Studies Of The Defect And Properties Of Electron In Semiconductor Using First-principles

Semiconductor device is a kind of electronic device that specified functions can be realized by the carrier characteristic of electron in semiconductor material. As the properties of semiconductor device are mainly determined by the type and distribution of defect and impurity, the studies of the effects of defects on the carrier characteristic of electron in semiconductor materials are a very importance field. As the development of nanotechnology, however, people find that when the size of the semiconductor device is reduced to as small as nano-size, many macroscopial properties with such a small semiconductor device disappear, and specially the thermal noise of semiconductor device has a very bad effect on the properties of device and consume the energy at the same time. Thus spin feature of electron becomes very important for the improvement of the properties of semiconductor device since spin has very small thermal noise and energy consuming. Consequently, a new subject named semiconductor spintronics is formed. Now the studies about the spin polarization of electrons in semiconductor have attracted a great deal of interest and become a very hot topic. However, people find that the interface between semiconductor layer and magnetic layer and dopant in semiconductor play a key role in the spin injection into semiconductor. Therefore, the studies of the effects of interface and dopant in semiconductor on spin injection into semiconductor is a very important subject in semiconductor spintronics.Our work mainly focus on the studies of the defects and the effects of defects on the carrier and spin of electron in semiconductor. Specifically, the effects of intrinsic defect, surface defect and dopant in CdTe and InN on the capacity and type of conductivity are studied, explaining the phenomena of the improvement of p-type conductivity of CdTe thin film after CdCl2 annealing process and how Ba or Zn dopants introduce hole conductivity in the bulk of InN thin film. Additionally, the effects of the dimer on the Si surface on spin polarization of electron at the Fermi level in Si are also studied. Followings are the main results of our researches:1) Studies of the stable states of intrinsic defects in CdTe. In CdTe thin film, the formation energy of Cd vacancy (Vcd) is very small, and its stable charge state is-2 indicating that Cd vacancy is an acceptor. Vcd is mainly responsible for the p-type conductivity of CdTe thin film. The substitutional defect TeCd also has a small formation energy that can be easily formed in CdTe thin film. However, TeCd is a kind of recombination center that can trap the carriers easily. As the trapped carriers are hard to be ionized and reduce the lifetime of carriers, TeCd is harmful for the CdTe thin film.2) Studies of the stable state of Cl ions in CdTe thin film after CdCl2 annealing process. The stable positions of Cl ions in CdTe thin film are substituting the positions of Te ions (ClTe). However, due to the high mobility of VCd, a complex defect of ClTe-Vcd can be easily formed in CdTe thin film. According to the results of mass-action law, we know that during the cool down process of CdTe thin film, it is not good for the formation of complex defect in the equilibrium condition while it is easy to form the complex defect in the non-equilibrium conduction. The isolated defects could completely form the complex defect when the temperature reduces to 350 K. Our work implies that the p-type conductivity of CdTe thin film can be improved in the non-equilibrium condition during the cool down process.3) Studies of Ba or Zn doping in InN thin film. As the exceeded electrons on the surface of InN thin film can compensate the holes introduced by Ba or Zn ions, the conductivity on the surface is mainly controlled by electrons. As the Ba or Zn ions diffuse to the bulk of InN thin film, the compensation of electron becomes smaller and even disappears when Ba or Zn ions is far away from the surface, then the conductivity of InN thin film is mainly controlled by holes.4) Studies of spin polarization at the Fermi level in Si. We find that increasing the dimer distance between Si atoms is a key factor for introducing spin polarized electrons in Si. In the clean Si substrate, the distance between the Si atoms is too small and there is no spin polarization at the Fermi level. In the LaMnO3/Si heterostructure, however, the attraction between interfacial Mn atoms and Si atoms make Si atoms move to these Mn atoms, resulting in an increase of Si dimer distance. As the spin up state of MnO2 layer reaches the inside of Si substrate, and there is a gap of spin down state of Si substrate, electrons at the Fermi level in Si substrate are completely spinpolaried. Our results might be very useful for semiconductor spintronics devices fabricated by manganite-based heterostructure grown on Si substrate.