Effects of impurities on the lattice parameters of GaN

The effects on the lattice parameters due to incorporation of impurities in GaN are evaluated using first-principles pseudopotential-density-functional calculations. Both the size effect, due to the relaxation of host atoms around the impurity, and the deformation-potential effect, due to placing free carriers in the conduction or valence band, are investigated. The incorporation of silicon and beryllium causes contraction of the lattice, while oxygen and magnesium cause expansion. Consequences for experimental observations are discussed; in particular, the presence of impurities in bulk crystals that are used as standards for the lattice parameters of GaN can have a sizable effect on the measured values.