First-principles study on the defect properties of Si and ZnO and the transport behavior of hydrogenated graphene

Abstract

Based on first-principles calculations, a number of interesting issues of the dopant stability and diffusion in semiconductors such as Si and ZnO and the transport properties in disordered graphene were discovered. First, we studied the effect of Ge atoms on the diffusion of a B dopant in Si. Due to the extremely small binding energy of a substitutional B and Ge pair, it is ruled out that Ge atom act as a trap for B atom. However, in nonequilibrium condition, which has a self-interstitial, we found that the presence of the Ge atom increases the migration barrier for B diffusion, although the mechanism for B diffusion is not changed. When the Ge atom is located around B diffusion pathway and a number of Ge atoms exist around B atom, the effect of the Ge atom becomes more significant. Thus it can be stated that the Ge atom plays a role in the retardation of B diffusion observed in SiGe alloys. Next, we investigated the diffusion and thermal stability of hydrogen in ZnO. The migration energy of an interstitial hydrogen is around 0.4-0.5 eV, while substitutional hydrogen has a much higher energy barrier for diffusion about 1.7 eV. Thus, interstitial hydrogen is very mobile specie even in the room temperature, but substitutional hydrogen is thermally more stable. Using the calculated energy barriers for hydrogen diffusion, we performed kinetic Monte Carlo simulations to study the annealing effect on the stability of hydrogen. The results show that the thermal stability of substitutional hydrogen is maintained up to 475 $^{\rm o}$C, while interstitial hydrogen diffuses out at low temperature, in good agreement with the annealing experiments. In addition, our calculations suggest that injected hydrogen from air turns into substitutional hydrogen, causing $\It{n}$-type conductivity. The stable position of interstitial hydrogen in ZnO depends on the type of adjacent impurity, either occupying a bond center site or an antibonding site to the impurity atom. As the next ...