Structural and Electronic Properties of Ge-based Nanostructures through Density Functional Theory Calculation

Abstract

The performance of silicon based microelectronic circuit is reaching its end of the roadmap with the shrinking down of transistor critical size. Thus, new material systems are required for further improvements in speed and power consumption of the device. Among many candidate materials, Germanium is a possible candidate to substitute silicon for microelectronic devices, as its hole mobility is the highest of all semiconductor materials. Together with its lower band gap, it could be an ideal material for energy-saving devices. This thesis is dedicated to first principles studies of the Ge devices through first principle study. We begin the study of density functional theory calculations in conjunction with thermodynamic modeling to investigate oxygen adsorption on Ge(100) c(4×2) surface and initial oxidation process. For several possible adsorption sites, the adsorption energy of atomic oxygen as well as the atomic configuration and electronic properties of the adsorbed structure were examined. Then, the effect of the surface coverage of oxygen from 1/64 to 1/4 monolayers on the adsorption energy was considered. Through surface Gibbs free energy as a function of temperature and oxygen partial pressure, the (T,P) surface stability diagram was predicted for the O/Ge(100) c(4×2) surface. Our theoretical prediction well reproduces previous experimental observations. Next, first-principles calculations were performed to systematically study the atomic and electronic structures of Ge/a-GeO2 interfaces with various surface orientations of Ge. The study shows that the Ge(111)/a-GeO2 and Ge(100)/a-GeO2 interfaces have the lowest and highest interface energies, respectively. The stability of the Ge/a-GeO2 interface is governed by the interfacial bond density and the minimization of the dangling bonds. We find that the interface region, composed of the Ge suboxides, dominates the electronic structures of the Ge/a-GeO2. The Ge atoms with uncompensated dangling bonds result in various trap states within the band gap of Ge, which is related to the charge neutrality level of the Ge defect. The band offsets between Ge and a-GeO2 show little dependence on the original Ge orientation. For Ge/high-K systems, the effects of the atomic configuration of the epitaxial Ge(111)/La2O3(001) interface on the electrical properties of the structure were studied. The interface stability of this heterostructure is susceptible to the atomic configuration of the interface. The Ge-O-bonded interface without interfacial gap states is generally more stable than the Ge-La-bonded interface, which involves interfacial gap states. The band alignment is affected by the charge transfer depending on the interface atomic configuration, and the band bending across the La2O3 region was observed due to the electronic dipole inside the La2O3. Finally, atomic and electronic properties of Ge nanowire with different factors are investigated. For several possible adsorption sites, the adsorption energy of atomic oxygen as well as the atomic configuration and electronic properties of the adsorbed structure were examined. The adsorption stability is not much affected by the nanowire diameter ranging from 0.8 nm to 2 nm, while the electronic structures like band gap and gap states varies depending on the nanowire diameter. The interface between Ge nanowire and amorphous GeO2 (a-GeO2) were also investigated. The interface region is composed of Ge suboxides as that between Ge bulk slab and a-GeO2. The valence band offsets between Ge nanowire and a-GeO2 considerably decreases as the diameter of nanowire shrinks, while the conduction band offsets show less dependence on the diameter of nanowire.