Theoretical Study Of The New Wide Band Gap Semiconductor Photoelectric Materials

In recent years, wide band gap semiconductor materials such as Ga N, Zn O, Si C, Zn Se, diamond and other representatives developed rapidly in the third generation of new semiconductor materials after elemental Si and Ga As as the representative of the first generation, second-generation semiconductor material. As a well-known II-VI semiconductor, Zn O posses a direct band gap of 3.37 e V at room temperature and has a large exciton binding energy of 60 me V. So it’s expected to achieve UV exciton luminescence at room temperature. However, due to the lack of efficient luminescence p-n junction, efficient Zn O electric pump laser have been very slow to realize. This is caused by the fact that it is easy to realize n-type doping but hard to realize p-type doping for Zn O. Over the years, high quality p-type Zn O has been a technical problem in the optoelec-tronic application of Zn O. Moreover, how to improve the luminous efficiency of Zn O bansed photoelectric device has been a popular concern. Two chapters in the main text are mainly dedicated to these two problems, and the following results are obttained:(1) We put forward a new strategy of N doped p-type Zn O, which can suppress the spontaneous compensation from the intrinsic donors. By analyzing the reaction path of nitrogen at the Zn O surface, we proved that the complex defects NZn-VO and NO-VZn are essential for obtaining p-type conductivity in N doped Zn O. The hybrid functional calculation containing a mix of the exact exchange(36%) and the PBE functional proved that the most shallow acceptor level position of NO-VZn locates 0.23 e V above the valence band maximum and the defect transition level ε(0/-1) is 0.16 e V. This theoretical result provides a new possible path to obtained stable and reliable p-type Zn O experimentally.(2) What’s more, our team has been exploring new ideas which can enhance Zn O luminous efficiency including the strategy of graphene-plasmon-enhanced ultraviolet photoluminescence of Zn O. So far, some encouraging results have been obtained in improving the luminous efficiency of Zn O based devices through graphene. However how does the graphene work is unclear, it’s nessceary to further study the enhancing mechanism. In addition, access of high-quality graphene is an important factor in improving the luminous efficiency of Zn O based devices. As a wide-bandgap material, Si C is particularly suitable for the production of high pressure, high temperature, high-power electronic devices due to its saturation drift velocity, the thermal conductivity. Moreover, it can be used as a good substrate because of its higher critical breakdown field, and low leakage current. For example, graphen can be directly grown on Si C substrate and there’s no need to transfer the as-grown graphene to other sbustrate. Therefore, the epitaxial growth of high-quality graphene on Si C substrates has been studied extensively. In this research, how to reduce the growth temperature of graphene on Si C is an open problem to be solves. One chapter in the main text is mainly dedicated to the problems, and the following results are obttained: for the epitaxial growth of graphene on Si C(0001) surface, we propose a surface vacancy assisted Si-C flip mechanism for the first time.We demonstrate that there are several critical stages during the growth of graphene: the formation of surface Si vacancy、Si-C flipping、sublimation process of surface Si atoms、accumulation process of C atoms、eventually delamination and rotation of monolayer graphene. The energy intervals at various growth stages of graphene on Si C are actually different. Based on the identified energy intervals, we propose an optimized energetic-beam enhanced growth method for fabricating graphene with the desired size and patterns at a much lower temperature.(3)As another well-known II-VI semiconductor, Zn Se has similar structure properties with that of Zn O. They have two common structure polytypes, namely zinc-blend and wurtzite. And the most of the studies on doping of Zn Se is also N doping. Under normal conditions the as-grown bulk Zn Se exists in the zinc-blend(ZB) phase. However, from a crystallography point of view, the high-symmetry ZB structure contains many glide planes and glide dislocations which limit the lifetime of Zn Se-based light-emitting devices. While the low-symmetry WZ structure contains only one primary glide plane and the diffusion of glide dislocations in the epitaxial Zn Se layer is suppressed, so the luminescence lifetime of the light emitting devices can be effectively improved. Therefore how to obtain a stable Zn Se WZ structure has been extensively studied. According to this problem, we have studied the lattice dynamics of Zn Se, and the results obtained are as follows. Based on the density-function perturbation theory, we found the phonon vibration modes at Γ can be characteristic to distinguish between ZB-Zn Se and WZ-Zn Se. For ZB-Zn Se, the optical phonon frequency at Γ is linearly dependent on the lattice strain, while the dependence is more complex for WZ-Zn Se. We also found that the introduction of a nitrogen atom to Zn Se lattice may result in three new lattice vibration modes with the frequencies above 500 cm-1, and the splitting of the three new vibration modes in WZ structure is greater than in ZB structure.