Ryan McClintock

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.

Hole-initiated multiplication in back-illuminated GaN avalanche photodiodes

Avalanche p-i-n photodiodes were fabricated on AlN templates for back illumination. Structures with different intrinsic layer thicknesses were tested. A critical electric field of 2.73MV∕cm was estimated from the variation of the breakdown voltage with thickness. From the device response under back and front illumination and the consequent selective injection of holes and electrons in the junction, ionization coefficients were obtained for GaN. The hole ionization coefficient was found to be higher than the electron ionization coefficient as predicted by theory. Excess multiplication noise factors were also calculated for back and front illumination, and indicated a higher noise contribution for electron injection.

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.

Back-illuminated separate absorption and multiplication GaN avalanche photodiodes

The performance of back-illuminated avalanche photodiodes with separate absorption and multiplication regions is presented. Devices with an active area of 225μm2 show a maximum multiplication gain of 41 200. The calculation of the noise equivalent power yields a minimum value of 3.3×10−14WHz−1∕2 at a gain of 3000, increasing to 2.0×10−13WHz−1∕2 at a gain of 41 200. The broadening of the response edge has been analyzed as a function of bias.

Surface plasmon enhanced light emission from AlGaN-based ultraviolet light-emitting diodes grown on Si (111)

We report on the development of surface plasmon (SP) enhanced AlGaN-based multiple quantum wells (MQWs) ultraviolet (UV) light-emitting diodes (LEDs) grown on silicon (111) substrates. In order to generate SP-coupling with the radiating dipoles in MQWs, an aluminum layer is selectively deposited in holes etched in the top p-AlGaN to p-GaN layers. After flip-chip bonding and substrate removal, an optical output power of ∼1.2 mW is achieved at an emission wavelength of 346 nm; the output power of these UV LEDs with Al layer is increased by 45% compared to that of conventional UV LEDs without Al layer. This enhancement can be attributed to an increase in the spontaneous emission rate and improved internal quantum efficiency via resonance coupling between excitons in MQWs and SPs in the aluminum layer.

Geiger-mode operation of back-illuminated GaN avalanche photodiodes

The authors report the Geiger-mode operation of back-illuminated GaN avalanche photodiodes. The devices were fabricated on transparent AlN templates specifically for back illumination in order to enhance hole-initiated multiplication. The spectral response in Geiger-mode operation was analyzed under low photon fluxes. Single photon detection capabilities were demonstrated in devices with areas ranging from 225to14063μm2. Single photon detection efficiency of 20% and dark count rate <10kHz were achieved in the smallest devices.

Near milliwatt power AlGaN-based ultraviolet light emitting diodes based on lateral epitaxial overgrowth of AlN on Si(111)

We report on the growth, fabrication, and device characterization of AlGaN-based thin-film ultraviolet (UV) (λ ∼ 359 nm) light emitting diodes (LEDs). First, AlN/Si(111) template is patterned. Then, a fully coalesced 7-μm-thick lateral epitaxial overgrowth (LEO) of AlN layer is realized on patterned AlN/Si(111) template followed by UV LED epi-regrowth. Metalorganic chemical vapor deposition is employed to optimize LEO AlN and UV LED epitaxy. Back-emission UV LEDs are fabricated and flip-chip bonded to AlN heat sinks followed by Si(111) substrate removal. A peak pulsed power and slope efficiency of ∼0.6 mW and ∼1.3 μW/mA are demonstrated from these thin-film UV LEDs, respectively. For comparison, top-emission UV LEDs are fabricated and back-emission LEDs are shown to extract 50% more light than top-emission ones.

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.