| Along with developing of spintronics, dilute magnetic semiconductors (DMS) have attracted much attention in the past decade because of their compatibility with normal nonmagnetic semiconductor devices and the possibility of manipulating the spin and charge of electron. Recently, the prototype DMS such as Mn-doped InAs and GaAs have a low ferromagnetic Curie temperature (Tc < 172k). Thus, the most difficult challenge of DMS as practical device is how to substantially increase the ferromagnetic Curie temperature. Interestingly, magnetic element (ME)-doped ZnO system was predicted to be promising as DMS. In this thesis, based on the density functional theory, we explore the Cu modulated role in (Ni,Cu)-codoped ZnO systems (Zn(Ni,Cu)O) and investigate magnetic variations of C-doped ZnO (ZnO:C) systems in the presence of defects.We first explore the Cu modulated role in (Ni,Cu)-codoped ZnO systems. It is found that dopant Cu displays long-range modulation for the Ni-Ni ferromagnetic coupling and enhances the stability of ferromagnetism. Furthermore, Cu-induced magnetism is necessary for the enhanced ferromagnetism. We demonstrate that the spin polarized Cu is analogous to a new channel for facilitating the spin percolation between Ni magnetic moments and thereby enhances the ferromagnetism.Secondly, we have systemically studied the variations of the magnetic properties induced by the defects in ZnO:C systems. Our results clearly show that O and Zn vacancy play important roles in modulating the C-induced magnetism. The presence of O vacancy in the vicinity of C atom suppresses the magnetic moment of ZnO:C system, while the presence of Zn vacancy enhances the magnetic moment. This result explains well the experimental observation of the magnetic moment variation from 1.3 to 3.0μB. Our calculations also show that the magnetic properties of ZnO:C can be modulated through Li/Al doping. Specifically, the Curie temperature of ZnO:C can be further raised by implanting Al dopant. |
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