United States

Abstract Body: Energy-saving photodetectors have recently attracted intensive research interest to meet the great demands of future photonic systems. Particularly, according to the large photocurrent, high sensitivity, environment-sensitivity, and lithography-free features, self-powered photoelectrochemical-type photodetectors (PEC PDs) have attracted tremendous attention in the optoelectronics community. However, owing to their poor crystal quality and low carrier separation efficiency of chemically synthesized semiconductor materials, PEC PDs based on such materials demonstrate slow response and low responsivity. In our work, we construct PEC PD using defect-free AlGaN nanowires photoelectrode which synthesized through molecular beam epitaxy (MBE)[1][2]. After decorating AlGaN nanowires with noble metal species (Pt or Ru), our PEC PDs feature the combined benefits of effective carrier separation and fast interfacial kinetics, resulting in superior performance compared to previous reported solar-blind PEC PDs (Responsivity of 45 mA/W and response/recovery time of 47/20 ms for Pt decorated p-type AlGaN nanowires; Responsivity of 48.8 mA/W and response/recovery time of 83/19 ms for Ru decorated n-type AlGaN nanowires;). Besides, according to the attractive surrounding sensitivity of PEC PDs, their photoresponse behaviors can be effectively modulated by varying the applied bias potential, light intensity, and the concentration of the electrolyte further promotes its adaptability in different working conditions. Our work suggests that embracement of defect-free wide-bandgap semiconductor nanowires with appropriate surface decoration offers an unprecedented opportunity for designing future energy-efficient and large-scale optoelectronic systems on the silicon platform. Reference: [1] D. Wang, X. Liu, S. Fang, C. Huang, Y. Kang, H. Yu, Z. Liu, H. Zhang, R. Long, Y. Xiong, Y. Lin, Y. Yue, B. Ge, T. K. Ng, B. S. Ooi, Z.Mi, J. H. He, and H. Sun “Pt/AlGaN Nanoarchitecture: Toward High Responsivity, Self-Powered Ultraviolet-Sensitive Photodetection,” Nano Lett., 2021, 21, 1, 120–129. [2] D. Wang, C. Huang, X. Liu, H. Zhang, H. Yu, S. Fang, B. S. Ooi, Z. Mi, J. H. He, and H. Sun, “Highly Uniform, Self-Assembled AlGaN Nanowires for Self-Powered Solar-Blind Photodetector with Fast-Response Speed and High Responsivity,” Adv. Optical Mater., 2020:202000893.

EMC 2021

AlGaN-Based Nanowires for Self-Powered Ultraviolet Photodetectors with High Responsivity

solar-blind photodetection; photoelectrochemical photodetector; algan

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DRC and EMC Joint Conference Day

technical paper

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Abstract Body: Lead chalcogenide (PbX, X = S, Se, Te) colloidal quantum dots (CQDs) support rich optoelectronic phenomena, offering an attractive platform for advanced optoelectronics based on low-dimensional materials. Experimental results have shown a change in CQD shape and optical characteristics as the CQDs grow in size. This work aims at understanding the effect of size, shape, and composition on the sensitive optical behavior in this particular class of CQDs using density functional theory (DFT) code Octopus. Resonant optical transitions and energy relaxation pathways are elucidated, with emphasis towards activity in the 3-5 µm mid-wave infrared (MWIR) spectrum. Optoelectronic results are presented for the bulk material, isolated slabs representative of specific crystalline facets, and for the octahedral through cubic morphologies that evolve with CQD growth.

Ab Initio Studies of the Optical Properties of Lead Chalcogenide Nanocrystals

Abstract Body: There is continued interest in the combination of Si and Ge in unique geometries to further improve the performance of transistors [1]. A novel diffusion mechanism of Ge along an oxidizing Si/SiO2 interface was reported, which resulted in the formation of strained Si nanowires [2]. By taking advantage of this diffusion mechanism, there is the potential of forming site-controlled nanowires, quantum dots, and arbitrary nanoshapes. However, there is limited understanding of the diffusion mechanism that results in the formation of these nanostructures. To investigate this mechanism, alternating 15nm thick layers of Si and Si0.7Ge0.3were grown in a superlattice and patterned into fins via electron beam lithography. The final pattern contained multi-layered Si/SiGe fins with widths varying from 70 to 280nm with a 100nm layer of Si substrate exposed at the base. This extended Si surface enabled the observation Ge’s diffusion down the fin’s sidewall during high temperature oxidation. Ge’s lateral diffusion was observed over a range of times at temperatures between 800 and 950°C. Cross-sectional TEM samples were prepared after each anneal and the diffusion length was measured through analysis of HAADF-STEM images, utilizing z-contrast. The diffusion length was measured after four different oxidation times at each temperature and the diffusivity was extracted. An Arrhenius plot of the data indicated the activation energy of Ge diffusion down the oxidizing interface is 2.50 +/- 0.3 eV. This is approximately half of activation enthalpy for Ge’s movement into bulk Si or through the oxide [3], [4]. We propose that the Ge may be diffusing through the suboxide along the Si/SiO2 interface and is subsequently reinserted back into the Si during oxidation. This is further supported by the linear correlation between the thickness of the thermally grown oxide and the distance the Ge diffuses down the Si sidewall. The difference in the activation energy compared to bulk interdiffusion helps to explain why the nanowires and quantum dots are formed within these materials during oxidation. In addition to dry oxidation anneals, anneals in a wet oxidizing ambient (steam) will be discussed. A model for the diffusion using the Florida object-oriented process and device simulator (FLOOXS) was developed. This model was used to assist in the extraction of diffusivities and is being further developed to help predict possible structures using this phenomenon. Preliminary results using the process simulator’s model will be compared to the experimentally observed evolution of the nanostructures. This work is supported by Sandia National Laboratories, which is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. [1] T. David et al., “New strategies for producing defect free SiGe strained nanolayers,” Sci. Rep., vol. 8, no. 1, pp. 1–10, Feb. 2018, doi: 10.1038/s41598-018-21299-9. [2] W. M. Brewer, Y. Xin, C. Hatem, D. Diercks, V. Q. Truong, and K. S. Jones, “Lateral Ge Diffusion During Oxidation of Si/SiGe Fins,” Nano Lett., vol. 17, no. 4, pp. 2159–2164, Apr. 2017, doi: 10.1021/acs.nanolett.6b04407. [3] R. Kube et al., “Composition dependence of Si and Ge diffusion in relaxed Si1−xGex alloys,” J. Appl. Phys., vol. 107, no. 7, p. 073520, Apr. 2010, doi: 10.1063/1.3380853. [4] Y. Dong, Y. Lin, S. Li, S. McCoy, and G. Xia, “A unified interdiffusivity model and model verification for tensile and relaxed SiGe interdiffusion over the full germanium content range,” J. Appl. Phys., vol. 111, no. 4, p. 044909, Feb. 2012, doi: 10.1063/1.3687923.

Rapid Ge Diffusion Along Si/SiO2 Interfaces During High Temperature Oxidation for Quantum-Scale Structures

Abstract Body: With the discovery of topological insulators, a wide list of potential applications for their unique spin-momentum locked surface states have been proposed. Among these possibilities, it has been posited that electrons occupying these surface states might be useful as information carriers, since they cannot be scattered by a non-magnetic perturbation. A quantum dot-like system with discrete states that features these properties is of interest. To explore the potential of producing such a device, we first must learn more about how to make topological insulator quantum dots, and then study whether the topologically-protected surface states exhibit discretization while maintaining their unique properties. This study focuses on the growth and characterization of self-assembled, selenized bismuth particles on GaAs via droplet epitaxy in a molecular beam epitaxy system. In droplet epitaxy, we first deposit a small amount of bismuth which has a high surface tension and does not wet the substrate, resulting in the formation of bismuth clusters. These clusters are then exposed to a selenium flux, forming Bi2Se3 nanoparticles. For this study, growth time and substrate temperature during deposition were varied, and final particle density, average height and area were measured via a combination of atomic force microscopy and scanning electron microscopy. Within the substrate temperature (250-285 οC) and bismuth deposition time (20-100 seconds) ranges, we observed variations in average height, area, particle density, and polydispersity. Within the growth window, particles are generally 5-15 nm tall, and 500-3000 nm2 in area (equivalent circular radius ~12-30 nm). When the amount of bismuth initially deposited increases, the nanoparticle density also increases, though this effect becomes less significant at higher substrate temperatures. Above 275 οC, the average particle height and area settle around 10 nm and 500-1000 nm2 (equiv. circular radius ~12-17 nm). Polydispersity in particle height and area tends to drop relatively significantly above 275 οC as well. There also appears to be some correlation between particle height and area when changing growth conditions: in general, if particles are becoming taller on average, they are also becoming larger in radius. We demonstrated that adding a long anneal step between the bismuth deposition and selenium exposure steps resulted in smaller, shorter, and more numerous particles. These observations support our over-arching theory that the nanoparticle size is determined by bismuth adatom mobility and rate of bismuth removal from an existing nanoparticle rather than nanoparticle mobility. We have demonstrated the ability to affect the properties of particle height, area, density, and polydispersity by changing the initial bismuth deposition time and substrate temperature. These results should prove useful to those interested in creating topological insulator devices through epitaxial growth. Acknowledgments: National Science Foundation, Division of Materials Research under Award No. 1838504 and the Brookhaven National Laboratory/University of Delaware Seed Program under Award No. 20A00145. We also acknowledge the use of the Materials Growth Facility (MGF) at the University of Delaware, which is partially supported by the National Science Foundation Major Research Instrumentation under Grant No. 1828141 and is a member of the Delaware Institute for Materials Research (DIMR).

Growth of Topological Insulator Bi2Se3 Particles on GaAs via Droplet Epitaxy

Abstract Body: Alloy quantum dots (aQD) represent a new class of low-dimensional materials whose electron confinement is determined not exclusively by size, but also by composition field. While homogeneous quantum dots have been more widely explored for applications in photonics, optoelectronics, and photovoltaics, aQDs remain less investigated. The differences in both size and composition field across aQDs lead to differences in material properties. These variations in material properties present compelling potential applications, specifically in the field of energy generation and photovoltaics. This is primarily because of the complexity in characterizing nonuniform fields and establishing the correlation between inhomogeneity and confinement. In order to effectively analyze the varied electron confinement of these materials, we employ a combination of density functional theory based quantum simulations, and finite element analysis based k.p calculations to determine the electron energy levels for a number of uniform and nonuniform physical variables. As a part of the analysis, composition maps were constructed for each of the different aQD shapes, namely, the steep cone, normal cone, hut, and dome. Each of these compositions maps was fitted with an analytical function that was then used to analyze the electronic properties of the aQDs. Results show that a generic expression for all four of the aQDs exists such that modification of this equation allows each of the disparate aQDs to be modeled. The steep cone shaped aQD with a 15° sidewall angle corresponds to a composition map where spatial decay occurs from the tip of the cone to the bottom. The normal cone shaped aQD exhibits a similar behaviour, but the sidewall angle is instead 30°, and the fitting function is modified to a product of a quadratic and a linear function. For the hut shaped aQD, the composition map takes the shape of a “hut” where decay occurs from the top edges towards the bottom edges in a non-linear fashion. For the final shape, the dome, spatial decay emanates from the top tip of the dome. The dome is fitted by two sidewalls per side that have angles of 11° and 30° respectively. For each of the quantum dots above, composition maps obtained are independent of size to determine the singular effects of the compositional variation. Our results suggest that inhomogeneity in composition field has a consequential role in controlling the confinement energy levels. Thus, the shape of aQDs can play a critical role in tailoring the effective electronic behavior. Furthermore, a larger alloy quantum dot, can behave electronically like a smaller quantum dot, if the composition field is inhomogeneous. By engineering quantum dots with certain composition maps, the photon absorption properties of the respective quantum dot can thus be tailored substantially. In particular, the most apparent application of this is to solar cells and spintronics; engineering the properties of the quantum dot allows us to increase the absorption efficiency of the photovoltaic cell, which directly leads to greater potential for green energy generation.

Shape-Effect on Quantum Confinement in Alloy Quantum Dots and Their Applications for Photovoltaic Green Energy

Abstract Body: AlInN has gathered attention as a candidate material for power electronics and optoelectronics. Historically, there were many challenges in growing AlInN by traditional MBE or MOVCD due to conflicting issues of a) phase separation for high temperature growth, and b) low adatom surface mobility for low temperature growth. Metal Modulated Epitaxy (MME) has proven useful for this challenging material by improving adatom kinetics at low temperature and was shown to improve X-Ray diffraction figures of merit by as much as 11X1. However, the source of the film improvement has been unknown up to now. TEM is a useful tool to investigate defect structures but can itself cause damage and induce phase separation if not performed under proper conditions. Of the few TEM reports of AlInN, most focus attention to AlInN around 18% Al and electron-beam induced degradation on AlInN is not investigated. Few reports exist for imaging high In AlInN. The present work focuses on the investigation of MME grown high In AlInN/GaN heterojunctions, explanation of MME’s improved quality compared to prior literature and its potential false defect observation because of damage induced by the electron beam in the TEM. Unintentionally doped (UID) MME AlInN films were grown on MME GaN on hydride vapor phase epitaxy (HVPE) GaN templates on c-plane sapphire substrates as described elsewhere1, using plasma-assisted molecular-beam-epitaxy (PA-MBE) with a target composition of 70% indium. Strain relaxation at a heterojunction of AlInN/GaN in this epitaxial film is investigated by TEM and Scanning TEM (STEM). The cross-section sample is prepared by standard mechanical polishing methods, followed by argon ion milling. Lattice-expansion mapping (or strain mapping) obtained by STEM nano-beam diffractions showed an intermediate lattice constant between AlInN and GaN. This implies that there is a thin intermediate layer where gradual strain relaxation happens from GaN towards AlInN. Moire fringes are detected in the TEM with two-beam condition of g = [1 -1 0 0], indicating full strain relaxation beyond ~10 nanometers via the generation of misfit dislocations at the interface. Moire fringes in MME grown InGaN/GaN resulted from strain exceeding the critical thickness in the first monolayer2,3. Thus, like MME grown InGaN which also has high adatom mobility at low temperatures, high In AlInN appears to relax via the same misfit dislocation mechanism resulting in higher film quality above the misfit array. In order to validate the results, it was also necessary to ensure the electron beam did not damage the films under examination. It was found that a low beam dose of 5.7 A/cm2 was necessary to prevent damage in thicker regions of the AlInN but still appreciably damaged thin sections of the AlInN even for less than 20 seconds exposure. This beam damage threshold is far lower than the ~35 A/cm2 work on InGaN/GaN4 suggesting high In AlInN is more sensitive to beam damage than InGaN. In-situ TEM measurements on g = [0 0 0 2] detected decomposition and In clustering in/on the AlInN layer. The beam-induced damage was not observable in the low-resolution strain map obtained by STEM micro-beam diffraction in comparably thicker regions. This extremely low beam damage threshold complicates TEM analysis of high In AlInN and results in less clear images and quicker analysis time requirements. Further work quantifying the degradation from the electron beam as well as varied compositional studies are ongoing. 1Engel, et. al., J. Appl. Phys., 127, 125301, 2020 2Fischer, et. al., Appl. Phys. Lett., 103, 131101, 2013 3Clinton, et. al,. Solid-State Electronics, vol.136, pp. 3, 2017 4Smeeton, et. al., Appl. Phys. Lett., 83, 5419, 2003

Observation of Interfacial Strain Relaxation in High Indium, AlInN/GaN Heterostructures by Transmission Electron Microscope

Abstract Body: The ultraviolet-C (UVC) light with wavelength from 180 nm to 280 nm has been recognized as a preferable avenue to disinfect virus because it can effectively destroy the chemical bonds inside the DNA or RNA of the microorganisms. III-nitride based UVC light-emitting diodes (LEDs) have been recognized as one of the ideal candidates to replace the toxic mercury-based UV-lamps for the application of air/water purification, disinfection/sterilization and biochemical environmental sensing. However, the optical performance of the UVC LEDs is still limited by the poor external quantum efficiency (EQE) [1, 2]. The size and geometry of the active emitting elements are extremely important considerations in optimizing LEDs. As the InGaN-based LED topic, it has been found that the micro-LED approach can offer the potential to improve overall LED efficiency and efficiency droop. However, a systematic investigation on size-dependent AlGaN-based UVC LED on device performance and detailed analysis of the underlying changing mechanisms have not been studied as much as InGaN based visible LEDs, which have longer history. In this work, the size-dependence of the optical and electrical properties of 265 nm wavelength AlGaN based UVC LED grown on c-plane sapphire is systematically investigated. The LED devices with diameters ranging from 50 μm to 300 μm have been well fabricated. It is shown that the series resistance of the UVC LED increased with the diameter of the device decreased. Moreover, smaller UVC LEDs can deliver higher power densities despite the relatively lower absolute output powers and sustain higher current densities. Our results show that at low injection current densities, the efficiency of smaller LEDs reduces significantly via the non-radiative recombination induced by sidewall defects. At higher injection current densities, the current crowding effect dominates and the efficiency of smaller LEDs becomes higher. Reference: [1] H. Yu, Z. Ren, H. Zhang, J. Dai, C. Chen, S. Long, and H. Sun, "Advantages of AlGaN-based deep-ultraviolet light-emitting diodes with an Al-composition graded quantum barrier," Optics express, 27 (20), A1544-A1553, 2019. [2] Z. Ren, H. Yu, Z. Liu, D. Wang, C. Xing, H. Zhang, C. Huang, S. Long, and H. Sun, "Band engineering of III-nitride-based deep-ultraviolet light-emitting diodes: A review," Journal of Physics D: Applied Physics, 53 (7), 073002, 2019.

Micro-Scale AlGaN-Based Deep Ultraviolet Light-Emitting Diodes Emitting at 265 nm

Abstract Body: Electronic structure calculations of 4H-SiC doped with various transition metals reveal dilute magnetic semiconductor (DMS) behavior in a material suitable for high-power and high-frequency semiconductor devices. Our results are consistent with prior work on V, Cr, and Mn doping and explore additional metals: Fe, Co, and Ni. Charge-state calculations show that the latter maintain amphoteric semi-insulating properties while offering a higher stable spin polarization and also greater asymmetry in the spin density of states than previously studied dopants. This indicates possible enhanced half-metal properties. We then use these discoveries to inform the first calculations of the phonon-assisted optical properties of doped 4H-SiC systems, using the approach first published by Noffsinger et al. [1] based on Maximally Localized Wannier Functions to assist overcoming the numerical challenge. [1]: Phonon-Assisted Optical Absorption in Silicon from First Principles Jesse Noffsinger, Emmanouil Kioupakis, Chris G. Van de Walle, Steven G. Louie, and Marvin L. Cohen Phys. Rev. Lett. 108, 167402 – Published 20 April 2012

Transition Metal Defects 4H-SiC—Optical and Electronic Levels from First Principles

Abstract Body: As a wide bandgap semiconductor, silicon carbide (SiC) possesses unique properties that make it an advantageous replacement for silicon in high power, high frequency, and high temperature electronic devices. Ion implantation followed by high temperature annealing (>1600oC) is often used to introduce and activate dopants in the material. However,the high concentration of point defects created from implantation evolve into extended defects upon annealing, resulting in device failure. In order to address this issue, elevated temperature implants have been used extensively to prevent amorphization but little is known about higher temperature implant effects on defect evolution and how this correlates with dopant activation efficiency. In this study, 60 keV Al+ was implanted into single crystal n-type 4H-SiC wafers to a fluence of either 5x1013 or 1x1015 cm-2. The implant temperature was varied between 300°C and 700°C. High temperature furnace annealing was performed with a carbon cap from 1300°C to 1680°C for 30 minutes. Structural characterization using cross-sectional and plan-view BF-TEM and DF-TEM was performed for extended defect analysis. Ohmic contacts were created from sputter deposition of a Ti/Al/Ni metal stack followed by alloying at 1000oC for 1 minute. Electrical characterization using variable-temperature Hall Effect was performed to study dopant activation. As implanted cross-sectional TEM analysis showed amorphization is avoided for the low fluence (5x1013 cm-2) implants at all implant temperatures. For the high fluence case (1x1015 cm-2), amorphization is avoided above 400°C due to the increasing effects of dynamic annealing. The reduction of lattice damage with increasing implant temperature can be seen in UV/Vis/NIR measurements for the unannealed samples. The absorption peaks corresponding to lattice damage show a consistent reduction as the implant temperature is increased up to 700°C. A carbon cap was introduced to reduce surface decomposition during the high temperature annealing process with a variety of capping methods studied. Graphitized photoresist was shown by AFM measurements to result in the least amount of surface roughening upon annealing at around 1 nm. Extended defects were observed by TEM for the high fluence implants at all implant temperatures after annealing but were absent for the low fluence case. The loops are seen to coarsen with implant temperature as expected but are not eliminated by the anneal. TEM results suggest the concentration of trapped interstitials in the extended defect decreases with increasing implant temperature. Further analysis on the effect of implant temperature on the evolution of these extended defects will be presented. In addition to structural characterization, electrical measurements were carried out. Ohmic contacts were successfully created from the combination of an effective annealing cap and from the optimal metal thickness ratios in the metal stack. However, initial Hall effect measurements at room temperature were not successful in determining the dopant activation values even after 1680°C annealing. This is thought to be due to the relatively deep acceptor level associated with Al in SiC. Subsequently, variable field measurements at 400°C showed a dopant type change from the n-type substrate to the p-type implant enabling dopant activation values to be determined. These electrical results will be compared with structural images to highlight correlations between extended defect evolution and dopant activation efficiency.

Investigation of Implant Temperature on Dopant Activation Efficiency for Single Fluence Al+ Implanted 4H-SiC

Abstract Body: Superior material qualities of 4H-SiC such as wide bandgap, high thermal conductivity, and large breakdown electric field make devices capable of operating with high power at high temperature. In this work, the behavior of p-channel 4H-SiC metal-oxide-semiconductor field-effect transistor (MOSFET), where holes are the channel charge carriers; has been investigated. The experimental results confirm that post oxidation Nitric Oxide (NO) annealing reduces SiO2/4H-SiC interface trap densities (Dit) in the upper half of the 4H-SiC bandgap as widely reported. In addition, Dit is also significantly reduced in the lower half, which is critical for p-channel MOSFETs. This result is corroborated by capacitance-voltage (CV) measurements conducted on p- and n-type 4H-SiC MOS capacitors from room temperature to 300°C, as well as under UV illumination, which allows estimation of the Dit profile throughout the large 4H-SiC bandgap. Thereafter, p-channel 4H-SiC MOSFETs were fabricated and measured from room temperature through 300°C. The MOSFETs without NO annealing turn on at very large negative voltages with high dielectric leakage currents, making it difficult to get reliable high temperature data. In contrast, the NO annealed MOSFETs turn on at a much lower voltage, with low leakage, consistent with the reduction of Dit due to NO annealing. With the rise of temperature, the threshold voltage drops as the number of occupied interface traps reduce with the increase of temperature. However, the temperature dependence of the hole channel mobility is not obvious from these measurements. The weak temperature dependence can be explained considering the behavior of the phonon and Coulomb scattering components. The phonon scattering limited hole mobility reduces from ~120 cm2/Vs at room temperature to less than 25 cm2/Vs at 300°C in lightly doped bulk p-type 4H-SiC. On the other hand, the Coulomb scattering limited surface hole mobility increases weakly with temperature. The competition between the two mechanisms increases at higher temperatures and the overall field-effect mobility shows an almost temperature-independent behavior in the 25°C - 300°C range. While NO annealing realizes high temperature operation, the negative bias temperature instability (NBTI) is a major shortcoming. This is a result of hole injection and trapping in the oxide under high temperature and negative bias stress. In this presentation, the results from the NBTI experiments will be reported and the main mechanisms related to high temperature NBTI will be discussed. The authors gratefully acknowledge the funding support from US Army Research Laboratory grant ARMY-W911NF-18-2-0160 and National Renewable Energy Laboratory/ US Department of Energy grant NREL-AHL-9-92362-01.

High Temperature Characteristics of NO Annealed P-Channel 4H-SiC MOSFET

Abstract Body: III-Nitride (AlInGaN) avalanche photodiodes (APDs) have merits such as high breakdown field, high electron drift velocity, high thermal conductivity, and low dark current. The III-nitride material bandgap is tunable from 0.7 (1772nm) to 6.2eV (200nm), offering good UV sensitivity in both visible blind and solar blind UV regions. The Geiger-mode operation of III-Nitride APDs have numerous applications in UV single-photon detection, such as quantum-key distribution, optical time-domain reflectometry, positron-emission tomography, biomedical research, and UV communications. In this work, we report low-temperature Geiger mode measurements of GaN p-i-n APDs by using a thermoelectric cooling system. The GaN p-i-n APD structures were grown on c-plane hydride-vapor-phase-epitaxy (HVPE) free-standing (FS) GaN or ammonothermal bulk GaN. The epitaxially grown structure consists of a 0.6 mm unintentionally doped GaN, followed by a 2.3mm thick Si-doped GaN n-layer with a free-electron concentration of 4×1018 cm-3. The i-layer of 280nm thickness is grown on top of the n-layer, with a background Si doping of 2×1016 cm-3, targeting breakdown voltage of 96V. The p-layer stack consists of two layers of 100nm Mg-doped GaN and 15nm of highly Mg-doped GaN as the contact layer. The Mg-doped p-layer has a free-hole concentration of 1×1018 cm-3, and the highly Mg-doped p-layer has Mg concentration of 1×1020 cm-3. The fabrication of APD was performed in the following sequence: 1) mesa etching, 2) n-type metal deposition, 3) p-type metal deposition, 4) passivation and via-hole, and 5) metal interconnect deposition. A gated quenching circuit was used in the Geiger-mode measurement. In detail, a noninverting amplifier with gain of 2 and a transimpedance amplifier are inserted before and after the GaN APD in order to isolate the GaN APD and prevent reflection. An oscilloscope measures and collects the avalanche signals. A computer is utilized to perform signal data processing. For spectral response evaluation, the APD device is illuminated with the UV light from a UV LED with the peak wavelength of 375nm. The UV light is split by a bifurcated fiber: one end is fed to GaN APD, and the other end goes to an attenuator before being fed to a silicon photon counter for reference. For the low-temperature control, the thermoelectric cooler is installed directly under the GaN APD. The thermoelectric cooler is installed in an aluminum cage which serves as water-cooled heat sink. To prevent icing, the aluminum cage is sealed and filled with nitrogen. The Geiger-mode setup is installed in a optically isolated dark box. The method of measurement employed is the Δt method based on Poisson statistics. From the collected avalanche signal, the computer determines time interval between the rising edge of each pulse and its corresponding avalanche signal for 10,000 pulse events, then processes these time interval data into one histogram. Finally, by fitting the exponential function to the histogram, the dark count rate R is obtained. With this low-temperature Geiger-mode measurement system, the breakdown voltage was measured at various temperature from -10°C to 10°C with an increment step of 5°C. The breakdown voltage is proportional to the temperature, monotonically varying from 96.9V to 97.25V when the temperature increases from -10°C to 10°C. From this data, we obtained a temperature coefficient of the breakdown voltage of 0.0177±0.0005V/K. Based on the breakdown voltage vs. temperature, the dark count rate of various temperatures at 1V overvoltage above breakdown was measured. The dark count rate decreases as the temperature decreases from 19MHz to 11MHz when temperature decreases from 10°C to -10°C. By fitting Arrhenius equation, the obtained activation energy is . More detailed description will be presented in the conference. This work is funded by Support from ARO, DoE, NASA, Georgia Tech IEN, and the NSF National Nanotechnology Coordinated Infrastructure (NNCI)

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