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Research
High-efficiency laser for silicon chips
For tomorrow’s technology, large amounts of data must be transmitted extremely quickly in the smallest of spaces. Electrical transmission reaches its limits here. Light is seen as the solution of the future. Prospectively, light could be generated by a new laser that can be integrated directly into silicon chips.
Data can be transmitted much faster with light than with electronic methods, and optical transmission requires less energy. For distances of more than one metre, optical lines such as fibre optic cables are already standard. The situation is different for very short distances, such as from microchip to microchip in a computer. One problem is the minute light source that has to be placed on the chips. Researchers have been looking for solutions to this for a long time. One approach is to insert a laser directly into the chips that are made of the standard material silicon. A semiconductor laser made of a germanium-tin compound is promising for this purpose. Since germanium and silicon have similar properties, the two can be combined well – unlike silicon and conventional semiconductor lasers such as gallium arsenide lasers.
This is how fast data is transferred
(gigabit/second)
WLAN
9.7
Internal computer interface
63
Fibre optic cable
200,000
WLAN
9.7
Internal computer interface
63
Fibre optic cable
200,000
Germanium-tin laser
Scanning electron micrograph of the germanium-tin laser (left). The germanium-tin layer, which is only a few micrometres wide, is applied to a so-called silicon nitride stressor layer and an aluminium base for better heat dissipation, and then coated with silicon nitride (right). The orientation of the germamium-tin compound to the other atomic spacings in the crystal lattice of the silicon nitride creates a tension in the embedded material, which ultimately results in optical amplification. Already in 2015, an international team led by Dr. Dan Buca from the Peter Grünberg Institute (PGI-9) demonstrated that a laser can be made from germanium and tin and integrated directly into silicon chips during production. The Jülich scientists have now further optimised the laser with French partners from the Centre de Nanosciences et de Nanotechnologies, the STMicroelectronics company and the CEA-Leti Institute.
Preparation of the germanium-tin compound
Production of the highly concentrated germanium-tin compound (chemical vapour deposition, CVD): germanium and tin are inserted in the form of gaseous compounds such as G2H6 or SnCl4 and decomposed into reactive radicals that have a strongly exothermic reaction to the heated substrate, releasing hydrochloric acid (HCl). The resulting heat of reaction contributes locally to the deposit of germanium and tin into the crystal lattice. The process takes place below the actual crystallisation temperature. If the temperature were higher, the tin would “perspire” again above the saturation point. Previously, the problem has been that a laser source with germanium only works with a high dose of tin. At the same time, however, this reduces efficiency. “We were now able to reduce the tin concentration by additionally bracing the material. This makes our laser around 100 times more efficient than other lasers made from this combination of materials. The efficiency values are even comparable to those of gallium arsenide lasers,” reports Dan Buca. What is more, the new laser can also produce a continuous beam of light, not just short pulses – and it is cost-effective. “With the improvements to the laser, potential applications are one step closer,” explains the division’s Director Prof. Detlev Grützmacher. One example is artificial intelligence (AI) in autonomous driving: here, sensors provide large amounts of data with which AI is trained within a very short time. The laser technology, which is already patented, could also be used in infrared and night vision systems or in gas sensors for monitoring environmental and respiratory gases.
At present, however, the new laser is still limited to low temperatures in the range of minus 200 to minus 170 degrees Celsius. In addition, it is currently stimulated with light. Excitation by electrical signals would be ideal – which is another major challenge for transmitting data with light in computers.
Christian Hohlfeld
How a laser works
In a laser (light amplification by stimulated emission of radiation), energy is supplied to the laser medium by a pumping process. Pumping can be done optically, by irradiating light, or electrically; the required pumping rate can vary greatly depending on the laser. This way, the excited electrons are “pumped” to a higher metastable energy level. These states should last as long as possible so that an “occupation inversion” can be built up, during which a large number of the atoms or molecules are in the excited state.As soon as one of the excited states falls back into its ground state, a photon is emitted. If this photon encounters other excited states, these are also triggered to fall back into their ground state and emit an additional photon at the same time. This process is called “stimulated emission”. Through this doubling of the stimulating photon, the laser medium acts like a light amplifier. The “freshly created” second photon then, in turn, excites other excited atoms or molecules to emit. A chain reaction occurs in which a standing wave forms between the two mirrors on the sides of the laser medium where laser radiation is being emitted on one side through the semi-transparent mirror.
Grafics: Forschungszentrum Jülich / Tobias Schlößer