Mobility enhancement and yellow luminescence in Si-doped GaN grown by metalorganic chemical vapor deposition technique

We have studied the Si-doping characteristics in GaN on (0 0·1) sapphire substrates. The films were grown by horizontal metalorganic chemical vapor deposition technique at 100 Torr. Prior to Si-doping, achievement of optimum growth conditions was preceded in order to insure doping controllability. For undoped film, background carrier concentration was as low as 4 x 10¹⁷ cm^-3. 10 K-photoluminescence (PL) in undoped film was dominated by the luminescence from two sharp free-excitons and one bound-exciton. n-Type doping with SiH₄ was carried out and electron concentration of up to 2 x 10¹⁹ cm^-3 was achieved. As the electron concentration increased, the activation efficiency of Si donor also increased. In spite of the increased carrier concentration, Hall mobility in the doped films was increased by a factor of up to two (220 cm²/V s) compared to the undoped films. The mobility behavior is attributed to a transport mechanism involving defect band conduction and does not correlate with the line width of the X-ray rocking curve. In addition, as the doping concentration increased, yellow luminescence occurring at around 2.2 eV gradually dominated PL spectra at 10 K and room temperature. Our PL study suggests that gallium vacancy is responsible for the yellow luminescence.