Investigation of the suitability of wide bandgap dilute magnetic semiconductors for spintronics.

摘要

New semiconductor materials may enable next-generation 'spintronic' devices which exploit both the spin and charge of an electron for data processing, storage, and transfer. The realization of such devices would benefit greatly from room temperature ferromagnetic dilute magnetic semiconductors. Theoretical predictions have suggested that room temperature ferromagnetism may be possible in the wide bandgap semiconductors Ga1--xMnxN and Zn1--xMnxO, though the existing models require input from the growth of high-quality materials. This work focuses on an experimental effort to develop high-quality materials in both of these wide bandgap materials systems.;Zn1--xMnxO and Zn1--xCo xO single crystals have been grown by a modified melt growth technique. X-ray diffraction was used to examine the structural quality and demonstrate the single crystal character of these devices. Substitutional transition metal incorporation has been verified by optical transmission and electron paramagnetic resonance measurements. No indications of ferromagnetic hysteresis are observed from the bulk single crystal samples, and temperature dependent magnetization studies demonstrate a dominant antiferromagnetic exchange interaction. Efforts to introduce ferromagnetic ordering were only successful through processing techniques which significantly degraded the material quality, suggesting a defect-related origin to the observed ferromagnetism.;Ga1--xMnxN thin films were grown by metalorganic chemical vapor deposition on two inch sapphire substrates. Good crystalline quality and a consistent growth mode with Mn incorporation were verified by several independent characterization techniques. Substitutional incorporation of Mn on the Ga lattice site was confirmed by electron paramagnetic resonance. Mn acted as a deep acceptor in GaN, suggesting that hole mediated ferromagnetism in this material is not possible. Nevertheless, ferromagnetic hysteresis was observed in the Ga1--xMnxN films. The apparent strength of the magnetization correlated with the relative ratio of Mn 3+ to Mn2+. Valence state control through codoping with additional donors such as silicon was observed. Additional studies on Ga1--xFexN also showed a magnetic hysteresis. A comparison with implanted samples showed that the common origin to the apparent strong ferromagnetic hysteresis is really a by-product of a change within the paramagnetic ordering from isolated Mn2+/3+ substitutional ions. The observed magnetic hysteresis is due to the formation of Mn-rich regions during the growth process, which can be controlled through the use of antisurfactants during growth. Qualitative modeling of the magnetic signature using a combination of Mn-configurations replicated the observed magnetic behavior. Through careful and detailed studies of high-quality materials in the wide bandgap system, it was shown that the original intrinsic models for room temperature ferromagnetism in the wide bandgap semiconductors do not hold and the room temperature ferromagnetism in these materials results from extrinsic contributions.