Wladek Walukiewicz and Kin Man Yu at Berkeley Lab’s Rutherford Backscattering Spectrometry laboratory. |
A
long-standing controversy regarding the semiconductor gallium manganese
arsenide, one of the most promising materials for spintronic
technology, looks to have been resolved. Researchers with the U.S.
Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley
Lab) in collaboration with scientist from University of Notre Dame have
determined the origin of the charge-carriers responsible for the
ferromagnetic properties that make gallium manganese arsenide such a hot
commodity for spintronic devices. Such devices utilize electron spin
rather than charge to read and write data, resulting in smaller, faster
and much cheaper data storage and processing.
Wladek
Walukiewicz, a physicist with Berkeley Lab’s Materials Sciences
Division and Margaret Dobrowolska, a physicist at Notre Dame, led a
study in which it was shown that the holes (positively-charged energy
spaces) in gallium manganese arsenide that control the Curie
temperature, the temperature at which magnetism is lost, are located in
an impurity energy band rather than a valence energy band, as many
scientists have argued. This finding opens the possibility of
fabricating gallium manganese arsenide so as to expand the width and
occupation of the impurity band and thereby boost the Curie temperature
to improve spintronic potential.
“Our
results challenge the valence band picture for gallium manganese
arsenide and point to the existence of an impurity band, created by even
moderate to high doping levels of manganese,” Walukiewicz says. “It is
the location and partially localized nature of holes within this
impurity band that drives the value of the Curie temperature.”
The results of this study have been published in the journal Nature Materials in
a paper titled “Controlling the Curie temperature in (Ga,Mn)As through
location of the Fermi level within the impurity band.” Co-authoring the
paper with Walukiewicz and Dobrowolska were Kritsanu Tivakornsasithorn,
Xinyu Liu, Jacek Furdyna, Mona Berciu and Kin Man Yu.
As
a commercial semiconductor, gallium arsenide is second only to silicon.
Substitute some of the gallium atoms with atoms of manganese and you
get a ferromagnetic semiconductor that is well-suited for spintronic
devices. While it has been established that the ferromagnetism of
gallium manganese arsenide is hole-mediated, the nature of the
hole-states, which has a direct and crucial bearing on its Curie
temperature, has been vigorously debated.
In
semiconductors and other solid-state materials, the valence band is the
range of energies in which the movement of charge is determined by
availability of holes. Doping gallium arsenide with manganese can create
an impurity band that depletes the valence band and shifts the Fermi
level, the energy level at which the electronic states below are filled
and those states above are empty.
“The
question has been whether the holes mediating the interactions of
manganese spins reside in a delocalized valence band, or in a
manganese-derived partially localized impurity band,” Walukiewicz says.
“The valence band model assumes that a separate impurity band does not
exist for manganese concentrations higher than about 2%.”
Researchers with Berkeley Lab and Notre Dame University found that the spintronic properties of gallium manganese arsenide arise from holes in an impurity band, created by manganese doping, that depletes the valence band and shifts the Fermi level. |
Walukiewicz
and his co-authors addressed the issue through channeling experiments
that measured the concentrations of manganese atoms and holes relevant
to the ferromagnetic order in gallium manganese arsenide. These
experiments were carried out at Berkeley Lab’s Rutherford Backscattering
facility, which is operated under the direction of co-author Kin Man
Yu. The results of these experiments were then combined with
magnetization, transport and magneto-optical data performed at the
University of Notre Dame.
“We
were able to determine where the manganese atoms were located, what
fraction of this total replaced gallium and acted as electron acceptors
(meaning they created ferromagnetic-mediating holes), and what fraction
was in the interstitial sites, acting as positively-charged double
donors compensating for a fraction of manganese acceptors,” Walukiewicz
says. “Taking all our data together, we find that the Curie temperature
of gallium manganese arsenide can be understood only by assuming that
its ferromagnetism is mediated by holes residing in the impurity band,
and that it is the location of the Fermi level within the impurity band
that determines the Curie temperature.”
Electron
spin is a quantum mechanical property arising from the magnetic moment
of a spinning electron. Spin carries a directional value of either “up”
or “down” and can be used to encode data in the 0s and 1s of the binary
system. Walukiewicz says that understanding the factors that control the
Curie temperature can serve as a guide for strategies to optimize
ferromagnetic materials for spintronic applications.
“For
example, with appropriate control of the manganese ions, either
co-doping with donor ions, or modulation doping, we can engineer the
location of the Fermi level within the impurity band to best the
advantage,” he says.
Walukiewicz
says the findings of this study further suggest that it should be
possible to optimize magnetic coupling and Curie temperature for the
whole family of ferromagnetic semiconductors by tuning the binding
energy of the acceptor ions.
This
research was supported in part by the DOE Office of Science, and by
grants from the National Science Foundation, the Natural Sciences and
Engineering Research Council of Canada, and the Canadian Institute for
Advanced Research.
