Dilute Magnetic Bulk GaN Semiconductor and Nanostructuress

Thursday, October 12, 2006
7:00 PM
Free and open to the public

New semiconductor materials may enable next-generation electronic devices which exploit both the spin and charge of an electron for data processing, storage, and transfer. Proposed "spintronic" devices include integrated chip-based magnetic memories, reconfigurable logic elements, polarized light emitting diodes, and building blocks for solid state quantum computing. The realization of these devices requires new materials which can support both the storage and transport of spin-polarized electrons. Diluted magnetic semiconductors are ideal in this regard, as they can exhibit ferromagnetism in materials compatible with those used for modern solid state electronics and optoelectronics. To date, the most promising ferromagnetic semiconductor, Ga1-xMnxAs, has a maximum Curie temperature of only 170 K. For practical spintronic applications, room temperature ferromagnetic dilute magnetic semiconductors are required. Theoretical predictions have suggested that room temperature ferromagnetism may be possible in the wide bandgap semiconductors Ga1-xMnxN and Zn1-xMnxO.

In this work, Ga1-xMnxN was grown by metalorganic chemical vapor deposition, the commercial technique of choice for high quality GaN suitable for incorporation into optoelectronic devices. High resolution X-ray diffraction studies demonstrate that the material is high quality single phase similar in quality and lattice parameter to unalloyed GaN. Incorporation of Mn on the Ga lattice site has been verified by electron paramagnetic resonance. The as-grown epitaxial films exhibit ferromagnetic hysteresis at room temperature as measured by SQUID magnetometry at dilute alloy concentrations (~1%); the strength of the observed magnetization varies with Mn concentation and addition of Si and Mg codopants. Three sets of Raman modes appeared to be sensitive to Mn incorporation; the most prominent additional mode is attributed to nitrogen vacancy-related local vibrational modes of the GaN host lattice. The formation of a Mn-related midgap impurity band is observed via optical transmission measurement in Ga1-xMnxN with strong magnetic signatures. The critical importance of controlling the Fermi level relative to the Mn2+/3+ acceptor level in GaN and its influence on the materials properties will be discussed relative to the prevailing theories of ferromagnetism in Ga1-xMnxN. In the wide bandgap diluted magnetic semiconductors, compensating defects play a critical role in determining not only the electronic and optical properties, but the observed magnetic behavior as well.

Ultimately, the utility of this material will be measured not by the observed properties, but how successful it is incorporated into devices with novel or improved functionalities. To that end, results will be presented on the incorporation of Mn-doped GaN into self-assembled nanostructures. A novel growth method using MOCVD has been developed to produce optically active GaN-based nanostructures using a two-step process (GaN deposition followed by an activation step) and low growth temperatures (<850oC) and V-III ratios (<30). Stranski-Krastanow å_ like growth is demonstrated for these nanostructures. GaMnN nanostructeres were grown by introducing Mn (0-2%) to GaN flows under optimal conditions for the formation of nanostructure without the need of an activation step. Manganese incorporation into these structures, with the intent of producing ferromagnetic Ga1-xMnxN quantum dots that could be forerunners for solid state quantum computing elements, has the tangential benefit that Mn acts as an antisurfactant, promoting the low-temperature nucleation of these nanostructures. Preliminary experiments have been done in which Iron was introduced to bulk GaN layers to introduce ferromagnetic properties. It is determined that as the percentage of Fe incorporation increases the surface quality degrades significantly. Weak room temperature magnetic hysteresis was observed when small percentages of Fe were introduced to GaN. In addition, experiments have been conducted in which Fe was incorporated into GaN nanostructures at 8001-4C, and room temperature ferromagnetic behavior of GaN:Fe nanostructures was observed. Atomic force microscopy measurements revealed that Fe, like Mn, affected the surface morphology of the nanostructures. A larger amount of Fe incorporation reduces nanostructure size, enhances quantum confinement as well as ferromagnetism in the GaN:Fe nanostructures.

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Ian Ferguson

Georgia Institute of Technology