Gallium-arsenide nanowires are on a silicon surface. Credit: Thomas Stettner/Philipp Zimmermann / TUM
Physicists at the Technical University of Munich (TUM) have developed a nanolaser, a thousand times thinner than a human hair. Thanks to an ingenious process, the nanowire lasers grow right on a silicon chip, making it possible to produce high-performance photonic components cost-effectively. This will pave the way for fast and efficient data processing with light in the future. Ever smaller, ever faster, ever cheaper – since the start of the computer age the performance of processors has doubled on average every 18 months. 50 years ago already, Intel co-founder Gordon E. Moore prognosticated this astonishing growth in performance. And Moore’s law seems to hold true to this day.
But the miniaturization of electronics is now reaching its physical limits. “Today already, transistors are merely a few nanometers in size. Further reductions are horrendously expensive,” says Professor Jonathan Finley, Director of the Walter Schottky Institute at TUM. “Improving performance is achievable only by replacing electrons with photons.
Although silicon-based photonics chips exist, the sources of light for the transmission of data must be attached to the silicon in complicated and elaborate manufacturing processes. Now TUM scientists have developed a process to deposit nanolasers directly onto silicon chips. Growing a III-V semiconductor onto silicon requires tenacious experimentation. “The two materials have different lattice parameters and different coefficients of thermal expansion. This leads to strain,” explains Koblmüller. “For example, conventional planar growth of gallium arsenide onto a silicon surface results therefore in a large number of defects.” They solved this problem by depositing nanowires that are freestanding on silicon their footprints merely a few square nanometers. The scientists could thus preclude the emerging of defects in the GaAs material.
But how do you turn a nanowire into a vertical-cavity laser? To generate coherent light, photons must be reflected at the top and bottom ends of the wire, thereby amplifying the light until it reaches the desired threshold for lasing. “The interface between gallium arsenide and silicon does not reflect light sufficiently. We thus built in an additional mirror – a 200 nanometer thick silicon oxide layer that we evaporated onto the silicon,” explains Benedikt Mayer. “Tiny holes can then be etched into the mirror layer. Using epitaxy, the semiconductor nanowires can then be grown atom for atom out of these holes.”
Only once the wires protrude beyond the mirror surface they may grow laterally – until the semiconductor is thick enough to allow photons to jet back and forth to allow stimulated emission and lasing. “This process is very elegant because it allows us to position the nanowire lasers directly also onto waveguides in the silicon chip,” says Koblmüller.
Currently, the new gallium arsenide nanowire lasers produce infrared light at a predefined wavelength and under pulsed excitation. “In the future we want to modify the emission wavelength and other laser parameters to better control temperature stability and light propagation under continuous excitation within the silicon chips,” adds Finley.
“We want to create an electric interface so that we can operate the nanowires under electrical injection instead of relying on external lasers,” explains Koblmüller. “The work is an important prerequisite for the development of high-performance optical components in future computers,” sums up Finley. https://www.tum.de/en/about-tum/news/press-releases/short/article/32934/
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