Characterization of N-doped ZnO layers grown on (0 0 0 1) GaN/Al₂O₃ substrates by molecular beam epitaxy

We investigated the optical properties and electrical properties of N-doped ZnO layers grown on (0 0 0 1) GaN/Al₂O₃ substrates by molecular beam epitaxy, employing 10 K photoluminescence (PL) measurements, current-voltage (IV) measurements, capacitance-voltage (CV) measurements, and 100 K photocapacitance (PHCAP) measurements. 10 K PL spectra showed that excitonic emission is dominant in N-doped ZnO layers grown after O-plasma exposure, while overall PL emission intensity is significantly reduced and deep level emission at around 2.0 ∼ 2.2 eV is dominant in N-doped ZnO layers grown after Zn exposure. IV and CV measurements showed that N-doped ZnO layers grown after Zn exposure have better Schottky diode characteristics than O-plasma exposed samples, and an N-doped ZnO layer grown at 300 °C after Zn exposure has best Schottky diode characteristics. This phenomenon is presumably due to lowered background electron concentration induced by the incorporation of N. PHCAP measurements for the N-doped ZnO layer revealed several midgap trap centers at 1.2 ∼1.8 eV below conduction band minimum.