Silicon (Si) is the heart of the electronics industry however the optical characteristics are very poor compared to group III-V semiconductors. The main reason is – all elements in group IV elements are indirect bandgap materials. As a result, non-radiative recombination is the main electron-hole pair recombination process and energy lost in the form of phonon or heat. Among all group IV semiconductors, Germanium (Ge) is a promising material due to its unique properties compared to other group IV members. It has the direct valley (г valley) conduction band which is only 140 meV higher than the indirect valley (L valley). Therefore, with the help of strain or alloying one can easily tune the band structure of Ge and make it a direct bandgap material. The added benefits are- the new material will be compatible with Si CMOS technology.
First Germanium LASER has been fabricated by a research group at MIT (Link). With the combination of both high doping and strain engineering, the group was the first one to make us believe that it is possible to realize Group-IV semiconductor LASER. Unfortunately, the strained Ge LASER is terrible when it comes to efficiency because the threshold current was very high compared to III-V LASER.
There have been major efforts in developing GeSn (Germanium-Tin) alloy. It has high electron and hole mobilities and with 6% or more of Sn, GeSn becomes a direct bandgap material. Despite being a direct bandgap GeSn LASER is also from high threshold current. Till this date, only no electrically pumped GeSn direct bandgap laser has been fabricated.
Our research group (EPEE) was the first group to demonstrate that GeC can be an optically active direct bandgap material with 1% of C. EPEE has proofed it’s claims both theoretically (VASP DFT Modeling) and experimentally (MBE growth). My research is to optimize the GeC growth even more by adding surfactants.
EPEE has two MBE systems at Texas State University- Vecco Gen 930 and Gen II. Gen II is the hybrid gas chamber and Gen 930 is the solid III-V chamber. Hybrid MBE chamber has its own gas cabinet where the precursor stored and controlled with different valves. It has Vecco Sn, Ge, Ga, dopant effusion cells along with atomic H cell, hot injector and cold injector for gas source. Gen 930 has Vecco Arsenic, Antimony cracker cells along with other solid source cells. Both systems and gas cabinet has its own Residual Gas Analyzer (RGA) to monitor the residual particles. Turbo, ion and cryo are the main pumping systems along with respective roughing pump system e.g. scroll. Gen II and Gen 930 are connected through a buffer chamber.
Texas State University’s ‘Shared Research Operation‘ has a vast of analysis equipment which I mostly use in my research project. Mostly used analysis equipment are- Scanning Electron Microscopy (SEM), Xray diffraction (XRD), Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS). Besides this EPEE group has its own Photoluminescence (PL) system for studying the optical behaviour of the device.
Texas State University’s ‘Shared Research Operation‘ also has a class 100, 1000, 10000 cleanroom which I used for device fabrication.
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