Masao Kita

Wurtzite CuGaO2: A New Direct and Narrow Band Gap Oxide Semiconductor Applicable as a Solar Cell Absorber

An oxide semiconductor β-CuGaO2 with a wurtzite-derived β-NaFeO2 structure has been synthesized. Structural characterization has been carried out by Rietveld analysis using XRD and SAED, and it was shown that the lattice size is very close to that of zinc oxide. The optical absorption spectrum indicated that the band gap is 1.47 eV, which matches the band gap required to achieve the theoretical maximum conversion efficiency for a single-junction solar cell. The thermoelectromotive force indicated p-type conduction in its intrinsic state. Density functional theory calculations were performed to understand the electronic structure and optical properties of the semiconductor. These calculations indicated that β-CuGaO2 is a direct semiconductor and intense absorption of light occurs near the band edge. These properties render this new material promising as an absorber in solar cells.

Wurtzite-derived ternary I–III–O2 semiconductors

Abstract Ternary zincblende-derived I–III–VI2 chalcogenide and II–IV–V2 pnictide semiconductors have been widely studied and some have been put to practical use. In contrast to the extensive research on these semiconductors, previous studies into ternary I–III–O2 oxide semiconductors with a wurtzite-derived β-NaFeO2 structure are limited. Wurtzite-derived β-LiGaO2 and β-AgGaO2 form alloys with ZnO and the band gap of ZnO can be controlled to include the visible and ultraviolet regions. β-CuGaO2, which has a direct band gap of 1.47 eV, has been proposed for use as a light absorber in thin film solar cells. These ternary oxides may thus allow new applications for oxide semiconductors. However, information about wurtzite-derived ternary I–III–O2 semiconductors is still limited. In this paper we review previous studies on β-LiGaO2, β-AgGaO2 and β-CuGaO2 to determine guiding principles for the development of wurtzite-derived I–III–O2 semiconductors.

Electronic transition responsible for size-dependent photoluminescence of colloidal CuInS2 quantum dots

Colloidal CuInS2 quantum dots with an average size of 2.9–4.1 nm were synthesized by the one-pot heating of an organometallic solution. The size-dependent optical gaps and photoluminescence (PL) emission energies were analyzed based on the finite-depth well effective mass approximation. The PL emission was attributed to the recombination of electrons in the donor level and holes in the quantized hole state. Indium atom defects at copper sites or sulfur vacancies were proposed to be the defects acting as donors at ∼0.1 eV below the conduction band.

Systematic investigation of the growth rate of β-Ga2O3(010) by plasma-assisted molecular beam epitaxy

β-Ga2O3(010) homo-epitaxial growth was performed by plasma-assisted molecular beam epitaxy. Under Ga-rich conditions and for growth temperatures above 650 °C, the growth rate was independent of the Ga/O ratio (>1). A high growth rate of 2.2 nm/min for β-Ga2O3(010) was achieved by optimizing the O flux between 650 and 750 °C. Under Ga-rich conditions between growth temperatures of 500–900 °C, smooth surfaces with rms roughness below 1 nm were realized. We found that the slightly Ga-rich conditions between 650 and 750 °C were optimal for β-Ga2O3(010) growth with a smooth surface and a high growth rate.

Structural and Thermal Properties of Ternary Narrow-Gap Oxide Semiconductor; Wurtzite-Derived β-CuGaO2

The crystal structure of the wurtzite-derived β-CuGaO2 was refined by Rietveld analysis of high-resolution powder diffraction data obtained from synchrotron X-ray radiation. Its structural characteristics are discussed in comparison with the other I-III-VI2 and II-VI oxide semiconductors. The cation and oxygen tetrahedral distortions of the β-CuGaO2 from an ideal wurtzite structure are small. The direct band-gap nature of the β-CuGaO2, unlike β-Ag(Ga,Al)O2, was explained by small cation and oxygen tetrahedral distortions. In terms of the thermal stability, the β-CuGaO2 irreversibly transforms into delafossite α-CuGaO2 at >460 °C in an Ar atmosphere. The transformation enthalpy was approximately -32 kJ mol(-1), from differential scanning calorimetry. This value is close to the transformation enthalpy of CoO from the metastable zincblende form to the stable rock-salt form. The monovalent copper in β-CuGaO2 was oxidized to divalent copper in an oxygen atmosphere and transformed into a mixture of CuGa2O4 spinel and CuO at temperatures >350 °C. These thermal properties indicate that β-CuGaO2 is stable at ≤300 °C in both reducing and oxidizing atmospheres while in its metastable form. Consequently, this material could be of use in optoelectronic devices that do not exceed 300 °C.

Zn2LiGaO4, Wurtzite-Derived Wide Band Gap Oxide

The phase that appeared in the pseudo-binary LiGaO2–ZnO system was investigated by powder X-ray diffraction (XRD), selected area electron diffraction by transmission electron microscopy (TEM-SAD) and Raman spectroscopy, especially focusing on the 0.5(LiGaO2)1/20.5ZnO composition. A new quaternary wurtzite-derived Zn2LiGaO4 phase was found in this system. The TEM-SAD indicated that the phase possesses an incommensurately modulated ordering. The optical energy band gap of Zn2LiGaO4 was determined to be ~4.0 eV from its diffuse reflectance and photoluminescence spectra; and it was suggested that the Zn2LiGaO4 was a direct semiconductor.

Pseudo-binary alloying system of ZnO-AgGaO2 reducing the energy band gap of zinc oxide

Pseudo-binary oxide semiconductor alloy films of (1-x)ZnO-x(AgGaO2)1/2 were fabricated using a conventional rf-magnetron sputtering. The wurtzite-type single phases were obtained in the wide composition range of x ≤ 0.33 because the terminal β-AgGaO2 that corresponds to the composition with x = 1 possesses a wurtzite-derived β-NaFeO2 structure. The energy band gap of ZnO decreased with increasing AgGaO2 concentration, falling to 2.55 eV at x = 0.33. This alloy system enables to use ZnO-based semiconductors in optoelectronic devices working in visible region.

First principles calculations of ternary wurtzite β-CuGaO2

The electronic structure of β-CuGaO2 was studied by first principles calculations and X-ray photoelectron spectroscopy (XPS), and the expected electrical and optical properties of this material were discussed. Density functional theory calculations using the local density approximation with corrections for on-site Coulomb interactions (LDA + U) with U = 5–7 eV reproduced well the experimentally obtained crystal structure and valence-band XPS spectrum. The calculated electronic structure indicates that β-CuGaO2 is a direct band gap semiconductor and its conduction band minimum and valence band maximum consist mainly of highly delocalized Ga 4s and Cu 4s states and relatively localized Cu 3d and O 2p states, respectively. The effective electron mass obtained under parabolic approximation is small (me*/m0 = 0.21), similar to common n-type oxide semiconductors, and the effective hole mass is relatively large (mh*/m0 = 1.7–5.1) although p-type conduction is experimentally observed. The direct and allowed band ...

First-Principles Study of CuGaO2 Polymorphs: Delafossite α-CuGaO2 and Wurtzite β-CuGaO2

The electronic structures of delafossite α-CuGaO2 and wurtzite β-CuGaO2 were calculated based on density functional theory using the local density approximation functional including the Hubbard correction (LDA+U). The differences in the electronic structure and physical properties between the two polymorphs were investigated in terms of their crystal structures. Three major structural features were found to influence the electronic structure. The first feature is the atomic arrangements of cations. In the conduction band of α-CuGaO2 with a layered structure of Cu2O and Ga2O3, Cu and Ga states do not mix well; the lower part of the conduction band mainly consists of Cu 4s and 4p states, and the upper part consists of Ga 4s and 4p states. By contrast, in β-CuGaO2, which is composed of CuO4 and GaO4 tetrahedra, Cu and Ga states are well-mixed. The second feature is the coordination environment of Cu atoms; the breaking of degeneracy of Cu 3d orbitals is determined by the crystal field. Dispersion of the Cu 3d valence band of β-CuGaO2, in which Cu atoms are tetrahedrally coordinated to oxygen atoms, is smaller than those in α-CuGaO2, in which Cu atoms are linearly coordinated to oxygen atoms; this results in a larger absorption coefficient and larger hole effective mass in β-CuGaO2 than in α-CuGaO2. The interatomic distance between Cu atoms-the third feature-also influences the dispersion of the Cu 3d valence band (i.e., the effective hole mass); the effective hole mass decreases with decreasing interatomic distance between Cu atoms in each structure. The results obtained are valuable for understanding the physical properties of oxide semiconductors containing monovalent copper and silver.

Variation of crystal structure and optical properties of wurtzite-type oxide semiconductor alloys of β-Cu(Ga,Al)O2

Band-gap engineering of β-CuGaO2 was demonstrated by the alloying of gallium with aluminum, that is, Cu(Ga1−xAlx)O2. The ternary wurtzite β-NaFeO2-type alloys were obtained in the range 0 ≤ x ≤ 0.7, and γ-LiAlO2-type phase appeared in the range 0.7 ≤ x ≤ 1. The energy band gap of wurtzite β-CuGaO2 was controlled in the range between 1.47 and 2.09 eV. A direct band gap for x < 0.6 and indirect band gap for x ≥ 0.6 were proposed based on the structural distortion in the β-NaFeO2-type phase and density functional theory (DFT) calculation of β-CuAlO2. The DFT calculation also indicated that the γ-LiAlO2-type phases appeared in 0.7 ≤ x ≤ 1 are also indirect-gap semiconductors.