Plasma-Assisted Chemical Vapor Deposition of TiO<sub>2</sub> Thin Films for Highly Hydrophilic Performance

doi:10.4236/csta.2013.21001

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Crystal Structure Theory and Applications, 2013, 2, 1-7

http://dx.doi.org/10.4236/csta.2013.21001 Published Online March 2013 (http://www.scirp.org/journal/csta)

Plasma-Assisted Chemical Vapor Deposition of TiO 2 Thin

Films for Highly Hydrophilic Performance

Satoshi Yamauchi1*, Yoh Imai2

1Department of Biomolecular Functional Engineering, Ibaraki University, Hitachi, Japan

2Department of Electric and Electronic Engineering, Ibaraki University, Hitachi, Japan

Email: *[email protected]

Received October 31, 2012; revised December 8, 2012; accepted December 21, 2012

ABSTRACT

Titanium-oxide layer was grown on glass substrate by plasma-assisted chemical vapor deposition (PCVD) using oxygen

gas plasma excited by radio-frequency power at 13.56 MHz in the pressure as low as 3mtorr at relatively low tempera-

ture below 400˚C, and studied on the crystallographic properties with the hydrophilic behavior comparing to the layer

deposited by low-pressure chemical vapor deposition (LPCVD). Raman spectra indicated anatase-phase TiO2 layer

without amorphous-phase could be formed above 340˚C by simultaneous supply of plasma-cracked and non-cracked

titanium-tetra-iso-propoxide (TTIP) used as preliminary precursor. Surface Scanning Electron Microscope images indi-

cated the PCVD-layer consists of distinct nanometer-size plate-like columnar grains, in contrast to rugged microme-

ter-size grains in the LPCVD-layer. Extremely small water contact angle about 5˚ in dark and the quick conversion to

super-hydrophilicity by UV-irradiation with a light-power density as low as 50 W/cm2 were observed on the PCVD-

layer grown at 380˚C, while the large initial contact angle was above 40˚ and the response for the UV-irradiation was

gradual on the LPCVD-layer.

Keywords: PCVD; Titanium-Oxide Films; Anatase-TiO2; Hydrophilicity

1. Introduction

Titanium dioxide (TiO2) has been extensively investi-

gated in view of photo-induced applications using the

photo-catalytic reactions and the hydrophilicity on the

surface [1,2], in addition to electronic and optoelectronic

applications [3,4]. Commonly, anatase-phase TiO2 is pre-

ferred for the photo-induced applications because of the

efficient surface reaction by UV-irradiation compared to

another crystal phases (brookite, rutile) [5]. In addition,

the defects in the crystal should be also taken into ac-

count to control the reaction because it is recognized that

the Ti3+ sites reduced from Ti4+ at the surface by photo-

excited electrons accompanying oxygen vacancies gen-

erated by the photoexcited holes play an important role in

the photo-induced surface reaction [6]. Therefore, a low

of wet or dry process to fabricate anatase-TiO2 layer has

been advanced to control the crystallinity such as stoi-

chiometric composition, impurity concentration, surface

morphology, crystal orientation and so on. In such proc-

esses, wet process such as dip-coating, spray or sol-gel

has been widely used for the photocatalytic and hydro-

philic coating [7,8] with advantages of the low-cost and

the wide-area coatings. In contrast, dry process such as

reactive sputtering [9], electron beam evaporation [10]

and metalorganic chemical vapor deposition [11] has

been attractively studied to control the growth behavior.

Plasma-assisted chemical vapor deposition (PCVD) has

been candidate for the TiO2 synthesis process for low

temperature deposition, control of the grain structure and

the surface morphology [12-14]. In the PCVD process,

titanium tetra-iso-propoxide (TTIP) has been widely used

as the preliminary precursor in order to reduce contami-

nation in the layer [15], where O2 gas is simultaneously

used as the oxidant gas for the efficient reduction. The

metal oxide dissociated in the plasma [13] brings about

the low temperature deposition below 300˚C and the

formation of highly dense columnar grains compared to

the CVD-layers. However, the temperature to grow ana-

tase-phase TiO2 is increased above 450˚C, whereas the

anatase-TiO2 can be grown at the temperature above

300˚C by CVD [13]. The unfavorable increase of the

growth temperature is probably come from hindering the

crystal-phase formation by ion bombardment of excess

energy particles [10] and poor surface migration of the

metal-oxides on the growth surface. It is noted that the

PCVD had been demonstrated in relatively high pressure

above 0.1 torr by relatively high rf-power above 100 W

[14] aiming at enough dissociation of the precursor in the

*Corresponding author.

S. YAMAUCHI, Y. IMAI

plasma. On the other, the deposition enhanced chemical

dissociation by reactive oxidants such as atomic-oxygen

results in reduction of the TTIP-dissociation energy and

the high growth rate as demonstrated by remote plasma-

enhanced CVD [16]. The useful effects brought about the

low-temperature growth including ana-tase-TiO2, how-

ever, the layer grown at 350˚C was formed by relatively

low dense grains with poor crystallinity compared to the

CVD-layer. It is considered that the results indicated the

thermal-dissociation of TTIP without enhancement by

the reactive oxygen is more effective to growth anatase-

phase TiO2.

In this paper, PCVD of TiO2 supporting the crystalli-

zation in anatase-phase by thermal-dissociation of TTIP

is demonstrated at low-temperature below 400˚C and

hydrophilic property on the anatase-phase layer is shown

with the crystallinity and the surface morphology.

2. Experimental

Figure 1 shows a bell-jar type PCVD apparatus consists

of a diffusion-pump (D.P.) and a rotary-pump (R.P.) for

TiO2 deposition. The back-pressure in the chamber was

under 1 × 10−5 torr. An inductively coupled electrode to

introduce radio-frequency (rf) power at 13.56 MHz was

equipped in the chamber between a substrate holder and

gas inlets. A Coil to apply DC-magnetic-field of 3000

gauss at the center of the chamber was also settled

around the bell-jar to stabilize gas plasma excited by the

rf-power, because PCVD in this study was performed in

the pressure as low as 3mtorr and the low density plasma

excited by 10 W rf-power. In the case of LPCVD, the

rf-power was not induced during the growth. The tem-

peratures of substrate holder and gas inlet for preliminary

precursor were increased by resistive-heating and con-

trolled by PID-systems.

Titanium tetra-iso-propoxide (Ti(O-i-C3H7)4 : 97%-pu-

Figure 1. An apparatus of PCVD for TiO2 deposition.

rity) was used as preliminary precursor. The liquid-phase

TTIP was charged in a quartz-cell and then purified in

vacuum at 50˚C for 3hrs to remove volatile solvent in the

liquid. The purified TTIP was vaporized at 70˚C and

introduced into the chamber without any carrier gas

through a stainless tube and a variable-valve at the tem-

peratures about 90˚C. Pure oxygen gas (99.9999%-purity)

was also introduced through a stainless tube and a vari-

able-valve. The supply ratio of TTIP/O2 was controlled

by monitoring the chamber pressure when the TTIP and

the O2 were introduced into the chamber.

Quartz plates with mirror-surface used as substrates

were rinsed in deionized water and dried by a spinner

after removal of contaminations from the surface by or-

ganic solvents and a hot H2SO4 + H2O2, and then ther-

mally cleaned at 400˚C in the chamber in low pressure

below 1 × 10−5 torr for 30 min before the growth. Prior to

the growth, inner-wall of the chamber was cleaned by

oxygen plasma excited by 100 W rf-power to remove

residual gas, and then titanium-oxide layers were grown

at low temperatures ranging from 150˚C to 400˚C.

Thickness of the layer was checked by a contact-type

surface profiler (DEKTAK150). The surface morphology

was observed by SEM (HITACHI S-800) and the crys-

tallinity was investigated using Raman spectrometer

(JASCO NR-1100) using the 514.5 nm line of an Ar+

laser (100 mW) as the excitation source. Hydrophilicity

on the layer was evaluated by contact angle of water

when deionized water (10 μl) was dropped on the layer in

air at 20˚C with 50%-humidity. The photo-induced hy-

drophilicity was examined as a function of UV-irradia-

tion time, in which a black-light peak at 365 nm with the

low light power density of 50 μW/cm2 was used as the

light source.

3. Results and Discussions

3.1. Growth Rate

Figure 2 shows growth rates of titanium-oxide layers by

Figure 2. Growth rates of Titanium-oxide layer by PCVD

(closed circles), LPCVD (open circles) and thermal-disso-

ciation in PCVD (closed triangles).

S. YAMAUCHI, Y. IMAI 3

PCVD (closed circles) and LPCVD (open circles) at va-

rious temperatures, where the O2/TTIP supply ratio was

controlled to 1. The rate by LPCVD was increased with

the growth temperature above 320˚C according to Ar-

rhenius relationship and the activation energy was ob-

tained as 164 kJ/mol. The energy value was seemed to be

larger than the other reports [13,16] using TTIP and O2

but similar to the value of 150 kJ/mol by CVD using

TTIP in N2-gas with the high flow rate [17]. It has been

recognized that oxygen reduces the dissociation energy

of TTIP, but the large activation energy indicates the

dissociation of TTIP by LPCVD in such low pressure of

3mtorr with the small supply ratio of O2/TTIP was owing

to thermal-dissociation as follow [18],

3742 3637Ti(OC H )TiO2C H2HOC H 

On the other hand, two activation energies were observed

by PCVD. The activation energy in the low temperature

below 300˚C was 4.5 kJ/mol which was in good agree-

ment with the previous report by PCVD [13] but much

lower than that by thermal-dissociation enhanced by re-

active-oxygen [16]. The significant low energy indicated

that the preliminarily precursor was dissociated to tita-

nium-oxides in the plasma as described elsewhere [13].

In contrast, the activation energy between 300˚C and

380˚C was determined as 163 kJ/mol (closed-triangles in

Figure 2) after removal the growth rate extrapolated

from the rate in the low temperature region performed by

the plasma-cracked precursors. The activation energy

above 300˚C was coin-cident to that by LPCVD. It was

reported that activation energy of TTIP-dissociation was

reduced by plasma-excited oxygen [16], however, the

enhanced dissociation was not observed in this work be-

cause the collision probability between TTIP and oxygen

was relatively small in such low pressure with the small

O2/TTIP sup-ply ratio. The decreased growth rate above

400˚C was believed to be caused by the depletion of the

precursor at the growth surface due to the high desorp-

tion coefficient or the volumetric dissociation in the

gas-phase. It should be concluded from these results that

the PCVD above 300˚C was performed by supply of the

metal-oxides dissociated in the plasma, which was ex-

pected to form highly dense grain growth by the high-

sticking coefficient, and thermal dissociation of TTIP,

which was expected to enhance the crystallization into

the anatasephase.

3.2. Raman Spectra

Figure 3 shows Raman spectra of 600 nm-thick tita-

nium-oxide layers grown by PCVD at various tempera-

tures. Typical peaks corresponding to anatase-TiO2 were

appeared at 144, 399, 514 and 639 cm−1 in the layers

grown above 340˚C, where the peaks were attributed to

Eg, B1g, B2g and Eg vibration modes respectively [19].

However, any peaks except a weak broadband around

450 cm−1, which was recognized due to amorphous phase

of TiOx [20], could not be observed for the layer grown

at 250˚C.

The broadband was decreased with the deposition

temperature and disappeared in the layer grown above

340˚C. The broadband was appeared for the layer grown

at the temperature ranging from 300˚C to 330˚C but the

weak Eg-band could be also observed. It is noted here

that the low-temperature about 300˚C to form anatase-

TiO2 was much lower than 450˚C by PCVD reported

previously [13], but consistent with the temperature for

anatase-TiO2 growth by LPCVD using TTIP as single

precursor [21]. The significantly low temperature to form

anatase-TiO2 suggested that the crystallization could be

supported by the non-cracked TTIP which was simul-

taneously supplied with the plasma-cracked precursors

and thermally dissociated on the growth surface. Figures

4(a) and (b) show growth rate ratio by the CVD-mode

(GRthermal) and the PCVD-mode (GRplasma) thermal-disso-

ciation as a function of the growth temperature and inte-

grated intensity of Eg-mode peak at 144 cm−1 as a func-

tion of the GRthermal/GRplasma ratio, respectively. The ratio

of GRthermal/GRplasma except the open circle (grown at

400˚C) was rapidly increased with the growth tempera

Figure 3. Raman spectra of PCVD-layers with 600 nm-

thickness grown at various temperatures.

(a) (b)

Figure 4. (a) Rel ative growth rate by thermal dissociation

(GRtherma l) compared to growth by plasma cracked pre-

cursors (GRplasma) as a function of the growth tempera-

ture; and (b) Eg-intensity as a function of GR thermal/GRplasma,

where the open circles shows for the layer grown at

400˚C.

S. YAMAUCHI, Y. IMAI

ture above 300˚C according to rapid increase of the

thermal dissociation of TTIP. In contrast, the intensity of

Eg-mode was rapidly increased and gradually saturated

with the ratio of GRthermal/GRplasma, which indicated crys-

tallization in anatase-phase was enhanced by relatively

small amount of the thermally dissociated TTIP com-

pared to the supply of the plasma-cracked precursors. In

the case of the growth at 400˚C, decrease of the Eg-band

intensity (open circle in Figure 4(b)) and the weak

broadband due to amorphous-phase (Figure 3) suggested

the dissociated precursor in the gas-phase was simulta-

neously supplied on the growth surface.

Figure 5 shows the Eg-band spectra of PCVD- and

LPCVD-TiO2 layers grown at 380˚C with 600 nm-

thickness, where the background was numerically re-

moved in the spectra. The spectrum with the FWHM of

10.7 cm−1 of the PCVD-layer was slightly sharper than

that of the LPCVD-sample with the FWHM of 12.0 cm−1.

Additionally, Raman-spectrum shift of the PCVD-TiO2

was small below 1 cm−1, while the shift of the LPCVD-

layer was about 2 cm−1. It is recognized that the Ra-

man-spectrum shift is originated from residual stress in

the layer [22] on the substrate with different thermal-

expansion coefficient. Previously, Alhomoudi et al. re-

ported Eg-band of anatase-TiO2 layer was shifted toward

higher wavenumber above 3 cm-1 broadening about 23

cm−1 in the thick layer about 600 nm, where the layer

was grown by reactive-sputtering around 300˚C, and

concluded the spectrum shift and broadening were

caused by residual compressive stress and large distribu-

tion of the orientation of grains in the layer, respectively

[23]. In contrast, both of the shift and the broadening

were much smaller in the PCVD-TiO2 layer as shown

above, which clearly indicated the residual stress in the

PCVD-layer was relaxed and speculated the layer con-

sisted of uniform grains.

3.3. Surface Morphology

Figures 6(a) an d (b) show surface SEM images of 600

nm-thick PCVD-layer and LPCVD-layer grown at 380˚C,

Figure 5. Eg-band spectra of PCVD-(solid-line) and LP-

CVD-layer (dotted-line), where the both layers were grown

at 380˚C.

respectively. Relatively large and rugged grains with

sub-micrometer size were found in the LPCVD-layer,

which indicated the growth was performed by the low

density nucleation at the initial stage and/or the secon-

dary nucleation in the deep grain boundaries. In contrast,

the PCVD-layer was formed by highly dense plate-like

nano-grains with the width around 45 nm. It can be easily

recognized that the significant difference of the grain

feature compared to the LPCVD-layer was caused by

highly dense nucleation at the initial stage and prefe-

rential growth along the thickness. The nucleation with

high density could be performed by the plasma cracked

metal-oxides with the high sticking coefficients, then the

grains were grown by simultaneous supply of the plas-

ma-cracked precursors and non-cracked TTIP, where the

grain growth was governed by supply of the plasma-

cracked precursors in PCVD-mode as shown in Figure

4(b). In addition, it is considered that the distinct nano-

grains in the PCVD-layer resulted in relaxation of the

bi-axial stress observed in the Raman spectrum.

3.4. Hydrophilicity

Figure 7 shows contact angle of water on the PCVD-

layers grown at various temperatures ranging from 250˚C

to 380˚C as a function of irradiation time of UV-light

Figure 6. Surface SEM images of 600 nm-thick TiO2 layers

grown by (a) PCVD and (b) LPCVD at 380˚C.

Figure 7. Contact angle of water on TiO2 layers grown at

various temperatures by PCVD as a function of UV-irra-

diation time. The inset shows variation of the angle on

CVD-TiO2 layer grown at 380˚C for UV-irradiation time.

S. YAMAUCHI, Y. IMAI 5

peak at 365 nm-wavelength with the low power density

of 50 μW/cm2. The inset shows the dependence of

LPCVD-TiO2 layer grown at 380˚C. Here, the grown

layer was initially treated in atmosphere by the UV-irra-

diation for a few hours required to achieve super-hy-

drophilicity and exposed in air in dark for 1 month, and

then the contact angle on the layers irradiated by the

UV-light for each time was evaluated after the initial

contact angle was checked before the UV-irradiation.

The hydrophilicity on the PCVD-layer including the

initial angle and the hydrophilization feature by the

UV-irradiation was significantly dependent on the gro-

wth temperature. The contact angle was around 40˚ be-

fore the UV-irradiation and not responsible to the irradia-

tion on the layer grown at low temperature below 250˚C.

Nakamura et al. previously showed the amorphous tita-

nium-oxide deposited at low temperature below 200˚C

by PCVD included hydroxyl which plays an important

role of the hydrophilicity and the contact angle on the

layer was responsible for UV-irradiation [24]. However,

absorption due to -OH bond could not be observed in

FTIR spectrum of the amorphous-phase PCVD-layer

grown at 250˚C, and the contact angle was not responsi-

ble for the UV-irradiation. On the other, the contact angle

on the layer consists of anatase- and amorphous-phase

mixture grown at 300˚C was reduced by the UV-irradia-

tion but the initial contact angle was similar to the amor-

phous-sample. In contrast, the initial contact angle on the

layer grown above 340˚C, which consists of anatase-

phase grains without amorphous-phase, was obviously

decreased with the growth temperature and the prompt

hydrophilization was observed by the UV-irradiation.

Especially, the contact angle on the PCVD- layer grown

at 380˚C was extremely small about 5˚ before the UV-

irradiation and quickly converted to super-hydrophilicity

within 5 min by the UV-irradiation. It is recognized that

Ti3+ sites reduced from Ti4+ in TiO2 crystal cause the

hydrophilic conversion. The influence of surface mor-

phology should be also considered in the hydrophilicity,

however, the grain structure as shown in Figure 3 was

scarcely dependent on the growth temperature. Therefore,

it can be concluded that the hydrophilic property de-

pendent on the growth temperature was originated from

the density of Ti3+ sites at the surface. On the other, the

large initial contact angle about 40˚ and the gradual re-

sponse for the UV-irradiation were observed on LPCVD-

layer grown at 380˚C (inset of Figure 7), while the layer

consists of anatase-TiO2 without the amorphous-phase.

Capillary effect in addition to chemical property on the

surface should be taken into account to recognize the

significantly different feature of hydrophilic properties.

Previously, Katsumata et al. showed the hydrophilic

conversion during UV-irradiation was enhanced on the

sol-gel derived TiO2 layer consists of nano-grains with

the size around 50 nm by comparison of the property on

the layer consists of relatively large 150 - 200 nm grains

[25]. They found out that the difference of hydrophili-

cizing rate by UV-irradiation was originated from ho-

mogeneity of the surface and suggested the hydrphiliciz-

ing was enhanced by two-dimensional capillary effect on

the homogeneous microstructure. Although the grain-

structure in the PCVD layer as shown in Figure 6(a) was

different from the sol-gel derived layer, the surface con-

sists of nano-grains was homogeneous compared to re-

markably heterogeneous surface of the LPCVD as shown

in Figure 6(b). It has been also recognized that the hy-

drophilicity after UV-irradiation is gradually degraded in

dark since the life-time of -OH bond at the surface is not

so long, however, long-time storage behavior for the su-

per-hydrophilicity above 5 hrs was observed on the

PCVD-TiO2 layer. This useful behavior was speculated

by percolation of water into the nano-scale slits between

the grains as shown in Figure 6(a), where the percolated

water was probably kept in the pores for long-time

avoiding the evaporation compared on the surface. Of

course, the storage behavior could not be observed on the

LPCVD-layer, in which the contact angle was gradually

increased to 40˚ within 1 hr after exposure in dark.

4. Conclusion

Titanium-oxide layers were grown on quartz substrates

by PCVD using TTIP and oxygen mixed gas plasma with

O2/TTIP supply ratio of 1 in the pressure as low as 3

mtorr at low temperatures below 400˚C, and then char-

acterized by the growth behavior and the hydrophilicity

comparing to the layer by LPCVD. The PCVD was per-

formed by plasma-cracked precursors and thermally dis-

sociated TTIP, while the LPCVD was owing to thermal

dissociation of TTIP. Raman spectra investigated ana-

tase-TiO2 could be grown above 300˚C by the PCVD.

Further, it was revealed that the crystallinity of ana-

tase-TiO2 was significantly dependent on the ratio of the

PCVD-mode and the CVD-mode during the growth,

where non-cracked TTIP played a role to enhance the

crystallization of the plasma-cracked precursors on the

growth surface. SEM observations showed that the

PCVD anatase-phase TiO2 layer was formed by homo-

geneous nano-size columnar grains, whereas the LPCVD

layer consists of heterogeneous micro-size grains. Initial

contact angle of water before UV-light irradiation was

dependent on the growth temperature and drastically de-

creased on the layer grown above 340˚C. The initial con-

tact angle about 5˚ on the PCVD layer grown at 380˚C

showed the excellent hydrophilicity without UV-irradia-

tion was performed by the PCVD. The contact angle on

the PCVD-layer was quickly reduced and showed su-

per-hydrophilicity within 5 min by UV-light irradiation

S. YAMAUCHI, Y. IMAI

with the low light power density of 50 μW/cm2, while the

long time about 3 hrs was required for the LPCVD layer.

The PCVD-layer also showed interesting storage behav-

ior of the super-hydrophilicity as long as 5 hrs in dark

after the UV-irradiation, whereas the contact angle on the

LPCVD-layer was increased up to 40˚ within 1 hr in

dark.

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