Hydrogen effusion: a probe for surface desorption and diffusion

Hydrogen effusion results are discussed for hydrogenated amorphous silicon (a-Si:H) and related alloys as well as for crystalline silicon (c-Si). It is demonstrated that depending on the microstructure of the material, hydrogen effusion gives information on hydrogen diffusion or surface desorption. The results suggest for compact a-Si:H and for ion implanted c-Si a similar hydrogen diffusion process, which is a trap limited motion of atomic hydrogen. Hydrogen effusion from defect-free c-Si and from void-rich amorphous semiconductors is limited by surface desorption. Both hydrogen diffusion and desorption depend on the Fermi energy if hydrogen bonds to the host material are broken.     

Hyperfine interactions of muonium and hydrogen in silicon and diamond: Quantum chemical calculations

The electronic structure of a hydrogen-like atom located at interstitial sites of the silicon and diamond crystals is calculated by the intermediate neglect of differential overlap (INDO) method. Calculations of the electronic g- and hyperfine interaction tensors of the impurity atom are performed. The results obtained are compared with the experimental properties of both “anomalous” muonium and hydrogen centers. It is shown that the most likely model for these centers in silicon and diamond is that in which interstitial neutral hydrogen-like atom locates at the bond-center site.     

Theoretical study of the Cox-Symons model for normal muonium in silicon

A theoretical study of the Cox-Symons model for normal muonium in Si is presented. The calculations are performed using polarized basis set ab-initio Hartree Fock calculations followed by corrections for electron correlation. It is shown that, if lattice relaxations are included, the antibonding site becomes a minimum of the potential energy surface (PES) for neutral interstitial hydrogen. The energy at this minimum is lower than that at the undistorted tetrahedral interstitial site. The spin density changes from being almost entirely on the muon (for Mu at the T site) to being almost entirely on a three-fold coordinated Si atom (for Mu in the AB configuration). The mechanism required to explain the isotropy and magnitude of the observed hyperfine tensor of Mu in c-Si is complicated. Large displacements of some host atoms are needed, and the system must be dynamic. However, this model is the first able to produce a minimum of the PES together with an isotropic hyperfine interaction and a delocalized spin density.     

Molecular hydrogen and oxygen interactions with armchair Si nanotubes

Molecular hydrogen and oxygen adsorptions on a (6, 6) armchair silicon nanotube have been studied by optimizing the distances of the admolecules from both inside and outside the tube. Full geometry and spin optimizations have been performed without any symmetry constraints with an all electron 3-21G* basis set and the B3LYP functional. The molecule is originally placed perpendicular or parallel to the tube axis. Hydrogen adsorption with the molecular axis aligned parallel to the surface of the nanotube is less favorable. Hydrogen molecule does not dissociate while oxygen molecule dissociates after optimization. The on-top site is the only preferred site for hydrogen molecule with an adsorption energy of 3.71 eV and an optimized distance of 3.31 for external adsorption whereas the on-top site is the most preferred site with adsorption energy of 3.69 eV for internal adsorption. For oxygen, the molecule dissociates and the most preferred sites are the two bridge sites with an adsorption energy of 9.64 eV, the optimized distance being 1.65/1.68 Å when it is adsorbed from the outside of the tube. When oxygen molecule is originally placed at on-top site it will hold as a molecule after adsorption with a slightly increased bond length. For the internal adsorption of oxygen, the molecules also dissociate in most cases and the zigzag bridge site is the most preferred site. After molecular adsorption for both hydrogen and oxygen, the buckling of the nanotubes increased. Frustration effects in the nanotube due to molecular adsorption are also noted.     

氢化纳米硅薄膜中氢的键合特征及其能带结构分析

对氢化纳米硅薄膜中氢的键合特征和薄膜能带结构之间的关系进行了研究.所用样品采用螺 旋波等离子体化学气相沉积技术制备，利用Raman散射、红外吸收和光学吸收技术对薄膜的 微观结构、氢的键合特征以及能带结构特性进行了分析.Raman结果显示不同衬底温度下所生 长薄膜的微观结构存在显著差异，从非晶硅到纳米晶硅转化的衬底温度阈值为200℃.薄膜中 氢的键合特征与薄膜的能带结构密切相关.氢化非晶硅薄膜具有较高的氢含量，因键合氢引 起的价带化学位移和低衬底温度决定的结构无序性，使薄膜呈现较大的光学带隙和带尾宽度 .升 关键词： 氢化纳米硅 螺旋波等离子体 能带结构     

Voltage-tunable photo- and electroluminescence of porous silicon

Experimental results showing two electrically induced phenomena, namely the voltage-tunable electroluminescence (VTEL) and the voltage-induced quenching of porous silicon photoluminescence (QPL) are given. In both cases, a spectral shift as large as 300 nm can be recorded for an external bias variation of only 0.5 V. This spectral shift is characterised by a blue-shift of the whole EL line in the case of the VTEL whereas it results from a progressive and selective quenching starting by the low-energy part of the luminescence line in the case of the QPL experiments. The origin of this spectral shift is discussed in relation with an electrically induced selective carrier injection into the silicon nanocrystallites accompanied with an enhancement of the non-radiative recombination taking place by an Auger relaxation process. Finally, it is shown that a partial oxidation of the porous silicon layer leads to a complete loss of the selectivity of these two phenomena. This result is qualitatively discussed by considering the voltage drop distribution between the substrate and the silicon nanocrystallites. The voltage drops are modified by the growth of the oxide layer on the nanocrystallite surface leading to a modification of the energy barriers at the crystallite boundaries.     

Electron paramagnetic resonance of hydrogen in silicon

A review of the investigations by means of electron paramagnetic resonance (EPR) of hydrogen and hydrogen-related defects in crystalline silicon is presented. The main features of the EPR center Si-AA9 (bond-centered hydrogen), which is known as the hydrogenic analogue of the anomalous muonium (Mu^*) in silicon, are discussed. It was found that the process of annealing the AA9 center is characterized by an activation energy, E = 0.48 ± 0.04 eV with a second-order pre-exponential factor, K₀ = (1.25 ± 2.5) × 10^-7 cm³/s.

A detailed investigation by EPR of the defect (Si-AA1), which we identify as the hydrogen-related shallow donor in a positive charge state, is also presented. In particular it is shown that the H-related shallow donor is a helium-like center and its wave function has C_2v symmetry. Moreover, the main features of the series of EPR spectra in silicon characteristic for the implantation of hydrogen are presented.     

真空条件下不同波长固体激光烧蚀单晶硅的实验研究

通过倍频Nd：YAG固体激光的基波得到波长分别为532、355和266 nm的激光,研究了单晶硅（Si）对不同波长固体激光的吸收规律和3种不同波长激光在真空条件下烧蚀单晶Si的烧蚀特征。结果表明,单晶Si对波长为100~370 nm的紫外激光具有很好的吸收效果;在其他条件相同时,532 nm波长激光烧蚀单晶Si所需最低单脉冲能量（Ep=30 μJ）是355和266 nm波长激光烧蚀单晶Si所需最低单脉冲能量（Ep=15 μJ）的2倍;532、355和266 nm的激光烧蚀单晶Si的烧蚀阈值随着波长的变短而变小。     

单轴应变对H在α-Fe中占位及扩散的影响

采用基于密度泛函理论的第一性原理方法,研究了H在不同单轴应变下α-Fe中的间隙占位,计算了H原子的溶解能、态密度、电荷差分密度和电荷布居.结果表明:不同单轴拉压应变作用下,H原子优先占据四面体间隙(Ts)位,且随着压应变减小、拉应变增加,H原子越易溶于α-Fe.压应变使得Ts位的H获得更多的电子,而拉应变减少了这种电荷转移.应用LST/QST过渡态搜索计算垂直应变方向的扩散.八面体间隙位是邻近Ts位H的扩散过渡态.扩散激活能与应变呈线性关系,且随着压应变的增加,扩散激活能降低,扩散更容易.     

Correlation between the energy of SiGe nanoislands and their shape and size

The total energy of self-assembled SiGe nanoislands on a silicon substrate is investigated theoretically as a function of their geometric and physical parameters. It is demonstrated that the growth temperature and the silicon content in nanoislands affect the minimum of their energy. The results of numerical calculations for nanoislands are compared with experimental data obtained by atomic-force microscopy.     

The chemisorption of hydrogen on silicon and silicon carbide (1 0 0) surfaces

The chemisorption of hydrogen onto semiconductor surfaces is examined. The hydrogen bonds to the dangling bond of a surface atom. These dangling bonds also dictate the reconstruction of the crystal surface. The chemisorbed hydrogen therefore modifies the reconstructed surface topology. In this work theoretical calculations of the surface structures of both covalent elemental silicon and polar silicon carbide are presented. The periodic MINDO method is employed to determine the topologies for the 2 × 1 reconstructed (1 0 0) surfaces. These topologies are obtained from the minimisation of the total energy, for silicon and silicon carbide films of 14 layer thickness, with respect to the atomic co-ordinates of the hydrogen adsorbate and the first four layers of the substrate. The results show that the formation of the hydrogen bond to the substrate leads to a general lengthening of the surface dimer bond. In addition, the buckling of the silicon dimer determined for silicon terminated SiC is removed by hydrogen chemisorption.     

稀土元素在硅表面的化学吸附

本文对几种稀土元素(铈、钐、镱)在硅表面的化学吸附进行了理论计算。用定域密度泛函-离散变分方法(LDF-DVM)对顶位和三度开位两种集团模型进行了自洽计算,包括总结合能的计算,给出了吸附位能曲线,从而确定出最佳吸附构型和位置。讨论了电荷转移情况、吸附能的大小等,与一些实验结果进行了比较。并给出了三种元素在硅表面稳定吸附时的态密度图,分析讨论了其电子结构的差异和特点。 关键词：     

硅铝酸银分子筛吸附氖原子的第一性原理计算

采用基于第一性原理的密度泛函理论(DFT)和局域密度近似(LDA)方法,优化计算硅铝酸银分子筛吸附Ne原子体系的几何结构,能量,电子能带和电荷密度分布。结果表明,硅铝酸银为层状的周期结构,具有直径为a=5.390 Å的孔道。在分子晶体孔道的轴线上,桥O原子附近(I)和表面Ag+离子附近(II)的能量均有利于对Ne原子的吸附。尽管Ne(I)的能量最低,但是SiO4四面体排斥产生的能垒在动力学上不利于Ne原子的吸附。电子能带和电荷分布显示,Ne(II)原子主要受库仑极化的影响,其电子能带的能量较高,Ne(I)原子与桥O原子之间的共价作用能够降低对应的电子能带能量。     

硅铝酸银分子筛吸附氖原子的第一性原理计算

采用基于第一性原理的密度泛函理论(DFT)和局域密度近似(LDA)方法,优化计算硅铝酸银分子筛吸附Ne原子体系的几何结构,能量,电子能带和电荷密度分布。结果表明,硅铝酸银为层状的周期结构,具有直径为a=5.390 Å的孔道。在分子晶体孔道的轴线上,桥O原子附近(I)和表面Ag+离子附近(II)的能量均有利于对Ne原子的吸附。尽管Ne(I)的能量最低,但是SiO4四面体排斥产生的能垒在动力学上不利于Ne原子的吸附。电子能带和电荷分布显示,Ne(II)原子主要受库仑极化的影响,其电子能带的能量较高,Ne(I)原子与桥O原子之间的共价作用能够降低对应的电子能带能量。     

Oscillating cracks in glassy films on silicon substrates

Cracks that propagate with near-perfect sinusoidal form are reported in amorphous silicon-rich silica films deposited onto (001) silicon substrates by plasma-enhanced chemical vapour deposition and subjected to thermal annealing. The cracks are shown to result from high tensile stresses that develop in the film during thermal annealing at temperatures in the range up to 700°C, a process shown to be correlated with the loss of hydrogen from the films. Two distinct modes of crack propagation are reported: straight cracks that propagate along directions parallel to [100] cube-edge directions in the substrate, and oscillating cracks that propagate with sinusoidal form parallel to [110] diagonal directions. Sections through the cracks show that the oscillating cracks have a complex three-dimensional structure that extends through the glassy film and into the underlying silicon substrate. This involves a correlated oscillation between the crystallographic orientation of the crack in the surface plane and that of the crack extension into the substrate. Whereas a complete theoretical treatment of this behaviour would be extremely complicated, a simple theory is developed to demonstrate that an oscillating crack has a minimum energy per unit length for a particular wavelength and amplitude that depends upon the physical parameters of both film and substrate. The energy at this minimum is shown to be lower than that of a straight crack for certain parameter ranges so that the oscillating geometry is preferred.     

Effect of hydrogen dilution on the optical properties of hydrogenated amorphous silicon-nitrogen films prepared by plasma deposition

The effect of the dilution of silane and nitrogen with hydrogen on the optical properties of hydrogenated amorphous silicon-nitrogen films prepared by plasma deposition has been investigated as functions of the gas-volume ratio γ (= ([SiH₄] + [N₂])/([SiH₄] + [N₂] + [H₂]) and the substrate temperature. The prepared films are characterized by the values of the deposition rate, the optical gap, the Urbach energy, the defect density, the integrated infrared absorption intensity and the refractive index, and by correlations between these parameters and the type of hydrogen- and nitrogen-bonding configurations estimated from infrared absorption spectra. The hydrogen dilution effect is discussed in terms of the above and compared with that in hydrogenated amorphous silicon reported in a previous paper by the present authors. It is pointed out that nitrogen atoms incorporated into the silicon network cause more disorder than incorporated hydrogen atoms, from the γ dependence of the Urbach energy and the integrated infrared intensities associated with the hydrogen and nitrogen bondings.     

Evaluation of different models for the dissociation of silane on the Si(1 1 1)7 × 7 surface using the extended Brenner empirical potential

Several models have been proposed in the literature for the initial stages of the dissociative chemisorption of silane (SiH₄) on the Si(1 1 1)7 × 7 surface. In this paper, geometry optimisation calculations using the extended Brenner empirical potential have been performed to determine which of these models yields the minimum energy structure. The lowest energy configurations are found to correspond to the dissociation of silane into SiH₂ and two hydrogen atoms. The minimum energy structure involves the adsorption of the two hydrogen atoms onto the dangling bonds of an adjacent adatom and rest atom, and the insertion of the remaining SiH₂ fragment into one of the adatom backbonds. These results are discussed in the light of the existing experimental data.     

中子辐照含氢硅中的与氢有关的深能级缺陷

利用深能级瞬态谱(简称DLTS),恒温下瞬态电容技术及红外吸收光谱,研究了中子辐照氢气氛中生长的n型区熔硅。相应于间隙氢的红外吸收谱带中子辐照后强度减弱。首次观察到未经退火就出现了能级为E_c=0.20eV的一个新的缺陷——Z中心,由于该中心的能级很接近于A中心,而浓度又较A中心低得多,通过改变中子剂量使费密能级处于A中心以下几个kT处,才能精确地测定Z中心的DLTS峰所在的温度。根据实验判断Z中心很可能是氢与空位的复合物,讨论了它的可能电子结构。 关键词：     

Nuclear magnetic resonance investigation of H, H2 and dopants in hydrogenated amorphous silicon and related materials

Nuclear magnetic resonance has been successfully applied to the study of the microstructure of hydrogenated amorphous silicon and related materials. It has been used to determine the local bonding and structural environment of the host atoms, the hydrogen, and the dopants. First, we review some of these NMR experimental results on the hydrogen microstructure in hydrogentaed amorphous semiconductors and compare the results on plasma deposited hydrogenated amorphous silicon (a-Si:H), remote hydrogen plasma deposited a-Si:H, thermally annealed a-Si:H, doped a-Si:H, microcrystalline Si and amorphous (Si, Ge):H alloys. A common feature is that these materials exhibit a heterogeneous distribution of hydrogen bonded to the semiconductor lattice in dilute and clustered phases. In addition, the lattice contains voids of varying number and size that contain non-bonded molecular hydrogen whose quantity is altered by deposition conditions and thermal treatment. Second, we review some aspects of the local bonding structure of dopants in a-Si:H. A significant fraction of the dopants are found to be in dopant-hydrogen clusters similar to those proposed to explain hydrogen passivation in crystalline silicon. Implications of the determined local structure on the doping efficiency are discussed.     

Laser short-pulse heating of silicon film with the presence of metallic substrate

Laser interaction of silicon film located at he top of metallic substrate is examined and energy transport in electron and lattice sub-systems are formulated using the electron kinetic theory approach. The simulations are repeated for different substrate materials, namely gold, silver, and copper. It is found that electron temperature in the silicon film rises in the vicinity of the silicon–metallic substrate interface, despite the fact that energy absorption from the irradiated filed is significantly low in the silicon film. Lattice site temperature rises rapidly in the early heating period at the interface. In addition, lattice site temperature increase is higher in the silicon film than that corresponding to the metallic substrate.