The dielectric functions of Cu(In, Ga)Se2(CIGS)-based polycrystalline layers with different Ga and Cu compositions have been determined by applying spectroscopic ellipsometry (SE) in a wide energy range of 0.7–6.5 eV. To suppress SE analysis errors induced by rough surface and compositional fluctuation, quite thin CIGS layers (<60 nm) with high uniformity toward the growth direction have been characterized using a self-consistent SE analysis method. We find that the optical model used in many previous studies is oversimplified particularly for the roughness/overlayer contribution, and all the artifacts arising from the simplified analysis have been removed almost completely in our approach. The CIGS dielectric functions with the variation of the Ga composition [x = Ga/(In + Ga)] revealed that (i) the whole CIGS dielectric function shifts toward higher energies with x, (ii) the band gap increases linearly with x without the band-gap bowing effect, and (iii) the overall absorption coefficients are significantly smaller than those reported earlier. Furthermore, the reduction of the Cu composition [y = Cu/(In + Ga)] leads to (i) the linear increase in the band-edge transition energy and (ii) the decrease in the absorption coefficient, due to the smaller interaction of the Cu 3d orbitals near the valence band maximum in the Cu-deficient layers. When y > 1, on the other hand, the free-carrier absorption increases drastically due to the formation of a semi-metallic CuxSe phase with a constant band gap in the CIGS component. In this study, by using a standard critical-point line-shape analysis, the critical point energies of the CIGS-based layers with different Ga and Cu compositions have been determined. Based on these results, we will discuss the optical transitions in CIGS-based polycrystalline materials.

We investigated the effects of Cr and Nb doping on the metal-insulator transition (MIT) of single-oriented VO 2 films deposited on Al 2 O 3 substrates. The MIT temperature (T MI ) increased with trivalent Cr doping, whereas it decreased with pentavalent Nb doping. The temperature coefficient of resistance (TCR) and the thermal hysteresis width of the MIT (ΔT MI ) were suppressed by Cr and Nb doping, and Nb doping was about twice as effective as Cr doping for decreasing TCR and ΔT MI . We found that the maximum TCR and ΔT MI for the doped VO 2 films have a correlation with the lattice constant irrespective of doping elements, suggesting that the lattice deformation caused by metal-ion doping is involved in the decreases in TCR and ΔT MI . There is also an apparent correlation between the maximum TCR and ΔT MI for the doped VO 2 films. The present findings suggest that the combination of metal-ion doping with other techniques such as strain control is required to achieve nonhysteretic MIT with a large TCR in VO 2 films.

firms the applicability Li § gel electrolyte for optical devices. In addition, this electrolyte has a very wide electrochemical stability window (i.e., ranging from 0 to about 5 V vs. Li, see Fig. 9), which gives assurance of an unusual 1657 (1990).

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We have investigated the active-layer-thickness dependence of exciton-photon interactions in planar CuCl microcavities with HfO 2 /SiO 2 distributed Bragg reflectors. The active layer thickness was changed from λ/32 to λ/4, while the cavity length was fixed at λ/2. We performed angle-resolved reflectance measurements and clearly detected three cavity-polariton modes, originating from the lower, middle, and upper polariton branches, in a strong-coupling regime of the Z 3 and Z 1,2 excitons and cavity photon. The incidence-angle dependence of the cavity-polariton modes was analyzed using a phenomenological Hamiltonian for the strong coupling. It was found that the interaction energies of the cavity-polariton modes, the so-called vacuum Rabi splitting energies, are systematically controlled from 22(37) to 71(124) meV for the Z 3 (Z 1,2 ) exciton by changing the active layer thickness from λ/32 to λ/4. The active-layer-thickness dependence of the Rabi splitting energy is quantitatively explained by a simple theory for quantum-well microcavities.