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ZnO is a semiconductor with a bandgap of 3.4eV and has a number of properties that render it suitable for electro-optical applications. Devices such as detectors, lasers and diodes operating in the blue and ultra-violet (UV) spectrum have been demonstrated. As in all semiconductors, defects play an important role in the properties of devices. Although the defects in bulk-grown ZnO have been studied in some detail, much less is known about the defects present in ZnO grown by pulsed-laser deposition (PLD).
High quality Pd/ZnO Schottky diodes were realized on ZnO thin films grown heteroeptaxially on a-plane sapphire substrates by PLD. First, a 50 nm thick n++ ZnO:Al layer was deposited. The main layer (thickness: 1 µm) deposited on top is nominally undoped and was grown at a temperature of 650°C at an oxygen partial pressure of 0.016 mbar. A sample was annealed ex-situ for 2h at 750°C in oxygen (700 mbar) prior to metal deposition. The Schottky contacts were realized by thermally evaporation of Pd. The circular contacts have areas ranging from 4 × 10-4 to 5 × 10-3 cm2. The n++ ZnO:Al layer contacted by sputtered Au serves as ohmic back contact [1].
We have used deep level transient spectroscopy (DLTS) and thermal admittance spectroscopy (TAS) to characterize four shallow defect levels in this ZnO. These defects all have DLTS peaks below 100K. From DLTS measurements and Arrhenius plots we have calculated the energy levels of these defects as 22 meV, 80meV, 110meV and 145meV below the conduction band respectively. The 80meV defect exhibits a strong electric field assisted emission, indicating that it may be a donor. The 110meV defect, on the other hand, displayed a metastable behaviour. Annealing under reverse bias at temperatures of above 130K introduces it while annealing under zero bias above 110K removes it. This introduction and removal processes are reproducible. We have also determined the complete introduction and removal kinetics of this defect.
[1] H. von Wenckstern, G. Biehne, R. A. Rahman, H. Hochmuth, M. Lorenz, and M. Grundmann, Appl. Phys. Lett. 88, 092102 (2006).
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