0040-6090/02/$ - see front matter䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 6 7 5 - 3
Preferential growth of thin rutile TiO films upon thermal oxidation of
2sputtered Ti films
Chu-Chi Ting , San-Yuan Chen *, Dean-Mo Liu
a a, bDepartment of Materials Science and Engineering, National Chiao-Tung University Hsinchu 300, Taiwan, ROC a
Department of Metals and Materials Engineering The University of British Columbia Vancouver, British Columbia V6T 1Z4, Canada b
Received 22 March 2000; received in revised form 20 September 2001; accepted 9 October 2001
Abstract
Thin rutile TiO films with2 (200) preferred orientation were fabricated by thermal oxidation of sputtered Ti metal films on a
fused silica substrate. Experimental results indicate that the preferential crystal growth of (200)-oriented TiO is determined by2
the competition between surface free energy and strain energy. The highly crystalline Ti film with(002) orientation has a greater
tendency to promote the growth of(200)-oriented TiO . However, for the amorphous and low crystalline Ti films, orientation of2
the crystallites evolved in the resulting TiO film tends to be randomly distributed. The extent of preferential crystal growth of2
TiO2(200) plane can be enhanced by decreasing the annealing temperature or the thickness of Ti film. 䊚 2002 Elsevier Science
B.V. All rights reserved.
Keywords: Sputtered Ti film; Thermal oxidation; TiO film; Crystal growth; Surface free energy2
1. Introduction
Control of preferential growth of a specific crystal plane within a film matrix allows the final properties such as optical, mechanical and electrical properties of the film to be manipulated in such a manner that further improves applications w1–3x. In general, the crystal plane with the highest atomic density exhibits the lowest surface free energy that favors a subsequent preferential growth. Preferential crystal growth, on the other hand, is also strongly dominated by the synthesis parameters or lattice mismatch between film materials and sub-strates. For instance, (004)-oriented anatase TiO films2
fabricated from reactive sputtering process can be obtained by the control of substrate temperature or O2
partial pressure w4x. Rutile TiO films prepared by ion2
beam-assisted deposition can grow preferentially with
(100), (101) or (002) orientation depending on the
* Corresponding author. Tel.: q886-3-573-1818; fax: q886-3-572-4727.
E-mail address: [email protected](S.-Y. Chen).
impact angle and arrival ratio of ions to deposited atoms
(i.e. IyA ratio) w5,6x. By applying a sol–gel spin-coating
method, (004)-oriented anatase or (101)-oriented rutile
TiO2 films can be obtained through the selection of precursors and chelating agents w7x. In addition,
(100)-oriented rutile TiO and2 (001)-oriented Ti O films can2 3
be grown epitaxially on (001)-oriented a-Al O sub-2 3
strates because TiO2 or Ti O2 3 crystals have nearly identical hexagonal close-packed oxygen sublattices to the one of a-Al O w8,9x.2 3
Thin TiO films can be synthesized by several meth-2
ods including sol–gel processing, reactive magnetron sputtering, spray pyrolysis, and plasma enhanced chem-ical vapor deposition (PECVD) w10–14x. In addition to the afore-mentioned synthetic schemes, previous inves-tigation showed that the thermal oxidation of sputtered titanium metal film was found to be a rather simple technique to prepare TiO thin films w15x, which is also2
adopted in this work. It was found that the formation of
(200)-oriented or disoriented rutile TiO films depend2
on the crystallinity and orientation of Ti films initially prepared. The mechanisms affecting the preferred
ori-Fig. 1. XRD patterns of sputtered Ti films deposited at different work-ing pressuresysubstrate temperatures.
Fig. 2. The SEM photographs of sputtered Ti films deposited at different working pressureysubstrate temperatures:(a) 8 mtorryRT; (b) 8 mtorry 300 8C;(c) 8 mtorry500 8C; and (d) 32 mtorryRT.
entation of resultant TiO films during film growth will2
be discussed.
2. Experimental procedure
2.1. Film process
Titanium films were prepared using a d.c. magnetron sputtering system having a 30-dm cylindrical stainless-3
steel chamber with a base pressure of 5=10y6 torr. The target was a 3-inch-diameter titanium disk of 99.6% purity. Titanium was deposited at a constant d.c. power of 50 W onto a fused silica substrate. The target–
substrate distance was kept at 70 mm. The sputtering time was 30 and 40 min under a working pressure of 8 and 32 mtorr, respectively, in order to achieve the identical thickness of ;400 nm. The substrate temper-atures were controlled at room temperature (RT), 300
and 500 8C. Ultra-high purity (99.999%) Ar gas was used for sputter-deposition. Prior to Ti deposition, the target was pre-sputtered in an argon atmosphere for 8 min in order to remove surface oxides on the target. Finally, the sputtered Ti films were subjected to post-deposition annealing at temperatures ranging from 550 to 1000 8C for 1 h in air.
2.2. Film characterization
The crystal structure was determined by X-ray dif-fractometry (MAC Science, M18X). The thickness of
the Ti films and resultant TiO films were measured2
using a surface profilometer (Sloan, Dektak ST). Scan-3
ning electron microscopy (SEM; Hitachi, S4000) was used for microstructural examination.
3. Results and discussion
Fig. 1 shows the X-ray diffraction(XRD) patterns of
the sputtered Ti thin films prepared at different working pressuresy substrate temperatures (e.g. Ti-500 was
denoted as the sputtering parameters of 8 mtorry 500
8C). The diffraction peaks, i.e. (002), (100) and (101)
are characteristic of a-Ti. A decrease in sputtering pressure and an increase of substrate temperature mark-edly improve the crystallinity of resultant Ti films. Fig.
Fig. 3. XRD patterns of as-deposited Ti films with different sputtering conditions after thermal oxidation at(a) 800, and (b) 1000 8C for 1 h in air.
2 shows scanning electron microscopy (SEM)
photo-graphs of the sputtered Ti films, where the Ti film deposited at 8 mtorry 500 8C exhibited surface
mor-phology with large grain size and a few flat hexagonal crystal grains, whilst for those prepared at 32 mtorry RT
give rise to a fine-grained microstructure.
These results indicate that improved crystallinity and large grains were obtained at higher growth temperature and lower sputtering pressures. At 500 8C, the Ti film exhibits strong (001) preferred orientation, which is expected from surface free energy consideration. How-ever, amorphous structure was observed when the dep-osition was performed at 32 mtorry RT. This is due to
reduced adatom mobility at lower substrate temperature, associated with decreased kinetic energy of arriving species because of collisions with residual gas molecules.
Before further discussion, it should be pointed out that this oxidation process involves growth of the TiO2
crystals from the Tiy air interface(i.e. the portions of Ti
atoms first exposed to oxygen) to Tiysubstrate interface
(i.e. ‘downward growth’), rather than those growing
‘upwards’ from a ‘bare’ substrate by direct deposition methods w4–14x. Therefore, growth of the TiO crystals2
depends solely on the movement of the TiO –Ti inter-2
face w16–18x.
After thermal oxidation of the sputtered Ti films at 800 8C for 1 h, a typical rutile TiO2 structure was developed as identified by XRD patterns in Fig. 3 w19x. Surprisingly, no anatase TiO2 was detected over the entire annealing temperature range (550–1000 8C) of
study. This phenomenon appears to be attributed to the formation of a large quantity of oxygen vacancies in the oxide film that is known to significantly accelerate the phase transformation of anatase to rutile, as further evidenced in previous work w15x.
A randomly oriented rutile TiO film was formed for2
Ti-25 films. However, preferred orientation along the
TiO2 (200) plane was developed from the starting
Ti-500 film, having the highest degree (sharpest reflection peaks) of crystallinity among all the Ti films prepared. In other words,(200)-oriented TiO film can be induced2
on the Ti films with (002) preferred orientation. These
experimental observations strongly suggest a competi-tion between two driving forces, i.e. surface free energy and strain energy effects, on determining preferential growth of the resultant TiO crystals. The surface mor-2
phology of TiO films grown from the thermal oxidation2
of Ti-500 film at 800–1000 8C for 1 h are shown in Fig. 4, where the grain size is increased from ;100 to
;180 nm after annealing at 800 and 1000 8C,
respectively.
Numerous published reports have indicated that the metal and its oxide (e.g. TiO yTi, ZrO yZr, Cu OyCu,2 2 2
MoO3y Mo and Al O y Al2 3 ) display the characteristics of
oxidation anisotropy w20–25x. This phenomenon is
attributed to the epitaxial growth of metal oxide on parent metal. For example, Flower et al. used the transmission electron microscope to investigate the behavior of in situ oxidation reaction of a-titanium foil
w20x. They found that rutile TiO2 can easily grow
epitaxially on a clean, electro-polished Ti surface wi.e.
TiO2(100)y yTi (001) and TiO (100)y yTi (100)x, but2
those contaminated Ti grains will induce polycrystalline rutile TiO . As schematically illustrated in Fig. 5 for the2
atomic structures of some planes in rutile TiO and a-2
Ti, the most favorable matching occurs at the TiO2 (100)–Ti (100) interface. However, the lattice misfits
between TiO2 (100) and Ti (001) is as much as ;10%
along the c-axis. Likewise, the ZrO yZr system also2
exhibits epitaxial relationship, for instance ZrO2 (y
101)y yZr (100) and ZrO (001)y yZr (001) w21x.2
The XRD patterns of Ti-25 and Ti-500 films oxidized at low temperatures for different time frames were shown in Fig. 6. Obviously, the Ti(002) peak gradually shifts towards low angles (from 38.41 to 37.118) by increasing the oxidation time, and a similar observation
Fig. 4. The SEM photographs of TiO films fabricated by thermal2 oxidation of Ti-500 film at(a) 800, (b) 900 and (c) 1000 8C anneal-ing temperature for 1 h.
Fig. 5. Atomic structures of some planes of single crystal rutile TiO and Ti.2
Fig. 6. XRD patterns of as-deposited Ti films with different sputtering condition after low temperature thermal oxidation.
also occurs in Ti-25 films w15x. This phenomenon can be ascribed as a result of the incorporation of oxygen atoms into the Ti structure. It is known that the thermal oxidation process of titanium is an oxygen diffusion-controlled mechanism w15–17x. Initially, a fast oxygen adsorption on titanium surface causes the formation of lower oxides after subsequently, oxygen molecules dif-fuse through the oxide layer into a-Ti. The maximum solubility of oxygen atoms in a-Ti can be reached to a level of Oy Tis0.5 mol% (i.e. oxygen-enriched Ti,
where the oxygen atoms occupy the hole in every second layer of octahedral interstices, causing a considerable
expansion in the c-axis, but only a slight change in the a-axis) w26x. Therefore, the structure of Ti films has
changed to oxygen-enriched Ti (i.e. Ti O) before the2
rutile TiO2 was detected by XRD, Fig. 6. After a complete transformation from Ti to TiO , the oxygen-2
enriched Ti O phase is acting as a crystallographic2
intermediate template and produces strain energy due to lattice misfits between Ti O and TiO crystals2 2
In general, a growing film has the tendency to reduce the overall film energy by minimizing the strain energy, due to lattice mismatch and surface free energy. For thermal oxidation of the amorphous and poorly crystal-line Ti films (Ti-amorph and Ti-25), the XRD pattern
Table 1
The integrated intensity ratios, gsI (200)yI (110), of the rutile TiO films prepared under different oxidation temperatures for the Ti2 films deposited at different substrate temperatures
Oxidation g(Ti-25) g(Ti-300) g(Ti-500)
temp. (8C)
800 0.20 0.94 3.12
900 0.18 0.63 1.39
1000 0.11 0.60 1.15
Note that the I (200)yI (110) ratio for randomly oriented TiO2 crystals is 0.08.
of the resultant TiO is similar to that of the powder2
TiO , having completely randomly oriented crystals.2
However, the TiO from the well-crystalline Ti-500 film2
with stronger (002) preferred orientation exhibits a preferential growth along(200) orientation. These obser-vations suggest that the development of preferred or randomly distributed rutile TiO crystallites is strongly2
related to the crystallinity and orientation of initially-deposited Ti films. Although lattice mismatch between
Ti O2 (001) and TiO (100) is approximately 0.3% in2
the a-axis direction and 10% in the c-axis direction, a
localized coherent relationship between Ti O and TiO2 2
may exist, which further promotes the growth of TiO2
with(200) orientation. From an energy viewpoint, strain
energy can be minimized when (200)-oriented TiO2
(relative to (110)-oriented TiO ) developed on the2
(002)-oriented Ti crystallites whereupon the interfacial
mismatch is optimized.
A higher temperature oxidation()900 8C) of Ti-500 films facilitates the formation of TiO crystallites with2
random orientation, and the g-value wthe integrated intensity ratio of rutileI (200)yI (110)x decreased with
increasing oxidation temperature, as shown in Table 1 and Fig. 3b. When oxygen atoms (5–24 at.%) dissolve
in a-Ti, the structure of this oxygen-enriched Ti (i.e. hexagonal structure) can be stabilized at temperature as high as ;1720 8C without changing to b-Ti w27x. However, at higher oxidation temperatures, the lattice vibration will further interfere with the local coherency between oxygen-enriched Ti and TiO , leading to the2
formation of randomly distributed TiO crystallites. On2
the other hand, since the (110) plane has the lowest
surface free energy, high-temperature oxidation seems favorable to bring the TiO crystal growth towards a2
surface with the lowest surface free energy in order to reduce the overall energy. Additionally, by reducing the thickness of the Ti-500 film from 400 to 200 nm, a stronger rutile (200) peak appeared, resulting in an increased g-value (8.39). The enhanced preferential growth toward(200) plane of thinner TiO films can be2
attributed to reduced strain energy due to a decrease in film thickness w28x.
4. Conclusion
Rutile TiO2 films with random or (200) preferred orientations can be easily grown and tailored by the thermal oxidation of sputtered Ti metal films, having different degrees of crystallinity and preferred orienta-tion. The preferential growth of TiO film results from2
a competition between surface free energy effect and strain energy effect. The highly crystalline Ti film with
(002) orientation can induce the preferential growth of
the resultant TiO crystals with2 (200) orientation.
How-ever, a randomly oriented rutile TiO film is developed2
for the thermal oxidation of poorly crystalline Ti films. In addition, the preferential growth of (200)-oriented
TiO is enhanced by decreasing the film thickness where2
the strain energy is reduced. Higher temperature
oxida-tion (G900 8C) promotes the formation of rutile (110)
plane because of surface free energy effect.
Acknowledgements
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract no. NSC89-2216-E-009-034.
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