Epitaxial Semiconductor Films Grown By Laser Photochemical Vapor Deposition

Abstract

ABSTRACT Experiments in which epitaxial semiconductor films (Ge) have been grown by laser pho­ tochemical vapor deposition (LPVD) are described. The results provide a clear example of the ability of LPVD to grow epitaxial films under conditions in which growth is not attrib­ utable to substrate heating or adlayer photolysis but rather to species generated photo- chemically and in the gas phase. 1. INTRODUCTION Laser photochemical vapor deposition (LPVD) was first demonstrated in the late 1970s. A rapid survey of the literature will show that, in the intervening decade, most of the research has concentrated on the deposition of metal films by this approach. Considerably fewer examples can be found of the growth of semiconductor films by laser-assisted techniques and, in virtually all of these, transient or CW heating of the substrate and growing film played a significant or dominant role in the growth process. Furthermore, most experimental studies have been conducted in the so-called perpendicular geometry (i.e., substrate is directly illuminated by the optical source) for which it is extremely difficult to resolve the contributions of surface, adlayer and gas phase photochemical and thermal processes to film growth. The real promise of LPVD lies in the ability of a laser to produce highly non-equilibrium concentrations of gas phase species (atoms, radicals and even excited states) which thereby alter the reactor chemistry in a way favorable to the growth of epitaxial films. To our knowledge, no clear example exists in the literature of the use of a laser to photochemically produce, in the gas phase, a species which migrates to the substrate and alters the surface kinetics, resulting in the growth of epitaxial films while, in the absence of laser radiation, films of much poorer quality are grown.This paper describes the growth of epitaxial Ge films on [100] GaAs by LPVD. Upon pho- todissociating GeH4 at 193 nm in parallel geometry (i.e., laser does not irradiate the substrate), it was found that epitaxial films could be grown at substrate temperatures (Ts) as low 285°C. In the absence of laser radiation, the films were either amorphous or heavily-defected polycrystalline, depending upon Ts.The experimental apparatus has previously been discussed in detail