Magnetite Schottky barriers on GaAs substrates

Abstract

Carrier transport across Fe 3 O 4 / GaAs interfaces has been studied for n-and p-type GaAs͑001͒ substrates with medium ͑7.7ϫ 10 17 cm −3 ͒ to high ͑3.5ϫ 10 18 cm −3 ͒ carrier concentrations. Currentvoltage ͑I-V͒ measurements on medium-doped substrates show a rectifying behavior that is characteristic for thermionic emission/diffusion across a Schottky barrier. The n-type structure exhibits a low ideality factor of 1.3 and a Schottky barrier height of 0.58-0.63 eV. However, one of the main goals of spin electronics, to achieve spin injection from a ferromagnetic metal into a semiconductor, has been more difficult to attain. One particular obstacle is the conductivity mismatch between metals and semiconductors, which drastically limits the spin injection efficiency for ohmic contacts between metals and semiconductors. 4 For nonohmic contacts, however, the conductivity mismatch model of Schmidt et al. does not apply and it has recently been demonstrated that spin injection can be obtained by tunneling through a barrier at the semiconductor interface 5 or by using a magnetic tunnel transistor emitter. 6 An alternative route for spin injection is to use a halfmetallic ferromagnet, which has a completely spin-polarized conduction band, as the spin injector. For perfectly spinpolarized sources, the conductivity mismatch is circumvented even for ohmic contacts, and in the case of tunneling the higher polarization should give a much larger spin accumulation in the semiconductor compared to 3d transition metal injectors. Unfortunately the growth of half-metallic ferromagnets onto semiconductors tends to be more difficult than ferromagnetic transition metals, and very often the spin polarization will be determined more by disordered interfacial layers than by the bulk properties of the ferromagnet itself. Furthermore, perfect spin polarization is a zerotemperature property. The two spin states are mixed, to some extent, at finite temperature. Nonetheless, Fe 3 O 4 has a number of unusual properties that make it attractive as a spinpolarized source.As a class II B half-metal, Fe 3 O 4 is not truly a metal but it is rather a polaronic hopping conductor of minority spins. 7The resistivity increases with decreasing temperature and goes up sharply at the Verwey transition at about 120 K.

Hybrid Spintronic Structures With Magnetic Oxides and Heusler Alloys

“…This is about half of the Schottky barrier height of the metal/GaAs contact, such as Fe/GaAs, which lies within the range 0.69-0.72 eV. It is interesting to note that the Schottky barrier height of the MBE-grown magnetite is less than the 0.58-0.63 eV Schottky barrier height observed by Watts et al [36] in the magnetite thin films deposited on top of the GaAs(100) by reactive dc sputtering. This indicates that the Schottky barrier height of the Fe O /GaAs contact depends on the growth condition, which may be controlled to have the desired barrier heights for spin injection.…”