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 (substitutional 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 [Li]⁰ centers. The conductivity is thermally activated with an activation energy of (0.70±0.02)eV, which is independent of the [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 [Li]⁰-acceptor centers. In as-grown MgO:Li crystals (without [Li]⁰ centers) the electrical current increases with time as [Li]⁰ centers are being formed. When ample [Li]⁰ centers are formed, an activation energy of 0.7 eV was observed. At sufficiently high current, Joule heating thermally destroys the [Li]⁰ centers.