We report a theoretical study on the role of shallow d states in the screened-exchange local density approximation (sX-LDA) band structure of binary semiconductor systems. We found that inaccurate pseudo-wavefunctions can lead to 1) an overestimation of the screened-exchange interaction between the localized d states and the delocalized higher energy s and p states and 2) an underestimation of the screened-exchange interaction between the d states. The resulting sX-LDA band structures have substantially smaller band gaps compared with experiments. We correct the pseudo-wavefunctions of d states by including the semicore s and p states of the same shell in the valence states. The correction of pseudo-wavefunctions yields band gaps and d state binding energies in good agreement with experiments and the full potential linearized augmented plane wave sX-LDA calculations. Compared with the quasi-particle GW method, our sX-LDA results shows not only similar quality on the band gaps but also much better d state binding energies. Combined with its capability of ground state structure calculation, the sX-LDA is expected to be a valuable theoretical tool for the II-VI and III-V (especially the III-N) bulk semiconductors and nanostructure studies.

The band gap bowing and the electron localization of GaxIn1-xN are calculated using both the local density approximation (LDA) and screened-exchange local density functional (sX-LDA) methods. The calculated sX-LDA band gaps are in good agreement with the experimentally observed values, with errors of -0.26 and 0.09 eV for bulk GaN and InN, respectively. The LDA band gap errors are 1.33 and 0.81 eV for GaN and InN, in order. In contrast to the gap itself, the band gap bowing parameter is found to be very similar in sX-LDA and LDA. We identify the localization of hole states in GaxIn1-xN alloys along In-N-In chains. The predicted localization is stronger in sX-LDA.

We present a first principle investigation of the electronic structure and the band gap bowing parameter of zinc-blende \AlGaN using both local density approximation and screened-exchange density functional method. The calculated sX-LDA band gaps for GaN and AlN are 95 percent and 90 percent of the experimentally observed values, respectively, while LDA under estimates the gaps to 62 percent and 70 percent. In contrast to the gap itself, the band gap bowing parameter is found to be very similar in sX-LDA and LDA. Because of the difference in the conduction band structure, the direct to indirect band gap crossover is predicted to occur at different Al concentration.

We present a theoretical study of crystal and electronic structures of CaB6 within a screened-exchange local density approximation (sX-LDA). Our ab initio total energy calculations show that CaB6 is a semiconductor with a gap of > 1.2 eV, in agreement with recent experimental observations. We show a very sensitive band gap dependence on the crystal internal parameter, which might partially explain the scatter of previous theoretical results. Our calculation demonstrates that it is essential to study this system simultaneously for both crystal structures and electronic properties, and that the sX-LDA provides an ideal method for this problem.

The coupling of mechanical and optical properties in semiconductor nanostructures can potentially lead to new types of devices. This work describes our theoretical examination of the mechanical properties of CdSe tetrapods under directional forces, such as may be induced by AFM tips. In addition to studying the general behavior of the mechanical properties under modifications of geometry, nanocrystal-substrate interaction, and dimensional scaling, our calculations indicate that mechanical deformations do not lead to large changes in the band-edge state eigenenergies, and have only a weak effect on the oscillator strengths of the lowest energy transitions.

We present a systematic study on the exchange-correlation effects in screened-exchange local density functional method. To investigate the effects of the screened-exchange potential in the band gap correction, we have compared the exchange-correlation potential term in the sX-LDA formalism with the self-energy term in the GW approximation. It is found that the band gap correction of the sX-LDA method primarily comes from the downshift of valence band states, resulting from the enhancement of bonding and the increase of ionization energy. The band gap correction in the GW method, on the contrary, comes in large part from the increase of the conduction band energies. We also studied the effects of the screened-exchange potential in the total energy by investigating the exchange-correlation hole in comparison with quantum Monte Carlo calculations. When the Thomas-Fermi screening is used, the sX-LDA method overestimates (underestimates) the exchange-correlation hole in short (long) range. From the exchange-correlation energy analysis we found that the LDA method yields better absolute total energy than sX-LDA method.