EPFL Lecture #6 (Venables)

Notes for EPFL Lecture 6 (Venables)

Lecture notes by John A. Venables. Lecture given 30 Oct 97. Latest version 2 April 09.

The references for this lecture are here.

6. Continuation and Conclusions

In this final session, I am planning to discuss briefly: a) metal-semiconductor interactions during and after growth; b) defect nucleation and pattern formation and c) the choice of materials systems for future research. The first topic will assume that you know something about reconstructions at semiconductor surfaces, which I describe in my ASU lectures in section 1.4. I will describe some of the experiments and modelling we have done recently especially on the comparison of Ag/Si(111) and Ag/Ge(111) (Metcalfe et al. 1996, Venables et al. 1997).

The second topic arises from a general interest in nucleation, followed by a specific interest in nucleation on defect sites of various types. At ASU we have been interested in this topic in conjunction with nucleation of magnetic materials (Fe, Co, Ni etc) on both metals and insulators, and for which I have developed the nucleation models (Heim et al. 1996, Venables 1997a,b). At EPFL, several students and colleagues are working on related topics and have experiments and KMC simulations to match. Thus this session is an occasion to discuss what is wanted from such models.

The topic of 'pattern formation' is currently hot, and so many groups are currently working in this area. We can discuss what they have in common and in what way they differ. One of the differences is in the choice of materials systems to study. It is not be so easy to write this topic down in the form of web notes, but can usefully be discussed orally. It is one thing to write down one's own research proposals, but to write them down for someone else seems a bit too much- at best one looks stupid, and at worst one loses real money, this can‘t be right! So, I will not attempt to finalise the notes in advance of the lecture, but will put up a short summary of the discussion afterwards.

In the meantime, you may wish to consult some papers from the original literature. I have compiled a reference list which is now essentially complete. Chapter 5 of my book expands these references

Note added 10th December 1997: We did have a brief discussion on the choice of materials system to study, but of course did not reach any firm conclusions. The discussion really stems from a (mild?) anxiety about where the field is going, and how young faculty/ post-docs can build a career that will actually add up to something; i.e. isn't it all a bit ‘crowded’?

My own feeling is that it is important to do a version of applied physics in the University system, which is also fundamental, in that it explores possibilities, makes relatively simple models, and relates experiments back to the best possible theoretical work. In this way, one can develop new concepts, and close the experiment-theory circle, which is really what science is all about. There is very little sense in trying to do the jobs that Industry does better, even though there has been a lot of pressure in many countries, and personal inclination in some cases, to blur the University-Industry boundary.

So, for my own part, I have tried to choose sytems for study that could in principle be understood in scientific terms, while being technical enough to command an audience in more applied contexts: for each study of 2D melting of Xe/graphite, which interests directly only a handful of academics worldwide, there is one on Si/Ge/Si, Fe/Ag/Fe multilayers or equivalent which can play at the larger Materials Science conferences where industrial participation is high.

However, for the future, most younger people, in most situations, would be well advised not just to do structural studies, since the end point interest is nearly always electrical, magnetic or optical properties, as I pointed out at the beginning of lecture #4. This is despite that fact that the reason that more useful devices can’t be made reliably often has structural causes. Be prepared for plenty of ironies of this type, and for the necessity for working as part of a team, whether or not you would prefer to go it alone.

We discussed briefly a local case involving optical activity due to chiral molecules ordered on metal surfaces. The work is at an exploratory stage, but already some excellent theorists are involved, and this interaction is bound to be interesting and productive as the project proceeds. No direct applications are yet envisaged, but it seems that, in Switzerland at least, this pressure for short term ‘results’ is not yet at the counter-productive level. This is healthy, but of course requires a healthy economy, and also a good level of public awareness for it to be sustained. A certain amount of effort is therefore needed, in the ‘public understanding of science’, and results are not always encouraging. There is a great danger that it is seen as special pleading by scientists as just another interest group. Whatever else we do in future, I think we will all need to take this point seriously.