Deposition and characterization of CVD - Mo03 thin films

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J Phys. IV France 9 (1999). Pr8-453. Deposition and characterization of CVD - Mo03 thin films. K.A. Gesheva, T. Ivanova, A. lossifova, D. Gogova and R. Porat*.
J Phys. IV France 9 (1999)

Pr8-453

Deposition and characterization of CVD - Mo03 thin films K.A. Gesheva, T. Ivanova, A. lossifova, D. Gogova and R. Porat* Central Laboratory of Solar Energy and New Energy Sources at Bulgarian Academy of Sciences, Blvd. Tzarigradsko shosse 72, 1784 Sofia, Bulgaria * ISCAR Co. Ltd., P.O. Box 11, Tefen 24959, Israel

1.

INTRODUCTION

Chemical Vapor Deposition is a promising technique for deposition of electrochromic films of Mo oxide, being an example of the transition metal oxides, capable of going reversibly between a transparent state and an absorbing state by joint insertion/extraction of small ions and the electrons [I]. Plasma-assisted CVD amorphous molybdenum oxides have been produced from MO(CO)6 [2]. Atmospheric CVD from MO(CO)6 was used by other authors [3] for preparation of polycrystalline a-phase Mo03. The process has been accomplished in two steps - first Mo02 films were deposited at 300°C and then oxidized at 500 and 600°C. This paper presents results on molybdenum oxide films deposited by APCVD process from MO(CO)6 in a low-temperature range - 125-200°C. We first study the correlation between CVD-process growth parameters and the structure of Mo03 films, from one side, and the relation between their structure and optical properties, from the other side. This is important, since the growth of a wellperforming MoOx coatings as functioning film in electrochromic devices depends critically on the deposition parameters, since they affect film composition, structure, density and thus the electrochromic behavior. Concerning the CVD-process, the growth parameters that have to be analyzed are the substrate temperature and the oxygen flow, and the vapor pressure of the precursor (MO(CO)6). In this paper we present a study basically on the influence on the structure of the temperature - the deposition and the postdeposition annealing ones and the influence of the vapor pressure of MO(CO)6. The structure of the films is studied by different methods including infrared spectroscopy. The purpose is to see what kind of additional information can supply the IR measurements on the structure of the films in dependence of the CVD-process parameters.

2.

EXPERIMENTAL EQUIPMENT AND METHODS

The films were chemically vapor deposited in a quartz reactor, where inserted, an electrically isolated, SiC coated graphite susceptor was inductively heated by high-frequency generator. The precursor was MO(CO)6 - 99 %, Fluka type, situated in a sublimator, heated at 70-90°C. Argon (99.995%) flow was carrying the Mo(CO)6 vapors to the reactor, where through a separate line, oxygen (99.95%) was entering. The MO(CO)6 to O2 molar ratio in the gas stream was regulated by varying the quantity of the

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gas flowing through the sublimator maintained at 70-90°C. In the presented results this ratio was kept constant and equal to 15. Using our previous experience [4], this ratio was chosen as proper for obtaining of better optical quality of the films[ 4]. When investigating the infrared spectra of the films in dependence on the value of this ratio, we have shown samples for three different values of the ratio (see below Fig. 5 and 6). The substrates were Donnelly type Sn02:Sb covered glass substrates with sheet resistance of 8 nJO. Double-side polished c-Si was used as substrate for the infrared spectroscopy measurements, since the infrared spectra of the films were collected in transmittance mode only. The optical transmittance in the visible was measured by a Perkin-Elmer 330 UV-VIS spectrophotometer and 1430 Infrared spectrophotometer was used for the infrared spectral measurements. For structural characterization of the films, a Carl Zeiss Jena EF-5 RHEED-diffractometer, and a Philips X-Ray difractometer, using Cu Ka,radiation selected by a graphite focusing monochromator were used. The X-ray diffraction measurements were performed in 2Q configuration. Talystep profilometer was used for thickness determination. In the CVD reactor, close to the hot substrate (the substrate temperature was in the range of 125-2000q, the following reaction is expected to take place: Mo(CO)6 + O2 ~ Mo03 + C02 t 150-200°C Two sets of samples were prepared: the first one includes films deposited at substrate temperature of 150° C, and the second one - 200°C. The as-deposited samples were additionally annealed in air at 200°C, 300°C, 400°C and 500°C for one hour for obtaining of a better stoichiometry of the films. The thicknesses were in the range of 320-400 nm for the low-temperature set of samples and 120-240 nm for the 200°Cset of samples. The two sets were grown for one and the same deposition time. It was interesting that the lower-temperature set, the deposition was proceeding with higher rate (10 nm/min compared to 6 nm/min for the higher- temperature set), resulting in thicker films. Lower temperatures obviously favor higher deposition rates for the Mo(CO)6 - CVD-process. This behavior is to be associated with the thermal decomposition of molybdenum carbonyl, because the number of the molecules, available for oxidation decreases with increasing of the substrate temperature, through thermal decomposition in the vapor phase. The two sets of samples differ in their color. The samples of the lower-temperature look bluish, when the 200°C - set of samples are yellowish. In the text below we will refer to these definitions.

3.

RESULTS AND DlSSCUSION

Mo03 films, as-deposited on glass at 150°C and 200°C are preferably amorphous (RHEED patterns not shown, they are typical hallos), but when deposited on crystalline substrate, such as conductive glass, the structure of the films, especially of the 200°C-ones, has a certain degree of crystallinity. In Fig.I-A, the RHEED pattern of as-deposited 200°C-sample is shown together with RHEED pattern of the same sample, but annealed at 300°C (Fig. I-B). Although not clear, the RHEED pattern of 200°C-deposited Mo oxide film show weak reflexes. Crystallization in the film starts at temperatures above 150°C.

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Fig.l. RHEED diffraction micrographs of CVD-Mo03 thin film - A - as-deposited at 200°C, and B - the same, but annealed at 300°C.

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If annealed at 300°C, strong texturing occur. As we will see below XRD-spectra confirm this result. The authors in [5] reported that depending on the deposition technique, MoO) crystallization of the amorphous structure starts over 150°C. They observe that the film structure at T> 150°C is a mixture of a and ~ phase of MoO), expressed by (0 k 0) and (0 k 1) orientations. The observed d-spacings for Mo oxide film annealed at 300°C (derived from the RHEED pattern in Fig. I-B), namely d=3.78, 3.4, 2.86, 2.5,2.295, 1.84, 1.55, 1.458 A are referred to orthorhombic Mo03 characterized by d-spacings as follows: d=3.81, 3.463, 3.006, 2.527, 2.31, 1.85, 1.82, 1.569, 1.443 A - card 5-0513, JCPDS 1985. In our attempt to crossprove the results from the RHEED study by using the X-ray diffraction method, a difference in the results for the textured film was noted (this will be described below). For the as-deposited at 200°C films, and further annealed at temperatures 200, 300, 400 and 500°C, the results are shown in Fig. 2.

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Fig. 2. XRD patterns of films deposited at 200°C and annealed at: A - 200°C, B - 300°C C - 400°C and D - 500°C. The observed strong reflection in (060) orientation is connected with existing of MoO) crystalline phase. Mo oxide films obtained by other authors [5-7] have shown (0 k 0) and (0 k I) orientations, indicative for Mo03 in a-~ mixed phase. On transformation from mixed phase to a phase, Mo03 displays a very strong (0 k 0) preferred orientation. Authors [6] connect (020), (040) and (060) planes to a-MoO) The shown

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diffraction patterns are typical for polycrystalline material. The XRD diffraction spectra of annealed at 200°C for 1 hour Mo oxide film does not differ from the as-deposited at the same temperature one (not shown in the figure). Annealing at 300°C resulted in XRD spectra, showing a strong reflection (texture) in (211) orientation. The observed d-spacings (d = 4.133, 2.959, 2.181 and 1.797 A) differ from the dspacings derived from the RHEED pattern in Fig.1-B of the same film. The XRD-d spacings were related to a structure which is a mixture of suboxides, M08023 (d = 2.959 A) and M04011 (d = 4,00 A, d = 2.207 A and d = 1.810 A). One explanation of this may be the disposition of the sample towards the X-ray beam, which may result in appearance of different diffraction peaks. This we have noted in others not presented here, XRD-studies of similar samples. Anyway, the RHEED and XRD results show that Mo oxide films, deposited at 200°C and additionally annealed at 300°C are Mo03 mixed with suboxides. It is questionable why at 300°C films structure still contains suboxides. Some other authors observe Mo02 phase in the structure of their Mo oxide samples, annealed at 300°C [6]. Obviously this temperature is not enough high for fully oxidation of Mo oxide films at least when the oxidation goes for only one hour and in air (but not in oxygen flow media). Stable orthorhombic alpha-phase Mo03 is obtained by other authors [5] on thermal treatment at about 300°C in air for 4 h. The annealed samples at higher temperature (400 and 500°C) show only molybdenum trioxide lines - Fig. 2. - C and D. To understand the chemical structure and binding configuration, the IR transmission spectra of molybdenum oxide films were recorded in the range 200 - 1200 cm". In Fig. 3 and Fig. 4, the IR spectra of annealed at 200, 300, 400 and 500°C films are presented for the two sets of samples.

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Fig. 5. IR transmission spectra of Mo oxide films, deposited at 150°C for three different temperatures of the sublimation: 70, 80 and 90°C.

Fig. 6. IR transmission spectra of Mo oxide deposited at 200°C for three different temperatures ofthe sublimation: 70, 80 and 90°C.

In Fig. 5 and Fig. 6, the influence of the precursor vapor pressure on the IR spectra is shown. In these experiments, the oxygen flow was not changed (the argon flow through the sublimator was kept constant). It is seen that the 200°C-samples show their absorption band at 810 ern", while for Mo oxide films, deposited at 150°C, this peak is shifted and situated at 730 cm". For the different sublimation

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temperatures (which means higher vapor pressure or different amounts of MO(CO)6 molecules), the dips only change their shape, the position of the dips stay unchanged. The change of the dips shape is considered only qualitatively, since the film thickness is not equal. We want to note that on Fig. 6, the broad band around 800 cm-1 of the spectrum at 70°C exhibit more than one contribution. The broadening of this peak to the higher-energy side includes the small shoulder at 600 ern", and a peak at around 730 cm". Similar consideration is made for the absorption band around 730 cm", characteristic for the 150°Csamples. For the infrared spectrum at 70°C, the broadening of the basical band is from the lower-energy side of the spectrum. We refer this broadening to a contribution of vibrational modes, defining the absorption band at 810 ern". We note thatthe bands corresponding to the higher sublimation temperatures (Fig.'s 5 and 6) are not asymmetrical, their width decreases, which is a sign for increasing of crystallites size. All the samples presented in Fig. 5 to 8 are grown for one and the same deposition time, but because of the different Mo(CO)6/02 ratios ( in result of different oxygen flow and different sublimation temperature), the growth-rate is different and the thickness is different.Because ofthat,the infrared spectra are considered only qualitatively. In Fig. 7 and Fig. 8 the infrared transmittance spectra of bluish and yellowish samples of Mo oxides are shown for different oxygen contents, defined by different Mo(CO)6/02 ratios. It is seen that the position of the absorption bands stay unchanged (for the 150°C-samples the peak stays at 730 cm", and for the 200°C-samples at 810 cm'), The same effect of the broadening of the infrared spectra is observed for different Mo(CO)6/02 ratios. The same broadening of the main absorption bands is seen. 100.00

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