Zinc oxide (ZnO) is a promising material for ultra-violet optoelectronics applications due to its direct band gap and large exciton binding energy. Defect in semiconductor plays an important role in determining the optical and electrical properties. It is thus crucial to understand the defects‟ performance for realizing the device fabrication. Green luminescence (GL) having the peak at 2.4-2.5 eV is a defect related emission band commonly found in the luminescence spectra of many of the ZnO materials. Despite of the effort devoted for several decades, its origin and emission mechanism remain controversial. In this thesis, the origin of the GL emitted from the ZnO films grown by pulsed laser deposition (PLD) is studied using a comprehensive spectroscopic approach, including the Hall effect measurement, photoluminescence (PL), Raman spectroscopy, positron annihilation spectroscopy (PAS), and secondary ion mass spectroscopy (SIMS). ZnO thin films are grown by PLD method with the growth parameters (namely the substrate temperature and oxygen pressure during the growth) systemically varied. Annealing studies in argon atmosphere reveal the correlation between the free electron concentration and the hydrogen concentration in the samples. Two oxygen deficient defect related Raman modes are also identified and they anneal out after annealing at high temperature. We have investigated the introduction the GL systematically grown by different growth parameters, undergone different post-growth annealing treatment, and different methods of growth. Two kinds of GL’s are identified. The first kind of GLs has peak at 2.47 eV without the fine structure, and the other has the peak at 2.45 eV having the fine structure of separation of 0.07 eV. The GL with the fine structure is originated from the surficial region of the ZnO film. The GL without the fine structure is introduced after the annealing 900℃ irrespective of the initial growth conditions. PAS results show a strong correlation between the thermal introductions of a kind of Zn-vacancy and the GL without the fine structure. Moreover, a donor-acceptor-pair (DAP) emission is induced in the low temperature PL spectrum after the same annealing temperature of 900℃. The GL and the DAP emissions are thus associated with the involvement of the VZn. Furthermore by comparing the photon energies of the GL and DAP with the previous first principle calculated results, the GL is ascribed to the conduction band to the (-/2-) acceptor level of VZn, and the DAP involves the (0/-) acceptor level of VZn The presence of the conduction band to the (0/-) level transition is compatible with the results of the photoluminescence excitation (PLE) study. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

Zinc oxide (ZnO), as a Ⅱ-Ⅵ compound semiconductor with a wide direct band gap, has attracted great attention from the worldwide researchers for its potential application in the fields of spintronics and optoelectronics. At present research about the defects in ZnO and ZnO-based materials is still far from complete. The deep level defects in melted grown ZnO single crystal induced by helium ions implantation and electron irradiation, as well as their thermal evolution, were studied in this research using the technique of deep level transient spectroscopy (DLTS) and photoluminescence (PL). DLTS results indicated that, besides E3 (????~0.28 ????) trap which was widely observed in the as-grown ZnO samples, the deep level with ????~0.92 ???? was also indentified in the helium-implanted ZnO samples, which was introduced by the ion implantation and tentatively assigned to be the oxygen vacancy (VO). This deep level was removed after 350 oC annealing in argon gas. Annealing at 350 oC also brought along a new deep level with ????~0.66???? into helium-implanted samples which could be annealed out by 650 oC annealing in argon gas. The electron irradiation induced a deep level with ????~0.59 ???? into ZnO, which was probably associated with the singly charged state of VO. This deep level also tended to be removed at 350 oC annealing in argon gas. The PL spectra revealed that both helium implantation and electron irradiation could improve the bound-exciton peak. Helium implantation also introduced defects emission at 1.90 eV , which was the red luminescence band, into the ZnO single crystal materials. This red luminescence band peak might be associated with DAP recombination. Electron irradiation might restrain the green luminescence in ZnO single crystal. The fine structures could disappear as the measurement temperature increased, leaving the green luminescence band only. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

The fundamental physics and techniques employed in Laplace transform deep level transient spectroscopy (L-DLTS) are reviewed. A Laplace-DLTS system has been constructed. The high resolving power of this system has been demonstrated experimentally. The L-DLTS system was applied to characterize the defects in undoped n-type ZnO thin film grown by the pulsed laser deposition (PLD) method. A 0.3 eV deep trap has been identified. The formations of Ec-0.39eV and Ec-0.20eVcan be enhanced when the sample surface is seriously damaged by high temperature annealing.AnEc-0.25eV trap is identified in the freshly grown samples, but would disappear after the storage of 3 months. Copper doped n-type ZnO thin film samples with low carrier concentration (n~〖10〗^16 〖cm〗^(-3)) were investigated by using both conventional and Laplace DLTS techniques. Positive DLTS signal peaks were detected that are suspected to be contributed by the minority carrier (hole carrier) emission. A physics model involving the inversion layer of a metal-insulator-semiconductor contact has been invoked to interpret the hole carrier concentration existing near the metal-semiconductor interface. Expression for the defect concentration is determined as a function of the temperature of DLTS peaks. AnEv+0.6eV defect with high concentration (N_T~〖10〗^17 〖cm〗^(-3)) was detected. The concentration of Ev+0.6eVcan be enhanced when the annealing temperature was increased from 750 to 900 degree C. / published_or_final_version / Physics / Master / Master of Philosophy

Zinc Oxide (ZnO)is a wide band gap semiconductor which has attracted great attention because of its wide applicability. In order to obtain semiconductor devices with stable and reproducible properties further study of deep level defects is essential. DLTS (Deep level Transient Spectroscopy) is a direct and straightforward techniqueto determine the energy level of the deep level defects. Other information such as activation energy and capture cross section of the defect can also be obtained through this method. In our study ZnO single crystal samples were implanted by oxygen with the energy of 150keV. After the pretreatment of hydrogen peroxide, Schottky contacts were fabricated with Au film deposited by thermal evaporation. Deep level defects were studied by deep level transient spectroscopy (DLTS). Single peak spectra were observed in the as-implanted sample and samples anneal at 350oC, 650oC and 750oC with the corresponding activation energy decreasing with the annealing temperature from ~0.29eV as found in theas-implanted sample. Three peaks were identified in the DLTS spectra of the 900oC sample, with the activation energies of 0.11eV, 0.16eV and 0.37eV respectively.After analysis in detail we found some peaks in the DLTS spectra were the combination of two other peaks, dominated in different temperature range. The thermal evolutions of the deep levels up to the temperature of 1200oC were also investigated. / published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy