Band And Optical Properties Engineering Of Low Dimensional Structures Based On GaN And SnS₂

In recent years, the demand for energy is increasing due to the booming of economy, so new energy-saving optical materials have received much attention. In this paper, based on the effective-mass approximation and density functional theory, we investigate the exciton state, electronic structure and optical properties of some low dimensional quantum structures, such as the GaN-based quantum wells(QWs), ZnO/GaN superlattices and Tin dichalcogenides SnX2(X=S,Se) bilayers.First, based on the effective-mass approximation, we have calculated the exciton binding energy, the electron-hole recombination rate and the emission wavelength as functions of the structural parameters in the wurzite(WZ) GaN/In0.2Ga0.8N/InyGa1-yN/GaN symmetric and asymmetric staggered QWs. Numerical results show that the exciton states and the optical properties depend greatly on the structural parameters, such as the well width Lw and the In composition y in the staggered InGaN QWs. For the WZ InGa N symmetrical staggered QWs, when the well width and In concentration increase, the exciton binding energy decreases and then becomes insensitive to any variation of the In concentration y in the WZ InGaN symmetric staggered QWs with y > 0.1 for any well width, while the emission wavelength increases. In particular, our results show that the recombination rate approaches zero when the In content y of the InyGa1-yN well layer is large(y > 0.1) and the total well width Lw is large(Lw > 6 nm) in the WZ In Ga N staggered QWs. However, for the WZ GaN/In0.2Ga0.8N/InyGa1-yN/GaN asymmetric staggered QWs, the exciton binding energy and the electron-hole recombination rate reach a maximum value with increasing In0.2Ga0.8N well layer width. These findings are very important and indicate that, for high-efficiency blue and green LEDs based on the WZ InGaN staggered QWs as the active region, the WZ GaN/In0.2Ga0.8N/InyGa1-yN/GaN symmetric staggered QWs should have an In concentration y < 0.1 and a total well width Lw < 6 nm. We hope that our calculations can stimulate further investigations into the related physics, as well as device applications of group-III nitrides.Second, we have investigated the electronic structures of the polar and nonpolar WZ ZnO/GaN ultrathin superlattices(USLs) by means of first-principles methods. Numerical results show that the band gaps of WZ ZnO/GaN USLs depend highly on growth directions and quantum sizes of the USLs. In the polar case, the built-in electric fields induced by the spontaneous and piezoelectric polarizations strongly decrease the band gaps of the polar USLs. Compared with the nonpolar WZ ZnO/GaN USLs, the band gap values of the polar USLs are smaller due to the built-in electric field effects. In the polar case, the band gaps in polar WZ 1ZnO/nGaN USLs increase a maximum value when the GaN layer is 5 monolayers and then decrease with increasing GaN barrier layer thickness. However, for the nonpolar WZ ZnO/GaN USLs, the band gaps show less dependence on the GaN layer thickness when the ZnO layer is larger than 2 monolayers. The results show that the band gap can be tuned effectively by quantum size in the polar and nonpolar WZ ZnO/GaN USLs, which are interesting to design semiconductor photoelectric devices of high performance.Third, we also have investigated systematically the stabilities, electronic structures and optical properties of different SnX2 stacking bilayers by means of first-principles calculations. Numerical results show that crystal-type SnX2 stacking bilayer AA is the most stable structure with the least equilibrium interlayer distances. In addition, the band gap value can be tuned effectively from 2.083(0.964) to 2.229(1.137) eV for SnS2(SnSe2) bilayer by forming different stacking patterns. Moreover, the band gaps increase firstly and then become insensitive to the variation of the interlayer distances with increasing the interlayer distances for each stacking case. The studies of optical properties also show that the most stable AA stacking bilayer has the smallest static dielectric constants. We expect that these theoretical predications may shed light on the experimental investigations of SnX2 nanosheets.