The surface recombination velocity in n-type heteroepitaxial GaN is shown to decrease dramatically when the surface is treated with chemicals commonly used in device processing. The room-temperature photoluminescence (PL) intensity increases by up to a factor of 2.5 when treated with acid solutions such as BOE, HCl, HF, or H2SO4, while H2O2, NaOH, and KOH decrease the intensity by a factor as low as 0.7. To further investigate the surface, we tried (NH4)2Sx and Na2S×9H2O solutions and found that the PL intensity increases by a factor of up to 6-7. These results indicate that the surface recombination velocity has an important effect on the luminescence efficiency of air exposed GaN. Solutions of (NH4)2Sx and isopropanol, methanol, or butanol are explored; the aqueous (NH4)2Sx solution is the most effective treatment to increase the PL intensity. Parameters, such as treatment time and concentration, are explored. A 1:1 solution of ammonium sulfide and isopropanol is seen to be stable for at least a month.
GaN is a wide bandgap semiconductor that can be used for short wavelength LED’s, laser diodes, and photodetectors.1 These devices are needed for optical communication systems or optical displays. GaN also has applications in high power, temperature, and frequency devices. These GaN devices can function under extreme conditions, such as in high radiation environments. The performance of these devices depends critically on the surface and interface properties, but these properties are not well understood in GaN. In particular, it is not known if the PL efficiency of GaN epilayers is significantly affected by nonradiative surface recombination.
In GaAs, the PL efficiency does depend on nonradiative surface recombination. The surface recombination velocity in GaAs was found to decrease when the surface is treated with a sulfur based compound.
This result was first reported by Yablonovitch et al. in 1987. They found that the surface recombination velocity can be reduced by eliminating the oxides that form on the surface when a sample is exposed to air. The oxides create surface states, which pin the Fermi level and decrease device performance. It was latter shown that the sulfur treatment unpins the Fermi level.2 Since that time, many techniques have been developed for the passivation of GaAs.3-5 Similar experiments have also been done on InP6-7 and GaP.8 We report the first successful use of these methods to decrease the surface recombination velocity in GaN.9 We study the surface recombination velocity by measuring the room temperature band edge photoluminescence. We find that the surface defects degrade the photoluminescence properties of GaN by increasing the amount of nonradiative recombination and decreasing the PL intensity. We have shown that the surface recombination velocity can be decreased by treating the surface chemically. This indicates that the surface recombination velocity is significant in GaN. We tried various chemical treatments that increase the PL by a factor of 1.5 to 2.5. We found, though, that sulfide treatments ((NH4)2SX or Na2S×9H2O) can increase the PL by a factor of five to seven. In addition, the PL intensity at the band edge of sulfide treated samples is still three to four times its untreate