Probing the potential of hexagonal silicon-germanium for lasing
Marvin van Tilburg defended his PhD thesis at the Department of Applied Physics and Science 果冻传媒 on October 1st.
There is persistent drive to improve computing technologies. Using light rather than electricity would drastically improve the computer chip and bring the high-speed capabilities of fiber optics to the chip scale. However, a silicon-based laser is the last puzzle piece needed. For years, the idea of a silicon-based laser was just a dream, as conventional silicon is incapable of emitting light. However, in his PhD thesis, Marvin van Tilburg shows that a silicon laser can become a reality.
Conventionally, silicon is a semiconductor material that cannot emit light efficiently due to its inherent material properties. However, by slightly altering the way that atoms are stacked in their crystal structure (changing it from a cubic to a hexagonal structure) and introducing germanium, a semiconductor has been created that can efficiently emit light as well as any other state-of-the-art LED material.
This is material is known as hexagonal silicon-germanium (hex-SiGe). The big question though is if one can create a laser using hex-SiGe, as many other vital material properties related to lasing are still not known.
Chasing lasing
In his PhD thesis, investigated whether lasing is possible from hex-SiGe. Van Tilburg鈥檚 research involved growing hex-SiGe as tiny nano-scaled wire structures called nanowires.
Along with his collaborators, van Tilburg characterized several material properties by investigating how individual hex-SiGe nanowires emit light when optically pumped with a separate light beam. This showed him whether the inherent material properties of hex-SiGe allow for lasing to occur and under which circumstances lasing is then possible.
Room temperature
The researcher discovered that hex-SiGe is capable of efficient light emission even near room temperature, which is important for practical applications.
Crucially, the properties of the luminescence are in line with what is expected from an efficient light-emitting semiconductor. Additionally, the electrons in hex-SiGe can relax in energy rapidly, which is vital towards achieving the quick, constant light emission required for a laser. Van Tilburg determined that hex-SiGe did not underperform in either property in comparison to state-of-the-art lasing materials. Therefore, this is suitable for lasing applications.
Testing the potential
To test the potential for lasing, he designed two different nanostructures around a hex-SiGe nanowire. This enhanced the confinement of light which allows the light to interact with the material for a longer time, facilitating lasing.
One design involves a hex-SiGe nanowire lying flat on a substrate sticking out over the edge of a substrate into the air. The second was a more complex design where the hex-SiGe nanowire lies flat on top of a silicon stadium structure acting as an external resonator.
Optical investigations
Van Tilburg investigated both structures optically for any signatures of lasing. The suspended nanowire structure showed clear signs of stimulated emission, the driving mechanism behind lasing, but the lasing was prevented due to large losses caused by the excitation method.
However, from the microstadium resonator, the first signatures of lasing from hex-SiGe were observed. These included enhanced efficiency of light emission, which occurred within picoseconds, and emission at a specific wavelength.
While definitive proof of lasing from hex-SiGe was not achieved, van Tilburg determined that lasing is possible as he observed its first signatures. This marks a significant step toward developing a silicon-based laser and brings researchers closer to the goal of fully integrated silicon photonic circuits. It鈥檚 now only a matter of time as to when it will happen.
Title of PhD thesis: . Supervisors: Jos Haverkort and Erik Bakkers.