SURFACE AND INTERFACE ANALYSIS, VOL. 24, 675-678 (1996)

Investigation of the Interfacial Region Formed During ZnO Growth on Si(lO0) Substrate Using Single-source CVD

M. H. Koch, G. L. Mar, A. J. Hartmann and R. N. Lamb* Surface Science & Technology, School of Chemistry, University of New South Wales, NSW 2052, Sydney, Australia

The growth process of ZnO films on Si(100) by single-sour~e chemical vapour deposition (CVD) was investigated. During the initial stages of growth (film thickness <30A), oxidation of the Si substrate was observed ihich resulted in an interfacial region consisting of ZnO and Si oxides. For film thicknesses in excess of 40 A the composition of the film approaches that of a continuous ZnO film. It is suggested that the mixed interfacial region strongly influences the adhesion of the film on the substrate by providing Zn-Mi-type bonds. The oxidation of the substrate during the initial film growth may have direct implications on the type of contact layers that can be used in ZnO thin-film devices using single-source CVD techniques.

~~ ~

INTRODUCTION EXPERIMENTAL

Single-source chemical vapour deposition (CVD) pro- vides a simple alternative to the more established thin- film deposition techniques such as sputtering and multiple-source CVD for the growth of ZnO films. In this technique, a single metal-organic precursor con- taining both zinc and oxygen is deposited and subse- quently decomposed on a heated substrate. With this approach the quality of the resultant ZnO films is dependent on the optimization of the chemistry of the precursor and the related deposition conditions, such as substrate temperature and source flux.

For applications in thin-film devices, such as electro- optical modulators1 and acousto-optic phase modula- tors,2 the production of high-quality films has to be controlled over a wide range of thicknesses (from angs- troms to microns). Film characterization (e.g. structure, electrical properties, etc.) is generally performed ex situ, i.e. outside of the deposition apparatus after film deposi- tion. Thus, bulk properties of the films, such as film crystallinity, bandgap and chemical composition, can easily be obtained. However, the interface between the film and underlying substrate or contact layer (electrical or optical) plays an important role in device applica- tion. Changes such as oxidation of the contact layer during semiconductor film growth could alter the elec- trical properties of a device structure significantly.

In this work the initial growth stages and nature of the interface region of ZnO films grown on silicon(100) wafers using basic zinc acetate [Zn,0(CH3C00),] as the single-source precursor3 was investigated using in situ surface analytical methods.

* Author to whom correspondence should be addressed.

CCC 0142-2421/96/090675-04 0 1996 by John Wiley & Sons, Ltd.

A high-vacuum single-source CVD system3 was attached to a Kratos XSAM 800 PCi system to allow in situ x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The silicon( 100) substrate was initially cleaned by argon ion sputtering and subse- quently annealed in situ at 900 K. Photoelectron diffrac- tion measurements4 were performed to ensure that the silicon substrate was free of major structural damage, while cleanness of the substrate was monitored using XPS and AES. The films were deposited at a substrate temperature of 450"C, which was necessary for com- plete precursor decompo~ition,~ and exposed to a basic zinc acetate source partial pressure of - 1 x lo-' mbar during film growth. The film thickness was controlled by the time of exposure. The sample was transferred immediately after deposition into the Kratos analysis system. The XPS measurements were performed using an unmonochromated A1 Ka source (FWHM for Au 4f,,, = 1.25 eV). All spectra have been charge-corrected to an elemental Si 2p binding energy (EB) of 99.5 eV.6

RESULTS AND DISCUSSION ~~ ~ ~

The XPS scans of the Si 2p region after successive depo- sition steps are shown in Fig. 1. The upper scan [Fig. l(a)] is from the silicon substrate after cleaning and annealing but before deposition. Only one peak associ- ated with elemental silicon was detected. A second peak at a - 3 eV higher binding energy of EB - 102.5 eV as compared to the elemental Si peak was detected after the first deposition and throughout the initial film growth stages [Fig. l(b)-(g)]. This binding energy was found for the + 3 oxidation state of Si in Si,03 by Himpsel et aL6 and similar binding energies have been reported for silicon in metal-Si-0 compounds for dif-

Received 14 November 1995 Accepted 17 May 1996

676 M. H. KOCH ET AL.

T'

2 Y

x m Y .I

5 +A

I c

elemental - 7

\

165 160 S\j 1 ' '3'i Binding Energy / eV

Figure 1. Silicon 2p XPS region scans after successive (b-b) deposition steps-as a fuvction ofof i lm thicbness: (b) 14 A; (c) 20 A; (d) 24 A; ( e ) 30 A; (f) 43 A; (9) 55 A; (h) 66 A.

ferent oxidation state^.^ In this paper we will refer to this peak as the silicon oxide peak as compared to the elemental Si peak. Figure 2 shows a plot of the XPS silicon oxide 2p peak, the elemental Si 2p peak and the Zn 3s peak intensities as a function of film thickness. The intensities for each photoelectron (PE) peak have been normalized to the highest measured intensity. The attenuation of the elemental silicon PE peak was used to determine the thickness, d, of the film after each deposition step.

Assuming the growth of a continuous film, the inten- sity of the elemental Si photoelectron peak, Isub, should decrease according to8

d Isub = ' 0 , sub exp[ - .coF(o)]

where 10,sub is the intensity prior to the film growth,

A = 20 A is the attenuation length of the Si 2p photoelectrong and 0 is the detection angle off the surface normal. Equation (1) was used to calculate d. It has to be pointed out that Eqn (1) ignores elastic scat- tering of the PE. As a consequence, the accuracy of this method will be of the order of f 25%.'

The silicon oxide peak intensity increases with film thickness to a maximum at -24 A before decreasing exponentially, similar to the elemental silicon peak. The Zn 3s peak intensity increases with film thickness up to 43 A beyond which no further increase was observed. This was interpreted as the instance where the thickness of the film is greater than the attenuation length of the Zn 3s photoelectrons.

For oxidation of the elemental silicon oubstrate there has to be oxygen or an oxygen-containing species present at the surface. Since the film growth was per- formed under high vacuum conditions, it may be assumed that the amount of ambient oxygen species present is insignificant to rapidly oxidize the silicon. However, the basic zinc acetate precursor Zn,0(CH3COO), contains excess oxygen. During decomposition of the precursor on the substrate various oxygen-containing species are liberated. It has been reported previously that ketones and CO, are produc- ed." Recent mass spectroscopy studies and laser- induced pyrolysis studies" have shown that other species, such as ketenes and CO, are also formed during the decomposition of basic zinc acetate. Given that the decomposition and film growth is occurring on the sub- strate surface, the local density of the oxygen-type species could be sufficient to promote rapid oxidation of the silicon. The thickness of the resulting continuous, interfacial region is estimated to be 20-30 A from the maximum of the XPS silicon oxide 2p peak intensity in Fig. 2.

Further evidence for the above assumption of a con- tinuous overlayer was obtained by exposing a -14 A thick film to the atmosphere for 12 h. The XPS Si 2p region scans before and after the exposure are shown in Fig. 3. No change in the peak shape was observed, indi- cating that no further oxidation occurred. This implies that the Si substrate was covered by a continuous over- layer preventing any further oxidation (as opposed to three-dimensional islands with areas of uncovered ele-

-+- Si oxide --a - Si elem. - + - 2 n

\d

-a -33-

0 10 20 30 40 50 60 0 ( ~ ~ ' ~ " ' ~ ~ " ~ ' ~ " ~ ' ~ " ~ ~ " ~ ' ~ ~ ~ ~ " '

Film Thickness [A] Figure 2. Normalized intensities of the elemental Si 2p, the silicon oxide Si 2p and the Zn 3s XPS peak intensities. For each element the intensities were normalized to the highest intensity measured.

ZnO GROWTH ON Si(100) SUBSTRATE 677

mental Si). This is in agreement with previous results reported by Mar et al. for growth on a polycrystalline copper ~ubstrate.~

The XPS Zn 2 ~ , , ~ and the Auger Zn LMM region scans as a function of film thickness are shown in Fig. 4. There was only a small change of +0.2 eV observed in the modified Auger parameter,12 0: + hv = E(Auger) + E,(PE), between the first and the fourth deposition. For subsequent depositions it remains constant at 2010.2 eV. This value is in good agreement with values reported for Zn2+ in zinc oxides.lO*ll The shift of +0.2 eV during the initial growth is too small to assign it to a different oxidation state of the interfacial zinc species, as it lies well within the error probability of the data

Binding Energy / eV

obtained for Zn2 + from different labori~tories.'~ We suggest that the shift is caused by the change in the chemical environment from a mixed zinc and silicon oxide matrix to a continuous ZnO matrix for the bulk film.

The deconvoluted XPS 0 1s region scans for various film thicknesses are shown in Fig. 5. The peaks were fitted using a Caussian/Lorentzian peak shape (60%/ 40%) after a Shirley background subtraction. The peak at lower binding energy (530.7 eV) is attributed to ZnO. The shoulder at higher binding energy (532.1 eV) was observed to decrease in relative intensity as compared to the ZnO main peak with increasing film thickness, For thick films (d > 40 A) the shoulder is -35% of the

Figure 3. Silicon 2p XPS region scans for a 14w thick ZnO film before (a) and after exposure to air for 12 h (b).

i, 7b4 562 560 438 496 494 2

Bind ing Energy / eV

Figureo4. Zinc 2~+ photpelectron and LMM Auger region scans as a function of film thickness: (a) 14w; (b) 20A; (c) 24A; (d) 30w; ( e ) 43 A; (f) 55 A; (9) 66 A.

678 M. H. KOCH ET AL.

Binding Energy / eV

9

+ a m . L .- In L 6 6 c -

5

540 535 530 525 Binding Energy / eY

Binding Energy / SV

Figure 5. Oxygen 1s XPS region scans as a function of film thickness: (a) 14A; (b) 24A; (c) 30A; (d) 4 3 k (e) 5 5 k (f) 60A.

0 1s peak, whereas at the interface it is increased to -50%. Such a shoulder at higher binding energies in the XPS 0 1s region has previously been attributed to Zn hydroxides in thick ZnO films.15 However, for oxygen-treated ZnO, Grunze et al. attributed a higher binding energy 0 1s peak to oxygen in an oxygen-rich layer at the surface.I6 The fact that the relative intensity of the shoulder peak is higher at the interfacial region [Fig. 5(a) and (b)] than in the bulk film [Fig. 5(f)] sug- gests that there are additional oxygen species present in the interfacial region as compared to the resulting ZnO film. X-ray photoelectron spectroscopy C 1s region analysis showed only trace amounts (<1 at.%) of carbon bonded to oxygen throughout the film growth. As a consequence, any significant contribution to the 0 1s peak can be ruled out. As a result, we suggest that the additional shoulder intensity at the interface can be attributed to oxygen in a mixed Si,O,-Zn, environ- ment.

CONCLUSIONS

It has been shown that, during the initial film growth of

ZnO on silicon(100) using basic zinc acetate as the single-source CVD precursor at a substrate temperature of 450°C, an interfacial layer containing both zinc oxides and silicon oxides is formed. The thickness of t h s interfacial layer is estimated to be 20-30 A. The high local partial pressure of oxygen-containing frag- ments in the precursor decomposition is the likely source of oxygen for the obtained oxidation of the sub- strate. On a metal contact the formation of an oxide layer during the deposition would strongly affect the electrical properties of the substrate/film interface. This will have direct consequences on making electrical contact to such films in ZnO thin-film devices. The effect of the interfacial region produced is expected to be enhanced with decreasing contact layer thickness. Our single-source CVD results suggest that the contact layer thickness required in future devices will have to be sufficiently greater than that of the interface produced. It is therefore necessary to study the growth on less- reactive substrates such as silver, gold or platinum. Alternatively, substrate materials which remain highly conductive after oxidation may be used for making elec- trical contact to the ZnO film in devices.

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