One of the mysteries of ZnO materials is defect chemistry and its role in tailoring the material properties. Besides of its wide band gap and large exciton binding energy, ZnO has plenty of intrinsic defects which can strongly influenced the structural, optical, electrical and thermoelectric properties related with this material. Therefore the identification, control and effects of intrinsic defects on ZnO material properties is always very important topic of study among the researchers engaged in ZnO based research.
The modulation, control of intrinsic defects and their role in tailoring the structural, optical, electrical and thermoelectric of ZnO has been investigated comprehensively in this thesis. Three methods have been employed to vary the density of intrinsic defects; By changing the growth conditions of MBE reactor (Zn/O contents ratio from 1.08 to 1.22), annealing in different environments (oxygen, zinc, vacuum and successively annealed in vacuum and zinc), annealing in oxygen at different temperature (500 0C to 1000 0C) and using high pressure (MPa) high temperature condition of MBE grown samples on Si (100) substrate.
XRD, FTIR and PL measurements were performed on these three types of samples to investigate the structural and optical properties. All samples showing strong ZnO (002) plane suggested the c-axis growth of samples. The FWHM of this peak increased with increasing Zn/O contents ratio, while annealing in vacuum, zinc, successively annealed in vacuum-zinc and decreased as the annealing temperature increased from 500 0C to 800 0C. The PL measurements also confirmed the XRD results that crystal quality degrades as the Zn/O contents ratio increased. The detailed investigation on the origin of defect emission from was performed by annealing samples in different annealing environments and related it with transition involving zinc interstitial and oxygen interstitial.
Hall measurements suggested that carrier concentration increased with higher Zn/O ratio, annealing in vacuum, zinc and successively annealed in vacuum and zinc but decreased while annealed in oxygen environment at different temperature. A defect donor complex VO-Zni was identified as source of intrinsic n-type conductivity of ZnO.
In order to study the electrical properties, Au metal contacts were fabricated on all sample using similar evaporation conditions. DLTS study was suggested that all samples have similar kind of donor deep level defects having activation energy 0.53 eV below the conduction band. Therefore another sample with high oxygen ratio was chosen for further DLTS measurements. This sample consists of a deep acceptor level and after detail investigation; we related it with Zn-vacancy related complex. This defect also showed meta-stable behavior with time.
Thermoelectric properties of annealed samples were also performed using home-made Seebeck effect and Hall measurements. The results revealed that Seebeck coefficient, power factor and figure of merit were improved with annealing temperature.
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