Molecular-beam epitaxy growth and structural characterization of semiconductor-ferromagnet heterostructures by grazing incidence x-ray diffraction
Abstract (Summary)The integration of conventional semiconductors with ferromagnetic metals is important for the development of semiconductor spin based devices. The present work is devoted to the growth of the ferromagnetic metal MnAs on the semiconductor GaAs. The MnAs films are deposited on GaAs by molecular-beam epitaxy (MBE). We investigate the evolution of strain, morphology and interfacial structure during growth. In situ grazing incidence diffraction (GID) studies using synchrotron x-rays is carried out in real-time, during the MBE growth. In addition, reflection high-energy electron diffraction (RHEED) azimuthal scans are used to study the nucleation phase. Four stages of the nucleation process during growth of MnAs on GaAs (0 0 1) are revealed by RHEED azimuthal scans. A coherent adsorption of the Mn adatoms during the early stage of growth [? 0.3 monolayer (ML)] is observed. Then, randomly oriented structures with the nearest neighbor distance of MnAs form on the surface with neither the film nor the substrate periodicity. The epitaxial lock-in of the layer takes place out of this disordered phase between 1.1 ML to 2 ML in two steps: first along the [1 1 0] direction, in which there is a strong interaction between film and substrate, then in the orthogonal [1 ¯1 0] direction at a nominal coverage of 2 ML. GID shows that further growth of MnAs films proceed via the formation of relaxed islands at a nominal thickness of 2.5 ML which increase in size and finally coalesce to form a continuous film. Early on, an ordered array of misfit dislocations forms at the interface releasing the misfit strain even before complete coalescence occurs. The films are almost completely relaxed during formation. The relaxation process is thermally activated. A two stage coarsening of islands is seen. A fast increase in the island size is observed until the coalescence at about 20 ML, and a slower increase afterwards. The fascinatingly complex nucleation process of MnAs on GaAs (0 0 1) contains elements of both Volmer–Weber and Stranski– Krastanov growth. A nonuniform strain amounting to 0.66% along the c axis and 0.54% along the a axis is demonstrated from x-ray line profile analysis. A high correlation between the defects is found along the GaAs [1 1 0] direction in comparison to the GaAs [1 ¯1 0] direction. In accordance with this, an extremely periodic array of misfit dislocations with a period of 4.95±0.05 nm is found at the interface, using depth-tunable GID. The periodic array of dislocations release the misfit of 7.5% along the a direction. The inhomogeneous strain due to the periodic dislocations is confined at the interface within a layer of 1.6 nm thickness. The misfit along the c direction is released by the formation of a coincidence site lattice. A basic periodicity of the coincidence site lattice with a non-integer plane match of 4.4 MnAs planes to 6.4 GaAs planes is found at the interface along the c direction. In addition to this, a superperiod of 9 basic units exists at the interface along the c direction. The MnAs layers grow on GaAs (1 1 3)A surface via the formation of three-dimensional islands at 1 ML coverage. The same mismatch of 7.5% along the a direction is released again by periodic misfit dislocations, but with a two times smaller Burgers vector and two times smaller spacing, compared to MnAs/GaAs (0 0 1). The c axis of the hexagonal unit cell is tilted by 4? along the growth direction.
School Location:USA - Ohio
Source Type:Master's Thesis
Date of Publication: