Tanehiro Nakau

p-CuGa1-xInxS2/n-ZnSe Heterojunction by LPE Method from In Solution

p-CuGa1-xInxS2/n-ZnSe heterojunction was made by the LPE method from the In solution of solute CuGaS2 and solvent In, though a p-CuGaS2/n-ZnSe heterojunction was not obtained. The reflection electron diffraction pattern from the interface indicates an epitaxial growth. The measurements of EPMA have shown that Zn and Se decrease monotonically, and Cu, Ga and S increase monotonically, from the substrate to the over-growth layer. Indium from the solvent showed a monotonic increase from the substrate to the over-growth layer. The maximum temperature Tm of 650°C and cooling rate of 0.5°C/min were taken. The epitaxial layer is represented by the formula CuGa1-xInxS2, where x was 0.16 for Tm of 650°C. It is suggested that a complete series of solid solutions (CuGa1-xInxS2)1-y-(2ZnSe)y alloys is formed. The thickness of the mixedcrystal layer was 4.9 µm.

The Density of Localized States in Amorphous InxSe1-x Thin Films

The field effect method has been used to determine the density of localized states in amorphous InxSe1-x thin films for the range of the composition 0.05≤x≤0.4. This method provides a monotonic density distribution of localized states below the thermal equilibrium Fermi level for each specimen. Concerning the compositional dependence on the density of states, experimental results show that the density of states has a minimum value for the amorphous In0.2Se0.8 film and a maximum value for the amorphous In0.3Se0.7 film. These results may be explained by the random network model with the In-Se and Se-Se bonds.

Transient Photodischarge Characteristics of Photoconductors with Dispersive Transport

The transient photodischarge characteristics in a photoconductor displaying dispersive transport under an open-circuit condition are analyzed numerically. The transport parameters obtained from the present configuration are equivalent to those of a constant-voltage configuration within the small-signal limit. It is shown that, with increasing amounts of charge in transit, the post-transit-time slope of a double-logarithmic |dV(t)/dt| vs time plot increases algebraically, with the pre-transit slope and transit time being largely unaffected. Investigations of the effect of pre-existing trapped space charge indicate that the pre-transit slope depends upon the amount of this space charge. The results point the way to the use of the characteristics for determining transport parameters in a practical xerographic system.

LPE Growth of CuGa1-xInxS2 on ZnSe Substrate Using a Mixture of CuGaS2 and CuInS2 as a Solute

Liquid Phase Epitaxial (LPE) growth using (CuGaS2)1-Y–(CuInS2)Y as a solute, ZnSe as a substrate and In as a solvent has been studied by varying Y under the growth conditions of a maximum temperature Tm of 700 and 650°C, and constant cooling rate CR of 0.5°C/min. The over-growth layer is confirmed to be epitaxial CuGa1-xInxS2 by means of EPMA and electron diffraction under the growth conditions of Tm=700°C for all Y and Tm=650°C for Y0.7 for Tm=700°C, x is not proportional to Y and is almost constant for low Y. For Tm=650°C, the x-Y relation is similar to that for Tm=700°C, but the In content is higher than that for Tm=700°C. When Y>0.8, the growth rate and Cu content in the overgrowth layer rapidly decrease, where the ternary compound and/or ternary-ternary alloy growth layer could no longer be obtained. Thus, the range of epitaxial growth was Y0.7 for Tm=650°C. The above results are discussed and compared with those of our previous reports.

Isothermal Photocurrent Transient Spectroscopy of Gap States in Amorphous Chalcogenide Semiconductors

Gap state spectroscopy for determining the density and distribution of gap states in amorphous chalcogenide semiconductors is described. The method is based on the analysis of the transient current after a light pulse. The applicability of the method for determining the gap states in a-Se:Te films has been experimentally demonstrated. The experiments reveal that the gap states are relatively discrete states at energies of 0.42 eV and 0.34 eV above the valence band edge for 12 at.% Te:Se films and 18 at.% Te:Se films respectively.

Gap State Spectroscopy in Amorphous Selenium Photoreceptors

We propose a method useful for studying the energy distribution of deep gap states in amorphous semiconductors. The method is a sensitive and direct technique based upon the analysis of time dependence of residual voltage in photoreceptors under isothermal conditions. This enables us to map out the energy distribution of deep gap states in amorphous semiconductors. Experimental results using the method for amorphous selenium photoreceptors are demonstrated. Our results are in good agreement with those in the literature.

Electrical and Optical Properties of p ? ZnSnAs2/n?ZnSe Heterodiode

A pZnSnAs2/nZnSe heterodiode was prepared by LPE from Sn solution on a low-resistivity nZnSe substrate. The I-V characteristics of the diode were measured, and it was found to have a good rectification ratio of 104 at 1 V. The diode showed a photoresponse extending over a wide wavelength region between 0.4 and 1.9 µm. The measurements of the C-V characteristics showed that the diode had an abrupt junction with a diffusion potential of 0.60 V. The dielectric constant for ZnSnAs2 was first estimated from the analysis of the C-V characteristics: eA=12.0e0. The values of the minority carrier lifetime were obtained from the decay curves of EBIC as 70 ns and 0.4 ns for nZnSe and pZnSnAs2, respectively.

Effect of Ge Addition on Ga-Se-Te System Reversible Optical Recording Media

A reversible optical storage medium capable of more than 104 write/erase cycles has been realized using a Te0.8(Ga0.05Se0.95)0.2+5 wt% Ge film (~1000 A). For this composition, a peak temperature of an exothermic curve of 156°C and an activation energy from Kissingers plot of 2.24 eV were obtained using a differential scanning calorimeter. In this film, a peak of GeTe crystal was identified by X-ray diffraction. To investigate changes of the film surface induced through an annealing (200°C + 100 mW/cm2Xe, ~1 min), a high resolution SEM observation was carried out; while cracks preventing the high reversibility of write/erase cycles were generated for the GeTe film, the cracks were not recognized for the Te0.8(Ga0.05Se0.95)0.2+5 wt%Ge film.

A Theoretical Investigation of the Residual Voltage on Electrophotographic Plates

A numerical analysis of the residual voltage on an electrophotographic plate is presented for a photodischarge process characterized by carrier trapping and release parameters. The physical model is described by a set of one-dimensional transport equations with appropriate initial and boundary conditions. Examples of calculations are given for two models involving one and two hole trap levels. It is shown that a residual voltage can be derived which is valid for the decay process in amorphous Se film under typical electrophotographic conditions. For one trapping level, a trapping parameter of w 10-5 are needed to decrease the residual voltage Vr. With two trapping levels, the residual voltage is mainly influenced by the deep trap Et1.

Transient Photocurrent in Amorphous (As2Se3):Te Thin Films

Effects on the transport properties and gap states in a-As2Se3 films produced by the additive Te (2 at.%, 4 at.%, 6 at.%) have been studied by measuring transient photocurrents with a time-of-flight technique. The experimental results are interpreted in terms of a trap-controlled band transport as the mechanism of the dispersive carrier transport in these films. It is found that as the amount of Te increases, the hole drift mobilities decrease, the profiles of gap states remain exponential and their characteristic temperature increases slightly.