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High field microscopy of nickel on tungsten
SURFACE
SCIENCE 41 (1974) 559-580 0 North-Holland
HIGH
FIELD
MICROSCOPY
OF NICKEL
Publishing Co.
ON TUNGSTEN
J. P. JONES and A. D. MARTIN Division of Electrical Materials Science, School of Electronic Engineering Science, College of North Wales, Dean Street, Bangor, Caerns., U.K.
University
Received 6 June 1973: revised manuscript received 22 August 1973 Field emission and field ion microscopy have been used to study the properties of nickel layers adsorbed on tungsten, and the growth of nickel crystallites. The first monolayer of nickel has a maximum density of 0.97 i 0.05 x 1On atoms m-2 and results in an increase in the work function which can be attributed to the formation of dipoles of moment DUO = 1.70 i 0.08 x lo-a0 C m at zero coverage and polarizability a = 7.3 i 0.05 A3. Nickel desorbs from the tungsten surface with activation energy 4.22 i 0.01 eV and second layer atoms desorb with activation energy 3.2 f 0.02 eV. Surface diffusion of second and higher layers over clean tungsten layer is believed to proceed by the “unrolling carpet” mechanism, with activation energy 0.93 &to.03 eV in close agreement with measurements of surface self-diffusion of nickel. Nickel does not dissolve appreciably in single-crystal tungsten and we confirm that atomic disordering at the nickel-tungsten interface is confined within a few angstroms of the interface. Well-ordered crystallites can be grown from a central nuclear structure which develops on (110) W. Combination of field ion and field emission techniques indicate that the crystallites adopt the expected growth form, having surfaces comprising large low-index faces, and also serve to confirm that field emission images alone cannot be relied upon to give an indication of crystallite shape. Crystallites invariably form upon an adsorbed layer which is at least one atom thick but may be thicker depending upon conditions of growth. The growth of crystals in situ offers the possibility of generating well-ordered low-index planes of large area which are suitable for further study, but it has yet to be confirmed that they behave as surface planes of bulk nickel.
1. Introduction The growth
of nickel crystallites
upon clean tungsten
substrates
has been
studied using field emission and field ion microscopy by Smith and Andersonl) who reported mainly on the disposition and structure of the crystallites and their relationship to the underlying tungsten structure. The object of this paper is to report observations which complement their findings, and to describe some properties of the tightly-bound layer which appears to lie between the substrate surface and any overgrowth structure.
2. Experimental
techniques
Most field emission studies were made using a simple sealed-off microsscope, the design of which has been described previously2). Each vapour 559
5m
J.I'.JONESAND A. D. MIUiEN
source was formed by enclosing a 5 mm length of 0.5 mm diameter Johnson Matthey ‘“specpure” nickel wire in a ctosely wound coil of 0.25 mm diameter tungsten wire which was held on a 4-wire support structure to allow rapid outgassing of the supporting leads. Two orthogonai sources were sited approximately 3 cm from the tip and slightly forward of it so that the central I IO plane could receive a nickel vapour flux at an angle of about 30” to the ptane. Fiefd ion microscopy combined with some field emission studies was performed in an all-glass microscope of conventional design, which, after bakeout at 400°C routinely attained pressures of 2 x 10-iO torr. Helium and neon were introduced as imaging gases through quartz diffusers. No gaseous impurity could be detected in the imaging gases by monitoring the field emission current from a clean tungsten surface at 78°K before and after contact with gas at XOF4torr. Work function changes were computed from field emission current/voltage data, and least squares analysis of Fowler-Nordheim plots gave typica errors of kO.03 eV in the measured work function.
3.1.
~EA~U~EM~N~O~NI~KEL
COVERAGE
By measuring work function in the manner which has been describedz), it was first established that the amount of nickel condensed on the tip at 78 “K was the same as that obtained when condensation took place at 3OO”K, the sticking probability was assumed therefore to be unity at 300°K. The average work function of the emitting area increases with increasing coverage, passes, through a maximum #,,, of 4.85 eV falls to 4.52 eV remaining constant at that value until the coverage becomes so high that crystallite formation prevents measurement of Cp.This behaviour for thermally cleaned tungsten is depicted in fig. 1a and the corresponding data for the field evaporated end form in fig, I b. It had been reported earliera) that +,,, is attained at an average density of 2.7-t-0.3 x 1019 nickel atoms m-‘. Combined field emission and field ion microscopy has showna) that when the last monolayer is isolated by field evaporation of higher layers, the work function corresponds to Qfmax,and from the manner in which it meld-evaporates the final monolayer appears to be a coherent layer one atom thick. It is difficult to see how nickel atoms can pack to a mean density of 2.7 x 1019 atoms mm2 when the maximum density of potential wells in the tungsten surface is only 1.4 x 10”’ me2 and the mean density nearer I .Ox 1OXgrnm2_The coverage at #,,, was therefore re-determined in the following way:
HIGH FIELD MICROSCOPY
OF Ni ON
W
561
A simple field emission microscope was equipped with a tungsten field emitter tip on one side of which was a nickel source 4.1 cm from the tip, and on the other side, 8.0 cm from the source, was an AT cut quartz crystal oscillator. Nickel was evaporated simultaneously onto tip and crystal and the resulting change in the natural frequency of oscillation of the quartz crystal was measured by standard techniquess). The frequency change which resulted from condensation of sufficient nickel to give $,,x at the tip was measured 2b
4.90-
1
Fig. 1. Change in work function q5 with amount of nickel condensed and uniformly spread on the emitting area: (a) nickel on thermally cleaned tungsten; (b) nickel on tungsten field-evaporated end form. ArIowed points in la) correspond to field emission patterns in fig. 2.
562
3. P. JONES AND
A. 0.
MARTIN
and the crystal then removed and calibrated by establishing a relationship between frequency change and thickness of a condensed film which was measured using a Varian “A-scope” interferometer. In order to ensure that the measured change in crystal frequency contained no spurious thermal effects it proved necessary to allow the crystal to cool for a period between Q and 1 hr. After allowing for the attenuation of the initial deposit by l/rr on spreading over the tip structurc6) the atom density was found by triplicate
Fig. 2.
Field emission patterns of’ nickel layers on tungsten: (a) H= 0.45; (b) U- 1.04: (c) D = 1.40; (d) 19= 3.60; (e) Q= 4.50.
measurements to be 0.97iO.05 x 1019 atoms m-2. This technique is considered to be more reliable than theearlier method adopted for mesuring 0 which relied on chemical analysis of submicrogram quantities of films deposited on Pyrex glass. As before we define 0 as the ratio of the mean density of surface nickel atoms to that of surface tungsten atoms so that c$,,,,, occurs at 0=0.97 as indicated in fig. I. After equilibration, the presence of small amounts of adsorbed nickel had little effect on the field emission pattern. Fig. 2 shows field emission patterns which correspond to the arrowed points in fig. I, and indicate that at 0% 1 the pattern fig. 2b resembles that of clean tungsten which supports the assign-
HXGHFIELD
MICROSCOPYOF
NioN w
563
ment of one monolayer to the coverage at that point. Addition of more nickel increases the intensity of the (110) environs so that at 1.04 B the 110 plane is well delineated, fig. 2~. A gradual increase in brightness around (1 IO) becomes evident at 3.6 8, fig. 2d, and (1 I I> becomes relatively dark. Addition of a further 0.9 8 produces no change in work function, but small crystallites are formed around (1 lo), fig. 2e, and attempts to disperse these by heating resulted in their amalgamation to form larger structures facing the vapour source, fig. 2f. These were dispersed by heating at 800°K to form a surface like that of fig. 5c in which emission from the (110) environs dominates the pattern. The initial rise in work function can be ascribed to the formation of nickeltungsten dipoles as discussed earlier?) and a similar treatment gives a zerofield dipole moment of 1.7kO.08 x lO-3o C m, with a polarizability a=7.3f 40.05A3. 3.2. SURFACE D~F~SION Nickel spreads from a localised deposit on one side of the tip over the remaining imaged surface at measurable rates in the temperture range 350 to 460°K. Rates of spreading of a fixed dose of nickel were measured at five temperatures in this range by timing the transition between the two stages shown in figs. 3a and 3b. Attempts to time the rate of advance of the front over particular crystallographic regions, e.g. (Ill], led within experimental error to the value quoted below for the activation energy for surface diffusion Work function measurements served to show that the dose of nickel employed gave a final coverage of 1.4 8, thus the coverage before spreading was approximately 4.5 8. The maintenance of a sharp front to the nickel layer throughout its spreading suggested that the process was controlled by diffu-
Fig. 3. Stages in surface diffusionof a nickel deposit at 401“K. Initial coverage B= 4.5 Final coverage B = i .4. Rates of diffusionwere measured by timing the transition from fa) to (b)~ The diffusion front in (a) lies between the arrows.
564
sion
J. P. JONES
of second
and higher
layers
AND A. D. MARTIN
of nickel
over the first layer and pre-
cipitation of atoms onto the tungsten surface at the layer edge. Support for this “unrolling carpet” mechanism comes from the measured activation energy which was calculated by least squares analysis by Arrhenius plots to be 0.93kO.03 eV with pre-exponential 0.7 cm2 set-’ in agreement with the activation energy obtained by Melmeds) for surface self-diffusion of nickel. 3.3. BULK DIFFUSION All crystallites could be dispersed by heating the tip for one minute at 725”K, and in view of the reported solubility of nickel in tungsteng) it was thought that destruction of crystallites resulted from diffusive loss of nickel into the bulk of the tungsten tip. Evidence for loss into the bulk was sought in two ways : A dose of nickel equivalent to 200 8 was condensed onto the tip held at 300°K and the tip was then maintained at 700°K for 30 min to allow solution of nickel in the substrate. After heating at 2500°K for the minimum time necessary to produce a clean tungsten pattern and work function, which experiment showed to be approximately 8 set, attempts were made to detect possible dissolved nickel by heating for several minutes at temperatures in the range 300 to 700°K. No accumulation of nickel at the surface could be detected although as little as 1/200th of a monolayer would have produced a detectable change in the work function. Dissolved nickel could conceivably remain undetected by this method if the free surface cannot retain nickel for thermodynamic reasons”) in which case diffusive loss of nickel from the surface should be easily detectable. Accordingly in the second experiment 1 ~9of nickel was equilibrated on the surface and the support structure and when attenuation by surface diffusion was complete the tip was held at 600°K for 3 hr. Less than l/200 0 was lost from the tip surface, and we therefore conclude that nickel does not diffuse directly into bulk single crystal tungsten. It is interesting to observe that despite the possible loss of nickel into grain boundaries on the tip support structure this was not reflected in loss from the tip surface. Insolubility of nickel in single crystal tungsten has also been detected by measurements of the lifetime of nickel atoms on hot tungsten surfacesll). 3.4. THERMAL DESORPTION Following earlier practice activation energies for desorption of nickel were obtained by measuring the rate of removal of a fixed quantity 6 0 as a function of temperature by monitoring the change in applied Voltage VA required to give 1 x 10m6 A field emission current. Three regions could be
HIGH FIELD MICRDSCOPY OF
5800
0
a 10
’ 20
c 30
10
Ni
ON
565
w
1 i 1291.K
T - 1467-K
I[
I
20
30
40 Time
50 in minu:es
5
10
15
20
25
30
at T*K
Fig. 4. Thermal desorption of nickel. Curves were obtained by measuring the applied voltage PA required to draw 0.5 x 10-s A field emission current after heating for a fixed time at the specified temperature. Arrowed points correspond to field emission patterns shown in fig. 5. Point x is equivalent to the end point used in measuring the sticking probability. TABLE 1
Region
_-.
__.
&t W) . ~~_.. -.
Pre-exponential (set-I)
I
4.22 i 0.07
10”
II
3.18 iO.23
109
III
0.76 i 0.09
10-l
distinguished in this process, these are shown in fig. 4 and the related field emission patterns in fig. 5. Measured activation energies which are derived from least squares analysis on the Arrhenius plots are given in table 1. The fall in VAin region III, fig. 4, can be seen from the low value of E,,, in table 1 to be due to diffusion of nickel to form the strongly emitting accretions at the lattice plane edges close to (1 IO), figs. 5a and 5b. Figs. 5c to 5g show that the rise in VAin region III fig. 4 results from progressive removal of nickel, first from the outer region figs. 5d, 5e and 5f where removal is indicated by the growing dark segment (arrowed) and later from the inner ring, fig. 5g. In region II the emission pattern is smooth and devoid of features except for the rapid collapse of a plane at (110) shown in fig. 5(i).
566
Fig. 5.
J.
P. JONES AND A. D. MARTIN
Stages in thermal desorption of nickel. Note the removal of bright segments of material close to (110) (at-rowed), and the collapse of a plane at (110) on Ci).
XIC~HFIELD MICROSCOPYOF
Ni ON W
567
Desorption of the remaining nickel which is principally the last monolayer, proceeds by removal from {1lo}, (21 I} and finally from {100) as shown in figs. Sj, 5k and 51. Stage II is ascribed to removal of second and higher layers of nickel, and the activation energy will correspond to desorption from the most tightly bound sites on the surface and not to the observed decomposition of the structure around (110). The desorption energy is thus lower than the heat of sublimation of nickel [approximately 4.9 eV at 1000”K12)], and it seems therefore that the binding energy of nickeI atoms in the thick nickel layer is substantially less than that of nickel atoms in bulk metai. This situation has also been observed for copper on tungstena). Although it remains possible that a process having a low activation energy which proceeds simultaneously with desorption, might suppress the measured value of i?,,,, the evidence presented in section 3.5 makes this unlikely in the present case. It is therefore puzzling to observe that nickel crystallites, which presumably have the bulk structure, are unstable with respect to the thick nickel layer. Even if the crystallites are assumed to be highly curved (and evidence against this is presented in section 4.4), the degree to which their thermodynamic stability is lowered by this cause is probably not sufficient to render them stable on the nickel surface. It seems therefore that other factors such as misfit between crystallite and substrate or non-pairwise additivity of bonding contribute to the instability of small crystallites, or the measurement of desorption energies in this way is defective in some unidentified way. It should perhaps be noted that desorption energies for thick layers of gold2r) and silverss) measured in this way, are identical with bulk sublimation energies within experimental error. Stage III remains a puzzling process, apparently nickel is lost by surface diffusion, perhaps into grain boundaries in the shank and support structure even though the nickelcoated surface was pre-heated at 1020°K before each desorption run, to ensure equilibration of the deposit by surface diffusion. 3.5.
STICKING PROBABILITY
In section 3.3 it was assumed that the sticking probability S of nickel atoms on tungsten at 300 “K was unity from 8 =0 to 8 = 200. If however S decreased drastically with increasing nickel depth then the conclusions in section 3.3 regarding the insolubility of nickel in tungsten could be incorrect. The effect of increasing thickness and substrate temperature on S was therefore examined in the following way: An amount of nickel was condensed on the tip and held at a pre-selected temperature and then desorbed at a standard temperature of 1220& 6°K and the time t, required to attain the state which is equivalent to point x in
568
J. P.
3ONES AND
A. D. MARTIN
569
HIGH FIELD MICROSCOPY OF Ni ON W
Fig. 6. Stages in the formation and growth of a nickel crystallite: (a) central structure on (110); (b) sketch of central structure: (c) two-layer structure at (1 lo), 6 = 2.0; (d) enlarged view of (c); sketch of two-layer structure; (f) substrate temperature 372”K, 0 = 6.0; (g) substrate at 410”K, 6 = 3.4; (h) substrate at 439”K, 0 = 6.0; (i) 0 = 6; (j) 0- 12; (k) symmetrical crystallite B> 60; (1) defective crystallite; (m) sketch of (1); (n) crystallyte partly “dissolved” by heating at 700°K.
fig. 4 was measured. Time t, was found to be a reliable measure of thickness up to 3 8 condensed onto the tip at 294°K and from the relationship thus established between t, and thickness, t, was used to measure the amount of nickel condensed on the tip under other conditions. The findings which are summarised in table 2 show that the behaviour of this system compares with that of others when condensation occurs with practically unit efficiency at temperatures well below the desorption temperature, and differs from that of the silver/nickel system reported recently 2s).
TABLET
Amount of nickel condensed (0) 6
3 4 5 6 6 8 10 12
Substrate temperature (“K) 78 294 798 1011 1104 294
1109
s 1.O assumed 1.0 0.93 + 0.1 0.92 & 0.1 0.92 i 0.1 1.0 1.0 1.0 1.0 0.92 0.92 0.92 1.10~0.1
510
3. P. JONESAND A. D. MARTIN
The high value of S obtained for a dose of 12-OB probably results from a change in the rate of desorption due to the formation of crystailites on the surface. Thus as expected S appears to depend little upon the coverage and tem~rature of the substrate up to 1100°K and although the assumption that S= 1 under the conditions described in section 3.3 is not correct because s lies nearer to 0.92, the conclusions of that section are unaffected. 3.6. FORMATION ANDGROWTHOF
ANICKELCRYSTAL
The positions of the sources allowed an appreciable flux of nickel at the 110 plane which permitted a detailed examination of the morphology of a growing crystallite. The earliest detectable stage in formation is shown in fig. 6a and was obtained by spreading a dose (1.5 8 when equilibrated) until the structure was most readily seen, thus the coverage at which the structure developed is not accurately known but is probably greater than 8=2. The central region is bounded by comparatively straight lines as sketched in fig. 6b which lie normal to the (111) zone lines. Increasing the dose to 2 0 produced a more complex central structure, fig. 6c, 6d, which appears to consist of two elliptical layers as sketched in fig. 6e. The precise form taken by subsequent growth depended upon the temperature of the substrate during condensation. For example, condensation of 2.3 0 on the surface held at 372°K produced growth only on the side of the structure nearest to the source, fig. 6f, whereas deposition at 410°K produced a more even development of the structure, fig. 6g. Deposition of nickel at 439°K produced a slightly different growth form, fig. Sh, in which development occurs only towards (21 l> along the (1 1I> zone lines, but extension of the crystallite proceeded at all temperatures as shown in fig. 6i, the structure becoming relatively brighter, fig. 6j, and more extensive until, in favourable cases a symmetrical single crystal developed as the only visible feature, fig. 6k. A careful examination of the central region in fig. 6i shows a central plane which is smaller than that in 6g, This may be a third (11 I)Ni or it may be the originaf plane of 6g which appears smaller due to the accretion of surrounding material. In most cases attempts to extend the structure led to the formation of defective crystallites, fig. 61. The crystallites have the symmetry and appearance closely similar to those grown by Smith and Andersonwith(lll)Ni/) (IlO)W,(2ll)Ni~J(11O)Wand(110)Ni~~(1OO)W The boundary which is seen in fig. 61 to cross (1 101Ni is thought to be a step in the crystal surface resulting from partial growth of a new nickel crystallite which had formed on (11 l)Ni, as sketched in fig. 6m. No crystaliites are seen if nickel is condensed upon the tip held at 700°K although a structure forms at 13101, as observed earlierl), which cannot be enlarged by addition of nickel. Destruction of crystallites is rapid above
HIGH FIELD MICROSCOPY
OF
Nim
571
W
700°K. A partially “dissolved” crystallite is seen in fig. 6n and the surface shown in fig. 5b follows complete destruction of the crystallite. 3.7. THE GROWTH SEQUENCE The central structure at (1 lo), fig. 6a is closely similar to that in fig. Si and was formed by heating a deposit condensed at room temperature; therefore it is not a structure formed by growth of a smaller entity, and its shape may have been controlled by diffusive loss from the edges. Although it is not a nucleus in the accepted sense, nevertheless it has an interesting shape because its edges seem to lie along definable directions. If this structure is a small 111 plane of nickel then its edges, if close packed, should lie 5.6” off the
a
b
C
Fig. 7. Sketch interpreting figs. 6g, 6h and 6i. Close-packed rows of nickel atoms form along [I 111 W where their development is assisted by the row lines in the substrate structure. Development of similar rows along [IlO] W is inhibited by the absence of closepacked rows of tungsten atoms in the substrate.
572
I. P. JONES AND A. D. MARTIN
illustrated in fig. 7. Development of close packed nickel rows along the [I lo] W direction to complete the hexagon appears to be inhibited, perhaps by the absense of a close packed tungsten row beneath, which can act as a template for the nickel structure. When growth takes place at higher substrate temperatures the rows of nickel atoms are shorter and less well-developed, fig. 6h. The bright regions which accompany further extension of the crystallite, figs. 6d, 6e, result from field enhancement at the growing edge of the crystal which is probably atomically sharp in the early stages of growth (see section 3.9), and not the cap-shaped structure envisaged by Smith and Anderson. The sharp edges of the crystallite must gradually become rounded to minimise the surface free energy of the crystallite as growth proceeds, so that new crystal faces of low free energy appear in the larger crystals, figs. 6j and 6k. We therefore envisage growth as proceeding by formation of a stable atom cluster’s) at (110) on top of the adsorbed monolayer to form the nucleus of a crystal plane one atom thick, on top of which is formed one or two more atom layers of nickel crystal. As suggested earlier]) re-alignment of the growing crystal probably takes place. The second layer becoming correctly aligned at some undetermined stage before the 2 layer structure covers W( 110); whether the first atom layer of the crystal is also re-aligned remains unknown. At low fluxes (3 Ojmin) further nucleation at Ni(ll1) is not detectable and extension appears to proceed by outward growth of the close packed rows to give a normal growth form of crystal which is bounded by surfaces of low free energy. The surface of a large crystal, fig. 6j comprises extensive low-index faces joined by regions, the curvature of which, will be determined chiefly by the details of the y-plot at these orientations and the temperature at which growth takes place. 4. Experimental 4.1. DISORDER AT
THE
results II. Field ion microscopy
INTERFACE
The present findings confirm those of Smith and Anderson, that the nickel/ tungsten interface becomes disordered at temperatures in the region of 300°K The degree of disorder is strongly dependent upon temperature. Figs. 8a and 8b show the extent of interfacial disorder induced by heating for 30 see at 370 and 600°K respectively. These surfaces are thought to contain no nickel for two reasons, firstly the majority of imaged atoms survive weii beyond the field required to remove nickel, and secondly, the measured work functions of these surfaces and the fiela evaporated end form subsequently developed, were identical to within experimental error. In both cases field-evaporation of one central I 10 plane restored the end form and no evidence for penetration
HIGH FIELD MICROSCOPY
of nickel into the tungsten the belief that disorder
OF
Ni
lattice was detected.
is confined
ON
573
W
These observations
to the interface’)
confirm
and that solution
of
nickel in tungsten is inappreciable at 600°K. In fig. 8b, nickel has not been removed from the regions close to (110) and it is interesting that these regions display a good degree of replication despite the fact that subsequent removal of nickel reveals many displaced tungsten
Fig. 8. Atomic disorder produced in a field-evaporated tungsten end-form by covering with a thick nickel layer and heating for 30sec: (a) at 300°K; (b) at 6OO’K. Helium images at 54°K.
atoms. It seems that either displacement is induced by field evaporation of nickel, or, more likely, tungsten atoms are incorporated into the top layer and remain undetected in the image. 4.2. FIRST LAYER NICKEL Replication of the field-evaporated tungsten surface by the first monolayer of nickel has been reporteda), and the impression given by the ion micrograph is of a faithful replication at all points. This may not necessarily be the case; fig. 9a is a simulated image of (110) oriented tungsten which was computed in a manner similar to that described by Moore4,14), and contains 4046 image points. The effect of “desorption” of over 30% of the most prominent atoms is simulated in fig. 9b by reducing the thickness of the spherical shell within which all located atoms are printed. Despite this the pattern retains much of its superficial perfection except along [l lo], and would not readily be distinguished from fig. 9a in a poorly resolved micrograph. This indicates that an accurate assessment of the degree of replication requires highly resolved field ion micrographs of the adsorbed layer.
574
J. P. JONES
N = 430 PO = 0.065 4046 points
a
AND A. D. MARTIN
N = 430 PO = 0,065 2670 points
b
Fig. 9. Computed simulation of field ion images. (a) (ilO)-oriented tungsten, 430A radius, 4046 image points; (b) same surface after removal of the most prominent 30% of image points, 2670 image points.
4.3. SECOND LAYER NICKEL
Strong evidence that in some regions the second monolayer can also conform to the substrate structure is provided by an examination of a nickel layer evaporated onto the field evaporated surface at 300”K, fig. 10a. Diffusion of nickel across the tungsten surface was incomplete and the nickel/tungsten boundary was very sharp, as expected for diffusion by the “unrolling carpet” mechanism. The corresponding field emission image, fig. lob, shows a variation in intensity across the layer, which indicates that the thickness of the layer was greatest on the side facing the vapour flux. An incomplete second layer was present at (111) W (arrowed) but only first-layer nickel was seen at (ill) and the relatively high work function of the region (approximately Cp,,,) is reflected in the field emission image fig. lob. Removal of the plane at (111) by field evaporation revealed the monolayer structure which seems much more perfect than that at (ill). Removal of the first monolayer proceeded first from around (11 I). Fig. 10~ and then from the plane itself, fig. IOd. A comparison of figs. 10a and lob show that second layer nickel appears not to decrease the work function at (111). This is not in keeping with the data summarised in fig. 1, and the apparent anomaly may result from a reduction in the local field strength at (I 11) due to the presence of thicker layers of nickel on the adjoining surface facing the source, on which the
Hi0~ FIELD MICROSCOPY
0F NioN
W
575
b
Fig. 10. Field emission and neon ion images of second layer nickel at 78°K: (a) layer condensed at 3OO”K, 0.69 BIV; fb) field emission of (a); (c) 0.71 BIV; (d) 0.90 BIV; (e) layer condensed at 78°K; (f) 0.72 BIV. Note: dark dots in (b) result from ion damage of the phosphor screen and should be ignored.
516
J. P. JONES AND
A. D. MARTIN
flux density was higher during nickel condensation, This could also account for the fact that at all stages in fig. 10, (ill) and its environs are brighter in appearance and more advanced in field evaporation, than the (111) region. The effect of a thick nickel deposit on the image can be seen in fig. 10e which depicts a surface which has been partly field evaporated after condensation of a deposit at 78 “K. At the extreme right hand rim of the screen can be seen the imaged structure of second and higher layers, and between it and the monolayer surface can be seen a dark band (arrowed) lying parallel to the thicker deposit, which is thought to result from a decrease in the local field strength. Further field evaporation moves this band to the edge of the screen as expected, fig. 1Of. The ease with which second and higher layers of metal are field-evaporated despite the stability of nickel crystallites when imaged in helium, suggests that these layers do not have the normal nickel structure and are probably pseudomorphic layers of the kind observed in other systems by electron microscopyaO). The basic similarity between the field emission pattern of a thick nickel layer, fig. Ze, and that of clean tungsten also supports the idea that these layers are pseudomorphic. 4.4. THE CRYSTALLITE
IMAGE
Owing to the position of the nickel source, the vapour flux density at (110) was low and it therefore proved difficult to produce a crystallite in this region. Fig. 11a is the field emission image of one of the few crystallites formed in this region. Neon ion imaging revealed that the edge of this crystallite was probably atomically sharp, fig. 11b, and reversion to the field emission mode showed that neon ion imaging had not changed the crystallite image. This illustrates a point which has been made beforers), namely that it is possible to infer very little about the shape of the crystallite from its field emission image, It is sometimes concluded that the crystallite is cap-shapedr6), but it is unlikely that a small assembly of atoms will adopt a cap shape especially when formed at a temperature far below the melting point of the materia117*1s). The approximate shape of the crystallite can be reconstructed from the field evaporation sequence, figs. 1lb to 1 If. The sharp step edge, fig. 1lb, is first field evaporated to reveal a pair of edges, fig. llc, and this process continues until all constituent planes are revealed, fig. 1 le. The crystallite appears to be thinnest at its edges and consequently is reduced in width as field-evaporation proceeds. Measurement shows that each new terrace ledge is revealed on the (110) side of the original sharp corner, as expected if the crystallite forms an extension of the (110) plane. A central structure on the 110 plane was discernable but not recordable and its relationship to the
HIGH
FIELD
MiCROSCOPY
OF
Ni
ON w
577
Fig. 11. Images of a nickel crystallite at 78°K: (a) field emission image of a crystallite formed from - 68 condensed at 433°K; (b) neon ion image of (a), 0.61 BIV; (c) 0.71 BIV; Cd) 0.77 BIV; (e) 0.87 BIV; (f) 0.93 BIV
578
1. P. JONES
AND A. D. MARTIN
crystallite could not be ascertained. Only the last monolayer of nickel could be detected on (110) after removal of the crystallite. A schematic of the evaporation sequence is shown in fig. 12. The neon ion image indicates that the crystallite probably possesses a high degree of structural perfection on the 111 plane but has only a moderate degree of structural perfection in the bulk
b
a Fig. 12.
Fig. 13.
C
Sketch of crystallite in stages of field evaporation.
Large crystallite formed by condensation of - 20 0 at 500°K. Note the arrowed region which appears to be imperfect. Helium image at 78°K.
when formed at 430°K and so the structures shown in fig. 6 which were formed at temperatures in the range 350-420°K probably had no higher degree of perfection. The more extensively developed crystal shown in fig. 13 was formed by condensing approximately 20 8 on to the tip at 500°K and this too appears to be imperfect both at the 110 plane and in the arrowed region where the plane edges seem to crowd together possibly in some form of stacking fault which can be seen by field evaporation to persist in subsurface layers of the crystal.
HIGH FIELD MICROSCOPY OF
Ni
ON w
579
5. Summary and conclusions (1) Adsorbed
nickel increases
the work function
of a tungsten
surface by
forming dipoles with the nickel negatively charged, having a dipole moment at zero coverage p0 = 1.70+0.08 x 10m3’ C m and polarizability a=7.3+ +0.5 A3. The density of adsorbed atoms at 4,,, =0.97+0.05 x 1Ol9 atoms rnm2, these atoms can be imaged in helium and are bound to the tungsten substrate with energy 4.22 f 0.01 eV. Higher layers of nickel reduce the work function to -4.5 eV, are bound with energy 3.2 +0.2, and can only be imaged close to { 11 l} by neon ion microscopy. Higher layers of nickel diffuse over the tungsten surface probably by the “unrolling carpet” mechanism, with activation energy 0.93 f0.03 eV. (2) Nickel does not dissolve appreciably in single crystal tungsten. (3) Growth of nickel crystallites at (11O)W proceeds as described by Smith and Anderson except that growth commences at a nucleus centred on (110). The geometry of the growing “island” on (110) and later of the growing crystallite are inferred from high field microscopy to have the topography which is expected of a growth form, in which the shape is determined by the need to minimise the surface free energy of the crystal, or the total free energy in the case of the island structure. In contrast to the behaviour of second and higher layers of nickel, crystallites at (110) are readily imaged in helium. (4) By a suitable choice of evaporation conditions large areas of low-index planes can be generated and they probably possess a high degree of structural perfection. These might provide ideal substrates upon which to carry out further investigations as suggested by Smith and Anderson. However it remains to be proven that, for example, the (111) Ni surface at (110) W is supported upon a sufficiently thick layer of nickel to behave as true (111) Ni. Work function measurements indicate that beyond 3 monolayers the surface can be regarded as typical of bulk nickel, and the thickness at (111) is at least 3 monolayers, so there is ground for hope that these crystallites are typical of nickel, but confirmation by, for example, an examination of the spectrum of electrons field-emitted through these planes would be desirable as would supporting evidence of the type obtained for iron on tungsten19). Acknowledgements The authors are grateful to the Science Research Council for financial support of this project and also for the award of a maintenance grant to one of us (A.D.M.). We also wish to thank Mr. M. G. Le Due who carried out some of the field emission measurements.
580
J. P. JOKES
AND A. D. MARTIN
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23)
G. D. W. Smith and J. S. Anderson, Surface Sci. 24 (1971) 459-483. J. P. Jones, Proc. Roy. Sot. (London) A 284 (I 965) 469 J. P. Jones, Nature 211(1966) 47Y. A. D. Martin, Ph.D. Thesis, Univ. of Wales, 1970. G. Sauerhrey, Z Physik 155(1959) 206. L. D. Schmidt and R. Gomer, J. Chem. Phys. 42 (1965) 3573. J. P. Jones, Surface Sci. 32 (1972) 29. A. J. Melmed, J. Appl. Phys. 38 (1967) 1885. M. Hansen, Constitution ofBinary Alloys (McGraw Hill, New York, 1958). C. Herring, in: Stn!cture and Properties of Solid Surfaces, Eds. R. Gomer and C. S. Smith (Univ. of Chicago Press, 1952) p. 5. H. Shelton and A. Y. 1%.Cho, J. Appl. Phys. 37 (1966) 3544. C. J. Smithells, Metals Reference Booi, 4th ed. (Buttcrworlhs, London, 1967). D. W. Bassett, Surface Sci. 23 (1970) 240. A. J. W. Moore, J. Phys. Chem. Solids 23 (1962) 907. A. J. Melmed and S. C. Hardy, Surface Sci. 6 (1967) 481. R. D. Gretz, Surface Sci. 6 (1967), 481. P. A. Tick and A. F. Witt, Surface Sci. 26 (1971) 165. J. V. Dave and F. F. Abraham, Surface Sci. 26 (19fi) 557. A. J. Mclmed and J. 3. Carroll, Surface SCI. 19 (1970) 243. W. A. Jesser and J. W. Matthews, Phil. Msg. 20 (1969) 999. J. P. Jones, in: Svmp. on Surface Phenomena of Metal.~, Monograph No. 28 (Society of Cnemical Industry, 1967). A. Cetronio, Ph.D. Thesis, Univ. of Wales, 1972. P. C. Jackson, T. E. Gallon and A. Chambers, Surface Sci. 36 (1973) 381.
SCIENCE 41 (1974) 559-580 0 North-Holland
HIGH
FIELD
MICROSCOPY
OF NICKEL
Publishing Co.
ON TUNGSTEN
J. P. JONES and A. D. MARTIN Division of Electrical Materials Science, School of Electronic Engineering Science, College of North Wales, Dean Street, Bangor, Caerns., U.K.
University
Received 6 June 1973: revised manuscript received 22 August 1973 Field emission and field ion microscopy have been used to study the properties of nickel layers adsorbed on tungsten, and the growth of nickel crystallites. The first monolayer of nickel has a maximum density of 0.97 i 0.05 x 1On atoms m-2 and results in an increase in the work function which can be attributed to the formation of dipoles of moment DUO = 1.70 i 0.08 x lo-a0 C m at zero coverage and polarizability a = 7.3 i 0.05 A3. Nickel desorbs from the tungsten surface with activation energy 4.22 i 0.01 eV and second layer atoms desorb with activation energy 3.2 f 0.02 eV. Surface diffusion of second and higher layers over clean tungsten layer is believed to proceed by the “unrolling carpet” mechanism, with activation energy 0.93 &to.03 eV in close agreement with measurements of surface self-diffusion of nickel. Nickel does not dissolve appreciably in single-crystal tungsten and we confirm that atomic disordering at the nickel-tungsten interface is confined within a few angstroms of the interface. Well-ordered crystallites can be grown from a central nuclear structure which develops on (110) W. Combination of field ion and field emission techniques indicate that the crystallites adopt the expected growth form, having surfaces comprising large low-index faces, and also serve to confirm that field emission images alone cannot be relied upon to give an indication of crystallite shape. Crystallites invariably form upon an adsorbed layer which is at least one atom thick but may be thicker depending upon conditions of growth. The growth of crystals in situ offers the possibility of generating well-ordered low-index planes of large area which are suitable for further study, but it has yet to be confirmed that they behave as surface planes of bulk nickel.
1. Introduction The growth
of nickel crystallites
upon clean tungsten
substrates
has been
studied using field emission and field ion microscopy by Smith and Andersonl) who reported mainly on the disposition and structure of the crystallites and their relationship to the underlying tungsten structure. The object of this paper is to report observations which complement their findings, and to describe some properties of the tightly-bound layer which appears to lie between the substrate surface and any overgrowth structure.
2. Experimental
techniques
Most field emission studies were made using a simple sealed-off microsscope, the design of which has been described previously2). Each vapour 559
5m
J.I'.JONESAND A. D. MIUiEN
source was formed by enclosing a 5 mm length of 0.5 mm diameter Johnson Matthey ‘“specpure” nickel wire in a ctosely wound coil of 0.25 mm diameter tungsten wire which was held on a 4-wire support structure to allow rapid outgassing of the supporting leads. Two orthogonai sources were sited approximately 3 cm from the tip and slightly forward of it so that the central I IO plane could receive a nickel vapour flux at an angle of about 30” to the ptane. Fiefd ion microscopy combined with some field emission studies was performed in an all-glass microscope of conventional design, which, after bakeout at 400°C routinely attained pressures of 2 x 10-iO torr. Helium and neon were introduced as imaging gases through quartz diffusers. No gaseous impurity could be detected in the imaging gases by monitoring the field emission current from a clean tungsten surface at 78°K before and after contact with gas at XOF4torr. Work function changes were computed from field emission current/voltage data, and least squares analysis of Fowler-Nordheim plots gave typica errors of kO.03 eV in the measured work function.
3.1.
~EA~U~EM~N~O~NI~KEL
COVERAGE
By measuring work function in the manner which has been describedz), it was first established that the amount of nickel condensed on the tip at 78 “K was the same as that obtained when condensation took place at 3OO”K, the sticking probability was assumed therefore to be unity at 300°K. The average work function of the emitting area increases with increasing coverage, passes, through a maximum #,,, of 4.85 eV falls to 4.52 eV remaining constant at that value until the coverage becomes so high that crystallite formation prevents measurement of Cp.This behaviour for thermally cleaned tungsten is depicted in fig. 1a and the corresponding data for the field evaporated end form in fig, I b. It had been reported earliera) that +,,, is attained at an average density of 2.7-t-0.3 x 1019 nickel atoms m-‘. Combined field emission and field ion microscopy has showna) that when the last monolayer is isolated by field evaporation of higher layers, the work function corresponds to Qfmax,and from the manner in which it meld-evaporates the final monolayer appears to be a coherent layer one atom thick. It is difficult to see how nickel atoms can pack to a mean density of 2.7 x 1019 atoms mm2 when the maximum density of potential wells in the tungsten surface is only 1.4 x 10”’ me2 and the mean density nearer I .Ox 1OXgrnm2_The coverage at #,,, was therefore re-determined in the following way:
HIGH FIELD MICROSCOPY
OF Ni ON
W
561
A simple field emission microscope was equipped with a tungsten field emitter tip on one side of which was a nickel source 4.1 cm from the tip, and on the other side, 8.0 cm from the source, was an AT cut quartz crystal oscillator. Nickel was evaporated simultaneously onto tip and crystal and the resulting change in the natural frequency of oscillation of the quartz crystal was measured by standard techniquess). The frequency change which resulted from condensation of sufficient nickel to give $,,x at the tip was measured 2b
4.90-
1
Fig. 1. Change in work function q5 with amount of nickel condensed and uniformly spread on the emitting area: (a) nickel on thermally cleaned tungsten; (b) nickel on tungsten field-evaporated end form. ArIowed points in la) correspond to field emission patterns in fig. 2.
562
3. P. JONES AND
A. 0.
MARTIN
and the crystal then removed and calibrated by establishing a relationship between frequency change and thickness of a condensed film which was measured using a Varian “A-scope” interferometer. In order to ensure that the measured change in crystal frequency contained no spurious thermal effects it proved necessary to allow the crystal to cool for a period between Q and 1 hr. After allowing for the attenuation of the initial deposit by l/rr on spreading over the tip structurc6) the atom density was found by triplicate
Fig. 2.
Field emission patterns of’ nickel layers on tungsten: (a) H= 0.45; (b) U- 1.04: (c) D = 1.40; (d) 19= 3.60; (e) Q= 4.50.
measurements to be 0.97iO.05 x 1019 atoms m-2. This technique is considered to be more reliable than theearlier method adopted for mesuring 0 which relied on chemical analysis of submicrogram quantities of films deposited on Pyrex glass. As before we define 0 as the ratio of the mean density of surface nickel atoms to that of surface tungsten atoms so that c$,,,,, occurs at 0=0.97 as indicated in fig. I. After equilibration, the presence of small amounts of adsorbed nickel had little effect on the field emission pattern. Fig. 2 shows field emission patterns which correspond to the arrowed points in fig. I, and indicate that at 0% 1 the pattern fig. 2b resembles that of clean tungsten which supports the assign-
HXGHFIELD
MICROSCOPYOF
NioN w
563
ment of one monolayer to the coverage at that point. Addition of more nickel increases the intensity of the (110) environs so that at 1.04 B the 110 plane is well delineated, fig. 2~. A gradual increase in brightness around (1 IO) becomes evident at 3.6 8, fig. 2d, and (1 I I> becomes relatively dark. Addition of a further 0.9 8 produces no change in work function, but small crystallites are formed around (1 lo), fig. 2e, and attempts to disperse these by heating resulted in their amalgamation to form larger structures facing the vapour source, fig. 2f. These were dispersed by heating at 800°K to form a surface like that of fig. 5c in which emission from the (110) environs dominates the pattern. The initial rise in work function can be ascribed to the formation of nickeltungsten dipoles as discussed earlier?) and a similar treatment gives a zerofield dipole moment of 1.7kO.08 x lO-3o C m, with a polarizability a=7.3f 40.05A3. 3.2. SURFACE D~F~SION Nickel spreads from a localised deposit on one side of the tip over the remaining imaged surface at measurable rates in the temperture range 350 to 460°K. Rates of spreading of a fixed dose of nickel were measured at five temperatures in this range by timing the transition between the two stages shown in figs. 3a and 3b. Attempts to time the rate of advance of the front over particular crystallographic regions, e.g. (Ill], led within experimental error to the value quoted below for the activation energy for surface diffusion Work function measurements served to show that the dose of nickel employed gave a final coverage of 1.4 8, thus the coverage before spreading was approximately 4.5 8. The maintenance of a sharp front to the nickel layer throughout its spreading suggested that the process was controlled by diffu-
Fig. 3. Stages in surface diffusionof a nickel deposit at 401“K. Initial coverage B= 4.5 Final coverage B = i .4. Rates of diffusionwere measured by timing the transition from fa) to (b)~ The diffusion front in (a) lies between the arrows.
564
sion
J. P. JONES
of second
and higher
layers
AND A. D. MARTIN
of nickel
over the first layer and pre-
cipitation of atoms onto the tungsten surface at the layer edge. Support for this “unrolling carpet” mechanism comes from the measured activation energy which was calculated by least squares analysis by Arrhenius plots to be 0.93kO.03 eV with pre-exponential 0.7 cm2 set-’ in agreement with the activation energy obtained by Melmeds) for surface self-diffusion of nickel. 3.3. BULK DIFFUSION All crystallites could be dispersed by heating the tip for one minute at 725”K, and in view of the reported solubility of nickel in tungsteng) it was thought that destruction of crystallites resulted from diffusive loss of nickel into the bulk of the tungsten tip. Evidence for loss into the bulk was sought in two ways : A dose of nickel equivalent to 200 8 was condensed onto the tip held at 300°K and the tip was then maintained at 700°K for 30 min to allow solution of nickel in the substrate. After heating at 2500°K for the minimum time necessary to produce a clean tungsten pattern and work function, which experiment showed to be approximately 8 set, attempts were made to detect possible dissolved nickel by heating for several minutes at temperatures in the range 300 to 700°K. No accumulation of nickel at the surface could be detected although as little as 1/200th of a monolayer would have produced a detectable change in the work function. Dissolved nickel could conceivably remain undetected by this method if the free surface cannot retain nickel for thermodynamic reasons”) in which case diffusive loss of nickel from the surface should be easily detectable. Accordingly in the second experiment 1 ~9of nickel was equilibrated on the surface and the support structure and when attenuation by surface diffusion was complete the tip was held at 600°K for 3 hr. Less than l/200 0 was lost from the tip surface, and we therefore conclude that nickel does not diffuse directly into bulk single crystal tungsten. It is interesting to observe that despite the possible loss of nickel into grain boundaries on the tip support structure this was not reflected in loss from the tip surface. Insolubility of nickel in single crystal tungsten has also been detected by measurements of the lifetime of nickel atoms on hot tungsten surfacesll). 3.4. THERMAL DESORPTION Following earlier practice activation energies for desorption of nickel were obtained by measuring the rate of removal of a fixed quantity 6 0 as a function of temperature by monitoring the change in applied Voltage VA required to give 1 x 10m6 A field emission current. Three regions could be
HIGH FIELD MICRDSCOPY OF
5800
0
a 10
’ 20
c 30
10
Ni
ON
565
w
1 i 1291.K
T - 1467-K
I[
I
20
30
40 Time
50 in minu:es
5
10
15
20
25
30
at T*K
Fig. 4. Thermal desorption of nickel. Curves were obtained by measuring the applied voltage PA required to draw 0.5 x 10-s A field emission current after heating for a fixed time at the specified temperature. Arrowed points correspond to field emission patterns shown in fig. 5. Point x is equivalent to the end point used in measuring the sticking probability. TABLE 1
Region
_-.
__.
&t W) . ~~_.. -.
Pre-exponential (set-I)
I
4.22 i 0.07
10”
II
3.18 iO.23
109
III
0.76 i 0.09
10-l
distinguished in this process, these are shown in fig. 4 and the related field emission patterns in fig. 5. Measured activation energies which are derived from least squares analysis on the Arrhenius plots are given in table 1. The fall in VAin region III, fig. 4, can be seen from the low value of E,,, in table 1 to be due to diffusion of nickel to form the strongly emitting accretions at the lattice plane edges close to (1 IO), figs. 5a and 5b. Figs. 5c to 5g show that the rise in VAin region III fig. 4 results from progressive removal of nickel, first from the outer region figs. 5d, 5e and 5f where removal is indicated by the growing dark segment (arrowed) and later from the inner ring, fig. 5g. In region II the emission pattern is smooth and devoid of features except for the rapid collapse of a plane at (110) shown in fig. 5(i).
566
Fig. 5.
J.
P. JONES AND A. D. MARTIN
Stages in thermal desorption of nickel. Note the removal of bright segments of material close to (110) (at-rowed), and the collapse of a plane at (110) on Ci).
XIC~HFIELD MICROSCOPYOF
Ni ON W
567
Desorption of the remaining nickel which is principally the last monolayer, proceeds by removal from {1lo}, (21 I} and finally from {100) as shown in figs. Sj, 5k and 51. Stage II is ascribed to removal of second and higher layers of nickel, and the activation energy will correspond to desorption from the most tightly bound sites on the surface and not to the observed decomposition of the structure around (110). The desorption energy is thus lower than the heat of sublimation of nickel [approximately 4.9 eV at 1000”K12)], and it seems therefore that the binding energy of nickeI atoms in the thick nickel layer is substantially less than that of nickel atoms in bulk metai. This situation has also been observed for copper on tungstena). Although it remains possible that a process having a low activation energy which proceeds simultaneously with desorption, might suppress the measured value of i?,,,, the evidence presented in section 3.5 makes this unlikely in the present case. It is therefore puzzling to observe that nickel crystallites, which presumably have the bulk structure, are unstable with respect to the thick nickel layer. Even if the crystallites are assumed to be highly curved (and evidence against this is presented in section 4.4), the degree to which their thermodynamic stability is lowered by this cause is probably not sufficient to render them stable on the nickel surface. It seems therefore that other factors such as misfit between crystallite and substrate or non-pairwise additivity of bonding contribute to the instability of small crystallites, or the measurement of desorption energies in this way is defective in some unidentified way. It should perhaps be noted that desorption energies for thick layers of gold2r) and silverss) measured in this way, are identical with bulk sublimation energies within experimental error. Stage III remains a puzzling process, apparently nickel is lost by surface diffusion, perhaps into grain boundaries in the shank and support structure even though the nickelcoated surface was pre-heated at 1020°K before each desorption run, to ensure equilibration of the deposit by surface diffusion. 3.5.
STICKING PROBABILITY
In section 3.3 it was assumed that the sticking probability S of nickel atoms on tungsten at 300 “K was unity from 8 =0 to 8 = 200. If however S decreased drastically with increasing nickel depth then the conclusions in section 3.3 regarding the insolubility of nickel in tungsten could be incorrect. The effect of increasing thickness and substrate temperature on S was therefore examined in the following way: An amount of nickel was condensed on the tip and held at a pre-selected temperature and then desorbed at a standard temperature of 1220& 6°K and the time t, required to attain the state which is equivalent to point x in
568
J. P.
3ONES AND
A. D. MARTIN
569
HIGH FIELD MICROSCOPY OF Ni ON W
Fig. 6. Stages in the formation and growth of a nickel crystallite: (a) central structure on (110); (b) sketch of central structure: (c) two-layer structure at (1 lo), 6 = 2.0; (d) enlarged view of (c); sketch of two-layer structure; (f) substrate temperature 372”K, 0 = 6.0; (g) substrate at 410”K, 6 = 3.4; (h) substrate at 439”K, 0 = 6.0; (i) 0 = 6; (j) 0- 12; (k) symmetrical crystallite B> 60; (1) defective crystallite; (m) sketch of (1); (n) crystallyte partly “dissolved” by heating at 700°K.
fig. 4 was measured. Time t, was found to be a reliable measure of thickness up to 3 8 condensed onto the tip at 294°K and from the relationship thus established between t, and thickness, t, was used to measure the amount of nickel condensed on the tip under other conditions. The findings which are summarised in table 2 show that the behaviour of this system compares with that of others when condensation occurs with practically unit efficiency at temperatures well below the desorption temperature, and differs from that of the silver/nickel system reported recently 2s).
TABLET
Amount of nickel condensed (0) 6
3 4 5 6 6 8 10 12
Substrate temperature (“K) 78 294 798 1011 1104 294
1109
s 1.O assumed 1.0 0.93 + 0.1 0.92 & 0.1 0.92 i 0.1 1.0 1.0 1.0 1.0 0.92 0.92 0.92 1.10~0.1
510
3. P. JONESAND A. D. MARTIN
The high value of S obtained for a dose of 12-OB probably results from a change in the rate of desorption due to the formation of crystailites on the surface. Thus as expected S appears to depend little upon the coverage and tem~rature of the substrate up to 1100°K and although the assumption that S= 1 under the conditions described in section 3.3 is not correct because s lies nearer to 0.92, the conclusions of that section are unaffected. 3.6. FORMATION ANDGROWTHOF
ANICKELCRYSTAL
The positions of the sources allowed an appreciable flux of nickel at the 110 plane which permitted a detailed examination of the morphology of a growing crystallite. The earliest detectable stage in formation is shown in fig. 6a and was obtained by spreading a dose (1.5 8 when equilibrated) until the structure was most readily seen, thus the coverage at which the structure developed is not accurately known but is probably greater than 8=2. The central region is bounded by comparatively straight lines as sketched in fig. 6b which lie normal to the (111) zone lines. Increasing the dose to 2 0 produced a more complex central structure, fig. 6c, 6d, which appears to consist of two elliptical layers as sketched in fig. 6e. The precise form taken by subsequent growth depended upon the temperature of the substrate during condensation. For example, condensation of 2.3 0 on the surface held at 372°K produced growth only on the side of the structure nearest to the source, fig. 6f, whereas deposition at 410°K produced a more even development of the structure, fig. 6g. Deposition of nickel at 439°K produced a slightly different growth form, fig. Sh, in which development occurs only towards (21 l> along the (1 1I> zone lines, but extension of the crystallite proceeded at all temperatures as shown in fig. 6i, the structure becoming relatively brighter, fig. 6j, and more extensive until, in favourable cases a symmetrical single crystal developed as the only visible feature, fig. 6k. A careful examination of the central region in fig. 6i shows a central plane which is smaller than that in 6g, This may be a third (11 I)Ni or it may be the originaf plane of 6g which appears smaller due to the accretion of surrounding material. In most cases attempts to extend the structure led to the formation of defective crystallites, fig. 61. The crystallites have the symmetry and appearance closely similar to those grown by Smith and Andersonwith(lll)Ni/) (IlO)W,(2ll)Ni~J(11O)Wand(110)Ni~~(1OO)W The boundary which is seen in fig. 61 to cross (1 101Ni is thought to be a step in the crystal surface resulting from partial growth of a new nickel crystallite which had formed on (11 l)Ni, as sketched in fig. 6m. No crystaliites are seen if nickel is condensed upon the tip held at 700°K although a structure forms at 13101, as observed earlierl), which cannot be enlarged by addition of nickel. Destruction of crystallites is rapid above
HIGH FIELD MICROSCOPY
OF
Nim
571
W
700°K. A partially “dissolved” crystallite is seen in fig. 6n and the surface shown in fig. 5b follows complete destruction of the crystallite. 3.7. THE GROWTH SEQUENCE The central structure at (1 lo), fig. 6a is closely similar to that in fig. Si and was formed by heating a deposit condensed at room temperature; therefore it is not a structure formed by growth of a smaller entity, and its shape may have been controlled by diffusive loss from the edges. Although it is not a nucleus in the accepted sense, nevertheless it has an interesting shape because its edges seem to lie along definable directions. If this structure is a small 111 plane of nickel then its edges, if close packed, should lie 5.6” off the
a
b
C
Fig. 7. Sketch interpreting figs. 6g, 6h and 6i. Close-packed rows of nickel atoms form along [I 111 W where their development is assisted by the row lines in the substrate structure. Development of similar rows along [IlO] W is inhibited by the absence of closepacked rows of tungsten atoms in the substrate.
572
I. P. JONES AND A. D. MARTIN
illustrated in fig. 7. Development of close packed nickel rows along the [I lo] W direction to complete the hexagon appears to be inhibited, perhaps by the absense of a close packed tungsten row beneath, which can act as a template for the nickel structure. When growth takes place at higher substrate temperatures the rows of nickel atoms are shorter and less well-developed, fig. 6h. The bright regions which accompany further extension of the crystallite, figs. 6d, 6e, result from field enhancement at the growing edge of the crystal which is probably atomically sharp in the early stages of growth (see section 3.9), and not the cap-shaped structure envisaged by Smith and Anderson. The sharp edges of the crystallite must gradually become rounded to minimise the surface free energy of the crystallite as growth proceeds, so that new crystal faces of low free energy appear in the larger crystals, figs. 6j and 6k. We therefore envisage growth as proceeding by formation of a stable atom cluster’s) at (110) on top of the adsorbed monolayer to form the nucleus of a crystal plane one atom thick, on top of which is formed one or two more atom layers of nickel crystal. As suggested earlier]) re-alignment of the growing crystal probably takes place. The second layer becoming correctly aligned at some undetermined stage before the 2 layer structure covers W( 110); whether the first atom layer of the crystal is also re-aligned remains unknown. At low fluxes (3 Ojmin) further nucleation at Ni(ll1) is not detectable and extension appears to proceed by outward growth of the close packed rows to give a normal growth form of crystal which is bounded by surfaces of low free energy. The surface of a large crystal, fig. 6j comprises extensive low-index faces joined by regions, the curvature of which, will be determined chiefly by the details of the y-plot at these orientations and the temperature at which growth takes place. 4. Experimental 4.1. DISORDER AT
THE
results II. Field ion microscopy
INTERFACE
The present findings confirm those of Smith and Anderson, that the nickel/ tungsten interface becomes disordered at temperatures in the region of 300°K The degree of disorder is strongly dependent upon temperature. Figs. 8a and 8b show the extent of interfacial disorder induced by heating for 30 see at 370 and 600°K respectively. These surfaces are thought to contain no nickel for two reasons, firstly the majority of imaged atoms survive weii beyond the field required to remove nickel, and secondly, the measured work functions of these surfaces and the fiela evaporated end form subsequently developed, were identical to within experimental error. In both cases field-evaporation of one central I 10 plane restored the end form and no evidence for penetration
HIGH FIELD MICROSCOPY
of nickel into the tungsten the belief that disorder
OF
Ni
lattice was detected.
is confined
ON
573
W
These observations
to the interface’)
confirm
and that solution
of
nickel in tungsten is inappreciable at 600°K. In fig. 8b, nickel has not been removed from the regions close to (110) and it is interesting that these regions display a good degree of replication despite the fact that subsequent removal of nickel reveals many displaced tungsten
Fig. 8. Atomic disorder produced in a field-evaporated tungsten end-form by covering with a thick nickel layer and heating for 30sec: (a) at 300°K; (b) at 6OO’K. Helium images at 54°K.
atoms. It seems that either displacement is induced by field evaporation of nickel, or, more likely, tungsten atoms are incorporated into the top layer and remain undetected in the image. 4.2. FIRST LAYER NICKEL Replication of the field-evaporated tungsten surface by the first monolayer of nickel has been reporteda), and the impression given by the ion micrograph is of a faithful replication at all points. This may not necessarily be the case; fig. 9a is a simulated image of (110) oriented tungsten which was computed in a manner similar to that described by Moore4,14), and contains 4046 image points. The effect of “desorption” of over 30% of the most prominent atoms is simulated in fig. 9b by reducing the thickness of the spherical shell within which all located atoms are printed. Despite this the pattern retains much of its superficial perfection except along [l lo], and would not readily be distinguished from fig. 9a in a poorly resolved micrograph. This indicates that an accurate assessment of the degree of replication requires highly resolved field ion micrographs of the adsorbed layer.
574
J. P. JONES
N = 430 PO = 0.065 4046 points
a
AND A. D. MARTIN
N = 430 PO = 0,065 2670 points
b
Fig. 9. Computed simulation of field ion images. (a) (ilO)-oriented tungsten, 430A radius, 4046 image points; (b) same surface after removal of the most prominent 30% of image points, 2670 image points.
4.3. SECOND LAYER NICKEL
Strong evidence that in some regions the second monolayer can also conform to the substrate structure is provided by an examination of a nickel layer evaporated onto the field evaporated surface at 300”K, fig. 10a. Diffusion of nickel across the tungsten surface was incomplete and the nickel/tungsten boundary was very sharp, as expected for diffusion by the “unrolling carpet” mechanism. The corresponding field emission image, fig. lob, shows a variation in intensity across the layer, which indicates that the thickness of the layer was greatest on the side facing the vapour flux. An incomplete second layer was present at (111) W (arrowed) but only first-layer nickel was seen at (ill) and the relatively high work function of the region (approximately Cp,,,) is reflected in the field emission image fig. lob. Removal of the plane at (111) by field evaporation revealed the monolayer structure which seems much more perfect than that at (ill). Removal of the first monolayer proceeded first from around (11 I). Fig. 10~ and then from the plane itself, fig. IOd. A comparison of figs. 10a and lob show that second layer nickel appears not to decrease the work function at (111). This is not in keeping with the data summarised in fig. 1, and the apparent anomaly may result from a reduction in the local field strength at (I 11) due to the presence of thicker layers of nickel on the adjoining surface facing the source, on which the
Hi0~ FIELD MICROSCOPY
0F NioN
W
575
b
Fig. 10. Field emission and neon ion images of second layer nickel at 78°K: (a) layer condensed at 3OO”K, 0.69 BIV; fb) field emission of (a); (c) 0.71 BIV; (d) 0.90 BIV; (e) layer condensed at 78°K; (f) 0.72 BIV. Note: dark dots in (b) result from ion damage of the phosphor screen and should be ignored.
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A. D. MARTIN
flux density was higher during nickel condensation, This could also account for the fact that at all stages in fig. 10, (ill) and its environs are brighter in appearance and more advanced in field evaporation, than the (111) region. The effect of a thick nickel deposit on the image can be seen in fig. 10e which depicts a surface which has been partly field evaporated after condensation of a deposit at 78 “K. At the extreme right hand rim of the screen can be seen the imaged structure of second and higher layers, and between it and the monolayer surface can be seen a dark band (arrowed) lying parallel to the thicker deposit, which is thought to result from a decrease in the local field strength. Further field evaporation moves this band to the edge of the screen as expected, fig. 1Of. The ease with which second and higher layers of metal are field-evaporated despite the stability of nickel crystallites when imaged in helium, suggests that these layers do not have the normal nickel structure and are probably pseudomorphic layers of the kind observed in other systems by electron microscopyaO). The basic similarity between the field emission pattern of a thick nickel layer, fig. Ze, and that of clean tungsten also supports the idea that these layers are pseudomorphic. 4.4. THE CRYSTALLITE
IMAGE
Owing to the position of the nickel source, the vapour flux density at (110) was low and it therefore proved difficult to produce a crystallite in this region. Fig. 11a is the field emission image of one of the few crystallites formed in this region. Neon ion imaging revealed that the edge of this crystallite was probably atomically sharp, fig. 11b, and reversion to the field emission mode showed that neon ion imaging had not changed the crystallite image. This illustrates a point which has been made beforers), namely that it is possible to infer very little about the shape of the crystallite from its field emission image, It is sometimes concluded that the crystallite is cap-shapedr6), but it is unlikely that a small assembly of atoms will adopt a cap shape especially when formed at a temperature far below the melting point of the materia117*1s). The approximate shape of the crystallite can be reconstructed from the field evaporation sequence, figs. 1lb to 1 If. The sharp step edge, fig. 1lb, is first field evaporated to reveal a pair of edges, fig. llc, and this process continues until all constituent planes are revealed, fig. 1 le. The crystallite appears to be thinnest at its edges and consequently is reduced in width as field-evaporation proceeds. Measurement shows that each new terrace ledge is revealed on the (110) side of the original sharp corner, as expected if the crystallite forms an extension of the (110) plane. A central structure on the 110 plane was discernable but not recordable and its relationship to the
HIGH
FIELD
MiCROSCOPY
OF
Ni
ON w
577
Fig. 11. Images of a nickel crystallite at 78°K: (a) field emission image of a crystallite formed from - 68 condensed at 433°K; (b) neon ion image of (a), 0.61 BIV; (c) 0.71 BIV; Cd) 0.77 BIV; (e) 0.87 BIV; (f) 0.93 BIV
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1. P. JONES
AND A. D. MARTIN
crystallite could not be ascertained. Only the last monolayer of nickel could be detected on (110) after removal of the crystallite. A schematic of the evaporation sequence is shown in fig. 12. The neon ion image indicates that the crystallite probably possesses a high degree of structural perfection on the 111 plane but has only a moderate degree of structural perfection in the bulk
b
a Fig. 12.
Fig. 13.
C
Sketch of crystallite in stages of field evaporation.
Large crystallite formed by condensation of - 20 0 at 500°K. Note the arrowed region which appears to be imperfect. Helium image at 78°K.
when formed at 430°K and so the structures shown in fig. 6 which were formed at temperatures in the range 350-420°K probably had no higher degree of perfection. The more extensively developed crystal shown in fig. 13 was formed by condensing approximately 20 8 on to the tip at 500°K and this too appears to be imperfect both at the 110 plane and in the arrowed region where the plane edges seem to crowd together possibly in some form of stacking fault which can be seen by field evaporation to persist in subsurface layers of the crystal.
HIGH FIELD MICROSCOPY OF
Ni
ON w
579
5. Summary and conclusions (1) Adsorbed
nickel increases
the work function
of a tungsten
surface by
forming dipoles with the nickel negatively charged, having a dipole moment at zero coverage p0 = 1.70+0.08 x 10m3’ C m and polarizability a=7.3+ +0.5 A3. The density of adsorbed atoms at 4,,, =0.97+0.05 x 1Ol9 atoms rnm2, these atoms can be imaged in helium and are bound to the tungsten substrate with energy 4.22 f 0.01 eV. Higher layers of nickel reduce the work function to -4.5 eV, are bound with energy 3.2 +0.2, and can only be imaged close to { 11 l} by neon ion microscopy. Higher layers of nickel diffuse over the tungsten surface probably by the “unrolling carpet” mechanism, with activation energy 0.93 f0.03 eV. (2) Nickel does not dissolve appreciably in single crystal tungsten. (3) Growth of nickel crystallites at (11O)W proceeds as described by Smith and Anderson except that growth commences at a nucleus centred on (110). The geometry of the growing “island” on (110) and later of the growing crystallite are inferred from high field microscopy to have the topography which is expected of a growth form, in which the shape is determined by the need to minimise the surface free energy of the crystal, or the total free energy in the case of the island structure. In contrast to the behaviour of second and higher layers of nickel, crystallites at (110) are readily imaged in helium. (4) By a suitable choice of evaporation conditions large areas of low-index planes can be generated and they probably possess a high degree of structural perfection. These might provide ideal substrates upon which to carry out further investigations as suggested by Smith and Anderson. However it remains to be proven that, for example, the (111) Ni surface at (110) W is supported upon a sufficiently thick layer of nickel to behave as true (111) Ni. Work function measurements indicate that beyond 3 monolayers the surface can be regarded as typical of bulk nickel, and the thickness at (111) is at least 3 monolayers, so there is ground for hope that these crystallites are typical of nickel, but confirmation by, for example, an examination of the spectrum of electrons field-emitted through these planes would be desirable as would supporting evidence of the type obtained for iron on tungsten19). Acknowledgements The authors are grateful to the Science Research Council for financial support of this project and also for the award of a maintenance grant to one of us (A.D.M.). We also wish to thank Mr. M. G. Le Due who carried out some of the field emission measurements.
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AND A. D. MARTIN
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23)
G. D. W. Smith and J. S. Anderson, Surface Sci. 24 (1971) 459-483. J. P. Jones, Proc. Roy. Sot. (London) A 284 (I 965) 469 J. P. Jones, Nature 211(1966) 47Y. A. D. Martin, Ph.D. Thesis, Univ. of Wales, 1970. G. Sauerhrey, Z Physik 155(1959) 206. L. D. Schmidt and R. Gomer, J. Chem. Phys. 42 (1965) 3573. J. P. Jones, Surface Sci. 32 (1972) 29. A. J. Melmed, J. Appl. Phys. 38 (1967) 1885. M. Hansen, Constitution ofBinary Alloys (McGraw Hill, New York, 1958). C. Herring, in: Stn!cture and Properties of Solid Surfaces, Eds. R. Gomer and C. S. Smith (Univ. of Chicago Press, 1952) p. 5. H. Shelton and A. Y. 1%.Cho, J. Appl. Phys. 37 (1966) 3544. C. J. Smithells, Metals Reference Booi, 4th ed. (Buttcrworlhs, London, 1967). D. W. Bassett, Surface Sci. 23 (1970) 240. A. J. W. Moore, J. Phys. Chem. Solids 23 (1962) 907. A. J. Melmed and S. C. Hardy, Surface Sci. 6 (1967) 481. R. D. Gretz, Surface Sci. 6 (1967), 481. P. A. Tick and A. F. Witt, Surface Sci. 26 (1971) 165. J. V. Dave and F. F. Abraham, Surface Sci. 26 (19fi) 557. A. J. Mclmed and J. 3. Carroll, Surface SCI. 19 (1970) 243. W. A. Jesser and J. W. Matthews, Phil. Msg. 20 (1969) 999. J. P. Jones, in: Svmp. on Surface Phenomena of Metal.~, Monograph No. 28 (Society of Cnemical Industry, 1967). A. Cetronio, Ph.D. Thesis, Univ. of Wales, 1972. P. C. Jackson, T. E. Gallon and A. Chambers, Surface Sci. 36 (1973) 381.