E. Kuokstis

Visible light-emitting diodes using a-plane GaN–InGaN multiple quantum wells over r-plane sapphire

We report blue-purple pn-junction light-emitting diodes (LEDs) with a-plane GaN–InGaN multiple quantum well active region. The LEDs were grown over r-plane sapphire substrates. Our study has shown the low pump intensity photoluminencence and electroluminescence to be dominated by emission from the band-tail states which then saturates rapidly giving rise to band-edge emission.

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.

Two mechanisms of blueshift of edge emission in InGaN-based epilayers and multiple quantum wells

We present the results of a comparative photoluminescence (PL) study of GaN and InGaN-based epilayers, and InGaN/GaN multiple quantum wells (MQWs). Room-temperature PL spectra were measured for a very broad range of optical excitation from 10 mW/cm2 up to 1 MW/cm2. In contrast to GaN epilayers, all In-containing samples exhibited an excitation-induced blueshift of the peak emission. In addition, the blueshift of the emission in the InGaN epilayers with the same composition as the quantum well was significantly smaller. The comparison of the blueshift in the “bulk” InGaN and in the MQWs allowed us to separate two different mechanisms responsible for this effect: (i) filling of the localized states in In-rich areas and (ii) screening of the polarization electric field in strained MQW structures.

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.

Ultraviolet Light Emitting Diodes Using Non-Polar a-Plane GaN-AlGaN Multiple Quantum Wells

We report a pn-junction ultraviolet light-emitting diode (LED) with peak emission at 363 nm using a-plane GaN-AlGaN multiple quantum wells over r-plane sapphire. The peak emission wavelength does not shift with increasing pump currents-therefore establishing the feasibility of high-efficiency non-polar light emitting devices.

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.

Deep-ultraviolet emission of AlGaN/AlN quantum wells on bulk AlN

An approach for growing high-quality AlGaN/AlN multiple quantum wells (MQW) emitting in deep UV region is proposed. The structures are deposited on bulk AlN substrates. Structural analysis by using x-ray diffraction confirms high crystalline quality of these structures. Photoluminescence dependences on excitation intensity and temperature under band-to-band excitation of AlN barrier layers and under selective excitation of the quantum wells are presented. Al0.5Ga0.5N/AlN MQW grown on bulk AlN demonstrate emission at 260 nm with high emission intensity. Stimulated emission of these structures at 258 nm was observed. The results prove great potential of growing structures with high-aluminum-content layers on bulk AlN substrates.

Pulsed atomic layer epitaxy of quaternary AlInGaN layers

In this letter, we report on a material deposition scheme for quaternary AlxInyGa1−x–yN layers using a pulsed atomic layer epitaxy (PALE) technique. The PALE approach allows accurate control of the quaternary layer composition and thickness by simply changing the number of aluminum, indium, and gallium pulses in a unit cell and the number of unit cell repeats. Using PALE, AlInGaN layers with Al mole fractions in excess of 40% and strong room-temperature photoluminescence peaks at 280 nm can easily be grown even at temperatures lower than 800 °C.

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.

Lateral Epitaxial Overgrowth of Fully Coalesced A-Plane GaN on R-Plane Sapphire

Fully coalesced epitaxial laterally overgrown a-plane GaN films were characterized for their structural and optical quality. The films had a very smooth surface with a root mean square roughness as low as 4.6 ? for a 5 ?m ? 5 ?m atomic force microscope scan area. They exhibited a wing tilt of only 0.27? and optically pumped stimulated emission, which establish their high structural and optical quality. These non-polar films are ideal for fabricating high-efficiency optoelectronic and electronic devices.