Wet Oxidation of Si 1-x-y Ge x C y Layers on (100) Si
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A. E. BAIR*, Z. ATZMON*, T. L. ALFORD*, D. CHANDRASEKHAR**, DAVID J. SMITH** *Department of Chemical, Bio, and Materials Engineering; **Center for Solid State Science, Arizona State University, Tempe, Arizona 85287
ABSTRACT Single crystal Si0.63Ge0 .36C0.01 and amorphous Si0 .65Ge 0 .27C0 .08 layers have been oxidized in a wet ambient at 700 'C and 900 'C. The oxide growth has been studied using Rutherford backscattering spectrometry and transmission electron microscopy. A reference sample of Si 0 .63Ge 0 .37 was also oxidized in order to determine the influence of C on the oxidation behavior. The lower C content alloy behaved similar to the SiGe alloy. Uniform Sil-xGexO 2 was obtained at 700 'C whereas SiO 2 was formed at 900 'C, and Ge piled up underneath the oxide. In both cases, C was not detected in the oxide layer. The amorphous Si0.65Ge 0 .27C0 .08 alloy behaved significantly different at both oxidation temperatures in comparison with the crystalline Si 0.63Ge0 .36C0 .01 and 8i0 .63 Ge0 .37 . Negligible oxidation occurred at 700 'C whereas Si0 2 was obtained at 900 'C and the rejected Ge distributed uniformly throughout the SiGeC alloy. It is proposed that fast Ge diffusion during oxidation at 900 'C resulted from diffusion at grain boundaries, since crystallization of the amorphous SiGeC layer occurred in conjunction with oxidation, leading to nucleation of-5 nm nanocrystals. INTRODUCTION Group IV alloy heterostructures are of great interest due to their compatibility with current Si based manufacturing technology. The bandgap of Si can be decreased by the addition of Ge [1]. However, since the lattice of Ge is 4.2% larger than that of Si, compressive strains develop in the film which create stability problems limiting layer thickness. Carbon can be introduced substitutionally into the lattice at temperatures up to 700 "C without the formation of thermodynamically favored SiC phases [2-4]. Since the diamond phase of C is 46% smaller than Si, its presence could compensate for compressive strain in the film. Vegard's law predicts that complete compensation will result from a Ge:C ratio of 8.2:1 if 100% of the C in the film is incorporated substitutionally. Understanding the oxidation characteristics of a thin film alloy is important for its application in integrated circuit devices. Thermal oxidation of pseudomorphic and polycrystalline Sil.xGex has been studied previously by multiple investigators using wet and dry ambients [5-10]. The thermal oxidation of Sil.,Gex thin films depends on the temperature, the oxidizing ambient, the pressure, the Ge content, and the initial structure of the alloy layer. For alloys with a Ge content below 40 at.% oxidized at temperatures above 800 "C, nearly pure Si0 2 is obtained for both wet and dry ambients [5-7]. The Ge is rejected from the oxide and piles up at the oxide/film interface [9]. At annealing temperatures below 700 "C, uniform (Sil.xGe)O 2 is formed with the Si:Ge ratio in the oxide preserved from the alloy [8, 9]. Si0 2 oxidizes preferentially due to its lower
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