Donor Levels in ZnO

Abstract

We have investigated and compared the electrical properties of ZnO grown by four different methods. From temperature dependent resistivity and Hall measurements the parameters of the dominant donors are obtained. Deep level transient and thermal admittance spectroscopy have been applied to study deep states in ZnO. The energetic position of deep donors has been identified for ZnO thin films for the first time. We have found the levels E1 (E_d = 110 meV) and E3 (E_d = 290 meV). Semi-insulating ZnO thin films grown by PLD have been demonstrated by three methods: avoiding the incorporation of shallow donors, compensation with acceptors, and co-doping. Further we report a Schottky diode structure enabling the use of ohmic back contacts for ZnO grown on insulating substrates with cut-off frequencies > 10 MHz. Schottky contacts on ZnO thin films are characterized by current voltage measurements for temperatures ranging from 210 K to 293 K and the results are evaluated considering the lateral fluctuations of the barrier height. The Schottky barrier height of Pd/ZnO has been determined to be 1.1 eV with a standard deviation of the lateral fluctuations of 130 meV.