Amorphous Silicon Surface Research Articles

Surface diffusion kinetics on amorphous silicon

We obtain parameters for self-diffusion on the amorphous silicon surface by reanalyzing kinetic data for the formation of hemispherical grained silicon (HSG) previously published by another laboratory [J. Vac. Sci. Technol. A11 (1993) 2950]. Our mathematical model for individual HSG grain growth permits extraction of diffusivities for overall mass transport, and to our knowledge yields the first numbers for temperature-dependent self-diffusion on any amorphous surface. The activation energy for mass transfer diffusion is lower than that previously measured for crystalline Si(1 1 1).

Structural models of amorphous silicon surfaces

Structural models of amorphous silicon surfaces

Early stages of silicon nitride film growth studied by molecular dynamics simulations

The molecular dynamics method is used to simulate silicon nitride film growth through atomic deposition on an amorphous silicon surface, which is modeled by the stochastic classical trajectory-ghost theory. For the first time, realistic semi-empirical interatomic potential including both two-body and three-body interactions is employed to calculate the forces among deposited atoms. By the simulation we studied growth kinetics and the effects of substrate temperature and kinetic energy of vaporized atoms on the film growth. It is found that the film growth follows power law mechanism but with two different growth exponents in different growth stages, and changes of the substrate temperature and atomic kinetic energy lead to various film morphologies. This provides a possibility to precisely control the film properties, especially cluster size and compactness, by choosing suitable deposition parameters.

Surface Alkylation of Hydrogenated Amorphous Silicon by Grignard Reagent

Abstract Surface alkylation of hydrogenated amorphous silicon thin films has been achieved by Grignard reaction. IR absorption spectra indicate the existence of C–H bonds that are due to covalent bonded decyl group on the surface of amorphous silicon. This modification can be a first step of novel organic–inorganic interface formation.

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 dangling bonds, in plasma-deposited amorphous Si films through reactions of the clusters with the deposition surfaces.

Removal Efficiency of Metallic Impurities on Various Substrates in HF-Based Solutions

In this study, the removal efficiency of metallic impurities with various substrate surfaces has been investigated. It strongly depends on the substrate conditions as well as the kind of metallic impurities. Copper (Cu) impurities deposited on an amorphous silicon (a-Si) surface are hardly removed by a hydrofluoric acid-hydrogen peroxide mixture (FPM) cleaning, while they are completely removed from a single crystalline silicon (c-Si) surface at a level lower than the detection limit. A hydrofluoric acid-hydrogen peroxide mixture with nonionic surfactant (FPMS) cleaning with high HF concentration, however, can efficiently remove Cu contaminants from both c-Si and a-Si surfaces. We postulate that both etching of the silicon by the FPM solution and surface passivation by the surfactant are essential to removing Cu contaminants from a-Si surface. Moreover, the Cu removal efficiency of FPMS solution is highly dependent on the nature of a-Si films. The range of high removal efficiency on a-Si film surface with inactivated phosphorus, when changing the chemical mixing ratio of an FPMS solution with 50 ppm nonionic surfactant, is much broader than that on undoped a-Si film surfaces, whereas it is not depending on the presence of hydrogen in a-Si film. In order to remove metallic impurities from all substrates including various a-Si film surfaces, a cleaning solution should have the etching capability of substrates and high dissolution rate of metallic contaminants. Much more attention should be given for HF-based solution process of various substrates with high etching rate. © 2001 The Electrochemical Society. All rights reserved.

Surface Derivatization of Amorphous Silicon by Grignard Reagents

ABSTRACTSubstitution reactions of surface hydrogen in amorphous silicon at room temperature have been studied. After wet treatment of hydrogenated amorphous silicon thin films with Grignard reagent, surface hydrogen atoms have been substituted by organic groups. Wet treatment of amorphous silicon thin films in CH3(CH2)9-MgBr ether solution followed by quenching by diluted HCl changes infrared absorption spectra. After the treatment, new three peaks of C–H have been observed. In addition, the peaks did not disappeared after rising by hydrofluoric acid or various kinds of organic solvent. The spectra indicate that the Grignard reagent is enough reactive to form new covariant bonding of C and Si at the amorphous silicon surface by substitution reaction.

Multi-Scale Simulations of Silicon Etching by Halides: Effects of Surface Reaction Rates.

ABSTRACTWe investigate the reactive ion etching of amorphous silicon by halides using a hierarchy of models on different time and length scales. The feature evolution is modeled using a two- dimensional cell based Monte-Carlo feature scale simulator. The fluxes, the energy distributions, and the angular distributions of the wafer-incident particles are provided by a hybrid plasma sheath simulator. The relevant surface reaction rates are calculated by a molecular dynamics simulator using a Stillinger-Weber representation of the interatomic potential. Our investigations show that the surface reaction rates are strongly determined by the particular surface morpho- logy, which, in turn, is strongly influenced by the kinetic properties of the impinging particles. Thus, we link the molecular dynamics simulator into the model as a whole.As results, we present calculations for the etching of amorphous silicon by fluorine, chlorine, and bromine. A Stillinger-Weber representation of the bromine and the silicon-bromine potential which was not yet available in literature is additionally developed. We discuss the different morphologies of halogenated silicon surfaces as a consequence of the energy distri- bution and the angular distribution of the impinging particles. Comparisons of the sputter yield functions of bare amorphous silicon surfaces and corresponding halogenated surfaces exhibit considerable differences, qualitative as well as quantitative.

Abstraction of hydrogen by SiH radicals from hydrogenated amorphous silicon surfaces

The dynamics and energetics are presented of two Eley–Rideal reactions by which SiH radicals impinging at thermal energies on growth surfaces during plasma deposition of hydrogenated amorphous silicon (a-Si:H) films abstract hydrogen atoms from the surface and return to the gas phase as SiH 2 radicals. The reactions were observed during classical molecular-dynamics simulations of a-Si:H film deposition from SiH radicals impinging on an initially H-terminated Si(001)-(2×1) surface maintained at 500 K. The H-abstraction reaction may either produce a dangling bond on the surface or eliminate a surface coordination defect. The computed activation energy barriers for the two hydrogen abstraction reactions are 0.15 and 0.07 eV, respectively, and the corresponding exothermic reaction energies are 0.20 and 0.30 eV. The effects of both reactions on the growth surface are examined through detailed analysis involving the local structural configurations and the associated bond angle and bond length distributions in the vicinity of the surface hydrogen abstraction sites.

Optics of textured amorphous silicon surfaces

We have performed real time Stokes vector spectroscopy (also called polarimetry) during the preparation of hydrogenated amorphous silicon (a-Si:H) p–i–n solar cells on textured tin-oxide (SnO 2) surfaces. With this spectroscopy (1.5–4.0 eV), the irradiance, I r, and polarization parameters, {( Q, χ), p}, for the specularly reflected beam are obtained with 0.8 s resolution versus time during solar cell preparation. Here Q and χ are the tilt and ellipticity angles of the polarization state, respectively, and p is the degree of polarization. From an analysis of these spectra, we establish a model for textured a-Si:H intrinsic-layer growth that includes both microscopic and macroscopic structural components. Using such a model, the evolution of structural and optical properties of the solar cell components can be determined, and the results provide realistic inputs for optical modeling of solar cells.

Key Technologies for Next Generation Thin Film Silicon Solar Cells. Amorphous Silicon Solar Cells and Surface Science.

We achieved the highest conversion efficiency of 9.5% in a large area amorphous silicon (a-Si) solar cell (40 cm×30 cm) for the first time in the world. It was achieved by employing surface technologies, such as, the optical confinement structure, the high quality a-Si fabrication technique, and high quality a-Si germanium technology. Using the a-Si technology HIT solar cell that is a hybrid of a-Si and c-Si was also developed. The conversion efficiency of a module using HIT solar cells is 15.2% that is the highest for the products. In addition to high conversion efficiency the HIT structure solar cell has an improved temperature dependence compared to conventional c-Si cells.

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 either by forming two bonds with Si atoms of adjacent dimers in the same dimer row or through Si–Si bonds with one or both atoms of a surface dimer. In addition, the SiH2 radical can attach itself in the trough between dimer rows, forming two Si–Si bonds with second-layer Si atoms. The energetics and dynamics of these surface reactions are analyzed in detail. A reaction probability of approximately 70% is calculated for SiH2 radicals impinging on surfaces of hydrogenated amorphous silicon (a-Si:H) films with varying concentrations of hydrogen. Recent experimental measurements have reported a 60% loss probability for the SiH2 radical on the reactor walls through laser induced fluorescence. The experimentally obtained reaction probability falls within the range for the sticking coefficients on the H-terminated and amorphous film surfaces as determined by our atomistic calculations. Molecular-dynamics (MD) simulations of a-Si:H film growth by repeated impingement of SiH2 radicals have revealed adsorption reactions at early stages to occur with similar energetics as the corresponding reactions of isolated radicals on crystalline surfaces. The reaction probability of SiH2 on a-Si:H films deposited through MD simulations is approximately 30%. Finally, it is found that the SiH2 radical is much more mobile on surfaces of a-Si:H films than on crystalline surfaces, especially when the hydrogen concentration in the amorphous film and, thus, on the surface is high.

Thin films of hydrogenated amorphous silicon and polycrystalline silicon; oxygen and hydrogen interaction effects on electrical properties

The reversible interaction between molecular oxygen and hydrogenated amorphous silicon or polycrystalline silicon surfaces is studied. Coplanar configuration and Schottky structure are used to check the interaction effects. The interaction is reversible and may be explained by electric charge exchange between trapped gas species and semiconductors. The trapped gas species at the semiconductors surface may be considered as electronic centers at the surface. This assimilation explains the trapped and released process of gas species. We use a developed model to extract the desorption characteristics. The hydrogenated amorphous silicon in the metal/semiconductor structure interacts with oxygen and hydrogen. The interaction effects are observed on the courant–voltage ( I– V) characteristics which are fitted by using Schottky model. The characteristic parameters show a significant change. This phenomenon is reversible and may be explained by assuming an electric charge exchange between metal/semiconductor interface and the gas. The interface states formed by the exposure to the gas seem to play a similar role that those formed on the surface of the respective semiconductor films.

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

SiH 3 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 radical impingement designed to sample the high-symmetry adsorption sites on the surface. When SiH3 reacts on the H-terminated surface, it either inserts into the Si–Si dimer bond or returns to the gas phase as SiH4 after abstracting H from the surface. The insertion into the Si–Si bond occurs through a dissociative adsorption reaction mechanism that produces two surface SiH2 species after transfer of one of the H atoms from SiH3 to one of the dimer Si atoms. The energetics and dynamics of the surface reactions are analyzed in detail. During simulations of a-Si:H film growth, adsorption onto a dangling bond, dissociative insertion, and H abstraction reactions also were observed to occur with similar energetics as the corresponding reactions on crystalline surfaces. The radical is much more mobile on surfaces of a-Si:H films than crystalline surfaces, especially when the hydrogen concentration in the amorphous film and, thus, on the surface is high.

Effective energy-loss functions for oxygen-adsorbed amorphous silicon surfaces

Effective energy-loss functions were derived for oxygen-adsorbed amorphous silicon surfaces from a reflection electron energy-loss spectroscopy analysis based on the extended Landau theory. This study has revealed that the intensity of the surface-plasmon-loss peak for a clean surface decreases and its peak position shifts towards the lower-energy losses as oxygen exposure proceeds (⩽1000 L). To understand the above behavior of the surface-plasmon-loss peak, the distribution of the energy losses was calculated using the hydrodynamic model. The decrease and shift of the surface-plasmon-loss peak has been described with considerable success by assuming that the quasifree static electron density in the vicinity of the silicon surface decreases as oxygen adsorption proceeds owing to oxygen’s high electron affinity.

In situ observation of hydrogenated amorphous silicon surfaces in electron cyclotron resonance hydrogen plasma annealing

The surface reactions of hydrogenated amorphous silicon (a-Si:H) films exposed to electron cyclotron resonance (ECR) H2 plasma, which is called ECR H2 plasma annealing, has been investigated by in situ polarization modulation infrared reflection absorption spectroscopy (PM-IR-RAS). The surfaces of a-Si:H films were exposed to ECR H2 plasmas at various substrate temperatures. To distinguish the roles of ionic species and atomic hydrogens, permanent magnets were employed to shut out charged species incident on the substrate. The surface reactions of a-Si:H films exposed to only atomic hydrogens were observed by in situ PM-IR-RAS and the results were compared with those exposed to ECR H2 plasmas. It was found that atomic hydrogens reacted with the Si–H2 and (SiH2)n bonds in the vicinity of the surfaces and did not etch the bulk of the films at room temperature. Moreover, it was suggested that atomic hydrogens penetrated into the bulk of a-Si:H film and promoted the Si-network modification as the substrate temperature increased. On the other hand, it was found that ionic hydrogens with the energy of 50 eV had energy enough to etch a-Si:H films.

Novel Method for the Formation of Large‐Grained, Silicon Thin Films on Amorphous Substrates

We describe a novel, solid‐phase crystallization method for synthesizing large‐grained, textured silicon films on amorphous substrates at relatively low processing temperatures (⩽550°C). This method is based on the use of a roughened single‐crystal silicon seed which is pressed onto the amorphous silicon surface. The composite structure is then annealed at low temperatures (<550°C) for times ranging from 2 to 10 h in a furnace so that crystallization from the surface layers is achieved. X‐ray diffraction measurements showed that the films processed by the surface‐seeded crystallization (SSC) method exhibited a (110) texture. Transmission electron microscopy revealed the presence of very large [110] grains (>10 μm) for films crystallized by the SSC method. Hall measurements conducted on boron‐doped films, annealed at 800°C, showed excellent hole mobility with values exceeding 180 cm2/V s, which was almost a factor of six higher than that found in polycrystalline films obtained from standard annealing procedures (annealing temperature ∼600°C for 28 h).

Silicon hydride composition of plasma-deposited hydrogenated amorphous and nanocrystalline silicon films and surfaces

In situ attenuated total reflection Fourier transform infrared spectroscopy was used to study the H bonding on the surfaces of a-Si:H and nc-Si:H during plasma enhanced chemical vapor deposition from SiH4/H2/Ar containing discharges. Well-resolved SiHx (1⩽x⩽3) absorption lines that correspond to the vibrational frequencies commonly associated with surface silicon hydrides were detected. During deposition of a-Si:H films using SiH4 without H2 dilution, the surface coverage was primarily di- and trihydrides, and there are very few dangling bonds on the surface. In contrast, during deposition of nc-Si:H using SiH4 diluted with H2, the amount of di- and trihydrides on the surface is drastically reduced and monohydrides dominate the surface. Furthermore, the vibrational frequencies of the monohydrides on nc-Si:H film surfaces match well with the resonant frequencies of monohydrides on H terminated Si (111) and Si (100) surfaces. The decrease of higher hydrides on the surface upon H2 dilution is attributed to increased dissociation rate of tri- and dihydrides on the surface through reaction with dangling bonds created by increased rate of H abstraction from the surface. Results presented are consistent with SiH3 being at least one of the precursors of a-Si:H deposition.

Charge exchange and excitation in collision at medium energy (2-30 keV). Experiment and simulation

ions were scattered from an amorphous silicon surface (a:Si) at relatively low incident energy (2-30 keV) to investigate the charge exchange processes and the electronic excitation mechanisms. The study was carried out as a function of several experimental parameters: primary energy , incidence angle and scattering angle . We show in this study that neutralization in the subsurface region of Si plays a significant role. The electron spectra show the same predominant autoionizing peaks as observed in scattering from metallic surfaces and the core rearrangement mechanisms used for explaining the conversion are still valid. The conversion efficiency is an increasing function of the incidence angle and a decreasing one of the beam energy in the 1-5 keV range; however this efficiency remains nearly constant at higher energies. This behaviour is attributed to a direct production of states not assisted by core rearrangement processes.

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 highly reactive with surfaces of hydrogenated amorphous silicon films, independent of radical orientation and the location of impingement. Our simulations predict an average reaction probability of 95% for SiH with a-Si:H film surfaces, which is in excellent agreement with experimental data.