Transparent electronic materials will make it possible to build a new generation of smart objects.
After crash-landing on Mars in the 2000 movie 'Red Planet', Val Kilmer tries to work out where he and his team have wound up on the surface. So, he unrolls a see-through computer that tries to match the local landscape with the images collected by scores of unmanned Mars probes over the years.
It was a bomb at the box office. Ten years on, 'Red Planet' is not showing much sign of becoming a cult classic and ultimately profitable like 'Blade Runner'. But it's still inspiring engineers to work out how to make a roll-up, see-through map.
Tolis Voutsas, director of the materials and devices applications lab at Sharp Laboratories of America, says: ''Red Planet' was shown in 2000. And we still don't have technology to do this. But thanks to Hollywood we still have the vision.'
Director Antony Hoffman reckoned it might take a while to realise the transparent map. 'Red Planet' was set in 2056. Engineers such as Chris Bower, principal scientist at Nokia's research centre in Cambridge, are hoping that they can develop something similar much more quickly.
Working on morph
A couple of years ago, Nokia unveiled what it called the Morph concept. A set of videos showed what the portable computer and phone of the future might look like. Bower explained the idea at the Printed Electronics conference in Dresden in April: 'You can take a standard candy-bar phone and transform it. You can wrap it around your wrist so that it becomes a wearable device.
'We are working hard to enable the Morph concept. We are trying to build a library of functional surface materials that provide the ability to change colour or haptic feedback. We also need compliancy to reshape the device, with flexible and even stretchable displays. And transparency is something we require,' says Bower, showing a Photoshop-assisted mockup of Nokia's take on the transparent navigator.
The roll-up map is not the only applications for see-through electronics. Douglas Keszler of Oregon State University, a leading researcher into transparent metal oxides, reckons these materials will find uses in car dashboards and windows to provide extra real estate for computer circuits.
Carbon nanotubes and plastics are vying with metal oxides for a role in transparent electronics, but the metals have a solid lead historically.
'We believe metal oxides can enable transparent electronics and they have been around for some time,' says Flora Li, research associate at the University of Cambridge.
In the Second World War, aircraft makers used transparent conductive oxides to deliver heat to windshields to keep them free of ice. Indium tin oxide (ITO) has become the one material that appears almost everywhere as a conductive coating for flat-screens and touchscreens. Unfortunately, the key component, indium, is a very rare and expensive metal, giving researchers a strong incentive to find other options.
Peter Harrop, chairman of analyst firm IDTechEx, says: 'It's a defeat that indium tin oxide is still used for transparent electronics. There are replacements but they need to gain traction. That is a big opportunity for a lot of people.'
Li says ITO represents the first generation of transparent electronics, forming just passive conductors on the surface of screens. 'The phase we are in now, we consider the second generation, allowing us to fabricate discrete transparent components,' she claims. The coming third generation will put active transparent components into many more devices.
Thin-film transistors made out of metal oxides date back to the the 1960s but it's only since the late 1990s that research has shown that it is possible to create a library of standard components that you can see through. Keszler points to a paper on the creation of a p-type transistor by Hiroshi Kawazoe and colleagues at the Tokyo Institute of Technology in 1997 as the birth of modern transparent electronics. Up to that point, all the conductors were n-type. With the two types available, it became feasible to build thin-film diodes and transistors.
There is a reason why transparent metal oxides are not more widely used in electronics. As with the organic polymers used in printed electronics, electrons do not move easily through most of them. According to Keszler, the best materials have a conductivity more than ten times worse than the contact metals used today in silicon chips.
Li says even with this lower performance, there is still a useful role for these devices. She compares metal oxides to lower-grade forms of silicon used in flat-panel displays, such as polycrystalline and amorphous, non-crystalline silicon, often called alpha-silicon.
'Polysilicon gives you great mobility. But you need to use really high temperatures to get this. Alpha-silicon you can make at much lower temperatures but at the expense of lower mobility. This is where we believe transparent metal oxides fit in: filling a gap between organic materials and alpha-silicon in terms of cost and performance,' says Li.
Whereas alpha-silicon generally has a mobility of around 1cm/Vs, researchers have managed to achieve around 30cm'/Vs for the widely available material zinc oxide, which is still five times lower than the polysilicon used in high-end displays but is usable.
Mobility is only one of the concerns that researchers have with metal oxides. Sharp worked with startup Inpria, which Keszler co-founded, on indium gallium zinc oxide transistors. 'However, the current doesn't saturate,' says Voutsas, in the way that it should for a workable transistor. 'And the threshold voltage is high. That is why you don't see a product that uses amorphous-oxide TFTs.'
As with the the transistor, researchers are working with a range of metals in the hope of finding combinations that work. Li says many of these materials are binary oxides that are difficult to produce reliably using sputtering - the balance between the two metals in the oxide varies, disrupting its ability to conduct electricity.
Like silicon-based processes, the metals can migrate into other layers, which Li found with indium zinc oxide and hafnium oxide gates. 'We found the indium migrated into the hafnium layer and destroyed the device. What we found really works with indium zinc is aluminium,' says Li. On the other hand, zinc oxides seem to work well with hafnium oxide.
With work continuing on indium-based oxides, materials scientists have yet to find a genuine low-cost, easily available winner in transparent conductive oxides. 'But we think that this is one of the technologies that will emerge soon,' Voutsas concludes.