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Resonant low energy electrons and their impact on sampling depth during backscatter-electron ms̈sbauer spectroscopy
Nuclear
Instruments
and Methods
in Physics Research
B31 (1988) 576-583
North-Holland,
RESONANT LOW ENERGY ELECTRONS AND THEIR IMPACT ON SAMPLING DURING BACKSCATTER-ELECTRON MijSSBAUER SPECTROSCOPY J.S. ZABINSKI and B.J. TATARCHUK Department Received
of Chemical Engineering, 11 August
Auburn
Amsterdam
DEPTH
*
University, Auburn, Alabama
1987 and in revised form 26 January
36849, USA
1988
Collection of low energy electrons ( < 15 eV) during Conversion Electron Mossbauer Spectroscopy (CEMS) provides enhanced surface sensitivity. To identify and quantify the potential surface sensitivity provided by these species, spectra were collected from a 92.8% enriched 57Fe foil using retarding field energy analyzers in conjunction with spiraltron electron multipliers. Both resonant and nonresonant count rates decrease by as much as 50% at 10 eV bias potential establishing that a large fraction of low energy electrons is produced during CEMS. Surface enhancement due to low energy electrons was identified by observing inherent signal-to-background ratios from samples
with the topmost 1.0 nm chemically labeled. The area ratio of the 1.0 nm overlayer to the substrate was 1.43 at 0 eV bias potential while at 15 eV bias potential the area ratio decreased to 0.72. By vacuum evaporating a 5.0 nm copper coating on the sample, near complete attenuation of the excess low energy electrons from the 1.0 nm overlayer was achieved. These results suggest that some low energy electrons below 15 eV are formed as primary products of electronic relaxation following nuclear decay and that they are not the result of straggling or other scattering phenomena. Low energy resonant electron signals appear useful for surface science applications by permitting information from the topmost monolayers to be readily distinguished from those signals arising from deeper within the solid.
1. Intmduction Backscatter-M8ssbauer
spectra
are
obtained
by col-
electrons and photons from deexcitation processes following nuclear resonant absorption. The penetration and the energy loss of resonant electrons is largely dependent on their depth of origin so that spectra collected at various energies provide information weighted over various depths. Since resonant electrons have inherently shallow escape depths (ca. 300 nm for 7.3 keV K-shell conversion electrons), backscatterelectron spectra can provide nondestructive depth profiles from the surface region of infinitely thick absorbers. However, photons (14.4 keV gamma rays or 6.4 keV X-rays) which have comparatively larger escape depths (ca. 20 pm), reveal information more characteristic of the bulk. To obtain the depth information available from backscattered-electron spectra, much attention has been devoted to theories which allow deconvolution of energy resolved Conversion Electron Mossbauer Spectra (CEMS) [l-8]. These theories describe the generation and attenuation of backscattered electrons from resonant nuclei located at various depths in a specimen. However, most depth-selective (DCEMS) theories are based on the fate of 7.3 keV K-shell conversion eleclecting
* Author
to whom all correspondence
should be addressed.
0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
trons because electrons in the 6.4-7.3 keV range do not significantly overlap electrons originating from the 14.3 and 13.6 keV M-shell and L-shell internal conversion events, respectively [9]. In the case of 7.3 keV DCEMS, electrons emerging with their full complement of energy most likely originate from the topmost 10 nm, whereas electrons which have experienced significant energy loss most likely arise from deeper within the specimen. Since theories are available to deconvolute energy resolved spectra from backscattered 7.3 keV K-shell conversion electrons, most DCEMS experiments are conducted by collecting electrons in the 6.4-7.3 keV range. To provide adequate energy discrimination during these experiments, high resolution low luminosity spectrometers are typically used [7,10], and as a result, long counting times are generally required. The technique has nevertheless been successfully used [11,12] to depth-profile layered samples with 5 nm resolution. The theoretical aspects of modeling CEMS are not trivial and no complete theory is available which considers all the processes involved. Specifically, most theoretical treatments do not consider electrons with energies less than ca. 200 eV. Results from this paper provide evidence that low energy electrons ( < 200 eV) are numerous and can be used to provide depth-resolved information from close to the surface. Concurrent with this experimental effort is the development of a more complete theoretical model
J.S. Zabinski, B. J. Tatarchuk / Resonant low energy electrons in CEMS
for CEMS capable of treating both resonant and nonresonant electrons to energies of 50 eV [13]. Few experiments have been performed in the past which specifically utilize low energy electrons below 200 eV. However, indications that these electrons are indeed important have previously appeared in. the literature [14-B]. Tyliszczak, Sawicki and coworkers [15,17] conducted experiments demonstrating that large numbers of low energy electrons are produced following nuclear resonant absorption and subsequently used this effect to enhance counting rates in their experiments [15,17,19-241. To our knowledge, no investigations have been conducted to determine the origin of low energy electrons below 50 eV or to determine what information they may contain. In this paper, the surface chemical information provided from low energy resonant electrons is investigated. The data collected suggest that a portion of the low energy electron spectrum below 50 eV appears to arise as a result of primary deexitation events following nuclear resonant absorption and not, most likely, as a result of straggling or other scattering phenomena. To detect these low energy electrons, appropriate design considerations were incorporated in the construction of
I
511
our spectrometer. The spectrometer allows the inherent signal-to-background ratio from Mijssbauer phenomena to be determined in a relatively short period of time.
2. Experimental 2. I. Overview The spectrometer (see fig. 1) was constructed so as to combine the techniques of both backscattered-electron and backscattered-photon MSssbauer spectroscopy. A major design consideration was minimization of CEMS counting times and energy discrimination at low energies. To ensure that background contributions from the stainless steel chamber were minimized, the solid angle subtended by the collimated source radiation was restricted to allow direct irradiation of only the sample. The solid angle for the detectors was also restricted to allow a direct view of only the sample, thereby providing additional minimization of background interference. Experiments conducted without a resonantly absorbing specimen verified the absence of stainless steel contributions to the Mossbauer spectra collected in the cham-
k38mmy
Fig. 1. Cross section and top view of combined-backscatter-electron and backscatter-photon Mbssbauer spectrometer. (1) Side uiew: A = sample transfer, B = electron detector, C = Doppler velocity drive, D = gamma ray collimator, E = photon detector, F = sample manipulator, G = sample, H = vibration isolation bellows, I = to cryo-pump. (2) Top uiew. (3) Magnification of sample: J = lead/brass/stainless, K = “Co/Pd source, L = 0.125 mm Be window.
578
J.S. Zabinski, B.J. Tatarchuk / Resonant low energy electrons in CEMS
I25mm
1
Fig. 2. Schematic of backscatter-electron detector with retarding grids. A = high voltage instrumentation feedthrough, B = detector support, C = electron shield, D = spiraltron suppoit, E = spiraltron electron multiplier ( < 1 cps dark count), F = nickel grids: grid # 1,2, 3,4, G = silver coated teflon electron shield, H = O-200 V, I = 800 V, J = 3000 V, K = 3200 V.
ber. All components were housed in an UHV analysis chamber operated from 1 X lo-* Torr to 1 X lo-” Torr while the sample was affixed to a specimen manipulator allowing motion in the x, y and z directions along with rotary motion around the z axis. The sample could be maintained at temperatures from 90-873 K. The vacuum system was also equipped with a microcapillary array gas doser and a mass spectrometer for regulating gas dosages as well as allowing TPD measurements. 2.2. Detectors Seven electron detectors were constructed using Galileo model 4219 spiraltron electron multipliers with 2.54 cm diameter entrance cones (see fig. 2). Spiraltrons were selected for their high efficiency in counting low energy electrons [25,26]. Each detector was housed in an aluminum casing to shield the spiraltron from stray electrons while allowing only those electrons with direct trajectories from the sample to be counted. A series of 4 Buckbee-Mears nickel grids spaced 2 mm apart (20 lines/in. at 95% transmission) were mounted in front of each detector for electron retardation and to ensure that the electric fields in the vicinity of the specimen and the detector did not interact with one another. The grids were mounted on insulators and the outside of the insulators were held at ground potential by an Epotek H21D silver filled solventless epoxy coating. Spiraltrons and grid assemblies were mounted on Kurt J. Lesker 11 pin instrumentation feedthroughs rated to 11 kV (model EFT-16). A stainless steel vacuum chamber was constructed with eight equivalent positions available around
the chamber centroid. The angular position of these ports was optimized to allow 8 detectors to approach as closely as possible to the sample. Seven of these ports were dedicated to electron detectors and one to a germanium-based GLP-16195/10-S EG&G Ortec photon detector. Each detector and its grids were powered by individual supplies to permit independent operation. The signals from each detector were converted from charge pulses to TTL pulses by K&M model 2136 pulse nmplifier discriminators (PADS). TTL pulses were stored in a TN-4000, 8 memory multichannel analyzer. To decrease counting times, the detectors were operated simultaneously at equivalent conditions and the 7 spectra added together. Spectra from isotopically enriched foils could be obtained in minutes while spectra from naturally enriched foils required less than one hour Table 1 Spectral areas of an enriched a-iron foil (92.8%) obtained simultaneouslyusing seven electron detectors Detector #
% mm/s
1 2 3 4 5 6 7 Average = 217.7 IJ = 9.0 I%= 4.2
224.1 207.9 214.9 221.9 229.6 221.8 204.0
J.S. Zabinski, B.J. Tatarchuk / Resonant low energy eiectrons in CEMS
using a 100 mCi s7Co/Fd source. C~ibmtio~ runs were made to ensure that signal-tu-ba~k~aund ratios were equivalent for all seven detectors and dictated by the ratio of r~son~~ to nomeson~t events oc0rrririg in the specimen rather than the levels of the gain and thresholds on the PADS. (See table 1.) Aithou~ detector analyzer resolution was not sp~i~call~ dete~~~~ measurements were performed which ~n~~~~ the detectors were operatin a distortion free environment. Spectra collected detectors operated with a 5 eV negative bias potential on the number 2 grid (see fig. 2) were equivalent to spectra obtained with all grids at ground and a 5 eV positive bias potential applied to the specimen. Since the results of either measurement were the same, indications are that grid biasing procedures did not det~men~~ distort the electric fiehis su~o~d~n~ the spiraltrons.
The untrue-vacuum chamber used for these studies had 24 ports to allow attac~ent of: electron and phuton detectors, a mass spectrometer, a gas doser, a Cryo-Ton 10 cryogenic vacuum pump, an eight inch QD. vibration isolation bellows, a sample m~pulator, a he~sphe~~ anaIyzer, sample vibration stabihzers, an ion gauge, viewports, and other utilities. An adto mount the Doppler velocity drive unit and i gamma rays ~0~~ a colhmafrom stainless steel, brass and lead. The collimator, which extended into the chamber, allowed ~~a rays to pass into the chamber trout a 0.125 mm be~l~urn window affixed appro~matel~ 5.7 cm from the sample (see fig. 1). Three other vacuum fibers with complemental surfer analysis techniques were available from the ~~ssbauer chamber via a high vacuum shut~ecraft (ca, 10e8 Torr). Additional ~pabi~ties included an electron beam evaporation ~h~ber with a quartz crystal thickness mo~tor, a S~~S/~SS/LEED/A~S system, and a chamber equipped for XPS studies.
A 2 mm thick uaturall~ enriched (2.17 iron foil (> 99.99% purity, Alfa Ventron~ was electropolished and cleaned by successive solvent washings in t~~~or~~a~e, acetone, me~ano~ and water. A 1.0 nm overlayer of 93.0% enriched 57Fe was vacuum evaporated on top of the foil substrate and the specimen subsequently exposed to air. As a result of this procedure, the surfa~ layer was tagged (as an oxide) and easily separated from the metallic iron spectrum arising from the bulk. For other sets of exper~ents, a 5.0 nm copper ov~rlayer was vacuum evaporated onto the above noted s~~rnen~ SIMS scans, which examined the 5tFe/56Fe
579
ratio, were used to verify that int~r~ffusion did not occur between the foil substrate and 93.0% 57Fe overlayer. XPS exa~ations were also used to verify that a ~nt~uaus copper overlayer was formed Duane the copper coating procedure.
3. ResulfS
The first spectrum shown in fig. 3 was acquired with the retardation grids in front of each detectar held at ground potential. The spectrum was deconvoluted using a least squares procedure by fitting the data to Loren~~ ~n~ba~ using the program MFLT f27J Zero velocity was defined as the centroid of a metallic iron sextuplet and positive veIocity was defined as tbe source approa~~ng the absorber. A ~mbinat~o~ of a metallic iron sextuplet and an Fe3+ quadrupole doublet provided a good fit to these data. The total resonant area for this sp~trum was 9.90 %~/s and the area ratio of the 1.0 nm FeC3 overlayer to the metallic substrate was 1.43. FoBowing the above noted spectrum collection, a series of bias potentials were applied to the retarding grids as shown in fig. 3. Several noteworthy changes were observed: (I) At 15 V negative bias (i.e., electron r~~lsio~~ count rates were reduced by M. 50% for: (i) the nonresonant bat ound ~ase~ne~, (ii) the 1.0 nm r~on~t overlayer of 93.0% enriched *?Fe (i.e, Fe3*) and (iii> the resonant metallic iron substrate.
f -8
I
f
VELOCITY
-I
I
I
II
(min/sec)
Fig. 3. Back~tter~-~nve~~on electron spectra recorded from 1.0 nm 0.93 57Fe/W?217 “Fe sample versus potential on electron retardation grids.
J.S. Zabinski, B.J. Tatarchuk / Resonant low energy electrons in CEMS
580 (2) The count
rate from the 1.0 nm overlayer of Fe3+ decreased with respect to bias potential at a rate significantly greater than the nonresonant background or the resonant signal from the metallic iron substrate. At 15 V repulsive bias, the ratio of the resonant signal from the 1.0 nm Fe3+ overlayer to the metallic iron substrate decreased to 0.72. (3) Application of attractive bias potentials to the grids provided initial increases in the count rate of ca. 30% reaching a plateau at ca. + 5 eV. Results one and three are consistent with those of Tyliszczak and Sawicki [15,17,24] who also observed sharp decreases in count rate with application of negative bias potentials as well as increases in count rate following application of small positive bias potentials. The best way to demonstrate and compare the influences of bias potential on count rate for the various signals recorded in this study is to normalize each of these signals by the count rate obtained at zero bias potential as shown in fig. 4. From these plots it can be seen that the contribution from the 1.0 nm Fe3+ overlayer decreases more rapidly than the other spectral components. This result gives evidence for the fact that some low energy electrons (O-15 eV) are indeed generated in the surface layer as primary products of nuclear ana subsequent electronic relaxation. The low energy resonant electrons shown to be important in fig. 4 cannot be generated as a result of resonant X-ray photoelectrons (XPE) or resonant gamma ray photoelectrons (GPE) as suggested by Tricker [28] and subsequently refuted by Deeney and McCarthy [29] and Tatarchuk [16]. Indeed, resonant electron signals generated by either of these two processes are expected to be extremely small based on the thin
0.01
0.0
I,,
,
10.0
20.0 BIAS
I
I
I
I
I
I
50.0 30.0 40.0 POTENTIAL (-volts)
I
60.0
I
70.0
I
Fig. 4. Normalized signal intensity versus bias potential for the uncoated specimen A = Resonant signal from iron substrate (0.0217 “Fe) B = Nonresonant background signal from substrate and overlayer. C = Resonant signal from 1.0 nm overlayer of 0.93 “Fe. D = Resonant signal from a 1 pm thick 0.928 “Fe specimen.
0.01 0.0
I
,
10.0
,
,
20.0 BIAS
,
,
,
,
,
30.0 50.0 40.0 POTENTlAL (-volts)
,
60.0
,
,
70.0
Fig. 5. Normalized signal intensity versus bias potential for a copper coated specimen. A = Resonant signal from iron substrate (0.0217 57Fe). B = Nonresonant background signal from substrate and overlayer. C = Resonant signal from 1.0 nm layer of 0.93 57Fe.
nature of the Fe3+ overlayer while calculations [16,18,29] suggest that such signals comprise < 5% of the resonant signal from any spectral component. It also appears that straggling and other forms of multiple scattering which produce secondary electrons cannot explain the low energy resonant electrons observed from the Fe3+ surface layer. These effects would be expected to most enhance the resonant spectral features associated with the “thick” resonant substrate relative to the “thin” resonant overlayer. While the above noted considerations indicate that the unusually high levels of low energy electrons produced from thin surface layers cannot be totally assigned to straggling-type mechanisms, figs. 4 and 5 nevertheless show that all the signals examined do possess significant levels of low energy electrons which must arise from multiple scattering events originating from electrons at higher energies. It is the excess proportion of low energy resonant electrons from the Fe3+ overlayer which is the topic of this study since it arises from that part of the specimen least likely to produce scattered electron signals. The origin of the excess proportion of low energy electrons can be further verified by considering electron escape depth versus electron energy. Using a Bethe-Block analysis or a more complicated theory such as the generalized oscillator strength density, it is observed that the escape depth of electrons decreases with decreasing electron energy. However, so called “universal curves” of electron energy versus inelastic mean free path generally possess minima in the value of the inelastic mean free path at energies between 20 and 100 eV [30]. In either case, the deposition of additional overlayers on a resonant surface should result in the attenuation of low energy electrons generated at the
J.S. Zabinski, B.J. Tatarchuk / Resonant low energy electrons in CEMS resonant surface. This, in fact, may provide one means for dete~~g the origin of these species and is considered in the fo~o~ng section.
The specimen was transferred from the Mijssbauer chamber to the evaporation chamber via a high vacuum shuttlecraft and a 5.0 nm copper overiayer was vacuum evaporated onto the 1.0 mu 93.0% enriched S7Fe specimen. A series of Mbssbauer spectra were then recorded at various bias potentials in a fashion identical to that reported above. As can be seen in fig. 5, the intensity of the spectral contribution from the 93.0% enriched s7Fe3f layer behaved in a manner markedly different than that observed earlier in fig. 4. In the previous case, with no copper overlayer present, the resonant count rate from the Fe3’ decreased at a rate faster than all other signals as the bias potential was increased to 15 eV, however, in the case of the copper-coated specimen, fig. 5 demonstrates that all the signals behave in a fairly similar fashion. Therefore, it appears that the copper overlayer is effective in preferentially absorbing low energy resonant electrons from the Fe3’ overlayer as compared to the naturally enriched substrate. Comparison of the data in figs. 4 and 5 has been shown in fig. 6 where the normalized resonant intensity ratio from the metallic iron substrate (i.e. [1(eV)/1(0 eV)]r,o) has been divided by the no~~zed resonant intensity ratio from the Fe3+ overlayer (i.e., { I(eV)/I(O eV)]F,+s) and plotted as a function of bias potential, I? Comparison of this quotient as a function of bias potential provides a stationary value of ca. I.0 in the case of the copper coated specimen indicating that the normalized energy distributions from both the resonant natural iron substrate and the 1.0 nm 93.0% enriched 57Fe3+
Fig, 6. Ratio of normalized signal intensities versus bias potential A = Copper coated specimen. 3 = Uncoated specimen. Note: For details on no~~~tion procedures see discussions pertinent to figs. 4 and 5.
581
overlayer are nearly identical (see fig. 6). However, in the case of the uncoated specimen, a significant increase in this ratio ~lustrates that the iron substrate contains a si~fic~tly greater fraction of higher energy electrons This latter result once again demonstrates that the Fe3+ signal from the uncoated surface contains a larger fraction of resonant low energy electrons which do not appear to be the result of straggling or other scattering phenomena. 4. Discussion The results reported above can be rationalized in terms of electron escape depths. From the universal curve, it is observed that electrons between 10 and 1000 eV have inelastic mew-fre~paths on the order of 1.5 nm. The 5.0 nm copper overlayer may therefore attenuate a large fraction of resonant low energy electrons generated in the surface of the Fe3’ overlayer, but do little to enhance or attenuate low energy electrons which result from other scattering mechanisms (viz., stragglers and secondaries) which have their origin tied to internal conversion events located deeper within the specimen. Analytical expressions, such as the general oscillator strength density equations, indicate that the escape depth continually decreases as the primary electron energy decreases. ConverseIy, so called ‘“universal curves” indicate that as the primary electron energy decreases below ca. 15 eV the inelastic mean free path begins to increase exponenti~ly approaching values greater than 10.0 nm for electrons with s< 2 eV energy. A proposed explanation for the increase is that few mechanisms are available to attenuate these electrons, therefore, they have relatively large inelastic mean free paths 1301. Evidence provided in the context of this study, however, does not support this argument since the low energy electrons discussed above originate in the topmost 1.0 nm and are attenuated by a 5.0 mn overlayer. As a consequence of their surface sensitivity, low energy resonant electron spectroscopy offers complementary advantages to high energy resonant electron spectroscopy. Most depth-sel~tive CEMS experiments performed to date have involved the detection of relatively high energy electrons in the vicinity of the 7.3 keV K-shell conversion electron [7,9,10]. These electrons are usually detected by some type of energy or momentum dispersive device such as an electrostatic analyzer, a beta-ray magnetic spectrometer or an energy resolving gas-filled proportional counter [18,31-331. In cases where 7.3 keV conversion electrons are collected, the topmost layers of the specimen cannot be depth-profiled to an appreciable extent because the inelastic mean-free-pan of these electrons is on the order of 10 nm. However, as indicated in this study, low energy resonant electrons ( < 15 eV) are available which con-
582
J.S. Zabiaski, 3.J. Tatu~c~~
/ Remnant low energy electrons in CEMS
tain information from the surface layer to a depth of 5.0 nm. Other workers have shown that the LMM Auger electron at ca. 600 eV offers improved surface sensitivity due to its smaller inelastic mean free path [31,34J but not likely as great an improvement as electrons with energies < 15 eV. The significant advantages offered by resonant electrons below 15 eV inchtde: (i) they have relatively short inelastic mean free paths and shallow escape depths permitting dep~“~rofil~ng in the topmost 5.0 nm or less and (ii) they account for more than 50% of the integral GEMS signal. Two additional observations of import~ce to CEMS have been confirmed [13-15,171. First, both resonant and n~~esonant
count rates could be increased
by ca.
3~~ by applying a 10 eV negative potential to the sample. And second, count rates could be decreased in a manner similar to biasing with the detector grids by applying a 10 eV positive potential to the specimen, While these effects appear to mimic the action of the detector grids, distortions of the electric field within the analysis chamber can be introduced whenever surfaces within the chamber are held at different potentials. For this reason, these effects have not been explored in greater detail within the context of this study, although, s~g~fi~t increases in count rate can be obtained if this is desired.
Some low energy resonant electrons below 15 eV are generated as primary products of deexcitation following nuclear resonant absorption. They have shahow escape depths and appear to be detectable only if they originate somewhere within the topmost 5.0 nm of the surface. The impact of this discovery is that low energy electrons can be readily used to obtain dep~*selective information from within the topmost 5.0 nm of the surface. For low energy r~o~~t electron sp~tros~~y to become a working tool, further development of approp~ate theoretical models for low energy electron generation and transport through a solid are required. Development of such theories could be used to increase the near surface resolution of DCEMS and extend the effective range of DCEMS when used in conjunction with depth-profiling at higher electron and photon energies. Appropriate theories for the above noted effects are currently under development in this laboratory (351.
Support for this work is tefully ac~owl~ged from the Air Force Office of Scientific Research (AF~SR-8~~3~1).
References Zw. 3onchev, A. Jardanov, and A. Minkova, Nucl. Instr. and Metb. 70 (1969) 36. U. Baverstam C. Bohm, T. EkdahI, D. ~~j~~st, and 8. Ring&m, MSssbauer Effect Methodology, vol. 9 (Plenum, New York, 1974) p. 259. R.A. Krakowski and R.B. Miher, Nud. Iustr. and Meth. 100 (1972) 93. J. Bainbridge, Nucl. In&r. and Meth. 178 (1975) 531. G.P. Huffman, Nucl. Instr. and Meth. 137 (1976) 267. D. Liljequist, Nucl. Instr. and Meth. 179 (1981) 617. D. Liljequist, Scanning Electron Microscopy III (1983) 997. F. Salvat, and J. Parellada, Nucl. Instr. and Meth. Bl (I984~ 70. D. Liljequist~ T. Ekdahl, and B. Baverst~m, Nucl. Instr. and Meth. 255 (1978) 529. J.A. Saw&i, and B.D. Sawicka, Hyperfine Interactions 13 (1983) 199. U. Baverst’jim, T. Ekdahl, CH. Bohm, B. Ringstrijm, V. Stefansson and D. Liljequist, Nucl. Instr. and M&h. 115 (1974) 373, T. S~g~rna~u* H.-D. Pfannes, and W. Keune, Phya Rev. Lett. 45 (1980) 1206. T.S. Lee, T.D. Placek, J.A. Dumesic, and B.J. Tatarchuk, Nucl. Instr. and Meth. B18 (1987) 182. W. Jones, JM. Thomas, R.K. Thorpe, and M.J. Tricker, Appl. Surf. Sci. 1 (1978) 388. T. Tyliszczak, J.A. Sawicki, J. Stauek, and B.D. Sawicka, J. Pbys. Colloq. (Paris) 41 (1980) Cl-117. B.J. Tatarchuk, Ph.D. Thesis, University of Wisconsin (1981). J.A. Saw&i, T. Tyliszczak, Nucl. Instr. and Meth. 216 (1983) 501. B.J. Tatarchuk and J.A. Dumesic, Chemistry and Physics of Solid Surfaces, vol. 5 (Springer, Berlin, 1984) ch. 4. LA. Sawicki, T. Tyliszczak, and 0. Gzowski, Nucl. Instr. and Meth. 190 (1981) 433. T. ‘Tyhszczak~J.A. Sawicki, and W. Wilk, Hyperfine Interactions 15/16 (1983) 1001. J.A. Sawicki, and B.D. Sawicka, Hyperfine Int~actions 15/16 (1983) 1013. J.A. Saw&i, T. Typed, J. Stanek, B.D. Sawicka, and J. Kowalski, Nucl. Instr. and Meth. 215 (1983) 567. H. Binczycka, B. Miczko, J.A. Sawicki, J. Stauek, M. Drwiega, and T. Tyliszczak, Mater. Sci. and Engin. 69 (1985) 519. J.A. Sawicki, Nucl. Instr. and Meth. Bl6 (1986) 483. R. Oswald and M. Ohring, J. Vat. Sci. Teclmol. 13 (1976) 4Q. E.A, Kutz, American Laboratory 3 (1979). K. Sorensen, LTE. II. IntemaI Report No. 1 (1972), Laboratory of Applied Physics of Denmark II, Technical U~versity of Denmark, Lyugby. M.J. Tricker, L.A. Ash, and T.E. Cr~s~aw, Nucl. Instr. and Meth. 143 (1977) 307. F.A. Deeney and P.J. McCarthy, Nucl. In&. and Meth. 166 (1979~ 491. M. ~ornp~n, M.D. Baker, A. Christie, J.F. Tyson, Auger Electron Spectroscopy, vol. 74 (Wiley, New York, 1985).
J.S. Zabinski, B.J. Tatarchuk / Resonant few energy electrons in &EMS 1311 M. Domke, B. Kyvelos, and C. Kaindl, Hype&e
Interactions 10 (1981) 1137. [32] H. Nakagawa, Y. Ujihira, and M. Inaba, Nucl. Instr. and Meth. 196 (1982) 573. [33] Y. Yonekura, T. Toriyama, J. Itoh and K. Hisatake, Hype&e Interactions E/16 (1983) 1005.
583
1341 S. Staniek, T. S~gematsu, W. Keune, and H.-D. Pfannes, J. Magn. and Magnet. Mater. 35 (1983) 347. [35] T.S. Lee, J.S. Zabinski, and B.J. Tatarchnk, Nucl. Instr. and Meth. B, in press.
Instruments
and Methods
in Physics Research
B31 (1988) 576-583
North-Holland,
RESONANT LOW ENERGY ELECTRONS AND THEIR IMPACT ON SAMPLING DURING BACKSCATTER-ELECTRON MijSSBAUER SPECTROSCOPY J.S. ZABINSKI and B.J. TATARCHUK Department Received
of Chemical Engineering, 11 August
Auburn
Amsterdam
DEPTH
*
University, Auburn, Alabama
1987 and in revised form 26 January
36849, USA
1988
Collection of low energy electrons ( < 15 eV) during Conversion Electron Mossbauer Spectroscopy (CEMS) provides enhanced surface sensitivity. To identify and quantify the potential surface sensitivity provided by these species, spectra were collected from a 92.8% enriched 57Fe foil using retarding field energy analyzers in conjunction with spiraltron electron multipliers. Both resonant and nonresonant count rates decrease by as much as 50% at 10 eV bias potential establishing that a large fraction of low energy electrons is produced during CEMS. Surface enhancement due to low energy electrons was identified by observing inherent signal-to-background ratios from samples
with the topmost 1.0 nm chemically labeled. The area ratio of the 1.0 nm overlayer to the substrate was 1.43 at 0 eV bias potential while at 15 eV bias potential the area ratio decreased to 0.72. By vacuum evaporating a 5.0 nm copper coating on the sample, near complete attenuation of the excess low energy electrons from the 1.0 nm overlayer was achieved. These results suggest that some low energy electrons below 15 eV are formed as primary products of electronic relaxation following nuclear decay and that they are not the result of straggling or other scattering phenomena. Low energy resonant electron signals appear useful for surface science applications by permitting information from the topmost monolayers to be readily distinguished from those signals arising from deeper within the solid.
1. Intmduction Backscatter-M8ssbauer
spectra
are
obtained
by col-
electrons and photons from deexcitation processes following nuclear resonant absorption. The penetration and the energy loss of resonant electrons is largely dependent on their depth of origin so that spectra collected at various energies provide information weighted over various depths. Since resonant electrons have inherently shallow escape depths (ca. 300 nm for 7.3 keV K-shell conversion electrons), backscatterelectron spectra can provide nondestructive depth profiles from the surface region of infinitely thick absorbers. However, photons (14.4 keV gamma rays or 6.4 keV X-rays) which have comparatively larger escape depths (ca. 20 pm), reveal information more characteristic of the bulk. To obtain the depth information available from backscattered-electron spectra, much attention has been devoted to theories which allow deconvolution of energy resolved Conversion Electron Mossbauer Spectra (CEMS) [l-8]. These theories describe the generation and attenuation of backscattered electrons from resonant nuclei located at various depths in a specimen. However, most depth-selective (DCEMS) theories are based on the fate of 7.3 keV K-shell conversion eleclecting
* Author
to whom all correspondence
should be addressed.
0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
trons because electrons in the 6.4-7.3 keV range do not significantly overlap electrons originating from the 14.3 and 13.6 keV M-shell and L-shell internal conversion events, respectively [9]. In the case of 7.3 keV DCEMS, electrons emerging with their full complement of energy most likely originate from the topmost 10 nm, whereas electrons which have experienced significant energy loss most likely arise from deeper within the specimen. Since theories are available to deconvolute energy resolved spectra from backscattered 7.3 keV K-shell conversion electrons, most DCEMS experiments are conducted by collecting electrons in the 6.4-7.3 keV range. To provide adequate energy discrimination during these experiments, high resolution low luminosity spectrometers are typically used [7,10], and as a result, long counting times are generally required. The technique has nevertheless been successfully used [11,12] to depth-profile layered samples with 5 nm resolution. The theoretical aspects of modeling CEMS are not trivial and no complete theory is available which considers all the processes involved. Specifically, most theoretical treatments do not consider electrons with energies less than ca. 200 eV. Results from this paper provide evidence that low energy electrons ( < 200 eV) are numerous and can be used to provide depth-resolved information from close to the surface. Concurrent with this experimental effort is the development of a more complete theoretical model
J.S. Zabinski, B. J. Tatarchuk / Resonant low energy electrons in CEMS
for CEMS capable of treating both resonant and nonresonant electrons to energies of 50 eV [13]. Few experiments have been performed in the past which specifically utilize low energy electrons below 200 eV. However, indications that these electrons are indeed important have previously appeared in. the literature [14-B]. Tyliszczak, Sawicki and coworkers [15,17] conducted experiments demonstrating that large numbers of low energy electrons are produced following nuclear resonant absorption and subsequently used this effect to enhance counting rates in their experiments [15,17,19-241. To our knowledge, no investigations have been conducted to determine the origin of low energy electrons below 50 eV or to determine what information they may contain. In this paper, the surface chemical information provided from low energy resonant electrons is investigated. The data collected suggest that a portion of the low energy electron spectrum below 50 eV appears to arise as a result of primary deexitation events following nuclear resonant absorption and not, most likely, as a result of straggling or other scattering phenomena. To detect these low energy electrons, appropriate design considerations were incorporated in the construction of
I
511
our spectrometer. The spectrometer allows the inherent signal-to-background ratio from Mijssbauer phenomena to be determined in a relatively short period of time.
2. Experimental 2. I. Overview The spectrometer (see fig. 1) was constructed so as to combine the techniques of both backscattered-electron and backscattered-photon MSssbauer spectroscopy. A major design consideration was minimization of CEMS counting times and energy discrimination at low energies. To ensure that background contributions from the stainless steel chamber were minimized, the solid angle subtended by the collimated source radiation was restricted to allow direct irradiation of only the sample. The solid angle for the detectors was also restricted to allow a direct view of only the sample, thereby providing additional minimization of background interference. Experiments conducted without a resonantly absorbing specimen verified the absence of stainless steel contributions to the Mossbauer spectra collected in the cham-
k38mmy
Fig. 1. Cross section and top view of combined-backscatter-electron and backscatter-photon Mbssbauer spectrometer. (1) Side uiew: A = sample transfer, B = electron detector, C = Doppler velocity drive, D = gamma ray collimator, E = photon detector, F = sample manipulator, G = sample, H = vibration isolation bellows, I = to cryo-pump. (2) Top uiew. (3) Magnification of sample: J = lead/brass/stainless, K = “Co/Pd source, L = 0.125 mm Be window.
578
J.S. Zabinski, B.J. Tatarchuk / Resonant low energy electrons in CEMS
I25mm
1
Fig. 2. Schematic of backscatter-electron detector with retarding grids. A = high voltage instrumentation feedthrough, B = detector support, C = electron shield, D = spiraltron suppoit, E = spiraltron electron multiplier ( < 1 cps dark count), F = nickel grids: grid # 1,2, 3,4, G = silver coated teflon electron shield, H = O-200 V, I = 800 V, J = 3000 V, K = 3200 V.
ber. All components were housed in an UHV analysis chamber operated from 1 X lo-* Torr to 1 X lo-” Torr while the sample was affixed to a specimen manipulator allowing motion in the x, y and z directions along with rotary motion around the z axis. The sample could be maintained at temperatures from 90-873 K. The vacuum system was also equipped with a microcapillary array gas doser and a mass spectrometer for regulating gas dosages as well as allowing TPD measurements. 2.2. Detectors Seven electron detectors were constructed using Galileo model 4219 spiraltron electron multipliers with 2.54 cm diameter entrance cones (see fig. 2). Spiraltrons were selected for their high efficiency in counting low energy electrons [25,26]. Each detector was housed in an aluminum casing to shield the spiraltron from stray electrons while allowing only those electrons with direct trajectories from the sample to be counted. A series of 4 Buckbee-Mears nickel grids spaced 2 mm apart (20 lines/in. at 95% transmission) were mounted in front of each detector for electron retardation and to ensure that the electric fields in the vicinity of the specimen and the detector did not interact with one another. The grids were mounted on insulators and the outside of the insulators were held at ground potential by an Epotek H21D silver filled solventless epoxy coating. Spiraltrons and grid assemblies were mounted on Kurt J. Lesker 11 pin instrumentation feedthroughs rated to 11 kV (model EFT-16). A stainless steel vacuum chamber was constructed with eight equivalent positions available around
the chamber centroid. The angular position of these ports was optimized to allow 8 detectors to approach as closely as possible to the sample. Seven of these ports were dedicated to electron detectors and one to a germanium-based GLP-16195/10-S EG&G Ortec photon detector. Each detector and its grids were powered by individual supplies to permit independent operation. The signals from each detector were converted from charge pulses to TTL pulses by K&M model 2136 pulse nmplifier discriminators (PADS). TTL pulses were stored in a TN-4000, 8 memory multichannel analyzer. To decrease counting times, the detectors were operated simultaneously at equivalent conditions and the 7 spectra added together. Spectra from isotopically enriched foils could be obtained in minutes while spectra from naturally enriched foils required less than one hour Table 1 Spectral areas of an enriched a-iron foil (92.8%) obtained simultaneouslyusing seven electron detectors Detector #
% mm/s
1 2 3 4 5 6 7 Average = 217.7 IJ = 9.0 I%= 4.2
224.1 207.9 214.9 221.9 229.6 221.8 204.0
J.S. Zabinski, B.J. Tatarchuk / Resonant low energy eiectrons in CEMS
using a 100 mCi s7Co/Fd source. C~ibmtio~ runs were made to ensure that signal-tu-ba~k~aund ratios were equivalent for all seven detectors and dictated by the ratio of r~son~~ to nomeson~t events oc0rrririg in the specimen rather than the levels of the gain and thresholds on the PADS. (See table 1.) Aithou~ detector analyzer resolution was not sp~i~call~ dete~~~~ measurements were performed which ~n~~~~ the detectors were operatin a distortion free environment. Spectra collected detectors operated with a 5 eV negative bias potential on the number 2 grid (see fig. 2) were equivalent to spectra obtained with all grids at ground and a 5 eV positive bias potential applied to the specimen. Since the results of either measurement were the same, indications are that grid biasing procedures did not det~men~~ distort the electric fiehis su~o~d~n~ the spiraltrons.
The untrue-vacuum chamber used for these studies had 24 ports to allow attac~ent of: electron and phuton detectors, a mass spectrometer, a gas doser, a Cryo-Ton 10 cryogenic vacuum pump, an eight inch QD. vibration isolation bellows, a sample m~pulator, a he~sphe~~ anaIyzer, sample vibration stabihzers, an ion gauge, viewports, and other utilities. An adto mount the Doppler velocity drive unit and i gamma rays ~0~~ a colhmafrom stainless steel, brass and lead. The collimator, which extended into the chamber, allowed ~~a rays to pass into the chamber trout a 0.125 mm be~l~urn window affixed appro~matel~ 5.7 cm from the sample (see fig. 1). Three other vacuum fibers with complemental surfer analysis techniques were available from the ~~ssbauer chamber via a high vacuum shut~ecraft (ca, 10e8 Torr). Additional ~pabi~ties included an electron beam evaporation ~h~ber with a quartz crystal thickness mo~tor, a S~~S/~SS/LEED/A~S system, and a chamber equipped for XPS studies.
A 2 mm thick uaturall~ enriched (2.17 iron foil (> 99.99% purity, Alfa Ventron~ was electropolished and cleaned by successive solvent washings in t~~~or~~a~e, acetone, me~ano~ and water. A 1.0 nm overlayer of 93.0% enriched 57Fe was vacuum evaporated on top of the foil substrate and the specimen subsequently exposed to air. As a result of this procedure, the surfa~ layer was tagged (as an oxide) and easily separated from the metallic iron spectrum arising from the bulk. For other sets of exper~ents, a 5.0 nm copper ov~rlayer was vacuum evaporated onto the above noted s~~rnen~ SIMS scans, which examined the 5tFe/56Fe
579
ratio, were used to verify that int~r~ffusion did not occur between the foil substrate and 93.0% 57Fe overlayer. XPS exa~ations were also used to verify that a ~nt~uaus copper overlayer was formed Duane the copper coating procedure.
3. ResulfS
The first spectrum shown in fig. 3 was acquired with the retardation grids in front of each detectar held at ground potential. The spectrum was deconvoluted using a least squares procedure by fitting the data to Loren~~ ~n~ba~ using the program MFLT f27J Zero velocity was defined as the centroid of a metallic iron sextuplet and positive veIocity was defined as tbe source approa~~ng the absorber. A ~mbinat~o~ of a metallic iron sextuplet and an Fe3+ quadrupole doublet provided a good fit to these data. The total resonant area for this sp~trum was 9.90 %~/s and the area ratio of the 1.0 nm FeC3 overlayer to the metallic substrate was 1.43. FoBowing the above noted spectrum collection, a series of bias potentials were applied to the retarding grids as shown in fig. 3. Several noteworthy changes were observed: (I) At 15 V negative bias (i.e., electron r~~lsio~~ count rates were reduced by M. 50% for: (i) the nonresonant bat ound ~ase~ne~, (ii) the 1.0 nm r~on~t overlayer of 93.0% enriched *?Fe (i.e, Fe3*) and (iii> the resonant metallic iron substrate.
f -8
I
f
VELOCITY
-I
I
I
II
(min/sec)
Fig. 3. Back~tter~-~nve~~on electron spectra recorded from 1.0 nm 0.93 57Fe/W?217 “Fe sample versus potential on electron retardation grids.
J.S. Zabinski, B.J. Tatarchuk / Resonant low energy electrons in CEMS
580 (2) The count
rate from the 1.0 nm overlayer of Fe3+ decreased with respect to bias potential at a rate significantly greater than the nonresonant background or the resonant signal from the metallic iron substrate. At 15 V repulsive bias, the ratio of the resonant signal from the 1.0 nm Fe3+ overlayer to the metallic iron substrate decreased to 0.72. (3) Application of attractive bias potentials to the grids provided initial increases in the count rate of ca. 30% reaching a plateau at ca. + 5 eV. Results one and three are consistent with those of Tyliszczak and Sawicki [15,17,24] who also observed sharp decreases in count rate with application of negative bias potentials as well as increases in count rate following application of small positive bias potentials. The best way to demonstrate and compare the influences of bias potential on count rate for the various signals recorded in this study is to normalize each of these signals by the count rate obtained at zero bias potential as shown in fig. 4. From these plots it can be seen that the contribution from the 1.0 nm Fe3+ overlayer decreases more rapidly than the other spectral components. This result gives evidence for the fact that some low energy electrons (O-15 eV) are indeed generated in the surface layer as primary products of nuclear ana subsequent electronic relaxation. The low energy resonant electrons shown to be important in fig. 4 cannot be generated as a result of resonant X-ray photoelectrons (XPE) or resonant gamma ray photoelectrons (GPE) as suggested by Tricker [28] and subsequently refuted by Deeney and McCarthy [29] and Tatarchuk [16]. Indeed, resonant electron signals generated by either of these two processes are expected to be extremely small based on the thin
0.01
0.0
I,,
,
10.0
20.0 BIAS
I
I
I
I
I
I
50.0 30.0 40.0 POTENTIAL (-volts)
I
60.0
I
70.0
I
Fig. 4. Normalized signal intensity versus bias potential for the uncoated specimen A = Resonant signal from iron substrate (0.0217 “Fe) B = Nonresonant background signal from substrate and overlayer. C = Resonant signal from 1.0 nm overlayer of 0.93 “Fe. D = Resonant signal from a 1 pm thick 0.928 “Fe specimen.
0.01 0.0
I
,
10.0
,
,
20.0 BIAS
,
,
,
,
,
30.0 50.0 40.0 POTENTlAL (-volts)
,
60.0
,
,
70.0
Fig. 5. Normalized signal intensity versus bias potential for a copper coated specimen. A = Resonant signal from iron substrate (0.0217 57Fe). B = Nonresonant background signal from substrate and overlayer. C = Resonant signal from 1.0 nm layer of 0.93 57Fe.
nature of the Fe3+ overlayer while calculations [16,18,29] suggest that such signals comprise < 5% of the resonant signal from any spectral component. It also appears that straggling and other forms of multiple scattering which produce secondary electrons cannot explain the low energy resonant electrons observed from the Fe3+ surface layer. These effects would be expected to most enhance the resonant spectral features associated with the “thick” resonant substrate relative to the “thin” resonant overlayer. While the above noted considerations indicate that the unusually high levels of low energy electrons produced from thin surface layers cannot be totally assigned to straggling-type mechanisms, figs. 4 and 5 nevertheless show that all the signals examined do possess significant levels of low energy electrons which must arise from multiple scattering events originating from electrons at higher energies. It is the excess proportion of low energy resonant electrons from the Fe3+ overlayer which is the topic of this study since it arises from that part of the specimen least likely to produce scattered electron signals. The origin of the excess proportion of low energy electrons can be further verified by considering electron escape depth versus electron energy. Using a Bethe-Block analysis or a more complicated theory such as the generalized oscillator strength density, it is observed that the escape depth of electrons decreases with decreasing electron energy. However, so called “universal curves” of electron energy versus inelastic mean free path generally possess minima in the value of the inelastic mean free path at energies between 20 and 100 eV [30]. In either case, the deposition of additional overlayers on a resonant surface should result in the attenuation of low energy electrons generated at the
J.S. Zabinski, B.J. Tatarchuk / Resonant low energy electrons in CEMS resonant surface. This, in fact, may provide one means for dete~~g the origin of these species and is considered in the fo~o~ng section.
The specimen was transferred from the Mijssbauer chamber to the evaporation chamber via a high vacuum shuttlecraft and a 5.0 nm copper overiayer was vacuum evaporated onto the 1.0 mu 93.0% enriched S7Fe specimen. A series of Mbssbauer spectra were then recorded at various bias potentials in a fashion identical to that reported above. As can be seen in fig. 5, the intensity of the spectral contribution from the 93.0% enriched s7Fe3f layer behaved in a manner markedly different than that observed earlier in fig. 4. In the previous case, with no copper overlayer present, the resonant count rate from the Fe3’ decreased at a rate faster than all other signals as the bias potential was increased to 15 eV, however, in the case of the copper-coated specimen, fig. 5 demonstrates that all the signals behave in a fairly similar fashion. Therefore, it appears that the copper overlayer is effective in preferentially absorbing low energy resonant electrons from the Fe3’ overlayer as compared to the naturally enriched substrate. Comparison of the data in figs. 4 and 5 has been shown in fig. 6 where the normalized resonant intensity ratio from the metallic iron substrate (i.e. [1(eV)/1(0 eV)]r,o) has been divided by the no~~zed resonant intensity ratio from the Fe3+ overlayer (i.e., { I(eV)/I(O eV)]F,+s) and plotted as a function of bias potential, I? Comparison of this quotient as a function of bias potential provides a stationary value of ca. I.0 in the case of the copper coated specimen indicating that the normalized energy distributions from both the resonant natural iron substrate and the 1.0 nm 93.0% enriched 57Fe3+
Fig, 6. Ratio of normalized signal intensities versus bias potential A = Copper coated specimen. 3 = Uncoated specimen. Note: For details on no~~~tion procedures see discussions pertinent to figs. 4 and 5.
581
overlayer are nearly identical (see fig. 6). However, in the case of the uncoated specimen, a significant increase in this ratio ~lustrates that the iron substrate contains a si~fic~tly greater fraction of higher energy electrons This latter result once again demonstrates that the Fe3+ signal from the uncoated surface contains a larger fraction of resonant low energy electrons which do not appear to be the result of straggling or other scattering phenomena. 4. Discussion The results reported above can be rationalized in terms of electron escape depths. From the universal curve, it is observed that electrons between 10 and 1000 eV have inelastic mew-fre~paths on the order of 1.5 nm. The 5.0 nm copper overlayer may therefore attenuate a large fraction of resonant low energy electrons generated in the surface of the Fe3’ overlayer, but do little to enhance or attenuate low energy electrons which result from other scattering mechanisms (viz., stragglers and secondaries) which have their origin tied to internal conversion events located deeper within the specimen. Analytical expressions, such as the general oscillator strength density equations, indicate that the escape depth continually decreases as the primary electron energy decreases. ConverseIy, so called ‘“universal curves” indicate that as the primary electron energy decreases below ca. 15 eV the inelastic mean free path begins to increase exponenti~ly approaching values greater than 10.0 nm for electrons with s< 2 eV energy. A proposed explanation for the increase is that few mechanisms are available to attenuate these electrons, therefore, they have relatively large inelastic mean free paths 1301. Evidence provided in the context of this study, however, does not support this argument since the low energy electrons discussed above originate in the topmost 1.0 nm and are attenuated by a 5.0 mn overlayer. As a consequence of their surface sensitivity, low energy resonant electron spectroscopy offers complementary advantages to high energy resonant electron spectroscopy. Most depth-sel~tive CEMS experiments performed to date have involved the detection of relatively high energy electrons in the vicinity of the 7.3 keV K-shell conversion electron [7,9,10]. These electrons are usually detected by some type of energy or momentum dispersive device such as an electrostatic analyzer, a beta-ray magnetic spectrometer or an energy resolving gas-filled proportional counter [18,31-331. In cases where 7.3 keV conversion electrons are collected, the topmost layers of the specimen cannot be depth-profiled to an appreciable extent because the inelastic mean-free-pan of these electrons is on the order of 10 nm. However, as indicated in this study, low energy resonant electrons ( < 15 eV) are available which con-
582
J.S. Zabiaski, 3.J. Tatu~c~~
/ Remnant low energy electrons in CEMS
tain information from the surface layer to a depth of 5.0 nm. Other workers have shown that the LMM Auger electron at ca. 600 eV offers improved surface sensitivity due to its smaller inelastic mean free path [31,34J but not likely as great an improvement as electrons with energies < 15 eV. The significant advantages offered by resonant electrons below 15 eV inchtde: (i) they have relatively short inelastic mean free paths and shallow escape depths permitting dep~“~rofil~ng in the topmost 5.0 nm or less and (ii) they account for more than 50% of the integral GEMS signal. Two additional observations of import~ce to CEMS have been confirmed [13-15,171. First, both resonant and n~~esonant
count rates could be increased
by ca.
3~~ by applying a 10 eV negative potential to the sample. And second, count rates could be decreased in a manner similar to biasing with the detector grids by applying a 10 eV positive potential to the specimen, While these effects appear to mimic the action of the detector grids, distortions of the electric field within the analysis chamber can be introduced whenever surfaces within the chamber are held at different potentials. For this reason, these effects have not been explored in greater detail within the context of this study, although, s~g~fi~t increases in count rate can be obtained if this is desired.
Some low energy resonant electrons below 15 eV are generated as primary products of deexcitation following nuclear resonant absorption. They have shahow escape depths and appear to be detectable only if they originate somewhere within the topmost 5.0 nm of the surface. The impact of this discovery is that low energy electrons can be readily used to obtain dep~*selective information from within the topmost 5.0 nm of the surface. For low energy r~o~~t electron sp~tros~~y to become a working tool, further development of approp~ate theoretical models for low energy electron generation and transport through a solid are required. Development of such theories could be used to increase the near surface resolution of DCEMS and extend the effective range of DCEMS when used in conjunction with depth-profiling at higher electron and photon energies. Appropriate theories for the above noted effects are currently under development in this laboratory (351.
Support for this work is tefully ac~owl~ged from the Air Force Office of Scientific Research (AF~SR-8~~3~1).
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J.S. Zabinski, B.J. Tatarchuk / Resonant few energy electrons in &EMS 1311 M. Domke, B. Kyvelos, and C. Kaindl, Hype&e
Interactions 10 (1981) 1137. [32] H. Nakagawa, Y. Ujihira, and M. Inaba, Nucl. Instr. and Meth. 196 (1982) 573. [33] Y. Yonekura, T. Toriyama, J. Itoh and K. Hisatake, Hype&e Interactions E/16 (1983) 1005.
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1341 S. Staniek, T. S~gematsu, W. Keune, and H.-D. Pfannes, J. Magn. and Magnet. Mater. 35 (1983) 347. [35] T.S. Lee, J.S. Zabinski, and B.J. Tatarchnk, Nucl. Instr. and Meth. B, in press.