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We previously developed perovskite/graphene solar cells with improved long-term stability. In this study, we investigated the effect of the number of layers on the characteristics of a solar cell by changing the number of graphene layers and further determined the optimum number of graphene layers. We showed that the interfacial recombination due to the damaged layer of the outermost perovskite layer and the band bending of the perovskite layer due to the change of the work function of graphene had a significant influence on the characteristics of the solar cell, especially the short-circuit current. A conversion efficiency of up to 9.4% was obtained as a result of optimizing the formation conditions of the perovskite layer with seven graphene layers.

The two-dimensional materials have the thickness of an atomic layer level and are expected as alternative materials for future electronics and optoelectronics due to their specific properties. Especially recently, transition metal monochalcogenides and dichalcogenides have attracted attention. Since these materials have a band gap unlike graphene and exhibit a semiconductor property even in a single layer, application to a new flexible optoelectronics is expected. In this study, the photovoltaic characteristics of a GaSe/MoSe2 heterojunction device using two-dimensional semiconductors, p-type GaSe and n-type MoSe2, were investigated. The heterojunction device was prepared by transferring GaSe and MoSe2 onto the substrate which the titanium electrodes were fabricated through a mechanical peeling method. The current–voltage characteristics of the GaSe/MoSe2 heterojunction device were measured in a dark condition and under light irradiation using a solar simulator. The irradiation light intensity was changed from 0.5 to 1.5 sun. It was found that when the illuminance was increased in this illuminance range, both the short-circuit current and the open-circuit voltage increased. The open-circuit voltage and the energy conversion efficiency were 0.41 V and 0.46% under 1.5 sun condition, respectively.

This study was conducted with the objective of improving the stability of perovskite solar cells by using the unique characteristics of graphene in order to facilitate the widespread application of such solar cells, for example, in multijunction devices. We consequently developed a new transfer method for graphene using vacuum lamination and, using graphene, successfully fabricated a perovskite solar cell that does not require a hole-transport layer. Initial stability tests indicated that the new device has better stability than a control perovskite solar cell using spiro-OMeTAD. Although the new solar cell design exhibited poorer cell performance than the control, we determined via modeling that its performance can be improved by modifying the interface state between perovskite and graphene or by modulating the work function of graphene.

We developed a new perovskite film forming process based on solvent bathing method. The preheating of substrate before spin-coating of precursor solution drastically changed the quality of the obtained perovskite films. The perovskite films formed by modified solvent bathing method were much dense with large grains. The improvement of open circuit voltage implied that the modification succeeded to form the better interface between TiO2 and perovskite layer and to grow the higher quality perovskite layer. Finally, we obtained the best conversion efficiency of 12.6%. Although the conversion efficiency of this work is not sufficient compared with other reports, our modified solvent bathing method has a wider process window comparing to the commonly used anti-solvent dipping methods. We expect that our modified method can also applied to monolithic tandem solar cells.

Highly oriented CuInS2 films were successfully prepared on graphene electrodes for solar-cell applications. CuInS2 films were deposited on high-quality graphene via reactive sputtering using a system with facing targets. This high-quality graphene was grown via chemical vapour deposition. The obtained CuInS2 films on graphene were highly (112)-oriented, and their crystallite size was enhanced compared with that of films grown on conventional Mo electrodes. The graphene/CuInS2 interfaces clearly exhibited ohmic characteristics comparable to those of Mo/CuInS2. Our experimental results demonstrate the high potential of graphene electrodes for use in CuInS2 thin-film and multi-junction solar cells.

The optoelectrical properties of bromide-based perovskite thin films were evaluated to develop an optical wireless power transmission system. CH3NH3PbBr3 (MAPbBr3) films were deposited by the anti-solvent method after substrate preheating. The low-temperature exciton emission and discontinuous peak shift from phase transitions in the emission spectra indicate the high optical quality of MAPbBr3 films on TiO2. The Urbach energy and diffusion length of the MAPbBr3 thin films on TiO2 were 22.7 meV and 1.14 μm, respectively. The films exhibited good solar cell performance under monochromatic light with an efficiency over 20%.

We developed a method of patterning graphene by plasma etching to improve the performance of graphene transparent conductive film. We succeeded in patterning the graphene multilayer film into the porous shape of the mask by optimizing the plasma etching gas type, pressure and power. As the plasma irradiation time was lengthened, the transmittance became high and saturated at a transmittance corresponding to the aperture ratio of the mask. A possibility of independently controlling the optical and electrical properties of the graphene film by etching under appropriate conditions was indicated.

We developed a perovskite solar cell using graphene as a hole transport layer. We succeeded in transferring graphene onto perovskite layer by using a vacuum bonding machine. The conversion efficiency of the perovskite solar cells using graphene achieved power conversion efficiency of 7.1%. Graphene cells showed higher stability than spiro-OMeTAD cells after light irradiation at high humidity. This is because graphene blocks the penetration of H_{2}O and O_{2} to the perovskite layer and plays also a role as a blocking layer.

Epitaxial growth of CuInS2 films with various [Cu]/[In] ratios at 570 °C on GaP (001) substrates is presented with structural and luminescent investigations. CuIn5S8 phase with Cu-Au ordered CuInS2 dominate in almost stoichiometric films while Cu-Au ordering of CuInS2 was observed in Cu-rich films. Chalcopyrite ordering of CuInS2 was highly evident in the slightly Cu-rich films at a narrow range of 1.28 ≤ [Cu]/[In] ≤ 1.41. These slightly Cu-rich films exhibited streaky reflection high energy electron diffraction patterns corresponding to the c-axis oriented chalcopyrite structure without the other oriented structures were observed, indicating highly structural quality of CuInS2 epitaxial films. Broad emission bands in the band-edge region were observed in the films. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The preparation of CuYO2 films on quartz glass substrates was carried out through the solution method using copper and yttrium nitrates as sources. Firstly, dip-coated films on the quartz glass substrates were annealed in air, and then annealed again under nitrogen flow. Continuous films were formed on the substrate surfaces. The films annealed in air at 900–950 °C were almost completely dominated by the Cu2Y2O5 phase, except for slight appearances of CuO and Y2O3. After annealing under nitrogen flow at 900–950 °C, the films were almost completely dominated by the CuYO2 phase, except for slight appearances of Cu2O and Y2O3. The slight appearances of the impurity phases are possibly related to slight variations in the concentration ratio of the copper source to the yttrium source within the dip-coated films. The nitrogen-annealed films, as well as bulk CuYO2, exhibited a green emission band owing to the Cu+ interconfiguration transition from 3d94s1 to 3d10 with the Stokes shift, being consistent with the dominant appearance of the CuYO2 phase.

As-deposited films were prepared on sapphire substrates at 500–680 °C by alternately sputtering Cu and Al targets in Ar-diluted O2 gas atmosphere. The composition of the as-deposited films corresponded to that of the slightly oxygen-rich region of the CuO–CuAl2O4–Al2O3 system. The films as-deposited at 500 °C had an amorphous structure, while the films as-deposited at 680 °C had CuAl2O4 phase but no CuAlO2 phase. Annealing at 1050 °C in nitrogen flow caused a reduction in the molar fraction of oxygen, i.e., the composition of the annealed films with [Cu]/[Al] ≈1 corresponded to CuAlO2. The annealed films were predominated by the CuAlO2 phase. The preferential orientation of the films toward the c-axis normal to the substrate surface is due to the small lattice mismatch between the rhombohedral [010] of delafossite-type CuAlO2 and the hexagonal [1100] of the sapphire substrate. The annealed films had an absorption edge corresponding to the energy gap of CuAlO2 and exhibited p-type conductivity.

Cu(In,Ga)S 2 crystals have been grown from the melt using constituent elementary substances as sources. The chalcopyrite structure was confirmed by powder X-ray diffraction patterns. Lattice parameters a and c changed almost linearly corresponding to the composition ratio between In and Ga. The amplitudes of photo-acoustic signals changed greatly near the band gap energies. This phenomenon is due to the changes of the absorption coefficient. Band gap energies, estimated from the photo-acoustic spectra, also showed continuous change with the composition.

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.

Cu–Al–O films were prepared on quartz glass substrates at 500–700 °C by sputtering the Cu and Al targets alternately on atomic-layer scale under an Ar-diluted O2 (5–20%) gas atmosphere, and then annealed at 1050 °C under a nitrogen atmosphere. The [Cu]/[Al] ratio was controlled by changing the Cu and Al deposition periods. The composition of as-deposited films corresponded to the slightly oxygen-rich region of the CuO–CuAl2O4–Al2O3 system. Films as-deposited at 500 °C had an amorphous structure, while films as-deposited at 700 °C had CuAl2O4 and CuO phases. After thermal annealing in a nitrogen atmosphere, the composition of the films approached that of the Cu2O–CuAlO2–Al2O3 system line, causing a noticeable appearance of the CuAlO2 phase along with the disappearance of the CuAl2O4 and CuO phases. Cu- and Al-rich annealed films had in addition a Cu2O phase and an amorphous Al2O3 phase, respectively. All annealed films exhibited p-type conductivity. The annealed films with [Cu]/[Al]≈1 had an absorption edge corresponding to the energy gap of CuAlO2. These results indicate that the change in the Cu ion from divalent to monovalent through nitrogen annealing results in the preparation of transparent conductive films dominated by CuAlO2.

In delafossite-type oxides of CuR x Y 1-x O 2 (R = Eu, Tb or Tm) and CuR x Cu Y Y 1-x-y O 2 prepared through the conventional solid state reactions, the lattice constant dependence on the composition implies doping with the Tb 3+ , Eu 3+ and Tm 3+ cations on the Y 3+ site. Noticeable sharp emission lines due to the f-f transitions of Eu 3+ , Tb 3+ and Tm 3+ are observed at room temperature in CuR x Ca y Y 1-x-y O 2 as well as CuR x Y 1-x O 2 . P-type conductivity of CuR x C y Y 1-x-y O 2 increases with increasing Ca concentration, indicating the increase of hole concentration caused by doping with Ca 2+ acceptor cation on the Y 3+ site. These results indicate the controllability of the luminescence and conductivity in delafossite-type CuYO 2 oxides by co-doping with Eu, Tb or Tm rare-earth cation and Ca acceptor cation on the Y3 + sites.

We report the growth and preferred structures of heteroepitaxial CuInS2 films on (001)GaP grown by evaporation of elemental Cu, In and S in vacuum. Monocrystalline CuInS2 thin films were successfully grown at 500 and 570 °C substrate temperatures. The c -axis Cu-Au ordering was the preferred orientation for slightly Cu-rich films deposited at 500 °C as revealed by XRD and RHEED results. The slightly Cu-rich films grown at 570 °C exhibited prominent c -axis orientation of chalcopyrite structure. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Delafossite-type oxides of CuTbyY1−yO2, CuEuyY1−yO2, CuCaxTbyY1−x−yO2 and CuCaxEuyY1−x−yO2 have been prepared by solid state reactions. The lattice-parameter dependence on the composition implies substitution of the Tb3+, Eu3+ and Ca2+ cations for the Y3+ site. Noticeable sharp emission lines due to the f–f transitions (5D4→7FJ, J=3–6) of Tb3+ or due to the f–f transitions (5D0→7FJ, J=0–4) of Eu3+ are observed at room temperature. Electrical conductivities of CuCaxTbyY1−x−yO2 and CuCaxEuyY1−x−yO2 are larger than those of CuTbyY1−yO2 and CuEuyY1−yO2, indicating the increase of the hole concentration caused by the substitution of Ca2+ for the Y3+ site. These results indicate the controllability of the luminescence and conductivity in CuCaxTbyY1−x−yO2 and CuCaxEuyY1−x−yO2 delafossite-type oxides by simultaneous substitution of the rare earth Tb3+ or Eu3+ cation and the Ca2+ cation for the Y3+ site.

Organic light emitting diodes (OLEDs) with nanostructured ultrathin layers inserted at the interface between electron- and hole-transport layers were investigated. The fundamental structure of the OLEDs fabricated by a vacuum evaporation method was indium-tin-oxide (ITO) anode/copper phthalocyanine (CuPc)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (TPD)/8-hydroxyquinoline aluminum (Alq3)/LiF/Al cathode. Fullerene (C60) and rhodamine B (RhB) molecules were used as the nanosutructured ultrathin layers inserted at the interface between the Alq3 and TPD layers. The electroluminescent (EL) properties have been measured for the OLEDs with C60 and RhB ultrathin layers and the dependences on the thickness and the position of the inserted layers were examined. For the OLEDs with the C60 ultrathin layer, the improvements of the drive voltage and EL efficiency were observed. The OLED with the inserted C60 ultrathin film of a monolayer thickness showed the highest efficiency, which was twice as large as that without C60 layer. On the contrary, the improvements were not observed for the OLEDs with the RhB ultrathin layer.

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.

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.

Composition and structure of CuAlO 2 films, deposited through the dc-reactive sputtering method using Cu and Al elemental targets and Ar-diluted oxygen gas, were controlled by the Cu and Al deposition periods and the postannealing temperature. The delafossite CuAlO 2 films were successfully prepared by the postannealing of the films with [Cu]/[Al]=1 at temperatures higher than 700 °C in the nitrogen atmosphere. In comparison with the optical absorption edge of the CuAlO 2 films, those of Cu-rich and Al-rich films sifted to the longer and shorter wavelength regions, respectively. The shifts of the optical absorption edge in the off-stoichiometric films are interpretable to be due to the additional copper oxide or aluminum oxide phase. The resistivity of the high-temperature postannealed films with p-type was in the range of 10–10 2 Ωcm.

Organic light-emitting diodes (OLEDs) with a nanostructured fullerene (C60) layer at and near the interface between the electron- and hole-transport layers were fabricated using a vacuum evaporation method. The OLEDs had a fundamental structure of indium-tin-oxide (ITO) anode/copper phtalocyanine (CuPc)/N,N'-dephenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine (TPD)/8-hydroxyquinoline aluminum (Alq3)/LiF/Al cathode. The electroluminescent (EL) properties have been investigated for the OLEDs with a nanostructured C60 layer at the TPD/Alq3 interface and at the different positions in the Alq3 layer near the TPD/Alq3 interface. The dependences of the EL properties on the thickness of the inserted C60 layer and on the position from the interface were examined. The OLEDs with a C60 monolayer inserted at the TPD/Alq3 interface showed the highest efficiency.

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.

The crystallographic characteristics of twin-reduced ZnSe crystals grown by a high-pressure melting method and the approach to stoichiometric growth are given. The crystals were grown by the seeded vertical Bridgman method and gradient freezing with in-situ composition control by a defined Zn source temperature. One inch ZnSe single crystals having twin-reduced parts and an average etch pit density of 2.0 × 105cm−2 were successfully grown by the seeded Bridgman method. Multi-twin patterns, typically for melt-grown boules, were not detected from the observation by optical microscopy with crossed polaroids and by transmission X-ray topography. The crystallinity of the as-grown crystal was characterized by X-ray rocking curve analysis using a double crystal arrangement. A full width at half maximum of 24 arcsec was detected. The influence of the Zn source on the crystal composition was studied by chemical analysis and photoluminescence.

Transparent p-type NiO films were prepared by reactive sputtering using the facing-target system under Ar-diluted O2 gas at T sub of 30 and 200 °C. The increasing intensity of dominant X-ray diffraction (XRD) peaks indicates improvements in the crystallinity of NiO films upon Cu doping. In spite of the crystallographic and optical changes after Cu-doping, the electrical properties of Cu-doped NiO films were slightly improved. Upon Ag-doping at 30 °C under low O2 concentration, on the other hand, the intensity of the dominant (111) XRD peaks was suppressed and p-type conductivity increased from ∼10−3 to ∼10−1 S cm−1. Finally, our Ag-doped NiO films were applied as the back contact of CdTe solar cells. CdTe solar cells with a glass/ITO/CdS/CdTe/NiO structure exhibited an efficiency of 6.4%, suggesting the high potential of using p-type NiO for the back-contact film in thin-film solar cells.

Origins of lines appearing in the exciton region spectra of CuGaS2 crystals at low temperatures under 4579, 4765 or 4880 A excitation of an Ar+ laser are discussed in terms of emissions from upper- and lower-branch excitonic polaritons (UBPs and LBPs) and multi-phonon Raman processes. The line at 2.500 eV is an emission from LBPs. The line at 2.504 eV seen in our samples under 4765 A excitation is shown to be interpretable as a three-phonon Raman process in resonance with UBP states, while the line reported at the same energy by Shirakata is likely to be an emission from UBPs.

As the basic study to realize a blue-color light-emitting diode using CuAlxGa1-xS2 crystals, heterojunction diodes were fabricated by the vacuum deposition of an n-type ZnyCd1-yS thin film on the {112} surface of a p-type CuGaS2 single crystal grown by the iodine transport method. It was found that the {0001} surface of ZnyCd1-yS is oriented in parallel to the substrate when the substrate is not heated, that the film is transparent to green and blue color, and that the resistivity is extremely low when y = 0.2 to 0.4. When single crystalline CuGaS2 is annealed in saturated sulfur vapor, the resistivity decreases. Next, the current-voltage characteristic of a diode was investigated. When sulfur-annealed CuGaS2 single crystal was used, the series resistance of the diode was low and the forward characteristic could be explained by using series and parallel resistances in the equivalent circuit. On the other hand, the reverse current showed almost an ohmic characteristic at low voltage but it increased nonlinearly with an increase of the reverse voltage. When y ≠ 0, a yellowish-orange or red light emission was observed when the diode was forward-biased. When y = 0, no light emission was observed although the electrical property of the diode was excellent.

The structural properties of epitaxial CuInS2 thin films with various [Cu]/[In] ratios grown on GaAs(001) by the co-evaporation method with three element-sources are reported in comparison to those of the films on GaP(001). For the In-rich and almost stoichiometric films, the CuIn5S8 phase and metastable CuAu-type ordering of the CuInS2 phase were exhibited. The Cu-rich films had sphalerite-type ordering of the CuInS2 phase. The slightly Cu-rich films had the c-axis oriented chalcopyrite-type CuInS2 structure with traces of the CuAu-type ordering and twins. Taking account of the fact that the slightly Cu-rich films on GaP had only the c-axis oriented chalcopyrite-type structure, the traces are considered to be due to the larger lattice mismatch for GaAs than for GaP. Broad photoluminescence emission bands in the band-edge region of the films, which could be related to shallow levels, were observed at low temperature.

The effects of annealing on the chemical composition and other properties of ZnO thin films prepared on glass substrates by chemical bath deposition are described. The atomic ratio [Zn]:[O] of the unannealed film measured by electron probe X-ray microanalysis (EPMA) is 45:55, lower than the ideal ratio 50:50. It gradually increases toward the ideal ratio with increasing annealing temperature up to 500°C. The change in the atomic ratio [Zn]:[O] is interpreted to be due to the content of remaining Zn(OH)2, whose existence in films is determined by the analysis of the O1S peak in X-ray photoelectron spectroscopy (XPS) spectra, that decreases with increasing annealing temperature. The increase in electrical conductivity is observed on annealed films. This is also interpreted to be due to the decrease in the content of remaining Zn(OH)2. Notable changes in crystallinity and optical transmission by annealing are not observed.

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.

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.

Delafossite-type CuYO2 and CuY1-x Ca x O2 (x≤0.05) samples exhibited a green-PL-emission band due to the Cu+ interconfiguration transition from 3d94s1 to 3d10 at room temperature. With increasing Ca concentration, the Cu+ emission shifted slightly to the shorter-wavelength side with a slightly weaker emission intensity. This slight change in Cu+ emission property is considered to be caused by the increase in hole concentration within the Cu+ layer arising from the increase in Ca concentration. On the other hand, oxygen-annealed CuYO2 samples exhibited no emissions. The increase in nonradiative relaxation processes in the oxygen-annealed CuYO2 samples is possibly related to structural changes such as orthorhombic distortion during the oxygen annealing. In the photoluminescence of delafossite-type CuY1-y Tb y O2 (y=0.05) at room temperature, no Cu+ emission band was observed, but sharp emission lines were observed. All the emission lines were assigned to the f-f transitions of Tb3+ (5D4→7F J , J=3–6).

Reflectance anomaly and photoluminescence emission spectra of Ag(Al x Ga1-x )S2 epitaxial layers, which were grown in the range of xx. Because the oscillator energy and the damping constant coincided with the peak energy and the spectral full-width at half maximum of the emission respectively, the emission was identified as free exciton emission, indicating the good optical quality of the layers. Taking account of the lattice strain effect, the composition dependence of the A1-mode Raman scattering frequency of the layers coincided with that of the bulk crystals.

Epitaxial growth of Ag(Al,Ga)S2 layers with the [Al]/([Al]+[Ga]) ratio less than 0.3 was demonstrated on GaAs(100) substrate through multisource evaporation. The oriented domain with the a-axis normal to the substrate disappeared by stoichiometry control of the [Ag]/([Al]+[Ga]) ratio. The 180°-rotated {112} twin crystal regions slightly appeared in the range of [Al]/([Al]+[Ga])>0. Reflectance anomaly spectra due to the free exciton were observed in the layers.

The growth of chalcopyrite Ag(Al x Ga1-x )S2 crystals is demonstrated in the entire alloy composition range using the conventional iodine transport method. The composition of the grown crystals corresponds to that of starting materials. With increasing x, lattice constants decrease, following Vegard's law. The vibration modes for Raman scattering lines are assigned on the basis of the reported AgGaS2 data. The compositional dependence of the A1 mode spectrum is discussed in comparison with that for Cu–III–VI2 compounds. Two bands observed in photoluminescence excitation spectra for a dominantly deep emission at 77 K correspond to band-to-band transitions, which are related to the uppermost valence band and the lower two closed valence bands. The changes in photon energies of these photoluminescence excitation bands indicate an increase in the energy gap from 2.7 eV to approximately 3.6 eV with x. The spectral broadening of the A1 mode line and the two photoluminescence excitation bands around x=0.5 is considered to be due to the clustering effect.

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.

First- and second-order Raman scattering and two-phonon infrared (IR) transmission measurement results for Cu(Al x Ga1- x )S2 single crystals have been examined as a function of x. It is apparent that the highest frequency B2 and E modes at the zone center exhibit two-mode(persistence)-type behavior. The appearance of the two-mode-type behavior in a ternary alloy system is shown to be explainable by theoretical consideration based on a modified model of the type judging criterion. In addition, some of the two-phonon IR absorption bands and second-order Raman peaks are found to be due to phonons located at zone-boundary points. Compositional dependence of the A1(W1) mode Raman line shape and the appearance of a cross-coupled phonon between the two modes indicate that the clustering effect is essentially suppressed in this alloy system.

In the alternate feeding of metal chlorides (CuCl, GaCl3) and H2S sources, the growth rate saturation of CuGaS2 on GaP was observed for substrate temperatures between 530° C and 570° C, and also for some range of H2S flow rate. Together with the growth rate saturation behavior for metal chlorides reported in our previous paper, these results can be considered to indicate achievement of atomic layer epitaxy (ALE) of CuGaS2. The growth orientation change leading to c-axis growth was observed for a large H2S flow rate. Some possibility of stoichiometry control by ALE was suggested from photoluminescence spectra.

Scattering processes and the relevant intermediate state for intense Raman lines appearing in the exciton region spectrum of CuGaS2 under 4765 Å excitation of an Ar+ laser are discussed from measured intensity changes of the lines with temperature as well as photon energy of excitation. Experimental results are consistent with the previous interpretation that the line at 2.504 eV corresponds to a three-phonon Raman scattering in resonance with the upper-branch excitonic polariton (UBP) state. The line at 2.509 eV is thought to correspond to a two-phonon Raman scattering in resonance with the UBP state.

Investigations were made on band-edge-region photoluminescence and the electric resistivity of CuGaS2 epitaxial films on GaAs and GaP substrates, which were prepared by vapor phase epitaxy using metal chlorides and H2S gas. The similar temperature dependence was observed on the resistivity and the intensity of 2.40 eV emission, giving nearly the same activation energy of about 0.05 eV. This value suggests the existence of an acceptor shallower than the acceptor of effective mass approximation. A proposed candidate of this shallow acceptor is an interstitial chlorine atom coming from source materials.

In vapor phase epitaxial growth of CuGaS2 on GaP substrates under simultaneous feeding of H2S and metal chlorides, two-step growth was found to be useful for obtaining thick layers with single orientation of c-axis of the chalcopyrite normal to the substrate. Under alternate feeding of H2S and mixed gas of the chlorides, saturation behavior of growth rate was observed for the feeding amount of copper chloride and that of gallium chloride, and also for the supplying period of H2S gas. These results strongly suggest atomic layer epitaxy of CuGaS2. The layers grown under alternate feeding as well as layers by the two-step process under simultaneous feeding showed strong free exciton emission at low temperatures, indicating high quality of these layers.

The broad emission bands appearing at 6700 Å, 7500 Å and 8600 Å were discussed in terms of preparation processes of epitaxial films and bulk crystals. For the emission at 7500 Å showing spectral shifts during decay, characteristic to donor-acceptor-pair type recombinations, possible relation is pointed out to sulfur vacancies and interstitial halogen (Cl or I) atoms in films or bulk crystals.