Shigetaka Nomura

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.

Preparation of CuInSe 2 Thin Films on Mo-Coated Glass Substrates by Pulse-Plated Electrodeposition

CuInSe2 thin films are prepared on Mo-coated glass substrates by pulse-plated electrodeposition from an aqueous solution including CuCl2, InCl3 and SeO2. Film deposition with a stoichiometric composition and a smooth surface has been achieved by the control of the applied pulses with a duty cycle θ of 33% and a cathode potential during on-time of -0.7 V vs the saturated calomel electrode (SCE). The deposited films are annealed in nitrogen gas to be crystallized. The optimum annealing conditions have been determined using X-ray diffraction and Raman spectra measurement as: annealing temperature of 400°C, and annealing duration of 90 min.

Preparation of CuInSe2 thin films by the pulse-plated electrodeposition

CuInSe2 thin films have been prepared by the pulse-plated electrodeposition from aqueous solutions containing CuCl2, InCl3 and SeO2. The influence of the square pulse cathode potential on the film compositions, crystallinity and surface morphology has been studied. Homogeneous single phase CuInSe2 films with smooth surfaces have been obtained at a pulse potential of -0.8V vs the saturated calomel electrode (SCE), a duty cycle θ of 33%, and annealing treatment.

Compensated Electrical Properties of CuInSe2 Single Crystals

Electrical properties of stoichiometry-controlled single crystals of CuInSe2 are investigated by the Hall effect measurement over the temperature range of 20 to 300 K. Overshoot characteristics have been observed in the temperature dependence of the Hall coefficient for all p-type crystals. The compensating donor density is shown to be a dominant factor for the Hall overshoot rather than the acceptor density. If we assume a relation that µn/µp=Const.T-η with η around 1.5, the Hall overshoot is well explained.

Electrical and Optical Properties of the CuGa(S1-xSex)2 System

The mixed crystal system CuGa(S1-xSex)2 was prepared by the normal freezing method. The compositional dependence of the lattice constants and the optical band gap obeyed Vergards law. The phase diagram of this system was studied by DTA measurement. An orange photoluminescence was observed for x0.5 after annealing in vacuum. It was confirmed that the emission was due to D-A pair recombinations, and an increase of S-vacancy caused an increase of photoluminescence intensity. The electrical resistivity was also affected by S-vacancy.

Electron Spin Resonance Study of CuInSe2 Single Crystals

Imperfections relating to iron contamination in undoped CuInSe2 single crystals have been studied using Electron Spin Resonance at liquid He temperature. A fine structure of the S=5/2 state attributed to Fe3+ has been observed for crystals of Cu/In ~ 1. On the other hand, some of indium-rich (Cu/In<1) crystals show a single line of geff~10. Fe2+ is one of the possible candidates for this line. The two different ionization states of iron in CuInSe2 suggest that iron atoms substitute copper sites in Cu-deficient system rather than indium sites which are preferably occupied by Fe3+ in nearly stoichiometric system.

Chemical Reaction Processes in Synthesizing CuInSe2 from the Elements by the Melting Method

Chemical reaction processes in synthesizing CuInSe2 from the elements are investigated. The powder method of X-ray diffraction has been used to identify the reaction products formed in increasing temperature. Copper is selenized first to be Cu3Se2. Heated further, the Cu3Se2 changes gradually to another copper selenide which is supposed to be Cu7Se4. On the other hand, an indium selenide formed in an indium-deficient part inside the reaction tube is confirmed to be In6Se7, while in an indium-rich part, it is presumably In2Se. Above 500°C, the main body of CuInSe2 begins to be formed from Cu7Se4 as the basis. Cu3Se2 which has not participated in forming CuInSe2 begins to diffuse above 500°C, which is one of the main problems for the stoichiometric control of CuInSe2. The diffused copper selenide also changes to CuInSe2 by absorbing In6Se7 above 600°C.

Raman Spectra of Defect Stannite Phases in the Cu-(In,Ga)-Se System

Lattice vibration behaviors of defect stannite phases in the Cu-(In,Ga)-Se system are studied by the Raman spectrometry and the dispersion curve calculation in comparison with the chalcopyrite phases in this system. Bulk crystals in the alloy systems of Cu(In1-xGax)3Se5 and Cu2(In1-xGax)4Se7 are prepared by the normal freezing method. The characteristic two Raman peaks at 160 and 175 cm-1 for these crystals, apparently unrelated to the indium to gallium ratio, are assigned as two Γ1 modes of D2d symmetry. This can be explained from the dispersion curve calculation assuming the vacancies at 2a or 2b site of the defect stannite structure through the Cu-(In,Ga)-Se system.

Method of selenizing liquid Cu-In alloy in growing CuInSe2 single crystals

Stoichiometry-controlled growth of single-crystalline CuInSe 2 has been studied using a method of selenizing liquid Cu-In alloy. X-ray diffraction study reveals that selenization temperatures above the melting point of CuInSe 2 are indispensable for the single-phase synthesis with preventing binary additional phases such as Cu 7 Se 4 and In 2 Se. The electrical properties are controllable by adjusting the initial Cu/In ratio and /or applied selenium vapor pressure during the crystal growth. The application of selenium vapor pressure around 200 Torr is effective for the reduction of imperfections in p-type crystals

Dysonian Line in the Electron Spin Resonance of Highly Conductive CuIn2S4 Single Crystals

Electron spin resonance (ESR) spectra have been measured in CdIn2S4 single crystals with various resistivities (ρ=106-10-3 Ωcm). As the conductivity increases, samples vary from insulator-like semiconductors to degenerate ones. ESR spectra due to donor electrons are observed under illumination in high resistive samples, whereas Dysonian lines appear in low resistive ones. From the line shape analysis, we conclude that the variation in ESR line shapes with the concentration of donors arises from the motion of electrons in the paramagnetic centers.