PHYSICAL REVIEW B VOLUME 48, NUMBER 7 15 AUGUST 1993-I Photon emission in scanning tunneling microscopy: Interpretation of photon maps of metallic systems Richard Berndt* and James K. Gimzewski IBM Research Division, Zurich Research Laboratory, CH-8803 Ru'schlikon, Switzerland (Received 16 April 1993) We analyze maps of the integral photon intensity emitted from the tunneling gap of a scanning tunnel- ing microscope obtained simultaneously with topography from a variety of metal films and single-crystal surfaces in ultrahigh vacuum. The effects of adsorbates and structures created with the scanning tunnel- ing microscope on their local photon emission properties are investigated to explore the potential of the technique for chemical mapping. It is proposed that contrasts in photon maps on a scale of some tens of nanometers are attributable to local variations in the field strength of tip-induced plasmon modes which are determined by the surface geometry of the junction and its dielectric properties. On a (sub)nanometer scale, a second contrast mechanism is observed to occur, consistent with geometry- induced variations in the matrix element for inelastic tunneling. A comparison of electron spectroscopic data with bias-dependent photon maps indicates that contrasts on a subnanometer scale are further mediated by local modifications of the density of final states positioned one quantum of energy (hv) below the bottom of the elastic tunneling channel with respect to the Fermi level. These three mecha- nisrns provide a framework for the interpretation of photon maps obtained on metallic systems. I. INTRODUCTION Use of a scanning tunneling microscope (STM) to study surface geometry and electronic structures locally' is based primarily on the tunneling current and its variation with experimental parameters. The dominant portion of this current arises from elastic tunneling. Studies of in- elastic electron tunneling (IET) processes using the tun- neling current in a STM are hampered by noise in the measured tunnel current. Following earlier proposals, ' we have demonstrated that IET excitation of tip-induced plasmon modes on metal surfaces can be monitored con- veniently by studying photons emitted from the tunnel gap. In particular, detecting photons emitted from a STM allows access to a purely inelastic component of the tunnel current with a high signal-to-noise ratio. In a more general sense the electromagnetic interaction of two metallic objects in nanometer proximity may be probed using photon emission induced by tunneling. This ap- proach should permit elucidation of the collective prop- erties of nanostructured materials, clusters, and the role of electromagnetic coupling in surface-enhanced Raman scattering. In this paper we discuss simultaneous mea- surements of integral photon intensity (photon maps) and STM topographs of metals, and explore the geometric and electronic contributions that determine photon emis- sion induced by tunneling electrons. Photon emission from a STM has recently attracted the interest of a number of groups, and we shall subse- quently review their work brieAy. The first results for semiconductors were reported for Si(111) surfaces where an emission of ultraviolet photons was detected. The results showed similarities to conven- tional inverse photoemission and additional features that were assigned to optical transitions into field- induced states. A more useful technique was local catho- doluminescence (CL), where emission from optical transi- tions between the band edges was detected. Abraham et al. used the tip of the STM to excite CL in GaAs/Ga& Al As heterostructures and have obtained photon maps of the quantum wells. Recent extensions of this work have enabled local band offset profiling of the structures. The mechanism involved here occurs under- neath the surface due to recombination of injected elec- trons with holes in the GaAs. Luminescence for gold- passivated GaAs surfaces has been reported by Wen- deroth, Gregor, and Ulbrich. ' For CdS(1120) surfaces, CL was spectrally resolved for injecting either electrons or holes from the tip into the semiconductor. " The opti- cal spectra revealed both band-edge emission and deep trap states. More recently, polarization effects have been addressed using GaAs samples. ' Ushioda has reported light emission spectra from Si(111) surfaces as well as measurements of the polarization. ' The first attempts at low-temperature STM studies of luminescence from InP have been made by Montelius, Pistol, and Samuelson. ' Interpretations of CL rely partly on what is known from conventional CL studies. However, many new and useful aspects of the method exist, particularly regarding the in- vestigation of quantum-size effects, the mapping of CL from porous Si being a recent case. Photon emission from metal surfaces has also received much interest because the mechanism involved in the emission process is new and specific to the nanometer proximity and electromagnetic coupling of the tip to the surface. Understanding the underlying physics, however, is more complex and requires new approaches both from the theoretical and the experimental sides. Experiments are related to previous investigations of light emission from metal-oxide-metal (M-0-M) solid-state tunnel junc- 0163-1829/93/48(7)/4746(9)/$06. 00 48 4746 1993 The American Physical Society