p-Type semiconducting properties in lithium-doped MgO single crystals

The phenomenally large enhancement in conductivity observed when Li-doped MgO crystals are oxidized at elevated temperatures was investigated by dc and ac electrical measurements in the temperature interval 250-673 K. The concentration of ([Li]^{0}) centers (Li^{+} ions each with a trapped hole) resulting from oxidation was monitored by optical absorption measurements. Both dc and ac experiments provide consistent values for the bulk resistance. The electricalconductivity of oxidized MgO:Li crystals increases linearly with the concentration of ([Li]^{0}) centers. The conductivity is thermally activated with an activation energy of (0.70 +/- 0.01) eV, which is independent of the ([Li]^{0}) content. The \textit{standard semiconducting} mechanism satisfactorily explains these results. Free holes are the main contribution to band conduction as they are trapped at or released from the ([Li]^{0})-acceptor centers. In as-grown MgO:Li crystals, electrical current increases dramatically with time due to the formation of ([Li]^{0}) centers. The activation energy values between 1.3 and 0.7 eV are likely a combination of the activation energy for the creation of ([Li]^{0}) centers and the activation energy of ionization of these centers. Destruction of ([Li]^{0}) centers can be induced in oxidized crystals by application of an electric field due to Joule heating up to temperatures at which ([Li]^{0}) centers are not stable.Comment: LaTeX, 20 pages, 9 Encapsulated Postscript Format Figures, use the version 4.0 of REVTEX 4 macro packag

Theory of bound polarons in oxide compounds

We present a multilateral theoretical study of bound polarons in oxide compounds MgO and \alpha-Al_2O_3 (corundum). A continuum theory at arbitrary electron-phonon coupling is used for calculation of the energies of thermal dissociation, photoionization (optically induced release of an electron (hole) from the ground self-consistent state), as well as optical absorption to the non-relaxed excited states. Unlike the case of free strong-coupling polarons, where the ratio \kappa of the photoionization energy to the thermal dissociation energy was shown to be always equal to 3, here this ratio depends on the Froehlich coupling constant \alpha and the screened Coulomb interaction strength \beta. Reasonable variation of these two parameters has demonstrated that the magnitude of \kappa remains usually in the narrow interval from 1 to 2.5. This is in agreement with atomistic calculations and experimental data for hole O^- polarons bound to the cation vacancy in MgO. The thermal dissociation energy for the ground self-consistent state and the energy of the optically induced charge transfer process (hops of a hole between O^{2-} ions) have been calculated using the quantum-chemical method INDO. Results obtained within the two approaches for hole O$^-$ polarons bound by the cation vacancies (V^-) in MgO and by the Mg^{2+} impurity (V_{Mg}) in corundum are compared to experimental data and to each other. We discuss a surprising closeness of the results obtained on the basis of independent models and their agreement with experiment.Comment: 13 pages, 2 figures, 2 tables, E-mail addresses: [email protected], [email protected]

OPTICAL ABSORPTION OF V--TYPE CENTERS IN MgO AND CaO : SMALL POLARONS BOUND IN OCTAHEDRAL SYMMETRY

On explique l'absorption des centres V- (antimorphes des centres F+) et des centres affins dans les oxydes d'alcalino-terreux comme un transfert des trous entre sites O2- équivalents près des défauts du type lacune de cation. Le transfert est induit par la lumière et le trou est déplacé contre le potentiel piégeant. La compétition entre l'interaction trou-phonon, qui abaisse la symétrie, et l'interaction de résonance, qui maintient la symétrie cubique, est formulée comme un effet Pseudo- Jahn-Teller. Le modèle explique la largeur, la puissance d'oscillateur, la position et le dédoublement par résonance des bandes optiques. Les paramètres qui expliquent ces caractéristiques reproduisent la position d'un état relaxé et excité et l'interaction de résonance entre les états fondamentaux vibratoires, connus pour MgO : V-. Les transitions entre orbitales non équivalentes aux sites équivalents O2- conduisent à des bandes d'énergie élevée, observées pour tous les centres du type V-. La généralisation du traitement de ces systèmes simples à l'absorption de petits polarons libres sont donnés. La version détaillée de ce travail est publiée dans Zeitschrift für Physik B [l].The optical absorption of V--type centers (F+ antimorphs and related defects) in the alkaline-earth oxides is explained as a light induced transfer of holes between equivalent O2- sites next to cation vacancy type defects, the excitation acting against the self-induced trapping potentials. The interplay between symmetry-breaking hole-phonon- and symmetry-restoring resonance-interaction is formulated as a Pseudo-Jahn-Teller effect. The model explains the width, oscillator strength, position and resonance splitting of the bands. The parameters explaining these features also reproduce the position of a relaxed stable excited state and the resonance interaction between the vibrational ground States, known for MgO : V-. Transitions between nonequivalent orbitals at the equivalent O2- sites lead to bands at higher energies, observed for all V-.-type centers. Generalisations of the treatment of these simple systems to optical absorption of free small polarons are given. The detailed version of the paper is published in Zeitschrift für Physik B [1]

Bulk photovoltaic effect of LiNbO3 Fe and its small polaron based microscopic interpretation

Based on recent experimental evidence on the electronic and optical properties of Fe Li 2 and Nb Nb 4 in LiNbO3 Fe, both strongly determined by their small polaron character, a microscopic model is presented accounting for the main features of the bulk photovoltaic effect BPVE in this material. The relative sizes of the components of the photovoltaic tensor are explained on an atomic basis. Optical small polaron transfer from Fe Li 2 to Nb Nb 5 conduction band states and the subsequent coherent bandlike electron transport, terminated by the formation of Nb Nb 4 free small polarons within about 10 13 s, characterize the first steps of the BPVE. These free polarons, transported by thermally activated incoherent hopping, are then trapped by deeper defects such as Nb Li 5 and Fe Li 3 impurities. The model allows us to explain the strong increase of the ionization probability of Fe Li 2 and the coherent transport length with photon energy. The low mobility of the Nb Nb 4 conduction polarons appears to be the reason for the high open circuit photovoltaic fields attainable in LiNbO3. year 2011 volume 83 issue 16 page 165106 doi 10.1103 PhysRevB.83.16510