Molecular beam epitaxial growth and characterization of strain-compensated Al0.3In0.7P/InP/Al0.3In0.7P metamorphic-pseudomorphic high electron mobility transistors on GaAs substrates

A novel metamorphic high electron mobility transistor (HEMT) structure was grown on GaAs substrates by solid-source molecular-beam epitaxy for potential microwave power applications. The HEMT device layers were strain compensated with pseudomorphic (tensile-strained) Al0.3In0.7P donor-barrier layers and a pseudomorphic (compressive-strained) InP channel layer. Atomic force microscopy measurements of the metamorphic structure yielded a root-mean-square surface roughness of 8 Angstrom. Transmission electron micrographs of the device layers exhibited flat interfaces with the dislocation density estimated to be less than 1X10(6) cm(-2). Room temperature photoluminescence measurements of metamorphic AlInP layers indicated large direct band gaps up to 2.10 eV. Due to the larger conduction band discontinuity at the Al0.3In0.7P/InP heterojunction than the AlGaAs/InGaAs heterojunction in GaAs pseudomorphic HEMTs, significantly higher channel sheet densities were obtained. For Al0.3In0.7P/InP HEMTs, channel sheet densities (cm(-2)) exceeding 3 X 10(12) for single-pulse-doped, and greater than 4 X 10(12) for double-pulse-doped, structures were readily obtained. Hall measurements on a double-pulse-doped Al0.3In0.7P/InP/Al0.3In0.7P HEMT gave mobilities (cm(2)/V s) of 4450 at 300 K and 18 500 at 77 K, which are consistent with a high quality InP channel layer. Secondary ion mass spectroscopy depth profiles of a double-pulse-doped structure displayed sharp doping pulses and interfaces indicating that metamorphic growth was not leading to enhanced diffusion or migration. Initial and nonoptimized devices with a gate length of 0.15 mum exhibited a maximum current density of 500 mA/mm and a transconductance of 520 mA/mm, which compare favorably to mature AlGaAs/InGaAs pseudomorphic HEMTs. (C) 2001 American Vacuum Society.