Grazing incidence Mössbauer spectroscopy: new method for surface layers analysis

NOMB

Nuclear Instruments and Methods in Physics Research B74 (19931545-553 North-Holland

Beam Interactions with Materials 8 Atoms

Grazing incidence Miissbauer spectroscopy: layers analysis Part I. Instrumentation

new method for surface

Sobir M. Irkaev Institute for Analytical Instrumentation, Russian Academy of Sciences, 198103 St. Petersburg, Russian Federation

Marina A. Andreeva Department of Physics, Moscow State University, II 7234 Moscow, Russian Federation

Valentin G. Semenov, Genadii N. Belozerskii and Oleg V. Grishin Department of Geography, St. Petersburg State University, 199164 St. Petersburg, Russian Federation

Received 15 May 1992 and in revised form 30 December 1992

The aims of this series of papers are to describe a spectrometer for simultaneous investigation of Mijssbauer spectra from: (1) specularly reflected gamma-rays, (2) secondary electron, (3) characteristic X-ray, and (4) scattered gamma-rays (part I); to present a general theory of such spectra and to indicate some unusual characteristics and features of Mossbatter spectra at grazing angles (part II); and to give a quantitative analysis of experimental spectra for some 57Fe films which shows that grazing incidence mijssbauer spectroscopy (GIMS) is really a new method of surface investigation (part III).

1. Introduction Grazing incidence Mijssbauer spectroscopy (GIMS) can be a powerful technique for investigation of the interaction of nanometer wavelength region radiation with surface atoms. In such experiments grazing incidence angles are varied up to tens of milliradians, ensuring wide possibilities for depth selective analyses. The results obtained by X-ray grazing incidence studies are well known and supply information about many atomic and crystal phenomena [l-3]. A specific feature of Mijssbauer spectroscopy under grazing incidence angles is the fact that Mossbauer spectra are formed not only by radiation scattered on the electron shell, but also by nuclear scattering. The latter contribution may be varied experimentally by changing the gamma-ray energy. The dependence of specularly reflected gamma-ray intensity upon the incident energy in the neighbourhood of the exact resonance reveals interesting interference effects due to

Correspondence

too: S.M. Irkaev, Institute

strumentation, Prospect Ogorodnikova, 198103, Russian Federation. 0168-583X/93/$06.00

for Analytical In26, St. Petersburg

the coherent interaction of radiation with a flat mirror [4]. This can provide additional information about the atomic and electronic structure of surface layers (l-100 nm). Investigations at angles where total external reflection (TER) takes place are of great interest for the study of ultrathin surface layers. Such investigations are also important for the development of resonance wave guides, low-pass filters, supermonochromatization of radiation aimed at obtaining non-decaying, polarized, time-quantified narrow-beam gamma-rays using synchrotron radiation, etc. [5,6]. It is known that the susceptibility of the medium to electromagnetic waves incident on a smooth surface is: x = 2(n - 1) = -26 + Zip,

(1) where S and p are the real and imaginary parts, respectively, of the complex refractive index of the medium. Total external reflection takes place at a grazing angle B less than a critical angle BC defined by the relation: 9,’ = 26.

(2) At grazing angles 0 < BC and for a transparent medium (Im ,+J= 0) the wave vector of the refracted

0 1993 - Elsevier Science Publishers B.V. All rights reserved

S.M. Irkaev et al. / Grazing incidence Miissbauer spectroscopy, Part I

546

wave has a pure imaginary normal component and the reflection coefficient, R, is equal to 1. For real media and especially for resonant media there is always some absorption Urn X f O), so R < 1 and the reflected wave intensity is comparable to that of the incident wave and cannot be ignored. The angle region where f7*“X is described as the total external reflection region. For example, in the absence of nuclear resonant interactions the critical angle for a metal iron mirror is 0, = 3.8 mrad for radiation with energy 14.4 keV. The radiation penetration depth depends upon susceptibility and grazing angle. If we define it as the depth at which the intensity of the refracted wave in the medium decreases by “e” times then at grazing angles d I is equal to:

The dependence of penetration depth, d I, of Mossbauer radiation into a resonant medium vs grazing angle, 8, and the energy difference between incident radiation and exact resonance, X (in units of halfwidth), are presented in fig. la and b, respectively. Dotted lines represent d I in the absence of nuclear resonant scattering (i.e. electronic scattering alone). As we are dealing with a resonant medium, X includes two terms resulting from electronic scattering and from nuclear resonant scattering, X = xe, + Xnucl, where Xnuc,= ,ynuc,(Ey) and E, is the energy of the incident gamma radiation, and we should take into account that the penetration depth d I is a function of the incident radiation energy and this depth differs for energies less or greater than the resonance energy due to refraction effects. For the case of a metallic iron mirror we use for the electronic part:

I

0

is

ti

GRAZING

-al

-ICI

as

sb

7b

ANGLE, mrad

Q

%I

xl

(9

x - ENERGY,in units cd line halfwidth Fig. 1. Penetration depth d I of MGssbauer radiation into a resonant sample as a function of grazing angle 0 (a) and energy difference X between incidence radiation and exact resonance in units of halfwidth of resonant line (b). Dotted lines represent d I in the absence of ,ynucl.

where X is the energy difference between the incident radiation E, and exact resonant energy E, in units of halfwidth of resonant lines r/2. It can be seen from fig. la that the depth selectivity of the GIMS method is caused by the abrupt decrease of the penetration depth d I in the TER region. The abrupt decrease of the refracted wave may be explained by the creation of a specularly reflected wave above the surface. One can also see from fig. lb that the interaction of radiation with the medium is different for E, less or greater than E,. This should be observed in experimental spectra.

According to our calculation, the minimum penetration depth is about 2-3 nm for the case considered. Experimental confirmation of the Mossbauer radiation TER effect is given in refs. [7-91. In these experiments, Mossbauer spectra of the reflected gamma radiation were measured at a wide range of grazing angles and basic relationships for the formation of these spectra above and below the critical angle were established. Although the analyzed layer under TER conditions is very thin, the technique has not been widely used because of the very low luminosity at grazing angles. Furthermore, interference effects complicate interpretation of the experimental data.

Xel = (- 13.0 + 1.3i) X 10e6, and for the nuclear part: -48.0 xnuct= x+i

x IO-6,

S.M. Irkaev et al. / Grazing incidence Missbauer spectroscopy,

Substantial progress can be achieved by detecting the conversion of Auger electrons (secondary electrons) created by Massbauer radiation instead of the specularly reflected gamma-quanta as proposed in ref. [lOI. The first experimental observations of the M&sbauer spectra of secondary electrons under TEAR conditions were reported in refs. [11,12]. These experiments confirm the surface sensitivity of the method. However, there is no fult description of interaction anomalies of Miissbauer radiation with a resonant medium at grazing angles. A wide application of the TER technique has also been limited by two essential factors: the lack of commercially available equipment and the absence of good theoretical methods for analysis of Miissbauer spectra (reflected wave or secondary radiation). Some efforts to overcome these difficulties are made in this paper. We optimized experimental conditions and design of a spectrometer intended for such experiments. The specific feature of the spectrometer is that it allows registration of Mtissbauer spectra over a wide range of grazing angles based on: specularly reflected gammarays, secondary electrons, characteristic X-rays and scattered gamma-rays in four independent channels simultaneously. Simultaneous registration of all these types of radiation at grazing incidence angle complement one another and expand the information about the structure and peculiarities of the interaction of the radiation with ultrathin surface layers. For interpretation of our results we start from the Fresnel formula which was used in the first papers on the Massbauer TER effect [7-91 and arrive at the common theory of propagation of radiation in multilayered anisotropic media at grazing angles. On the basis of this theory we can analyze Mijssbauer spectra of reflected and secondary radiation under TER conditions, which allows us to obtain information about the depth profile of the hyperfine interactions in ultrathin surface layers. The possibilities of the GIMS method are illustrated by experimental results from investigations of some metallic iron films.

2. Spectrometer design The design of the analytical bench of the spectrometer intended for GIMS studies should ensure: (1) simple and reliable setting, measurement and determination of the grazing angle 8, (2) easy variation of angular beam divergence, (3) sample replacement without affecting the experimental layout, (4) accurate maintenance of the source-collimator-sample distances, and 6) sampIe rotability beyond the TER region angle range up to 90”.

Part I

547

Fig. 2. Block diagram of spectrometer for GIMS investigation.

In addition, the spectrometer should provide the necessary velocity range of scanning, linearity, longterm stability, and efficient acquisition of input data over four channels simult~eously. A block diagram of the spectrometer is shown in fig. 2. The spectrometer consists of an analytical bench and control and energy selection systems interfaced to a personal computer. Control and energy selection systems together with the necessary equipment consist OE a modulator driver (MD), four discriminators (pulse-height analyzer with built-in amplifier) (SCA), two dual-output high-voltage power supplies (HV), four special-purpose accumulators @A), CAMAC crate~~ntroller and other accessories, i.e. computer-CAMAC interface card, coaxial and power cables, connectors, etc. Control and energy selection systems are built into CAMAC Eechnology. An IBM PC XT/AT or any compatible computer can be used. The radioactive source, S, is placed on the moving part of a Doppler modulator, DM, while the sample under investigation, A, is placed inside the working volume of a combined detector consisting of two chambers: D2 for electron and D3 for gamma- and X-ray registration. The modulator driver produces the control voltage signals for the DM, thus making it vibrate according to a preset motion law. In addition, the modulator driver generates pulses to start and switch memory cells (channels) for overall system synchronization. The start and channel advance pulses are fed to the service inputs of special-purpose accumulators @A). The signals from the detectors are fed to the preamplifiers. After amplification and shaping the signals are applied to the input of the SCA. Amplitude-discriminated and normalized (TTL) pulses from the SCA outputs are fed to the data inputs of accumulators SA for scaling.

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Accurate correlation between the duty cycle of motion and memory cells of the SA is ensured by the start and channel advance pulses. The HV modules supply detectors with high-voltage power. The accumulation cycle is initiated by the start pulse and terminates with the last of the 4096 channel advance pulses. The main program sets high voltages, SCA parameters (input pulse polarity, amplifier gain, window width), displacement law, and the sign and range of velocity variation for DM. It also provides spectrum visualization and outputs to peripherals. The analytical bench of the spectrometer (fig. 3) comprises a rigid platform suspended on vibration dampers. The Doppler modulator, shielding screens, collimator forming a narrow plane-parallel radiation beam, combined detector for secondary electrons, gamma- and X-ray and scintillation detector for reflected gamma-radiation are placed on “wedge slide”type guides mounted on the platform. A narrow plane-parallel gamma-quantum beam from source S rigidly attached to Doppler modulator 1 is formed by collimator 2 and through the combined detector entrance window 3 falls on tested sample 4 placed within its working volume. The secondary electrons and gamma- and X-ray radiation are detected in separate volumes of the combined detector, while the gamma-radiation reflected from the sample surface and passed through the detector exit window and slotted mask 6 is detected by scintillation detector 7. The Doppler modulator used here is that of the commercial SM 2201 Miissbauer spectrometer [13]. The hollow rod of this modulator can be employed for aligning the optical system. The geometric dimensions and their variation intervals for the analytical bench of the spectrometer were optimized based on the following requirements: - the divergence value of the collimated y-quantum beam incident on the sample surface, 2u, must ensure stable Mossbauer spectra for angles ~9< 0, (0 N 3.8

mrad); as seen from fig. la, the 2u value should not exceed 0.5 mrad; - the luminosity of the experiment should be high enough for reasonable analytical bench sizes. As a starting point at the optimization stage, the geometric dimensions (2 x 15 mm’) of the active part of the radioactive source were used. Solution of the multiparameter optimization problem yielded the following values for the basic dimensions and their adjustment intervals for the -y-ray optical scheme of the spectrometer: (L,) - 600 mm, (L,) N 700 mm, ( L2) - 400 mm; maximum sizes, h, of collimation slits should be set < 1 mm to an accuracy not worse than 0.05 mm. The complete spectrometer includes two types of collimation systems: one has a fixed L, value equal to 600 mm and the other has L, variable from 408 to 600 mm. Details of the collimation systems are described in ref. [14]. To obtain a well-collimated gamma-ray beam, two slit diaphragms of identical construction are used. The range of diaphragm opening is (O-1) f 0.02 mm. For alignment of the collimator axis the diaphragms can be transversely displaced along the horizontal and vertical directions (within f5 mm), rotated between the transverse horizontal and longitudinal axes (within k5”). Alignment of the slit collimators is achieved by independent rotation in a plane normal to the collimator’s axis. The combined detector 3 is a dual-chamber gas proportional counter for detection of Auger and conversion electrons (electron chamber) and gamma- and X-rays (-yX-rays chamber). Gamma- and characteristic X-ray radiation emitted by the sample almost without attenuation passes through the sensitive volume of the electron chamber, 50 urn thick beryllium foil separating the chamber volumes, and is detected in the upper chamber. The detector employs optically transparent windows of resin glass. This permits adjustment of the radiation incidence angle to the sample surface and filtration of

Fig. 3. Gamma-optical scheme.

the X-ray component (6.4 keV1 out of the origina flux. The existence of the exit window offers the oppo~uni~ of detecting a specularly reflected g~ma-ray beam. The sample under investigation is placed inside the electron chamber of the detector so that the surface of interest faces the working volume of the chamber where a gas mixture of He + 8% CH, fed through a nipple flows at a rate of 2 cm3/min. The anode of this chamber consist of three tungsten wires (20 grn in diameter) mounted on an insulating base (3 mm from the sample surface). The anode in the yX-rays chamber is a tungsten filament GO &rn in diameter) attached to the insulating base. The working gas used for detection of gammaand X-rays is a mixture of Ar + 8% CH4 fed through the nipple at a rate of 2 cm3/min. The detector is provided with an additianal beryllium window so that the detector can also be used in the conventional backscattering geometry. The combined detector is rigidly mounted on a microlift and can be moved in a vertical direction. It may also be turned with respect to the collimated beam plane by a graduated microscrew with a wedge-shaped device in the O-30 mrad angle range.

3. Experimental pt-w&u-e The main difficulty in GfMS experiments is the initial orientation of the sample surface with respect to the incident gamma radiation. This is because the gamma-ray incidence angles, 0, should be set to a chosen value within 0.5 mrad with respect to the sample plane. To align the optical system, an alignment Screen with a coordinate grid of (30 x 301 mm2 area and a 100 W light source are introduced in the s~c~ometer, The alignment is ac~mplished optically. For this purpose, the y-source is replaced with the light source. The sample surface serves as a mirror for the light beam formed by the collimator. The beam width is large enough to observe two light reflexes on the screen 1 m away from the detector’s centre. The grazing angle 0 can be defined from the distance between the reflexes as follows:

e = D/2L,, where D is the distance between the reflexes on the screen, and L, is the distance between the detector’s centre and the screen.

3,74

i

Fig. 4. Conventional CEMS spectra of (a) the pure iron film, and (b) the iron film with a thin oxide layer.

SM. irkaev et al. / Grazing incidence Miissbauer spectroscopy, Part I

550

The gamma-quantum beam divergence is also easily defined. If the slit width is denoted by k, and the sample length along the radiation by I, the beam divergence 2a can be defined as: 2a = h/L,

+18/L,.

Application of this formula is limited by the condition I%Q h, i.e. the mirror projection on the plane

RELRTIUE

Fig. 5. MSsbauer

perpendicular to the beam direction should be less than the slit width h. Otherwise 2u = 2h/L,, where L, is the distance between the entrance slit of the collimation tube and the sample. The spectrometer construction has ensured a gala-ray divergence 2a = 0.5 mrad at a siit width of 0.2 mm. A significant advantage of the spectrometer is the

UELOCITY&m/s)

spectra of the pure iron film obtained from: (a) secondary electrons, (b) reflected gamma-radiation, characteristic X-ray radiation, and (d) scattered gala-mdiation.

(cl

optical method of sample alignment which ensures both reliable alignment before each measurement and monitoring during the measurement process. This is important as the inning time is very long: ten days or more. The alignment procedure also involves setting of the detector slit of the scintillation detector which detects the radiation reflected from the sample. Thus during a single run (preset 6 value) one can acquire four Mossbauer spectra simultaneously: three from the combined detector (conversion and Auger

electrons and characteristic X-rays and scattered gamma-rays) and one from the scintillation detector (reflected gala-radiation). Upon measuring a number of spectra at angles 8 < t?,, 3 _ 6, and 8 > @,, it is possible to define spectral components ~~es~nding to the resonant isotope atom nuclei in subsurface layers and hence to elucidate physical properties of these layers. It is obvious that for GIMS one needs a ribbon-type source. For this purpose we tried at first to use a

412lrrwl

1.18

Fig. 6. Miksbauer spectra of the iron film with a thin oxide layer at grazing angles: obtained from secondary electrons, (left) and specularly reflected g~a-radiation

(right). Grazing angle 0 is indicated.

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SM. Irkaev et al. / Grazing incidence Miissbauer spectroscopy Part I

chemical compound. We prepared the ribbon-type source (2 X 8) mm in size with 30 mCi of 57Co in a matrix of a chemical compound. But the line width was - 0.35 mm/s and this was the reason why it was replaced by another source. The next ribbon-type source was 200 mCi of 57Co(Rh) (2 X 15) mm in size. The line width of this source was 0.262 f 0.005 mm/s.

4. Spectrometer operation For testing of the spectrometer capabilities and revealing the essential features of experimental spectra we prepared two pilot samples. The first was a film of metallic iron 50 nm thick (enriched up to 90%) deposited on a polished beryllium disk (50 mm diameter and 10 mm thick). The second sample was the same, but after preparation it was heated in a thermochamber up to 150°C for 4 h. This was needed to produce a thin layer of Fe oxide on the surface of the original film. By using a beryllium substrate the non-resonant scattering intensity is reduced and this results in diminished background counts and an enhanced Miissbauer spectra effect. The Mijssbauer spectra of these samples measured by the detection of secondary electrons in conventional CEMS geometry are shown in fig. 4. The spectrum of the first sample (fig. 4a) represented an almost pure a-Fe sextet. The relative intensities of the spectrum lines (approximately 3 : 4 : 1) indicate that the orientation of the hyperfine magnetic field in the iron film is parallel to the surface plane. The spectrum of the second sample (fig. 4b) has an additional doublet corresponded to Fe3+ ions located in the surface layer. According to the result of least square fitting, the proportion of these ions is less than 5%. Calculations were made on the assumption of additive partial wntributions from the atoms at different depths. It can be seen from the figure that the contribution from the surface layer to the total spectrum is considerably shaded by a strong signal from the deeper layers. Fig. 5 shows four Mijssbauer spectra of the first sample measured simultaneously by the detection of secondary electrons (a), reflected gamma-radiation (b), characteristic X-ray radiation (c) and scattered gamma-radiation (d). These spectra measured at an incidence angle of 5 mrad show the capabilities of the spectrometer for obtaining more complete information about peculiarities of the Miissbauer radiation interaction with the surface under grazing angles. With the aim of observing the growth of the surface phase signal at the grazing angles, we carried out an investigation of the second sample. The incident beam in these experiments was strongly collimated (2~ = 0.5 mrad).

For example, fig. 6 shows the specularly reflected and secondary electron Miissbauer spectra obtained at angles 10, 4.2, 3.5 and 2.2 mrad. It can be seen that the intensity of the signal from the Fe3+ ions at the surface increases with decreasing grazing angle. The reflected gamma-radiation spectra are similar to those observed in refs. [5,6]. At angles 0 - Bc(0 = 4.2 and 3.5 mrad) each line in the reflected spectrum has a dispersive shape which is caused by a change of the refraction coefficient due to the medium resonant properties. At an angle less than the critical angle (0 = 2.2 mrad) the reflected spectrum looks like an ordinary transmission spectrum and the asymmetry is small. On the other hand, at angles greater than 0, the intensity of the reflected wave is small and it cannot be seen above the noise background. Fig. 6 clearly shows that specularly reflected gamma-radiation spectra can be used for surface analysis only when grazing angles are less than 0, (in our case Bc- 3.8 mrad); on the contrary, secondary electrons’ spectra give more clear information about surface states at any incident angle. Secondary electrons’ spectra have a shape similar to that of a wmmon CEMS spectrum and they clearly reveal that the surface layer signal increases with decreasing grazing angle, especially in the region of 0,. The asymmetry of the lines is due to the fact that the refraction angles and the penetration depth of refracted radiation differ for energies less or greater than the exact resonance energy (see fig. 1) and the line broadening is caused by saturation effects which arise even for very thin layers due to grazing angles of propagation of the refracted radiation [lo]. It should be pointed out that the baseline in secondary electrons’ spectra exhibits non-trivial behaviour: at 0 = 4.2 mrad the “tail” on the right side is higher than those on the left side and the situation is reversed at angles of 3.5 and 2.2 mrad. This effect was briefly described in ref. 1151. A more detailed theoretical description of the Mkbauer spectra shape and experimental results will be presented and discussed in parts II and III, respectively.

5. Conclusions A spectrometer for GIMS investigations of ultrathin surface layers was developed. The spectrometer allows data acquisition at a wide range of grazing incidence angles over four independent channels simultaneously: specularly reflected gamma-radiation, secondary electrons, characteristic X- and scattered gamma-radiations. The capabilities of the spectrometer are demonstrated by investigation of metallic iron films deposited

S.M. Irkaev et al. /

Grazinginhere

on a beryllium substrate. The obtained specularly reflected and secondary electrons Miissbauer spectra at different grazing angles exhibit features associated with the TER effect. Surface sensitivity is also revealed.

References

El1A.M. Afanasev, B.A. Alexandrov and R.M. Imamov, X-ray Diffraction Diagnostic of Submicron Layers (Nauka, Moscow, 1989). PI M.A. Andreeva, S.F. Etorisova and S.A. Stepanov, Poverkhnost 4 (1985) 5. 131A.V. Vinogradov, LA. Britov, A.Ya. Grudskii, M.T. Kogan, I.V. Kozhevnikov and V.A. Slemzin, Mirror Optics of X-rays, ed. A.V. Vinogradov (Mashinostroenie, Leningrad, 1989). r.41M.A. Andreeva and R.N. Kuzmin, Messbauerovskaya Gamma-optika (MGU, Moscow, 1982). El R.V. Pound and W.T. Vetterling, J. Phys. (Paris) C2-40 (1979) 2.

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[6] J.P. Hanon, G.T. Trammel, M. Mueller, E. Gerdau, R. Rutter and H. Winkler, Phys. Rev. B32 (1985) 6374. [7] S. Bernstein and E.C. Campbell, Phys. Rev. 132 (1963) 1625. [8] S. Bernstein, EC. Campbell and C.W. Nestor Jr., J. Phys. Chem. Solids 26 (19651883. [9] F.E. Wagner, Z. Phys. 210 (1968) 361. [lo] M.A. Andreeva and R.N. Kuzmin, Phys. Status Solidi 125 (1984) 461. Ill] J.C. Frost, B.C.C. Cowie, S.N. Chapman and J.F. Marshall, Appl. Phys. I.&t. 47 (1985) 581. 1121 MA Andreeva, G.N. Belozerski, SM. Irkaev, V.G. Semenov and A.Yu. Sokolov, Phys. Status Solidi Al27 (1991) 455. 1131 R.N. Gall, S.M. Irkaev and L.P. Romankov, Nauchnaya Apparatura 2 (1987) 71. 1141 M.L. Alexandrov, G.N. Belozerski, Yu.V. Gladkov, O.V. Grishin, S.M. Irkaev, V.I. Nikolaev and V.G. Semenov, Preprint N46, Institute for Analytical Instrumentation, USSR Academy of Science, 1991. [15] M.A. Andreeva, G.N. Belozerskii, O.V. Grishin, S.M. Irkaev, V.I. Nikolaev and V.G. Semenov, Sov. Phys. JETP L&t. 55 (1992) 62.