Growth and properties of n- and p-type ZnO

We will focus our attention on growth and properties of low-resistivity, n-type ZnO single crystals (μ_e ∼ 230 cm²/V-s, n_e ∼ 1 × 10¹⁷/cm³, at RT), including study and identification of their characteristic shallow residual donors that are thought to control the high electrical conductivity observed in as-grown, n-type ZnO crystals. We will also direct some of our attention to study of the electrical and optical properties of recently demonstrated, nitrogen-doped, homoepitaxial p-type ZnO films grown by molecular beam epitaxy (MBE), either on low-resistivity (ρ) substrates, or on Li-diffused, semi-insulating ZnO substrates. Hall mobilities, carrier concentrations and resistivities of the p-type ZnO films ranged from μ_p = 1-2 cm²/V-s, n_p = 9 × 10¹⁶/cm³ to 1-5 × 10¹⁸/cm³, and ρ ∼ 10-40 ohm-cm, respectively. SIMS profiles have shown that the p-type films, with thickness typically in the range of 1-2 μm, contained large concentrations of N-atoms (as high as 1 × 10¹⁹/cm³ in the films) against a background of N in the substrates ∼ 1 × 10¹⁷/cm³, and temperature-dependent Hall measurements of activation energies have shown that the binding energy (BE) of N-acceptors in ZnO is -146 meV, comparable to the BE of shallow Mg acceptors in GaN (∼ 165 meV); conductivity measurements have shown that at least one of our p-type films is prone to instability and type conversion at cryogenic temperatures and under illumination with room light. MBE growth conditions necessary to achieve p-type ZnO:N films/ZnO are discussed together with the characteristic bandedge photoluminescence (PL) signatures of the p-type layers.