Publication III

–We investigate spin-dependent interband tunnelling in a ferromagnetic (Ga,Mn)As/ GaAs Zener diode. The ferromagnetic pn-junction is fabricated with a Mn-doped p-type GaAs layer on top of a nonmagnetic n-type GaAs substrate. When both sides of the junction are heavily doped, a large magnetic-field–dependent tunnelling effect can be seen at low temperatures in the measured current-voltage (I-V ) characteristics. A model for the tunneling current in a ferromagnetic Zener diode is presented, and the observed effects are explained by the exchange interaction related splitting of the valence band states in (Ga,Mn)As. To our knowledge, this is the first time the effect of the band splitting and a large magnetoresistance related to the tunnelling processes have been observed experimentally in the I-V characteristics of a ferromagnetic Zener diode. In semiconductor physics the spin degree of freedom usually is neglected due to the almost degenerate energies of the spin-up and spin-down states of the charge carriers. However, recent advances [1] in the fabrication of ferromagnetic semiconductor thin films based on the diluted magnetic III-V semiconductors such as Mn-doped GaAs have opened a route to semiconductor spintronics, where spin-dependent phenomena in electrical and optical properties may become the dominant ones. The strong exchange interaction between the carrier spins and the spins of the magnetic 3d electrons is manifested, e.g., in a large splitting between the spin-up and spin-down band energies. Some basic semiconductor device structures such as pn-junctions, light-emitting diodes [2– 4], and resonant-tunnelling diodes [5] have been made of Mn-doped GaAs. The first ferromagnetic (Ga,Mn)As Zener diode was studied by Jonston-Halperin et al. [6], when they used the reverse biased diode as a spin emitter for a nonmagnetic quantum well. However, the c © EDP Sciences Article published by EDP Sciences and available at http://www.edpsciences.org/epl or http://dx.doi.org/10.1209/epl/i2005-10142-y 812 EUROPHYSICS LETTERS