Monte Carlo study of interfacial silicon suboxide layers and oxidation kinetics

A simple simulation scheme that simultaneously describes the growth kinetics of SiO₂ films at the nanometer scale and the SiO_x/Si interface dynamics (its extent, and spatial/temporal evolution) is presented. The simulation successfully applies to experimental data in the region above and below 10 nm, reproduces the Deal and Grove linear-parabolic law and the oxide growth rate enhancement in the very thin film regime (the so-called anomalous region). According to the simulation, the oxidation is governed mainly by two processes: (a) the formation of a transition suboxide layer and (b) its subsequent drift towards the silicon bulk. We found that it is the superposition of these two processes that produces the crossover from the anomalous oxidation region behavior to the linear-parabolic law.     

Structural studies of thin silicon layers repeatedly implanted by carbon ions

The composition and structure of homogeneous SiC_1.4 and SiC_0.12 layers produced by multiple implantation of 40-, 20-, 10-, 5-, and 3-keV carbon ions into silicon were studied by electron microscopy, x-ray diffraction, Auger spectroscopy, and IR spectroscopy. The temperature dependences of the IR transmittance peak parameters obtained in the range 200–1400°C indicate that the increase in the number of carbon atoms that are bound to silicon atoms and are involved in absorption is caused by the formation and breaking of hexagonal, near-tetrahedral, and multiple Si-C bonds and by the decomposition of optically active strong carbon clusters. The high crystallization temperature of SiC (1200°C) in the SiC_1.4 layer is explained by the presence of stable multiple Si-C bonds and strong carbon clusters. Strong carbon clusters are shown to exist in the implanted SiC_0.12 layer, and their decomposition is found to affect the formation of tetrahedral bonds in the temperature range 1200–1400°C.     

Porous silicon layers prepared by electrochemical etching for application in silicon thin film solar cells

In this paper, multilayer structures of porous silicon were fabricated by using electrochemical etching and characterized for its optical properties and surface morphology. Samples of monolayer of porous silicon were grown to study the characteristics of porous layer formation with respect to applied current density, etching time and hydrofluoric acid concentrations. Photoluminescence peaks of red emission at wavelength 695 and 650 nm were observed from multilayer porous silicon structures. By atomic force microscopy measurement, hillocks like surface were clearly observed within the host material, which confirmed the formation of pores.     

Elastic modulus-density relationship for amorphous boron suboxide thin films

Boron suboxide thin films have been deposited on Si(100) substrates by reactive RF magnetron sputtering of a sintered B target in an Ar/O₂ atmosphere. Elastic recoil detection analysis was applied to determine the film composition and density. Film structure was studied by X-ray diffraction and transmission electron microscopy. The elastic modulus, measured by nanoindentation, was found to decrease as the film density decreased. The relationship was affected by tuning the negative substrate bias potential and the substrate temperature during film growth. A decrease in film density, by a factor of 1.55, caused an elastic modulus reduction by a factor of 4.5, most likely due to formation of nano-pores containing Ar. It appears evident that the large scattering in the published data on elastic properties of films with identical chemical composition can readily be understood by density variations. These results are important for understanding the elastic properties of boron suboxide, but may also be qualitatively relevant for other B-based material systems. Received: 22 February 2002 / Accepted: 11 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +46-13/288-918, E-mail: denmu@ifm.liu.se     

Growth mechanism for thin autoepitaxial silicon layers grown in a low-temperature chloride process

Electron-microscope investigations show that in the SiCl₄,-H₂ system at sufficiently high purity of the substrate surface and the gas-phase mixture in a low-temperature process, morphologically perfect epitaxial layers of silicon grow by a step-layer mechanism. Estimates are given for a number of parameters of the silicon-layer growth surface.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 59–63, February, 1979.     

Enhancement of the room temperature oxidation of silicon by very thin predeposited gold layers

The oxidation of Si(111) surfaces covered with very thin layers of gold is studied by Auger and electron energy loss spectroscopies under ultra high vacuum conditions. It is found that by exposing the Au covered surface to an oxidizing atmosphere, formation of silicon dioxide occurs at room temperature on top of the substrate and the presence of SiO₄ tetrahedra is clearly seen on electron energy loss spectra. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the formation of an oxygen monolayer and no structure corresponding to Si-O bonds in SiO₄ tetrahedra are observed. This enhancement of the oxidation is attributed to a change in the hybridization state of Si atoms in a gold environment.     

Interaction of thin silicon layers with the (0001) surface of rare-earth metals

The electronic and crystalline structures of the systems formed upon deposition of silicon layers onto the Gd(0001) and Dy(0001) surfaces of single-crystal films annealed subsequently at T=450–500°C have been studied by low-energy electron diffraction (LEED) and also by the Auger electron and angle-resolved photoelectron spectroscopy of the valence band and the Si(2p) core level. It is shown that the systems thus produced can be described as starting single-crystal films of Gd and Dy, with 3D islands of the silicides of these metals on the surface of the corresponding metalfillms.     

多孔硅薄层的正电子湮灭寿命研究

用阳极氧化法按不同腐蚀条件制备的多孔硅，其薄层厚度仅为几十纳米，远小于^22Na源的正电子平均射程．文章提出了一种用控Na的正电子测量体寿命谱并扣除基片贡献的方法，来确定多孔硅薄层的平均孔径。     

Light emission from nanocrystalline silicon clusters embedded in silicon dioxide: Role of the suboxide states

Silicon clusters embedded in a silicon dioxide matrix were prepared by ultrasound-assisted implantation resulting in a modified concentration of suboxide states as revealed by high-resolution photoelectron spectroscopy. It is suggested that ultrasound treatment results in formation of different interface structure between silicon cluster and silicon dioxide matrix which is characterized by a distinctly reduced concentration of the suboxide states. It is observed that photoluminescence properties are strongly correlated with the concentration of the suboxide states thereby providing an evidence that besides a quantum confinement effect a closer look at the chemical composition of the nc-Si/SiO₂ system is important.     

Composition of nanocomposites of thin tin layers on porous silicon,formed by magnetron sputtering

Physics of the Solid State - Morphology and surface composition of the nanocomposite of thin tin layers on porous silicon, formed by magnetron sputtering, are investigated by scanning electron...     

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)     

Development of dislocation-free ion-doped silicon layers

Possible suppression of the formation of residual extended defects in boron-implanted silicon is studied using a method that combines the effects of a threshold dose and annihilation of point defects on substitutional impurities. Implantation conditions are determined under which dislocation-free ion-doped silicon layers are formed.     

Structure of Al2O3 thin layers synthesized on the silicon surface by atomic layer deposition

The distribution of the phase and chemical composition at an Al₂O₃/Si interface is studied by depth-resolved ultrasoft x-ray emission spectroscopy. The interface is formed by atomic layer deposition of Al₂O₃ films of various thicknesses (from several to several nanometers to several hundreds of nanometers) on the Si(100) surface (c-Si) or on a 50-nm-thick SiO₂ buffer layer on Si. L _2,3 bands of Al and Si are used for analysis. It is found that the properties of coatings and Al₂O₃/Si interfaces substantially depend on the thickness of the Al₂O₃ layer, which is explained by the complicated character of the process kinetics. At a small thickness of coatings (up to 10–30 nm), the Al₂O₃ layer contains inclusions of oxidized Si atoms, whose concentration increases as the interface is approached. As the thickness increases, a layer containing inclusions of metallic Al clusters forms. A thin interlayer of Si atoms occurring in an unconventional chemical state is found. When the SiO₂ buffer layer is used (Al₂O₃/SiO₂/Si), the structure of the interface and the coating becomes more perfect. The Al₂O₃ layer does not contain inclusions of metallic aluminum, does not vary with the sample thickness, and has a distinguished boundary with silicon.     

Surface morphology and reaction at Cu/Si interface—Effect of native silicon suboxide

Copper thin films are deposited by thermal evaporation on unetched and etched monocrystalline silicon. The study by alpha particles backscattering (RBS) raises a strong diffusion of copper in silicon substrates with and without native suboxide layer. On the other hand, the X-rays diffraction shows the formation and the growth of Cu₃Si and Cu₄Si silicides. Whereas the scanning microscopy underlines large crystallites growth surrounded by black zones of silicon coming from the uncovered substrate, independently to the surface state of the substrate, after annealing at high temperature. The presence of native silicon suboxide at Cu/Si interface, influences in a drastic way the minimal temperature to which the interfacial reaction occurs. The oxygen impurities detected by microanalysis, after heat treatment under vacuum, are closely related to the growth of silicides crystallites.     

Role of amorphous silicon domains of Er^3＋ emission in the Er—doped hydrogenated amorphous silicon suboxide film

An investigation on the correlation between amorphous Si (a-Si) domains and Er^{3+} emission in the Er-doped hydrogenated amorphous silicon suboxide (a-Si:O:H) film is presented. On one hand, a-Si domains provide sufficient carriers for Er^{3+} carrier-mediated excitation which has been proved to be the highest excitation path for Er^{3+} ion; on the other hand, hydrogen diffusion from a-Si domains to amorphous silicon oxide (a-SiO_x) matrix during annealing has been found and this possibly decreases the number of nonradiative centres around Er^{3+} ions. This study provides a better understanding of the role of a-Si domains on Er^{3+} emission in a-Si:O:H films.     

Laser transfer of thin mesotetraphenylporphyrin layers

The technique of forming mesotetraphenylporphyrin microstructures on the quartz substrate surface by laser-induced forward transfer from a target was developed. The target was a transparent substrate with a thin titanium coating onto which five mesotetraphenylporphyrin layers were deposited by the Langmuir-Schaefer method. The target was irradiated with single 500-ps pulses through a transparent substrate, which causes nondestructive thermal deformation of a metal film, resulting in efficient porphyrin emission from the target. The effect of the titanium film thickness and the laser emission wavelength on the transfer process was studied. An analysis of the optical absorption and fluorescence spectra of obtained surface microstructures suggests that the material structure is retained during laser transfer.     

Analysis of thin layers and discontinuities

A modification of the finite difference scheme for thin dielectric layers is presented and implemented into the method of lines. The thin layers may be placed between two consecutive discretisation lines, i.e. no line is inside the layer. The analysis is based on the treatment of dielectric boundaries. So, formulas for the finite difference scheme at interfaces are given, too. The expressions are tested by determining the effective index of a slab waveguide with a very thin film-layer. A comparison with the analytical solution proves the accuracy of the derived formulas. Another test was the analysis of a Bragg-grating, here only the treatment of interfaces was checked. The modified difference scheme works very well in the TE-case whereas there remain problems for the TM-polarisation.     

The role of Laplace pressure in the formation of the structure of thin layers based on silicon dioxide

Formation of the structure of thin layers deposited from the gas phase occurs not only during film deposition but also during the subsequent annealing. In the present work, porous films were deposited using the catalytic chemical vapor deposition (Cat-CVD) procedure. It was shown that the kinetics of a decrease in the film volume during annealing can be explained taking into account the role of Laplace pressure in the formed pores.     

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.     

Laser generation in thin inverse layers

Features of self-excitation in thin inverse layers, based on which a special type of laser device using reflection of light from an amplifying medium can be constructed, are considered. Such devices are promising for use in integrated optics. Corresponding thicknesses of the inverse layers and possible angles of generation that appear close to the limiting angles for total reflection are calculated for a number of practically important special cases.