SPIN-DEPENDENT TRANSPORT IN Mn DOPED GaAs AND GaN DIODES

OF DOCTORAL DISSERTATION HELSINKI UNIVERSITY OF TECHNLOGY P.O. BOX 1000, FI-02015 TKK http://www.tkk.fi Author Heikki Holmberg Name of the dissertation Spin-dependent transport in Mn Doped GaAs and GaN diodes Manuscript submitted 16.11.2007 Manuscript revised 4.2.2008 Date of the defence 14.3.2008 Monograph Article dissertation (summary + original articles) Faculty Faculty of Electronics, Communication and Automation Department Department of Micro and Nanosciences, Electron Physics Group Field of research Semiconductor Technology Opponent(s) Prof. Tuure Tuuva Supervisor Prof. Pekka Kuivalainen Abstract The main idea of the thesis was to study experimentally the potential and the possibilities of semiconductor spintronic devices. Spin dependent magnetotransport was studied in different Mn doped GaAs and GaN thin films and diode structures. The ferromagnetic thin films and pn, spin Esaki-Zener tunnel and resonant tunnelling diodes were fabricated from Mn doped GaAs and GaN using the Molecule Beam Epitaxy technique. The magnetotransport measurements consist of ,e.g., I-V, resistance and Hall measurements as a function of magnetic field and temperature. In this study also the magnetic field dependence of the current of the ferromagnetic diodes were modeled. In addition, the material studies were carried out using Secondary Ion Mass Spectroscopy and X-ray Diffraction techniques and the magnetization measurements using a magnetometer. The main result of this study was the observation of the tunnelling anisotropic magnetoresistance effect (TAMR) in Mn doped GaAs spin Esaki-Zener tunnel and resonant tunnelling diodes. In the Mn doped GaAs Esaki-Zener diode the TAMR effect was observed at low bias voltages, which makes it possible to fabricate ultra low power spintronic devices. Another important result was that a room temperature ferromagnet of Mn doped GaN can be fabricated by a solid state diffusion method. The effect of Mn doping on the resonant tunnelling diodes with magnetic Mn doped GaAs emitters was also characterized. In addition, the reason why the current of a pn-diode having a lightly doped region is independent of magnetic field, even when some part of the device is magnetic, is given. Also the negative magnetoresistance due to the spin disorder scattering was observed and modeled in Mn doped GaAs and GaN thin films.