Understanding conductivity anomalies in CuI-based delafossite transparent conducting oxides: Theoretical insights

Abstract

The Cu I -based delafossite structure, Cu I M III O 2 , can accommodate a wide range of rare earth and transition metal cations on the M III site. Substitutional doping of divalent ions for these trivalent metals is known to produce higher p-type conductivity than that occurring in the undoped materials. However, an explanation of the conductivity anomalies observed in these p-type materials, as the trivalent metal is varied, is still lacking. In this article, we examine the electronic structure of Cu I M III O 2 ͑M III =Al,Cr,Sc,Y͒ using density functional theory corrected for on-site Coulomb interactions in strongly correlated systems ͑GGA+ U͒ and discuss the unusual experimental trends. The importance of covalent interactions between the M III cation and oxygen for improving conductivity in the delafossite structure is highlighted, with the covalency trends found to perfectly match the conductivity trends. We also show that calculating the natural band offsets and the effective masses of the valence band maxima is not an ideal method to classify the conduction properties of these ternary materials.

Chemical bonding in copper-based transparent conducting oxides: CuMO2(M = In, Ga, Sc)

“…A plane-wave basis set was used to represent the wavefunctions, with the projected augmented wave (PAW) [56,57] The GGA was applied to the exchange-correlation functional within the Perdew-BurkeErnzerhof (PBE) [58] implementation using corrections for [12,66,21].…”

“…Recently, in a theoretical investigation of CuMO 2 (M = Al, Cr, Y, Sc) it was proposed that the Cr 3d states appear in the valence band of CuCrO 2 and mix with the O 2p, with this increased covalency in the system leading to increased hole mobility [21]. This analysis has been supported by calculations on the p-type chemistry of CuAlO 2 [42] and CuCrO 2 [43] which show that the thermodynamic transition levels for the copper vacancy in CuCrO 2 are much shallower than for CuAlO 2 , which is primarily mediated by the increased covalency of the Cr d states with the Cu and O states in the valence band of CuCrO 2 .…”

“…There have been many discussions in the literature about the influence of the size and electronic structure of the trivalent metal cation on conductivity [19][20][21]. Delafossites and Cu I oxides in general are considered to be polaronic [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36], and as such it is expected that the Cu-Cu distances should play a part in any conductivity, with holes expected to hop from Cu to Cu [36].…”

Comparison of the defective pyrochlore and ilmenite polymorphs of AgSbO3using GGA and hybrid DFT

“…However, as the bottom CB bands for the two structures are seen to be more parabolic in nature, the calculated electron effective masses should have a higher degree of accuracy than the hole effective masses. However, the calculated effective masses will serve as an approximate guide allowing comparisons to be drawn, as has been previously done for CuMO 2 (where M = Al, Sc, Y, Cr, and B), 82 As can be seen, the results show that in terms of the n-type conductivity, both structures have very similar properties. The electron effective masses of the CBM are also smaller than the hole effective masses of the VBM for both structure types, indicating that the n-type ability of the materials will be greater than the p-type ability.…”

“…83 The effective masses at the VBM and CBM were calculated as being 16m e and 0.24m e , respectively, and can be considered as being indicative of poor p-type and good n-type ability. The CBM electron effective masses of both AgSbO 3 82 In comparison, our calculated VBM hole effective masses suggests that the defective pyrochlore may also exhibit reasonable p-type properties.…”