Magneto-transport in (Ga,Mn)As-based alloys and hybrids

The work described in this thesis has to be seen in the context of developing semiconductor technology towards controlling the spin character of carriers in semiconductors. In this spintronic field, one of the present aims is to achieve a controllable ferromagnetic semiconductor at room temperature. One way is to enhance the Curie temperature in the MBE-grown II(Mn)-VI and III(Mn)-V DMS alloys. Another alternative are hybrid structures including a ferromagnetic component with Curie temperature above room temperature. Therefore, the spin-related transport properties were investigated in GaMnAs-based alloys and hybrids grown by MOVPE and MBE in this thesis. The obtained important results are as follows: 1)Unusual positive and negative MR, which are very sensitive to the Mn concentration, were observed at low temperatures in paramagnetic GaAs:Mn alloys grown by MOVPE.It is suggested that the interplay of two effects plays an important role. One is the magnetic field-dependent spin splitting of the valence band caused by the sp-d exchange between the Mn A0 (d5+h) centers and the band states. The other is the disorder effect induced by Mn incorporation. The competition of these two effects is responsible for the unusual positive and negative MR effects. 2)A theoretical calculation by a network and a mobility model based on a phenomenological description was successfully used to explain the experimental MR results in the II(Mn)-VI and III(Mn)-V DMS with low Mn concentration. By adjusting the weighting of the effects of occupation of the four spin-subbands and of disorder, the negative and positive MR behavior is obtained in these two models. The theoretical results are in good agreement with the experimental results. 3)By Te doping into paramagnetic GaAs:Mn with small Mn concentration, a control of the carrier concentration by Te incorporation is almost independent of the Mn concentration. The Hall measurements reveal the majority carrier type from p to n type with Te co-doping. Te co-doping causes a transition from VB transport to CB transport. In the case of conduction band transport, the s-d exchange interaction induced conduction band splitting and its contributions to the MR is of minor importance, confirming that the |N0b| >> |N0a| in (Ga,Mn)As.The MCD measurements reveal that due to Te co-doping the valence band filled with electrons consequently leads to Mn acting as A- centers and an AFM type coupling between VB states and the Mn spins. This result confirms that the sign and magnitude of exchange integral N0b indicating FM or AFM coupling between spins of the VB carriers and the Mn ions can be tuned by the local electronic structure of the Mn ion. 4)The series GaAs:Mn/MnAs paramagnetic-ferromagnetic hybrids prepared either by MOVPE directly or by post-growth annealing of GaMnAs alloys grown by low-temperature MBE are studied based on the understanding of GaAs:Mn alloys. Large unusual MR effects are found in the hybrids including MnAs clusters with NiAs structure grown by MOVPE, e.g. a 30% negative MR at low temperature and change to 160% positive MR with increasing temperature. The ferromagnetic MnAs cluster leads to a local localization process of the carriers around the clusters and act as a spin-filter, which is suggested to be responsible for the observed unusual coexistence of the large negative and positive MR effects. The MR and Hall effect investigated in these hybrids confirm that the interplay of the paramagnetic matrix and of the MnAs clusters dominates the spin-dependent transport behaviour. Both, the properties of the matrix and of the MnAs clusters strongly depend on the preparation procedure. 5)The FMR measurements indicate that two type of MnAs clusters form in the post-growth annealing of GaMnAs alloys grown by low-temperature MBE. One of them is confirmed as the same as the one observed in the hybrids grown by MOVPE with NiAs-structure (type I). The other one (type II) is of much lower Curie temperature and different structure. 6)The geometry dependence of the MR and Hall effect of the GaAs:Mn/MnAs hybrid reveals that the degree of microscopic interaction between the free carriers in the paramagnetic semiconductor host and the ferromagnetic MnAs clusters also strongly depends on the transport geometry and the resulting current path through the sample. The hydrostatic pressure dependence of the MR and the Hall effect are consistent with what happens in II-VI DMS and InMnAs, i.e., an enhanced p-d exchange interaction by increasing applied hydrostatic pressure. However, these effects are partly compensated by the pressure-induced variation of the carrier density. Therefore, the properties of GaAs:Mn/MnAs-based hybrids indicate that the paramagnetic-ferromagnetic structures exhibit large spin-dependent transport effects. These effects can be tuned by the growth conditions, by the growth procedure, and by external physical parameters