Shoichi Kurita

Photodoping sensitivity of Ag into amorphous As2S3 films

The photodoping of Ag into as‐evaporated, annealed, and pre‐exposed As2S3 films has been studied by measuring the resistance and the optical density of Ag films and the depth of Ag doping estimated by the refractive index change. The photodoping consists of a dissolution process and a diffusion process. In the dissolution process, the spectral sensitivity reflects the absorption spectra of As2S3 films and the sensitivity limit is at about 650 nm. The dissolution rate of Ag into As2S3 is largest in the pre‐exposed films and is smallest in the as‐evaporated films. In the diffusion process, the spectral sensitivity limit is at about 700 nm and the diffusion rate varies only slightly over the three sets of As2S3 film conditions.

Polycrystalline Zn3P2/Indium-Tin Oxide Solar Cells

Zinc phosphide (Zn3P2)/indium-tin oxide (ITO) heterojunction solar cells are fabricated by rf sputter depositing ITO on large grain polycrystalline Zn3P2 wafers. An AES analysis indicates that sputter etching of the Zn3P2 wafer surface before depositing ITO is of great importance in achieving a better performance as a solar cell. The passivation of Zn3P2 by reaction with atomic hydrogen is found to improve the cell performance significantly. Consequently, a power conversion efficiency of 2.1% (6 mm2 in active area) has been obtained without antireflection coating.

Zn3P2/ITO Heterojunction Solar Cells

Heterojunction solar cells of Zn3P2/ITO have been fabricated by rf sputter depositing ITO films onto Zn3P2 multiple crystal, and their electrical and photovoltaic properties are studied. Multiple crystal boules of Zn3P2, 12–16 mm in diameter and 3 cm long with grain size of 1–5 mm in diameter, and resistivity ρ of 40–105 Ω-cm have been formed by vapor phase transport. The ρ of as-grown Zn3P2 has been reduced significantly by Ag doping above 400°C. A Voc of 0.24–0.32 V and a Jsc of 13–26 mA/cm2 are measured and the power conversion efficiency of 1.1% is obtained without AR coating at simulated AM 1. The J-V characteristics suggest tunneling as the dominant conduction mechanism. A spectral response of Jsc shows a good band-pass behavior between 360 nm and 830 nm.

High‐speed light valve using an amorphous silicon photosensor and ferroelectric liquid crystals

A novel high‐speed response light valve composed of a hydrogenated amorphous silicon (a‐Si:H) photosensor and a chiral smectic C phase liquid crystal is presented for the first time. This device is optically addressed. The switching between on and off states is caused by reversing the polarity of the applied voltage across the liquid crystal due to the photocurrent from the a‐Si photosensor. The response time measured is about 400 μs. The switching speed of this device is one to two orders of magnitude faster than that of the nematic liquid‐crystal light valve. This device can be applied to optical bistable devices without optical feedback, using an electro‐optic memory effect of the ferroelectric liquid crystal.

Zinc phosphide thin films grown by RF sputtering

Abstract Zinc phosphide (Zn 3 P 2 ) thin films have been prepared by RF sputtering in either Ar, H 2 , or PH 3 atmospheres. The Zn 3 P 2 films were deposited on glass substrates at RF power P s of 50–300 W, substrate temperature T s of 60–250°C, and total pressure up to 1 Torr. The Zn 3 P 2 films grown in PH 3 show a strongly preferred orientation; the c -axis is aligned perpendicular to the glass substrates, whereas the films grown in Ar and H 2 are amorphous. The crystallinity of Zn 3 P 2 films grown in PH 3 depends strongly on both PH 3 pressure P p and RF power: below P s =150 W the films are amorphous, while they are polycrystalline above it; the Zn 3 P 2 films are polycrystalline below P p =0.6 Torr, whereas they are amorphous above it. The X-ray fluorescence analysis and EPMA indicate Zn 3 P 2.03 for the films deposited at P p =0.2 Torr, P s =200 W, and T s =100°C. Optical measurements show that the film has a 1.5 eV direct bandgap.

Properties of Zn3P2 Thin Films Grown by Ionized-Cluster Beam Deposition

Zinc phosphide (Zn3P2) thin films have been fabricated by ionized-cluster beam (ICB) deposition on glass substrates. Highly crystalline Zn3P2 films are obtained with a strong preferred orientation along the c-axis by optimizing the acceleration voltage and substrate temperature. The Zn3P2 films have extremely good stoichiometry compared with those by conventional vacuum evaporation: fluorescent X-ray spectroscopy indicates Zn3P2.05 for the Zn3P2 films deposited at an acceleration voltage of 2 kV and substrate temperature of 300°C. The absorption coefficient is deduced from the optical transmission and reflectivity data; the fundamental absorption edge is obtained as 1.55 eV at 300 K.

New double‐heterostructure indium‐tin oxide/InGaAsP/AlGaAs surface light‐emitting diodes at 650‐nm range

New double‐heterostructure indium‐tin oxide/InGaAsP/AlGaAs surface light‐emitting diodes have been fabricated by liquid‐phase epitaxy and rf sputtering methods. In this structure, indium‐tin oxide acts as both an n‐type cladding layer and a transparent conductor. Peak wavelength and full width at half maximum of the surface emitting spectrum were 653 and 17 nm, respectively. An output power of 1 mW was achieved at a current level of 66 mA, corresponding to a current density of 22 A/cm2 under pulsed operation for the diode with a 400 μm×450 μm emitting area. The optical emission was distributed over the entire emitting area.

Photoenhanced chemical vapor deposition of zinc phosphide

Thin films of zinc, phosphorus, and zinc phosphide (Zn3P2) have been deposited by photodissociation of dimethylzinc (DMZ) and phosphine (PH3) on various substrates using a low‐pressure mercury lamp as a light source. The substrate temperature was varied between room temperature and 250 °C. The deposition rates of the films were significantly affected by the UV light intensity, the density of gases, the PH3/DMZ molar ratio, and the substrate temperature. Zn3P2 microcrystallites were grown on Si(111) substrates at a temperature of 250 °C. Those crystallites were studied by using a scanning electron microscope and reflection high‐energy electron diffraction. A very weak photoluminescence spectrum at 808 nm (1.53 eV) was observed at room temperature.

Influence of Ag Photodoping on Photoinduced Changes in As2S3 Glass Films

The transient photoinduced optical absorption change (TOC) and the photodarkening have been studied in Ag photodoped As2S3 glass films. They were observed in all of the sample films which we prepared ((As0.4S0.6)100-XAgX: 0X24.4at.%). For the photodarkening, the degree of the change in optical-gap energy due to the light irradiation and the heat treatment decreases with the increasing of Ag content. This may be ascribed to the fact that the motion of S atoms is hindred by the Ag atoms, causing a decrease in spatial flexibility. For the TOC, the spectral distribution of the optical absorption change has a broad peak. The peak center shifts to a lower energy position with the increasing of Ag content.

Proton injection phenomena in WO3‐electrolyte electrochromic cells

Incorporation of protons from an aqueous solution into WO3 films at the coloration of electrochromic cells has been demonstrated directly by two methods: first by observing coloration from an aqueous anode in planar geometric cells, and second by detecting infrared absorption due to O–D radicals in WO3 previously colored with heavy water.