Fabrication of Self-Assembling AlGaN Quantum Dot on AlGaN Surfaces Using Anti-Surfactant
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lasers. Recently, we have succeeded in the fabrication of self-assembling GaN and InGaN QD on AlGaN surfaces using metal organic chemical vapor deposition (MOCVD) and observed a strong photoluminescence (PL) emission from QD structures [7][8]. We also achieved optical pumping stimulated emission from GaN QDs sandwiched with AlGaN optical confinement layers[9]. The QDs were fabricated using a growth mode change from 2-dimensional step-flow growth to 3dimensional island formation by modifying the surface energy balance of AlGaN with a Si antisurfactant. Contrary to Stranski-Krastanov (SK) growth mode, the QD formation using this method is based on the surface energy balance. Then, this method is especially useful for the QD formation on the interfaces of small lattice mismatch system. In this report, we demonstrate the first fabrication of self-assembling AlGaN QDs on AlGaN surfaces using MOCVD. The formation of AlGaN QDs on AlGaN surfaces is relatively difficult in comparison with the cases of GaN or InGaN QDs, because AlGaN easily forms a film on GaN or AlGaN surfaces due to large surface energy of AlGaN. The use of Si anti-surfactant was not enough to obtain a growth mode control to 3-dimensional nano-scale dot formation in high growth temperature. In addition to the use of anti-surfactant, we reduced the growth temperature of AlGaN QDs in order to control the migration of precursors on AlGaN surface, and as a result, we succeeded in the fabrication of nano-scale AlGaN QDs. EXPERIMENTS AND DISCUSSIONS The structures were grown, at 76 Torr on the Si-face of an on-axis 6H-SiC(0001) substrate, by a conventional horizontal-type MOVPE system. As precursors ammonia (NH3), tetraethylsilane (TESi), trimethylaluminum (TMAl), and trimethylgallium (TMGa) were used with H2 as carrier gas. N2 gas was also independently supplied by a separate line and mixed with the H2 just before the substrate susceptor. Typical gas flows were 2 standard liters per minute (SLM), 2 SLM, and 0.5 SLM for NH3, H2, and N2, respectively. The molar fluxes of TMG and TMA of Al0.38Ga0.62N growth for buffer and capping layers were 38 and 13 µmol/min, respectively. At this condition, the growth rate was approximately 2.5 µm/h. The molar fluxes of TMG and TMA of Al0.05Ga0.95N growth for fabrication of QD structure were 7.2 and 0.47 µmol/min, respectively. The growth rate used for QD formation was approximately 0.4 µm/h. The substrate temperature during the growth was measured with a thermocouple located at the substrate susceptor. The sample structure is shown in Fig.1. In order to achieve a surface suitable for growth of AlGaN QDs, first an approximately 400-nm-thick Al0.38Ga0.62N buffer layer was deposited on a 6H-SiC substrate at 1140°C. The buffer layer was found to provide a step-flow grown surface as confirmed by a
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