Structural and electronic properties ofFe3+andFe2+centers in GaN from optical and EPR experiments

Abstract

This work provides a consistent picture of the structural, optical, and electronic properties of Fe-doped GaN. A set of high-quality GaN crystals doped with Fe at concentrations ranging from $5\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ is systematically investigated by means of electron paramagnetic resonance and various optical techniques. ${\mathrm{Fe}}^{3+}$ is shown to be a stable charge state at concentrations from $1\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The fine structure of its midgap states is successfully established, including an effective-mass-like state consisting of a hole bound to ${\mathrm{Fe}}^{2+}$ with a binding energy of $50\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. A major excitation mechanism of the ${\mathrm{Fe}}^{3+}(^{4}T_{1}\ensuremath{\rightarrow}^{6}A_{1})$ luminescence is identified to be the capture of free holes by ${\mathrm{Fe}}^{2+}$ centers. The holes are generated in a two-step process via the intrinsic defects involved in the yellow luminescence. The ${\mathrm{Fe}}^{3+∕2+}$ charge-transfer level is found $2.863\ifmmode\pm\else\textpm\fi{}0.005\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the valence band, suggesting that the internal reference rule does not hold for the prediction of band offsets of heterojunctions between GaN and other III-V materials. The ${\mathrm{Fe}}^{2+}(^{5}E\ensuremath{\rightarrow}^{5}T_{2})$ transition is observed around $390\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at any studied Fe concentration by means of Fourier transform infrared spectroscopy. Charge-transfer processes and the effective-mass-like state involving both ${\mathrm{Fe}}^{2+}$ states are observed. At Fe concentrations from $1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, additional lines occur in electron paramagnetic resonance and photoluminescence spectra which are attributed to defect complexes involving ${\mathrm{Fe}}^{3+}$. With increasing Fe concentration, the Fermi level is shown to move from near the conduction band to the ${\mathrm{Fe}}^{3+∕2+}$ charge-transfer level, where it stays pinned for concentrations from $1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Contrary to cubic II-VI and III-V materials, both electronic states are effected by only a weak Jahn-Teller interaction.