Hydrogenated silicon fullerenes: effects of H on the stability of metal-encapsulated silicon clusters

Ab initio calculations of H interaction on Si12M, Si18M2 (M=Cr, Mo, and W), and Zr@Si(16) fullerene (f) show relatively weak binding of H in agreement with experimental results of H free Si12M and Si18M2 clusters. Adsorption of H enhances sp(3) bonding between the Si atoms, weakens the M-Si cage interactions, and leads to distortions in the cages. Si12CrH12 has 4 mu(B) magnetic moment in contrast to zero for Si12Cr. Removal of the M atom leads to stable empty cages of Si12H12, f-Si16H16, and f-Si20H20 with large highest occupied-lowest unoccupied molecular orbital gaps of 2.5-3.0 eV, making them attractive for optoelectronic applications.     

Hydrogenated carbon clusters produced by highly charged ion impact on solid

The emission of small (hydrogenated) carbon cluster ions C_nHm ⁺ (n =2-22) upon highly charged Xe^q+ (q =20-44) impact on C₈₄ surfaces is studied by means of time-of-flight secondary ion mass spectrometry. The respective stage of hydrogenation/protonation of a certain carbon cluster ion Cn ⁺ is a strong indication for its geometrical structure. From the cluster ion yield as a function of cluster size it can be concluded, that the hydrogenation takes place after the initial carbon cluster formation. The carbon clusters seem to be emitted as an entity in agreement with “equilibrium” and “shock wave” models. Received 4 February 2000     

Reactive force field potential for carbon deposition on silicon surfaces

In this paper a new interatomic potential based on the Kieffer force field and designed to perform molecular dynamics (MD) simulations of carbon deposition on silicon surfaces is implemented. This potential is a third-order reactive force field that includes a dynamic charge transfer and allows for the formation and breaking of bonds. The parameters for Si-C and C-C interactions are optimized using a genetic algorithm. The quality of the potential is tested on its ability to model silicon carbide and diamond physical properties as well as the formation energies of point defects. Furthermore, MD simulations of carbon deposition on reconstructed (100) silicon surfaces are carried out and compared to similar simulations using a Tersoff-like bond order potential. Simulations with both potentials produce similar results showing the ability to extend the use of the Kieffer potential to deposition studies. The investigation reveals the presence of a channelling effect when depositing the carbon at 45°?incidence angle. This effect is due to channels running in directions symmetrically equivalent to the (110) direction. The channelling is observed to a lesser extent for carbon atoms with 30°?and 60°?incidence angles relative to the surface normal. On a pristine silicon surface, sticking coefficients were found to vary between 100 and 73%, depending on deposition conditions.     

Stuffed fullerene structures for medium-sized silicon clusters

The structural properties of medium-sized silicon clusters (Si₄₀, Si₄₅ and Si₅₀) have been studied using an unbiased global genetic algorithm search incorporated with a tight-binding model, followed by gradient-corrected density functional calculations. Stuffed fullerene cages are obtained as energetically favorite structures. The stuffing/cage ratio (m/n for Si_m@Si_n) can be understood by a space filling picture. The present results, along with our recent works on Si_N (N=27-39) clusters [20], suggest that stuffed fullerene cages are the preferred structural growth pattern of medium-sized silicon clusters.     

Unexpected dynamics for self-interstitial clusters in silicon

Ab initio molecular-dynamics simulations of self-interstitial clusters in Si show that I2 and the most stable of the I3 (" I(a)(3)") clusters diffuse extremely fast. In these clusters, the I's share a single bond-centered (BC) site. The metastable I(b)(3) cluster involves three adjacent BC sites. Simulations show that the three I's exchange sites with each other, but the center of the defect remains at the same place. Simulations with I1 and I4 show no diffusion on the same time scale.     

Evidence for atomic recondensation on ion-bombarded solid surfaces

Direct evidence is presented that vapor-phase crystal growth is induced on solid surfaces bombarded with inert-gas ions. Surface cones formed on AlGaAs/GaAs superlattice targets exposed to a few keV Ar⁺ ions were characterized as polycrystalline cone tips protruding beyond the original target surface. Fine particles were also found to grow in close vicinity to the cone evolution sites, thus demonstrating that target atoms ejected from the impact area condensed into polycrystalline cone tips and particles, as a result of an enhanced particle supply to the projected areas. Since this finding is not within the current knowledge of ion-solid interactions, a new theoretical approach is required for its interpretation.     

RETRACTED ARTICLE: Hydrogenated nanocrystalline silicon germanium thin films

Hydrogenated nanocrystalline silicon germanium thin films (nc-SiGe:H) is an interesting alternative material to replace hydrogenated nanocrystalline silicon (nc-Si:H) as the narrow bandgap absorber in an a-Si/a-SiGe/nc-SiGe(nc-Si) triple-junction solar cell due to its higher optical absorption in the wavelength range of interest. In this paper, we present results of optical, structural investigations and electrical characterization of nc-SiGe:H thin films made by hot-wire chemical vapor deposition (HW-CVD) with a coil-shaped tungsten filament and with a disilane/germane/hydrogen gas mixture. The optical band gaps of a-SiGe:H and nc-SiGe:H thin-films, which are deposited with the same disilane/germane/hydrogen gas mixture ratio of 3.4 : 1.7 : 7, are about 1.58 eV and 2.1 eV, respectively. The nc-SiGe:H thin film exhibits a larger optical absorption coefficient of about 2–4 in the 600–900 nm range when compared to nc-Si:H thin film. Therefore, a thinner nc-SiGe:H layer of ∼500 nm thickness may be sufficient for the narrow bandgap absorber in an a-Si based multiple-junction solar cell. We enhanced the transport properties as measured by the photoconductivity frequency mixing technique. These improved alloys do not necessarily show an improvement in the degree of structural heterogeneity on the nanometer scale as measured by smallangle X-ray scattering. Decreasing both the filament temperature and substrate temperature produced a film with relatively low structural heterogeneity while photoluminescence showed an order of magnitude increase in defect density for a similar change in the process.      

Atomic structure of silver clusters on silicon

The atomic structure of silver clusters formed on a Si(111)-7×7 single crystal surface by annealing a silver thin film condensed at room temperature is investigated by the diffraction of medium-energy quasielastically scattered electrons. Simulation of the diffraction pattern obtained within a nearly complete electron reflection hemisphere shows that the silver islands formed on the silicon have an ordered structure and a fixed orientation relative to the substrate. Close-packed Ag(111) monolayers, which form a face-centered cubic structure, are arranged parallel to the surface, the direction of these microcrystals being antiparallel to the axis of the substrate. Zh. Tekh. Fiz. 67, 62–66 (December 1997)     

Hydrogenated amorphous silicon nitride deposited by DC magnetron sputtering

Hydrogenated amorphous silicon nitride thin films are deposited by DC magnetron sputtering in argon, and molecular hydrogen and nitrogen mixture. The samples are characterized by electrical measurements and by optical transmission. The physicochemical structure is studied by FTIR spectrum analysis. The results show that both nitrogen and hydrogen are incorporated. The ratio [N]/[Si] increases with increasing hydrogen partial pressure. The deposited films are used in MIS structures. The capacitance–voltage characteristics are carried out. The deposited samples present a large gap and show a nearly chemical stoichiometric composition.     

Electron beam-adsorbate interactions on silicon surfaces

The interactions which occur between electron beams in the energy range 0.5–2.5 keV, with currents of 0.1–1.0 microA and various adsorbates (H₂, CO, CH₄ and C₂H₄) on silicon surfaces have been investigated. The accumulation of beam induced dissociation products on the surface has been monitored by Auger spectroscopy, and the extent of electron stimulated desorption of neutral molecules has been determined mass spectroscopically. Thermal desorption spectra for various gases have also been obtained in order to compare adsorption behaviour with and without the presence of an electron beam. It is concluded that serious experimental errors may occur when LEED and AES are used in adsorption studies, particularly where comparatively weak binding energies are involved.     

Si(001)表面硅氧团簇原子与电子结构的第一性原理研究

在周期性边界条件下的k空间中,采用基于密度泛函理论的第一性原理广义梯度近似方法,对建立的规则对称型结构（A）、周期性非对称型结构（B）、周期性非对称型结构（C）、不规则型结构（D）四种可能的Si（001）表面硅氧团簇的结构模型进行了优化计算.结果表明优化后的表面结构呈无定形状,并且优化后的B,C,D三种模型的表面结构具有类似SiO₂的四面体结构的几何特征.此外,通过电子局域函数图以及Mulliken布居分析发现硅氧团簇中的Si—O键既有明显的离子键成分,也有一定的共价键成分. 关键词： Si（001）表面 硅氧团簇 密度泛函理论 第一性原理     

Charge neutrality condition for solid surfaces

The charge neutrality condition for the plane contact of the two media is derived, using the technique of the surface Green functions. For illustration, a simple semiinfinite one-dimensional Sommerfeld model with a-function is discussed.     

Microstructured silicon surfaces for field emission devices

Enhanced field emission of electrons from silicon surfaces was obtained by surface microstructuring, by means of electrochemical oxidation in organic solutions containing HF. Morphological characterisations showed the formation of cylindrical rods, randomly distributed with relative spacing of a few microns. They are originated at the top of silicon pyramids and have typical diameter in the 100 nm range. Variable length in the 1–50 μm range was obtained, by adjusting the process parameters. Electron field emission properties were characterised for several samples, prepared in different conditions: the emission threshold was found to be strongly correlated with the overall charge exchanged during electrochemical oxidation. In the most favourable conditions, the threshold field for the emission of an electron current I_th = 10⁻¹⁰ A was 11.1 V/μm.