Carrier concentration and shallow electron states in In-doped hydrothermally grown ZnO

Single-crystal ZnO has been hydrothermally grown with additional In2O3 in the solution. Schottky barrier contacts have been deposited by electron beam evaporation of Pd onto the (0001̄) face. Capacitance-voltage measurements have been performed to reveal the carrier concentration as a function of the In2O3 content in the solution, and secondary-ion mass spectrometry was used to measure the resulting In concentration in the samples. For an In2O3 content of 2×1019 cm−3, the average free electron concentration increased to 5×1018 cm−3 compared to 4×1017 cm−3 for the non-doped material. An increase of the In2O3 content to 4×1019 cm−3 leads to a measured carrier concentration of approximately 1×1019 cm−3; however, only up to a quarter of the incorporated In became electrically active. From thermal admittance spectroscopy measurements two prominent electronic levels are found, and compared with to the non-doped material case, the freeze-out of the shallow doping in the In-doped samples takes place at lower temperatures (below 80 K).