Ion-Induced Electron Emission from Crystalline Solids

Abstract

Ion-induced electron emission from solids reflects a variety of physical processes such as electronic excitation and ionization in the target material, the transport of energetic electrons through solids, and electron capture and loss of the projectile during the passage, etc. Over a hundred years many workers have studied this phenomenon not only from pure physical interest but also from practical purposes, for example, various applications to surface layer analysis of materials. These numerous experimental and theoretical studies were reviewed in a number of articles [1, 2, 3, 4, 5]. Also, atomic processes responsible for the ion-induced electron emission have been reviewed extensively [6, 7]. While the solid targets used in most of the studies were polycrystalline, the observations using single crystal targets have been reported since the early 1960s [8, 9, 10, 11, 12]. In these studies, the reduced electron yields were observed when the ions were incident in transparent, i.e., channel directions of the crystal lattice. It is notable that this period overlaps the early days of ion channeling which later became a key concept in charged-particle interactions with single crystals [13, 14, 15]. Furthermore, some groups reported observations of diffraction effects of ion-induced low-energy electrons emitted from a crystal target [16, 17, 18, 19]. At present, the diffraction and related effects should be discussed in a unified manner, for example, in connection with photoelectron diffraction which has been widely used for surface structure analysis. The next phase of the study of ion-induced electron emission from crystal targets started from about 1980 when several groups measured ion-induced Auger electrons under channeling incidence conditions [20, 21, 22]. Their interest includes the applicability of the channeling technique to ion-induced Auger electron spectroscopy for surface structure analysis. The observed Auger energy spectra typically exhibited reduced peak height and a short low-energy tail under channeling incidence conditions. These spectra were successfully accounted for in terms of the ion beam shadowing effect. However, the reduced continuum electron yields generally observed in the experiments were explained less quantitatively [20], or attracted only minor attention [22]. The subsequent experimental studies of the continuum electron yield from crystal targets has been carried out by the research group, involving the author, at the University of Tsukuba. These studies rely entirely on the well established channeling technique which enables control of impact parameters in the ion-atom collision. The measurements of the continuum electron yield under channeling as well as nonchanneling incidence conditions have been continued since the late 1980s mainly at the University of Tsukuba and Japan Atomic Energy Research Institute at Takasaski, using ion beams obtained from accelerators of the tandem or single-stage type, and from an ion implanter as well. The experimental data cover the ion energy range 0.1–10 MeV/u, ion species of atomic numbers from 1 to 17, and various charge states. Technically, the electron measurements at ∼180◦ with respect to the ion beam direction allow practical data acquisition for solid targets. The crystal targets used include metals, semiconductors, as well as insulators. The present understanding of the ion-induced electron emission from crystal targets is outlined as follows: