Growth of semiconducting cubic boron nitride thin films.

Abstract

This thesis details the growth mechanisms and deposition processes for semiconducting cubic boron nitride. Through experimental research a better understanding of the nucleation and growth mechanisms of the metastable cubic phase of boron nitride is achieved. A deposition process based on reduced-bias ion-assisted sputtering is developed. We found that relatively high nitrogen ion energy (&sim;100eV) necessary for initial nucleation and coalescence of c-BN islands can be reduced substantially (to &sim;60eV) leading to a dramatic improvement in the film quality in terms of fewer defects, larger grain size (&sim;2000A), lower residual stress, and well defined texture. It was found that the films grown at temperatures above 1050&deg;C are stable at ambient conditions and consist of pure-phase cubic boron nitride. An important aspect of the studies was the use of a novel technique for <italic>in-situ </italic> real-time stress monitoring which allowed us to produce films of cubic boron nitride with record thicknesses of more than 2mum. Cubic BN films prepared in this way are typically polycrystalline exhibiting partial crystallographic alignment relative to the substrate (i.e. textured morphology). A kinematic theory of reflection high energy electron diffraction (RHEED) is developed for textured polycrystalline thin films. Analytical expressions for RHEED patterns are derived for arbitrary texture situations and for any general crystallographic orientation that may be encountered in thin-film growth. We showed that the RHEED can be used as a fast and convenient tool for <italic>in-situ</italic> texture characterization. The approach also permits quantitative extraction of texture angular dispersion parameters which are useful for optimizing thin-film growth conditions. Analysis of RHEED patterns showed that c-BN films are textured with the texture axis along the (100) crystallographic direction perpendicular to the film surface with the texture spread of less than 20 degrees, a dispersion that can be reduced substantially by optimizing the growth conditions. The deposition technique devised in this research enables p-type semiconducting cubic boron nitride thin films with low a carrier activation energy of 60meV, high room temperature carrier mobility of &sim;500cm<super>2</super>/V-s and high room-temperature carrier concentrations of &sim;5 &middot; 10<super> 18</super>cm<super>-3</super>.