3000 rev. min -l, heat treatment temperature 600°C for 20min): 1, chamber cover open; 2, ..... 11 B. Morris Henry, U.S. Patent 4,200,474, 1978. 12 M.R. Kozlowski ...
Thin Solid Films, 207 (1992) 180-184
180
Sol-gel Ti02 films on silicon substrates K. A. Vorotilov, E. V. Orlova and V. I. Petrovsky MoscowInstitute of Radioengineering, Electronics and Automation,
Vernadsky Prosp. 78, Moscow (U.S.S.R.)
(Received November 5, 1990; accepted July 1, 1991)
Abstract Titanium dioxide (TiO,) thin films have been prepared on silicon substrates by the sol-gel method. The properties of the resultant films are process dependent. The effects of solution content (type of titanium alkoxide, type of solvent,
equivalent oxide concentration, [H,O]/[Ti(OR),] ratio), gas moisture during deposition and heat treatment temperature on the film properties are discussed.The X-ray diffraction spectra, optical and electrical properties and TiO,-Si interface quality have been studied.
1. Introduction
Titanium dioxide (TiO,) has many interesting physical properties which make it suitable for thin film applications. Because of their good transmittance in the visible region, high refractive index and chemical stability, TiO, films have found wide application for various optical coatings [IA]. The high dielectric constant E of TiO, (the largest E among simple metal oxides) opens prospects for the use of TiO, thin films in microelectronic devices, e.g. in capacitors or as a gate dielectric in metal-dielectric-semiconductor devices [5,6]. During the last decade TiO, films have been suggested as photoanodes in the process of photoelectrolysis of water in solar energy conversion systems [7-91 and as electrochromic materials for display devices [9, 101. TiO, films can be obtained by a large variety of methods: thermal [l l] or anodic [12] oxidation of titanium, electron beam evaporation [ 131,ion sputtering [ 141, chemical vapour deposition [5, 151, including plasma-enhanced chemical vapour deposition [ 161, and the sol-gel method [I, 3,4,6-lo]. The last method was first used for application of optical coatings more than 50 years ago, but in the last decade it has attracted much attention owing to the intensive development of sol-gel technology [17]. In comparison with other oxide film technologies the sol-gel process has certain advantages: (i) it gives wide possibility to vary the film properties by changing the composition of the solution (change in film microstructure, introduction of dopants, etc.); (ii) low process cost, especially for large-scale substrates (e.g. TiO, antireflective coatings of
0040-6090/92/$5.00
photovoltaic solar cells prepared by the sol-gel method cost 2&40 times less than those obtained by vacuumcoating techniques [3]; (iii) while for thin films applied by vacuum techniques it is difficult to provide a stoichiometric ratio of the elements [13] and appropriate quality of the dielectric-semiconductor interface (as a result of bombardment of the substrate by high energy particles), the sol-gel method allows one to overcome these problems. This paper deals with the preparation of TiO, thin films on silicon substrates by the sol-gel method.
2. Preparation of sol In order to prepare TiO, films, solutions of doubly distilled titanium ethoxide (Ti(OC,H,),) or titanium butoxide (Ti(OC,H,),) in absolute alcohol (ethanol or butanol) with addition of a certain amount of water for hydrolysis and some HCl as catalyst were used. The contents of the components in solution were characterized by the equivalent oxide concentration by weight and [H,O]/[ Ti(OR),] and [HCl]/[Ti(OR),] ratios. For preparaton of the sol the required amount of solvent was divided into two equal parts; one part was used for dissolution of the titanium alkoxide and the other was mixed with the necessary amount of water and HCl. Both parts were stirred mechanically and then placed in an ultrasonic bath at room temperature for 12 h. Prior dilution of the metal alkoxide by alcohol prevents uncontrolled hydrolysis during mixing. For example, when Ti(OC,H,), was added to butanol-water mixture equivalent TiO, concentration, (6% F-WI/ [Ti(OC,H,),] = 2) the solution immediately became
0
1992 ~ Elsevier Sequoia.
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K. A. Vortilov et al. / Sol-gel TiO2films on silicon
turbid due to sedimentation of Ti(OH)4. Addition of butanol-water mixture to the solution of Ti(OC4Hg)4 in absolute butanol caused no precipitation and resulted in clear and stable solutions. The extreme susceptibility of Ti(OC2Hs) 4 to hydrolysis by atmospheric moisture necessitated preparation of its solutions in a flowing nitrogen dry-box. While the Ti(OC4Hg) 4 sols were stable and could be successfully used for up to a month, the Ti(OC2Hs)4 sols were much less stable and could be used for only 5 days. Different amounts of water were required for ethoxide and butoxide hydrolysis. Figure 1 shows the regions of ~___~,9__01 ,a
2 4 6 8 'fO t2 ~# ",q'O,zwt,% Fig. 1. Regions of stable sols for TiO 2 film preparation: 1, Ti(OC4Hg) , in butanol solution; 2, Ti(OC2Hs) 4 in ethanol solution.
stable Ti(OC2Hs)4 and Ti(OC4H9)4 sols. Clear and stable solutions were obtained if the equivalent oxide concentration and [H20]/[Ti(OR)4 ] values lay under the appropriate curve. In solutions obtained with higher water content, precipitation of Ti(OH)4 occurred. The catalyst content [HCI]/[Ti(OR)4 ] was 0.04. It was almost impossible to obtain stable solutions in the absence of the catalyst. For instance, it is impossible to prepare stable sols with more than 10Vo equivalent oxide content if no HCI is added. In contrast to Ti(OC2Hs) 4 sols, Ti(OC4H9) 4 sols with greater oxide content can be prepared (Fig. 1).
3. Effect of processing parameters on film quafity and thickness
The deposition of TiO 2 films on silicon substrates was carried out by the spin-on process (whirler speed 3000revmin -1) in a chamber with fixed nitrogen moisture supply [18]. The film thickness and refractive index were meassured by multiangle ellipsometry at 632.8 nm [19]. Figure 2 shows the TiO 2 film thickness as a function of the equivialent oxide concentration of the sol. The thickness TiO 2 films prepared from Ti(OC4H9) 4 in butanol was much less than that of SiO 2 films prepared from Si(OC2Hs) 4 in ethanol or butanol [18]. This is mainly due to different degrees of polycondensation of alkoxides. TiO 2 layers with a thickness of more than 0.1 lam crack during heat treatment (broken portion of
d,
/,m
/
181
t
O.i
/ /
0,05 t
I
I
i
i
z ~ 6 8 ~b iz 7 7 0 ~ , 7 . Fig. 2. TiO 2 film thickness as a function of equivalent oxide concentration in sol (Ti(OC4Hg) 4 in butanol solution, whirler speed 3000 rev. min - l , heat treatment temperature 600°C for 20min): 1, chamber cover open; 2, gas flow rate 451 h - 1.
curves in Fig. 2). The thickness of the films also depends on the gas exchange rate in the deposition chamber. In the case of limited gas exchange (gas flow rate 451 h - 1 ) the films were thinner than in the absence of this limitation (chamber cover open) as shown in Fig. 2 . It is obvious that in the first instance the rate of solvent evaporation during deposition is lower; therefore the solution viscosity rises more slowly and convective outflow ceases in a longer time span, giving thinner films [20]. The gas moisture during deposition had negligible influence on the film quality if the equivalent oxide concentration was in the range 2~o--6~o and the water content in solution was near the maximum value (see Fig. 1). However, the quality of films prepared from sols with higher TiO 2 content depended significantly on the gas moisture during deposition. For example, the films prepared from Ti(OC4Hg)4 in butanol sols (8~o equivalent TiO2 content) were turbid with lots of defects when the gas moisture was lower than 50~o, while higher values of gas moisture led to clear and uniform films.
4. Heat treatment of the films
Figures 3 and 4 present the film thickness and percentage change in thickness (shrinkage) respectively as a function of heat treatment temperature in nitrogen for 20min. The shrinkage of TiO 2 films was strongly d, Ym 0.~6 O.f'q O,IZ
0.~0 t~0s 0.~ 0.~ ~ ,
la~
~o
i
i
',oo
Fig. 3. (continued).
I
i
6'00' 's00 "r;,'c-
182
K. A. Vortilov et al. / Sol-gel TiO2films on silicon
refractive index of the films prepared by multiple application with annealing of each layer at 600 °C for 20 min increases as well with increasing film thickness as shown in Fig. 6. This is probably associated with the fact
d~
j~m 0.08
0.06
O.OL/ 0.0,?. I
I
I
;LoO
I
I
I
#o0
I
I
600
I
800
7", ~'C
(h)
2.7.
Fig. 3. TiO 2 film thickness as a function of heat treatment temperature (time 20 min). (a) Ti(OC4Hg) 4 in butanol solution; equivalent TiO 2 concentrations (l) 2~o, (2) 4~o, (3) 6~o, (4) 8~o and (5) 12%. (b) Titanium alkoxide (equivalent TiO 2 concentration 2~o) : (l, 3) Ti(OC4Hg) 4 ; (2) Ti(OC2Hs) 4. Solvent: (1) butanol; (2, 3) ethanol.
fOO
2.00
-~00
600
600 ~ °C
I
I
0.1 0.2. ~$
~d,~m
Fig. 6. TiO 2 film refractive index as a function of film thickness (Ti(OC4H9) 4 in butanol solution, equivalent TiO 2 concentration 6/0,o/ [H20]/[Ti(OC4Hg) J = 2, heat treatment temperature 600 °C for 20 min for each layer).
that there are larger crystallites in the thicker TiO 2 films owing to the longer heat treatment time for thicker films. X-ray diffraction patterns of the TiO 2 films are presented in Fig. 7. The films annealed at 200 °C are
Fig. 4. TiO 2 film shrinkage as a function of heat treatment temperature (time 20min). Ti(OC4H9) 4 in butanol solution; equivalent TiO 2 concentrations (1) 2% and (2, 3) 8%; [H2OI/[Ti(OC4Hg),d ratios (I) 4, (2) 2 and (3) 0.
R
A~
i, 3 ,
influenced by the oxide and water content in the sol. The film shrinkage increased with increasing TiO2 content (see Fig. 4, curves I and 2) and decreasing [H20]/[Ti(OR)4 ] ratio (see Fig. 4, curves 2 and 3). This is possibly due to the higher porosity of films prepared from sols with higher equivalent oxide concentration and lower water content. The shrinkage of TiO 2 films were greater than that of S i O 2 films [18] owing to the higher molecular weight of the aikyl group of the alkoxide as well as to the crystallization behaviour of TiO 2. Shrinkage in the films obtained from Ti(OC2Hs)4 was far less pronounced than in the films obtained from Ti(OC4H9)4 . The type of solvent is also important: films prepared from butanol were thinner than those prepared from ethanol solutions (Fig. 3) owing to the low evaporation rate of butanol. The refractive index of the TiO 2 films increases with increasing temperature of heat treatment owing to crystallization of the films as shown in Fig. 5. The rt
A
A __k___z
A
j___
I z
A I
I
I
?,0
2~"
ZO
I
I
~5 O, de~,ees
Fig. 7. X-ray diffraction patterns of TiO 2 films (CuK~t radiation). T = (l) 401), (2) 600 and (3) 800 °C. A, anatase; R, futile.
amorphous to X-rays (for X-ray analysis as well as for electrical measurements the films were prepared by fivefold application with annealing of each layer for 20 min). All the peaks in the samples heat treated at 400 and 600 °C can be attributed to the anatase phase (A). The diffraction pattern for the films annealed at 800°C contained peaks for both the anatase and rutile (R) phases.
5. Characteristics of AI-TiO2-Si structures
z.z 2.1 2,,0 t.g f.8 t.7 I
I
ZOO
i
~
z40C
I
I
600
f
r
~00
I
~ •C
Fig. 5. TiO 2 film refractive index as a function of heat treatment temperature (time 20 min).
The dependence of the dielectric constant of TiO 2 films (measurement frequency 200 kHz) on the heat treatment temperature is shown in Fig. 8. The increase in dielectric constant with increasing temperature of heat treatment is in agreement with the change occurring in the crystalline structure of the films. The samples heat treated at 850 °C had e = 130-150, which is approximately equal to the dielectric constant reported for single crystals of futile.
183
K. A. Vortilov et al. / Sol-gel TiO2films on sificon
00
/
5O I
I
I
400
g0
700
I
"T', "C
Fig. 8, TiO2 film dielectric constant as a function of heat treatment temperature.
The resistivity of the TiO 2 films was about 1012 ~ cm but showed a marked decrease on heat treatment at T > 700°C 0011 ~ c m for the samples heat treated at 850 °C). The dielectric breakdown of the TiO 2 films was (1-5) x 106 V c m - 1. Figure 9 shows the effect of the heat treatment temperature on the interface state density derived by the
features. In the strong inversion region the capacitance had greater value than the theoretical one. This may be due to the effect of extending the gate area as a result of the presence of OH groups and adsorbed water on the TiO 2 surface [22]. TiO 2 films prepared on n - S i ( l l l ) substrates at 4(X)-500 °C had injection-type hysteresis (clockwise), the value of which was minimum at a bake temperature of 550 °C. The structures prepared at higher temperatures had polarization-type hysteresis (counterclockwise), the value of which increased with increasing bake temperature and amounted to 1 0 - a - 2 x l 0 -7 C cm-2. The films on p-Si(100) substrates demonstrated no hysteresis with the exception of the films annealed at 850 °C (polarization-type hysteresis, 10- 7 C c m - z). Table 1 shows the fixed charge in the oxide ( Q F ) and the changes in this value after positive (Q~+) and negative (Q~) bias-temperature stresses (the capacitor with an applied field of positive or negative polarity was annealed at 150 °C for about 30 min). TABLE 1. Charge properties of AI TiO-2Si structures
1~$$ , F
fO~z
I
t
qoo
550
t
f
700
I
"7,*C
Substrate type
Heat treatment temperature (°C)
QF (C c m - 2)
Q~+ (C c m - 2)
n-Si(111) n-Si(111) n-Si(lll) n-Si(111) p-Si(100) p-Si(100) p-Si(100) p-Si(100)
400 550 700 850 400 550 700 850
6xl0 -s -5x10 -s - 8 x 1 0 - s - 2 x 1 0 -7 -lxl0 7 4xl0-a l x l 0 -6 l x l 0 -6 2x10 -s 4 x 1 0 -7 6 x l 0 -8 1×10 7 3 × 10 -a 5 × 10 -6 8 × 1 0 -7 5 × 1 0 -7
Q~, (C c m - 2) 6 × 1 0 -9 3 x 1 0 -7 _lxl0-S - 5 x 1 0 -7 - 3 x 1 0 -7 _7xl0-a - l × 10 -6 - 2 x l 0 -7
Fig. 9. Effect of heat treatment temperature on interface state density: Ntss, interfaced state density at middle of band gap; ~ , interface state density at fiat band.
Terman method [21] at the middle of the band gap (N~ss) and at the flat band (A~B) for TiO2 films deposited on nS i ( l l l ) substrates. The high value of interface state density for films prepared at 400 °C is caused by the high content of organic chemical residuals and OH groups [22]. With an increase in bake temperature up to 550 °C the organic and hydroxyl content is diminished and the interface state density decreases. Further rise in heat treatment temperature causes intensification of crystallization during which heavy reconstruction of atomic structure, crystal growth and grain boundary formation occur. The processes lead to breaking and weakening of atomic bonds at the interface and therefore to increased interface trap density. Thus the minimum values NIss = 7 x 10 l° cm-2 eV -1 and ~=6xl011 c m - 2 eV -1 were observed at a bake temperature of 550 °C. The high frequency capacitance-voltage ( C - V ) characteristics (200 kHz) had a number of distinguishing
6. Conclusions In contrast to titanium ethoxide sols, titanium butoxide sols are more stable and can be prepared with higher oxide content. In order to prepare high quality TiO2 films, the equivalent oxide concentration must be in the range 2 ~ 6 ~ o and the water content in the sol must be near a certain maximum value (but no Ti(OH)4 precipitate should appear). Under these conditions the gas moisture during deposition has negligible influence on the film quality. Films annealed at temperatures lower than 400°C are amorphous and those annealed at higher temperatures have a crystalline structure consisting of anatase (for films annealed at 400-600 °C) or anatase and futile (for films annealed at more than 600 °C). The structural change causes an increase in dielectric constant and refractive index of the TiO 2 films with increasing heat treatment temperature. These values are e = 20 and n = 2.0 for a bake temperature of 450 °C and e = 150 and
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K. A. Vortilov et al. / Sol-gel T i O 2 f i l m s on silicon
n = 2.2 for films annealed at 850 °C. The highest quality of TiO2-Si interface is shown by the structures annealed at 550 °C. In this case the interface state density at the middle of the band gap is N~ss = 7 x 101° cm -2 e V - 1 and hysteresis of the C - V characteristics is minimal or absent. At higher heat treatment temperatures crystallization of the films leads to inferior quality of the TiO2-Si interface (high interface state density and substantial hysteresis of the C - V characteristics).
Acknowledgments The authors are greatly thankful to E. P. Turevskaya, N. Ya. Turova and M. I. Yanovskaya for helpful assistance and discussions during this study. We thank Yu. A. Kus'min for technical assistance.
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