| Ferromagnetic semiconductor alloy (Ga,Mn)As is studied using different fabrication methods, including doping. All (Ga,Mn)As samples were grown by either low temperature molecular beam epitaxy (LT-MBE) or ion implantation followed by pulsed laser melting (II-PLM). The structures of the samples are as follows: (i) (Ga,Mn)As/(Al,Ga)As heterostructures and (Al,Ga)As/(Ga,Mn)As/(Al,Ga)As quantum wells modulation-doped by Be atoms in (Al,Ga)As layers; (ii) (Ga,Mn)As epilayers grown on GaAs substrates by either LT-MBE or II-PLM; (iii) Si-doped (Ga,Mn)As films grown on GaAs substrates.;First, we study how the ferromagnetic coupling in (Ga,Mn)As films is determined by Mn spins and holes using the modulation-doped (Ga,Mn)As/(Al,Ga,Be)As structures. At small values of hole concentration p the Curie temperature TC of (Ga,Mn)As is seen to increase with increasing p, as expected from the p-d Zener model. However, as p continues to increase, this trend is reversed: at some point TC begins to decrease, and eventually the ferromagnetism of (Ga,Mn)As disappears altogether, in stark contrast with the p-d Zener model. Mechanisms which can lead to this behavior are discussed.;Second, we study the effect of by Mn interstitials and As antisites (defects which are inherent to LT-MBE-grown (Ga,Mn)As) on the magneto-crystalline anisotropy of this alloy; and we study the intrinsic magnetic anisotropy of (Ga,Mn)As using a nearly-freestanding ferromagnetic (Ga,Mn)As layers formed by II-PLM where these defects are virtually absent. In qualitative terms the material formed by II-PLM exhibits all magnetic anisotropy features commonly found in (Ga,Mn)As films fabricated by LT-MBE. Quantitatively, however, magnetic anisotropy of II-PLM (Ga,Mn)As is dominated by cubic anisotropy terms, which we attribute to smaller strain in the II-PLM material due primarily to the absence of Mn interstitials. One will note, however, that the II-PLM (Ga,Mn)As also exhibits a weak but finite uniaxial in-plane magnetic anisotropy similar to that observed in LT-MBE (Ga,Mn)As, the origin of which is not yet understood in either of these materials.;Last, we also investigate the effect of donor doping on Ga 1-xMnxAs films using Si. Si acts as a compensating donor for the Mn acceptors in Ga1-xMn xAs alloys. For a Ga1-xMn xAs alloy with low Mn content (e.g., x = 0.033) the presence of Si decreases TC compared to undoped Ga1-xMnxAs. At higher Mn concentrations, however (e.g., for x > 0.10) we find that Si doping has the desirable effect of improving ferromagnetic properties of Ga1-xMnxAs, including a significant increase of TC. Although Si doping decreases p in Ga1-xMn xAs, interestingly, we find that Si doping increases hole mobility at higher Mn concentration (e.g., x = 0.16). We ascribe this hitherto unknown effect to the effect of Si on the relative occupancy of holes in the impurity band. |
