Sunao Matsubara

Observation of p-n Junctions with a Flying-Spot Scanner Using a Chopped Photon Beam

A flying-spot scanner that employs a chopped photon beam emitted from a cathode ray tube is reported. The photon beam scanns a planar p-n junction put on a metal electrode through a In2O3-coated transparent electrode and a 15 µm-thick mylar spacer. The ac photovoltage is picked up with electrodes through a condensor formed by the spacer. The photovoltage signal modulates brightness of another cathode ray tube to form a scanning image. Mean wavelength and chopping frequency of the photon beam are 507 nm and 2 kHz. Scanning image analysis is done using photocurrent density equations based on a step-like junction model. Three kinds of junctions, a solar cell, a partly-deep junction and a non-uniformly ion implanted one, have been evaluated to show the validity of the present method.

Polycrystalline Indium Phosphide Solar Cells Fabricated on Molybdenum Substrates

Heterojunctions on polycrystalline indium phosphide films are fatbricated and characterized. The InP films are chemically deposited on molybdenum substrates. Cuprous selenide films, being cubic in crystal structure and degenerate in conduction, are prepared on n-type InP films. CuxSe/InP thin film solar cells have efficiencies of up to 1.7% with a short circuit current density of 11 mA/cm2 under AM 1 simulated irradiation. Furthermore, CdS/InP heterojunction solar cells are fabricated by evaporating indium-doped CdS films on zinc-doped InP surfaces. The efficiency directly increases with the annealing temperature of the cells up to 2.0% with a short-circuit current density of 18 mA/cm2. A collection efficiency of 70% is obtained at a wavelength of 0.65 µm from a spectral response curve. In addition, the electron diffusion length is calculated to be 0.2 µm, which determines the lower collection efficiency at longer wavelengths.

Integrated Radiation Detectors with a-Si Photodiodes on Ceramic Scintillators

Solid-state monolithic-type X-ray detectors were fabricated by preparing amorphous silicon photodiodes monolithically on a Gd2O2S ceramic scintillator. The X-ray detection sensitivity of the solid-state detectors was comparable to that of a xenon ionization detector. It could be improved by optimizing the amorphous silicon photodiode and the intermediate materials between the photodiodes and the scintillator. The performance of the X-ray detector did not deteriorate even after 500,000-roentgen irradiation.

Determination of electric field profiles in amorphous silicon solar cells

Time‐resolved photoconductivity measurements with subnanosecond time resolution are applied to study the electric field profile in amorphous silicon solar cells in the range from 0.3 V forward to 0.8 V reverse bias. The method is used for a comparison of state‐of‐the art devices with different junction design. Optical and electrical contributions to the device performance are discussed and the limitations in improving the performance by use of a‐SiC:H window layers are pointed out.

Very-low-temperature silicon epitaxy by plasma-CVD using SiH4-PH3-H2 reactants for bipolar devices

A novel plasma-CVD process has been developed for growing specular silicon epitaxial layers at very low temperatures of approximately 200°C. The epitaxial layers were deposited from SiH4-PH3 gases diluted with H2 on single-crystal substrates just after being etched in a HF dip. The highest electron mobility and lowest resistivity of the n+-layers were 30 cm2/V s and 4×10-4 Ωcm, respectively. This technology has been successfully applied to fabricate bipolar transistors with a high current gain of 370.

Electrical Properties of n‐Type Polycrystalline Indium Phosphide Films

Electrical properties of n-type polycrystalline indium phosphide deposited on molybdenum substrates are investigated as a function of deposition conditions. For undoped films, the electron concentration is found to be 10/sup 15/ to 10/sup 17/ cm/sup -3/ and tends to increase with the increase in deposition temperature. The electron mobility and resistivity are almost constant irrespective of varying deposition conditions. On the other hand, the electrical properties of sulfur-doped films are found to be strong functions of deposition conditions and doping level. A grain boundary carrier trapping model is applied to explain the electronic transport properties of the polycrystalline films. The barrier height and the density of trapping states at grain boundaries are calculated.

Czochralski Growth of Square Silicon Single Crystals

Square single crystal silicon ingots with 3-inch sides are grown by forming an essentially uniform temperature distribution around the growing ingots. The resistivity distribution pattern for wafers from these ingots is generally square. The square single crystals have etch pit densities of 1-2×103/cm2. The growth for square crystals is explained by a model in which supercooling occurs in the radial direction of the ingots.

Mobility-Lifetime Product in Hydrogenated Amorphous Silicon

We report an analysis of the carrier loss process in time-of-flight experiments on amorphous silicon solar cells. The electron mobility-lifetime ( µτ) product is determined by a transitory immobilization of the carriers in deep traps. The carriers can be recovered and collected when the experiment is extended into the ms time range. The collection efficiency is shown to depend on the occupation of gap states. We discuss the discrepancy between µτ values obtained from time-of-flight and photoconductivity measurements and present an analysis which relates the time-of-flight µτ products to the density of shallow and deep states in a-Si:H.

Characterization of Si Layers Deposited on (100) Si Substrates by Plasma CVD and Its Application to Si HBTs

Studies have been carried out on Si layers deposited on (100) Si substrates by plasma CVD, and on heterojunction devices fabricated with the layers. These Si layers vary from microcrystalline to epitaxial structures depending on substrate surface cleaning and deposition temperature conditions. The characteristics of Si HBTs using these layers as emitters depend greatly on the structures of the layers. Very thin interfacial oxides formed by RCA cleaning are effective in growing homogeneous microcrystalline emitters. Hetero-junction effects exemplified by high current gains at room and low temperatures demonstrated the usefulness of the microcrystalline emitters for HBTs and future BiCMOS technology.

Numerical analysis of amorphous silicon solar cells: A detailed investigation of the effects of internal field distribution on cell characteristics

This paper describes the application of a new iterative method to numerical calculation of the performance of amorphous silicon p‐i‐n solar cells. Using this method, the effects of gap‐state density, dopant impurities (B and P), and various cell parameters (diffusion length, interface recombination velocities, thickness of the i layer, etc.) on cell characteristics are investigated. The calculated results show that a strong electric field advantageously effects cell characteristics (especially through fill factor) in the i‐layer where many photocarriers are generated. However, a uniform field isn’t always adequate for high conversion efficiency. The improvement in conversion efficiency provided by boron doping is attributed to stretching of the diffusion length rather than rearrangement of the field distribution.