Crystallization and Melting in Multilayered Structures
- PDF / 1,153,890 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 33 Downloads / 208 Views
CRYSTALLIZATION AND MELTING IN MULTILAYERED STRUCTURES W. SEVENHANS, H. VANDERSTRAETEN, J.P. LOCQUET, Y. BRUYNSERAEDE, Laboratorium voor Vaste Stof-Fysika en Magnetisme, Katholieke Universiteit Leuven, B-3030 Leuven, Belgium H. HOMMA, Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A. IVAN K. SCIHULLER Physics Departement-B019, University of California-San Diego, LaJolla, California 92093, U.S.A. ABSTRACT The stability of Pb/Ge and Pb/C multilayers has been studied over a broad temperature range
by x-ray diffraction experiments. In the Pb/Ge system an amorphous to microcrystalline phase transformation of the Ge-layers was already observed at -_ 100 °C. This transition destroys the
modulation structure and improves the Pb(111) texture. In the Pb/C multilayers, the layered structure was still present at temperatures higher than the melting temperature of Pb. Contrary to recent publications, no depression of the melting temperature of the two-dimensional Pb layers could be observed. 1
Introduction
Metal/metal and metal/semiconductor multilayers and superlattices have received considerable attention in recent years [1]. This has been motivated by the development of novel preparation techniques which allow microscopic control of the thickness and structure of the layers, and by the prediction of unusual superconducting and magnetic properties [2]. More recently interesting phenomena where predicted and observed in the recrystallization [3] and melting [4] behaviour of those multilayered structures [4,5,6]. Amorphous Si and Ge in contact with certain metals appear to crystallize at temperatures much lower than the usual bulk crystallization temperature [7,8]. The precise mechanism for the nucleation and growth of a crystalline film starting from an amorphous layer is however not well understood. In an interesting experiment [4] it is claimed that the melting temperature of Pb in Pb/Ge multilayers is suppressed and the transition changes from first to second order as the Pb layer thickness is reduced along the (111) growth direction. Ion-shadowing and blocking measurements [9] reveal a reversible order-disorder transition at the (110) surface of a Pb crystal well below its melting point Tm. This transition is however not present in the Pb (111) surface [10]. The orientation dependence of the degree of positional disorder appears to be directly correlated with the large anisotropy in surface free energy. In this paper we report on a detailed study of the crystallization of a-Ge and the melting of Pb in Pb/Ge and Pb/C multilayers prepared by electron-beam evaporation. Both phenomena where studied using x-ray diffraction, transmission electron microscopy (TEM) and electron diffraction (ED).
2
Experimental techniques
The Pb/Ge and Pb/C multilayers were condensed on liquid nitrogen cooled sapphire substrates in a load-locked molecular beam epitaxy (MBE) apparatus equipped with two electron beam guns [11]. The evaporation rates were controlled using a quadrupole mass-spectrometer in
Data Loading...
