Up to speed - introducing chalcogenides to electronics
Dan Hewak, from the Optoelectronics Research Centre at the University of Southampton, UK, describes advances in new electronic and optoelectronic materials.
The UK has a long history of advancing the field of semiconductor materials and commercialising the resulting devices and products. This dates back to the birth of the transistor, when IBM established a UK manufacturing facility near Edinburgh, paving the way for Scotland’s Silicon Glen. Over the following decades, world leading companies, including Sun Microsystems, Motorola, Agilent, Microsoft, Raytheon, and Oracle Corporation, established themselves there. Meanwhile, the recruitment of silicon design engineers from across the UK continued, down to the southwest corner of England where INMOS and GEC-Plessey Semiconductor created a silicon chip industry.
While the term ‘silicon chip’ is well established, few people can name the new electronic and optoelectronic materials, which are the foundation of today’s research. One of these is a little-known family of glasses – materials based on sulphur and other group VIA elements – that has been establishing itself as the main material for many emerging applications. Used as the active layer in billions of rewritable CD’s and DVD’s, the glasses hold considerable promise as the next generation of memory in iPods, and as highly efficient solar cells.
A chalcogenide glass contains one or more chalcogenide elements, such as sulphur, selenium or tellurium, as a substantial constituent. Glass elements are covalently bonded materials and, although an amorphous or crystalline solid, may be classified as semiconductors with a band gap of approximately two electron volts.
In the UK, work on the application of chalcogenide glasses goes back several decades. Experiments in the 1970s at the Royal Signal and Radar Establishment, Malvern, UK, established the materials as passive optical component materials for infrared applications. At the same time, the Cavendish Laboratory at the University of Cambridge, UK, developed an understanding of the electronic properties of impurity doped amorphous chalcogenides.
After decades of R&D, chalcogenides are now commercially recognised as highly functional. Market leaders such as Intel and ST Microelectronics are committed to phase change based on chalcogenides. Robust investment in thin-film chalcogenide-based solar cell manufacturers shows confidence in these materials. Both memory and photovoltaics are multi-billion dollar industries and offer opportunities for this wide platform technology.
At the University of Southampton’s Optoelectronic Research Centre (ORC), chalcogenide materials and thin films have been studied since 1991 and their range of applications continues to grow. Initially investigated for their optical properties, current research interest is now driven by electrical properties. The ORC has been working on adding dopants to phase change memory materials, for example bismuth significantly reduces crystallisation time, and therefore the read/write speed, while copper doping raises resistivity and reduces power consumption in memory devices.
The ORC has been working closely with high-throughput materials discovery company Ilika Technologies Ltd on new phase change memory materials. Ilika specialises in high throughput, combinatorial R&D techniques and has extended the range of chalcogenides used in memory devices while optimising device performance.
Applications of chalcogenide materials go beyond thin film devices such as memory chips or solar panels. The original application of chalcogenide glass was as a passive infrared transmitting material. Chalcogenide glass can be easily cut and polished, extruded or moulded, making it a versatile optical tool. Chalcogenides based on sulphur have optimum transmission at wavelengths between two and five micrometres, while those based on selenides and tellurides extend to 10µm and beyond.
When used as optical fibres, chalcogenides transmit far beyond the transmission range of conventional silica fibres. These properties allow transmission of infrared laser power, for uses including medical, defence, optical sensing and industrial applications. When doped with rare earth ions, the optical fibres can amplify light or act as a laser host.
Laser action in a chalcogenide glass was first shown at the ORC in 1995 and, since then, bulk glass, thin film, optical fibre and, recently, microsphere lasers have been demonstrated. Other active properties include acousto-optic modulation, which is being studied by the Gooch and Housego Group for laser modulators and Q-switches.
Chalcogenides are a highly nonlinear optical medium and scientists now use this dependence of the refraction index on the applied electric field to produce harmonic generation or frequency shifting. Telecommunications will benefit through a new
generation of high-speed all optical switches.
Chalcogenides exhibit a range of properties that go beyond the electrical and optical. When illuminated, chalcogenide microscopic cantilevers will bend, demonstrating the optomechanical effect in a glass. Other photo effects include photobleaching and photodarkening, both temporary and permanent effects, which have been used in a range of applications.
Research goals are ultimately to introduce chalcogenides into all aspects of modern technology. Work is underway to bridge the gap between the material, conventional CMOS and silicon-based technology. Throughout history it is often the simple technology replacing the complex that enables revolutionary changes. Chalcogenides – materials that are at home as amorphous solids and semiconductors, and exist in multiple crystalline phases – have this potential.