Electronic devices are now so ubiquitous that it is impossible to imagine life without them. However, new types of electronics are emerging that rely on semiconductor materials other than silicon. For example, wearable, transparent, and flexible electronics can enable a new generation of devices and applications which were not possible using silicon.
We spoke to Husam N. Alshareef, of KAUST, about transparent electronic systems he is developing which use oxide semiconductors and the role thin-film deposition plays in his research.
Why are you interested in transparent electronics systems? What different systems have you developed?
My team works on semiconductor nanomaterials. This involves the design of new materials, the development of new processes to make them in a controlled fashion, and finally introducing them to new applications.
Electronics and Energy storage are what interests us most and, as the number of semiconductor materials is large, we focus specifically on oxides. Oxide semiconductors tend to be transparent which means we can build transparent circuits. This is exciting as it allows us to build transparent displays. Conventional displays currently rely on a back light which consumes a lot of power. Ambient displays reduce this power consumption and also will allow new technologies to be developed, such as windows which can turn into a display.
Another application of transparent circuits could be smart windows which change transparency upon changes in the environment (heat, light, voltage). This could take the form of a thermochromic window which, upon sensing the room is getting too hot, becomes increasingly opaque to help cool it down. Another scenario could be a window with invisible sensors embedded in it, which sense chemicals or radiation. These would be hidden in vital locations such as airports, improving public safety.
We have also been developing oxides for energy storage. Oxide semiconductors work well as electrodes in batteries, including in future battery technologies such as sodium ion and zinc ion. In addition to this, they can also be used in supercapacitors – high powered devices which play a vital role in applications such electric vehicles, power backup, and micropower units in self-powered sensors and sensor networks. We also try to integrate micro-batteries and micro-supercapacitors with transparent and/or wearable electronics to enable new generations of self-powered sensors.
Transparent oxide semiconductors will allow windows to become interactive displays. Shutterstock | metamorworks
What are the typical layers in a thin film transistor?
In a typical transistor there are three different layers – one conducting, one semiconducting and one insulating. The ability to deposit these with precise control in discrete layers can make or break your transistor.
The insulating layer must be strong enough to not allow any electron leakage, and the conducting layer must allow the most efficient electron transport possible. The oxide semiconducting layer sits between these and by modulating its conductivity you can produce electrical ‘1’ and ‘0’ signals. This is, essentially, how you build a transistor.
How do you build the different layers into a transistor?
There are several different approaches depending on what is being built. If you are building high-density electronics, which require large number of parallel transistors, a vacuum system in a clean room environment is required. This needs a system at low pressure, so there is very little contamination.
For new emerging areas such as large, cheap electronics, or flexible electronics we are developing methods of achieving layering by chemical processes; which can be carried out economically. These are for less demanding applications such as RFID tags, food labels and sensors, and toys.
Either of these two methods, one for high purity and density, and another that is more economical, can be used to make transistors. Which method to use depends on the requirements of the finished product.
What different materials have you been using for the conducting film layer? What are the advantages of each?
With semiconductor devices, the overall performance of the device is highly dependent of the materials used to construct it. In the case of transparent electronics, we have been focusing on a hole transporting layer for the conducting layer. This means that the layer conducts charge by transporting positive holes, which is essentially the opposite of transporting electrons.
Silicon semiconductors can transport both electrons and holes, and it’s this ability that has made silicon technologies so successful. Combining the two types of transport facilitates the building of functional, smart circuits which run at a low energy consumption.
Transparent oxide semiconductors lack good p-type (hole transporting semiconductor). There are many good electron transporters but we’re yet to make a robust hole transporting layer, however we have made some significant progress. We want to combine a transparent hole transporting layer with a transparent electron transporting layer to develop transparent circuits and sensor systems.
Additionally, we have been developing transparent metallic oxides which can serve as transistor and circuit contacts. Generally, we are targeting indium-free transparent oxide conductors since indium supplies are limited and it is becoming increasingly expensive.
The materials used for capacitive energy storage are different – the oxides must be conducting but also electrochemically active. Metal atoms that can easily exist in multiple oxidation (such as manganese which can access five different stable oxidation states) can readily participate in redox reactions with an electrolyte when compared to an element such as hafnium which only has one stable oxidation state.
For this reason, we have been developing oxides using elements with multiple stable oxidation states such as manganese, vanadium, molybdenum, and cobalt. Using such materials, we are making energy storage devices at large and microscale. That latter we want to integrate with our transparent electronics to make self-powered sensors and smart devices.
Integrated micro-supercapacitors in a wearable electronic bracelet. The capacitors are the serpentine looking pattern. On-chip integrated energy storage (such as micro-supercapacitors) are expected to play important role in deployment of millions of self-powered sensors for applications such as wearable medical sensors (for point of care diagnostics), remote sensor networks, and internet of things sensors.
What system do you use to deposit layers for your transparent electronics?
We have been using three approaches to make oxide semiconductors: (1) physical vapor deposition (sputtering), (2) atomic layer deposition, which uses chemical vapor transport, and solution processing. The first two are rather precise, but the latter is significantly cheaper.
Why do you choose to use a deposition system from Angstrom Engineering?
We choose to work with Angstrom for two reasons – the first is that they are known for building very reliable systems which provide high quality film deposition. The second, and most important reason to me, is that they have such excellent customer service.
They are extremely responsive and no matter what time of the day a problem arises we always promptly get the support we need.
Why is the deposition of a high quality film important?
As electronic circuits are now being scaled down to a nanometer scale, you cannot afford to have rough layers or rough, crooked interfaces. Instead, you have to have really flat, smooth layers with a precisely controlled thickness. The level of thickness control is very important as modern devices can be only a few atomic layers thick, meaning thickness variations can have a huge impact on device performance.
Some emerging semiconductors, such as 2D materials, can even be a monolayer thick which means you need to have a deposition system which precisely controls the material thickness.
What features must be considered when designing a viable transistor and a viable supercapacitor?
Transistors and supercapacitors have very different functions so it is natural that the materials used for each exhibit different properties.
In our capacitors the oxides we use must have good electronic and ionic conductivity, allowing the rapid movement of electrons and ions through them, which in turn facilitates a high energy density. In addition to this we also need the oxide to have a large surface area as this increase the potential interaction that a capacitor has with the electrolyte.
Additionally, for a supercapacitor, engineering pores into the oxide which match the size of ions in the electrolyte will also increase the potential. This can be achieved using a chemical method or a form of deposition, glancing angle sputter deposition, can also be used to create oxide substrates with pores of a controlled size and density.
For transistors, thickness control is very important factor. The material must also retain its properties, even when reduced down to a very thin layer. In addition to this to ensure stability at high operational voltages the oxide must be of a high purity with a low level of structural defects. This means that the deposition system used to create the oxide substrate must be extremely precise, and very reliable.
Shutterstock | Deni_M
Where do you see the science of electronics taking us in the future and what technologies do you think electronics systems will enable?
Conventional Silicon electronics are here to stay and I expect that they will continue to represent the bulk of electronics being used. However, there are many new technologies emerging which are going to have a transformative effect.
For example, a lot of research is currently being undertaken on 2D semiconductors. These are only one atom thick and display very high mobilities and excellent optical properties. Once methods have been developed to grow these materials efficiently over a large area we will be able to build a new generation of extremely fast and powerful multifunctional devices.
Wearable electronics is another emerging area. In just twenty years we have scaled electronics down from mainframe processors to ones you can now hold in your hand or wear on your wrist. Complex sensing and processing systems are being produced that can sense your vital signs and power themselves by harvesting energy from motion.
I expect this technology to evolve even further into bioelectronics where not just physical phenomena, such as pulse and temperature, are measured; but physiological measurements such as hormones and chemicals can be measured by wearables. This could even be extrapolated to devices which sense changes in a patient’s bloodstream and then deliver therapeutics as and when required.
Where can our readers find out more about your research?
The easiest place to find out more would be my website, here you can find out more about what we are doing.
About Husam N. Alshareef
Husam Alshareef is a Processor of Materials Science and Engineering at King Abdullah University of Science and Technology (KAUST). He obtained his PhD at North Carolina State University in 1996 followed by a post-doctoral Fellowship at Sandia National Laboratory, USA. He then embarked on a 10-year career in the semiconductor industry, holding positions at Micron Technology and Texas Instruments. There he worked on developing new materials and processes for the microelectronics industry.
In 2009 he joined KAUST, where he initiated an active research group focusing on energy storage and electronics. The author of nearly 360 articles, he has nearly 70 issued patents. He has won the UNDP Undergraduate Fellowship, Seth Sprague Physics Award, NC State Dean’s Fellowship, U.S. Department of Education Electronic Materials Fellowship, the SEMATECH Corporate Excellence Award (2006), two Dow Sustainability Awards (2011) and (2014), AH Shoman Award for Excellence in Energy Research (2016), and KAUST Distinguished Teaching Award (2018).
He is a fellow of the Royal Society of Chemistry and IEEE Distinguished Speaker in Nanotechnology. He was co-chair for the 2014 Materials Research Society (MRS) Fall Meeting in Boston.
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