Configuration coordinate energy level diagrams of intervalence and metal-to-metal charge transfer states of dopant pairs in solids

Configuration coordinate diagrams, which are normally used in a qualitative manner for the energy levels of active centers in phosphors, are quantitatively obtained here for intervalence charge transfer (IVCT) states of mixed valence pairs and metal-to-metal charge transfer (MMCT) states of heteronuclear pairs, in solid hosts.

POTENTIAL ENERGY SURFACES OF OXYGEN VACANCIES IN RUTILE TiO2: CONFIGURATION COORDINATE AND MIGRATION BARRIER SCHEMES

Applying the screened hybrid functional Heyd–Scuseria–Ernzerhof (HSE) method, we studied the polaronic degree of freedom of different charged oxygen vacancies V o in rutile TiO 2. The HSE method not only corrects the band gap, but also allows for correct polaron localization. Due to the important role of phonon in oxygen vacancy associated levels in the gap, we calculated configuration coordinate (CC) potential energy surfaces for all charged V o 's. Our calculated CC diagrams with effective impression on host states, show significant improvement of electron–lattice interaction compared to semi(local) DFT methods. The obtained values of stokes shifts for sequential transitions of charged vacancies agree well with experimental evidences which confirm Ti 3+ centers are responsible for photoluminescence. In addition, we explored the effect of polaron localization on diffusive mechanism of V o along most open [001] direction. Calculated values of migration barriers for [Formula: see text] are found to be in quantitative agreement with experimental migration energy [E. Iguchi and K. Yajima, J. Phys. Soc. Jpn.32 (1971) 1415] of 2.4 eV. These results highlight the small polaronic behavior of V o 's and is consistent with studies suggest the polaronic hopping model for electron transport of n-type conductivity in reduced TiO 2 [J.-F. Baumard and F. Gervais, Phys. Rev. B15 (1977) 2316–2323].

Feedback and Inflation Mechanism in Successive Multiphonon Carrier Captures at Deep-level Defects

ABSTRACTPossibility of feedback and inflation mechanism among carrier captures by a deep-level defect and transient induced lattice vibrations is discussed using proper configuration coordinate diagrams for many carriers. Treating the lattice motion classically we selfconsistently simulate the time evolution of the interaction mode and a series of athermal captures of electron(s) and hole(s). When both the activation energies Eacte and Eacth are small, a series of successive athermal captures is enhanced and probable for high carrier densities, however, we find that the possibility of inflation in the amplitude of the lattice vibration critically depends on the minority capture rate and the relative width of the phonon frequency distribution.

Experimental Evidence for Nitrogen as a Deep Acceptor in ZnO

ABSTRACTWhile zinc oxide is a promising material for blue and UV solid-state lighting devices, the lack of p-type doping has prevented ZnO from becoming a dominant material for optoelectronic applications. Over the past decade, numerous reports have claimed that nitrogen is a viable p-type dopant in ZnO. However, recent calculations by Lyons, Janotti, and Van de Walle [Appl. Phys. Lett. 95, 252105 (2009)] suggest that nitrogen is a deep acceptor. In our work, we performed photoluminescence (PL) measurements on bulk, single crystal ZnO grown by chemical vapor transport. Nitrogen doping was achieved by growing in ammonia. In prior work at room temperature, we observed a broad PL band at ∼1.7 eV, with an excitation threshold of ∼2.2 eV, consistent with the calculated configuration-coordinate diagram. In the present work, at liquid-helium temperatures, the PL emission increases in intensity and red-shifts by ∼0.2 eV. A peak is observed at 3.267 eV, which we tentatively attribute to an exciton bound to a nitrogen acceptor. Our experimental results indicate that nitrogen is indeed a deep acceptor and cannot be used to produce p-type ZnO.