Conduction mechanisms in bulk and thin-film crystalline chromia
Kwan, Chun Pui
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Nonvolatile magnetoelectric (ME) devices are emerging post-CMOS spintronic applications that promise ultralow power consumption, and faster switching, relative to existing (spin-transfer torque) technology. Switching of such devices is based upon the idea of exploiting voltage-controlled magnetic anisotropy (VCMA), arising at the surface of a ME material. Here we study the promising ME chromia (Cr2O3), which is one of the few MEs to exhibit VCMA at room temperature. While previous research has successfully demonstrated VCMA in heterostructures formed with bulk Cr2O3, its electrical characteristics is not well understood. Also, it remains a significant challenge to demonstrate VMCA in the thin films needed for device applications. Notably, the development of fully functional devices, in which the Cr2O3 exhibits high endurance in response to repeated switching, requires that current leakage be minimized at high electric fields (>100 kV/cm). In this thesis, we investigate the high-field current leakage in both high-quality single crystal and thin-film crystalline Cr2O3, synthesized on different substrates. By studying the variation of the leakage over a wide range of temperature (200 – 400 K), the physical mechanisms responsible for dielectric breakdown are identified. The breakdown in thin-film Cr2O3 is found to be strongly sensitive to the nature of the metallic substrate on which the crystalline film is formed. By proper control of these parameters, we show how it is possible to achieve high film resistivity (at the order of 1012Ωcm) with large breakdown fields. The important role of the substrate arises from its capacity to impact the propagation of twinning defects into the Cr2O3 itself, and to thereby impact the resistivity and breakdown characteristics exhibited by the resulting film. These results therefore suggest a highly-promising direction for the realization of spintronic devices based on this material.