Influence of non-uniformity of laser beam intensity on the surface layer structure of implanted silicon crystals

The influence of the non-uniformity of intensity distribution in a laser beam on the surface layer structure of As-implanted silicon crystals as well as a method for homogenizing a pulsed laser beam is described.     

Study of the amorphization of surface silicon layers implanted by low-energy helium ions

The structural changes in surface layers of Si(001) substrates subjected to plasma-immersion implantation by (2–5)-keV helium ions to a dose of D = 6 × 10¹⁵–5 × 10¹⁷ cm^–2 have been studied by highresolution X-ray diffraction, Rutherford backscattering, and spectral ellipsometry. It is found that the joint application of these methods makes it possible to determine the density depth distribution ρ(z) in an implanted layer, its phase state, and elemental composition. Treatment of silicon substrates in helium plasma to doses of 6 × 10¹⁶ cm^–2 leads to the formation of a 20- to 30-nm-thick amorphized surface layer with a density close to the silicon density. An increase in the helium dose causes the formation of an internal porous layer.     

Influence of <Emphasis Type="Italic">in situ</Emphasis> photoexcitation on structure of damaged layer in GaAs (001) substrates implanted with Ar<Superscript>3+</Superscript> ions

The influence of photoexcitation on the formation of the defect structure in GaAs crystals implanted with 200 keV Ar⁺ ions at doses of 1 × 10¹³, 3 × 10¹³, and 5 × 10¹³ cm^?2 has been studied by high-resolution Xray diffractometry. It was found that photoexcitation gives rise to annihilation of radiation-induced Frenkel pairs, and, thus, decreases the residual concentration of radiation-induced point defects. It is established that amorphization of the damaged layer proceeds via the formation and growth of clusters of radiation-induced point defects. The vacancy-and interstitial-type clusters are spatially separated—the former are located closer to the crystal surface than the latter. Photoexcitation hinders cluster growth and stimulates diffusion of interstitial defects into the substrate depth.     

Complementary study of the internal porous silicon layers formed under high-dose implantation of helium ions

The surface layers of Si(001) substrates subjected to plasma-immersion implantation of helium ions with an energy of 2–5 keV and a dose of 5 × 10¹⁷ cm^–2 have been investigated using high-resolution X-ray reflectivity, Rutherford backscattering, and transmission electron microscopy. The electron density depth profile in the surface layer formed by helium ions is obtained, and its elemental and phase compositions are determined. This layer is found to have a complex structure and consist of an upper amorphous sublayer and a layer with a porosity of 30–35% beneath. It is shown that the porous layer has the sharpest boundaries at a lower energy of implantable ions.     

X-ray diffraction investigation of structure perfection state of silicon crystals after irradiation by fast uranium ions

The results, obtained from U-implanted silicon crystals, are presented. Disclocation-free Si-wafers (200 μm thick) were implanted with 16 MeV/n U-ions using Darmstadt accelerator. The applied dose was of 4.4 · 10¹² i/cm². During implantation the crystal was covered by 2 mm thick copper mask with a circular hole of 8 mm diameter. We present the results which were obtained 4 years after performing the implantation. During this time the crystal was kept at the room temperature. Various X-ray diffraction methods have been applied to characterize the state of crystal perfection.     

Properties of hydrogenated amorphous silicon prepared by chemical vapor deposition

Properties of hydrogenated amorphous silicon (a-Si:H) prepared by chemical vapor deposition (CVD) are reported and compared to corresponding properties of glow discharge a-Si:H. The CVD material was produced from mixtures of silane, disilane, trisilane and higher polysilanes in hydrogen carrier gas at one atmosphere total pressure, at substrate temperatures from 420 to 530 °C. The photovoltaic properties of our present CVD a-Si:H are somewhat inferior to those of the best glow discharge a-Si:H. However, as discussed below, there are some indications that higher quality CVD a-Si:H may be possible.     

Growth of pure ZnO thin films prepared by chemical spray pyrolysis on silicon

Structural, morphological, optical and electrical properties of ZnO thin films prepared by chemical spray pyrolysis from zinc acetate (Zn(CH₃COO)₂ 2H₂O) aqueous solutions, on polished Si(1 0 0), and fused silica substrates for optical characterization, have been studied in terms of deposition time and substrate temperature. The growth of the films present three regimes depending on the substrate temperature, with increasing, constant and decreasing growth rates at lower, middle, and higher-temperature ranges, respectively. Growth rate higher than 15 nm min⁻¹ can be achieved at T_s=543 K. ZnO film morphological and electrical properties have been related to these growth regimes. The films have been characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy.     

Cracking assisted nucleation in chemical vapor deposition of silicon nanoparticles on silicon dioxide

Deposition of sub-monolayer silicon on SiO₂/Si(1 0 0) greatly facilitates nucleation in subsequent thermal chemical vapor deposition (CVD) of silicon nanoparticles. Sub-monolayer seeding is accomplished using silicon atoms generated via disilane decomposition over a hot tungsten filament. The hot-wire process is nonselective towards deposition on silicon and SiO₂, is insensitive to surface temperature below 825 K, and gives controlled coverages well below 1 ML. Thermal CVD of nanoparticles at 1×10⁻⁴ Torr disilane and temperatures ranging from 825 to 925 K was studied over SiO₂/Si(1 0 0) surfaces that had been subjected to predeposition of Si or were bare. Seeding of the SiO₂ surface with as little as 0.01 ML is shown to double the nanoparticle density at 825 K, and densities are increased twenty fold at 875 K after seeding the surface with 30% of a monolayer.     

TEM investigation on the structure of amorphous silicon monoxide

Commercially available powder samples of silicon monoxide have been investigated by methods of transmission electron microscopy: electron scattering, electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). Pair distribution functions (PDFs) as well as EEL spectra can be shown to be a composition of the PDF and EEL spectra of elemental silicon and amorphous SiO₂. The distribution of the elements silicon and oxygen calculated from ESI images proof the silicon monoxide to be inhomogeneous, i.e. it consists of amorphous silicon and amorphous SiO₂. The phase separated regions measure ≈3–4 nm. One maximum in the PDF at 2.95 Å does not stem from either a-Si or a-SiO₂, and it is assigned to atomic configurations at the interphase boundary layer between Si and SiO₂. The portion of the interphase domain in the total composite material is estimated to be between 20% and 25%.     

X-ray and electron microscopy studies of arsenium implanted silicon crystals after a pulsed laser annealing

The structural changes obtained due to laser annealing in the surface layer of As-implanted silicon crystals were investigated by means of a triple crystal spectrometer. The rocking curves of implanted crystals do not show any changes compared with non-implanted crystals. It was caused by the large dose of ions which amorphized the surface layer. Laser annealing process influenced on the broadening of rocking curve. The comparative studies were carried out by using the HTEM technique. They showed that the laser annealing caused formation of quasi-mosaic structure, planar defects and dislocation clusters. These defected structure and local strains influence on the broadening of rocking curves.     

Improvement of amorphous silicon n-i-p solar cells by incorporating double-layer hydrogenated nanocrystalline silicon structure

We develop a double-layer p-type hydrogenated nanocrystalline silicon (p-nc-Si:H) structure consisting of a low hydrogen diluted i/p buffer layer and a high hydrogen diluted p-layer to improve the hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells. The electrical, optical and structural properties of p-nc-Si:H films with different hydrogen dilution ratio (R_H) are investigated. High conductivity, low activation energy and wide band gap are achieved for the thin films. Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the thin films contain nanocrystallites with grain size around 3-5 nm embedded in the amorphous silicon matrix. By inserting a p-nc-Si:H buffer layer at the i/p interface, the overall performance of the solar cell is improved significantly compared to the bufferless cell. The improvement is correlated with the reduction of the density of defect states at the i/p interface.     

Influence of chemical composition on dislocation structure and its change by plastic deformation of V3Si single crystals

The dislocation structure and its change by plastic deformation of V₃Si single crystals has been studied by an etch technique. The solution hardening effect established elsewhere is interpreted in terms of the observed dependence of the dislocation density ϱ(r) and its increment with the plastic deformation on the chemical composition within the range of homogeneity.     

Influence of europium ions on structure and crystallization properties of bismuth borate glasses and glass ceramics

Glasses of the xEu₂O₃ · (100?x)[2Bi₂O₃ · B₂O₃] system with 0 ? x ? 25 mol% have been characterized by X-ray diffraction and FTIR spectroscopy measurements. Melting at 1100 °C and the rapid cooling at room temperature permitted us to obtain glass samples. In order to improve the local order and to develop crystalline phases, the glass samples were kept at 625 °C for 24 h. After heat treatment two crystalline phases were put into evidence. One of the crystalline phases was observed for the host glass matrix, the x = 0 mol% sample, and belongs to the cubic system. The second one was observed for the x = 25 mol% sample and was find to be orthorhombic with two unit cell parameters very close to each other. For the samples with 0 < x < 25 mol% there is a mixture of the two mentioned phases. FTIR spectroscopy data suggest that both Bi₂O₃ and B₂O₃ play the glass network former role while the europium ions play the network modifier role in the studied glasses.     

Controlling the quality of nanocrystalline silicon made by hot-wire chemical vapor deposition by using a reverse H2 profiling technique

Hydrogen profiling, i.e., decreasing the H₂ dilution during deposition, is a well-known technique to maintain a proper crystalline ratio of the nanocrystalline (nc-Si:H) absorber layers of plasma-enhanced chemical vapor-deposited (PECVD) thin film solar cells. With this technique a large increase in the energy conversion efficiency is obtained. Compared to PECVD, the unique characteristics of hot-wire CVD (HWCVD), such as the catalytic reactions, the absence of ion bombardment, the substrate heating by the filaments and filament aging effects, necessitate a different strategy for material and device optimization. We report in this paper the results of using a reverse H₂ profiling technique, i.e., increasing the H₂ dilution of silane instead of decreasing it, to improve the quality of HWCVD intrinsic nc-Si:H and the performance of this material in single junction n-i-p cells. Thus far, the efficiency of nc-Si:H n-i-p cells made on a stainless steel substrate with an Ag/ZnO textured back reflector has been improved to 8.5%, and the efficiency of triple junction solar cells with a structure of proto-Si:H(HWCVD) top cell/proto-SiGe:H (PECVD) middle cell/nc-Si:H (HWCVD, with reverse H₂ profiling) bottom cell has reached 10.9%. These efficiency values show the viability of n-i-p cells comprising HWCVD nanocrystalline i-layers.     

Crystal structure of the growth surface of silicon carbide obtained by sublimation

Structural study of polycrystalline silicon carbide obtained by sublimation performed via X-ray luminescence and X-ray diffraction analysis. It is shown that chemical vapor deposition of silicon carbide results in the formation of grains with the (00.1), (01.1), and (12.3) crystallographic planes parallel to the growth surface. The grains with the (00.1) growth planes are characterized by perfect structure and by red luminescence. Domains with yellow luminescence have a mosaic structure with the (01.1) and (12.3) growth planes.     

Influence of the electric double layer on glass leaching

When a glass comes into contact with water, an electric double layer forms at the glass-water interface. An analytical model that calculates the electric field and the resulting velocity of univalent alkali ions migrating in the glass matrix is developed. The ion migration velocity is calculated for an 85% SiO₂–15% Na₂O glass and for more complex glasses. The calculations indicate that the process of ion migration leads to normalized leach rates which are small compared to glass network dissolution rates.     

The X-ray method for the determination of damaged layer thickness of the Si crystal surface

In this paper the X-ray section topograph method was used for determination of damaged layer thickness of silicon crystal surface. For measurements such reflections should be chosen for which the “margine” effect is very sharp. It was used the fact that in the ranges of strong perturbances the approximation of kinematic theory could be applied.     

Influence of impurity atmosphere on the deformation of silicon crystals

The Alexander–Haasen theory, which describes the deformation kinetics of silicon crystals, has been generalized for impurity crystals. The deformation kinetics of an impurity sample is calculated in a wide range of parameters, including the cases of partial and complete entrainment of impurities by moving dislocations. The developed model, despite its simplicity, adequately describes the qualitative transformation of the stress–strain curves of impurity silicon crystals in dependence of the impurity concentration and other material parameters. The manifestation of negative velocity dependence of the yield stress, observed in natural experiments, is analyzed.     

Characterization of chromium silicide thin layer formed on amorphous silicon films

A detailed investigation of the compositional, optical and electrical properties of a chromium silicide layer grown at room temperature on top of doped amorphous silicon films is presented. The formation of the layer is promoted only when phosphorous atoms are present in the film. The deposition of a very thin n-type doped layer (around 5 nm) on top of a p-type doped film has allowed us to achieve the chromium silicide formation also on p-type material without changing its doping properties. Angle resolved X-ray photoelectron spectroscopy measurements demonstrate the presence of chromium-oxide, chromium silicide and metallic chromium in similar percentages for both p- and n-type doped layers. From the ellipsometric analysis, the refractive index spectra have been extracted, and the layer thickness has been estimated to be 5 nm for both p- and n-type doped layers. From planar conductivity measurements, we have found that the chromium silicide promotes an activation energy reduction from 0.24 eV down to 0.017 eV for the n-type layer and from 0.36 eV down to 0.14 eV for the p-type film.     

Hydrogenated amorphous silicon produced by laser induced chemical vapor deposition of silane

a-Si:H films deposited by laser induced CVD (LICVD) have been characterized and the growth process modelled. Growth rates are exponentially dependent on gas temperature and film properties follow the equilibrium hydrogen content, exponentially dependent on substrate temperature.