On the band gap of CuAlO2 delafossite

Abstract

In this letter, we discuss the electronic structure of copper aluminate ͑CuAlO 2 ͒ on the basis of absorption measurements at low temperature and under high pressure in single crystals and thin films, combined with ab initio electronic structure calculations. The indirect character of the fundamental transition could be clearly established through the photon energy dependence of the absorption edge as measured in single crystals, yielding a band gap ͑plus a phonon͒ of 2.99± 0.01 eV at room temperature. An indirect excitonic structure is also observed at low temperature. The lowest energy direct allowed transition occurs near 3.53± 0.01 eV and is identified through its low-energy tail in single crystals and its excitonic peak in thin films. Comparing our results with ab initio calculations available in the literature, we conclude that CuGaO 2 and CuInO 2 can hardly be considered as wide gap semiconductors, even if they are transparent in thin films.

Electronic structure of CuAlO2 and CuScO2 delafossites under pressure

“…Band structure calculations predict a negative pressure coefficient for the CuScO 2 direct transition of -4.9 meV/GPa, which agrees fairly well with the experimental value. A similar value for the theoretical pressure coefficient of the direct gap is also obtained for the direct transition in CuAlO 2 (-6 and -4 meV/GPa at the L-and P-points respectively [7]), in contrast to the small, but positive experimental value of its absorption tail. This discrepancy can be explained by taking into account the resonant nature of the direct exciton in this compound.…”

“…X-ray diffraction shows that the volume of the 3R unit cell of CuAlO 2 is 22% smaller than that of CuScO 2 [5,6], which is consistent with the size of the trivalent cation. It has been shown that CuAlO 2 has an indirect gap (plus a phonon) of 2.99 ± 0.01 eV at room temperature and pressure, with the lowest energy direct transition occurring at 3.53 ± 0.01 eV [7]. These experimental results are supported by several band structure calculations [7,8].…”

“…It has been shown that CuAlO 2 has an indirect gap (plus a phonon) of 2.99 ± 0.01 eV at room temperature and pressure, with the lowest energy direct transition occurring at 3.53 ± 0.01 eV [7]. These experimental results are supported by several band structure calculations [7,8]. This is in contrast to CuScO 2 , which is a direct semiconductor with a band gap of 4.24 ± 0.05 eV [9], found by a direct analysis of the excitonic absorption edge.…”

Lattice dynamics of CuAlO2 under high pressure from ab initio calculations

“…A considerable amount of work was developed in the last years in the study of CuAlO 2 , mostly focused in the growth and the optical and electrical characterization (see reference [4]), but many basic properties of CuAlO 2 remain unknown, for example very recently has been proved experimentally that the fundamental gap is indirect [3].…”

10.3726/978-3-653-04937-4/14

“…Also Pellicer Porres et al [393] questioned the interpretation of the low energy peaks as indirect transitions, as the absorption coefficient is more than two orders of magnitude larger than in typical indirect absorption edges. The most promising defects that could be responsible are oxygen interstitials O i , as DFT calculations within the LDA predict low formation energies and the introduction of states in the gap at 0.7 and 1.4 eV [409].…”