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A new measurement of the 429 keV 15N(p,αγ)12C resonance. Applications of the very narrow width fouNd to 15N and 1H depth location
Nuclear Instruments and Methods in Physics Research 218 (1983) 159-164 North-Holland, Amsterdam
A NEW MEASUREMENT O F T H E 429 keV tSN(p,~x~,)lZC R E S O N A N C E . APPLICATIONS O F T H E V E R Y N A R R O W W I D T H F O U N D T O 15N A N D I H D E P T H
159
LOCATION
*
1. Resonance width measurement B. M A U R E L
and G. AMSEL
Groupe de Physique des Solides de I'Ecole Normale SupOrieure, Unit,ersitO Paris VIL Tour 23, 2, Place Jussieu, 75251 Paris COdex 05, France
High precision nuclear reaction resonance width measurement techniques were applied to the 429 keV resonance in 15N(p, a), )12C yielding a result which may be stated at present as: F = (120_+ 30) eV. Work is in progress to improve precision further. The measurements were performed using a 2 MV Van de Graaff equipped with both slit and capacitive pick-off plate feedback stabilization and fitted with an automatic energy scanning system. Energy resolutions as low as - 40 eV fwhm were reached. Thick, nearly stoichiometric niobium nitride targets highly enriched in 15N were prepared on silicon backings by reactive sputtering in a 15N2 and argon mixture. The measurements were carried out using a - 1 ~tA beam. at room temperature, in 10 -6 Torr liquid nitrogen trapped vacuum. The characterization of the targets as well as the fitting procedures used to extract F from the thick target yield curves (including Doppler effects) are presented.
l. Introduction T h e n a r r o w r e s o n a n c e at 429 keV of the 15N(p, acy)12C reaction has been widely used for many years for tSN d e p t h profiling, as it presents marked advantages with respect to stronger resonances at higher energies. It is the first strong resonance, well isolated, with a very low off-resonance cross section: the excitation curves are hence free of appreciable pedestals even for relatively thick targets, a n d the sensitivity is excellent. Moreover this resonance is the narrowest occurring in this reaction. This small width, c o m b i n e d with the high p r o t o n stopping powers at a comparatively low energy, yield optimal d e p t h resolutions for tSN profiling. A m o n g recent work using this resonance for 15N d e p t h profiling we may quote refs. 1 to 4. For some years the reversed reaction has been used systematically for hydrogen d e p t h profiling, the resonance occurring at 6.435 MeV 15N energy [5,6]. Here again the low energy at which this resonance occurs is a great advantage as it limits the size of the accelerators required by the experim e n t s and as a r o u n d the resonance energy the 15N stopping power is near its m a x i m u m in most materials. While most authors working on 15N analysis assume for the width of this resonance the classical value F = 0.9 keV reported in the Ajzenberg-Selove compilation [7], those using 15N beams soon began to question this value: the corresponding width FN,5 = 13.5 keV ap-
* Work supported by the Centre National de la Recherche Scientifique, RCP No. 157. 0 1 6 7 - 5 0 8 7 / 8 3 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
peared too large c o m p a r e d to the experimental results. T h u s Lanford [5] quotes an estimate of FN~ = 6 keV (i.e. F = 0.4 keV) a n d T h o m a s et al. [6] Ey,~ = 7 keV. More recently La Marche et al. [8] estimated this width to be F y , = (3.5 +_ 1) keV corresponding to F = (0.25 +_ 0.07) keV. La Marche [9] carried out a direct width m e a s u r e m e n t with a p r o t o n b e a m and a TilSN target a n d found F = (0.3 + 0.1) keV. The aim of this and its c o m p a n i o n paper is to present the results of a new m e a s u r e m e n t of this reson a n c e width using the high precision resonance width m e a s u r e m e n t techniques also presented at this conference [10], and to show some possible applications following from the low value found for F.
2. Targets 2.1. Target production
Our aim was to produce a 15N labelled " t h i c k target" as uniform as possible in composition across its thickness and in particular near its surface region [10]. There is no a priori reason for nitridation techniques to produce uniform films; in contrast sputtering techniques should in principle produce such films if the discharge conditions remain stable during deposition. We hence chose the latter method. N i o b i u m nitride films were deposited on 3 cm diameter silicon wafers by reactively sputtering n i o b i u m in an a r g o n - n i t r o g e n mixture, using a V E E C O triode sputtering system. The tungsten filament was heated by
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B. Maurel, G. Amsel / A new measurement of the g29 keV /SN(p, y)12(" resonance
a 7.6 A current; the cathode potential was 2.25 kV a n d the current 50 mA, kept constant by adjusting the a n o d e current. The limiting vacuum in the system was 2 to 5 10 -7 Torr with a 2000 l / s liquid nitrogen trapped diffusion p u m p ; prior to deposition the p u m p valve was partially closed so as to reduce the p u m p i n g speed to a b o u t 100 1/s. In these conditions the residual pressure was 10 6 Torr. D u r i n g sputtering the overall gas pressure was kept at 1.2 × 10 3 T o r t by servo control of the argon inlet. The target t e m p e r a t u r e was not controlled but did not exceed 200°C. Two - I 1 glass bottles containing high purity ~SN enriched a n d natural nitrogen gas at a pressure of 130 g / c m 2 were connected to the system through high precision leaks. Prior to argon injection the leak corresponding to the desired gas is first set so as to produce a well chosen partial pressure in the sputtering chamber, then closed. The discharge is initiated in pure argon and n i o b i u m is presputtered for 10 rain, the target being protected by a mobile shutter. Nitrogen is then let in; this produces no noticeable change in the discharge. Deposition on the target is started after a further 5 rain presputtering period. In a preliminary experiment films were deposited for 15 min at 1.2, 2 a n d 4 × 10 -4 T o r t nitrogen partial pressure in natural nitrogen. As will be shown in the next section it was found that nearly stoichiometric nitride N b N is o b t a i n e d for a partial pressure of 1.6 × 10 4 Torr which was subsequently used for the ~SN enriched targets. The high purity ~SN enriched gas contained 94.8% 15N, the natural a b u n d a n c e of 15N being 0.37%. NblSN films were deposited for times from 6 s to 15 rain. In o u r experimental conditions the 15N 2 gas c o n s u m p t i o n was a r o u n d 0.5 c c / m i n NPT, allowing us to carry out 15 depositions with 150 cc N P T 15N 2 gas, thus keeping the enriched gas price per sample at reasonable values.
2.2.1. a) N_, partial pressure dependence of stoichiometrv It was found that for increasing nitrogen partial pressures P the nitrogen deposition rate is practically c o n s t a n t whereas the niobium deposition rate decreases markedly. For an overall formula NbN~ x = 0.87 for P=I ×10-4Torrandx=l.4forP=4×10 4Torr. A nearly stoichiometric nitride is obtained for P = 1.6 x 10 4 Tort, the pressure used subsequently. ~
2.2.2. Deposition rate In these conditions films containing - 270 × 1015 N b a t o m s / e r a 2 were obtained reproducibly in 15 rain. Using the classical density of 8.4 for N b N we get a corresponding film thickness of - 600 A, the deposition rate being - 4 0 A / m i n . However fihns deposited for various times t did not have thicknesses precisely proportional to t. This observation is not understood, the so more because, as will be shown in {}3.2, the 15 rain deposits were highly uniform across their thickness. 2.2.3. Oxygen contamination The spontaneous oxide layer on the silicon backings c o n t a i n e d - 15 × 10 ~ 5 0 / c m 2 corresponding to - 35 ,~ SiO 2, originating in a very long storage time of the wafers. This layer was welcome as it constituted a good barrier against possible low temperature silicide formation, in particular during subsequent heat treatments of the samples. The additional oxygen found in the 600 ,~ NblSN films was a r o u n d 25 x 10 ~ 5 0 / c m 2 ~ i.e. - 9% of the nitrogen content. This a m o u n t did not decrease m u c h for thinner deposits. The first interpretation of this observation is that the surface layer is oxidized either just after deposition or in the atmosphere. The possible locations of this oxygen c o n t a m i n a t i o n will be discussed in {}3.3. The presence of this ill-defined oxygen c o n t a m i n a t i o n is the main defect of our targets as for resonance width measurements. It could not be overcome.
2.2. Target characterization Target thicknesses and composition were measured by 4He RBS for n i o b i u m and nuclear reactions for 14N, ~SN, a n d for possible ~60 contamination. 14N was measured using the 14N(d, a0)lZC reaction at 1.75 MeV where the cross section is high; pile up due to n u m e r o u s d e u t e r o n induced reactions in the silicon backing was reduced by using fast (20 ns) electronics. 15N was measured using the 15N(p, a)~2C reaction at 620 keV a n d 160 with the 160(d, p)170* reaction at 600 keV. This latter low energy allows one to reduce the nitrogen peaks, which interfere with the 160 peak, to negligible contributions. Detection was at 150 ° in classical conditions, as described in ref. 1l. Our standard reference targets had an accuracy of a few per cent for RBS and 160 a n d - 10% for 14N a n d 15N.
2.2.4. lk~N enrichement of the N b N deposits The overall composition of the sample used for the resonance width m e a s u r e m e n t s was, to within the precision of the reference targets~ in 10 ~5 a t o m / c m 2 units: N b : 266; 15N: 253; 14N: 28; 160: 25. The films cont a n e d neither argon nor tungsten to within the sensitivity of the RBS measurements. The ~SN isotopic concentration was hence 90%, to be c o m p a r e d to the gas enricbement: 94.8%. The additional - 5 % of 14N originates p r o b a b l y from the walls of the sputtering c h a m b e r : it seems that the previously deposited 14N nitride layers which cover the walls desorb nitrogen d u r i n g sputtering due to plasma action, as evidenced by the appreciable nitrogen c o n t a m i n a t i o n of N b films subsequently sputtered in pure argon. In conclusion the two main difficulties to be overcome for o b t a i n i n g ideal ~SN labelled targets are the
B. Maurel. G. Amsel / A new measurement of the 429 k e V /SN(p, y)I-'C resonance
160 and 14N contaminations. However as will be shown in §3.3 our present targets allowed us to observe the resonance u n d e r study in fairly good conditions.
3. Excitation curves and resonance shape analysis 3.1. Recording the excitation curve." e x p e r i m e n t a l
The ion b e a m analysis facility of the G r o u p e de Physique des Solides, based on a 2 MV A N 2000 type H V E C Van de G r a a f f is described in ref. 11. The most i m p o r t a n t feature of the accelerator, of basic importance here, is its stabilization system: it is based on a double feedback loop, the error signals for correcting the high voltage fluctuations being derived both from the classical slit b e a m position sensor and from a capacitive pick-off plate in the tank. The various factors which must be overcome for obtaining high beam energy resolutions, with overall energy spreads below 100 eV fwhm are discussed in refs. 10, 12 and 13. The latter reference describes an automatic hysteresis free energy scanning device coupled to our accelerator, which allowed us to record with great detail the excitation curves presented here. The short term high voltage fluctuation probability density is measured directly with a multichannel analyser by sampling the calibrated signal from the capacitive pick off plate [10,13] (ripple monitor). This yields the b e a m energy spread to within the ion energy spread at the output of the ion source, which is negligible with respect of the voltage fluctuations. Energy spread is continuously monitored in this way; its probability density is nearly Gaussian [10,13]. This spread may be b r o a d e n e d by slow drift effects, which were estimated as described in ref. 10. The best measured short term energy resolution reached during our experiments was - 40 eV fwhm a r o u n d 430 keV. Such a high resolution was exceptional: typical values were rather between 60 to 100 eV. These very low energy spreads seem to depend critically on such ill-controlled parameters as the state of the accelerator belt etc. [10]: work is in progress, attempting to bring the ultimate energy resolution of our accelerator u n d e r control. Drift effects during the m e a s u r e m e n t of the rise of the yield curve (see next paragraph), were estimated to be a r o u n d 30 eV fwhm. The overall energy resolution for the results shown below, recorded while the measured short term energy spread was - 4 0 eV, will thus b e taken as 50 eV fwhm. The measurements were carried out in a scattering c h a m b e r located at a b e a m deflection angle of 18030 '. The c h a m b e r is fitted with a Balzers turbo p u m p and with an internal liquid nitrogen trap. The role of this trap is crucial. In fact one of the main difficulties in such measurements arises from c a r b o n c o n t a m i n a t i o n
161
of the samples. While it is most difficult to get rid from the h y d r o c a r b o n c o n t a m i n a t i o n from the atmosphere it is essential to eliminate further c a r b o n build up due to b o m b a r d m e n t by the b e a m [10,12]. Reduction of hydroc a r b o n c o n t a m i n a t i o n is possible only by using ultra high vacuum type in situ sample cleaning procedures. This is being set up but could not be attempted for the experiments presented here. The limiting vacuum in our c h a m b e r was - 10 6 Torr and carbon build up during a 30 rain measurement was small when all the precautions were taken. The 4.43 MeV y-rays were detected around 90 ° from the beam with a 3 " × 3" NaI(T1) scintillator at 12 cm from the impact point. The y-ray energy window was set from - 2.6 to - 5 MeV. Beam currents were from - 0.5 to - 1 /~A. Typical measurement times for recording an excitation curve a r o u n d the resonance were - 30 rain. To minimize any possible carbon build up at the most critical stage of the measurements the energy scanning was started near the resonance for a fresh impact point o n the target; off-resonance parts of the yield curve were recorded subsequently. For very broad yield curves covering the whole thickness of the target the impact point on the sample could be changed: this had no bearing on the thick target yield plateau even if the target film was slightly non uniform. 3.2. R e s u l ~
Fig. l a shows a b r o a d scan corresponding to the target described in §2.2. The b a c k g r o u n d level off-reson a n c e is very low; it corresponds mainly to cosmics. The plateau of the yield curve is very flat: this is the best p r o o f of the uniformity of our target, meaning that stoichiometry, 160 c o n t a m i n a t i o n in the bulk as well as the aSN isotopic concentration remain stable through the film. The very slow decrease of the curve reflects the effects of energy straggling of the b e a m particles in the film. Fig. l b shows in detail the fast rise of the thick target excitation curve. The striking feature of this rise is that it begins very slowly, the increase above backg r o u n d level starting at ~ 700 eV before the mid point of the rise while the 12% to 88% rise takes place over - 300 eV. Such measurements were repeated independently several times, using energy steps down to 40 eV and yielding similar results. However these measurements could be carried out only with somewhat larger b e a m energy spreads (see §3.1). We analyze here hence the results shown in fig. 1 ; the larger energy step, 80 eV, has no appreciable influence on the results, the slow rise of the curve playing a major role in the determination of F, as will be shown next. It should be emphasized here that the word " c h a n n e l " in the figures is used improperly for our energy scanning device. In fact the analyser works in the multiscaler mode a n d the energy is varied
B. Maurel, G. Amsel / A new measurement of the 429 keV lSN(p, "y)12C resonance
162
i
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i
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3.3. Interpretation and best fit
Z
0
100
200
CHANNELS B
i
r
(~
/
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rise of Gaussian type, with a 12% to 88% increase within 140 eV. It thus appeared to us that the slow start and sharp rise of the curve in fig. l b does correspond to a physical reality.
2
80 eV/channel 0
10
20
::50
40
CHANNELS 15N(p, o~y)12Cexcitation curves
Fig. 1. Typical near the 429 keV resonance for a - 6 0 0 ,~ thick NblSN target. 3"×3" NaI(TI) scintillator at - 90 ° and 12 cm: current - 1 p,A; 10 /~C per point; (a) overall curve with best fit. (b) Rise observed with optimal beam energy resolution. Solid line: best fit calculated for a 26 A thick hydrocarbon contamination layer, F = 120 eV, beam energy spread: 50 eV, Doppler broadening: 90 eV (fwhm) at room temperature. Dashed line: theoretical rise calculated as before but in the absence of surface contamination. A small shift, a steeper rise and a marked Lewis effect show up.
in discrete steps, no smearing effect being associated with the channel widths. We kept the terminology associated with multichannel analyzers for convenience. We never observed such a narrow resonance at comparatively low energies with our facility, our main work having been devoted to the 100 eV wide resonance of 2VAl(p,y)ZSSi at 992 keV and to the 50 eV wide resonance of lSO(p,a)XSN at 1167 keV [12,14]. Around 430 keV beam stability and slowing d o w n effects in possible carbon contamination layers present on the samples could have larger effects than at higher energies. We checked therefore whether the rise observed in fig. l b could not be due to some effect unaccounted for by our above described technique to estimate beam energy spread or to contamination effects. The closest extremely narrow resonance to 430 keV, that of 27Al(p, 7)2aSi at 632 keV ( F < 30 eV) was hence studied with a thick A1203 target [10]. We observed a sharply starting
High resolution excitation curves near very narrow resonances must be interpreted by resorting to the stochastic theory of slowing down, which takes into consideration energy straggling effects and in particular the so-called Lewis effect. These aspects are discussed in ref. 10; they were developed in detail in ref. 12 and reviewed in ref. 14. We give here only the results of our calculations carried out according to principles described in these references. The following factors were taken into consideration for the calculations related to fig. 1: (i) the overall beam energy spread was taken to be F B = 50 eV fwhm. (ii) The actual energy spread as seen by the reacting nuclei is broadened by the Doppler effect due to the thermal vibrations [10]: this effect is Gaussian with a width calculated for the 15N nuclei of F D = 90 eV at room temperature. The total instrumental broadening with Gaussian shape is hence F v -- 100 eV. (iii) The intrinsic line shape of this very narrow resonance at energy E a is a Lorentzian [10] of width F, i.e. a B r e i t - W i g n e r element of the form o (E)
constant
( F / Z ) 2 +( E - ER) 2" It follows that an ideally thin target should give a yield curve which would be the convolution ofa Gaussian of width 100 eV with a Lorentzian [10] of width F, to be determined. As an ideally thin target is not physically realisable we had to use the thick target technique [10,12,14]. (iv) Stoichiometry, and in particular oxygen contamination, were considered as uniform through the film. The latter assumption will be critically discussed in
§3.4. The statistical aspects of slowing down in the thick target were taken into account using our special computer program [12,14]. A simplified collision spectrum (i.e. the energy loss spectrum in a single encounter between the incoming proton and an electron in the medium) was used [12]. The parameters of the latter for our N h N targets may be determined from their known composition (§2.2.) and from the mean energy loss and energy straggling per unit length in the film. These two quantities are deduced from a best fit of the width and slow fall of the broad excitation curve in fig. la, the details of the rise of the curve being irrelevant at this stage. The corresponding fit is shown in fig. l a (where the fit of the steep rise corresponds to the more detailed
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B. Maurel, G. Amsel / A new measurement of the 429 k e V 15N(p, y)12C resonance
calculations shown in fig. lb). It should be noted that the measured width of the curve in fig. l a was found to be in full agreement with the value of the mean energy loss calculated for our target from the tables of Andersen and Ziegler [15]. In this way the following parameters could be deduced from the data: stopping power of the film: d E / d x = 17 e V / , ~ ; straggling p a r a m e t e r (spread variance = s2x): s = 60 eV/A1/2; effective p r o t o n m e a n free p a t h between two electronic collisions [12]: 3.5 A; maxim u m effective energy transfer [12] in a collision: 935 eV; m i n i m u m effective energy transfer [12] in a collision: 13.7 eV. A typical result of the calculations carried out with these parameters, is shown in fig. l b , using the value F = 120 eV. The curve exhibits a marked Lewis effecL i.e. an overshoot, which is not observed experimentally. However the very slow rise at the start of the curve, underlined above, is well reproduced: this is due in fact to the particularly long tails of the Lorentzian, which induce the most peculiar properties of resonance line shapes [10]. For a better interpretation of our data we must take into account a n o t h e r factor, not considered until now in the calculations i.e. c a r b o n contamination, mentioned in {}3.1. In ref. 12 it was established that most samples handled in atmosphere are covered by a h y d r o c a r b o n film of approximate global composition C H 4 and thickness typically a r o u n d 30 ~. (with a density assumed to be - 1). Our fitting procedure consisted thus in choosing the c o m b i n a t i o n of F a n d of hydroc a r b o n thickness leading to a best fit. The energy spread spectrum induced by various thicknesses of hydroc a r b o n films a r o u n d 30 A was hence calculated separately using k n o w n data a n d folded in the results of the above calculations. The exact a m o u n t of h y d r o c a r b o n c o n t a m i n a t i o n actually present during the resonance measurements is difficult to establish. However from our calculations it appears that the range of possible thicknesses is limited. A too thin film does not suppress the unobserved overshoot of the Lewis effect; a too thick film leads to a too slow rise of the curve around its mid point, while it does not reproduce the very slow start of the rise at lower energies if F is too small. On the other h a n d this slow start can be reproduced only if F is also in a relatively narrow range of values. The best fit was o b t a i n e d for the values: h y d r o c a r b o n thickness 26 ,~ (1016 at. C / c m 2) a n d F = 120 eV. The corresponding fit is shown in fig. lb. It should be noticed that while at these low energies h y d r o c a r b o n c o n t a m i n a t i o n masks the Lewis effect, strong overshoots were observed experimentally on resonances occurring at energies a r o u n d 1 MeV, demonstrating the i m p o r t a n c e of taking into account statistical effects in slowing down for interpreting narrow reson a n c e yield curves [12-14].
3.4. Precision
It appears that, provided our accelerator works properly, the main error on F does not arise from the b e a m energy spread. The latter would d o m i n a t e the Doppler b r o a d e n i n g only for measurements on cooled targets. The peculiar properties of the convolution of a gaussian with a Lorentzian [10] are such that in our case a total gaussian width F T up to - 150 eV would not appreciably reduce the precision with respect to that corresponding to the value used here: 100 eV. Thus accelerators with an energy spread of a r o u n d 150 eV, and even more, at 430 keV may be used quite efficiently for high resolution depth profiling with the resonance studied here, if ultimate precision is not required. While for the results shown here the gaussian comp o n e n t of the source of error on F was d o m i n a t e d by the Doppler effect, when the accelerator is less well behaved the two effects are of comparable magnitude. On the other h a n d c a r b o n c o n t a m i n a t i o n is a major source of uncertainty on F. This, by the way, demonstrates the high sensitivity d e p t h profiling ability of this resonance near the surface, as illustrated by fig. lb, which in fact shows the effect of a very thin absorber present on the surface of a clean sample. By carrying out calculations with various thicknesses of h y d r o c a r b o n a n d various values of F we found that, in the present state of our experiments, fits could be obtained, still compatible with the statistical scatter of our data, for values of F between 90 and 150 eV. However these extreme values do not seem very likely a n d we hop that the statement: r : (120__ 30) eV will not be strongly contradicted by future experiments. The question arose whether the oxygen c o n t a m i n a n t
I
I
I
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8 0 eV/channel
Z
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I 30
I 40
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CHANNELS Fig. 2. Excitation curve rises calculated with and without hydrocarbon contamination, with the same parameters as in fig. 1 but for a target which would consist in a 65 ,~ NblSN O layer on a thick NblSN layer. These assumptions preclude any fit of the results in fig. lb.
164
B. Maurel, G. Amsel , / A new measurement of the 429 ke V ]~NIp, y)]e(, re~onance
in the films is really in the bulk or whether it is rather near the surface. If so it might form an oxynitride of formula N b N O a n d to the a m o u n t of 160 measured there should correspond a 65 A thick N b N O film. Pure n i o b i u m oxide on the surface, without 15N, c a n n o t exceed the m o n o l a y e r range, as this would destroy the steep rise of the curve in fig. lb. Fig. 2 shows calculated curves with and without h y d r o c a r b o n c o n t a m i n a t i o n , assuming 65 * N b N O near the surface. It is clear that such a curve c a n n o t fit our results. A recent p a p e r by F r a n k e n t h a l et al. [16] also seems to suggest that the presence of the oxygen in the bulk of our N b N films is likely.
We t h a n k E. G i r a r d for assistance with the electronics and G. G e n i n and B. Thiel in c o m p u t e r calculations. J. C h a u m o n t is gratefully acknowledged for providing us with the ion implanted ~SN reference target. N o t e a d d e d after the conference
The value I ' = ( 1 2 0 _ + 3 0 ) eV, i.e. for a 15N beam /'N,~ = (1.8 ± 0.45) keV, was fully confirmed in a paper presented at the conference by Damjantschitsch et al. [17]. These authors deduced FN, from a careful meas u r e m e n t of the resonance line shape tails over several decades using a low b a c k g r o u n d detection system. From the observed Lorentzian tail shapes they conclude to I'Nt, - 1.85 keV, with an estimated precision of + 10%.
4. Conclusions The low value found for F shows that d e p t h profiling of 15N is possible with much better depth resolutions than thought until now. In fact the n o m i n a l ultimate d e p t h resolution near the surface corresponding to F = 120 eV is, in N b N : R e = F / ( d E / d x ) = 7 A,. Similarly the corresponding value of FN,~ for hydrogen depth profiling with tSN beams is 1.8 keV, a low value which might lead to further i m p r o v e m e n t s in this technique. Possible applications are illustrated in the second part of this paper. We insisted in this paper on the description of our target fabrication procedures, on their characterization a n d on their various possible contaminations, as those factors set the precision a n d credibility of a very narrow resonance width measurement. We plan to further improve the precision and reliability of these measurem e n t s by operating in ultra high vacuum, by developing cleaning procedures which do not perturb the composition of the samples in the first few angstroms, by cooling the targets and possibly by improving their purity. We shall also try to improve the reliability of our accelerator for very low energy spreads. Such i m p r o v e m e n t s are by no means sheer sport: they are the key to absolute, high precision, non-destructive d e p t h profiling of light nuclei near the surface of solids, including isotopic tracing experiments. We are most grateful to A. Laurent who developed with us the delicate reactive sputtering techniques for the fabrication of 15N labelled N b N targets. We thank S. Rigo for advice related to this field. The help of E. d ' A r t e m a r e in r u n n i n g the accelerator in optimal stability conditions and for data collection was invaluable.
References [1] W.J.M.J. Josquin and Y. Tamminga, J. Electrochem. Soc. 129 (1982) 1803. [2] M. Luomaj~rvi, J. Keinonen, M. Bister and A. Anttila, Phys. Rev. B18 (1978) 4657. [3] M. Hautala, R. Paltemaa, A. Anttila and M. Luomaj~irvi, Nucl. Instr. and Meth. 209/210 (1983) 37. [4] G. Marest, C. Skoutarides+ Th. Barnavon, J. Tousset, S. Fayeulle and M. Robert+ Nucl. Instr. and Meth. 209/210 (1983) 259. [5] W.A. Lanford, Nucl. Instr. and Meth. 149 (1978) 1. [6] J.P. Thomas, C. Pijolet and M. Fallavier. Rev. Phys. Appl. 13 (1978) 433. [71 F. Ajzenberg-Selove, Nucl. Phys. A281 (1977) 1. [8] P.H. La Marche, W.A. Lanford and R. Golub. Nucl. Instr. and Meth. 189 (1981) 533. [9] P.H. La Marche, PhD Thesis, Yale University (1981) (University Microfilms, Ann Arbor, Michigan). [10] G. Amsel and B. Maurel, these Proceedings (IBA-6), p. 183. [11] G. Amsel, J.P. Nadai, E. d'Artemare, D. David, E. Girard and J. Moulin, Nucl. Instr. and Meth. 92 (1971) 481. [12] B. Maurel, Thesis, University of Paris VII (October 1980). [13] G. Amsel, E. d'Artemare and E. Girard, Nucl. Instr. and Meth. 205 (1983) 5. [14] B. Maurel, G. Amsel and J.P. Nadai, Nucl. Instr. and Meth. 197 (1982) 1. [15] H.H. Andersen and J.F. Ziegler, Hydrogen stopping power and ranges in all elements (Pergamon Press, New York, 1977). [16] R.P. Frankenthal, D.J. Siconolfi, W.R. Sinclair and D.D. Bacon, J. Electrochem. Soc., in press. [17] H. Damjantschitsch, W. Weiser, G. Heusser, S. Kalbitzer and H. Mannsperger, these Proceedings (IBA-6), p. 129.
A NEW MEASUREMENT O F T H E 429 keV tSN(p,~x~,)lZC R E S O N A N C E . APPLICATIONS O F T H E V E R Y N A R R O W W I D T H F O U N D T O 15N A N D I H D E P T H
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LOCATION
*
1. Resonance width measurement B. M A U R E L
and G. AMSEL
Groupe de Physique des Solides de I'Ecole Normale SupOrieure, Unit,ersitO Paris VIL Tour 23, 2, Place Jussieu, 75251 Paris COdex 05, France
High precision nuclear reaction resonance width measurement techniques were applied to the 429 keV resonance in 15N(p, a), )12C yielding a result which may be stated at present as: F = (120_+ 30) eV. Work is in progress to improve precision further. The measurements were performed using a 2 MV Van de Graaff equipped with both slit and capacitive pick-off plate feedback stabilization and fitted with an automatic energy scanning system. Energy resolutions as low as - 40 eV fwhm were reached. Thick, nearly stoichiometric niobium nitride targets highly enriched in 15N were prepared on silicon backings by reactive sputtering in a 15N2 and argon mixture. The measurements were carried out using a - 1 ~tA beam. at room temperature, in 10 -6 Torr liquid nitrogen trapped vacuum. The characterization of the targets as well as the fitting procedures used to extract F from the thick target yield curves (including Doppler effects) are presented.
l. Introduction T h e n a r r o w r e s o n a n c e at 429 keV of the 15N(p, acy)12C reaction has been widely used for many years for tSN d e p t h profiling, as it presents marked advantages with respect to stronger resonances at higher energies. It is the first strong resonance, well isolated, with a very low off-resonance cross section: the excitation curves are hence free of appreciable pedestals even for relatively thick targets, a n d the sensitivity is excellent. Moreover this resonance is the narrowest occurring in this reaction. This small width, c o m b i n e d with the high p r o t o n stopping powers at a comparatively low energy, yield optimal d e p t h resolutions for tSN profiling. A m o n g recent work using this resonance for 15N d e p t h profiling we may quote refs. 1 to 4. For some years the reversed reaction has been used systematically for hydrogen d e p t h profiling, the resonance occurring at 6.435 MeV 15N energy [5,6]. Here again the low energy at which this resonance occurs is a great advantage as it limits the size of the accelerators required by the experim e n t s and as a r o u n d the resonance energy the 15N stopping power is near its m a x i m u m in most materials. While most authors working on 15N analysis assume for the width of this resonance the classical value F = 0.9 keV reported in the Ajzenberg-Selove compilation [7], those using 15N beams soon began to question this value: the corresponding width FN,5 = 13.5 keV ap-
* Work supported by the Centre National de la Recherche Scientifique, RCP No. 157. 0 1 6 7 - 5 0 8 7 / 8 3 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
peared too large c o m p a r e d to the experimental results. T h u s Lanford [5] quotes an estimate of FN~ = 6 keV (i.e. F = 0.4 keV) a n d T h o m a s et al. [6] Ey,~ = 7 keV. More recently La Marche et al. [8] estimated this width to be F y , = (3.5 +_ 1) keV corresponding to F = (0.25 +_ 0.07) keV. La Marche [9] carried out a direct width m e a s u r e m e n t with a p r o t o n b e a m and a TilSN target a n d found F = (0.3 + 0.1) keV. The aim of this and its c o m p a n i o n paper is to present the results of a new m e a s u r e m e n t of this reson a n c e width using the high precision resonance width m e a s u r e m e n t techniques also presented at this conference [10], and to show some possible applications following from the low value found for F.
2. Targets 2.1. Target production
Our aim was to produce a 15N labelled " t h i c k target" as uniform as possible in composition across its thickness and in particular near its surface region [10]. There is no a priori reason for nitridation techniques to produce uniform films; in contrast sputtering techniques should in principle produce such films if the discharge conditions remain stable during deposition. We hence chose the latter method. N i o b i u m nitride films were deposited on 3 cm diameter silicon wafers by reactively sputtering n i o b i u m in an a r g o n - n i t r o g e n mixture, using a V E E C O triode sputtering system. The tungsten filament was heated by
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B. Maurel, G. Amsel / A new measurement of the g29 keV /SN(p, y)12(" resonance
a 7.6 A current; the cathode potential was 2.25 kV a n d the current 50 mA, kept constant by adjusting the a n o d e current. The limiting vacuum in the system was 2 to 5 10 -7 Torr with a 2000 l / s liquid nitrogen trapped diffusion p u m p ; prior to deposition the p u m p valve was partially closed so as to reduce the p u m p i n g speed to a b o u t 100 1/s. In these conditions the residual pressure was 10 6 Torr. D u r i n g sputtering the overall gas pressure was kept at 1.2 × 10 3 T o r t by servo control of the argon inlet. The target t e m p e r a t u r e was not controlled but did not exceed 200°C. Two - I 1 glass bottles containing high purity ~SN enriched a n d natural nitrogen gas at a pressure of 130 g / c m 2 were connected to the system through high precision leaks. Prior to argon injection the leak corresponding to the desired gas is first set so as to produce a well chosen partial pressure in the sputtering chamber, then closed. The discharge is initiated in pure argon and n i o b i u m is presputtered for 10 rain, the target being protected by a mobile shutter. Nitrogen is then let in; this produces no noticeable change in the discharge. Deposition on the target is started after a further 5 rain presputtering period. In a preliminary experiment films were deposited for 15 min at 1.2, 2 a n d 4 × 10 -4 T o r t nitrogen partial pressure in natural nitrogen. As will be shown in the next section it was found that nearly stoichiometric nitride N b N is o b t a i n e d for a partial pressure of 1.6 × 10 4 Torr which was subsequently used for the ~SN enriched targets. The high purity ~SN enriched gas contained 94.8% 15N, the natural a b u n d a n c e of 15N being 0.37%. NblSN films were deposited for times from 6 s to 15 rain. In o u r experimental conditions the 15N 2 gas c o n s u m p t i o n was a r o u n d 0.5 c c / m i n NPT, allowing us to carry out 15 depositions with 150 cc N P T 15N 2 gas, thus keeping the enriched gas price per sample at reasonable values.
2.2.1. a) N_, partial pressure dependence of stoichiometrv It was found that for increasing nitrogen partial pressures P the nitrogen deposition rate is practically c o n s t a n t whereas the niobium deposition rate decreases markedly. For an overall formula NbN~ x = 0.87 for P=I ×10-4Torrandx=l.4forP=4×10 4Torr. A nearly stoichiometric nitride is obtained for P = 1.6 x 10 4 Tort, the pressure used subsequently. ~
2.2.2. Deposition rate In these conditions films containing - 270 × 1015 N b a t o m s / e r a 2 were obtained reproducibly in 15 rain. Using the classical density of 8.4 for N b N we get a corresponding film thickness of - 600 A, the deposition rate being - 4 0 A / m i n . However fihns deposited for various times t did not have thicknesses precisely proportional to t. This observation is not understood, the so more because, as will be shown in {}3.2, the 15 rain deposits were highly uniform across their thickness. 2.2.3. Oxygen contamination The spontaneous oxide layer on the silicon backings c o n t a i n e d - 15 × 10 ~ 5 0 / c m 2 corresponding to - 35 ,~ SiO 2, originating in a very long storage time of the wafers. This layer was welcome as it constituted a good barrier against possible low temperature silicide formation, in particular during subsequent heat treatments of the samples. The additional oxygen found in the 600 ,~ NblSN films was a r o u n d 25 x 10 ~ 5 0 / c m 2 ~ i.e. - 9% of the nitrogen content. This a m o u n t did not decrease m u c h for thinner deposits. The first interpretation of this observation is that the surface layer is oxidized either just after deposition or in the atmosphere. The possible locations of this oxygen c o n t a m i n a t i o n will be discussed in {}3.3. The presence of this ill-defined oxygen c o n t a m i n a t i o n is the main defect of our targets as for resonance width measurements. It could not be overcome.
2.2. Target characterization Target thicknesses and composition were measured by 4He RBS for n i o b i u m and nuclear reactions for 14N, ~SN, a n d for possible ~60 contamination. 14N was measured using the 14N(d, a0)lZC reaction at 1.75 MeV where the cross section is high; pile up due to n u m e r o u s d e u t e r o n induced reactions in the silicon backing was reduced by using fast (20 ns) electronics. 15N was measured using the 15N(p, a)~2C reaction at 620 keV a n d 160 with the 160(d, p)170* reaction at 600 keV. This latter low energy allows one to reduce the nitrogen peaks, which interfere with the 160 peak, to negligible contributions. Detection was at 150 ° in classical conditions, as described in ref. 1l. Our standard reference targets had an accuracy of a few per cent for RBS and 160 a n d - 10% for 14N a n d 15N.
2.2.4. lk~N enrichement of the N b N deposits The overall composition of the sample used for the resonance width m e a s u r e m e n t s was, to within the precision of the reference targets~ in 10 ~5 a t o m / c m 2 units: N b : 266; 15N: 253; 14N: 28; 160: 25. The films cont a n e d neither argon nor tungsten to within the sensitivity of the RBS measurements. The ~SN isotopic concentration was hence 90%, to be c o m p a r e d to the gas enricbement: 94.8%. The additional - 5 % of 14N originates p r o b a b l y from the walls of the sputtering c h a m b e r : it seems that the previously deposited 14N nitride layers which cover the walls desorb nitrogen d u r i n g sputtering due to plasma action, as evidenced by the appreciable nitrogen c o n t a m i n a t i o n of N b films subsequently sputtered in pure argon. In conclusion the two main difficulties to be overcome for o b t a i n i n g ideal ~SN labelled targets are the
B. Maurel. G. Amsel / A new measurement of the 429 k e V /SN(p, y)I-'C resonance
160 and 14N contaminations. However as will be shown in §3.3 our present targets allowed us to observe the resonance u n d e r study in fairly good conditions.
3. Excitation curves and resonance shape analysis 3.1. Recording the excitation curve." e x p e r i m e n t a l
The ion b e a m analysis facility of the G r o u p e de Physique des Solides, based on a 2 MV A N 2000 type H V E C Van de G r a a f f is described in ref. 11. The most i m p o r t a n t feature of the accelerator, of basic importance here, is its stabilization system: it is based on a double feedback loop, the error signals for correcting the high voltage fluctuations being derived both from the classical slit b e a m position sensor and from a capacitive pick-off plate in the tank. The various factors which must be overcome for obtaining high beam energy resolutions, with overall energy spreads below 100 eV fwhm are discussed in refs. 10, 12 and 13. The latter reference describes an automatic hysteresis free energy scanning device coupled to our accelerator, which allowed us to record with great detail the excitation curves presented here. The short term high voltage fluctuation probability density is measured directly with a multichannel analyser by sampling the calibrated signal from the capacitive pick off plate [10,13] (ripple monitor). This yields the b e a m energy spread to within the ion energy spread at the output of the ion source, which is negligible with respect of the voltage fluctuations. Energy spread is continuously monitored in this way; its probability density is nearly Gaussian [10,13]. This spread may be b r o a d e n e d by slow drift effects, which were estimated as described in ref. 10. The best measured short term energy resolution reached during our experiments was - 40 eV fwhm a r o u n d 430 keV. Such a high resolution was exceptional: typical values were rather between 60 to 100 eV. These very low energy spreads seem to depend critically on such ill-controlled parameters as the state of the accelerator belt etc. [10]: work is in progress, attempting to bring the ultimate energy resolution of our accelerator u n d e r control. Drift effects during the m e a s u r e m e n t of the rise of the yield curve (see next paragraph), were estimated to be a r o u n d 30 eV fwhm. The overall energy resolution for the results shown below, recorded while the measured short term energy spread was - 4 0 eV, will thus b e taken as 50 eV fwhm. The measurements were carried out in a scattering c h a m b e r located at a b e a m deflection angle of 18030 '. The c h a m b e r is fitted with a Balzers turbo p u m p and with an internal liquid nitrogen trap. The role of this trap is crucial. In fact one of the main difficulties in such measurements arises from c a r b o n c o n t a m i n a t i o n
161
of the samples. While it is most difficult to get rid from the h y d r o c a r b o n c o n t a m i n a t i o n from the atmosphere it is essential to eliminate further c a r b o n build up due to b o m b a r d m e n t by the b e a m [10,12]. Reduction of hydroc a r b o n c o n t a m i n a t i o n is possible only by using ultra high vacuum type in situ sample cleaning procedures. This is being set up but could not be attempted for the experiments presented here. The limiting vacuum in our c h a m b e r was - 10 6 Torr and carbon build up during a 30 rain measurement was small when all the precautions were taken. The 4.43 MeV y-rays were detected around 90 ° from the beam with a 3 " × 3" NaI(T1) scintillator at 12 cm from the impact point. The y-ray energy window was set from - 2.6 to - 5 MeV. Beam currents were from - 0.5 to - 1 /~A. Typical measurement times for recording an excitation curve a r o u n d the resonance were - 30 rain. To minimize any possible carbon build up at the most critical stage of the measurements the energy scanning was started near the resonance for a fresh impact point o n the target; off-resonance parts of the yield curve were recorded subsequently. For very broad yield curves covering the whole thickness of the target the impact point on the sample could be changed: this had no bearing on the thick target yield plateau even if the target film was slightly non uniform. 3.2. R e s u l ~
Fig. l a shows a b r o a d scan corresponding to the target described in §2.2. The b a c k g r o u n d level off-reson a n c e is very low; it corresponds mainly to cosmics. The plateau of the yield curve is very flat: this is the best p r o o f of the uniformity of our target, meaning that stoichiometry, 160 c o n t a m i n a t i o n in the bulk as well as the aSN isotopic concentration remain stable through the film. The very slow decrease of the curve reflects the effects of energy straggling of the b e a m particles in the film. Fig. l b shows in detail the fast rise of the thick target excitation curve. The striking feature of this rise is that it begins very slowly, the increase above backg r o u n d level starting at ~ 700 eV before the mid point of the rise while the 12% to 88% rise takes place over - 300 eV. Such measurements were repeated independently several times, using energy steps down to 40 eV and yielding similar results. However these measurements could be carried out only with somewhat larger b e a m energy spreads (see §3.1). We analyze here hence the results shown in fig. 1 ; the larger energy step, 80 eV, has no appreciable influence on the results, the slow rise of the curve playing a major role in the determination of F, as will be shown next. It should be emphasized here that the word " c h a n n e l " in the figures is used improperly for our energy scanning device. In fact the analyser works in the multiscaler mode a n d the energy is varied
B. Maurel, G. Amsel / A new measurement of the 429 keV lSN(p, "y)12C resonance
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3.3. Interpretation and best fit
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rise of Gaussian type, with a 12% to 88% increase within 140 eV. It thus appeared to us that the slow start and sharp rise of the curve in fig. l b does correspond to a physical reality.
2
80 eV/channel 0
10
20
::50
40
CHANNELS 15N(p, o~y)12Cexcitation curves
Fig. 1. Typical near the 429 keV resonance for a - 6 0 0 ,~ thick NblSN target. 3"×3" NaI(TI) scintillator at - 90 ° and 12 cm: current - 1 p,A; 10 /~C per point; (a) overall curve with best fit. (b) Rise observed with optimal beam energy resolution. Solid line: best fit calculated for a 26 A thick hydrocarbon contamination layer, F = 120 eV, beam energy spread: 50 eV, Doppler broadening: 90 eV (fwhm) at room temperature. Dashed line: theoretical rise calculated as before but in the absence of surface contamination. A small shift, a steeper rise and a marked Lewis effect show up.
in discrete steps, no smearing effect being associated with the channel widths. We kept the terminology associated with multichannel analyzers for convenience. We never observed such a narrow resonance at comparatively low energies with our facility, our main work having been devoted to the 100 eV wide resonance of 2VAl(p,y)ZSSi at 992 keV and to the 50 eV wide resonance of lSO(p,a)XSN at 1167 keV [12,14]. Around 430 keV beam stability and slowing d o w n effects in possible carbon contamination layers present on the samples could have larger effects than at higher energies. We checked therefore whether the rise observed in fig. l b could not be due to some effect unaccounted for by our above described technique to estimate beam energy spread or to contamination effects. The closest extremely narrow resonance to 430 keV, that of 27Al(p, 7)2aSi at 632 keV ( F < 30 eV) was hence studied with a thick A1203 target [10]. We observed a sharply starting
High resolution excitation curves near very narrow resonances must be interpreted by resorting to the stochastic theory of slowing down, which takes into consideration energy straggling effects and in particular the so-called Lewis effect. These aspects are discussed in ref. 10; they were developed in detail in ref. 12 and reviewed in ref. 14. We give here only the results of our calculations carried out according to principles described in these references. The following factors were taken into consideration for the calculations related to fig. 1: (i) the overall beam energy spread was taken to be F B = 50 eV fwhm. (ii) The actual energy spread as seen by the reacting nuclei is broadened by the Doppler effect due to the thermal vibrations [10]: this effect is Gaussian with a width calculated for the 15N nuclei of F D = 90 eV at room temperature. The total instrumental broadening with Gaussian shape is hence F v -- 100 eV. (iii) The intrinsic line shape of this very narrow resonance at energy E a is a Lorentzian [10] of width F, i.e. a B r e i t - W i g n e r element of the form o (E)
constant
( F / Z ) 2 +( E - ER) 2" It follows that an ideally thin target should give a yield curve which would be the convolution ofa Gaussian of width 100 eV with a Lorentzian [10] of width F, to be determined. As an ideally thin target is not physically realisable we had to use the thick target technique [10,12,14]. (iv) Stoichiometry, and in particular oxygen contamination, were considered as uniform through the film. The latter assumption will be critically discussed in
§3.4. The statistical aspects of slowing down in the thick target were taken into account using our special computer program [12,14]. A simplified collision spectrum (i.e. the energy loss spectrum in a single encounter between the incoming proton and an electron in the medium) was used [12]. The parameters of the latter for our N h N targets may be determined from their known composition (§2.2.) and from the mean energy loss and energy straggling per unit length in the film. These two quantities are deduced from a best fit of the width and slow fall of the broad excitation curve in fig. la, the details of the rise of the curve being irrelevant at this stage. The corresponding fit is shown in fig. l a (where the fit of the steep rise corresponds to the more detailed
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B. Maurel, G. Amsel / A new measurement of the 429 k e V 15N(p, y)12C resonance
calculations shown in fig. lb). It should be noted that the measured width of the curve in fig. l a was found to be in full agreement with the value of the mean energy loss calculated for our target from the tables of Andersen and Ziegler [15]. In this way the following parameters could be deduced from the data: stopping power of the film: d E / d x = 17 e V / , ~ ; straggling p a r a m e t e r (spread variance = s2x): s = 60 eV/A1/2; effective p r o t o n m e a n free p a t h between two electronic collisions [12]: 3.5 A; maxim u m effective energy transfer [12] in a collision: 935 eV; m i n i m u m effective energy transfer [12] in a collision: 13.7 eV. A typical result of the calculations carried out with these parameters, is shown in fig. l b , using the value F = 120 eV. The curve exhibits a marked Lewis effecL i.e. an overshoot, which is not observed experimentally. However the very slow rise at the start of the curve, underlined above, is well reproduced: this is due in fact to the particularly long tails of the Lorentzian, which induce the most peculiar properties of resonance line shapes [10]. For a better interpretation of our data we must take into account a n o t h e r factor, not considered until now in the calculations i.e. c a r b o n contamination, mentioned in {}3.1. In ref. 12 it was established that most samples handled in atmosphere are covered by a h y d r o c a r b o n film of approximate global composition C H 4 and thickness typically a r o u n d 30 ~. (with a density assumed to be - 1). Our fitting procedure consisted thus in choosing the c o m b i n a t i o n of F a n d of hydroc a r b o n thickness leading to a best fit. The energy spread spectrum induced by various thicknesses of hydroc a r b o n films a r o u n d 30 A was hence calculated separately using k n o w n data a n d folded in the results of the above calculations. The exact a m o u n t of h y d r o c a r b o n c o n t a m i n a t i o n actually present during the resonance measurements is difficult to establish. However from our calculations it appears that the range of possible thicknesses is limited. A too thin film does not suppress the unobserved overshoot of the Lewis effect; a too thick film leads to a too slow rise of the curve around its mid point, while it does not reproduce the very slow start of the rise at lower energies if F is too small. On the other h a n d this slow start can be reproduced only if F is also in a relatively narrow range of values. The best fit was o b t a i n e d for the values: h y d r o c a r b o n thickness 26 ,~ (1016 at. C / c m 2) a n d F = 120 eV. The corresponding fit is shown in fig. lb. It should be noticed that while at these low energies h y d r o c a r b o n c o n t a m i n a t i o n masks the Lewis effect, strong overshoots were observed experimentally on resonances occurring at energies a r o u n d 1 MeV, demonstrating the i m p o r t a n c e of taking into account statistical effects in slowing down for interpreting narrow reson a n c e yield curves [12-14].
3.4. Precision
It appears that, provided our accelerator works properly, the main error on F does not arise from the b e a m energy spread. The latter would d o m i n a t e the Doppler b r o a d e n i n g only for measurements on cooled targets. The peculiar properties of the convolution of a gaussian with a Lorentzian [10] are such that in our case a total gaussian width F T up to - 150 eV would not appreciably reduce the precision with respect to that corresponding to the value used here: 100 eV. Thus accelerators with an energy spread of a r o u n d 150 eV, and even more, at 430 keV may be used quite efficiently for high resolution depth profiling with the resonance studied here, if ultimate precision is not required. While for the results shown here the gaussian comp o n e n t of the source of error on F was d o m i n a t e d by the Doppler effect, when the accelerator is less well behaved the two effects are of comparable magnitude. On the other h a n d c a r b o n c o n t a m i n a t i o n is a major source of uncertainty on F. This, by the way, demonstrates the high sensitivity d e p t h profiling ability of this resonance near the surface, as illustrated by fig. lb, which in fact shows the effect of a very thin absorber present on the surface of a clean sample. By carrying out calculations with various thicknesses of h y d r o c a r b o n a n d various values of F we found that, in the present state of our experiments, fits could be obtained, still compatible with the statistical scatter of our data, for values of F between 90 and 150 eV. However these extreme values do not seem very likely a n d we hop that the statement: r : (120__ 30) eV will not be strongly contradicted by future experiments. The question arose whether the oxygen c o n t a m i n a n t
I
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I 40
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CHANNELS Fig. 2. Excitation curve rises calculated with and without hydrocarbon contamination, with the same parameters as in fig. 1 but for a target which would consist in a 65 ,~ NblSN O layer on a thick NblSN layer. These assumptions preclude any fit of the results in fig. lb.
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B. Maurel, G. Amsel , / A new measurement of the 429 ke V ]~NIp, y)]e(, re~onance
in the films is really in the bulk or whether it is rather near the surface. If so it might form an oxynitride of formula N b N O a n d to the a m o u n t of 160 measured there should correspond a 65 A thick N b N O film. Pure n i o b i u m oxide on the surface, without 15N, c a n n o t exceed the m o n o l a y e r range, as this would destroy the steep rise of the curve in fig. lb. Fig. 2 shows calculated curves with and without h y d r o c a r b o n c o n t a m i n a t i o n , assuming 65 * N b N O near the surface. It is clear that such a curve c a n n o t fit our results. A recent p a p e r by F r a n k e n t h a l et al. [16] also seems to suggest that the presence of the oxygen in the bulk of our N b N films is likely.
We t h a n k E. G i r a r d for assistance with the electronics and G. G e n i n and B. Thiel in c o m p u t e r calculations. J. C h a u m o n t is gratefully acknowledged for providing us with the ion implanted ~SN reference target. N o t e a d d e d after the conference
The value I ' = ( 1 2 0 _ + 3 0 ) eV, i.e. for a 15N beam /'N,~ = (1.8 ± 0.45) keV, was fully confirmed in a paper presented at the conference by Damjantschitsch et al. [17]. These authors deduced FN, from a careful meas u r e m e n t of the resonance line shape tails over several decades using a low b a c k g r o u n d detection system. From the observed Lorentzian tail shapes they conclude to I'Nt, - 1.85 keV, with an estimated precision of + 10%.
4. Conclusions The low value found for F shows that d e p t h profiling of 15N is possible with much better depth resolutions than thought until now. In fact the n o m i n a l ultimate d e p t h resolution near the surface corresponding to F = 120 eV is, in N b N : R e = F / ( d E / d x ) = 7 A,. Similarly the corresponding value of FN,~ for hydrogen depth profiling with tSN beams is 1.8 keV, a low value which might lead to further i m p r o v e m e n t s in this technique. Possible applications are illustrated in the second part of this paper. We insisted in this paper on the description of our target fabrication procedures, on their characterization a n d on their various possible contaminations, as those factors set the precision a n d credibility of a very narrow resonance width measurement. We plan to further improve the precision and reliability of these measurem e n t s by operating in ultra high vacuum, by developing cleaning procedures which do not perturb the composition of the samples in the first few angstroms, by cooling the targets and possibly by improving their purity. We shall also try to improve the reliability of our accelerator for very low energy spreads. Such i m p r o v e m e n t s are by no means sheer sport: they are the key to absolute, high precision, non-destructive d e p t h profiling of light nuclei near the surface of solids, including isotopic tracing experiments. We are most grateful to A. Laurent who developed with us the delicate reactive sputtering techniques for the fabrication of 15N labelled N b N targets. We thank S. Rigo for advice related to this field. The help of E. d ' A r t e m a r e in r u n n i n g the accelerator in optimal stability conditions and for data collection was invaluable.
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