Review of Recent Efforts on the Growth and Characterization of Nitride-based Diluted Magnetic Semiconductors

摘要

Wide bandgap nitrides and oxides have been heralded as a possible platform for future semiconductor spintronics applications based on the inherent compatibility of these materials with existing semiconductors as well as theoretical predictions of room temperature ferromagnetism. Experimental reports of room temperature ferromagnetism in these materials are complicated by disparate crystalline quality and phase purity in these materials, as well as conflicting theoretical predictions as to the nature of ferromagnetic behavior in this system. A complete understanding of these materials, and ultimately intelligent design of spintronic devices, will require an exploration of the relationship between the processing techniques, resulting transition metal atom configuration, defects, and electronic compensation as related to the structure, magnetic, and magneto-optical properties of this material. This work explores the growth and properties of Ga_(1-x)Mn_xN films by metalorganic chemical vapor deposition on c-plane sapphire substrates with varying thickness, Mn concentration, and alloying elements. Homogenous Mn incorporation throughout the films was verified with Secondary Ion Mass Spectroscopy (SIMS), and no macroscopic second phases were detected using X-ray diffraction (XRD). SQUID and vibrating sample magnetometry measurements showed an apparent room temperature ferromagnetic hysteresis, whose strength can be altered considerably through annealing and introduction of either Si or Mg during the growth process. Three sets of Raman modes appeared to be sensitive to Mn incorporation. The intensities of a broad band around 300cm~(-1) and sharper modes near 669cm~(-1) increased with increasing Mn concentration. The rise of the former is attributed to a decrease in long-range lattice ordering for higher Mn concentration. The second mode is due to nitrogen vacancy-related local vibrational modes of the GaN host lattice. Si co-doped Ga_(1-x)Mn_xN results in shallow donor states in GaN suppress the formation of nitrogen vacancies by compensating the p-tupe deep level defects introduced by substitutional Mn. The formation of a Mn-related midgap impurity band is observed via optical transmission measurement in Ga_(1-x)Mn)xN with strong magnetic signatures, but not for Si co-doped samples. Initial studies on light emitting diodes (LEDs) containing a Mn-doped active region have also been produced. Devices were fabricated with different Mn-doped active layer thicknesses, and Ⅰ-Ⅴ characteristics show that the devices become highly resistive as thickness of the Mn-doped active layer increases. The electroluminescence of these devices is dominated by a high suppressed band-edge recombination and a midgap defect-related emission, leading to an orange-colored but weakly emitting LED. These results suggest that traditional theoretical and device approaches akin to those realized in Ga_(1-x)Mn_xAs may be difficult to realize in Ga_(1-x)Mn_xN, and exploitation of these materials will require further novel device approaches taking into account the nature of this material.