Internal Photoemission Spectroscopy

Fundamentals and Recent Advances

Valeri V. Afanas'ev

• Publisher: Elsevier
• Year: 2014
• Edition: 2
• eBook ISBN:
  9780080999302
• Topic: Physical Sciences
• Subtopic: Spectroscopy & Spectrum Analysis

The second edition of Internal Photoemission Spectroscopy thoroughly updates this vital, practical guide to internal photoemission (IPE) phenomena and measurements. The book's discussion of fundamental physical and technical aspects of IPE spectroscopic applications is supplemented by an extended overview of recent experimental results in swiftly advancing research fields. These include the development of insulating materials for advanced SiMOS technology, metal gate materials, development of heterostructures based on high-mobility semiconductors, and more. Recent results concerning the band structure of important interfaces in novel materials are covered as well. Internal photoemission involves the physics of charge carrier photoemission from one solid to another, and different spectroscopic applications of this phenomenon to solid state heterojunctions. This technique complements conventional external photoemission spectroscopy by analyzing interfaces separated from the sample surface by a layer of a different solid or liquid. Internal photoemission provides the most straightforward, reliable information regarding the energy spectrum of electron states at interfaces. At the same time, the method enables the analysis of heterostructures relevant to modern micro- and nano-electronic devices as well as new materials involved in their design and fabrication. - First complete model description of the internal photoemission phenomena - Overview of the most reliable energy barrier determination procedures and trap characterization methods - Overview of the most recent results on band structure of high-permittivity insulating materials and their interfaces with semiconductors and metals

1

Preliminary Remarks and Historical Overview

Similar to electron photoemission into vacuum, which is universally observed at the surfaces of condensed phases, the phenomenon of internal photoemission (IPE) represents an equally common property of solid/solid or solid/liquid interfaces. It consists of the optically assisted transition of a charge carrier, an electron or a hole, from one phase into another across the interface energy barrier. This chapter reviews observations of IPE phenomena at interfaces of different types, including interfaces between metals and semiconductors with wide bandgap insulators, metal–semiconductor contacts, semiconductor heterojunctions, interfaces of molecular and organic materials, and solid–electrolyte interfaces. Specific features of the IPE reproducibly observed in these systems are compared and the underlying physical mechanisms are discussed in order to be developed into analytical models in subsequent chapters.

Keywords

Photoemission; internal photoemission; photoinjection; interface barrier; optical excitation; charge injection

1.1 General Concept of IPE

In the most simple terms, internal photoemission (IPE) can be defined as a process of optically induced transition of a mobile charge carrier, electron or hole, from one solid (the emitter) into another condensed phase (the collector) across the interface between these phases. Except for the fact that electrons do not leave the condensed phase, IPE is quite similar to the classic photoemission of electrons from a solid into vacuum (external photoemission) because the optical excitation of a carrier in the emitter and its transport to the emitting surface (or interface) are basically the same. This similarity allows the use of the multi-step model developed for external photoemission as the departure point in describing IPE, as will be done in Chapter 2.

Differences between external and IPE processes are predominantly related to the different nature of the potential barriers at the surface and at the interface of a solid, respectively, which require modifications of several descriptions. First, there are differences in carrier transport associated with the different nature of wave function in collectors and different barrier properties. Second, the photon energy hν required for IPE transition may be significantly (sometimes by one order of magnitude) lower than for photoemission into vacuum, as illustrated in Fig. 1.1. This figure shows schematically the transitions corresponding to photoemission of electrons from a metal (Au) into vacuum, a wide bandgap insulator (SiO₂), and a semiconductor (Si) in panels (a–c), respectively. The energy onsets of electron emission correspond to the experimentally determined photoemission threshold (work function) of the metal Φ_vac (Rhoderick, 1978), and the barrier heights

(Deal et al., 1966) and Φ_Si (Tung, 2001). Finally, thanks to the presence of occupied electron states in the collector material, IPE of holes becomes possible, which has no analogue in vacuum photoemission. The corresponding electron transitions are schematically shown in Fig. 1.1(d) using the barrier parameters pertinent to the case of a PtSi/p-Si Schottky diode (Mercer and Helms, 1989).

Fig. 1.1 Schematic of optically excited transitions corresponding to photoemission of electrons from the states near the Fermi level of a metal (E_F) into vacuum (a), insulator (b) and semiconductor (c). The shown threshold energies of transitions correspond to experimentally determined values for the surface of Au (the energy level of an electron resting in vacuum is indicated as E_VACUUM), and Au/SiO₂ and Au/n-type Si interfaces. The energies E_C and E_V correspond to the edges of the conduction and the valence bands, respectively. Zero on the energy scale is placed at the Fermi level of the metal. (d) Photoemission of holes from the states near the Fermi level of PtSi into the valence band of p-type silicon.

Despite the aforementioned close similarity between IPE and external photoemission, the general understanding of the IPE process and, related to it, the development of IPE-based spectroscopic methods came almost half a century after the classic photoemission picture had been established. The most significant difficulty in IPE is the need for sufficient understanding of the spectrum of electron states inside a solid to clarify the origin of the energy barriers at interfaces. These barriers are generically related to the occurrence of forbidden energy gaps (bandgaps) in a solid. Therefore, transport of charge carriers across the interface could only be adequately addressed when the quantum theory of solids had been sufficiently developed. In fact, the concept of IPE was first introduced by Mott and Gurney to illustrate formation of conduction bands in rock salt crystals by comparing energy thresholds of electron photoemission from metallic sodium into the salt and into vacuum (Mott and Gurney, 1946) (cf. Fig. 1.1). Since then, thanks to its extremely rapid development (for an overview of early work, see, e.g., Mead, 1966 and Williams, 1970), IPE spectroscopy has emerged as the most physically sound and reliable tool for characterizing energy barriers between condensed phases and for determining the transport properties of excited charge carriers in the near-interface region. The “older sister” of IPE, external photoemission, gave numerous hints to the development of modern physics ranging from the quantum theory of light to the band theory of electronic states in condensed phases. In its turn, IPE deals with intricate electron transfer interactions at interfaces of solids, which in many cases still cannot be adequately described even at the present level of quantum theory because the atomic structure of interfaces is not known precisely. Thus, by using this kind of spectroscopy one often addresses fundamentally novel elements in condensed matter physics.

1.2 IPE and Materials Analysis Issues

In addition to fundamental physics, great impetus to development of IPE spectroscopy came from the practical application of solid-state electronics, primarily semiconductor-based heterostructures. Electron transport through and near semiconductor interfaces plays a crucial role in the operation of most solid-state electronic and optoelectronic devices. Essential features of this transport are determined by the density, relative energy and quantum–mechanical coupling between electron states in the contacting materials, which ultimately determine the rate of electron transition(s), i.e., the electric current. Therefore, to understand the details of electron transport phenomena in device-relevant heterostructures, the energy spectrum of electron states at the interface requires quantitative characterization so one can control technologically the electronic properties of the interfaces. Furthermore, knowledge of interface barrier properties is often needed for numerical simulations of electronic transport, which become increasingly important during device design.

The results of studies carried out over past 50 years strongly indicate that the spectrum of electron states at an interface cannot be immediately derived from the known bulk band structure of two contacting solids. Moreover, in many cases the properties of solid materials in the vicinity of their interfaces appear to be very different from the corresponding bulk parameters. These differences indicate the significance of interface chemistry and bonding configurations on composition and structure of the near-interfacial layers of a solid (for a review, see, e.g., Mönch, 2004). With the continuing trend to reduce the size and dimensionality of functional elements in solid-state electronic devices, incorporation of new, often surface-stabilized materials in the device design, as well as the extension of the solid-state electronics to new areas of functionality, the need to understand interface properties of solid materials and related nanostructures is acute as never before.

This need, in turn, raises question about reliable sources of information concerning electron states at interfaces of solids. More specifically, physical methods capable of probing the interface-relevant portion of electron-state energy distribution appear to have the focus of attention. As the picture of the observed physical process or phenomenon must be unambiguous and transparent to enable straightforward and reliable interpretation of the results, the experimental characterization of electron states at the interfaces must go far beyond the conventional electrical characterization of the interface commonly applied in the semiconductor industry. This brings up the issue of designing experimental physical methods suitable for detecting and characterizing the interface-specific portion of electron-state density.

When developing a characterization technique of this type, one might follow two different paths to isolate interface-related contributions to the electron density of states (DOS). As the partial DOS is proportional to the number of atoms encountered in a particular bonding configuration, the bulk component(s) of DOS will be dominant (at least in the energy range outside the fundamental bandgap) unless the analysis is confined to a narrow near-interface layer of a solid. To enhance the sensitivity of the technique to electron states at the interface, the studied volume of the sample can be limited to its very surface layer by using surface-sensitive measurements. The best known example of this approach is provided by electron spectroscopy methods in which inelastic scattering of electrons in a solid limits the mean electron escape depth to values in the range of a few nanometres (Feuerbacher et al., 1978; Briggs and Seah, 1985). By combining this surface-sensitive analysis with a gradual growth of substrate coverage with the second component of the heterostructure, initial stages of interface formation and the related evolution in electron DOS can be studied in great detail. This kind of analysis is able to provide information about atomic composition and chemical features of the interface as well as about electron DOS in a straightforward manner, delivering in this way the most complete picture of the DOS development as a function of the overlayer thickness. Moreover, electron spectroscopy analysis can be complemented by other surface characterization techniques ranging from optical spectroscopy to scanning probe microscopy, thus enabling reliable cross-checking of the results.

Although electron spectroscopy of surfaces represents the most successful approach to experimental DOS characterization, the small depth of analysis determined by the inelastic mean free path of electrons (typically <5 nm) leaves open the question of the relevance of the results obtained at the initial stages of inte...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Preface
• List of Abbreviations
• List of Symbols
• 1. Preliminary Remarks and Historical Overview
• 2. Internal Versus External Photoemission
• 3. Photoemission into Insulators: Physical Model
• 4. Internal Photoemission Spectroscopy Methods
• 5. Injection Spectroscopy of Thin Layers of Solids
• 6. Analysis of the Charge Trapping Kinetics
• 7. Silicon–Insulator Interface Barriers
• 8. Barriers at Interfaces of High-Mobility and Compound Semiconductors
• 9. Electron Energy Barriers Between Conducting Materials and Insulating Oxides
• 10. Conclusions