Calculation of desorption and migration of hydrogen on SiGe(100)-2×1 surface using density functional theory

Ab initio calculations have been carried out to investigate the H-atom migration and H2 desorption on the mixed SiGe(100)-2×1 surface using the Si9-xGexH13, Si14GeH19 and Si13Ge2H19 cluster models. The H2 recombinative desorption is the rate-determining step in hydrogen migration and desorption on SiGe(100) surface since the energy barrier of H-atom migration is generally lower than that of H2 desorption. Nonetheless H-atom migration into the interdimer position or onto the Ge site still affect the overall pathway and the surface reactivity because its following desorption step is made easier. The Ge presence on surface is interesting, the energy barriers for H2 desorption from the interdimer, the Si-Ge pair 52.8 kcal/mol and the Ge-Ge pair 45.1 kcal/mol, are lower than that for the Si-Si pair by 7.5 and 15.2 kcal/mol. In other words, the SiGe(100)-2×1 surface in chemical vapor deposition provides more dangling bonds than the Si(100)-2×1 surface owing to the inclusion of Ge. Introduction Owing to the benefit of achieving band-gap engineering at lower costs through employing Si-based technology, Si1-xGex epitaxial growth has been integrated into the fabrication processes for high-speed devices, which were restricted to -V technology previously. In the literature, Si1-xGex epitaxial techniques, using ultrahigh vacuum chemical vapor deposition (UHVCVD), rapid thermal chemical vapor deposition (RTCVD) and gas-source molecular beam epitaxy (GSMBE), have been extensively studied. The growing surface in Si1-xGex CVD is generally covered with chemisorbed hydrogen since Si and Ge hydrides are precursors and H2 gas is commonly employed as the carrier gas. The chemisorbed hydrogen is known to play an important role in the surface reactions of Si growth, which proceeds with the precursor impingement on the site without being covered by hydrogen. The interaction between hydrogen and SiGe surfaces is not only a crucial process in SiGe epitaxies but also one of the most well-defined adsorbate/surface systems. Still many details of hydrogen interaction with SiGe crystalline surfaces remain unknown. Cluster models are frequently used in studying various processes involving the interactions between gas molecules and crystalline surface. An example which attracts considerable experimental and theoretical interest over the past years is the dissociative adsorption and associative desorption on the Si(100)-2×1 surface. The Si(100)-2×1 surface consists of parallel rows of dimers, with each Si atom of a dimer pair having a dangling bond or a chemisorbed H. The migration of H-atom on the Si(100)-2×1 surface is intimately related to which reaction sites are available. Theoretical calculations using the cluster model have reported activation energies of hydrogen migration on the Si(100)and Ge(100)-2×1. Owing to the difference in implemented constraints and cluster sizes, the calculated results are dissimilar. Thomsen et al. have summarized the experimental results of SiH4 growth rates on Si(100), and showed that at low temperature the growth rate is limited by hydrogen desorption and independent of the partial pressure of silane. An activation energy 42± 6 kcal/mol of hydrogen desorption has been suggested. This value of hydrogen desorption is quite close to the results measured using the temperature programmed desorption technique under high vacuum condition, 47±3 kcal/mol. The Si1-xGex epitaxial growth mechanism at low Ge contents in CVD has been suggested to be also controlled by the hydrogen desorption reaction. Meanwhile, the correlated activation energy of Si1-xGex growth rate was reduced by the Ge content. Jang and Reif have published results on Si1-xGex epitaxial layer growth. They have deposited Si1-xGex layers using silane-germane gas mixtures at very low pressure. These researchers have suggested a surface-related growth mechanism controlled by hydrogen desorption in the range of low germanium contents. Additional Ge incorporation into Si1-xGex layer would also change the growth rate activation energy of Si1-xGex alloy. In this article, the H-atom migration and H2 desorption are simulated using density functional theory (DFT) on SiGe clusters of two sizes. The energetics of these processes are calculated and steps of relatively low energy barriers are identified and connected to be the probable path in the interaction between hydrogen and SiGe surface. Comparison of the results on SiGe and Si surfaces reveals the influence of Ge involvement. Methodology Various sizes clusters Si9-xGexH13(x=0,1,2), Si14GeH19, Si13Ge2H19(I), Si13Ge2H19(II), Si13Ge2H19(III), illustrated in Fig. 1, are used in modeling hydrogen surface migration and combinative desorption. These seven clusters which provide different types of surface atom pairs represent the reconstructed SiGe(100)-2×1 surface. Hydrogen atoms are used to terminate the subsurface dangling bonds of the cluster and keep the tetrahedral structure. The clusters shown in Fig. 1 have been optimized twice. The first optimization is done on the clusters with all dangling bonds being H-terminated. The second optimization is done on the optimized clusters with the one or three hydrogen atoms removed from the surface Si or Ge atom, which represent the surfaces of high and low hydrogen coverage. In the first optimization, the initial bond lengths of the H-atoms to the Si or Ge-atoms have been set to be 1.48 and 1.51 Å. The Si-Si bond length is set to that in bulk silicon, 2.35 Å. a Si9-xGexH13(x=1) b Si14GeH19 c Si13Ge2H19(I) d Si13Ge2H19(II)