Silicon is a prodigious material for electronics. Its useful electronic properties, high abundance, low cost and excellent processability helped to stimulate a revolution in silicon technology: the development of mass-produced silicon chips, which allow computing capabilities to be integrated into almost any device. But, alas, silicon is an inefficient absorber and emitter of light, preventing it from being used in many photonics applications. Writing in Nature, Fadaly et al.1 report the development of silicon–germanium alloys that have excellent optoelectronic properties, and could thus aid the development of photonics technologies that are compatible with currently available silicon electronic devices.

Read the paper: Direct-bandgap emission from hexagonal Ge and SiGe alloys

Silicon’s lack of useful optoelectronic capabilities is due to its electronic properties — it is said to be an indirect-bandgap semiconductor. As an example of the problem, solar cells based on silicon must be at least 100 times thicker than those based on gallium arsenide (a direct-bandgap semiconductor, which absorbs and emits light efficiently) to collect the same amount of light, but they still convert the light into electricity much less efficiently2. And silicon-based lasers remain an unrealized dream, even after decades of intense research efforts. Instead, lasers are typically made using ‘compound’ semiconductors, which incorporate costly elements such as indium or gallium. The components used to absorb or emit light in currently available silicon photonics schemes are also mostly made from compound semiconductors, and are usually bonded to the silicon or used off-chip3.

Several generations of scientists have tried to convert silicon and silicon-containing alloys into materials suitable for optoelectronics (optoelectronic-grade materials) by modifying the electronic band structure of silicon in different ways. Fadaly et al. make use of a strategy known as zone folding, which was originally outlined4 in the 1970s. The idea is that the presence of a periodic electric potential in an indirect-bandgap semiconductor could transform it into a direct-bandgap semiconductor.

Until now, the best example of this approach was the production of a pseudodirect-bandgap semiconductor (which absorbs and emits light more efficiently than do indirect-bandgap semiconductors, but less efficiently than do direct-bandgap ones), in a special type of silicon–germanium alloy known as a superlattice, reported5 in 1992. This was achieved by alternating atomic layers that have different compositions of atoms, but the resulting material still couldn’t absorb or emit light efficiently enough for potential applications.

Nearly three decades later, Fadaly and colleagues have taken a different approach. Instead of modifying the atomic potential by alternating layers of different composition, they alternate two types of atomic stacking in germanium and in silicon–germanium alloys. This changes the symmetry of the materials’ crystal lattices from a cubic form to a hexagonal one (Fig. 1).

Figure 1 | Cubic and hexagonal crystal lattices. a, The silicon used for electronics has a cubic crystal lattice, which causes the material to be a poor absorber and emitter of light — limiting its use in optoelectronics. b, Fadaly et al.1 report a way of producing germanium and silicon–germanium alloys that have a hexagonal lattice. The resulting materials are good light absorbers and emitters, and would be compatible with existing silicon electronics technology, potentially opening the way to the development of new optoelectronic devices.

The unit cell (the smallest repeating unit) of the hexagonal lattice contains twice as many atoms as the unit cell of the cubic form. This halves the size of the Brillouin zone — a unit cell of the abstract ‘momentum space’ that is used to describe the properties of electrons in semiconductors. This size reduction, in turn, results in the folding of the materials’ electronic bands in momentum space, moving the position at which the energy value of the conduction band is at a minimum to the centre of the Brillouin zone, and thus generating a direct bandgap. Fadaly et al. use quantum-mechanical calculations to determine the exact band structure of germanium and silicon–germanium alloys in the hexagonal crystal structure, thereby confirming that these materials have a direct bandgap.

Most importantly, the authors demonstrate that hexagonal germanium behaves as an optoelectronic-grade direct-bandgap semiconductor. Moreover, by alloying hexagonal germanium with different amounts of silicon, they find that they can tune the energy of photons emitted from the resulting materials from 0.3 to 0.67 electronvolts. These emission energies are extremely relevant for chemical sensing and optical-communication technologies3,6.

Electronics and photonics united

All of this work was made possible by producing the materials in the form of nanowires, filamentary crystals that have a tailored diameter of between a few and a few hundred nanometres. In simple terms, the high surface-to-volume ratio of nanowires enables the formation of metastable crystalline phases such as hexagonal silicon or germanium7,8. Fadaly et al. are the first to report that defect-free hexagonal germanium and silicon–germanium alloys can be made in a scalable manner using this approach.

The nanowire structure also offers another advantage: it causes light to interact with the nanowire material in a way that is ideal for photonic applications9,10. For example, nanowire shape can be engineered to ensure efficient light absorption and to prevent light from being trapped in the nanostructure by internal reflection. Nanowires could potentially also be used in light detectors for the ultra-rapid collection of the charge carriers produced from incoming photons, an effect that might be extremely useful for high-speed telecommunications.

Fadaly and colleagues’ findings could potentially lead to the development of the first silicon-based laser, or be used to make mid-infrared light detectors, both of which would be compatible with the complementary metal-oxide semiconductor (CMOS) silicon technology that underpins much of the circuitry in computers. Such mid-IR detectors could be used in a scalable and economic lidar platform — a laser-based surveying technology that could be used by self-driving vehicles to detect objects. Mid-IR light does not damage the human eye, which means that mid-IR lasers could be used at high power in lidar systems; this enables object detection at long distances, thus allowing self-driving vehicles to travel safely at high speeds11. More broadly, the development of silicon-based alloys that have optoelectronic functionality could spark a second revolution in silicon technology, this time in silicon photonics.