Comparative research on the electronic structures and optical properties of Mg-interstitial doped and Mg-substitution doped Al0.25Ga0.75N

Differences of electron structures and optical properties between Mg-interstitial doped and Mg-substitution doped Al0.25Ga0.75N were investigated by First-principles calculations based on density functional theory and the pseudopotential method. The results of these calculations demonstrate that it is more stable for a substitution Mg atom to replace a Ga atom than a interstitial Mg atom located at the center of Al0.25Ga0.75N crystal. Mg substitution doping increases the band gap of Al0.25Ga0.75N and makes crystal exhibit p-type properties. While an interstitial Mg impurity narrows the band gap of Al0.25Ga0.75N and makes crystal become n-type semiconductor. Analysis of E-Mulliken charge distribution indicates that a substitution Mg atom acts as an acceptor impurity, and an interstitial Mg atom acts as a donor impurity. There is surface state for Al0.25Ga0.75N with a Mg atom interstitial doping, which means that crystal possesses metal properties. The comparison results show that Al0.25Ga0.75N with Mg atoms substitution doping has higher absorption coefficient, lower reflectivity and energy loss function than with Mg atoms interstitial doping. This work provides a profound guiding for the ways of Mg doping at Al0.25Ga0.75N.