ZnO MESFETs for application to Intelligent Windows

Lead Research Organisation: University of Liverpool
Department Name: Electrical Engineering and Electronics

Abstract

The central aim of the research is to produce electronic circuits into glass windows for visual display, monitoring and control purposes: 'transparent electronics'. The circuits are made from a very thin semiconductor-like material (ZnO: zinc oxide) and thin metal contacts. The materials are so thin (10's nm) that most of the light passes straight through. Thus 'intelligence' can be built into the window. The material used also offers optical advantages in that is 'low emissivity'; its refractive index serves to enhance the capture of solar energy and reduces the heat loss from the building. The added-value of introducing electronics capability into the glass coatings can open up a new generation of glass windows in buildings, cars and aeroplanes; especially for 'heads-up' displays. The last application is currently available using films containing indium. This is a very expensive material so its replacement with ZnO will bring down the cost and so open up the availabilty of such products. ZnO also has low-toxicity.

The devices we will use are called metal-semiconductor field effect transistor (MESFET). These devices are chosen because they are very simple and easy to fabricate. They also operate at lower voltages than the more commonly used metal-oxide field effect transistor (MOSFET), which is found in computers and phones etc. The use of MESFETs avoids the need to develop a gate oxide technology which is very challenging technologically. The downside is that the digital circuits need to more complicated and so take up more space. However, the footprint of the circuits is less critical that for Si based chips, as the electronics is on large area panels. The project is then concerned with the establishment of a MESFET fabrication process, device design for the intended circuitry and finally design and realisation of basic circuit blocks and gate arrays.

Planned Impact

The aim of the project is to establish a transparent electronics (TE) device capability on commercial, flat glass substrates, building on an atomic layer deposition, thin-film process already established by the team. The establishment of a circuit framework technology will open the door to further built-in capability such as gas sensors, energy harvesters, most of which have been studied for feasibility in the literature. The pathways to impact look to a number of potential beneficiary and market areas, namely the glass, construction, electronics and electronics materials industries. The basic technology should be transferable to other forms of substrate particularly those used for displays and plastic electronics. The area of 'Green Buildings' is of great interest in the drive to reduce energy consumption and intelligent windows can play a significant part in achieving the aims. The building sector accounts for 42% of the EU (our major trading partner) final energy consumption and about 35% of all greenhouse gas emissions. The inorganic transparent electronics global sector was nearly $76.4 billion in 2010, but is expected to increase to $123 billion in 2015, for a 5-year compound annual growth rate of 10. The reports make clear that with the exception of the transparent conductor space, it is still too early to pick real winners and losers in the transparent electronics materials space so the project is timely as the field is only just opening up. The UK has a strong position in the flat glass industry and the added value of built-in electronics and sensors has the potential for a new generation of intelligent glass. A further major outcome of this project lies in the development of a number of chemical precursors for atomic layer deposition. These precursors will have applications in other market sectors, but in particular the mainstream IT industry and the broader field of thin film nanotechnolgy (photovoltaics, catalysts, plastic electronics and others). SAFC Hitech are a major supplier of precursors and support the project. Equally, Silvaco will benefit from improved knowledge of materials and device properties and parameters. The activity on circuit design will also benefit the organic electronics area where there is a dearth of information and experience.

Publications

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Description The modification of the semiconductor ZnO by alloying using two different routes, namely Mg and Nb has been exploited to make transparent transistors by the ALD manufacturing process for the first time. These materials are much lower cost, more abundant and more environmentally friendly than existing routes which involve indium and gallium; elements that are toxic with indium expensive and difficult to source.
Exploitation Route The research will be exploitable where electronics are built into glass screens (e.g. phones and tablets) or into electronically controlled energy saving glazing, for instance.
Sectors Construction,Electronics,Energy,Manufacturing, including Industrial Biotechology