NH3 as a photosensitizer in the epitaxial growth of Ge on GaAs by laser photochemical vapor deposition

Abstract

The viability of ammonia as a sensitizer for the growth of epitaxial Ge on GaAs (100) by laser photochemical vapor deposition (LPVD) has been investigated. Specifically, NH3/GeH4/He (0.8/55.0/95 sccm, 5.5 Torr total pressure) mixtures have been irradiated by an excimer laser beam (ArF, 193 nm) in parallel geometry and for substrate temperatures (Ts) in the range 25≤Ts<400 °C. As evidenced by a more than an order of magnitude acceleration in the Ge film growth rate (factor of ∼33 improvement at 305 °C), even trace amounts of NH3 added to the gas stream efficiently couple the laser radiation to the Ge precursor (GeH4) without incorporating significant concentrations of N and H into the film. Auger, secondary-ion mass spectrometry (SIMS), and x-ray photoelectron spectroscopy analyses of the films show the nitrogen content to be negligible (<1%). Also, the slight amount of hydrogen detected by SIMS appears to arise from hydrocarbon contaminants rather than from the ammonia. Transmission electron microscopy studies demonstrate that films grown at Ts≥300 °C are epitaxial with an amorphous overlayer. Embedded within the amorphous matrix are Ge microcrystals which are textured, having their 〈112〉 axes parallel to the substrate normal. The beneficial effect of NH3 on the growth rate of LPVD Ge films is attributed to the photolytic production of hydrogen atoms which efficiently decompose GeH4 by hydrogen abstraction collisions.