Scott A. McPherson

Back-illuminated GaN/AlGaN heterojunction photodiodes with high quantum efficiency and low noise

Back-illuminated GaN/AlGaN ultraviolet (UV) heterojunction photodiodes with high quantum efficiencies are demonstrated. Photovoltaic (zero bias) responsivity of 0.2 A/W at 355 nm was achieved. The improved efficiencies primarily arise from the use of AlGaN/GaN heterojunction in which photons are absorbed within the p-n junction thus eliminates carrier losses due to surface recombination and diffusion processes in previously reported homojunction devices. Very high dark impedance and large visible rejection ratio were obtained. These results indicate high quality GaN/AlGaN interface and efficient photocarrier collection in the photodiode.

LASER ACTION IN GAN PYRAMIDS GROWN ON (111) SILICON BY SELECTIVE LATERAL OVERGROWTH

Laser action was observed in GaN pyramids under strong optical pumping at room temperature. The pyramids were laterally overgrown on a patterned GaN/AlN seeding layer grown on a (111) silicon substrate by metal–organic chemical vapor deposition. Each pyramid had a 15-μm-wide hexagonal base and was on average 15 μm in height. The pyramids were individually pumped, imaged, and spectrally analyzed through a high-magnification telescope system using a high-density pulsed excitation source. Under high levels of optical pumping, multimode laser at room temperature was observed. The integrated emission intensity for both spontaneous and lasing peaks was studied as a function of excitation power density. The effects of pyramid geometry and short-pulse excitation on the multimode nature of laser oscillations inside of the pyramids is discussed. This study suggests that GaN microstructures could potentially be used as pixel elements and high-density two-dimensional laser arrays.

Time-resolved photoluminescence studies of GaN, InGaN, and AlGaN grown by metalorganic chemical vapor deposition

Time-resolved photoluminescence (PL) measurements are reported for GaN, InGaN, and AlGaN thin films, as well as InGaN/GaN multiple quantum wells (MQWs) with 3-nm-thick InGaN wells and 4.5-nm-thick GaN barriers. All the samples were grown on c-plane sapphire substrates by metalorganic chemical vapor deposition. We observed that the carrier recombination lifetime of the ternary InGaN and AlGaN alloys becomes longer with decreasing emission energy. An anomalous temperature-dependent emission behavior was observed for InGaN-related PL with increasing temperature. The actual temperature dependence of the PL emission was estimated with respect to the bandgap energy determined by photoreflectance spectra. The temperature-induced S-shaped PL shift is explained by a change in the carrier dynamics with increasing temperature due to inhomogeneity and carrier localization in the alloys. We also investigated the influence of Si doping on the optical characteristics of InGaN/GaN MQWs. The 10 K radiative recombination lifetime was observed to decrease from approximately 30 ns to approximately 4 ns with increasing Si doping concentration from < 1 X 1017 to 3 X 1019 cm-3. A reduced Stokes shift between PL and PL excitation spectra, a reduction in a pump-density-induced blueshift, and an increase in the interface quality with increasing Si doping were observed. From these results, we conclude that Si doping results in a decrease in carrier localization caused by potential fluctuations in the MQWs.

GaN/AlGaN UV photodiodes and phototransistors

Using MOCVD grown GaN and AlGaN alloys and heterostructures, we realized heterojunction UV photodiodes and phototransistors. We achieved photovoltaic internal quantum efficiency in excess of 90 percent and large UV-visible rejection ratio in the GaN/AlGaN PIN photodiodes. The results indicate high quality heterojunction interface and efficient carrier collection. We demonstrated an n-p-i-n GaN/AlGaN heterojunction bipolar phototransistor with gain in excess of 105, and 360 nm to 400 nm rejection ratio of 108. The phototransistor features a rapid electrical quenching of photoconductivity therefore can be operated without DC drift, an option not available in unipolar photoconductors. The electrical bandwidth of the phototransistor can be changed to accommodate particular applications by simply adjusting the repetition rate of the quenching pulses. The operation and evaluation of these devices are compared with alternative devices including SiC photodiode and GaN photoconductors.