A quantitative study of the carrier localization in GaN under large hydrostatic pressure is presented using infrared reflection and Raman spectroscopy. The free-carrier concentration in as-grown n-type GaN crystals is determined optically from the phonon-plasmon--coupled mode and an analysis of the dielectric function. A strong decrease from 1×{}10¹⁹ cm^-3 at ambient pressure to only 3×{}10¹⁷ cm^-3 at a pressure of 27 GPa is observed. This free-carrier reduction is attributed to a strongly localized donor present at a concentration of 1×{}10¹⁹ cm^-3 and it is in agreement with previous qualitative results. From our quantitative data we determine the position of the neutral-donor level to be 126_-5⁺²⁰ meV below the conduction band at 27 GPa. We present a model for the pressure dependence of the localized defect and predict its neutral level at 0.40±{}0.10 eV above the conduction-band edge at ambient pressure. {} 1996 The American Physical Society.
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Wetzel et al. (1996) studied this question.
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