We report on the incorporation of In during growth of InxGa1−xN by molecular beam epitaxy under varying In/Ga flux ratios and with different film thicknesses. The incorporation efficiency studied by energy dispersive x-ray microanalysis, high-resolution x-ray diffraction and photoluminescence spectroscopy is strongly affected by the chosen fluxes of Ga and N and is limited by the excess of nitrogen compared to gallium. Furthermore, thick films exhibit a decrease of the In content in growth direction. The behavior can be explained by considering the different stabilities of the two binary compounds InN and GaN.

Magnesium doping of GaN was found to generate extended defects with a pyramidal shape. Transmission electron micrographs of layers with different doping levels typically showed a defect-free region at the start of doping and a modulation of the defect density in the subsequent film. We developed a rate equation model based on the segregation of Mg to explain the formation process of these defects. The model explains the dependence of the defect-free thickness on the doping level and yields a criterion to avoid the defect formation. Hall measurements show a significant reduction of the free hole concentration for samples grown at doping levels beyond defect formation.

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CdSe/ZnSe quantum structures grown on GaAs͑001͒ by molecular-beam epitaxy were systematically investigated by high-resolution x-ray diffraction and high-resolution transmission-electron microscopy. Half of the initial Cd deposit redesorbs when migration-enhanced epitaxy is used instead of conventional molecular-beam epitaxy for the overgrowth of the CdSe by ZnSe. This result is explained by a segregation model accounting for an enhanced redesorption of Cd due to Cd segregation and replacement of Cd by Zn in the topmost surface layers. The observed intermixing of CdSe/ZnSe can be explained by this model. DOI: 10.1103/PhysRevB.64.193311 PACS number͑s͒: 68.35.Dv, 68.35.Fx, 81.05.Dz The system CdSe/ZnSe is in the center of interest because of its high lattice mismatch of about 7%, which is expected to result in self-assembled quantum dots during epitaxial growth 1-4 and its possible application for optoelectronic devices in the yellow/green/blue spectral range. Recently, a strong intermixing leading to broadened ternary quantum structures was reported by different groups, 5-11 but the origin of this intermixing is still under discussion. From the widely studied system InAs/GaAs, which has a similar lattice mismatch, the importance of surface segregation for intermixing is well known.12 Surface segregation was also observed in the system CdSe/ZnSe ͑Refs. 5 and 10͒ but the rather symmetrical depth profiles of composition found by highresolution transmission-electron microscopy ͑HRTEM͒ are commonly interpreted in terms of interdiffusion.11 However, this results in Cd diffusion constants, which are orders of magnitude higher than those determined by annealing experiments. 9,11 In order to clarify the role of surface segregation, we have varied the method of ZnSe-cap-layer deposition under controlled conditions. Conventional molecularbeam epitaxy ͑MBE͒ and migration-enhanced epitaxy ͑MEE͒ have been used for cap-layer growth. The situation at the growing surface is changed drastically in the latter case due to alternate supply of group-II and -VI elements. The influence of the overgrowth parameters is systematically studied by high-resolution x-ray diffraction ͑HRXRD͒, grazing incidence x-ray diffraction ͑GIXRD͒, and HRTEM.HRXRD as well as HRTEM give information about the incorporated amount of CdSe. Additionally, HRTEM provides knowledge on the depth distribution of Cd, which is hardly accessible by HRXRD due to the small scattering volume of CdSe quantum-dot structures. Structural information on extremely thin layers at the sample surface can be obtained by GIXRD despite the small scattering volume. This enables to investigate the structural properties of CdSe quantum dots before overgrowth by ZnSe.The samples were grown at 280°C on GaAs͑001͒ substrates in a twin-chamber MBE system ͑EPI 930͒ equipped with Zn, Se, and Cd elemental sources for II-VI layer growth. The CdSe layers were deposited by MEE at 0.029 ML per second and are embedded in a 40-50-nm-thick ZnSe buffer layer and a 20-25-nm ZnSe cap layer. The intend...