Skip to main content

Über dieses Buch

The study of condensed matter using optical techniques, where photons act as both probe and signal, has a long history. It is only recently, however, that the extraction of surface and interface information, with submonolayer resolution, has been shown to be possible using optical techniques (where "optical" applies to electromagnetic radiation in and around the visible region of the spectrum). This book describes these "epioptic" techniques, which have now been quite widely applied to semiconductor surfaces and interfaces. Particular emphasis in the book is placed on recent studies of submonolayer growth on well-characterised semiconductor surfaces, many of which have arisen from CEC DGJGII ESPRIT Basic Research Action No. 3177 "EPIOPTIC", and CEU DGIII ESPRIT Basic Research Action No. 6878 "EASI". Techniques using other areas of the spectrum such as the infra-red region (IR spectroscopy, in its various surface configurations), and the x-ray region (surface x-ray diffraction, x-ray standing wave), are omitted. The optical techniques described use simple lamp or small laser sources and are thus, in principle, easily accessible. Epioptic probes can provide new information on solid-gas, solid-liquid and liquid-liquid interfaces. They are particularly suited to growth monitoring. Emerging process technologies for fabricating submicron and nanoscale semiconductor devices and novel multilayer materials, whether based on silicon or compound semiconductors, all require extremely precise control of growth at surfaces. In situ, non-destructive, real-time monitoring and characterisation of surfaces under growth conditions is needed for further progress. Both atomic scale resolution, and non-destructive characterisation of buried structures, are required.



Chapter 1. Introduction

The penetration depth of optical radiation into condensed matter is large, in general, which makes the isolation of a surface or interface contribution difficult (“interface”, from now on, is understood to include the surface, which is the condensed matter-ambient interface). Even in the near ultra-violet (UV) region, the interface contribution to the linear reflectivity from a semiconductor will only be a few percent. However, deeper understanding of the underlying physics of the optical response, combined with advances in instrumentation, have allowed the contribution from the interface to be identified. Particularly important has been the recognition that symmetry differences between the bulk and interface can be exploited, as can interface electronic and vibrational resonances [1]. The special efforts and techniques required to obtain this interface optical response, together with the increasing activity in the area, justify distinguishing it from more conventional optical studies. The term “epioptics” has been coined for this special area (from the Greek “epi” meaning “upon”) [2].
John McGilp

Chapter 2. The Linear Optical Response

The linear optical response theory considered in this chapter relates to the experimental techniques of reflectance anisotropy spectroscopy (RAS) [1–6] and surface differential reflectance (SDR) [7–15]. These experimental techniques were outlined in Ch. 1, and are described in detail in Ch. 4. This chapter describes calculations of the RAS and SDR spectra from clean and adsorbate-covered Si, GaAs and GaP surfaces. The experiments aim to extract information about how the narrow selvedge region at a surface contributes to the reflected electromagnetic field in vacuum. Thus, the field amplitudes in this region at the surface are a very important ingredient in the calculation. What provides an interesting challenge for the theorist in this problem is that the region where the field amplitudes need to be known accurately is also the region where they are changing very rapidly (over ~1 nm) from their vacuum values to the bulk values. We recognise three ill-defined regions (in the sense that their edges are not sharp), which are important in determining the optical response of a surface: I the vacuum region; II the (narrow) selvedge region; HI the bulk. Since the advent of modern surface science, probably the first attempt to include these three regions was in the work of Mclntyre and Aspnes [16]. This has come to be known as the three-phase model (see Sect. 1.3).
Rodolfo Del Sole, Anatolii Shkrebtii, Jiang Guo-Ping, Charles Patterson

Chapter 3. Spectroscopic Ellipsometry

Ellipsometry dates back to the middle of the last century [1, 2], and it has been used since then for the determination of the optical properties of metals, semiconductors and insulators [3]. Ellipsometry is nowadays quite intensively employed in semiconductor characterisation and has the potential for in situ diagnostics of surfaces. The principle of ellipsometry is based on the fact that the status of light polarisation is changed when light is reflected from a surface. This change can be related to the dielectric function of the reflecting material, as discussed in Sect. 1.3.
Uwe Rossow

Chapter 4. Reflection Difference Techniques

Reflectance measurements are conventionally used to derive the optical constants of solid materials, i.e. the refractive index, N, and the extinction coefficient, K. At near normal incidence, for example, the reflectance, R, (the ratio of the reflected to incident EM intensity: see Sect. 1.3) is given by:
$$R = \frac{{{{(n - 1)}^2} + {\kappa ^2}}}{{{{(n + 1)}^2} + {\kappa ^2}}}$$
The optical constants are related to the dielectric function, ε = ε’ + iε” by:
$$\varepsilon ' = {n^2} - {\kappa ^2}{\text{ and }}\varepsilon '' = 2n\kappa $$
Dietrich Zahn

Chapter 5. Raman Spectroscopy

Raman spectroscopy (RS) is one of the linear optical techniques widely applied in the analysis of solids, as well as for liquids and gases. It is one of the experimental techniques based on inelastic scattering of light. If the frequency difference between incident and scattered light is in the range below (ħω = 10-7 eV, light scattering is termed Rayleigh scattering and Fourier analysis of the detector current yields the change of frequency caused by the inelastic scattering. For larger frequency differences (ħω = 10-7 to 10-4 eV), interferometric devices, usually Fabry-Perot interferometers, are used to determine the frequency change. The inelastic light scattering is then termed Brillouin scattering. Finally, Raman scattering describes larger frequency differences (above ħω = 10-4 eV) where grating monochromators can be utilised for frequency analysis of the scattered light.
Wolfgang Richter

Chapter 6. Photoluminescence Spectroscopy

It is only fitting that a chapter on photoluminescence (PL) should be included in a book which ascribes to reflect the present role of optical techniques in the characterisation of surfaces and interfaces. Whilst the phenomenon of luminescence has been investigated for more than 100 years, it is only within the last 10 to 15 years that luminescence spectroscopies have been applied to semiconductor surfaces and interfaces. This late maturing, of a well-established experimental technique, is linked almost exclusively to improvements in material quality, which have resulted from the ability to deposit epitaxial layers with atomic precision. In particular, the confinement of carriers in a potential well, afforded by growing a thin layer of a material with a bandgap less than that of the surrounding material, means that luminescence measurements become probes of the local electronic environment. Moreover, information can be gathered on specific surfaces and interfaces by monitoring the coupling which occurs between the quantised well levels and the electronic states at the surface or interface itself.
Zbig Sobiesierski

Chapter 7. On the Theory of Second Harmonic Generation

In Sect. 1.5, second-harmonic and sum-frequency generation (SHG and SFG) at surfaces and interfaces was introduced. Experimental results are discussed in Chapter 8. Various aspects of the theory of SHG at surfaces and interfaces are discussed in this chapter.
Andrea d’Andrea, Michele Cini, Rodolfo Del Sole, Lucia Reining, Claudio Verdozzi, Raffaello Girlanda, Edoardo Piparo, David Hobbs, Denis Weaire

Chapter 8. Second Harmonic and Sum Frequency Generation

As mentioned in Sect. 1.5, the lowest order nonlinear optical response of materials produces three-wave mixing phenomena, which include SHG and SFG, and these phenomena may be surface sensitive at non-destructive power densities. For centrosymmetric materials, an order of magnitude calculation shows that the surface effect should be at least comparable in size to the higher order nonlocal bulk effects [1]. Initial surface SHG studies in the 1960s under non-UHV conditions detected a surface signal but found no dependence on adsorbate or surface structure, and this held back the development of the field until the early 1980s, when Shen in Berkeley established the potential of SHG as a surface probe [2].
John McGilp

Chapter 9. Conclusions

Epioptics is now well established, and is making a major contribution to the study of surfaces and interfaces in general, and those of semiconductors in particular. Significant advantages over conventional surface spectroscopies have been demonstrated: all pressure ranges and transparent media are accessible; insulators can be studied without the problem of charging effects; buried interfaces can be studied due to the large penetration depth of optical radiation. Epioptic techniques offer micron lateral resolution and femtosecond temporal resolution. Nondestructive, in situ characterisation of thin films, surface and interfaces in all pressure regimes is central to the development of new materials and processes, particularly in this evolving era of nanoscale structures. It is these advantages over conventional surface spectroscopies which have driven the development of epioptics.
John McGilp


Weitere Informationen