Variation of lattice parameters in GaN with stoichiometry and doping

Lattice parameters were measured for different GaN samples, undoped as well as doped with Zn or some iron-group metals. Very large variations in values of $a$ and $c$ were obtained, the difference between extreme values being as large as 1%. It appears that nitrogen vacancies $V_{\mathrm{N}}$ cause a decrease in lattice parameters of GaN approximately according to $\frac{\Delta a}{a}\approx \frac{\Delta c}{c}\approx -\frac{V_{\mathrm{N}}}{N_{\mathrm{GaN}}}$. An additional increase in lattice parameters at high growth rates is the interpreted as due to self-interstitials in undoped materials. High doping with Zn and some iron-group metals (Fe, Cr, Ni) also causes a large increase of lattice parameters, possibly due to a substantial incorporation of these elements at interstitial sites and at N sites.