Self-Organized GaN/AlN Superlattice Nanocolumn Crystals Grown by RF-MBE
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E8.39.1
Self-Organized GaN/AlN Superlattice Nanocolumn Crystals Grown by RF-MBE Kouji Yamano, Akihiko Kikuchi and Katsumi Kishino Department of Electrical and Electronics Engineering, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, JAPAN. ABSTRACT GaN nanocolumns including a GaN/AlN superlattice (SL) region were grown by rf-plasma assisted molecular beam epitaxy. The photoluminescence (PL) peak intensity of the GaN/AlN SL nanocolumns was 300~500 times stronger than that of conventional GaN continuous films with a dislocation density of 3~5x109 cm-2 and thickness of 3.75 µm grown by metalorganic chemical vapor deposition (MOCVD). The peak wavelengths of the GaN (10.2 ML)/AlN (15.2 ML) SL and GaN (7.7 ML)/AlN (12.4 ML) SL were 420 and 380 nm, respectively. The theoretically calculated transition wavelength agreed well with experimental values, suggesting that GaN/AlN SL nanocolumns involve a large built-in electrostatic field of about 5.8 MV/cm. The effect of the surface morphology of nanocolumns on the PL intensity was studied using GaN/AlN SL nanocolumns with different surface morphologies but with the same nanocolumn structure. Integrated PL intensity was increased by a factor of 2.2 upon changing the surface morphology from continuous to columnar. Introduction The GaN/AlN heterojunction with a large conduction band offset (~2eV) is attractive for various quantum effect devices such as ultrafast intersubband transition (ISBT) modulators at optical communication wavelengths, 1.55-µm-wavelength quantum cascade lasers, and resonant tunneling diodes. However, due to large mismatches in the lattice parameters and thermal expansion coefficients between the sapphire substrate, GaN and AlN, this GaN/AlN heterosystem tends to a have high density of threading dislocations and cracks. To avoid these problems, we have proposed the use of nanocolumns [1, 2] for a large-lattice-mismatch system. Because the nanocolumns are almost dislocation free and have a large aspect ratio with a small diameter, the nanocolumns can prevent threading dislocation, interface distortion and cracks in a large strain system. In 1966, we reported the self-organized growth of GaN nanocolumns by rf-plasma-assisted molecular beam epitaxy (RF-MBE) on (0001) sapphire substrates [1, 2]. Subsequently, Garcia and coworkers reported the growth of GaN nanocolumns on (111) Si by RF-MBE and characterized their optical properties [3, 4]. Mamutin et al. revealed that GaN nanocolumns with a diameter of less than 50 nm are almost dislocation free, by transmission electron microscope (TEM) observation [5]. Other reports on the growth of nitride nanocolumns (nanorods, nanowires and nanopillars) by different growth methods, such as electron-cyclotron resonance plasma-excited (ECR-) MBE [6] and hydride vapor phase epitaxy (HVPE) [7] have also been presented. Recently, we reported optically pumped stimulated emissions with the very low threshold optical power density of 198 kW/cm2 for GaN nanocolumns grown on an (0001) sapphire substrate [8] and 300 kW/cm2
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