Koichiro Oishi

Growth and characterization of CuGaS2 thin films on (100) Si by vacuum deposition with three sources

Abstract The vacuum evaporation technique, using constituent elementary substances as sources, has been examined in growing CuGaS 2 on (100) Si wafers. The RHEED patterns of the grown films show that two different orientational types coexist in the film, i.e., the type with the c -axis of CuGaS 2 along the two a -axes of the Si substrate and the type with the c -axis normal to the substrate surface. Additional spots are observed in the RHEED patterns, which propose that the {112} twin 180° rotated about 〈221〉 also exists in the film.

Growth of Cu(In,Ga)S2 on Si(100) substrates by multisource evaporation

Abstract Cu(In,Ga)S 2 films were grown on Si(100) by vacuum evaporation using the constituent elementary substances as sources. It was found from X-ray diffraction studies that orientational growth occurred in a limited temperature region. The chalcopyrite structure was confirmed by studying RHEED patterns. The existence of c -axis growth, two kinds of a -axis growth and four kinds of {112} twins which can be classified as two symmetrical pairs is proposed.

Growth of CuInS2 and CuIn5S8 on Si(001) by the Multisource Evaporation Method

Thin films of CuInS2, CuIn5S8 and a mixture of these were epitaxially grown on Si(001) wafers at 400°C using a multisource evaporation method in which the In source temperature was varied. X-ray diffraction and reflection high energy electron diffraction observation showed that the CuInS2 film did not crystallize in the chalcopyrite structure, but in the Cu–Au structure with the c-axis normal to the substrate coexisting with the sphalerite structure. The CuIn5S8 film crystallized in the relaxed spinel structure with fairly good orientation. No evidence of CuIn3S5 was found from the X-ray diffraction patterns.

Orientation of CuGaS2 thin films on (1 0 0) GaAs and GaP substrates

Abstract CuGaS 2 films have been grown on (1 0 0) GaAs and GaP wafers by the vacuum evaporation technique using constituent elementary substances as sources. RHEED patterns of the films that use 〈0 1 1〉 azimuths in a substrate show two different patterns in mutually orthogonal directions. In one direction, the observed pattern is a single pattern which illustrates that CuGaS 2 grows in the [0 0 1] orientation direction (the c -axis growth). In another direction, however, additional patterns appeared. This phenomenon indicates that one pair of the 180 rotated {1 1 2} twins exists in addition to c -axis growth in a film, and it also indicates that additional orientations except for c -axis growth are reduced in films grown on compound substrates as compared with ones on (1 0 0) Si.

Preparation and characterization of AgGaS2 thin films by vacuum evaporation

Abstract AgGaS 2 thin films have been prepared by the vacuum evaporation method on GaAs(100). Elemental silver, gallium and sulfur have been used as the source materials. The X-ray diffraction and the RHEED analysis show that the good epitaxial films with the c -axis normal to the GaAs(100) substrate are grown for the stoichiometric composition. The non-stoichiometric films show that the two different orientational types coexist in a film. One type is that the c -axis is normal to the substrate. The other type is that the c -axis is parallel to the two a -axes of GaAs.

Crystal Structure and Optical Properties of Defect-Chalcopyrite-Type MnGa2S4

The crystal structure of MnGa2S4 grown by the iodine transport method was refined by Rietveld analysis of its powder X-ray diffraction data, and it was confirmed to be the defect chalcopyrite structure. No emissions can be observed near the absorption edge region, but a red broad emission band having an asymmetric spectral shape with a low-energy tail is observed in the deep region at room temperature. The red broad emission band is considered to be due to the transition from the lowest excited state 4T1(4G) to the ground state 6A1(6S) of the Mn2+ ion, because five photoluminescence excitation bands corresponding to five optical absorption bands are assigned to transitions from the ground state 6A1(6S) to the excited states 4T1(4G), 4T2(4G), 4E(4G)+4A1(4G), 4T2(4D) and 4E(4D). The role of the Mn2+ ion with respect to the luminescence property of the defect-chalcopyrite-type MnGa2S4 is similar to its role with respect to the luminescence property of zinc-blend-type Mn-doped ZnS. This is reasonable because the Mn2+ ion is tetrahedrally coordinated by four S2- ions in both the defect-chalcopyrite-type MnGa2S4 and the zinc-blend-type Mn-doped ZnS.

Growth of CuInS2 Epitaxial Films on Si(001) by Multisource Evaporation Method

CuInS2 thin films were epitaxially grown on Si(001) wafers at 500°C using multisource evaporation method. The Cu source temperature was varied keeping the In and S source temperatures constant. The obtained films belonged to the Cu2S–In2S3 system with little extra phase over the range of 0.9≤[Cu]/[In]≤2.9. X-ray diffraction and reflection high energy electron diffraction measurements showed that the films were composed of Cu–Au, sphalerite and chalcopyrite structures, although the last was not always observed. The characteristic structures in the photoluminescence spectrum of a film with [Cu]/[In]=1.4 measured at 23 K was interpreted as the weak broad exciton emission and the donor-acceptor pair emission with a phonon replica comparing to the bulk crystals. The position and the broadening of the exciton emission are reasonable considering the coexistence of various crystal structures.

Ordering and Orientation of Epitaxial CuInS2 Films Grown on GaP(001) by Three-Source Evaporation

The analyses on the ordering and orientation of epitaxial CuInS2 thin films on GaP(001) grown by three-source evaporation method at 500 °C are reported here in comparison to the films on Si(001). From the X-ray diffraction and the reflection high energy electron diffraction structural investigations, the improved quality of epitaxial film on GaP is demonstrated. For the In-rich and the stoichiometric CuInS2 films, the coexistence of the spinel CuIn5S8 ordered structure and the Cu–Au ordering of CuInS2 with a- and c-axes orientations normal to the GaP(001) substrate are exhibited. It is shown that the slightly Cu-rich film reveals both a- and c-axes orientations of chalcopyrite and Cu–Au orderings. However, the fairly Cu-rich film show dominant twin-free monocrystalline c-axis orientated Cu–Au ordered structures. The existence of sphalerite ordering is probable in all films. Comparing with the films on Si(001), the absence of the growth with random orientation and unknown phases is noticeable. The decrease of twin structures is also observed.

Effects of sputtered buffer layer on the characteristics of ZnO:Al films grown on glass substrates using high-temperature H2O generated by a catalytic reaction

The improvement in the quality of ZnO:Al films grown on glass substrates was investigated using a sputtered buffer layer inserted between a glass substrate and a ZnO:Al CVD film. ZnO:Al layers were grown at 773 K using dimethylzinc (DMZn), trimethylaluminum (TMAl), and high-temperature H2O generated by a catalytic reaction as zinc, aluminum, and oxygen sources, respectively. The electron mobility increased by approximately 20 cm2 V−1 s−1 with the use of a buffer layer with a thickness of approximately 40 nm. In addition, the optical transmittance in the wavelength range of 380–600 nm increased with the insertion of the buffer layer. For the growth of ZnO films on a sputtered buffer layer, the average surface roughness was reduced, and as a result the fluctuation in crystal orientation along the c-axis became smaller than that of the film grown directly on the glass substrate by the proposed CVD. This resulted in improvements in the optical transmittance and electron mobility of the ZnO:Al films.

Lattice Strain in AgGaS2 Epitaxial Layers Grown on GaAs (100) by Multisource Evaporation

Lattice strain in AgGaS2 epitaxial layers of different thicknesses, which are grown on GaAs (100) by the multisource evaporation method, is described. Biaxial compressive mismatch strain in thin layers relaxes with increasing layer thickness, and biaxial tensile thermal strain becomes dominant in thick layers. The thin AgGaS2 epitaxial layers are completely oriented toward the c-axis, but the thick layers contain 180°-rotated { 112 } twin crystals. The generation of twin crystals is considered to cause partial relaxation of the thermal strain. The observed variation in the A1-mode frequency with thickness is fully explained by considering the layer thickness dependence of the lattice strain. Full width at half-maximum of the A1-mode Raman scattering line is affected by the spatial inhomogeneity of the strain magnitude in the depth direction. Raman scattering measurement of the A1 mode can be used as a sensitive technique to evaluate the lattice strain in AgGaS2.