Pyrolytic and Laser Photolytic Growth of Crystalline and Amorphous Germanium Films from Digermane (Ge 2 H 6 )
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PYROLYTIC AND LASER PHOTOLYTIC GROWTH OF CRYSTALLINE AND AMORPHOUS GERMANIUM FILMS FROM DIGERMANE (Ge2H6) DJULA ERES, D. H.LOWNDES, J. Z.TISCHLER, J. W. SHARP, D. B. GEOHEGAN, and S. J. PENNYCOOK; Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6056 ABSTRACT High-purity digermane (Ge 2 H6 , 5% in He) has been used to grow epitaxially oriented crystalline Ge films by pyrolysis. Amorphous Ge:H films also have been deposited by pyrolysis and ArF (193 nm) laser-induced photolysis. The amorphous-to-crystalline transition and the film's morphology was studied as a function of deposition conditions. The film's microstructure, strain and epitaxial quality were assessed using x-ray diffraction curves and scanning and transmission electron microscopy. Itwas found that commensurate, coherently strained epitaxial Ge films could be grown pyrolytically 0on (100) GaAs at low (0.05-40 m Torr) Ge2H6 partial pressures, for substrate temperatures above 380 C. INTRODUCTION The epitaxial Ge/GaAs interface is a subject of considerable research interest because it is almost perfectly lattice-matched (the relative difference in room temperature lattice constants is less than 8 x 10-4) and it is relatively simple to grow [1]. Heterostructures involving Ge or Ge alloy buffer layers on other semiconductor materials (e.g., Si) are technologically interesting since epitaxial Gebased systems seem to be the most suitable for lattice matching to GaAs and other Ill-V compounds. Ge epitaxial film growth methods have included evaporation, sputtering [2], pyrolysis [31, and more recently MBE [1], ion beam deposition [4] and laser-induced chemical vapor deposition [5]. 0Epitaxial growth has been claimed in a rather wide temperature range, from room temperature to 700 C. However, substrate temperatures between 400 and 5000 C seem to be necessary for high quality epitaxy. In this paper we report what are believed to be the first published results of pyrolytic and ArF laser photolytic chemical vapor deposition (CVD) experiments using pure digermane. Comparisons with germane (GeH4) CVD also are reported. Systematic investigations of deposition conditions and of GaAs substrate preparation procedures were carried out in order to define a variable space for high quality epitaxial Ge film growth. EXPERIMENTAL The deposition chamber [6] was a six-way turbo-pumped stainless steel cross, configured as a flowing-gas reactor. Itwas evacuated typically to the low 10-6 Torr range prior to deposition. The source gases entered the chamber through a rectangular slit with a 30 x 0.2 mm 2 cross-section, positioned about 5 mm above and 25 mm away from the substrate. The source gases were mixtures of either GeH 4 (10%) or Ge 2 H6 (5%) in helium. The chamber was equipped with 25 mm diameter Suprasil windows that permitted photolysis of source gas molecules with the laser beam parallel to the substrate surface. We eliminated the problem of film deposition on the windows through which the laser beam traversed the chamber by internally flushing each wi
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