Hideyuki Takakura

A new type of high efficiency with a low‐cost solar cell having the structure of a μc‐SiC/polycrystalline silicon heterojunction

A new type of high‐efficiency solar cell has been developed by a simple production process only with electron cyclotron resonance plasma‐assisted chemical vapor deposition of highly conductive microcrystalline silicon carbide (μ c ‐SiC) on polycrystalline silicon (poly‐Si). The device consists of a p ‐type μ c ‐SiC/ n ‐type poly‐Si heterojunction where the window material is a specially made wide‐band gap and highly conductive μ c ‐SiC. At the present stage, a conversion efficiency of 15.4% with V oc=556 mV, J sc=35.7 mA/cm2, and F. F.=77.4% has been achieved. Also employing this device as a bottom cell in a four‐terminal amorphous silicon ( a ‐Si) tandem‐type solar cell, 16.8% efficiency has been obtained. A series of technical data on the fabrication technology and device performance is presented and discussed.

Photoreflectance characterization of surface Fermi level in as‐grown GaAs(100)

Photoreflectance (PR) spectroscopy has been used to study the surface Fermi level in GaAs by taking account of photovoltaic properties at the semiconductor surface. The PR signal amplitude ‖ΔR/R‖ is proportional to a modulating photovoltage Vm generated by a modulation light irradiation. To modify the surface potential, the sample was irradiated by a third perturbing light, i.e., a continuous bias light Pb, together with the modulation light. The modulation light power dependence of ‖ΔR/R‖ is extremely sensitive to bias light intensity, surface Fermi level, and temperature. From the analysis of the temperature dependence of ‖ΔR/R‖ on modulation‐light power, the surface Fermi level of 0.47±0.09 eV below the conduction band was determined for a molecular beam epitaxially‐grown GaAs(100).

Fourier-transformation analysis of deep level transient signals in semiconductors

Abstract A new processing method for DLTS characterization is proposed through an analysis by means of Fourier transformations. Emission time constants for one or two traps at a given temperature can be obtained from an analysis of the Fourier spectrum of the isothermal-capacitance transient signal without any determination of spectral peak positions. Thus it is easy to automate DLTS measurements and signal processing with this analysis. This method is also advantageous in judging whether a measured signal includes single or plural traps.

Deep‐level characterization of n‐type GaAs by photoreflectance spectroscopy

Photoreflectance (PR) signal of n‐type GaAs grown by molecular‐beam epitaxy has been systematically studied as a function of modulation frequency in the PR measurements. The trap activation energy obtained from analysis of the frequency response of PR signal with its temperature dependence is 0.34 eV. The dramatic change in the frequency response has been observed as a function of the modulation (ac) and bias (dc) light intensities. The theoretical analysis has been made by assuming a single level electron trapping model. The calculated result shows a good agreement with the experimental data. Furthermore, photoluminescence spectrum associated with the deep level of the molecular‐beam‐epitaxy‐grown GaAs was measured and compared with the PR data.

Preparation of composition-controlled silicon oxynitride films by sputtering; deposition mechanism, and optical and surface properties

Silicon oxynitride films have been grown on silicon by current-controlled reactive sputtering. The content of oxygen in the films could be well controlled by regulating the sputtering current under the reactive gas of Ar+ N2 with an oxygen content of around 3%. The atomic ratio of oxygen to nitrogen in the silicon oxynitride film became larger with increasing sputtering current. It has been found that electron irradiation of the silicon substrate induces adsorption of oxygen and nitrogen. The degree of oxygen adsorption was about ten times larger than that of nitrogen. This phenomenon is a key mechanism in controlling the film composition. The adsorptive mechanism might be explained by the phenomenon of surface activation by the electron bombardment. Utilizing this technique, wettability by germanium of silicon oxynitride films could be controlled by varying their oxygen and nitrogen contents. A better wetting condition was obtained from films with large atomic ratio of nitrogen to oxygen in the silicon oxynitride film.

Graphoepitaxial growth of ZnS on a textured natural crystalline surface relief foreign substrate

A new type of graphoepitaxial growth of zinc sulfide (ZnS) crystalline thin films has been investigated. The substrate is polyimide coated with various thin films. It has an inverted pyramidal replica pattern taken from textured (100) single‐crystalline silicon. Crystallinity and growth orientation of films were examined by scanning electron microscopy and x‐ray pole figures. It has been confirmed that the crystal was grown from the bottom of the inverted pyramids. The results show that the graphoepitaxial effects are strongly sensitive to the ability of the semiconductor to wet the substrate coating materials at the nucleation temperature. The controllability of the crystallographic orientation normal to the substrate by the synthetic pattern is more than 85% in the present technology status.

Characterization of the optical properties of LPE InxGa1−xAsyP1−y thin layers grown on InP

Abstract A series of In x Ga 1− x As y P 1− y single-crystal thin layers have been grown on an InP substrate in a vertical liquid phase epitaxy furnace with a rotating slide boat system. The optical properties of these LPE quaternary alloys lattice-matched to InP have been investigated mainly by photoluminescence and electroreflectance measurements. Photoluminescence spectra of In x Ga 1− x As y P 1− y epitaxial layers are dominated by a strong luminescence line due to band-edge emission. At low temperatures, around 4.2 K, we have observed complicated luminescence bands with many fine structures. Electroreflectance spectra for the LPE In x Ga 1− x As y P 1− y layers are sufficiently broad to fulfil the low-field condition, and the analysis enabled us to determine precisely the band gap energy.

Graphoepitaxial growth of germanium by laser recrystallization

A systematic experimental approach has been performed to achieve orientation‐controlled nucleation and graphoepitaxial growth of germanium on the replica substrate with a textured single‐crystalline silicon (001) surface. The recrystallization of germanium was successfully obtained by scanning a cw argon‐ion laser on a germanium/metal eutectic alloy system. It was found that the crystallographic orientation of germanium is strongly dependent on the recrystallization conditions such as laser power, scan direction of laser beam, size of surface relief, and wetting condition to crystallized material. The 〈001〉 directions of the recrystallized germanium are normal to the substrate, and the 〈110〉 directions are parallel to the valleys among the pyramidal relief, i.e., the crystallographic orientation coincides with one of original silicon (001) single crystal. It has been confirmed that the crystallite sizes range from 2 to 10 μm, and the deviation of the [001] crystallographic orientation distribution is less...

Epitaxial vapor growth of GaAs1−xPx on germanium substrates and their opto-electronic properties

A series of GaAs1−xPx-Ge heterojunctions has been fabricated by a vapor phase epitaxial growth technique to investigate the opto-electronic properties of the wide-to-narrow gap heterojunction, and particularly, their wide-range of spectral photosensitivity. Epitaxial GaAs1−xPx crystal layers over a wide-range of composition x were grown on Ge substrates using the Ga-AsCl3-PCl3-H2 system. A new fabrication technology suitable for making good quality heterojunctions has been developed by separating the reaction process into two steps (mixed source deposition and epitaxial growth). This remarkably reduced the vapor etching reaction of the Ge substrate and minimized the contamination by cross diffusion. The stoichiometric and crystalline quality examinations of epitaxial layers were made by fluorescent X-ray analysis, EPMA, SEM and a four circle diffractometer. A series of electrical and opto-electronic measurements were made to evaluate the junction properties. The n-p heterojunction fabricated has a good rectification characteristics with a step-like impurity profile. A very wide spectral photoresponse having a wave length region from 0.35 μm to 2.0 μm has been obtained with a sensitivity about 10 times or more as great as that of a Ge p-n homojunction in the higher energy region (>2.0 eV).

Graphoepitaxial growth of germanium on the textured natural crystalline surface relief duplicated on a foreign substrate

A series of experimental trials was made on the oriented nucleation in the graphoepitaxial growth of germanium. The graphoepitaxial growth was performed from a germanium/metal eutectic alloy solution by scanning a cw argon‐ion laser on a silicon‐dioxide/tantalum/nickel substrate having a surface of pyramidal relief duplicated from the textured surface of (100) single‐crystalline silicon. In the germanium/metal system, the germanium can be grown at relatively low temperatures in the 300–400 °C range by the eutectic effect. The laser‐induced recrystallization of an eutectic alloy started at the bottom of the valleys and moved up to the top through the germanium/metal alloy phase. Then the metal was left on the top of the pyramids. When the laser scan was repeated, the surface became flat, and its crystallographic orientation was epitaxially controlled by the surface relief of the substrate.