Defect physics and doping engineering in semiconductor optoelectronic materials

Abstract

Semiconductor materials play a central role in the integrated circuits, photovoltaics, information and communication, microelectronic devices, lighting, and so on. However, the performance of semiconductor optoelectronic devices depends critically on the dopability and defect engineering of semiconductor materials. In this paper, firstly, we introduce the doping properties in semiconductors and the theoretical progress of charged defect calculations over the past decades, including the traditional jellium model. To overcome the disadvantages and limitations of the traditional jellium model, recently, we propose a straightforward and universal theory, i.e., transfer real state model (TRSM), which can calculate directly the charged defect properties in both bulk and low-dimensional semiconductors. In the jellium model for a finite supercell size calculation, it suffers a serious problem to determine the defect properties in low-dimensional semiconductors due to charge distribution in the vacuum region. However, our TRSM method by putting the ionized electrons or holes on a real host band edge states naturally keeps the supercell neutral and provides clear physical meaning. For three-dimensional bulk materials, the defect formation energy and transition energy level calculated by our TRSM method are almost the same as the results obtained by using the traditional jellium model. For low dimensional semiconductors, however, the TRSM method cures the divergence issue that occurred in the jellium model due to long-range electrostatic Coulomb interactions. Secondly, we discuss the wide band gap semiconductors. By analyzing the doping limit law, we elucidate the effective methods for defect engineering in oxide wide band gap semiconductors, and then how their p-type conductivity can be improved. Those methods are based on two rules: (1) Reducing the ionization energy of acceptors; (2) suppressing the formation of compensating donors. Further, the physical mechanism of the difference in conductivity between ionic and covalent compounds in the amorphous wide band gap semiconductors is introduced. A band coupling model is employed to clarify the difference between pseudo-hydrogen passivated and real-hydrogen passivated low dimensional wide band gap semiconductors. Thirdly, we discuss the semiconductor alloys with high doping concentration. In the nonisovalent semiconductor alloys, due to strong wave function localization of the band edge states, the physical properties of nonisovalent alloys do not meet the statistical average law, distinct from the isovalent alloys. The band crossing phenomenon in large mismatched isovalent semiconductor alloys is often misinterpreted by a phenomenological two-level band anti-crossing model. Fortunately, this phenomenon has now been properly explained by the band broadening picture. When doping small concentration of Bi or N impurities into GaAs, the defect levels are localized due to the weak interactions between impurities. The band edges of GaAs1− x Bi x and GaAs1− x N x consist of the host atoms. The impurity level gradually broadens out with the increase of the impurity concentration, and thus the band edges of defective GaAs are dominated by impurities. Finally, we focus on the diffusion of metal impurities in semiconductors. The fundamental reason for the differences in the diffusion of Ag and Cu atoms between ionic and covalent semiconductors is clarified. The s-d coupling between the d orbitals of the diffusors and the s orbitals of the host materials are responsible for the behavior of diffusive atoms. The deep understanding of metal impurities in semiconductor materials offers effective theoretical guidance for controlling the diffusion properties of impurities in different types of semiconductor materials.