DESC:
TECHNICAL FIELD
[0001] The present disclosure relates generally to ultraviolet light detection devices. In particular, the present disclosure relates to tuneable, broadband ultraviolet light detection devices.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Ultraviolet (UV) region of the electromagnetic spectrum ranges from approximately 10 nm to 400 nm of wavelength. It is further classified into UV-A light (315-400 nm), UV-B light (280-315 nm) and UV-C light (200-280 nm). The ozone layer and gases such as sulphur dioxide and nitrogen dioxide naturally filter the UV-C light and hence, light detecting devices operating in this wavelength range are free from any solar background and are referred to as being “solar blind”.
[0004] Many applications like astronomical imaging, dosimetry during UV phototherapy, bio-aerosol detection, non-destructive forensic examination, flame detection etc. utilize such photodetectors. For specific applications, the detecting devices must show broadband spectral response and wavelength tuneability.
[0005] In order to realise UV light detecting devices with broadband detection and wavelength tuneability capabilities, semiconductors with wide and tuneable energy band gaps are utilised which are intrinsically solar blind and display highly efficient UV detection. Some popular materials for this are group-III oxides and nitrides.
[0006] However, achieving multi-spectral or broadband sensing capabilities is a challenge due to the nature of absorption characteristics of the material. Generally, the spectral response of the absorbing material peaks near the band edge wavelength corresponding to the energy band gap of the material and falls off sharply on either side of the peak wavelength.
[0007] Vertical device designs utilising heterojunctions of tuneable, wide band gap materials like AlGaN/GaN have been explored as broadband detecting devices. Material and process limitations such as cracking of thick AlGaN layers on GaN due to lattice mismatch and a high conduction band discontinuity at the AlGaN/GaN interface have however posed serious limitations to the development of these materials as viable broadband detecting devices. Particularly, the high conduction band discontinuity impedes the flow of photo-generated electrons even if GaN is grown on AlGaN to circumvent the cracking issue, in a back-illuminated approach.
[0008] Some broadband detecting devices using n-p-n based, p-n junction device architecture in a back-illuminated approach have also been reported. However, complexities arising from an inability to efficiently dope aluminium rich AlGaN gives rise to limitations on the performance and scalability of the approach.
[0009] AlGaN/GaN and GaN/AlGaN based multiple quantum wells or triangular tunnel-barriers have also been employed to realise a broadband UV detecting device. A high interface quality across thin quantum wells or barriers and a precise control over quantum well thickness is required for such a device to be efficient. Due to the bottlenecks as described, large area focal plane arrays based on these device designs have not been realised.
[0010] There is therefore a need in the art to develop a UV detecting device capable of multi-spectral or broadband operation along with wavelength tuneability, to be suited for specific applications. Also required by such a detecting device is a high efficiency and potential for scale-up.
[0011] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[0012] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about”. Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
[0013] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0014] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all groups used in the appended claims.

OBJECTS
[0015] A general object of the present disclosure is to provide an ultraviolet (UV) light detecting device.
[0016] Another object of the present disclosure is to provide a broadband UV light detecting device sensitive to a range of UV wavelengths.
[0017] Another object of the present disclosure is to provide a tuneable, broadband UV light detecting device.
[0018] Another object of the present disclosure is to provide a tuneable, broadband UV light detecting device with a scalable design.
[0019] Another object of the present invention is to provide a tuneable, broadband UV light detecting device which can be integrated into a large area focal plane array for UV imaging.
[0020] Another object of the present disclosure is to provide a tuneable, broadband UV light detecting device that can be integrated into a CMOS platform.

SUMMARY
[0021] The present disclosure relates generally to ultraviolet light detection devices. In particular, the present disclosure relates to tuneable, broadband ultraviolet light detection devices.
[0022] In an aspect, the present disclosure provides an ultraviolet light detecting device comprising: a semiconducting substrate layer with one or more buffer layers disposed epitaxially on the semiconducting substrate layer; a first absorption layer having a first energy band gap, disposed on top of one or more buffer layers, the first absorption layer configured to receive and absorb incident light of a first wavelength; a second absorption layer having a second bandgap and disposed on top of the first absorption layer, the second absorption layer configured to receive and absorb incident light of a second wavelength, wherein the second absorption layer has a favourable band offset with the first absorption layer to allow transport of photo-generated carriers across a junction between the second absorption layer and the first absorption layer; a first contact is provided on the first absorption layer; and a second contact is provided on the second absorption layer, such that the first contact and the second contact transport carriers across the junction between the first absorption layer and the second absorption layer, respectively to an external circuit; wherein the first absorption layer and the second absorption layer selectively generate carriers based on the wavelength of incident light they absorb, to enable selective response of the device to a range of incident light wavelengths.
[0023] In an embodiment, the junction between the second absorption layer and the first absorption layer is formed by an oxide material and a nitride material. In another embodiment, the oxide material is a Gallium Oxide (Ga2O3) based alloy. In another embodiment, the nitride material is a Gallium Nitride (GaN) based alloy.
[0024] In another embodiment, in case of front illumination, the second band gap is higher than the first band gap.
[0025] In another embodiment, in case of back illumination, the second band gap is lower than the first band gap. In another embodiment, the semiconducting substrate and the one or more buffer layers disposed over the semiconducting substrate are transparent to the incident light.
[0026] In another embodiment, the first contact and the second contact are metallic and made of any of identical and non-identical metals.
[0027] In another embodiment, a capping metal is provided on each of the first contact and the second contact, to prevent oxidation of the contact metal.
[0028] In another embodiment, each of the first contact and the second contact is passivated by deposition of a wide band gap dielectric.
[0029] In an aspect, the present disclosure provides a method for fabricating an ultraviolet light detecting device, said method comprising the steps of: depositing a first absorption layer having a first bandgap on top of a base comprising a semiconducting substrate followed by one or more buffer layers, the first absorption layer configured to receive and absorb an incident light of a first wavelength; depositing a second absorption layer having a second bandgap on top of the first absorption layer, the second absorption layer configured to receive and absorb incident light of a second wavelength, wherein the second absorption layer has a favourable band offset with the first absorption layer to allow transport of photo-generated carriers across a junction between the second absorption layer and the first absorption layer; depositing a first contact on the first absorption layer; and depositing a second contact on the second absorption layer, such that the first contact and the second contact transport carriers across the junction between the first absorption layer and the second absorption layer respectively to an external circuit, wherein the first absorption layer and the second absorption layer selectively generate carriers based on the wavelength of incident light they absorb to enable selective response of the device to a range of incident light wavelengths.
[0030] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF DRAWINGS
[0031] The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present invention and, together with the description, serve to explain the principles of the present invention.
[0032] FIG. 1 illustrates a schematic representation of an epi-stack consisting of a semiconducting substrate and different absorbing layers required to realise an exemplary broadband UV detecting device, in accordance with an embodiment of the present disclosure.
[0033] FIG. 2 illustrates an exemplary process flow for the steps such as photoresist (PR) patterning and contact metallization, involved in fabricating the proposed light detecting device, in accordance with an embodiment of the present disclosure.
[0034] FIG. 3A illustrates a simulated equilibrium energy band diagram of an exemplary ß-Ga2O3/GaN heterojunction, relevant to an example prototype of the embodiment of the present disclosure, utilizing GaN and ß-Ga2O3 as the first and second absorbing semiconducting layers, respectively.
[0035] FIG. 3B illustrates a schematic representation of the exemplary ß-Ga2O3/GaN heterojunction-based epi-stack on silicon (111) semiconducting substrate, utilized to realize the proposed light detecting device.
[0036] FIG. 3C illustrates an optical micrograph of the fabricated exemplary ß-Ga2O3/GaN heterojunction-based light detecting device.
[0037] FIG. 4A illustrates the Bragg reflections corresponding to the various semiconducting layers in the ß-Ga2O3/GaN based exemplary detecting device epi-stack, in accordance with an embodiment of the present disclosure.
[0038] FIG. 4B illustrates the phi (F) scan of the epi-stack depicting the epitaxial orientation of the active layers with respect to one another, in accordance with an embodiment of the present disclosure.
[0039] FIG. 4C illustrates the omega (?) scans of the symmetric (-201) and the asymmetric (111) planes of monoclinic ß-Ga2O3.
[0040] FIG. 5 illustrates the current voltage characteristics of the ß-Ga2O3/GaN based exemplary detecting device under dark and light conditions, for different bias conditions.
[0041] FIGs. 6A and 6B illustrate variation of the spectral responsivity (SR) of the exemplary detecting device with wavelength, in linear and semi-log scale, respectively, for forward-bias condition which is defined as the condition when the Ga¬2O3 contact is biased positive with respect to the GaN contact.
[0042] FIGs. 6C and 6D illustrate variation of spectral responsivity (SR) of the exemplary detecting device with wavelength, in linear and semi-log scale respectively, for reverse-bias condition which is defined as condition when the Ga¬2O3 contact is biased negative with respect to the GaN contact.
[0043] FIG. 7 illustrates the comparison of the spectral responsivity (SR) curve of the exemplary detecting device with that of a GaN-only detecting device under a forward bias of 5V.
[0044] FIGs. 8A – 8D illustrate the energy band diagrams of the ß-Ga2O3/GaN interface under different bias and illumination conditions.
[0045] FIG. 9 illustrates a generic schematic of the exemplary epi-stack with contacts, essential to realize the proposed tuneable, broadband light detecting device, in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION
[0046] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0047] If the specification states a component or feature “may”, “can”, “could”, or “might” be included or have a characteristic, that particular component or feature is not required to be included or have the characteristic.
[0048] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0049] Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. These exemplary embodiments are provided only for illustrative purposes and so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those of ordinary skill in the art. The invention disclosed may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Various modifications will be readily apparent to persons skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Moreover, all statements herein reciting embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future (i.e., any elements developed that perform the same function, regardless of structure). Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.
[0050] The use of any and all examples, or exemplary language (e.g., “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non – claimed element essential to the practice of the invention.
[0051] Embodiments described herein relate generally to the field of ultraviolet (UV) light detecting devices. In particular, the present invention relates to an ultraviolet light detecting device for the detection of broadband ultraviolet light, whose spectral response can be tuned for different wavelengths in the ultraviolet range of wavelengths.
[0052] In an aspect, the proposed detecting device has an architecture based on epitaxial integration of alloys of two dissimilar semiconductor material systems: Gallium Oxide ((InmAlnGa1-m-n)2O3) and Gallium Nitride ((InxAlyGa1-x-y)N) on a single platform to realise a detecting device with a broadband spectral response.
[0053] In another aspect, the proposed detecting device has a vertical topology and utilises both oxide and nitride-based layers as active absorbers. The favourable band alignment of the two layers allows for an efficient carrier transport across the heterojunction between the two absorber layers.
[0054] In another aspect, by varying the composition of the oxide layer and or the nitride layer, the cut-on and cut-off wavelengths in the spectral response of the broadband light detecting device can be tuned.
[0055] FIG. 1 illustrates a schematic representation of an epi-stack consisting of a semiconducting substrate and different absorbing layers required to realize an exemplary broadband UV detecting device, in accordance with an embodiment of the present disclosure.
[0056] In an aspect, the structure of the proposed epi-stack comprises: a substrate 108; one or more buffer layers 106 disposed epitaxially upon the substrate, to manage the strain between the substrate and the absorber layers; an absorber layer 104 (A1); and a second absorber layer 102 (A2).
[0057] In an embodiment, the substrate 108 is made of a material such as, but not limited to, Gallium Nitride (GaN) bulk substrate, sapphire (Al2O3), (111) oriented silicon, silicon carbide (SiC), (-201) oriented beta Gallium Oxide (ß-Ga2O3) single crystals etc.
[0058] In an instance, the use of silicon enables easier CMOS integration of the detecting devices. However, use of GaN bulk substrate or silicon carbide as a substrate helps realise low defect density films, and this is due to less lattice mismatch between the substrate and the epi-layer.
[0059] In another embodiment, the buffer layer 106 manages the strain between the epilayer and the substrate 108. The buffer layer 106 can be Aluminium Gallium Nitride (AlGaN) epi-layers.
[0060] In another embodiment, the active layers, that is, the light absorbing layers comprise of two independent layers with different energy band gaps.
[0061] In another embodiment, one of the absorbing layers (A1) 104 is a semiconductor nitride composed of one or more group-III elements.
[0062] In an exemplary implementation of the above embodiment, the material can be an alloy (InxAlyGa1-x-y)N, where ‘x’ and ‘y’ indicate the mole-fractions of Indium (In) and Aluminium (Al) in the alloy, respectively.
[0063] In another embodiment, one of the absorbing layers (A2) 102 is a semiconductor oxide composed of one or more group-III elements.
[0064] In an exemplary implementation of the above embodiment, the material can be an alloy (InmAlnGa1-m-n)2O3, where ‘m’ and ‘n’ indicate the mole-fractions of Indium (In) and Aluminium (Al) in the alloy, respectively.
[0065] In an instance, band gaps of absorber layers integrated in the detecting device can help determine cut-on and cut-off wavelengths of the spectral response of said detecting device. To tune the spectral response of the absorber layers, In, Ga and Al mole fraction(s) in the alloy(s) can be varied and tuned.
[0066] It can also be appreciated by one skilled in the present art that successful integration of the two absorber layers depends on the composition of the layers and the energy band offset between them. If band offset at the hetero-interface is very large, transport of carriers across the heterojunction can be impeded.
[0067] It can further be appreciated by one skilled in the present art that to measure detecting device response under a condition of front illumination, A2 102 should have a wider band gap than A1 104 such that both layers can absorb the incident light and contribute to the spectral response. In the reverse case, when detecting device response is measured under a condition of back illumination i.e., illumination from the substrate side, A1 104 should have a wider band gap than A2 102. Further, in the latter’s case, the substrate is required to be transparent in the wavelength range of the detecting device response.
[0068] FIG. 2 illustrates an exemplary process flow for the steps involved to fabricate the detecting device, in accordance with an embodiment of the present disclosure.
[0069] In an embodiment, the metal contact regions on the first and second absorber layers are defined using i-line optical lithography using a photoresist (PR) to pattern the contacts.
[0070] In an embodiment, mesa region on the top absorber layer can be defined using an inductively coupled plasma reactive ion etching (ICP-RIE) process using Cl2/BCl3/Ar based gas chemistry. Though wet chemical etch processes can also be used for defining the mesa region, the advantage of using RIE of the specified chemistry is that both nitride and oxide epi-layers are etched at stable rates and with smooth anisotropic etch profiles.
[0071] In another embodiment, high work-function metals are used to create Schottky contacts 204 to both the epi-layers A1 104 and A2 102. The metals can include, but not be limited to, Platinum (Pt), Palladium (Pd), Nickel (Ni), Gold (Au) etc. Any or a combination of said high work-function metals can be deposited by various methods like e-beam evaporation, sputtering etc.
[0072] In another embodiment, the contact metal can be capped by a stable metal such as gold to prevent oxidation of the contact. Depending on the thickness of the capping metal deposited, thickness of the metal stack varies accordingly. Further, the contacts 204 can be annealed for instance in a nitrogen atmosphere, which can improve its adhesion with the semiconductor layers.
[0073] In an exemplary instance of the above embodiment, the metal contact and capping metal combination is Ni (30 nm) /Au (100 nm).
[0074] In another embodiment, the contact 204 metal stack can further be passivated by depositing a wide band gap dielectric layer, which decreases leakage current arising due to surface states and etch induced damage.
[0075] In an exemplary instance of the above embodiment, the wide band gap dielectric can be any or a combination of SiO2, Al2O3, HfO2 etc., deposited by either of the methods like e-beam evaporation, atomic layer deposition (ALD), plasma-enhanced chemical vapor deposition (PECVD), sputtering etc.
[0076] In an embodiment, in the event of front illumination of the detecting device assembly, the light of wavelength corresponding to band gap of layer A2 102 will lead to generation of photo-generated carriers in both absorber layers A1 104 and A2 102.
[0077] In another embodiment, in the event of front illumination of the detecting device assembly, the light wavelength corresponding to band gap of layer A1 104 will lead to carrier generation in the layer A1 104 only since the layer A2 102 will be transparent to the particular wavelength.
[0078] In another embodiment, depending on the direction of bias, a preferential carrier separation occurs across the junction. This results in detecting devices with an asymmetrical bias-dependant broadband spectral response between two wavelengths.
[0079] FIG. 3A illustrates a simulated equilibrium energy band diagram of an exemplary ß-Ga2O3/GaN heterojunction.
[0080] In an embodiment, the exemplary heterojunction is a ß-Ga2O3/GaN vertical metal-heterojunction-metal (MHM) UV detecting device on a (111) silicon substrate. The band alignment at the interface of ß-Ga2O3 (band gap of 4.9 eV)/GaN (band gap of 3.4 eV) is depicted by FIG. 3A.
[0081] In another embodiment, in order to realise an asymmetric spectral response with respect to an applied bias, a small conduction band offset (?Ec) at the heterojunction of the two dissimilar semiconductors, measuring 0.1 eV is utilised in the device design, as illustrated by the curve 302. The ?Ec is depicted against the fermi level 304. The valence band offset (?Ev) at the hetero-junction is large (1.4 eV), as depicted by curve 306 and this impedes flow of holes across the junction.
[0082] FIG. 3B illustrates a schematic representation of the exemplary ß-Ga2O3/GaN heterojunction-based light detecting device.
[0083] In an embodiment, (111) silicon is used as a substrate 340, which is cleaned ex-situ using a hydrofluoric acid dip and desorption in a hydrogen atmosphere in-situ prior to growth.
[0084] In another embodiment, using metal organic chemical vapour deposition (MOCVD) technique, a two-temperature step Aluminium Nitride (AlN) (338, 340) layer is epitaxially grown on the substrate along with an AlGaN layer 336, in which the Al-composition is graded in steps. Together, they serve as strain-relieving buffer layers and also help in the deposition of a smooth, low defect density GaN layer 334.
[0085] In another embodiment, on the (111) silicon substrate 340 and AlN/AlGaN buffer layers (336, 338, 340), highly crystalline GaN epi-layers 334 are grown, using MOCVD technique.
[0086] In another embodiment, a ß-Ga2O3 film 332 is grown epitaxially over the MOCVD-grown GaN layers, using a pulsed laser deposition (PLD) technique.
[0087] FIG. 3C illustrates an optical micrograph of the fabricated exemplary ß-Ga2O3/GaN heterojunction-based light detecting device.
[0088] In an embodiment, Ni/Au Schottky contacts 204 are patterned and deposited on the GaN epi-layers (334) by means of optical lithography-based patterning and electron beam evaporation, respectively.
[0089] In another embodiment, the ß-Ga2O3 layer undergoes an RIE process using Cl2/BCl3/Ar based gas chemistry, and the Ni/Au Schottky contacts are also deposited by electron beam evaporation on the ß-Ga2O3 film 332.
[0090] In another embodiment, the crystalline quality of the nitride and oxide-based absorber layers was studied using high resolution X-Ray diffraction measurements.
[0091] FIG. 4A illustrates the Bragg reflections corresponding to the various semiconducting layers in the ß-Ga2O3/GaN based exemplary detecting device epi-stack on Si (111), in accordance with an embodiment of the present disclosure. In an embodiment, the ?-2? scan of the epi-stack illustrates the Bragg reflections corresponding to ß-Ga2O3, GaN and silicon.
[0092] In an exemplary instance of the above embodiment, the peaks at 2? values of 18.90°, 38.32° and 59.18° correspond to the (-201) family of planes of ß-Ga2O3.
[0093] In another exemplary instance of the above embodiment, the peaks at 2? values of 34.53° and 36.11° correspond to (002) planes of GaN and AlN respectively. The Bragg reflection corresponding to AlN layer is marked as 404.
[0094] In another exemplary instance of the above embodiment, the peaks at 2? values of 34.88°, 35.19° and 35.56° correspond to the step graded AlGaN buffer, marked as 402. In an embodiment the AlGaN buffer is beneath the GaN layer.
[0095] In another exemplary instance of the above embodiment, the peaks at 2? values of 28.31° and 58.74° correspond to the (111) and (222) planes of silicon respectively.
[0096] In another embodiment, it is inferred from FIG. 4A that ß-Ga2O3 is phase pure and the epi-stack is oriented such that (-201) Ga2O3 || (002) GaN || (111) Si.
[0097] FIG. 4B illustrates the phi (F) scan of the epi-stack depicting the orientation of the active layers with respect to one another, in accordance with an embodiment of the present disclosure.
[0098] In an embodiment, six peaks are evident, separated by 60°, which is inferred to indicate a six-fold symmetry between (011) GaN and (111) ß-Ga2O3.
[0099] In an aspect, since ß-Ga2O3 exhibits a monoclinic structure, the (111) planes exhibit a two-fold rotational symmetry and the underlying wurtzite GaN exhibits three-fold rotational symmetry, leading to the growth of three crystallographic variants along each direction at the same growth rate, and thus showing the six-fold symmetry in the F scan.
[0100] In another embodiment, from the ?-2? scan and F scan it is inferred that the orientation relationship between the ß-Ga2O3 and GaN is: (-201) Ga2O3 || (001) GaN, (102) Ga2O3 || (100) GaN, (010) Ga2O3 || (110) GaN, (102) Ga2O3 || (-110) GaN and (102) Ga2O3 || (-100) GaN.
[0101] FIG. 4C illustrates the omega (?) scans of the symmetric (-201) and the asymmetric (111) planes of monoclinic ß-Ga2O3. The full width at half maxima (FWHM) values of the symmetric and asymmetric planes of the first and second absorber layers are indicative of the crystalline quality of the layers and are compiled in Table 1.

GaN (002) GaN (102) ß-Ga2O3 (-201) ß-Ga2O3 (111)
FWHM (degrees) 0.24 0.40 1.05 5.80

Table 1: FWHM values for symmetric and asymmetric planes of GaN and ß-Ga2O3.

[0102] In another embodiment, experimental data in the form of current voltage measurements under dark as well as under UV illumination conditions were performed on a Quantum Efficiency (QE) setup consisting of a broadband spectrum Xenon lamp that is calibrated using a standard UV enhanced Silicon photodiode, along with a sourcemeter to bias the devices.
[0103] FIG. 5 illustrates the current voltage characteristics of the exemplary detecting device assembly under dark and light conditions, for different bias voltages.
[0104] In an embodiment, for the exemplary MHM devices, forward bias implies that the Ga2O3 is biased positive with respect to GaN and reverse bias implies that Ga2O3 is biased negative with respect to GaN.
[0105] In another embodiment, current voltage characteristics of the device are measured under UV wavelengths of 256 nm and 365 nm, which correspond to the ß-Ga2O3 and GaN band edge wavelengths respectively.
[0106] In another embodiment, under different bias conditions, an asymmetry is observed between the dark and light current levels.
[0107] In another embodiment, the dark current (Idark) across the detecting devices is always limited by the current across the reverse biased Schottky contact and hence, under forward bias configuration, Idark is limited by the current across the reverse-biased Ni/GaN Schottky barrier and under reverse bias, by the Ni/ Ga2O3 barrier. The Ni/Ga2O3 barrier is marginally higher than Ni/GaN, thus leading to higher dark current under forward bias.
[0108] In another embodiment, the light current (Ilight) at 256 nm wavelength of light is lower than 365 nm wavelength of light due to the lower optical power emitted by the lamp, at lower wavelengths. In an exemplary instance, the lamp can be a broadband source like Xenon lamp.
[0109] In another embodiment, the ratio of Ilight to Idark is always greater than two orders of magnitude.
[0110] In an instance, the responsivity values at any given applied bias and wavelength are calculated by obtaining the ratio of the measured photo-current (Ilight - Idark) with the input optical power (Pop) at that wavelength.
[0111] In another instance, the UV-to-visible rejection ratio is defined as the ratio of the responsivity measured at 500 nm to that measured at 365 nm.
[0112] FIGs. 6A and 6B illustrate the responsivity (SR) variation of the exemplary detecting device with wavelength, under a forward bias. Under forward bias, it is observed that the device exhibits a broadband UV-A/UV-C response, peaking at wavelengths of 256 nm and 365 nm, which corresponds to the band gaps of the absorbing layers, with a high spectral responsivity of 3.7 A/W at both 256 nm and 365 nm, under an applied bias of 5V.
[0113] FIGs. 6C and 6D illustrate SR variation of the exemplary detecting device with wavelength, under a reverse bias. Under reverse bias, the response at 365 nm is enhanced in comparison to the response at 256 nm, the responsivity measuring 0.6 A/W and 0.75 A/W at 256 nm and 365 nm, respectively, under a bias of 40V.
[0114] In another embodiment, the UV-visible rejection ratio for forward and reverse bias conditions exceeds three orders of magnitude.
[0115] FIG. 7 illustrates the comparison between the SR of the exemplary detecting device and a similar geometry detecting device fabricated only on GaN, under a bias of 5V. FIG. 7A depicts the exemplary MHM detecting device. FIB. 7B depicts a similar geometry device, fabricated on the GaN-only region of the same sample, achieved by masking a small portion of the sample during Ga2O3 growth.
[0116] In an aspect, the SR of the GaN only device peaked at 365 nm and fell on both higher on lower wavelengths, as depicted by curve 704. There is also no peak at 256 nm. This is in contrast to the 256 nm enhanced response of the exemplary MHM device, as depicted in curve 702. This confirms that the high SR of the exemplary MHM device originates from ß-Ga2O3.
[0117] In another aspect, the sharp fall in SR below 365 nm wavelength is not observed in the exemplary MHM device, and this can be attributed to a reduced surface recombination velocity at the GaN surface. This implies that Ga2O3 acts as an absorber layer as well as a surface passivation layer for GaN.
[0118] FIGs. 8A – 8D illustrate the energy band diagrams of the ß-Ga2O3/GaN interface under different bias and illumination conditions.
[0119] As illustrated in FIG. 8A, under a forward bias and an illumination of 256 nm wavelength light, approximately 63 % of light is absorbed in the ß-Ga2O3 layer and photo-carriers are generated in both absorber layers. It is assumed here, that the absorption co-efficient for ß-Ga2O3 is 105 cm-1. The low conduction band offset at the heterojunction allows the photo-electrons in GaN to get collected at the positively biased Ga2O3 contact. Photo-generated holes in GaN get collected at the negatively biased GaN contact. The high hole effective mass and the consequent low hole mobility in the Ga2O3 layer does not allow for collection of photo-generated holes at the GaN contact and therefore, photocurrent under 256 nm illumination is mainly contributed by electron collection at the Ga2O3 contact.
[0120] As illustrated in FIG. 8B, under a reverse bias, the photo-generated holes generated in GaN cannot get collected due to the high valence band offset at the hetero-interface. However, the photo-generated electrons get collected at the positive biased GaN contact.
[0121] FIGs. 8C and 8D illustrate observed device characteristics for 365 nm illumination. In an aspect, photo-carriers will only be generated in the GaN layer as the Ga2O3 layer is transparent to said wavelength. Under forward bias, the carriers get efficiently collected. Under reverse bias, however, the carriers face high lateral sheet resistance.
[0122] In another aspect, the higher SR at 365 nm illumination under reverse bias can arise due to the fact that photo-carriers generated in the Ga2O3 layer have to traverse a larger distance before they can be collected, and this can lead to their recombination or their being trapped before collection at the GaN contact.
[0123] FIG. 9 illustrates the exemplary epi-stack with contacts. In an aspect, the relatively high dark current can be decreased by improving the crystalline quality of the epi-layers, improving the quality of the interface the epi-layers, a smoother etched surface morphology, as well as sidewall passivation of the epi-layers and annealing of contacts.
[0124] It can be appreciated by those skilled in the art that design modifications such as, but not limited to, doping the GaN layer, replacing Schottky GaN contact with an ohmic contact and the use of a thicker ß-Ga2O3 layer can enable a self-powered, dual-band UV-A/UV-C detecting device with the exemplary MHM architecture, exhibiting a tuneable, broadband response even without the application of an external bias voltage.
[0125] It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive patient matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “includes” and “including” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refer to at least one of something selected from the group consisting of A, B, C ….and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc. The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practised with modification within the spirit and scope of the appended claims.
[0126] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES
[0127] The present disclosure provides an ultraviolet (UV) light detecting device.
[0128] The present disclosure provides a broadband UV light detecting device sensitive to a range of UV wavelengths.
[0129] The present disclosure provides a tuneable, broadband UV light detecting device.
[0130] The present disclosure provides a tuneable, broadband UV light detecting device with a scalable design.
[0131] The present disclosure provides a tuneable, broadband UV light detecting device which can be integrated into a large area focal plane array for UV imaging.
[0132] The present disclosure provides a tuneable, broadband UV light detecting device that can be integrated into a CMOS platform.

,CLAIMS:
1. An ultraviolet light detecting device comprising:
a semiconducting substrate layer with one or more buffer layers disposed epitaxially on the semiconducting substrate layer;
a first absorption layer having a first energy band gap, disposed on top of one or more buffer layers, the first absorption layer configured to receive and absorb incident light of a first wavelength;
a second absorption layer having a second bandgap and disposed on top of the first absorption layer, the second absorption layer configured to receive and absorb incident light of a second wavelength, wherein the second absorption layer has a band offset with the first absorption layer to allow transport of photo-generated carriers across a junction between the second absorption layer and the first absorption layer;
a first contact is provided on the first absorption layer; and
a second contact is provided on the second absorption layer, such that the first contact and the second contact transport carriers across the junction between the first absorption layer and the second absorption layer, respectively to an external circuit,
wherein the first absorption layer and the second absorption layer selectively generate carriers based on the wavelength of incident light they absorb, to enable selective response of the device to a range of incident light wavelengths.
2. The device as claimed in claim 1, wherein the junction between the second absorption layer and the first absorption layer is formed by an oxide material and a nitride material.
3. The device as claimed in claim 2, wherein the oxide material is a Gallium Oxide (Ga2O3) based alloy.
4. The device as claimed in claim 2, wherein the nitride material is a Gallium Nitride (GaN) based alloy.
5. The device as claimed in claim 1, wherein, in case of front illumination, the second band gap is higher than the first band gap.
6. The device as claimed in claim 1, wherein, in case of back illumination, the second band gap is lower than the first band gap.
7. The device as claimed in claim 6, wherein the semiconducting substrate and the one or more buffer layers disposed over the semiconducting substrate are transparent to the incident light.
8. The device as claimed in claim 1, wherein the first contact and the second contact are metallic and made of any of identical and non-identical metals.
9. The device as claimed in claim 1, wherein a capping metal is provided on each of the first contact and the second contact, to prevent oxidation of the contact metal.
10. The device as claimed in claim 1, wherein each of the first contact and the second contact is passivated by deposition of a wide band gap dielectric.
11. A method for fabricating an ultraviolet light detecting device, said method comprising the steps of:
depositing a first absorption layer having a first bandgap on top of a base comprising a semiconducting substrate followed by one or more buffer layers, the first absorption layer configured to receive and absorb an incident light of a first wavelength;
depositing a second absorption layer having a second bandgap on top of the first absorption layer, the second absorption layer configured to receive and absorb incident light of a second wavelength, wherein the second absorption layer has a band offset with the first absorption layer to allow transport of photo-generated carriers across a junction between the second absorption layer and the first absorption layer;
depositing a first contact on the first absorption layer; and
depositing a second contact on the second absorption layer, such that the first contact and the second contact transport carriers across the junction between the first absorption layer and the second absorption layer respectively to an external circuit,
wherein the first absorption layer and the second absorption layer selectively generate carriers based on the wavelength of incident light they absorb to enable selective response of the device to a range of incident light wavelengths.