The notion of converting electronically generated information into light emissions and vice versa might initially appear somewhat abstract. However, this concept – known as optoelectronics or optronics – has long been part and parcel of our daily lives.
Whether it’s lasers, screens, computers or optical storage solutions – they all rely on components that combine optics with semiconductor electronics. Nowadays, the technology for generating, recording and controlling light – optoelectronics – is used in a wide variety of applications. The most obvious optoelectronic components are doubtless light-emitting diodes (LED). Thanks to their energy efficiency, they aren’t just a popular solution for lighting systems in buildings, they can also be found in high-resolution displays on smartphones, in TV sets, in vehicle lighting systems, in the telecommunications industry and in industrial production processes. An LED is a semiconductor device that emits light when an electric current flows through it.
Although the biggest leaps in the development of LED technology have already been made, there is still potential for innovation in this field. For example, chemists from the University of Jena have discovered a fluorescent aluminium complex with the highest quantum yield known to date: for virtually every photon absorbed by the substance, a photon is emitted. This could be of benefit in applications such as LED technology. “The record so far for aluminium complexes is around 70 per cent,” explains Robert Kretschmer, Junior Professor for Inorganic Chemistry of Catalysis at Friedrich Schiller University Jena. “This means that with this quantum yield, for every ten light particles absorbed, seven new ones are emitted by the substance. In our complex, however, almost every light particle is converted into a new one.”
Disinfection using light
During the Covid-19 pandemic, one particular use case for LED technology attracted particular attention: LEDs that emit ultraviolet (UV) radiation can be used to disinfect surfaces, air and water quickly, in an eco-friendly way, without chemicals. For example, the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik (FBH) has developed UV-LED-based radiation systems that will in future be able to inactivate multi-resistant pathogens such as MRSA and coronaviruses like SARS-CoV-2 directly on humans with no harmful effect on the skin. Each system is equipped with 120 LEDs that emit light at a wavelength of 233 nanometres. Thanks to optimised semiconductor epitaxy and chip process technology, these new-generation LEDs can be operated with currents that are twice as high as before – they provide more than 3 milliwatts of output power at 200 milliamperes.
A new technology for displays
Miniaturisation is also all the rage in the field of optoelectronics – just look at micro-LEDs, for example. They measure less than 50 micrometres and require much less space to generate pixels. According to market analysts from MarketsAndMarkets, the market for micro-LEDs is set to reach 21,169 million US dollars in 2027 – equivalent to annual growth of 81.5 per cent between 2021 and 2027.
Full-colour micro-displays can be created by combining red, green and blue (RGB) micro-LEDs. Compared with current display technology, micro-LEDs provide a greater pixel density, a longer service life, increased brightness, a higher switching speed and a wider colour spectrum. What’s more, micro-LEDs also stand out from the pack with their very low energy consumption – making them ideal for future generations of small mobile devices where space for batteries is at a premium.
Like conventional LEDs, micro-LEDs are also manufactured using the MOCVD (metal organic chemical vapour deposition) method, where the semiconductors are applied to the substrate material, atomic layer by atomic layer, each just one atom thick. However, the requirements for the production process are much higher.
Light from silicon
Despite the aforementioned progress, the ultimate challenge in the field of optoelectronics is finding a way to combine silicon and photonics directly. If this were achieved, future microchips would be able to transmit signals and information not by current, but instead by light pulses – which do not generate any waste heat and would enable much faster data transfers. As a result, researchers have been working for 50 years on building light-emitting components from silicon or germanium. Their efforts have, so far, been unsuccessful. Silicon, the workhorse of the chip industry, normally crystallises in a cubic crystal lattice. In this form, it is unsuitable for converting electrons into light.
However, researchers at Eindhoven University of Technology have now managed to develop alloys made of germanium and silicon capable of emitting light. The crucial step was to produce germanium and alloys made from germanium and silicon with a hexagonal crystal lattice. “This material has a direct band gap, and can therefore emit light itself,” says Prof. Jonathan Finley, Professor of Semiconductor Nanostructures and Quantum Systems at the Technical University of Munich. “If we can implement on-chip and inter-chip electronic communications by optical means, speeds can be increased by a factor of up to 1000,” says Finley. “In addition, the direct combination of optics and electronics could drastically reduce the cost of chips for laser-based radar in self-driving cars, chemical sensors for medical diagnostics, and air and food quality measurements.”