Photoreflectance spectroscopy for the qualification of GaAsSb alloys introduced in ultrafast heterojunction bipolar transistors on InP

摘要

GaAsSb is an important III-V semiconductor ternary alloy which is currently used as the thin base layer in high speed InP Heterojunction Bipolar Transistors (HBT). Up to now, very little is known about physical properties of GaAsSb alloys, such as surface Fermi level pinning and Schottky barrier heights with different contact metals. However such parameters impact directly the cut-off frequency of HBT devices. Photoreflectance spectroscopy is applied to specific samples differing by their Sb concentration. The exploitation of Franz-Keldysh Oscillations (FKO) spectra allows determining the Fermi level pinning at the free surface of GaAsSb as a function of the alloy composition. Micro-electroreflectance spectroscopy is also applied to Schottky diodes performed on the same structures, with Ti deposited at the GaAsSb surface. The Schottky barrier height between GaAsSb and Ti is derived from the FKO system recorded in the micro-eleetro-reflectance spectra.