Wide-bandgap III-N-based PIN photodiodes are being developed to provide enhanced UV light detection compared to Sibased photodiodes. We present data on GaN-based top-illuminated ultraviolet PIN avalanche photodiodes (APDs) with shallow-bevel mesa (SBM) edge termination, grown by metalorganic chemical vapor deposition (MOCVD) and having different epitaxial structures with different p-type layer designs and devices grown on three different (0001) n-type GaN substrates. The performance of these APDs is compared.

Recent advances in the development of ultraviolet avalanche p-i-n photodiodes composed of the III-N and SiC compound semiconductors have established that these materials can provide distinct advantages over the current Si-CMOS-based photodetector technology. We review the early development of semiconductor photodetectors and the current understanding and state-of-the-art for the III-N and SiC photodetectors. Particular emphasis is devoted to considering the potential use of these materials for single-photon avalanche ultraviolet photodetectors operating in Geiger-mode.

GaN is rapidly gaining attention for implementation in power electronics but is still impacted by its high density of threading dislocations (TDs), which have been shown to facilitate current leakage through devices limiting their performance and reliability. Here, we discuss a novel implementation of photoluminescence (PL) imaging to study TDs in regions within vertically structured p-i-n GaN (PIN) diodes consisting of metalorganic chemical vapor deposition (MOCVD) epitaxial layers grown on ammonothermal GaN (am-GaN) substrates. PL imaging with a sub-bandgap excitation energy (3.1 eV) reveals TDs with excellent clarity in three dimensions within the am-GaN substrate. Galvanometric-driven PL imaging allows the microstructure of hundreds of devices to be characterized in a single session, enhancing the screening process through the addition of device specific TD location tracking and density mapping. The visibility, structural characteristics, luminescent nature and evolution of TDs through the GaN growth process are described, potentially providing the ability to define TD structures associated with leakage current.

This work reports high-performance homojunction gallium nitride (GaN) p-i-n (PIN) avalanche photodiodes (APDs) grown on a low-defect GaN substrate and fabricated with an $\sim$ 1.8 $^{\circ}$ shallow-bevel-mesa edge termination. High-quality p-i-n GaN epitaxial layers were grown using metalorganic chemical vapor deposition (MOCVD), and the device edge termination was achieved by multistep pseudo gray-scale lithography followed by photoresist reflow and dry etching of the bevel mesa. Fabricated GaN APDs showed an ultralow dark current density $<$ 10 $^{-\text{9}}$ A/cm $^{\text{2}}$ in the low-mid reverse bias voltage region prior to avalanche breakdown and achieved a current-limited photocurrent gain of $>$ 10 $^{\text{7}}$ under deep-ultraviolet (DUV) illumination at $\lambda $ $=$ 280 nm and a reverse bias $-$ 107 V. The total current density without illumination was in the order of 10 $^{-\text{5}}$ A/cm $^{\text{2}}$ at the onset of avalanche breakdown. The avalanche breakdown characteristic was confirmed by temperature-dependent J – V studies, with a derived positive temperature coefficient of 0.02 V/K for the breakdown voltage (BV). The analysis of the temperature-dependent reverse-bias characteristics indicated that the reverse leakage current components before avalanche could be attributed to trap-assisted tunneling (TAT) and phonon-assisted tunneling (PAT) processes.

Al0.6Ga0.4N deep-UV p-i-n avalanche photodiodes (APDs) are demonstrated grown by metalorganic chemical vapor deposition (MOCVD) on a (0001) c-plane aluminum nitride (AlN) bulk substrate and fabricated both with and without an ion implantation process for mesa-edge electric-field termination. Three design considerations are taken into account: the optical absorption of the AlN bulk substrate, the photosensitivity of the APD, and the breakdown electric field. The MOCVD growth conditions of the AlGaN APD layers are optimized. Then, a detailed description of the seven device fabrication steps of the APDs is provided, which includes nitrogen ion implantation. The APDs fabricated by the ion implant process exhibit a dark-current density under low reverse bias approximate to 1 x 10-9 A cm-2. This is one order of magnitude lower than the APD fabricated without ion implantation from the same wafer. The breakdown voltage of the APD is approximate to-140 V. The calculated optical gain of the ion-implanted APD beyond avalanche breakdown is approximate to 5.2 x 105 (current limited) and the device has an average zero-bias photoresponse of approximate to 68 mA W-1 at a wavelength of 250 nm.

This work reports analysis of the reverse-bias breakdown characteristics of homojunction gallium nitride (GaN) p–i–n (PIN) rectifiers fabricated on bulk GaN substrates. Sub-bandgap photoluminescence mapping at room temperature as a contactless, non-destructive wafer inspection method was performed to analyze the impact of material properties on grown GaN PIN diodes and to study the correlation between defect types and breakdown characteristics of vertical GaN PIN rectifiers. Under the sub-bandgap excitation, yellow luminescence is dominant. The premature breakdown characteristics of the fabricated kV-class vertical GaN PIN rectifiers with nitrogen-implanted floating guard rings are found to be associated with material defects and deep level complexes. Photoluminescence mapping has demonstrated its effectiveness in quantitative analysis of dislocations and other types of defects.

•MOCVD was used to grow Mg-doped GaN layers to study Mg impurity diffusion.•The effect of precursor prepurge and growth temperature on Mg memory effect and back diffusion were studied.•It was determined that the Mg precursor molar flow rate is the strongest factor in determining the Mg diffusion effects.

Non-planar growth of AlxGa1-xN epitaxial layers with an average alloy composition up to Al-mole-fraction of x similar to 0.21 was performed on patterned c-plane GaN on (0001) sapphire substrates with stripe-shaped mesa structures. This approach successfully realized the growth of crack-free AlGaN layers on the top of mesas with layer thicknesses greater than the expected critical layer thickness by relaxing the in-plane stress on the top of the mesa. The effectiveness of the relaxation strongly depends on the width and depth of the stripe mesa. The relaxation of in-plane stress and resultant suppression of cracks in AlGaN layer are qualitatively discussed.

A deep-ultraviolet Al0.6Ga0.4N p-i-n avalanche photodiode (APD) structure was grown on a (0001) AlN bulk substrate by metalorganic chemical vapor deposition. The wafer was fabricated into 20 mu m diameter mesa APD devices both with and without ion-implantation with nitrogen ions on the periphery of the p-type region of the diode mesa and tested. The dark current density vs bias, photoresponse, and the optical gain of the APDs with and without ion implantation were compared. The devices fabricated with ion implantation showed improved performance, exhibiting lower dark current densities of similar to 1 x 10(-9) A/cm(2) and a higher optical gain of similar to 5.2 x 10(5) at a current density limit of 0.3 A/cm(2). The average temperature coefficients of the reverse-bias breakdown voltage were also compared. Although the data showed negative coefficients for APDs fabricated both with and without ion implantation, the ion-implanted APDs showed an improvement relative to the devices fabricated without ion-implantation.

This work reports on the fabrication and properties of a homojunction gallium nitride (GaN) p-i-n (PIN) rectifier fabricated on a free-standing GaN substrate. Uniform device performance is achieved with breakdown voltage (BV) $ > {1.2}$ kV and low ON-resistance $\times $ area ( ${R}_{{ \mathrm{\scriptscriptstyle ON}}} {A}$ ). The statistics of the BV measurements show 58.5% of devices achieve BV $ > {1.3}$ kV, and 71.1% of devices achieve BV $ > {1.2}$ kV, as attributed to high quality and control in both epitaxial growth and device process. At room temperature, ${R}_{{ \mathrm{\scriptscriptstyle ON}}} {A}$ is 0.23 $\text{m}\Omega \cdot $ cm2 at a current density ( ${J}$ ) of 6.9 kA/cm2. The corresponding Baliga’s figure of merit is ${>}{5.97}$ GW/cm2. Temperature-dependent reverse ${I}$ – ${V}$ measurements were performed and show a positive temperature coefficient of 0.42 V/K, indicating the avalanche capability of reverse breakdown. Further analysis with the Poole-Frenkel model on the temperature-dependent measurement suggested that a trap-assisted tunneling process contributed to the reverse leakage current. Floating guard rings (FGRs) formed by nitrogen implantation serve as an effective edge termination technique in these GaN PIN rectifiers, resulting in uniform performance in both forward and reverse bias.

The non-planar growth of UV-A laser diode heterostructures composed of AlGaN layers with high Al-mole-fractions and thicknesses exceeding the critical layer thickness was performed on patterned c-plane GaN (0001) substrates with stripe-shaped mesa structures. This approach suppressed the surface cracking at the top of the mesas via an anisotropic relaxation of the in-plane strain along the direction normal to the mesa stripes. Stimulated emission and laser operation at room temperature with emission in the UV-A range under pulsed current injection were demonstrated for laser diodes fabricated on the mesas.

The unintentional impurity incorporation in GaN epitaxial layers impacts the electrical conductivity and optical properties of the films grown by metalorganic chemical vapor deposition (MOCVD). It is critical to control impurity-related states for device structure development. The aim of this work is to contribute to the under-standing of the reasons for the presence of certain background impurities. In this paper, the unintentional im-purity incorporation of carbon, hydrogen, oxygen, and silicon in undoped (u-) GaN films grown by low-pressure MOCVD were studied. Background impurity concentrations were evaluated by secondary ion mass spectroscopy (SIMS) to optimize growth parameters including V/III ratio, growth temperature (Tg), growth pressure (Pg), and gallium (Ga) precursors of trimethylgallium (TMG) or triethylgallium (TEG). Unintentional [C] and [Si] in-corporations are found to be highly dependent on the growth parameters. For the same growth conditions, u-GaN films grown with a TEG precursor exhibited a lower background concentration of C compared to that of GaN grown with TMG. However, the lowest background [C] achieved was grown with TMG using optimized con-ditions and exhibited [C] < 4 x 1015 cm-3 which was at the detection limit of the SIMS measurement. These results were measured for a u-GaN layer grown with TMG at 200 Torr, 1030 degrees C, and a V/III of 3700. The lowest background [Si] of 4 x 1015 cm-3 was achieved by growing with TMG at 200 Torr, 1000 degrees C, and V/III of 650.

We present the low-temperature Geiger-mode characteristics of GaN (gallium nitride) p-i-n avalanche photodiodes (APDs). The devices have a breakdown voltage of −95V and a temperature dependence of $\sim $ 0.0159 ± 0.0034 V/K near 300K. The room-temperature (300 K) dark-count rate (DCR) is 23.8 MHz for a $75\times 75\,\,\mu \text{m}^{2}$ device biased at 1 % overvoltage. The DCR halves when lowering the temperature by 50°C. Based on the temperature-dependent characteristics of the DCR, we identify band-to-band tunneling as the dominant DCR generation mechanism. At 4.65 % overvoltage and 375 nm, the photon detection efficiency (PDE) is 0.82 %- limited by a low breakdown probability of 1.7 %. We discuss the measurement setup and the method to extract count rates, which is based on the Poisson distribution of the time intervals between Geiger-mode breakdowns of the APD. The setup includes a custom circuit to bias the diode and amplify its signals, a steady ultraviolet (UV) light source, and a system to control the temperature of the APD with a thermoelectric element in the range from −40 to 20°C.

Solar-blind (<280nm) deep-ultraviolet (DUV) avalanche photodetectors (APDs) are of importance in various applications such as quantum communication, biomedical, defense, and non-line-of-sight (NLOS) communication. This makes the detectors from AlxGa1-xN materials attractive for such applications owing to their wide direct-bandgap characteristics. In this work, top-illuminated DUV Al0.6Ga0.4N p-i-n APD structures were designed, grown by metalorganic chemical vapor deposition on bulk AlN substrates, and fabricated. The devices showed distinctive avalanche breakdown behavior, with breakdown voltages of -150V, and low-leakage current density of <10-8A/cm2. The peak spectral response is 141mA/W at the wavelength of 245nm under 0V.

Wide-bandgap III-nitride heterostructures are required for a variety of device applications. However, this alloy system has a large lattice constant and thermal expansion coefficient mismatch that limits the alloy composition and layer thickness for many heteroepitaxial device structures. Consequently, various methods have been devised to allow the heteroepitaxial growth of AlInGaN heterostructures to accommodate this inherent strain. In this work, we describe a non-planar-growth approach that enables the deposition of crack-free high-Al-mole-fraction AlxGa1−xN on patterned GaN/sapphire templates and bulk GaN substrates with large-area mesas. We have studied the effects of the patterned mesa width, the mesa etch depth, and the gap between the mesas on the heteroepitaxy of AlxGa1−xN superlattices with an average Al molar fraction 0.11 < x¯ < 0.21 and non-planar overgrowth growth thicknesses up to 3.5 μm. Similar to the planar growth approach, increasing the thickness and Al mole fraction of the AlxGa1−xN superlattices leads to surface cracking when exceeding the critical layer thickness. However, limiting the mesa dimension in one direction enables strain mitigation and drastically increases the critical layer thickness. Additionally, larger etch depths of the mesas increase the Al alloy composition and thickness for crack-free AlGaN heteroepitaxy whereas the gap in between the mesas seems to have no crucial influence. We demonstrate that the Al alloy composition and layer thicknesses of such heterostructures can be increased far beyond the critical layer thickness for planar growth and demonstrate the growth of a crack-free full AlxGa1−xN/GaN quantum-well laser heterostructure designed for operation at ∼370 nm.

A top-illuminated deep-ultraviolet Al0.6Ga0.4N p-i-n avalanche photodiode (APD) structure was designed and grown by metalorganic chemical vapor deposition on an AlN bulk substrate and on two different quality AlN/sapphire templates, and APDs were fabricated and tested. The APD devices with a circular diameter of 20 μm have demonstrated a distinctive reverse-bias breakdown behavior. The reverse breakdown voltage of the APDs is approximately −140 V, which corresponds to a breakdown electric field of 6–6.2 MV/cm for the Al0.6Ga0.4N material as estimated by Silvaco TCAD simulation. The APDs grown on the AlN bulk substrate show the lowest leakage current density of <1 × 10−8 A/cm2 (at low reverse bias) compared to that of the devices grown on the AlN templates. From the photocurrent measurement, a maximum gain (current limited) of 1.2 × 104 is calculated. The average temperature coefficients of the breakdown voltage are negative for APD devices fabricated from both the AlN bulk substrate and the AlN templates, but these data show that the coefficient is the least negative for the APD devices grown on the low-dislocation-density AlN bulk substrate.

III-Nitride laser diodes (LDs) emitting in the near ultraviolet spectral region can enable various important applications such as high-precision chip-scale atomic clocks. However, III-N LDs emitting near 369nm suffer from material and heterostructure design challenges including stress-induced layer cracking and p-type doping limitations. We will present a detailed study on the influence of the Al mole fraction and thickness on the occurrence of surface cracks of heterostructures using nonplanar growth by metalorganic chemical vapor deposition on macro-patterned GaN/sapphire templates and bulk GaN substrates. Data on the nonplanar growth of full III-N UV LD structures will be presented.

Characterization of operational AlGaInN heterostructure light emitting diodes (LEDs) is critical to their performance optimization and time-to-failure analysis. Typically, device performance data needs to be corroborated with structural information such as layer thicknesses, charge profiles, and the absolute location of the pn-junction. Here, non-destructive testing by capacitance-voltage profiling is being applied to AlGaInN LED structures. Within a large set of samples with different active layer geometry, we observe distinct layers of high mobile charge accumulation. We correlate those with layer thicknesses derived from an x-ray diffraction analysis of the corresponding epiwafers. In this way, we identify the charge maxima as the upper and lower interfaces of the p-type AlGaN electron blocking layer to the neighboring GaN layers. By means of this successful analysis, we now have the opportunity to monitor epi process performance and stability as well as device degradation progress quasi-continuously over the device lifetime in a non-destructive mode.