Application to Materials Science

Application to IVIaterials Science Paul Dhamelincourt and Shin-ichi Nakashima

1. INTRODUCTION When Raman spectroscopy is applied to the analysis of microscopic particles or microscopic volumes within a heterogeneous sample, it is usually assumed that all of the physical phenomena involved in the Raman scattering of Hght from macroscopic samples are the same. However, it is well known that scattering from samples whose dimensions become comparable to the wavelength of the excitation represents a special case. Calculations based on the Lorenz-Mie formahsm (Kerker, 1969) show that a strong increase in the internal electric field may occur as a result of morphology-dependent resonances when microsamples of well-defined geometries (e.g. spheres, spheroids and cylinders) are illuminated by a plane wave at certain wavelengths. Physically, these resonances result when a wave traveling inside the microsamples is in phase with itself after having been internally reflected at the interface with the surrounding medium. The analysis of such resonances, which occur for certain values of the size parameter X= ITTYIX, where r is the radius of the microsample, is very well documented (Owen et al., 1982). Calculations show that these resonances should lead to very sharp peaks in the Raman and fluorescence spectra of |xm-sized samples (dimensions up to several tens of juim) that are not predicted in bulk samples of the same composition. These peaks would result either from resonance-induced enhancement of the Raman scattering efficiency itself and/or enhancement of the fluorescent background when fluorescent species are embedded in the sample. Fortunately, the resonances described above have never been observed in the usual micro-Raman experiments. Their absence is explained both by imperfect particle or heterogeneity geometries (complex structures inhibit phase relations between travelUng waves) and the strong optical coupling into the sample substrate, which is for a heterogeneous sample that part of the

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P. Dhamelincourt and S. Nakashima

sample which is not illuminated. Both factors preclude the observation of the morphology-dependent effects. Hence, resonances in the inelastic scattering (Raman and fluorescence) spectra of juim-sized samples have only been observed when special experimental conditions (e.g. levitated liquid droplets, glass and polystyrene perfect spheres supported by smooth substrates, and ends of cylindrical optical fibers) have been employed. In these cases the samples have well-defined geometries with virtually no optical coupling into the substrate, if present (Thurn and Kiefer, 1985). Morphology-dependent effects have not, thus far, impaired the analytical usefulness of micro-Raman spectroscopy. However, at fxm sample dimensions orientation effects are far more important (see Chapter 1, Section VIII). Indeed at that scale most of the microcrystals or crystalline domains have well-defined crystallographic axes. Hence, the relative intensities of the Raman bands (compared with those observed in polycrystalUne bulk samples) depend strongly on the orientation of the samples with respect to the incident polarization, as defined by the electric field direction of the laser beam at the sample and the polarization vector of the scattered fight. If no analyzer is used, the latter parameter is determined by the axis of the entrance slit of the monochromator. Care must be taken when qualitative (and quantitative) comparisons of the spectra of microsamples are made with reference spectra obtained from polycrystalline bulk samples, especially for the Raman bands which correspond to totally symmetric modes of vibration. Finally, for samples which contain crystallites whose dimensions are far below the exciting wavelength (nanophases ranging from several tens to a few hundred nm), band shifts and broadening are expected due to finite crystalUte size (relaxation of the K = 0 selection rules), surface pressure and nonstoichiometry. New bands may also be observed which are due to surface and aggregation effects (Bobovich and Tsenter, 1982; Pigenet and Frevet, 1980).

II. INORGANIC SOLIDS A. Catalysts L

Introduction

Metal oxide yAl203-supported catalysts to which an NiO or CoO promoter is added have been extensively studied because of their important industrial applications. In particular, after activation, they are employed in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of petroleum and coal products. The precursor oxides, which are the catalysts before activation by sulfidation, are generally prepared by the pore-filUng method with the use

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of yAl203 extrudates according to the following steps (Payen et al., 1986): (i) Impregnation with ammonium heptamolybdate solution, (ii) Drying at temperatures above 373 K, followed by calcination at 773 K for several hours, and (iii) Promoter impregnation with Ni or Co nitrate solution, drying and final calcination at 773 K. The metal and promoter loadings are given in oxide (i.e. M0O3, NiO, CoO) weight per cent. 2. Characterization by Vibrational Spectroscopy In addition to surface characterization techniques (see Chapter 5) such as X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS), vibrational spectroscopy has proven to be a very useful method for characterizing the supported phases themselves. As these catalysts are not transparent below 1000 cm"^ due to absorption by the 7AI2O3 support, Raman spectroscopy is preferred to infrared spectroscopy for obtaining data. Conventional laser Raman spectroscopy has been used to analyze oxide-supported catalysts, but a large number of the results reported in the literature have been obtained with the use of micro-Raman spectroscopy. The advantages of a Raman microprobe for the study of 7Al203-supported catalysts are as follows: (i) In the case of absorbing samples the efficiency of the excitation and collection of hght is far higher than in the conventional instrument (see Chapter 3), (ii) The confocal configuration used is very efficient in reducing background emission, and (iii) The micro-Raman instrument offers the possibihty of employing controlled temperature and controlled atmosphere cells. Such cells for optical microscopy are currently available or can be made in the laboratory. They facihtate the study of sohds in situ under a wide variety of atmospheric conditions and at temperatures ranging from that of Uquid nitrogen to 1500 K. 3. Raman Spectroscopic Analysis of Precursor Oxides The understanding of the function of alumina-supported oxomolybdate catalysts in their oxide states owes much to results obtained by the use of Raman spectroscopy. It was shown by Brown et al. as early as 1977 that even

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for loadings lower than that required to saturate the first monolayer, good Raman spectra could be obtained. According to Payen et al. (1986), the observed Raman spectra are complex in nature, as they involve mainly surface effects and loading inhomogeneity. A significant amount of work has appeared in the literature and an overall interpretation of the Raman bands associated with supported oxides has emerged (Payen et al., 1987), namely: (i) At loadings of <5.5 wt% M0O3, only monomeric (M) molybdate species are present on alumina supports. Transformation of the heptamolybdate anions occurs during the wetting of the support, (ii) At higher loadings (>14wt% M0O3), heptamolybdate (HM) aggregates are present. Similar results are observed when loading 7AI2O3 with tungsten and vanadium. In situ measurements at temperatures up to 773 K show that the species M and HM are stable; this stability is thought to result from interaction with the support. During thermal treatment the dehydration and rehydration of the supported species can be followed by the shift of the stretching vibration of the Mo-Ot bond, which occurs between 900 and 1090 cm~^. (Ot denotes a terminal oxygen atom which is not Unked to other atoms.) In the VI"^ oxidation state, molybdenum may adopt various configurations between tetrahedral and octahedral. Hence the shift of the Mo-Ot bond stretching vibration which corresponds to the monomer or heptamer species will depend not only on the ligand heterogeneity (relative to the number of bridging oxygens), but also on the coordination heterogeneity (unsaturated Mo^^ due to ligand vacancies). Both the ligand and coordination heterogeneity effects lead to an increase in the Mo-Ot stretching frequency (see Fig. 1). Thus, by recording spectra in the 900-1090 cm~^ range, it is shown that: (i) Calcination around 773 K reinforces the link between the H and HM species and the support through dehydration. The presence of a promoter (Ni or Co) enhances the phenomenon by faciUtating dehydration. (ii) Calcination at higher temperatures leads to the formation of AI2 (M004)3.

(iii) Ageing of the catalysts after calcination is a hydration process which always leads to the supported hydrated species

Movjf °' . OH (iv) This hydration-dehydration scheme seems to be general and has been observed in other yAl203-supported oxides such as WO3 and V2O5.

Application nb 0

nb nb nb 0 0 0 \ 1/

/ l \ 0 0 0 nb nb nb

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247

b nb b 0 0 0 \ l /

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to IVIaterials Science

/

// 00 1 1

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1 1

X X

1

_>

M06 +

//l\\

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X X X

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Figure 1 Characteristic Raman frequencies of the molybdenum ion in various oxidation states. The subscripts b and nb denote bridging and nonbridging oxygen atoms, respectively.

4. Bronsted Acidity of Supported Oxides Pyridine is a model molecule which is often used to measure the surface acidity of soHds. In the hydrated form, precursor oxides possess a hydroxyl group bonded to the supported metal. Pyridine chemisorption through the formation of the pyridinium ion is a good way to demonstrate the acidic

248

P. Dhamelincourt and S. Nakashima 1014

(c)

b)

(a) N)cm-i

Figure! Pyridine chemisorption on a Ni-W catalyst, (a) Fresh catalyst, (b) after increasing time of contact with pyridine, (c) after desorption of physisorbed pyridine by N2 purging. character of the OH group. With the use of a controlled atmosphere and controlled temperature cell, Raman measurements can be made during the flow of the N2-pyridine mixtures on powder catalyst (Payen et al., 1982). On molybdenum- or tungsten-supported catalysts, the same observations have been made (see Fig. 2). At first, a band at 1014 cm"^ appears which is characteristic of the chemisorbed pyridine (pyridinium ion). Then, on increasing the contact time, bands at 1032 and 990cm~^ appear which are characteristic of physisorbed pyridine. Subsequent purging with pure N2 leads to the desorption of physisorbed pyridine only (the bands at 1032 and 990cm~^ disappear, whereas that at 1014 cm~^ is unchanged). Therefore, the supported polymolybdate or polytungstate species have acidic Bronsted sites if no pretreatment to desorb water has been performed prior to the absorption experiments. 5. Sulfidation of Precursor Oxides The precursor oxides have to be sulfided (activation step) before being used in the catalytic reactor. This activation process yields supported sulfide

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(a) (cm - i i

15

M-

380

10J

385

yJ

\

J

L.

L 12

8

% M0O3 (b)

L (nm) 4

-L

J-

8 %Mo03

12

Figure 3 (a) Wavenumber and width of the E2g band and (b) average length of an M0S2 crystaUite versus Mo loading.

particles. The same controlled atmosphere and controlled temperature cell can be used to make in situ Raman analyses of the surface phases appearing during the sulfidation of oxides by H2/H2S or N2/H2S mixtures. During sulfidation intermediate sulfides (MS3) and complex oxysulfides [(MoS2(S2)n)^~] have been identified (Payen et at., 1989). After complete sulfidation, nanocrystallites of M0S2 and WS2 are formed whose dimensions correlate well with the oxide loading for hydrated catalysts (see Fig. 3).

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6. Conclusion Raman spectroscopy is a sensitive method of monitoring the state of 7Al203-supported catalysts before (precursor oxides) and after activation by sulphidation.

B. Ceramics

1.

Introduction

The Raman microprobe is particularly well adapted to the analysis of ceramics, as it allows the investigation of highly localized volumes in ceramic microstructures. The dimensions of these volumes are comparable to typical grain sizes. This situation is in contrast to that of conventional X-ray diffraction techniques, where the probed volume cannot be localized. Moreover, microphases and inclusions can usually be observed directly under the microscope with the use of conventional illumination techniques (because of differences in reflectivity or color). These elements are normally not visualized by scanning electron microscopy. 2. Polyphase Ceramics (a) Silicon and boron nitride Two polymorphs of silicon nitride (Si3N4) are known to exist {a and 0). The powdered a form is usually the starting product employed in the manufacture of hot-pressed or injection-moulded silicon nitride alloys. The j8 structure is formed during hot pressing. Thus the a-to-j8 phase transition is often used as a monitor of the extent of the hot pressing process. As the spectra of the two structures are completely different (a-Si3N4 is characterized by bands at 262, 365, 514, 670 and 850 cm~^; j8-Si3N4 exhibits a triplet with components at 185, 210 and 230 cm~^), they provide a convenient means of distinguishing the structures. In the same way silicon oxynitride (Si2N20), which often coexists with the nitrides, is easily identified by its Raman spectrum (bands at 187 and 254cm~i). The ultra-hard borazon ceramic is the cubic phase of boron nitride (j8-BN or Z-BN) produced at high pressure and temperature. This material is characterized by the TO and LO modes of the cubic lattice which appear at 1055 and 1306 c m ~ \ respectively. Conversion of the cubic phase to the hexagonal one can be easily seen because the latter, which is isoelectric with graphite, has only a single band at 1367 cm~^.

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(b) Partially stabilized zirconia (PSZ) Zirconia (Zr02) is normally monoclinic at room temperature and transforms to the tetragonal and cubic forms at higher temperatures. However, these higher-temperature structures can be stabiUzed by mixing the starting material with additives such as alumina, magnesia, ceria or yttria. Multiphase ceramics based on the constrained, metastable, tetragonal zirconia constitute a class of technically important materials due to their mechanical properties (flexural strength, toughness, wear resistance, etc.). In recent years considerable effort has been employed in the research and development of these ceramic materials for use as engine components. The toughening mechanism in PSZ depends on a volume expansion and sheer strain that occur when tetragonal zirconia transforms irreversibly into the monocHnic form. This transformation is initiated by the stress fields that form ahead of any crack propagating in the material. The size of the transformed zone is an important parameter used to model the enhanced toughness derived from the transformation. Thus, the determination of the size of the transformation zone is of prime importance and imphes that the relative concentration of the monoclinic phase should be obtained with a good spatial resolution. The monochnic and tetragonal polymorphs of zirconia have distinct and characteristic Raman spectra. In particular, over the range 100-300 cm"^ the monoclinic doublet at 181 and 192 cm~^ is well separated from the tetragonal bands appearing at 148 and 264 cm""^. This result permits the monoclinic concentration to be measured with the use of a relation proposed by Clarke and Adar (1982), namely tl81_j_2l92 ^m

77/0^148 , Ci264\

, ^ 1 8 1 , ^^192

V-^^'

where 3 ^ and 3^ are the integrated intensities of the characteristic bands of each phase (refer to superscripts) and F is a correction factor to allow for the increased Raman cross-section of the monochnic phase with respect to the tetragonal one. Most of the experiments reported in the hterature use the Vickers hardness tester to produce cracks in the ceramic material. The focused laser beam is first positioned on the indentation cracks and then at successively greater distances from the crack. At each point, a Raman spectrum is recorded and the monochnic concentration is evaluated. Plots of the relative monochnic concentration as a function of distance from the stress-induced crack can be estabhshed for different indentations and materials. With the use of a Raman imaging technique, Veirs et al. (1990) obtained maps which give the monoclinic fraction in phase-transformed zones surrounding cracks induced in magnesia-stabihzed zirconia.

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3. High-Tc

and S. Nakashima

Ceramics

(a) Introduction Since the high critical temperature superconductors were discovered in 1986, there has been intense interest in their investigation and characterization by Raman spectroscopy. Many superconducting oxides are now known with various critical temperatures. However, the most interesting ones are those with Tc above the liquid N2 temperature (Tc > 77 K) because of their practical applications (see Fig. 4). Their technology is the same between 77 K and room temperature. (b) Structure of high-T^ superconductors and Raman spectroscopy The common feature of all high-r^ superconductors is that these compounds are copper-oxide-based ceramics for which CUO2 planes are present in a more-or-less oxygen-deficient perovskite structure (see Fig. 4). In these CUO2 planes the electrons which are missing from the closed oxygen shell are responsible for the superconductivity. Thus, most of the Raman work on high-Tc materials has been devoted to phonon characterization because of its possible application to the investigation of the mechanism of superconductivity and, more particularly, to the study of vibrations in the *superconducting' CUO2 planes. [For a review in the field see Ferraro and Maroni (1990).]

T{K) 130 A2M2Ca„_^Cu„02„44 n = 1-3 A = BiorTI M = Sr or Ba

120 110

r

oj

100 O

YBa2Cu30y

90 80 LIQUID

N2 BARRIER

70 k

(a) Figure 4 Superconducting oxides with T^ above liquid nitrogen temperature, (a) Composition and (b) structure.

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In addition to the fundamental studies of phonon and related electronic properties, Raman spectroscopy has been extensively used to evaluate the quality of variously prepared high-Tc materials (e.g. bulk powders, thin films and single crystals). (c) Interest in Raman microspectroscopy The potential appUcations of high-r^ superconducting materials are mainly in the field of superconducting microelectronics [superconducting, quantuminterference devices (SQUIDs), superconducting microwave and sub-mm devices, etc.]. But this possibility impUes the fabrication of very-high-quality superconducting thin films. Micro-Raman spectroscopy cannot be used directly as a test of superconductivity, as unequivocal connections between specific vibrational modes and superconductivity have not been made. However, this technique is particularly well adapted for controUing the microchemical structure of the films (compositional heterogeneity and impurity phases), as well as the quality of the epitaxy. The MBa2Cu30(7_;^.) compounds, which have been the most investigated, provide a good illustration of the power of micro-Raman spectroscopy as a controlling technique. These compounds are synthesized with the use of the ternary system BaO(BaC03)-M203-CuO, where M may be any of the rare-earth metals (Y, Gd, Ln, etc.). The yttrium compounds are the best-known members of the series, where Yi 2,3 is the common name given to the yttrium-based compounds (YiBa2Cu3). When x is close to zero, these compounds are superconducting at Tc = 90 K and have an orthorhombic structure. On the other hand, when x is close to unity they are semiconductors with a tetragonal structure. There is now a general agreement that the wave numbers of the five Raman-active Ag modes of the orthorhombic phase are: 502 cm-^' 436 cm-^^ 335 cm" ^ 146 cm-^^ 115 cm-^^

0(IV) 0(II)-Cu(2)-0(III) 0(II)-Cu(2)-0(III) Cu(2) Ba

(axial motion) (in-phase bending motion) (out-of-phase bending motion) (axial motion) (axial motion)

In the early stages of this work many studies were made on poorly controlled materials. Thus, the assignments of phonon symmetries were often ambiguous, or even erroneous, due to the presence of impurities (see below). (d) Characterization of impurity phases Impurity phases are byproducts of the processes which lead to the preparation of superconducting MB CO materials. Micro-Raman spectroscopy may

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Table 1 Observed Raman wavenumbers of MB CO materials. Phase

Color

Wavenumbers (cm~^)

Y2CU205

Blue-green Green Black Black

210, 315, 390, 480, 605 265, 330, 395, 516 640 640, 585

YzBaCuOj BaCu02 BaCuO(2+J

be used to characterize impurity phases inside the targets used in sputtering techniques (e.g. DC or RF diode, DC magnetron, or laser), as well as inside the superconducting thin film itself. Though many of the impurity phases have a number of Raman Hnes close to, or coincident with those of MBCO, they are now well characterized (Etz et aL, 1991), as shown in Table 1. (e) Stoichiometry

monitoring

The success of sputtering techniques is a result of their ability to produce homogeneous, stoichiometric thin films on substrates. Post-deposition annealing is generally made in order to optimize the superconducting properties of these films. The critical temperature T^ is very sensitive to the oxygen stoichiometry. Furthermore, thermal annealing under atmospheric-oxygen pressure ensures the reoxygenation of the oxygen-deficient sputtered films. Some modes of vibration of the sample are very sensitive to the oxygen content. In particular, the mode at 500 cm~^ can be used to monitor the homogeneity of the stoichiometry of the films. A number of Raman studies have shown that the frequency shift of this mode is well correlated with the oxygen content (Burns, 1991; Huong, 1991). This observation has been used to test in situ the homogeneity of the film on the jjim scale. (f) Epitaxial quality Oriented superconducting thin films present a very selective anisotropy in their polarized Raman spectra. In particular, spectra recorded with incident and scattered polarization along the c axis {zz spectrum) are quite different from those with polarization along the a ov b axes {xx or yy spectra). With the aid of polarization measurements, it is possible to determine the orientation of any surface and thus to establish the orientation of the film on the substrate. (g) Conclusion Although micro-Raman spectroscopy does not provide a direct test for superconductivity, it is an excellent tool for characterizing the quality of thin superconducting films deposited on substrates.

Application to Materials Science 255 C. Protective Coatings

1. Polycrystalline Diamond Coatings The fabrication of diamond films by chemical vapor deposition (CVD) and, more recently, by plasma-enhanced, chemical vapor deposition (PECVD) at low pressure, has opened potential applications in numerous hightechnology areas. A considerable effort has been made in the perfection of these techniques (Bachmann et al., 1991). Diamond films are produced in order to take advantage of the well-known properties of this substance, which include high thermal conductivity, hardness, chemical inertness and electrical resistance. However, in optical applications it is their transparency that is important, not only from the UV to the far IR, but also in the X-ray region. Micro-Raman spectroscopy provides several key advantages for the investigation of carbon films deposited with the use of any of the CVD techniques. In addition to its spatial resolution, which permits the study of individual microcrystals as well as thin films, Raman spectroscopy can distinguish the various forms of carbon. Thus, carbon with sp^-type bonding (diamond), carbon with sp^-type bonding (graphite and carbonaceous materials) and carbon in mixtures of these two types of bonding (diamondlike carbon) can be characterized by their Raman spectra (Sarvides, 1986). Films prepared by vapor deposition (evaporated or sputtered carbon) are typically diamond-Uke, amorphous carbon films (DLC). On the other hand, films prepared by PECVD methods (DC, RF or microwave plasmas) are either crystalline diamond or DLC films, depending on the conditions of deposition, i.e. nature of the plasma, nature and temperature of the substrate, flow rate and current density (Piano and Adar, 1987). Diamond and perfect graphite are each characterized by a single Raman line which appears at 1332 and 1580 cm "^, respectively. However, when the graphite lattice is disordered, a second line appears at 1360 cm~^ which grows in intensity with increasing disorder (Beny-Bassez and Rouzaud, 1985). Furthermore, both bands broaden as the disorder increases (see Fig. 5). The Raman spectra of DLC differ notably from those of graphite and amorphous carbon (Sarvides, 1986). The DLC spectra are characterized by a very broad band centered at 1530 cm~^, with a more-or-less distinct shoulder at about 1400 cm" 1 (see Fig. 5). An important feature of the Raman spectrum is that it is very sensitive to carbon materials having sp^-type bonding. The Raman cross-section of these materials is far higher than that of diamond, thus small amounts of graphite or diamond-Uke carbon mixed with diamond are easily detected. For example, films that appear to be purely polycrystalline diamond on the basis of electron diffraction results often exhibit bands that correspond to disordered carbon (graphitic or DLC).

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1500 Wavenumber

Figures Characteristic Raman spectra of carbon materials, (a) Pyrolitic carbon (highly oriented graphite), (b) polycrystalline graphite, (c) amorphous carbon and (d) diamond-like carbon.

2000

1500 Wavenumber

1000

500

(cm-l)

Figure 6 Raman spectra of a diamond coating on an Si substrate, (a) Single microcrystal, (b) grain boundary. An example of the analysis of a polycrystalline diamond film deposited on an Si substrate is shown in Fig. 6. The spectrum recorded from one microcrystal (Fig. 6a) exhibits the well-characterized sharp diamond line at 1332 c m ~ \ together with very weak bands which are characteristic of graphitic carbon. On the other hand, the spectrum recorded from an area where microcrystals are not adjacent (Fig. 6b) is characteristic of DLC mixed

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with diamond. It is worth noting that the spectum of DLC always exhibits an increase in the background due to luminescence emission induced by diamond-lattice imperfections (Etz et al., 1988). Micro-Raman spectroscopy is thus a rapid and sensitive method of characterizing the quality of diamond films and other carbon coatings. The Raman microprobe technique was apphed to the characterization of diamond films by Bonot (1990), who measured the Raman spectra of individual crystaUites of various shapes. For diamond crystallites which are well faceted, the spectra show only the Raman component, but the bands are broader than those obtained from natural diamond. Ager et al. (1991) have studied the frequency and shape of Raman bands for a number of crystallites in diamond films grown by chemical vapor deposition. With the use of a two-dimensional detector they obtained 500 data points from different positions on each of the single films grown under different conditions. It was found that the Raman frequencies and bandwidths are correlated and that the films with higher frequencies have larger bandwidths. 2. Silica Coatings EthylsiHcate paints, charged or not with zinc particles, provide excellent protection against corrosion of steel structures attacked by water or chemicals. The sol-gel transformation of ethylsilicate leads to amorphous sihca, a very inert material. However, it can be apphed only to steel which has been previously sand-blasted in order to permit a mechanical linkage between the silica coating the steel surface. Recently, a new process has been developed (Dhamehncourt et al., 1989; Mayot et al., 1989) which permits both unpolished and polished steel to be coated with ethylsilicate paints. After dipping thlesteel structure in a bath of phosphoric acid, an ethylsilicate prehydrolyzate is vaporized at ambient temperature, resulting in bonding of an amorphous-silica coating to the metal. The chemical phosphatation pretreatment insures that the sol-gel transformation starts from the metal surface. By introducing the correct water vapor pressure in the medium surrounding the film, the reactions are carefully controlled to ensure that the film becomes dense without bursting as residual solvents are released during the densification. The coatings obtained in this manner (with thicknesses varying from 10 to 100 |xm according to the conditions of deposition) offer exceptional electrical insulation and thermal shock strength. Micro-Raman spectroscopy can be used to monitor the extent of silica formation and to characterize the nature of the compound formed at the substrate-coating interface (Mayot et al., 1990). Ethyl residues are well characterized by sharp bands appearing between 3000 and 1000 cm" ^, whereas amorphous silica exhibits wide bands near 500 and 1100 cm~^.

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3500

3000

_L

_L

2500

2000

Wavenumber

1500

1000

500

( cm-i)

Figure 7 Micro-Raman study of the interface between sheet steel and siHca coatings showing the strong condensation of ethyl polysilicate induced by the phosphatation pretreatment. Figure 7 presents the results of an analysis of a section of polished sheet-steel a few hours after the ethylsilicate paint deposition. It shows that at the surface the dissolution of the iron phosphate [Vivianite, Fe3(P04)2. 8H2O] has induced a strong densification of ethylsihcate in the first few ixm above the surface. In this region the spectra of the ethyl residues are barely observable. The process creates an interphase between the metal and the silica network which is responsible for the adhesion of the coating. Near the surface the densification process is not achieved. This result is clearly evidenced by the presence of the strong Raman bands of the ethyl residues.

III. MICROELECTRONICS AND SEMICONDUCTORS A. Introduction The Raman microprobe provides a powerful technique for the investigation of semiconductor materials and the analysis of problems in microelectronic devices. This method is a nondestructive one which is important for the characterization of semiconductors with composite structures, ceramics consisting of grains, heterogeneous and device structures, etc. A Raman microprobe measurement is not limited to the study of a local

Application to Materials Science 259

point in bulk materials and small particles. Recent developments in Raman technology have enabled one- or two-dimensional images to be obtained (see Chapter 4). Raman imaging provides information on the spatial distribution of physical quantities in materials such as strain, atomic fraction in mixed crystals, impurity concentrations, free carrier concentrations and local crystallographic orientation. This information is useful not only to evaluate the quahty of a sample, but also to infer the relevant dynamical processes, e.g. growth of crystallites, atomic diffusion and reactions at interfaces or surfaces. In an earher report (Nakashima and Hangyo, 1989) some results were presented on semiconductor characterization with the use of Raman microscopes. This section describes some further developments in this area, focusing attention on the Raman imaging technique. B. Raman Microprobe Measurements

Some precautions are necessary in the application of Raman microprobe measurements. Therefore, several of the problems which are relevant to micro-Raman studies will be briefly described in the following paragraphs. 1. Heating Effects The temperature rise in materials due to laser illumination under a microscope presents a serious problem for the evaluation of strain from observed Raman frequency shifts. The temperature variation not only produces shifts of Raman peaks, but local expansion of the heated region also causes additional strain (Liarokapis and Anastassakis, 1988). The frequency variation of the first-order Raman line of Si is about 0.02 cm~^ per degree at room temperature. This shift corresponds to that of the Si Raman line under a pressure of 0.1 GPa. Accordingly, the determination of the local strain in crystals requires that the Raman microprobe measurements be carried out at minimum laser powers. A point-illumination method is widely used for Raman microprobe measurements of semiconductors because high spatial resolution can be obtained. However, this method results in heating, and possible sample degradation, even when low laser power levels are used. Particular care should be exercised in the measurement of powders and thin films on insulators, because their thermal diffusion is poor. As the laser power level is decreased, Raman peaks shift in general toward lower frequencies. Optimum laser power can be determined if a level can be found below which the Raman peak does not shift. Huang et al. (1990) observed the Raman spectrum of Si with the use of a power of 0.05 mW fxm~^ in order to avoid the heating effect.

260

P. Dhamelincourt and S. Nakashima

2. Oblique Incidence When a wide-aperture objective lens is used a large fraction of the laser beam enters the sample at large angles with respect to the surface normal; this oblique incidence results in an apparent breakdown of the Raman polarization selection rules (Turrell, 1984; Mizoguchi and Nakashima, 1989). This situation is the same for the scattered hght (see Chapter 2). Therefore, these effects should be taken into account in the determination of crystallographic orientation of crystals by Raman microprobe polarization measurements. The use of an objective lens with a small numerical aperture or the rejection of the light at large oblique incidence with the use of a suitable diaphragm is desirable for polarization measurements. 3. Depth Profiling The depth resolution for semiconductors which are opaque to laser Ught is limited by the optical penetration depth. In order to obtain Raman spectra with high depth resolution, the following methods have been used: (i) One observes Raman spectra with the use of various exciting Unes which have different penetration depths. The resulting bandshape change is analyzed with the aid of a model based on the convolution of the penetrating depth of the light and the depth dependence of the Raman bandshape (Shen and Pollak, 1984; Hang et al., 1987). (ii) With the use of beveled samples, Raman spectra of the beveled edge are observed as a function of position by translating the sample under the microscope. The spatial variation of the spectrum is analyzed by the convolution method similar to method (i). Depth profihng of a beveled specimen has been studied with strain and disorder in the GaAs epitaxial layer on Si (Huang et al., 1987), strain in Ge;».Sii_;»;/Si-strained superlattices (Chang et al., 1988), strain in laser-annealed amorphous Si (Inoue et al., 1986), interface disorder of a GaAs/Si heterostructure (Mlayah et al., 1990) and composition in Al;^.Gai_;^. as mixed-crystal layers (Abstreiter et al., 1978). (iii) Depth profiles are obtained from Raman measurements by the use of successively etched surfaces (Kakimoto and Katoda, 1982; Holtz et al., 1988; Roughani et al., 1989). C. Ion Implantation and Annealing Ion beams are widely used in the fabrication of semiconductor devices. Ion implantation is an important technology for impurity doping. The damage to crystals resulting from ion implantation, as well as the recovery of crystallinity after annealing, has been studied by Raman spectroscopy.

Application

1

"1

r

r

to IVIaterials ~i

Science

261

r

CrystdUine Si Implanted with As* 2x10^0111^2

1.5X10 Cm^,.. ^..^u;,;;^,..:^^.,^.

..^..A. 7U \ < ^ O

n

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-2

9x10 cm

6x10 cm

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-z, 2 < <

Q:

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/ ; ;\

x2.5 ^'•*V^<_...,s vC/)

x1

Pure Si J AOO

I A20

I A40

^60

WAVE

'^80

500

520

SAO

NUMBER ( c m - ' )

Figure 8 Raman spectra of As'^-implanted Si for various dosages. The implantation of energetic ions produces disorder in crystals and eventually converts the crystalline state into an amorphous one. The Raman intensity of the crystaUine component decreases v^ith increasing damage. With increasing dosage levels of implantation, the crystaUine component

262

P. Dhamelincourt

and S. Nakashima

disappears completely and a broad amorphous band is observed, as shown in Fig. 8. This observation enables the dosage to be evaluated from Raman intensity measurements. Recently, focused ion beams (FIB) have been used to carry out highresolution Hthography and impurity doping in selected areas without the usual photomask and resist steps. The Raman microprobe technique has been applied to the characterization of local regions damaged by FIB implantation. Silicon crystals implanted with a 200 keV FIB of diameter 0.1-0.2 jxm were prepared by Mizoguchi et al. (1987). Using a scanning microscope, they measured successively the integrated intensity of the 520 cm"^ component by translating the sample in such a way that the incident laser beam crossed the implanted and unimplanted regions (Fig. 9a). The Raman intensity was found to vary sharply at the boundary between the implanted and unimplanted regions. From this measurement the normalized intensity, which is defined as 3 ^ = (3cryst ~ 3imp)/3cryst? was obtained (Morhange et al., 1974); 3cryst and Simp are the intensities of the unimplanted and implanted regions, respectively. The normalized intensity for Au^"*", Si^"*" and Be^"^ implanted samples is plotted against the dosage in Fig. 9b. It increases with increasing dosage and saturates ( 3 ^ = 1) above a certain level at which the implanted region is completely amorphous. The onset of the rise of the normahzed intensity depends on the mass of the implanted ion for a constant acceleration energy. These results indicate that dosage levels and the degree of damage can be quantitatively determined once the relation between the normalized intensity and the dosage is experimentally obtained. The damage to the surface layer is not uniform and varies with depth. The depth profile of the damage is inferred from the Raman measurements with the use of laser light of different wavelengths. The penetration depth of visible light in Si ranges from 0.5 to 1 fxm; it is comparable to the projected range of the conventional ion implantation in silicon. The normalized intensity ( 3 N ) as a function of dosage is measured for different wavelengths of the laser light. The results are shown in Fig. 10. For the Au^"^ implantation, 3^f does not vary with wavelength, but does change for the Si^"^ and Be^^ implantations. The results of Fig. 10 lead to the conclusions that: (i) The region damaged by the Au^"^ implantation is very close to the surface and the projected range is smaller than the penetration depth of the laser light in undamaged crystals. (ii) For the Si^"^ and Be^^ implantations, the undamaged or partially damaged regions remain at the surface and the projected range is comparable to the penetration depth. The crystallinity of the implanted layers is recovered by post-annealing with laser or flashlamp irradiation and heat treatment in furnaces. One-dimensional images of the Raman spectrum have been

Application to Materials Science 263

W

\ ^ ^

tto 2 LU »2

"^^^

\ \

\

Damaged region \

-A^Av

2

< < Q:

POSITION

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A:AU2+

I

o : Si2+ n : Be2+

10

10'^

10'^

10'"

10,16

1017

D O S E (ions cm"^) Figure 9 (a) Structure of a silicon crystal implanted with focused ion beams, (b) Normalized intensity as a function of dosage for Au^^, Si^^ and Be^"^ with the use of 200 keV ions (Mizoguchi et al., 1987).

obtained for Si crystals which were amorphized by P"^ implantation and subsequently annealed by flashlamp irradiation for 10 s at 900°C (Mizoguchi et al., 1995). The Raman images are obtained with the use of the Hne-illumination method (see Chapter 4) and a CCD camera as detector.

264

P. Dhamelincourt and S. Nakashima (a)

.fii—^aS^-

z

-a

n

51A5A

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A880A

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1

1

.1 . . j

10'*

Ion dose (ions cnn" —I

(c)

.i

T

0.5'

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Dose (ions cnn )

Figure 10 The normalized intensity of the siHcon band in implanted regions measured at different wavelengths: 4579, 4880 and 5145 A. (a) Au^"^ ion implantation (FIB), (b) Si^"^ ion implantation (FIB) and (c) Be^"^ (Mizoguchi et al., 1987).

Application

to Materials Science

265

(a) 25.0

c-Si band

20.0

3

B

15.0 H

a o .*^ 10.0-1

cluster band

O

C

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P^ ^'

c-c -

!•

480

^

• • ^ > ^ -SH^

500 520 Raman Shift (cm')

540

Figure 11 (a) One-dimensional Raman micrograph of the sample annealed at 900°C. (The depth indicates the Raman intensity, black corresponding to high intensity.) (b) Raman spectra at typical areas inside and outside the cluster and near the cluster edge. Illumination time: 10 s; intensity measured at 4880 A (Mizoguchi et aL, 1995). The crystallinity is recovered in almost all areas when the specimens are annealed at temperatures higher than 800°C. However, in sparse regions of a few U | Lm in size, a broad band shifted to the low-frequency side of the crystalline band (520 cm~^) is observed (see Fig. 11). These regions correspond to clusters with a high density of defects. The existence of the defect clusters was confirmed by TEM measurements. The downshift of the Raman band of a cluster may be due to the local stress produced by the high density of defects. The broadening of the cluster band could be caused by the decrease in the phonon lifetimes (lifetime broadening), as well as the nonuniform distribution of the stress field (inhomogeneous broadening).

266

P. Dhamelincourt

and S. Nakashima

(a) JKK

*"-%

Scan 50 cm s"^ 1 O(K/) • Kyz)

c n n

1 K//

>

hC/)

2

LU 1-

2 Z

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V——

1

•

1

1

DISTANCE(5^m/ldiv)

DISTANCE(5^m/1div)

DISTANCE(5^m/1div)

DISTANCE(5^m/ldiv)

Figure 12 Raman intensity profiles of laser-recrystallized silicon-on-silicon structures (SOSI) at two different geometries x(yy)x and x(y2)jc, where x||
The inhomogeneous recrystallization of amorphous silicon by pulsed laser irradiation was studied by Huang et al. (1990). Regrowth processes under laser heating have also been studied in polycrystalline silicon and amorphous sihcon films. Figure 12 shows one-dimensional Raman intensity profiles of laser-recrystallized stripes in a silicon film which was directly deposited on the (100) surface of a siHcon single crystal and then amorphized (Nakashima et al., 1984). The Raman intensity was measured at 1 jjim intervals for two

Application to IVIaterials Science 267

different polarizations, x{yy)x and x{yz)x, where x||(100), y||(011) and zII(Oil). The crystals axes are referred to those of the substrate. For samples annealed at lower power levels Raman signals are observed at both geometries, while at high anneaUng power, e.g. 7W, the Raman spectra are very weak in the central region of the stripe for the x{yz)x geometry but are measurable at the boundary regions. The following quantity can be introduced: 5=l - ^ = l - | p ,

(2)

yy

where p is the depolarization ratio. This quantity becomes zero for an ensemble of small grains with perfectly random orientations, like randomly oriented molecules in a gas or liquid. It is equal to unity for an annealed stripe having the same crystallographic orientation as that of the crystalline substrate, because 3^^ = 0. Accordingly, the value of S can be used as a monitor of the orientational ahgnment. In Fig. 13, the quantity S is plotted against position. At an annealing power of 3 W, S is less than 0.5 and is almost constant over the whole annealed region. A steep rise occurs in the central region of the annealed zone at 4.0 W. The width of the single-crystalline region increases as the annealing power is increased. Figure 14 shows the two-dimensional images of the Raman intensity for laser-recrystallized silicon films (Mizoguchi et aL, 1986). A nearly flat intensity distribution along the scanning direction of the annealing laser is observed for the two different polarization geometries. This result indicates that uniform recrystallization occurs along the scanning direction and that the crystallographic orientation of the annealed region is the same as that of the underlying crystal. Irradiation by a focused laser beam easily produces the local conversion of an amorphous state into a crystaUine state. The compositional analysis of Ge-Si alloy microstructures formed by this laser-writing technique has been performed by Herman and Magnotta (1987). Damage induced by pulsedlaser irradiation has been observed in siHcon by Fauchet et al. (1985).

D. Determination of Crystallographic Orientation

Attempts to grow thin films of crystaUine semiconductors on insulators have been made by various methods. However, it was found that it is not easy to obtain large-area single-crystal films. The formation of small grains or misalignment of the crystal orientation often occurs. The determination of the local orientation of the crystallites is required in order to understand the growth mechanism of the thin films and to evaluate the crystal quaHty.

268

P. Dhamelincourt and S. Nakashima (a) 1.0

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DISTANCE(5^m/1div)

DISTANCE(5^m/ldiv)

lid)

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)

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1

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I •

K •

S 0.5

•

• •

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• I—1

1

1

1

DISTANCE(5|im/ldiv)

r\

....

1

1

1

1

DISTANCE(5^m/ldiv)

1 1 S 1 which i 1 Figure 13 Profiles of the value are calculated from the data of Fig. 12. (a) Power of annealing laser 3W, 50cms~^ scan, (b) 4W, (c) 5W and (d) 7W (Nakashima et aL, 1984).

The determination of crystal axes by Raman measurements is based on the fact that Raman scattering intensity depends on the polarization directions of the incident and scattered light relative to the crystal axes (see Chapter 1). Several methods have been proposed for the determination of crystallographic axis from Raman polarization measurements, e.g. those by Hopkins and Farrow (1986), Nakashima et al. (1986), Mizoguchi and Nakashima (1989), Huasheng et al. (1986) and Huasheng et al. (1989). In what follows, one of the procedures which was applied to the diamond structure is briefly described (Mizoguchi and Nakashima, 1989).

Application

to Materials Science

\ ^ ^

^

•^

X

269

X(KK)X \ .

^

> cut a c ^^

/ z

K ^

^

^

(a)

^/

X

0\^

x(yz)x

>

o c o c K

\^<^^^^^^A\

^

/ z

^

^

(b) Figure 14 Two-dimensional images of the Raman intensity of SOSI obtained from (a) xiyy)! and (b) xiyz)^ geometries. The power and scanning speed of the anneaUng laser are 6W and 12.5 cm s~^, respectively (Mizoguchi et al., 1986).

270

P. Dhamelincourt

and S. Nakashima

The relative intensity of the Raman-scattered Hght is given by [see Chapter 1, Eq. (7)] 35-|ee«esP,

(3)

where the unit vectors Cg and Cs define the directions of the electric fields of the exciting and scattered radiation, respectively. The Raman tensor components for the F2g mode in the diamond structure are given by ^0 0 0\

0 dL ,0 d 0/

/O 0 d\

/O d 0\

a^ = I 0 0 0 I and a^ = \d

d 0/

\0

d 0 0 .

(4)

0 0/

As the backscattering geometry is usually used in Raman microprobe measurements, the wave vectors of the incident and scattered hght are given by Kg = (—sini^coscp, —sini^sin(^, -cos<^)

and

kg = —kg,

(5)

where ^ is the angle between kg and the (001) axis of the crystal and ip is the angle between the (100) axis and the projection of k^ onto the (001) surface. In a laboratory coordinate system let i/^o t>e the angle between the projection of the (001) axis on the X-Y plane and the X axis and i// {ifj') be the angle between Ce (Cs) and the X axis. The relative scattering intensity can then be expressed in the form 3 = A{^, (p, iljQ.^) + 5 ( d , (p, i/fo:iA) cos2(A' + C(i^, (p, (Ao:
(6)

where A, B and C are functions of i^, cp, ipo and ip. The Raman intensity is measured as a function of the angle for a fixed polarization direction of the incident light. In the experiment the angle if/' is varied by rotation of a Glan-Thompson prism polarizer mounted in the path of the scattered light. Taking i^, (p, and i/^o as adjustable parameters, the fitting of Eq. (6) to the experimental intensity versus ifj' curve can be carried out. The crystallographic orientation is determined from the best-fit values of these parameters. The accuracy of the orientational analysis obtained from the measurements on Si single crystals was better than 2°. However, surface roughness may result in an error in the determination of the orientation. This effect was discussed by Kolb et al. (1991) and Mizoguchi and Nakashima (1989). The method described above has been applied to laser-recrystalHzed Si films on insulators (SOI). Two types of sample, as shown in Fig. 15, were prepared. The first is seeded SOI, for which a polycrystalline silicon film is directly attached to an underlying Si crystal at an opening in the Si02 film. The film is melted with a scanning continuous wave Ar"^ laser (Fig. 15a). The oriented crystallite at the seeded region extends along the direction of laser scanning. The second type of sample is unseeded SOI, for which there

Application to Materials Science 271 LASER SCANNING >

(a) SEEDED SOI

-Si02

UNSEEDED SOI

Si02

(b)

Figure 15 Structure of laser-recrystallized silicon-on-insulator (SOI), (a) Seeded SOI and (b) unseeded SOI. is no opening in the Si02 film (Fig. 15b). The Seco-etched samples showed that there are a number of grains with small and large areas. The large grains lie in the central region of the recrystallized stripes and the small grains are in the circumference of the stripes. The average size of the large grains is about 20 X 200 |xm^ and that of the small grains is a few |xm. Crystallographic orientations of the laterally seeded and unseeded SOI are determined by the polarization Raman microprobe technique. As shown in Fig. 16, the seeded SOI exhibits a variation of the crystal axes with distance from the seeded region along the direction of laser scanning. At the seeded region, the orientation of the recrystallized film is the same as that of the substrate. The (001) axis is normal to the surface. Going away from the seeded region, the crystal axis varies gradually and at the point B, which is 2 mm from the seeded region, the (001) axis of the film is inclined by about 45° with respect to the surface normal. Figure 17 shows the orientations of vectors normal to the surface for various small grains in unseeded SOI. It can be seen that the normal vectors gather in a certain region. One of the reasons for this tendency may be related to the interface interaction between the silicon film and Si02 layer. Local crystallographic orientations have also been measured for laser-recrystallized silicon films by Kolb et al. (1991), Hopkins et al. (1984), Nakashima et al. (1983) and Nakashima and Hangyo (1989).

272

P. Dhamelincourt

and S. Nakashima

Figure 16 Variation of the (001) axis of the recrystalhzed film along the scanning direction of the laser. The measurement was made at intervals of 100 |xm.

Figure 17 Distribution of the surface orientations of small grains in unseeded SOL The open circles show the unit vector along the normal to the surface.

Application to Materials Science 273 Yoshikawa et al. (1991) applied the technique of Raman microprobe determination of crystal orientation to diamond films grown on cubic BN. They confirmed by this experiment that a single crystal of diamond grows on the (100) surface of cubic BN.

E. Distribution of Free Carriers The control of conductivity (carrier concentration, mobility) in semiconductors is important in device fabrication. Contactless and nondestructive characterization of the concentration and mobility of free carriers is desirable. Raman spectroscopy is a potential technique for this purpose. However, plasmons formed by the free carriers do not couple with Raman active modes in centrosymmetric semiconductor crystals such as Ge and Si. Plasmons were observed in highly doped Ge, but their intensities were low (Cerdeira et al., 1984). In noncentrosymmetric crystals such as zincblende and wurtzite-type semiconductors, the plasmon and an LO phonon form a hybridized mode, the so-called LO-phonon plasmon-coupled (LOPC) mode. This mode is Raman active and has two branches, L+ and L_. The Raman frequency, intensity and shape of the LOPC mode depend strongly on the plasma frequency o)^ and the damping constant y. Both o)^ and y are related to the carrier concentration n and mobility ^t, respectively, through the relations w^ = ATTn^l{eo,nf)

(7)

y=T-i = e/(m»,

(8)

and

where e^ is the high-frequency dielectric constant and m* is the effective mass of the carriers. The analysis of the Raman band of the LOPC mode, therefore, enables the carrier concentration and mobility in semiconductors to be measured. The optically determined carrier concentrations and mobihties are consistent with the values obtained from Hall effect measurements (Irmer et al., 1983; Yugami et al, 1987). The Raman microprobe technique can be applied to the characterization of the nonuniform distribution of dopants in compound semiconductors, which are introduced by atomic diffusion, heteroepitaxial growth at high temperatures and ion implantation. The method is also useful in the evaluation of electrical activity of dopants introduced by ion implantation and subsequent annealing. The distribution of carrier concentration and carrier mobility in GaP hght-emitting diodes (LED) have been obtained by Nakashima et al. (1988). The LED diode used had a p"^-n-n"^ junction structure, as shown in Fig. 18.

274

P. Dhamelincourt

and S. Nakashima

20

1

1

AO 60 DISTANCE ( p m )

1

1——1

1

r

I

l

l

1A

- (b)

-

j

12

r 10

M

^ 8-

^ 6 S 4 u. 2 n

-

j

\

H \ ^ \

\

1

rp—O-O—O——O——O

1

20

20

1

1

1

1

_ J

C

J

1

80

100

AO 60 80 DISTANCE ( j j m )

100

AO DISTANCE

60 (urn)

Figure 18 (a) The intensity, (b) bandwidth and (c) peak frequency of the plasmonLO phonon-coupled mode plotted as a function of distance from the outer surface of the GaP LED (Nakashima et aL, 1988).

Application

to Materials Science — 1

1

-|

1

(b)

275 \—

-

o

300

'>

1

\

1 ^

e

\ \ / >o-o-o

u

>• =1-200 1— _j

p*

CQ O

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1 "•

_

> n 1 20

AO 60 80 OISTANCE(pm)

100

L 20

^

_!_

1 _ _

AO 60 80 DISTANCE ( p m )

1

100

Figure 19 Distributions of (a) carrier concentration and (b) mobility in GaP LED obtained from the analysis of the results in Fig. 18 (Nakashima et at., 1988).

The Raman spectra were measured at various points in a cross-section of the diodes. In a GaP carrier, damping is large (wpT
F. Strain in Materials

Strain occurs often in materials with heterogeneous structures and in devices fabricated by various processes. The strain in crystals can be evaluated from the observed frequencies of Raman bands compared with those of strain-free

276

P. Dhamelincourt

and S. Nakashima

crystals. The strains have been measured by Raman spectroscopy in the following materials: (i) Semiconductor heteroepitaxial layers and strained superlattices, (ii) Heterostructures consisting of elements with different thermal expansion coefficients, such as silicon on insulators (SOI structures), and (iii) Semiconductor surfaces prepared by poHshing, ion implantation, ion etching, etc. The Raman measurement of strain in semiconductors has attracted much attention because the strain affects electrical properties and sometimes induces defects or dislocations. The distribution of strain depth has been measured with the use of the Raman microprobe and bevelled samples. The crystal quality and strain in laser-annealed, amorphous Si films have been examined by Inoue et al. (1986). The Raman spectra of the beveled portion were measured as a function of the position. The variation of the Raman feature with depth was analyzed by the deconvolution of Raman components arising from the crystalline substrate and the annealed layer. The variation of the residual strain and crystal perfection with depth in the annealed layer was estimated by this procedure. The distribution of strain depth has also been measured for GaAs epitaxial films on Si with the use of a bevelled specimen (Huang et al., 1987). Silicon films grown on insulators have residual strains owing to the differences in the thermal expansion coefficients between silicon and the underlying insulators. The strain varies steeply at the edges and interfaces of these composite materials. Zorabedian and Adar (1983) have measured the strain distribution in laser-recrystallized silicon films on silicon oxide. The strain in silicon with a LOCOS (local oxidation of silicon) structure has been evaluated by Kobayashi et al. (1990). The nonuniform distribution of the strain has been measured for many materials: silicon-on-sapphire (SOS) device structures (Breck et al., 1982), patterned SOS (Yamazaki et al., 1984), Ge islands on Si02 (Takai et al., 1984), Ge on Si and Si02 (Fauchet et al., 1987), GaAs on Ge (Fauchet et al., 1987) and a thermally oxidized Si substrate (Miura et al., 1990). The Raman microprobe measurement of trench-isolated silicon islands has been reported by Tomozawa et al. (1991), who obtained the strain distribution in the cross-section of an Si island. The strain distribution in Si substrates with tungsten electrodes has been measured with a sub-|xm spatial resolution by Sakata et al. (1990). They observed Raman spectra as a function of position with the use of a focused laser beam with a 1 |xm spot size. The resulting spectral changes were analyzed assuming that a Raman band is the convolution of the straindependent Raman band and the intensity distribution of the laser beam. The result is shown in Fig. 20. There is a compressive strain in the Si substrate near the edge of the W film. The strain decreases on a sub-|xm scale as the distance from the edge of the W film increases.

Application

to IVIaterials Science

211

300 250 CL

If) 0)

I I 0

0.2

0.4

0.6

0.8

JO 1.0

Distance from the edge, x (//m)

Figure 20 Raman shift of the Si band and the compressive strain versus distance from the edge of a thin W film. The full and open circles indicate the values of the shift of the Raman peak for the two polarization directions (Sakata et al., 1990).

Cheong et al. (1987) have measured residual strains at Si quartz interfaces with the use of a Raman microscope. In order to improve the spatial resolution in the axial direction, an aperture was placed at an intermediate image plane between the Raman microscope and the entrance slit of the monochromator (see Chapter 2, Section VI). It was inferred from the results that the residual strain exists within a small region of the substrate (quartz) near the interface, about twice the thickness of the Si film.

G. Thermal Conversion of SiC Polytypes

Zincblende-structured j8-SiC is converted into a-SiC polytypes by maintaining the samples at high temperatures. This thermal conversion of the polytype does not occur uniformly at the early stages of the process. A nonuniform spatial distribution of the Raman spectra has been observed in thermally converted SiC crystals (Yoo and Matsunami, 1991). Figure 21 shows the Raman spectral profile of j8-SiC after annealing at 2080°C for 3 h . The Raman bands corresponding to the 6H polytype are clearly observed at 789 and 768 cm~^. The Raman band at 789 cm~^ is due to the TO band of j8-SiC, and the TO band of the 6H polytype, which can be observed in this experimental geometry.

278

P. Dhamelincourt and S. Nakashima

760

780 800 WAVENUMBER (cm-^)

Figure 21 One-dimensional spectral profile of an SiC polytype converted at 2080°C (anneal time: 3h).

The Raman intensity ratio of the 768 and 789 cm"^ bands changes from place to place, indicating that the polytype conversion is not uniform. Raman bands corresponding to the 15R polytype bands are found at some locations. These results indicate that the polytype conversion at 2080°C is not spatially uniform. Raman spectral images were also measured for samples annealed at 2200°C for 2h. They show^ed only slight intensity variation Wiih position.

H. Raman Microprobe Measurements of Inorganic Conducting Materials Raman microprobe measurements have been made for SbCls, Br2 and FeCl3 graphite intercalation compounds by McNeil et al. (1985). Depletion of the intercalate was observed within 10 ^Jim of the sample edge in the SbClsgraphite compound. Those authors concluded that the intercalate contracts thermally within graphite as the sample is cooled after the intercalation reaction is completed. Micro-Raman spectra of graphite intercalated with SO3 were obtained by Ladjadj et al. (1985). They observed the Raman spectra as a function of the intercalation time to obtain information on intercalation kinetics. Raman microprobe spectra of graphite fibres with a diameter of 7.6 |xm were measured under uniaxial stresses by Sakata et al. (1988). The polarization-dependent splitting and shift of the graphite Raman peak were observed under tension along the fiber axis.

Application to IVIaterials Science 279 Tungsten silicide films formed on silicon were studied with a Raman microprobe (Codella et al., 1985). The Raman spectra were obtained from siHcide films 8 |xm wide by 20 nm thick.

IV. POLYMERS AND FIBERS A. Introduction For many years the study of industrial polymers with the use of conventional Raman spectrometers was limited by the high fluorescence induced by the visible excitation used. In most cases the weak Raman lines were completely masked by a strong fluorescence spectrum. More recently, FT-Raman instruments which employ exciting lines in the near-IR spectral region have overcome the problem of fluorescence (see Chapter 3). This technique allows the Raman spectra of bulk industrial polymers to be obtained without difficulty. However, when high spatial resolution is required (e.g. analysis of luim-sized defects or of single textile filaments), FT-Raman instruments equipped with a microscope have thus far been limited to a sensitivity which is far lower than that of modern Raman microspectrometers equipped with multichannel detectors and which employ visible excitation. Consequently, FT-Raman microscopes often yield deceiving results. Although micro-Raman spectrometers use visible excitation, when they are well designed the use of a confocal configuration almost eliminates the fluorescence which falls outside of the focal volume. Thus, the Raman spectrum of most industrial polymers can be successfully recorded. B. Identification of Defects The prime consideration of a manufacturer is the continuing satisfactory and trouble-free production of a good-quality product. These criteria are especially important in the manufacture of a textile from the synthetic fibers produced by spinning from the molten polymer. Defects or breaks in the textile filament can perturb the production process and impair the quality and appearance of the product. Typically, the textile yarn is composed of several tens of filaments spun and wound together. For economic reasons the spinning speeds are now on the order of 6000 m per minute (Ogilvie and Addyman, 1980) which implies that the diameter of an individual filament ranges from 5 to 20 \xm. Thus, at this scale the filament breaks caused by particulate contamination may be important when the contaminant becomes a substantial proportion of the material flowing through the spinneret. Filters are mounted before the spinneret which remove particulate matter down to the 5 |uim range. Accordingly, the analytical tools employed must function

280

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in the spatial domain which is less than 5 |xm. A large variety of microanalytical techniques exist which determine the elemental composition of particulates, but molecular characterization is often necessary in order to identify the contaminant material and establish its source. Micro-Raman spectroscopy is particularly well adapted to this kind of analysis because it permits a filament contaminant to be directly identified by focusing the laser beam on it and recording the Raman spectrum. There are two types of inclusion arising from particulate contamination of the feedstock polymer (Ogilvie and Addyman, 1980): (i) Internal contamination from substances which are part of the polymer recipe or produced during processing. For example, titanium dioxide, which is used as an antilustre agent in the textile yarn, and carbonaceous residues produced in the thermal degradation of the polymer, have been identified. (ii) External contamination from various substances acquired during the material-handling processes. Materials identified by micro-Raman spectroscopy include packaging materials (polythene, polyvinyls), insulation materials (fiberglass) and miscellaneous substances (polyamide fibers from workers' clothing, airborne particles such as quartz and calcium carbonate, etc.). C. Analysis of Chemical Composition

The intensity of Raman scattering by a functional group is proportional to its concentration. This relation is invaluable for the determination of the relative concentration of compounds which exist in several isomeric forms. For example, polybutadiene has three isomeric forms (d5-l,4, trans-l,A and syndiotactic-1,2). This material is often used as a blend or copolymer with polystyrene in order to improve the impact strength of the polystyrene. The relative concentration of the three isomers of polybutadene is an important parameter which governs the mechanical properties of the polymer. In the 1600-1700 cm~^ spectral range, the C=C stretching modes of the three isomers appear separately in the Raman spectrum. This fact enables the ratio of these isomers to be determined in situ and microscopic inhomogeneities can thus be detected. Figure 22 shows the analysis of a poly butadienepolystyrene copolymer in which the variation in the concentration of the cis isomer is followed. D. Analysis of Morphology in Polyester Fibers

Polyester textile fibers are made of polyethyleneterephtalate (PET), whose molecular geometry is shown in Fig. 23. In the crystalline state the

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1665 (frans)

V (cm ^)

Figure 22 Raman spectra of polybutadiene in the C=C stretching region, (a) At the center of the sample and (b) at the sample boundary.

Trans

Gauche Figure 23 Molecular geometry in PET fibers.

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arrangement of the polymer chain corresponds to a trans conformation of the glycol unit, as well as coplanarity of the carbonyl group with the benzene ring. In the amorphous state, the coplanarity and the trans conformation of the glycol units do not exist; thus, the gauche conformation of the glycol unit is prevalent in the material. However, some trans conformation may still be present. The macroscopic properties of a polyester textile are governed by amorphous and crystalline structural arrangements in the fibers. Thus, in order to improve the mechanical properties of these fibers, they are stretched or drawn during production. This orients the molecules so that the chain axes are preferentially oriented towards the fiber axis. However, the stretching or drawing of the fiber modifies its morphology, as it changes molecular alignment, conformation and crystallization. With the use of polarization measurements and band assignments it is possible to monitor independently molecular alignment and crystallization (Cook and Ogilvie, 1982). MicroRaman spectroscopy allows information on both of these characteristics to be obtained from a single filament. This technique is very important in the detection of drawing faults in fibers which result when they are subjected to the stretching and shrinking processes of the textile industry. Orientation measurements have been well described by Purvis et al. (1973) for PET fibers. For transversely isotropic polymers, such as in a uniaxially drawn PET fiber, a relationship exists between the values of cos^ 6 and cos"*^, where 0 is the angle between the molecular chain axis and the drawing direction. These two functions of 6 can be determined by refractive index measurements (for cos^ 6) and from polarized Raman spectra (for both cos^ 8 and cos^^). In practice, the degree of orientation in polymers is defined by the two averaged Legendre polynomials F2 = 5 ( 3 c o s ^ - l )

(9)

P 4 = f c o s ^ - f c o s ^ + i.

(10)

and

For a completely nonoriented (isotropic) polymer ^2 = 0 (since cos^ 6 = 1/3), while for a perfectly oriented fiber P2= ^ (since cos^^ = 1). The development of orientation can be monitored by studying the 1615 cm~^ band, which corresponds to the C—C stretching vibration of the benzene ring. It is sensitive only to the molecular chain orientation. Polarization measurements on a single filament require it to be positioned with its draw axis Z along the larger dimension of the microscope shde, and the laser beam to propagate along the Y axis. An analyzer is set to transmit the Raman radiation which is polarized perpendicular to the entrance sUt of the instrument and a rotating microscope stage is employed. It is then

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A. Y{ZZ) Y A':Y{ZX)Y

A: Y{XX) Y

Figure 24 Polarization configurations for orientation measurements in polyester fibers. possible to measure three different intensities for ^he 1615 cm ^ band; 3zz? 2zx and 3xx, which correspond to the Y(ZZ)Y, Y(ZX)Y and Y(XX)Y polarization configurations, respectively (see Fig. 24). With the use of the method of Purvis and Bower (1976), Cook and Ogilvie (1982) derived the equations which have to be solved to obtain the orientation functions P2 and P4, namely 3zz = ^(0.169 4- O.512F2 + O.3I8P4),

(11)

2zx = ^(0.093 + O.O66P2 - O.I6OF4)

(12)

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and

3 ^ ^ = ^(0.169 - 0.256F2 + O.II9P4),

(13)

where yl is a constant. When the laser beam illuminates a filament, polarization scrambling due to reflection and refraction can lead to errors in intensity measurements. These errors can be effectively eliminated by immersing the sample in a liquid with a similar refractive index. Ogilvie and Cook (1982) have reported values of P2 obtained from Raman measurements on fibers having different draw ratios, which correlate well with those obtained from birefringence measurements. The degree of crystallinity in PET is obtained by measurement of the bandwidth of the carbonyl-stretching vibration located at about 1730 cm~^. Generally, with increasing crystallinity, the associated conformational bands show decreasing bandwidth. The bandwidth of the carbonyl band in PET is indeed a measure of the conformational disorder due to the rotations of the carbonyl group around the carbonyl phenylene bond (Melveger, 1972). Recent work reported by Adar et al. (1990) shows that with the use of band-fitting procedures the carbonyl band is in fact complex and is composed of two or three bands. The central band at 1726 cm~^ is Unked to crystalHne PET, whereas the other two bands, at 1721 and 1735 c m ~ \ characterize the amorphous phase. The intensity increase of the central band at the expense of the lateral bands explains the profile change and the clear correlation of the width of the multicomponent carbonyl band with the crystallinity of the polymer. Nevertheless, the detailed analysis of this carbonyl band provides more information on the amorphous, oriented polymer (i.e. spin-oriented fiber).

E. Conclusion

Chemical compositions, molecular configurations and conformations in polymers are identified through their vibrational frequencies. Thus, this information can be obtained by FT-IR techniques and, more recently, with the use of FT-Raman accessories proposed by IR instrument manufacturers. However, Raman microscopy remains as a unique, invaluable tool for the analysis of polymers at the microvolume level (a few \LW?). Thus, the nature of defects or inhomogeneities can be readily identified. In the same way, investigation of polymer morphology and quantitative measurements of localized molecular symmetry in oriented polymers are possible from Raman polarization measurements.

Application to Materials Science 285 V. GENERAL CONCLUSIONS Raman spectroscopy was not widely applied to the characterization of materials until the advent of Raman microprobe techniques. With conventional Raman spectroscopy it was not possible to reduce and localize the analysis volume to dimensions commensurate with grain or phase size in microstructures or with the size of the analyzed object itself (e.g. microelectronic devices and fibers). Now the ability to investigate regions as small as 1 |xm in diameter with a tool which yields molecular information enables Raman microspectroscopy to complement existing microanalysis tools (scanning electron microscopy, X-ray microanalysis, etc.) and to be a unique technique for the investigation of organic substances. The examples given in this chapter have been selected in order to illustrate what Raman microspectroscopy can offer in the investigation of both organic and inorganic materials. These examples do not, however, represent an exhaustive hst of applications. Indeed, with today's changing technologies and the rapid appearance of newly engineered materials, the future of Raman microspectroscopy for materials characterization is extremely promising.

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