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p-type transparent conducting oxide researchDefect engineering in Cr2O3This joint research project with the experimental group of Prof. Igor Shvets considers the suitability of an insulating material Cr2O3 (chromia) as a potential host matrix for a p-type TCO. The geometric and electronic properties of pure, intrinsically defective, and Mg-doped chromia are studied computationally with DFT+U and compared to the results of high resolution x-ray diffraction (HRXRD) and x-ray photoelectron spectroscopy (XPS). While the O-rich/Cr-poor regime favours the formation of p-type defects and the O-poor/Cr-rich conditions promote the formation of n-type carriers, an unusual result in that most TCOs favour one polarity regardless of growth conditions, the formation energy of all intrinsic defects under these growth conditions is too high to result in any significant conductivity. When Mg is incorporated into the material in the O-rich regime, the cause of the improved conductivity observed in Mg-doped Cr2O3 is immediately apparent. The formation energy of Mg on a Cr site is 0.66 eV, considerably lower than that of the Cr vacancy (2.82 eV) or peroxide ion (2.79 eV), and the ionisation level of 0.42 eV for the 0/-1 transition is also shallower than that of the dominant acceptor, the Cr vacancy, in the undoped system (0.57 eV). Additionally, the transition level diagrams show that n-type defects are dominant under O-poor conditions in intrinsically defective chromia, giving rise to the potential of a pn junction in a single host matrix.   Related references:
Copper-based delafossite structured TCOsThe first ever material observed to exhibit both transparency and p-type conductivity was CuAlO2 as reported by the group of Hideo Hosono in 1997. The Al site in this copper-based delafossite structure can be substituted by a wide range of trivalent metal ions, leading to a signifcant number of materials with potential for p-type TCO behaviour. We have investigated the structural and electronic properties of several of these systems, including those containing group 3 ions (Sc and Y) and group 13 ions (B, Ga, In). For each material, electronic density of states, band structure, and effective mass calculations provide insight into the chemical bonding, the valence band composition, and the conductivity. In particular, the nature of the trivalent cation and its interactions with the oxygen ions is highlighted with respect to the trends in conductivity across the series. The role of the Cu-O bonding is also considered, with the electronic structure greatly influenced by the Cu-O bond length, and the bonding and antibonding interactions between Cu 3d and O 2p states contributing significantly to the valence band features. In all cases, it is found the the valence band is not dispersed enough to give rise to sufficient p-type conductivity to fulfil the expectations of a p-type TCO.  Related references:
Acceptor levels in p-type Cu2O: rationalizing theory and experimentCuprous oxide (Cu2O) is a prototypical p-type conducting oxide with applications in photovoltaics, dilute magnetic semiconductors, low cost solar cells, gas sensors and catalysis. Cu2O is also the parent compound of many p-type transparent conducting oxides (TCOs) such as CuMO2 delafossites (M = Al, Cr, B, In, Ga, etc.) and SrCu2O2, which are thought to retain the valence band features and conduction mechanisms of Cu2O. Understanding conduction in Cu2O is therefore vital to the optimization of Cu-based materials for photovoltaics applications. An accurate description of the polaronic nature of p-type defects in Cu2O is not possible using present generalized-gradient- and local-density-approximation or using GGA corrected for onsite Coulomb interactions. Using a screened hybrid-density-functional approach we have investigated the formation of p-type defects in Cu2O, giving rise to single-particle levels that are deep in the band gap, consistent with experimentally observed activated, polaronic conduction. Our calculated transition levels for simple and split copper vacancies explain for the first time the source of the two distinct hole states seen in DLTS experiments.   Related references:
Electronic structure of CuAl1-xCrxO2Recently we have investigated the electronic structure of CuAlO2 and CuCrO2, in collaboration with the group of Prof. Russ Egdell in Oxford. We have used detailed GGA + U analysis of the density of states and band structures, in synergy with high resolution XPS, to elucidate for the first time the cause for the different valence band features seen for both materials. CuCrO2 possesses more states at the top of the VB than CuAlO2, but this feature was found to not be the Cr 3d states. Our results indicate that the occupied Cr 3d states interact covalently with the neighboring O atoms and hence indirectly modify the Cu 3d states, an effect which the 2p6 Al atoms are unable to produce.  Related references:
SrCu2O2 and PbCu2O2While CuAlO2 and SrCu2O2 are the two most high profile p-type materials, they both fail to match the performance of their n-type counterparts. Hole conductivity in oxides is limited by the high electronegativity of oxygen, and introducing Cu decreases the localization of the topmost valence band due to the low binding energy Cu d10 states. The electronic structures of SrCu2O2 and PbCu2O2 have been studied by density functional theory calculations in conjunction with high resolution x-ray photoemission spectroscopy (XPS) and electron paramagnetic resonance spectroscopy (EPR). Our aim was to supplement the properties of Cu with Pb, whose low binding energy s states may serve to further decrease hole localization (and hence aid carrier mobility). Unfortunately this is not the case. Hybridization between Pb s and O 2p is observed, but the Cu d states still dominate the topmost valence band. It is concluded that replacement of Sr with Pb does not facilitate delocalisation of the holes.  Related references:
Email: watsong AT tcd.ie Last updated: Apr 14 2012 Back to Top |
