WO2024072325A1

WO2024072325A1 - Ultra-low voltage ultraviolet photodetector

Info

Publication number: WO2024072325A1
Application number: PCT/SG2023/050642
Authority: WO
Inventors: Yuhan PU; Yung-Chii LIANG
Original assignee: National University of Singapore
Current assignee: National University of Singapore
Priority date: 2022-09-26
Filing date: 2023-09-22
Publication date: 2024-04-04
Anticipated expiration: 2025-03-26

Abstract

Ultra-low voltage AlGaN/GaN ultraviolet (UV) photodetectors (21) are provided. The UV photodetector (21) comprises an anode electrode (17), fin trenches (15) filled with a Schottky contact ITO layer to form fin-shaped cathode electrodes (19), and partial sidewall oxide passivation (16) around the fin trenches (15) split an intrinsic 2DEG plane (11) located in the GaN layer into islands; the 2DEG plane is isolated from the cathode electrodes (19) and transformed into field plates when biased by the anode electrode (17). Owing to the intrinsic polarization field and fin-shaped cathode electrodes (19), photo-carriers collections are effected at a bias voltage as low as about 200 mV, with a 365-nm photocurrent-to-dark current ratio above 105 and a peak UV responsivity around 103. As a result, enhanced frequency response is exhibited up to about 1 kHz on 365-nm UV switching. The photodetector (21) is suitable for future ultra-low voltage III-V circuits integration.

Description

ULTRA-LOW VOLTAGE ULTRAVIOLET PHOTODETECTOR

RELATED APPLICATION

The present invention claims priority to Singapore patent application no. 10202251157B filed on 26 September 2022, the disclosure of which is incorporated in its entirety.

FIELD OF INVENTION

The present invention relates to the field of ultraviolet (UV) photodetectors formed with fin-shaped heterojunction structures, which are operable at low voltages or consume low power.

BACKGROUND OF THE INVENTION

A great deal of effort has been directed in developing UV photodetectors on AlGaN/GaN heterojunction structure in the past three decades. Profiting from the improved material growth technique and smart device design, desired performances such as ultra-high photo- to-dark current ratio (PDCR) and high responsivity have been realized in recent years. However, the working bias of the AlGaN/GaN-based UV photodetectors is in a common range from 5 V to 15 V. The publications Y.M. Zhao, et al. IEEE J. Quantum Electron. 56(3), (2020) and M. Martens, et al. Appl. Phys. Lett. 98(21), (2011) 211114 are two examples. H. Zhang, et al. Appl. Phys. Lett. 118, (2021) 242105, teaches about a phototransistor design but that requires an additional gate bias of around 5 V. According to the International Technology Roadmap for Semiconductors 2.0 (ITRS) (2015), the integrated circuit supply voltage will continue to scale down from 0.65 V in 2021 to 0.4 V in digital circuits with the advancement of III-V and germanium devices. The frequency response can only realize tens of Hertz at room temperature in recent reported AlGaN/GaN-based photodetector works.

As published in Appl. Phys. Lett. 121, 062105 (2022) and illustrated in Figures 1 and 2, a two-dimensional electron gas (2DEG) is blocked by a shallow trench in front of the cathode electrode. The anode and the cathode electrodes are parallel plates. Herein the distance between the anode and the cathode electrodes needs to be large, thus preventing low voltage operation. It operates above 3 V bias. Patent No. US 10,734,537 B2 teaches about a surface graphene-based comb-shaped anode electrode, as illustrated in Figures 3 and 4. The photo-carrier collection is in the top region, with electron dominated collection. The 2DEG channel remains active, thereby limiting the operation range.

Patent No. US 11,302,835 B2 teaches about an infrared (IR) and UV filtering stack which controls the photo-carrier generation in the AlGaN/GaN layers, as illustrated in Figures 5 and 6. It is a standard AlGaN/GaN HEMT structure with large dark current. The anode and cathode electrode contact layouts are planar.

Publication. No. US 2019/0165032 Al teaches about a structure that is formed by selectively etching or disabling the 2DEG region, and to form the comb-shaped regions for anode and cathode electrodes, as illustrated in FIGs. 7 and 8. When the bias is applied, an electric field is formed for photo-carrier collection. The distance between regions 115 and 135 cannot be large, and the material 220 can be intrinsic conductive. Also, the hole carriers may have difficulty to be collected by the 2DEG plate.

P. F. Satterthwaite, et.al, ACS Photonics 5, 4277 (2018), teaches a photodetector with a trenched interdigital structure. In this prior art. the collection comb is fabricated by patterning AlGaN layer, so the 2DEG underneath becomes partitioned into anode and cathode regions, which allow a shallow collection between these two regions. This will leave a large PPC (Persistent Photo Conduction) effect in the GaN region, and thus it limits the frequency response of the device; 5 Hz frequency response had been demonstrated in this paper. Also, the prior art operates at a higher voltage of 5V bias for photocurrent collection. In the recently reported works such as X. Tang, et al. Appl. Phys. Lett. 119, (2021) 013503, the frequency response can only realize tens of Hertz at room temperature.

It can thus be seen that there exists a need to provide other UV photodetectors that are advantageous over the existing prior art, such as, exhibiting high photo-to-dark current ratio (PDCR), high frequency response or low bias voltage operations.

SUMMARY OF THE INVENTION

The following presents a simplified summary to provide a basic understanding of the present invention. This summary is not an extensive overview of the present invention, and is not intended to identify key features of the invention. Rather, it is to present some of the inventive concepts of this invention in a generalised form as a prelude to the detailed description that is to follow.

According to an embodiment of the present invention, a trenched-fin-shaped AlGaN/GaN UV photodetector is provided for ultra-low voltage operations. The active region features UV photodetector cells comprising a sinker anode metal structure to function as the sinker anode electrode, a trenched-fin structure filled with a Schottky contact metal such as indium tin oxide (ITO) to serve as the sinker cathode electrode, and partial sidewall oxide passivation along the trenches to split the two-dimensional electron gas (2DEG) plane into islands; as a result, the 2DEG plane is isolated from the sinker cathode electrode and is transformed into field plates to be biased by the sinker anode electrode. The Schottky contact metal extends to the top of the UV photo-current collection cells and over the AlGaN layer. The sinker cathode electrodes 19 are connected together by a layer of the Schottky contact metal to form a top cathode electrode. As there exists a Schottky barrier junction at the sinker cathode electrodes and the GaN layer interface, when the photodetector is reversely biased by the supply voltage, the leakage current is low.

In other words, this invention discloses a novel trench-etching scheme that is filled with a Schottky contact metal for the sinker cathode electrodes to form the fin-shaped photodetector cell structure, thereby relying on an innovative mechanism for photocarriers collection.

Owing to the intrinsic polarization field and the fin-shaped field plates, the collection of photo-carriers is effective at a bias voltage as low as or lower than about 200 mV, with the 365-nm photocurrent-to-dark current ratio (PDCR) above 10⁵ and the peak UV responsivity around 10³, as measured on an embodiment of the UV photodetector. With the low bias voltage, the power consumption is reduced and an enhanced frequency response is observed up to about 1 kHz on 365-nm UV switching. The 2DEG islands are narrow in dimensions, hence allowing very low bias voltage to operate the effective photocarriers collection. Under the reverse-biased condition, besides the polarization field intrinsically existing near the 2DEG junction, two sets of 3-D electric fields are produced respectively in the AlGaN layer and GaN layer. When incident UV light exceeds the AlGaN/GaN bandgap energy, electron and hole pairs are generated within an UV penetration depth of around 250 nm. These carriers then drift under the influence of the two electric fields in the collection cells. Electrons drift towards the 2DEG islands; holes in the AlGaN layer drift upwards to the top cathode electrode over the photodetector cells and holes in the GaN layer drift to the deep bulk to be collected by the sinker cathode electrode at the two sides. The enhanced frequency response is a result of effective photocarriers collection mechanism.

According to an embodiment, the sinker anode electrode comprises layers of Ti/Al/Ni/TiN that form ohmic connection with the 2DEG islands for collecting the electron carriers.

According to another embodiment of the present invention, a method of fabrication of the photodetector is provided. The photodetector obtained can be integrated with a low system voltage integrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be described by way of non-limiting embodiments of the present invention, with reference to the accompanying drawings, in which:

FIG. 1 illustrates a prior art photodetector where a two-dimensional electron gas (2DEG) is blocked by a shallow trench in front of the cathode electrode;

FIG. 2 illustrates the directions of movements of the electrons and the holes under bias for the photodetector shown in FIG.1 ;

FIG. 3 illustrates a prior art photodetector with a surface graphene -based comb-shaped anode electrode;

FIG. 4 illustrates corresponding photocurrent response for the photodetector shown in FIG. 3;

FIG. 5 illustrates a prior art device with an infrared (IR) and UV filtering stack which controls the photo-carrier generation in the AlGaN/GaN layers.

FIG. 6 illustrates a planar anode electrode and cathode electrode contact layout for the device shown in FIG. 5;

FIG. 7 illustrates a planar view of a prior art device;

FIG. 8 illustrates a corresponding structure of the device at FIG.7 that is formed by selectively etching or disabling the 2DEG region and forming the comb-shaped regions for anode and cathode electrodes;

FIG. 9 is a 3-D schematic illustration of an embodiment of the present invention showing a trenched-fin-shaped AlGaN/GaN ultraviolet photodetector;

FIG. 10 in a planar view illustrates an optical microscope image of the trenched-fin-shaped UV photodetector shown in FIG. 9.

FIG. 11 on the left hand side illustrates the x-x’ cross-sectional view at FIG. 9 showing the sinker anode electrode connection with the 2DEG islands. The right hand side illustrates the y-y’ cross-sectional view at FIG. 9 showing the trenches and the cathode electrode connections;

FIG. 12 is a schematic representation of the photocurrent collection in a photodetector cell and illustrates the electric field distribution under a bias for the photodetector cell as shown by the dashed region at FIG.11 ;

FIG. 13 in a schematic representation illustrates the drift path of the photo-generated carriers under influence of the electric fields in the photodetector cell as shown by the dashed region at FIG.11 ;

FIG. 14 provides the steady state log scale measured I-V of the dark current, the photocurrents under 0.7 mW/cm² 365-nm UV light and visible light (hollow symbols), and the PDCR (solid dots) for the photodetector shown in FIG. 9;

FIG. 15 provides the photo-responsivity as a function of 365-nm UV light intensity at various bias voltages for the photodetector shown in FIG. 9;

FIG. 16 shows the photocurrent frequency response for the photodetector shown in FIG. 9, as detected and displayed by an oscilloscope;

FIG. 17 describes a fabrication process flow with masking steps for forming the photodetector shown in FIG. 9 according to an embodiment;

FIG. 18 illustrates the starting wafer specification for the fabrication process flow shown in FIG. 17;

FIG. 19 illustrates the mesa and sinker anode electrode region formation with MaskO;

FIG. 20 illustrates the sinker cathode electrode region formation with Maskl;

FIG. 21 illustrates the sidewall passivation layer deposition;

FIG. 22 illustrates the passivation layer etching to open the sinker anode electrode region with Mask2; FIG. 23 illustrates the passivation layer etching to open the sinker cathode electrode region and etching the trenches deeper with Mask3;

FIG. 24 illustrates the deposition of the metal for sinker anode electrode with Mask4;

FIG. 25 illustrates removal of the passivation layer on the active regions with Mask5;

FIG. 26 illustrates ITO deposition and region definition with Mask6; and

FIG. 27 illustrates metal pad deposition over the top cathode electrode using Mask7.

DETAILED DESCRIPTION OF THE INVENTION

The following description presents several preferred embodiments of the present invention in sufficient detail so that those skilled in the art can make and use the present invention.

FIGs. 1 to 8 have been referred to in the background section.

FIG. 9 depicts a 3-D schematic of a trenched-fin-shaped ultraviolet photodetector according to an embodiment of the present invention; as shown in FIG. 9, the trenched- fin-shaped ultraviolet photodetector 21 is formed in a stack 24 of an AlGaN layer 10, a GaN layer 12, and a substrate formed with a buffer layer 13. A heterojunction is formed between the AlGaN layer 10 and the GaN layer 12, and a two-dimensional electron gas (2DEG) plane 11 is intrinsically formed in the GaN layer 12 near the heterojunction. The active region features multiple UV photo-current collection cells 22 comprising a sinker anode 17 metal structure to function as the sinker anode electrode 17, trenches 15 filled with a Schottky contact metal to serve as the sinker cathode electrode 19, and partial sidewall passivation layer 16 formed around the trenches 15 to split the two-dimensional electron gas (2DEG) plane 11 into islands; as a result, the 2D EG plane 11 is isolated from the cathode electrodes 19 and is transformed into field plates when biased by the sinker anode electrode 17. On the top of the UV photo-current collection cells 22 and over the AlGaN layer 10, the sinker cathode electrodes 19 are connected together by a layer of the Schottky contact metal to form a top cathode electrode 23. In one embodiment, the Schottky contact metal used is Indium tin oxide (ITO). Surrounding the sidewall of the trenches 15, a substantially 80-nm thick oxide passivation layer 16 is deposited, which splits the 2DEG plane 11 into islands and transforms it into a field plate to be biased by the sinker anode electrode 17. The metal for the sinker anode electrode 17 forms an ohmic contact with the GaN layer 12. In one embodiment, a metal stack of Ti/Al/Ni/TiN is used to form the anode electrode 17 that connects with the 2DEG islands 11 to function as a collection plate for the electron carriers. A cathode metal pad 20 is formed by a metal stack of Ni/TiN over the top cathode electrode 23, which functions as a collection plate for the hole carriers. As there exists a Schottky barrier junction at the sinker cathode electrode 19 and the GaN 12 interface, when the photodetector 21 is reversely biased by the supply voltage, leakage current is suppressed low. FIG. 10 is the optical microscopy picture of the planar view of the fabricated trenched-fin-shaped the UV photodetector 21. Herein the top cathode electrode 23 is connected to the sinker cathode electrodes 19 in the trenches 15 to form a comb or fin-shaped cathode electrodes 19; in one embodiment, the comb or fin-shaped cathode electrodes 19 comprise six fingers, which are about 20 pm in width and about 20 pm in separation distance. On the left hand side, FIG. 11 shows the x-x’ cross-sectional view of the connection of the sinker anode electrode 17 with the 2DEG plane 11. On the right hand side, FIG. 11 shows the y-y’ cross-sectional view of the trenches 15, the passivation layer 16, the sinker cathode electrode 19 and the top cathode electrode 23. FIG. 12 is a schematic representation of the photocurrent collection cell 22 as shown by the dashed region in FIG. 11 and illustrating the electric field distribution under a bias condition. Under the reverse-biased condition, besides the polarization field intrinsically existing near the 2DEG plane 11, two sets of 3-D electric fields are produced respectively as shown, in the AlGaN layer 10 and in the GaN layer 12. FIG. 13 is a schematic representation of photocurrent collection cell 22 illustrating the drift paths of the photogenerated carriers under influence of the electric fields. When the incident UV light exceeds the AlGaN/GaN bandgap energy, electron and hole pairs are generated within the UV penetration depth of around 250 nm. These carriers then drift under the influence of the two electric fields in the collection cell 22. The electrons drift towards the 2DEG islands 11 and are collected by the sinker anode electrode 17. The holes in the AlGaN layer 10 drift upwards to the top cathode electrode 23, and the holes in the GaN layer 12 drift to the deep bulk and are collected by the sinker cathode electrode 19 at the two sides.

FIG. 14 provides a steady state log scale I-V of the dark current, the photocurrents under about 0.7 mW/cm² 365-nm UV light and visible light (hollow symbols), and the PDCR (solid dots). The steady-state currents of the fin-shaped photodetector 21 were measured using the probe station and semiconductor analyzer (Agilent B1500A). Due to the small dimensions of each fin-shaped photocurrent collection cell 22, a considerably large electric field that enables effective photo-carrier collection can be formed even under a low biasing voltage. A high photocurrent of about 1.26 mA/mm at 2.0 V bias, and a peak responsivity of about 5080 AAV at 2.0 pW/cm² are achieved. The insignificant response observed with the visible light indicates that the trenched-fin-shaped photodetector 21 can be regarded as insensitive to the visible wavelength. The normalized PDCR against the bias voltage is shown by the right vertical axis. FIG. 15 shows the photo-responsivity as a function of 365-nm UV light intensity at various bias voltages. The peak values and the falling ratio correlated to the voltage suggests that the photodetector 21 is capable of functioning at ultra-low bias conditions at as low as about 200 mV.

FIG. 16 shows the photocurrent response as detected and displayed by an oscilloscope. The steady-state photo-responses of the fin-shaped photodetector 21 was measured using the probe station and semiconductor analyzer (Agilent Bl 500 A) under the exposure of a 365 nm-centered UV light at about 0.7 mW/cm². For the transient performance, the 365 nm-LED is driven by square waves at a frequency ranging from 10 Hz to 1 kHz. Persistent photoconductivity (PPC) effect where the photocurrent commonly exhibits a severely long decay time after the light stimulus is turned off is a common issue, thus limiting the transient performance of GaN-based photodetectors. Enhanced transient performance was observed in the trenched-fin-shaped UV photodetector 21 at a low biasing voltage of as low as about 0.2V. Arising from the narrow width of the photocurrent collection cell 22, the drift lengths of the photo-carriers are shortened, and hence, the photocurrent collection process is enhanced. Although the inevitable peak-to-peak magnitude degradation occurs in the transient performance, FIG. 15 demonstrates a good frequency response of up to about 1 kHz level at room temperature or about 300 K. For the present invention, the enhanced transient performance was observed in the fin-shaped UV photodetector 21 even at a low biasing voltage of about 0.2 V. During transient performance testing, a bias voltage of 1.5V was used, so as to reduce the PPC effect and to yield sufficient signal-to-noise ratio.

FIG. 17 describes a fabrication process for forming the photodetector 21 with masking steps according to an embodiment of the present invention. The mask patterns are defined by a laser writer and used for various purposes. In one embodiment, steps of reactive ion etching are conducted by ICP-RIE. In another embodiment, ohmic anode and cathode metal depositions are conducted by electron beam evaporation followed by lift-off in acetone, whilst the ITO is deposited by sputtering.

FIG. 18 illustrates a start of the above wafer fabrication process. As shown in FIG. 18, the wafer is fabricated from a substrate with the buffer layer 13, the GaN layer 12 and the AlGaN layer 10 formed according to respective thicknesses. For eg., the Alo.25Gao.75N/GaN epitaxy wafer used is commercially available from DOWA Electronics Materials Co. Ltd.

FIG. 19 illustrates the definition of a mesa with MaskO by etching the AlGaN/GaN layers up to a depth of dl below the 2DEG plane for forming the sinker anode electrode region 14. With reference to FIG. 11, the view on the left-hand side corresponds to the x-x' cross sectional view, while the view on the right-hand side corresponds to the y-y' cross sectional view. FIG. 20 illustrates the formation of the trenches 15 with Maskl by etching the AlGaN/GaN layers up to a depth of d2 for forming the sinker cathode electrodes 19 subsequently. Herein, the depth d2 allows the bottom of the trenches 15 to extend substantially below the 2DEG plane 11. FIG. 21 illustrates the passivation layer 16 deposition of about 80 nm thick silicon dioxide by plasma enhanced chemical vapor deposition (PECVD). FIG. 22 illustrates that through the Mask2, selectively the passivation layer 16 is etched from the anode electrode region 14 by the ICP-RIE followed by a wet etching. FIG. 23 illustrates the selective opening for the trenches 15 through Mask3. Herein, the ICP-RIE etches the silicon dioxide passivation layer 16 at the bottom surface and thereafter etches about 200nm of the GaN layer 12 through the bottom surface to deepen the depth of the trenches 15 to a depth d3, which is substantially 250 nm deep. Thus, the trenches 15 have the sidewall passivation oxide 16 only up to the depth of d2 (which extends below the 2D EG plane 11). Hence, the trenches 15 extend below the 2DEG plane 11 and are surrounded by the sidewall passivation layer 16 only partially. In one embodiment, the depth dl is substantially 80 nm, whilst the depth d2 is substantially 200 nm.

FIG. 24 illustrates the metal stack deposition for the sinker anode electrode 17 with Mask4. Through the mask pattern, an ohmic metal stack of Ti/Al/Ni/TiN (substantially 25/125/45/60nm) is deposited by e-beam evaporation followed by lift-off in acetone. After the mask pattern is removed, a step of rapid thermal annealing of the metal stack follows in nitrogen at about 850 degrees centigrade for about 30 secs. FIG. 25 illustrates etching of the oxide of the passivation layer 16 on the active regions 18 over the AlGaN layer 10 through the Mask5 for the following Schottky contact metal deposition. FIG. 26 illustrates ITO deposition of about lOOnm thickness by sputtering and patterning by laser writing, followed by dry etching with RIE to remove the unwanted ITO, and a rapid thermal annealing in nitrogen at about 600 degrees centigrade for about 5 minutes. FIG. 27 illustrates a step for depositing the cathode metal pad 20 through Mask7. In one embodiment, the cathode metal pad 20 is formed by a metal stack of Ni/TiN (substantially 20/80nm) using an e-beam evaporator, followed by lift-off in acetone. A step of annealing follows thereafter in nitrogen.

The above fabrication process teaches fabricating the fin-shaped photodetector 21 of the present invention according to some embodiments; also, as described, the photodetectors obtained exhibit high photo-to-dark current ratio (PDCR), high frequency response or low bias voltage operations; in other words, the photodetectors obtained exhibit these advantages:

• Effective electron-hole pair collection under low biasing voltage (as low as about 200mV) due to shorter drift lengths;

• Reduction in power consumption and thus providing stable operations with frequency responses of up to about 1kHz; and

• Compatible with production lines of existing foundries for quick launching of new devices incorporating these UV photodetectors.

In addition, performance of this fin-shaped photodetector 21 in terms of the photo-to-dark- current ratio and the responsivity can be further optimized by tuning the device dimensions and by selecting other suitable Schottky contact materials for the cathode electrode. The disclosed fin-shaped UV photodetector is very suitable for future ultra-low voltage III-V integrated circuits, in which the semiconductor materials respond to different photowavelengths (such as, UV, visible or IR) without detracting the above fin-shaped photocurrent collection mechanism. In addition, this non-gold CMOS compatible fabrication is compatible with most of the existing semiconductor foundries. While the foregoing description presents preferred embodiments of the present invention along with many details set forth for purposes of illustration, it will be understood by those skilled in the art that many variations, modifications or combinations of variations disclosed in the text description and drawings thereof could be made to the design, construction and operation without departing from the scope of the present invention.

Claims

CLAIMS:

1. An ultraviolet photodetector operable at a low voltage comprising: a stack of an AlGaN layer over a GaN layer on a buffer and substrate layer, thereby forming a heterojunction of AlGaN/GaN with an intrinsic two-dimensional electon gas (2DEG) plane at the heterojunction; a sinker anode electrode formed to a predetermined depth so as to make ohmic contact with the AlGaN layer, the 2DEG plane and the GaN layer; fin-shaped trenches formed to a predetermined depth extending through the AlGaN layer, the 2DEG plane and the GaN layer, and are filled with a Schottky contact material to form fin-shaped cathode electrodes; and partial sidewall passivation layer formed around each of the fin-shaped cathode electrode, with the passivation layer having a predetermined depth that extends below the 2D EG plane and a distal end of the fin-shaped cathode electrodes make Schottky contact with the GaN layer; wherein, when the UV photodetector is illuminated with UV light that exceeds the AlGaN/GaN bandgap energy, electrons and holes pairs are generated, such that electrons drift towards both the 2DEG plane and the anode, whilst holes drift towards the fin-shaped cathode electrodes.

2. The UV photodetector according to claim 1, further comprising: a top cathode electrode formed over the AlGaN layer by a Schottky contact metal that interconnects the fin-shaped cathode electrodes and forms Schottky contact with the AlGaN layer.

3. The UV photodetector according to claim 2, further comprising: a metal pad layer formed over part of the top cathode electrode to provide an ohmic contact.

4. The UV photodetector according to claim 2 or 3, wherein the fin-shaped cathode electrodes comprise indium tin oxide (ITO).

5. The UV photodetector according to any one of claims 1-4, wherein the sinker anode electrode comprises Ti/Al/Ni/TiN. The UV photodetector according to any one of claims 3-5, wherein the metal pad layer comprises Ni/TiN. The UV photodetector according to any one of claims 1-6, wherein the fin-shaped cathode electrodes are substantially about 20 pm in width and about 20 pm in separation distance. A method for fabricating the UV photodetector defined in claims 1-7, the method comprises: forming or selecting a silicon substrate with a buffer layer, a GaN layer and an AlGaN layer; defining a sinker anode electrode region with a depth dl by etching through a first mask; defining sinker cathode electrode trenches to a depth d2 by etching through the first mask, wherein the depth d2 being less than dl but extending below an intrinsic 2DEG plane; depositing a passivation layer over the partially formed silicon substrate; etching the passivation layer at the sinker anode electrode region through a second mask; etching the passivation layer at the bottom of the cathode electrode trenches through a third mask; etching the GaN layer to deepen the cathode electrode trenches through the third mask, so that d2 is substantially as deep as dl; forming the sinker anode electrode through a fourth mask by depositing a metal to form an ohmic contact with the AlGaN layer and the GaN layer; annealing the sinker anode electrode metal; etching the passivation layer over the AlGaN layer through a fifth mask; filling a Schottky metal into the sinker cathode electrode trenches through a sixth mask to form sinker cathode electrodes and a top cathode electrode extending over a top of the sinker cathode electrodes; annealing the Schottky metal of the cathode electrodes; and depositing a metal pad over the top cathode electrode using a seventh mask; wherein the sinker anode electrode and the metal pad deposited over the top cathode electrode provide external electrical connections to the UV photodetector.

9. A method of configuring a UV photodetector defined in claims 1-7, the method comprises: forming or selecting a silicon substrate with a buffer layer, a GaN layer and an AlGaN layer; forming an anode electrode in contact with both the GaN and the AlGaN layers; forming fin-shaped trenches through the AlGaN layer and into the GaN layer; depositing a sidewall passivation layer around part of the trenches to split an intrinsic 2DEG plane into islands; filling the trenches with indium tin oxide (ITO) to form fin-shaped cathode electrodes; depositing an ITO layer over a top of the fin-shaped electrodes; and forming a cathode metal pad over a part of the ITO layer; wherein the anode electrode and the cathode metal pad provide external electrical connections to the UV photodetector.