Ferromagnetic III-Mn-V semiconductors: Manipulation of magnetic properties by annealing, extrinsic doping, and multilayer design

We present a systematic study on the growth and properties of ferromagnetic (FM) III-V-based semiconductor GaMnAs, including extrinsically-doped GaMnAs:Be and GaMnAs/ZnMnSe double layers. All specimens used in this study were grown by low-temperature molecular beam epitaxy (MBE). a method required for preventing the formation of MnAs precipitates. We carried out systematic annealing studies on the GaMnAs epilayers with Mn concentration ranging from 0.03 to 0.08. We found significant annealing-induced changes in the magnetic properties of these materials, that depended on the Mn concentration and on annealing conditions. In particular, annealing samples with higher Mn content at a low temperature (typically at T = 280 °C) was consistently observed to result in a dramatic increase of the Curie temperature (T_c), typically to 110 K. Channeled Rutherford back scattering (c-RBS) and channeled particle-induced x-ray emission (c-PIXE) experiments carried out on GaMnAs shed important light on the mechanism responsible for this systematic and reproducible improvement in the FM properties of GaMnAs. By their ability to identify the position of Mn atoms in the GaMnAs lattice, these experiments clearly pointed to the crucial role which interstitial Mn atoms (Mn₁) play in determining the FM parameters. In an attempt to further improve the T_c of GaMnAs, we have undertaken a systematic program of extrinsic p-doping of this material. In GaMnAs with low x (x = 0.02), T_c is indeed seen to increase monotonically with increased Be doping. However, we have observed a systematic decrease of T_c with increasing Be doping in the case of GaMnAs with higher Mn content (x = 0.06). This surprising result can be understood in terms of a thermodynamic limit imposed on the hole concentration by the band structure of the material. Finally, we have demonstrated that the FM properties of GaMnAs epilayers can be significantly modified by the so-called "proximity effects" achieved by epitaxial deposition of other magnetic systems on GaMnAs. Specifically, we have observed that the deposition of ZnMnSe overlayers on GaMnAs leads to a clear increase of both the coercive field and of the Curie temperature of this FM alloy.