Low Temperature Si Homoepitaxy by a Reactive CVD with a SiH₄/F₂ Mixture

Abstract

Single crystalline Si thin films on insulating substrates (SOI) have a variety of potential applications to such as high mobility TFT and to high efficiency and low cost solar cells. Since the SOI is limited to a thin layer, it is needed to develop a low temperature epitaxial growth technology to form active layers thicker than several micorns at low temperatures. The purpose of this study is to develop a deposition technique of single crystalline Si thin films by a reactive CVD method [1] at temperatures less than 600○C utilizing gas-phase reaction (SiH₄, F₂). Deposition of Si films was performed on a single crystalline Si (100) wafer. Substrate-temperature was varied between 100 and 700○C, reaction-pressure 1 and 500mTorr, flow-rate between SiH₄/F₂ = 1/1 and 1/3, and the geometry of the substrate and the gas-outlet were optimized. First, it was found that deposition rate was sensitive to the distance between the gas-outlet and the substrate and to the total pressure. For four different combinations of pressures, 250 and 500 mTorr and distances, 50 and 150 mm. The deposition took place only for the combination of 500 mTorr and 50 mm, and otherwise the deposition rate was significantly lower or etching of Si wafer was observed. The deposition rate for gas flow ratio, SiH₄/F₂ of 1/1 was 1.7 nm/s at a substrate-temperature of 400○C, while for higher F2 flow rate ratio, SiH₄/F₂ = 1/2 and 1/3, the deposition rates were 8.3×10^-3 nm/s and etching, respectively. Raman measurements show that crystallinity depends on the substrate-temperature; broad amorphous signal appears at 300, microcrystalline signal at 300 and 500○C and sharp crystalline at 400○C. RHEED observation shows a halo-pattern of amorphous-Si at 200○C, a mixed pattern of streak and spot without 2×1 superstructure at 300○C, a 2×1 streak-pattern at 400○C and a spot-pattern at 500○C. The reason of the narrow temperature window for epitaxial layer is a characteristic feature of low temperature epitaxy as reported before [2]. It is noteworthy the deposition rate of epitaxy obtained in this work is quite high, 1.7 nm/s even at 400○C. These observations are ascribed to the gas phase reaction between SiH₄ and F₂ and successive surface reactions. The SiH₄ and F₂ cause an exothermic reaction in the gaseous phases to generate radicals such as SiHx, H and F. The SiHx acts as a film precursor and others act as etchant. Under the conditions which radical density ratio SiHx/F increases, therefore, the deposition rate decreases or etching occurs. The material properties also will be discussed in relation to the growth mechanism. [1]J. Hanna et al., J. Non-Crst. Solids 114 (1989) 172-174 [2]T. Kitagawa, M. Kondo et al, Appl. Surf. Sci. 159-160 (2000) 30-34