Electrical Characterization of Bulk and Thin Film Zinc Oxide

Abstract

Zinc oxide (ZnO) has been used for many years in a wide range of products, but not for its semiconducting properties. Recent improvements made in the production of ZnO generated a new interest towards this material. Indeed, the direct wide band gap (3.37 eV at room temperature) and the high exitonic binding energy (60 meV) make ZnO attractive as a transparent conducting oxide (TCO) in photovoltaic and in optoelectronic applications. In addition, the non-toxicity, the Zn availability and abundance, the possibility to grow single crystal bulk material and to deposit thin ZnO film on wide range of substrates at low temperature make this material very appealing. However, in order to reach the full potential of ZnO, several issues must be resolved, such as a full control of the inherent n-type conductivity and the achievement of p-type doping. The work contained in this thesis is focused on the investigation of the electrical properties of hydrothermally grown ZnO and ZnO/Si heterojunctions. In particular, current-voltage (I-V), capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) techniques have been employed to characterize Pd/ZnO Schottky contact and ZnO/Si heterostructure properties. Especially, defect levels in the band gap of ZnO and at the interface of ZnO/Si heterostructures have been investigated. Firstly, electrically active defects in hydrothermally grown ZnO have been investigated by DLTS and thermal admittance spectroscopy (TAS) through Pd Schottky contacts. Defect levels with energy positions of 0.19 (E2), 0.30 (E3) and 0.57 (E4) eV below the conduction band edge (Ec) have been observed. In addition, the different annealing conditions reveal strong influence on several defect levels. For example, E2 and E4 appear as O-rich and Zn-rich defects, respectively. Moreover, improvement made on the Pd Schottky contacts has allowed us to explore deeper region in the band gap of ZnO by using DLTS in the temperature range up to 600K. Thus, two new defect levels have been observed with energy positions of 1.0 (E5) and 1.2 (E6) eV below Ec. These defects are strongly influenced by mechanical polishing and are tentatively assigned to vacancy type defects. Moreover, similar defect levels arise after Zn implantation, which supports the intrinsic nature of E5 and E6. Secondly, ZnO/Si heterostructures prepared by DC magnetron sputtering and atomic layer deposition (ALD) have been investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) have been used to investigate the structure and morphology of the ZnO layers and the interface of the heterostructures. The electronic properties of the ZnO/Si junctions have been studied by I-V, C-V and DLTS measurements. The junctions show rectifying behavior on both n-type and p-type Si and defect levels have been observed close to the interface. Moreover, the barrier heights of ZnO/Si heterostructures prepared by ALD have been investigated by IV and C-V versus temperature. The results give an estimate of the work function of n-type ZnO, ΦZnO = 4.65 eV. Finally, preliminary work has been done to compare ALD and RF magnetron sputtering and the results demonstrate that ALD is a soft deposition technique which induces less interfacial defects than sputtering.