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The influence of thermal annealing on the structural, electrical and optical properties of TiO2-x thin films
Applied Surface North-Holland
Science 65/66
(1993) 227-234
applied surface science
The influence of thermal annealing on the structural, electrical and optical properties of TiO,_, thin films M. Radecka,
K.
Zakrzewska, H. Czternastek,
T. Stapiriski
Institute of Electronics, Academy of Mining and Metallurgy, al.Mickiewicza 30, 30-059 Krako’w, Poland
and S. Debrus Laboratoire de Physique des Solides, Universitt! P. et M. Curie, 4 place Jussiey Received
29 June
1992; accepted
for publication
75252 Pan’s Cedex 05, France
24 July 1992
Amorphous thin films of TiO,_, with a departure from stoichiometry x in the range from 0.08 to -0.2 were deposited by plasma-emission-controlled DC magnetron and conventional RF reactive sputtering. The influence of the post-deposition annealing at 573-1373 K on the structural, electrical and optical properties was studied. It was established that substoichiometric films with x = 0.08 when subjected to the annealing changed their overall spectral characteristics while in the case of overoxidized samples (x = -0.2) the transmission spectra were strongly modified in the region of the absorption edge (3.0-3.7 eV). It was found that the refractive index of TiO,_, films increased with annealing temperature up to a saturation level of about 2.39. The analysis of the absorption data revealed that direct allowed transitions at the energy of 3.44-3.51 eV prevailed in rutile samples.
1. Introduction Titanium dioxide is one of the most extensively studied transition-metal oxides. The increased interest in both the application and the fundamental research of TiO, observed in the last decade stems from its remarkable optical and electronic properties. TiO, crystallizes in three forms: brookite, anatase and rutile [l-31, the latter being the most dense (4.22-4.26 g/cm3) and thermodynamically stable modification. Tetragonal rutile is a particularly desirable oxide phase as far as the optical properties are concerned. In spite of the fact that rutile is formed above 800°C according to the phase diagram [4], both anatase and r-utile have been reported for vacuum-deposited thin films [5--71. From the electronic point of view, rutile TiO, is a wide-band-gap non-conducting material in which the valence band is formed by the O(2p) 0169.4332/93/$06.00
0 1993 - Elsevier
Science
Publishers
wave functions, while the narrow 3d conduction band with extremely high effective mass of about 30-50m, 131 is constituted by Ti states. The progressive transition from an insulator, through semiconductor to metal can be accomplished by reduction [2]. There is no agreement in the literature concerning the nature of the dominant points defects responsible for the semiconducting properties of TiO, [S-101. It is generally accepted, however, that the electronic disorder resulting in the electrical conductivity of TiO, is introduced by oxygen non-stoichiometry. The recent development in the field of deposition methods of nonstoichiometric, oxygen-deficient TiO,_, thin films has stimulated the research of the correlation between their microstructure, composition, defect formation, conduction mechanism and optical properties [4,5,11,12]. High electrical sensitivity of TiO,_, to the ambient gases such as hydrogen, oxygen and car-
B.V. All rights reserved
gations is to point out the potential applications of titanium oxides films in gas-sensor technology CondenserC
[1X1. 2. Experimental
I- PC I Fig. 1. Experimental
set-up for optical
spherical quartz lens. L? - cylindrical - diaphragms,
measurements:
quartz lens. D,,
L,
-
D?, D,
T,, T,,, T,,, are the images of the light source
formed hy C, L, and Lz, respectively.
bon dioxide has made this material particularly useful for applications in gas-sensing devices [ 101. Rutile TiO, satisfies as well the requirements of optical technology due to the high refractive index (2.3-2.6) and transparency (T = 0.8-0.9) over the wide spectral range (350-3000 nm). Up to now, TiOz thin films have been commonly used as highly refractive components in the multilayer dielectric stacks while recently non-stoichiometric absorptive TiO,_, films have been considered as potential candidates for antireflecting coatings in solar systems [ 131. Photosensitive properties of TiO, have opened up quite a number of applications in photoelectrolysis [14,151. The properties of titanium dioxide films are known to be easily affected by the technological conditions of the deposition process such as the substrate temperature and oxygen partial pressure as well as by the post-deposition heat-treatment. In our previous papers we have reported the preliminary results of the structural, electrical and optical properties of TiO,_, thin films deposited by plasma-emission-con&&led magnetron [16] and conventional RF reactive sputtering [ 171. Here we have undertaken systematic studies of the correlation between the film structure, deviation from stoichiometry, and electrical and optical properties of TiO,_,. The influence of the annealing temperature on the refractive index and position of the absorption edge was studied for substoichiometric (X = 0.08) and overoxidized (X = - 0.2) films. The ultimate aim of our investi-
Two deposition techniques were used to proreactive TiO, ~,, films. RF conventional sputtering from a 99.99% pure titanium target in Ar + 15%02 gas atmosphere resulted in slightly overoxidized films. Plasma-emission-controlled DC magnetron sputtering from the same target allowed suboxidized films with x = 0.08 to be deposited as well overoxidized films with x = - 0.02. In order to obtain TIO,_, with well-defined and reproducible departure from stoichiometry x, one has to utilize the unstable mode of magnetron sputtering, i.e. the mode that lies in the region of an abrupt transition from one to the other stable (metallic/ reactive) mode. Stabilization of the operating point in the transition region is achieved in our system by means of controlling the intensity I, of the Ti emission lint at A = 500 nm. The intensity of the metal emission state of the titaline I M reflects the oxidation nium target (0 is the target coverage by the duce
,
2.3
(1000
T
T\\
1.8
I
0.05
I
0.10
1
0.15
0.20
I
0.25
I 0.1
0.30
S (nm/s) Fig. 2. Atomic
ratio
scale) and electrical
O/Ti
of oxygen to titanium
conductivity
u (right-hand
function of the growth rate .Y of TiOz_, guide the eye.
(left-hand scale)
as a
films; lines drawn to
M. Radecka et al. / Structural, electrical and opticul properties
products of 0-Ti reactions) and is proportional to the sputtering rate of the target. Hence, the automatic control of I, provides a constant deposition rate s and deviation from stoichiometry x in the sample during the growth process. The principle of the operation of the system as well as the details of the sputtering conditions are presented elsewhere [ 161. The films with thicknesses of 100-800 nm where deposited onto optical quartz and carbon foil at the substrate temperature 300-500 K. As determined by X-ray diffraction (CuKa radiation) all as-sputtered films were amorphous independently of x. The film thickness was measured by a Talysurf 4 profilometer with an accuracy of * 10 nm. The atomic ratio of O/Ti was analysed by means of the Rutherford backscattering (RBS) technique. The DC four-point-probe method was used in the electrical resistivity measurements. Post-deposition annealing in air at 573-1373 K was performed on all specimens for 15 h. The crystallographic structure and film resistivity were monitored during annealing. The optical properties of TiO,_, films were studied by means of spectrophotometric measurements of transmission, T, and reflection, R, coefficients in the wavelength range 190-2500 nm. A double-beam Perkin-Elmer Lambda 9 UV-VISNIR spectrophotometer was used in the visible and infrared region. In the vicinity of the absorption edge the absorption spectroscopy studies were performed. The experimental set-up (fig. 1) fitted with a Jarrell-Ash spectrometer gives the spectral resolution AA = 0.1 nm. The refractive index, n, and absorption coefficient, cy, were computed from the spectral dependence of T and R using a procedure described in ref. [19].
3. Results and discussion The correlation was found between the film composition (O/Ti atomic ratio), electrical conductivity (T and the film growth rate s (fig. 2). As expected, plasma-emission-controlled DC magnetron sputtering yields substoichiometric TiO,-, (x = 0.08) at high sputtering rates, i.e. when the
1.8-I 1
.a
I
I 1.9
of TiO,
x
.
I
I 2.0
O/Ti
229
I 2.1
I
23
2.F
Fig. 3. Refractive index n (left-hand scale) and absorption coefficient (Y (right-hand scale) of TiO,_, films as a function of the atomic ratio O/Ti of oxygen to titanium.
target coverage 8 is low. The considerable electrical conductivities of these films remain consistent with earlier observations that rutile equilibrated in an atmosphere of low oxygen activity behaves like a typical semiconductor 191. The increase in O/Ti and the rapid decrease in u clearly seen at lower growth rates suggest that TiO,-, becomes overoxidized and insulating at the limit of high 0. The optical properties of TiO,-, produced by plasma-emission-controlled deposition (fig. 3) support this conclusion. The refractive index II increases steadily with O/Ti while the absorption coefficient a decreases. Similar&haviour of n and k (extinction coefficient) as a function of the departure from stoichiometry was reported by Schiller et al. 1111, however we failed to observe the further stabilization and decrease in II for overoxidized samples. Independently of the initial state, all X-ray diffraction patterns of TiO,-, films are featureless up to about 700 K. The influence of the annealing temperature on the film structure is shown in fig. 4. The first traces of the long-range order appear at approximately 750 K. The welldeveloped rutile structure is produced at 1173 K and no significant changes occur at higher temperatures. Formation of the anatase reported by many authors [5-7,201 was not brought into evidence here. Dynamic changes in the electrical resistivity p with x = -0.2 in the course of annealof TiO,-,
230
M. Radecka
et al. / Structural.
electrical
and optical properties
of TX),
*
1.oo b-
5
go.75 0 k ! zo.50 0
Ti02_. d= 145nm
, -
/
\ \ \ L
-
-
t
heat-treated
\
z 5
$0.25 w__
673K
Fig. 4. X-ray diffraction patterns of TiO, x (x = -0.2) thin film at different annealing temperatures. The top curve represents a spectral purity TiO, powder sample crystallized in the rutile structure.
ing are presented in fig. 5. The first remarkable drop in p at about 750 K correlates quite well with the onset of the structural ordering. The subsequent rise in the electrical resistivity is probably due to competing processes: further development of the crystal structure which tends to reduce p and oxidation which has the opposite effect. The second decrease in the film resistivity at approximately 970 K and its stabilization at higher temperatures can be attributed to the formation of the well-developed rutile.
.
/-
/
,‘.’ / ’
,
. .
_*
-
. . .* .
II-.,---n
*-I,“‘--
160
+
-300 TiO&. d=290
0
. -600g
a.
260 TIME (min)
“m II”““’
300
Fig. 5. Dynamic changes in the electrical resistivity p of as-sputtered TiO,_, (x = -0.2) thin film during annealing in air. Dashed line indicates the temperature versus time behav-
c as-sputtered x=0.08
Fig. 6. Spectral dependence of the transmission coefficient 7 for TiOZmr film: ( -) T for as-sputtered film with x = 0.08: (-) T for the same film annealed in air at I I73 K.
Post-deposition annealing strongly affects the optical transmission spectra of both initially substoichiometric (X = 0.08) and overoxidized (X = - 0.2) samples. However, its influence on T(h) manifests itself in quite a different way for those two initial-state limits. Annealing in air changes the overall transmission characteristics of assputtered substoichiometric film, as shown in fig. 6, affecting not only the average transmission coefficient over the entire visible range but also producing well-pronounced oscillations due to the interference. Therefore, it may be concluded that high-temperature heat-treatment of initially substoichiometric films leads to their complete oxidation. In the case of overoxidized samples (X = - 0.2) the spectral characteristics of the transmission are strongly modified by annealing only in the region of the fundamental absorption edge (fig. 7), while the long-wavelength transmission coefficient remains almost intact. These observations suggest that the oxidation process is of no significance in the case of initially overoxidized samples. The major contribution comes from the recrystallization that may be responsible not only for the changes in band structure of the material but also for the increased scattering losses due to the surface modifications.
231
M. Radecka et al. / Structural, electrical and optical properties of TiO, _ x
2.5 -\ c
2 2.4 L z 0.50 .-
;
0
a
TiO2-. d=220nm
w
I $ 0.25 ._ _
l
l l l
l
h 2.2 lx
as-sputtered
bbbbb773 ~~~~~ 973
2.3
Y
K K
m-#
a+-
;:j
a~~~~1173 K •~~~~ 1263 K
d-220nm
0.5
1 .o
I
,
1.5
2.0
I_
h bm)
hv (eV)
Fig. 7. Spectral dependence of the transmission coefficient T for TiO,_r film (X = -0.2) annealed in air at different temperatures.
Fig. 9. Refractive index n as a function of wavelength A for film (X = - 0.2) annealed in air at different temperaTiO,-, tures.
The refractive indices calculated from the transmission spectra (figs. 6 and 7) increase upon annealing independently of the initial film stoichiometry as shown in figs. 8 and 9 for x = 0.08 and -0.2, respectively. The systematic change in n with the annealing temperature accompanied by the saturation at about 2.39 remains in good qualitative agreement with the results reported by Anderson et al. [21].
The absorption coefficient (Y in the case of substoichiometric films is quite high for photon energies below the fundamental absorption edge (fig. 10). These strong “absorption tails” that are due to the high density of states in the forbidden band gap tend to disappear upon heat-treatment. The absorption edge for the overoxidized film (x = -0.2) moves towards lower photon energies as the annealing temperature increases (fig. 11). Starting from 1173 K there is no significant difference in the optical properties of the samples.
2.6 TiO&. d=145nm
2.5 c x 2.4
0
: ,,., 2.3 t . 12.2 0:
. *.
**...
as-sputtered, *a.*
x=0.06
. . . . . . .
2.1
2.0 -0.35
0.40
0.45
,
0.50
a hv (eV)
Fig: 8. Refractive index n versus wavelength of light A for film with x = 0.08; (0) n TiO, --x film: (0) n for as-sputtered for the same film annealed in air at 1173 K.
Fig. 10. Absorption coefficient a versus photon energy hv for film: (.) n for as-sputtered film with x = 0.08; (0) n TiO,_, for the same film annealed in air at 1173 K.
7 E 25-
3
01
a
1
3.2
1
3.3
3.4
I
3.5
3.6
hv
Fig. 11. Spectral for TiO,_,
dependence
film 0; = -0.2)
3.7
3.0
3.9
4
(eV)
hv
of the absorption annealed
coefficient
in air at different
a
tem-
Fig.
13. Analysis
(w = ~0.2) experimental
of the optical
annealed
line (left-hand
peratures.
(eV)
transitions
in air at different
scale) represents
the best fit of (tulrv)’
data for films annealed
Solid lines (right-hand the experimental
for Ti02
temperatures.
I
film
Dashed to the
in 12X3 K and II73
K.
scale) are the best fits of (cY~v)‘/ ’ to
data for as-aputtered
and hca-treated
(773
K) films.
The values of the optical band gap EC,p, were found by fitting (cyhv)* versus hv to the experimental data of thin films with well-developed rutile structure (figs. 12 and 13). For the assputtered amorphous films, similarly to the results presented earlier [17], the best fits are ob-
‘j
I TiO z-i d=145nm L,,=3.51
+o.o5ev
heat-treated
hv
(A’)
Fig. 12. Analysis of optical transitions of photon sputtered annealed
energy film with
/IV for TiO,
,
x = 0.08: (0)
in air at II73
- (cuhv)
(uhv)’ data.
for as-
for the same film
K, solid line represents
the experimental
as a function
film: (@) (uhv)’
the best fit to
mined with the Taut plot, i.e. (cuhv)“’ versus Irv dependence. As far as the band gap of TiOz rutilc is concerned, there arc still contradictory data in the literature [2,22-301. In the photon energy range extending below 4 eV, the experimental as well as the theoretical data suggest three major optical transitions for single crystals: two indirect allowed transitions centered around 3.00 cV [2,2224,26-291 and 3.11 eV [23,26-291 and/or one direct allowed in the range of 3.3-3.5 eV [2,22,24.25,27]. It should be pointed out that the transition at 3.3-3.4 eV is the most frequently reported for thin films [20,25,31,32]. It stems possible that due to the well pronounced defect structure of thin films the lower-energy transition cannot bc detected by absorption spectroscopy measurements. Our results indicate direct allowed transitions at 3.44-3.51 CV for fully crystallized rutile. The transition energy of about 3.32-3.35 eV is observed for amorphous and poorly crystallized samples, but in this case the general conclusions regarding the character of optical transitions and the band gap should not be drawn. Similar obser-
M. Radecka et al. / Structural, electrical and optical properties of TiO,
vations have recently been made by Halley et al. [30] for anodically grown TiO, films claiming that the transition at 3.3 eV is a spurious effect. We believe that the transformation from the amorphous state to the rutile structure affects significantly the shape of the fundamental absorption edge, which accounts for the change in the power dependence from (ahv) ‘1’ to (ahvj2. The energy of the optical transition does not undergo such a dramatic change.
4. Conclusions (i) Plasma-emission-controlled DC magnetron and conventional RF reactive sputtering were used to deposit nonstoichiometric TiO,_, thin films where x varied from 0.08 to -0.2. (ii> All as-sputtered films were amorphous independently of X. The well-developed rutile structure was detected after annealing at 1173 K. (iii) The electrical conductivity u, the refractive index n, and the absorption coefficient (Y were found to be correlated with the deviation from stoichiometry X. (iv> The optical transmission spectra were found to be strongly modified by post-deposition annealing in both limits of x. The oxidation process is definitely marked in the case of initially substoichiometric films (X = 0.08), while for the overoxidized samples (X = - 0.2) the structural modification is responsible for drastic changes in the shape of the fundamental absorption edge. (v) The refractive index increases up to 2.39 with annealing temperature. (vi) The predominant direct allowed optical transitions at the energy of 3.44-3.51 eV were observed for well-developed rutile samples.
Acknowledgements We would like to express our deepest thanks for Prof. M. May from the Laboratoire de Physique des Solides, Universite P. et M. Curie, Paris, where the absorption spectroscopy studies were carried out. We are also grateful to Prof. F. Demichelis from Dipartimento di Fisica, Politec-
233
x
nice di Torino, where the measurements were made.
spectrophotometric
References [l] R.W.G. Wyckoff, Crystal Structures Handbook (Interscience, New York, 194X). [2] D.C. Cronemeyer, Phys. Rev. 87 (1952) 876. [3] R.G. Breckenridge and W.R. Hosler, Phys. Rev. 91 (1953) 793. [4] K.G. Geraghty and L.F. Donaghey, Thin Solid Films 40 (1977) 375. [5] S. Schiller, G. Beister, W. Sieber, G. Shirmer and E. Hacker. Thin Solid Films 83 (1981) 239. [6] L.M. Williams and D.W. Hess, J. Vat. Sci. Technol. A 1 (1983) 1810. [7] K. Fukushima and I. Yamada, Appl. Surf. Sci. 43 (1989) 32. [8] A. von Hippel, J. Kalnajs and W.B. Westphal, J. Phys. Chem. Solids 23 (1962) 779. [9] U. Balachandran and N.G. Eror, J. Mater. Sci. 23 (1988) 2676. [IO] U. Kirner, K.D. Schierbaum, W. Gopel, B. Leibold. N. Nicoloso. W. Weppner, D. Fischer and W.F. Chu, Sensors Actuators B 1 (1990) 103. [II] S. Schiller, G. Beister. S. Schneider and W. Sieber, Thin Solid Films 72 (1980) 475. [12] M. Ghanashyam Krishna, K. Narasimha Rao, M. Adinarayana Murthy and S. Mohan, Mater. Sci. Eng. B 5 (1990) 427. [I31 S. Yoshida, Thin Solid Films 56 (1979) 321. [14] A.A. Soliman and H.J.J. Seguin, Sol. Energy Mater. 5 (1981) 95. [15] N. Nakao, R. Schumacher and R.N. Schindler, J. Electrochem. Sot. 133 (1986) 2308. [16] A. Brudnik, H. Czternastek, K. Zakrzewska and M. Jachimowski, Thin Solid Films 199 (1991) 45. [17] M. Radecka, K. Zakrzewska, T. Stapinski and M. Rqkas, Le Vide, les Couches Minces, Suppl. No. 259 (1991) 116. [18] M. Radecka, Z. Szklarski and K. Zakrzewska, in: Proc. Electron. Tech. ELTE’YO. Ksi@, Poland, 1990, p, 369. [19] J. Szczyrbowski and A. Czapla, J. Phys. D 12 (1979) 1737. [20] E. Gofuku, Y. Toyoda, Y. Uehara, M. Kohara and M. Nunoshita, Appl. Surf. Sci. 48/49 (1991) 343. [21] 0. Anderson, C. Ottermann and K. Bange. Le Vide, les Couches Minces, Suppl. No. 259 (199 I J 32. [22] P. Moth, M. Balkanski and P. Aigrain, CR. Acad. Sci. (Paris) 251 (1960) 1373. [23] F. Arntz and Y. Yacobi, Phys. Rev. Lett. 17 (1966) 857. [24] A. Frova, P.J. Boddy and Y.S. Chen. Phys. Rev. 157 (1967) 700. [25] S.K. Deb, Solid State Commun. 11 (1972) 713. [26] K. Vos and H.J. Krusemeyer, Solid State. Commun. 15 (1974) 949.
2.34 [27] N. Daude,
M. Radecka
et al. / Structural,
electrical and optical properties
C. Gout and C. Jouanin, Phys. Rev. B IS (1977) 3229. [2X] J. Pascual, J. Camassel and H. Mathieu, Phys. Rev. B I8 (1978) 5606. [291 J.L. Jourdan, C. Gout and J.P. Albert, Solid State Commun. 31 (1070) 1023.
of TiOz
I
[30] J.W. Halley, M. Kozlowski. M. Michalewicz, W. Smyrl and N. Tit, Surf. Sci. 256 (1991) 397. [31] M. Takeuchi, Phys. Status Solidi (a) 5.5 (1979) 653. [32] A. Shibata, Jpn. J. Appl. Phys. 30 (1991) L650.
Science 65/66
(1993) 227-234
applied surface science
The influence of thermal annealing on the structural, electrical and optical properties of TiO,_, thin films M. Radecka,
K.
Zakrzewska, H. Czternastek,
T. Stapiriski
Institute of Electronics, Academy of Mining and Metallurgy, al.Mickiewicza 30, 30-059 Krako’w, Poland
and S. Debrus Laboratoire de Physique des Solides, Universitt! P. et M. Curie, 4 place Jussiey Received
29 June
1992; accepted
for publication
75252 Pan’s Cedex 05, France
24 July 1992
Amorphous thin films of TiO,_, with a departure from stoichiometry x in the range from 0.08 to -0.2 were deposited by plasma-emission-controlled DC magnetron and conventional RF reactive sputtering. The influence of the post-deposition annealing at 573-1373 K on the structural, electrical and optical properties was studied. It was established that substoichiometric films with x = 0.08 when subjected to the annealing changed their overall spectral characteristics while in the case of overoxidized samples (x = -0.2) the transmission spectra were strongly modified in the region of the absorption edge (3.0-3.7 eV). It was found that the refractive index of TiO,_, films increased with annealing temperature up to a saturation level of about 2.39. The analysis of the absorption data revealed that direct allowed transitions at the energy of 3.44-3.51 eV prevailed in rutile samples.
1. Introduction Titanium dioxide is one of the most extensively studied transition-metal oxides. The increased interest in both the application and the fundamental research of TiO, observed in the last decade stems from its remarkable optical and electronic properties. TiO, crystallizes in three forms: brookite, anatase and rutile [l-31, the latter being the most dense (4.22-4.26 g/cm3) and thermodynamically stable modification. Tetragonal rutile is a particularly desirable oxide phase as far as the optical properties are concerned. In spite of the fact that rutile is formed above 800°C according to the phase diagram [4], both anatase and r-utile have been reported for vacuum-deposited thin films [5--71. From the electronic point of view, rutile TiO, is a wide-band-gap non-conducting material in which the valence band is formed by the O(2p) 0169.4332/93/$06.00
0 1993 - Elsevier
Science
Publishers
wave functions, while the narrow 3d conduction band with extremely high effective mass of about 30-50m, 131 is constituted by Ti states. The progressive transition from an insulator, through semiconductor to metal can be accomplished by reduction [2]. There is no agreement in the literature concerning the nature of the dominant points defects responsible for the semiconducting properties of TiO, [S-101. It is generally accepted, however, that the electronic disorder resulting in the electrical conductivity of TiO, is introduced by oxygen non-stoichiometry. The recent development in the field of deposition methods of nonstoichiometric, oxygen-deficient TiO,_, thin films has stimulated the research of the correlation between their microstructure, composition, defect formation, conduction mechanism and optical properties [4,5,11,12]. High electrical sensitivity of TiO,_, to the ambient gases such as hydrogen, oxygen and car-
B.V. All rights reserved
gations is to point out the potential applications of titanium oxides films in gas-sensor technology CondenserC
[1X1. 2. Experimental
I- PC I Fig. 1. Experimental
set-up for optical
spherical quartz lens. L? - cylindrical - diaphragms,
measurements:
quartz lens. D,,
L,
-
D?, D,
T,, T,,, T,,, are the images of the light source
formed hy C, L, and Lz, respectively.
bon dioxide has made this material particularly useful for applications in gas-sensing devices [ 101. Rutile TiO, satisfies as well the requirements of optical technology due to the high refractive index (2.3-2.6) and transparency (T = 0.8-0.9) over the wide spectral range (350-3000 nm). Up to now, TiOz thin films have been commonly used as highly refractive components in the multilayer dielectric stacks while recently non-stoichiometric absorptive TiO,_, films have been considered as potential candidates for antireflecting coatings in solar systems [ 131. Photosensitive properties of TiO, have opened up quite a number of applications in photoelectrolysis [14,151. The properties of titanium dioxide films are known to be easily affected by the technological conditions of the deposition process such as the substrate temperature and oxygen partial pressure as well as by the post-deposition heat-treatment. In our previous papers we have reported the preliminary results of the structural, electrical and optical properties of TiO,_, thin films deposited by plasma-emission-con&&led magnetron [16] and conventional RF reactive sputtering [ 171. Here we have undertaken systematic studies of the correlation between the film structure, deviation from stoichiometry, and electrical and optical properties of TiO,_,. The influence of the annealing temperature on the refractive index and position of the absorption edge was studied for substoichiometric (X = 0.08) and overoxidized (X = - 0.2) films. The ultimate aim of our investi-
Two deposition techniques were used to proreactive TiO, ~,, films. RF conventional sputtering from a 99.99% pure titanium target in Ar + 15%02 gas atmosphere resulted in slightly overoxidized films. Plasma-emission-controlled DC magnetron sputtering from the same target allowed suboxidized films with x = 0.08 to be deposited as well overoxidized films with x = - 0.02. In order to obtain TIO,_, with well-defined and reproducible departure from stoichiometry x, one has to utilize the unstable mode of magnetron sputtering, i.e. the mode that lies in the region of an abrupt transition from one to the other stable (metallic/ reactive) mode. Stabilization of the operating point in the transition region is achieved in our system by means of controlling the intensity I, of the Ti emission lint at A = 500 nm. The intensity of the metal emission state of the titaline I M reflects the oxidation nium target (0 is the target coverage by the duce
,
2.3
(1000
T
T\\
1.8
I
0.05
I
0.10
1
0.15
0.20
I
0.25
I 0.1
0.30
S (nm/s) Fig. 2. Atomic
ratio
scale) and electrical
O/Ti
of oxygen to titanium
conductivity
u (right-hand
function of the growth rate .Y of TiOz_, guide the eye.
(left-hand scale)
as a
films; lines drawn to
M. Radecka et al. / Structural, electrical and opticul properties
products of 0-Ti reactions) and is proportional to the sputtering rate of the target. Hence, the automatic control of I, provides a constant deposition rate s and deviation from stoichiometry x in the sample during the growth process. The principle of the operation of the system as well as the details of the sputtering conditions are presented elsewhere [ 161. The films with thicknesses of 100-800 nm where deposited onto optical quartz and carbon foil at the substrate temperature 300-500 K. As determined by X-ray diffraction (CuKa radiation) all as-sputtered films were amorphous independently of x. The film thickness was measured by a Talysurf 4 profilometer with an accuracy of * 10 nm. The atomic ratio of O/Ti was analysed by means of the Rutherford backscattering (RBS) technique. The DC four-point-probe method was used in the electrical resistivity measurements. Post-deposition annealing in air at 573-1373 K was performed on all specimens for 15 h. The crystallographic structure and film resistivity were monitored during annealing. The optical properties of TiO,_, films were studied by means of spectrophotometric measurements of transmission, T, and reflection, R, coefficients in the wavelength range 190-2500 nm. A double-beam Perkin-Elmer Lambda 9 UV-VISNIR spectrophotometer was used in the visible and infrared region. In the vicinity of the absorption edge the absorption spectroscopy studies were performed. The experimental set-up (fig. 1) fitted with a Jarrell-Ash spectrometer gives the spectral resolution AA = 0.1 nm. The refractive index, n, and absorption coefficient, cy, were computed from the spectral dependence of T and R using a procedure described in ref. [19].
3. Results and discussion The correlation was found between the film composition (O/Ti atomic ratio), electrical conductivity (T and the film growth rate s (fig. 2). As expected, plasma-emission-controlled DC magnetron sputtering yields substoichiometric TiO,-, (x = 0.08) at high sputtering rates, i.e. when the
1.8-I 1
.a
I
I 1.9
of TiO,
x
.
I
I 2.0
O/Ti
229
I 2.1
I
23
2.F
Fig. 3. Refractive index n (left-hand scale) and absorption coefficient (Y (right-hand scale) of TiO,_, films as a function of the atomic ratio O/Ti of oxygen to titanium.
target coverage 8 is low. The considerable electrical conductivities of these films remain consistent with earlier observations that rutile equilibrated in an atmosphere of low oxygen activity behaves like a typical semiconductor 191. The increase in O/Ti and the rapid decrease in u clearly seen at lower growth rates suggest that TiO,-, becomes overoxidized and insulating at the limit of high 0. The optical properties of TiO,-, produced by plasma-emission-controlled deposition (fig. 3) support this conclusion. The refractive index II increases steadily with O/Ti while the absorption coefficient a decreases. Similar&haviour of n and k (extinction coefficient) as a function of the departure from stoichiometry was reported by Schiller et al. 1111, however we failed to observe the further stabilization and decrease in II for overoxidized samples. Independently of the initial state, all X-ray diffraction patterns of TiO,-, films are featureless up to about 700 K. The influence of the annealing temperature on the film structure is shown in fig. 4. The first traces of the long-range order appear at approximately 750 K. The welldeveloped rutile structure is produced at 1173 K and no significant changes occur at higher temperatures. Formation of the anatase reported by many authors [5-7,201 was not brought into evidence here. Dynamic changes in the electrical resistivity p with x = -0.2 in the course of annealof TiO,-,
230
M. Radecka
et al. / Structural.
electrical
and optical properties
of TX),
*
1.oo b-
5
go.75 0 k ! zo.50 0
Ti02_. d= 145nm
, -
/
\ \ \ L
-
-
t
heat-treated
\
z 5
$0.25 w__
673K
Fig. 4. X-ray diffraction patterns of TiO, x (x = -0.2) thin film at different annealing temperatures. The top curve represents a spectral purity TiO, powder sample crystallized in the rutile structure.
ing are presented in fig. 5. The first remarkable drop in p at about 750 K correlates quite well with the onset of the structural ordering. The subsequent rise in the electrical resistivity is probably due to competing processes: further development of the crystal structure which tends to reduce p and oxidation which has the opposite effect. The second decrease in the film resistivity at approximately 970 K and its stabilization at higher temperatures can be attributed to the formation of the well-developed rutile.
.
/-
/
,‘.’ / ’
,
. .
_*
-
. . .* .
II-.,---n
*-I,“‘--
160
+
-300 TiO&. d=290
0
. -600g
a.
260 TIME (min)
“m II”““’
300
Fig. 5. Dynamic changes in the electrical resistivity p of as-sputtered TiO,_, (x = -0.2) thin film during annealing in air. Dashed line indicates the temperature versus time behav-
c as-sputtered x=0.08
Fig. 6. Spectral dependence of the transmission coefficient 7 for TiOZmr film: ( -) T for as-sputtered film with x = 0.08: (-) T for the same film annealed in air at I I73 K.
Post-deposition annealing strongly affects the optical transmission spectra of both initially substoichiometric (X = 0.08) and overoxidized (X = - 0.2) samples. However, its influence on T(h) manifests itself in quite a different way for those two initial-state limits. Annealing in air changes the overall transmission characteristics of assputtered substoichiometric film, as shown in fig. 6, affecting not only the average transmission coefficient over the entire visible range but also producing well-pronounced oscillations due to the interference. Therefore, it may be concluded that high-temperature heat-treatment of initially substoichiometric films leads to their complete oxidation. In the case of overoxidized samples (X = - 0.2) the spectral characteristics of the transmission are strongly modified by annealing only in the region of the fundamental absorption edge (fig. 7), while the long-wavelength transmission coefficient remains almost intact. These observations suggest that the oxidation process is of no significance in the case of initially overoxidized samples. The major contribution comes from the recrystallization that may be responsible not only for the changes in band structure of the material but also for the increased scattering losses due to the surface modifications.
231
M. Radecka et al. / Structural, electrical and optical properties of TiO, _ x
2.5 -\ c
2 2.4 L z 0.50 .-
;
0
a
TiO2-. d=220nm
w
I $ 0.25 ._ _
l
l l l
l
h 2.2 lx
as-sputtered
bbbbb773 ~~~~~ 973
2.3
Y
K K
m-#
a+-
;:j
a~~~~1173 K •~~~~ 1263 K
d-220nm
0.5
1 .o
I
,
1.5
2.0
I_
h bm)
hv (eV)
Fig. 7. Spectral dependence of the transmission coefficient T for TiO,_r film (X = -0.2) annealed in air at different temperatures.
Fig. 9. Refractive index n as a function of wavelength A for film (X = - 0.2) annealed in air at different temperaTiO,-, tures.
The refractive indices calculated from the transmission spectra (figs. 6 and 7) increase upon annealing independently of the initial film stoichiometry as shown in figs. 8 and 9 for x = 0.08 and -0.2, respectively. The systematic change in n with the annealing temperature accompanied by the saturation at about 2.39 remains in good qualitative agreement with the results reported by Anderson et al. [21].
The absorption coefficient (Y in the case of substoichiometric films is quite high for photon energies below the fundamental absorption edge (fig. 10). These strong “absorption tails” that are due to the high density of states in the forbidden band gap tend to disappear upon heat-treatment. The absorption edge for the overoxidized film (x = -0.2) moves towards lower photon energies as the annealing temperature increases (fig. 11). Starting from 1173 K there is no significant difference in the optical properties of the samples.
2.6 TiO&. d=145nm
2.5 c x 2.4
0
: ,,., 2.3 t . 12.2 0:
. *.
**...
as-sputtered, *a.*
x=0.06
. . . . . . .
2.1
2.0 -0.35
0.40
0.45
,
0.50
a hv (eV)
Fig: 8. Refractive index n versus wavelength of light A for film with x = 0.08; (0) n TiO, --x film: (0) n for as-sputtered for the same film annealed in air at 1173 K.
Fig. 10. Absorption coefficient a versus photon energy hv for film: (.) n for as-sputtered film with x = 0.08; (0) n TiO,_, for the same film annealed in air at 1173 K.
7 E 25-
3
01
a
1
3.2
1
3.3
3.4
I
3.5
3.6
hv
Fig. 11. Spectral for TiO,_,
dependence
film 0; = -0.2)
3.7
3.0
3.9
4
(eV)
hv
of the absorption annealed
coefficient
in air at different
a
tem-
Fig.
13. Analysis
(w = ~0.2) experimental
of the optical
annealed
line (left-hand
peratures.
(eV)
transitions
in air at different
scale) represents
the best fit of (tulrv)’
data for films annealed
Solid lines (right-hand the experimental
for Ti02
temperatures.
I
film
Dashed to the
in 12X3 K and II73
K.
scale) are the best fits of (cY~v)‘/ ’ to
data for as-aputtered
and hca-treated
(773
K) films.
The values of the optical band gap EC,p, were found by fitting (cyhv)* versus hv to the experimental data of thin films with well-developed rutile structure (figs. 12 and 13). For the assputtered amorphous films, similarly to the results presented earlier [17], the best fits are ob-
‘j
I TiO z-i d=145nm L,,=3.51
+o.o5ev
heat-treated
hv
(A’)
Fig. 12. Analysis of optical transitions of photon sputtered annealed
energy film with
/IV for TiO,
,
x = 0.08: (0)
in air at II73
- (cuhv)
(uhv)’ data.
for as-
for the same film
K, solid line represents
the experimental
as a function
film: (@) (uhv)’
the best fit to
mined with the Taut plot, i.e. (cuhv)“’ versus Irv dependence. As far as the band gap of TiOz rutilc is concerned, there arc still contradictory data in the literature [2,22-301. In the photon energy range extending below 4 eV, the experimental as well as the theoretical data suggest three major optical transitions for single crystals: two indirect allowed transitions centered around 3.00 cV [2,2224,26-291 and 3.11 eV [23,26-291 and/or one direct allowed in the range of 3.3-3.5 eV [2,22,24.25,27]. It should be pointed out that the transition at 3.3-3.4 eV is the most frequently reported for thin films [20,25,31,32]. It stems possible that due to the well pronounced defect structure of thin films the lower-energy transition cannot bc detected by absorption spectroscopy measurements. Our results indicate direct allowed transitions at 3.44-3.51 CV for fully crystallized rutile. The transition energy of about 3.32-3.35 eV is observed for amorphous and poorly crystallized samples, but in this case the general conclusions regarding the character of optical transitions and the band gap should not be drawn. Similar obser-
M. Radecka et al. / Structural, electrical and optical properties of TiO,
vations have recently been made by Halley et al. [30] for anodically grown TiO, films claiming that the transition at 3.3 eV is a spurious effect. We believe that the transformation from the amorphous state to the rutile structure affects significantly the shape of the fundamental absorption edge, which accounts for the change in the power dependence from (ahv) ‘1’ to (ahvj2. The energy of the optical transition does not undergo such a dramatic change.
4. Conclusions (i) Plasma-emission-controlled DC magnetron and conventional RF reactive sputtering were used to deposit nonstoichiometric TiO,_, thin films where x varied from 0.08 to -0.2. (ii> All as-sputtered films were amorphous independently of X. The well-developed rutile structure was detected after annealing at 1173 K. (iii) The electrical conductivity u, the refractive index n, and the absorption coefficient (Y were found to be correlated with the deviation from stoichiometry X. (iv> The optical transmission spectra were found to be strongly modified by post-deposition annealing in both limits of x. The oxidation process is definitely marked in the case of initially substoichiometric films (X = 0.08), while for the overoxidized samples (X = - 0.2) the structural modification is responsible for drastic changes in the shape of the fundamental absorption edge. (v) The refractive index increases up to 2.39 with annealing temperature. (vi) The predominant direct allowed optical transitions at the energy of 3.44-3.51 eV were observed for well-developed rutile samples.
Acknowledgements We would like to express our deepest thanks for Prof. M. May from the Laboratoire de Physique des Solides, Universite P. et M. Curie, Paris, where the absorption spectroscopy studies were carried out. We are also grateful to Prof. F. Demichelis from Dipartimento di Fisica, Politec-
233
x
nice di Torino, where the measurements were made.
spectrophotometric
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electrical and optical properties
C. Gout and C. Jouanin, Phys. Rev. B IS (1977) 3229. [2X] J. Pascual, J. Camassel and H. Mathieu, Phys. Rev. B I8 (1978) 5606. [291 J.L. Jourdan, C. Gout and J.P. Albert, Solid State Commun. 31 (1070) 1023.
of TiOz
I
[30] J.W. Halley, M. Kozlowski. M. Michalewicz, W. Smyrl and N. Tit, Surf. Sci. 256 (1991) 397. [31] M. Takeuchi, Phys. Status Solidi (a) 5.5 (1979) 653. [32] A. Shibata, Jpn. J. Appl. Phys. 30 (1991) L650.