Related Applications

The present application claims priority to provisional application No. 62/485,792 filed on April 14, 2017 which is herein incorporated by reference in its entirety.

Government Support

The present application was made with United States government support under the Grant No. FA9550-14-1-0150 from the U.S. Air Force Office of Scientific Research and

National Defense and Engineering Graduate Fellowship DGE 0946799 by the Department of Defense. The United States government has certain rights in this invention.

Background

The present invention relates generally to a germanium (Ge) photodetector and its methods of fabrication, and more particularly to a Ge photodetector that employs a highly doped Ge layer for converting light energy into electrical signals.

The detection of radiation in the mid-IR range of the electromagnetic spectrum, e.g., in a wavelength range of about 2 μπι to about 10 μπι, is gaining importance for a variety of industrial and medical applications, such as imaging, sensing and surveillance. Currently, narrow band- gap semiconductors, such as, Hei _-x Cd _x Te, Pbi _-x Se _x , or Ini _-x Ga _x As, are mainly employed for mid- IR sensing. However, such semiconductors can be expensive, toxic, and chemically incompatible with standard silicon (Si) microelectronics manufacturing. These disadvantages can severely limit the use of such semiconductors in the development of potential applications and their commercial development.

Conventional Ge detectors are not capable of detecting mid-IR radiation due to the photon energy being below the Ge band-gap and intrinsic absorption edge. It has been shown possible, however, to engineer Ge to detect sub-band gap mid-infrared radiation via extrinsic, dopant-mediated photoconductivity. These extrinsic detectors were used before the development of narrow band-gap detectors, but they exhibit poor performance due to their very weak sub- band gap defect mediated absorption. Further, these detectors require bulky and expensive liquid nitrogen cooling systems to reduce dark currents and increase signal-to-noise ratio for mid-IR radiation detection.

Accordingly, there is a need for enhanced Ge mid-IR photodetectors and methods of their fabrication, e.g., to achieve room temperature, high performance operation.

Summary

In one aspect, a photodetector is disclosed, which comprises a germanium substrate having a doped layer configured for exposure to external radiation and having a thickness in a range of about 10 nm to about 1 micron. The doped layer includes at least one deep-level dopant distributed therein such that a concentration of the dopant in the doped layer is at least about 10 ¹⁶ atoms/cm ³ . The doped layer forms a diode junction with an underlying portion of the germanium substrate.

In some embodiments, the concentration of the dopant in the doped layer can be in a

16 3 20 3

range of about 10 atoms/cm to about 10 atoms/cm . In some embodiments, the

concentration of the dopant in the doped layer can be in a range of about 10 ¹⁷ atoms/cm ³ to about 10 ²⁰ atoms/cm ³ in the doped layer. In some embodiments, the concentration of the dopant in the

18 3 20 3

doped layer can be in a range of about 10 atoms/cm to about 10 atoms/cm in the doped layer. In some embodiments, the concentration of the dopant in the doped layer can be in a range of about 10 19 atoms/cm 3 to about 1020 atoms/cm 3.

In some embodiments, the photodetector can exhibit an external quantum efficiency of at least about 10 ^"5 for radiation having a wavelength in a range of about 1750 nanometers to about 3,000 nanometers. By way of example, the photodetector can exhibit an external quantum efficiency in a range of about 10 ^"5 to about 0.9, e.g., in a range of about 10 ^"3 to about 10 ^"1 . For example, the photodetector can exhibit such an external quantum efficiency for radiation having a wavelength in a range of about 2,000 nm to about 3,000 nm.

In some embodiments, the photodetector can exhibit such an external quantum efficiency upon application of a reverse bias voltage in a range of about 0 V to about -10 V thereto. Further, in some embodiments, the photodetector can exhibit such an external quantum efficiency at an operating temperature in a range of about 18 °C to about 25 °C, which can be achieved in many embodiments without any active cooling of the detector.

In some embodiments, the photodetector can exhibit a responsivity in a range of about 0.1 A/W to about 0.9 A/W for incident radiation having a wavelength in a range of about 1750 nm to about 3000 nm. By way of example, the photodetector can exhibit such a responsivity upon application of a reverse bias voltage in a range of about 0 V to about -10 V thereto.

In some embodiments, the deep-level dopant generates an energy defect level characterized by an energy level separated from any of a minimum of conduction band or a maximum of valence band of the germanium substrate by a value of at least about 0.12 eV, or at least about 0.13 eV, or at least about 0.14 eV, or at least about 0.15 eV.

In some embodiments, the dopant can be a transition metal. In some embodiments, the dopant can be any of gold (Au), boron (B), sulfur (S), selenium (Se) and tellurium (Te).

In some embodiments, the photodetector can include at least two electrical contacts disposed on portions of the germanium substrate for accessing an electrical current generated as a result of formation of electron-hole pairs in vicinity of said diode junction.

In some embodiments, an antireflective coating can cover at a least a portion of the top surface (i.e., the light incident surface) of the doped layer. In some embodiments, the antireflective coating can be any of silicon nitride or silicon oxide.

In some embodiments, the photodetector can include a passivation layer disposed on at least a portion of the top surface of the doped layer and/or surrounding outer surfaces of the device (e.g., on all surrounding surfaces of the Ge substrate). In some embodiments, the passivation layer can include any of oxisilicon nitride and germanium oxide.

In some embodiments, the dopant can exhibit a concentration gradient from a top surface of the doped layer to a depth below said surface.

In some embodiments, the top surface of the doped layer can have a roughness characterized by a root-mean-square (rms) height variation in a range of about 1 nm to about 500 nm, e.g., in a range of about 1 nm to about 100 nm, or in a range of about 10 nm to about 50 nm, or in a range of about 1 nm to about 5 nm.

In some embodiments, the doped Ge layer is a substantially crystalline layer

characterized, for example, by a crystallinity free of long-range extended defects or amorphous regions.

In some embodiments, a photodetector is disclosed, which includes a germanium substrate having a doped layer having a top surface configured for exposure to external radiation and having a thickness in a range of about 10 nm to about 1 micron, a transition metal dopant distributed in said doped layer such that an average concentration of the dopant in the doped layer is at least about 10 ¹⁶ atoms/cm ³ , wherein said doped layer forms a diode junction with an underlying portion of the germanium substrate.

In some embodiment, the transition metal dopant can be gold.

In some embodiments, such a photodetector can exhibit a quantum efficiency, e.g., an external quantum efficiency, of at least about 10 ^"5 for incident radiation having a wavelength in a range of about 1750 nm to about 3000 nm. For example, the photodetector can exhibit such a quantum efficiency for incident radiation having a wavelength in a range of about 2000 nm to about 3000 nm. In some embodiments, the photodetector can exhibit such a quantum efficiency at an operating temperature in a range of about 18 °C to about 25 °C, which can be obtained in many embodiments without active cooling of the photodetector.

In another aspect, a method of fabricating a germanium photodetector is disclosed, which includes bombarding a top surface of a germanium substrate with an ion beam comprising dopant ions so as to implant at least a portion of said dopant ions in a surface layer of the substrate at a concentration in a range of about 10 ¹⁶ to about 10 ²⁰ atoms/cm ³ , and irradiating the top surface of the substrate with a plurality of laser pulses having a pulse width in a range of about 1 ns to about 500 ns so as to anneal said doped surface layer. In many embodiments, the concentration of the dopant ions in the doped surface layer is greater than the solid equilibrium solubility limit of the dopant in germanium. In some embodiments, the ion beam can have an energy in a range of about 10 keV to about 500 keV, e.g., in a range of about 100 keV to about 300 keV.

In some embodiments, the laser pulses can have an energy in a range of about 5 mJ to about 10 J, e.g., in a range of about 100 mJ to about 5 J, or in a range of about 1 J to 3 J. Further, the fluence of the laser pulses at the top substrate surface can be, for example, in a range of about 0.3 J/cm ² to about 0.9 J/cm ² . In some embodiments, the laser pulses can have a central wavelength in a range of about 193 nm to about 1064 nm. By way of example, the central wavelength of the laser pulses can be any of 193 nm, 248 nm, 266 nm, 308 nm, 355 nm, 532 nm, or 1064 nm.

The dopant can be a deep-level dopant. Some examples of suitable dopants include, without limitation, Au, B, P, As, Sb, S, Se and Te.

In some embodiments, the above method further includes forming one or more Ohmic contacts on the top surface of the germanium substrate. For example, a photoresist can be deposited on the top surface of the germanium substrate and a mask, e.g., a copper mask, can be applied to the photoresist. The masked photoresist layer can be exposed to ultraviolet (UV) radiation. The mask can be removed and an appropriate solvent can be applied to remove the portions of the photoresist layer that were exposed to the UV radiation, thereby forming a photoresist pattern on the germanium surface. A metal can then be deposited on the patterned surface. The photoresist portions with metal deposited thereon can then be removed, e.g., via application of an appropriate solvent, to form one or more electrical contacts on the surface.

The method can further include depositing an Ohmic contact on a surface of the substrate opposed to the top surface (herein referred to as the back surface). By way of example, such an Ohmic contact can be deposited in a manner similar to deposition of the Ohmic contact on the top surface. In many embodiments, an ion implantation is performed on the back surface prior to deposition of metal for forming the Ohmic contact in order to increase the electrical conductivity of the substrate proximate to the back Ohmic contact.

In another aspect, a photodetector is disclosed, which includes a germanium substrate having a doped layer configured for exposure to external radiation and having a thickness in a range of about 10 nm to about 1 micron. The doped layer includes a deep-level dopant distributed therein such that a concentration of the dopant in the doped layer is at least about 10 ¹⁶ atoms/cm ³ , where the doped layer forms a diode junction with an underlying portion of the germanium substrate.

The dopant can be any of a donor or an acceptor. In some embodiments, the dopant can

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exhibit a concentration in a range of about 10 atoms/cm to about 10 atoms/cm in said doped layer. For example, the dopant concentration can be in a range of about 10 ¹⁷ atoms/cm ³ to about

10 20 atoms/cm 3 or in a range of about 1018 atoms/cm 3 to about 1020 atoms/cm 3 or in a range of about 10 19 atoms/cm 3 to about 1020 atoms/cm 3 in said doped layer.

In some embodiments, the dopant can be a transition metal. Some examples of suitable deep-level dopant include, without limitation, any of Au, B, P, As, Sb, S, Se and Te. In some embodiments, the photodetector exhibits an external quantum efficiency of at least about 10 ^"5 for radiation having a wavelength in a range of about 1750 nanometers to about 3,000 nanometers.

In some embodiments, the photodetector can exhibit a quantum efficiency, e.g., an external quantum efficiency, of at least about 10 ^"5 , e.g., in a range about 10 ^"5 to about 0.9, for radiation having a wavelength in a range of about 2,000 nm to about 3,000 nm. By way of example, the photodetector can exhibit such an external quantum efficiency upon application of a reverse bias voltage in a range of about 0 V to about -10 V thereto. In some embodiments, the photodetector can exhibit such an external quantum efficiency at an operating temperature in a range of about 18 °C to about 25 °C.

In some embodiments of the above photodetector, the photodetector can exhibit a responsivity in a range of about 0.1 A/W to about 0.9 A/W for incident radiation having a wavelength in a range of about 1750 nm to about 3000 nm. In some cases, the photodetector can exhibit such a responsivity upon application of a reverse bias voltage in a range of about 0 V to about -10 V thereto.

In some embodiments, the deep-level dopant can generate an energy defect level characterized by an energy level separated from any of a minimum of conduction band or a maximum of valence band of said germanium substrate by a value of at least about 0.12 eV, or at least about 0.13 eV, or at least about 0.14 eV, or at least about 0.15 eV. By way of example, the dopant can be a transition metal, such as gold.

The photodetector can further include at least two electrical contacts disposed on portions of the germanium substrate for accessing an electrical current generated as a result of formation of electron-hole pairs in vicinity of the diode junction. In some embodiments, an antireflective coating and/or a passivation layer can be disposed on a top surface of the doped layer. The passivation layer can also coat surrounding outer surfaces of the device. Some examples of suitable materials for forming an antireflective coating include silicon nitride and silicon oxide. Further, some examples of suitable materials forming a passivation layer include oxisilicon nitride and germanium oxide.

In some embodiments of the above photodetector, the dopant can exhibit a concentration gradient from a top surface of the doped layer to a depth below that surface. Further, in some embodiments, the top surface of the doped layer has a roughness characterized by a root-mean- square (rms) height variation in a range of about 1 nm to about 500 nm, e.g., in a range of about 1 nm to about 100 nm, or in a range of about 1 nm to about 50 nm, or in a range of about 1 nm to about 5 nm.

In some embodiments, the doped layer is a substantially crystalline layer characterized, for example, by a crystallinity free of extended defects.

In some embodiments, a photodetector is disclosed, which includes a germanium substrate having a doped layer with a top surface configured for exposure to external radiation and having a thickness in a range of about 10 nm to about 1 micron, and a transition metal dopant, such as gold, distributed in the doped layer such that a concentration of the dopant in the doped layer is at least about 10 ¹⁶ atoms/cm ³ , where the doped layer forms a diode junction with an underlying portion of the germanium substrate.

In some embodiments, the photodetector can exhibit a quantum efficiency, e.g., an external quantum efficiency, of at least about 10 ^"5 , e.g., in a range of about 10 ^"5 to 0.9, for incident radiation having a wavelength in a range of about 1750 nm to about 3000 nm. In some embodiments, the photodetector can exhibit such a quantum efficiency for radiation having a wavelength in range of about 2000 nm to about 3000 nm. In some embodiments, such a quantum efficiency can be achieved upon application of a bias voltage in a range of about 0 to about -10 volts to the photodetector. In some embodiments, the photodetector can exhibit such a quantum efficiency at an operating temperature in a range of about 18 °C to about 25 °C.

In some embodiments, a photodetector is disclosed, which comprises a germanium substrate having a doped layer configured for exposure to external radiation, said doped layer comprising at least one deep-level dopant and at least one shallow-level dopant distributed therein, where said doped layer forms a diode junction with an underlying portion of the germanium substrate. In some embodiments, the deep level dopant can be any of Au, B, S, Se and Te, and the shallow-level dopant can be any of antimony (Sb), gallium (Ga), phosphorus (P), and arsenic (As). In some embodiments, any of the deep-level dopant and the shallow-level dopant has a concentration in a range of about 10 ¹⁴ to about 10 ²⁰ dopants/cm ³ , or in a range of about 10 15 to about 1020 , or in a range of about 1016 to about 1020 , or in a range of about 10 IV to

20 18 20 19 20 about 10 , or in a range of about 10 to about 10 , or in a range of about 10 to about 10 dopants/cm ³ , in said doped layer. In some embodiments, the thickness of the doped layer can be, for example, in a range of about 10 nm to about 1 micron.

In another aspect, a method of fabricating a germanium photodetector is disclosed, which includes bombarding a top surface of a germanium substrate with an ion beam comprising dopant ions (e.g., dopant ions having an energy in a range of about 10 keV to about 500 keV) so as to implant at least a portion of said dopant ions in a surface layer of the substrate at a concentration in a range of about 10 ¹⁶ to about 10 ²⁰ atoms/cm ³ , and irradiating the top surface of the substrate with a plurality of laser pulses having a pulse width in a range of about 1 ns to about 500 ns so as to anneal said doped surface layer. In some embodiments, the pulse energy can be in a range of about 5 mJ to about 10 J. Further, the fluence of the laser pulses at the top substrate surface can be, for example, in a range of about 0.3 J/cm ² to about 0.9 J/cm ² . In some embodiments, the central wavelength of the pulses can be, for example, in a range of about 198 nm to about 1064 nm. In some embodiments, the dopant is a deep-level dopant. Some examples of suitable dopants include, without limitation, any of Au, B, S, Se and Te. In some embodiments, the method can further include depositing Ohmic contacts on the top and the back surface of the substrate. For example, as discussed above, a photoresist can be deposited on the top surface of the germanium substrate and a mask, e.g., a copper mask, can be applied to the photoresist. The masked photoresist layer can be exposed to ultraviolet (UV) radiation. The mask can be removed and an appropriate solvent can be applied to remove the portions of the photoresist layer that were exposed to the UV radiation, thereby forming a photoresist pattern on the germanium surface. A metal can then be deposited on the patterned surface. The photoresist portions with metal deposited thereon can then be removed, e.g., via application of an appropriate solvent, to form one or more electrical contacts on the surface.

The method discussed above can be used to deposit Ohmic contacts on the top and the back surface of the substrate.

Further understanding of various aspects of the invention can be obtained by reference to the following detailed description in conjunction with the associated drawings, which are described briefly below.

Brief Description of the Drawings

FIG. 1 schematically depicts a Ge photodetector according to an embodiment of the present teachings having a hyperdoped crystalline layer,

FIG. 2 schematically depicts a photodetector according to the present teachings connected to an external circuit including a resistor,

FIG. 3 schematically depicts a Ge photodetector according to an embodiment, which includes a passivation layer disposed on a top surface of a hyperdoped Ge layer,

FIG. 4A schematically depicts a Ge photodetector according to an embodiment, which includes an antireflective coating disposed on a top surface of a hyperdoped Ge layer,

FIG. 4B schematically depicts a Ge photodetector according to an embodiment, which includes a coating on a top surface of a hyperdoped Ge layer functioning as both a passivation and an anti -reflection layer, FIG. 5 is a flow chart depicting various steps in a method for fabricating a photodetector according to the present teachings,

FIG. 6 shows measured electronic energies of a number dopants in Ge,

FIG. 7 is a schematic diagram depicting a laser system for irradiating a hyperdoped Ge layer to laser pulses,

FIG. 8A schematically depicts a photodetector according to an embodiment in which the photodetector includes a hyperdoped Ge layer in which at least one deep-level dopant and at least one shallow-level dopant are incorporated,

FIG. 8B shows measured absorptance of a number of Ge wafers, including two wafers having a hyperdoped layer, for wavelengths extending from about 500 nm to about 2500 nm, and

FIG. 9 shows measured photocurrent for a number of Ge photodetectors fabricated using methods disclosed herein with different dopants.

Detailed Description

The present invention relates generally to a Ge photodetector that can exhibit a high quantum efficiency for detecting radiation wavelengths in a range of about 1750 nm to about 3000 nm. In many embodiments, the Ge photodetector can be operated at room temperature, e.g., at a temperature greater than about 18 °C. As discussed in more detail below, in many embodiments, the photodetector includes a doped Ge layer that includes a dopant with a concentration greater than the solid equilibrium solubility limit of the dopant in germanium. As discussed in more detail below, a variety of deep level dopants can be used for forming the doped Ge layer. It has been discovered that the use of gold as the dopant can be particularly advantageous in forming the doped Ge layer. In general, a photodetector according to the present teachings can be formed by using donor or acceptor dopants. As such, the teachings discussed herein apply equally to both donor and acceptor dopants, though certain embodiments may describe one type of dopant or another.

It has also been discovered that a combination of ion implantation with deep level dopants followed by pulsed laser melting of the doped layer can provide a hyperdoped, highly crystalline layer, which can extend absorption of Ge beyond the its bandgap, which is about 1870 nm. In particular, such a hyperdoped layer can exhibit significant absorption for wavelengths in a range about 2000 nm to about 3000 nm. Further, such a hyperdoped Ge layer can form a diode junction with the underlying portion of the substrate and hence can be used to fabricate a photodetector, e.g., a photodetector capable of operating at room temperature.

As discussed in more detail below, in many embodiments, ion implantation allows a non- equilibrium concentration of dopants to be inserted into the germanium lattice with control over the dopant dose and depth profile. Pulsed laser melting can heal the implantation damage to achieve non-equilibrium dopant concentrations in a crystalline structure, which are several orders of magnitude greater than the solid solubility of the dopant in germanium. Without being limited to any particular theory, during the resolidification of the melted doped Ge layer, the re- solidification front solidifies the molten layer slowly enough for epitaxial regrowth from the underlying crystalline substrate to occur but fast enough to prevent the dopant atoms from diffusing away from the melt front interface so quickly as to establish equilibrium solid concentrations.

Various terms are used herein consistent with their ordinary meanings in the art. For additional clarity, the following terms are defined:

The term "a deep level dopant" refers to a dopant that once incorporated in a Ge substrate generates at least one electronic level that is separated from any of the maximum of the conduction band or the minimum of the valence band of Ge by at least about 0.12 electron volts (eV).

The term "a shallow level dopant" refers to a dopant that once incorporated in a Ge substrate generates at least one electronic level that is separated from any of the maximum of the conduction band or the minimum of the valence band of Ge by an energy in a range of about 0.001 eV to about 0.05 eV.

The term "quantum efficiency" as used herein refers to the number of charge carriers generated within a photodetector relative to the number of photons that are incident on a radiation-receiving surface of the photodetector. The term "external quantum efficiency" as used herein refers to the number of charge carriers generated within a photodetector and extracted from the photodetector into an external circuit relative to the number of photons incident on a radiation-receiving surface of the photodetector.

The term "equilibrium solid solubility limit" of a dopant in Ge is defined as the maximum concentration of the dopant that can be incorporated in the host Ge lattice using thermodynamic equilibrium processing methods without the dopant precipitating as multiple phases, creating segregation or forming extended defects.

The term "substantially crystalline layer" refers to a layer that has a crystalline structure free of extended defects or extended amorphous regions, such as defects or amorphous regions having at least one linear dimension greater than about 10% of the thickness of the layer, e.g., greater than about 100 nm. In some embodiments, at least 95% of the volume of such a substantially crystalline layer exhibits long-range crystalline order.

The term "about" as used herein to modify a numerical value is intended to denote a variation of less than 5%.

The concentration of a dopant refers to the number of dopants divided by the volume of the doped layer.

FIG. 1 schematically depicts a photodetector 100 according to an embodiment of the present invention, which includes an n-type germanium substrate 102 and a p+-type hyperdoped germanium (Ge) layer 104, and a Sb-doped n+ layer 106. In this embodiment, the hyperdoped Ge layer 104 extends from a bottom surface 104a to a top surface 104b. A portion 104bb of the top surface of the hyperdoped Ge layer is configured for exposure to incident radiation. In other embodiments, the photodetector can include a p-type germanium substrate and n+-type hyperdoped Ge layer.

In this embodiment, the hyperdoped Ge layer can have a thickness (T) in a range of about 10 nm to about 1 micrometer (micron), e.g., in a range of about 20 nm to about 80 nm, or in a range of about 30 nm to about 70 nm. In this embodiment, the hyperdoped Ge layer includes gold (Au) atoms as a dopant, though in other embodiments other dopants, such as those listed above, can be employed. In this embodiment, the concentration of the gold dopant atoms can be, for example, in a range of about

10 16 atoms/cm 3 to about 1020 atoms/cm 3 , e.g., in a range of about 1017 ^" atoms/cm 3 to about 1020 atoms/cm 3 , or in a range of about 1018 atoms/cm 3 to about 1020 atoms/cm 3.

In this embodiment, the hyperdoped Ge layer can have a substantially crystalline structure. By way of example, in some embodiments, at least about 95% of the volume of the hyperdoped Ge layer can have a long-range crystalline order.

While in some embodiments the distribution of the gold atoms within the doped Ge layer is substantially uniform, in other embodiments it may be non-uniform. For example, in some embodiments, the concentration of the dopant gold atoms can vary from the top surface 104b of the doped Ge layer to the bottom surface 104a thereof.

In this embodiment, the underlying n-type Ge substrate can have a thickness in a range of about 100 nm to about 300 nm, though other thicknesses can also be employed. Further, the n-type Ge substrate can have an electrical resistivity in a range of about 0.01 ohm-cm to about 50 ohm-cm.

With continued reference to FIG. 1, the photodetector 100 further includes two Ohmic contacts 108 and 110. The Ohmic contact 108 is formed of aluminum and is applied to a portion of the top surface of the hyperdoped Ge layer 104. The Ohmic contact 110 is formed of nickel and is applied across the entire bottom surface of the n+-type Sb-implanted layer. As discussed in more detail below, the two Ohmic contacts allow extracting charged particles generated in the photodetector 100 in response to the exposure of the hyperdoped Ge layer 104 to incident radiation.

In some embodiments, the photodetector 100 exhibits an external quantum efficiency of at least about 10 ^"5 for radiation having a wavelength in a range of about 1750 nanometers to about 3,000 nanometers. By way of example, the photodetector 100 can exhibit an external quantum efficiency in a range of about 10 ^"5 to about 0.9 for radiation having a wavelength in a range of about 1750 nm to about 3,000 nm. In some embodiments, the photodetector 100 can operate at room temperature and exhibit an external quantum efficiency in the above range. For example, the photodetector 100 can operate at a temperature in a range of about 18 °C to about 25 °C and provide an external quantum efficiency in the above range.

In some embodiments, the photodetector 100 can exhibit a responsivity in a range of about 0.1 AAV to about 0.9 AAV for incident radiation having a wavelength in a range of about 1750 nm to about 3000 nm. By way of example, the photodetector 100 can exhibit such a responsivity upon application of a reverse bias voltage in a range of about 0 V to about -10 V thereto.

By way of illustration, FIG. 2 schematically depicts the photodetector 100 according to the present teachings connected to an external circuit. The external circuit includes a resistor 200 and an ammeter 202 for measuring a current flowing through the resistor. A voltage source 204 allows the application of a bias voltage, e.g., a bias voltage equal to or less than about 10 V, to the detector. Upon exposure of the hyperdoped Ge layer of the photodetector 100 to incident radiation, electron-hole pairs can be created, which can in turn result in the generation of a current flow in the external circuit. The external quantum efficiency of the photodetector can be determined by measuring the intensity of the incident light and the current flowing through the external circuit.

In some embodiments, the top surface 104b of the doped Ge layer, or at least the portion 104bb thereof that is configured for exposure to incident radiation, has a roughness characterized by a root-mean-square (rms) height variation in a range of about 1 nm to about 500 nm, e.g., in a range of about 20 nm to about 400 nm, or in a range of about 30 nm to about 300 nm, or in a range of about 50 nm to about 200 nm. In some embodiments, the top surface of the doped Ge layer, or at least the portion 104bb thereof that is configured for exposure to incident radiation, exhibits a roughness characterized by a height variation in a range of about 1 nm to about 100 nm, e.g., in a range of about 10 nm to about 50 nm, or in a range of about 1 nm to about 10 nm, or in a range of about 1 nm to about 5 nm.

In some embodiments, a passivation layer can be disposed on at least a portion of a top surface of the doped Ge layer (i.e., the surface of the doped Ge layer at least a portion of which is configured for exposure to radiation) of a photodetector according to the present teachings. For example, FIG. 3 schematically depicts a photodetector 300 according to an embodiment of the present teachings, which has the same structure as that of the photodetector 100 discussed above except for a passivation layer 302 disposed on the top surfacel04b of the doped Ge layer. A variety of different materials can be employed to form the passivation layer. By way of example, the passivation layer 302 can be formed of any of oxisilicon nitride and germanium oxide. In some embodiments, the passivation layer can have a thickness in a range of about 10 nm to about 1 μπι.

In some embodiments, the passivation layer 302 can advantageously reduce the surface active defect density, surface recombination velocity and the dark current of the photodetector 300.

In some embodiments, an anti -reflective coating can be applied to a top surface of the doped Ge layer. By way of example, FIG. 4A schematically depicts a photodetector 400 according to such an embodiment, which has the same structure as that of the photodetector 100 discussed above except for an antireflective coating 402 disposed on the top surface 104b of the doped Ge layer. A variety of different materials can be employed to form the antireflective coating. By way of example, the antireflective coating can be formed of silicon nitride or silicon oxide.

In some embodiments, a layer that functions both as a passivation layer as well as an anti- reflection layer is disposed on at least a portion of the hyperdoped Ge layer. By way of example, FIG. 4B shows an exemplary embodiment of such a photodetector 401, which has the same structure as the photodetector 100 discussed above except for a coating 402 disposed on the top surface 104b of the hyperdoped Ge layer. In this embodiment, the coating 403 functions concurrently as both a passivation layer and an anti -reflection layer. Some suitable materials for forming the coating 402 include, without limitation, germanium oxide, silicon nitride, silicon oxide, an epitaxial silicon passivation layer or sulfur passivation.

Referring again to FIG. 1, the p+-type doped Ge layer 104 forms a diode junction 111 with the underlying n-type Ge substrate. Such a diode junction 111 can be characterized by a charge depletion region supporting an electric field. In use, upon exposure of the doped Ge layer to incident electromagnetic radiation, the absorption of at least a portion of the incident photons can excite some of the electrons from the valence band of doped Ge layer to its conduction band, thereby generating a plurality of electron-hole pairs. The electron-hole pairs generated in the vicinity of the diode junction 111 will drift under the influence of the electric field associated with the junction (as well as that associated with a bias voltage, if applied) to the upper and lower Ohmic contacts 108 and 110 and can be extracted from the photodetector into an external circuit, such as that shown in FIG. 2.

FIG. 5 is flow chart depicting various steps in one method for fabricating a Ge photodetector according to the present teachings, such as the photodetectors described above. A flat zone germanium (Ge) wafer grown in the (100) direction is obtained. A backside of the Ge wafer is implanted with one or more dopants to create an N ⁺ region (step 1). The Ge wafer can be grown in the (100) direction and can be an n-type wafer having an electrical resistivity in a range of about 5 ohm-cm to about 10 ohm-cm. The concentration of the dopant can be, for

15 3 20 3

example, in a range of about 10 cm ^" to about 10 cm ^" . By way of example, the backside of the Ge wafer can be implanted with antimony (Sb). In some embodiments, such an ion implantation of the backside of the Ge wafer can generate a doped layer having a thickness in a range of about 10 nm to about 1 μπι with a dopant concentration in a range of about 10 ¹⁶ cm ^"3 to about 10 ²⁰ cm ^"3 .

Subsequently, the Ge wafer is subjected to a rapid thermal anneal (RTA) in an inert atmosphere, e.g., in an atmosphere of nitrogen (N ₂ ) or argon (Ar) (step 2). By of example, the annealing can be performed at an elevated temperature of about 500 °C and for a time duration of about 30 seconds. In some embodiments, the doped Ge wafer can be annealed via exposure to an elevated temperature in a furnace, e.g., exposed to an elevated temperature in a range of about 300 °C to about 900 °C for a time duration in a range of about 30 minutes to about 2 hours.

A metal contact can then be formed on the back (bottom) surface of the Ge wafer (step 3). In particular, a patterned mask of a photoresist can be applied to the back surface of the Ge wafer and cured using, e.g., photolithographic techniques. Subsequently, a metal, e.g., aluminum or nickel, can be deposited over the back surface to coat both the masked and the exposed portions of that surface. The photoresist can then be removed using, e.g., an appropriate solvent, to leave behind the metal on the exposed portions of the surface, thereby forming a metallic Ohmic contact on the back surface of the Ge wafer. The metallic contact layer can have a thickness, for example, in a range of about 5 nm to about 1 μιη. For example, a nickel contact layer having a thickness of about 200 nm can be formed in this manner.

In some embodiments, after the formation of the back contact layer, the Ge wafer can be subjected to a rapid thermal annealing in an inert atmosphere (e.g., in an atmosphere of N ₂ or Ar) (step 4), e.g., at a temperature of 500 °C for about 30 seconds.

The front surface of the Ge wafer can then be exposed to a beam of a p-type dopant (gold ions in this embodiment) so as to implant a dose of the dopant in a top surface layer of the Ge wafer (step 5). The implantation of the dopant (e.g., gold) can be achieved, for example, by exposing the front surface of the Ge wafer to a beam of dopant ions having an energy, for example, in a range of about 10 keV to about 500 keV. In some embodiments, the concentration of the implanted dopant in the top Ge surface layer can be, for example, in a range of about 10 ¹⁶

20 3 17 3 19 to about 10 atoms/cm , for example, in a range of about 10 atoms/cm to about 10

atoms/cm ³ .

A variety of commercial ion implanters can be employed to implant a dopant into the Ge layer.

In many embodiments, the dopant is a deep-level dopant. As noted above, a deep level dopant refers to a dopant characterized by an electronic energy level that is separated from any of the maximum of the conduction band or the minimum of the valence band of Ge by at least about 0.12 electron volts (eV). By way of illustration, FIG. 6 schematically depicts the approximate calculated electronic energy of a number of dopants in Ge. As shown in this figure, gold can function as deep-level dopant.

Referring again to the flow chart of FIG. 5, the implantation of the dopant in the top layer of the Ge wafer can be followed by exposing the top surface of the implanted layer to one or more laser pulses to melt at least a portion (and preferably the entire) implanted layer followed by its resolidification (step 6). In many embodiments, such melting and resolidification of the doped layer can enhance the crystallinity of the doped layer. By way of example, the annealing laser pulses can have a pulse width in a range of about 1 ns to about 500 ns, e.g., in a range of about 10 ns to about 400 ns, or in a range of about 50 ns to about 300 ns. Further, the laser pulses can have an energy in a range of about 5 mJ to about 10 J, e.g., in range of about 10 mJ to about 100 mJ, or in a range of about 20 mJ to about 200 mJ, or in a range of about 1 J to about 10 J, or in range of about 2 J to about 5 J. The fluence of the applied laser pulses can be, for example, in a range of about 0.3 J/cm ² to about 0.9 J/cm ² , e.g., in a range of about 0.5 J/cm ² to about 0.8 J/cm ² . The central wavelength of the applied laser pulses can be, for example, in a range of about 193 nm to about 1064 nm.

By way of example, FIG. 7 schematically depicts a laser system suitable for exposing a doped Ge layer to laser pulses for melting the layer followed by its resolidification. The exemplary system 700 includes a Nd: YAG laser 701 that generates 20-ns laser pulses with a center wavelength of 355 nm. A plurality of mirrors 702 and 703 direct the pulses to a lens 704, which in turn focuses the laser pulses onto a sample 705, e.g., a Ge substrate having a

hyperdoped layer, which is mounted on a sample holder 706. The sample holder 706 can be moved in x, y, and z dimensions, and hence can allow exposing different portions of the sample to the laser pulses. In this embodiment, the system 700 further includes an Ar-He laser 707 generating continuous laser radiation at a wavelength of 488 nm. A mirror 708 directs the laser radiation onto a polarizer 709, which generates a polarized beam. The polarized beam is focused by a lens 710 at an oblique angle onto the sample 705. The reflected radiation is directed by a mirror 711 onto a lens 712, which focuses the reflected beam onto a photodiode detector 713. In this embodiment, the signals generated by the photodiode detector can be monitored and analyzed via an oscilloscope 714. By way of example, the signals can be used to perform time- resolved reflectivity measurements, which can detect a change in the reflectivity of the film as melting occurs and its return to initial state after resolidification. Such reflectivity measurements can allow for the determination of the in-situ melt time duration and the calculation of fluence in the layer.

Referring again to the flow chart depicted in FIG. 5, a metal contact is formed on the top surface of the doped Ge layer (step 7). The formation of the metal contact can be achieved, for example, by depositing a photoresist on the top (front) surface of the doped Ge layer and applying a mask, e.g., copper mask, to the photoresist followed by curing the photoresist, e.g., via exposure to UV radiation. The mask can be removed and appropriate solvent can be applied to remove the portions of the photoresist layer that were exposed to the UV radiation.

Subsequently, a metal, e.g., aluminum or nickel, can be deposited over the top surface of the doped Ge layer to coat both the masked and the exposed portions of the surface. The photoresist can then be removed using, e.g., an appropriate solvent, to leave behind the metal on the exposed portions of the surface, thereby forming a metal contact on the front surface of the doped layer.

As noted above, in some embodiments, a photodetector according to the present teachings can include a passivation layer and/or an antireflective layer disposed on at least a portion of its radiation-receiving surface. In such embodiments, such a passivation and/or antireflective layer can be deposited on the top surface of the doped Ge layer subsequent to melting and resolidification via exposure to one or more laser pulses (step 6a). In some such embodiments, the top metal contact can be deposited on a portion of the hyperdoped layer.

In some embodiments, a photodetector according to the present teachings can include a hyperdoped layer in which at least one deep-level dopant and at least one shallow-level dopant are incorporated. By way of example, FIG. 8A schematically depicts such a photodetector 800 that includes a hyperdoped layer 801 in which a deep-level dopant 802 and a shallow-level dopant 803 are incorporated. Similar to the deep-level dopant, the shallow-level dopant can be either a donor or an acceptor. By way of example, the deep-level dopant can be any of the deep- level dopants discussed above, such as Au, B, S, Se and Te, and the shallow-level dopant can be, for example, any of antimony (Sb), gallium (Ga), phosphorus (P), and arsenic (As).

With continued reference to FIG. 8A, the remainder of the photodetector 800 is similar to a number of photodetectors discussed above. In particular, the photodetector 800 includes an N- type Ge substrate 102 forming a junction at a top surface thereof with a bottom surface of the hyperdoped layer 801 and is contact at a bottom surface thereof with an Sb-doped N+ layer 106. The photodetector 800 further includes top and bottom Ohmic contacts 108 and 110,

respectively.

In some embodiments, the concentration of any of the deep-level dopant and the shallow- level dopant in the hyperdoped layer can be, for example, in a range of about 10 ¹⁴ to about 10 ²⁰

3 15 20 3 16 dopants/cm , or in a range of about 10 to about 10 dopants/cm , or in a range of about 10 to 20 3 17 20 3

about 10 dopants/cm or in a range of about 10 to about 10 dopants/cm , or in a range of about 10 ¹⁹ to about 10 ²⁰ dopants/cm ³ . Similar to the deep-level dopant, the shallow-level dopant can be incorporated in a Ge substrate via exposing a surface of the Ge substrate to an ion beam, for example, an ion beam having an energy in a range of about 10 keV to about 500 keV, e.g., in a range of about 100 keV to about 300 keV.

In some embodiments, the incorporation of both a deep-level dopant as well as a shallow- level dopant in the hyperdoped layer can improve the signal/noise (S/N) ratio of the detector. By way of example, in some embodiments, the incorporation of both the deep-level and the shallow- level dopants in the hyperdoped layer can reduce the detector's dark current, thus improving the detector's S/N ratio.

By way of example, in some embodiments, the deep-level dopant can be an electron donor and the shallow-level dopant can be electron acceptor, or vice versa. In some

embodiments, such counter-doping of the hyperdoped layer can be useful in reducing the photodetector' s dark current. In some such embodiments, a shallow-level dopant can be selected to modulate a shallow energy level of the deep-level dopant. For example, in some

embodiments, the deep-level dopant, once incorporated in Ge, can exhibit at least one deep energy level (e.g., an energy level separated from the maximum of the valence band or the minimum of the conduction band by at least about 0.12 eV) and at least one shallow energy level (e.g., an energy level separated from the maximum of the valence band or the minimum of the conduction band by an energy in a range of about 0.001 eV to about 0.05 eV). In some such embodiments, the shallow-level dopant can be selected so as to modulate the shallow electronic level of the deep-level dopant. By way of example and without being limited to any particular theory, in such an embodiment, if the deep-level dopant is an electron donor, a shallow-level dopant that is an electron acceptor can be used to reduce, and in some cases substantially "neutralize," the contribution of the electron in the shallow electronic level of the deep level dopant to the dark current, e.g., via recombination of the electron in the shallow electronic level of the deep-level dopant with a hole associated with the shallow-level dopant. The following examples are provided for further elucidation of various aspects of the invention and are not intended to indicate necessarily the optimal ways of practicing the invention and/or the optimal results that may be obtained Examples

Example 1

A Ge wafer (5-10 ohm-cm Sb doped n-type wafer) was implanted with gold (Au) ions at a dose of about 10 ^"15 cm ^"2 . The implantation was achieved by exposing a surface of the Ge wafer to a gold ion beam having an energy of about 110 keV. The gold implanted wafer was not subjected to laser annealing.

A Ge wafer was implanted with gold (Au) ions at a dose of about 10 ¹⁵ cm ^"2 by exposing a surface thereof to a beam of gold ions having an energy of about 110 keV to generate a doped Ge layer. Subsequent to the implantation, the doped Ge layer was exposed to laser pulses having a pulsewidth of about 4 ns and a fluence of about 0.22 J/cm ² to cause melting and resolidification of the doped Ge layer.

Another Ge wafer was implanted with gold (Au) ions at a dose of about 10 ¹⁵ cm ^"2 by exposing a surface thereof to a beam of gold ions having an energy of about 110 keV to generate a doped Ge layer. Subsequent to the implantation, the doped Ge layer was exposed to laser pulses having a pulsewidth of about 4 ns and a fluence of about 0.22 J/cm ² to cause melting and resolidification of the doped Ge layer.

FIG. 8B compares the absorptance of a virgin germanium (Ge) wafer over a wavelength range extending from 500 nm to about 2500 nm with those of the above three dopant-implanted Ge wafers as measured by a UV-NIR spectrophotometer. The absorptance data shows that the virgin Ge sample does not exhibit any sub-bandgap absorption in the wavelength region extending from about 2000 nm to about 2500 nm.

The Ge wafer that was implanted with gold but not laser annealed shows a weak sub- bandgap absorption in the wavelength range of about 2000 nm to about 2500 nm. This wafer also exhibits a decrease for the absorption of radiation wavelengths in a range of about 1000 nm to about 2000 nm, which may be due to defects introduced in the wafer as a result of dopant implantation.

In contrast, the two Ge wafers that were hyperdoped with gold and laser annealed exhibit an appreciable sub-bandgap absorptance, e.g., in the wavelength range of about 2000 nm to about 2500 nm. Further, these Ge wafers exhibit an above bandgap absorptance that is very similar to that exhibited by the virgin Ge wafer, indicating a recovery of crystallinity after the laser processing. Without being limited to any particular theory, the significant below bandgap absorptance of these two Ge wafers can be due to the introduction of the Au deep level donors as well as the recovery of the crystallinity of the doped layer via laser processing.

Example 2

Several Ge photodiodes each having a doped Ge layer were fabricated. In each case, a fabrication method similar to that discussed above was employed. In particular, a doped Ge layer was generated via ion implantation and the doped layer was then melted and resolidified via exposure to laser pulses having a pulse width of about 4 ns and a fluence of about 0.22 J/cm ² .

For two of the photodiodes, Boron (B), which is a shallow level acceptor, was used as the dopant. For one of these photodiodes, the dose of the implanted B was about 10 ¹⁴ cm ^"2 and for the other the dose of the implanted B was about 10 ¹⁵ cm ^"2 .

For two other photodiodes, gold (Au), which is a deep level donor, was used as the dopant. For one of these gold-implanted photodiodes, the dose of the implanted gold was about 10 ¹⁴ cm ^"2 and for the other the dose of the implanted gold was 10 ¹⁵ cm ^"2 .

FIG. 9 shows the measured photocurrent for each of these photodiodes as a function of incident laser radiation over a selected wavelength range. The photocurrent data in the wavelength range of 2 microns to 3 microns shows that the sub-bandgap photoresponse of the Au hyperdoped wafers is above the noise level in this wavelength range. The B implanted photodiodes also exhibit certain sub-bandgap photoresponse in the wavelength range of about 2 microns to about 3 microns. However, the Au-implanted photodiodes exhibit a much enhanced photoresponse relative to the B-implanted photodiodes.

Those having ordinary skill in the art will appreciate that various changes can be made to the above embodiments without departing from the scope of the invention.