CHARACTERIZATION OF DEEP LEVEL DEFECTS IN N-TYPE ZINC OXIDE LAYERS GROWN BY HYDROTHERMAL TECHNIQUE

Abstract

Zinc oxide (ZnO) is a promising wide-bandgap semiconductor due to its favorable properties for a variety of demanding device applications such as UV light emitters/detectors, high-power and high-temperature devices. The presence of defects in the material can considerably change the electrical properties of the semiconductors. However, recently it has been found that the terminated face of the material significantly alter the characteristics of such devices. The defects in ZnO have been studied in last decades, but no clear consensus has been made. This dissertation investigates the electrical properties of defects in ZnO grown by hydrothermal and molecular beam epitaxy techniques using deep level transient spectroscopy (DLTS). Among the growth techniques available to grow the thin film, the hydrothermal is one of the most cheap and user friendly technique. DLTS provides a sensitive method for identifying defects and for determining their parameters. The main findings are as follow: A. Several circular Schottky contacts (1mm diameter) with Pd metal on the Znface and O-face on n-type ZnO grown by hydrothermal and Ohmic contact of nickel-gold on the backside were deposited by e-beam technique. The asobtained samples were labeled as group A and B samples, respectively. The present literature on n-type ZnO has highlighted a defect, labeled as E3 irrespective of growth technique, which is also studied thoroughly in this research project. The respective summary of each group A and B of samples is explained below:  DLTS has been carried out on the group A samples to study deep level defects. Its result showed two electron trap level E1 having activation energy Ec-0.22 ±0.02 eV and E2 with activation energy Ec-0.49 ±0.05 3 eV. E1 level has time-delayed transformation of shallow donor defects zincinterstitial and vacancyoxygen (Zni-VO) complex. It is observed through X-ray differaction that the preferred direction of ZnO growth is along (10 1 0) plane i.e. VO-Zni complex, assuming that under favourable condition (Zni-VO) complex is transformed into a zinc antisite (ZnO). Consequently, the trap concentration increases with decreasing free carrier concentration. Hence, the ZnO is correlated to E1 level demonstrating the increase in concentration.  Several renowned research groups have revealed different points defects in bulk ZnO like naming oxygen vacancy, zinc interstitial, and/or zinc antisite. These defects having activation energy (free carrier concentration) in the range of 0.32–0.22 eV (1014 -1017 cm-3 ) below conduction band. The results of group A and B samples also showed activation energy (free carrier concentration) as observed by other renowned research groups. This result is due to activation energy of the level while it is not conceivable by with Vincent et al.,[ J. Appl. Phys. 50 (1979) 5484]. They believed that data should be carefully interpreted obtaining by capacitance transient measurement of diodes having carrier concentration greater than 1015 cm -3 . Thus the influence of background free-carrier concentration, ND induced field on the emission rate signatures of an electron point defect in ZnO Schottky devices has been studied by using deep level transient spectroscopy. Many theoretical models were tested on the experimental data to understand the mechanism. Our findings were supported by PooleFrenkel model based on Coulomb potential. It is revealed by 4 investigation that Zn related charged impurities were found to be responsible for electron trap. Results were also tested through qualitative measurements like current-voltage and capacitance-voltage measurements. B. Several Schottky contacts of 1mm diameter with silver were prepared on ZnO grown by molecular beam epitaxy. These samples were labeled as group C samples, DLTS measurements revealed a hole trap exhibiting metastability effect in the emission rates of trap with storage time. We determined that hole trap transfers from one configuration to other with storage time. As a result the activation energy of the acceptor level varied in the range of 0.31 eV to 0.49 eV above the valance band at different measurement time. Impurities cannot be removed in the growth procedure. SIMS results showed the presence of nitrogen. During the growth process nitrogen occupies O site and produces Zn-N complex. But Zn-N bond is not stable because of its large bonding energy and consequently results into metastable nature of the defect. All experimental findings and available literature support the conclusion that the observed hole trap arise from Zn-N complex. C. The ZnO nanorods were grown on glass substrate coated with different metal (Ni, Al, Ag and Au) by aqueous chemical growth. These samples were labeled as D, E, F and G, respectively. The structural properties of ZnO nanorods were investigated by X-Ray diffraction (XRD) and scanning electron microscopy (SEM). The intensity of ZnO (0 0 2) diffraction peak in X-ray diffraction pattern is maximum of sample D because of nucleation of Ni metal coated on substrate. SEM measurements strongly support our observation that thin layer Ni metal increases the growth of nanorods.