Kumayasu Yoshii

Crack healing and fracture strength of silicon crystals

The influence of annealing at 700 to 1100° C on fracture strength of pre-cracked silicon wafers was examined by four-point bending tests at room temperature. The fracture strengths of the specimens annealed in oxygen increased significantly with increasing annealing temperature. On the other hand, annealing in vacuum showed little influence on the fracture strength. The strength increase by the annealing in oxygen was found to be caused by crack healing. Utilizing transmission electron microscopy, it is suggested that the crack surfaces were rebonded by the formation of a thin oxide layer at the crack interface. The activation energy for the crack healing was determined to be 2.0±0.1 eV, which was consistent with that of the reaction-limited growth of thin oxide film.

Atmospheric pressure plasma chemical vapor deposition system for high-rate deposition of functional materials

The atmospheric pressure plasma chemical vapor deposition (CVD) system has been developed to fabricate functional thin films at very high deposition rate. The atmospheric pressure plasma, in which high-density radicals are created, has been effectively used to deposit thin films. Combination of the newly designed rotary electrode and the 150 MHz very high frequency power supply makes it possible not only to generate the high-density atmospheric pressure plasma but also to avoid ion bombardment against the film. By virtue of these noble characteristics of the system, high quality films can be fabricated at an unprecedented high deposition rate. In order to demonstrate the effectiveness of the atmospheric pressure plasma CVD system, hydrogenated amorphous silicon (a-Si:H) films were prepared in gas mixtures containing He, H2, and SiH4. The results showed that homogeneous a-Si:H films grew when substrates were heated at 200 °C. Extremely high deposition rate, which was more than 100 times faster than that of...

Crystallization behaviour of amorphous SiC films prepared by r.f. sputtering

Abstract The crystallization behaviour of amorphous SiC (a-SiC) films prepared by r.f. diode sputtering was studied using IR measurements and transmission electron microscopy. The absorption band at around 800 cm -1 in the IR spectrum became sharper and more intense when a sample was annealed above 1000°C, corresponding to the phase transition in the film from amorphous to polycrystalline β-SiC. From cross-sectional transmission electron microscopy observations, crystallization occured homogeneously in the film. The crystallization behaviour was independent of the film thickness and the substrate temperature during preparation. The measured overall activation energy of crystallization of a-SiC films is about 5.0 eV.

Crystallization behaviour of amorphous Si1-xCx films prepared by r.f. sputtering

Abstract The crystallization behaviour of amorphous Si1-xCx films with x = 0.11, 0.38, 0.48 and 0.75 prepared by r.f. sputtering was studied using IR spectroscopy and transmission electron microscopy, where films were annealed isothermally at 700–1100°C for 1 h. The broad transmission bands in IR spectra at 700–800 cm-1 for as-deposited films became sharper and their peak positions shifted to the higher frequency side with increasing annealing temperature. The wavenumber of the peak positions reached finally 818 cm-1 for x = 0.11 and 800 cm-1 for x = 0.38, 0.48 and 0.75, the corresponding microstructures being polycrystalline β-SiC and silicon, and β-SiC and amorphous graphite respectively. The crystallization temperature of stoichiometric SiC film was the highest, and the greater the deviation of the composition from stoichiometry, the lower the crystallization temperature. From the kinematic analyses of the results of IR measurements for the films with x = 0.11, the activation energy of the crystallization process was found to be about 7.1 eV.

Molecular beam epitaxial growth of AlN single crystalline films on Si (111) using radio-frequency plasma assisted nitrogen radical source

We have grown AlN films on Si (111) using a molecular beam epitaxy (MBE) approach in which reactive nitrogen species are generated in a remote 13.56 MHz radio-frequency plasma discharge, nitrogen radical source. Effects of the Al/Si (111) γ-phase on epitaxial growth of AlN on Si (111) has been studied. Successive processes of forming stable Al/Si (111) γ-phase at 800 °C followed by MBE growth without interrupting Al beam exposure was effective to prevent amorphous SiNx formation on Si substrate. The Al/Si (111) γ-phase was found effective to fix the orientation relationship between AlN and Si for epitaxial growth of single crystal AlN. Single crystal AlN was grown on the Al/Si (111) γ-phase at the temperature as low as 400 °C. By optimizing growth conditions (substrate temperature, V/III ratio and plasma conditions), growth of single crystal AlN films in a quasi-layer-by-layer fashion was observed for the thickness up to 60 nm. The optimized value on the growth temperature was 800 °C and that on the V/III...

Fracture of GaAs Wafers

Fracture characteristics of undoped and several kinds of doped GaAs single-crystal wafer were studied. The fracture toughness value determined by four-point bending fracture test of specimens precracked by indentation at room temperature showed no difference for In-, Si-, Cr- and undoped crystals. Indentation microcracking characteristics of In-, Si- and undoped crystals and probability distribution functions of the fracture stresses of In-doped and undoped crystals were found not to have meaningful differences. The polarity of the indentation cracking in relation to the indentation rosette extension and the temperature dependence of the fracture toughness value in relation to the crack tip plasticity were also investigated.

Tensile strength of Ni/Cu/(001)Ni triple layer films

Tensile properties of Ni/Cu/(001)Ni triple layer films and single crystal films of (00l)Cu and (00l)Ni were studied. The specimen films having the thickness of 50 to 500 nm were prepared by the epitaxial evaporations on (001)NaCl substrates. The stress-strain curves of tensile tests in the direction of [100] of Ni/Cu/(001)Ni triple layer films were almost linear until fracture and showed extremely small plastic deformation. The yield stresses of these specimens were 600 to 700 MPa independently of the layer thicknesses, which were 2.5 to 5 times higher than those of Ni or Cu single crystal films, and were also higher than the values given by the mixture rule for Ni and Cu single crystal films. The increase of yield stress was evaluated from the repulsive force acting on the mobile dislocations originating from the stress fields of misfit dislocation arrays at the Ni-Cu interface.

High-rate growth of epitaxial silicon at low temperatures (530-690 °C) by atmospheric pressure plasma chemical vapor deposition

Abstract High-rate growth of epitaxial Si films at low temperatures by atmospheric pressure plasma chemical vapor deposition has been investigated. Si films are deposited on (001) Si wafers in gas mixtures containing He, H 2 and SiH 4 at substrate temperatures ranging from 530 to 690 °C. The films are characterized by reflection high-energy electron diffraction, atomic force microscopy and cross-sectional transmission electron microscopy. High quality Si films with excellent crystallinity and surface flatness similar to or better than those of commercial Si wafers are grown in the area where the deposition gap between the substrate and rotary electrode is small. Especially, in epitaxial Si film grown at 610 °C with an input plasma power of 2000 W, no lattice defects are observed by transmission electron microscopy. The maximum growth rate is approximately 6.6 μm/min at 690 °C with 1500 W and 1.2 μm/min at 610 °C with 2000 W, which is approximately 20–30 and 4–6 times faster than that obtained by thermal chemical vapor deposition at approximately 1100 °C, respectively.

High-Rate Deposition of Intrinsic Amorphous Silicon Layers for Solar Cells Using Very High Frequency Plasma at Atmospheric Pressure

We have investigated the electrical and optical properties of hydrogenated amorphous silicon (a-Si:H) films prepared with extremely high deposition rates using very high frequency (VHF) plasma of gas mixtures containing He, H2, and SiH4 at atmospheric pressure. VHF power is a very important deposition parameter governing the dissociation of SiH4 molecules and the structural relaxation of the film. When the deposition parameters are optimized, the optical gap of the film can be controlled by the H2/SiH4 ratio without appreciable deterioration in photoconductivity. On the basis of the results, the a-Si:H films have been applied to the intrinsic layers (i-layers) of p–i–n single-junction solar cells. An initial efficiency of 8.25% is obtained for the cell with an i-layer prepared at a very high deposition rate of 128.1 nm/s. However, the results suggest that unknown factors limiting cell performance still exist in the fabrication process of a-Si:H solar cells.

Characterization of hydrogenated amorphous Si1 xCxfilms prepared at extremely high rates using very high frequency plasma at atmospheric pressure

Using the atmospheric pressure plasma chemical vapour deposition (CVD) technique, hydrogenated amorphous Si1 − xCx(a-Si1 − xCx : H) films were deposited at extremely high deposition rates. The films were prepared on Si(001) wafers at atmospheric pressure in a very high frequency (150 MHz) plasma of gas mixtures containing He, H2, SiH4 and CH4. Film properties (structure, density and composition of a-Si1 − xCx : H) were studied as functions of CH4 concentration and substrate temperature by transmission electron microscopy, Auger electron spectroscopy and infrared (IR) absorption spectroscopy. The relation between IR absorption spectra and chemical resistance of the films to 15% KOH solution was also investigated. The maximum deposition rate was 50 nm s−1, which was more than ten times faster than that achieved by the conventional plasma CVD technique. It was found that when the CH4 concentration was more than ten times higher than SiH4, the interaction of CH4 with SiH4 was saturated, and carbon-rich a-Si1 − xCx : H films were formed. The density of the a-Si1 − xCx : H films was about 1.5 g cm−3, being less than half of the crystalline value of SiC. The a-Si1 − xCx : H film was not etched by a KOH solution, which was supported by IR analysis of the Si–C stretching vibration mode.