Enhanced annealing, high Curie temperature, and low-voltage gating in (Ga,Mn)As: A surface oxide control study

The essential role of carefully optimized synthesis for elucidating intrinsic material properties of (Ga,Mn)As

“…Because of the highly non-equilibrium nature of the heavily-doped ferromagnetic (Ga,Mn)As, the growth and post-growth annealing procedures have to be individually optimized for each Mn-doping level to obtain films that are as close as possible to idealized uniform (Ga,Mn)As mixed crystals with the minimal density of compensating and other unintentional defects. An extensive set of characterization measurements has to accompany the synthesis to guarantee that the materials show systematic doping trends; monitoring the thermodynamic Curie point singularities is essential for assuring the uniformity and high magnetic quality of the materials 11,15,54,55 . When omitting the above procedures, extrinsic impurities and sample inhomogeneities can yield nonsystematic doping trends and conceal the intrinsic magnetic properties of (Ga,Mn)As.…”

“…The next important factor determining the quality of the resulting (Ga,Mn)As materials are post-growth annealing conditions 4,15,54 . The common trends in annealing (at temperatures close to the growth temperature) suggest the presence of competing mechanisms.…”

“…One mechanism yields the increase of T C and is ascribed in a number of reports to the removal of charge and moment compensating interstitial Mn impurities 57,58 . The removal is slowed down by the growth of an oxide surface layer during annealing 54 and an additional mechanism can eventually yield reduction of T C after sufficiently long annealing times, depending on the annealing temperature. The origin of this detrimental mechanism may be in Mn clustering or in the competition between the nonequilibrium (Ga,Mn)As phase and the equilibrium MnAs second phase.…”

“…Because of the competing mechanisms, the absolutely highest Curie temperature for the given nominal doping is achieved at intermediate annealing temperature and time. The remaining critical parameter of the synthesis is the epilayer thickness 54 . For a given nominal doping, the highest attainable T C is reached only in thin films, typically thinner than B50 nm.…”

Identifying the character of ferromagnetic Mn in epitaxial Fe/(Ga,Mn)As heterostructures

“…From spectroscopic results, it is generally agreed that the two prevailing Mn electronic structures in ͑Ga,Mn͒As are Mn Ga ͑Mn substitutional on the Ga site͒ and Mn forming oxides or aggregates near the surface, with a hybrid d 4 -d 5 -d 6 and a purely d 5 electronic configuration ͑localized Mn͒, respectively. The latter configuration has a more structured XAS spectrum, located at higher BE with respect to the d 4 -d 5 -d 6 one.…”