In this thesis we investigate atomic scale imperfections and fluctuations in thequantum transport properties of novel semiconductor nanostructures. For thispurpose, we have developed a numerically efficient supercell model of quantumtransport capable of representing potential variations in three dimensions. Thisflexibility allows us to examine new quantum device structures made possiblethrough state-of-the-art semiconductor fabrication techniques such as molecularbeam epitaxy and nanolithography. These structures, with characteristic dimensionson the order of a few nanometers, hold promise for much smaller, fasterand more efficient devices than those in present operation, yet they are highlysensitive to structural and compositional variations such as defect impurities, interfaceroughness and alloy disorder. If these quantum structures are to serve ascomponents of reliable, mass-produced devices, these issues must be addressed.
In Chapter 1 we discuss some of the important issues in resonant tunneling devicesand mention some of thier applications. In Chapters 2 and 3, we describe oursupercell 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 tunnelingstructures with neutral impurities. We find that an isolated attractive impurity ina single barrier can produce a transmission resonance whose position and strengthare sensitive to the location of the impurity within the barrier. Multiple impuritiescan lead to a complex resonance structure that fluctuates widely with impurityconfiguration. In addition, impurity resonances can give rise to negative differentialresistance. In Chapter 5, we study interface roughness and alloy disorder in doublebarrier structures. We find that interface roughness and alloy disorder can shiftand broaden the n = 1 transmission resonance and give rise to new resonancepeaks, especially in the presence of clusters comparable in size to the electrondeBroglie wavelength. In Chapter 6 we examine the effects of interface roughnessand impurities on transmission in a quantum dot electron waveguide. We find thatvariation in the configuration and stoichiometry of the interface roughness leadsto substantial fluctuations in the transmission properties. These fluctuations arereduced by an attractive impurity placed near the center of the dot.
