Controlling 2d/3d Growth of GaN by Molecular Beam Epitaxy: From Superlattices to Quantum Dots
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ABSTRACT By controlling the Stranski-Krastanov growth mode of strained GaN deposited by molecular beam epitaxy on AIN, it is shown that islands with nanometric dimensions can be elaborated. The luminescence energy of the wetting layer is 4.6 eV, determined by cathodoluminescence experiments. The luminescence of dots with a mean size of 14 nm in diameter and 2nm in height is 3.8 eV and remains constant in intensity with increasing temperature, as an evidence of 0-dimension confinement effect. Theoretical analysis of the wetting layer luminescence energy is carried out, taking into account the non parabolicity in the conduction band. INTRODUCTION Due to the lack of adapted substrates, a major problem faced when growing III-nitrides is strain relaxation which results in defective layers, with high dislocations densities detrimental to optical properties. More generally, realization of high quality III-nitrides heterostructures implies to know critical thickness and to control the strain relaxation mode which depends on the growth mode. As concerns A1N/GaN heterostructures, it has been shown previously that the molecular beam epitaxy of AIN under tensile stress on GaN is of Frank-Van der Merwe type. In this case, relaxation occurs through formation of misfit dislocations [1]. In the reverse case of GaN grown under compressive stress on AIN, a Stranski-Krastanov (SK) growth mode is observed [2]. The SK mode corresponds to bidimensional growth of a few monolayers followed by 3D island formation. In this case, strain relaxation first occurs by elastic relaxation at the free surface of 3D islands and is followed, for further deposition, by plastic relaxation. The 3D GaN islands obtained when stopping the growth just after the 2D/3D transition are small enough to expect quantum effect. This opens the way to devices with quantum dots in the active layer [3]. One aim of the present work was to characterize the structural properties of GaN quantum dots by high resolution electron microscopy (HREM). The optical properties of the dots were also studied by cathodoluminescence, allowing the identification of the contribution assigned to the wetting layer. 205
Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
EXPERIMENT Growth of GaN dots The samples were grown by molecular beam epitaxy (MBE) in a commercial Meca 2000 chamber. Active nitrogen was produced by an EPI rf plasma cell. As usual in MBE, growth was in situ controlled by reflection high energy electron diffraction (RHEED) observation. Before growth, sapphire substrates were exposed to nitrogen flux. During this nitridation step, extra streaks appeared in the RHEED pattern, superimposed to streaks characteristic of sapphire surface. After 60 minutes, the extra streaks alone were visible, as an evidence of conversion of the first layers of sapphire substrate into a material with lattice parameter close to the one of AIN [4]. After completion of the nitridation stage, a low temperature (550°C) AIN buffer, about 30 nm thick, was grown. Then, a thick AIN l
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