| dc.description.abstract | ZnO is a promising material for modern technological applications. Due to its large direct bandgap (3.4 eV) and large exciton binding energy (60 meV), the compound is suggested as a material for optoelectronic devices, such as light emitting diodes, lasers etc. Moreover, the piezoelectric and pyroelectric properties make ZnO a promising candidate for sensors, actuators and energy generators. ZnO is also suggested for spintronics, and in other application areas in science like biotechnology and for medical applications. ZnO is a transparent wide bandgap semiconductor, and the electronic conductivity can be controlled by doping. External defects, however, can have both positive and negative effects on the material properties. Thus, understanding the defect and dopant properties in ZnO is vitally important for developing ZnO-based devices. In this thesis, bulk ZnO has been analysed and effects due to defects/dopants in ZnO are considered. The calculations have been performed by first-principles atomistic modeling within the density functional theory (DFT), employing the local density approximation (LDA) as well as the generalized gradient approximation (GGA) to describe the effective potential in the Kohn-Sham equation. The presence of three elements, hydrogen, lithium and nickel in ZnO is investigated individually, with a primarily focus on Li in ZnO. The vibrational frequencies have been calculated for H and Li, and the results are consistent with earlier reports. However, in this work it is found that the choice of the effective potential affects the vibrational frequencies. Calculations of the frequencies within the GGA show better accuracy relative to the experimental values. Interstitial Li (Lii) is most stable in octahedral position in ZnO. In this work the diffusion barriers of axial diffusion and basal diffusion have been calculated to ∆E_(a,∥) = 0.67 eV and ∆E_(a,⊥) = 0.72 eV, respectively. As a result, Lii shows slightly anisotropic transition rates as the diffusion barriers are directional dependent. The corresponding diffusion coefficients are also carried out. Lii is found to be mobile at room temperature and can easily diffuse in the material at normal conditions. Substitutional Li (LiZn) is suggested as an acceptor in ZnO to establish stable p-type ZnO. However, the presence of donors, like interstitial H (Hi) and interstitial Li, are compensating dopants making the p type behaviour difficult. We find that LiZn can act as a trapping center for Lii diffusion. In this work, the kick-out barrier for a LiZn – Lii pair has been calculated to ∆E_(a,〖Li〗_Zn-〖Li〗_i )≤ 0.37 eV, a significant decrease from the direct diffusion of Lii. This indicates that if a LiZn is present, the Lii will more likely diffuse through a kick-out mechanism than through a direct interstitial diffusion. Dependent on the Li concentration, the kick-out mechanism is expected to be present to large extent at room temperature. The kick-out mechanism where a Lii atom kick out a host Zn atom has been investigated. However, the configuration of an interstitial Zn is unstable and the kick-out mechanism exhibits a large barrier. The kick-out mechanism is found to be reversible at normal conditions. The present study indicates that transport of LiZn needs to be assisted by the presence of interstitial Li atoms or by intrinsic Zn vacancies in the structure. The relaxed structure of the LiZn – Lii pair and the LiZn – Zni pair has been investigated. In both cases, the formation energy of the two-defect pair were lower than the separated single-defects formation energy, implying formation of pairs, with the interstitial atoms located closer to the substitutional Li. The formation energy is calculated to be ∆H_f^rel=- 1.62 eV for the LiZn – Lii pair relative to formation of the two defects separately, which strongly indicates the formation of a Li-pair in the structure. Ni is a magnetic element and hence it provides magnetic properties in ZnO. Vibrational frequencies of substitutional Ni (NiZn) have been studied as well as investigation of Ni-doped ZnO co-doped with LiZn. LiZn is most stable close to the substitutional Ni for the formation of a NiZn - LiZn - O cluster. The vibrational mode of NiZn is found to be modified to higher frequencies by the presence of LiZn. This thesis can lead to a better understanding of the defect properties of Li in ZnO. The slightly anisotropic transition rates indicate that interstitial diffusion is preferred through the axial direction for Lii, and Lii is found to be mobile at room temperature. However, it is suggested that LiZn acts as a trapping center for the Lii and prevent further diffusion as a result of the low kick-out barrier for the LiZn – Lii pair. | eng |