Nucleation and epitaxy of gold deposits on sodium chloride substrates during electron bombardment

Thin Solid Films - Elsevier Sequoia S.A., L a u s a n n e - Printed in the N e t h e r l a n d s

NUCLEATION AND EPITAXY OF GOLD DEPOSITS ON SODIUM CHLORIDE SUBSTRATES DURING ELECTRON BOMBARDMENT B. LEWIS AND M. R. JORDAN

Allen Clark Research Centre, The Plessey Company Limited, Caswell, Towcester, Northants. (Gt. Britain) (Received April 4, 1970)

SUMMARY

Electron bombardment of cleaved sodium chloride substrates during deposition of gold causes an increase in the nucleation density at early stages of growth, and an improvement in the epitaxial orientation of the deposit nuclei. An experimental study of the variation of nucleation density with temperature, in conjunction with a theoretical analysis of nucleation behaviour on a substrate with both deep and shallow adsorption sites, shows that electron bombardment causes virtually complete coverage by sites of depth 0.47 eV. Without electron bombardment, some small substrate areas contain only shallow adsorption sites of depth approximately 0.1 eV, but most areas contain a high density of deep sites. Competition for incident material by growth of existing nuclei allows nucleation at only a small proportion of the deep adsorption sites. The nature of the deep sites and their significance in epitaxial growth are discussed.

]. INTRODUCTION

The effects of electron bombardment of the substrate during or before the deposition of metal deposits on cleaved alkali halides have been described by Stirland 1-3, Palmberg e t al. 4'5 and others 6'7. During the early stages of growth, while the deposit comprises independent islands or nuclei, electron irradiation increases the island density by roughly an order of magnitude, and this leads to the film becoming continuous at a smaller thickness than without electron bombardment. Electron bombardment also affects the crystallographic orientation of the deposit. For example, gold deposited on vacuum-cleaved rocksalt at 200 °C without electrons has a mixture of the orientations (001) [1 lO]gu//(O01) [110]N.Cl and (111) [110]a.//(001) [110]NRClwith multiple positioning and multiple twinning of the Thin Solid Films, 6 (1970) 1-15

2

B. LEWIS~ M. R. J O R D A N

(111) material. With electron bombardment, the proportion of (111) gold is drastically reduced and twinning is eliminated. These bombardment effects are substantially independent of electron flux and energy above threshold levels of approximately 2 x 1010 electrons cm-2secand 75 volts. The obvious explanation of the increased nucleation density and improved epitaxy, which has been discussed in some detail by Palmberg et al. 5 and by Stirland 3, is that electron bombardment produces preferred adsorption sites, probably halide vacancies, which act as nucleation centres and which favour (100) orientation. The objective of the present investigation was to determine the density and depth of preferred nucleation sites produced by electron bombardment.

2.

NUCLEATION THEORY

2.1. Adsorption sites o f uniform depth and spacing

Atoms arriving on a substrate may be desorbed, may form new stable nuclei, or may be captured by existing stable nuclei. The essence of the problem is the division of adatoms between these three competing processes. Lewis 8 has expressed the probability p that one of these events will occur at each migration hop of an adatom, as the sum of the desorption probability Pa, the nucleation probability p., and the capture probability Pc. Nucleation behaviour on a substrate with a density no of adsorption sites each with adsorption energy Ea, and with activation energy E d for migration from site to site, can then be described by the following equations: P = P.+p.WPc E a -- E d

p~ = e x p - - - - -

kT

i*+1 Pn --

ni*bi*

(1) (2)

(3)

no

1 i max Pc =

~

nibl

(4)

n o i*+ 1

d = R. bq wq = d t (Mq3) hoP ± K l ( 2 q p ~) bq = rcqp ~ Ko(2qp~) Thin Solid Films, 6 (1970) 1 - 1 5

(5)

(6)

NUCLEATION AND EPITAXY OF A u ON R El I

=

3

Ee

pv exp k T

Ell = no

Ji -

NaC1 DURING ELECTRON BOMBARDMENT

nl exp--Ei \no~ kT

dni,+i R p, dt - i * + l p

(7) i ~ i*

(8)

(9)

T is the substrate temperature and R is the incidence rate of adatoms, nl and nl are the densities of single atoms and of nuclei containing i atoms, respectively.b~ is the capture factor of a nucleus containing i atoms and is weakly dependent on p and on the radius q. bq and wq, the capture rate, for a nucleus of radius q are derived from a relation given by Halpern 9. Ko, K1 are modified zero and first-order Bessel's functions, b~ is of order unity for small nuclei, M is a shape factor relating the number of atoms in a nucleus to the radius, Equations (1)-(4) are derived directly from the definitions of the respective event probabilities, in conjunction with the capture equations (5) and (6). Equations (7) and (8) are the equilibrium equations for adatoms (v is the adatom migration rate pre-exponential) and for nuclei up to the critical size, following Walton 1o. E~ is the binding energy of a cluster of i atoms. The critical size i* is the size with the lowest density n~,. The directly measured dissociation energy of an isolated pair of gold atoms is 3.9 eV and the mean binding energy per atom of bulk gold is 0.6 eV 11. Taking either of these values for/?2, all reasonable experimental conditions give i* = 1 and ni, = nl. Thus for gold eqns. (3) and (19) become 2Rbl Ed P" -- nopv exp k-T

'-/2

dn 2 R p, . . . . . . dt 2 p

(10) (11)

We are principally concerned with the calculation of the density of stable nuclei ns = ~ nl for i ~ 2 as a function of time. On a bare substrate ns = 0 and Pc = 0. p is found by solving a quadratic equation and used in the calculation of bl and J2. As stable nuclei form and grow, ns, pC and p increase, while nl, p , and J2 decrease. These changes, which are consequential on one another, can be followed in numerical calculations over successive small increments of time. The surface coverage of nuclei, F = ~ 7rq~2n~ for all stable nuclei also increases with time, and the loss rate of nuclei by coalescence due to growth J¢ can be estimated as ns(dF/dt). Since nucleation and growth occur only on bare substrate the net nucleation rate is ( J z - J ~ ) ( 1 - F ) , and when this becomes zero ns has reached its maximum value nmax. The effects of loss of mobile stable nuclei are discussed in Section 2.3 below. When Pa ~ P. very little desorption occurs. The initial value of J2 is R/4 when Pa = P, and R/2 when Pa '~ P," nmax is independent of E, and is roughly Thin Solid Films, 6 (1970) 1-15

4

B. LEWIS, M. R. JORDAN

proportional to R ~ exp (Ed/3kT). (The dependence of bq on p and q precludes analytical solutions.) When pa > Pn the nucleation and growth rates are reduced by desorption; nm,x becomes roughly proportional to R e x p ( E , / k T ) a n d falls steeply with increasing temperature. To determine material parameters from experimental data, trial values of the parameters are put into the equations. Calculated and experimental values of ns as a function of time are then compared and the parameter values adjusted until agreement is reached. The primary parameters are E a and Ed when desorption occurs, and E d alone when desorption does not occur.

2.2. Preferred adsorption sites Let us now consider a substrate with a density np of preferred sites, which are deeper than the remaining no-np adsorption sites by an energy Ep, as indicated in Fig. 1. The adatom density nl includes atoms on both types of site. For VACUUM LEVEL

EQ TAL

Fig. 1. Potential energy variations with position of an adatom on a substrate with normal and preferred adsorption sites, showing the energies E,, Ea, Ep and the total preferred site depth Ed-t- E..

those on preferred sites, the desorption probability p, per migration hop is e x p - [ ( E a + E v ) - ( E d + E p ) ] / k T = e x p - ( E , - E a ) / k T , i.e. eqn. (2) still applies. Equation (3) (with a new value for ni.) and eqn. (4) also apply, since in their derivation the migration rate has cancelled out. Equation (7) can be written nt = R/pv, where v is the mean hop rate given by v = holt; z is the transit time over no sites and is now equal to the sum of' Ta --

n o -- np Ea v exp k--7'

(12a)

for transit over n o - n p adsorption sites and rp = n p exp E d + E p

v

(12b)

kT

for transit over np preferred sites. Hence

R

Ed ( n°--nP + -np -exp Ep) ,,~ no -kT

nl = p~exPkT\

Thin Solid Films, 6 (1970) 1-15

(13)

N U C L E A T I O N A N D E P I T A X Y OF A l l o n

NaC1

DURING ELECTRON BOMBARDMENT

5

replaces eqn. (7). When i* = I, eqn. (3) then gives Pn --

2Rbl E d (n o - n o Ep) nopv e x p ~ \ no _ + nPnoexp ~-T

(14)

which should be compared with eqn. (10). When either//p = 0 o r E p = 0 eqn. (14) reduces to eqn. (10), as it should. Also when np = no eqn. (14) becomes Pn -

2R b 1

nopv

Ed + Ep exp - -

kT

05)

which is the same as eqn. (10) with Ed+E v replacing Ea. This again is expected because this case has all sites preferred and the migration energy is now E d + E v. When Pc = 0 and Pa '~ P,, J = R/2 as before. However, when p~ = 0 the increase Ofpn compared withPa also increases the temperature above which desorption occurs. When (np/no) exp (Ep/kT) ~ 1, eqn. (14) simplifies to

2R b 1

Ea "~ Ep

Pn -- noZpv no exp

kT~

(16)

Thus experimental results over a range of temperature should give values of np and E d + Ep.

2.3. Mobile stable clusters When only single atoms are mobile, the formation rate of pairs, as given by eqn. (11), is equal to the nucleation rate. However, when pairs or larger clusters are mobile some will be captured by other nuclei and this loss decreases the effective nucleation rate. It has been shown by Lewis ~z and by Zinsmeister 13 that under certain conditions nmax can be markedly reduced by stable cluster mobility. To enable mobility of all stable clusters to be taken into consideration, over a range of temperature, Lewis postulated a size- and temperature-dependent mobility function. In the present case the relevant experimental data is scanty and we will therefore employ Zinsmeister's assumption that only pairs are mobile and that their mobility is vl/p, where vl is the mobility of single atoms and p is a constant. Let us now consider event probabilities per pair migration hop. The formation rate of triplets by capture of migrating single atoms is nlnzbl 2(vl + v2)/no, and writing this as the probability per pair migration hop P2n that a pair will form a triplet givesp2n = nlb~2(p + 0/no. The competing process is capture by other stable nuclei and the capture probability of a pair per migration hop is i max P2c =

(I/no) ~

nlb21

3

The capture factor ba2, for capture of migrating single atoms by pairs, is Thin Solid Films, 6 (1970) 1 - 1 5

6

B. LEWIS, M. R. JORDAN

evaluated by eqn. (6) using p from eqn, (1). b21 for capture of migrating pairs by larger stable clusters is evaluated by eqn. (6) using p = loss probability per pair migration hop = pza+pzn+p2e. Since pairs are stable, Pza is negligible. The net nucleation rate J3 is J3 =

']2

P2n

(17)

Pzn + P2c

If we write b12 = bzi = 1, which is a good approximation, J3/J2 simplifies to n~p(nlp + ns). Thus if nma× < n~ when n ..... is reached, then pair mobility is not significant. Otherwise, when n~ = n~p the effective nucleation rate is halved and its further decrease as n~ increases rapidly reduces the net nucleation rate (including coalescence) to zero. The effect is most likely to occur at high temperatures which give a low value of nl. When there are preferred sites, we take the effective mobility of pairs as lip times the effective mobility of adatoms, and eqn. (17) stands, n~ now has the higher value given by eqn. (13), so that Pz, is increased by the factor (no-np)/no + (np/no) e x p (Ep/kT). Pzc is unchanged. Hence pair mobility is less important when there are preferred site~.

3.

EXPERIMENTAL DETAILS

To ensure reliability of comparisons between gold films grown on rocksalt with and without electron bombardment, the technique described by Stirland 2 was employed in which two matching vacuum-cleaved faces of rocksalt are obtained. The system enables two evaporations, one with and one without electron bombardment, to be performed simultaneously under identical conditions of evaporation rate, residual gas pressure, incident quantity of gold, substrate purity and temperature. The vacuum chamber was of stainless steel with gold wire gaskets and was pumped by a 6-in. mercury diffusion p u m p through a liquid-nitrogen-cooled Z baffle. After prolonged baking and degassing of filaments and heaters a pressure of 5 x 10- 9 torr could be maintained during evaporation. Comparison experiments using a Viton " O " ring for the main seal between chamber and baseplate showed no significant differences, although the operating pressure during evaporation was then 2 x 10-7 tort. Most experiments were carried out using this latter arrangement. The experimental procedure was as follows. The jig was set up using a Harshaw Chemical Co. rocksalt bar of dimensions 25 x 3 x 2 ram. The tungsten filament source was loaded with 99.99 o~ gold wire. The system was evacuated and the tungsten filament, electron gun filament and quartz iodine substrate heater lamps were degassed until a pressure in the low 10- 7 tort range was obtained with the jig at the required temperature. Thin Solid Films, 6 (1970) 1-15

N U C L E A T I O N A N D EPITAXY OF

Au

ON

NaC1

DURING ELECTRON BOMBARDMENT

7

The electron gun was then operated to give the required electron current and energy and the tungsten filament was heated to give the required evaporation rate of gold; the evaporation rate was monitored by a rotating-disc type rate meter 14 and was held constant by manually adjusting the filament current. The rocksalt bar was cleaved and the two freshly cleaved surfaces exposed to deposition of gold as they swung downwards. Only one surface was exposed to electron bombardment. The deposition was terminated by switching off the filament, and when the substrate temperature had fallen to 100°C a 200 A film of carbon was deposited on both substrates. Finally the specimens were removed from the vacuum system and the carbon-backed gold deposits were removed by water dissolution of the rocksalt for examination in the electron microscope. Gold nuclei visible on the micrographs were counted on areas away from obviously stepped regions of the substrate, and the nucleation density was calculated. An alternative experimental system was employed if a growth sequence, i.e. three thicknesses of gold deposit on one rocksalt substrate was required. This system consisted of a steel clamp which held a rocksalt bar; the bar was cleaved by a rotating blade, which also acted as a complete or partial shutter between the substrate and gold source. Thus by moving the shutter across the substrate during deposition a film of varying thickness was obtained. The blade was driven by a "wobble-drive" unit. The experimental procedure was similar to that outlined before, except that now only films either with or without electron bombardment could be prepared in one pump-down cycle.

4. RESULTS 4. l. Epitaxy

Micrographs of deposits at 30°C and 300°C are shown in Fig. 2. The deposition rate was 0.14 ,~ sec-1 and the electron flux on the bombarded specimens was 1015 electrons cm -2 sec -1 at an energy of 240 eV. The bombarded specimens shewed the (001) parallel orientation even for substrate temperatures as low as 55°C, whereas the non-bombarded deposits were polycrystalline at the lower temperatures and showed mixed orientations up to 300°C, at which temperature the single parallel orientation occurred. The upper limit of 300°C was imposed by the onset of thermal etching of the rocksalt substrates. 4.2. Nucleation density

The deposition time was normally 60 sec. Longer deposition times generally gave slightly lower densities of nuclei, e.9. about 10% lower after 120 sec than Thin Solid Films, 6 (1970) 1-15

8

B. LEWIS, M. R. JORDAN

0

2

0 .cz ,4

AZ

0

~5 o

..2

p. ca

0 ¢rj

<)

~aO iX UJ

t~ e~

~E ee.

0

0

3

eat~

Z~ rq (o .,r

Thin

Solid

Films,

6

(1970) 1-15

NUCLEATION AND EPITAXY OF

013 I '

3OO

200 I

?

Au ON NaCI DURING ELECTRON BOMBARDMENT

'T (°C)

150 i

10(5

50

I

25 '

I

i I • ELECTRONBOMBARDED,2,40V160#A¢la"2 o NO ~OMBARDMENT.NORMALDENSITY

r I np ~ n o ~

9

2,

(cm-~I

o ~'~

"P - i ~

. . . . . . . . . . . i01o j . ~ o J / I / I V" I

[/

] O*

15

n p ~i _ _ t ~" ' ~ ' ~

•

OHL~S~NGi~L" ATOMSMOBILE Ea= ~9¢V Ed. os=v

Ed =.moor

Ep=.42=v

Ep ..37=V

.... ,

2

1

PAIR. . . . MOBILE o = 2.000 Ea =.64¢V .,o..o,

2.5

~ [oK")

[

3

3.5x10-3

Fig. 3. Variation of nucleation density n, at t = 60 sec with temperature for gold on rocksalt with and without electron b o m b a r d m e n t . The points are experimental and the lines are theoretical, calculated with the parameter values shown.

after 60 sec. Thus the observed densities are close to the maximum values. For our analyses we will compare experimental and theoretical values of ns at t = 60 sec. The experimental values are the points plotted against lIT in Fig. 3. The values of ns at t = 60 sec for the bombarded specimens were reasonably reproducible, and the plotted points show an approximately linear variation with reciprocal temperature. The unbombarded specimens showed a wide scatter of ns and no systematic variation with temperature. However, careful re-examination of the micrographs revealed a few areas in which the island density was much lower. An example is shown in Fig. 4. The variation normally occurs over a larger distance, and is not easily illustrated in a single micrograph. The solid circles in Fig. 3 are values of ns at t = 60 sec taken from the low density areas. At 300°C there was a sharp drop in ns and the island sizes were smaller, indicating that desorption was occurring. Experimental values of ns as a function of time for a low density unbombarded specimen at 300°C are given in Fig. 5, and show that nm~x is reached after about 600 sec.

4.3. Condensation coefficient Estimates were made of the condensation coefficient c~ by measuring the numbers and sizes of nuclei, calculating the total volume of visible gold deposit, Thin Solid Films, 6 (1970) 1-15

10

B. LEWIS, M. R. JORDAN

O

O

~

4

O.2At m Fig. 4. Areas of high and low nucleation density on an u n b o m b a r d e d substrate.

assuming that the islands had hemispherical shapes and dividing by the known incident volume of gold. Calibration of the incidence rate depended on Talysurf measurements of gold deposits approximately 1000 A thick. The low density unbombarded specimen at 300°C gave ~ = 0.03 after 60 sec, rising to 0.1 after 180 sec. This indicates significant desorption. All the other specimens gave values of ~ between 0.2 and 0.4, with no systematic variation with temperature or with nucleation density. This is unexpected, since desorption should be eliminated at high densities or low temperatures. A condensation coefficient near 0.3 has also been reported for gold deposited at 150°C on vacuumcleaved rocksalt over a range of incidence rates 15 and for platinum deposited on rocksalt over a wide range of temperature 6'~2. The suggested explanation is that only a fraction, 0.3, of the incident gold is thermally accommodated and adsorbed, the remainder being either reflected or diffusing into the substrate. Accepting this conclusion, the effective incidence rate in these experiments was 0.3×0.14 = 0.042 A sec- 1. The possibility of diffusion into the rocksalt was investigated with radioThin Solid Films, 6 (1970) 1-15

NUCLEATION AND EPITAXY OF A u ON id a

NaC1 DURING ELECTRON BOMBARDMENT

l1

! ONLY SINGLE ATOMS MOBILE Ea = ' 5 9 e V Ed = . O 5 f V Ep = . 4 2 e v = 1013se¢'l

1012 - -

30°C

PAIRS ALSO MOBILE = 2000 Ea = 6 4 •

,,.... ~

v

ns

~

~0-,o,v

c~, .~)

/

~

~170°c '="

%=% = 6.7 x 1014cm - 2

~'~-----*'~ I _...---

,o,O

J~--

7I

to s

I(~ 2

U//

/I/.

-

i

ro-l

/1

~

I0

lO

I0

Fig. 5. Variation o f nucleation density with time a n d t e m p e r a t u r e for gold on rocksalt with a n d without electron b o m b a r d m e n t . T h e points s h o w experimental values for low density areas o f u n b o m b a r d e d specimens. T h e lines are theoretical, calculated with the p a r a m e t e r values s h o w n .

active Au deposited on air- and vacuum-cleaved NaCI. Stripped carbon-coated deposits were found to have 8 0 ~ of the activity measured before stripping, i.e. 20 ~ of the gold was lost when the film was stripped. With uncoated film3, changes of activity produced by gradual dissolution of the substrate in a water-alcohol mixture indicated that any penetration of the gold into the rocksalt was very slight. Measurements of the activity of deposits received on a collector by reflection from the rocksalt surface exposed to evaporating gold were qualitatively in accordance with expectations. No attempt was made to measure the distribution, and hence no quantitative estimate of the total amount of reflected material could be made.

4.4. Adsorption site density and form factor Values of no, the adsorption site density, and M, the form factor, are required for the calculations. In a (001) surface of NaC1 the most likely adsorption site is above a Na + ion and between four C I - ions. This gives no = 6.7 x 1014 cm -3. The site spacing a = no- ÷ = 3.9/k is used in the calculations as a unit of size in connection with nucleus growth. A hemispherical cluster of radius qa has a volume Thin Solid Films, 6 (1970) 1-15

12

B. LEWIS, M. R. JORDAN

2rcq3a3/3 and contains 27tq3a3/3 V atoms, where V = 17 ~3 is the atomic volume of gold. Hence the shape factor M, defined by i = Mq 3, is 2rca3/3V = 7 for hemispherical clusters. The precise values of no and M have only a small influence on the calculated nucleation densities.

4.5. Analysis The low density results for unbombarded specimens are presumed to represent randem nucleation, with no preferred sites. The broken line in Fig. 3 gives calculated values ofns for t = 60sec, np = 0, v = 1013 sec- 1, Ed = 0.05 eV and E, = 0.59 eV. These figures show that, even with the lowered value of incidence rate, allowing for incomplete thermal accommodation, a high value of single atom mobility is needed to give the low densities observed. The calculated variation with time of the number of nuclei larger than 20 A diameter for the parameter values given above is shown in Fig. 5. At 300°C the value of E~ which is required to account for the density observed at 60 sec gives calculated densities at 180 and 600 sec which are higher than the experimental values. The observed sharp reduction of nucleation rate as ns rises suggests depletion by capture of mobile stable nuclei. Trials with a number of values of p, the ratio of single atom to pair mobility, gave the full lines shown in Figs. 3 and 5 with np = 0, v = 1013 s e c - 1 , E d = 0.10 eV, E a = 0.64 eV and p = 2000. The agreement with the experimental points is now satisfactory. The pair mobility given by these figures corresponds to the effective mobility of stable nuclei of all sizes and is surprisingly low. A higher mobility would give lower values of n . . . . u n l e s s np ~ 0. It may be that the low density areas have some preferred sites. In this case the mobility values obtained correspond to the effective mobility of atoms and stable nuclei, including the effects of preferred sites. The calculated condensation coefficients at 300°C, including the allowance for lack of thermal accommodation, range from 0.2 after 60 sec to 0.03 after 30 sec in reasonable agreement with the experimental values. For the electron-bombarded specimens we assume a temperature-independent density n v of preferred sites of total depth E a + E v. np and E d + Ep are then found by matching calculated and experimental values of ns at t -- 60 sec as a function of temperature. The results are shown in Fig. 3. The values used for the other parameters are E, = 0.59 eV for the broken line (no pair mobility) and E, = 0.64 eV, p = 2000 for the full line. Desorption is insignificant, even at 300°C, and the effect of pair mobility is small. The values E d + Ep = 0.47 eV and n v = no are used for both lines. A slightly lower value of Ed + Ep would have given better agreement with the experimental temperature variation, but the absolute values of ns would then be too low. This confirms the evaluation of np a s nO, the highest possible value. We conclude that electron bombardment produces a saturation density of preferred sites, of total depth 0.47 eV. Thin Solid Films, 6 (1970) 1-15

NUCLEATIONAND EPITAXYOF Au ON NaC1 DURING ELECTRONBOMBARDMENT

13

The calculated variation with time of the density of nuclei larger than 20 A diameter is given in Fig. 5 for the bombarded specimens. At 30°C, with only single atoms mobile, nmax occurs at t = 60 sec. With mobile pairs the maximum is at 30 sec. (The total number of nuclei goes through a maximum before any nucleus is visible. The growth time to 20 A. diameter is about 20 sec at this density.) The corresponding micrograph in Fig. 2 shows that some coalescence is occurring. This agreement, which depends largely on the theoretical estimates of surface coverage, derives from and justifies the thermal accommodation factor of 0.3 allowed for in the incidence rate used in the calculations. At high densities, the surface coverage and coalescence rate at t = 60 sec are expected to be strongly dependent on incidence rate, and this may well account for the scatter of experimental points at low temperatures. The high density results without bombardment are assumed to represent nucleation with a variable density of preferred sites. Calculated values ofnmax using the parameter values already found and np = no/100 are shown in Fig. 3. The line goes through the scattered points. The preferred sites responsible for these densities may well be different to the bombardment-produced sites and may have a different value of Ep, or several different values. Nevertheless, the calculation gives an estimate of the magnitude of the preferred site density needed to explain the observed results.

5. CONCLUSIONS In all cases, nucleation of gold on vacuum-cleaved rocksalt substrates is controlled by the established processes of adsorption and migration of adatoms and formation and growth of nuclei. Depletion of mobile atoms and nuclei by capture by growing nuclei causes a progressive decrease in nucleation rate leading to an effective saturation nucleation density. The results show that vacuum-cleaved rocksalt substrates commonly have a high density (10tz-1013 cm -z) of preferred adsorption sites, but the effects of depletion are that only a small proportion of these sites are decorated by nuclei. On areas free from preferred sites, the substrate has shallow adsorption sites over which the atomic mobility is very high. The experimental data is insufficient to determine separate values of single atom and stable cluster mobility or an accurate value for the adsorption energy, but estimates have been made. For deposition during electron bombardment, the analysis shows that the substrate is substantially completely covered by preferred adsorption sites of depth 0.47 eV. The data and calculations do not give a precise figure for the density, and the preferred sites are not necessarily at the same lattice points as the shallow sites on unbombarded substrates. 0.47 eV is the estimated activation energy for migration from the preferred sites. Thin Solid Films, 6

(1970) 1-15

14

B. LEWIS, M. R. JORDAN

Regarding the identity of the preferred sites, Auger spectroscopy 5't7 suggests that electron bombardment produces halide surface vacancies. The postulates of isolated vacancies 5 or of an equilibrium density of vacancies 3, produced by electron bombardment, raised severe difficulties with regard to the experimental independence of electron flux and energy over broad limits, and the weak temperature dependence despite rapid decay of site effectiveness above 80 °C in the absence of irradiation. These difficulties are removed by the finding that electron bombardment induces a saturation coverage of preferred sites. Although the preferred sites present on unbombarded rocksalt, or produced by air exposure v, are effective nucleation centres they have no apparent effect on orientation. This suggests they are different to those produced by electron bombardment. Stirland 3, discussing the epitaxial behaviour, has pointed out that isolated vacancies would not be expected to favour the parallel orientation, since the atomic fit is much better for the 45 ° orientation, (001) [100]au // (100) [ll0]NaO, which is never observed. Halpern 18 has defined a critical size above which nuclei have a stable configuration and below which no orientation with respect to the substrate can exist. (Palmberg et al. 5 have also used the term critical nucleus in this sense.) This size may well be large enough for the atomic fit with the substrate to be irrelevant. The significant feature affecting epitaxy would then be the interaction of symmetry elements of the deposit and substrate lattices, and electron bombardment may be effective in enhancing the polar symmetry field of the substrate by creation of an array of ion vacancies.

ACKNOWLEDGEMENTS

The authors would like to thank D. J. Stirland for the electron microscopy and D. C. Newton and J. P. Whitworth for the radioactivity measurements. The work was supported by the Ministry of Technology and is published by permission of The Plessey Company Limited.

REFERENCES 1 2 3 4 5 6 7 8 9 10

D . J . STIRLAND,Appl. Phys. Letters, 8 (1966) 326. D . J . STXRLAND, Thin Solid Films, 1 (1967) 447. D . J . STIRLAND,Appl. Phys. Letters, 15 (1969) 86. P . W . PALMBERG, T. N. RHODIN AND C. J. TODD, Appl. Phys. Letters, 10 (1967) 122. P . W . PALMBERG, C. J. TODD AND T. N. RHOD~N, J. Appl. Phys., 39 (1968) 4650. A. CHAMBERS AND M. PRUTTON, Thin Solid Films, I (1967) 235. K . M . KUNZ, A. K. GREEN AND E. BAUER, Phys. Status Solidi, 18 (1966) 441. B. LEWlS, Surface Sci., to be published. V. HALP~RN, J. Appl. Phys., 40 (1969) 4627. D. WALTON, J. Chem. Phys., 37 (1962) 2182.

Thin Solid Films, 6 (1970) 1-15

NUCLEATION AND EPITAXY OF ALl ON

NaC1

DURING ELECTRON BOMBARDMENT

15

G. VERI-IAEGAN,F. E. STAFFORD, P. GOLDFINGERAND M. ACKERMANN, Trans. Faraday Soe., 58 (1962) 1926. 12 B. LEWIS, SurJace Sci., to be published. 13 G. ZfNSMEISTER, Thin Solid Films, 4 (1969) 363. 14 A . R . BEAVITT, J. Sci. Instr., 43 (1966) 182. 15 B. LEWIS AND D. S. CAMPBELL,J. Vac. Sci. TechnoL, 4 (1967) 209. 16 G . G . SUMNER,PhiL May., 12 (1965) 767. 17 P . W . PALMaERG AND T. N. RHOD1N, J. Phys. Chem. Solids, 29 (1968) 1917. 18 V. HALPERN, Brit. J. Appl. Phys., 12 (1967) 163.

11

Thin Solid Films, 6 (1970) 1-15