1 Introduction

1.1 What is optical characterization?

The study of solids is a rich scientific area, which depends on a great variety of experimental probes. Techniques using electromagnetic radiation are among the most fruitful of these. The very short wavelengths of X-rays are instrumental, even essen­tial, in examining the atomic lattices that define crystalline solids. Wavelengths from the ultraviolet through the visible and infrared to the millimeter range have enormous power to examine all aspects of solids, especially semiconductors: the crystal lattice, through its quantized vibrations, the phonons; the electronic band structure, including the band gap, where light is absorbed; free electrons and holes, which also absorb light as they move; impurities and defects, which interact with light through their own vibrations in the lattice, or by ionization processes. And when semiconductors are formed into the microstructures that increasingly define their use in devices, electro­magnetic radiation also measures the dimensions and explores the interfaces of these artificial systems.

Light, therefore, examines most of the properties of semiconductors and their structures; or to put it another way, to know how a semiconductor reacts to light, we must understand its basic properties. But of equal importance is the ability of optical analysis to contribute to applications of semiconductors. Such analysis can measure those properties which determine whether a semiconductor will serve well in a specific use—that is, optical methods can characterize semiconductors.

There is no strict line which marks off fundamental measurements from characteriz­ation measurements. The same optical measurement, at the same level of sophisti­cation, might determine a quantity for its innate interest, or to help create better devices. To decide, for instance, whether a new semiconducting alloy or microstruc-ture can be useful, the designer must know its intrinsic properties, the constants of the material such as band gap and phonon frequencies. But extrinsic properties which vary from sample to sample or even within a sample are equally important. The materials maker must know sample-dependent quantities such as type and density of impurities, and film thickness. Especially with complex growth methods like metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy ( M B E ) , these parameters are essential to track sample quality, to trace sources of contami­nation, and to meet specific device requirements. Moreover, rapid feedback of measured values is needed so that successive cycles of growth can rapidly converge to high-quality material, enhancing the yield from expensive facilities. After the material is made, further processing and fabrication into devices also needs careful monitoring. The device itself may require evaluation. Optical characterization can meet these needs, from routine day-to-day analysis, to solving difficult problems in new ma­terials.

The very qualities that make semiconductors interesting and useful make it difficult

2 Introduction

to determine their intrinsic and extrinsic characteristics. The flexibility to choose design parameters, by selecting from the large family of semiconducting materials or creating new ones, requires equal flexibility in characterization methods. The sensi­tivity to small amounts of impurities which makes semiconductors essential to the electronic and photonic industries means that small inadvertent changes—minute contamination during growth, for instance—have large effects. The performance of semiconductor microstructures depends on the accurate creation of layers only nano­meters thick. Varied and sensitive characterization methods are essential.

Fortunately, there are many characterization techniques, each with its advantages and disadvantages. Consider, for instance, the electronic properties due to free car­riers—resistivity, carrier density, and mobility—which can vary from sample to sample, and must be optimized for devices. The standard electrical measurements which determine these, such as van der Pauw-type Hall and resistivity measurements, are widely used and thoroughly understood. They do not need expensive facilities, or highly trained personnel, for the actual measurements. But they require that electrical contacts be attached to the sample, a time-consuming process, and a difficult one for high resistivity material (Henisch, 1984). The need for physical contacts also limits possibilities to spatially probe a sample, say to determine inhomogeneity in the carrier density. Traditional Hall and resistivity measurements can also be difficult to interpret for a multilayer system. Non-electrical characterization techniques such as trans­mission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS) do not need contacts, but require other types of sample preparation, or alter the speci­men as it is examined.

Among characterization methods, optical techniques stand out because they require little sample preparation—the time-consuming process of creating electrical contacts is eliminated, for instance. Hence the sample is generally unaltered, nor does the measurement itself cause damage (unless a probing laser beam is too intense, which is usually avoidable). Because an optical beam is easily manipulated, these methods can examine different parts of a structure, at spatial resolutions determined by the wavelength of the light. Visible to near-infrared light can probe the finest details of a semiconductor microstructure or device. This means that optical measure­ments can create two-dimensional maps of properties in the plane of the sample, such as impurity distribution or layer thickness. This is difficult or impossible with electrical contacts fastened in place (although there are non-optical methods, notably scanning electron microscopy (SEM), which achieve this). It is also possible to differentiate properties along the third dimension, as the light propagates into the sample with a component perpendicular to its surface. The penetration depth of the light depends on its wavelength and on the sample properties, so that the region examined can range from nanometers to micrometers deep.

These are the motivations to explore optical characterization methods as valuable additions to the array of analytical tools for semiconductors. To do so in this book, I examine three of the many optical methods: photoluminescence, where light separ­ates charge carriers within the band or impurity structure of a semiconductor, whose later recombination produces characteristic emissions; Raman scattering, where the energy of an incoming photon is altered by non-linear interaction with phonons, carriers, or impurities in the material, to produce a frequency-shifted outgoing pho­ton; and infrared absorption, where long-wavelength light is directly absorbed at band features, and by phonons, free charge carriers, or impurities.

Infrared, Raman, and photoluminescence spectroscopy 3

1.2 Infrared, Raman, and photoluminescence spectroscopy

I have selected infrared, Raman and photoluminescence methods for a combination of reasons: they are widely used in research, in materials development, and in the semiconductor industry; they are spectroscopic in nature, meaning that intensity is measured versus wavelength, which provides the capacity for quantitative analysis; they offer breadth in the properties they measure, and in the wavelengths they use. There are of course other techniques of great importance, such as optical microscopy in its different forms, and fixed-wavelength ellipsometry. These are also widely used, but lack the spectroscopic character. Some methods, such as spectroscopy in a mag­netic field, or time-resolved photoluminescence, have enormous capability for analy­sis, but are not yet widespread and accessible for general use. I comment on these other techniques in the final chapter of this book.

The three techniques I have chosen probe every important intrinsic and extrinsic semiconductor property—band characteristics, transport behavior, lattice and pho-non features, impurities and defects—as well as the geometry and interface behavior of semiconductor microstructures, from single epitaxial films to complex superlattices. The methods also span virtually the entire wavelength range used for electromagnetic spectroscopy, from 0.4 (xm to 1000 |xm, excluding only the X-ray and deep ultraviolet regions. Photoluminescence and Raman scattering are generally excited in the near ultraviolet, visible or near-infrared regions, over 0 .4-1 fxm. The infrared, as I define it here, covers a very broad range, spanning the near infrared (approximately 1-5 |xm), the middle infrared (5-50 |xm), and the far infrared (50-1000 (xm = 0.05-1 mm).

Table 1.1 shows in concise form what each technique best characterizes. Photolumi­nescence is perhaps the most widespread of the three. It has become a routine characterization tool in many materials laboratories. It is especially sensitive to im­purities, is important in the analysis of Al^Ga!_ x As-GaAs microstructures, and requires relatively simple and inexpensive instrumentation. Raman scattering charac­terizes lattices and their stresses, impurities, and free carriers. It is not so widely used as photoluminescence spectroscopy, partly because it is a more difficult measurement which needs more elaborate facilities. It offers great power to examine the lattice properties of A\xGai-xAs-based superlattices and other microstructures, and a unique capability to measure lattice temperature. Spectroscopy in the middle infrared is widely used to measure the concentration of interstitial oxygen and substitutional carbon impurities in silicon, and the thickness of epitaxial layers. Far-infrared spectra identify shallow impurities and measure carrier properties. Infrared spectroscopy is the most quantitative of the three methods, because simple theories can be used to analyze the data. This permits detailed examination of microstructures on a layer-by-layer basis.

A striking feature of the summary in Table 1.1 is the degree of redundancy among the techniques; for instance, each method can examine semiconductor impurities. This gives freedom to consider other factors, such as the spatial resolution and the pen­etration depth of the radiation. All three methods can provide two-dimensional maps of semiconductor properties, but the short wavelengths of photoluminescence and Raman methods give them the best spatial resolution. These wavelengths also pen­etrate semiconductors less deeply than does long-wavelength infrared light. Photo­luminescence and Raman scattering tend to operate as surface probes, penetrating only tens to hundreds of nanometers into a semiconductor. Mid- to far-infrared

4 Introduction

T a b l e 1.1 Semiconductor properties which can be charac­terized by the optical methods treated in this book: photo-luminescence ( P L ) , Raman scattering, and infrared ( IR) spectroscopy.*

Semiconductor property Optical method

PL Raman IR

Band Gap Effective mass Band offset

Free carrier Concentration Mobility Scattering time Resistivity

Lattice Alloy composition Orientation Crystallinity Stress

Impurity and defect Presence and type Concentration

Microstructure Layer thickness Surface behavior Interface behavior Layer-by-layer behavior

Other Homogeneity mapping

* A bullet at the intersection of a method (column) and a property (row) indicates that method is especially useful to measure that property. Some measurements require auxiliary information. Where more than one method is available, other factors may deter­mine the choice; see text. The discussions and case studies in this book present and illustrate most of the applications shown.

radiation is useful to examine an entire layered structure, because its penetration depth is micrometers.

It is useful to think of the three techniques, not as entirely separate approaches, but as a family of characterization tools, each with special capabilities which suit it for given tasks. When I turn to case studies of optical characterization later in this book, it will be clear that it is often fruitful to combine optical methods. (The benefits of combining optical and nonoptical characterization will also emerge in many cases, for instance, in determining effective mass from infrared data.) In addition, the overlap between the methods means that nontechnical factors such as cost and ease of use can be considered in decisions about the best way to characterize.

The three techniques cover so broad a range of wavelengths and of applications that this book amounts to an overview of the science and technology of optical measure-

Infrared, Raman, and photoluminescence spectroscopy 5

ments in semiconductors. After briefly reviewing elements of electromagnetic theory and semiconductor physics, I discuss in Chapter 3 the fundamental interactions be­tween light and the different elements of a semiconductor, from lattice and band structure, to carriers and impurities. I link each light-semiconductor interaction to the technique (photoluminescence, Raman, or infrared spectroscopy) that measures it, to emphasize the connection between basic physics and spectroscopic result. This presentation is illustrated with experimental results from the literature, again to make the connection between microscopic behavior and measurement outcome. Much optical characterization is a matter of examining spectra, and there is value in showing how the important quantities appear in them. Then the discussion shifts to technical analysis of each of the three methods, which shows how measurements are actually made. I discuss instrumentation including sources, wavelength analyzers, and detec­tors—from the relatively simple systems which serve for photoluminescence, to sophisticated Fourier transform and triple monochromator arrangements. I also comment on sample requirements and sample cooling. A single book cannot describe every technique, but the coverage here is meant to convey general principles and methods that carry over to other methods.

A most important part of the book follows, i.e. many illustrative examples or case studies from the literature. These show actual application of the three techniques to semiconductors, to link theory, experiment, and practice. The cases are chosen to illustrate the range of what can be measured and how it is done, and include a variety of materials. A majority of the examples draw on the most widely used semiconduc­tors, silicon, G a A s , and A l ^ G a x _ x A s ; but I do not hesitate to include other materials such as InP and Hgx_ x Cd^Te when these applications are especially informative or novel, and to show the diversity of the methods. Materials are considered in bulk, film, and microstructure form. The case studies include technical details of the measurements, especially when these are unusual or illustrate a point. It is not, of course, possible to give full details of the cases, but the extensive set of references provides further information. The references have been selected partly because they offer experimental details. Some also provide good overviews of the characterization application. These case studies and references should be a most useful part of the book for those who wish to use optical characterization.

M y discussion of the theory behind the optical behavior of semiconductors is pur­posely limited, because there are excellent works which cover the material in depth. A full description of the optical properties of semiconductors can be found in Pankove (1975), a standard and useful work, and Balkanski (1980). A more general work is Wooten (1972), which deals with the optical behavior of solids, not only semiconduc­tors. On the other hand, Willardson and Beer (1967) focuses on the optical behavior of I I I - V materials exclusively. I give other foundation references elsewhere in the book, as appropriate.

Books about the physics and optical properties of semiconductor provide much information essential for optical characterization; but I know of no book completely devoted to the use of optical methods to characterize semiconductors. There are, however, other useful sources. A set of conference proceedings from SPIE, the International Society for Optical Engineering, gives what is probably the broadest coverage to be found. These include Aspnes et al. (1981), Pollak and Bauer (1983), Pollak (1985), Cheung and Nicolet (1986), Glembocki etal (1987,1988), and Brillson and Pollak (1989). Other SPIE proceedings which bear on optical characterization

6 Introduction

include Izatt (1986), Alfano (1987,1988), and Baars and Longshore (1989). There are also chapter-length overviews of optical characterization in specific areas. Palik and Holm (1979), Perkowitz (1983), Carr et al. (1985), and Krishnan et al (1990) review different aspects of infrared characterization. Pollak (1990) treats Raman characteriz­ation of semiconductors. Lightowlers (1990) discusses the use of photoluminescence spectroscopy to detect impurities and measure their concentration.