Pulsed CO2 laser-driven production of ultrafine silicon,silicon carbide,silicon nitride and silicon oxynitride powders

Summary Ultrafine Si, Si₃N₄, SiC and silicon oxynitride powders have been produced by irradiating gas-phase reactants by means of a CO₂ laser. The mechanism of SiH₄ CO₂ laser-induced absorption and dissociation is discussed on the basis of the results of the spectral and time-resolved measurement of fragment chemiluminescence. The role played by the SiH₂ radical in the powder formation is investigated. The quality of Si, Si₃N₄, SiC and silicon oxynitride powders is checked by means of several off-line diagnostics (IR spectroscopy, X-ray diffraction at wide and small angle, BET analysis). The possibility of controlling powder stoichiometry and doping from the gas-phase reactant concentration is discussed.     

Paramagnetic defects in silicon/silicon dioxide systems

Paramagnetic defect centers in Si/SiO₂ systems have been observed by direct ESR, optically-induced ESR, and NMR relaxation of liquids at the outer oxide surface. In general, all the defects reported elsewhere were confirmed, but with some significant discrepancies in character. The P_B center was observable even at room temperature. The P_C center was found to exist much deeper in the silicon than previously determined, and it is tentatively identified to be neutral iron. Surface liquid relaxation is very strong on oxidized crushed silicon, is not dependent on liquid composition, and suggests a strong wide-line spin center in the outer oxide surface. The optically activated spin center created by HF/HNO₃ etches was found not to involve H₂O or OH functionalities, and appears to be a nitrogenous radical. The optical defect center lies within the silicon, and its presence warrants caution in use of HNO₃-based etches in wafer processing. Oxides prepared at elevated pressures show fewer P_A and P_C defects than those produced by conventional processing, which indicates potential merit in pressure oxidation methods.     

Metastability of amorphous silicon from silicon network rebonding

We propose a network rebonding model for light-induced metastability in amorphous silicon, involving bonding rearrangements of silicon and hydrogen atoms. Nonradiative recombination breaks weak silicon bonds and generates dangling bond-floating bond pairs, with very low activation energies. The transient floating bonds annihilate, generating local hydrogen motion. Charged defects are also found. Support for these processes is found with tight-binding molecular dynamics simulations. The model accounts for major experimental features of the Staebler-Wronski effect including electron-spin resonance data, the t(1/3) kinetics of defect formation, two types of metastable dangling bonds, and hysteretic annealing.     

Excimer-laser-induced micropatterning of silicon dioxide on silicon substrates

Highly resolved micropatterns induced on SiO₂-coated Si sample surfaces have been investigated using a KrF excimer laser (λ: 248 nm and τ: 23 ns). Uniform micropatterns were observed to form in the oxide layer after laser-induced melting of interfaces. The pattern size can be controlled either by the laser parameters or even by the oxide layer thickness. SEM analysis identified that the micropatterns were virtually initiated at the molten interface and the oxide layer followed the interface patterning to change its profile. Simulation of laser interaction with double-layered structures indicated that the oxide layer could melt or be ablated due to interface superheating when it was deposited on a highly absorbing Si substrate. IR analysis has demonstrated that the structural properties of the SiO₂ layer undergo no appreciable changes after laser radiation. This process provides a possible basis for its application in micropatterning of transparent materials using excimer lasers. Received: 4 September 2000 / Accepted: 13 September 2000 / Published online: 30 November 2000     

Cathodoluminescence from nanocrystalline silicon films and porous silicon

Received: 4 May 1998 / Accepted: 30 October 1998     

Secondary ion emission from silicon and silicon oxide

The emission of Si⁺, Si²⁺, Si³⁺, Si₂⁺, SiO⁺ and B⁺ from boron doped silicon has been studied at oxygen partial pressures between 2 × 10^?10 and 2 × 10^?5 Torr. Sputtering was done with 2 to 15 keV argon ions at current densities between 3 and $\text{40}\text{μ}\text{A}\text{cm}{}^2$. The relative importance of the different ionization processes could be deduced from a detailed study of the yield variation at varying bombardment conditions. Comparison with secondary ion emission from silicon dioxide allows a rough determination of the composition of oxygen saturated silicon surfaces.     

Mechanism of silicon epitaxy on porous silicon layers

The mechanism of silicon epitaxy on porous Si(111) layers is investigated by the Monte Carlo method. The Gilmer model of adatom diffusion extended to the case of arbitrary surface morphology is used. Vacancies and pendants of atoms are allowed in the generalized model, the activation energy of a diffusion hop depends on the state of the neighboring positions in the first and second coordination spheres, and neighbors located outside the growing elementary layer are also taken into account. It is shown that in this model epitaxy occurs by the formation of metastable nucleation centers at the edges of pores, followed by growth of the nucleation centers along the perimeter and the formation of a thin, continuous pendant layer. Three-dimensional images of surface layers at different stages of epitaxy were obtained. The dependence of the kinetics of the epitaxy process on the amount of deposited silicon is determined for different substrate porosities. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 512–517 (10 April 1998)     

Hydrogen implantation and diffusion in silicon and silicon dioxide

Depth profiles of hydrogen implanted into crystalline silicon in random direction at different fluences have been measured by the¹⁵N technique and by SIMS. Whereas hydrogen implanted at a fluence of 10¹⁵ ions/cm² shows some limited mobility, no such mobility is observed for higher implantation fluences. In these cases, ballistic computer codes describe the depth distributions well, within the ranges of both experimental and theoretical accuracy. Annealing up to 510 K does not change the hydrogen distributions.Furthermore, high-fluence hydrogen implantation into silicon dioxide has been examined. There is some indication for radiation-enhanced diffusion during the implantation process. Upon subsequent thermal annealing, the hydrogen is found to diffuse, probably via a trapping/detrapping mechanism associated with an OH/H₂ transformation of the hydrogen bonding.     

Low-porosity porous silicon nanostructures on monocrystalline silicon solar cells

In this work we present a study of low-porosity porous silicon (PS) nanostructures stain etched on monocrystalline silicon solar cells. The PS layers reduce the reflectance, improve the diffusion of dopants by rapid thermal processes, and increase the homogeneity of the sheet resistance. Some samples were subjected to chemical oxidation in HNO₃ to reduce the porosity of the surface layer. After the diffusion process, deposition of a SiN_x antireflection layer, and screen printing of the samples, an efficiency of 15.5% is obtained for low-porosity PS solar cells, compared with an efficiency of 10.0% for standard PS cells and 14.9% for the reference Cz cells.     

Positron annihilation in diamond,silicon and silicon carbide

Recent positron lifetime and doppler broadening results on silicon, diamond and silicon carbide are presented in this contribution. In as-grown Czochralski Si ingols vacancies are found to be retained after growth at concentrations typically around 3×10¹⁶/cm³. 10 MeV eleciron irradiation of variously doped Si wafers shows that only high doping concentrations well in excess of the interstitial oxygen concentration causes an increase in the amount of monovacancies retained.In porous silicon very long-lived positronium lifetimes in the range 40–90 ns are found. Polycrystalline diamond films contain various types of vacancy agglomerates but these are found to be inhomogeneously distributed from crystallite to crystallite. Electron irradiation of silicon carbide results in two vacancy-related lifetimes which are interpreted as resulting from carbon and silicon vacancies.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

利用硅光电池测量硅单晶半导体材料的禁带宽度

以白炽灯为光源照射单晶硅光电池,测量在硅光电池前加不同截止波长的滤色片时的短路电流.通过短路电流和截止波长的关系,经拟合得到单晶硅材料的长波限,再利用半导体材料的长波限与半导体的禁带宽度Eg的关系,即Eg=hc/λ,计算得出其禁带宽度.     

Photoluminescence characterization of silicon nanostructures embedded in silicon oxide

This paper describes a simple method to analyze the photoluminescent characteristics of materials based on embedded light-emitting nanoclusters. Photoluminescence spectra of deposited silicon sub-oxide layers with the same composition and different thicknesses have been obtained. A saturation of the total luminescence intensity is observed with increase in thickness. By analyzing the photoluminescence spectra several optical and structural parameters can be evaluated. We thus propose a model in which the absorption of light from a nanostructure layer implies the possibility of subsequent luminescence and affects the underlying layers as well. By fitting the data to the developed model, two fundamental parameters are extracted: nanostructures absorption probability, which is independent of the emission energy and the spectra of emission probability of an excited nanostructure which fits a Gaussian shape.     

Analysis of thin thermal silicon nitride films on silicon

We have investigated the chemical and electrical properties of very thin (<32 Å thick) silicon nitride films grown by rapid thermal nitridation of silicon. These films were of interest as a possible means of tailoring the barrier heights of silicon Schottky barrier diodes. Auger and XPS analysis showed that the level of oxygen contamination in the films was very low ([N]/[N]+[O]) =0.85 to 0.95). The oxygen is located primarily at the surface and interface of the films. Metal-nitride-silicon devices were characterized by I-V and C-V techniques. These measurements indicated an increase in barrier heights to p-type substrates and a decrease in barrier heights to n-type substrates compared to values measured in the absence of the nitride layers. The magnitude of the change in barrier height increases with increasing nitride thickness. The barrier height can be varied reproducibly over a wide range. For molybdenum on p-type, this range is greater than half the bandgap. For titanium and molybdenum on p-type diodes, barrier heights higher than 1.0 V can be achieved. These measurements could be explained by a reduction in the density of silicon interface states with increasing nitride thickness or by the presence of positive fixed charge in the nitride layer.     

Influence of silicon cluster on epitaxial growth of silicon carbide

Precursor concentration dependences of growth rate, doping concentration and surface morphology have been investigated in the epitaxial growth of 4H-SiC(0001) epilayers with horizontal hot-wall CVD system using various precursor concentrations under constant C/Si ratio. Form the experimental data it is found that silicon cluster which is formed through gas phase nucleation plays an important role in controlling the doping concentration and epitaxial growth rate of the silicon carbide. It was observed that t...     

Penetration of electronic states from silicon substrate into silicon oxide

The depth profiling of O 1s energy loss in silicon oxide near the SiO₂/Si interface was performed using extremely small probing depth. As a result, the energy loss of O 1s photoelectrons with threshold energy of 3.5 eV was found. This value of 3.5 eV is much smaller than the SiO₂ bandgap of 9.0 eV, but quite close to direct interband transition at Γ point in energy band structure of silicon. This can be explained by considering the penetration of electronic states from silicon substrate into silicon oxide up to 0.6 nm from the interface. In addition, the penetrating depth is larger than the thickness of the compositional transition layer.     

Boron implanted in silicon: Segregation at angular configurations of the silicon/silicon dioxide oxidation boundary

Based on computer simulation of the physicochemical segregation processes involving dopants implanted into a host material (silicon), the details of boron injection were investigated for four types of angular configurations (direct and inverse kinks and cavities of the “trench” and “square” types) of the “silicon/silicon dioxide” oxidation boundary. A complicated picture of the B distribution inside the Si and SiO₂ regions and at the SiO₂/Si front was obtained and analyzed in general terms.     

Electron trapping levels in silicon dioxide thermally grown on silicon

Photocurrents associated with optical release of photoinjected electrons trapped in thin films of amorphous silicon dioxide have been studied. Temporal and spectral variation of the photocurrents were examined in detail: the effects on spectrally resolved response caused by variations in applied electrical field, wavelength sweep rate, and optical belaching are reported. All measurements were made on metal-oxide-semiconductor capacitors. The experiments were interpreted in terms of a straightforward model of optical excitation and transport of electrons out of localized energy levels in the silicon dioxide band gap. Semi-quantitative analysis indicated that a distribution of states peaked approximately 2·1 eV below the conduction band edge was associated with an electron trapping center distributed rather uniformly throughout the oxide film. In the wet thermal oxide specimens examined, the average density of trapping centers was greater than 10¹⁴ cm⁻³. A time-stable spread in energy of approximately 0·5 eV was measured, and was attributed to local disorder in the amorphous insulator. The existence of an optically inactive charge distribution in the oxide films, with bulk average density greater than 10¹⁵ cm⁻³, was indicated by collected charge vs. applied field data.     

Experimental evidence of the vacancy-mediated silicon self-diffusion in single-crystalline silicon

We have determined silicon self-diffusivity at temperatures 735-875 degrees C based on the Raman shift of longitudinal optical phonon frequencies of diffusion annealed 28Si/30Si isotope superlattices. The activation enthalpy of 3.6 eV is obtained in such low temperature diffusion annealing. This value is significantly smaller than the previously reported 4.95 eV of the self-interstitial mechanism dominating the high temperature region T>855 degrees C and is in good agreement with the theoretical prediction for the vacancy-mediated diffusion. We present a model, containing both the self-interstitial and the vacancy terms, that quantitatively describes the experimentally obtained self-diffusivity between 735 and 1388 degrees C, with the clear crossover of the two diffusion mechanisms occurring around 900 degrees C.     

Flower-like silicon nanostructures

In this paper we present a flower-like silicon nanostructure grown by combining the oxidation-assisted growth (OAG) mechanism and the vapor–liquid–solid (VLS) growth mechanism. It is found that the flower-like silicon nanostructures are nucleated initially via the VLS mechanism and then grown on silicon wafer via the OAG mechanism. Furthermore, light emission was observed, which is considered to be the enhanced photothermal effect.