Shyam Ramalingam

Atomistic simulation study of the interactions of SiH3 radicals with silicon surfaces

SiH3 radicals created by electron impact dissociation of SiH4 in reactive gas discharges are widely believed to be the dominant precursor for plasma deposition of amorphous and nanocrystalline silicon thin films. In this article, we present a systematic computational analysis of the interactions of SiH3 radicals with a variety of crystalline and amorphous silicon surfaces through atomistic simulations. The hydrogen coverage of the surface and, hence, the availability of surface dangling bonds has the strongest influence on the radical–surface reaction mechanisms and the corresponding reaction probabilities. The SiH3 radical reacts with unit probability on the pristine Si(001)-(2×1) surface which has one dangling bond per Si atom; upon reaction, the Si atom of the radical forms strong Si–Si bonds with either one or two surface Si atoms. On the H-terminated Si(001)-(2×1) surface, the radical is much less reactive; the SiH3 radical was reflected back into the gas phase in all but two of the 16 simulations of...

Interactions of SiH radicals with silicon surfaces: An atomic-scale simulation study

A comprehensive study is presented of the interactions of SiH radicals originating in silane containing plasmas with crystalline and amorphous silicon surfaces based on a detailed atomic-scale analysis. The hydrogen concentration on the surface is established to be the main factor that controls both the surface reaction mechanism and the reaction probability; other important factors include the location of impingement of the radical on the surface, as well as the molecular orientation of the radical with respect to the surface. On the ordered crystalline surfaces, the radical reacts in such a way as to maximize the number of Si–Si bonds it can form even if such bond formation requires dissociation of the radical and introduction of defects in the crystal structure. The radical is established to be fully reactive with the pristine Si(001)-(2×1) surface. This chemical reactivity is reduced significantly for the corresponding H-terminated surface with a hydrogen coverage of one monolayer. SiH is found to be ...

Evolution of structure, morphology, and reactivity of hydrogenated amorphous silicon film surfaces grown by molecular-dynamics simulation

The relationship between the structure, H coverage, morphology, and reactivity of plasma deposited hydrogenated amorphous silicon (a-Si:H) film surfaces was investigated using molecular-dynamics simulations. Surfaces of a-Si:H films grown with SiH3 as the sole deposition precursor are found to be remarkably smooth due to a valley-filling mechanism where mobile precursors, such as SiH3 and Si2H6, diffuse and react with dangling bonds in the valleys on the surface. Surface valleys are reactive due to the increased concentration of dangling bonds and decreased H coverage in these regions. The previously speculated physisorbed configuration, where SiH3 is weakly bound to the surface through a H atom, is highly unlikely to be the mobile precursor state.

Mechanism and energetics of dissociative adsorption of SiH3 on the hydrogen-terminated Si(001)-(2×1) surface

Abstract A comprehensive theoretical study is presented of the adsorption mechanism and energetics of the silyl (SiH3) radical on the Si (0 0 1) -(2×1) surface terminated by one monolayer of hydrogen atoms. SiH3 adsorbs on the surface dissociatively through an insertion reaction that involves breaking of the Si–Si surface dimer bond and subsequent transfer of an H atom from the radical to an atom of the broken dimer, thus leading to formation of two surface dihydride (SiH2) species. The structure and energies of the corresponding equilibrium and transition-state configurations are presented, as well as detailed energetics along the reaction pathway.

Theoretical study of the interactions of SiH2 radicals with silicon surfaces

Silylene (SiH2) radicals created by electron impact dissociation of silane in reactive gas discharges can play an important role in plasma deposition of amorphous and nanocrystalline silicon thin films. In this article, we present a systematic computational analysis of the interactions of SiH2 radicals with a variety of crystalline and amorphous silicon surfaces based on atomistic simulations. The hydrogen coverage of the surface and, hence, the availability of surface dangling bonds is shown to exert the strongest influence on the radical-surface reaction mechanisms and the corresponding reaction probabilities. The SiH2 radical reacts with unit probability on the pristine Si(001)-(2×1) surface which has one dangling bond per Si atom; upon reaction, the Si atom of the radical forms strong Si–Si bonds with either one or two surface Si atoms. On the H-terminated Si(001)-(2×1) surface, the radical is found to react with a probability of approximately 50%. The SiH2 radical attaches itself to the surface eithe...

Atomistic simulation of SiH interactions with silicon surfaces during deposition from silane containing plasmas

A systematic atomic-scale analysis is presented of the interactions of SiH radicals from a silane plasma with amorphous hydrogenated silicon (a-Si:H) film surfaces based on molecular dynamics and Monte Carlo simulations. The coordination of the silicon surface atoms and the hydrogen surface concentration determine the mechanisms by which the SiH radical interacts with the surface and attaches itself to the film, as well as the “sticking” probability of the radical. On a-Si:H surfaces with low hydrogen concentration, SiH radicals remain on the surface and attach to silicon dangling bonds. In contrast, on a-Si:H surfaces with high hydrogen concentration and lower density of dangling bonds, radicals penetrate into the bulk film and attach to subsurface silicon atoms. An average reaction probability of 95% is predicted, which is in excellent agreement with recent experimental data.

In Situ Probing and Atomistic Simulation of a-Si:H Plasma Deposition

Hydrogenated amorphous silicon thin films deposited from SiH4 containing plasmas are used in solar cells and thin film transistors for flat panel displays. Understanding the fundamental microscopic surface processes that lead to Si deposition and H incorporation is important for controlling the film properties. An in situ method based on attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy was developed and used to determine the surface coverage of silicon mono-, di-, and tri-hydrides as a function of deposition temperature and ion bombardment flux. Key reactions that take place on the surface during deposition are hypothesized based on the evolution of the surface hydride composition as a function of temperature and ion flux. In conjunction with the experiments, the growth of a-Si:H on H-terminated Si(001)-(2×1) surfaces was simulated through molecular dynamics. The simulation results were compared with experimental measurements to validate the simulations and to provide supporting evidence for radical-surface interaction mechanisms hypothesized based on the infrared spectroscopy data. Experimental measurements of the surface silicon hydride coverage and atomistic simulations are used synergistically to elucidate elementary processes occurring on the surface during a-Si:H deposition.

Molecular dynamics study of the interactions of small thermal and energetic silicon clusters with crystalline and amorphous silicon surfaces

An atomic-scale analysis based on molecular dynamics simulations of the interactions of small thermal and energetic SinHm, n>1, clusters observed in various plasmas with crystalline and amorphous Si surfaces is presented. The experimental literature has assumed and employed a unit reaction probability for clusters of various sizes on all Si surfaces in phenomenological models for obtaining hydrogenated amorphous Si film growth rates, while the reaction mechanisms of clusters with the deposition surfaces have remained unexplored. In addition, it is widely speculated that clusters have a detrimental effect on the film quality. Our study shows that the clusters react with high (>85%) probability with crystalline surfaces and with surfaces of amorphous Si films. The structure and energetics of the corresponding adsorbed cluster configurations on these surfaces are analyzed and discussed. Furthermore, the simulations provide insight into possible mechanisms for the formation of defects, such as voids and dangl...

Visualizing radical-surface interactions in plasma deposition processes: reactivity of SiH/sub 3/ radicals with Si surfaces

There is a need for improving our fundamental understanding of the radical surface interactions during plasma enhanced chemical vapor deposition of Si. Toward this goal, we have calculated the interaction energy between SiH/sub 3/ radicals and various Si surfaces, and mapped the changes in this energy as a function of position on the surface. Application of this analysis to the SiH/sub 3/ radical impinging on a variety of Si surfaces, coupled with detailed investigation of the radical impingement dynamics, proved valuable in understanding and visualizing the driving forces that determine the nature of the radical-surface interactions during plasma deposition of Si.