Magnetic Semiconductor - Materials

Materials

The manufacturability of the materials depend on the thermal equilibrium solubility of the dopant in the base material. E.g., solubility of many dopants in zinc oxide is high enough to prepare the materials in bulk, while some other materials have so low solubility of dopants that to prepare them with high enough dopant concentration thermal nonequilibrium preparation mechanisms have to be employed, e.g. growth of thin films.

A flurry of research in the past few years has shed some light on the crucial factors that are needed to achieve high-Curie temperature (above room temperature) ferromagnetic semiconductors, which can explain the so-called controversy in the field and lack of reproducibility in the magnetic properties for the same materials. Indeed, the first great discovery in the field was in 1986 by T. Story and co-workers where they demonstrated that the ferromagnetic Curie temperature of Mn2+-doped Pb_1-xSn_xTe can be controlled by the carrier concentration. The theory proposed by Dietl required charge carriers in the case of holes to mediate the magnetic coupling of manganese dopants in the prototypical magnetic semiconductor, Mn2+-doped GaAs. If there is an insufficient hole concentration in the magnetic semiconductor, then the Curie temperature would be very low or would exhibit only paramagnetism. However, if the hole concentration is high (>~1020 cm−3), then the Curie temperature would be higher, between 100-200 K.

Recent research by the University of Washington group led by Daniel Gamelin has shed some light for instance on the importance of interstitial zinc (a shallow donor) for controlling the ferromagnetism in a high-Curie temperature, Co2+-doped ZnO.

Several examples of ferromagnetic semiconductor materials are e.g.:

• Manganese-doped indium arsenide and gallium arsenide (GaMnAs), with Curie temperature around 50–100 K and 100–200 K, respectively
• Manganese-doped indium antimonide, which becomes ferromagnetic even at room temperature and even with less than 1% Mn.
• Oxide semiconductors
  • Manganese- and iron-doped indium oxide, ferromagnetic at room temperature
  • Zinc oxide
    • Manganese-doped zinc oxide
    • n-type cobalt-doped zinc oxide
  • Titanium dioxide:
    • Cobalt-doped titanium dioxide (both rutile and anatase), ferromagnetic above 400 K
    • Chromium-doped rutile, ferromagnetic above 400 K
    • Iron-doped rutile and iron-doped anatase, ferromagnetic at room temperature
    • Nickel-doped anatase
  • Tin dioxide
    • Manganese-doped tin dioxide, with Curie temperature at 340 K
    • Iron-doped tin dioxide, with Curie temperature at 340 K
• Nitride semiconductors
  • Chromium doped aluminium nitride