Three-dimensional supercell simulation of novel semiconductor nanostructures

In this thesis we investigate atomic scale imperfections and fluctuations in the quantum transport properties of novel semiconductor nanostructures. For this purpose, we have developed a numerically efficient supercell model of quantum transport capable of representing potential variations in three dimensions. This flexibility allows us to examine new quantum device structures made possible through state-of-the-art semiconductor fabrication techniques such as molecular beam epitaxy and nanolithography. These structures, with characteristic dimensions on the order of a few nanometers, hold promise for much smaller, faster and more efficient devices than those in present operation, yet they are highly sensitive to structural and compositional variations such as defect impurities, interface roughness and alloy disorder. If these quantum structures are to serve as components of reliable, mass-produced devices, these issues must be addressed.

In Chapter 1 we discuss some of the important issues in resonant tunneling devices and mention some of thier applications. In Chapters 2 and 3, we describe our supercell model of quantum transport and an efficient numerical implementation. In the remaining chapters, we present applications.

In Chapter 4, we examine transport in single and double barrier tunneling structures with neutral impurities. We find that an isolated attractive impurity in a single barrier can produce a transmission resonance whose position and strength are sensitive to the location of the impurity within the barrier. Multiple impurities can lead to a complex resonance structure that fluctuates widely with impurity configuration. In addition, impurity resonances can give rise to negative differential resistance. In Chapter 5, we study interface roughness and alloy disorder in double barrier structures. We find that interface roughness and alloy disorder can shift and broaden the n = 1 transmission resonance and give rise to new resonance peaks, especially in the presence of clusters comparable in size to the electron deBroglie wavelength. In Chapter 6 we examine the effects of interface roughness and impurities on transmission in a quantum dot electron waveguide. We find that variation in the configuration and stoichiometry of the interface roughness leads to substantial fluctuations in the transmission properties. These fluctuations are reduced by an attractive impurity placed near the center of the dot.

The Art of Madness

The nineteenth century was not an entirely kind time for the female artist. Coming of age as the 1800’s bridged into its latter half, literary artists Elizabeth Stuart Phelps Ward, Charlotte Perkins Gilman, and Kate Chopin were all well aware of their uncharitable culture. Equipped with firm feminist bents and creative visions, each of three women produced a seminal work – The Story of Avis, “The Yellow Wallpaper,” and The Awakening, respectively – taking that atmosphere to task. In these stories, each of the three women produces a female protagonist who struggles for having been born simultaneously an artist and a woman. The writers pit their women’s desires against the restrictive latitude of their time and show how such conditions drive women to madness, as a result of which they are forced to either escape into the blind mind of insanity or deal daily with their pain and inescapable societal condemnation. In an age where “hysteria” was a frequent hit in the vernacular, Phelps, Gilman and Chopin use art and literature as mediums to show that, indeed, there is a method behind the madness.