Electronic and optical properties of unstrained and strained wurtzite GaN

A semiempirical tight-binding method in the sp³s* model is used to investigate the electronic and optical properties of strained and unstrained wurtzite GaN (α-GaN). The calculated unstrained fundamental gap of α-GaN is 3.45 eV, which is in agreement with experiment. The empirical scaling rule has been used in the strained band-structure calculation, where the strains cover -5-5%. The band gap at the Γ point increases with the absolute value of strains. GaN has an indirect band gap when strains reach 5%. The unstrained and strained density of states and imaginary part of the dielectric function [ε₂(ω)] are studied. There are mainly three peaks at 6.4, 7.5, and 8.4 eV dominating the unstrained ε₂(ω) spectrum, whose two components, ε2xy(ω) and ε2z(ω), are also calculated. Both the shape and energy position of the peaks of the ε₂(ω) successively change with the strains. The real part of the dielectric function, reflectivity, refractive index, and the effects of the strains on them are all researched.