Advanced boundary condition method in quantum transport and its application in nanodevices
Modern semiconductor devices have reached critical dimensions in the sub-20nm range. During the last decade, quantum transport methods have become the standard approaches to model nanoscale devices. In quantum transport methods, Schrödinger equations are solved in the critical device channel with the contacts served as the open boundary conditions. Proper and efficient treatments of these boundary conditions are essential to provide accurate prediction of device performance. The open boundary conditions, which represent charge injection and extraction effects, are described by contact self-energies. All existing contact self-energy methods assume periodic and semiinfinite contacts, which are in stark contrast to realistic devices where the contacts often have complicated geometries or imperfections. On the other hand, confined structures such as quantum dots, nanowires, and ultra-thin bodies play an important role in nanodevice designs. In the tight binding models of these confined structures, the surfaces require appropriate boundary treatments to remove the dangling bonds. The existing boundary treatments fall into two categories. One is to explicitly include the passivation atoms in the device. This is limited to passivation with atoms and small molecules due to the increasing rank of the Hamiltonian. The other is to implicitly incorporate passivation by altering the orbital energies of the dangling bonds with a passivation potential. This method only works for certain crystal structures and symmetries, and fails to distinguish different passivation scenarios, such as hydrogen and oxygen passivation. In this work, an efficient self-energy method applicable for arbitrary contact structures is developed. This method is based on an iterative algorithm which considers the explicit contact segments. The method is demonstrated on a graphene nanoribbon structure with trumpet shape contacts and a Si0.5Ge0.5 nanowire transistor with alloy disorder contacts. Furthermore, a new surface passivation method for passivation of arbitrary crystal structures and symmetries is developed. This method is based on a selfenergy treatment of the passivation materials and the parameters are validated with abinitio calculations therefore it can distinguish different passivation scenarios. The method is illustrated on the Si/SiO2 interface with different oxidation configurations. These methods are applied to study the surface roughness and its impact in the transport properties of black phosphorus transistors. The calculated mobilities due to roughness scattering agree well with measurement suggesting that roughness might be the essential scattering mechanism in black phosphorus transistors. The calculated transport properties along armchair direction outperform the zigzag direction, further demonstrating the experimentally observed anisotropic behavior.
Klimeck, Purdue University.
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