No abstract

No abstract

Car-Parrinello simulations and static density-functional theory calculations reveal how hydrogen promotes growth of epitaxial, ordered Si films in plasma-enhanced chemical vapor deposition at lowtemperature conditions where the exposed Si 001 -2 1 surface is fully hydrogenated. Thermal H atoms, indeed, are shown to selectively etch adsorbed silyl back to the gas phase or to form adsorbed species which can be easily incorporated into the crystal down to T 200 C and start diffusing around T 300 C. Our results are well consistent with earlier experiments. DOI: 10.1103/PhysRevLett.100.046105 PACS numbers: 81.15.Gh, 68.43.Bc, 68.47.Fg Plasma-enhanced chemical vapor deposition (PECVD) is a very popular technique for growing silicon films. The possibility of reaching high growth rates at moderate temperatures has made PECVD particularly appealing for generating amorphous (a), microcrystalline ( c), and nanocrystalline (nc) films, of particular importance, e.g., in the production of solar cells [1]. By reducing the growth rate [2] or by raising the substrate temperature [3] also epitaxial silicon films of good quality can be obtained. This versatility surely makes PECVD of extreme scientific interest. Among the large amount of information coming out of decades of experiments and production, one phenomenon has captured particular attention, leading to several, sometimes controversial, interpretations. Increasing amounts of hydrogen impinging the surface were indeed shown to enhance crystallinity both during growth, and/or in post-growth treatments [2,4,5]. The observation of a simultaneous decrease in the growth rate, and/or of the thinning of the samples during hydrogen exposure, strongly suggested hydrogen to be able to preferentially etch noncrystalline regions [2,6]. A further, possibly complementary channel for H-induced crystallinity in postgrowth exposure was also proposed [5]. While very many indications for possible competing mechanisms were given based on experimental observations, the complexity of the problem, in terms of geometry, chemistry, and the multitude of species involved, has prevented so far the development of a parameter-free quantitative modeling of the process at the atomic scale able to shed full light on the phenomenon.In this Letter we shall focus on the role played by hydrogen during silicon-film growth starting from a fully hydrogenated Si 001 -2 1 representative of the initial stages of low-temperature deposition, where thermal H desorption is not activated [7]. Our study has been motivated by the wide interest in hydrogen-induced crystallization, and, more specifically, by the seminal paper of Tsai et al. [2] in which it is demonstrated the possibility of growing epitaxial Si films with PECVD at a very low temperature T 200 C, provided that silane is introduced in the reactor strongly diluted with H 2 . In their paper, Tsai et al. suggested preferential etching of nonepitaxial adsorbed species to cause the improved quality of the grown sample. Hydrogen etching is proposed to be selecti...

Diffusion and decomposition of SiH x species adsorbed on the clean Si͑100͒ surface are processes of relevance for the growth of crystalline silicon by plasma-enhanced chemical vapor deposition. In this work, we report an extensive search of diffusion and decomposition pathways for SiH 3 , SiH 2 , and SiH by means of combined ab initio metadynamics simulations and optimization of minimum-energy reactions paths. We find that on the clean surface SiH 3 undergoes stepwise decompositions into Si and H adatoms according to SiH 3 → SiH 2 +H→ SiH+ 2H → Si+ 3H with an overall reaction barrier of the order of 0.8 eV, consistent with the scenario inferred from secondary ion mass spectroscopy data. The lifetime of SiH 3 at room temperature calculated within transition state theory in the harmonic approximation is in agreement with experiments. The lifetime of SiH 2 turns out to be similar to that of SiH 3 . Possible trap states for SiH 2 are proposed, based on energetics and by comparing calculated scanning tunneling microscope images with experimental data.

No abstract