CARRIER LOCALIZATION OF AS-GROWN N-TYPE GALLIUM NITRIDE UNDER LARGE HYDROSTATIC-PRESSURE

A quantitative study of the carrier localization in GaN under large hy drostatic pressure is presented using infrared reflection and Raman sp ectroscopy. The free-carrier concentration in as-grown n-type GaN crys tals is determined optically from the phonon-plasmon-coupled mode and an analysis of the dielectric function. A strong decrease from 1x10(19 ) cm(-3) at ambient pressure to only 3x10(17) cm(-3) at a pressure of 27 GPa is observed. This free-carrier reduction is attributed to a str ongly localized donor present at a concentration of 1x10(19) cm(-3) an d it is in agreement with previous qualitative results. From our quant itative data we determine the position of the neutral-donor level to b e 126(-5)(+20) meV below the conduction band at 27 GPa. We present a m odel for the pressure dependence of the localized defect and predict i ts neutral level at 0.40 +/- 0.10 eV above the conduction-band edge at ambient pressure.