Crystal structures and stabilities of AlxGa1-xN and P-Type AlxGa1-xN with different Al compositions: A density functional study

The crystal equilibrium structures, stabilities and electronic structures of AlxGa1-xN and p-type AlxGa1-xN with different Al compositions were investigated by the plane wave pseudo-potential method which is based on density functional theory. The calculation results show that the higher the Al composition in AlxGa1-xN and AlxGa(1-x)N:Mg, the more stable the structure, and the main factor influencing the stability is the type of atom rather than the position of the atom. By analyzing the electronic density of states, charge density difference and Mulliken overlap population, the bonding mechanism of M-N bond (M = Al, Ga or Mg) and their bond strength (Al-N > Ga-N > Mg-N) are obtained. Moreover, it was also found that an increase in the number of Al-N bonds has a "squeezing effect" on the AlxGa1-xN and AlxGa1-xN Mg crystal and causes an increase in Ga-N and Mg-N bond energy. The higher the Al component, the better the stability is due to the increase of the number of Al-N bonds with strong bond energy and the increase of the bond energy of Ga-N and Mg-N bonds due to the "squeezing effect". More importantly, the essential reason was inferred for the difficulty in growing AlGaN single crystal films with high Al content and p-type doping.