Structural and electrical characterization of GaN based HEMT heterostructures on SiC
Protik Parvez Sheikh
Date of Issue2017-08-29
School of Electrical and Electronic Engineering
Group III-nitride semiconductors and its alloys has been the subject of intense research because of its wide range of applications in the field of microelectronics and optoelectronics. They are used in light emitting diodes, lasers, detectors, next generation wireless network base stations, satellite communication systems and compact digital radar applications where GaN based devices can multiply the efficiency of amplifiers. Among the group III-nitrides, GaN is unique with excellent properties such as wide band gap, mechanical and thermal stability, high breakdown field and high mobility that can be used to fabricate high power transistors on GaN based heterostructures. Although GaN based heterostructures have been widely demonstrated on silicon and sapphire substrates, silicon carbide is still considered to be the best option in terms of higher thermal conductivity and smaller lattice mismatch with respect to GaN. This research project involves the structural and electrical characterization of AlGaN/GaN high electron mobility transistor (HEMT) heterostructures grown on SiC substrate by plasma-assisted molecular beam epitaxy (PA-MBE). The growth parameters such as III/V ratio, layer thickness and metal composition were optimized to obtain a smooth surface morphology and improved electrical characteristics of the HEMTs. Structural characterization techniques such as optical microscopy and atomic force microscopy (AFM) were used to analyze the surface morphology and roughness of the AlGaN/GaN HEMT heterostructures. The AlN layer of the AlGaN/GaN HEMT heterostructure grown in the intermediate growth regime (Al/N ratio ~ 1) exhibited a smooth surface morphology with root mean square (RMS) roughness of 0.7 nm but with the presence of some cracks on its surface. The heterostructure grown using thin AlN layer of 50 nm thickness in the intermediate growth regime also resulted in an improved surface morphology with RMS roughness of 0.9 nm but without the presence of any cracks on its surface. The GaN layer of the AlGaN/GaN HEMT heterostructure grown in the Ga-rich growth regime (Ga/N ratio > 1) showed a smooth, crack-free, pit-free surface filled with Ga droplets. It had an RMS roughness value of 0.7 nm in this growth regime. Hall Effect measurement technique was used for the electrical characterization of the AlGaN/GaN HEMT heterostructures. The AlN layers of the AlGaN/GaN HEMT heterostructures that were grown in the N-rich growth regime (Al/N ratio < 1) resulted in improved 2DEG properties such as a high mobility of 1020 cm2/V.s and a sheet carrier density of 1.1 x 1013 cm-2. The variation of the AlN layer thickness however did not have any effect on the electrical properties of the heterostructures. The GaN layer of the AlGaN/GaN HEMT heterostructure that was grown in the intermediate growth regime (Ga/N ratio ~ 1) also resulted in a high mobility of 1040 cm2/V.s and a sheet carrier density of 1 x 1013 cm-2 for the HEMT. Lastly, the Al composition in the AlGaN barrier layer and its thickness effect on the electrical properties of AlGaN/GaN HEMT heterostructure was systematically studied. A high mobility of 1220 cm2/V.s was achievable when AlGaN barrier layer had an Al composition of 29% and thickness of 19 nm whereas a high sheet carrier density of 10.9 x 1012 cm-2 and moderate sheet resistivity of 540 Ω/sq was obtained with an AlGaN barrier layer having Al composition of 28% and thickness of 26 nm.
DRNTU::Engineering::Electrical and electronic engineering