FORMATION OF COMPOUND SEMICONDUCTORS BY ELECTROCHEMICAL ATOMIC LAYER EPITAXY

A method for the electrochemical formation of epitaxial deposits of compound semiconductors is being developed. It is referred to as electrochemical atomic layer epitaxy (ECALE). The method is the electrochemical analog of atomic layer epitaxy (ALE), where ALE is a method used to form compounds by alternately depositing atomic layers of the constituent elements. Atomic layers are formed in ECALE by using underpotential deposition (UPD). UPD is a phenomena where an atomic layer of an element deposits at a potential prior to that needed to deposit the bulk element, due to the increased stability afforded by reaction with a second element present at the substrate surface. This paper describes the structure of the first monolayer of Te formed on an Au(100) surface and the structure of a monolayer of CdTe, subsequently formed by deposition of an atomic layer of Cd. Deposits have been formed and analyzed in an ultrahigh vacuum (UHV) surface analysis instrument directly coupled to an electrochemical cell. Low-energy electron diffraction (LEED) and Auger electron spectroscopy have been used to follow the structures and compositions of deposits after various steps in the ECALE cycle. As well, some initial studies of the atomic arrangements have been performed using scanning tunneling microscopy. Reductive UPD of Te on the Au(100) plane resulted in a series of well ordered Te structures at increasing coverages: (2 X 2), (2 X square-root 37), (2 X 4), and (square-root 2 X square-root 5)R45-degrees. Oxidative UPD of Te also resulted in a well-ordered structure, a (2 X square-root 10) at 1/3 coverage of Te. Subsequent reductive UPD of Cd on this surface resulted in a well ordered c(2 X 2)-CdTe structure.