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 $[\mathrm{Li}{]}^0$ centers (substitutional ${\mathrm{Li}}^{+}$ ions each with a trapped hole) resulting from oxidation was monitored by optical absorption measurements. At low electric fields, dc measurements reveal blocking contacts. At high fields, the $I-V$ characteristic is similar to that of a diode connected in series with the bulk resistance of the sample. Low-voltage ac measurements show that the equivalent circuit for the sample consists of the bulk resistance in series with the junction capacitance connected in parallel with a capacitance, which represents the dielectric constant of the sample. Both dc and ac experiments provide consistent values for the bulk resistance. The electrical conductivity of oxidized MgO:Li crystals increases linearly with the concentration of $[\mathrm{Li}{]}^0$ centers. The conductivity is thermally activated with an activation energy of $(0.70\mathrm{}\pm 0.02)\mathrm{eV},$ which is independent of the $[\mathrm{Li}{]}^0$ content. The standard semiconducting mechanism satisfactorily explains these results. Free holes are the main contribution to band conduction as they are released from the $[\mathrm{Li}{]}^0$-acceptor centers. In as-grown MgO:Li crystals (without $[\mathrm{Li}{]}^0$ centers) the electrical current increases with time as $[\mathrm{Li}{]}^0$ centers are being formed. When ample $[\mathrm{Li}{]}^0$ centers are formed, an activation energy of 0.7 eV was observed. At sufficiently high current, Joule heating thermally destroys the $[\mathrm{Li}{]}^0$ centers.