Thermal Effects in Silicon-Metal Interface Formation: A Photoemission Study of Si/Gd and Si/Yb

Abstract

In the field of metal-semiconductor interfaces it has been experimentally established that many interfaces are reacted along the scale of several atomic layers when they are grown at room temperature ( RT ) or even at lower temperatures, such as around 80 °K (conventionally called LNT, Liquid Nitrogen Temperature ). This fact resulted intriguing in consideration of the rather high energy needed to break a bond in many semiconductor substrates. For instance, the spontaneous reaction of Si with Pd at RT, in which a Pd2Si-like interfacial compound forms over several atomic layers, was unexpected when it was observed A similar reaction was also observed at LNT ³. The discussion of the reaction mechanisms became one of the central issues in interface physics due to the interest in the reaction mechanisms “per se” and for the very important connections with Schottky barrier physics. In fact, this last problem is well defined only if one takes into account the interface chemical processes which determine the real nature of the solids constituting the interface. Moreover, a better understanding of interface chemistry can be greatly useful in order to identify those systems in which the reaction compounds have a well defined stoichiometry. A well defined chemical composition, along with a knowledge of interface morphology, would make these systems the best candidates for a theoretical interpretation of their electrical properties. In this perspective the problem of interface reactivity is, in our opinion, to a large extent still open and deserves future attention from spectroscopical and structural investigations. A good example is a recent Scanning Tunneling Microscopy (STM) work on the Si (111) (7x7)-Pd interface ⁴ in which the authors extensively use the available knowledge, obtained through electron spectroscopy, on the chemical nature of the interface reaction products. In this respect we belive that the role of synchrotron radiation (SR) photoemission in interface reaction studies must be regarded in a new way. Instead of using photoemission data in order to model interface growth, photoemission should be devoted to study in detail the role of some key parameter in determining the nature of the interface electron states. This approach would increase the chance of a cross fertilization with structural investigations. In this perspective an important possibility consists in studiing the role of thermal effects on interface formation. The present paper is a step forward in this direction and has two main purposes. The first is to stress the usefulness of photoemission studies vs. temperature when the low temperature range is covered, i.e., the usefulness of preparing and measuring the interfaces at LNT, and to subsequently cycle the samples between LNT and RT. We will show that this approach is very interesting in discussing of the relative weight of the possible reaction mechanisms. The second is to show the interest of the comparison between the thermal behavior of the Si (111) (2x1)-Gd interface compared to the Si (111) (2x1)-Yb interface, as seen with photoemission. Our results show interesting differences between these two cases and point out these systems as good candidates for future structural investigations.