Amorphous superlattice structures with carbon as a wide bandgap component

Abstract

Since the pioneering work of Esaki and Tsu, superlattice (SL) systems have been intensively studied. There are two main difficulties in the technology of crystalline superlattices. One consists in a necessity of using expensive and hazardous deposition processes such as MBE. The second difficulty deals with problem of lattice mismatch between different materials used for the superlattice preparation. Both these difficulties gave rise to an alternative approach to the preparation of superlattice systems from amorphous semiconductors. In this case the problem of lattice mismatch disappears due to the very nature of amorphous state of matter. In this work we present technology used for the deposition of the superlattices based on amorphous hydrogenated carbon-germanium semiconductors (a-Ge/sub x/C/sub y/:H) using plasma decomposition of an organogermanium compound. It has been shown in our previous papers that the optical gap E/sub opt/ (defined by the Tauc law) of a-Ge/sub x/C/sub y/:H materials strongly depend on RF power input into glow discharge. It is obvious then that by application of an alternating power profile one should obtain a structure with periodically varied E/sub opt/. The time-power profile suitable to fulfill this condition was proposed and superlattice structures were deposited in a typical parallel-plate 13.56 MHz glow discharge deposition system. Variable angle-of-incidence spectroscopic ellipsometry (VASE) was used to study both topology and optical properties of the samples. The results of VASE measurements not only confirmed a layered structure of the SL systems but revealed a blue shift effect as well - a phenomenon which can be attributed to quantum confinement of charge carriers in superlattices.