Letter

Morphological differences in ZnO ®lms deposited by the pyrosoltechnique: effect of HCl

AgneÁs Smitha,*, Rafael Rodriguez-Clementeb

aGroupe d'Etude des MateÂriaux HeÂteÂrogeÁnes, Ecole Nationale SupeÂrieure de CeÂramique Industrielle, 47 aÁ 73 Avenue Albert Thomas,

87065 Limoges, Cedex, FrancebInstitut de Cienca dels Materials, Campus de la UAB, 08193, Bellaterra, Spain

Received 5 October 1998; accepted 9 February 1999

Abstract

In this letter, the surface aspect of crystallized ZnO ®lms deposited by the pyrosol technique is examined. Depending on whether the

starting solution contains chlorine species or not, the crystal development is either in a pyramidal form or in a prismatic form. The in¯uence

of HCl vapor is used to explain these morphological differences. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Pyrosol; Pyrolysis; Surface morphology; Zinc oxide

1. Introduction

Zinc oxide based ®lms can exhibit a wide variety of prop-

erties. They are essentially electrically conductive as well as

optically transparent in the visible wavelengths [1±10].

With respect to applications, piezoelectric characteristics

[11±13], non-linear electrical behaviour [14,15], and gas

sensing ability [16,17] have all been demonstrated. High

quality ZnO ®lms can be deposited by techniques such as

sputtering [3,10±13,15,18±23], pulsed laser deposition [24],

chemical vapor and beam deposition [4,5], reactive

evaporation [25], cathodic evaporation [26], sol gel [7],

spray pyrolysis [27,28] and the pyrosol method [29,30]. In

all applications, the ®lm response to a chemical or physical

stimulus is dictated by its microstructure. In this letter, we

present a morphological study of ZnO ®lms deposited by the

pyrosol technique with a special emphasis on the role of

chlorine.

2. Experimental

The pyrosol process is a chemical vapor deposition tech-

nique which operates at atmospheric pressure [31±33]. A

solution consisting of a salt of the material to be deposited

dissolved in an appropriate solvent (water, alcohol or

ketone) is nebulized with a piezoelectric transducer func-

tioning at a ®xed frequency (800 kHz). The produced aero-

sol consists of monodispersed droplets, carried by a gas (air

¯ow of 8 l/min) towards a heated substrate where pyrolysis

and ®lm deposition occur through a heterogeneous nuclea-

tion mechanism.

Two zinc oxide precursors have been tested, namely zinc

acetate, Zn(CH3CO2)2´2H2O, and zinc chloride, ZnCl2,

dissolved in methanol (concentration of 0.1 mol/l). In

some cases, chlorine, in the form of hydrochloric acid,

HCl, or indium chloride, InCl3, has been added to the zinc

acetate solution. InCl3 was selected because indium is a

dopant which can enhance the electrical conductivity and

also the optical transmission when the ZnO ®lms are depos-

ited on a glass substrate [30,34]. The substrate temperature

for deposition was set to 4508C in order to obtain well

crystallized layers [35].

3. Results and discussion

ZnO crystallizes in the hexagonal system (space group

P63mc) with lattice parameters a � 3:24950 AÊ and c �5:2069 AÊ , respectively [36]. It has a wurtzite type crystal

structure where each zinc atom is surrounded by four

oxygen atoms forming a distorted tetrahedron with three

Zn±O(1,2,3) bonds at distance 2.0423 AÊ and one bond Zn±

O0 at 1.7963 AÊ (Fig. 1). The angles O1±Zn±O2, O2±Zn±O3

and O3±Zn±O1 are equal to 105.48, and the angles O0±Zn±

O3, O0±Zn±O2 and O0±Zn±O1 are 115.58. In a similar

Thin Solid Films 345 (1999) 192±196

0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.

PII: S0040-6090(99)00167-4

* Corresponding author. Tel.: 1 33-5-5545-2229; fax: 1 33-5-5579-

0998.

E-mail address: a.smith@ensci.fr (A. Smith)

manner, each oxygen atom is surrounded by a distorted

tetrahedron of zinc atoms, but the orientation of the

deformed tetrahedron is antiparallel to that of Zn. These

distortions are due to the sp3 hybridisation of the two

elements. Since the structure is not centrosymmetric, it

has a polar axis along the [0001] direction.

Polar crystal morphology with the wurtzite structure has

been studied using the periodic bond chain (PBC) theory

[37,38]. This indicated that the equilibrium (less reactive,

slow growing) forms, also called ¯at forms, are: {0001},

{0001Å}, {101Å0}, {101Å1} and {101Å1Å}. They can be observed

during the initial stages of crystal formation. The faces can

also be classi®ed according to their polarity [39±42]. Forms

which consist of only zinc or only oxygen atoms are polar,

while planes which contain both zinc and oxygen atoms are

less polar. For ZnO the polar forms are the base of the

wurtzite cell, i.e. {0001} and {0001Å} and the pyramidal

forms {101Å1} and {101Å1Å}. The prismatic form {101Å0} is

less polar. The predominance of polar or less polar forms is

largely determined by the chemical nature of the surround-

ing atmosphere during ®lm growth.

Fig. 2a shows a typical morphology of a ®lm deposited

from zinc acetate, i.e. from a solution which does not

contain chlorine based raw materials. It consists of hexago-

nal columns on top of which there is a spiral. Such a

morphology corresponds to the predominance of {0001}

or {0001Å}and {101Å0} forms (Fig. 2b). Similar crystal struc-

tures were observed after deposition of aluminium nitride by

CVD [38] where the crystallites had grown with the {0001}

and the {101Å0} forms parallel and perpendicular to the

substrate, respectively. On the micrograph, spirals can be

seen at the top of the crystallites; these can be explained by

the emergence of a screw dislocation [43,44].

When the ®lm is deposited from solutions which contain

chlorine, namely zinc chloride and zinc acetate solutions

containing HCl or InCl3, the ®lm crystallites present ¯at

faces some of which are pyramidal (Figs. 3a and 4a).

Comparison between the observed (Fig. 3a) and simulated

(Fig. 3b) morphologies for ZnO deposited from ZnCl2

shows that {101Å1} or {101Å1Å} as well as the {101Å0}prismatic

form have persisted in the ®nal crystals and that they have

probably grown with {0001} or {0001Å}parallel to the

substrate. Cope [45] obtained the development of {101Å1}

or {101Å1Å}while etching {0001Å}forms of ZnO single crystals

with HCl. He concluded that {0001Å} progressively

A. Smith, R. Rodriguez-Clemente / Thin Solid Films 345 (1999) 192±196 193

Fig. 1. (a) ZnO structure (black circles: zinc; grey circles: oxygen). (b)

Deformed coordination tetrahedron.

Fig. 2. (a) Surface of a ZnO ®lm (thickness: 1.2 mm) deposited from

Zn(CH3CO2)2.2H2O (concentration: 0.1 mol/l) dissolved in methanol; bar:

0.3 mm. (b) Simulated crystal.

converted into {101Å1} or {101Å1Å}. In our case, the persis-

tence of {101Å1} or {101Å1Å} may have a different origin.

During the transformation of ZnCl2 into ZnO, HCl vapor

is produced in the pyrolysis region. The {101Å1} or {101Å1Å}

polar forms are able to adsorb this gas and as a consequence

their growth will be slowed down.

In a similar manner, it is interesting to examine the

morphology of ®lms obtained from zinc acetate solutions

containing HCl (Fig. 4a). The crystallites are well facetted

and some consist of twin-pyramids. A simulation of the

crystallite shapes can be obtained using exclusively the

{101Å1} or {101Å1Å} polar forms (Fig. 4b). These forms corre-

late with the presence of large amounts of HCl vapor in the

vicinity of the growing ®lm. The remaining issue is to

understand why large quantities of gaseous HCl favor the

predominance of {101Å1} or {101Å1Å} forms instead of

{0001}or {0001Å}. A ®rst reason can be found by comparing

the two dimensional lattice of {101Å1}and {0001} (Fig. 5). In

the ®rst case, where there is an alternative packing of dense

planes made of oxygen and zinc atoms, we have a rhomboid

of sides equal to 3.2495 and 6.1376 AÊ , respectively, and an

angle of 74.658 between them. In the other case, we have

alternative layers of zinc and oxygen atoms forming an

equilateral rhombohedron with side 3.2495 AÊ and angle of

608. The HCl molecule has an ionic character and consider-

ing that chlorine has an ionic diameter of 3.62 AÊ , while the

ionic diameters of zinc and oxygen are equal to 1.48 and

2.80 AÊ , respectively, [46], it is deduced that HCl molecules

are probably too large to be absorbed on the {0001} faces.

Another reason can be considered by looking at the ionicity

of ZnO which is equal to 59% according to Pauling's rule

[47]. The Zn±O0 bond along the [0001] direction in Fig. 1 is

A. Smith, R. Rodriguez-Clemente / Thin Solid Films 345 (1999) 192±196194

Fig. 3. (a) Surface of a ZnO ®lm (thickness: 1 mm) deposited from ZnCl2

(concentration: 0.1 mol/l) dissolved in methanol; bar: 1 mm. (b) Simulated

crystal.

Fig. 4. (a) Surface of a ZnO ®lm (thickness: 1 mm) deposited from

Zn(CH3CO2)2.2H2O (concentration: 0.1 mol/l) and HCl (concentration:

0.01 mol/l) dissolved in methanol; bar: 1 mm. (b) Simulated crystal.

Fig. 5. Top views of a {101Å1} type plane (a) and a {0001} type plane (b).

more covalent than the three Zn±O(1,2,3) bonds. The interac-

tion between the arriving growth units of zinc and oxygen

and the {0001} faces is mainly covalent. By contrast, the

{101Å1} faces are made of rows of atoms of a single element,

either zinc or oxygen depending on the chosen plane, and

separated by a distance of 5.91 AÊ . This spacing is large

enough to accomodate HCl molecules in between the rows.

The case of ®lms obtained from zinc acetate solution

containing indium chloride is shown on Fig. 6a. Some of

the crystallites are truncated hexagonal pyramids, which

correspond to the combination of the {1000} forms with

the {101Å1} forms (Fig. 6b). Some of the crystallites seem

to be twinned with six (T6 on Fig. 6a) or ®ve edges (T5 on

Fig. 6a). The {101Å1} and {112Å2} planes are twin planes in

ZnO [48]. Fig. 6c,d show the effect of twinning on crystals

made of {1000} and {101Å1} forms. The {101Å1} twin plane

(Fig. 6c) gives a simulated crystallite which has six edges.

Similarily, if {112Å2} is the twin plane, the simulation gives

crystallites with ®ve edges. Twinning is therefore a plausi-

ble explanation for the development of T6 and T5 type

crystallites.

4. Conclusion

In conclusion, this letter has shown that the ZnO ®lms

develop (i) pyramids mostly made of {101Å1} or {101Å1Å}

forms or (ii) hexagonal columns which consist of {0001}

or {0001Å} and {101Å0} forms depending on whether HCl

vapors are present or not during deposition. HCl vapor

can act as a surfactant during ®lm growth by lowering the

speci®c surface energy of polar faces, thus slowing down

their growth. This phenomenon does not seem to be speci®c

to ZnO deposition since it has already been observed for tin

oxide based ®lms deposited by the pyrosol technique [31].

Acknowledgements

The authors are grateful to J.M. Laurent, G. Plassart, and

S. Roeser for their valuable contribution in ®lm preparation

and characterization.

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