Cum laude for light-emitting silicon
No longer a 'holy grail' in microelectronics
Chips based on light would revolutionize microelectronics. Elham Fadaly laid the basis for this ‘holy grail’. She managed to let silicon emit light, by forcing it into a new shape. Her work was proclaimed Breakthrough of the Year by Physics World and she received the Nanotechnology Young Researcher runner-up award. Fadaly obtained her PhD cum laude on April 16th at the department of Applied Physics.
Silicon is a powerful material pervading our everyday lives. Silicon-based microchips are the basis of nearly every electronic device used in our houses, cars, smart gadgets, and even in the human body. The material is cheap, has superior electronic properties and a mature processing technology.
But silicon has been widely known as optically handicapped. It is an extremely inefficient light emitter, hindering it from being employed in laser devices: the basis of fast computing and high-speed communications. Solving this holy grail would revolutionize computing, making chips faster than ever before.
From cubic to hexagonal
Fadaly: “If we could force SiGe to emit light, it would be an ideal material for uniting the electronic and optoelectronic functionalities on a single chip, opening new frontiers towards silicon-based integrated device concepts.”
Efficient light emission for optical telecommunications needs an infrared wavelength range of 3.5-1.8 μm. But silicon germanium (SiGe) alloys exist naturally in an optically inactive cubic structure. Efficient light emission has been predicted to be possible, when forcing the silicon into a hexagonal structure. It is, however, extremely challenging to achieve the hexagonal structure in this class of materials.
Hexagonal nanowires as template
During her PhD, Fadaly developed high-quality hexagonal Si-based alloys in big volumes. These alloys proved to be capable of emitting light efficiently and had excellent optoelectronic properties. Fadaly: “I changed the arrangement of the atoms of the natural cubic silicon structure to the promising hexagonal one. To do so, I used hexagonal nanowire templates to transfer the crystal structure to SiGe in a core-shell geometry by utilizing the crystal transfer technique.”
In collaboration with her colleagues, Fadaly has examined the quality of the fabricated material via several structural and optical characterization techniques, confirming its premium quality. She therefore identified an unconventional type of crystal defect in this novel material and understood its formation mechanism, which helped to avoid its occurrence and to produce high-quality crystals.
But that is not all. She made it possible to tune the emitted wavelength over a broad range while preserving the superior optical properties by controlling the Si with Ge alloy composition.
Erik Bakkers, Elhams supervisor, says; “The achieved experimental findings of Elham Fadaly are in excellent quantitative agreement with theoretical calculations. Furthermore, the Hex-SiGe emission yield is similar to that of direct-bandgap group-III–V semiconductors, the current state-of-the-art laser materials, a result nobody expected on forehand. With this great work, Elham easily belongs to the top few per cent of her peer PhD fellows in the international research community”.
Next step: integrated laser
Having proved efficient light emission in silicon, demonstrating lasing in this novel material is the next important milestone. The finding of this work could potentially lead to the development of the first silicon-based laser or mid-infrared light detectors, both of which would be compatible with the current silicon technology.
These lasers could be deployed in several applications such as telecommunications, LiDAR, a radar with laser for self-driving cars, and chemical sensors for medical diagnosis or measuring air and food quality.
Title of PhD-thesis: Epitaxy of Hexagonal SiGe Alloys for Light Emission. Supervisors: Erik Bakkers (TU/e), Marcel Verheijen (TU/e and Eurofins), and Jos Haverkort (TU/e). Other main parties involved: University of Jena, TU Munich, Johannes Kepler University Linz, University of Oxford, IBM-Zurich.
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