High optical quality self-organized GaN nano-columns were successively grown on (111) Si substrates by an rf-plasma assisted molecular beam epitaxy. The intensity of room temperature photoluminescence (PL) of the nano-columns was 500 times stronger than that of typical GaN film grown by MOCVD. Furthermore, room temperature stimulated emission was observed with a very low threshold excitation power density of 350kW/cm². This superior optical property of GaN nano-columns on Si is comparable to that of the optimized GaN nano-columns on (0001) sapphire reported recently [1].
GaN nano-columns with a diameter, density and height of ~100nm, 5~12x10⁹/cm², and ~1.6μm, respectively, were grown on (111) Si substrate without native oxide (sample A), and with native oxide (sample B). After a Ga deposition, active nitrogen was irradiated for 60s to form GaN dots as nucleation layer at 530^oC, then GaN nano-columns were grown at high temperature for 60min. The c-axis of nano-columns was perpendicular to the substrate surface for sample A and randomly tilted to the perpendicular of the substrate surface for B.
Strong PL emission was observed at 363~364nm from both samples for low excitation condition (1.3W/cm²). The peak intensity was 500 times stronger than that of a 3.7μm-thick GaN films grown by MOCVD on a (0001) sapphire with a dislocation density of 3~5x10⁹/cm². For higher excitation condition, stimulated emission was observed at 369nm with a threshold of 350kW/cm² for sample A. While no stimulated emission was observed up to 6MW/cm² for sample B. For reference, the threshold of stimulated emission of the MOCVD-GaN film and optimized GaN nano-columns on (0001) sapphire were 1.6MW/cm² and 200kW/cm², respectively.
These results suggesting that high performance light emitting devices on low-cost conductive substrate can be realized by GaN-based nano-columns on Si substrates.
[1] A. Kikuchi and K. Kishino, Int. Symp. on Blue Laser and LED, B3-1, March 2004.

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1. InGaN/GaN nanocolumn LEDs and selective area growth of GaN nano-crystals by rf-plasma assisted molecular beam epitaxy