Bulletin Spring‧Summer 1994

In a superconductor, an electrical current can flow without being pushed by a voltage. Ceramics have other properties that make them useful. In many ceramics, the centres of positive charge and negative charge are separated, exh i b i t i ng an electric 'dipole momen t '. By applying an external electric field, the dipole moment can be turned one way or another, and w i ll stay there when the external field is switched off. Ceramics which can remember the external electric field are called ferroelectric ceramics and can be used for developing computer memory devices. These ferroelectric ceramics are also 'piezoelectrics': they deform when an electric voltage is applied across them. A rapidly changing vo l t age w i l l lead to a r ap i d ly changing deformation, wh i ch can then generate an ultrasonic wave in the f l u i d in wh i ch it is immersed, or in the air in which it is exposed. These piezoelectrics are therefore key elements in ultrasonic cleaners and toxic gas sensors. Some other ceramics are transparent, and have good electro-optic properties. They are useful for making optical switches and infrared sensors. Ceramic Thin Films In many of these applications, the ceramics are in the form of thin films, a layer with a thickness under 1/10,000 of a centimetre. A thin kite flies when driven by the lightest wind, and can also flutter rapidly when suitably disturbed. So in the same way, thin films have the decided advantage that they can be driven into motion by very low voltages, and when suitably driven, can respond at high frequencies. For example, when an electric voltage is applied across some ferroelectric ceramics, the refractive index of the ceramics is modified and they can be used to make optical switches. Scientists have demonstrated that with a driving voltage of under 5 volts, these ceramics can perform switching at a rate of over a billion times a second, which would be extremely useful in optical communication. The challenge in research on ceramic thin films is to find better ways of making these films, to characterize the structure of the thin films, and i f possible to relate the properties to the structure, and the structure to the process of making the film. The Physics Department at The Chinese University of Hong Kong has several research projects wh i ch study mode rn materials o f technological importance. The study of ceramic thin films by Dr. H. K. Wong and his students has been going on for a number of years, and won competitive funding from the Research Grants Council in 1990. Some significant results were recently published in the prestigious Applied Physics Letters and the Journal of Applied Physics. How the Thin Films Are Synthesized Dr. Wong and his students have set up two simple but reliable facilities to synthesize these thin films. In the first method, high-power ultraviolet laser light is shone onto a sample of oxide (some ceramic material). The bombarded area of the oxide target is heated up to extreme temperatures in a very short time, and the material evaporates in a plume. The plume, which consists of atoms, molecules and small atomic clusters, impinges upon a carefully polished, cleaned and heated crystal surface. The surface acts as a substrate on which the thin film is formed. Under optimized conditions, the deposited atomic species can move and adjust themselves to form an organized array with a composition nearly identical to that of the oxide target. A schematic diagram of the thin film preparation equipment using laser bombardment Research Projects 12