Growth kinetics of Cd3As2 films on sodium chloride substrates

167

Journal of Crystal Growth 62 (1983) 167- 172 North-Holland Publishing Company

GROWTH

KINETICS

OF Cd,As,

FILMS

ON SODIUM

CHLORIDE

SUBSTRATES

S. MIOTKOWSKA

Imtttute of Ph.rsics. Polish Academy OJ Sctences, Al.Lotnikdw Received

19 March

1982: manuscript

32/46,

02 -668 Warsaw, Poland

received in final form 27 December

1982

The growth morphology of thin Cd,As, films on NaCl substrates has been described. It was found that growth of the films begins with the formation of three-dimensional nuclei. The dependence of both number density and size of such nuclei on condensation number densities of nuclei. saturation densities and initial parameters was investigated. In addition, nucleus size distributions, nucleation rates were analysed on the basis of theoretical expressions. Values for the activation energies of adsorption and surface diffusion and for the binding energy of a pair of atoms were estimated.

1. Introduction

Investigations of the initial stages of thin vacuum evaporated films of Cd,As, have been carried out for several years. In previous papers [ 1,2] the substrate temperature dependence of the crystallographic orientation of the growth layer was investigated. Those phenomena were associated with various shapes of nuclei. It was found that at low temperatures nuclei of a hemispherical shape were obtained. At higher temperatures, nuclei in the form of trihedral and tetrahedral pyramids, of sharply defined crystallographic orientation, were observed. The present paper deals with the growth kinetics of thin Cd,As, films.

2. Experimental Thin Cd,As, films were deposited on NaCl substrates by evaporation techniques. The equipment and experimental set-up used in this work have been described in detail previously [2]. In general, the depositions were carried out at pressures of lops Torr. The Cd, As, was evaporated from a molybdenum crucible onto substrates held at constant temperature. Two different incidence rates and a range of substrate temperatures were 0022-0248/83/0000-0000/$03.00

0 1983 North-Holland

used. In order to investigate the consecutive stages of growth a moving shutter was used to produce a sequence of deposition times along the substrate. After deposition of the Cd,As, film the surface was coated with a carbon-platinum layer. This composite film was then floated off the rock-salt and examined using a JEM-120 transmission electron microscope. Different parts of each sample were examined in order to improve the reliability of the data.

3. Results and discussion 3. I. Growth morphology The growth morphology of Cd,As, films is illustrated in the form of electron micrographs obtained for the successive stages of growth. Figs. 1 and 2 show typical micrographs of the films and illustrate the general sequence of growth for substrate temperatures of 293 and 493 K, respectively. It is seen that the growth of the films begins with the formation of three-dimensional nuclei. The number density and the size of such nuclei increase as time progresses, and also depend on both condensation temperature and incidence rate. It may be concluded that the condensation temperature has a more pronounced effect on the number

density

and size of nuclei

eters

of the

hand.

the

con&nation

shape

of

the

than

do the other

process. nuclei

does

On

the not

param-

other change

from that in the initial xtage for each individual sequence of growth although three distinct shapes of the three-dimen.sional nuclei wxrt‘ ohxrvd. With increased deposition time the nuclei Join and a continuous layer is eventually formed. These results have been published previously [ 1.21. Fig. 3 shows the decorated surface of a NaCI sub.\trate. Two types of nucleation are visible. namely. nucleation in linear arrays on cleavage and slip steps. and nucleation on apparentI\ featureless flat areas. Nucleation on the steps should occur more easilv since adsorbed atoma are

S. Mwrkon~sko ,’ Grouth krnrtic,s

in contact with two crystal faces, whereas, on the flat areas, they will be in contact with only one crystal face. Along these lines the nucleus density is high. The distribution of nuclei on the flat areas between steps does not appear to be uniform on either side of the step edge, which results from the effect of step faces normal to the substrate surface. At lower temperatures the distribution is asymmetric. whilst at higher temperatures it is symmetric [3]. The nucleation number densities shown in the figures and used in the analysis were determined on flat areas remote from step edges. 2.2. Nucleus si:e distribution The nucleus size distribution can be seen in figs. 1 and 2 which illustrate the general sequence of growth of Cd,As, films. The plot of nucleus size distribution versus deposition time is shown in fig.

of Cd,As,

Jilms on NaCI

.su/w/rcrre.~

169

4. The appearance of this distribution is quite typical of the whole temperature range covered by the present experiments: a sharp peak at large diameters, with slowly rising and steeply falling sides, corresponding to a well defined maximum diameter. With increasing growth times the following distinct features are to be noted: (i) The maximum is displaced towards the region of large nucleus size. This shifting of the maximum is due to the general growth of all nuclei. (ii) The left side of the distribution drops below the previously measured va!ues. This,means that in the later stages of deposition the number density of small nuclei is less than that of nuclei of the same size at earlier times. (iii) The total nucleus number density, given by the total area under the distribution curves, is increased. 3.3. Number

density of nuclei

The number density of nuclei, N,, in the successive stages of growth was measured from the electron micrographs. It was found that as time progressed the number of nuclei per unit area increased as is shown in figs. 5 and 6. This increase in the number density was monatomic with time to

293K

f

.... .._.,

6 :’

,.:

3 ‘i:

200 -2r

20

600

[A]-

Fig. 4. Plot of nucleus size distribution at various times (T= 373 K. R = 9~ 10” atoms cm-’ s-l).

deposition

40

60

Fig. 5. Variation of nucleus density IV, with deposition time f at various temperatures for CdjAs, on NaCl at an incidence rate of 2X IO” atoms cm-* s-‘.

r

IId”-j

323 u

8

CT

-

‘E L

The initial

nucleation

rates,

J. Lvere determined

by estimating the initial slopes of the figa. 5 and 6. directly from the plotted a broad found higher

maximum.

On

that N, was larger incidence

rates

the

other

for lower if all

hand.

it \\;I>

temperatures

other

factors

or \vere

equal.

similar

The time to reach substrate temperature much what

and

plot of In J as a function of reciprocal is shown in fig. X. This semi-logarithmic incidence rate of 2 X IO’” atoms cm

longer shorter

for

saturation depended on both and incidence rate: being

higher

for higher

temperatures incidence

and

some-

rates.

from NY versus t plots, according to Donohoe and Robins [4] and Robinson and Robins [5]. It was found that for nuclei present on (OOl)NaCl, the maximum number of nuclei depends on the substrate temperature and the incidence rate. In fig. 7. nuclei

the saturation

density

as a function semi-logarithmic

of the reciprocal temperature. plot demonstrates that

is plotted

transition is in evidence. N, increases from 10”’ cm -2 as the condensation temperature from 523 maintained

X

pcjint tion found

The maximum number density of nuclei. ;V’. the initial nucleation rate, J. were determined

of Cd3Asz

9

to 293 K, while the incidence constant. On the other hand,

This some 10” to drops rate is if the

substrate temperature is constant, the saturation density of nuclei increases with a rise of the incidence rate.

to that

10”

atoms

for an incidence

cm

‘_ From

of the asymptotes temperature,

r,,.

of these

the curbes

in A

temperature plot. at an ’ 5 ’ ia ver\

obser\,ed ’ s

curves pointa.

rate

of

intersection the tranxi-

could be deduced. It V.\;IS that J is proportional to H’ over the temper-

S. Mwthowka

/ Grow,th

krnettcsof Cd., As,

ature range from room temperature to the transition temperature TD, and is proportional to R’ for temperatures above T,.

4. Analysis Figs. 7 and 8 provide data for the evaluation of activation energies above and below the transition temperature. According to the theoretical expressions [4-81 for N,( R, T) and J( R, T) the following features can be determined for one-component deposition systems. (i) The semi-logarithmic plot of N, versus l/T. for low temperatures. is a straight line with slope E,/3k. (ii) In the temperature range between room temperature and the transition temperature. the low temperature asymptote of the semi-logarithmic plot of .I versus l/T is a straight line with slope (2 E, E, )/k. (iii) In the high temperature region above the transition temperature, T> TD, the slope of the asymptote of a J versus l/T plot is ( E2 + 3E, E, )/‘k. Here E, is the activation energy of absorption, E, is the activation energy for surface diffusion, El is the binding energy of a pair of atoms and k is the Boltzmann constant. Using the slopes from figs. 7 and 8 the values for E,. E, and E, for Cd,As, on NaCl substrates were obtained: E, = 0.28 eV, E, = 0.21 eV and El = 0.66 eV. Whether the three energies estimated above are in fact the adsorption energy. binding energy of a pair of atoms and the activation energy for surface diffusion for Cd and As atoms or the resultant value for these atoms on the NaCl substrates remains to be proved although they do have magnitudes which are reasonable and they are of the same order as related values for other condensed materials [4,5,9. lo]. The fact remains that the subject of investigation in this paper is a two-component material. But, according to Westmore et al. and Lyons and Silvestri [ 11,121, during the evaporation process a complete thermal dissociation of Cd,As, molecules occurs, the Cd’, Cd;, As:, As_: and Asi appearing in stoichiometric composition in the

fibs

on

NaCI

suhstrutes

171

vapour phase. During the adsorption of such ions on a substrate surface there is rather convincing evidence that dissociation accompanies adsorption [ 131. In addition, the relation 3E, > Es, between binding energy and energy of adsorption was found to be satisfied in the present investigation, which is in accordance with theory for the three-dimensional nucleation.

5. Conclusions For the deposition of cadmium arsenide, Cd,Asz, on NaCl substrates it was found that for all the assumed parameters of condensation, growth of thin films was initiated by three-dimensional nuclei. The temperature of the substrate during growth has a marked influence on the resulting nucleation. However. it is evident that the nucleation occurs rapidly and a saturation density, N,, is quickly achieved. When plotted on a logarithmic scale versus reciprocal temperature, N, was found to linearly increase in accordance with theory but showed a change of slope at the transition temperature T,. The initial nucleation rate as determined from the experimental curves was found to be a function of the incidence rate and substrate temperature. Below the transition temperature (T < T,) the nucleation rate was found to be proportional to R2 whilst above the transition temperature (T > T,) it was proportional to R3. A value for E, was determined from the low temperature dependence of the saturation density and a value for E, was subsequently derived from this value for Ed and the temperature variation of the nucleation rate. The interpretation to be applied to these energy terms in the case where a two-component material is being deposited has not yet been resolved.

Acknowledgments

The author would like to express his thanks to Dr. J.L. Robins of the Department of Physics of the University of Western Australia for useful discussions and comments on the paper.

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