Electronic and Magnetoresistive Properties of InMnAs Films and Heterojunctions

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The electronic transport and magnetoresistive properties of the ferromagnetic semiconductor In1-xMnxAs were investigated in order to determine the nature of the transport and ferromagnetism in the films. p-InMnAs/n-InAs and metal/oxide/InMnAs heterojunctions were fabricated and characterized to elucidate the transport and magnetoresistance mechanisms at these technologically important interfaces. The electronic transport properties of InMnAs films deposited by metal-organic vapor phase epitaxy were measured to investigate the coupling between the ferromagnetic and semiconducting properties in the material. From Hall effect measurements the presence of two hole species was inferred: itinerant holes that are responsible for conduction and tightly bound holes that are localized within Mn clusters known to form in the films. The intercluster exchange, which leads to global ferromagnetism, is attributed to overlap of the tightly bound hole wavefunctions between neighboring clusters. The itinerant holes do not play an active role in mediating the ferromagnetic order between clusters. The properties of the itinerant holes were studied by applying a two band conduction model to itinerant hole concentrations measured from 5 to 300 K. Using this model, the number of itinerant holes in the valence band and shallow Mn impurity band was quantified as a function of temperature. The depth of the shallow Mn acceptor level was also determined. Ferromagnetic p-InMnAs/n-InAs heterojunctions were fabricated and characterized in order to study the magnetoresistance in these structures. The conduction mechanisms were determined as a function of bias and temperature from current-voltage measurements. A large, positive magnetoresistance was measured in the junctions, the magnitude of which depended on the conduction mechanism. The room temperature magnetoresistance increased linearly with field from 1.5 to 9 T, exceeding 1280 % at 9 T. The magnetoresistance is attributed to enhanced scattering of minority carrier electrons in the p-InMnAs layer due to structural or chemical inhomogeneities. The large, linear magnetoresistance makes the junctions excellent candidates for high-field magnetic sensors. Metal/oxide/InMnAs heterojunctions were fabricated and characterized. Conductance-bias measurements confirmed that tunneling was the dominant conduction mechanism in the junctions. A negative magnetoresistance was measured in the junctions persisting up to 150 K. The magnetoresistance is attributed to the tunneling anisotropic magnetoresistance (TAMR) effect, demonstrating a coupling between the magnetization and electronic band structure of InMnAs.

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  • 05/14/2018
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