Wenhong Sun

AlGaN Deep-Ultraviolet Light-Emitting Diodes with External Quantum Efficiency above 10%

Improvements of the internal quantum efficiency by reduction of the threading dislocation density and of the light extraction by using UV transparent p-type cladding and contact layers, UV reflecting ohmic contact, and chip encapsulation with optimized shape and refractive index allowed us to obtain the external quantum efficiency of 10.4% at 20 mA CW current with the output power up to 9.3 mW at 278 nm for AlGaN-based deep-ultraviolet light-emitting diodes grown on sapphire substrates.

250nmAlGaN light-emitting diodes

We report AlGaN deep ultraviolet light-emitting diodes (LEDs) at 250 and 255nm that have short emission wavelengths. For an unpackaged 200×200μm square geometry LED emitting at 255nm, we measured a peak power of 0.57mW at 1000mA of pulsed pump current. For a similar device emitting at 250nm the peak output power of 0.16mW was measured at 300mA of pulsed pump current. Progress is based on the development of high quality AlGaN cladding layers with an Al content up to 72%, which were grown over AlGaN∕AlN superlattice buffer layers on sapphire substrates. These n-Al0.72Ga0.28N layers were doped with Si up to about 1×1018cm−3 and electron mobilities up to 50cm2∕V∙s were estimated. High resolution x-ray diffraction studies gave a narrow (002) rocking curve with full width at half maximum of only 133arcsec.

Pulsed atomic-layer epitaxy of ultrahigh-quality AlxGa1−xN structures for deep ultraviolet emissions below 230 nm

In this letter, we report the pulsed atomic-layer epitaxy of ultrahigh-quality AlN epilayers and AlN/Al0.85Ga0.15N multiple quantum wells (MQWs) on basal plane sapphire substrates. Symmetric and asymmetric x-ray diffraction (XRD) measurements and room-temperature (RT) photoluminescence (PL) were used to establish the ultrahigh structural and optical quality. Strong band-edge RT PL at 208 and 228 nm was obtained from the AlN epilayers and the AlN/Al0.85Ga0.15N MQWs. These data clearly establish their suitability for sub-250-nm deep UV emitters.

Polarization effects in photoluminescence of C- and M-plane GaN/AlGaN multiple quantum wells

Polarization effects have been studied in GaN/AlGaN multiple quantum wells (MQWs) with different c-axis orientation by means of excitation-dependent photoluminescence (PL) analysis. Quantum structures were grown on [0001]-oriented sapphire substrates (C plane) and single-crystalline [1100]-oriented freestanding GaN (M plane) using the metalorganic chemical vapor deposition technique. Strong PL spectrum line blueshifts (up to 140 meV) which are correlated with the excitation intensity have been obtained for C-plane MQWs, whereas no shift has been observed for M-plane MQWs. Theoretical calculations and comparison with the PL data confirm that the built-in electric field for C-plane structures is much stronger than the field present for M-plane MQWs. In the former case, the excitation-induced blueshift of the PL line is due to the screening of the built-in electric field by photoinjected carriers, which is consistent with the field strength of 1.23 MV/cm in the absence of excitation.

Reduction of threading dislocation densities in AlN∕sapphire epilayers driven by growth mode modification

A strategy to reduce the density of threading dislocations (TDs) in AlN epilayers grown on sapphire substrates is reported. The TDs experience a redirection of their line orientation which is found to coincide with imposed increases in both of V/III ratio and overall flux rate leading to the formation of an internal subinterface delineated by the changes in dislocation orientation. Threading dislocations either experience large kinks and then redirect into threading orientation or form dipole half loops via annihilation of redirected threading segments of opposite sign with the latter leading to a significant dislocation density reduction. These phenomena can be accounted for by a transition of growth mode from atomic step flow to two-dimensional layer-by-layer growth which accompanies the imposed changes in V/III ratio and flux. As this occurs, macrosteps (several atomic layers thick) laterally overgrow pre-existing dislocation outcrops. Image forces initiate the redirection processes and create trailing...

GaN homoepitaxy on freestanding (11̄00) oriented GaN substrates

We report homoepitaxial GaN growth on freestanding (1100) oriented (M-plane GaN) substrates using low-pressure metalorganic chemical vapor deposition. Scanning electron microscopy, atomic-force microscopy, and photoluminescence were used to study the influence of growth conditions such as the V/III molar ratio and temperature on the surface morphology and optical properties of the epilayers. Optimized growth conditions led to high quality (1100) oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Since for III-N materials the (1100) crystal orientation is free from piezoelectric or spontaneous polarization electric fields, our work forms the basis for developing high performance III-N optoelectronic devices.

High power AlGaN ultraviolet light emitters

We present the analysis of the external quantum efficiency in AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) on sapphire substrates and discuss factors affecting the output power of DUV LEDs. Performance of the LED is related to optimization of the device structure design and improvements of the epitaxial material quality.

Submicron gate Si 3 N 4 /AlGaN/GaN-metal-insulator-semiconductor heterostructure field-effect transistors

We present the characteristics of a quarter-micron gate metal-insulator-semiconductor heterostructure field-effect transistor (MISHFET) with Si/sub 3/N/sub 4/ film as a gate insulator. A detailed comparison of the MISHFET and an identical geometry HFET shows them to have the same radio frequency (RF) power gain and cut-off frequency, while the MISHFET has much lower gate-leakage currents and higher RF powers at operating frequencies as high as 26 GHz. The MISHFET gate-leakage currents are well below 100 pA at gate bias values from -10 V to +8 V. At zero gate bias, the drain saturation current is about 0.9 A/mm and it increases to 1.2 A/mm at +8 V gate bias. The output RF power of around 6 W/mm at 40 drain bias was found to be frequency independent in the range of 2 to 26 GHz. This power is 3 dB higher than that from HFET of the same geometry. The intrinsic cutoff frequency is /spl sim/63 GHz for both the HFET and the MISHFET. This corresponds to an average effective electron velocity in the MISHFET channel of 9.9/spl times/10/sup 6/ cm/s. The knee voltage and current saturation mechanisms in submicron MISHFETs and heterostructure field-effect transistors (HFET) are also discussed.

AlGaN-based 280nm light-emitting diodes with continuous wave powers in excess of 1.5mW

We report on AlGaN-based light-emitting diodes over sapphire with peak emission at 280nm. A modified active layer structure consisting of four multiple quantum wells, addition of an electron blocking magnesium doped p-AlGaN layer, improved contacts along with flip-chip packaging resulted in a cw power of 0.7mW at 230mA for a single 200μm×200μm device. Flip-chipping four 100μm×100μm devices in a parallel configuration improved the dc saturation current and enabled us to obtain a cw power of 1.53mW (at 450mA) and a pulse power as high as 24mW (at 1.5A). These powers translate to values of 0.36% and 0.12% for the external quantum efficiency and the wall plug efficiency.

A New Selective Area Lateral Epitaxy Approach for Depositing a-Plane GaN over r-Plane Sapphire

We report a new epitaxy procedure for growing extremely low defect density a-plane GaN films over r-plane sapphire. By combining selective area growth through a SiO2 mask opening to produce high height to width aspect ratio a-plane GaN pillars and lateral epitaxy from their c-plane facets, we obtained fully coalesced a-plane GaN films. The excellent structural, optical and electrical characteristics of these selective area lateral epitaxy (SALE) deposited films make them ideal for high efficiency III-N electronic and optoelectronic devices.