From defects to alloys: Computational design of non-stoichiometric materials

Non-stoichiometry provides an important route to materials by design. Using an extensive toolset of first principles computations, we predict the evolution of materials properties with changes of the composition. These approaches span the range from isolated point defects, to highly doped materials with dopant-defect interactions, to alloy systems. Highlighted examples include: (a) Doping and defect phase diagram in Ga2O3, an ultra-wide band gap material receiving high current interest. (b) Heterostructural MnO-ZnO alloys, where the composition induced rock-salt to wurtzite transformation enables photo-electrochemical water splitting applications [1]. (c) Disorder and extended anti-site defects in the photovoltaic Cu chalcogenides Cu2SnS3 and Cu2ZnSnS4 [2,3]. [1] H. Peng, P. Ndione, D.S. Ginley, A. Zakutayev, S. Lany, Phys. Rev. X 5, 021016 (2015). [2] P. Zawadzki, A. Zakutayev, S. Lany, Phys. Rev. Appl. 3, 034007 (2015). [3] P. Zawadzki, A. Zakutayev, S. Lany, Phys. Rev. B 92, 201204(R) (2015)

Pathway to oxide photovoltaics via band-structure engineering of SnO

The prospects of scaling current photovoltaic technologies to terawatt levels remain uncertain. All-oxide photovoltaics could open rapidly scalable manufacturing routes, if only oxide materials with suitable electronic and optical properties were developed. A potential candidate material is tin monoxide (SnO), which has exceptional doping and transport properties among oxides, but suffers from a low adsorption coefficient due to its strongly indirect band gap. Here, we address this shortcoming of SnO by band-structure engineering through isovalent but heterostructural alloying with divalent cations (Mg, Ca, Sr, Zn). Using first-principles calculations, we show that suitable band gaps and optical properties close to that of direct-gap semiconductors are achievable in such SnO based alloys. Due to the defect tolerant electronic structure of SnO, the dispersive band-structure features and comparatively small effective masses are preserved in the alloys. Initial Sn1-xZnxO thin films deposited by sputtering exhibit crystal structure and optical properties in accord with the theoretical predictions, which confirms the feasibility of the alloying approach. Thus, the implications of this work are important not only for terawatt scale photovoltaics, but also for other large-scale energy technologies where defect-tolerant semiconductors with high quality electronic properties are required

Manybody GW calculation of the oxygen vacancy in ZnO

Density functional theory (DFT) calculations of defect levels in semiconductors based on approximate functionals are subject to considerable uncertainties, in particular due to inaccurate band gap energies. Testing previous correction methods by many-body GW calculations for the O vacancy in ZnO, we find that: (i) The GW quasi-particle shifts of the VO defect states increase the spitting between occupied and unoccupied states due to self-interaction correction, and do not reflect the conduction versus valence band character. (ii) The GW quasi-particle energies of charged defect states require important corrections for supercell finite size effects. (iii) The GW results are robust with respect to the choice of the underlying DFT or hybrid-DFT functional, and the (2+/0) donor transition lies below mid-gap, close to our previous prediction employing rigid band edge shifts.Comment: revision date 02/08/201

Polaronic hole localization and multiple hole binding of acceptors in oxide wide-gap semiconductors

Acceptor-bound holes in oxides often localize asymmetrically at one out of several equivalent oxygen ligands. Whereas Hartree-Fock (HF) theory overly favors such symmetry-broken polaronic hole-localization in oxides, standard local density (LD) calculations suffer from spurious delocalization among several oxygen sites. These opposite biases originate from the opposite curvatures of the energy as a function of the fractional occupation number n, i.e., d2E/dn2 0 in LD. We recover the correct linear behavior, d2E/dn2 = 0, that removes the (de)localization bias by formulating a generalized Koopmans condition. The correct description of oxygen hole-localization reveals that the cation-site nominal single-acceptors in ZnO, In2O3, and SnO2 can bind multiple holes

Nonstoichiometry and hole doping in NiO

ABSTRACT: We have study by means of DFT+U and thermodynamic calculations the doping response of the p-type transparent oxide NiO. We have found from the calculated defect formation enthalpies that Ni vacancy, not the O interstitial, is the main source of nonstoichiometry in NiO. On the other hand, the calculated free-hole concentration at room temperature of pure NiO remains very low compared to the concentration of Ni vacancies; this is due to the too large ionization energy of the Ni vacancy. The free-hole concentration can be strongly increased by extrinsic dopants with a more shallow donor as it is illustrated for the case of Li