Growth and characterization of GeSn and SiGeSn alloys for optical interconnects [electronic resource]
Keywords: Materials Science and Engineering

Over the past few decades, the microelectronics industry has achieved previously unimagined success. Following Moore's law, the number of transistors in a chip has approximately doubled every 18 months and the size of a transistor has reduced down to 32nm. However, as the device size scales down, one of the major limitations in today's silicon integrated circuits comes from the electrical interconnects. In order to increase the interconnect density and decrease the interconnect energy, optical interconnects between chips or even on-chip have been proposed and widely investigated. The big challenge of integrating optics onto a Si chip is the compatibility, so group IV materials are considered. Optoelectronic devices for optical interconnects, such as modulators and detectors, have been demonstrated using both Si and Ge. A major issue now is the lack of a Si compatible light source. Because Si itself has very poor light emitting efficiency, current research focuses on Ge based semiconductors. There are two promising approaches to modify the band structure of Ge and make it a direct band gap material. This dissertation focuses on both of these approaches: applying biaxial tensile strain and alloying with Tin (Sn). In addition, combining these two methods is expected to achieve the goal with a more stable materials system. Therefore the ability to decouple these two effects and investigate the material properties independently is critical in our research. Ge1-xSnx alloys were grown by molecular beam epitaxy (MBE) machines at low growth temperatures (150-200⁰C) on InGaAs buffer layers on GaAs substrates. Ge1-xSnx alloys with up to 10.5% Sn have been demonstrated with high crystal quality in this dissertation. Crystal quality of Ge1-xSnx layers was characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Composition and strain were studied by X-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS). The optical properties of Ge1-xSnx alloys were determined by photoreflectance (PR) and photoluminescence (PL). The advantage of using InGaAs buffer layers is the separate control of the strain and composition effects on the material properties of Ge1-xSnx alloys. In this dissertation, Ge1-xSnx alloys with different Sn compositions and various levels of strain were grown. Room temperature PR measurements were used in this work to determine the direct bandgap from the maxima of the light- and heavy-hole bands to the bottom of [upper case Gamma] valley. The energy bowing parameter (bGeSn) was calculated from the bandgap of unstrained Ge1-xSnx alloys to describe the composition effect. The indirect to direct band gap transition for unstrained Ge1-xSnx alloys was estimated to be around 6~7% Sn composition from low-temperature PL studies. The effect of biaxial strain on the direct band gap, described by two deformation potentials (a and b), was investigated for the first time as well. These basic parameters are very useful for the design of optoelectronic devices based on strained Ge1-xSnx alloys. Additionally, the strain and composition contributions to Raman shift of Ge-Ge LO peak in Ge1-xSnx alloys were quantified separately as well. Moreover, Ge1-xSnx/ SixGe1-x-ySny quantum well (QW) structures are of great interest for photonic devices. Due to the large direct band gap of Si, SixGe1-x-ySny alloys have larger bandgap energies than Ge1-xSnx alloys by design, so as barrier layers they can confine injected carriers inside the active Ge1-xSnx well. Good crystal quality of SixGe1-x-ySny alloys were grown by MBE at low temperatures and annealed by RTA at 500⁰C in a forming gas ambient. The decoupling of the direct band gap and lattice constant of SixGe1-x-ySny alloys were demonstrated. This feature simplifies the strain engineering in QW designs. Finally, PL from SixGe1-x-ySny/Ge1-xSnx/ SixGe1-x-ySny double heterostructure was demonstrated experimentally for the first time. The measured PL peak energy matches the calculated value at room temperature, indicating that the basic materials properties determined in this dissertation are accurate. The observation of PL proves that these group IV alloys are promising candidates to make a Si-compatible laser for on chip optical interconnects.