Growth and characterization of undoped and doped silicon thin films using supersonic molecular jets.

Abstract

This thesis work has involved studying epitaxial growth of single crystal silicon thin films using a unique approach. Pulsed Supersonic Jet Epitaxy utilizes high kinetic energy ($\approx$1.6 eV) jets of a disilane $\rm(Si\sb2H\sb6$)--hydrogen mixture incident on a silicon substrate to conduct growth. The necessary activation energy for epitaxy is provided by the inherent energy of the precursors, precluding the need for a high substrate temperature. We have successfully demonstrated film growth of single crystal silicon (without external activation) at temperatures as low as 400$\sp\circ$C. This technique has potential applications in processes requiring low thermal budgets, including abrupt-interface multilayer film growth and flat panel display manufacturing with glass substrates. We have been able to accurately control the film thickness with sub-Angstrom resolution by effectively utilizing the self limiting nature of this hydrogen desorption limited system. An order of magnitude increase in sticking coefficients $\rm(S\approx0.3$ at T = 450$\sp\circ$C) is observed compared to conventional gas source molecular beam epitaxy. This technique has also simplified the surface kinetics and we have been able to develop a fundamental reaction scheme for a pulsed system to predict growth rate dependencies on various parameters. Silicon epitaxy from pulsed jets has also been analyzed using a Monte Carlo simulation to study the effects of high kinetic energy jets on the surface morphology of the growing epilayer. We have been able to predict temperature independent atomic layer epitaxy for incident energy values exceeding the hydrogen desorption activation energy. We have successfully applied this technique for in situ n-type doping of silicon using supersonic jets of phosphine (PH$\sb3).$ The high flux, kinetic energy and low growth temperature associated with this technique have enabled us to obtain active carrier concentrations up to $\rm5\times10\sp{19}\ cm\sp{-3}$ in silicon films (with no subsequent annealing) at substrate temperatures of 550$\sp\circ$C, hitherto only possible with ion implantation. The thin films exhibit uniformity in doping levels with Hall mobilities of 90 cm$\rm\sp2V\sp{-1}s\sp{-1}$ at the highest doping concentration, comparable to bulk layers. This method also provides good control of doping level by variation of substrate temperature and pulse parameters.