Detailed photoemission studies of the prototype diluted magnetic semiconductor (Ga,Mn)As
Doctoral thesis, 2012
Magnetic semiconductors are materials that combine the key features needed in information technology, namely magnetism and controllable charge transport. The prospects of integrating these properties and of using the spin as informa-tion carrier have motivated large efforts to produce such materials for imple-mentation in future spin-based electronics. This has turned out to be a very difficult task, and despite all endeavor, there is still no material produced that could be used in common devices. One of the main hurdles is the low tempera-ture that is needed for the ferromagnetic state: (Ga,Mn)As, the material with the highest transition temperature to the ferromagnetic state, still requires cooling to around 200 K. As the basic principles about the mechanisms underlying ferromagnetism in (Ga,Mn)As are still under debate, there is an obvious need for detailed experimental information that can provide important clues towards an improved understanding. This is the goal of the work in the present thesis.
(Ga,Mn)As is prepared by low temperature molecular beam epitaxy (MBE), which is a means of obtaining a material with a composition far outside the thermodynamic equilibrium. Although the solubility of Mn in GaAs is below 0.1%, it is possible to grow layers with concentrations up to 20%. However, such layers are thermo-dynamically unstable, and phase segregation occurs rapidly under heating above 300 °C. At the same time it is established that controlled heating at lower tempe-ratures has very positive effects on the magnetic properties, based on removal of Mn atoms in defect crystallographic positions. The heating is thus a delicate process that requires deeper understanding. One aspect in this context is to clarify why the diffusion of Mn does not take place across the boundary between (Ga,Mn)As and GaAs. This question was addressed in the present work. By studying diffusion through GaAs overlayer films, it was demonstrated that the range of diffusion is about 10 atomic layers, and is limited by the build-up of an electrostatic potential barrier.
The thesis is also focused on details in the electronic structure that can be relevant for the magnetic properties. One important question in this context is which electron states mediate the magnetic coupling between the Mn atoms. Having access to a dedicated MBE system directly connected to a high-resolution photoelectron spectrometer at the synchrotron radiation laboratory MAX-lab, the conditions for new detailed investigations of this problem were excellent. One of the main results in this thesis is thus the discovery of a strongly dispersive Mn-induced electron energy band, which is suggested to play the key role in spin alignment. Results of studies of very dilute samples also show that each Mn atom in the (Ga,Mn)As system influences a relatively large volume around it, far beyond the distance between nearest neighbors in the lattice. Furthermore, evidence is provided for a locally nonuniform distribution of Mn atoms. The picture of ferromagnetism in (Ga,Mn)As emerging from the present experiments is consistent with an earlier proposed magnetic polaron model, rather different from the currently most applied theories.