Pair Spectra Involving the Shallow Acceptor Mg in GaP

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Abstract
Type II donor-acceptor pair spectra involving the shallow group IIA acceptor Mg and the group VI donors S and Te have been observed from GaP crystals grown from Ga solution in an open-tube furnace system. Magnesium can be an optically active inadvertent contaminant in high-purity GaP. Crystals containing ∼1017 cm−3 of these desired impurities, added intentionally, exhibited sharp structure due to electron-hole recombinations at discrete donor-acceptor pairs, with negligible contamination from unwanted spectra. The transition energies in these spectra can be closely fitted for shell numbers m≳20 (pair separation ≳17 Å) by an expression which includes only the coulomb monopole term for the final state interaction and a van der Waals term for the initial state interaction, using low-temperature static dielectric constant. This analysis yields the activation energy of the Mg acceptor in GaP, (EA)Mg=53.5±1 meV, substantially different from previous estimates derived from electrical transport and impurity Raman scattering measurements. Trends in the activation energies of the three Ga-site acceptors and three P-site acceptors known in GaP are discussed qualitatively.