The influence of charged point defects and contamination of substrate surfaces on nucleation

Thin Sohd Fdms, 116 (1984) 55-74 PREPARATION AND CHARACTERIZATION

55

THE INFLUENCE OF CHARGED POINT DEFECTS AND CONTAMINATION OF SUBSTRATE SURFACES ON NUCLEATION* M. HARSDORFF Institutfur Angewandte Physik, UmversztiitHamburg, Jungiusstrafle 11, D 2000 Hamburg 36 (F.R.G.) (Received October 18, 1983; accepted May 31, 1984)

Point defects are created in many different ways such as the cleavage of alkali halides to obtain uncontaminated substrate surfaces. It is also possible to create point defects by heating a substrate such as mica to high temperatures. Moreover, the bombardment of the substrate surface with charged or neutral particles arising from an evaporation or sputtering source can generate point defects in sensitive substrates. In contrast with steps which can be made visible by decoration, point defects are not immediately evident. Their existence is recognizable on account of characteristic changes they cause in the nucleation process. Examples will be presented of all the above-mentioned processes yielding point defects, and their various effects on the nucleation process will be discussed. The influence of surface contamination is much more difficult to determine especially if more than one contaminant is present as must be assumed for experiments carried out in a high vacuum. It is possible to obtain an overall picture of the influence of contaminants in nucleation experiments by letting gas into ultrahigh vacuum and thus producing a known contaminated layer on which nucleation experiments are then carried out. Some examples will be discussed.

1. INTRODUCTION

In the last 10 years some progress has been made towards a better understanding of nucleation and growth processes. The application of ultrahigh vacuum techniques made it possible to condense onto surfaces free from contaminants. Thus the results oftbe different research groups became comparable and were no longer dependent on the particular vacuum systems with different residual gas compositions and different adsorption layers. Moreover, a simple atomic model of two-dimensional polymerization has been found to be applicable in many cases. The use of an atomic model was necessary because of the very high supersaturations, leading to extremely small critical nuclei, that made it impossible to use the thermodynamic theory. * Paper presented at the Second International Summer School on Thin Film Formation, Hajdfszobosz16, Hungary, Septanber 18-24, 1983. 0040-6090/84/$3.00

© Elsewer Sequoia/Printed in The Netherlands

56

2.

M. HARSDORFF

NUCLEATION

ON DEFECT-FREE

SURFACES

Firstly a short review will be given of nucleation and growth processes in general and especially the Volmer-Weber growth process on clean and defect-free surfaces. Volmer-Weber growth is the nucleation and growth of three-dimensional islands on a surface without an intermediate adsorption layer on top of the substrate. The atoms arriving from the vapour source strike the substrate surface with an energy distribution related to the source temperature. The energy is dissipated in the substrate and the atoms are in thermal equilibrium with the crystal. Changes in the adsorption sites are possible if the activation energy for surface diffusion is not too high. The time constant is given by ~'p = '1~0

exp

Re-evaporation is also possible and is described by the Frenkel equation z l = % exp (~-~) In these equations T is the substrate temperature, Ep is the activation energy for surface diffusion, E1 is the adsorption energy and z0 is approximately the reciprocal Debye frequency. The diffusing atoms can collide with each other to form diatomic molecules. Diatomic molecules can collide with single atoms to form triatomic molecules etc. If i* is the number of atoms in the critical nucleus, defined as having a probability of ½ to grow or to decay, then a cluster containing i* + 1 atoms is the smallest stable cluster and the formation rate of these clusters is the so-called nucleation rate I: J oc R '*+1 where R is the deposition rate. Investigations by SchmeiSer and Harsdorff2 and Robinson and Robins 3 have yielded the result J oE R 2

(Fig. 1), i.e. the smallest stable cluster is a diatomic molecule, if Zinsmeister's theory4 in its simplest form is applied. It is assumed that this molecule is fixed at the surface and is growing by the addition of diffusing atoms. For further treatment some simplifications have to be assumed. (1) Only atoms are mobile. (2) The growth of clusters takes place only by atom addition. (3) Diatomic and larger clusters do not decay. (4) No surface defects are present. (5) Each collision between atoms or between atoms and clusters leads to a binding. Under these assumptions the condensation can be described by the differential

INFLUENCE OF DEFECTS AND CONTAMINATION ON NUCLEATION

57

3 c m "IS -i

10 ~o

a/

b

10 9

10 s

Io'

~ '

Fig. 1. Nucleation rate c, T = 390 °C.

i 1' vs.

~

1'o

3b

R "lO~cr~ 2 s :'

deposition rate for Au/NaCI: curve a, T --- 270 °C; curve b, T = 330 °C; curve

equations for a second-order kinetic process:

dNl = R - N ' - 2K t N12- NI "~" K,N, dt TI ~=2 dN2 = K I N 1 2 - K2NIN 2 dt dN~ dt = Kt-INxNI-1--KININi where N~ is the concentration of clusters containing i atoms, n ~ is the number of atoms in the largest cluster and K~ is the collision factor for the addition of an atom to a cluster of size i. These collision factors K~ are defined as the product of the diffusion velocity of the atoms and the cross section or in the two-dimensional case a

58

M. HARSDORFF

"cross length" tr,. In only a first approximation tr, is the sum of the diameters of the cluster of size i and of the atom. Thus K,

=

VdtffG ~

The diffusion velocity is the product of the number of diffusion jumps per unit time and the jumping distance 4. Experiments show that the re-evaporation of atoms is dominant compared with the nucleation and growth, i.e. the condensation coefficient ~ is much less than unity. For this case the steady state solution of dNx = R dt

N1 zl

is N1 =

Rzt

The formation rate of diatomic molecules is the nucleation rate J = KINt 2

or using the definitions ofzx and K1 J=CI

R2

exp~{2E1~ -- E p ~)

This equation describes the formation of diatomic molecules, i.e. particles which are not visible in the electron microscope. At equilibrium, using the second and third differential equations, we obtain K~N, = Kl1~ 1

where/Vl and/V~ are the saturation densities of atoms and of clusters of size i. This formula shows that the total cluster density up to a size i is a constant. The consequence is that the rate of appearance of clusters in the microscope is equal to the rate of formation of diatomic molecules. The validity of this prediction was proved experimentally by Schmeil3er and Harsdorff 2. They found that the diameter distribution near the resolution power of the instrument was constant and not a function of the deposition time. A plot of the nucleation rate versus the reciprocal temperature yields an Arrhenius line, as predicted by the model (Fig. 2). After nucleation the stable clusters grow by atom addition. The number density is limited by coalescence of the particles. If the nucleation rate is equal to the coalescence rate, a maximum in the number density versus deposition time curves is reached. The nucleation stage is followed by the coalescence and filling-in stage, leading to a completed film. 3.

INFLUENCE OF DEFECTS ON NUCLEATION

The nucleation process is modified if defects are present at the surface. Point defects are always present and are additionally generated with a high density by the cleavage of crystals 5, by the irradiation of ionic crystals with electrons ~, by the heating of mica 7 or by the bombardment of surfaces with high energy neutral particles s (see also ref. 9). All these eases will be discussed in detail.

INFLUENCE OF DEFECTS AND CONTAMINATION ON NUCLEATION

450 400

350

300

.

250 -''To 'C

59

101o

cm-2 S-1

109 -I

1o8-I

V

Io'-I /

10 6

I

1,3

,~/

a/

/

I- Io8

I

/

.

1,4

.

.

1.5

I-10 9

I-1o'

.

I 10 6

.

1,6

1.7

1.8 10 3

1~9 2,0 K

T Fig. 2. Nucleation rate vs. reciprocal temperature for A u / N a C I : curv¢ a, R = 4 x ]013 cm - 2 s - ' ; curve b, R = 1.3x 1013cm-2 s-1.

3.1. Defect generation by the cleavage of alkali halides In nucleation studies with the system gold on sodium chloride Bohn 5 observed an unexpectedly high density of clusters if the cleavage of the crystals was performed in a vapour beam of gold. The number density was about ten times higher than that in experiments with a waiting time of 30 s between cleavage of the crystals and exposure of the cleavage plane to the gold vapour. He considered a high defect concentration at the surface to be responsible for this observation (see also ref. 10). The defects are decorated immediately by atoms diffusing on the substrate. The waiting time between the cleavage and the start of condensation was varied systematically. At a waiting time of 20 s the number densities measured were as normal. (The deposition rate was 2.2 x 1013 cm -2 s - 1 the substrate temperature T = 603 K, the deposition time t -- 35 s and the pressure p = 1.5 × 10 -9 Torr.)

M.HARSDORFF

60

The dependence of the number density on the waiting time is plotted in Fig. 3. An exponential decrease is observed until a constant minimal value is reached. The exponential behaviour shows that an accidental decay or rearrangement process after the cleavage of the crystals must play an important role. The two curves are for samples obtained by the use of different cleavage tools. For sample a the razor blade scratched the cleavage plane, leading to higher cluster densities. The time dependence of the observed cluster density can be described by N(zsp) = N(0) e x p ( - ~ ,p) where Zsp is the waiting time between cleavage of the substrate crystal and the start of the gold deposition and T* a time constant. The nucleation rate on surfaces with point defects is1

J = K1RNdefZ 1 For constant deposition time, deposition rate and substrate temperature the observed cluster density is N = constant x Nd,f From this it follows that

N('t'sP)~----Ndef('~sP)~~-~exp(--~,P) This means that the measured time constant is the decay constant of the point defect concentration. The experimental values are z*(603K) = 5.5 +0.5 s and z*(573 K) = 12+ 1.5 s. Using the Frenkel equation, the activation energy for the decay of defects can be estimated as E = 0.78 + 0.15 eV. This value shows a remarkable agreement with the activation energy for the movement of a positively charged vacancy in sodium chloride. It is possible that the cleavage-generated point defects will diffuse into the bulk and will behave there as other Schottky defects generated thermally, but a much more probable decay process will be the annihilation at surface steps.

3.2. Defects generated by the impact of charged particles As an example of this case the nucleation of gold on sodium chloride cleavage planes irradiated with electrons will be investigated. The crystals were cleaved in an ultrahigh vacuum environment and irradiated with electrons for a fixed time (energy, 50 eV; current, 1 mA; irradiation time, 30 s; waiting time between radiation and deposition, 180 s). After condensation of the gold and its stabilization with carbon the Au/C sandwiches were floated offin distilled water and investigated in a high resolution electron microscope. The micrographs were analysed with an image analysing computer 12 to determine the number of clusters, the fractional area covered with particles, the mean cluster diameter and the diameter distribution. In Fig. 4 the dependence of the nucleation rate on the vapour flux rate is shown. At low rates defects are predominantly decorated and the nucleation rate is proportional to the vapour flux rate. At higher deposition rates statistical nucleation is predominant and an R 2 dependence is observed, because in this case the diatomic molecule and

61

INFLUENCE OF DEFECTS AND CONTAMINATION ON NUCLEATION

J

J-RZl

crfiZs-I

!

/

N I 0 )( 4O

3o!

//J/

20

10

10 9

75" iI

3-

"/~"

R 1 01½rfi2;~ 1

I

2-

I

/

g

lb

fs

2b

2~

3'0.~,

I

I

I

Fig. 3. Dependence ofthe number densRy of clusters on the interval between cleavage and deposition. Fig. 4. Nucleation rate v s . deposition rate for Au/NaCl in the case of Irradiated surfaces (T = 330 °C).

not the atom is the stable nucleus. It is difficult to estimate the concentration of defects on the surface. It is only possible to hold constant the acceleration voltage, the emission current and the radiation time to ensure reproducible conditions, but the charging of the irradiated crystals makes it impossible to obtain the density of defects. A rough estimation is possible by measuring the increased number density of clusters in comparison with that obtained in experiments with unirradiated crystals. The irradiation conditions were defined to produce point defects or better defects with atomic dimensions. A higher accelerating voltage of 200 V and a longer exposure time at a higher current (120 s; 20 mA) lead to very high cluster densities at decorated structures. The micrograph of the replica of such a specimen shows an extensively destroyed surface (Fig. 5). In contrast, the micrograph for a specimen irradiated at a lower dose does not show any marked alterations, except that the number density ofcrystallites is higher than that for unirradiated surfaces (Fig. 6). In many papers and discussions it has been supposed that charged particles, being present in the vacuum system, lead to surface damage and to an increased crystallite density even in the case of unirradiated crystals13. To test this assumption screening grids were installed to keep back electrons and ions from the substrate crystal. The lower grid was charged negatively to prevent electron impact; the upper positive grid was to prevent the impact of ions. The efficiency of this arrangement was tested by the use of an isolated collector for current measurement in the place of the substrate crystal. Applied voltages of - 30 V and + 150 V respectively suppressed the charged particles arriving from the vapour source. Two series of condensation experiments were carried out, one with and one without the applied voltages. The results are shown in Fig. 7. Without the electrostatic filter the cluster density is higher by a factor of 2 than that with the filter. If only electrons are trapped, the

Fig. 5. Replica of a damaged NaC! surface. (Magnification, 6000x.)

Fig. 6 Rephca of an NaC1 surface after low dose irradiation (Magmficatlon, 6000x )

o

>

~e

Ox I'O

63

INFLUENCE OF DEFECTS AND CONTAMINATION ON NUCLEATION

reduction in the cluster density is nearly the same. The impact of electrons seems to play an important role in the generation of defects. The role of positive ions could not be determined within the accuracy of these investigations. In condensation experiments on defect-free surfaces a characteristic diameter distribution of clusters is observed. A typical example is shown in Fig. 8 for three different deposition times. For gold on sodium chloride at T = 350 °C the cluster density for a given diameter interval is plotted against the cluster diameter. N cm 2 109

aN / ad 10Scm-2]k"1

18 16

12

1/,

10.

12 10

8-

8

6-

,]

6

42.

2 20

/,0

60 t s -1

2's 5b

loo

F~g. 7. Cluster density v s deposition Ume with and without the screening of charged particles (T = 3 3 0 ° C ; R = 2.6 x I013 cm -2 s - 1 ) ' O , grid 1 a t 0 V, g n d 2 a t 0 V ; A , grid 1 a t 0 V, g n d 2 a t - 3 0 V ; x, grid 1 at + 150 V, grid 2 at - 30 V. Fig. 8. Cluster diameter distributions for Au/NaCI (defect-free surface; R = 9.1 x 1013 cm -2 s - l ) . r, t = 1 0 s ; N = 8.3 x 1 0 9 c m - 2 ; ~ , t = 30s, N = 1 7 6 x 1 0 9 c m - 2 , ~ , t = 50s, N = 35.2x 109era -2.

The indicated regions of confidence are the standard deviations. The distributions show a similar behaviour under all experimental conditions if defect-free surfaces are used. A sharp maximum is observed with a subsequent steep decrease. The increase up to the maximum is relatively gradual. A determination of the full width at half-maximum of the diameter distributions yields a value of approximately 25 ~, independent of the deposition parameters (1013 cm -2 s -1 < R < 5 x 1014 c m - 2 s- 1 ; 270 °C < T < 390 °C). The question is what alteration in the full width at half-maximum is to be expected if surfaces with a high concentration of point defects are used for the experiments. Figure 9 shows the result of such an investigation. The full width at half-maximum is reduced to 10_+ 2 tit. This result is not unexpected and gives an insight into the nucleation processes on surfaces with a high defect concentration. Obviously the defects are preferred nucleation sites. This means that at the beginning of the condensation process the defects become occupied by atoms. These atoms are no longer able to move and are consequently the smallest stable

64

M. HARSDORPF

"clusters". This fact is demonstrated by proportionality of the nucleation rate to the deposition rate. Statistical nucleation with the diatomic molecule as the smallest stable cluster plays no important role. The atoms, remaining in the defects, are decorated by impacts with diffusing atoms and grow into clusters. If high concentrations of clusters are present, many atoms are trapped in this way. This mechanism of the decoration of defects leads to clusters of nearly identical age, a reason for the very narrow size distribution of crystallites. An extended deposition time leads only to a shift in the maximum towards greater diameters with no alteration in the full width at half-maximum. At very high deposition times the width of the distribution is enlarged by coalescence (Fig. 10). AN/Ad 108 ~I cm-2 40

20

20

40

60

80 d ~-1

Fig. 9. Cluster & a m e t e r d i s t r i b u t i o n s for A u / N a C I (surface w i t h defects): © , t = 10 s; I--1, t = 40 s; x , t = 120s.

,,N

109cm'2-" ~'1/~

10

,"

"

..." ...."" , 7

•

,b

so

50

70

90

Fig. 10. C l u s t e r d i a m e t e r d i s t r i b u t i o n in the coalescence stage (R = 28.5 x 1012 e r a - 2 s - t ; T = 603 K).

INFLUENCE OF DEFECTS AND CONTAMINATION ON NUCLEATION

65

A very important result of these investigations should be stated. If very small cluster diameter distributions are observed in nucleation experiments, the presence of a high concentration of point defects must be taken into account. However, it is possible to obtain island layers with a high cluster concentration and a small diameter distribution if the alkali halide cleavage planes used are irradiated with electrons prior to metal deposition.

3.3. Defects generated by the heating of mica substrates The annealing of suitable substrates at high temperatures is another way of generating point defects of high density. Mica is such a substrate. Point defects are generated by simply heating the mica to 550 °C. At this temperature potassium is evaporated from the surface, as clearly detected with a quadrupole mass filter. Using Auger electron spectroscopy it was confirmed that the uppermost potassium layer is depleted at the applied temperature. This fact was also reported by Poppa and Elliott 14. The question is what nucleation process is predominant in this case. Statistical nucleation leads to an equation in which the nucleation rate is proportional to the square of the deposition rate. If the nucleation takes place at point defects of high density, the single atom is the stable nucleus and the nucleation rate should be directly proportional to the deposition rate. Figure 11 shows that at least for low deposition rates nucleation at defect sites is predominant. At high deposition rates a deviation towards a statistical nucleation behaviour is observed. Obviously the high deposition rate leads to a high population of atoms on the surfaces. In this case short diffusion lengths are sufficient to lead to collisions between diffusing atoms prior to the decoration of a defect site.

3.4. Defect generation by impact with neutral particles This process will play an important role if nucleation experiments are performed by r.f. sputtering. Unfortunately not only neutrals but also electrons and 1012.

/

/

,' J-R 2

/;-R

//

/"

I[/~ ii I

II

1011

#~"

/I / III

.'IY / .1 / , ~"

i

i ; i,b

Fig. 11. Nucleation rate

vs.

2'o

,-'oio~oioo

deposition rate for ku/mica (T~ = 475 °C).

66

M. HARSDORFF

ions reach the substrate surface. This leads to a rise in the substrate temperature (Fig. 12) and the erosion of the surface. In this case nucleation experiments are not possible. To eliminate this effect, so that only the sputtered atoms have to be considered, the surface was shielded from the bombardment of charged particles by placing magnets between the substrate and the plasma. Successful shielding was demonstrated by the constant substrate temperature even at high sputtering times and the normal nucleation and growth of gold islands on sodium chloride. One difference between thermal sources and sputter sources remains, as seen in Fig. 13 which shows the energy distribution of gold for thermal evaporation and for sputtering with helium.

-,f'/j K

25O

20O

150

100

50

°o '&~

i~o

360 _

Fig. 12 Rise m the substrate temperature vs. deposition time for different sputter powers (To = 290 K): II, 100W; O, 300 W; A, 500 W; O, 800 W ~N

I

I

I

I

AE

0

(

•

I

2

I

4

I

6

I

8

E_ eV

Fig 13 Energy distribution funcUon for gold: curve a, thermal evaporation; curve b, sputtenng with hehum.

INFLUENCE OF DEFECTS AND CONTAMINATION ON NUCLEATION

67

In the vapour beam highly energetic neutrals are present leading to point defects at sensitive surfaces. It can be assumed that a part eR of the particle flux with < 1 leads to defect generation. With the usual assumptions that (a) atoms can desorb from surfaces and (b) diatomic molecules are stable, the nucleation in sputtering experiments can be described as a decoration of defects, generated by some of the condensed atoms themselves. The condensation process can be described in the following manner. High energy atoms reaching the surface generate defects and remain at the defects with an increased stay time. If an atom is not decorated within a certain interval it can leave the defect and diffuse across the surface until it collides with other atoms or reevaporates. At the defect site the diatomic molecule is the smallest stable nucleus as for nucleation on defect-free surfaces. The time dependence of the concentration of defects is described by the following differential equation: dNdcf Ndef dt - RD---KDNdefNI ~D

where R v is the rate of incidence of atoms leading to defects, xv is the stay time of an atom at the defect site, KD is the collision factor and N 1 is the concentration of diffusing atoms. Given the experimental observations that (1) pair production on defect-free parts of the surface is negligible and (2) re-evaporation of atoms is predominant, i.e. the condensation coefficient ~ <~ 1, and with the assumptions ~~ 1

R o = eR

K D = a ( a + d l ) ' C o - ' exp ( - ~-~)

the residence time of atoms in defects is zD=Zo exp(~--~) with Zo much less than the deposition time. In this case Ndef -~ KDNdef NI

The simplified differential equation dNdef

Ndef

dt

zD

is solved for the equilibrium case Na©f = e R T n

The nucleation rate is J = KDNaefN 1

68

M. HARSDORFF

The result is that J oc R 2 and E I + E D - E p = 1.6_+_0.2 eV for gold on sodium chloride. The value of E 1 - E p = 0.37 +_0.04 eV is known from nucleation studies on defect-free surfaces. Hence the activation energy ED for the trapped atoms is 1.2 + 0.3 eV. The use of plausible values in the equation leads to an estimate for e of 0.01, showing that the above assumption that ~ ~ 1 is valid. In Fig. 14 a plot of the nucleation rate v e r s u s the condensation rate is shown. It gives a slope of 2, as for statistical nucleation, but the absolute values are higher than those in the statistical case. The observation of a J oc R 2 dependence is obviously no proof of statistical nucleation, as frequently assumed. Also for cases in which the defect sites are generated by the condensing atoms themselves a similar behaviour can be observed. In Fig. 15 the dependence of the nucleation rate on the substrate temperature is plotted. The slope of the Arrhenius plot is greater than that for experiments using thermal sources. The reason is the contribution of the activation energy to the movement of trapped atoms. Also the absolute values of the nucleation rate are remarkably high compared with those for the statistical case. At very high deposition rates a deviation in the J oc R e law is apparent. The slope is less than 2. The reason is that the defects caused by the impact of highly energetic atoms have no time to heal. In this case the trapped atoms are the smallest stable clusters and a J oc R law, as in the other cases of defect decoration, has to be expected. The Arrhenius plot also shows a deviation in slope towards lower values at low temperatures. In this case again the decay of defects can be neglected and a

o:i

/

:f 10 s t "

'

I 2

I 3

I 5

I 10

I 20

i 3O

R

1012cm-2s -I

Fig. 14 Nucleation rate vs. deposition rate for AufNaCl (r.f.sputtenng; T = 603 K): curve a, ref. 5; curve b. ref. 3; curve c, ref.6; curve d, ref.2.

INFLUENCE OF DEFECTS AND CONTAMINATION ON NUCLEATION

T v~

360 330 300 270 I

J

I

I

69

I

cm-2s-1

+ 10~o

109

108

I

1

15

I

I

17

I

1.9

I

I

~03 K T

Fig. 15. Nucleation rate vs. reciprocal temperature for Au/qqaC! (r.f. sputtering; R = 6.6x l012 cm - z s - t). curve a, ref. 3; curve b, ref. 9; curve c, ref. 2.

decoration of fixed atoms takes place. The energy term in the exponential function is decreased by the value of ED. Summarizing all the results concerning the behaviour of the nucleation rate w e can distinguish the following cases: (a) statistical nucleation at defect-free surfaces, for which

J = cxg 2exp.-f2E ~1- Ev.) (10)nucleation on defects with a long lifetime, for which

{E,-~p\

J = C2No.fR exp~---k-~- }

(c) nucleation at defects generated by the condensing atoms, for which J

ED--Ep~ CaeR 2 exp(El + -Ff /

It should be pointed out that the dependence of the nucleation rate on the vapour flux density and the substrate temperature is considered only to be an example of the application of the kinetic nucleation theory to substrates with a high point defect density. Furthermore, the behaviour of the condensation coefficient, the

70

M. HARSDORFF

growth of small clusters, the induction time and the growth of larger crystallites can be discussed on the basis of this atomic model. These aspects will be discussed elsewhere 8. An interesting case of nucleation on defects will be added as a supplement. Stenzel and Bethge 15 used alkali halide crystals doped with divalent impurities to obtain a defined defect density for nucleation studies. Gold was deposited onto calcium-doped sodium chloride. The result was an increase in the cluster density with increasing concentration of calcium ions. Unfortunately, no dependence of the nucleation rate on the vapour flux and the substrate temperature was reported. A quantitative comparison with other results is not possible. 4.

THE INFLUENCE OF CONTAMINATION OF SURFACES ON NUCLEATION

A systematic investigation of the effect of contamination on nucleation does not exist. This means that no dependence of the nucleation rate, the condensation coefficient, the coverage of the surface, the induction time and the growth diameter of crystallites on the deposition rate and the substrate temperature has been reported. However, it is a well known fact that the presence of contaminants on for instance alkali halides leads to a marked increase in the number density of clusters compared with that for experiments in ultrahigh vacuum. It is possible that impurity atoms adsorbed at the surface could act as preferred sites for nucleation. That may be valid in special cases but in general this explanation is improbable. One of these special cases was reported by Lee et al.16 for the system Au/mica. The assumption of impurity adatoms was based on a discrepancy between the energy parameters computed by the use of growth and nucleation data. A direct proof was not given by Lee et al. and the nature of the adsorbed material remained unknown. A much more probable explanation is the greater mobility of atoms, leading to a higher nucleation rate. The higher mobility is caused by the reduction in the activation energy for surface diffusion by the adsorbed species. Hence two competing processes can be the reason for a high cluster density: (1) decoration of point defects as described in the preceding sections; (2) statistical nucleation on a substrate with a reduced activation energy for surface diffusion by adsorbed material. High cluster densities are the reason for an early onset of heavy coalescence. It is well known that oriented overgrowth or epitaxy is influenced by different growth processes, as follows. (1) Epitaxial nucleation may take place, which means that a fraction of the crystallites is oriented with respect to the crystallographic axes of the substrate depending on the rate and the substrate temperature. (2) An improvement in the orientation may occur during the coalescence stage. In many cases the coalescence of an oriented cluster with a randomly positioned cluster leads to a larger oriented cluster especially if the diameter of the clusters is small. (3) The orientation may be improved by recrystallization in the filling-in stage of the condensation process. The presence of adsorbed layers improves the mobility of atoms and small clusters as well as initiating coalescence. It may be expected that the growth

INFLUENCE OF DEFECTS AND CONTAMINATION ON NUCLEATION

71

processes, epitaxial nucleation and the improvement in the orientation by coalescence should be greatly influenced by adsorbed material, lfepitaxial growth is affected by contamination ofthe substrate surfaces, it should be possible to study the influence of adsorbed layers on the nucleation and growth process by studying the orientation of the deposits. 4.1. Experiments with substrates covered with an adsorbed layer o f unknown material Before the use of ultrahigh vacuum environments for surface studies was recognized to be necessary for reproducible results, many investigations concerning epitaxial layers were performed in the pressure range 10 -6 Torr < p < 10 -4 Torr. The composition of the residual gases was unknown. The first investigation of this type was reported by Lassen 17. A so-called epitaxial temperature was defined to be dependent only on the materials combination. For instance, an air-cleaved sodium chloride crystal has to be heated to 150°C to obtain a perfect parallel-oriented silver film. For a perfect gold film a temperature of 400 °C was required. The idea of a materials constant, the so-called epitaxial temperature, was undisputed for many years until Ino et al. 18 reported that a much lower epitaxial temperature resulted if the substrate crystals were cleaved in vacuum rather than in air. It was suspected that the reduced adsorption of gases would lead to a better orientation of the deposits if the cleavage was performed in a high vacuum. Consequently an attempt to achieve a further reduction in the epitaxial temperature was made by cleaving the substrate crystals in an ultrahigh vacuum environment 19. It was a great disappointment that in this case the critical temperatures for perfect orientation were increased drastically. For the system gold on potassium chloride no perfect orientation could be obtained even at temperatures exceeding 400 °C. Adam 2° showed for different materials combinations that in the case of ultrahigh vacuum experiments the epitaxial temperature is dependent not only on the substrate-metal combination but also on the deposition rate. High deposition rates correspond to high critical temperatures for the perfect orientation of deposits. The result of all these investigations is that the critical temperature is high if the crystals are cleaved in air or in ultrahigh vacuum. In the first case a thick adsorption layer, in the second case an extremely clean surface must be assumed. A qualitative explanation was given by Harsdorff and Raether 21. For the system silver on sodium chloride it could be demonstrated that a certain adsorption layer thickness provides the most favourable conditions for epitaxy. This was proved by varying the time between the cleavage of the crystal and the beginning of the vapour deposition. In Fig. 16 the relative orientation is plotted against the waiting time. After a delay of about 7 s the adsorption layer present allows the maximum degree of orientation. Another example is presented in Fig. 17. The perfection of the film orientation is plotted against the substrate temperature for two different deposition rates and a constant pressure of 2 x 10 -4 Torr. At low deposition rates the adsorption can reach the optimum thickness and the epitaxial temperature has the very low value of about 40 °C. Ifthe deposition rate is increased by a factor of 6 the required thickness of the adsorption layer is not reached and the critical temperature is about 160 °C. In this paper the composition of the residual gases was stated to have an important influence on the orientation. In

72

M. HARSDORFF

S 10

S 10-

08

(18-

06

06-

04

04-

02

0.2"

.~

io

f8

2"o

2's

3"o t S

2b ~b

~o ~

160 1~o 1~o T

Fig. 16. Degree of epltaxlal order vs, time between cleavage of the NaCl/crystal and the start of condensation of silver (T, = 60 °C, p = 10- s Torr). Fig. 17. Degree of epRaxial order v s substrate temperatures: A , R = 7 x 1 0 i s c m - 2 s - 1 ; O, R = 4 . 5 x 1014cm-2 s - t

Fig. 18 the perfection of the foil orientation is plotted against the deposition rate. The substrate temperature and the total pressure were held constant. The parameter of the curves is the residual gas composition. In curve a air of normal humidity was used to adjust the pressure to 2 x 10 -'~ Torr. (The background pressure p was less than 10 -6 Torr.) In curve b dry air was used, in curve c hydrogen and in curve d helium. A very strong dependence on the residual gas composition is apparent, leading to the question what parts of the residual gas are responsible for the orientation phenomena. S

tO

°c

c

02

06

10

t4

R

1014cm-2 s-1 Fig. 18. Degree of epitaxial order vs. deposition rate for various residual gases (T, = 60 °C; p = 2 x 10 - 4 Torr): curve a, air of normal humidity; curve b, dry air; curve c, hydrogen; curve d, helium.

4.2. Substrates covered with special contaminants The investigations described in Section 4.1 show that the nature of the adsorbed layers or the substrate contaminants plays an important role in the condensation processes. Many papers were published in the years after the first investigations. As an example the paper of Matthews and Griinbaum 22 will be discussed. As a wellknown example of epitaxy the growth of single crystals of f.c.c, metals on the (100) cleavage plane of sodium chloride was investigated. The result is the necessity for contaminants to be present for the epitaxial growth of gold on rock salt. In the first investigations in an ultrahigh vacuum environment for a wide range of substrate temperatures the formation of polycrystalline or textured films was shown. All the

INFLUENCE OF DEFECTS AND CONTAMINATION ON NUCLEATION

73

textures show a (111) contact plane to be parallel to the substrate surface. In the subsequent experiments the crystals were cleaved at pressures of 10-9 Torr and then exposed to vapours and gases or gas mixtures, e.g. water vapour, for 1 h. The deposition of gold was performed at a very low pressure. The use of moist oxygen was observed to be very effective in the formation of a nearly perfect parallel orientation. The question is how this curious behaviour can be explained. Obviously in the case of ultrahigh vacuum experiments the mobility of very small clusters, containing three or four atoms, is too low to allow rotation into the minimum of the Gibbs free energy before the nuclei grow by atom addition and become immobile. If a very thick adsorption layer is present between the crystal surface and the deposit, the periodic modulation of the surface is too weak to influence the cluster orientation markedly. A certain amount of adsorbed material seems to be necessary to permit the alignment of clusters of orientation-specific size in a sufficiently modulated periodic potential. 5. CONCLUSIONS

In the previous sections the influence of defects and contaminants of substrate surfaces on the condensation processes was discussed. The influence of defects depends strongly on the generation process. Different types of defect formations were discussed. It was demonstrated that the only way to distinguish between nucleation on defects and statistical nucleation is to compare the full width at halfmaximum of the diameter distributions of the clusters. An unexpectedly high value of the cluster density may be an indication of the existence of a large number of defect sites, but this result is not clear. The presence of contaminants may also be the origin of high cluster densities because the reduced activation energy for surface diffusion causes a high diffusion velocity of adsorbed atoms, leading to a high nucleation rate. The dependence of the nucleation rate on the vapour flux density and the substrate temperature is not suitable to distinguish between nucleation at defect sites and statistical nucleation on a clean and defect-free surface. The investigation of the influence of surface contaminants on the condensation process shows the absolute necessity to employ ultrahigh vacuum techniques for all surface studies. At higher pressures a complicated system of substrate, adsorption layer and condensed material must be treated. If for instance the temperature dependence of the nucleation rate or of other parameters is to be studied, it is necessary to pay attention to the fact that with increasing temperature the behaviour of the substrate will also alter because of modifications in the partially desorbing contamination layer. These statements are important for basic research. For the application of thin films it is indisputable that the formation of a high density of point defects or of adsorption layers of a particular nature on surfaces can be used to produce thin films with special properties. REFERENCES 1 D. Walton, J. Chem. Phys., 37 (1962) 2182. 2 H. SchmeiBer and M. Harsdorff, Z Naturforsch, 25a (1970) 1896.

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M. HARSDORFF

V.N.E. Robinson and J. L. Robins, Thin SohdFdms, 20 (1974) 155. G. Zinsmeister, Vacuum, 16 (1966) 259. H. Bohn, to be published. M. Grote, to be published. J. Cardoso and M. Harsdorff, Z. Naturforsch., 33a (1978) 442. W. Jark, to be pubhshed. G.E. Lane and J. C. Andersen, Thin SohdFilms, 26 (1975) 5 T.E. Gallon, J. G. Higgmbotham, M. Prutton and H. Tokutaka, Surf. Sct., 21 (1970) 224. J . L RobmsandT. N. Rhodin, Surf. Sci.,2(1964)364. M. Harsdorff, Thin Solid Films, 90(1982) 1 J.L. Robins, Surf. Sci., 86 (1979) 1. H Poppa and A. Grant Elliott, Surf. Scz., 24 (1971) 149 H. Stenz¢l and H. Bethge, Thin Sohd Fdms, 32 (1976) 267. E.H. Lee, H Poppa and G. M. Pound, Thin SolidFUms, 32 (1976) 229. H Lassen, Phys. Z.,35(1934) 172 S. Ino, D. Watanabe and S Ogawa, J Phys Soc Jpn., 17. S. Ino, D. Watanabe and S. Ogawa, J. Phys. Soc Jpn, 19 (1964) 881. R. W Adam, Z. Naturforsch., 23a (1968) 1526. M. Harsdorlfand H. Raether, Z. Naturforsch., 19a (1964) 1497 J.W. Matthews and E. Grfinbaum, Appl. Phys Left., 5 (1964) 106