Preparation and Characterization of the Magnetic Semiconductor (GaMn)As
Sammanfattning: The diluted ferromagnetic semiconductor (GaMn)As has been grown by low temperature molecular beam epitaxy and the effects of post growth treatments on the electronic, magnetic and structural properties have been studied. In situ low temperature annealing of (GaMn)As layers has been performed. A clear correlation between annealing temperature and the Fermi level position of the annealed layers is found by photoelectron spectroscopy. The Curie temperature is also seen to depend on the position of the Fermi level as is expected from mean field Zener model according to which it is predicted that TC∝p1/3, where p is the density of of holes in the magnetic layer. After these first annealing experiments in vacuum, much of the efforts have been devoted to an alternative method of heat treatments utilizing an amorphous arsenic capping layer as an effective sink for diffusing Mn interstitial (MnI) atoms. These MnI defects are known to act as double donors and to form antiferromagnetically coupled complexes with the substitutional Mn (MnGa) atoms and thereby causing a deterioration of the ferromagnetic state. Thus, by removing MnI by the present annealing procedure TC and the saturation magnetization are found to be considerably enhanced after only short heat treatments, ~3h, at low temperatures, ~180ºC. This short annealing time is also found to give optimal improvement of the magnetic state of the (GaMn)As layers independently of layer thickness. Several different techniques have been used to characterize the modifications of (GaMn)As layers annealed under As-capping. XRD and Hall effect measurements have given direct evidences for that MnI defects are effectively removed from the (GaMn)As and that this process primarily occurs at a time scale of only a few hours after which the rate of change is considerably slowed down. The surface reaction between the diffusing Mn atoms and arsenic from the capping layer has also been thoroughly investigated by several synchrotron radiation based spectroscopic techniques as well as by microscopy such as AFM and MFM. It is found that the number of available defects, i.e. primarily the (GaMn)As layer thickness, determines the character of the annealed surface. Upon annealing, initially a monolayer of epitaxial MnAs will be formed at the (GaMn)As/As-caping interface as the MnI reach the interface. Further out diffusion of MnI will also occur after the formation of this MnAs layer and will result in self organized quantum dots at the (GaMn)As surface. However, this process is much slower than the initial reaction forming the continuous MnAs layer. Keywords: UPS, XAS, Magnetic Semiconductor, (GaMn)As.
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