Controlling the Curie temperature in (Ga,Mn)As through location of the Fermi level within the impurity band

Abstract

The ferromagnetic semiconductor (Ga,Mn)As has emerged as the most studied material for prototype applications in semiconductor spintronics. Because ferromagnetism in (Ga,Mn)As is hole-mediated, the nature of the hole states has direct and crucial bearing on its Curie temperature T(C). It is vigorously debated, however, whether holes in (Ga,Mn)As reside in the valence band or in an impurity band. Here we combine results of channelling experiments, which measure the concentrations both of Mn ions and of holes relevant to the ferromagnetic order, with magnetization, transport, and magneto-optical data to address this issue. Taken together, these measurements provide strong evidence that it is the location of the Fermi level within the impurity band that determines T(C) through determining the degree of hole localization. This finding differs drastically from the often accepted view that T(C) is controlled by valence band holes, thus opening new avenues for achieving higher values of T(C).

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

“…The materials have no measurable charge or moment compensation of the substitutional Mn Ga impurities and have a large degree of uniformity reflected by sharp Curie point singularities. We point out that this phenomenology is in striking contrast to the nonsystematic doping trends in semiconducting and magnetic properties reported, for example, in Dobrowolska et al, 9 on the mixed series of as-grown and annealed, B100-nm-thick (Ga,Mn)As epilayers prepared without individually optimizing T G , T A and the annealing time.…”

“…Inferring doping trends in basic material properties of (Ga,Mn)As from sample series mixing as-grown and annealed materials, as has been often the case in the literature 7,9 , is unsuitable as the quality of the samples may vastly vary in such a series. Chosing one a priori-fixed T G , T A and annealing time for a range of Mn dopings, as is also common in the literature 7,9,16,17 , is unlikely to produce a high-quality, uniform and uncompensated (Ga,Mn)As material even for one of the considered dopings and is bound to produce lower quality samples for most of the studied Mn dopings. Finally, reliable measurements of intrinsic semiconducting and magnetic properties on optimized (Ga,Mn)As samples require exceedingly long annealing times for film thicknesses \50 nm and are unachievable in B100 nm and thicker films by the known approaches to the (Ga,Mn)As epilayer synthesis.…”

“…In a recurring alternative proposal, the Fermi level in ferromagnetic materials with B10 20 -10 21 cm À 3 Mn-acceptor densities is assumed to still reside in a narrow impurity band detached from the valence band, that is, the band structure keeps the form closely reminiscent of a single isolated Mn Ga impurity level [7][8][9] . Several phenomenological variants of the impurity band model have been proposed whose assumed spectroscopic characteristics are, however, mutually inconsistent and have not been recreated from microscopic band structure theories 6 .…”

“…Since to date no framework has been introduced that would allow to test the validity of the impurity band model by comparing microscopic atomic orbital based theory calculations to experiment, it is primarily the experimental findings which have served as a basis for postulating the impurity band picture. A common feature of these experimental reports is the limited attention paid to the details of the synthesis of this highly nonequilibrium alloy 7,9 . As we show in this paper, careful material optimization is, however, of central importance.…”

Theory of Mn-doped II-II-V semiconductors

“…However, despite four decades of intensive work, challenges remain and materials complexity often hinders theoretical understanding. The origin of magnetic ordering [1][2][3]5] and paths to higher T C remain strongly debated [3,9,10]. Doping with Mn is the usual method for synthesizing DMS, and in the most common III-V DMS (the best studied example is GaAs:Mn) this leads to both spin and carrier doping.…”