Haruo Itoh

Design of integrated radiation detectors with a-Si photodiodes on ceramic scintillators for use in X-ray computed tomography

An integrated radiation detector with a-Si photodiodes on ceramic scintillators is designed for use in X-ray computed tomography. The output signal is estimated using Monte Carlo simulation. Results show that this detector has higher sensitivity than a c-Si photodiode/scintillator detector when a transparent layer between the scintillator and the thin film semiconductor is optimized. The noise characteristic is also considered when an integrated detector is coupled with an operational amplifier. Good agreement is shown between predicted and measured parameters. >

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.

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.

Non-mass analysed ion implantation using microwave ion source

Abstract To evaluate the usefulness of high-current non-mass-analysed (NMA) ion implantation using a new microwave ion source, the photovoltaic properties of silicon solar cells were studied. When phosphorus vapour is introduced into the ion source, it provides implant current of more than 10 mA for the 5–15 keV energy range. The fraction of neutral phosphorus particles implanted together with ions is typically less than 10% of the total dose. The conversion efficiency of solar cells fabricated by NMA implantation is about 10% without any anti-reflection coating. Changes in implantation energy do not significantly change the values of these efficiencies. Continuous and stable oxygen ion beams of about 110 mA were obtained at 5.0 kV, which is high enough to apply to material modification in metals, insulators and semiconductors.

Characterization of silicon layers epitaxially grown on metallurgical-grade polycrystalline substrates

The structural and electrical properties of silicon layers epitaxially grown on metallurgical-grade polycrystalline silicon substrates are examined to clarify the effect of grain boundaries, crystal defects and impurities in the substrates. Chemical etching of the epitaxial layer reveals that all the grain boundaries continue from the substrate into the epitaxial layer, whereas lines of high density etch pits do not always continue. The polycrystalline thin film solar cells are fabricated on the metallurgical-grade silicon substrates by successive deposition of p and n+ layers. These cells show short circuit current densities around 70% of that of the conventional single crystal cell. This reduction of the short circuit current is caused mainly by the short minority carrier diffusion length in the grains probably due to impurities involved in the epitaxial layers. The origins of such impurities are discussed by considering autodoping and solid-state diffusion from the substrate during growth of epitaxial layers.

Watermarks: Generation, Control, and Removal

After dilute HF cleaning, when a wafer is rinsed and dried naturally stains arise on the parts where there were water droplets. These are called watermarks or drying spots. During the typical wafer drying process in semiconductor manufacturing, watermarks are generated for a variety of reasons. In spin-drying, it has been reported that they are generated between the lines and in U- and V-grooves. Watermarks become masks in successive etching processes and hinder the film from depositing or forming normally, and they damage several device characteristics. Therefore, in the semiconductor fabrication process, how to reduce the number of watermarks to zero is an issue of wafer cleaning and drying technology.

Photoelectric and structural properties of a-Si1-xGex: H alloys prepared using Si2H6 and GeH4

Abstract High-rate deposition of a-Si 1- x Ge x : H alloys was performed in order to both improve quality and to lower the fabrication cost of narrow-gap materials for use in amorphous solar cells. This paper presents the characteristics of a-Si 1- x Ge x : H alloys prepared by glow discharge decomposition of a Si 2 H 6 and GeH 4 gas mixture. An excellent quality alloy with E g =1.55 eV, a photoconductivity of σ p =3X10 -5 Ω -1 cm -1 and a dark conductivity of σ d =4X10 -9 Ω -1 cm -1 is obtained under the high deposition rate of 4 A/s. In addition, it is proposed that at least two types of heterostructure, which deviate from a randomly mixed homogeneous structure, exist. Therefore, reduction of these heterostructure are necessary to improve alloy quality.

Transport parameters and electric field profile in amorphous silicon solar cells

Abstract We present a detailed study of the electronic transport parameters in amorphous silicon based single junction solar cells. Employing the time-of-flight technique we determine the drift mobility and mobility-lifetime product for electrons and holes, as well as the electric field profile in optimized devices having conversion efficiencies between 10% and 12%. We have also studied the trends in these parameters due to alterations in the device design and material. Here we discuss the importance of buffer layers, and the changes induced by light-soaking. Finally, we discuss our results with respect to future improvements in the conversion efficiency.

A New Ion Implanter for Solar Cell Fabrication

A high-current ion implanter based on a continuous wafer-processing concept has been newly developed, in order to apply the implant technique to solarcell p-n junction formation. In this implanter, a single magnet carries out mass separation as well as ion-beam scanning, and wafers mechanically traverse the scanned-beam region only once, perpendicularly to the beamscanning direction. The implanter provides the maximum implant current of 20mA at 30kV with the use of a coaxial microwave ion source. Wafer throughput is about 400 wafers/hr for 100mm-diameter wafers, at a 3.0x1015 ions/cm2 dose. Dose non-uniformity is a standard deviation of 3% of the sheetresistivity distribution. Energy-conversion efficiency is 13.5% with an anti-reflection coating. It is shown experimentally that the newly developed implanter is applicable to low-cost solar-cell fabrication.

Electric Field Profile and Charge Collection in a-Si:H Solar Cells

We have studied the electric field profile in single-junction amorphous silicon solar cells. Our results allow an estimate of the space charge density at the pi- junction and a simple quantitative evaluation of the electrical properties of the junction. We find the field profile to be approximately exponential, and to be largely determined by deep defect states in the intrinsic a-Si:H absorber layer.