Tem Investigation of Al_0.5Ga_0.5As_1-y Sb_y Buffer Layer Systems

Abstract

We report on a TEM study of Sb-adjusted quaternary Al_0.5Ga_0.5As_1-y Sb_y buffer-layers grown on <001> GaAs substrates. A series of structures were grown by MBE at 470°C that utilize a multilayer grading scheme in which the Sb content of Al_0.5Ga_0.5As_1-ySb_y is successively increased in a series of eight 125 nm thick layers. Post growth analysis using conventional bright field and weak beam dark field imaging of these buffer layers in cross-section reveals that the interface misfit dislocations are primarily of the 60° type and are distributed through out the interfaces of the buffer layer. Plan view studies show that the threading dislocation density in the active regions of the structure (approximately 2 fum from the GaAs substrate) is 10^5-6/cm² which is comparable to equivalent In_xGa_1-x As buffers. Weak Sb-As compositional modulations with a period of 1.8 nm were observed that provide a marker for establishing the planarity of the growth process. These features reveal that the growth surface remains planar through out the buffer layer growth sequence.