This book is about processes that occur at surfaces and in thin films; it is based on teaching and research over a number of years. Many of the experimental techniques used to produce clean surfaces, and to study the structure and composition of solid surfaces, have been around for about a generation. Over the same period, we have also seen unprecedented advances in our ability to study materials in general, and on a microscopic scale in particular, largely due to the development and availability of many new types of powerful microscope.

The combination of these two fields, studying and manipulating clean surfaces on a microscopic scale, has become important more recently. This combination allows us to study what happens in the production and operation of an increasing number of technologically important devices and processes, at all length scales down to the atomic level. Device structures used in computers are now so small that they can be seen only with high resolution scanning and transmission electron microscopes. Device preparation techniques must be performed reproducibly, on clean surfaces under clean room conditions. Ever more elegant schemes are proposed for using catalytic chemical reactions at surfaces, to refine our raw products, for chemical sensors, to protect surfaces against the weather and to dispose of environmental waste. Spectacular advances in experimental technique now allow us to observe atoms, and the motion of individual atoms on surfaces, with amazing clarity. Under special circumstances, we can move them around to create artificial atomic-level assemblies, and study their properties.

To convert a quantity expressed in the units listed in the first column to those listed in the third column, multiply by the number given in the second column. The SI unit is given in brackets for each quantity. The values with (±1σ) error bars are taken or deduced from the CODATA 1986 report (Cohen & Taylor 1987, 1998) as recommended by NIST in the USA and NPL in the UK. Note that it is not required to keep the full accuracy of these data to do the typical calculations encountered in this book, but it is helpful to have the extra decimal places were one to need them.

Mutiplication factors

The standard prefix is used for multiples and sub-multiples of units. These are shown in Table C1 below.

This chapter discusses surface and near-surface processes that are important in the context of the production and use of various types of thin film device. In section 8.1 the role of band bending at semiconductor surfaces is considered along with the importance, and the perfection, of oxide layers and metal contacts on silicon surfaces. Section 8.2 describes models which have been developed to understand electronic and optical devices based on metal–semiconductor and semiconductor–semiconductor interfaces. Then section 8.3 describes conduction processes in both non-magnetic and magnetic materials, and discusses some of the trends which are emerging in new technologies based on thin films with nanometer length scales. The final section 8.4 discusses chemical routes to manufacturing, including novel forms of synthesis and materials development. The treatment in this chapter is rather broad; my aim is to relate the material back to topics discussed in previous chapters, so that, in conjunction with the further reading and references given, emerging technologies may be better understood as they appear.

Metals and oxides in contact with semiconductors

This section covers various models of metals in contact with semiconductors, and the oxide layers on semiconductors. Such topics are important for MOS (metal–oxide–semiconductor) and the widely used CMOS (complementary-MOS) devices. Models in this field have been extremely contentious, so sometimes one has felt that little progress has been made. However, recent developments and some new experimental techniques have shed light on what is happening.

Band bending and rectifying contacts at semiconductor surfaces

Band bending can occur just below free semiconductor surfaces, and when metals or oxides come into contact with semiconductors.

Chemi- and physisorption

A qualitative distinction is usually made between chemisorption and physisorption, in terms of the relative binding strengths and mechanisms. In chemisorption, a strong ‘chemical bond’ is formed between the adsorbate atom or molecule and the substrate. In this case, the adsorption energy, Ea, of the adatom is likely to be a good fraction of the sublimation energy of the substrate, and it could be more. For example, in chapter 1, problem 1.2(a), we found that in a nearest neighbor pair bond model, Ea = 2 eV for an adatom on an f.c.c. (100) surface when the sublimation energy L0 = 3 eV. In that case the atoms of the substrate and the ‘adsorbate’ were the same, but the calculation of the adsorption stay time, τa, would have been valid if they had been different. Energies of 1–10 eV/atom are typical of chemisorption.

Physisorption is weaker, and no chemical interaction in the usual sense is present. But if there were no attractive interaction, then the atom would not stay on the surface for any measurable time – it would simply bounce back into the vapor. In physisorption, the energy of interaction is largely due to the (physical) van der Waals force. This force arises from fluctuating dipole (and higher order) moments on the interacting adsorbate and substrate, and is present between closed-shell systems. Typical systems are rare gases or small molecules on layer compounds or metals, with experiments performed below room temperature. Physisorption energies are ∼50–500 meV/atom; as they are small, they can be expressed in kelvin per atom, via 1 eV ≡ 11604 K, omitting Boltzmann's constant in the corresponding equations.

This chapter gives, in section 6.1, some generally accessible models of metallic behavior, and tabulates the values of work function and surface energies of selected metals. In section 6.2 we discuss electron emission properties of metals, concentrating on the role of low work function, high surface energy materials as electron sources; we also show that electron emission and secondary electron microscopy can be used to study diffusion of adsorbates. An introduction to magnetism in the context of surfaces and thin films is given in section 6.3.

The electron gas: work function, surface structure and energy

Free electron models and density functionals

Free electron models of metals have a long history, going back to the Drude model of conductivity which dates from 1900 (Ashcroft & Mermin 1976). The partly true, partly false predictions of this classical model were important precursors to quantum mechanical models based on the Fermi–Dirac energy distribution. If words in the following description don't make sense, now is the time to take a second look at section 1.5. Modern calculations start from a description of the electron density, ρ−(r) (ρ−(z) in 1D) in the presence of a uniform density ρ+(r or z) of metal ions. This is the jellium model, where the positive charge is smeared out uniformly. At a later stage we can add the effects of the ion cores Δρ+(r) by pseudopotentials or other approximations.

This appendix gives a few indicators of suitable materials for use in a UHV environment. The main point is simply to emphasize that the materials need to have low outgassing rates per unit area exposed, and that they need to be stable at the temperatures not only of use, but also during bakeout. Some of the obvious candidates in the different categories are as follows. Much of this information can be gleaned from talking to practitioners, from vacuum technology books such as Dushman & Lafferty (1992), O'Hanlon (1989), Roth (1990) or from reading between the lines in design handbooks such as Yates (1997), or increasingly from the web. A page giving properties and some sources for materials is at http://venables.asu.edu/grad/ appmat1.html

Structural materials

The most widely used structural material is 304 stainless steel, which is used to make chambers, flanges, etc., and can also be used for stages and other parts of the experiment itself. At very low temperatures this austenitic (largely f.c.c.) 18–20%Cr, 8–10%Ni Fe-based alloy transforms in part to the b.c.c. (martensitic) structure, and thereby becomes magnetic. If this could be important, then more technical details are needed, such as would be obtained from ASSDA – the Australian Stainless Steel Development Association – or AVS – the American Vacuum Society. Aluminum alloys are used for experimental pieces inside the vacuum system, and have also been used for whole chambers on occasion.

Various excited state processes, in addition to radiative decay, are important to laser performance. The quenching of luminescence reduces the excited state lifetime and can cause sample heating, thereby contributing to photothermal effects such as thermal lensing and thermal shock. Luminescence quenching also results in reduced laser slope efficiency, as discussed briefly here. Other excited state processes that require consideration are excited state absorption and energy transfer. In excited state absorption (ESA) a photon excites an electronic centre from the ground state to an excited state, which then relaxes to some lower lying metastable level. A second photon promotes the centre to an even higher energy state. Energy transfer arises when the optical centres are close enough together to interact, and this occurs when the concentration exceeds some lower bound, which need not be large. Although the energy levels of the interacting ions can be unaffected at such concentrations, the interion interaction is strong enough to enable excitation to be transferred between them. Prior to 1966, energy transfer was understood to involve excited states of donors (|D*〉) interacting with the ground states of acceptors (|A〉). Auzel (1966) pointed out that excited acceptors (|A*〉) also receive energy from excited donors (|D*〉), and that energy differences can be exchanged as well as absolute energies. Energy transfer from excited donors to the metastable levels of acceptors can be treated by generalization of the Förster–Dexter theory outlined in §7.1.

Principles and objectives

Crystal-field engineering seeks to use present knowledge to establish appropriate design principles for the development of new laser and nonlinear optical materials. First, the wavelength range of the optical device and its possible application (e.g. CW, ultrashort pulse, single frequency or tunable) are specified. This determines the chemical nature of the optical centre. The host environment is then selected, guided by historical knowledge of gain media or intuition of novel hosts with potentially beneficial properties, and then the theoretical and experimental techniques outlined in earlier chapters are invoked. The numerous objectives of crystal-field engineering include shifting the wavelength ranges of optical transitions, increasing the rates of radiative transitions and minimizing loss by nonradiative decay and excited state absorption. In addition, there may be reason to minimize or maximize energy transfer between centres, to avoid concentration quenching and to enhance laser efficiency respectively. Such objectives may be achieved by manipulating the unit cell containing the optical centre using such external perturbations as hydrostatic pressure, uniaxial stress or electric field. More usually, however, manipulating the unit cell is accomplished by changing its chemical composition.

Manipulating the unit cell

Hydrostatic pressure shortens bond lengths, reducing the unit cell dimensions without changing its symmetry. Such hydrostatic pressures will enhance the crystal field and, in consequence, shift spectra to shorter wavelengths. Studies of the Cr3+-doped elpasolites and garnets under pressure demonstrate the continuous tuning of the crystal field and of the coupling of the 2E and 4T2 states of the Cr3+ ion [Dolan et al. (1986), Hommerich and Bray (1995)].

Introduction

The distinguishing feature of crystalline solids is their symmetry, manifest microscopically in their X-ray diffraction patterns and macroscopically in crystal morphology. An ideal crystal is an infinite regular repetition in space of identical structural units. The symmetry of a particular ideal crystal is specified by the set of symmetry elements, comprising rotations, reflections and translations, which leave it invariant. Real crystals are not only of finite extent, but also contain a variety of imperfections such as inclusions of minority phases, grain boundaries, dislocations, impurities and point defects; the latter two are especially relevant in the present context. An isolated impurity or point defect in an otherwise ideal crystal obviously removes translational symmetry. It may also reduce the residual point symmetry in some circumstances, exemplified by ions with degenerate electronic states (the Jahn–Teller effect), impurities in the form of small molecules, small substitutional cations which move off-centre, vacancy pairs, bipolarons, etc. A principal theme of the present monograph is exploration of the ways in which the properties of a laser-active centre are controlled or affected by its crystalline surroundings, including their residual point symmetry. We shall find that it is often useful to distinguish a dominant site symmetry, determined by coordination alone, which is somewhat higher than the actual point symmetry.

The formal description of symmetry exploits a branch of mathematics called ‘Group Theory’, which is not a physical theory in the sense of ‘Quantum Theory’, but is rather a collection of principles deduced from a chosen set of axioms.

Natural minerals and gemstones

There are several thousand naturally occurring minerals, inorganic compounds of fixed chemical composition and regular sub-microscopic structure. Not more than 100 or so of these are recognized as gemstones although the number may vary as fashions change and new sources are found. Gemstones are hard and durable to withstand regular use without damage. But most particularly, they are beautiful, in their colour and lustre, especially when cut, faceted or polished for personal adornment. Beauty notwithstanding, the most important quality of a gemstone is scarcity. To be rare is to be greatly valued.

Minerals that are the basis of gems are found in rocks which form over aeons in time: where they are found reflects the process of continuous formation. There are three main classes of gem-bearing rocks. Sedimentary rocks are formed by the accumulation of eroded rock fragments, which settle over time, are compressed and again harden into rock. They are set down in layers which eventually emerge from below the earth's surface. Gypsum is a typical sedimentary rock with important variations in alabaster and selenite. Opal and tourmaline occur as veins in such sedimentary rocks as shale. Igneous rocks solidify from molten rock deep beneath the earth's surface, sometimes escaping in lava flows from erupting volcanoes. The slower the rate at which the molten rocks cool, the larger are the gems that grow within them. In consequence, gems grow at high temperature, under huge hydrostatic pressures.

Physical principles

The present chapter is concerned with transitions between optical levels of point imperfections in solids in which energy is conserved by emission of phonons rather than photons. Such transitions can introduce an undesired loss mechanism in laser materials which competes with stimulated emission, but they also play an essential role in the performance of laser materials, since excited states reached in electric-dipole-allowed pumping transitions subsequently relax to metastable lasing states by a sequence of radiationless processes.

Radiationless transitions are, paradoxically, both ubiquitous and elusive. They are the rule, rather than the exception, following optical excitation; yet, they are inaccessible to direct observation. Radiationless relaxation is generally inferred from its consequences, including such familiar phenomena as radiant heating and the absence of luminescence. In favourable circumstances, the observation of luminescence with diminished intensity and duration provides more detailed information about radiationless processes. Quantitative understanding of some aspects of radiationless relaxation has also proved elusive, in that theory has been more successful in elucidating trends than in the prediction of absolute transition rates. Radiationless transitions generally belong to one of two categories: static processes which are thermally activated from a metastable state, and dynamic processes which occur during rapid relaxation immediately following excitation [Bartram (1990)]. Point imperfections in solids provide examples of both categories.

Prepared state

Radiationless transitions can only occur between non-stationary states of a system; thus the radiationless transition rate depends critically on the sort of non-stationary state which is prepared in a given experiment.