Photoemission studies of quantum well states in thin films

Confinement of electrons in small structures such as a thin firm results in discrete quantum well states. Such states can be probed by angle-resolved photoemission, as first predicted by theory in 1983 and later observed experimentally in 1986. Since then, numerous advances have been made in this field, and the purpose of this report is to review the basic physics and applications of quantum well spectroscopy. The energies and lifetime widths of quantum well states in a film depend on the him thickness, the dynamics of electron motion in the film, and the confinement potential. A detailed study allows a determination of the bulk band structure of the film material, the lifetime broadening of the quasiparticle, and the interfacial reflectivity and phase shift, as will be demonstrated with simple examples. Quantification of the photoemission results can be achieved via a simple phase analysis based on the Bohr-Sommerfeld quantization rule. Explicit forms of wave functions can also be constructed for additional information regarding the spatial distribution of the electronic states. From such studies, a detailed understanding of the behavior of simple quantum wells including the effects of lattice mismatch can be developed, which provides a useful basis for investigating the properties of multilayers. Examples of multilayer systems including coupled quantum wells and superlattices will be presented and discussed. An important recent development in this field is the preparation of atomically uniform films, as demonstrated for Ag(1 0 0) grown on Fe(1 0 0) whisker substrates. The elimination of atomic layer fluctuation allows a precise measurement of the intrinsic line widths of quantum well states, which are related to the quasiparticle lifetime and the interfacial reflectivity. The underlying physics can be described in terms of electron interferometry, and a Fabry-Perot analysis yields the quasiparticle lifetime. It consists of three major contributions including electron-electron scattering, electron-phonon scattering, and defect scattering. The experimental results are compared with theoretical predications based on the Fermi-liquid theory and a perturbative treatment of the electron-phonon coupling. Since the absolute film thickness in terms of monolayers is known from layer counting, the same interferometric analysis yields an electronic band structure of Ag with unprecedented accuracy (< 30 meV). The resulting Fermi wave vector challenges the de Haas-van Alphen value. Film structures containing magnetic materials can exhibit spin-split quantum well states, and their magnetic sensitivity makes them suitable candidates for device applications. Quantum well spectroscopy is useful for clarifying the relationship between quantum confinement and translayer magnetic coupling effects. (C) 2000 Elsevier Science B.V. All rights reserved.