Theory of optical properties in semiconductor nanostructures

with Huaxiang Fu

Semiconductor nanostructures (e.g., quantum dots, wires, and films) exhibit many novel optical properties different from their bulk counterparts. Such properties include, among others, the localization of electron wavefunctions, the enhanced oscillation strength, controllability of single electrons in transport, and unusual carrier dynamics mechanism, as well as the size-tunable photoluminescence wavelength. Because of their superior materials properties, nanostructures are so important that they have become and will continued to be a subject of focus penetrating very scientific domain. Understanding different aspects of the properties of nanostructures from the fundamental solid state theory is a good starting point for students to expand their scientific horizons, and will motivate students in pursuing their advanced studies. Students are motivated to do science only after they understand them. Training students on the theoretical side serves this purpose well.

We propose to study the electronic structure (including single-particle energy levels, wavefunctions) and optical transitions in semiconductor quantum wires. The materials to be investigated will be Si, CdS, CdSe, and InP. Silicon is a material of adequate abundance, and many technologies have been well established in very large-scale integrated circuits. InP, on the other hand, is a III-V material with high carrier mobility. The single-particle orbital energy and wave function will be studied by solving the fundamental Schroedinger equation. Each atom inside the nanostructures will be counted in the theory by including its contribution in the potential. More specifically, we will use screened atomic pseudopotential approach to solve for the electronic structure. This approach proved to give accurate and reliable results, not only compared well with experimental observations, but also being able to predict physics quantities unknown in experiments. We will study how the indirect band gap semiconductor may turn into a direct band gap when made into nanostructure. Depending on the wire direction, the electronic structures will vary significantly, and it will be very interesting to find out which direction has the highest oscillation strength for a given size of wire. The carrier mobility along wires is a quantity of importance in application, we will calculate the effective masses for differently oriented wires.

The project not only helps students learning semiconductor physics, optical properties, nanostructure physics, it also helps them realizing the importance of computational physics. Computational physics has evolved into an important branch in physics science, independent of experimental inputs. This project, being practicable for the senior undergraduate students within the amount of time allowed, will improve students' scientific capabilities in many aspects, and will largely enhance their confidence in pursuing higher degree.