Alireza Yasan

Electroluminescence at 375nm from a ZnO∕GaN:Mg∕c-Al2O3 heterojunction light emitting diode

n-ZnO∕p-GaN:Mg heterojunction light emitting diode (LED) mesas were fabricated on c-Al2O3 substrates using pulsed laser deposition for the ZnO and metal organic chemical vapor deposition for the GaN:Mg. High crystal quality and good surface morphology were confirmed by x-ray diffraction and scanning electron microscopy. Room temperature (RT) photoluminescence (PL) showed an intense main peak at 375nm and a negligibly low green emission indicative of a near band edge excitonic emission from a ZnO layer with low dislocation/defect density. The LEDs showed I-V characteristics confirming a rectifying diode behavior and a RT electroluminescence (EL) peaked at about 375nm. A good correlation between the wavelength maxima for the EL and PL suggests that recombination occurs in the ZnO layer and that it may be excitonic in origin. This also indicates that there is significant hole injection from the GaN:Mg into the ZnO.

High-power 280 nm AlGaN light-emitting diodes based on an asymmetric single-quantum well

We demonstrate high-power AlGaN-based ultraviolet light-emitting diodes grown on sapphire with an emission wavelength of 280 nm using an asymmetric single-quantum-well active layer configuration on top of a high-quality AlGaN/AlN template layer. An output power of 1.8 mW at a pulsed current of 400 mA was achieved for a single 300 μm×300 μm diode. This device reached a high peak external quantum efficiency of 0.24% at 40 mA. An array of four diodes produced 6.5 mW at 880 mA of pulsed current.

4.5 mW operation of AlGaN-based 267 nm deep-ultraviolet light-emitting diodes

We demonstrate 4.5 mW output power from AlGaN-based single quantum well ultraviolet light-emitting diodes at a very short wavelength of 267 nm in pulsed operation mode. The output power in continuous-wave mode reaches a value of 165 μW at an injected current of 435 mA. The measurements were done on arrays of four devices flip chip bonded to AlN submounts for thermal management.

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.

Avalanche multiplication in AlGaN based solar-blind photodetectors

Avalanche multiplication has been observed in solar-blind AlGaN-based p-i-n photodiodes. Upon ultraviolet illumination, the optical gain shows a soft breakdown starting at relatively low electric fields, eventually saturating without showing a Geiger mode breakdown. The devices achieve a maximum optical gain of 700 at a reverse bias of 60 V. By modeling the device, it is found that this corresponds to an electric-field strength of 1.7MV∕cm.

Photoluminescence study of AlGaN-based 280 nm ultraviolet light-emitting diodes

We investigated optical properties of single quantum well AlGaN-based UV 280 nm light-emitting diodes using temperature-dependent photoluminescence (PL) measurement. We found an “S-shaped” temperature dependence of the peak energy. From the Arrhenius plot of integrated PL intensity, we speculate that dislocations as well as thermal emission of carriers out of the quantum well are responsible for the PL quenching behavior. Also a second nonradiative channel with much lower activation energy was found, the origin of which we believe to be quenching of the bound excitons.

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.

Future of AlxGa1-xN materials and device technology for ultraviolet photodetectors

Thanks to advances in the quality of wide bandgap AlxGa1-xN semiconductors, these materials have emerged as the most promising approach for the realization of photon detectors operating in the near ultraviolet from 200 to 365 nm. This has in turn spurred the need for such devices in an increasing number of applications ranging from water purification to early missile threat warning systems. Nevertheless, the control of the material quality and doping, and the device technology remain tremendous challenges in the quest for the realization of high performance photodetectors. Design of the photodetector structure is one of the key issues in obtaining high performance devices; especially the thickness of the intrinsic region for p-i-n photodiodes is a crucial value and needs to be optimized. We compare the performance of the p-i-n photodiodes with different widths for the depletion region, which shows a trade-off between speed and responsivity of the devices. Furthermore, another challenge at present is the realization of low resistivity wide bandgap p-type AlxGa1-xN semiconductors. We present here recent advances and propose future research efforts in the enhancement of the AlxGa1-xN p-type conductivity through the use of polarization fields in AlxGa1-xN/GaN superlattice structures.