S. Ishizuka

Photoluminescence characterization of Zn1−xMgxO epitaxial thin films grown on ZnO by radical source molecular beam epitaxy

The authors report that high-quality Zn1−xMgxO alloys are very brilliant light emitters, even more brilliant than ZnO, particularly in the high-temperature region; both the emission bandwidth and the oscillator strength of the photoluminescence from Zn1−xMgxO alloys increase remarkably with increasing Mg composition ratio x. The authors have revealed that the increase in the oscillator strength is mainly due to the increase in the activation energy required for the nonradiative recombination processes. Therefore, it is suggested that the localization of excitons, because of the compositional fluctuation, takes place in Zn1−xMgxO alloys and that the degree of the localization increases with increasing x.

Photoluminescence characterization of excitonic centers in ZnO epitaxial films

Photoluminescence properties of nominally undoped ZnO thin films grown by radical-source molecular-beam epitaxy have been investigated as a function of (i) sample growth conditions, (ii) excitation laser power density, and (iii) measurement temperatures. Altogether four excitonic emission peaks were observed at photon energy of 3.3646, 3.3606, 3.3572, and 3.3331 eV, which are tentatively denoted as emission peaks A, D, F, and G, respectively. We have classified the defect types responsible for the emission peaks into the following two groups; (i) D and F, whose responsible defect types are suggested to be residual impurities such as aluminum and indium, respectively, and (ii) A and G, whose responsible defect types are suggested to be intrinsic defects such as oxygen vacancies, interstitial zinc, and extended structural defects particularly for G.

Impact of Cu/III ratio on the near-surface defects in polycrystalline CuGaSe2 thin films

Polycrystalline CuGaSe2 thin films grown with various Cu/III(=Cu/Ga) ratios were investigated by positron annihilation spectroscopy (PAS). The line-shape parameter S of the spectra was used to characterize defects in CuGaSe2 films. The S-parameter in positron annihilation spectra increased with decreasing bulk Cu/III ratio in the CuGaSe2 film. Experimental results combined with theoretical calculation show the formation of multiple vacancy-type defect complexes in the near-surface region of the CuGaSe2 film when Cu-content in the film is decreased. These point defects appear to cause the higher S-parameter in PAS measurement.

Impact of Se flux on the defect formation in polycrystalline Cu(In,Ga)Se2 thin films grown by three stage evaporation process

Cu(InxGa1−x)Se2 (CIGS) films, grown under various Se fluxes, have been investigated by the positron annihilation spectroscopy. The line-shape parameter (S) of the positron annihilation spectra was used to characterize the defects in the CIGS films. When Se flux was decreased, the S parameter at the surface and subsurface region of the films increased. This phenomenon was attributed to the increased concentration of the defect complex formed between Se and Cu vacancies. S parameter at the surface region was significantly higher than that in the bulk region of each film. It was explained with the model of compositional inhomogeneities along the depth of the film. Solar cell performance strongly correlated to the S parameter at the surface region of the CIGS films. Sufficient Se flux was found to be effective to reduce the S parameter, thereby, to suppress the defects in the films.

Cu-dependent phase transition in polycrystalline CuGaSe2 thin films grown by three-stage process

The Cu-dependent phase transition in polycrystalline CuGaSe2 thin films has been studied by an electron probe micro-analyzer (EPMA) and the synchrotron x-ray diffraction method. A Cu-deficiency parameter, Z, defined as (1 − Cu/Ga) was used to study the phase transition. Upon increasing the Z-value, the composition of the films on the Cu2Se-Ga2Se3 pseudo binary tie line was found to shift from the stoichiometric CuGaSe2 (1:1:2) (Z = 0) to the Ga-rich composition through the formation of several ordered defect compounds.The structural modification in the Cu-poor CuGaSe2 film has been investigated by the synchrotron x-ray diffraction method. The existence of the Cu-poor surface phase over the near-stoichiometric bulk CuGaSe2 film was confirmed by the fitting of the accelerated voltage dependent EPMA data.The Cu-dependent phase transition in polycrystalline CuGaSe2 thin films has been studied by an electron probe micro-analyzer (EPMA) and the synchrotron x-ray diffraction method. A Cu-deficiency parameter, Z, defined as (1 − Cu/Ga) was used to study the phase transition. Upon increasing the Z-value, the composition of the films on the Cu2Se-Ga2Se3 pseudo binary tie line was found to shift from the stoichiometric CuGaSe2 (1:1:2) (Z = 0) to the Ga-rich composition through the formation of several ordered defect compounds.The structural modification in the Cu-poor CuGaSe2 film has been investigated by the synchrotron x-ray diffraction method. The existence of the Cu-poor surface phase over the near-stoichiometric bulk CuGaSe2 film was confirmed by the fitting of the accelerated voltage dependent EPMA data.

Fabrication of integrated CIGS modules using the in-line three-stage process

We have attempted an in-line three-stage process for the deposition of CIGS (CuInGaSe2) absorber layers. The allowed growth area of CIGS deposition apparatus is 30 × 30 cm2, and the compositional uniformity and distribution of cell performance have been investigated. Large-size grains and band-grading typical for the three-stage process have been observed. The composition ratios and the performance of small area cells (0.5 cm2) have shown good uniformity over the whole deposition area, with an average efficiency of 15.8 % without anti-reflection coating. By improving CIGS deposition process, efficiency of small cells enhanced up to 17.3 %. Minimodules with aperture area of 76.5 cm2 were also fabricated on 10 × 10 cm2 SLG substrates. Preliminary results showed efficiency as high as η = 14.2 % with anti-reflection coating.

Water Vapor Introduction During Cu(In1-xGax)Se2 Thin-Film Deposition and its Effect on Solar Cell Performance

We have developed a novel technique to improve Cu(In1-xGax)Se2 (CIGSe) cell performance by means of water vapor introduction during CIGSe deposition. We have examined thus far the effectiveness of water vapor introduction on CIGSe with particular emphasis on a Ga/(In+Ga) ratio x of around 0.5. In this study, the effects of water vapor introduction on thin film properties and solar cell performance for other x-compositions, specifically CuInSe2 (CISe) and CGSe2 (CGSe) were studied. Variations in the electrical properties observed in CIGSe (x~0.5), that is, increasing hole density and conductivity with water vapor introduction, were also observed in CISe and CGSe. Water vapor introduction affected solar cell performance as well. The observed improvements in cell performance are thought to be related to the annihilation of donor defects arising from Se vacancies by incorporation of oxygen from the water vapor. In addition, the sodium content in the CIGSe layers was found to increase with increasing oxygen content with water vapor introduction. This result suggests that the mechanism behind the improvement is also related with an enhancement of so-called Na effects

Study of Changes of Electronic and Structural Nature of CBD-CDS/CIGS Interface with Ga Concentration

Band alignments and microscopic structures at interfaces between CdS buffer layer grown by chemical bath deposition and Cu(In1-x Gax)Se2 (CIGS) absorbing one by three stage co-evaporation have been studied as a function of Ga substitution ratio x by means of photoemission, inverse photoemission spectroscopy, Kelvin probe force microscopy and in-situ X-ray photoemission spectroscopy (XPS). For the interfaces over the In-rich CIGS, conduction band offset (CBO) was positive, where conduction band minimum of CdS was higher than that of CIGS. The CIGS region adjacent to the interface of these specimens had band gap energy much wider than that of corresponding bulk. In in-situ XPS measurement, as-grown surfaces of the vacuum-transported In-rich CIGS showed a wide band gap feature with a surface composition around Cu:(ln+Ga):Se = 1:3.5:5.5~1:2.5:3.5, which indicates that the CIGS region adjacent to the interface should be modified in the final stage of the growth of CIGS. An increase of x resulted in a decrease of CBO. On the specimen with x = 0.40~0.45, an almost flat conduction band alignment was realized. This decreasing tendency of CBO continued even in the Ga rich region. Further rise of x resulted in an inversion of sign of CBO; for the specimens with x = 0.60 and 0.75, CBO were about -0.2 and -0.3, respectively. Around a center region of the interface of the Ga rich samples, a decrease of Fermi level, which corresponded to a rise of conduction band minimum, was locally observed together with a local rise of oxygen impurities in this region. This phenomenon also enhanced a rise of CBM of CIGS beyond that of CdS. The observed changes of the band alignments are consistent with Ga substitution ratio dependence of performances of the cells fabricated over the studied interfaces

Properties of deep-level defect in Cu(In, Ga)Se 2 thin films

The properties of the deep-level defect which locates at around 0.8 eV above the valence band in Cu(In, Ga)Se2 thin films with different Ga content was investigated by transient photo-capacitance measurements and the time-resolved photoluminescence. It was deduced that the 0.8 eV deep defect plays as a carrier recombination center and influences the efficiency of CIGS solar cells. The influence of the deep defect becomes more significant as the Ga content increases.

Growth and properties of Cu(In, Ga)(S, Se) 2 films

Cu(In, Ga)(S, Se)2 (CIGSSe) films were grown on Mo/soda-lime glass substrates in a three-stage process using a molecular beam epitaxy apparatus equipped with an rf-cracked S-radical beam source. CIGSSe films have been studied for their application in the development of solar cells with wide-bandgap semiconductors. The S/(S+Se) composition ratios in the fabricated films were determined from electron probe microscopy analysis (EPMA), and scanning electron microscopy (SEM) images of the films were obtained. The results showed that the grain sizes in the films decreased with increasing S concentrations. The surfaces of the films with higher S/(S+Se) composition ratios had greater roughness. To determine the compositional depth profile of the films, the acceleration voltage dependencies of EPMA on the S/(S+Se) ratios, and the group-VI atoms (S and Se) were obtained. The fabricated CIGSSe solar cell achieved an efficiency of 15% and had a fill factor of 0.72. However, since the open circuit voltage was lower than the expected value improvement in the band profile is necessary.