Electron Microscopy and Diffraction of the Early Stages of Metal Heteroepitaxy
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ELECTRON MICROSCOPY AND DIFFRACTION OF THE EARLY STAGES OF METAL HETEROEPITAXY E. BAUER Physikalisches Institut, Technische UniversitHt Clausthal, D-3392 Clausthal-Zellerfeld, Federal Republic of Germany
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ABSTRACT The application of some characterization methods to the study of heteroepitaxy is illustrated by recent work in the authors laboratory on the epitaxy of Cu on Mo and W(110). In particular, the use of PEEM and LEEM in the study of the epitaxy of Cu, Au and Co on Si(lil) is discussed.
INTRODUCTION There is now a large arsenal of surface-analytical tools which can be used in the study of heteroepitaxy. The standard method for the study of the growth mode is the measurement of the specular beam intensity Ioo in reflection high energy electron diffraction (RHEED) [1]. This method is based on interference and can be used in any stage of growth. The amplitudes of the I oscillations are a good indicator of the quality of the layer-by-layer growth. Methods which are based on the small inelastic mean free paths of slow electrons, such as Auger electron spectroscopy (AES), are limited to the study of the early stages of the growth mode and have also other limitations as will be shown below. The epitaxial relationship and the evolution of the lateral periodicity is, in general, still determined by low energy electron diffraction (LEED). LEED and other diffraction techniques average over a large area and give thus only average information on the microstructure. Whenever information on the variation of the microstructure across the surface is needed, some type of microscopy has to be used. Amongst the many modes of microscopy, low energy electron microscopy (LEEM), supplemented by photoemission microscopy (PEEM) [2], has emerged as particularly useful for the study of heteroepitaxy because it can be easily combined with LEED, can be done over a wide temperature range and allows to study processes such as epitaxy in situ in real time. The application of these various techniques will be illustrated in the next chapter in the case of the growth of Cu on Mo and W(11O). The subsequent chapter will demonstrate the information on heteroepitaxy which can be obtained with LEEM and PEEM.
EPITAXY OF Cu ON W(110) AND Mo(11O) Fig. 1 shows the RHEED I00 oscillations during the growth of Cu on W(110) at 100 K and 300 K [3]. Each period corresponds to one monolayer (ML). The slow decay of the amplitude at 100 K indicates ML-nucleation and ML-growth over a significant thickness range while the fast decay at 300 K signals a rapid transition to a steady state growth mode whose nature cannot be deduced from RHEED without a detailed spot profile analysis. The early growth stages are better studied by AES, provided that the energy analyzer has a sufficiently wide angular acceptance and Auger electrons with sufficiently short mean free inelastic paths are used (see e.g. ref. [4]). In this case, sudden slope changes of the AES signal as a function of deposition time are usually due to the start of a new monolayer. If the en
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