Basic Principles The hndamental principles of XRF can be found in the literature.1-3 Briefly, X rays are electromagnetic radiation of very high energy (or short wavelength).The unit of measurement for X rays is the angstrom (A),which is equal to lo4 cm.When an Xray photon strikes an atom and knocks out an inner shell electron, if the incident photon has energy greater than the binding energy of the inner shell electron, a readjustment occurs in the atom by filling the inner shell vacancy with one of the outer electrons and simultaneously emitting an X-ray photon The emitted photon (or fluorescent radiation) has the characteristicenergy of the difference between the binding energies of the inner and the outer shells The penetration depth of a highenergy photon into a material is normally in the range (Another method commonly used to produce X rays is electron-beam excitation; the penetration depth of an electron beam is about an order of magnitude smaller than that of X rays See the articles on EDS and EPMtL) Measurements of the characteristic X-ray line spectra of a number of elements were first reported by H G J Moseley in 1913.He found that the square root of the frequency of the various X-ray lines exhibited a linear relationship with the atomic number of the element emitting the lines This hdamental “Moseleylaw” shows that each element has a characteristic X-ray spectrum and that the wavelengths vary in a regular fashion form one element to another The wavelengths decrease as the atomic numbers of the elements increase In addition to the spectra of pure elements, Moseley obtained the spectrum of brass, which showed strong Cu and weak Z n X-ray lines; this was the first XRF analysis The use of XRF for routine spectrochemical analysis of materials was not carried out, however, until the introduction of modern X-ray equipment in the late 1940s Instrumentation The instrumentation required to carry out XRF measurements normally comprises three major portions: the primary X-ray source, the crystal spectrometer, and the detection system A schematicX-ray experiment is shown in Figure Fluorescent X rays emitted from the specimen are caused by high-energy (or short-wavelength) incident X rays generated by the X-ray tube The fluorescentX rays fiom the specimen travel in a certain direction, pass through the primary collimator The andyzing crystal, oriented to reflect from a set of crystal planes of known dspacing, reflects one X-ray wavelength (A) at a given angle (e) in accordance with Bragg’s law: = 2dsin0, where n is a small positive integer giving the order of reflection By rotating the analyzing crystal at one-half the angular speed of the detector, the various wavelengths from the fluorescent X rays are reflected one by one as the analyzing crystal makes the proper angle for each wavelength The intensity of at each wavelength is then recorded by the detector This procedure is known also as the 6.1 XRF 339 X-ray tube Analyzing crystal Figure Schematic of XRF experiment wavelength-dispersive method (The wavelength-dispersive method is used extensively in EPMA, see the EPMA article in this volume.) X-RaySoums A sealed X-ray tube having a W, Cu, Rh,Mo, Ag,or Cr target is commonly used as the primary X-ray source to directly excite the specimen A secondary target material located outside the X-ray tube is used sometimes to excite fluorescence This has the advantages of selecting the most efficient energy close to the absorption edge of the element to be analyzed and of reducing (or not exciting) interfering elements (The intensity is much reduced, however.) X-ray sources, including synchrotron radiation and radioactive isotopes like 55Fe(which emits Mn K X rays) and AM-24 (Np L X rays) are used in place of an X-ray tube in some applications Analyzing Crystals Crystals commonly used in XRF are: LiF (200) and (220), which have 2Cspacings of 4.028 and 2.848 A, respectively; pyrolytic graphite (OO2), spacing 6.715 A; PET(OO2), spacing 8.742 A; TAP(OOl), spacing 25.7 A; and synthetic multilayers ofW/Si, W/C, V/C,Ni/C, and Mo/B&, spacing 55-160 A The lowest-Zelement that can be detected and reflected efficientlydepends on the Cspacing of the analyzing crystal selected The crystals are usually mosaic, and each reflection is spread over a small angular range It is thus important that the crystal used be of good quality to obtain intensive and sharp XRF peaks The angular spread of the 340 X-RAY EMISSION TECHNIQUES Chapter FeKa 60 50 "20 XRF spectrum of MnFe/NiFe thin film Figure2 peaks, or the dispersion, de/& = n/(2dcose), increases with decreasing d The dispersion thus can be increased by selecting a crystal with a smaller d X-Ray Detection Systems The detectors generally used are scintillation counters having thin Be windows and NaI-T1 crystals for short wavelengths (above A or kev), and gas-flow proportional counters having very low absorbing windows and Ar/CH* gas for long wavelengths (below A or kev) A single-channel pulse amplitude analyzer is used to accept fluorescent X rays within a selected wavelength range to improve peak-to-background ratios and to eliminate unwanted high-order reflections The counting times required for measurement range between a few seconds and several minutes per element, depending on specimen characteristicsand the desired precision A typical XRF spectrum of a FeMn/NiFe thin film is plotted in Figure The Ka and Kp XRF fluorescent peaks from the fdm are identified, and the remaining peaks are those from the spectrum of the X-ray tube The experimental conditions included a Mo target X-ray tube operated at 45 kV, a LiF (200) analyzing crystal, and a scintillation counter with a single-channel pulse amplitude analyzer The energy resolution of the Mn K a peak at 5.89 keV was 24 eV, compared to 145 eV for a Si (Li) solid-state energy-dispersivesystem (see EDS article) The high spectral resolution of the wavelength-dispersive method made possible the measurements of Ni, Fe, and Mn fiee of interference from adjacent peaks Analytical Capabilities Elemental Depth Profiling The X-ray penetration depth in a material depends on the angle of incidence It increases from a few tens of A near the total reflection region to several pm at large 6.1 XRF 341 incidence angles (a few tens of degrees) The XRF beam, which originates from variable depths, can be used for elemental depth analysis For example, the grazing incidence XRF method has been used for studies of concentration profiles of a dissolved polymer near the air/liquid intehce? Langmuir-Blodgett multilayer^,^ and multiple-layer fdms on substrates.6 This type of analysis requires a parallel-incidence beam geometry, which currently is not possible with a conventional spectrometer Chemical State Analysii The XRF wavelengths and relative intensities of a given element are constant to first approximation Small changes may occur when the distribution of the outer (or valence) electron changes A major area of research in XRF involves the use of "soft" X-ray emission (or long-wavelength XRF) spectra for chemical state analysis Soft X-ray peaks often exhibit fine structure, which is a direct indication of the electronic structure (or chemical bonding) around the emitting atom Thus the shift in peak position, change in intensity distribution, or appearance of additional peaks can be correlated with a variety of chemical factors, including the oxidation state, coordination number, nature of covalently bound ligands, etc The equipment ot required for s f X-ray analysis is almost identical to that required for conventional XRF, with one major exception Since it is a study of transitions involving the outer orbits and therefore long wavelengths, s f X-ray analysis employs a long-waveot length X-ray source such as Al(8.34 A for Al Ka)or Cu (13.36 for Cu La).Special analyzing crystals or gratings for measuring wavelengths in the range 10-1 00 A also are needed.7 QuantitativeAnalysis In addition to qualitative identification of the elements present, XRF can be used to determine quantitative elemental compositions and layer thicknesses of thin films In quantitative analysis the observed intensities must be corrected for various factors, including the spectral intensity distribution of the incident X rays, fluorescent yields, matrix enhancements and absorptions, etc Two general methods used for making these corrections are the empirical parameters method and the fundamental parameters methods The empirical parameters method uses simple mathematical approximation equations, whose coefficients (empirical parameters) are predetermined from the experimental intensities and known compositions and thicknesses of thin-film standards A large number of standards are needed for the predetermination of the empirical parameters before actual analysis of an unknown is possible Because of the difficulty in obtaining properly calibrated thin-film standards with either the same composition or thickness as the unknown, the use of the empirical parameters method for the routine XRF analysis of thin films is very limited 342 X-RAY EMISSION TECHNIQUES Chapter The fundamental parameters method uses XRF equations derived directly from first principles Primary and secondary excitations are taken into account Primary excitations are caused directly by the incident X rays from the X-ray source, while the secondary excitations are caused by other elements in the same film, whose primary fluorescent X-ray radiation has sufficient energy to excite the characteristic radiation of the anal+ element Higher order excitations are generallyconsidered insignificant because of their much lower intensities XRF equations relate intensiry; composition, and thickness through physical constants (fundamental pararneters) like fluorescent yields, atomic transition probabilities, absorption coefficients, etc For example, the XRF equations for single-layer films were reported by Laguitton and Parrish,' and for multiple-layer films by M a ~ ~ t l e r ~ equations for-thin The films are very complex, and the values of composition and thickness cannot be determined directly from the observed intensities They are obtained by computer iteration using either linear or hyperbolic approximation algorithm The h d a mental parameters technique is suitable for the analysis of thin films because it requires a minimum number of pure or mixed element and bulk or thin-film standards Applications The principle application of XRF thin-film analysis is in the simultaneous determination of composition and thickness The technique has been used for the routine analysis of single-layer films' since 1977 and multiple-layer filmsio since 1986 Two main sources of publications in the fields are the annual volumes of Advances in X-RayAm&s by Plenum Press, New York, and the Journal o X-RaySpectromef t y by Heyden and Sons, London Typical examples on the analysis of single-layer r films and multiple-layer films are used to illustrate the capabilities of the technique Single-Layer Films Evaporated FeNi films with a large range of compositions were selected because of the strong absorption of Ni and enhancement of Fe K X rays in the films XRF compositions of FeNi films deposited on quartz substrates are listed in Table and are compared to those obtained by the Atomic Absorption Spectroscopy (AAS) and the Electron Probe Microanalysis (EPMA) Since the strong X-ray absorption and enhancement effects are severe for both XRF and EPMA but not present in AAS, a comparison between the XRF results and the two non-XRF techniques provide a useful evaluation ofXRF." As shown in Table ,there is good agreement between results of XRF and AAS or EPMA, and the average deviation is 0.9% between XRF and AAS and is 1.1% between XRF and EPMA It is worth noting that the compositions of more than half of the FeNi films obtained by XRF, AAS, and EPMA are significantly different fiom the intended compositions (see values inside the parentheses listed in column of Table 1) The discrepancy shows the '* 6.1 XRF 343 Fe (5)-Ni (95) 4.2 5.0 2.5 Fe (10)-Ni (90) 9.2 9.0 6.2 Fe (20)-Ni (80) 19.4 19.2 19.4 Fe (34)-Ni (66) 47.3 48.4 44.5 Fe (50)-Ni (50) 59.1 61.7 59.1 Fe (66)-Ni (34) 78.9 79.8 78.4 Fe (80)-Ni (20) 89.2 89.6 89.2 Table Fe concentrations 1% wt.) for FoNi films risk of using intended composition and the important of determining composition experimentallyby XRF or other reliable techniques im The volume density p and thickness tof a fl appear together as a single parameter ptin the XRF equations, the value of pt, the areal density (not the thickness) is determined directly by iteration From the areal density, the film thickness can be calculated when the volume density is known experimentally or theoretically Using the volume densities calculated fiom the film composition and the published volume densities of pure elements, the thicknesses of 12 FezoNigo films were calculated from the XRF areal densities and are compared to those obtained by a nonXRF technique (i.e., AAS or a deposition monitor) As shown in Table 2, good agreement between XRF and non-XRF thicknesses are obtained with average and maximum deviations of 2.95% and 6.7%, respectively (see the last column of Table 2) The volume density can also be calculated from the XRF areal density when the thickness of a film is known For example, the volume densities of Fe19Ni81 permalloy films with known thicknesses of 5O-10,OoO A were calculated from the XRF areal densities The calculation shows that the volume density of the permalloy is not constant and changes systematicallywith the film’s thickness It is equal to the bulk value of 8.75 g /cm3 for films of 1000 A or greater thickness, decreases to 94% of the bulk value for the 500-A film, and to 81% for the 50-A tilm.12 Multiple-layer F i h s XRF analysis of multiple-layer films is very complex because of the presence of XRF absorption and enhancement effects, not only between elements in the same layer but also between all layers in the fdm Equations fbr the calculation of XRF intensities for multiplelayer films are avaiIabIe from the Iiterature.9’ l3 Proper correc344 X-RAY EMISSION TECHNIQUES Chapter F h m 825 848 28 858 848 12 1117 1142 22 3180 2967 6.7 3215 3011 63 3558 3473 24 3579 3524 15 4090 4070 05 5533 5452 15 10 5550 5237 56 11 5601 5655 o 12 6283 6053 37 Non-XRF' A/XRF () %b a Either AAS or monitor b A = IXRF - non-XRFI Table mi- (A) for wifilms tions for intralayer and interlayer effects are essential for a successful XRF analysis of multiple-layer films The accuracy of XRF compositions and thicknesses for multiple-layer films was bund to be e q d to those for single-layer films For example, XRF was used successfully to analyze two triple-layer films of Cry l4 The two films, T and T2, have Cu, and FeNi deposited on Si substrates." identical individual CryCu and FeNi layers but different order In T ,the FeNi layer is on top, the Cu layer in the middle, and the Cr layer at the bottom; in T2, the positions of the Cr and FeNi layers are reversed, with Cr on top and FeNi at the bottom; meanwhile the Cu layer remains in the middle Because of this reversal of layer order, interlayer absorption and enhancement effects are grossly different between these two films This led to large differences in the observed intensities between these two films The differences between T and T were -17%, +2%, +20%, and +15%, respectively, for the CryCu, Feyand Ni K a observed intensities.12 Using the same set of observed XRF intensities, rhe results obtained by two different analysisprograms: LAMA-I11from the US12and DF270 fiom Japan'* are essentially the same within a relative deviation of 0.2% in composition and 1% in 6.1 XRF 345 T (FeNilCulG) T (CrlCdFeNi) s1 (Cr) s2 (Mi) s3 (cu) Fe (% wt.) 10.25 10.25 - 10.50 - Ni (% wt.) 89.75 89.75 - 89.50 - - - tc,(4 1652 1698 1674 tFeNi(& 2121 2048 - tCu@) 2470 2457 - Table 2115 - 2416 XRF mutts for films o Cr, FeNi, and Cu f thickness The results obtained by the LAMA-I11 program are listed in Table In spite of the large differences in the observed intensities of Cr, Fe and Ni, the compositions and thickness of all three layers determined by XRF are essentially the same for T1 and T2 For comparison, an XRF analysis was also done on three single-layer Cr, FeNi, and Cu films (S 1, S2, and S3) prepared under identical deposition conditions to the two triplelayer films As shown in Table 3, good agreement was obtained between the single- and triple-layer films This indicates that the severe interlayer enhancement and absorption effects observed in T1 and T2 were corrected properly It is also worth noting that the deviations between the results of the triple- and single-layer films are within the accuracy reported for the single-layer fhs In multiple-layer thin films, it is possible that some of the elements may be present simultaneously in two or more layers XRF analysis of this type of f h can be complicated and cannot be made solely from their observed intensities Additional information, such as the compositions or thickness of some of the layers is needed The amount of additional non-XRF information required depends on the complexity of the fdm For example, in the analysis of a FeMn/NiFe double-layer film, the additional information needed can be the composition or thickness of either the FeMn or NiFe layer Using the composition or thickness of one of the film predetermined from a single-layer film deposited under identical conditions, XRF analysis of the FeMn/NiFe film was ~uccessful.~~ Related Techniques XRF is closely related to the EPMA, energy-dispersiveX-Ray Spectroscopy (EDS), and total reflection X-Ray Fluorescence (TRXF), which are described elsewhere in this encyclopedia Brief comparisons between XRF and each of these three techniques are given below 346 X-RAY EMISSION TECHNIQUES Chapter EPMA Both XRF and EPMA are used for elemental analysis of thin films XRF uses a nonfocusing X-ray source, while EPMA uses a focusing electron beam to generate fluorescent X rays XRF gives information over a large area, up to cm in diameter, while EPMA samples small spots, pm in size An important use of EPMA is in point-topoint analysis of elemental distribution Microanalysis on a sub-prn scale can be done with electron microscopes The penetration depth for an X-ray beam is norrange, while it is around ~ITI for an electron beam There is, mally in the 10-~un therefore, also a difference in the depth of material analyzed by XRF and EPMA EDS EDS is another widely used elemental analysis technique and employs a solid state detector with a multichannel analyzer to detect and resolve fluorescent X rays according to their energies EDS uses either X rays or an electron beam as a source to excite fluorescence Unlike XRF, which uses the wavelength-dispersive method to record X-ray intensities one by one, EDS collects all the fluorescent X rays from a specimen simultaneously.A limitation of EDS is its energy resolution, which is an order of magnitude poorer than that of the wavelength-dispersive method For example, the Ka peaks of transition elements overlap the KP peaks of the next lighter element, which cause analytical difficulties The poorer resolution also causes relatively lower peak-to-background ratios in EDS data TXRF XRF at large incident angles, as described in this article, is normally used for elemental analysis of major concentrations of 0.1 % or higher Total Reflection X-Ray Fluorescence (TXRF) with grazing-incidence angles of a few tenths of a degree is used for trace-element analysis Detectable limits down to lo9 atoms/cm2 are now attainable using a monochromatic X-ray source Examples of the use of this technique in d e r technology are given in the article on TXRF in this volume Conclusions XRF is one of the most powerful analysis technique for the elemental-composition and layer-thickness determination of thin-film materials The technique is nondestructive, inexpensive, rapid, precise and potentially very accurate XRF characterization of thin films is important for the research, development, and manufacture of electronic, magnetic, optical, semiconducting,superconducting, and other types of high-technology materials Future development is expected in the area of microbeam XRF, scanning XRF microscopy, grazing-incidenceXRF analysis of surfices and buried interfaces, long-wavelength XRF and chemical state analysis, and synchrotron XRF 6.1 XRF 347 Figure Schematic layout of a high-sensitivity PL system incorporating a laser and photon-countingelectronics A dispersive element for spectral analysis of PL This may be as simple as a filter, but it is usually a scanning grating monochromator For excitation spectroscopy or in the presence of much scattered light, a double or triple monochromator (as used in Raman scattering) may be required An optical detector with appropriate electronics and readout Photomultiplier tubes supply good sensitivity for wavelengths in the visible range, and Ge, Si, or other photodiodes can be used in the near infrared range Multichannel detectors like CCD or photodiode arrays can reduce measurement times, and a streak camera or nonlinear optical techniques can be used to record ps or sub-ps transients A schematic of a PL system layout is shown in Figure This optical system is very similar to that required for absorption, reflectance, modulated reflectance, and Raman scattering measurements Many custom systems are designed to perform several of these techniques, simultaneously or with only small modifications Conclusions Photoluminescence is a well-established and widely practiced tool for materials analysis In the context of surface and microanalysis, PL is applied mostly qualitatively or semiquantitatively to exploit the correlation between the structure and composition of a material system and its electronic states and their lifetimes, and to identify the presence and type of trace chemicals, impurities, and defects Improvements in technology will shape developments in PL in the near future PL will be essential for demonstrating the achievement of new low-dimensional quantum microstructures Data collection will become easier and Edster with the continuing development of advanced focusing holographic gratings, array and imaging detectors, sensitive near infrared detectors, and tunable laser sources 7.1 PL 383 Related Articles in the Encyclopedia CL, Modulation Spectroscopy,Raman Spectroscopy,and FTIR References I? J Dean Prog CrystaLGrowth Charat 5,89,1982 A review of PL as a diagnostic probe of impurities and defects in semiconductorsby an important progenitor of the technique L T Canham, M R Dyball, and K G Barrad0ugh.J Appl Pbys 66, 920,1989 G E Stillman, B Lee, M H Kim, and S S Bose h e Elcchochem Soc 88-20,56, 1988 Describes the use of PL for quantitative impurity analysis in semiconductors K D Mielenz, ed Measurement ofPhotoluminescence.vol of Optical Radiation Measuremena (F Grum and C J Bartleson, eds.) Academic Press, London, 1982 A thorough treatment of photoluminescence spectrometry for quantitative chemical analysis, oriented toward compounds in solution R J Hurtubise Solid Su$ae Luminescence Analysis Marcel Dekker, New York, 1981 Practical aspects of analysis for organics adsorbed onto solids H B Bebb and E W Williams in Semiconductorsand Semimetah (R K Willardson and A C Beers, e&.) Academic Press,vol 8,1972 An extensive review of PL theory and technique, with emphasison semiconductors Some of the experimental aspects and examples are becoming outdated H J Queisser Appl Pbys 10,275, 1976 Describes PL measurements of a variety of semiconductor properties K Mettler Appl Pbys 12,751977 PL measurements of surhce state densities and band bending in GaAs L Zlatkevich, ed Luminescence Techniquesin Solid-state Polymer Research Marcel Dekker, New York, 1989 Practical emphasis on polymers in the solid state rather than in solution 384 VISIBLE/UV EMISSION, REFLECTION, Chapter 7.2 Modulation Spectroscopy F R E D H POLLAK Contents Introduction Basic Principles Instrumentation Line Shape Considerations Applications and Examples Conclusions Introduction Modulation Spectroscopy is an analog method for taking the derivative of an optical spectrum (reflectance or transmittance) of a material by modifying the measurement conditions in some manner.14 This procedure results in a series of sharp, derivative-like spectral features in the photon energy region corresponding to electronic transitions between the filled and empty quantum levels of the atoms that constitute the bulk or surfice of the material Using Modulation Spectroscopy it is possible to meas-ure the photon energies of the interband transitions to a high degree of accuracy and precision In semiconductors these band gap energies are typically eV, and they can be determined to within a few meV, even at room temperature The energies and line widths of the electronic transitions are characteristic of a particular material or surfice The energies are sensitive to a variety of internal and external parameters, such as chemical composition, temperature, strains, and electric and magnetic fields The line widths are a function of the quality of the material, i.e., degree of crystallinity o dopant concentration r The ability to measure the energy of electronic transitions and their line widths accurately, in a convenient manner, is one of the most important aspects of serniconductor characterization The former can be used to evaluate alloy compositions 7.2 Modulation Spectroscopy 385 (including topographical scans)? near-surface temperatures? process- or growthinduced strainsY8 surface or intehce electric fields associated with surface or interhce states and metallization (Schottky barrier formation),8 carrier types,'" topographical variations in carrier concentrations? and trap states.8 The broadening parameter at a given temperature is a measure of crystal quality and hence can be used to evaluate the influence of various growth, processing and annealing procedures These indude ion implantation, reactive-ion etching, sputtering, and laser or rapid annealing7s8In real device structures, such as heterojunction bipolar transistors, certain features of the Modulation Spectroscopy spectra have been correlated with actual device performance.6 Thus, this method can be employed as an effective screening tool to select materials having the proper device characteristics before undertaking an expensive fibrication process Various forms of Modulation Spectroscopy can be employed for in-situ monitoring of growth by molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), or gas-phase MBE (GPMBE) at elevated temperatures." 3-"Modulation Spectroscopyhas been used extensively to study semiconductorshaving diamond (Ge and Si), zincblende (GaAs, GaAlAs, InP, CdTe, and HgCdTe), and m i t e (CdS) crystal structures There also has been some work in the area of metals, including alloys The characteristic lines observed in the absorption (and emission) spectra of nearly isolated atoms and ions due to transitions between quantum levels are extremelysharp As a result, their wavelengths (photon energies) can be determined with great accuracy The lines are characteristic of a particular atom or ion and can be used for identification purposes Molecular spectra, while usually less sharp than atomic spectra, are also relatively sharp Positions of spectral lines can be determined with sufficient accuracy to verify the electronic structure of the molecules The high particle density of solids, however, makes their optical spectra rather broad, and often uninteresting from an experimental point of view The large degeneracy of the atomic levels is split by interatomicinteractions into quasicontinuous bands (valence and conduction bands) The energy difference between the highest lying valence and lowest lying conduction bands is designated as the fundam e n d band gap Penetration depths for electromagnetic radiation are on the order of 500 A through most of the optical spectrum Such small penetration depths (except in the immediate vicinity of the hndamental gap), plus other considerations to be discussed later, make the reflection mode more convenient for characterization purposes, relative to absorption measurements These aspects of the optical spectra of solids are illustrated in the upper portion of Figure 1,which displays the reflectance curve (R) at room temperature for a typical semiconductor, G A The hndamental absorption edge around 1.4 eV proas duces only a weak shoulder Some structure is apparent in the two features around eV and the large, broad peak near eV However, the dominant aspect of the line shape is the slowly varying background The derivative nature of Modulation Spectroscopy suppresses the uninteresting background effects in hvor of sharp, deriva386 VISIBLE/UV EMISSION, REFLECTION, Chapter I I I I I I I ENERGY (eV) Figure Reflectance(R)and electroreflectance( A R I R )spectra of GaAs at 300 K tive-like lines corresponding to the shoulders and peaks in Figure Also, weak structures that may go unseen in absolute spectra are enhanced Band gaps in semiconductors can be investigated by other optical methods, such as photoluminescence, cathodoluminescence, photoluminescence excitation spectroscopy, absorption, spectral ellipsometry, photocurrent spectroscopy, and resonant Raman spectroscopy Photoluminescence and cathodoluminescence involve an emission process and hence can be used to evaluate only features near the fundamental band gap The other methods are related to the absorption process or its derivative (resonant Raman scattering) Most of these methods require cryogenic temperatures For applied work, an optical characterization technique should be as simple, rapid, and informative as possible Other valuable aspects are the ability to perform measurements in a contactless manner at (or even above) room temperature Modulation Spectroscopy is one of the most useful techniques for studying the optical proponents of the bulk (semiconductorsor metals) and surface (semiconductors)of technologically important materials It is relatively simple, inexpensive, compact, and easy to use Although photoluminescence is the most widely used technique for characterizing bulk and thin-film semiconductors, Modulation Spectroscopy is gaining in popularity as new applications are found and the database is increased There are about 100 laboratories (university, industry, and government) around the world that use Modulation Spectroscopy for semiconductor characterization 7.2 Modulation Spectroscopy 387 Basic Principles The basic idea of Modulation Spectroscopy is a very general principle of experimental physics Instead of measuring the optical reflectance (or transmittance) of a material, the derivative with respect to some parameter is evaluated The spectral response of the material can be modified directly by applying a repetitive perturbation, such as an electric field (electromodulation), a heat pulse (thermomodulation), or stress (piezomodulation).This procedure is termed external modulation The change may also occur in the measuring system itself, e.%., the wavelength or polarization conditions can be modulated or the sample reflectance (transmittance) hs can be compared to a reference sample Ti mode has been labeled intemalmodulation Because the changes in the optical spectra are typically small, in some cases part in lo6, phase-sensitive detection or some other signal-processing procedure is required To illustrate the power of Modulation Spectroscopy, displayed in the lower part of Figure is the electromodulated reflectance spectra ( A R / R ) of the semiconductor GaAs at 300 K in the range 0-6 eV Although the fundamental direct absorption edge (E,)at about 1.4 eV produces only a weak shoulder in R it is observed as a sharp, well-resolved line in AR/ R There are also other spectral features, labeled + Ao, El, E + AI, &, and E2 that correspond to transitions between other quantum levels in the semiconductor In the region of the features at E, and E, + A0 the penetration depth of the light (the sampling depth) is typically several thou1 sand A, while for the peaks at El and E + the light samples a depth of only a few hundredA For characterizationpurposes of bulk or thin-film semiconductorsthe features at E, and El are the most useful In a number of technologically important semiconductors (e.g., Hgl+Cd,Te, and In,Gal-&) the value of E, is so small that it is not in a convenient spectral range for Modulation Spectroscopy, due to the limitations of light sources and detectors In such cases the peak at E can be used! The features at & and are not useful since they occur too far into the near-ultraviolet and are too broad Instrumentation Gttemal Modulation For characterization purposes the most useful form of external modulation is electromodulation, because it provides the sharpest structure (third derivative of R in bulk or thin films) and is sensitive to surfice or interhce electric fields.'-5 The most widely used contactless mode of electromodulation is termed Photoreflectance (PR).53 388 ' VISIBLE/UV EMISSION, REFLECTION, Chapter LAMP LASER (OR QTHER SECONDARY LIGHT SOURCE) Figure Schematic representationof a photoreflectanceapparatus A schematic representation of a PR apparatus is shown in Figure 2.’ In PR a pump beam (laser or other light source) chopped at frequency a, creates photoinjected electron-hole pairs that modulate the built-in electric field of the semiconductor The photon energy of the pump beam must be larger than the lowest energy gap of the material A typical pump beam for measurements at or below room temperature is a 5-mW He-Ne laser (At elevated temperatures a more powerhl pump must be employed.) Light from an appropriate light source (a xenon arc or a halogen or tungsten lamp) passes through a monochromator (probe monochromator) The exit intensity at wavelength A, I&), is focused onto the sample by means of a lens (or mirror) The reflected light is collected by a second lens (mirror) and hcused onto an appropriate detector (photomultiplier, photodiode, etc.) For simplicity, the two lenses (mirrors) are not shown in Figure For modulated transmission the detector is placed behind the sample The light striking the detector contains two signals: the dc (or average value) is given by I&)R(A), where R(A)is the dc reflectance of the material, while the mod) , where AR(h) is the change in refleculated value (at frequency Q is IO(h)AR(h), tance produced by the modulation source The ac signal from the detector, which is 7.2 Modulation Spectroscopy 389 proportional to IOAR, is measured by a lock-in amplifier (or using another signalaveraging procedure) Typically IoAR is 104-104 IoR To evaluate the quantity of interest, i.e., the relative change in reflectance, AR/R, a normalization procedure must be used to eliminate the uninteresting & common feature I) In Figure the normalization is performed by the variable neutral density frlter (VNDF) connected to a servo mechanism The dc signal from the detector, which is proportional to Io(h)R(h),is introduced into the servo, which = moves the VNDF in such a manner as to keep I&)R(A) constant, i.e., I,(h)R(h) C Under these conditions the ac signal I&)AR(h) = C&R(A)/R(h) Commercial versions of PR are available Other contactless methods of electromodulation are Electron-Beam Electro-reflectance (EBER)l2 and Contactless Electroreflectance (CER)13 In EBER the pump beam of Figure is replaced by a modulated low-energy electron beam (- 200 ev) chopped at about kHz However, the sample and electron gun must be placed in an ultrahigh vacuum chamber Contactless electroreflectance uses a capacitor-like arrangement An example of a contact mode of electromodulation would be the semiconductor-insulator-med configuration, which consists of a semiconductor, about 200 of an insulator like AlZO3, and a semitransparent metal (about 50 A of Ni or Au) Modulating (ac) and bias (dc) voltages are applied between the front semitransparent metal and a contact on the back of the sample T o employ this mode the sample must be conducting In temperature modulation, the sample may be mounted on a small heater attached to a heat sink and the temperature varied cyclically by passing current pulses through the heater.' If the sample is properly conducting, the current can be must be kept passed through the sample directly Generally, for this method below 10-20 Hz, and hence there are often problems with the l/f noise of the detector In piezoreflectance (PzR), modulation is achieved by mounting the sample on a piezoelectric transducer that varies the lattice constant of the material, producing a band gap modulation l4 Although PzR is contactless it requires special mounting of the sample, as does thermomodulation a a, Internal Modulation Differential Reflectivity A commonly used form of internal modulation is differential reflectometry, in which the reflectance of the sample under investigation (or a portion of it) is compared to a standard material This can be accomplished either by holding the sample stationary and scanning the probe beam between two region^'^ or by holding the light spot fixed and moving the sample." 390 VISIBLE/UV EMISSION, REFLECTION, Chapter Reflection Difference Spectroscopy In Reflection Difference Spectroscopy (RDS) the difference between the normalincidence reflectance R of light polarized parallel and perpendicular to a principal crystallographica x i s in the plane of the crystal is measured experimentally as a funcr Because of The cubic symtion of time, photon energy, o surfice conditi~ns.~-'l metry of zincblende semiconductors, the bulk is nearly isotropic (i.e., there is no distinction between parallel and perpendicular), while regions of lower symmetry, like the surface or interfaces can be anisotropic In the case of (001) surfices of zincblende semiconductors, the contribution from the bulk is expected to vanish Thus, RDS is sensitive to both the chemical and structural state of the surface Sensitivities to surface species of 0.01 monolayer have been demonstrated, with averaging times of 100 ms Being an optical probe, RDS is well suited either to the reactive, relatively high-pressure sample environments in MOCVD reactors or to the ultrahigh-vacuum environment of MBE chambers Moreover, the presence of a film deposited on the viewport can be overcome Line Shape Considerations One of the great advantages of Modulation Spectroscopy is its ability to fit the line shapes of sharp, localized structures, as illustrated in the lower part of Figure These fits yield important relevant parameters, such as the value of the energy gap and the broadening parameter Electromodulation The most complicated form of Modulation Spectroscopy is electromodulation, since in certain cases it can accelerate the electron-hole pairs created by the light If the electric field is not too large the quantity AR/ R can be written as: where A is the amplitude of the signal, @ is phase angle that mixes together the real and imaginary parts of dielectric function, E is the photon energy, Eg is the energy gap and r is a parameter that describes the broadening of the spectral line The parameter m = 2.5 or 3.0 for the & and E1 optical features, respectively , Equation (1) is related to the third derivative of R At low temperatures the electron and hole created by the probe light beam can form a bound state (called an exciton) because of the Coulomb interaction between them In this case the exponent m in Equation (1) becomes and the line shape is only a first For sufficiently high built-in electric fields the electromodulation spectrum can 7.2 Modulation Spectroscopy 391 display an oscillatory behavior above the band gap; these are called Franz-Keldysh oscillations (FK oscillations) In the presence of the field F the energy bands are tilted by an amount eFz, where e is the electronic charge and z is in the direction of F Resonances appear whenever an integral number of de Broglie wavelengths fit into the triangular well formed by the electric field The de Broglie wavelength is bp, equal to 4n2/ where h is Planck’s constant and p is the momentum of the electron (hole) The energy of the mh resonance E,, is proportional to p Thus the periods of these resonances, or FK oscillations, are a direct measure of the built-in electric Piezo- and Thermomodulation These modulation methods not accelerate the electron-hole pairs and hence produce only a first-derivative Modulation Spectroscopy Their line shapes are given by Equation (l), with m = Applications and Examples Alloy composition Among the most important parameters for materials characterization are the compositions of binary A,, B, (e.g., Gel, Si,) alloys, ternary A,-, B, C (e.g., Gal, Al, As ,Hg l,Cdx Te alloys, and quaternary A,, B, CyD, (e.g., Inl-xGa&+’l-y) alloys The spectral features in Figure 1, e.g., and El vary with alloy composition Modulation Spectroscopy thus can be employed conveniently for this purpose even at 300 K of Shown in Figure is the variation of the fundamental direct band gap (4) Gal-Jil& as a function of Al composition (x) These results were obtained at 300 K using electromodulation.Thus it would be possible to evaluate the Al composition of this alloy from the position of b The case of Gal-fi& alloy determination is an example of the importance of the reflectance mode in relation to transmittance In almost all cases the Gal,Al,Asmaterial is an epitaxial film (0.1-lpm) grown on a GaAs substrate (-0.5 mm thick) Since the band gap of GaAs is smaller than that of Gal-A&, the reflectance mode must be used have a value of in certain composition Some materials, such as Hgl-.$d,Te, regions that is too fir into the infrared to be conveniently observed using Modulation Spectroscopy In such circumstances other higher lying features, such as the peaks at E ,can be used more readily The compositional variation of or higher lying features has been reported for a large number of alloys, including GeSi, GaAlAs, GaAlSb, G A P , InGaAs, InAsSb, InAsP, GaInSb, HgCdTe, HgMnTe, CdMnTe, CdZnTe, ZnMnTe, CdMnSe, InGaAsP lattice-matched to InP, GaAlInAs lattice-matched to InP, and 392 VISIBLE/UV EMISSION, REFLECTION, Chapter w 0.2 0.4 0.6 0.8 Composition x Figure Aluminum composition dependence of E, of Ga,A, , I+ at 300 K (solid line) GaAlInPAs lattice-matched to GaAs The alloy composition x can be evaluated with a precision of h = f 0.005 By using a high-quality lens to focus the light from the probe monochromator onto the sample (see Figure 2) a spot size of about 100 can be achieved By mounting the sample on an x-y stage it is possible to perform topographical scans with a spatial resolution of 100 pm Growth or Process-Induced Strain or Damage Modulation Spectroscopy can be very useful in evaluating strains induced by growth (lattice-mismatched systems) or processing procedures, such as reactive-ion etching or oxide formation The size and magnitude of the strain can be evaluated from the shifts and splittings of various spectral lines, such as or El Device Structures Certain features in the PR spectra at 300 K from GaAs/Gal-a& heterojunction bipolar transistor structures have been correlated with actual device performance; thus PR can be used as an efkctive screening tool.6 From the observed FK oscillations it has been possible to d u a t e the built-in dc electric fields Fdc in the Gal-$& emitter, as well as in the n-GaAs collector region The behavior of Fdc ( G U ) has been found to have a direct relation to a c t d device performance, i.e., dc current gain Shown in Figures 4a and 4b are the PR spectrum at 300 K for MBE and MOCVD fabricated samples, respectively There are a number of FK oscillations in the vicinity of both the GaAs (-1.42 eV) and Gal-$& 7.2 Modulation Spectroscopy 393 E E \ a 1.5 I 2.I 2.4 Energy (eV) Figure Photoreflectance spectra for two GaAs/Ga,-&As heterojunction bipolar transistor structures fabricated by MBE and MOCVD, respectively, at 300 K band gaps The Gal-$& portions of the two samples are 1.830 eV and 1.670 eV, which corresponds to x = 0.28 and 0.17, respectively, as shown in Figure The most important aspects of Figure are the FK oscillations associated with the Gal-A& band gap From these features it is possible to evaluate Fdcin the emitter-base p-n junction The electric fields, as deduced from the GaAlAs FK osdllaGaAlAs), were compared with fabricated heterojunction bipolar tions (Fdc, transistor MBE samples Below electric field values of about x lo5 V/cm high current gains were obtained Shown in Figure is FdC(in GaAlAs) as a hnction of dc current gain at mA Note that there is a sudden drop when Fdc (in GaAlAs) > x lo5 V/cm The explanation of this effect is the redistribution of the Be dopant in the p-region in these MBE samples When the redistribution moves the p n junction into the emitter, there is an increase in the electric field in this region; i.e., the value of Fdcbecomes greater The movement of the Be has been verified by Secondary Ion M s Spectroscopy (SIMS) When the p n junction and the as GaAs /G& heterojunction are not coincident, carrier recombination occurs, reducing the current and the performance of fabricated heterojunction bipolar transistors These observations have made it possible to use PR as a contactless screening technique to eliminate wafers with unwanted characteristics before the costly fabrication step 394 VISIBLE/UV EMISSION, REFLECTION, Chapter 601 00 m [I 40-c 20 0 O n OO I n , Fdc (GaAIAs) [IO5 V/cm] Figure Electricfield F (GaAlAs) in the p n junction as evaluatedfrom the GaAlAs FK ' oscillations as a function of the dc current gain of a fabricated heterojunction bipolar transistor In-Situ Monitoring of Growth RDS and PR are proving to be very useful methods for in-situ characterization of semiconductor thin-film growth by MBE, MOCVD, and GPMBE RDS was first applied to study GaAs growth in an MBE environment Results showed that the maximum surfice anisotropy between (2 x 4) As-terminated and (4 x 2) Ga- and AI-terminated surfaces of GaAs and ALAS occur in the photon energy region between 2.0-2.5 eV and 3.5 eV, respectively The strong dependence of this anisotropy on photon energy makes it possible to spectrally distinguish between AI-AI and Ga-Ga surface dimer bonds The time dependence of RDS and simultaneously measured reflection high-energy electron diffraction (RHEED) signals for changes in surface conditions revealed that the RDS measurements follow surface structure The RDS-RHEED correlation gives a valuable reference when RDS is applied to nonultrahigh-vacuum techniques, such as MOCVD, where RHEED cannot be used A commercial model of an RDS system is available MBE Growth Studied by RDS Figure shows typical RDS (bottom) and RHEED (top) responses for an As-toGa-to-As surface stabilization sequenc-from As-stabilized (2 x 4) to Ga-stabilized (4 x 2) (001) surface reconstructions and return-generated by interrupting and resuming the As flux at times t = s and 10 s, respectively, during otherwise normal growth of GaAs at a rate of GaAs monolayer per 4.6 s ~ ,lo The As growthsurface pressure of x lo4 torr provided 2.6 times the amount needed to consume the arriving Ga The differences between the RDS data on the left and those on the right are due to the differences in energy of the photons used to obtain them The differences in the RHEED data are due to small angle-of-incidence drifts of the electron beam in the time interval between the recording of successive sets of data 7.2 Modulation Spectroscopy 395 Figure RHEED (upper)and reflection anisotropy (lower)transients obtained by interrupting and resumingAs flux during otherwise normal growth (001) GaAs at semiconductor ML per 4.6 s Data are shown for photon energies near the Ga RD peak at 2.5 eV (right)and minimum at 3.5 eV (left) The maximum change of the 2.48-eV RDS signal is nearly 1%, with Rl increasing relative to R 10 as the surface becomes increasingly covered with Ga I As soon as the As flux is terminated, the RDS signal begins to change nearly linearly in time, and saturates near t = s; i.e., it tracks the amount of excess Ga accumulating on the surface up to one monolayer Since RDS responds only to surface species that are in registry with the crystallographic axes of the substrate (i.e., have already reacted with it), and since it is insensitive to the presence of randomly oriented species, t h i s time dependence implies that the excess Ga atoms are forming Ga-Ga dimer bonds instantly on arrival, with respect to laboratory time scales, and that the 2.48-eV RDS signal directly follows the chemistry of the (001) GaAs growth surface It also implies that Ga diffusion lengths under Ga-stabilized surface conditions are large, in particular, hundreds of times greater than under As-stabilized conditions 396 VISIBLE/UV EMISSION, REFLECTION, Chapter The 3.54-eV RDS response is completely different, exhibiting a striking similarity to the W E E D signal shown above it Clearly, at this photon energy the RDS signal, as W E E D , is determined by surface structure Thus RDS data either can complement or supplement W E E D data, depending on the measurement wavelength As the saturation RDS signal at 3.54 eV is about an order of magnitude smaller than that at 2.48 eV, it follows that the small inflection in the otherwise linear initial 2.48-eV RD transient is due to the contribution of the structuresensitive component, which is relatively minor at the lower photon energy Substrate Temperatureand Alloy Composition by PR It has been demonstrated that PR can be used to measure E, of technologically important materials, such as GaAs, InP, Ga()@0.18h, and InxGal-& (x= 0.06 and 0.15), to over 600" C.6*7 Such temperatures correspond to growth conditions for thin-film methods like MBE, MOCVD, and gas-phase MBE The value of can be evaluated to f meV at these elevated temperatures Thus, the temperature of GaAs and InP substrates can be evaluated to f10" C to within a depth of only several thousand A from the growth surface In addition, the alloy composition of epilayers of Gal+Al& and InxGal-& can be determined during actual growth Measurements have been performed under actual growth conditions, including the case of a rotating substrate Topographical scans can be performed to evaluate temperature or compositional homogeneity Figure shows for GaAs and Ga0&&18h as a function of temperature T to about 900 K Additional measurements on samples having differing A contents I would generate a family of curves The solid line is a least-squares fit to a semiempirical relation that describes the temperature variation of semiconductor energy gaps: aTL E ( T ) = E ( ) P+ T In Equation (2) E(0) is the energy gap at T =0, while a and p are materials parameters to be evaluated from experiment Once the GaAs substrate temperature is I measured from the position of E,(GaAs), the A composition of an epilayer can be determined readily from the position of (GaAlkr) at that temperature Conclusions Modulation Spectroscopy has proven to be an important characterization method for semiconductors and semiconductor microstructures The rich spectra contain a wealth of information about relevant materials, surfaces and interfaces, as well as device characteristics In general, the apparatus is relatively simple, compact (except EBER), inexpensive (except EBER), and easy to use One of the main advantages of Modulation Spectroscopy is its ability to perform relevant measurements at room 7.2 Modulation Spectroscopy 397 ... s2 (Mi) s3 (cu) Fe (% wt.) 10.25 10.25 - 10.50 - Ni (% wt.) 89 .75 89 .75 - 89.50 - - - tc,(4 1652 1698 1 674 tFeNi(& 2121 2048 - tCu@) 2 470 24 57 - Table 2115 - 2416 XRF mutts for films o Cr, FeNi,... Fe (5)-Ni (95) 4.2 5.0 2.5 Fe (10)-Ni (90) 9.2 9.0 6.2 Fe (20)-Ni (80) 19.4 19.2 19.4 Fe (34)-Ni (66) 47. 3 48.4 44.5 Fe (50)-Ni (50) 59.1 61 .7 59.1 Fe (66)-Ni (34) 78 .9 79 .8 78 .4 Fe (80)-Ni (20)... elemental-composition and layer-thickness determination of thin- film materials The technique is nondestructive, inexpensive, rapid, precise and potentially very accurate XRF characterization of thin