Saburo Endo

Electrical Properties of p- and n-Type CuInSe2 Single Crystals

The electrical resistivity and Hall effect of p- and n-type CuInSe2 single crystals are measured in the temperature range from 80 K to 500 K. p- and n-type samples are prepared by doping with excess Se and excess In, respectively. The acceptor levels at 0.020 eV and 0.028 eV above the valence band and the donor levels at 0.012 eV and 0.18 eV below the conduction band are identified. The mobility data are analysed assuming scattering by acoustic, polar optical, and nonpolar optical phonons and by ionized impurities. For some of the n-type samples, the measurements are extended to liquid helium temperature and the result is analysed by the existing theories of impurity band conduction.

Transport properties of CdIn2S4 single crystals

Abstract Several transport properties have been studied on CdIn 2 S 4 single crystals with different degrees of deviation from stoichiometry. The energy gap at 0 K was determined from the electrical measurements to be 2.2 eV. The anisotropy of the magnetoresistance effect was found and it was suggested that the minima of the conduction band were located at points along the [100] directions in k space. From an analysis of the mobility data it was found that the high resistivity of the samples is due to compensation of donors by acceptors introduced by excess sulphur. Several band parameters, the carrier scattering mechanisms and the impurity levels were determined. The thermal conductivity was measured from 4 K to 300 K and analysed by Callaways formalism.

Raman-Scattering Properties of I-III-VI2 Group Chalcopyrite Semiconductors

The Raman-scattering spectra of I-III-VI2 group chalcopyrite semiconductors and their solid solutions were measured. The frequency relationship for the A1 vibrational mode agrees with the simple ratio of the anion atomic weights, although it also depends on the mean atomic weights of the cations. Compositional change of the A1 mode frequency for their solid solutions with anion substitution can be explained by the probability of formation of two pairs of anion atoms.

Optical and Electrical Properties of CuIn5S8 and AgIn5S8 Single Crystals

The energy gaps at 300 and 96 K of CuIn5S8 and AgIn5S8 single crystals were determined from the optical absorption measurements. The peaks of the photoconductive spectra observed at 300 and 77 K were in good agreement with the energy gaps. The resistivity and the mobility for the samples with less sulphur than the stoichiometric one were determined from the electrical measurements.

Preparation and some properties of CuInSe2 single crystals

Abstract A brief review of our previous research on the electrical properties and p-n junction characteristics of CuInSe2 single crystals prepared by the normal freezing method is presented. Recently the optical absorption spectra have been measured as functions of temperature and pressure. The temperature and pressure coefficients of the absorption edge are found to be −1.1 × 10 −4 eV K −1 and +2.8 × 10 −11 eV Pa −1 respectively. Some trends may be seen in the distribution of the absorption edge energies measured for various n- and p-type samples.

Thermodynamical Properties of I-III-VI2-Group Chalcopyrite Semiconductors

Melting points and transition points of I-III-VI2-group compounds and their solid solutions were determined exactly. The heats of fusion and transition were also estimated. The melting points of I-III-VI2-group compounds and solid solutions depend on mean atomic weight and ionicity. It is found that the melting points are influenced by lattice strain. Heats of fusion and transition of I-III-VI2-group compounds depend on mean atomic weight, ionicity and u-parameter, the trends of which are in contrast to those for III-V-group compounds. Heats of fusion and transition of their solid solutions are much smaller than those for the end-member compounds.

Preparation and Characterization of Electrodeposited CuInSe2 Thin Films

CuInSe2 thin films were prepared on SnO2-coated glass by electrodeposition from aqueous solution containing CuCl2, InCl3 and SeO2. The deposition was carried out by varying cathode potential. It was found that the Cu/In ratio which influences CuInSe2 characteristics greatly can be controlled by the cathode potential. Annealing of as-deposited films in N2 atmosphere at 350°C was found to result in the formation of CuInSe2 films having a chalcopyrite structure. With the use of these annealed CuInSe2 thin films, CdS/CuInSe2 thin-film solar cells were formed as a preliminary step and the conversion efficiency of 1.49% was obtained.

Valence band densities of states of CdIn2S4 and In2S3 from x-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectra of the valence band of CdIn 2 S 4 and In 2 S 3 single crystals have been measured. The spectrum of CdIn 2 S 4 has a strong resemblance to its synthesized spectrum from the In 2 S 3 and CdS spectra, which is in good agreement with the theoretical density of states (DOS). The contribution of constituent atoms to the valence band DOS in CdIn 2 S 4 is corresponding to those in In 2 S 3 and CdS.

Raman Spectra of Ordered Vacancy Compounds in the Cu-In-Se System

Raman spectra of ordered vacancy compounds (OVC) of CuIn2Se3.5 and CuIn3Se5 have been measured at room temperature. Similar nonpolarized Raman spectra are obtained for CuIn2Se3.5 and CuIn3Se5. However, polarized Raman spectra are not available for CuIn3Se5 because of its unfavorable crystallinity. A signal at 154 cm-1 is assigned to be Γ1 mode of D2d symmetry, originating from the vibration of selenium atoms in the x-y (a-axes) plane. This signal corresponds to the signal at 178 cm-1 for CuInSe2 of chalcopyrite-type in the Cu-In-Se system. The lowering of Γ1 mode frequency for the defect structure is attributed to looser-bound selenium atoms on account of vacancies in the cation sites. Signals assigned as Γ3 mode are unexpectedly observed in the geometries allowing the longitudinal optical (LO) modes of Γ4 and Γ5 symmetries. The respective mode peaks are found to be more dispersive in frequency for CuIn2Se3.5 than for CuInSe2.

Low-Frequency Photocurrent Oscillations in CdIn2S4 Single Crystals

Low-frequency photocurrent oscillations in CdIn2S4 single crystals when a high electric field is applied at low temperature have been studied through simultaneous observations of the photocurrent and the photoluminescence. Several parameters necessary for analysis of the oscillations have been determined from additional experiments. We found that the pulse-type oscillations disappeared when light of photon energies between 1.97 and 2.54 eV was superposed. The distribution of the electric field in the sample was measured and it was found tbat the region of high electric field, the so-called domain, does not move in space but that the electric field varies periodically, leading to the current oscillation. The mechanism of the pulse-type photocurrent oscillations in CdIn2S4 single crystals is well-explained by the three-center model considering the thermal quenching process.