Research Group
Amorphous Materials
This group was responsible for the two major discoveries which made possible the commercial exploitation of amorphous silicon thin films. They demonstrated how to dope this material, thereby producing p-n junctions, now widely used in solar cells, and they made the first thin film field effect transistor from amorphous silicon, thereby opening up its use in flat screen displays. Current research on amorphous silicon includes memory devices; field-effect transistors and image sensors. Much of this work is done in close collaboration with industry.
This research group is part of the Organic Materials Research Group.
Research Staff
Academic Staff: Dr R A G Gibson, Dr D I Jones, Prof M J Rose, Dr D J Keeble
Research Technicians: Mr Stuart Anthony, Mr Keith Duncan
Research Students: T Tantbirojn
Research Projects
- Deposition of Amorphous Materials
- Basic Electrical and Optical Properties of a-Si and its Alloys
- a-Si Memory Devices
- a-Si Field Effect Transistors
- Organic Semiconductors
- Real-time X-ray Detection
Background
The last 30 years has seen a revolution in the field of electronics and a tremendous increase in the number of applications of crystalline semiconductors such as silicon. However, there are many areas where the expense of preparing these crystals and the limited size to which they can be grown have limited any large scale applications. For example, crystalline silicon solar cells are used in space vehicles for converting sunlight into electricity, but the economics of their production is such that their use on earth is very limited.
Silicon can be prepared in large areas, but the material is then amorphous (disordered) rather than crystalline. However, it was generally believed that amorphous silicon (a-Si) could not be doped to control its electrical properties and was not therefore suitable for use in modern electronics.
The Amorphous Materials Group at the University of Dundee were responsible for two major discoveries which have made possible the commercial exploitation of a-Si thin films.
The Group showed first how it is possible to dope a-Si to produce n- and p- type layers and p-n junctions. This has enabled a-Si to be used in the following applications:
- Solar cells: a-Si is presently used in 40% of the world's solar cells, mainly in consumer products. One company alone makes 5 million a-Si solar cells every month! You probably already own an a-Si solar cell if you have a calculator that does not require a battery.
- Image sensors: a-Si is used for the "read-head" of 10% of the world's FAX machines and is predicted to take 50% of the market in the near future.
- Photocopying: One company makes a range of photocopiers using a-Si and its alloys for the copying drum. This can make a million copies or more without degrading its performance. The Group also fabricated the first a-Si thin-film field effect transistor (FET) and demonstrated the potential use of these devices in large area arrays for addressing flat-screen liquid crystal displays and televisions.
- Flat screen displays and televisions: As many as 2 million FETs are deposited across a screen. These are on sale as displays in numerous personal computers and small hand-held TVs and video players. Displays up to fourteen inches (35 cm) in size and colour with high definition TV quality have been fabricated and some of these can hang on the wall like a picture. There are even plans for 1 metre displays.
Deposition of Amorphous Materials
Investigators: Dr R A G Gibson, Dr D J Keeble
Research Student: Mr T Tantbirojn
A basic understanding and optimisation of the Plasma-Enhanced Chemical Vapour Deposition process used to deposit amorphous thin films is being obtained by optical emission spectroscopy. This should ultimately provide improved thin film materials for an even wider range of applications.
Basic Electrical and Optical Properties of a-Si and its Alloys
Investigators: Dr D I Jones, Dr R A G Gibson, Dr D M Goldie
Measurements of the optical and electrical properties of a material not only yields a better understanding of the fundamental science but is vital before any device incorporating the material can be designed and fabricated.
The characterisation of amorphous silicon and its alloys has resulted in the development at Dundee of several techniques for the measurement of the properties of thin films. Optical absorption is measured using three separate techniques, the conventional reflection/transmission method, photothermal deflection spectroscopy (PDS) and the constant photocurrent method (CPM). Both CPM and PDS are particularly appropriate for measuring values of the absorption coefficient as low as 10-1cm-1 on 1mm thick films. Electrical measurements include the field effect and the temperature dependences of conductivity, photoconductivity, Hall effect, thermoelectric power and drift mobility. These measurements provide information on the density of state distribution and the predominant conduction paths in the amorphous films.
a-Si Memory Devices
Investigator: Prof M J Rose
This section is awaiting content.
a-Si Field Effect Transistors
Investigator: Dr R A G Gibson
Research Student: Mr G Masterson
This project is concerned with the fundamental reasons for a small degree of instability that can occur under special conditions in these polycrystalline thin film field effect transistors.
Organic Semiconductors
Investigator: Dr D M Goldie
The properties of organic semiconductors have much in common with amorphous materials and we are currently using our considerable experience in the latter to provide additional insight into the properties of organic semiconductors. These are used in a wide range of photocopiers and this work is supported by a large industrial company with an interest in this field.
Thin-film amorphous organic semiconductors have been the focus of considerable theoretical study since the 1960s and have established increasingly important commercial status in the areas of large-area imaging and displays over the last 5-10 years. The materials research group has expertise in characterising the fundamental electronic transport properties of these materials and has established industrial links to optimise the response of Organic Semiconducting molecule dual-layer organic photoreceptor systems for xerographic applications. Knowledge gained from a fundamental understanding of the processes which limit charge transferral across organic layer interfaces is currently being used to design novel electronic devices and sensors.
Organic layers ranging from 0.1m m to 10m m in thickness may be deposited under cleanroom conditions using standard techniques such as thermal evaporation and spin coating from solution.
The fundamental movement of electronic charge through single organic layers may be experimentally probed using transient photoconductivity whereby excess charge is instantaneously photogenerated on one side of the film and its movement towards the opposing side under the action of an applied electric field is monitored. Such measurements allow the mobilities and lifetimes of electron and hole carriers to be determined. By extending the technique to dual-layer structures, the efficiency of charge transfer between different organic materials may be quantified. A detailed knowledge of the electronic transport properties of organic semiconductors and the response of organic interfaces is critical for the subsequent design of electronic devices.
Examples of applications requiring both a rapid and efficient displacement of charge through multiple organic layers include plastic photoreceptors (for photocopier and laser printing) and electroluminescent diodes (for large-area, flexible displays).
Real-time X-ray Detection
Investigator: Prof M J Rose
Research Students: Mr B Henley, Mr D McGuigan
Novel large area 2D medical imaging devices are being developed that offer high resolution, high sensitivity and significant dose reductions. A collaboration has been set up between the Universities of Dundee and Surrey (Dr E. Morton) to address issues such as novel thin film devices, fabrication, readout electronics and image processing.
del.icio.us
digg
reddit
facebook
stumbleupon