Modeling analysis of core-shell Si/SiGe nanowires

Abstract

(cont.) a composition that results in a high mobility has a very promising thermoelectric performance. Lastly, the thermoelectric-related transport properties for a Si/SiGe core-shell nanowire are compared with the related properties for a Si nanowire and a SiGe nanowire. The Si/SiGe core-shell nanowire shows a better thermoelectric performance than its Si nanowire counterpart. On the other hand, by relaxing the harsh conditions imposed on the carrier mobility of the Si/SiGe core-shell nanowire structure in this thesis, the Si/SiGe core-shell nanowire structure is also expected to have a better thermoelectric performance than its SiGe nanowire counterpart.

In this thesis, I present a theoretical model for the Si core/SiGe shell core-shell nanowire system. A model for the single carrier pocket core-shell nanowire is first developed, along with the boundary conditions of a circular wire and sharp interfaces between the two media. A numerical scheme is then developed for the core-shell nanowire system, along with educated approximations for the numerical boundary conditions. The numerical model is designed such that low energy levels have higher accuracy than the high energy levels. The core-shell nanowire model is applied to a Si core/SiGe alloy shell structure, which is considered as a core-shell nanowire building block containing multiple carrier pockets. Based on the 2D band structure of strained SiGe on a Si substrate, the strained SiGe layer of the Si core/SiGe shell core-shell nanowire is modeled. The effect of different parameters (the interface offset energy V, the total core-shell diameter e, and the core diameter d) on the energy levels of the Si/SiGe core-shell nanowire system is investigated. It is found that the core-shell nanowire system with the greatest quantum mechanical effect is the one with a small e, a relatively small magnitude V, and a d that results in a secondary confinement effect in the lower potential energy region. A 1D semi-classical transport model for the core-shell nanowire structure based on the Boltzmann transport equation is developed. Applying the 1D semi-classical model to the Si core/SiGe shell core-shell nanowire system, the thermoelectric properties of this particular system and the effect of doping on these properties are investigated. It is found that the system with an optimal doping concentration (n[opt] or p[opt]), a small V, a small e, a small d, and a shell