Albert Chawanda

A comparative study of the electrical properties of Pd/ZnO Schottky contacts fabricated using electron beam deposition and resistive/thermal evaporation techniques

very low reverse currents of the order of 10 � 10 A at a reverse voltage of 1.0 V whereas the e-beam deposited contacts have reverse currents of the order of 10 � 6 A at 1.0 V. Average ideality factors have been determined as (1.43 60.01) and (1.66 60.02) for the resistively evaporated contacts and e-beam deposited contacts, respectively. The IV barrier heights have been calculated as (0.721 60.002) eV and (0.624 60.005) eV for the resistively evaporated and e-beam deposited contacts, respectively. Conventional DLTS measurements reveal the presence of three prominent defects in both the resistive and e-beam contacts. Two extra peaks with energy levels of 0.60 and 0.81 eV below the conduction band minimum have been observed in the e-beam deposited contacts. These have been explained as contributing to the generation recombination current that dominates at low voltages and high leakage currents. Based on the reverse current at 1.0 V, the degree of rectification, the dominant current transport mechanism and the observed defects, we conclude that the resistive evaporation technique yields better quality Schottky contacts for use in solar cells and ultraviolet detectors compared to the e-beam deposition technique. The 0.60 eV has been identified as possibly related to the unoccupied level for the doubly charged oxygen vacancy, Vo 2þ . V C 2011 American Institute of Physics. [doi:10.1063/1.3658027]

A study of the T2 defect and the emission properties of the E3 deep level in annealed melt grown ZnO single crystals

We report on the space charge spectroscopy studies performed on thermally treated melt-grown single crystal ZnO. The samples were annealed in different ambients at 700 °C and also in oxygen ambient at different temperatures. A shallow donor with a thermal activation enthalpy of 27 meV was observed in the as-received samples by capacitance-temperature, CT scans. After annealing the samples, an increase in the shallow donor concentrations was observed. For the annealed samples, E27 could not be detected and a new shallow donor with a thermal activation enthalpy of 35 meV was detected. For samples annealed above 650 °C, an increase in acceptor concentration was observed which affected the low temperature capacitance. Deep level transient spectroscopy revealed the presence of five deep level defects, E1, E2, E3, E4, and E5 in the as-received samples. Annealing of the samples at 650 °C removes the E4 and E5 deep level defects, while E2 also anneals-out at temperatures above 800 °C. After annealing at 700 °C, t...

Effects of high temperature annealing on single crystal ZnO and ZnO devices

We have systematically investigated the effects of high-temperature annealing on ZnO and ZnO devices using current voltage, deep level transient spectroscopy (DLTS) and Laplace DLTS measurements. Current–voltage measurements reveal the decrease in the quality of devices fabricated on the annealed samples, with the high-temperature annealed samples yielding devices with low barrier heights and high reverse currents. DLTS results indicate the presence of three prominent defects in the as-received samples. Annealing the ZnO samples at 300 °C, 500 °C, and 600 °C in Ar results in an increase in reverse leakage current of the Schottky contacts and an introduction of a new broad peak. After 700 °C annealing, the broad peak is no longer present, but a new defect with an activation enthalpy of 0.18 eV is observed. Further annealing of the samples in oxygen after Ar annealing causes an increase in intensity of the broad peak. High-resolution Laplace DLTS has been successfully employed to resolve the closely spaced ...