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.

Keywords: 72.80.Ey; 78.40.Fy; 81.05.Dz; II–VI semiconductor; I–III–VI2 semiconductor; oxide semiconductor.

Figure 1.

Schematic of (a) β-NaFeO₂ and (b) wurtzite structures.

Figure 2.

Phase variation in the (1 − x)ZnO–x(LiGaO₂)_1/2 pseudo-binary system together with the selected area electron diffraction (SAED) of ZnO, Zn₂LiGaO₄ and β-LiGaO₂. The SAED patterns were recorded for the 〈11〉 zone axis of the hexagonal wurtzite structure for ZnO and Zn₂LiGaO₄ and the 〈01〉 zone axis of orthorhombic β-LiGaO₂. The arrows in the SAED of Zn₂LiGaO₄ indicate superlattice diffractions.

Figure 3.

Band gap variation as a function of x in the (1 − x)ZnO–x(LiGaO₂)_1/2 pseudo-binary system. The red dots and green dots indicate the band gaps determined for the ceramics and the films, respectively. The blue rectangle indicates a two phase region of Zn₂LiGaO₄ and β-LiGaO₂ solid solutions. Although the boundary of the wurtzite-type phase and Zn₂LiGaO₄-type phase is between x = 0.1 and 0.2, all data are connected by one line, because both the phases are based on the wurtzite structure.

Figure 4.

Variation of optical band gap of the (1 − x)ZnO–x(AgGaO₂)_1/2 alloys (red dots and line) as a function of the alloying level, x, together with that reported for the (1 − x)–xCdO alloys (green dots and line) for comparison.

Figure 5.

Schematic illustration of the chemical bond between an oxide ion and a monovalent silver or copper ion in (a) β-AgGaO₂ and (b) β-CuGaO₂.

Figure 6.

Optical absorption spectrum, F(R_d) of β-CuGaO₂ obtained from the diffuse reflection, R_d, using the Kubelka–Munk function. Insets are a picture of powdered β-CuGaO₂ and the theoretical conversion efficiency of a single-junction solar cell as a function of the band gap energy based on the Shockley–Queisser limit using the AM1.5G solar spectrum as the illumination source.

Figure 7.

Electronic band structure of wurtzite-derived β-CuGaO₂ calculated using the sX-LDA functional. (a) The band structure along the symmetry line and (b) the corresponding total and partial density of states.

Figure 8.

Band gap versus pseudo-wurtzite lattice parameter, a₀, for binary and ternary wurtzite-type oxide semiconductors.