Characterization of GeSn Semiconductors for Optoelectronic Devices

Germanium-tin alloys with Sn compositions higher than 8 at. % to 10 at. % have recently attracted significant interest as a group IV semiconductor that is ideal for active photonics on a Si substrate. The interest is due to the fact that while at a few percent of Sn, GeSn is an indirect bandgap semiconductor, at about 8 to 10 at. % Sn, GeSn transitions to a direct bandgap semiconductor. This is at first surprising since the solid solubility of Sn in Ge under equilibrium growth conditions is limited to only about 1 at. %. However, under non-equilibrium growth conditions, Sn concentrations in GeSn of more than 20 at. % are reported. At these high concentrations several problems arise due to severe lattice mismatch and chemistry that can have serious impact on the optical quality and stability of GeSn optical devices. As a result, it is important to understand and perhaps control the growth of GeSn semiconductors containing such high Sn concentrations. This requires an investigation of the measurement, interplay, and role of composition, strain, defects, structure, segregation, and precipitation in GeSn/Ge/Si(001) heterostructures and is the subject of this dissertation More specifically, in this study, the experimental and theoretical x-ray diffraction analysis is shown as a reliable technique for non-destructive and precise characterization of composition, strain, defects, structure, segregation, and precipitation in GeSn. As a result, under an annealing treatment of GeSn/Ge/Si(001) heterostructures, the interplay of these properties was correlated with the stability and optical quality. For example, the density of misfit dislocations of ~2 × 105 cm-1 in the as-grown GeSn layers was shown to be correlated to the onset of Sn segregation. These results provide insight that can be used for the growth of metastable GeSn alloys with improved Sn content, thermal stability, and optical quality

Electron Accumulation Tuning by Surface-to-Volume Scaling of Nanostructured InN Grown on GaN(001) for Narrow-Bandgap Optoelectronics

The existence of an uncontrolled electron accumulation layer near the surface of InN thin films is an obstacle for the development of reliable InN-based devices for use in narrow-bandgap optoelectronics. In this article, we show that this can be regulated by modulating the surface of the InN grown on GaN(001). By increasing the surface-to-volume ratio, we can demonstrate a reduction in the surface carrier concentration from ∼1018 to ∼1017 cm–3. These controlled changes are despite the idea that donor-type surface states, which contribute to conduction band electrons are reported to be the main origin of the surface charge density. Additionally, by evaluating the surface carrier concentration through modeling of photoluminescence (PL) spectroscopy, we have found a failure of the Burstein–Moss theory. Conversely, modeling of the longitudinal optical phonon–plasmon coupled modes measured using Raman spectroscopy, simulations of InN structures using the k·p method, and Hall effect measurements, where possible, showed an excellent correlation of the surface electron concentrations. The large inhomogeneous broadening in the PL, which overwhelms any broadening due to the Burstein–Moss effect, is understood to be the result of varying Stark shifts due to varying strain throughout high surface-to-volume nanostructures, which dramatically affects the spatially indirect nature of the electron–hole recombination. Finally, our findings demonstrate how the electron population of 2D and 3D InN nanostructures can be tuned by structural features, such as porosity and/or the surface-to-volume ratio

Growth of Germanium Thin Films on Sapphire Using Molecular Beam Epitaxy

Germanium films were grown on c-plane sapphire with a 10 nm AlAs buffer layer using molecular beam epitaxy. The effects of Ge film thickness on the surface morphology and crystal structure were investigated using ex situ characterization techniques. The nucleation of Ge proceeds by forming (111) oriented three-dimensional islands with two rotational twin domains about the growth axis. The boundaries between the twin grains are the origin of the 0.2% strain and tilt grains. The transition to a single-grain orientation reduces the strain and results in a better-quality Ge buffer. Understanding the role of thickness on material quality during the Ge(111)/Al2O3(0001) epitaxy is vital for achieving device quality when using group IV material on the sapphire platform

Electron Accumulation Tuning by Surface-to-Volume Scaling of Nanostructured InN Grown on GaN(001) for Narrow-Bandgap Optoelectronics

The existence of an uncontrolled electron accumulation layer near the surface of InN thin films is an obstacle for the development of reliable InN-based devices for use in narrow-bandgap optoelectronics. In this article, we show that this can be regulated by modulating the surface of the InN grown on GaN(001). By increasing the surface-to-volume ratio, we can demonstrate a reduction in the surface carrier concentration from ∼1018 to ∼1017 cm–3. These controlled changes are despite the idea that donor-type surface states, which contribute to conduction band electrons are reported to be the main origin of the surface charge density. Additionally, by evaluating the surface carrier concentration through modeling of photoluminescence (PL) spectroscopy, we have found a failure of the Burstein–Moss theory. Conversely, modeling of the longitudinal optical phonon–plasmon coupled modes measured using Raman spectroscopy, simulations of InN structures using the k·p method, and Hall effect measurements, where possible, showed an excellent correlation of the surface electron concentrations. The large inhomogeneous broadening in the PL, which overwhelms any broadening due to the Burstein–Moss effect, is understood to be the result of varying Stark shifts due to varying strain throughout high surface-to-volume nanostructures, which dramatically affects the spatially indirect nature of the electron–hole recombination. Finally, our findings demonstrate how the electron population of 2D and 3D InN nanostructures can be tuned by structural features, such as porosity and/or the surface-to-volume ratio