The solid solutions of $\gamma {\text{-Si}}_3{\text{N}}_4$ and $\gamma {\text{-Ge}}_3{\text{N}}_4$, $\gamma \text{−}{\left({\text{Ge}}_x{\text{Si}}_{1-x}\right)}_3{\text{N}}_4$ with $x=0.000$, 0.178, 0.347, 0.524, 0.875, and 1.000, are studied. The band gap values of the solid solutions measured with soft x-ray spectroscopy have a range of $3.50–5.00\pm 0.20\text{eV}$. The hardness values of these solid solutions estimated using an empirical relationship have a range of 22.2–36.0 GPa. We use the generalized gradient approximation of Perdew-Ernzerhof-Burke (GGA-PDE) within density functional theory and obtained a calculated band gap value range of 2.20–3.56 eV. The simulated N absorption and emission spectra agree very well with our measurements and the compositional trend among the calculated band gap values corresponds well with the measured values. The agreement between experimental and theoretical spectra indicates that Ge prefers the site with tetrahedral bonding symmetry. The band gap and hardness estimates have two approximately linear regimes, when $0\le x\le 1/3$ and $1/3\le x\le 1$. The band gap decreases as Ge replaces Si on octahedral sites and this suggests that the type of cation in the octahedral sites is mainly responsible for decreasing the band gap in these spinel nitrides. Our results indicate that solid solutions of $\gamma \text{−}{\left({\text{Ge}}_x{\text{Si}}_{1-x}\right)}_3{\text{N}}_4$ provide a class of semiconductors with a tunable wide band gap suitable for UV laser or LED applications.

• Received 18 January 2010

DOI:https://doi.org/10.1103/PhysRevB.81.155207