Low-pressure Cvd of Germanium-silicon Films Using Silane and Germane Sources

Polycrystalline GexSi1-x can be grown from a mixture of SiH4 and GeH4 as reactive gases in a conventional LPCVD system. Several major problems make the growth of polycrystalline GexSi1-x more difficult with respect to poly-Si growth. Firstly, the initial formation of the GexSi1-x nuclei on the SiO2 surface is retarded. The incubation time, depending on deposition conditions, varies from a few seconds up to tens of minutes. The second important problem is relevant to the texture of the films. To obtain a uniform dopant distribution, a fast diffusion along the grain boundaries is necessary. For this purpose, a (110) orientation of the grains resulting in its near-cylindrical columnar structure is desired. Thirdly, a higher reactivity of GeH4 compared to SiH4 leads to a depletion of the gas mixture from GeH4 in the downstream direction of the reactor. This gradual change in the gas composition causes differences in deposition rate and germanium content measured for the films placed in a long waferboat. The film properties (texture, morphology) depend strongly on the composition and, therefore, are influenced by the germane depletion.