gallium (III) oxide ( LGaiOs) has gained much interest recently due to its large bandgap (Eg ~ 4.7 eV), the availability of large-area substrates, and the ability to increase the conductivity through doping. Plasma-assisted molecular beam epitaxy has recently been explored as a technique for growing films on substrates, with Silicon (Si),

Germanium (Ge), and Tin (Sn) used as possible source materials.

[0006] However, achieving low carrier densities can be challenging due to oxidation of the source material in the typically oxygen-rich MBE environment, especially when a Si or Ge source material is used. For example, it has been observed that Si doping of /-GazOa is relatively uncontrolled and unresponsive to source material temperature change, which may be due to “active” and/or “passive” oxidation. “Active” oxidation refers to a Si surface being responsive when exposed to O, with the Si layer of the surface being etched through the formation of the volatile, sub-oxide SiO. Conversely, in “passive” oxidation etching does not occur due to the surface being passivated by the formation of SiO2. The “active” and/or “passive” oxidation may occur when O is present in the MBE growth chamber, depending on the particular growth conditions therein. Under active oxidation, the volatile, sub-oxide SiO may be formed on the Si surface. SiO is then desorbed from the Si surface and incorporates into the film. Under passive oxidation the SiO is further oxidized to SiO2 at which point doping ceases. The delta-like doping behavior may be due to instability of the active oxidation process and, after prolonged periods of time, the SiO fully oxidizes to SiO2, which results in the Si profile in the film reverting back to the background level.

SUMMARY

[0007] According to an aspect of the present disclosure, an effusion cell of a molecular beam epitaxy system may include a conical crucible and an endplate. The conical crucible may be configured to receive a source material. The endplate may be configured to be inserted through an opening of the conical crucible to a location below the opening and above the source material. Additionally, the endplate may include a plurality of apertures defined therethrough.

[0008] In some embodiments, the endplate may include a circular top profile. Additionally, in some embodiments, the plurality of apertures may include 10 or more, 100 or more, or at least 350 apertures. Each aperture of the plurality of apertures may have a diameter that is less than a thickness of the endplate. Additionally, in some embodiments, each aperture of the plurality of apertures may be angled relative to a top surface of the endplate. The plurality of apertures may or may not be uniformly distributed across the endplate. For example, the plurality of apertures may have a density gradient in at least one direction across the endplate. [0009] In some embodiments, the apertures may be arranged in rows and columns on the endplate. Additionally, in some embodiments, each aperture of the plurality of apertures may a circular cross-section, a rectangular cross-section, or a square cross-section. Additionally, in some embodiments, each aperture of the plurality of apertures may have a linear top-profile, a curved top-profile, or a complex top-profile.

[0010] Additionally, in some embodiments, the endplate may have a diameter that determines the location of the endplate within the conical crucible when the endplate is inserted within the conical crucible. Additionally, the effusion cell may include a retainer ring in some embodiments. In such embodiments, the retainer ring may have a diameter greater than a diameter of the endplate. The retainer ring may be configured to be inserted through the opening of the conical crucible to a location below the endplate. In such embodiments, the endplate may contact the retainer ring when the retainer ring and the endplate are inserted into the conical crucible. Additionally, the retainer ring may include a single aperture defined therethrough and the plurality of apertures of the end ring may be located over the single aperture of the retainer ring when the retainer ring and the endplate are positioned into the conical crucible.

[0011] In some embodiments, the endplate may be embodied as a first endplate, and the effusion cell may include a second endplate configured to be inserted through the opening of the conical crucible. The second endplate may have a diameter different from the first endplate such that the first endplate and the second endplate are positioned in different locations within the conical crucible. Additionally, the second endplate may include a set of apertures arranged in a pattern different from a pattern of the set of apertures of the first endplate. Additionally, in some embodiments, the effusion cell may further include a pump configured to control an Oxygen partial pressure of the conical crucible.

[0012] Additionally, in some embodiments, the conical crucible may include an inner wall that defines a first compartment to receive a first source material and a second compartment to receive a second source material. In such embodiments, the endplate may be configured to contact the inner wall when the endplate is inserted into the conical crucible. Additionally, in such embodiments, the plurality of apertures of the endplate may include a first set of apertures positioned over the first compartment and a second set of apertures positioned over the second compartment. The first set of apertures may have a different shape and/or size from the second set of apertures. Additionally or alternatively, in some embodiments, the first set of apertures may have a different density gradient from the second set of apertures. [0013] According to an aspect of the present disclosure, an endplate of an effusion cell of a molecular beam epitaxy system may include a circular body and a plurality of apertures defined through the circular body. The circular body may have a top surface and a bottom surface opposite the top surface. Each of the plurality of apertures may extend from the top surfaced to the bottom surface. The circular body may a diameter sized to be received through an opening of a conical crucible of the effusion cell. The diameter of the circular body may determine the location of the endplate within the conical crucible when the endplate is inserted within the conical crucible.

[0014] In some embodiments, the endplate may include a circular top profile. Additionally, in some embodiments, the plurality of apertures may include 10 or more, 100 or more, or at least 350 apertures. Each aperture of the plurality of apertures may have a diameter that is less than a thickness of the endplate. Additionally, in some embodiments, each aperture of the plurality of apertures may be angled relative to a top surface of the endplate. The plurality of apertures may or may not be uniformly distributed across the endplate. For example, the plurality of apertures may have a density gradient in at least one direction across the endplate.

[0015] In some embodiments, the apertures may be arranged in rows and columns on the endplate. Additionally, in some embodiments, each aperture of the plurality of apertures may a circular cross-section, a rectangular cross-section, or a square cross-section. Additionally, in some embodiments, each aperture of the plurality of apertures may have a linear top-profile, a curved top-profile, or a complex top-profile.

[0016] According to a further aspect of the present disclosure, a method of operating a molecular beam epitaxy system may include positioning a source material trough an opening of a conical crucible an effusion cell of the molecular beam epitaxy system to position the source material into the conical crucible, inserting an endplate through the opening of the conical crucible and positioning the endplate within the conical crucible below the opening and above the source material, and operating the molecular beam epitaxy system to form a positive pressure in the effusion cell using the endplate. The endplate may include a plurality of apertures defined throughout.

[0017] In some embodiments, positioning the source material may include positioning a silicon source material into the conical crucible. Additionally, in some embodiments, operating the molecular beam epitaxy system may include creating a pressure difference between a volume of the conical crucible defined by the endplate and a chamber of the molecular beam epitaxy system. In some embodiments, creating the pressure difference between a volume of the conical crucible defined by the endplate and a chamber of the molecular beam epitaxy system may include reducing the Oxygen partial pressure within the volume of the conical crucible defined by the endplate.

[0018] Additionally, in some embodiments, positioning the endplate into the conical crucible may include positioning an endplate including 10 or more, 100 or more, or at least 350 apertures. In some embodiments, the apertures may be arranged in rows and columns on the endplate. Additionally, in some embodiments, each aperture of the plurality of apertures may a circular cross-section, a rectangular cross-section, or a square cross-section. Additionally, in some embodiments, each aperture of the plurality of apertures may have a linear top-profile, a curved top-profile, or a complex top-profile.

[0019] Tn some embodiments, each aperture of the plurality of apertures may have a diameter that is less than a thickness of the endplate. Additionally, in some embodiments, each aperture of the plurality of apertures may be angled relative to a top surface of the endplate. The plurality of apertures may or may not be uniformly distributed across the endplate. For example, the plurality of apertures may have a density gradient in at least one direction across the endplate. Additionally, in some embodiments, the endplate may have a diameter that determines the location of the endplate within the conical crucible when the endplate is inserted within the conical crucible.

[0020] Additional features of the present disclosure will become apparent to those skilled in the art upon consideration of illustrative embodiments exemplifying the best mode of carrying out the disclosure as presently perceived.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The concepts described herein are illustrated by way of example and not by way of limitation in the accompanying figures. For simplicity and clarity of illustration, elements illustrated in the figures are not necessarily drawn to scale. Where considered appropriate, reference labels have been repeated among the figures to indicate corresponding or analogous elements.

[0022] FIG. 1 is simplified diagram of an embodiment of a molecular beam epitaxy (MBE) system that includes at least one effusion cell having a conical crucible and an endplate positioned in the conical crucible; [0023] FIG. 2 is a perspective view of an embodiment of a conical crucible of the MBE system of FIG. 1 having the endplate being inserted through an opening of the conical crucible;

[0024] FIG. 3 is a perspective view of the conical crucible of FIG. 2 having the endplate seated within the conical crucible below the opening;

[0025] FIG. 4 is a cross-sectional view of the conical crucible of FIG. 3 taken generally along the section line 4-4 of FIG. 3 and showing the endplate positioned within the conical crucible above a source material;

[0026] FIG. 5 is a plan view of an embodiment of the endplate of FIG. 3 showing a set of circular apertures defined therethrough;

[0027] FIG. 6 is a partial cross-section view of the endplate of FIG. 3 taken generally along the section 6-6 of FIG. 5;

[0028] FIG. 7 is an elevation view of an embodiment of the endplate of FIG. 5 having a curved top side;

[0029] FIG. 8 is an elevation view of another embodiment of the endplate of FIG. 5 having curved top and bottom sides;

[0030] FIG. 9 is a partial elevation view of an embodiment of the endplate of FIG. 5 having a flat planar sidewall;

[0031] FIG. 10 is a partial elevation view of an embodiment of the endplate of FIG. 5 having an angled sidewall;

[0032] FIG. 11 is a partial cross-sectional view of an embodiment of the endplate of FIG.

5 showing a vertical aperture relative to a top side of the endplate;

[0033] FIG. 12 is a partial cross-sectional of another embodiment of the endplate of FIG.

5 showing an angled aperture relative to a top side of the endplate;

[0034] FIG. 13 is a plan view of another embodiment of the endplate of FIG. 5 having set of apertures defined therethrough and positioned to define a density gradient of the apertures;

[0035] FIG. 14 is a plan view of another embodiment of the endplate of FIG. 5 having set of apertures defined therethrough with non-uniform density gradients;

[0036] FIG. 15 is a plan view of another embodiment of the endplate of FIG. 5 having set of square apertures defined therethrough;

[0037] FIG. 16 is a plan view of another embodiment of the endplate of FIG. 5 having set of rectangular apertures defined therethrough; [0038] FIG. 17 is a plan view of another embodiment of the endplate of FIG. 5 having set of curved or non-linear apertures defined therethrough;

[0039] FIG. 18 is a plan view of another embodiment of the endplate of FIG. 5 having set of differently shaped apertures defined therethrough;

[0040] FIG. 19 is another cross-sectional view of another embodiment of the conical crucible of FIG. 3 taken generally along the section line 4-4 of FIG. 3 and showing an endplate having a diameter that determines a reference depth at which the endplate is seated within the conical crucible;

[0041] FIG. 20 is a cross-sectional view of the conical crucible of FIG. 19 showing a different endplate having a different diameter that determines a deferent reference depth at which the endplate is seated within the conical crucible relative to the endplate of FIG. 19;

[0042] FIG. 21 is another cross-sectional view of another embodiment of the conical crucible of FIG. 3 taken generally along the section line 4-4 of FIG. 3 and showing two endplates positioned within the conical crucible with each endplate having a different diameter that determines different reference depths at which the corresponding endplate is seated within the conical crucible;

[0043] FIG. 22 a cross-sectional view of another embodiment of the conical crucible of FIG. 3 that includes an inner wall defining a two compartments configured to received different source material;

[0044] FIG. 23 is another cross-sectional view of another embodiment of the conical crucible of FIG. 3 taken generally along the section line 4-4 of FIG. 3 and showing an endplate assembly including an endplate retainer ring and an endplate having a smaller diameter than the endplate retainer ring;

[0045] FIG. 24 is a plan view of the endplate retainer ring of the conical crucible of FIG. 23;

[0046] FIG. 25 is a plan view of the endplate of the conical crucible of FIG. 23 ;

[0047] FIG. 26 is a simplified diagram of another embodiment of an effusion cell having a conical crucible including an endplate located therein and a pump configured to control the Oxygen (O) partial pressure of the effusion cell; and

[0048] FIG. 27 is a simplified flow diagram of a method for operating a molecular beam epitaxy (MBE) system having an effusion cell including a conical crucible and an endplate configured to be positioned into the conical crucible at a desired position. [0049] FIG. 28 is a graph illustrating secondary ion mass spectrometry (SIMS) measured results of Si-doped layers grown on a Ga2C>3 substrate sample using a typical MBE system;

[0050] FIG. 29 is a graph illustrating SIMS measured results of Si-doped layers grown on a Ga2O3 substrate sample using the system and techniques described herein with stepped temperature increases;

[0051] FIG. 30 is a graph illustrating SIMS measured results of Si-doped layers grown on a Ga2O3 substrate sample using the system and techniques described herein with stepped temperature decreases;

[0052] FIG. 31 is a graph illustrating the SIMS measured average doping concentration as a function of 1000/T _si for each of the doped substrate samples of FIGS. 28, 29, and 30;

[0053] FIG. 32 is a graph illustrating resistivity vs. effusion cell temperature measured results of a substrate sample grown using a typical MBE system;

[0054] FIG. 33 is a graph illustrating resistivity vs. effusion cell temperature measured results of a substrate sample grown using the system and techniques described herein;

[0055] FIG. 34 is a table of growth conditions for three test substrate samples, along with associated Hall effect data and other relevant data;

[0056] FIG. 35 is a graph illustrating the carrier density vs. temperature measured results for three substrate samples grown using the system and techniques described herein;

[0057] FIG. 36 is a graph illustrating SIMS measured results of the carrier density of the three test substrate samples of FIG. 34;

[0058] FIG. 36 is a graph illustrating the carrier density vs. inverse temperature from which activation energy can be extracted for each of the three test substrate samples of FIG. 34;

[0059] FIG. 37 is a graph of the extracted activation energies for each of the three test substrate samples of FIG. 34 with indicia for reported values of other techniques; and

[0060] FIG. 38 is a graph illustrating measured mobility vs temperature of the three test substrate samples of FIG. 34.

DETAILED DESCRIPTION OF THE DRAWINGS

[0061] While the concepts of the present disclosure are susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will be described herein in detail. It should be understood, however, that there is no intent to limit the concepts of the present disclosure to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives consistent with the present disclosure and the appended claims.

[0062] References in the specification to “one embodiment,” “an embodiment,” “an illustrative embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may or may not necessarily include that particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. Additionally, it should be appreciated that items included in a list in the form of “at least one A, B, and C” can mean (A); (B); (C): (A and B); (B and C); or (A, B, and C). Similarly, items listed in the form of “at least one of A, B, or C” can mean (A); (B); (C): (A and B); (B and C); or (A, B, and C).

[0063] In the drawings, some structural or method features may be shown in specific arrangements and/or orderings. However, it should be appreciated that such specific arrangements and/or orderings may not be required. Rather, in some embodiments, such features may be arranged in a different manner and/or order than shown in the illustrative figures. Additionally, the inclusion of a structural or method feature in a particular figure is not meant to imply that such feature is required in all embodiments and, in some embodiments, may not be included or may be combined with other features.

[0064] Referring now to FIG. 1, an illustrative molecular beam epitaxy (MBE) system 100 includes an MBE growth chamber 102, including a sample mount 104, and one or more effusions cells 110. Additionally, although not shown in FIG. 1, the MBE system 100 may include additional or other devices found in typical MBE systems, such as various measuring devices (e.g., a reflection high-energy electron diffraction (RHEED) “gun, a beam flux monitor, etc.) and other structures and subsystems (e.g., cryo panels, controllers, motors, and heating systems). Those additional devices and structures are not illustrated in FIG. 1 for clarity of the description, but may be included in functional systems.

[0065] Each of the effusion cells 110 includes a corresponding conical crucible 200 having an endplate 202 positioned therein as discussed in more detail below. As indicated in FIG. 1, the effusion cells 110 are generally directed toward the sample mount 104. In use, a sample substrate may be positioned on the sample mount 104, and each of the effusion cells may be controlled (e.g., heated) to produce a flux beam of material, which deposits on the sample substrate located on the sample mount 104 over a growth period.

[0066] As discussed in more detail below, by positioning the endplate 202 within the conical curable 200 (i.e., below an opening of the conical crucible 200), a positive pressure may be established within the conical crucible 200, relative to the MBE growth chamber 102. The pressure difference between the conical crucible 200, wherein the source material is located, and the MBE growth chamber 102 reduces the O partial pressure experienced within the conical crucible 200 by the source material (e.g., Si material). The reduction of the O partial pressure within the conical crucible 200 allows a larger temporal window over which the MBE system 100 can operate while reducing or otherwise avoiding SiO: formation, and lowers the minimum effusion cell temperature, Tsi, under which active oxidation occurs. As such, the use of the endplate 202 within the conical crucible 200 allows for control of donor density in the growth layers of the substrate sample.

[0067] Referring now to FIGS. 2 and 3, as discussed above, each of the effusion cells 110 includes a conical crucible 200 having an endplate 202 received therein. That is, the endplate 202 is sized and shaped to be received through an opening 204 of the conical crucible 200 as indicated in FIG. 2 and seated or otherwise positioned in a location below the opening 204 as shown in FIG. 3. In the illustrative embodiment, the endplate 202 is formed from pyrolytic boron nitride (PBN) and is shaped as a circular disk having a diameter 212 that is less than a diameter 210 of the opening 204 of the conical crucible 200.

[0068] Referring now to FIGS. 4 and 5, the diameter 212 of the endplate 202 is configured to define or otherwise determine the placement (i.e., depth) of the endplate 202 inside the conical crucible 200. That is, due to the conical shape of the crucible 200 and the diameter of the endplate 202, the endplate 202 is configured to “drop” or otherwise be received into the conical crucible 200 to a reference height 400. In use, a source material 402 (e.g., Si) is positioned within conical crucible 200 and, as such, the endplate 202 is positioned above the source material 402 and below the opening 204 of the conical crucible 200. The endplate 202 may be held in place via gravitational forces or other via corresponding securing mechanisms.

[0069] As shown in FIG. 5, the illustrative endplate 202 includes a set of apertures or holes 500 defined therethrough. The set of apertures 500 may include any number of suitable apertures greater than one, such as two or more, ten or more, 100 or more, or a 1,000 or more apertures. The particular number of apertures incorporated into the endplate 202 may depend on one or more criteria such as, for example, the desired partial pressure experienced by the source material 402, the source material used, the semiconductor material, the growth settings of the MBE system, the desired crystalline structure, and/or other criteria.

[0070] In the illustrative embodiment of FIG. 5, the each of the apertures 500 is embodied as a circular aperture having a circular top-profile; however, aperture 500 have other shapes may be used in other embodiments as discussed in more detail below in regard to FIGS. 13-16. In the illustrative embodiment, the size of each of the top profile opening of each of the apertures is significantly less than the thickness of the endplate 202. For example, as shown in FIG. 6, each of the apertures of the endplate 202 of FIG. 5 has a diameter 600 that is less than a thickness 602 of the endplate 202. In an illustrative embodiment, the endplate 202 includes 350 apertures having a diameter of about 0.203 mm.

[0071] As shown in FIG. 6, the endplate 202 has a body 610 having a top surface 612 and a bottom surface 614, opposite the top surface 612. In the illustrative embodiment of FIG. 6, each of the top surface 612 and the bottom surface 614 is substantially planar or flat. However, in other embodiments, one or both of the surfaces 612, 614 may be curved or otherwise non-planar. For example, as shown in FIG. 7, the top surface 612 may be curved in a concave direction and the bottom surface may be planar. Alternatively, in other embodiments as shown in FIG. 8, both of the top and bottom surfaces 612, 614 may be curved in the concave direction. In other embodiments, however, one or both of the surfaces 612, 614 may be curved or otherwise non-planar. For example, in some embodiments, one or both of the surfaces 612, 615 may be convex.

[0072] Referring now to FIG. 9, the endplate 202 includes a sidewall 900 extending from the bottom surface 614 to the top surface 612 of the body 610. In the illustrative embodiment of FIG. 9, the sidewall 900 is substantially vertical. That is, the sidewall 900 extends approximately 90 degrees relative to the top and bottom surfaces 612, 614. However, in other embodiments, the endplate 202 may include an angled sidewall 1000 as shown in FIG. 10. That is, the sidewall 1000 may be angled relative to the top and bottom surfaces 612, 615. By angling the sidewall 1000, the endplate 202 may better contact and seat with the angled inner walls of the conical crucible 200. As such, in some embodiments, the angle of the side wall 1000 may match or otherwise be similar to the angle of the inner walls of the conical crucible 200.

[0073] As shown in FIG. 11 , in some embodiments, the apertures 500 are vertically defined through the body 610 of the endplate 202. That is, the aperture 500 may extend through the endplate 202 at a 90 degree angle relative to the top and bottom surfaces 612, 614. Alternatively, in other embodiments as shown in FIG. 12, the apertures 500 may be defined through the endplate 202 at an angle relative to the top and bottom surfaces 612, 614. That is, the apertures 500 may extend through the endplate 202 at an angle, relative to the surfaces 612, 614, different from a 90 degree angle. It should be appreciated that by angling the apertures 500 of the endplate 202, relative to the top and bottom surfaces 612, 614, the apertures 500 may define a direction of the flux beam of the effusion cell 110. For example, each aperture 500 may be oriented such that the aperture points toward a chamber wall of the MBE growth chamber 102, rather than directly at the sample substrate held by the sample mount 104.

[0074] It should be appreciated that by changing the distribution and the angle of the apertures 500, the flux beam of the effusion cell 110 can be effectively steered. Additionally, as discussed above, by changing the diameter and overall number of apertures 500, the partial pressure experienced by the source material 402 can be controlled. For example, in some embodiments as shown in FIG. 5, the endplate 202 may include a set of aperture 500 that are uniformly distributed across the endplate 202. For example, as shown, the apertures 500 may be arranged in a series of rows and columns or other uniform distribution pattern. Alternatively, as shown in FIG. 13, the density of the set of apertures 500 may be graded or varied in one or more directions in order to engineer a desired flux profile. In other embodiments, as shown in FIG. 14, the set of apertures 500 may have areas of localized increased density than other areas of the endplate 202. Additionally, as discussed above, each aperture 500 may be defined through the endplate 202 at an angle relative to a top and bottoms surfaces 612, 614 of the endplate 202 to further steer the flux.

[0075] As discussed above, in the illustrative embodiment of FIG. 5, each of the apertures 500 has a generally circular top profile such that the corresponding apertures form a cylinder through the endplate 202. However, in other embodiments, the particular shape of each aperture 500 may be varied to further adjust the flux. For example, as shown in FIG. 15, each aperture 500 may have a square top profile. Alternatively, as shown in FIG. 16, each aperture 500 may have a rectangular top profile. Further still, in some embodiments, the apertures 500 may have more complex top profiles. For example, as shown in FIG. 17, the apertures 500 may have a curved top profile. Additionally, in some embodiments, each aperture 500 may have the same or a different top profile. For example, as shown in FIG. 18, the endplate 202 may include apertures 500 having circular, square, and rectangular top profiles (e.g., cross-sectional geometries) to further control the resulting flux beam. [0076] As discussed above, by controlling the depth at which the endplate 202 is positioned within the conical crucible 200, the partial pressure developed within the conical crucible 200, relative to the MBE growth chamber 102, can be controlled. As such, it should be appreciated that the position of the endplate 202 within the conical crucible 200 (i.e., the depth at which the endplate 202 is seated or otherwise positioned below the opening 204 of the conical crucible 200) can be adjusted by using endplates 202 of different diameters. For example, as comparatively show in FIGS. 19 and 20, the endplate 202 of FIG. 19 has a larger diameter than the endplate 202 of FIG. 20. As such, the endplate 202 of FIG. 19 is positioned at a distance 1900 above the source material 402 that is greater than a distance 2000 above the source material 402 at which the endplate 202 of FIG. 20 is positioned. As such, by selecting an endplate 202 having a correct diameter, the position within the conical crucible 200 of the endplate 202, and thereby the developed partial pressure of the conical crucible 200, may be controlled.

[0077] Referring now to FIG. 21, in some embodiments, two endplates 202 may be used. In such embodiments, the two endplates 202 each have different diameters such that the each endplate 202 is located at a different position within the conical crucible 200 relative to the other endplate 202 (i.e., at difference distances above the source material 402. Additionally, each endplate 202 may have apertures 500 that are different from one another (e.g., different top profiles or gradients). Although the conical crucible 200 of FIG. 21 is shown as including only two endplates 202, it should be appreciated that additional endplates 202 may be used in other embodiments, each having a different diameter and similar or different apertures 500 defined therethrough.

[0078] Additionally or alternatively, as shown in FIG. 22, the conical crucible 200 may be partitioned in some embodiments. In such embodiments, the conical crucible 200 may include an inner wall 2200 separating the crucible 200 into two or more compartments 2212, 2214. Different source materials 412, 414 may be placed into each compartment 2212, 2214 defined by the inner wall 2200. Additionally, in some embodiments, the endplate 202 may have different configurations relative to each compartment 2212, 2214 of the conical crucible 200. For example, the shape and arrangement of the set of apertures 500 of the endplate 202 may be different for each corresponding compartment 2212, 2214 of the conical crucible 200. One application for such a design would be in those situations in which two source materials 412, 414 have different vapor pressures. At the same temperature, each material 412, 414 would produce a different flux due to the vapor pressure differences. However, by designing the endplate 202 differently on each side covering the corresponding compartment 2212, 2214, a desired flux from each source material 412, 414 can be achieved even though the two source materials 412, 414 are at the same temperature.

[0079] Referring now to FIGS. 23-25, in some embodiments, a retainer ring 2400 may be used to position and hold the endplate 202. In such embodiments, an endplate 202 having too small a diameter for a desired position within the conical crucible 200 may still be used with use of the retainer ring 2400. For example, as shown in FIG. 24, the retainer ring 2400 may have a diameter 2402 that defines the location within the conical crucible 200 at which the retainer ring 2400 will be positioned as discussed above. As shown in FIG. 25, the endplate 202 may have a diameter 2502 that is smaller than the diameter 2402 of the retainer ring 2400 and, as such, cannot be located at the desired position within the conical crucible 200. However, by use of the retainer ring 2400, the endplate 202 can be positioned at the desired location, which is determined based on the diameter 2402 of the retainer ring 2400. It should be appreciated that the retainer ring 2400 includes an inner opening 2410 having an inner diameter 2412 to allow the flow of the flux beam therethrough. The diameter 2502 of the endplate 202 is greater than the inner diameter 2412 of the retainer ring 2400 such that the retainer ring 2400 can support the endplate 202 within the conical crucible 200. As such, a single endplate 202 may be used at various positions within the conical crucible 200 by using different retainer rings 2400 (i.e., retainer rings 2400 having different diameters 2402). In use, it is ensured that any gaps between the retainer ring 2400 and the conical crucible 200 or between the endplate 202 and the retainer ring 2400 are limited or non-existent. The limiting of any such gaps ensures that the flux goes through the set of apertures 500 of the endplate 202.

[0080] It should be appreciated that in some embodiments, the effusion cell 110 may include additional structures and devices to improve the overall operation of the conical crucible 200. For example, as shown in FIG. 26, the effusion cell 1 10 may include a pump 2600 configured to further control the O partial pressure developed in the conical crucible 200 and experienced by the sample material 402.

[0081] Referring now to FIG. 27, a method 2700 of operating the molecular beam epitaxy system 100 begins with block 2702 in which a source material 402 is positioned in the conical crucible 200 of the effusion cell 110. In embodiments in which the conical crucible 200 includes multiple compartments, such as the embodiment illustrated in FIG. 22, a different source material 402 may be positioned into each corresponding chamber 2212, 2214 in block 2704. [0082] Subsequently, in block 2706, the endplate 202 is positioned into the conical crucible 200 above the source material 402. To do so, in block 2708, the endplate 202 is inserted into the opening 204 of the conical crucible 200 and positioned below the opening 204 and above the source material 402 in block 2708. In embodiments in which multiple endplates 202 are used such as the embodiment of FIG. 21, the additional endplates 202 may be positioned into the conical crucible 200 by inserting those endplates 202 through the opening 204 of the conical crucible 200 to their determined positioned in block 2710. Additionally, in block 2712, in those embodiments in which a retainer ring 2400 is used (see FIG. 23), the retainer ring 2400 is initially inserted through the opening 204 and positioned in the conical crucible 200 at its determined location, followed by the endplate 202 having the smaller diameter.

[0083] After the source material 402 and the endplate(s) 202 have been positioned within the conical crucible 200 in block 2706, the MBE system 100 is operated in block 2714 to form a positive pressure within the effusion cell 110 and eject the flux beam therefrom. In doing so, a pressure differential between the chamber of the conical crucible 200 defined by the endplate 202 (i.e., the chamber in which the source material 402 is located) and the MBE growth chamber 102 is established in 2716. The epitaxy growth process can then be continued using the MBE system 100.

[0084] Referring now to FIGS. 28-39, the MBE system 100 and techniques described herein were used to produce test substrate samples, which were measured for various properties and compared, in some cases, against samples grown using typical MBE systems (i.e., MBE systems without the disclosed effusion cell 110. The graphs and charts of FIGS. 28-39 illustrate those measured results. All samples were grown on 5 x 5 mm ² or 10 x 10 mm ² , semi-insulating, bulk Ga2Os substrates in the (010) orientation. The samples were cleaned with a standard solvent process before being loaded into a Veeco GEN 930 plasma-assisted (PA) MBE system. The samples were heated to 900 °C, in-situ and under vacuum (10 ^-9 torr), for 30 minutes to desorb chemical impurities. A ~ 250 nm, unintentionally doped (UID), ?-Ga2O3 buffer layer was grown (unless stated otherwise) to spatially separate the Si doped layers from the Fe diffused from the substrate. It should be appreciated that because Fe is a deep acceptor, it compensates free carriers. Within the Si doped layer, the Fe density was found to be at the detection limit, ~10 ¹⁵ /cm ³ . The surface roughness was measured by atomic force microscopy. X-ray diffraction, aligned to the 020 peak with a symmetric 20 - co scan, revealed no additional peaks, indicating that the samples were phase-pure. [0085] The control samples, were indicated as such, were grown with a conventional Si effusion cell with a conical crucible. As indicated above, the test samples were grown using the MBE system 100 with an endplate inserted into the opening of the corresponding conical crucible as discussed above. In the testing, the endplate was embodied as pyrolytic boron nitride (PBN) endplate having 350 apertures drilled into the surface, each of diameter 0.203mm. The apertures were drilled at an angle relative to a top surface of the endplate, and the endplate was oriented such that the apertures are pointed toward the MBE chamber wall, rather than directly at the sample, in an effort to reduce the SiC) _x flux experienced by the film. Using the described MBE system, Si concentrations ranging from ~1 x 10 ¹⁷ /cm ³ to - l x 10 ²⁰ /cm ³ were achieved in />- Ga?O3 as the effusion cell temperature, Tsi,was varied from 890 °C to 1100 1100 °C.

[0086] Referring to FIG. 28, a graph 2800 illustrates secondary ion mass spectrometry (SIMS) measured results of Si-doped layers grown on a Ga2O3 substrate sample using a typical MBE system (i.e., without an endplate) with a Ga flux of 1. 1 atoms/cm ² »s and an O flux was 2.0 atoms/cm ^2, s. As shown in FIG. 2800 by comparison of the results at 1050 °C and 1000 °C, a change from 1.5 to 1.2xl0 ² °/cm ³ was measured. At 950 °C, a lower concentration (~4 x 10 ¹⁹ /cm ³ ) was measured, but as peak rather than a plateau. Below an effusion cell temperature, Tsi, of 950°C, intentional doping was indistinguishable from the background. Such result may be due to passive and active oxidation. For example, below 7s = 950°C, SiOz formation on the Si source surface (i.e. passive oxidation) may hinder the desorption and incorporation of the dopant. As Tsi heats to 950°C, the source undergoes a disproportionation reaction, which may results in SiO on the surface. The SiO , if present, may then be desorbed and incorporated into the -GazOg film.

[0087] Referring now to FIG. 29, a graph 2900 illustrates SIMS measured results of Si- doped layers grown on a GazCh substrate sample using an MBE system including the features described herein, such as MBE system 100. For that substrate sample, the substrate growth temperature was T _su b ~ 515 °C, which was measured using a pyrometer pointed at the substrate. The Oz flow rate for the samples with results illustrated in FIGS. 29 and 30 (and the samples 1 and 3 of FIGS. 34-37 and 38) was 1.4 seem, which resulted in in an MBE chamber pressure of 2.5-3.2 x 10“ ⁵ torr. A ~ 150 nm undoped buffer layer was grown, followed by alternating 1 hour layers of UID and intentionally doped GazOs layers. The effusion cell temperature, Tsi, was increased from 700 °C to 900 °C in 100 °C steps. The SIMS measured results indicated the Si concentration was at the background level while the effusion cell was heated from 700 °C to 900 °C. However, as Tsi was increased from 900 °C to 1000 °C with the shutter closed, the Si concentration significantly increased, with a shoulder at 2 X 10 ¹⁷ /cm ³ . At Tsi = 1000 and 1100 °C, clear steps of Si concentration were indicated as the concertation increased from 10 ¹⁹ cm ^-3 to IO ²⁰ cm ^-3 .

[0088] As shown in FIG. 30, a graph 3000 illustrates the SIMS measured results of another Ga2C>3 substrate sample grown using an MBE system including the features described herein. For that substrate sample, a 0.5 hour UID buffer layer was grown, followed by alternating layers of doped and UID Ga2O3. To clearly demarcate the doping density in the film when the source shutter is closed, the doped layers were grown for 1 hour while the UID layers were grown for 1.5 hour. Additionally, the effusion cell temperature, Tsi, was stepped down from 950°C to 875 °C in 25 °C steps. Based on a comparison of the results of graph 3000 to the graph 2900 of FIG. 29, a Si doping plateau at 900°C is indicated in the substrate sample of FIG. 29. That plateau may be due to the passive oxidation of the Si in the substrate sample of graph 2900. For the substrate sample of graph 2900, the O plasma source was ignited when the Si effusion cell was at 700°C. Additionally, the source material was exposed to active O for 7.2 hours during the growth of the buffer layer and the alternating UID/doped layers until finally the effusion cell was heated from 900°C to 1000°C. Before that temperature ramp of 7si, SiO2 was formed on the Si surface, resulting in Si being at the background level within the film. When 7si was finally heated up from 900°C, the source material underwent a disproportionation reaction, changing from passive to active oxidation. Conversely, in the substrate sample of graph 30, the source material was already in the active oxidation state, which enabled doping at 900°C. It should be appreciated that when the O plasma is struck, even with the effusion cell at 890°C, SIMS measured results indicate uniform doping densities after 5.5 hours of active O exposure. That result may indicate that the condition for the Si source to enter the passive oxidation state, is more dependent on the 7si when the O plasma is struck than on the total time the source is exposed to the active O.

[0089] Referring now to FIG. 31, a graph 3100 illustrates the SIMS measured average doping concentration of each substrate sample of graphs 2800, 2900, and 3000 as a function of a function of 1000/T _S i, with the control sample substrate of graph 2800 (FIG. 28) indicated by triangles and the test sample substrates of graph 2900 (FIG. 29) and graph 3000 (FIG. 30) indicated by squares. The concentration results indicated by diamonds is the average doping concertation of the substrate sample of graph 3000 measured with the effusion cell shutter closed. As indicated by graph 3100, as the effusion cell temperature, Tsi, was changed activation energy of 3.6 eV was measured for those test samples produced using the MBE system including the features described herein, which is greater than the control sample produced using a typical MBE system.

[0090] It should be noted that the Si activation energy of the substrate sample of graph 3000 measured with the effusion cell shutter closed is based on the Tsi = 875 °C, 900°C, and 925°C data presented in graph 3000. When the effusion cell shutter is closed for other temperatures in the measured results, the Si concertation profile is not constant but rather achieves a minimum at some point. That results may be due to the Si diffusion from the doped region into the undoped region. As such, the measured values may not accurately represent the true UID Si concatenation.

[0091] With consideration of the above measurement limitation, it should still be appreciated that the background doping obtained when the shutter is closed may be acceptable if the desired doping densities are relatively low, such as 10 ¹⁸ /cm ³ and 10 ¹⁷ /cm ³ . Such targets may result in mid-to-low background densities, such as 10 ¹⁶ /cm ³ . If, however, higher doping densities are desired, the background doping level may also rise. For example, for intentional doping densities in the mid-10 ¹⁹ /cm ³ , the background density could be around lxl0 ¹⁷ /cm ³ . Such an increase in background doping may be due to SiO leaking from around the shutter, or such results could be due to the SiO _x desorption from the MBE chamber wall. As such, should higher doping densities be desired, a growth interrupt while the effusion cell temperature is changed may be beneficial.

[0092] Referring now to FIGS. 32 and 33, uniformly-doped control (FIG. 32) and test sample substrates (FIG. 33) were grown and Hall effect measurements were performed to evaluate the transport properties of Si-doped P-GaaOa. It should be appreciated that the critical Mott donor density (n _c ) above which a semiconductor exhibits metallic conductivity is n, = 0.26 ao" ³ , where ao = 1.93 nm is the effective Bohr radius, and for GtvOi is n _c ~ 2.4x 10 ¹⁸ cm“ ³ . The Hall effect measurements were performed in a Lakeshore Hall system with a 1 T magnet, with the temperature varied from 300 K down to 20 K.

[0093] Tn the graph 3200 of FIG. 32, the sample substrate grown using a typical MBE system produced non-consistent doping, with several produced samples being too resistive to measure. Conversely, the test sample grown using an MBE system including the features described herein had increase in conductivity as the effusion cell temperature, Tsi, was increased. As indicated in graph 3300 of FIG. 33, the resistivity changes by 5 orders of magnitude, from 149 Q-cm at Tsi = 865 °C to 2.67 x 10“ ³ Q-cm at Tsi = 1050°C. However, it should be noted that not all test samples presented in graph 3300 were grown with the same Ga flux or T _SU b. The Ga flux was increased to increase the growth rate, and the T _SU b was adjusted to reduce the surface roughness in some test samples.

[0094] From the results presented in graph 3300 of FIG. 33, three test samples were grown with the growth conditions set forth in the table 3400 of FIG. 34. The growth rate was the samples was about 93 nm/h. However, sample 2 was grown with lower Ga and O fluxes relative to samples 1 and 3, while approximately maintaining the Ga/O ratio. The growth rate of sample 2 was about 60 nm/h.

[0095] As indicated in graph 3500 of FIG. 35, all three test samples of the table 3400 produced carrier freeze-out at low temperature, which indicates the doping density is below the Mott criterion. The SIMS measured results of the carrier density of the three test samples are shown in graph 3600 of FIG. 36, with the donor densities obtained from the SIMS measurements being provided in table 3400 of FIG. 34. As indicated in graph 3600, the SIMS-measured density profiles indicate each of the three test substrate samples achieved uniform doping, despite the effusion cell being exposed to O for 5.5 hours, as discussed above.

[0096] The graph 3700 of FIG. 37 is an Arrhenius plot illustrating the carrier density freeze-out of the three test substrate samples, with donor activation energies extracted from the carrier density freeze-out being provided in the table 3400 of FIG. 34. The graph 3800 of FIG. 38 illustrates the extracted activation energies for each of the three test substrate samples and includes indicia for reported values of other fabrication techniques (e.g., HVPE, MOCVD, EFG, LP{CVD, etc.), with the measured results from an MBE system according to the present disclosure included. The activation energies measured for the test sample substrates produced using the technologies disclosed herein ranged from 15.3 meV to 41.9 meV.

[0097] In FIG. 39, a graph 3900 presents the measured Hall effect mobilities for each of the three test sample substrates as a function of temperature. As indicated, the samples 2 and 3 displayed RT electron mobilities over 100 cm ² /V • s. The peak RT mobility was ~ 129 cmW s with a corresponding doping density of 1.07 x 10 ¹⁷ cm ^-3 . All three samples demonstrated a mobility enhancement as the temperature was decreased to 100 K due to the reduction in the polar optical phonon scattering rate. The peak mobility among the 3 samples was measured at about 390 cmA' s at a temperature of 97 K and a density of 2.44 x 10 ¹⁶ cm ^-3 . Additional control samples were grown using a typical MBE system and without Si, but with the same growth conditions used for samples 1 and 3. Hie resistivity of the contacts of those control samples was too large to measure, indicating that the technologies disclosed here can achieve the same low-carrier densities and high mobilities, as well as insulating films, with the same growth conditions.

[0098] While the disclosure has been illustrated and described in detail in the drawings and foregoing description, such an illustration and description is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments have been shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected.

[0099] There are a plurality of advantages of the present disclosure arising from the various features of the methods, apparatuses, and systems described herein. It will be noted that alternative embodiments of the methods, apparatuses, and systems of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of the methods, apparatuses, and systems that incorporate one or more of the features of the present invention and fall within the spirit and scope of the present disclosure as defined by the appended claims.