Photoluminescence related to Ca in GaN

Abstract

The ${\mathrm{Ca}}_{\mathrm{Ga}}$ acceptor in GaN was studied using photoluminescence (PL) experiments and first-principles calculations. The experimentally found $\text{--}/0$ transition level of the ${\mathrm{Ca}}_{\mathrm{Ga}}$ at 0.50 \ifmmode\pm\else\textpm\fi{} 0.02 eV above the valence band reasonably agrees with the value calculated using the Heyd-Scuseria-Ernzerhof functional tuned to fulfill the generalized Koopmans condition (0.70 eV). The ${\mathrm{Ca}}_{\mathrm{Ga}}$ acceptor is responsible for the green luminescence band (labeled ${\mathrm{GL}}_{\mathrm{Ca}}$) with a maximum at 2.5 eV. The electron- and hole-capture coefficients for the ${\mathrm{Ca}}_{\mathrm{Ga}}$ acceptor are obtained from PL experiments: ${C}_{n}=9\ifmmode\times\else\texttimes\fi{}{10}^{\text{--}14}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}/\mathrm{s}$ and ${C}_{p}=6\ifmmode\times\else\texttimes\fi{}{10}^{\text{--}7}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}/\mathrm{s}$. The ${\mathrm{GL}}_{\mathrm{Ca}}$ band is an efficient radiative recombination channel in GaN samples implanted with Ca and in unintentionally doped GaN grown by molecular-beam epitaxy at relatively low temperatures, where Ca is a common contaminant.