Shyh-Chiang Shen

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.

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 ~1.8° 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 $\lt 10^{-{9}}$ A/cm2 in the low-mid reverse bias voltage region prior to avalanche breakdown and achieved a current-limited photocurrent gain of $\gt 10^{{7}}$ 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 $10^{-{5}}$ A/cm2 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.

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 μm 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 ∼1 × 10−9 A/cm2 and a higher optical gain of ∼5.2 × 105 at a current density limit of 0.3 A/cm2. 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.

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 Al_xGa_1−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 Al_xGa_1−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 Al_xGa_1−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 Al_xGa_1−xN/GaN quantum-well laser heterostructure designed for operation at ∼370 nm.

We report high-performance homojunction GaN avalanche photodiodes (APDs) grown on a low-defect GaN substrate and fabricated with an ion-implantation isolation method. High-quality p-i-n GaN layers were grown using metalorganic chemical vapor deposition (MOCVD), and an effective device isolation method using optimized nitrogen ion implantation was developed to provide significant leakage current suppression on the etched mesa sidewalls. Fabricated GaN APDs showed an ultralow dark current density <; 10 ^-9 A/cm ² up to 50% of the avalanching breakdown voltage region and achieved a photocurrent gain of >106 at a reverse bias of 71.5 V under deep-ultraviolet (DUV) illumination at λ = 280 nm. A further temperature-dependent study of the dark current indicated that a trap-assisted tunneling process predominates the leakage current component that also contributes to the carrier multiplication process near the avalanching breakdown.

We determined the optical and electrical characteristics of GaN-based, ultraviolet micro light emitting diodes (microLEDs). Such microLEDs are essential to next-generation high-resolution micro-displays. Square-shaped microLEDs of different sizes (side lengths: 5-50 μm) were designed. The peak emission wavelength of these devices shifted <0.15 nm during the current injection. The 50 μm device had a 3.8 times greater relative illumination intensity than did the 5 μm device, suggesting a degradation in quantum efficiency in small devices. Measurements of temperature-dependent reverse leakage current indicated (1) thermal activation from deep centers and (2) a high percentage of components with surface recombination current in the small devices.

We report vertical GaN p-i-n rectifiers grown on a bulk GaN substrate. These planar GaN p-i-n rectifiers were implemented with Schottky field plates and buried-junction field terminations using ion implantation as a mean of device isolation. The 1130-μm-diameter GaN p-i-n rectifiers achieved a blocking voltage (BV) of 1.2 kV and an ON-state current drive of 10 A. A temperature-dependent OFF-state current study showed that the variable-range-hopping (VRH) conduction is dominant for devices operating at the temperature of less than 160 K, and multistep electron transition is responsible for the reverse-biased leakage at higher temperature. Further study suggested the leakage current arises from the deep centers formed through the ion-implantation damage that led to perimeter-dependent leakage currents in GaN p-i-n rectifiers using ion-implanted isolation approach.

There is an error in Fig. 4 of [1] in the TE ₀₁ optical mode profile. The error is due to the author forgetting to rescale the x-axis when plotting the optical mode profile on the right y-axis. This oversight occurred because the optical mesh is different from the electrical mesh, and the correct figure is shown below. We wish to emphasize that this was merely a plotting error, and hence all of the results and conclusions presented in the manuscript remain unchanged. We sincerely regret the error.