Article - Issue 31, June 2007
Professor Richard De La Rue FREng
The optoelectronic semiconductor revolution has already changed lives in many ways and had an impact on numerous aspects of late 20th century living. Professor Richard De La Rue FREng describes how the revolution continues and spreads its influence wider with the discovery of new materials and the design of new semiconductor structures.
The Clifton Suspension Bridge in Bristol is now lit up using Luxeon light-emitting diodes from Crescent Lighting. These semiconductor devices have a very low maintenance cost, minimal power consumption and a very long life expectancy © Pinniger and Partners
Optoelectronic devices are everywhere today. Displays on alarm clocks; the remote control for televisions; mobile phones; CD players; the new traffic lights based on light-emitting-diodes; speed cameras at the side of the road; flat-screen displays; the list is endless.
The one unifying feature in all these applications is that the primary sources of light, as well as the sensitive detectors required, are semiconductor devices. This optoelectronic semiconductor revolution has been enabled by the unique electronic properties of semiconductors that can be manipulated in several ways to emit or absorb light. This revolution is not yet over, in fact it has only just begun. We are still learning about what semiconductors can do and what they can be used for.
Increasing demand and development
The semiconductor laser diode and the light-emitting diode (LED) are the two most important and widely used semiconductor light sources. When compared with traditional incandescent light bulbs, for example, LEDs use a fraction of the electricity and are virtually immune to shock and vibration, greatly reducing replacement and maintenance costs. This advance has seen LEDs go into several markets including replacing the conventional light bulb (see photo of Clifton Suspension Bridge).
The current annual production volumes for LEDs and laser diodes are many tens of billions and about a billion per year respectively. Demand for these devices is expanding rapidly, with annual production growth rates in some countries as high as 50%.
This state-of-play shows that, while semiconductor optoelectronic devices have now been around for a while, there is still plenty of room for growth and improvement. For example, when the CD and DVD replaced audio and video tapes, this was deemed a huge leap forward in data storage and retrieval. But even this has now been improved upon by the invention of Sony’s Blu-Ray technology. By simply changing to a different wavelength of laser (from infra-red to blue), a DVD can store four times as much data.
Experimenting with silicon
The semiconductor optoelectronics industry is not only making incremental improvements. Indeed, some breakthroughs demonstrate that what seems impossible today will become possible in the near future. For example, until recently it was thought that silicon, the most widely used semiconductor of all, could not produce stimulated emission and show optical gain – the key properties of a laser. Because most of today’s electronic circuits are based on silicon, and manufacturing in silicon is therefore cheaper, a silicon laser would be considered a huge breakthrough and enable all-optical computing and communication. Today a number of research groups around the world, including one from industry giant Intel, are working on making silicon lasers and working devices have been reported.
It remains true that silicon is not a ‘natural’ optical gain medium, but one way to extract optical gain from silicon is to hit it very hard with intense short wavelength optical pulses that excite strong high-frequency vibrations in the crystalline lattice while simultaneously launching new photons at longer wavelengths. The latter can then gain power from the former – this process is known as Raman scattering and harnessing it has given us ‘the silicon Raman laser’. But what does not happen in this laser is the direct conversion of electrical current into light, such as happens in conventional LEDs and lasers that exploit compound semiconductors such as gallium arsenide and indium phosphide.
In more recent work a thin layer of a light-emitting compound semiconductor has been ‘welded’ onto a silicon base to make a ‘silicon laser’, but again the efficient conversion of electric current into light occurs in the III-V semiconductor, not in the silicon. Neither of these examples is intended to deny the possible future significance of the silicon laser. They do show, however, how scientists across the world are continually striving to improve the performance of semiconductor materials.
As well as improving on current semiconductor technology, new semiconductors are also emerging. Not long ago it was discovered that polymers can also be semiconductors. Traditionally thought of as insulators, some polymeric materials can conduct electricity and they do so using a conduction process characterised by flows of either electrons or holes, just like traditional semiconductors.
The main advantage that polymers have over traditional semiconductors is their processability; they can simply be painted or spin-coated onto a substrate to make products of various flexible shapes and sizes. When selectively doped structures of conducting and light-emitting organic material are brought together, the result is the organic light-emitting diode (OLED). OLEDs have begun to make an impact in commercial displays, although their device lifetime does not match those for conventional LEDs.
In comparison with LCDs, OLED-based displays have a relatively simple arrangement because they can be organised as arrays of dots (pixels) that emit light in each of the three primary colours. In contrast LCDs require a separate, evenly distributed source of white light that is selectively filtered locally in the array. LCDs are also much more complicated to manufacture as they require additional polarising layers, transparent semiconducting layers and an array of driving transistors.
Organic semiconductors are not only of potential interest for light emission purposes. As with their inorganic counterparts, organic semiconductors can also be used to convert light into electricity, forming photovoltaic elements or solar cells. The economics of applying photovoltaic cells are influenced by their manufacturing costs, their working lifetime, and by their efficiency. If their working lifetime is long enough, low-efficiency voltaic devices that can cheaply cover large areas may be preferable to more efficient devices that are relatively expensive to manufacture and install over large areas. So the organic semiconductor photovoltaic cell may find a role to play.
Once device working lifetimes in the region of a hundred thousand hours become routine, together with electrical to optical conversion efficiencies of about 10%, organic semiconductors seem very likely to become widely used in optoelectronics. Production volumes measured in many millions and market values in many billions of dollars will rapidly become the norm, opening up applications as large as replacing the common light bulb.
Building new structures
As well as incrementally improving existing semiconductors and discovering new ones, researchers are also building optical structures to behave like semiconductors. Photonic crystals are man-made structures that behave photonically rather like semiconductors behave electronically. Made by creating periodic structures of holes in a material, photonic crystals exhibit ‘band-structure’ and can be used to manipulate light by changing its direction or by slowing it down.
Invented around 20 years ago, these structures are beginning to have a major impact on modern day optoelectronics and new developments are appearing almost daily. They are already used in many applications, such as in liquid crystal displays to improve the efficiency of backlighting, in optical switches to transmit data and now even to make an optical buffer – a way of storing light for a short time or providing short-term optical memory. In the particular form known as coupled resonator optical waveguides (CROWs), photonic crystal devices have been shown to be capable of storing light for up to 50 nanoseconds. This may not seem like a long time by some standards, but it corresponds to the passage of 500 bits of data at a rate of 10 Gbit/second which is a useful time interval for reading information packet labels and making switching or routing decisions in high bit-rate communications.
In such applications, a vital objective is to avoid having to transfer signals and information between the optical and electronic domains and back into the optical domain. The aim is to perform the logic operations that are the bread and butter of conventional electronics, but without the waste of time and energy that the optical-electronic-optical conversion processes imply. The objective is thus appropriately called ‘all-optical’ processing. In digital telecommunications networks, the ‘header’ is the coded sequence of pulses that precedes a data stream and identifies where the data signal has come from, indicates how the data that follows the header is encoded (meaning the characteristics of the data), and identifies where the data is meant to be directed or delivered. ‘Recognising’ the header sequence, or understanding it and acting on the information that it contains, is a vital task.
Photonic crystals are just one application of nanophotonics, a new and rapidly growing discipline of photonics that involves the interaction of light with nanoscale particles and structures. Nanophotonics has a huge range of potential applications. Examples include colloidal quantum dots or ‘nanodots’ (nanometre-scale solid particles of semiconducting crystal formed out of a chemical solution) that can be used in medicine; novel photovoltaic structures that behave more like photosynthesisers than like semiconductor solar-cell devices; and displays made from carbon nanotubes.
A further area of fascinating scientific activity has emerged from the recognition that living organisms (‘in nature’) exploit sub-micron scale structurie for functional optical effects. The brilliant blue-green reflectivity produced by the wings of the morpho butterfly is only one example of natural nanophotonics. In the future we may even see direct links being made between genetic engineering and nanophotonics – situations where the information that controls the detailed photonic nanostructures in a living organism (such as the structure of a butterfly wing) may be modified deliberately through changes in the genes of that organism.
Our computers are likely to exploit optoelectronics ever more widely as the preferred means of linking electronic computational zones. This can involve the use of optical fibre cables between entire computers or, on a massively reduced scale but much more densely, communicating information from one part of a silicon chip to another. It may be several decades before the industry switches from electronic interconnection to optoelectronic interconnection at silicon chip level, but when it does happen, it could come about in the space of a year. As we have seen in the past, clever physics and engineering can bring about the seemingly impossible.
Glimpsing the future
Semiconductor optoelectronics will continue to grow in importance in this century and the optoelectronic devices that we encounter in our daily lives will undoubtedly look different to today’s devices. Our computer systems and telecoms networks may well become more secure because they will send information using quantum cryptography – an encryption method that draws on the inherent properties of photons, which become slightly altered if they are observed by an intruder. Flexible display screens will be integrated into car dashboards.
Semiconductor lasers integrated into lab-on-a-chip systems could help to tell us in seconds if we have cancer or not. Our computer screens will be based on carbon nanotubes and will cost less than plasma or LCD screens, consume less energy and deliver a better picture. Our walls could be covered with optoelectronic wallpaper made using synthetic opal and quantum dots, this wallpaper could change to suit the mood of the people in a room or change to provide a different function for a space in a domestic or business situation.
All these predictions may seem like science fiction, but they are all products that are being developed today using semiconductor technology and are being designed to be cheaper, more efficient, more functional and simply better than today’s devices. While some of them may not make it to market, one thing is for sure: the optoelectronic semiconductor revolution is certainly not over – it has only just begun.
What is a semiconductor laser?
The basic ingredients for a laser are the same, no matter whether the laser is the size of a whole building or the size of a grain of sand. A laser cavity consists of a gain medium (gas, liquid or solid) that amplifies light; and feedback structures that reflect the light back through the gain medium.
Semiconductor lasers, or laser diodes,work on the same principle as for bigger lasers. They function as an optical oscillator by stimulating a chain reaction of photon emission inside a tiny cavity. They are made in clean-rooms on crystalline semiconductor wafers by depositing thin films of materials to form different layers. The most common semiconductors used in laser diodes are compounds based on gallium arsenide (with emission wavelengths of 750 to 900 nm in the infrared), indium gallium arsenide phosphide (1200 to 1700 nm in the infrared) and gallium nitride (near 460 nm in the blue or 405 nm in the violet).
In the semiconductor diode laser electrical charge carriers, that is negatively charged electrons (n) and positively charged holes (p), flow towards and are forced together in what is called a p-n junction. Semiconductor lasers now come in many shapes and sizes. The design of the laser and the wavelength that it emits depend on its intended application. For example, the distributed feedback laser (DFB) is the most common transmitter type in large-capacity fibreoptical telecommunications systems. To stabilise the lasing wavelength, a diffraction grating is etched close to the p-n junction – in other words, the optically active region of the diode. This grating acts as an optical filter, causing a single wavelength to be fed back to the gain region to build up laser oscillation. The DFB laser has a stable wavelength that is set during manufacturing by the pitch of the grating, but which can be tuned by a small amount with temperature and by changing the electrical current flow through the diode. Such lasers are the workhorse of modern fibre-optical communications.
Advances in semiconductor lasers
Like many semiconductor lasers, the DFB laser emits from its edge. A relatively new laser design is the vertical cavity surface emitting laser (VCSEL). As its name suggests, this type of laser emits from its surface – and this brings several advantages over edge-emitting lasers. The VCSEL is cheaper to manufacture in quantity, is easier to test, and is more efficient in some situations. In addition, the VCSEL typically requires less electrical current to produce a useful coherent light output. The VCSEL emits a narrower, less divergent and more nearly circular light-beam than do traditional edge emitters, making it easier to get the optical power from the device into the multi-mode optical fibres used in short-distance communications.
Another new type of semiconductor laser is the quantum cascade laser (QCL). Unlike conventional semiconductor lasers, the optical transitions in a QCL occur between electronic sub-bands produced by a repeating multilayer epitaxial structure, rather than between the conduction band and valence bands of the bulk semiconductor. The ’cascade‘ is a series of equal energy steps built into the material matrix as a multi-layer structure as the crystal is being grown on top of its parent wafer. When the electrons are transmitted through the laser crystal cascade, they emit one photon at each cascade step, unlike diode lasers which only emit, at most, one photon per electron transmitted.
The independence of cascade laser operation from the conduction and valence band edge characteristics of the bulk semiconductor allows much greater flexibility in the emission wavelengths obtainable than from conventional semiconductor materials such as the GaAs/AlGaAs material system. Furthermore, laser operation from indirect bandgap materials such as the Si/SiGe material system has now become possible. The quantum cascade laser’s high optical power output, tuning range and room temperature operation make it useful for spectroscopic applications like the remote sensing of environmental gases and pollutants in the atmosphere. It may eventually be used for vehicular cruise control in conditions of poor visibility, collision avoidance radar, industrial process control, and in medical diagnostic applications such as breath analyzers.
For more information about semiconductor lasers, see http://en.wikipedia.org/wiki/Diode_laser
BIOGRAPHY – Professor Richard De La Rue FREng
Richard De La Rue is Professor of Optoelectronics at the University of Glasgow. His research is focused, in particular, on the creation of novel optoelectronic devices – using a variety of nanophotonic and photonic crystal based technologies.
Additional reporting by Nadya Anscombe, freelance science journalist (http://www.nadya-anscombe.com/)