As Einstein revealed, light is the fastest thing that exists and conveniently, it can be controllably transferred in waveguides. For example, this is why the fibre connected internet is so fast. Waveguides can be also fabricated of silicon in silicon oxide forming small photonic integrated circuits. Most of the components of microchips can also be fabricated as their optical counterparts in photonic integrated circuits. These photonic circuits might offer a huge technology leap in computing in large data centers, that use huge amounts of energy globally.
Even though the photonic circuits are industrially scalable and well‒known process, the last pieces of the puzzle are still missing. Silicon is a poor material for fabrication of light sources or detectors suitable for the integrated circuits. The current light sources are fabricated of III-V semiconductors as separate chips and then transferred on the integrated circuits. Unfortunately, these materials can not be directly grown on silicon due to large lattice mismatch which means that their atoms want to remain at different distances than in silicon. This is one of the issues that semiconductor nanowires developed in Eero Koivusalo’s dissertation could solve. The nanowires are over thousand times thinner than a human hair and can thus release the strain caused by the lattice mismatch laterally.
III-V semiconductor nanowires have been already used to fabricate lasers, LEDs, solar cells and optical detectors. However, the fundamental research around their growth mechanisms is still an active field of research. Koivusalo’s dissertation focuses on these fundamentals utilizing molecular beam epitaxy technology.
“A novel nanowire growth method is introduced in my dissertation. It is used for example to grow nanowires with record size uniformity, which is very important as the nanowire’s size determines how it interacts with light. I also present a method to controllably switch the nanowire growth direction. This method could be used to fabricate complex nanowire devices used to demonstrate quantum phenomena,” explains Koivusalo.
“The most important achievements in my dissertation are related to the composition of the nanowires and incorporation mechanisms of different elements into them. Doping nanowires to conduct electricity and tuning their emission wavelength by changing the elemental composition is challenging due to their peculiar growth mechanism. This makes advancing the fundamental aspects of dopant incorporation and achieving light emission at telecommunication wavelengths the greatest results in my dissertation,” lists Koivusalo.
The doctoral dissertation of MSc (tech.) Eero Koivusalo in the field of photonics titled Advancing the Fundamentals in Molecular Beam Epitaxy growth of GaAs-based Nanowires will be publicly examined in the Faculty of Engineering and Natural Sciences at Tampere University Hervanta Campus, auditorium S2 of Sähkötalo building, Korkeakoulunkatu 3, Tampere on the 20th of November at 12 o’clock. The opponent will be professor Lucia Sorba from the Institute of Nanoscience, CNR, Italy. The custos will be professor Mircea Guina from the Faculty of Engineering and Natural Sciences, Tampere University.
The dissertation is available at http://urn.fi/URN:ISBN:978-952-03-1761-4
The public defence can be participated remotely.
Photo: Riku Isoaho