A model for the low-temperature growth of epitaxial Ge and Si films from GeH2 and SiH2 radicals produced by UV photolysis of GeH4 and SiH4

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Abstract
Recent experimental results have demonstrated low‐temperature growth of polycrystalline and single‐crystal Ge films by UV laser‐induced chemical vapor deposition (UV‐LCVD) from GeH4. It is also known from experiment that the initial photofragments produced during the UV photolysis of GeH4 and SiH4 are GeH2 and SiH2 radicals, respectively. In this paper, we present an idealized model for the low‐temperature epitaxial growth of Ge and Si films from the initial GeH2 and SiH2 radicals. This work is meant to serve as a first step in the development of a more detailed model which can also account for the role of other radicals, such as GeH3 and SiH3, which are formed in subsequent gas‐phase reactions. The rate‐limiting step for film deposition in the present case was found to be the formation and desorption of H2 molecules through bond switching from M‐H (M=Ge or Si) to H‐H. A cluster description of GeH2‐adsorbed Ge(100) surfaces was combined with molecular orbital calculations to estimate the activation energy for H2 desorption as 40 meV, a value which is in reasonable agreement with the measured activation barrier, 85±20 meV, for UV‐LCVD growth of polycrystalline Ge from GeH4.