Electromagnetic radiation

Besides what is commonly called light, electromagnetic radiation includes radiation of longer (infrared, microwave) and shorter (ultraviolet, X-ray) wavelengths (see p. xvii). As the name implies electromagnetic (EM) radiation contains both electric field E and magnetic field B components. The use of the bold typeface indicates that these are vector quantities. The relationship between the electric and magnetic fields is best illustrated by considering plane-polarized radiation. Here the electric vector is confined to a single plane. Figure B.1 depicts such radiation of wavelength λ travelling with phase velocity c (the velocity at which the crests of the wave travel) in a vacuum (c = 2.998 × 108 m s-1) along the x-axis. The electric component of the radiation is in the form of an oscillating electric field and the magnetic component is an oscillating magnetic field. These fields are orthogonal and are also at right angles to the direction of propagation of the radiation. The plane of polarization is conventionally taken to be the plane containing the direction of the electric field. Unpolarized radiation, or radiation of an arbitrary polarization, can always be resolved into two orthogonally polarized waves. If the two electric field components possess a constant phase difference and equal amplitudes, the resultant EM wave is said to be circularly polarized.

The gas—liquid interface

Certain organic molecules will orient themselves at the interface between a gaseous and a liquid phase (or between two liquid phases) to minimize their free energy. The resulting surface film is one molecule in thickness and is commonly called a monomolecular layer or simply a monolayer. In the previous chapter the individual properties of bulk phases were outlined. The interface region will now be examined.

The boundary between a liquid and a gas (e.g., the air/water interface) marks a transition between the composition and properties of the two bulk phases. A surface layer will exist with different properties from those of either bulk phase (Adamson, 1982; Gaines, 1966). The thickness of this region is very important. If the molecules are electrically neutral, then the forces between them will be short-range and the surface layer will be no more than one or two molecular diameters. In contrast, the Coulombic forces associated with charged species can extend the transition region over considerable distances.

The microscopic model of a real interface is one of dynamic molecular motion as molecules move in and out of it. However, for the interface to be in equilibrium, as many molecules must diffuse from the bulk of the liquid to its surface per unit time as leave the surface for the bulk.

Organization in long-chain materials

Long-chain aliphatic materials pack together with their hydrocarbon chains parallel. The simplest scheme is an hexagonal array, with the molecules freely rotating as rigid rods about their long axes. The diameter of the cylinder into which one molecule fits is about 0.48 nm (Kitaigorodskii, 1961). Such a plastic crystalline state (section 1.2), originally known as a rotator phase (Ungar, 1983), may be exhibited by straight chain alkanes and some monolayers just below their melting point. There may even be some similarity with the LS phase in floating monolayer films (section 2.4.3). However, there is still considerable debate about this (Ulman, 1991).

For infinite aliphatic molecules, the hydrocarbon chain takes the form of a zig-zag, repeating at 0.254 nm intervals along the chain axis. In the most stable state, all the CH2 group carbon atoms lie in a plane to give a flat zigzag (appendix A). Close-packed structures result from the hydrogen atoms in a CH2 group on one molecule fitting into depressions between hydrogen atoms on adjacent molecules. Different packing arrangements of the C2H4 repeat units define the crystallographic nature of the subcell or sublattice. There are three possible close-packed structures with similar packing densities: orthorhombic (R), monoclinic (M) and triclinic (T) (appendix C) (Kitaigorodskii, 1961).

The crystal lattice

An ideal crystal contains atoms arranged in a repetitive three-dimensional pattern. If each repeat unit of this pattern, which may be an atom or group of atoms, is taken as a point then a three-dimensional point lattice is created. A space lattice, such as that shown in figure C.1, is obtained when lines are drawn connecting the points of the point lattice. The space lattice is composed of box-like units, the dimensions of which are fixed by the distances between the points in the three noncoplanar directions x, y and z. These are known as unit cells and the crystal structure has a periodicity (based on the contents of these cells) represented by the translation of the original unit of pattern along the three directions x, y and z. These directions are called the crystallographic axes. Any directions may, in principle, be chosen as the crystallographic axes. However, it is useful to select a set of axes which bears a close resemblance to the symmetry of the crystal. This can result in x, y and z directions that are not at right angles to one another. In figure C.1, the angle between the y and z axes is designated α, between the z and x axes, β, and between the x and y axes, γ.

Fatty acids and related compounds

A simple long-chain fatty acid such as n-octadecanoic acid (stearic acid) consists of a linear chain (CnH2n+1) — an alkyl chain — terminating in a carboxylic acid group (COOH). The polar acid head confers water solubility while the hydrocarbon chain prevents it (section 2.2). It is the balance between these two opposing forces that results in the formation of an insoluble monolayer at the air/water interface. Any change in the nature of either the alkyl chain or the polar end group will affect the monolayer properties.

The solubility of fatty acids in water decreases as the length of the alkyl chain is increased. To obtain an insoluble monolayer of a nonionized fatty acid (i.e., the situation at sufficiently low pH values), the molecule must contain at least 12 carbon atoms. For example, n-dodecanoic acid (lauric acid — C11H23COOH) forms a slightly soluble gaseous monolayer at low temperatures. The addition of two more carbon atoms, to form n-tetradecanoic acid (myristic acid), causes the gas phase to condense at low surface pressures and an expanded monolayer phase to be formed (Stenhagen, 1955). If this monolayer is held at a surface pressure of 10 mM m-1 and a temperature of 20°C, then the loss in monolayer area due to solubility in the water subphase is 0.1% min-1.

Langmuir—Blodgett (LB) films have been the subject of scientific curiosity for most of the twentieth century. However, interest has grown significantly since the 1970s — a direct result of the work of Hans Kuhn and colleagues on energy transfer in multilayer systems. This introduced the idea of molecular engineering, i.e., using the LB technique to position certain molecular groups at precise distances to others. In this way new thin film materials could be built up at the molecular level and the relationship between these artificial structures and the natural world explored.

There are already several books that cover LB and related thin films. So why another? My own background is in electronics. While I have been involved in LB film research I have spent many hours pondering on chemical formulae, struggling with biological nomenclature and trying to understand the finer points of thermodynamics. The scope of the subject is continuing to grow and anyone now starting work in the area must assimilate an enormous amount of information. My intention therefore has been to provide a gentle introduction to newcomers with an emphasis on the multidisciplinary and interdisciplinary nature of the field.

Each chapter addresses a different issue. Chapter 1 describes the various bulk phases of matter and outlines physical principles that can be used to model these. Monolayer phases are introduced in chapter 2.

Introduction

The presence of metastable defects in device materials can have a variety of effects, some of them slight and some very important; but because the emphasis in this book is material phenomena, device effects are only summarized here. There are cases in which optically (or electronically) induced changes in defects degrade device performance, and some other cases in which similar processes speed the annealing of existing degradation. Either way, the inherent properties of metastability make its effects more prominent at lower temperatures, with the metastable-barrier energy determining the maximum temperature at which significant effects are observed. Since the energy gap of the host material limits the size of this barrier, Si devices operating at room temperature (Eg= 1.1 eV) are not much affected; GaAs (Eg = 1.4 eV) devices are affected, generally at lower temperatures; and compounds with larger gaps show the greatest effects. By far the greatest device impact is in photovoltaic cells of a-Si:H, whose degradation by bright light has been discussed at length in Chapters 4 and 5 and is the motivation for a great deal of the research reviewed there. There are effects in devices using crystalline III–V materials, but their impacts are less pervasive, and in other devices of a-Si:H they are not critical. For stable deep-level defects, the most prominent effect is as lifetime “killers” in minority-carrier devices, and indeed that is also the dominant effect of metastable defects in a-Si:H. In majority-carrier devices or semiconductor lasers other effects are more important.

Amorphous Semiconductors and Their Defects

For the reader unfamiliar with amorphous semiconductors, we present a brief summary of some of the essential principles needed as a context for the treatment of metastable defects in hydrogenated amorphous Si (a-Si:H). For more detailed discussions of properties of amorphous solids the reader is referred to books by Elliott (1983) and Zallen (1983); for theory of tetrahedral semiconductors to that by Overhof and Thomas (1989); and for broad descriptions of a-Si:H to a four-part set edited by Pankove (1984), a two-volume set edited by Fritzsche (1989), and a book by Street (1991b). For additional historical background there is the landmark book of Mott and Davis (1979).

It is essential to recognize from the start that, although amorphous materials lack the long-range order of crystals, atoms in amorphous structures do not have random locations as occur in a gas. Rather, they retain the same shortrange order that characterizes the local atomic relationships of a crystal of the same material; in this respect amorphous and liquid semiconductors have much in common. When some liquids are cooled sufficiently to solidify, they may form a glassy state, which is defined by such high viscosity as to provide structural rigidity (i.e., shear strength) although crystallization does not occur.

Most semiconductors, when cooled from the melt, form crystallites rather than glasses; but other more rapid cooling techniques can force them to become amorphous solids, although these are then limited to thin films.

CdS and CdSe

Descriptions of exotic effects as the result of photoexcitation of II–VI materials, particularly CdS and CdSe, can be found extending back to when Boer, Borchardt, and Borchardt (1954) reported temperature-dependent, slow decreases of photoconductivity with time of photoexcitation of CdS crystals. In this section we review some examples of these effects and the kinds of models that have been suggested. It must be admitted that genuinely authenticated models do not exist for many of these early photoinduced defect effects, and so we primarily call attention to the suggestions that investigators have made.

Apparently related effects were found by measurements of thermally stimulated conductivity depending on the temperature of photoexcitation before making the measurement, and in the effect of photoexcitation on impure CdS, particularly on CdS:Cu crystals showing both a decrease in bulk photoconductivity of the material and a reversible degradation of the junction collection properties of a CuxS/CdS heterojunction. The phenomenon of persistent photoconductivity has also been observed. In many cases decreases of luminescence with time under photoexcitation have been observed, paralleling the decrease in photoconductivity. A photoinduced defect interaction model for some of these kinds of effects has been described by Tscholl (1968).

Decrease in Photoconductivity with Time in CdS and CdSe

The experimental results at 100 °C of the original experiment of Boer and coworkers (Boer 1954, 1990 [p. 1101]; Boer, Borchardt, and Oberlander 1959) are shown in Figure 3.1.

Introduction

Among defects in semiconductors the DX center is unique in the extent and detail of its studies and in the advanced present state of its understanding. It (or similar centers) has been observed in n-type III–V compounds doped with a variety of species of atoms and in some II–VI compounds; it may also be related to the dominant defect in a-Si:H. The donor-related DX center in AlGaAs alloys (the most widely studied) can thus serve as a model that can usefully be invoked in the study of other semiconductor defects; the EL2 defect in GaAs – a native defect in undoped material now thought to be an AsGa antisite defect (an As atom on a site that normally would have a Ga atom) – has some similar properties, and is discussed in Section 2.6. We show later in this chapter that this model provides a new and convincing explanation for a major limitation on the ability to dope some III–V materials. This limitation has caused even more serious impediments to the applications of II–VI materials, and (as discussed in Chapter 3) the DX model is useful there too. Present understandings followed nearly twenty-five years of efforts to elucidate metastability in III–V compounds, and the literature is large; we therefore present here only a brief summary of the early work, which is described well in reviews by Lang (1992), Langer (1980), and Mooney (1990, 1991), who also discussed the impacts of such deep donors on the properties of devices using these materials.

A number of observations of anomalous effects have been reported for these centers, with the consequent conclusion that the familiar one-electron picture of deep-level centers is inadequate.

Over the past few decades a fascinating revolution has taken place with regard to the way that defects in semiconductors and their properties are regarded. Traditional understandings of a variety of effects have used models of isolated point defects with fixed properties, operating independently in the matrix of the solid, and describable in terms of one-electron energy bands and defect levels. More recently we have come to realize that this classic picture of a simple defect with fixed properties may be the exception – that in real semiconductors there is a wide variety of interactions between defects with deep energy levels and the lattice that leads to variations in local atomic configurations comprising the defect, and in some cases to complexes between defects. The central distinguishing feature of most of the defect processes of interest is that they inextricably couple the electronic system and local structural configurations. Consequently, processes involving these defects cannot be fully described by either electronic transitions or defect states alone.

Some of these new configurations are relatively stable, but many others are not: They can be brought into metastable existence in the solid by the action of photoexcitation, other high-energy excitation, injection of excess carriers, and/or thermal energy, and they can be removed once again by thermal or optical anneal that returns them to their ground state. The metastability of their existence and the radical changes in electronic properties that can exist between their ground state and metastable state provide interesting new possibilities for semiconductors, but often they also provide mechanisms for instability and degradation that handicap the applications of semiconductors.