Structural and Optical Properties of Al 0.30 Ga 0.70 N/AlN Multiple Quantum Wells Grown on Vicinal 4H p-SiC Substrates b
- PDF / 848,974 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 42 Downloads / 181 Views
Structural and Optical Properties of Al0.30Ga0.70N/AlN Multiple Quantum Wells Grown on Vicinal 4H p-SiC Substrates by Molecular Beam Epitaxy Gordie Brummer1, Denis Nothern2, and T.D. Moustakas1,2 1 Department of Electrical and Computer Engineering and Photonics Center, Boston University, Boston Massachusetts, 02215, U.S.A. 2 Department of Materials Science and Engineering and Photonics Center, Boston University, Boston Massachusetts, 0215, U.S.A ABSTRACT AlGaN based multiple quantum wells (MQWs) were grown on 8o vicinal 4H p-SiC substrates by plasma-assisted molecular beam epitaxy. The MQWs were designed to emit near 300 nm using the wurtzite k.p model. The MQW periodicity and strain state were measured with X-ray diffraction. The optical properties were characterized with temperature dependent photoluminescence (PL). The internal quantum efficiency was estimated from the ratio of room temperature to 18K integrated PL intensity. Internal quantum efficiency up to 48% was achieved. These data are encouraging for future vertical and inverted ultraviolet light emitting diodes grown on p-SiC substrates. INTRODUCTION Ultraviolet (UV) light-emitting diodes (LEDs) are ideal for applications ranging from epoxy curing to sterilization and phototherapy. Todays UV LEDs are fabricated from AlGaN alloys grown on sapphire or AlN substrates, as shown schematically in Figure 1A. The external quantum efficiency of these devices is limited by low internal quantum efficiency (IQE), poor UV light extraction, and poor hole injection from resistive epitaxial layers [1,2,3]. Furthermore, resistive n-AlGaN current spreading layers necessitate high forward voltage. As a result, selfheating degrades performance at high drive currents [4]. These limitations are mitigated with a vertical and inverted device geometry. Vertical devices, grown on sapphire substrates and fabricated with the laser-liftoff process, show lower series-resistance and self-heating than traditional lateral-conduction devices [5]. In an inverted device, the p-layer is grown before the n-layer (assuming metal-polar nitride films). Computational models of inverted nitride LEDs have shown the negative polarization charge in the electron blocking layer facilitates hole injection and reduces electron overshoot, and thus carrier density in the active region is increased [6]. Furthermore, the exposed n-AlGaN can be easily patterned or roughened in order to increase light extraction efficiency [4,7]. A vertical and inverted UV LED can be grown on a p-SiC substrate, as shown schematically in Figure 1B. The active region in this device is composed of AlGaN multiple quantum wells (MQWs) grown on a SiC/AlN template. Our group has previously reported AlGaN-based MQWs grown by molecular beam epitaxy (MBE) on semi-insulating 6H and 4H SiC substrates with IQE as high as 68% [8]. These MQWs were grown on a 500 nm AlN template, with estimated screw/edge threading dislocation density in the 5x107/5x109 cm-2 range. The high IQE was attributed to potential fluctuations, which red-shift emissio
Data Loading...
