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Clustering of krypton in tungsten observed by helium desorption spectrometry
Clustering of krypton in tungsten helium desorption spectrometry received
8 December
A van Veen. Netherlands
1978;
A Warnaar
in revised and
form
L M Caspers,
12 November Physics
observed
by
1979 Department,
Deb?
University
of Technology,
Mekelweg
15.2628
RT De/H.
A helium desorption spectrometer with small desorption volume incorporated in a large multipurpose uhv target chamber is described. With the spectrometer results have been obtained on substitutional Kr in W ( 100) that are consistent with previous work. Further experimental results obtained with higher Kr dose indicate the occurrence of Kr vacancy clustering. Reduction of these clusters to pairs of substitutional Kr atoms by annealing to temperatures above 1500 K can be observed by the occurrence of new characteristic helium desorption peaks.
1. Intraduction The fechnique of thermal desorption spectrometry (THDS), developed by Kornel&’ and Carter2 has proven to be a useful tool for the identification of defects in metals and the understanding of the defect behaviour. So far THDS has been applied to vacancies3 (V) in nickel and molybdenum and krypton vacancy complexes (KrV,; n denotes the number of vacancies) in molybdenum and tungsten3-‘. Earlier the authors together with Kornelsen have shown6 that KrV,, complexes in tungsten, obtained with a dose of 8.x 10” cms2 5 keV Kr+ ions, can be reduced to substitutional Kr (denoted KrV) by annealing to a temperature above 1400 K. In this work higher doses of 5 keV Kr implanted ions have been used in order to promote krypton clustering. Annealing measurements showed that with such doses stable krypton pairs of substitutional krypton (Kr2V2) are formed which bind a helium atom with a higher energy than does a single Kr atom. The details of this work are given in Section 3 followed by a discussion in Section 4.
The measurements ‘have been carried out in a new apparatus meant to evaluate the effects of ion implantation not only with THDS but also with other damage probing techniques, e.g. proton backscattering. THDS requires a small target chamber volume whereas the other techniques require a large target chamber to accomodate in vacua rotatable analysers (see v d Berga who encountered the same problem). In Section 2 will be described how these two requirements are met and further experimental details are given. 2. The apparatus and its operation 2.1. The target chamber and the vacuum system. The multipurpose stainless steel uhv target chamber with the equipment in use for helium desorption measurements is shown in Figure 1. The crystal and the electron gun for crystal heating is housed in a half open cylinder (A) attached to a plateau, which can be rotated (B), so that the crystal can be positioned either facing the ion source (C) or the mass analyser. The quadrupole mass
Figure 1. The interior of the target chamber with the target housing (A), the rotational drive of the target(B), the ion source(C), the quadrupole analysersystemwith the channeltron and dynode multiplier (D) and the translational drive of the quadrupole system @). Vacuum/volume @ Pergemon
30fnumber Press Ltd/Pri&ed
3.
0042-207X/80/0301-0109$02.00/0 in Greet Britein
109
A van
Veen et a/: Clustering
of Kr in W observed
by the desorption
spectrometry
analyser, also enclosed in a cylinder, can be moved towards the crystal housing by a linear motion drive (E) over a distance of 5 cm. The enclosure of the mass analyser and the crystal housing together constitute a volume of 1.4 I used in the desorption experiment. The conductance between this volume and the large volume (53 I) of the vacuum chamber can be regulated by varying the separation between the analyser and the crystal. The target chamber is pumped (see Figure 2) by a titanium getter pump, a turbomolecular pump (Balzers, 450 I s-l), an oil diffusion pump and a fore pump in series. The oil diffusion pump (100 I s-l) has been included in this series of pumps to improve the partial helium pressure in the target chamber during the helium desorption experiment. It reduces the partial pressure of helium at the backing side of the turbomolecular pump to a value of
tained at a total pressure <2.10m8 Pa while the filaments of the ion sources are operating. The gas supply system (see Figure 2 at E) consistsof two gasbottles from which the gas can be led to a leak valve connectedwith the ion source.The systemcan be pumped(in the caseof changingthe gasspecies) by the fore pump and the oil diffusion pump.
energyfrom 100eV to 6 keV. The ion sourceoperatesat a low pressure(lo-’ Pa). The gas is directly led into the ion source which is only pumpedby the smallaperture (0 3 mm) of the extraction slit; therefore the pressurein the vacuum chamber risesonly to about 5 x 10v5 Pa during the operation of the ion source. Ions are extracted from the plasma,that is constricted to the axis of the sourceby a parallel magneticfield, by an extraction electrodewith Piercegeometry(seethe scheme of the electrode configuration in Figure 5). The formed ion beamfurther can be focusedby a 3-elementeinzellens.At the target the beamdiameteris4 mm; the beamdivergenceamounts to 1”. To ensurea uniform bombardmentof a selectedpart of the target surfacethe beamis sweptby two pairs of deflection platesin two directions over a collimating slit (diameter4 mm and separated5 mm from the target surface) in front of the target. Secondary electrons from the target are collected by another electrodewhich isnot exposedto the primary ion beam. The slitsanddeflectionplatesaremounted(electrically insulated) on a plate (Figure 3) which movesaway from the target when the target housingis turned to the desorptionposition. In this
2.2. The ionsource.The electron impact ion source (of our own design)iscapableof delivering up to IO-’ A noble gasionswith
Figure 3. Positions of the slit (S), the ion source (B), the target housing (K) and the quadrupole (Q) during (a) the ion bombardment and (b) the desorption cycle.
way the slits are excluded from the desorption volume and so (earlier injected) helium desorbingfrom the slits cannot contribute to the helium desorption spectra. The ion gun design permits the inclusion of a Wien filter. In the presentmeasurements,however, sucha filter has not been used; this omission is believednot to affect the results(seeSection 3.2). Figure 4 shows the mounting of the monocrystallinetarget which can be heated by electron bombardment.Electronsfrom the filament (J) at earth potential are acceleratedtowards the crystal at a positive potential 2 kV. The cylindrical electrode (H) and the grid (I) are at a negative potential with respectto the crystal to focus the electronson the crystal. The crystal (C) is kept in position via a ceramiccylinder (G) by three tungsten pins (B) which fit in three spark eroded holes.The W-Re 5%, W-Re 23% thermocouple(K,D) fits in a fourth hole. The molybdenumflange(E) on the front sideof the metal target housing(F) can be brought into closecontact with an identical flange on the front side of the quadrupole housingto connect both volumes.
2.3. The target housing.
Figure 2. Scheme of the vacuumsystem with the uhv target chamber
pumpedby a titaniumsublimation(A), turbomolecular (B), diffusion (C) androughingpump(D), and the gasinlet system(E). 110
A van Veen et al: Clusteringof Kr in W observedby the desorptionspectrometry
r
ABC
DE
F
G.li
IJ
K
Figure 4. Targethousing(seethe text). x
2.4. The helium detection system. The helium detection system
consistsof a quadrupolemassspectrometerand a parallel plate ion deflecting device to direct the ions to one of the available ion detectors: a Faraday cup, a dynode multiplier and a channeltron multiplier (see Figure 1). The desorbedhelium atoms are ionized in the ion source and acceleratedinto the quadrupole system.The analysedions impinge on either the Faraday cup where they are detectedor passon through the deflectingdevice(seealsothe schemein Figure 5). The deflecting deviceconsistsof three parallel plates,which form two electrostatic analysersin series.This enablesus to guide the massanalysedions which have a certain energy to the channeltron (usingthe first analyser)or to the dynode multiplier (usingboth analysers). Figure 5 showsa schemeof the electronic equipment; the lower part representsthe data handlingsystem. Pulsesfrom the channeltron multiplier (Section 2.4) are amplified and are fed into a rate meter and two scalerscontrolled by a computer. The signalof the rate meter and the thermocouple signal are recorded with a two-pen recorder. The computer storesthe number of pulsesaccumulatedby the scalerduring adjustable sampling periods (> 10ms). The two scalersare used in alternating mode in order to reduce the dead time involved in the transfer of numbersto the computer. In each samplingperiod the (analogue)thermocouplesignal is fed, via an ADC (analogue to digital converter) into the computer simultaneouslywith the pulse signal. After completion of the measurement(at maximum 1024 sampling periods) the data are stored in a buffer memory for subsequentprocessing (correcting for the vacuum time constant, curve smoothingand plotting). 2.5. Data handling.
2.6. The vacuum time constant. The relation betweenthe helium desorption rate t(r) and the measuredpartial helium pressure f’(r) during heating of the target is given by
L(r) = v g+’ T> (
.l
Y
PLOT ioi
L_DaT~HA_N~LING__.-.~-_-----_--_J Figure 5. Scheme of theelectronicequipment.
where 7 is the vacuum time constant = V/S, with V = desorption volume and S = the pumpingspeedfor helium. The vacuumtime constantT isan important parameter,determining the designand use of a desorption spectrometer.Generally T ischosento beeither very smallor very largesothat L(t) follows by differentiating of J’(r) if T --t a3 or is proportional to P if 7 = 0. In both methodsof operation a small V is favourable for obtaining high signals.A disadvantageof the first method (T+ m) is the accumulationof gasin the system.When high dosesof helium and other implanted gasatomsare desorbed pressureswill be reachedat which the proper operationof the massspectrometeris disturbed.A secondproblem is the detection of smallamountsof desorbinggasat the end of the desorption cycle. The measuredsignalalwaysincludesd/N noise(N = number of pulses).When the pressureis high, due to earlier desorbedgas,a smallincreaseof the numberof pulseswill drown in the background noise.Sincewe wanted to usehelium doses up to lOi cmw2,we prefer the secondmethod (T f~ 0). The arrangementmentionedin Section2.1 enabledusto adjustT between0.1 and I s. The valueof the vacuumconstantT is derived from the exponentialdecreaseofP after the heliumreleasefrom the crystal is stoppedabruptly (by turning off the electron bombardment).It is of interestto mentionherethat Kornelsen,who usesmuch lower helium doses
on krypton clustering in W
Annealing of the W (100)crystal betweenthe damageand He injection stagegave
3.1. High dose krypton annealing experiment.
111
,4 van l/een et
a/:
Clustering of Kr in W observed by the desorption
spectrometry
the results presented in Figure 6. For each of the indicated anneal temperatures T,, the following procedure was repeated: (1) annealing of the crystal to T = 2500 K to make the crystal defect free, (2) creation of defects with a dose of 1013 cm-’ ions (normally incident), (3) annealing
to temperature
5 keV Kr+
T,,
(4) low energy helium injection with 8 x 10” cmm2 250eV He+ ions,
the
(5) recording
heating to 2500 K.
of the helium release during
standard
dose
In all cases the heating was done in an identical way. The heating rate fi was 35 K s-t from T = 300 to 1200 K; after 1200 K /3 decreased to 25 K s-t (see also Section 3.3). Besides the desorption peaks known from low dose krypton experiments, i.e. A, B, C, H’, H, I in refs 1 and 6, new peaks are observed. Two of them, which we named P and Q peak, have relatively low desorption temperatures; they appear after annealing above 1600 K. From the growth of the P and Q peak content with the helium dose it is found (Section 3.2) that the P and Q peaks are similar to the A, B and C peaks which are caused by interstitial clustering of helium around substitutional krypton6. The high temperature peaks are caused by release of helium trapped by larger krypton clusters; see Sections 3.2 and 3.3. 3.2. Peak assignments. HI, HII, HII,. Most peaks of Figure 6 have already earlier been obtained by Kornelsen3 who used a Wien filtered ion beam. The agreement for corresponding peaks of his results with ours makes us confident that the omission of a Wien filter has not influenced our measurements. The complex of peaks in Figure 6 between T = 1400 and 1600 K in the helium desorption spectra can be deconvoluted into three separate peaks, here called H,, H,,, HI,,. Kornelsen denoted these peaks in ref I the H’, H and I peak. We prefer the new nomenclature because all peaks are not exclusively caused by helium release from vacancies and vacancy complexes but also from krypton vacancy complexes, as will be discussed below. Table 1 summarizes the reactions in earlier
Table 1 Peak A B C HI (H’)
Reaction
He,KrV+KrV He,KrV+ KrV He,KrV + KrV HeKrVz-+ He +He HII W HeV HI,, (I) He2VZ” + 2He
Described in reference $ He 7 2He + iHe (i = 3,6) i- KrV2 SV t 2V
1 1 1 6 6 6
publications’u6 associated with various peaks. We expect that after the krypton ion bombardment has been completed the defect population consists of krypton vacancy complexes, denoted KrV,, and vacancies or vacancy complexes well separated from the implanted krypton. As regards the helium release from these defects, if filled with helium, it should be 112
Figure 6. Annealing of the damage created by 5 keV Kr+ ions (dose lOI cm-*) in W (100) observed by helium desorption spectrometry (THDS). The helium desorption spectra are obtained after filling of the damage with 250 eV He+ ions at a dose 8 x 10’2cm-2. T,, indicates the anneal temperature (see definition in Section 3.3). noted that in general dissociation. Krypton 2 helium atoms will excess vacancies (i.e. thereafter helium will
vacancy dissociation competes with helium vacancy complexes containing, e.g. 1 or upon heating first reduce the number of not occupied by krypton) to about 1 or 2, be released from the reduced Kr vacancy
A
van
Veen
et
al:
Clustering of Kr in W observed by the desorption spectrometry
complex. Therefore the desorption peaks Hr. H,,, HI,,, which we assign to Kr vacancy complexes with more than one excess vacancy, are found close to the peak temperature of helium release from monovacancies. The small differences in release temperature are caused by the presence of krypton in the vacancy complexes during the helium release. Annealing of the sample, between Kr ion bombardment and helium filling, results in a change of the HI, Htt, Hut peak populations and their final disappearance in the temperature interval TA = 1300and 1400K. From this anneal behaviour further information can be obtained on the nature of the defect complexes which contribute to the HI, HII, Hm peaks in the helium desorption spectrum(seeSection 3.3).
to the suggestionthat the P and Q peaksare alsocausedby an interstitial helium clustering process.Since the helium dissociation energy in the P and Q peaksis higher than the dissociation energy in the A, B and C peaks interstitial helium clusteringto Kr2V2 seemsvery probable (seealso Section 3.4 for the Kr clusteringmechanism). High temperature (> 1800 K) peaks. The peakswith very high desorption temperaturesare causedby large krypton vacancy complexes(number of krypton atoms >2) with more space than one vacancy available for trapping helium. The exact assignmentof thesepeaksrequiresfurther experimenting. stages. The annealingmethodusedin this study, i.e. heating the samplefast (dT/df = 20-40 K s-r) leadsto a shift of annealingstagesto higher temperatureascomparedto isochronalannealing.To enablecomparisonwith, e.g.resistance recovery and Mossbauer measurements,in which often isochronal annealingis used, we first derive an expressionfrom which the above shift can be estimated. A first order dissociationprocessis describedby
3.3. Annealing A, B, C, P, Q peaks. In an earlier study A, B and C peaks
have been assignedto helium release from substitutional krypton (KrV). The assignmentwas based6on (i) the growth of the A, B and C peaks with helium doses which differed fundamentally from the growth of vacancy type releasepeaksupon helium filling and (ii) the appearanceof the peaks at an anneal temperature correspondingto the releaseof the excessvacanciesof the KrV, complexes(KrV, -+ KrV). Since P and Q peaks,like the A, B and C peaks,appear at high anneal temperature we investigated the helium dose dependencyof the P and Q peaks,the result of which is shown in Figure 7.
dN dt = -NV exp( - Q/kT)
where N is the number of atoms still in the sample,Y is the frequency factor (RZlOI s-l), k is the Boltzmann constant, Q is the activation energy and T is the absolutetemperature. When the sampleis heated linear in time (dT/dt = fi) the maximumof the dissociationrate appearsat T = T, according to d2N = 0 + exp( -
dT2
Q/H',,,)= PQ/vkT,$
(3)
The relation betweenT,,, and Q, calculatedfor fi = 35 K S- ‘, is given in Figure 8. At the peak temperatureT,,, the fraction of
50-
t
(2)
-
t- loEl I- -z5 * 52 aw _
HELIUMDOSE
x 10’2/cm2-c
Figure 7. Normalizedheliumcontents of the peaksA, B, C and
A
P,Q vs the heliumfilling dose.
0
1
2 ACTIVATION
With low helium dosepeak P, which has the lowest binding energy, is the most prominent peak. With increasinghelium dosethe Q peak, which has the higher binding energy grows faster than the P peak. This P/Q peak helium dosedependency is identical to the behaviour of the A, B and C peaks.This leads
3 ENERGY
4
3
P (~‘4)
Figure 8. The temperature at whicha thermallyactivateddissociation process (frequency factor v = IO” s- ’ and activation energy Q :V) is evolved to thesamestage(M/No = 0.43)in caseof linearanneahng and isochronal annealing indicated by T,,, and 7’,, respectively. The dashed curve refers to the 35/25 KS -’ heating cycle used in this
work.
113
A van
Veen
et al: Clustering
Figure 9. Helium
contents
of Kr in W observed
by the desorption
of the A, B. C, P, Q and
Hr. H,,,
H,,,
defects not yet dissociated amounts to N/No = 0.43, where No is the number of defects present prior annealing. When isochronal annealing is applied (anneal time t,, = I h) the same stage is reached at a temperature T, which is always lower than T, and is found from =
0.43,
wheref(T,) = v exp( - Q/kr,). between T, and Q. We now discuss the annealing 6. As shown in Figure 9 the distinguished.
Figure
N/N,
(I)
= exp(-
36OOf(T,))
(4) 8 also shows
the relation
of the various peaks in Figure following anneal stages can be
The HII peak decreases at 700 K. This stage can be ascribed to the migration of the vacancies (stage III). These vacancies migrate to the surface or form vacancy clusters with or without a krypton atom. This also explains the simultaneous increase of the HI fraction which will be ascribed to KrV, in this temperature range.
(2)
At 1350 K, corresponding with an activation energy 3.6 & 0.05 eV (u = lOI s-l), the Ht peak decreases and the A, B and C peaks increase. Since the KrVz defect causes the HI peak and the KrV defect is the origin of the A, B and C peaks complex it seems reasonable to ascribe this anneal stage to reaction KrV2d KrV + V.
(3)
At 1500 K (3.80 eV) the Hut peak decreases and the P and Q peaks start to develop as discussed before (Section 3.2). The P and Q peaks are formed by helium trapped by the Kr2V2 cluster. We suggest therefore, that the decrease of the Httr peak in this temperature region is described by the ceactton Kr2V, + Kr2V, + V.
The Kr2V, complexes (which when filled with helium cause the Hm peak in the 1300-1500 K temperature range) are most likely formed by the clustering of mobile KrV. complexes, i.e. reactions like KrV. -1 KrV,KrZVn+.,. This clustering takes place in a temperature region where KrVz defects still exist and are not dissociated to substitutional krypton. From model calculations follows that the migration 114
spectrometry
peaks
of Figure
6 vs the annealing
temperature
T*.
energy of KrVz is some 0.4 eV lower than the dissociation energy of KrV,; therefore clustering by the reactions KrV, i KrVKr2V, or KrVz $ KrV,Kr2V, can be expected. These clustering reactions are then followed by the vacancy dissociation at the temperature where, in the helium desorption spectrum, the P peak appears. We finally note that the length of the temperature interval in which the Htt and HI,, peaks disappear and the development of the A, B, C, P, Q peaks indicate a first order desorption process. On the contrary the faint slope of the decrease of the P, Q, A, B and C peaks indicate a diffusional release process of the Kr,V. complexes 01 = I,Z). This process starts at 1800 K. At this temperature the thermal vacancy concentration is large enough to make diffusion by the vacancy mechanism possible.
4. Concluding
remarks
The measurements described in Section 2 have shown that clusters of substitutional Kr in W can be made by implantation of 5 keV Kr+ ions at a dose higher than lOI Kr cm-’ followed by heating to temperatures > 1500 K so as to enable migration of KrV,, (II > I) and clustering to Kr2V, complexes with II > 2. Subsequently the number of vacancies is reduced to 2 leaving a pair of substitutional Kr atoms (Kr2V,). Helium binds to Kr2V2 with higher binding energy than it does to KrV; the binding mechanism is characteristic for interstitial clustering, i.e. further added helium is bound stronger. It would be interesting to know whether, for example, Kr,V, complexes with II > 2 can also be made with the above procedure. We expect that vacancies become more strongly bound when II increases. So, for example, KJV, may only be stable if II > 3. Future work will include these larger clusters and will deal with the mobility of the KrV. complexes.
References ’ E V Kornelsen, Radioior Efl, 13, 1972, 227. ’ G Carter and J S Colligon, /OU Bumbardnreur Educational Books, London, chap. 8 (1968).
of Solids.
Heineman
A
van
Veen
et
a/:
Clustering of Kr in W observed by the desorption spectrometry
3 E V Kornelsen and D E Edwards, Jr, Applications of Ion Reams fo Me/ah. Edited bv A T Picreaux. E P Eernisse and F L Vook. Plenum Press, New York, p. 521 (1973). J S E Donelly, Yacmm, 28, 1978, 163. 5 A van Veen and L M Caspers, Proc 7th Int Vat Congr and 3rd Int Conf Solid Surfaces. Edited by Dobrozemsky er al, Vienna, p. 2637 (1976).
6 A van Veen, L M Caspers, E V Kornelsen, R Fastenau, A van Gorkum and A Warnaar, Phys Sratus Solidi, A40, 1977, 235. ’ L M Caspers, A van Veen,.A van Gorkum, A van den Beukel and D M van Baa], Plrj~ Sram Solidi, A37, 1976, 371. ’ J A van der Berg, D G Armour and L K Verheij, Inst Phys Conf Ser No. 38. Edited by K G Stephens er al. The Institute of Physics, Bristol and London, p. 298 (1977).
115
8 December
A van Veen. Netherlands
1978;
A Warnaar
in revised and
form
L M Caspers,
12 November Physics
observed
by
1979 Department,
Deb?
University
of Technology,
Mekelweg
15.2628
RT De/H.
A helium desorption spectrometer with small desorption volume incorporated in a large multipurpose uhv target chamber is described. With the spectrometer results have been obtained on substitutional Kr in W ( 100) that are consistent with previous work. Further experimental results obtained with higher Kr dose indicate the occurrence of Kr vacancy clustering. Reduction of these clusters to pairs of substitutional Kr atoms by annealing to temperatures above 1500 K can be observed by the occurrence of new characteristic helium desorption peaks.
1. Intraduction The fechnique of thermal desorption spectrometry (THDS), developed by Kornel&’ and Carter2 has proven to be a useful tool for the identification of defects in metals and the understanding of the defect behaviour. So far THDS has been applied to vacancies3 (V) in nickel and molybdenum and krypton vacancy complexes (KrV,; n denotes the number of vacancies) in molybdenum and tungsten3-‘. Earlier the authors together with Kornelsen have shown6 that KrV,, complexes in tungsten, obtained with a dose of 8.x 10” cms2 5 keV Kr+ ions, can be reduced to substitutional Kr (denoted KrV) by annealing to a temperature above 1400 K. In this work higher doses of 5 keV Kr implanted ions have been used in order to promote krypton clustering. Annealing measurements showed that with such doses stable krypton pairs of substitutional krypton (Kr2V2) are formed which bind a helium atom with a higher energy than does a single Kr atom. The details of this work are given in Section 3 followed by a discussion in Section 4.
The measurements ‘have been carried out in a new apparatus meant to evaluate the effects of ion implantation not only with THDS but also with other damage probing techniques, e.g. proton backscattering. THDS requires a small target chamber volume whereas the other techniques require a large target chamber to accomodate in vacua rotatable analysers (see v d Berga who encountered the same problem). In Section 2 will be described how these two requirements are met and further experimental details are given. 2. The apparatus and its operation 2.1. The target chamber and the vacuum system. The multipurpose stainless steel uhv target chamber with the equipment in use for helium desorption measurements is shown in Figure 1. The crystal and the electron gun for crystal heating is housed in a half open cylinder (A) attached to a plateau, which can be rotated (B), so that the crystal can be positioned either facing the ion source (C) or the mass analyser. The quadrupole mass
Figure 1. The interior of the target chamber with the target housing (A), the rotational drive of the target(B), the ion source(C), the quadrupole analysersystemwith the channeltron and dynode multiplier (D) and the translational drive of the quadrupole system @). Vacuum/volume @ Pergemon
30fnumber Press Ltd/Pri&ed
3.
0042-207X/80/0301-0109$02.00/0 in Greet Britein
109
A van
Veen et a/: Clustering
of Kr in W observed
by the desorption
spectrometry
analyser, also enclosed in a cylinder, can be moved towards the crystal housing by a linear motion drive (E) over a distance of 5 cm. The enclosure of the mass analyser and the crystal housing together constitute a volume of 1.4 I used in the desorption experiment. The conductance between this volume and the large volume (53 I) of the vacuum chamber can be regulated by varying the separation between the analyser and the crystal. The target chamber is pumped (see Figure 2) by a titanium getter pump, a turbomolecular pump (Balzers, 450 I s-l), an oil diffusion pump and a fore pump in series. The oil diffusion pump (100 I s-l) has been included in this series of pumps to improve the partial helium pressure in the target chamber during the helium desorption experiment. It reduces the partial pressure of helium at the backing side of the turbomolecular pump to a value of
tained at a total pressure <2.10m8 Pa while the filaments of the ion sources are operating. The gas supply system (see Figure 2 at E) consistsof two gasbottles from which the gas can be led to a leak valve connectedwith the ion source.The systemcan be pumped(in the caseof changingthe gasspecies) by the fore pump and the oil diffusion pump.
energyfrom 100eV to 6 keV. The ion sourceoperatesat a low pressure(lo-’ Pa). The gas is directly led into the ion source which is only pumpedby the smallaperture (0 3 mm) of the extraction slit; therefore the pressurein the vacuum chamber risesonly to about 5 x 10v5 Pa during the operation of the ion source. Ions are extracted from the plasma,that is constricted to the axis of the sourceby a parallel magneticfield, by an extraction electrodewith Piercegeometry(seethe scheme of the electrode configuration in Figure 5). The formed ion beamfurther can be focusedby a 3-elementeinzellens.At the target the beamdiameteris4 mm; the beamdivergenceamounts to 1”. To ensurea uniform bombardmentof a selectedpart of the target surfacethe beamis sweptby two pairs of deflection platesin two directions over a collimating slit (diameter4 mm and separated5 mm from the target surface) in front of the target. Secondary electrons from the target are collected by another electrodewhich isnot exposedto the primary ion beam. The slitsanddeflectionplatesaremounted(electrically insulated) on a plate (Figure 3) which movesaway from the target when the target housingis turned to the desorptionposition. In this
2.2. The ionsource.The electron impact ion source (of our own design)iscapableof delivering up to IO-’ A noble gasionswith
Figure 3. Positions of the slit (S), the ion source (B), the target housing (K) and the quadrupole (Q) during (a) the ion bombardment and (b) the desorption cycle.
way the slits are excluded from the desorption volume and so (earlier injected) helium desorbingfrom the slits cannot contribute to the helium desorption spectra. The ion gun design permits the inclusion of a Wien filter. In the presentmeasurements,however, sucha filter has not been used; this omission is believednot to affect the results(seeSection 3.2). Figure 4 shows the mounting of the monocrystallinetarget which can be heated by electron bombardment.Electronsfrom the filament (J) at earth potential are acceleratedtowards the crystal at a positive potential 2 kV. The cylindrical electrode (H) and the grid (I) are at a negative potential with respectto the crystal to focus the electronson the crystal. The crystal (C) is kept in position via a ceramiccylinder (G) by three tungsten pins (B) which fit in three spark eroded holes.The W-Re 5%, W-Re 23% thermocouple(K,D) fits in a fourth hole. The molybdenumflange(E) on the front sideof the metal target housing(F) can be brought into closecontact with an identical flange on the front side of the quadrupole housingto connect both volumes.
2.3. The target housing.
Figure 2. Scheme of the vacuumsystem with the uhv target chamber
pumpedby a titaniumsublimation(A), turbomolecular (B), diffusion (C) androughingpump(D), and the gasinlet system(E). 110
A van Veen et al: Clusteringof Kr in W observedby the desorptionspectrometry
r
ABC
DE
F
G.li
IJ
K
Figure 4. Targethousing(seethe text). x
2.4. The helium detection system. The helium detection system
consistsof a quadrupolemassspectrometerand a parallel plate ion deflecting device to direct the ions to one of the available ion detectors: a Faraday cup, a dynode multiplier and a channeltron multiplier (see Figure 1). The desorbedhelium atoms are ionized in the ion source and acceleratedinto the quadrupole system.The analysedions impinge on either the Faraday cup where they are detectedor passon through the deflectingdevice(seealsothe schemein Figure 5). The deflecting deviceconsistsof three parallel plates,which form two electrostatic analysersin series.This enablesus to guide the massanalysedions which have a certain energy to the channeltron (usingthe first analyser)or to the dynode multiplier (usingboth analysers). Figure 5 showsa schemeof the electronic equipment; the lower part representsthe data handlingsystem. Pulsesfrom the channeltron multiplier (Section 2.4) are amplified and are fed into a rate meter and two scalerscontrolled by a computer. The signalof the rate meter and the thermocouple signal are recorded with a two-pen recorder. The computer storesthe number of pulsesaccumulatedby the scalerduring adjustable sampling periods (> 10ms). The two scalersare used in alternating mode in order to reduce the dead time involved in the transfer of numbersto the computer. In each samplingperiod the (analogue)thermocouplesignal is fed, via an ADC (analogue to digital converter) into the computer simultaneouslywith the pulse signal. After completion of the measurement(at maximum 1024 sampling periods) the data are stored in a buffer memory for subsequentprocessing (correcting for the vacuum time constant, curve smoothingand plotting). 2.5. Data handling.
2.6. The vacuum time constant. The relation betweenthe helium desorption rate t(r) and the measuredpartial helium pressure f’(r) during heating of the target is given by
L(r) = v g+’ T> (
.l
Y
PLOT ioi
L_DaT~HA_N~LING__.-.~-_-----_--_J Figure 5. Scheme of theelectronicequipment.
where 7 is the vacuum time constant = V/S, with V = desorption volume and S = the pumpingspeedfor helium. The vacuumtime constantT isan important parameter,determining the designand use of a desorption spectrometer.Generally T ischosento beeither very smallor very largesothat L(t) follows by differentiating of J’(r) if T --t a3 or is proportional to P if 7 = 0. In both methodsof operation a small V is favourable for obtaining high signals.A disadvantageof the first method (T+ m) is the accumulationof gasin the system.When high dosesof helium and other implanted gasatomsare desorbed pressureswill be reachedat which the proper operationof the massspectrometeris disturbed.A secondproblem is the detection of smallamountsof desorbinggasat the end of the desorption cycle. The measuredsignalalwaysincludesd/N noise(N = number of pulses).When the pressureis high, due to earlier desorbedgas,a smallincreaseof the numberof pulseswill drown in the background noise.Sincewe wanted to usehelium doses up to lOi cmw2,we prefer the secondmethod (T f~ 0). The arrangementmentionedin Section2.1 enabledusto adjustT between0.1 and I s. The valueof the vacuumconstantT is derived from the exponentialdecreaseofP after the heliumreleasefrom the crystal is stoppedabruptly (by turning off the electron bombardment).It is of interestto mentionherethat Kornelsen,who usesmuch lower helium doses
on krypton clustering in W
Annealing of the W (100)crystal betweenthe damageand He injection stagegave
3.1. High dose krypton annealing experiment.
111
,4 van l/een et
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Clustering of Kr in W observed by the desorption
spectrometry
the results presented in Figure 6. For each of the indicated anneal temperatures T,, the following procedure was repeated: (1) annealing of the crystal to T = 2500 K to make the crystal defect free, (2) creation of defects with a dose of 1013 cm-’ ions (normally incident), (3) annealing
to temperature
5 keV Kr+
T,,
(4) low energy helium injection with 8 x 10” cmm2 250eV He+ ions,
the
(5) recording
heating to 2500 K.
of the helium release during
standard
dose
In all cases the heating was done in an identical way. The heating rate fi was 35 K s-t from T = 300 to 1200 K; after 1200 K /3 decreased to 25 K s-t (see also Section 3.3). Besides the desorption peaks known from low dose krypton experiments, i.e. A, B, C, H’, H, I in refs 1 and 6, new peaks are observed. Two of them, which we named P and Q peak, have relatively low desorption temperatures; they appear after annealing above 1600 K. From the growth of the P and Q peak content with the helium dose it is found (Section 3.2) that the P and Q peaks are similar to the A, B and C peaks which are caused by interstitial clustering of helium around substitutional krypton6. The high temperature peaks are caused by release of helium trapped by larger krypton clusters; see Sections 3.2 and 3.3. 3.2. Peak assignments. HI, HII, HII,. Most peaks of Figure 6 have already earlier been obtained by Kornelsen3 who used a Wien filtered ion beam. The agreement for corresponding peaks of his results with ours makes us confident that the omission of a Wien filter has not influenced our measurements. The complex of peaks in Figure 6 between T = 1400 and 1600 K in the helium desorption spectra can be deconvoluted into three separate peaks, here called H,, H,,, HI,,. Kornelsen denoted these peaks in ref I the H’, H and I peak. We prefer the new nomenclature because all peaks are not exclusively caused by helium release from vacancies and vacancy complexes but also from krypton vacancy complexes, as will be discussed below. Table 1 summarizes the reactions in earlier
Table 1 Peak A B C HI (H’)
Reaction
He,KrV+KrV He,KrV+ KrV He,KrV + KrV HeKrVz-+ He +He HII W HeV HI,, (I) He2VZ” + 2He
Described in reference $ He 7 2He + iHe (i = 3,6) i- KrV2 SV t 2V
1 1 1 6 6 6
publications’u6 associated with various peaks. We expect that after the krypton ion bombardment has been completed the defect population consists of krypton vacancy complexes, denoted KrV,, and vacancies or vacancy complexes well separated from the implanted krypton. As regards the helium release from these defects, if filled with helium, it should be 112
Figure 6. Annealing of the damage created by 5 keV Kr+ ions (dose lOI cm-*) in W (100) observed by helium desorption spectrometry (THDS). The helium desorption spectra are obtained after filling of the damage with 250 eV He+ ions at a dose 8 x 10’2cm-2. T,, indicates the anneal temperature (see definition in Section 3.3). noted that in general dissociation. Krypton 2 helium atoms will excess vacancies (i.e. thereafter helium will
vacancy dissociation competes with helium vacancy complexes containing, e.g. 1 or upon heating first reduce the number of not occupied by krypton) to about 1 or 2, be released from the reduced Kr vacancy
A
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Veen
et
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Clustering of Kr in W observed by the desorption spectrometry
complex. Therefore the desorption peaks Hr. H,,, HI,,, which we assign to Kr vacancy complexes with more than one excess vacancy, are found close to the peak temperature of helium release from monovacancies. The small differences in release temperature are caused by the presence of krypton in the vacancy complexes during the helium release. Annealing of the sample, between Kr ion bombardment and helium filling, results in a change of the HI, Htt, Hut peak populations and their final disappearance in the temperature interval TA = 1300and 1400K. From this anneal behaviour further information can be obtained on the nature of the defect complexes which contribute to the HI, HII, Hm peaks in the helium desorption spectrum(seeSection 3.3).
to the suggestionthat the P and Q peaksare alsocausedby an interstitial helium clustering process.Since the helium dissociation energy in the P and Q peaksis higher than the dissociation energy in the A, B and C peaks interstitial helium clusteringto Kr2V2 seemsvery probable (seealso Section 3.4 for the Kr clusteringmechanism). High temperature (> 1800 K) peaks. The peakswith very high desorption temperaturesare causedby large krypton vacancy complexes(number of krypton atoms >2) with more space than one vacancy available for trapping helium. The exact assignmentof thesepeaksrequiresfurther experimenting. stages. The annealingmethodusedin this study, i.e. heating the samplefast (dT/df = 20-40 K s-r) leadsto a shift of annealingstagesto higher temperatureascomparedto isochronalannealing.To enablecomparisonwith, e.g.resistance recovery and Mossbauer measurements,in which often isochronal annealingis used, we first derive an expressionfrom which the above shift can be estimated. A first order dissociationprocessis describedby
3.3. Annealing A, B, C, P, Q peaks. In an earlier study A, B and C peaks
have been assignedto helium release from substitutional krypton (KrV). The assignmentwas based6on (i) the growth of the A, B and C peaks with helium doses which differed fundamentally from the growth of vacancy type releasepeaksupon helium filling and (ii) the appearanceof the peaks at an anneal temperature correspondingto the releaseof the excessvacanciesof the KrV, complexes(KrV, -+ KrV). Since P and Q peaks,like the A, B and C peaks,appear at high anneal temperature we investigated the helium dose dependencyof the P and Q peaks,the result of which is shown in Figure 7.
dN dt = -NV exp( - Q/kT)
where N is the number of atoms still in the sample,Y is the frequency factor (RZlOI s-l), k is the Boltzmann constant, Q is the activation energy and T is the absolutetemperature. When the sampleis heated linear in time (dT/dt = fi) the maximumof the dissociationrate appearsat T = T, according to d2N = 0 + exp( -
dT2
Q/H',,,)= PQ/vkT,$
(3)
The relation betweenT,,, and Q, calculatedfor fi = 35 K S- ‘, is given in Figure 8. At the peak temperatureT,,, the fraction of
50-
t
(2)
-
t- loEl I- -z5 * 52 aw _
HELIUMDOSE
x 10’2/cm2-c
Figure 7. Normalizedheliumcontents of the peaksA, B, C and
A
P,Q vs the heliumfilling dose.
0
1
2 ACTIVATION
With low helium dosepeak P, which has the lowest binding energy, is the most prominent peak. With increasinghelium dosethe Q peak, which has the higher binding energy grows faster than the P peak. This P/Q peak helium dosedependency is identical to the behaviour of the A, B and C peaks.This leads
3 ENERGY
4
3
P (~‘4)
Figure 8. The temperature at whicha thermallyactivateddissociation process (frequency factor v = IO” s- ’ and activation energy Q :V) is evolved to thesamestage(M/No = 0.43)in caseof linearanneahng and isochronal annealing indicated by T,,, and 7’,, respectively. The dashed curve refers to the 35/25 KS -’ heating cycle used in this
work.
113
A van
Veen
et al: Clustering
Figure 9. Helium
contents
of Kr in W observed
by the desorption
of the A, B. C, P, Q and
Hr. H,,,
H,,,
defects not yet dissociated amounts to N/No = 0.43, where No is the number of defects present prior annealing. When isochronal annealing is applied (anneal time t,, = I h) the same stage is reached at a temperature T, which is always lower than T, and is found from =
0.43,
wheref(T,) = v exp( - Q/kr,). between T, and Q. We now discuss the annealing 6. As shown in Figure 9 the distinguished.
Figure
N/N,
(I)
= exp(-
36OOf(T,))
(4) 8 also shows
the relation
of the various peaks in Figure following anneal stages can be
The HII peak decreases at 700 K. This stage can be ascribed to the migration of the vacancies (stage III). These vacancies migrate to the surface or form vacancy clusters with or without a krypton atom. This also explains the simultaneous increase of the HI fraction which will be ascribed to KrV, in this temperature range.
(2)
At 1350 K, corresponding with an activation energy 3.6 & 0.05 eV (u = lOI s-l), the Ht peak decreases and the A, B and C peaks increase. Since the KrVz defect causes the HI peak and the KrV defect is the origin of the A, B and C peaks complex it seems reasonable to ascribe this anneal stage to reaction KrV2d KrV + V.
(3)
At 1500 K (3.80 eV) the Hut peak decreases and the P and Q peaks start to develop as discussed before (Section 3.2). The P and Q peaks are formed by helium trapped by the Kr2V2 cluster. We suggest therefore, that the decrease of the Httr peak in this temperature region is described by the ceactton Kr2V, + Kr2V, + V.
The Kr2V, complexes (which when filled with helium cause the Hm peak in the 1300-1500 K temperature range) are most likely formed by the clustering of mobile KrV. complexes, i.e. reactions like KrV. -1 KrV,KrZVn+.,. This clustering takes place in a temperature region where KrVz defects still exist and are not dissociated to substitutional krypton. From model calculations follows that the migration 114
spectrometry
peaks
of Figure
6 vs the annealing
temperature
T*.
energy of KrVz is some 0.4 eV lower than the dissociation energy of KrV,; therefore clustering by the reactions KrV, i KrVKr2V, or KrVz $ KrV,Kr2V, can be expected. These clustering reactions are then followed by the vacancy dissociation at the temperature where, in the helium desorption spectrum, the P peak appears. We finally note that the length of the temperature interval in which the Htt and HI,, peaks disappear and the development of the A, B, C, P, Q peaks indicate a first order desorption process. On the contrary the faint slope of the decrease of the P, Q, A, B and C peaks indicate a diffusional release process of the Kr,V. complexes 01 = I,Z). This process starts at 1800 K. At this temperature the thermal vacancy concentration is large enough to make diffusion by the vacancy mechanism possible.
4. Concluding
remarks
The measurements described in Section 2 have shown that clusters of substitutional Kr in W can be made by implantation of 5 keV Kr+ ions at a dose higher than lOI Kr cm-’ followed by heating to temperatures > 1500 K so as to enable migration of KrV,, (II > I) and clustering to Kr2V, complexes with II > 2. Subsequently the number of vacancies is reduced to 2 leaving a pair of substitutional Kr atoms (Kr2V,). Helium binds to Kr2V2 with higher binding energy than it does to KrV; the binding mechanism is characteristic for interstitial clustering, i.e. further added helium is bound stronger. It would be interesting to know whether, for example, Kr,V, complexes with II > 2 can also be made with the above procedure. We expect that vacancies become more strongly bound when II increases. So, for example, KJV, may only be stable if II > 3. Future work will include these larger clusters and will deal with the mobility of the KrV. complexes.
References ’ E V Kornelsen, Radioior Efl, 13, 1972, 227. ’ G Carter and J S Colligon, /OU Bumbardnreur Educational Books, London, chap. 8 (1968).
of Solids.
Heineman
A
van
Veen
et
a/:
Clustering of Kr in W observed by the desorption spectrometry
3 E V Kornelsen and D E Edwards, Jr, Applications of Ion Reams fo Me/ah. Edited bv A T Picreaux. E P Eernisse and F L Vook. Plenum Press, New York, p. 521 (1973). J S E Donelly, Yacmm, 28, 1978, 163. 5 A van Veen and L M Caspers, Proc 7th Int Vat Congr and 3rd Int Conf Solid Surfaces. Edited by Dobrozemsky er al, Vienna, p. 2637 (1976).
6 A van Veen, L M Caspers, E V Kornelsen, R Fastenau, A van Gorkum and A Warnaar, Phys Sratus Solidi, A40, 1977, 235. ’ L M Caspers, A van Veen,.A van Gorkum, A van den Beukel and D M van Baa], Plrj~ Sram Solidi, A37, 1976, 371. ’ J A van der Berg, D G Armour and L K Verheij, Inst Phys Conf Ser No. 38. Edited by K G Stephens er al. The Institute of Physics, Bristol and London, p. 298 (1977).
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