# Patent

## Title
Solar blind ultra violet (UV) detector and fabrication methods of the same

## Abstract

Described herein is device configured to be a solar-blind UV detector comprising a substrate; a plurality of pixels; a plurality of nanowires in each of the plurality of pixel, wherein the plurality of nanowires extend essentially perpendicularly from the substrate.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is related to U.S. patent application Ser. Nos. 61/266,064, 61/357,429, 61/360,421, 12/204,686 (granted as U.S. Pat. No. 7,646,943), Ser. Nos. 12/270,233, 12/472,264, 12/472,271, 12/478,598, 12/573,582, 12/575,221, 12/633,297, 12/633,305, 12/633,313, 12/633,318, 12/633,323, 12/621,497, 12/648,942, 12/910,664, 12/945,492, 12/966,514, 12/966,535, 12/966,573, 12/967,880, 12/974,499, 12/982,269 and 13/047,392, the disclosures of which are hereby incorporated by reference in their entirety.


BACKGROUND
The ozone layer of the Earth atmosphere strongly absorbs ultra violet (UV) radiations in the 300 nm to 200 nm wavelength region. Therefore, below the ozone layer, which is mainly located in the lower portion of the stratosphere from approximately 13 to 40 kilometers above Earth surface, UV radiations from the sun in the 300 nm to 200 nm wavelength region are essentially absent.
To be able to reject the visible wavelengths and to detect the solar blind UV (SBUV) light, wide band-gap materials such as AlGaN, SiC, ZnO have often been used in SBUV detectors. All of these materials are difficult to grow, not compatible with conventional semiconductor processing, and are difficult for integrate with silicon-based readout electronics.
On the other hand, to increase sensitivity of an SBUV detector, an avalanche multiplication is often implemented by using the avalanche photodiode (APD) structure. For most of direct band-gap compound materials, however, multiplication noise also increases at the same rate as the avalanche gain due to the almost unity ratio of the hole to electron impact ionization coefficients.
An APD is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. APDs can be thought of as photodetectors that provide a built-in first stage of gain through avalanche breakdown. By applying a high reverse bias voltage (typically 100-200 V in silicon), APDs show an internal current gain (around 100). Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents to flow within materials which are otherwise good insulators. It is a type of electron avalanche. The avalanche process occurs when the carriers in the transition region are accelerated by the electric field to energies sufficient to free electron-hole pairs via collisions with bond electrons.
SUMMARY
According to an embodiment, a device comprises a substrate; a plurality of pixels; a plurality of nanowires in each of the plurality of pixel, wherein the plurality of nanowires extend essentially perpendicularly from the substrate, wherein the plurality of nanowires extend essentially perpendicularly from the substrate, and the device is configured to be a solar-blind UV detector to detect UV radiation in the solar-blind UV region.
According to an embodiment, the plurality of nanowires are configured to react only to radiations in the solar-blind UV region.
According to an embodiment, each of the plurality of nanowires comprises a core and a cladding surrounding the core, wherein the core has a higher refractive index than the cladding.
According to an embodiment, each of the plurality of the nanowires comprises a coupler disposed on an end of each of the nanowire away from the substrate, the coupler being functional to guide radiation into the nanowires.
According to an embodiment, the nanowires have a diameter from about 5 nm to about 30 nm, a height from about 0.1 μm to about 5 μm; the cladding has a thickness of about 10 nm to about 200 nm. According to an embodiment, the nanowires have a diameter from about 5 nm to about 150 nm.
According to an embodiment, the nanowires have a pitch from about 0.2 μm to about 2 μm.
According to an embodiment, the nanowires detect UV radiation in the solar-blind UV region by converting UV radiation in the solar-blind UV region to an electrical signal.
According to an embodiment, the solar-blind UV detector further comprises electrical components configured to detect the electrical signal.
According to an embodiment, the solar-blind UV detector is functional to detect the electrical signal from the nanowires in different pixels separately.
According to an embodiment, each of the nanowires comprises a photodiode or forms a photodiode with the substrate, wherein the avalanche photodiode is functional to convert at least a portion of UV radiation in the solar-blind UV region impinged on the nanowires. According to an embodiment, the photodiode can be an avalanche photodiode.
According to an embodiment, each of the nanowires comprises a first heavily doped semiconductor layer, a lightly doped semiconductor layer or an intrinsic semiconductor layer, a second heavily doped semiconductor layer, and a metal silicide layer; wherein the first heavily doped semiconductor layer is disposed on the lightly doped semiconductor layer or the intrinsic semiconductor layer; the lightly doped semiconductor layer or the intrinsic semiconductor layer is disposed on the second heavily doped semiconductor layer; the second heavily doped semiconductor layer is disposed on the metal silicide layer; the metal silicide layer is disposed on the substrate; the first heavily doped semiconductor layer is of an opposite type from the second heavily doped semiconductor layer; and wherein the first heavily doped semiconductor layer, the lightly doped semiconductor layer or the intrinsic semiconductor layer, and the second heavily doped semiconductor layer form the photodiode. An intrinsic semiconductor, also called an undoped semiconductor or i-type semiconductor, is a substantially pure semiconductor without any significant dopant species present. A heavily doped semiconductor is a semiconductor with such a high doping level that the semiconductor starts to behave electrically more like a metal than as a semiconductor. A lightly doped semiconductor is a doped semiconductor but not have a doping level as high as a heavily doped semiconductor. In a lightly doped semiconductor, dopant atoms create individual doping levels that can often be considered as localized states that can donate electrons or holes by thermal promotion (or an optical transition) to the conduction or valence bands respectively. At high enough impurity concentrations (i.e. heavily doped) the individual impurity atoms may become close enough neighbors that their doping levels merge into an impurity band and the behavior of such a system ceases to show the typical traits of a semiconductor, e.g. its increase in conductivity with temperature.
According to an embodiment, the solar-blind UV detector further comprises a common electrode disposed on and electrically connected to ends of all the nanowires, wherein the common electrode is substantially transparent to UV radiation in the solar-blind UV region.
According to an embodiment, the common electrode is made of graphene.
According to an embodiment, space between the nanowires is filled with an oxide layer.
According to an embodiment, the solar-blind UV detector further comprises a metal grid on the common electrode, the metal grid configured to provide mechanical support for the common electrode.
According to an embodiment, each of the nanowires comprises a core of lightly doped semiconductor, an intermediate shell of intrinsic semiconductor and an outer shell of doped semiconductor; wherein the intermediate shell is conformally disposed over the core; the outer shell is conformally disposed over the intermediate shell; the outer shell is of an opposite type from the core; and the outer shell, the intermediate shell and the core form the photodiode.
According to an embodiment, each nanowire further comprises a heavily doped semiconductor layer of the same type as the core, and a metal silicide layer; wherein the heavily doped semiconductor layer and a metal silicide layer are sandwiched between the core and the substrate; the intermediate shell and the outer shell do not contact the heavily doped semiconductor layer and the metal silicide layer; and the metal silicide layer is in contact with the substrate and forms electrical contact to the substrate.
According to an embodiment, a solar-blind image sensor comprises the solar-blind UV detector above and electronic circuitry functional to detect electrical signals generated by the nanowires of the solar-blind UV detector.
According to an embodiment, the electronic circuitry comprises a high voltage supply.
According to an embodiment, a method of fabricating an image sensor comprising a substrate, a plurality of pixels, a plurality of nanowires in each of the plurality of pixel, wherein the plurality of nanowires extend essentially perpendicularly from the substrate; the method comprises bonding the plurality of nanowires onto the substrate. The term “image sensor” as used herein means a device that converts an optical image to an electric signal. An image sensor can be used in digital cameras and other imaging devices. Examples of image sensors include a charge-coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS) active pixel sensor.
According to an embodiment, the method further comprises conducting lithography, ion implantation, annealing, evaporation, atomic layer deposition, chemical vapor deposition, dry etch or a combination thereof.
According to an embodiment, the image sensor is a solar-blind UV detector or a color image sensor. The term “color image sensor” as used herein means an image sensor capable of converting an optical image in the visible spectrum (i.e. a color image) to an electric signal.



BRIEF DESCRIPTION OF FIGURES
FIG. 1A shows one of the plurality of nanowires on a substrate.
FIG. 1B shows simulated absorptance of the nanowire of FIG. 1A.
FIG. 1C shows optional couplers on the nanowires.
FIG. 2A and 2B show a perspective view and a top view of an image sensor according an embodiment.
FIG. 3 shows an SBUV detector according to one embodiment.
FIG. 4 shows an exemplary fabrication process of an image sensor such as the SBUV detector in FIG. 3.
FIG. 5 shows an SBUV detector according to one embodiment.
FIG. 6 shows an exemplary fabrication process of an image sensor such as the SBUV detector in FIG. 5.
FIG. 7 shows an SBUV detector according to one embodiment.
FIG. 8 shows an exemplary fabrication process of an image sensor such as the SBUV detector in FIG. 7.
FIG. 9 shows a block diagram of a solar-blind image sensor.
FIG. 10 shows a schematic of the SBUV detector being used as fore optics.



DETAILED DESCRIPTION
In the following detailed description, reference is made to the accompanying drawings, which form a part thereof. In drawings, similar symbols typically identify similar components, unless the context dictates otherwise. The illustrate embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here.
This disclosure is drawn to, among other things, methods of use, methods of fabrication, apparatuses, systems, and devices related to an SBUV detector for detecting UV radiation in the solar-blind UV region. The SBUV region is the range of wavelengths in which the earth atmosphere absorbs essentially all UV radiation from the sun to the earth. For example, the wavelength region of 300 nm to 200 nm is within the SBUV region. A device that detects radiation only in the SBUV region and rejects visible and infrared radiation is called an SBUV detector or an SBUV sensor.
Sunlight does not interfere or blind a SBUV detector of the embodiments herein. Therefore, the SBUV detector of the embodiments herein is particularly useful for detecting manmade radiation sources that emit in the SBUV region and enjoys a very low false alarm rate. Such manmade radiation sources can include exhaust plumes of shoulder launched surface to air missiles (SLSAM), fire and flame, or any other UV emitting source including hydrogen flames. An SBUV detector of the embodiments herein can function at all lightning conditions including full daylight, without interference from the solar radiation and with very high signal to background ratio.
According to an embodiment, the SBUV detector comprises a substrate having a plurality of regions defined thereon (hereafter referred to as “pixels”). In each pixel, the SBUV detector comprises a plurality of nanowires extending essentially perpendicularly from the substrate. The term “nanowire” used herein means a structure whose dimension in a direction perpendicular to the substrate (hereafter referred to as the “normal direction”) is substantially greater than dimensions of the structure in any direction perpendicular to the normal direction (hereafter referred to as a “transverse direction”) and the dimensions in the transverse directions are less than 1000 nm. The plurality of nanowires can be equally or unequally spaced from each other, arranged in a regular array or in random. The plurality of nanowires are configured to react only to radiations in the SBUV region and not to react to radiations outside the SBUV region. Here, the term “react” is meant to broadly encompass absorbing, reflecting, coupling to, detecting, interacting with, converting to electrical signals, etc.
FIG. 1A shows one of the plurality of nanowires 100 on an SBUV detector 10. For brevity, only one nanowires is shown but the SBUV detector 10 can comprise a plurality of nanowires 100, such as more than 1000, or more than 1000000. The nanowires 100 extend essentially perpendicularly from a substrate 130. Each of the nanowires 100 preferably comprises a core 110, preferably made of silicon, and a cladding 120 surrounding the core 110. The substrate 130 is preferably a metal. The core 110 preferably has high refractive index than the cladding 120. The term “cladding” as used herein means a layer of substance surrounding the core 110.
FIG. 1B shows simulated absorptance of the SBUV detector 10 of UV light with wavelengths between 100 and 400 nm, incident in the normal direction. FIG. 1B clearly shows that absorptance of the SBUV detector 10 of UV light with wavelengths between 140 and 290 nm is greater than 50% and absorptance of UV light drops quickly to almost zero outside wavelength region of 140-290 nm. FIG. 1B indicate that the SBUV detector 10 indeed only reacts to radiations in the SBUV region. As shown in FIG. 1C, each of the nanowires 100 can further comprise a coupler 140 disposed on an end of the nanowire 100 away from the substrate 130 (hereafter referred to as the “receiving end”). The term “coupler” as used herein means a layer effective to guide light into the nanowires.
According to an embodiment, the nanowires 100 have a diameter of from about 5 nm to about 30 nm, preferably about 20 nm. According to an embodiment, the nanowires 100 have a diameter of from about 5 nm to about 150 nm. The cladding 130 has a thickness of about 10 nm to about 200 nm, preferably about 30 nm. Pitch of the nanowires 100 (i.e. center-to-center distance of one of the plurality of nanowires to a nearest neighbor thereof) is from about 0.2 μm to about 2 μm. Height (i.e. dimension in the normal direction) of the nanowires 100 is from about 0.1 μm to about 5 μm. Each pixel can have one or more nanowires 100.
FIG. 2A and FIG. 2B shows a perspective view and a top view, respectively, of the SBUV detector 10. For brevity, only four pixels (regions enclosed by dotted rectangles) are shown. The nanowires 100 preferably detect UV radiation in the SBUV region by converting it to an electrical signal. For example, each nanowire 100 can comprise a photodiode (such as an avalanche photodiode) or form a photodiode (such as an avalanche photodiode) with the substrate 130. The SBUV detector 10 can further comprise electrical components configured to detect the electrical signal from the nanowires 100, for example, Readout Integrated Circuits (ROIC) in the substrate 130, an electrode 150 disposed between and electrically connected to all the nanowires 110 on each pixel, a common electrode 160 disposed on and electrically connected to the receiving ends of all the nanowires 110 in the entire SBUV detector 10. Preferably, the SBUV detector 10 can detect electrical signals from the nanowires 100 in different pixels separately.
FIG. 3 shows an SBUV detector 20 according to one embodiment. For brevity, two pixels 20a and 20b of a substrate 230 are illustrated. The SBUV detector 20, however, can comprise a plurality of pixels such as more than 100, more than 1000, more than 1000000. The pixels preferably have a pitch of about 0.5 μm micron to 100 μm (more preferably 2 μm). In each of the pixels 20a and 20b, the SBUV detector 20 comprises a plurality of nanowires 200, respectively. Each of the nanowires 200 comprises an APD, the APD being functional to convert at least a portion of UV radiation in the SBUV region impinged the nanowires 200 to an electrical signal. The nanowires 200 do not generate any electrical signal from any radiation outside the SBUV region. Each nanowire 200 comprises a first heavily doped semiconductor layer 222, a lightly doped semiconductor layer or an intrinsic semiconductor layer 221, a second heavily doped semiconductor layer 225, and a metal silicide layer 231. The first heavily doped semiconductor layer 222 is disposed on the lightly doped semiconductor layer or the intrinsic semiconductor layer 221. The lightly doped semiconductor layer or the intrinsic semiconductor layer 221 is disposed on the second heavily doped semiconductor layer 225. The second heavily doped semiconductor layer 225 is disposed on the metal silicide layer 231. The metal silicide layer 231 is disposed on the substrate 230. The first heavily doped semiconductor layer 222 is of an opposite type from the second heavily doped semiconductor layer 225. As used herein, one semiconductor having an opposite type from another semiconductor means the former is n type if the latter is p type or, the former is p type if the latter is n type. The first heavily doped semiconductor layer 222, the lightly doped semiconductor layer or the intrinsic semiconductor layer 221, and the second heavily doped semiconductor layer 225 form an APD. The metal silicide layer 231 forms electrical contact to any electrical components of the substrate 230. A common electrode 240 is disposed on the receiving ends of and electrically connected to all the nanowires 200. The common electrode 240 is substantially transparent to UV radiation in the SBUV region. For example, the common electrode 240 can be made of graphene. Space between the nanowires 200 is preferably filled with an oxide layer 236. The term “transparent” as used herein means a transmittance of at least 70%.
FIG. 4 shows an exemplary method of fabrication of the SBUV detector 10.
In step 1000, a silicon substrate 224 is provided, wherein the silicon substrate 224 comprises a silicon oxide layer 223 thereon and the first heavily doped semiconductor layer 222 (e.g. heavily doped p type silicon) on the silicon oxide layer 223. A substrate of semiconductor material other than silicon (e.g. III-V or II-VI group compound semiconductor) can also be used.
In step 1001, the second heavily doped semiconductor layer 225 (e.g. heavily doped n type silicon) is fabricated on the layer 221 by a method such as ion implantation and subsequent annealing. An exemplary n type dopant suitable for use in the ion implantation is phosphorous, arsenic, antimony, or a combination thereof.
In step 1002, a resist layer 226 (e.g. a photoresist or an e-beam resist) is deposited on the second heavily doped semiconductor layer 225, by a suitable method such as spin coating.
In step 1003, a pattern is formed in the resist layer 226 using a lithography technique (e.g. photolithography or e-beam lithography) by removing portions 227 of the resist layer 226. The second heavily doped semiconductor layer 225 is exposed under the removed portions 227. The pattern corresponds to shapes and positions of the nanowires 200.
In step 1004, two metal layers 228 and 229 are deposited on the resist layer 226 and the exposed portions of the second heavily doped semiconductor layer 225, using a suitable technique such as thermal evaporation, e-beam evaporation, and sputtering. Exemplary metal suitable for use in the metal layers 228 and 229 are aluminum, gold, chromium, silver, copper, titanium, nickel or a combination thereof.
In step 1005, remainder of the resist layer 226 and portions of the metal layers 228 and 229 thereon are lift-off by a suitable technique such as plasma ashing and dissolution in a suitable solvent.
In step 1006, the nanowires 200 are formed by etching into the layers 225, 221 and 222, the using a suitable technique, such as dry etching with remainder of the metal layer 229 as etch mask, until portions of the silicon oxide layer 223 not directly below the remainder of the metal layer 229 are exposed. The nanowires 200 now comprise remainder of the layers 225, 221 and 222.
In step 1007, remainder of the metal layer 229 is removed by a suitable method such as etching with a suitable metal etchant. The metal layer 228 remains.
In step 1008, a silicide layer 231 is formed from the second heavily doped semiconductor layer 225 and the metal layer 228 thereon by annealing (e.g. at about 850° C. for about 30 minutes). Any remainder of the metal layer 228 is removed by a suitable technique such as etching with a suitable metal etchant.
In step 1009, an oxide layer 232 (e.g. HfO2, SiO2, Al2O3) is deposited isotropically over the nanowires 200 and the exposed portions of the silicon oxide layer 223, using suitable technique such as atomic layer deposition (ALD) and chemical vapor deposition (CVD). The oxide layer 232 is functional to passivate surfaces of nanowires 200. For example, the oxide layer 232 has a thickness of about 30 nm. The terms “passivation” and “passivate” as used herein means a process of eliminating dangling bonds (i.e., unsatisfied valence on immobilized atoms).
In step 1010, portions of the oxide layer 232 above the silicide layer 231 and the exposed portions of the silicon oxide layer 223 are removed by a suitable technique such as anisotropic dry etch. Now the silicide layer 231 is exposed.
In step 1011, a sacrificial layer 233 is deposited by pouring , spin coating or evaporation to fill space between the nanowires 200. The sacrificial layer 233 can be a suitable material such as polydimethylsiloxane, polyimide or oxide.
In step 1012, the substrate 224 is removed using a suitable technique such as etching with potassium hydroxide, until the silicon oxide layer 233 is exposed.
In step 1013, a glass substrate 234 is bonded to the exposed silicon oxide layer 233, using a suitable technique such as using a UV removable glue. The glass substrate 234 can provide mechanical support.
In step 1014, the sacrificial layer 233 is removed by a suitable method such as wet etching. For example, polyimide can be removed by a suitable photoresist developer.
In step 1015, the nanowires 200 are bonded to ROIC in the substrate 230 using a tin-silver alloy layer between the substrate 230 and the silicide layer 231 and annealing at about 220° C.
In step 1016, the glass substrate 234 is released from the silicon oxide layer 233 by illumination with UV light.
In step 1017, the silicon oxide layer 233 is removed by a suitable method such as etching with HF followed by rinsing.
In step 1018, the oxide layer 236 is deposited by spin coating, evaporation or CVD to fill space between the nanowires 200. The insulating material 236 preferably is transparent to UV radiation in the SBUV region. The insulating material 236 can be any suitable material such as silicon oxide, aluminum oxide, and graphene oxide.
In step 1019, the oxide layer 236 is planarized using a suitable technique such as chemical mechanical polishing (CMP) until the first heavily doped semiconductor 222 of the nanowires 200 is exposed.
In step 1020, an oxide layer 238 and a metal layer 239 are sequentially deposited on a sacrificial silicon substrate 237 using a suitable method such as thermal evaporation, e-beam evaporation, and sputtering. The metal layer 239 can be of any suitable material, such as nickel and iron.
In step 1021, a transparent conductive oxide (TCO) layer 240 is deposited on the metal layer 239 by a suitable method such as CVD using C2H4 or C2H2 gas. The TCO layer 240 can be any suitable material such as graphene. The TCO layer 240 is substantially transparent to UV radiation in the SBUV region.
In step 1022, a plastic layer 241 is deposited on the TCO layer 240 by a suitable method such as spin coating. The plastic layer 241 can be any suitable material such as poly(methyl methacrylate) (PMMA).
In step 1023, the sacrificial silicon substrate 237, the oxide layer 238 and the metal layer 239 are removed by a suitable method such as wet etching with KOH, HF, metal etchant, respectively.
In step 1024, the TCO layer 240 and the plastic layer 241 are transfer onto the receiving ends of the nanowires 200 under a wet condition, followed by annealing at about 220° C. for about 20 minutes to form electrical contact between the TCO layer 240 and the receiving ends of the nanowires 200.
In step 1025, the plastic layer 241 is removed by a suitable method such dry etch or wet etch.
In step 1026, a resist layer 242 (e.g. a photoresist or an e-beam resist) is deposited on the TCO layer 240 (which functions as the common electrode 240), by a suitable method such as spin coating. A pattern is formed in the resist layer 242 using a lithography technique (e.g. photolithography or e-beam lithography) by removing portions of the resist layer 425 above boundaries of pixels of the substrate 230.
In step 1027, a metal layer 243 is deposited, using a suitable method such as thermal evaporation, e-beam evaporation, and sputtering. Exemplary metal suitable for use in the metal layer 243 are aluminum, gold, chromium, silver, copper, titanium, nickel or a combination thereof.
In step 1028, remainder of the resist layer 242 and portions of the metal layer 243 thereon are lift-off by a suitable technique such as plasma ashing and dissolution in a suitable solvent.
FIG. 5 shows an SBUV detector 30 according to one embodiment. For brevity, two pixels 30a and 30b of a substrate 330 are illustrated. The SBUV detector 30, however, can comprise a plurality of pixels such as more than 100, more than 1000, more than 1000000. The pixels preferably have a pitch of about 0.5 μm micron to 100 μm (more preferably 2 μm). In each of the pixels 30a and 30b, the SBUV detector 30 comprises a plurality of nanowires 300, respectively. Each of the nanowires 300 comprises an APD, the APD being functional to convert at least a portion of UV radiation in the SBUV region impinged on the nanowires 300 to an electrical signal. The nanowires 300 do not generate any electrical signal from any radiation outside the SBUV region. Each nanowire 300 comprises a first heavily doped semiconductor layer 322, a lightly doped semiconductor layer or an intrinsic semiconductor layer 321, a second heavily doped semiconductor layer 325, and a metal silicide layer 331. The first heavily doped semiconductor layer 322 is disposed on the lightly doped semiconductor layer or the intrinsic semiconductor layer 321. The lightly doped semiconductor layer or the intrinsic semiconductor layer 321 is disposed on the second heavily doped semiconductor layer 325. The second heavily doped semiconductor layer 325 is disposed on the metal silicide layer 331. The metal silicide layer 331 is disposed on the substrate 330. The first heavily doped semiconductor layer 322 is of an opposite type from the second heavily doped semiconductor layer 325. The first heavily doped semiconductor layer 322, the lightly doped semiconductor layer or the intrinsic semiconductor layer 321, and the second heavily doped semiconductor layer 325 form an APD. The metal silicide layer 331 forms electrical contact to any electrical components of the substrate 330. A common electrode 340 is disposed on the receiving ends of and electrically connected to all the nanowires 300. The common electrode 340 is substantially transparent to UV radiation in the SBUV region. For example, the common electrode 340 can be made of graphene. Space between the nanowires 300 is preferably not filled. A metal grid on the common electrode 340 can provide mechanical support for the common electrode.
FIG. 6 shows an exemplary method of fabrication of the SBUV detector 30.
In step 2000, a silicon substrate 324 is provided, wherein the silicon substrate 324 comprises an silicon oxide layer 323 thereon and the first heavily doped semiconductor layer 322 (e.g. heavily doped p type silicon) on the silicon oxide layer 323. A substrate of semiconductor material other than silicon (e.g. III-V or II-VI group compound semiconductor) can also be used.
In step 2001, the second heavily doped semiconductor layer 325 (e.g. heavily doped n type silicon) is fabricated on the layer 321 by a method such as ion implantation and subsequent annealing. An exemplary n type dopant suitable for use in the ion implantation is phosphorous, arsenic, antimony, or a combination thereof.
In step 2002, a resist layer 326 (e.g. a photoresist or an e-beam resist) is deposited on the second heavily doped semiconductor layer 325, by a suitable method such as spin coating.
In step 2003, a pattern is formed in the resist layer 326 using a lithography technique (e.g. photolithography or e-beam lithography) by removing portions 327 of the resist layer 326. The second heavily doped semiconductor layer 325 is exposed under the removed portions 327. The pattern corresponds to shapes and positions of the nanowires 300.
In step 2004, two metal layers 328 and 329 are deposited on the resist layer 326 and the exposed portions of the second heavily doped semiconductor layer 325, using a suitable technique such as thermal evaporation, e-beam evaporation, and sputtering. Exemplary metal suitable for use in the metal layers 328 and 329 are aluminum, gold, chromium, silver, copper, titanium, nickel or a combination thereof.
In step 2005, remainder of the resist layer 326 and portions of the metal layers 328 and 329 thereon are lift-off by a suitable technique such as plasma ashing and dissolution in a suitable solvent.
In step 2006, the nanowires 300 are formed by etching into the layers 325, 321 and 322, the using a suitable technique, such as dry etching with remainder of the metal layer 329 as etch mask, until portions of the silicon oxide layer 323 not directly below the remainder of the metal layer 329 are exposed. The nanowires 300 now comprise remainder of the layers 325, 321 and 322.
In step 2007, remainder of the metal layer 329 is removed by a suitable method such as etching with a suitable metal etchant. The metal layer 328 remains.
In step 2008, a silicide layer 331 is formed from the second heavily doped semiconductor layer 325 and the metal layer 328 thereon by annealing (e.g. at about 850° C. for about 30 minutes). Any remainder of the metal layer 328 is removed by a suitable technique such as etching with a suitable metal etchant.
In step 2009, an oxide layer 332 (e.g. HfO2, SiO2, Al2O3) is deposited isotropically over the nanowires 300 and the exposed portions of the silicon oxide layer 323, using suitable technique such as atomic layer deposition (ALD) and chemical vapor deposition (CVD). The oxide layer 332 is functional to passivate surfaces of nanowires 200. For example, the oxide layer 332 has a thickness of about 30 nm.
In step 2010, portions of the oxide layer 332 above the silicide layer 331 and the exposed portions of the silicon oxide layer 323 are removed by a suitable technique such as anisotropic dry etch. Now the silicide layer 331 is exposed.
In step 2011, a sacrificial layer 333 is deposited by pouring, spin coating or evaporation to fill space between the nanowires 300. The sacrificial layer 333 can be a suitable material such as polydimethylsiloxane, polyimide or oxide.
In step 2012, the substrate is removed using a suitable technique such as etching with potassium hydroxide, until the silicon oxide layer 333 is exposed.
In step 2013, a glass substrate 334 is bonded to the exposed silicon oxide layer 333, using a suitable technique such as using a UV removable glue. The glass substrate 334 can provide mechanical support.
In step 2014, the sacrificial layer 333 is removed by a suitable method such as wet etching. For example, polyimide can be removed by a suitable photoresist developer.
In step 2015, the nanowires 300 are bonded to ROIC in the substrate 330 using a tin-silver alloy layer between the substrate 330 and the silicide layer 331 and annealing at about 220° C.
In step 2016, the glass substrate 334 is released from the silicon oxide layer 333 by illumination with UV light.
In step 2017, the silicon oxide layer 333 is removed by a suitable method such as etching with HF followed by rinsing.
In step 2018, an oxide layer 338 and a metal layer 339 are sequentially deposited on a sacrificial silicon substrate 337 using a suitable method such as thermal evaporation, e-beam evaporation, and sputtering. The metal layer 339 can be of any suitable material, such as nickel and iron. The metal layer 339 preferably has a thickness at least 100 nm. A transparent conductive oxide (TCO) layer 340 (which functions as the common electrode 340) is then deposited on the metal layer 339 by a suitable method such as CVD using C2H4 or C2H2 gas. The TCO layer 340 can be any suitable material such as graphene. The TCO layer 340 is substantially transparent to UV radiation in the SBUV region. A resist layer 342 (e.g. a photoresist or an e-beam resist) is deposited on the TCO layer 340, by a suitable method such as spin coating.
In step 2019, a pattern is formed in the resist layer 342 using a lithography technique (e.g. photolithography or e-beam lithography) by removing portions 343 of the resist layer 342. The TCO layer 340 is exposed under the removed portions 342. The removed portions 343 preferable have a width about 30 nm. The pattern can be a grid pattern in which the removed portions 343 separate the resist layer 342 into discrete square regions, which preferably have a width of about 120 nm.
In step 2020, two metal layers 345 and 346 are deposited on the resist layer 342 and the exposed portions of the TCO layer 340, using a suitable technique such as thermal evaporation, e-beam evaporation, and sputtering. Exemplary metal suitable for use in the metal layers 345 and 346 are aluminum, gold, chromium, silver, copper, titanium, nickel or a combination thereof.
In step 2021, remainder of the resist layer 342 and portions of the metal layers 345 and 346 thereon are lift-off by a suitable technique such as plasma ashing and dissolution in a suitable solvent.
In step 2022, a plastic layer 341 is deposited on the TCO layer 340 by a suitable method such as spin coating. The plastic layer 341 can be any suitable material such as poly(methyl methacrylate) (PMMA) and polydimethylsiloxane.
In step 2023, the sacrificial silicon substrate 337, the oxide layer 338 and the metal layer 339 are removed by a suitable method such as wet etching with KOH, HF, metal etchant, respectively.
In step 2024, the TCO layer 340, and the metal layers 345 and 346 are transfer onto the receiving ends of the nanowires 300 under a wet condition, followed by annealing at about 220° C. for about 20 minutes to form electrical contact between the TCO layer 340 and the receiving ends of the nanowires 300.
In step 2025, the plastic layer 341 is removed by a suitable method such dry etch or wet etch.
FIG. 7 shows a device 40 according to one embodiment. For brevity, two pixels 40a and 40b of a substrate 430 are illustrated. The SBUV detector 40, however, can comprise a plurality of pixels such as more than 100, more than 1000, more than 1000000. The pixels preferably have a pitch of about 0.5 μm micron to 100 μm (more preferably 2 μm). In each of the pixels 40a and 40b, the SBUV detector 40 comprises a plurality of nanowires 400, respectively. Each of the nanowires 400 comprises an APD, the APD being functional to convert at least a portion of UV radiation in the SBUV region impinged on the nanowires 400 to an electrical signal. The nanowires 400 do not generate any electrical signal from any radiation outside the SBUV region. Each nanowire 400 comprises a core 421 of lightly doped semiconductor, an intermediate shell 431 of intrinsic semiconductor and an outer shell 432 of doped semiconductor. The intermediate shell 431 is conformally disposed over the core 421. The outer shell 432 is conformally disposed over the intermediate shell 431. The outer shell 432 is of an opposite type from the core 421. The outer shell 432, the intermediate shell 431 and the core 421 form the APD. Each nanowire 400 can further comprise a heavily doped semiconductor layer 419 of the same type as the core 421, and a metal silicide layer 418 wherein the heavily doped semiconductor layer 419 and a metal silicide layer 418 are sandwiched between the core 421 and the substrate 430. As used herein, one semiconductor having a same type as another semiconductor means the former is n type if the latter is n type or, the former is p type if the latter is p type. The intermediate shell 431 and the outer shell 432 do not contact the heavily doped semiconductor layer 419 and the metal silicide layer 418. The metal silicide layer 418 is in contact with the substrate 430 and forms electrical contact to any electrical components of the substrate 430.
FIG. 8 shows an exemplary method of fabrication of the SBUV detector 40.
In step 3000, a silicon substrate 410 is provided, wherein the silicon substrate 410 comprises a lightly doped semiconductor layer 421 (e.g. epitaxial n type silicon), and a silicon oxide layer 423 sandwiched between the silicon substrate 410 and the lightly doped semiconductor layer 421. A substrate of semiconductor material other than silicon (e.g. III-V or II-VI group compound semiconductor) can also be used.
In step 3001, the heavily doped semiconductor layer 419 (e.g. heavily doped n type silicon) is fabricated on the lightly doped semiconductor layer 421 by a method such as ion implantation and subsequent annealing. An exemplary n type dopant suitable for use in the ion implantation is phosphorous, arsenic, antimony, or a combination thereof.
In step 3002, a resist layer 426 (e.g. a photoresist or an e-beam resist) is deposited on the heavily doped semiconductor layer 425, by a suitable method such as spin coating.
In step 3003, a pattern is formed in the resist layer 426 using a lithography technique (e.g. photolithography or e-beam lithography) by removing portions 427 of the resist layer 426. The heavily doped semiconductor layer 425 is exposed under the removed portions 427. The pattern corresponds to shapes and positions of the nanowires 400.
In step 3004, two metal layers 428 and 429 are deposited on the resist layer 426 and the exposed portions of the heavily doped semiconductor layer 425, using a suitable technique such as thermal evaporation, e-beam evaporation, and sputtering. Exemplary metal suitable for use in the metal layers 428 and 429 are aluminum, gold, chromium, silver, copper, titanium, nickel or a combination thereof.
In step 3005, remainder of the resist layer 426 and portions of the metal layers 428 and 429 thereon are lift-off by a suitable technique such as plasma ashing and dissolution in a suitable solvent.
In step 3006, the nanowires 400 are formed by etching into the layers 425 and 421, the using a suitable technique, such as dry etching with remainder of the metal layer 429 as etch mask, until portions of the silicon oxide layer 423 not directly below the remainder of the metal layer 429 are exposed. The nanowires 400 now comprise remainder of the layers 425 and 421.
In step 3007, remainder of the metal layer 429 is removed by a suitable method such as etching with a suitable metal etchant. The metal layer 428 remains.
In step 3008, a silicide layer 418 is formed from the heavily doped semiconductor layer 425 and the metal layer 428 thereon by annealing (e.g. at about 850° C. for about 30 minutes). Any remainder of the metal layer 428 is removed by a suitable technique such as etching with a suitable metal etchant.
In step 3009, a sacrificial layer 433 is deposited by pouring , spin coating or evaporation to fill space between the nanowires 400. The sacrificial layer 433 can be a suitable material such as polydimethylsiloxane, polyimide or oxide.
In step 3010, the substrate 410 is removed using a suitable technique such as etching with potassium hydroxide, until the silicon oxide layer 423 is exposed.
In step 3011, a glass substrate 434 is bonded to the exposed silicon oxide layer 423, using a suitable technique such as using a UV removable glue. The glass substrate 434 can provide mechanical support.
In step 3012, the sacrificial layer 433 is removed by a suitable method such as wet etching. For example, polyimide can be removed by a suitable photoresist developer.
In step 3013, the nanowires 400 are bonded to ROIC in the substrate 330 using a tin-silver alloy layer between the substrate 330 and the silicide layer 418 and annealing at about 220° C.
In step 3014, the glass substrate 434 is released from the silicon oxide layer 423 by illumination with UV light.
In step 3015, the silicon oxide layer 423 is removed by a suitable method such as etching with HF followed by rinsing.
In step 3016, a resist layer 435 is deposited on the lightly doped semiconductor layer 421 by a suitable method such as imprinting.
In step 3017, an oxide layer 436 is deposited on the substrate 430 and the resist layer 435 by a suitable method such as thermal evaporation, e-beam evaporation, and sputtering. The oxide layer 436 has a thickness greater than a total thickness of the heavily doped layer 419 and the silicide layer 418.
In step 3018, remainder of the resist layer 435 and portions of the oxide layer 436 thereon are lift-off by a suitable technique such as plasma ashing and dissolution in a suitable solvent.
In step 3019, an intrinsic amorphous silicon (a-Si) layer (is deposited isotropically over the lightly doped semiconductor layer 421 and the oxide layer 436 to form the intermediate shell 431, using a suitable technique such as ALD or CVD. ALD is preferred.
In step 3020, a doped a-Si layer is deposited isotropically over the intermediate shell 431 to form the outer shell 432, using a suitable technique such as ALD or CVD. The SBUV detector 40 is then annealed in a suitable atmosphere (e.g. forming gas) at about 450° C. for about 30 minutes.
In step 3021, an oxide layer 437 (e.g. HfO2, SiO2, Al2O3) is deposited isotropically over the outer shell 432, using suitable technique such as ALD or CVD. The oxide layer 437 is functional to passivate surfaces of the outer shell 432.
According to one embodiment as shown in FIG. 9, the SBUV detector 10, 20, 30 or 40 can be integrated with electronic circuitry into a solar-blind image sensor. The electronic circuitry can include address decoders, a correlated double sampling circuit (CDS), a signal processor, a multiplexor and a high voltage supply (e.g. a DC high voltage supply capable of supplying at least 50 V, 100 V, 200 V or higher) for driving the APDs in the nanowires 100, 200, 300 or 400. The electronic circuitry is functional to detect the electrical signal generated by the nanowires 100, 200, 300 or 400.
The SBUV detector 10, 20, 30 or 40 can also be used as fore optics in a light detector apparatus as shown in the schematic in FIG. 10.
The foregoing detailed description has set forth various embodiments of the devices and/or processes by the use of diagrams, flowcharts, and/or examples. Insofar as such diagrams, flowcharts, and/or examples contain one or more functions and/or operations, it will be understood by those within the art that each function and/or operation within such diagrams, flowcharts, or examples can be implemented, individually and/or collectively, by a wide range of hardware, software, firmware, or virtually any combination thereof.
Those skilled in the art will recognize that it is common within the art to describe devices and/or processes in the fashion set forth herein, and thereafter use engineering practices to integrate such described devices and/or processes into data processing systems. That is, at least a portion of the devices and/or processes described herein can be integrated into a data processing system via a reasonable amount of experimentation.
The subject matter described herein sometimes illustrates different components contained within, or connected with, other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermediate components.
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.
All references, including but not limited to patents, patent applications, and non-patent literature are hereby incorporated by reference herein in their entirety.
While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

## Claims

1. A device comprising:
a substrate;
a plurality of pixels;
a plurality of nanowires in each of the plurality of pixel, the nanowires comprising silicon;
wherein the plurality of nanowires extend essentially perpendicularly from the substrate and have a diameter from about 5 nm to about 30 nm, and the device is configured to be a solar-blind UV detector to detect UV radiation in the solar-blind UV region.

2. The device of claim 1, wherein the plurality of nanowires are configured to react only to radiations in the solar-blind UV region.

3. The device of claim 1, wherein each of the plurality of nanowires comprises a core and a cladding surrounding the core, wherein the core has a higher refractive index than the cladding.

4. The device of claim 1, wherein each of the plurality of the nanowires comprises a coupler disposed on an end of each of the nanowire away from the substrate, the coupler being functional to guide radiation into the nanowires.

5. The device of claim 1, wherein the nanowires have a height from about 0.1 μm to about 5 μm; the cladding has a thickness of about 10 nm to about 200 nm.

6. The device of claim 1, wherein the nanowires have a pitch from about 0.2 μm to about 2 μm.

7. The device of claim 1, wherein the nanowires detect UV radiation in the solar-blind UV region by converting UV radiation in the solar-blind UV region to an electrical signal.

8. The device of claim 7, wherein the device further comprises electrical components configured to detect the electrical signal.

9. The device of claim 7, wherein the device is functional to detect the electrical signal from the nanowires in different pixels separately.

10. The device of claim 1, wherein each of the nanowires comprises a photodiode or forms a photodiode with the substrate, wherein the photodiode is functional to convert at least a portion of UV radiation in the solar-blind UV region impinged on the nanowires.

11. The device of claim 10, wherein each of the nanowires comprises
a first heavily doped semiconductor layer,
a lightly doped semiconductor layer or an intrinsic semiconductor layer,
a second heavily doped semiconductor layer, and
a metal silicide layer;
wherein the first heavily doped semiconductor layer is disposed on the lightly doped semiconductor layer or the intrinsic semiconductor layer;
the lightly doped semiconductor layer or the intrinsic semiconductor layer is disposed on the second heavily doped semiconductor layer;
the second heavily doped semiconductor layer is disposed on the metal silicide layer;
the metal silicide layer is disposed on the substrate;
the first heavily doped semiconductor layer is of an opposite type from the second heavily doped semiconductor layer; and
wherein the first heavily doped semiconductor layer, the lightly doped semiconductor layer or the intrinsic semiconductor layer, and the second heavily doped semiconductor layer form the photodiode.

12. The device of claim 11, wherein the device further comprises a common electrode disposed on and electrically connected to ends of all the nanowires, wherein the common electrode is substantially transparent to UV radiation in the solar-blind UV region.

13. The device of claim 12, wherein the common electrode is made of graphene.

14. The device of claim 12, further comprising a metal grid on the common electrode, the metal grid configured to provide mechanical support for the common electrode.

15. The device of claim 10, wherein each of the nanowires comprises
a core of lightly doped semiconductor,
an intermediate shell of intrinsic semiconductor and
an outer shell of doped semiconductor;
wherein the intermediate shell is conformally disposed over the core;
the outer shell is conformally disposed over the intermediate shell;
the outer shell is of an opposite type from the core; and
the outer shell, the intermediate shell and the core form the photodiode.

16. The device of claim 15, wherein each nanowire further comprises
a heavily doped semiconductor layer of the same type as the core, and a metal silicide layer;
wherein the heavily doped semiconductor layer and a metal silicide layer are sandwiched between the core and the substrate;
the intermediate shell and the outer shell do not contact the heavily doped semiconductor layer and the metal silicide layer; and
the metal silicide layer is in contact with the substrate and forms electrical contact to the substrate.

17. The device of claim 10, wherein the photodiode is an avalanche photodiode.

18. The device of claim 1, wherein space between the nanowires is filled with an oxide layer.

19. A solar-blind image sensor, comprising the device of claim 1 and electronic circuitry functional to detect electrical signals generated by the nanowires of the device.

20. The solar-blind image sensor of claim 19, wherein the electronic circuitry comprises a high voltage supply.

21. A method of detecting UV radiation, comprising obtaining the device of claim 1, and detecting UV radiation with the device of claim 1.

22. The method of claim 21, wherein UV radiation in the solar-blind UV region.

23. The device of claim 1, wherein the device is configured to detect UV radiation in only the solar-blind UV region.