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ABSTRACT Light detection system combined with a UHV-scanning tunneling microscope (STM) was applied to the study of silver films deposited on Si(111) surfaces. Photon maps clearly show single atom height steps and terraces on an Ag(111) surface with high spatial resolution of nanometer scale. Chemical reaction on the Ag surface with residual gas was clearly revealed in the photon map. In the photon map of the thin Ag film of 2-3 ML in thick, no contrast appears between the terraces, and a characteristic bright contrast appears at the single atom height steps. The local plasmon model does not readily explain those contrasts. INTRODUCTION Light emission from STM was first reported by Cooms et al.[1], who observed enhanced light emission from an interface region between a tip and a metal surface. This enables us to study optical properties of surfaces in high spatial resolution, in addition to information obtained from STM such as surface topography and local electronic state. A silver (Ag) film has been frequently used for the STM-light emission study, because of the high emission efficiency. Gimzewski et al. [2] observed the dependence of emission spectrum on tip bias voltage using a polycrystalline silver film. They used an iridium tip with a constant tunneling current of 300 nA_ Berndt et al. [3] observed an Ag film with a tungsten tip and an Ag covered tungsten tip, which gave different emission spectra. They also observed photon maps of several metal surfaces such as Cu(l1l) and Ag films, and interpreted the contrast using the tip-induced plasmon mode [4]. Recently we developed a light detection system combined with an UJHV-STM and applied it to the Ag film deposited on the Si substrate [5]. The emission spectra were taken under various sample bias voltages, in which many peaks appear and change their intensity with bias voltage. It was revealed that those peaks in the spectra are assigned as the multipole modes of the local plasmon formed between the tip and the surface using the approximated dispersion relation. However, some properties are not clearly explained from the theory based on the local plasmon. In this study we further investigate the light emission from an Ag film of different thickness to find the mechanism of the contrast formation in the photon map. This is important in obtaining a correct information by the STM-light emission technique. EXPERIMENT We used an UHV-scannmg tunneling microscope (JSTM-4610). The pressure in the STM chamber is kept at 2x10 Pa. Samples can be cooled down to 80 K using a Liq.N 2 cold stage during the measurement. The ellipsoidal mirror is designed so as to maximize a collection efficiency, and a solid angle sustained by the mirror is 61% of 2 7t, the solid angle of hemisphere. 43 Mat. Res. Soc. Symp. Proc. Vol. 588 © 2000 Materials Research Society

A Liq.N2 cooled CCD detector is used for measurement of emission spectra, and a photo multiplier tube is used for photon mapping. We used etched tungsten tips heated by radiation from a Si substrate in UHV chamber. In this