Bulletin Number Four 1985

polymer films is an important factor in various uses. Many food items are enclosed in packages to prevent contact w ith water vapour. On the other hand, plastic bottles for carbonated drinks must have the ability to block the flow o f carbon dioxide to the outside. One o f the most effective means to improve barrier proper­ ties is through the process o f orientation. It was found that the diffusion rate was reduced fifty times when polyethylene and poly­ propylene films were drawn to ten times their original length. In conclusion, it can be seen that over the past decade the polymer physics group has set up a research programme covering a wide range o f topics o f both theoretical and practical interest. It is hoped that, w ith the recent addition o f powerful tools such as nanosecond pulsed laser, the research activities would be further enhanced. Amorphous Semiconductor Research —K.P. Chik In recent years, we have concentrated on the study o f amorphous silicon films (a-Si) and on the development o f new methods o f producing good quality a-Si:H films. Our research is now being extended to the study o f Si alloy films and micro- crystalline films. This research is carried out in collab oration w ith the Amorphous Silicon Research Group o f the University o f Western Ontario, Canada under Professor B.Y. Tong. As a result o f the collaboration, we have access to more sophisticated equipments such as secondary ion mass spectroscopy (SIMS) which are not available at this University. An amorphous semiconductor differs from a crystalline semiconductor in that the former does not have long range order, whereas the latter does. Hence one does not expect to be able to dope the amorphous semiconductors just as for crystalline semiconductors. But the recent discovery that doping is possible i f a-Si contains a reasonable amount o f hydrogen has stimulated active research in this field worldwide. Moreover, a-Si:H has already found its way to industrial application and more important applications are expected in the near future. A common method o f producing a-Si:H is by the decomposition o f silane gas by a high frequency source. This method o f film production has several drawbacks. One o f them is that the film quality degrades w ith time and w ith illum ination. In our laboratory, we approach the problem in another way and have developed in the past years a novel method o f post-hydrogenating evaporated films, using a theta-pinch plasma. The post-hydrogenated films are found to have good photoconducting properties and very high stability . This method has been patented in the United States two years ago. Further research is now being carried out on the use o f plasma gun for post-hydrogenation. We have also developed another method o f producing Si films using low pressure chemical vapour deposition (LPCVD) technique, w ith which we can produce a-Si films w ith low hydrogen content, boron- doped and phosphorous-doped silicon films. By varying the deposition conditions, films can be doped from low to very high concentration. SIMS analysis shows that some o f our B-doped films has a boron concentration o f more than fo rty atomic percent in the solid phase, while X-ray analysis shows that they are still amorphous. We would rather call such films a-Si-B alloy. These alloy films have some interesting properties. The electrical conductivity is three orders o f magnitude higher than that o f slightly doped samples and the thermopower increases w ith rising temperature. Hall mobility measurements are still underway and we have some indications that Hall mobility can have unexpectedly high value. On the other extreme, we can also produce B-doped and P- doped Si films which, according to X-ray analysis, can be regarded as microcrystalline. The electrical conduc tiv ity and thermopower measurements show that they are already degenerated. The electrical conduc tiv ity o f B-doped microcrystalline Si films is at least one order o f magnitude higher than similar films produced in other laboratories. We expect that this material has good potential in device application. We have tried using such films to produce a primitive heterojunction a-Si:x-Si solar cell w ith a conversion efficiency reaching ten percent. A t present, besides carrying out detailed study o f the physical properties o f our new films, we are also trying to improve our LPCVD deposition technique in order to produce better-controlled films. Electronics - H.H. Ho In the Physics Department, research projects in the area o f electronics are also undertaken. One o f the projects worth mentioning and that may influence the future design o f computer hardware is called 'multiple-valued logic'. Present digital systems are designed w ith binary approach. Its development is based on purely philosophical logic concepts o f 'true' and 'false' (Aristotelian logic) and the mathematical foundations by Boole (Boole, G.: The Mathematical Analysis o f Logic, Blackwell, 1948) and, the estab lishment o f a switching theory by Nakasima and Shannon based on Boole's mathematical foundations. (Nakasima, A.: 'Theory o f Equivalent Transformation o f Simple Partial Paths in the Relay Circuit ’， J. Inst. RECENT DEVELOPMENTS 27