R. McClintock

Top-emission ultraviolet light-emitting diodes with peak emission at 280 nm

We demonstrate light emission at 280 nm from UV light-emitting diodes consisting of AlInGaN/AlInGaN multiple quantum wells. Turn-on voltage of the devices is ∼5 V with a differential resistance of ∼40 Ω. The peak emission wavelength redshifts ∼1 nm at high injection currents.

High quantum efficiency AlGaN solar-blind p-i-n photodiodes

We report AlGaN-based back-illuminated solar-blind ultraviolet p-i-n photodetectors with a peak responsivity of 136 mA/W at 282 nm without bias. This corresponds to a high external quantum efficiency of 60%, which improves to a value as high as 72% under 5 V reverse bias. We attribute the high performance of these devices to the use of a very-high quality AlN and Al0.87Ga0.13N/AlN superlattice material and a highly conductive Si–In co-doped Al0.5Ga0.5N layer.

Comparison of ultraviolet light-emitting diodes with peak emission at 340 nm grown on GaN substrate and sapphire

Based on AlInGaN/AlInGaN multiquantum wells, we compare properties of ultraviolet light-emitting diodes (LED) with peak emission at 340 nm grown on free-standing hydride vapor phase epitaxially grown GaN substrate and on sapphire. For the LED grown on GaN substrate, a differential resistance as low as 13 Ω and an output power of more than one order of magnitude higher than that of the same structure grown on sapphire are achieved. Due to higher thermal conductivity of GaN, output power of the LEDs saturates at higher injection currents compared to the devices grown on sapphire.

320×256 solar-blind focal plane arrays based on AlxGa1−xN

We report AlGaN-based backilluminated solar-blind ultraviolet focal plane arrays operating at a wavelength of 280 nm. The electrical characteristics of the individual pixels are discussed, and the uniformity of the array is presented. The p–i–n photodiode array was hybridized to a 320×256 read-out integrated circuit entirely within our university research lab, and a working 320×256 camera was demonstrated. Several example solar-blind images from the camera are also provided.

Delta-doping optimization for high quality p-type GaN

Delta (δ−) doping is studied in order to achieve high quality p-type GaN. Atomic force microscopy, x-ray diffraction, photoluminescence, and Hall measurements are performed on the samples to optimize the δ-doping characteristics. The effect of annealing on the electrical, optical, and structural quality is also investigated for different δ-doping parameters. Optimized pulsing conditions result in layers with hole concentrations near 1018 cm−3 and superior crystal quality compared to conventional p-GaN. This material improvement is achieved thanks to the reduction in the Mg activation energy and self-compensation effects in δ-doped p-GaN.

Performance enhancement of GaN ultraviolet avalanche photodiodes with p-type δ-doping

High quality δ-doped p-GaN is used as a means of improving the performance of back-illuminated GaN avalanche photodiodes (APDs). Devices with δ-doped p-GaN show consistently lower leakage current and lower breakdown voltage than those with bulk p-GaN. APDs with δ-doped p-GaN also achieve a maximum multiplication gain of 5.1×104, more than 50 times higher than that obtained in devices with bulk p-GaN. The better device performance of APDs with δ-doped p-GaN is attributed to the higher structural quality of the p-GaN layer achieved via δ-doping.

AlxGa1-xN-based back-illuminated solar-blind photodetectors with external quantum efficiency of 89%

We report on high performance AlxGa1−xN-based solar-blind ultraviolet photodetector (PD) array grown on sapphire substrate. First, high quality, crack-free AlN template layer is grown via metalorganic chemical vapor deposition. Then, we systematically optimized the device design and material doping through the growth and processing of multiple devices. After optimization, uniform and solar-blind operation is observed throughout the array; at the peak detection wavelength of 275 nm, 729 μm2 area PD showed unbiased peak external quantum efficiency and responsivity of ∼80% and ∼176 mA/W, respectively, increasing to 89% under 5 V of reverse bias. Taking the reflection loses into consideration, the internal quantum efficiency of these optimized PD can be estimated to be as high as ∼98%. The visible rejection ratio measured to be more than six orders of magnitude. Electrical measurements yielded a low-dark current density: <2 × 10−9 A/cm2, at 10 V of reverse bias.

High power frequency comb based on mid-infrared quantum cascade laser at λ ∼ 9 μm

We investigate a frequency comb source based on a mid-infrared quantum cascade laser at λ ∼ 9 μm with high power output. A broad flat-top gain with near-zero group velocity dispersion has been engineered using a dual-core active region structure. This favors the locking of the dispersed Fabry-Perot modes into equally spaced frequency lines via four wave mixing. A current range with a narrow intermode beating linewidth of 3 kHz is identified with a fast detector and spectrum analyzer. This range corresponds to a broad spectral coverage of 65 cm−1 and a high power output of 180 mW for ∼176 comb modes.

Geiger-mode operation of ultraviolet avalanche photodiodes grown on sapphire and free-standing GaN substrates

GaN avalanche photodiodes (APDs) were grown on both conventional sapphire and low dislocation density free-standing (FS) c-plane GaN substrates. Leakage current, gain, and single photon detection efficiency (SPDE) of these APDs were compared. At a reverse-bias of 70 V, APDs grown on sapphire substrates exhibited a dark current density of 2.7×10−4 A/cm2 whereas APDs grown on FS-GaN substrates had a significantly lower dark current density of 2.1×10−6 A/cm2. Under linear-mode operation, APDs grown on FS-GaN achieved avalanche gain as high as 14 000. Geiger-mode operation conditions were studied for enhanced SPDE. Under front-illumination the 625-μm2-area APD yielded a SPDE of ∼13% when grown on sapphire substrates compared to more than 24% when grown on FS-GaN. The SPDE of the same APD on sapphire substrate increased to ∼30% under back-illumination—the FS-GaN APDs were only tested under front illumination due to the thick absorbing GaN substrate.

Type-II superlattice photodetectors for MWIR to VLWIR focal plane arrays

Infrared sensors utilizing Type II superlattice structures have gained increased attention in the past few years. With the stronger covalent bonds of the III-V materials, greater material uniformity over larger areas is obtained as compared to the weaker ionic bonding of the II-VI materials. Results obtained on GaSb/InAs Type II superlattices have shown performance comparable to HgCdTe detectors, with the promise of higher performance due to reduced Auger recombination and dark current through improvements in device design and material quality. In this paper, we discuss advancements in Type II IR sensors that cover the 3 to >30 μm wavelength range. Specific topics covered will be device design and modeling using the Empirical Tight Binding Method (ETBM), material growth and characterization, device fabrication and testing, as well as focal plane array processing and imaging. We demonstrate high quality material with PL linewidths of ~20 meV, x-ray FWHM of 20-40 arcsec, and AFM rms roughness of 1~.2 Å over a 20 μm×20μm area. Negative luminescence at 10 μm range is demonstrated for the first time. Device external quantum efficiency of >30%, responsivity of ~2A/W, and detectivity of 1011 Jones at 77K in the 10 μm range are routinely obtained. Imaging has been demonstrated at room temperature for the first time with a 5 μm cutoff wavelength 256×256 focal plane array.