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Effects of Nb on the photo- and thermal sensing characteristics of Sr0.98La0.02TiO3 thin-film resistor
Sensors and Actuators A 116 (2004) 178–182
Effects of Nb on the photo- and thermal sensing characteristics of Sr0.98La0.02 TiO3 thin-film resistor Y.R. Liu a,1 , P.T. Lai b,∗ , G.Q. Li a , B. Li a , M.Q. Huang a , J. Luo a b
a Department of Applied Physics, South China University of Technology, Guangzhou, China Department of Electrical and Electronic Engineering, University of Hong Kong, Pokfulam Rd., Hong Kong, China
Received 3 September 2003; received in revised form 8 March 2004; accepted 11 March 2004 Available online 4 May 2004
Abstract Nb-doped Sr0.98 La0.02 TiO3 thin-film resistor is fabricated on a SiO2 /Si substrate by argon ion-beam sputtering technique. Photo- and thermal sensing characteristics of the thin-film resistor are investigated, and the results show that besides superior sensitivity to visible light, the thin-film resistor has good thermal sensitivity with negative temperature coefficient from room temperature to 200 ◦ C, and a maximum value of −6.0%◦ C−1 at 30 ◦ C. With increasing Nb concentration, the photoconductive gain of the thin-film resistor increases, while the temperature coefficient firstly decreases, and then increases. It is proposed that the effects of Nb on thermal sensitivity should be closely related to the grain-boundary defects and impurities of the film, while the effects of Nb on photo-sensing should be associated with the acceptor defects in the grain boundaries. © 2004 Elsevier B.V. All rights reserved. Keywords: Nb-doped Sr1−x Lax TiO3 thin film; Photosensitivity; Thermal sensitivity
1. Introduction Since the positive temperature coefficient of resistance (PTCR) characteristics of semiconducting BaTiO3 materials was first observed in 1955, a lot of research has been performed to improve their thermal sensitivity and understand their conduction mechanisms [1–4]. Furthermore, some composite systems such as (Ba, Sr) TiO3 and (Ba, Pb) TiO3 ceramics have been studied to broaden the application of BaTiO3 -based thermistors for wider temperature range [5,6]. However, with the development of automatic control techniques, miniaturization of sensors and integration of sensing elements have become important issues, and their realizations require the investigation and exploitation of thin-film sensors on silicon substrate. Recently, SrTiO3 -based and BaTiO3 -based thin-film sensors have received much attention because of their multisensing properties, such as humidity, thermal and photo sensitivities [7]. Moreover, the fabrication of these thin-film
∗ Corresponding author. Tel.: +86-852-2859-2691; fax: +86-852-2559-8738. E-mail addresses: [email protected] (Y.R. Liu), [email protected] (P.T. Lai). 1 Tel.: 86-20-87110449.
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.03.031
devices is easily compatible with existing integrated-circuit (IC) processing. Therefore, they could be applied in a wide field covering temperature control, humidity control and light monitor. So far, it has been extensively reported that the PTCR effect of BaTiO3 ceramics can be improved by addition of some metal elements [8–10]. However, the influence of dopant on the negative temperature coefficient (NTC) effect and photoelectric properties of SrTiO3 -based ceramic film is rarely reported. Based on the past works on La-doped SrTiO3 and Nb-doped SrTiO3 [11,12], this study looks at the combined effects of La and Nb on the photo- and thermal sensing characteristics of the SrTiO3 thin-film ceramics. Sr0.98 La0.02 TiO3 thin films with different Nb-doping concentrations deposited on a SiO2 /Si substrate by the argon ion-beam sputtering technique are used to fabricate thin-film resistors by standard IC technology. The optical absorption spectrum of the film is measured and the bandgap of the film is extracted. Resistance–temperature characteristics of the resistors are studied. The carrier lifetime of the Nb-doped films is calculated by measuring the dependence of photocurrent of the Nb-doped films on the modulation frequency of a light source. Lastly, the influences of Nb impurities on the resistance-temperature characteristics and photoelectrical properties of the samples are investigated.
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2. Experimental N/n+ type (111) silicon epitaxial wafer with a 0.3– 0.6 cm resistivity was used as the substrate material for fabrication of the devices. A SiO2 layer with a thickness of 25 nm was grown by thermal oxidation at 900 ◦ C for 25 min in dry oxygen. Nb-doped Sr0.98 La0.02 TiO3 thin film was deposited on the oxide layer by an argon ion-beam sputtering equipment at room temperature under a vacuum of 1.33 mPa, and then the wafer was annealed at 400 ◦ C for 20 min in oxygen. High-purity aluminum was evaporated on the thin film to form electrodes, and post-metallization annealing was done at 400 ◦ C for 15 min in nitrogen to make better contacts to the sensors. Finally, dice of the wafer were attached to headers for subsequent testings. The sputtering targets chosen were three sintered-type semiconducting SrLaTiO3 ceramics with donor dopant of 1.0, 2.5 and 5.0 mol.% Nb2 O5 , respectively. During the sintering process, trivalent La3+ ion and pentavalent Nb5+ ion were substitutionally incorporated at the Sr and Ti sites in the SrTiO3 lattice respectively as donor dopants [13,14], resulting in an n-type semiconductor. X-ray diffraction spectrum showed that the deposited films had polycrystalline perovskite structure. The optical absorption spectra of the films were measured by an ultraviolet-visible spectrophotometer. Device current versus light intensity was characterized by a DC power supply, while device current versus modulation frequency was measured under an illumination intensity of 200 lux by a modulated pulse light source system. Device resistance versus temperature characteristics was measured from room temperature up to 200 ◦ C with a computer-controlled oven. The schematic experimental set-up is shown in Fig. 1.
Fig. 1. The schematic experimental set-up: (a) for measurement of photosensitivity characteristics; (b) for measurement of resistance–temperature characteristics.
Fig. 2. Absorption spectrum of Nb-doped Sr0.98 La0.02 TiO3 film with different Nb concentrations (light-source energy is the same for various wavelengths).
3. Results and discussions The absorption spectrum of the film in Fig. 2 shows that the intrinsic absorption peak is at a wavelength of 320 nm and the long-wave cut-off is at 410 nm. Hence, the bandgap calculated from the latter is 3.05 eV, which is smaller than that of bulk SrTiO3 (3.25 eV) [15]. The intrinsic absorption cut-off shifts towards the long-wave region, possibly due to excessive donor addition, which causes the Fermi level rising, and even going into the conduction band. At the same time, since the Ti–O interaction is disturbed by the (La, Nb) impurity atoms, the edges of the valence band and conduction band would be distorted, resulting in a reduction of the bandgap. The detailed formation mechanism of the absorption band is not yet well understood at present. But the absorption in the long-wave region should be due to the formation of hole polarons bound at the charge compensators, because incorporation of Nb5+ in the film creates some charge compensators [16]. Thus, as the Nb concentration is increased, the absorption intensity increases. Fig. 3 shows the current of the Nb-doped Sr0.98 La0.02 TiO3 film resistor versus visible-light intensity at room temperature. It can be seen that the dark current of the samples firstly increases, and then reduces with increasing Nb concentration. The samples are highly sensitive to the light intensity and present good linearity for photoconductor applications. For a test voltage Vt of 4 V, when the light intensity increases from 0 to 1000 lux, the current increases by 775, 790 and 1825% for Nb concentration of 1.0, 2.5 and 5.0 mol.%, respectively. Moreover, further measurements prove that this photosensitive property is basically unchanged between 15 and 30 ◦ C (i.e., the variation of the current with temperature is very small). Therefore, the device can be used as a light sensor under normal temperature range.
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Fig. 3. Photocurrent of the thin-film resistors vs. visible-light intensity at room temperature (applied voltage = 4 V).
Fig. 5. Resistance–temperature curve of Nb-doped Sr0.98 La0.02 TiO3 thinfilm resistors with different Nb concentrations (applied voltage = 1 V).
In order to estimate the carrier lifetime of the Nb-doped films, the effect of modulation frequency of light illumination on the photocurrent of the Nb-doped films is investigated. The experimental results are shown in Fig. 4. Normally, for light-sensitive resistor, the photocurrent reduces with increasing modulation frequency, and is given by
above equation, when f is equal to f0 , |I (f)| = 0.707|I (0)|. Hence, the carrier lifetime of Nb-doped films can be calculated from f0 to be 11, 20 and 27 ms for Nb concentration of 1.0, 2.5 and 5.0 mol.%, respectively. The results indicate that the photoconductive gain and response time of Nb-doped film increase with increasing Nb concentration. This may be explained as follows. When the Nb5+ ions are incorporated, there are vacancies created at the Sr sites in the film, forming acceptor defects which are distributed along the grain boundaries, and increase with increasing Nb concentration. These acceptor defects can effectively trap the holes generated by photo-excitation, and hence increase the carrier lifetime and the photoconductive gain. Fig. 5 gives the resistance–temperature (R–T) characteristics of the samples with different Nb concentrations. It can be seen that these thin-film resistors exhibit an NTC effect. The resistance slightly decreases in the low-temperature range, but sharply decreases in the high-temperature range. The room-temperature resistance of the thin-film resistor firstly decreases, and then increases with increasing Nb concentration. Normally, the parameters used to characterize NTC materials are the material constant (B) and the temperature coefficient (α), which are given as follow:
I(f) = I (0) f0 =
1 1 + (f/f0 )j
(1)
1 2πτ
where I (0) is the photocurrent under steady light source, f the modulation frequency of light source, f0 the cut-off frequency, and τ the carrier lifetime. According to the
B=
ln(R1 ) − ln(R2 ) (1/T1 ) − (1/T2 )
α=−
Fig. 4. Normalized photocurrent vs. modulation frequency of incident light under an illumination intensity of 200 lux (applied voltage = 5 V).
B T2
(2) (3)
where R1 and R2 represent the resistances of the material at temperatures T1 and T2 , respectively, and B is related to the activation energy EA of conduction as B = EA /2k, with k the Boltzmann’s constant. B is determined by the chemical composition and microstructure of the film, and can be regarded as a constant within the ordinary temperature
Y.R. Liu et al. / Sensors and Actuators A 116 (2004) 178–182 Table 1 Material constant (B) and temperature coefficient (α) of the thin-film resistors with different Nb concentrations. Nb concentration (mol.%)
B (K)
α at 30 ◦ C (%◦ C−1 )
0 1.0 2.5 5.0
1565 5495 5033 5327
−1.7 −6.0 −5.5 −5.8
range. According to Eq. (2), the slope of the curve in Fig. 5 is equal to the material constant. Using linear fit method, B of the Nb-doped films can be extracted from these curves, and then from Eq. (3), α of the films can be calculated. The results are summarized in Table 1, with the temperature coefficients of the Nb-doped films much higher than that of undoped Sr1−x Lax TiO3 film under the same conditions [7]. Normally, (La, Nb) co-doped SrTiO3 ceramic films are n-type semiconductor, and La3+ and Nb5+ ions can substitute Sr and Ti in the SrTiO3 perovskite structure to form defects La· Sr and Nb· Ti . This process causes net charges in the crystal lattice, and so Ti4+ would accept the residual charges and form Ti4+ ·e to keep charge neutrality in the SrTiO3 film. The trapping of the residual electrons to the Ti4+ ions is weak, and hence these electrons are highly mobile [17]. The electrical conduction of perovskite materials significantly depends on the leaping of electrons among the Ti4+ ions under an electric field. Therefore, the crystalline structure and defects in the SrTiO3 ceramic films would significantly affect the trapping of the electrons to the Ti4+ ions, and hence change the electrical properties of the SrTiO3 ceramic films. According to the above views, it is proposed that when Nb concentration is low, there is little vacancy compensation of Sr2+ ions in the film, and the electrical conduction in the film becomes stronger with increasing Nb concentration. But when Nb concentration increases beyond a critical value, there is high vacancy compensation of Sr2+ ions in the film, forming acceptor defects (V"Sr ) which weaken the electrical conduction. Therefore, the room-temperature resistance of the sample with 2.5 mol.% Nb is smaller than those of the samples with 1.0 and 5.0 mol.% Nb (Fig. 5). On the other hand, the NTC effect of the film resistors firstly decreases, and then becomes strong with increasing Nb concentration. This may be explained by the fact that donor defects increase in the grain boundary layer with increasing Nb concentration in the low-concentration region, resulting in a smaller barrier height. On the other hand, acceptor defects increase in the grain boundary layer with increasing Nb concentration in the high-concentration range and affect the trapping of the electrons to the neighboring Ti4+ , thus enhancing the barrier height. In addition, acceptor defects in the grain boundaries can trap the holes generated by photoexcitation near the grain boundaries, and hence decrease the barrier height, resulting in an increase of photocurrent (Fig. 3).
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The thermal sensitivity characteristics of the Nb-doped Sr1−x Lax TiO3 film resistor is superior to that of Sr1−x Lax TiO3 film resistor, because of the volume effect of the unit cell in the film. The enlargement of the unit-cell volume causes an increase of activation energy of the film [18]. In the case of Sr1−x Lax TiO3 , the radius of La cations (rLa 3+ = 0.116 nm) is smaller than that of Sr cations (rSr 2+ = 0.126 nm). The substitution of Sr by La in the film reduces the lattice constant and the cell volume. In the case of Nb-doped Sr1−x Lax TiO3 , the radius of Nb cations (rNb 5+ = 0.069 nm) is larger than that of Ti cations (rTi 4+ = 0.064 nm). The substitution of Ti by Nb in the film enlarges the lattice constant and the cell volume. Therefore, the activation energy of the Nb-doped Sr1−x Lax TiO3 film is higher than that of the undoped Sr1−x Lax TiO3 film, resulting in superior thermal sensing characteristics.
4. Conclusions Nb-doped Sr0.98 La0.02 TiO3 thin film is deposited on a SiO2 /Si substrate by argon ion-beam sputtering technique. Resistors made from the film display superior photo- and thermal sensing characteristics. Due to the incorporation of Nb in the material, the thin-film resistor exhibits a much larger negative temperature coefficient as compared to undoped Sr1−x Lax TiO3 film, with a maximum value of −6.0%◦ C−1 within the measurement range. The effects of the Nb impurities on the photo- and thermal sensitivity characteristics of the resistors are analyzed, and the amount of Nb is found to be critical in optimizing the photo- and thermal sensing characteristics of the film.
Acknowledgements This work was partially supported by a RGC Research Grant, Hong Kong; CRCG Research Grant, The University of Hong Kong; and Sci-Tech Committee Grant of Guangdong, China.
References [1] W. Heywang, Resistivity anomaly in doped BaTiO3 , J. Am. Ceram. Soc. 47 (1964) 484–490. [2] G.V. Lewis, C.R.A. Catlow, R.E.W. Casselton, PTCR effect in BaTiO3 , J. Am. Ceram. Soc. 68 (1985) 555–558. [3] D. Nagano, M. Sugiura, N. Wakiya, Conduction mechanism of La-, Nb-doped BaTiO3 thin films by doping MOCVD, Key. Eng. Mat. 216 (2002) 87–90. [4] S. Saha, S.B. Krupanidhi, Study of electrical properties of pulsed laser ablated (Ba0.7 Sr0.3 )TiO3 thin films, Mater. Sci. Eng. B57 (1999) 135–146.
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[5] J.K. Lee, J.S. Park, K.S. Hong, Role of liquid phase in PTCR characteristics of (Ba0.7 Sr0.3 )TiO3 ceramics, J. Am. Ceram. Soc. 85 (5) (2002) 1173–1179. [6] J.G. Kim, W.S. Cho, K.S. Park, Effect of atmosphere on the PTCR characteristics of porous (Ba0.7 Sr0.3 )TiO3 ceramics, Mater. Sci. Eng. B83 (2001) 123–129. [7] B. Li, P.T. Lai, G.Q. Li, S.H. Zeng, M.Q. Huang, Photoelectrical properties of Ba1−x Lax TiO3 thin-film resistor, Sens. Actuators A 86 (2000) 226–230. [8] S. Jidda, T. Miki, Electron paramagnetic resonance of Nb-doped BaTiO3 ceramics with positive temperature coefficient of resistivity, J. Appl. Phys. 80 (1996) 5234–5239. [9] A. Fujimoto, S. Jida, An evidence of trap activation for positive temperature coefficient of resistivity in BaTiO3 ceramics with substitutional Nb and Mn as impurities, J. Appl. Phys. 83 (1998) 1592– 1603. [10] J.C. Zhao, L.T. Li, Z.L. Gui, A study on the properties of (Y, Mn) co-doped (Ba0.7 Pb0.3 )TiO3 thermistors, Mater. Sci. Eng. B94 (2002) 202–206. [11] G.Q. Li, P.T. Lai, S.H. Zeng, M.Q. Huang, Y.C. Cheng, Photo-, thermal and humidity sensitivity characteristics of Sr1−x Lax TiO3 film on SiO2 /Si substrate, Sens. Actuators A 63 (1997) 223–226. [12] G.Q. Li, P.T. Lai, S.H. Zeng, M.Q. Huang, B. Li, A new thin-film humidity and thermal micro-sensor with Al/SrNbx Ti1−x O3 /SiO2 /Si structure, Sens. Actuators A 75 (1999) 70–74. [13] B.C. Lacoure, V.R.W. Amarakon, Characterization of the firing schedule for positive temperature coefficient of resistance BaTiO3 , J. Am. Ceram. Soc. 78 (12) (1995) 3352–3356. [14] I.C. Ho, Semiconducting barium titanate ceramics prepared by boron-containing liquid-phase sintering, J. Am. Ceram. Soc. 77 (3) (1994) 829–832. [15] A. Kahn, A. Layendecker, Electronic energy bands in strontium titanate, Phys. Rev. A 135 (1964) 11321–11325. [16] D. Millers, L. Grigorjeva, V. Pankratov, V.A. Trepakov, S.E. Kapphan, Pulsed electron beam excited transient absorption in SrTiO3 , Nucl. Instr. Meth. Phys. Res. B 194 (2002) 469–473. [17] C. Lee, I.-N. Lin, C.-T. Hu, Evolution of microstructure and V-shaped positive temperature coefficient of resistivity of (Pb0.6 Sr0.4 )TiO3 materials, J. Am. Ceram. Soc. 77 (5) (1994) 1340–1344. [18] L. Zhuang, W.L. Zhong, Y.P. Peng, A correlation on between the ferroelectric phase transition and the cell volume in barium strontium titanate, Chin. Phys. Soc. 49 (7) (2000) 1371–1376.
Biographies Y.R. Liu was born in Jiangxi, China, in 1968. He received the MSc in condensed physics from the University of Sichuan, Chengdu, China, in 1996. He is a lecturer at the South China University of Technology. His current research interests are semiconductor integrated sensor, thin-film ceramic sensor and solar cell. P.T. Lai got his PhD from the University of Hong Kong and worked as a post-doctoral fellow at the University of Toronto on advanced poly-emitter bipolar process with emphasis on self-alignment and trench isolation. He is now an Associate Professor at the University of Hong Kong. Current interests are on thin dielectrics for VLSI devices, micro-sensors and high-power devices. G.Q. Li was born in Guangdong, China, in 1940. He graduated from the South China University of Technology, Guangzhou, China, in 1965. He is a Professor at the South China University of Technology. His current research interests are semiconductor integrated sensor and thin-film ceramic sensor. B. Li was born in Beijing, China, in 1967. She received the BSc and MSc degrees in microelectronics from the South China University of Technology, Guangzhou, China, in 1989 and 1992, respectively, and the PhD degree in microelectronics from the University of Hong Kong, Hong Kong, in 2001. She is an Associate Professor at the South China University of Technology. Her current research interests are semiconductor integrated sensor and thin-film ceramic sensor. M.Q. Huang was born in Guangdong, China, in 1946. He graduated from the South China University of Technology, Guangzhou, China, in 1969. He is an Associate Professor at the South China University of Technology. His current research interests are semiconductor device and physics. J. Luo was born in Guangdong, China, in 1979. He received the BSc degrees in electronics from South China Normal University, Guangzhou, China, in 2001. He is a technician at the South China University of Technology. His current research interests are fabrication of thin-film ceramic sensor and IC design.
Effects of Nb on the photo- and thermal sensing characteristics of Sr0.98La0.02 TiO3 thin-film resistor Y.R. Liu a,1 , P.T. Lai b,∗ , G.Q. Li a , B. Li a , M.Q. Huang a , J. Luo a b
a Department of Applied Physics, South China University of Technology, Guangzhou, China Department of Electrical and Electronic Engineering, University of Hong Kong, Pokfulam Rd., Hong Kong, China
Received 3 September 2003; received in revised form 8 March 2004; accepted 11 March 2004 Available online 4 May 2004
Abstract Nb-doped Sr0.98 La0.02 TiO3 thin-film resistor is fabricated on a SiO2 /Si substrate by argon ion-beam sputtering technique. Photo- and thermal sensing characteristics of the thin-film resistor are investigated, and the results show that besides superior sensitivity to visible light, the thin-film resistor has good thermal sensitivity with negative temperature coefficient from room temperature to 200 ◦ C, and a maximum value of −6.0%◦ C−1 at 30 ◦ C. With increasing Nb concentration, the photoconductive gain of the thin-film resistor increases, while the temperature coefficient firstly decreases, and then increases. It is proposed that the effects of Nb on thermal sensitivity should be closely related to the grain-boundary defects and impurities of the film, while the effects of Nb on photo-sensing should be associated with the acceptor defects in the grain boundaries. © 2004 Elsevier B.V. All rights reserved. Keywords: Nb-doped Sr1−x Lax TiO3 thin film; Photosensitivity; Thermal sensitivity
1. Introduction Since the positive temperature coefficient of resistance (PTCR) characteristics of semiconducting BaTiO3 materials was first observed in 1955, a lot of research has been performed to improve their thermal sensitivity and understand their conduction mechanisms [1–4]. Furthermore, some composite systems such as (Ba, Sr) TiO3 and (Ba, Pb) TiO3 ceramics have been studied to broaden the application of BaTiO3 -based thermistors for wider temperature range [5,6]. However, with the development of automatic control techniques, miniaturization of sensors and integration of sensing elements have become important issues, and their realizations require the investigation and exploitation of thin-film sensors on silicon substrate. Recently, SrTiO3 -based and BaTiO3 -based thin-film sensors have received much attention because of their multisensing properties, such as humidity, thermal and photo sensitivities [7]. Moreover, the fabrication of these thin-film
∗ Corresponding author. Tel.: +86-852-2859-2691; fax: +86-852-2559-8738. E-mail addresses: [email protected] (Y.R. Liu), [email protected] (P.T. Lai). 1 Tel.: 86-20-87110449.
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.03.031
devices is easily compatible with existing integrated-circuit (IC) processing. Therefore, they could be applied in a wide field covering temperature control, humidity control and light monitor. So far, it has been extensively reported that the PTCR effect of BaTiO3 ceramics can be improved by addition of some metal elements [8–10]. However, the influence of dopant on the negative temperature coefficient (NTC) effect and photoelectric properties of SrTiO3 -based ceramic film is rarely reported. Based on the past works on La-doped SrTiO3 and Nb-doped SrTiO3 [11,12], this study looks at the combined effects of La and Nb on the photo- and thermal sensing characteristics of the SrTiO3 thin-film ceramics. Sr0.98 La0.02 TiO3 thin films with different Nb-doping concentrations deposited on a SiO2 /Si substrate by the argon ion-beam sputtering technique are used to fabricate thin-film resistors by standard IC technology. The optical absorption spectrum of the film is measured and the bandgap of the film is extracted. Resistance–temperature characteristics of the resistors are studied. The carrier lifetime of the Nb-doped films is calculated by measuring the dependence of photocurrent of the Nb-doped films on the modulation frequency of a light source. Lastly, the influences of Nb impurities on the resistance-temperature characteristics and photoelectrical properties of the samples are investigated.
Y.R. Liu et al. / Sensors and Actuators A 116 (2004) 178–182
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2. Experimental N/n+ type (111) silicon epitaxial wafer with a 0.3– 0.6 cm resistivity was used as the substrate material for fabrication of the devices. A SiO2 layer with a thickness of 25 nm was grown by thermal oxidation at 900 ◦ C for 25 min in dry oxygen. Nb-doped Sr0.98 La0.02 TiO3 thin film was deposited on the oxide layer by an argon ion-beam sputtering equipment at room temperature under a vacuum of 1.33 mPa, and then the wafer was annealed at 400 ◦ C for 20 min in oxygen. High-purity aluminum was evaporated on the thin film to form electrodes, and post-metallization annealing was done at 400 ◦ C for 15 min in nitrogen to make better contacts to the sensors. Finally, dice of the wafer were attached to headers for subsequent testings. The sputtering targets chosen were three sintered-type semiconducting SrLaTiO3 ceramics with donor dopant of 1.0, 2.5 and 5.0 mol.% Nb2 O5 , respectively. During the sintering process, trivalent La3+ ion and pentavalent Nb5+ ion were substitutionally incorporated at the Sr and Ti sites in the SrTiO3 lattice respectively as donor dopants [13,14], resulting in an n-type semiconductor. X-ray diffraction spectrum showed that the deposited films had polycrystalline perovskite structure. The optical absorption spectra of the films were measured by an ultraviolet-visible spectrophotometer. Device current versus light intensity was characterized by a DC power supply, while device current versus modulation frequency was measured under an illumination intensity of 200 lux by a modulated pulse light source system. Device resistance versus temperature characteristics was measured from room temperature up to 200 ◦ C with a computer-controlled oven. The schematic experimental set-up is shown in Fig. 1.
Fig. 1. The schematic experimental set-up: (a) for measurement of photosensitivity characteristics; (b) for measurement of resistance–temperature characteristics.
Fig. 2. Absorption spectrum of Nb-doped Sr0.98 La0.02 TiO3 film with different Nb concentrations (light-source energy is the same for various wavelengths).
3. Results and discussions The absorption spectrum of the film in Fig. 2 shows that the intrinsic absorption peak is at a wavelength of 320 nm and the long-wave cut-off is at 410 nm. Hence, the bandgap calculated from the latter is 3.05 eV, which is smaller than that of bulk SrTiO3 (3.25 eV) [15]. The intrinsic absorption cut-off shifts towards the long-wave region, possibly due to excessive donor addition, which causes the Fermi level rising, and even going into the conduction band. At the same time, since the Ti–O interaction is disturbed by the (La, Nb) impurity atoms, the edges of the valence band and conduction band would be distorted, resulting in a reduction of the bandgap. The detailed formation mechanism of the absorption band is not yet well understood at present. But the absorption in the long-wave region should be due to the formation of hole polarons bound at the charge compensators, because incorporation of Nb5+ in the film creates some charge compensators [16]. Thus, as the Nb concentration is increased, the absorption intensity increases. Fig. 3 shows the current of the Nb-doped Sr0.98 La0.02 TiO3 film resistor versus visible-light intensity at room temperature. It can be seen that the dark current of the samples firstly increases, and then reduces with increasing Nb concentration. The samples are highly sensitive to the light intensity and present good linearity for photoconductor applications. For a test voltage Vt of 4 V, when the light intensity increases from 0 to 1000 lux, the current increases by 775, 790 and 1825% for Nb concentration of 1.0, 2.5 and 5.0 mol.%, respectively. Moreover, further measurements prove that this photosensitive property is basically unchanged between 15 and 30 ◦ C (i.e., the variation of the current with temperature is very small). Therefore, the device can be used as a light sensor under normal temperature range.
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Fig. 3. Photocurrent of the thin-film resistors vs. visible-light intensity at room temperature (applied voltage = 4 V).
Fig. 5. Resistance–temperature curve of Nb-doped Sr0.98 La0.02 TiO3 thinfilm resistors with different Nb concentrations (applied voltage = 1 V).
In order to estimate the carrier lifetime of the Nb-doped films, the effect of modulation frequency of light illumination on the photocurrent of the Nb-doped films is investigated. The experimental results are shown in Fig. 4. Normally, for light-sensitive resistor, the photocurrent reduces with increasing modulation frequency, and is given by
above equation, when f is equal to f0 , |I (f)| = 0.707|I (0)|. Hence, the carrier lifetime of Nb-doped films can be calculated from f0 to be 11, 20 and 27 ms for Nb concentration of 1.0, 2.5 and 5.0 mol.%, respectively. The results indicate that the photoconductive gain and response time of Nb-doped film increase with increasing Nb concentration. This may be explained as follows. When the Nb5+ ions are incorporated, there are vacancies created at the Sr sites in the film, forming acceptor defects which are distributed along the grain boundaries, and increase with increasing Nb concentration. These acceptor defects can effectively trap the holes generated by photo-excitation, and hence increase the carrier lifetime and the photoconductive gain. Fig. 5 gives the resistance–temperature (R–T) characteristics of the samples with different Nb concentrations. It can be seen that these thin-film resistors exhibit an NTC effect. The resistance slightly decreases in the low-temperature range, but sharply decreases in the high-temperature range. The room-temperature resistance of the thin-film resistor firstly decreases, and then increases with increasing Nb concentration. Normally, the parameters used to characterize NTC materials are the material constant (B) and the temperature coefficient (α), which are given as follow:
I(f) = I (0) f0 =
1 1 + (f/f0 )j
(1)
1 2πτ
where I (0) is the photocurrent under steady light source, f the modulation frequency of light source, f0 the cut-off frequency, and τ the carrier lifetime. According to the
B=
ln(R1 ) − ln(R2 ) (1/T1 ) − (1/T2 )
α=−
Fig. 4. Normalized photocurrent vs. modulation frequency of incident light under an illumination intensity of 200 lux (applied voltage = 5 V).
B T2
(2) (3)
where R1 and R2 represent the resistances of the material at temperatures T1 and T2 , respectively, and B is related to the activation energy EA of conduction as B = EA /2k, with k the Boltzmann’s constant. B is determined by the chemical composition and microstructure of the film, and can be regarded as a constant within the ordinary temperature
Y.R. Liu et al. / Sensors and Actuators A 116 (2004) 178–182 Table 1 Material constant (B) and temperature coefficient (α) of the thin-film resistors with different Nb concentrations. Nb concentration (mol.%)
B (K)
α at 30 ◦ C (%◦ C−1 )
0 1.0 2.5 5.0
1565 5495 5033 5327
−1.7 −6.0 −5.5 −5.8
range. According to Eq. (2), the slope of the curve in Fig. 5 is equal to the material constant. Using linear fit method, B of the Nb-doped films can be extracted from these curves, and then from Eq. (3), α of the films can be calculated. The results are summarized in Table 1, with the temperature coefficients of the Nb-doped films much higher than that of undoped Sr1−x Lax TiO3 film under the same conditions [7]. Normally, (La, Nb) co-doped SrTiO3 ceramic films are n-type semiconductor, and La3+ and Nb5+ ions can substitute Sr and Ti in the SrTiO3 perovskite structure to form defects La· Sr and Nb· Ti . This process causes net charges in the crystal lattice, and so Ti4+ would accept the residual charges and form Ti4+ ·e to keep charge neutrality in the SrTiO3 film. The trapping of the residual electrons to the Ti4+ ions is weak, and hence these electrons are highly mobile [17]. The electrical conduction of perovskite materials significantly depends on the leaping of electrons among the Ti4+ ions under an electric field. Therefore, the crystalline structure and defects in the SrTiO3 ceramic films would significantly affect the trapping of the electrons to the Ti4+ ions, and hence change the electrical properties of the SrTiO3 ceramic films. According to the above views, it is proposed that when Nb concentration is low, there is little vacancy compensation of Sr2+ ions in the film, and the electrical conduction in the film becomes stronger with increasing Nb concentration. But when Nb concentration increases beyond a critical value, there is high vacancy compensation of Sr2+ ions in the film, forming acceptor defects (V"Sr ) which weaken the electrical conduction. Therefore, the room-temperature resistance of the sample with 2.5 mol.% Nb is smaller than those of the samples with 1.0 and 5.0 mol.% Nb (Fig. 5). On the other hand, the NTC effect of the film resistors firstly decreases, and then becomes strong with increasing Nb concentration. This may be explained by the fact that donor defects increase in the grain boundary layer with increasing Nb concentration in the low-concentration region, resulting in a smaller barrier height. On the other hand, acceptor defects increase in the grain boundary layer with increasing Nb concentration in the high-concentration range and affect the trapping of the electrons to the neighboring Ti4+ , thus enhancing the barrier height. In addition, acceptor defects in the grain boundaries can trap the holes generated by photoexcitation near the grain boundaries, and hence decrease the barrier height, resulting in an increase of photocurrent (Fig. 3).
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The thermal sensitivity characteristics of the Nb-doped Sr1−x Lax TiO3 film resistor is superior to that of Sr1−x Lax TiO3 film resistor, because of the volume effect of the unit cell in the film. The enlargement of the unit-cell volume causes an increase of activation energy of the film [18]. In the case of Sr1−x Lax TiO3 , the radius of La cations (rLa 3+ = 0.116 nm) is smaller than that of Sr cations (rSr 2+ = 0.126 nm). The substitution of Sr by La in the film reduces the lattice constant and the cell volume. In the case of Nb-doped Sr1−x Lax TiO3 , the radius of Nb cations (rNb 5+ = 0.069 nm) is larger than that of Ti cations (rTi 4+ = 0.064 nm). The substitution of Ti by Nb in the film enlarges the lattice constant and the cell volume. Therefore, the activation energy of the Nb-doped Sr1−x Lax TiO3 film is higher than that of the undoped Sr1−x Lax TiO3 film, resulting in superior thermal sensing characteristics.
4. Conclusions Nb-doped Sr0.98 La0.02 TiO3 thin film is deposited on a SiO2 /Si substrate by argon ion-beam sputtering technique. Resistors made from the film display superior photo- and thermal sensing characteristics. Due to the incorporation of Nb in the material, the thin-film resistor exhibits a much larger negative temperature coefficient as compared to undoped Sr1−x Lax TiO3 film, with a maximum value of −6.0%◦ C−1 within the measurement range. The effects of the Nb impurities on the photo- and thermal sensitivity characteristics of the resistors are analyzed, and the amount of Nb is found to be critical in optimizing the photo- and thermal sensing characteristics of the film.
Acknowledgements This work was partially supported by a RGC Research Grant, Hong Kong; CRCG Research Grant, The University of Hong Kong; and Sci-Tech Committee Grant of Guangdong, China.
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Biographies Y.R. Liu was born in Jiangxi, China, in 1968. He received the MSc in condensed physics from the University of Sichuan, Chengdu, China, in 1996. He is a lecturer at the South China University of Technology. His current research interests are semiconductor integrated sensor, thin-film ceramic sensor and solar cell. P.T. Lai got his PhD from the University of Hong Kong and worked as a post-doctoral fellow at the University of Toronto on advanced poly-emitter bipolar process with emphasis on self-alignment and trench isolation. He is now an Associate Professor at the University of Hong Kong. Current interests are on thin dielectrics for VLSI devices, micro-sensors and high-power devices. G.Q. Li was born in Guangdong, China, in 1940. He graduated from the South China University of Technology, Guangzhou, China, in 1965. He is a Professor at the South China University of Technology. His current research interests are semiconductor integrated sensor and thin-film ceramic sensor. B. Li was born in Beijing, China, in 1967. She received the BSc and MSc degrees in microelectronics from the South China University of Technology, Guangzhou, China, in 1989 and 1992, respectively, and the PhD degree in microelectronics from the University of Hong Kong, Hong Kong, in 2001. She is an Associate Professor at the South China University of Technology. Her current research interests are semiconductor integrated sensor and thin-film ceramic sensor. M.Q. Huang was born in Guangdong, China, in 1946. He graduated from the South China University of Technology, Guangzhou, China, in 1969. He is an Associate Professor at the South China University of Technology. His current research interests are semiconductor device and physics. J. Luo was born in Guangdong, China, in 1979. He received the BSc degrees in electronics from South China Normal University, Guangzhou, China, in 2001. He is a technician at the South China University of Technology. His current research interests are fabrication of thin-film ceramic sensor and IC design.