(2006) 63–68www.elsevier.com/locate/tsf

Thin Solid Films 514

Growth of SnO2 thin films by atomic layer deposition and chemical vapourdeposition: A comparative study

Jonas Sundqvist a,1, Jun Lu b, Mikael Ottosson a, Anders Hårsta a,⁎

a Department of Materials Chemistry, The Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Swedenb Materials Research Laboratory, The Ångström Laboratory, Uppsala University, Box 521 SE-751 21 Uppsala, Sweden

Received 31 October 2005; received in revised form 10 February 2006; accepted 14 February 2006Available online 6 March 2006

Abstract

Thin films of the tetragonal rutile-type SnO2 phase have been deposited by both atomic layer deposition (ALD) and chemical vapourdeposition (CVD) using the SnI4–O2 precursor combination. Depositions were carried out in the temperature region of 350–750°C on α-Al2O3

(0 1 2) substrates. In both cases the films were found to grow epitaxially with the in-plane orientation relationships [0 1 0]SnO2|| [1 0 0]α-Al2O3

and[1 0 1̄]SnO2

|| [1̄ 2̄ 1]α-Al2O3. Films grown by ALD were found to be close to perfectly single crystalline, contained a low density of defects and were

almost atomically smooth. The CVD films were found to have a much rougher film morphology, and exhibited both grain boundaries and twinformation.© 2006 Elsevier B.V. All rights reserved.

Keywords: Tin oxide; Chemical vapor deposition; Atomic layer deposition; X-ray diffraction; Transmission electron microscopy

1. Introduction

Tin oxide is a transparent conducting oxide, which hasimportant applications such as gas sensing material [1], ion-sensitive field effect transistors [2], dye-sensitized solar cells [3],etc. There are several reports on efforts to grow epitaxial rutile-type SnO2 films in the literature available today [4–19]. For thispurpose different growth methods have been investigated:reactive radio-frequency magnetron sputtering [8,10–12,14,15,19], pulsed laser deposition [17,18], halide chemicalvapour deposition (CVD) [4,6,7], metal-organic CVD[5,9,12,13], and halide atomic layer deposition (ALD) [16]. Inthe latter three approaches, the metal precursors employed havebeen SnI4 [4], SnCl4 [6,7,16], dibutyl tin diacetate [5], tetramethyl

⁎ Corresponding author.E-mail address: harsta@mkem.uu.se (A. Hårsta).

1 Presently at Infineon Technologies SC300 GmbH & Co. OHG, Königs-brücker Straβe 180, 01099 Dresden, Germany.

0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.02.031

tin [9,12], or tetraethyl tin [13] and the oxygen precursors havebeen H2O [4,16] or O2 [5–7,9,12,13]. The depositions wereusually performed using α-Al2O3 substrates, which were either(0 0 1) [5,8,10,12–15,17–19] or (0 1 2) [5,8–12,16–18] oriented.Although the growth of SnO2 on these substrates has beensuccessful, further improvement of the film quality and moredetailed studies about the epitaxial quality and microstructure ofthe films are of interest.

Recently two new methods to deposit epitaxial rutile-typeSnO2(1 0 1) on α-Al2O3(0 1 2) substrates with the epitaxial inplane relation [0 1 0]film || [1 0 0]substrate and [1 0 1̄]film ||[1̄ 2̄ 1]substrate were presented: ALD [20,21] and CVD [22]employing the precursor combination SnI4 and O2. It can bementioned that for gas sensing applications, a major advantageof ALD over CVD should be the ability for conformal coating.In the present paper, structural characterisation by X-raydiffraction (XRD) and transmission electron microscopy(TEM) has been carried out and the results from the twodifferent deposition methods will be compared. The mainreason for employing the current precursor combination is thatthe resulting films most probably will have a lower halidecontamination level due to the fact that metal iodides andoxyiodides have a lower thermal stability compared to the

0

100

200

300

400

500

600

700

800

SnI2(g)SnI

4(g)

IIII II

Gro

wth

Rat

e [n

m h

-1]

Deposition Temperature [oC]

Deposition Temperature [oC]350 400 450 500 550 600 650 700 750 800

350 400 450 500 550 600 650 700 750 800

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Gro

wth

Rat

e [n

m/c

ycle

]

a)

b)

SnI4 SnI2

I II III

Fig. 1. The growth rate of SnO2 on α-Al2O3(0 1 2) substrates in a) ALD andb) CVD as a function of the deposition temperature.

(-1 0 1)

(0 1 1)(0-1 1)

0o 30o 60o 90o

(1 0 1)

(-1 0 1)

(0 1 1)(0-1 1)

0o 30o 60o 90o

(1 0 1)

a)

b)

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corresponding metal chlorides and oxychlorides and also thatno residual hydrogen will be present in the films since ahydrogen free oxygen precursor is used.

20 30 40 50 60 70 80 90 100

(b) CVD

(a) ALD

OnS

2)2 0 2(

OnS

2)2 0 2(

OnS

2)1 0 1(

lA

2O3

)4 2 0(

OnS

2) 1 0 1(

lA

2O3

)4 2 0(

lA

2O3

lA

) 2 1 0(2O

3)2 1 0(

700 oC, 44 nm

700 oC, 120 nm

400 oC, 22 nm

400 oC, 40 nm

2θ [ o ]

Inte

nsity

[arb

. uni

ts]

Fig. 2. θ–2θ scans showing the diffraction pattern for SnO2 films deposited onα-Al2O3(0 1 2) substrates by a) ALD and b) CVD. The thickness and thedeposition temperature of the films are indicated in the figure. Please note thelogarithmic intensity scale.

2. Experimental details

All film depositions were carried out in a home-built flow-type ALD-CVD reactor [20]. SnI4 (Strem Chemicals Inc., 95%purity) was evaporated from a quartz crucible held at 115 °C.

(-1 0 1)

(0 1 1)(0-1 1)

(1 0 1)

T3

0o 30o 60o 90o

T3

T3

T3

T3 T3

T1 T1

T1

T1

T2 T2

T2

T2

T2

T2

T2

T2

T2 T2

T2 T2

T2

T2

T2

T2

c)

Fig. 3. {1 0 1}-pole figures for films deposited by a) ALD at 600 °C, b) CVD at400 °C and c) CVD at 600 °C. The arrows indicate substrate reflections.

Fig. 4. TEM picture of a film deposited on α-Al2O3(0 1 2) substrate at 600 °C by ALD (middle). The insert represents the SAED pattern. High-resolution images of thefilm surface (top) and film/substrate interface (bottom) are also included.

65J. Sundqvist et al. / Thin Solid Films 514 (2006) 63–68

The SnI4 vapour was transported to the reaction zone by a N2

(99.9999%) flow and O2 (99.998%) was used as oxygen source.The pulsing sequences of the ALD process and the CVDprocess were computer-controlled. In the case of ALD the pulsetimes were 4s for SnI4, 6s for the first N2 purge, 4s for O2 pulseand 6s for the second N2 purge in all depositions while bothSnI4 and O2 were supplied simultaneously in the CVD process.All film depositions in this study were made on α-Al2O3(0 1 2)substrates which prior to deposition were cleaned in heated(75 °C) methanol in an ultrasonic bath for 5 min. Growth wasinvestigated from 350 to 750 °C at a total pressure of 1.3 kPaand a linear gas flow velocity of 0.5 m/s for both processes.

The relative Sn content of the films and the residual iodinecontent were determined by X-ray fluorescence spectroscopy(XRFS) using a Spectrolab X2000 spectrometer. Phase analyses

were performed by standard θ–2θ scans, the epitaxial relation-ships were determined by φ-scans and thickness measurementswere made by X-ray reflectivity at low angles, using a SiemensD5000 X-ray diffractometer with CuKα-radiation. The relativefilm thickness was calculated by comparing the XRFS Sn Lα1

signal with the results from the XRR measurements of atemperature series deposited from 500 to 750 °C by ALD [20].

A Philips X-pert Pro MRD diffractometer using CuKαradiation and with a parallel beam set-up was used to record thepole figures. The cross-sectional specimens for TEM examinationwere prepared in the following way: the investigated sampleswere cut into 3×5mm2 pieces by a low speed diamond saw. Pairsof pieces with the deposited sides face to face were bondedtogether using epoxy, polished into a rod with 2.5 mm diameterand subsequently inserted into a brass tube. The rod was cut to a

Fig. 5. TEM picture taken along the [0 1 0]film//[1 0 0]substrate direction of a film deposited by CVD at 375 °C.

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0.5-mm thick slice by a diamond saw. The specimens were thenground to 0.1 mm thickness from both sides, dimpled at the centreto 10μm thickness and finally ion-milled to electron transparency.The high resolution TEM characterisation was carried out using afield emission gun TECNAI F30 ST operated at 300 kV with apoint resolution of 2.05Å. The low magnification images andselected area electron diffraction (SAED) patterns were obtainedusing a JEOL2000FXII at a working voltage of 200 kV.

3. Results

3.1. Film growth

For both ALD and CVD, growth of SnO2 on α-Al2O3(0 1 2)substrates was investigated between 350 and 750 °C. No iodinecould be detected by XRFS in any of the deposited films. In thecase of ALD no film growth was detected at 350 °C. However,from 400 °C the growth rate increased up to 500 °C andwas foundto saturate at around 0.10–0.12 nm/cycle (18–22 nm/h) at 500–750 °C (Fig. 1a). The growth rate in CVD was as expected muchhigher, and increased exponentially with temperature from 5 nm/h at 350 °C to 735 nm/h at 475 °C (Fig. 1b, regions I and II). Inthese temperature regions, the growth is controlled by surfacekinetics, but with two different rate-limiting steps, one involving

Fig. 6. High-resolution TEM picture of the film/substrate interface taken along the [0corresponding schematic atomic arrangement at the interface (right).

SnI4 at lower temperatures and one involving SnI2 at highertemperatures [22]. Above 475 °C the growth rate was found to becontrolled by the mass flow of SnI4 into the reactor [22].

3.2. Epitaxial relationship

As can be seen in Fig. 2, films deposited by bothALD (Fig. 2a)and CVD (Fig. 2b) were grown with a pronounced [1 0 1]-orientation in the examined temperature range, since only the 1 0 1and 2 0 2 reflections of SnO2 were visible in the XRD patterns.However, it should be mentioned that for the highest depositionrates (N 600 nm/h) in the CVD process, a weak 2 0 0 reflectioncould also be detected.

A typical SnO2 {1 0 1}-pole figure for a film deposited byALD is shown in Fig. 3a. It can be seen that only reflections fromthe {1 0 1} set of planes appear. The corresponding in planarorientational relationship (obtained fromφ-scans not shown here)is [0 1 0]film//[1 0 0]substrate and [1 0 1̄]film || [1̄ 2̄ 1]substrate. TheSnO2 {1 0 1}-pole figure has a similar appearance for films grownby CVD in the beginning of the kinetically controlled region (I),i. e., only reflections from the {1 0 1} set of planes appear with thesame in planar orientational relationship with the substrate as inthe case of ALD grown films (Fig. 3b). However, at the highergrowth rates occurring above 450 °C, both in the kinetically

SnO2

[010]

Al2O3

[100]

4.74 Å

4.76 Å

OSnAl

1 0]film//[1 0 0]substrate direction of a film deposited by CVD 375 °C (left) and the

SnO2

Al2O3

[10-1]

[-1-21]

5.13 Å

5.71 Å OSnAl

Fig. 8. Schematic atomic arrangement at the film/substrate interface along the[1 0 1̄]film || [1̄ 2̄ 1]substrate direction.

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controlled (II) and mass flow controlled regions (III), additionalreflections appear in the pole figures, which is exemplified by afilm grown at 600 °C (Fig. 3c). The additional sets of reflectionsdenoted T1, T2 and T3 belong to {0 1 1}-twins, which arecommon in the rutile-type structure [23]. Such twinning was alsoobserved for SnO2 films grown by ALD when SnCl4 was used asthe tin precursor [21].

3.3. Microstructure

As can be seen from Fig. 4, the ALD-grown films have auniform thickness, a flat surface and a low density of defects. Thefilms are close to perfectly single crystalline without any twinstructure. The surface is almost atomically flat (Fig. 4, Top) andthe SnO2/α-Al2O3 interface is sharp (Fig. 4, Bottom).

In contrast, the films grown by CVD have a much roughersurface morphology compared to films grown by ALD. Toexemplify this, a TEM cross section of a film grown at lowtemperature (375 °C) in the kinetically controlled growth region(I) is shown in Fig. 5. According to the pole figure (Fig. 3b) thisfilm does not contain any type of twin formation and only givesthe same set of reflections as the ALD grown film. Nevertheless,in the TEM cross section a rough facetted surface morphology isevident.

When preparing the SnO2 CVD samples for the HRTEMstudy, care was taken to cut the samples along the directionsindicated by the epitaxial relationships, viz., the [0 1 0] and [1 0 1̄]directions of the SnO2 film. For the [0 1 0]film direction, themismatch towards the [1 0 0]substrate direction is only − 0.42%.The film/substrate interface can be seen in the high resolutionTEM picture in Fig. 6 (left) and the corresponding schematicatomic arrangement of the interface (right). It is here obvious thata continuous transition without dislocations between the twolattices prevail. The low value for the lattice mismatch in thisdirection is the probable reason why no such defects are seenneither in this figure nor in Fig. 5.

In the perpendicular direction [1 0 1̄]film || [1̄ 2̄ 1]substrate, themismatch is much larger (11.32%). In the high resolution TEM ofthe film/substrate interface in this direction a large number of

10 Å Al2O3

SnO2

Fig. 7. High-resolution TEM picture of the film/substrate interface along the[1 0 1̄]film || [1̄ 2̄ 1]substrate direction of a film deposited by CVD at 375 °C.The vertical bars correspond to dislocations at the film/substrate interface.

misfit dislocations, marked with black vertical bars, can be seen(Fig. 7). At approximately every 10th interface atom such adislocation in the film is induced, which agrees quite well with thevalue of the lattice mismatch. The large mismatch in this directionis also visualised by the schematic atomic arrangement of the filmand substrate in Fig. 8.

4. Discussion

It can generally be assumed that since ALD is a relatively slowsurface-controlled growth method [24], the resulting films arelikely to be of higher quality than films deposited by faster growthmethods like conventional CVD. However, this is not always truesince the precursor chemistry is also of large importance. In thiscontext it is worth mentioning that the chloride based SnO2 ALDprocess has a slower growth rate than the corresponding iodidebased processes [21]. Nevertheless, the speed factor does not inthis case lead to higher quality films. In contrast, the SnO2 filmsgrown by the chloride based ALD process show high surfaceroughness, a high amount of twin formation and grain boundaries.This was suggested to be due to a competing etching process bychloride-containing species during growth [21].

In the present study, SnO2 films grown by ALD and CVDusing the same precursors, SnI4 and O2, are compared. Bothmethods are able to produce films of excellent epitaxial quality asdemonstrated by the pole figures (Fig. 3). However, in CVD this istrue only for films grown at rather low growth rates. When highergrowth rates are employed, defects in the form of twinning start toappear. The occurrence of twinning thus seems to be connected toa high growth rate. The most plausible reason for this is that forhigh growth rates twin nucleation is more likely, since there is notenough time for energy minimisation at the growing film surface.

It could also be argued that a reason why the ALD filmscontain less twinning and a smoother filmmorphology could be aheat treatment effect during growth, due to the longer time the

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films are held at elevated processing temperature. However, thediffusion rates of species have to be regarded as several order ofmagnitudes higher at the surface between the solid and thegaseous ambient than inside the bulk of the film at thesecomparatively moderate processing temperatures. This explana-tion is thus most probably of less importance.

A striking difference between ALD and CVD is the fact thatthe morphology of the films is considerably improved for ALD-grown films. For ALD-grown films the surface is almostatomically flat (Fig. 4), whereas for CVD-grown films the surfaceis much rougher (Fig. 5). This is true also for the lowest growthrates in CVD. A possible explanation might be the occurrence ofhomogeneous gas phase reactions in CVD. Due to the separationin time and space of the metal and oxygen precursors in ALD,such reactions cannot take place in ALD. The homogeneous gasphase reactions could then result in nucleation of SnO2 in the gasphase and these nuclei can then deposit on the growing surfaceresulting in a rougher film surface.

From the calculated lattice mismatches, the observed highepitaxial quality especially for the ALD-grown films, is notnecessarily to be expected. Even if the lattice mismatch along the[0 1 0]film direction is quite small (− 0.42%), the correspondingmismatch in the perpendicular direction is much larger, 11.42%.As can be seen in Fig. 6, in the [1 0 1] direction the oxygen atomscontinue across the SnO2/α-Al2O3 interface with low deformationenergy. In contrast, in the perpendicular direction where the latticefit is muchworse, dislocations are introduced to accommodate thelarge mismatch (Fig. 7). This has earlier been reported to lead to aone-dimensional feature for the epitaxial growth of SnO2 on theα-Al2O3(012) substrate [21].

5. Concluding remarks

In the present study, the growth of SnO2 films by ALD andCVD using the same precursor combination, SnI4 and O2, iscompared. The growth rate in ALD is slow but produces highquality films.When comparing the current precursor combinationfor growth of high quality epitaxial films it can be concluded thatthe ALD process produces films that are close to perfectly singlecrystalline and with a low number of defects according to theTEM investigation. Epitaxial films without twin formation canalso be grown at low temperatures by CVD. However, these filmscontain grain boundaries and have a high surface roughness.Furthermore, for these CVD-grown films the growth rate was

similar to ALD-grown films. At elevated temperatures when thegrowth rate is enhanced, the CVD-grown SnO2 films containtwins belonging to the {0 1 1} rutile twin system.

Acknowledgement

This study was supported by the Swedish Research Council.

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