Quantum transport model bridging from atomistic-scale to micro-scale devices

Roksana Golizadeh-Mojarad, Purdue University

Abstract

As the size of CMOS devices keeps shrinking more and more, power dissipation has become one of the major challenges for scaling. In fact, it has been suggested that the scaling may stop long before we reach any fundamental barriers because of our inability to remove the heat generated in these devices. This has stimulated significant efforts to look for novel devices that are fundamentally different in operating principle. In this thesis, we have explored the physics underlying non-equilibrium phenomena recently observed in both spintronics and graphene-based electronics, while examining their potential applications for practical devices. Understanding new phenomena thoroughly would lead us to the possibility of designing a new generation of device with better characteristics. The Non-equilibrium Green function (NEGF) method is being widely used to model coherent quantum transport where it is equivalent to the Landauer approach. However, exploring the physics of novel phenomena and devices needs a model considering dephasing processes which bridges the entire range from the atomistic to diffusive regime. As part of this thesis, a novel dephasing model in quantum transport has been developed which provides the flexibility of adjusting the degree of phase, momentum and spin relaxation independently while retaining the conceptual and numerical simplicity of other phenomenological models. Using this dephasing model, a number of devices and phenomena such as spin-Hall effect have been explored continuously from ballistic to diffusive regime. In diffusive regime our simulation results are in good agreement with the experimental results. We have also developed a correspondence between intrinsic spin-Hall effect and ordinary Hall effect. For graphene-based electronics device, we have benchmarked our model successfully in Quantum Hall regime with experiments. From our work on graphene-based electronics, we have predicted the importance of contact-induced states in extracting transport properties of graphene. Recently, our predictions in this regard have been confirmed by two sets of experiments. We have also analyzed transport in doped graphene field effect transistors. Our analysis has revealed the origin of the doping-induced asymmetry observed experimentally in electron and hole conductance. We have predicted two different types of conductance asymmetry based on the nature of doping.

Degree

Ph.D.

Advisors

Datta, Purdue University.

Subject Area

Electrical engineering

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