Stable silicon layer makes flat-panel display cheaper

In a joint project between the Technology Foundation STW and the energy agency Novem at Utrecht University, researchers have developed new silicon layers which are more stable and cheaper than the present amorphous silicon layers. The electronic properties of the present layers in laptop screens and solar cells deteriorate if the material is under ‘stress’, for example due to sunshine or a voltage.

Flat-panel displays and solar cells have a substrate of glass or plastic, which is coated with a thin layer of amorphous silicon. The silicon layer is a semiconductor which, under the influence of a brief local voltage, becomes conductive for a fraction of a second. This property makes it possible to create a potential difference via the amorphous silicon which addresses separate pixels in an active-matrix LCD display. However, the disadvantage of amorphous silicon is its instability. The threshold voltage needed to make the silicon conducting, changes if a prolonged gate voltage is applied. This is the case in thin film transistors, the devices which address the pixels in a flat-panel display. The microscopic mechanism of this ‘metastability’ is still not understood. Accordingly the application of amorphous silicon for thin film transistors in flat-panel displays and solar cells has not yet reached its full potential.
In the research project from STW and Novem, the Utrecht researchers tried to improve the electronic material properties of the silicon layers. They developed silicon layers which are more stable than the commonly used layers of amorphous silicon. By means of a simple method, hot-wire chemical vapour deposition, they also managed to deposit this layer at a rate ten times higher than conventional techniques. This considerably reduces the production costs of flat-panel displays and solar cells. This could be interesting for manufacturers of displays and solar cells and for the semiconductor industry.

For further information please contact Dr Bernd Stannowski (Debye Institute, Utrecht University), tel. + 31(0) 30 2532964, fax +31 (0)30 2543165, e-mail b.stannowski@phys.uu.nl. Information is also available on the Internet at www1.phys.uu.nl/wwwgf. The doctoral thesis was defended on 27 February 2002. Mr Stannowski’s supervisors were Prof. R.E.I. Schropp and Prof. W.F. van der Weg.