Behavior of Refractory Metal Surfaces in Ultrahigh Vacuum as Observed by Low-Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy

Abstract

Surface phenomena of both refractory metal single crystals and polycrystalline foils in ultrahigh vacuum environments (1×10−9–2×10−11 Torr) are discussed. Some of the more common refractory metals which have been studied are Ti, V, Cr, Fe, Zr, Nb, Mo, Rh, Hf, Ta, W, Re, and Au. Surface studies are initiated once the sample has been placed in the ultrahigh vacuum chamber, and the entire system has been baked out for several hours around 250°C. The first technique employed is Auger electron spectroscopy which involves measuring the energies of certain inelastically (secondary) emitted electrons by means of any one of several available energy analyzing devices. In our case, the low-energy electron diffraction (LEED) optics of a standard Varian unit were modified to serve as the required retarding potential energy analyzers. This technique enables the composition of the sample surface to be monitored during the procedures necessary to produce the clean metal surface. Specifically, some of the cleaning procedures are high temperature anneals (to near the melting point), reaction with H or O, and Ar ion bombardment. Some of the various contaminants which segregate on several refractory metal surfaces during cleaning procedures will be identified and techniques for their removal discussed. The sample surface is considered clean when its Auger spectrum corresponds very closely to published spectra taken from surfaces thought to be clean to the limit of present detection techniques. Once clean and in the 10−10–10−11 Torr range, the surface of the sample will remain clean for extended periods of time. If the clean specimen is then exposed to somewhat higher pressures (10−6–10−8 Torr) of common gases such as O2, H2, N2, and CO, the LEED pattern will in most instances change from that characteristic of the clean surface to another pattern due to the particular structure and species of adsorbed gas. One or more coverage states may exist for the same gas being chemisorbed onto the same single crystal metal surface. In most cases, a new coverage state will give rise to a different LEED pattern. And, except for H, the particular adsorbed species can again be identified using Auger spectroscopy. Specific cases to be discussed will be the adsorption of H, O, and CO on the three most densely packed planes of Mo, namely the (110), (100), and (112) planes, the adsorption of O on the Re (0001) plane, and other selected results on W, Ta, Hf, and Zr.