Ng Rachel Qiao-Ming, Tok E S, Kang H Chuan
Department of Chemistry, National University of Singapore, Singapore 117543, Singapore.
J Chem Phys. 2009 Jul 28;131(4):044707. doi: 10.1063/1.3191780.
At low temperatures, hydrogen desorption is known to be the rate-limiting process in silicon germanium film growth via chemical vapor deposition. Since surface germanium lowers the hydrogen desorption barrier, Si(x)Ge((1-x)) film growth rate increases with the surface germanium fraction. At high temperatures, however, the molecular mechanisms determining the epitaxial growth rate are not well established despite much experimental work. We investigate these mechanisms in the context of disilane adsorption because disilane is an important precursor used in film growth. In particular, we want to understand the molecular steps that lead, in the high temperature regime, to a decrease in growth rate as the surface germanium increases. In addition, there is a need to consider the issue of whether disilane adsorbs via silicon-silicon bond dissociation or via silicon-hydrogen bond dissociation. It is usually assumed that disilane adsorption occurs via silicon-silicon bond dissociation, but in recent work we provided theoretical evidence that silicon-hydrogen bond dissociation is more important. In order to address these issues, we calculate the chemisorption barriers for disilane on silicon germanium using first-principles density functional theory methods. We use the calculated barriers to estimate film growth rates that are then critically compared to the experimental data. This enables us to establish a connection between the dependence of the film growth rate on the surface germanium content and the kinetics of the initial adsorption step. We show that the generally accepted mechanism where disilane chemisorbs via silicon-silicon bond dissociation is not consistent with the data for film growth kinetics. Silicon-hydrogen bond dissociation paths have to be included in order to give good agreement with the experimental data for high temperature film growth rate.
在低温下，通过化学气相沉积生长硅锗薄膜时，氢脱附是已知的限速过程。由于表面锗降低了氢脱附势垒，Si(x)Ge((1 - x))薄膜的生长速率随表面锗含量的增加而提高。然而，在高温下，尽管进行了大量实验工作，但决定外延生长速率的分子机制仍未完全明确。我们在乙硅烷吸附的背景下研究这些机制，因为乙硅烷是薄膜生长中使用的一种重要前驱体。特别地，我们想要了解在高温 regime 下，随着表面锗含量增加导致生长速率下降的分子步骤。此外，需要考虑乙硅烷是通过硅 - 硅键解离还是通过硅 - 氢键解离进行吸附的问题。通常认为乙硅烷吸附是通过硅 - 硅键解离发生的，但在最近的工作中，我们提供了理论证据表明硅 - 氢键解离更为重要。为了解决这些问题，我们使用第一性原理密度泛函理论方法计算乙硅烷在硅锗上的化学吸附势垒。我们使用计算得到的势垒来估计薄膜生长速率，然后将其与实验数据进行严格比较。这使我们能够在薄膜生长速率对表面锗含量的依赖性与初始吸附步骤的动力学之间建立联系。我们表明，普遍接受的乙硅烷通过硅 - 硅键解离进行化学吸附的机制与薄膜生长动力学数据不一致。为了与高温薄膜生长速率的实验数据取得良好一致，必须考虑硅 - 氢键解离路径。
