Copyright
IIT Kharagpur
Abstract
Zinc oxide (ZnO) is a wide band gap (Eg = 3.3 eV at 300 K) semiconductor with large exciton binding energy at room temperature (~ 60 meV). As-grown ZnO is always n-type though p-type doping is indispensable to fabricate opto-electronic devices. This thesis contains studies on p-type doping in ZnO by lithium (Li) and nitrogen (N). After chemical synthesis of doped and as-prepared ZnO, the films were deposited by pulsed laser deposition and characterized by structural, optical and electrical techniques. It was observed from the x-ray diffraction (XRD) and Raman spectroscopy that Li had substituted Zn in ZnO lattice. The change in a- and c-parameter with Li incorporation was found to be due to lattice strain. It was also observed from photoluminescence (PL) measurements that no deep level was formed prohibiting p-type conductivity. The type of conductivity in both Li and N doped ZnO was analyzed by Hall measurements and hot probe, and it was found that those samples were p-type. The effect of ageing on the electrical, optical and structural properties on both as-prepared and doped ZnO was also analyzed. With aging time of two months, N-doped ZnO films were found to be more stable than Li-doped ZnO and the doped films showed high resistive p-type conductivity.
Defects induced due to Li and N in ZnO were analyzed by positron annihilation lifetime spectroscopy. The optimum doping was found to be achieved by 3% of Li and 5% of N to get better results in achieving p-type ZnO. The variation of electrical resistivity with temperature in the range 10 – 300 K was analyzed for both Li and N doped ZnO to study the carrier transport mechanisms in those samples. It was found that the mechanisms were different for different temperature regime. Three types of devices were fabricated to study the (i) room temperature behavior of p-ZnO/p-Si heterojunction, (ii) low temperature studies on p-ZnO/n-Si heterojunction and (iii) p-ZnO/p-Si based moisture sensor. N-doped p-ZnO/n-Si thin film heterojunction showed almost linear variation of resistance with relative humidity (RH) in the range 30–97% with a response and recovery time of 12 s and 36 s, respectively.
Keywords: Zinc oxide, pulsed laser deposition, thin film, p-type conductivity, heterojunction, carrier transport.
