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Optical and electrical properties of SnO2:Sb thin films deposited by oblique angle deposition
Applied Surface Science 256 (2010) 1636–1640
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Optical and electrical properties of SnO2:Sb thin films deposited by oblique angle deposition Xiudi Xiao a,b,*, Guoping Dong a,b, Jianda Shao a, Hongbo He a, Zhengxiu Fan a a b
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. BOX 800-211, Shanghai 201800, PR China Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 May 2009 Received in revised form 23 September 2009 Accepted 23 September 2009 Available online 2 October 2009
The antimony doped tin oxide (SnO2:Sb) (ATO) thin films were prepared by oblique angle electron beam evaporation technique. X-ray diffraction, field emission scanning electron microscopy, UV–vis–NIR spectrophotometer and four-point probe resistor were employed to characterize the structure, morphology, optical and electrical properties. The results show that oblique angle deposition ATO thin films with tilted columns structure are anisotropic. The in-plane birefringence of optical anisotropy is up to 0.035 at a = 708, which means that it is suitable as wave plate and polarizer. The electrical anisotropy of sheet resistance shows that the sheet resistance parallel to the deposition plane is larger than that perpendicular to the deposition plane and it can be changed from 900 V/& to 3500 V/& for deposition angle from 408 to 858, which means that the sheet resistance can be effectively tuned by changing the deposition angle. Additionally, the sandwich structure of SiO2 buffer layer plus normal ATO films and oblique angle deposition ATO films can reduce the resistance, which can balance the optical and electrical anisotropy. It is suggested that oblique angle deposition ATO thin films can be used as transparent conductive thin films in solar cell, anti-foggy windows and multifunctional carrier in liquid crystal display. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Thin films Physical vapor deposition Anisotropy Optical spectroscopy
1. Introduction Transparent conductive thin films attract unceasing attention due to its extensive application in flat panel displays [1], solar cells [2], anti-foggy windows [3], etc. A variety of transparent conductive materials, such as In2O3:Sn (ITO), SnO2:Sb (ATO), SnO2:F (FTO), CdOx, CuS, ZnO:Al (AZO), etc. are investigated. Sndoped In2O3 (ITO) thin film, which is one of the most mature transparent conductive thin films, has been commercially used due to its low resistivity and high transparency [4]. However, ITO contains the rare and expensive indium element, thus it is necessary to develop new transparent conductive materials, which are composed of abundant and low-cost elements [5,6]. SnO2 is one of the traditional, low-cost and multifunctional materials, which can be used as gas sensors, oxide catalyst, and transparent conductor because of its oxygen vacancy and dual valence [7]. Recently, SnO2 matrix doped materials attract great attention for their potential application as one of the ITO substitutions. SnO2:Sb (ATO) is one of the popular transparent conductive materials. A
number of methods such as chemical vapor deposition [8], physical vapor deposition [9], spray pyrolysis [10] and sol–gel spin coating [11], have been used to prepare ATO thin films. Oblique angle deposition (OAD) is a convenient method to fabricate thin films with a variety of nanostructures and unique optical properties [12– 15]. Recently, transparent conductive thin film prepared by OAD technique is confirmed to improve the optical, electrical and light emitting properties of liquid crystal display (LCD) and light emitting diode (LED), which shows greatly potential application in photoelectrical field [16–18]. However, to our best knowledge, as one of important transparent conductive thin films, the research on ATO thin films prepared by OAD technique has not been reported. Hence, the systematical and fundamental research on the optical and electrical properties of OAD ATO thin films is significant and meaningful. In the present paper, we prepare the ATO thin films by OAD technique and investigate the optical and electrical properties of ATO thin films deposited at different deposition parameters. 2. Experimental 2.1. Preparation
* Corresponding author at: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. BOX 800-211, Shanghai 201800, PR China. Tel.: +86 21 69918476; fax: +86 21 69918476. E-mail address: [email protected] (X. Xiao). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.09.084
ATO thin films were prepared by oblique angle electron beam (EB) evaporation technique with base pressure of 1.7 103 Pa. The substrate tilted angle a was measured as the direction of the
X. Xiao et al. / Applied Surface Science 256 (2010) 1636–1640
incident flux with respect to the substrate normal. Two motors were used to control the tilted angle a and rotation speed of the substrate. BK7 glass (Ø 30 mm 3 mm) and n-Si (1 0 0) substrates were ultrasonically cleaned in acetone and ethanol before introduced into the vacuum system. The substrates were not heated during deposition. ATO bulk with 5 at% Sb2O3 was evaporated from an EB source located 27 cm from the substrate with an O2 pressure of 2.7 102 Pa. The deposition rate monitored by the fixed quartz crystal oscillator was 0.3 nm/s. In our experiment, ATO thin films were prepared at different deposition angles and the sample was labeled according to the deposition angle as follows: (a) a = 408, (b) a = 608, (c) a = 708, (d) a = 758, (e) a = 808, (f) a = 858. For comparison, we also prepared the samples g–i. Sample g is dual layers with one layer SiO2 and one layer tilted ATO thin films deposited at a = 808, sample h is the sandwich structure thin films with SiO2, ATO deposited at a = 08 and ATO deposited at a = 808, sample i is the zigzag structure ATO thin film deposited at a = 858. The nominal thickness was controlled by the quartz crystal sensor. After deposition, all of the films were annealed in the muffle with atmosphere at 400 8C for 4 h. 2.2. Characterization Crystal structure of films was characterized by X-ray diffraction (XRD) using a Rigaku D/MAX-2550 with Cu Ka (l = 1.5418 A´˚ ). The surface and cross-sectional microstructure were observed by field emission scanning electron microscopy (FE-SEM) in Hitachi S-4700
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Fig. 1. The XRD patterns of ATO thin films for samples e and h.
microscope. For cross-sectional observation, thin films were cleaved along the deposited plane. The composition of thin films was analyzed by energy-dispersive X-ray (EDX) spectroscopy equipped on SEM. The UV–vis–NIR spectra of thin films were measured by Lambda 900 spectrophotometer. For the polarization measurement, the polarizer was introduced into the light path. The incident light was two orthogonal polarization lights normal to the substrate surface plane. The wavelength range was 300–1200 nm.
Fig. 2. The top-view (left) and cross-section (right) morphologic images for samples e, f and i.
X. Xiao et al. / Applied Surface Science 256 (2010) 1636–1640
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The sheet resistance was measured by the four-point probe resistor. 3. Results and discussion 3.1. The structure and morphology of ATO thin films The XRD patterns of ATO thin films for samples e and h are shown in Fig. 1. It can be found that there is no evident diffraction peak, indicating that the ATO thin films are amorphous. Due to oblique angle deposition, the crystallization is decreasing with increasing of deposition angles [19], which indicate that all of the OAD ATO thin films are all amorphous. This may be due to the limited adatom diffusion of room temperature deposition and the shadowing effect, which inhibit the diffusion of deposited atoms even in the post-annealing process. The typical surface and cross-sectional microstructure of samples e, f and i are shown in Fig. 2. Through the images, it can be seen that thin films are porous with isolated columns. The intercolumnar pore is open and increases with the increase of flux incident angle. The tilted sample exhibits slanted columns (see Fig. 2e and f), while rotates the substrate 1808 during deposition, the zigzag structure is obtained (see Fig. 2i), which is consistent with the results obtained by Motohiro and Taga [20]. The microstructures are formed due to the self-shadowing effect and limited atom mobility during the oblique deposition [21]. Column angle b defined as the angle between substrate normal and the long axis of slanted column. The measured column angle b for samples e, f and i are 458, 478 and 458, respectively, which are smaller than those predicted by tangent rule tan b = 1/2tan a (718, 808, 808) [22] or cosine rule b = a-arcsin[(1 cos a)/2] (558, 588, 588) [23]. The deviation from the empirical equation may result from the difference of material, deposition parameters (deposition pressure, rate and temperature) and flux vapor angular distribution. The columnar angle difference of tilted column and zigzag structure deposited at a = 858 may be caused by the angle error of the controlled motor. 3.2. The optical properties of ATO thin films The composition of ATO thin films was measured by EDX and shown in Table 1 (not including C and Na, C is from conductive layer and Na migrates from substrate). It can be found that the content of oxygen is increasing with the increase of deposition angle, while the content of tin and antimony shows no significant changes, which is consistent with the results shown on ITO thin films [24]. This can be attributed to growth rate and porosity of thin film. With the increase of deposition angle, the deposition rate on crystal oscillator is constant while the sticking coefficient and growth rate of thin film on the substrate is decreasing [25]. Gas phase oxygen introduced into the vacuum chamber during deposition can more fully react with thin film at lower growth rate and higher deposition angle. Simultaneously, due to the shadowing effect, the porosity and specific surface area is increasing with the increase of deposition angle, which also improves gas phase oxygen react with thin film. The transmittance spectra of ATO thin films are shown in Fig. 3. The OAD ATO thin films show high transmittance in the visible range. The transmittance of ATO thin films increases gradually as Table 1 The compositional analysis of ATO thin films. Sample No.
Oxygen (wt%)
Tin (wt%)
Antimony (wt%)
a d f
17 19 23
60 61 62
8 7 7
Fig. 3. The transmission spectra of samples deposited at different deposition angles; the insert is the transmission spectra for samples e, g and h.
the flux incident angle increases. Simultaneously, with the increase of flux incident angle, the ultraviolet cut-off wavelength shifts to short wavelength. There may be two factors attributed to the blue shift. One factor of the blue shift maybe result from the Burstein– Moss effect due to the increase of oxygen content in the ATO thin film [24,26]. As the deposition angle increases, ATO thin films may become degenerate semiconductors due to the higher oxygen content so that the Fermi level shifts to higher position from the bottom of the conduction band. Another factor may result from the increase of columnar angle. As the deposition angle increases, the obliquity of columnar structure ATO thin films is increasing and the thin films is thinner due to shadowing effect. Hence, the blue shift probably results from the quantum confinement effect caused by the exciton quantization in the ATO thin films [27,28]. Based on the transmittance spectra of natural light, the average refractive indices (nav) of ATO thin films were calculated according to Swanepoel’s method [29]. The dispersive curves were then fitted by the Cauchy dispersion equation. The effective refractive index (neff = nav) at l = 600 nm are compiled in Table 2. At l = 600 nm, the effective refractive index changes from 1.96 to 1.45, which is far less than that of the bulk materials (n = 2.05). The refractive index of porous material is determined by the porosity and the refractive index of dense material. The decrease of refractive index in OAD ATO thin films can be ascribed to the increasing porosity as shown in SEM images (see Fig. 2). Base on the Bruggeman effective medium approximation [18], pA
eA e eB e þ pB ¼0 eA þ 2e eB þ 2e
(1)
where e, eA, eB are the dielectric functions of effective medium, material A and material B, respectively. pA and pB represent the packing density of material A (air) and B (ATO bulk), respectively. The relationship between packing density and effective refractive index of ATO thin films can be obtained, as shown in Table 2. The packing density of ATO thin films decreases with the increase of flux incident angle, which result in the decrease of effective refractive index. By adjusting the flux incident angle, the effective index, packing density and ultraviolet cut-off wavelength of ATO thin films can be engineered in a continuous range. It is known that ATO thin film is a direct band gap semiconductor. The relationship between photon energy (hn) and absorption coefficient (a), which can be evaluated from the transmission spectra, is given by [30] 2
ðahnÞ ¼ Aðhn Eg Þ
(2)
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Table 2 The average refractive index, in-plane birefringence at l = 600 nm and the band gap of ATO thin films at different deposition angles. Sample No.
Average refractive index, nav
Packing Density, r
In-plane birefringence, Dn = ns np
Eg (eV)
a b c d e f
1.96 1.80 1.62 1.59 1.48 1.45
0.916 0.769 0.61 0.582 0.488 0.46
0.005 0.024 0.035 0.019 0.018 0.012
3.86 3.93 3.95 3.97 3.98 4.01
where A is a constant and Eg is the direct band gap energy. The linear relationship visualized by these plots indicated a direct transition. Thus the direct band gaps Eg were obtained by extrapolating the linear portion of the curve to the axis of abscissa. According to Eq. (2), the Eg for ATO thin films are obtained in Fig. 4. For convenience, the Eg are compiled and shown in Table 2, it can be found that the band gap is increasing from 3.86 to 4.01 eV with the increase of deposition angle from 408 to 858, which means that the band gap can be tuned by controlling deposition angle. 3.3. The optical and electrical anisotropy of ATO thin films Through the SEM images, it can be seen that the columns incline towards the direction of the incident flux. The higher the flux angle is, the greater the column inclination is. It is well known that the anisotropic structure will introduce the anisotropic dependence into the optical, electrical, magnetic and thermal properties of thin films [31]. The highly orientated nanostructure of the slanted columns indicates that OAD ATO thin films are anisotropic. According to transmittance spectra of two orthogonal polarized lights, the refractive indices along the directions parallel (np) and perpendicular (ns) to the deposition plane are extracted using the envelope method. The linear birefringence Dn = ns np [32,33] at l = 600 nm are obtained in Table 2. It can be found that birefringence is firstly increasing and then decreasing with the increase of deposition angle, which is ascribed to the competition between anisotropic structure and packing density with the increasing of deposition angle [34]. The maximum of birefringence is up to 0.035 at a = 708. The result is consistent with previous results about ZrO2, TiO2 and Ta2O5 thin films fabricated by OAD technique [20]. The maximum birefringence of OAD ATO thin films is higher than that of the common bulk materials, such as quartz (Dn = 0.009) and MgF2 (Dn = 0.012) [35]. It is suggested that OAD technique may offer an effective method to obtain large birefringence, and it seems particularly flexible to create designed devices such as retardation plate and polarizer.
Fig. 4. The band gap for samples deposited at different deposition angles.
Considering the conductive and anisotropic properties, the sheet resistances measured along directions parallel (x-direction) and perpendicular (y-direction) (see the insert in Fig. 5 at the rightdown) to deposition plane are shown in Fig. 5. Due to the oblique angle deposition, the thin films are porous, so the sheet resistance of OAD ATO thin films is larger than that of normal ATO thin films. The sheet resistance is increasing with deposition angle due to the increasing porosity. Because of the anisotropic column structure, the sheet resistance parallel to the deposition plane is larger than that perpendicular to the deposition plane. The similar results have been reported for oblique angle deposited ITO thin films [16]. As the flux incident angle becomes larger, more voids are introduced into the thin films. The grain boundary potential also increases. The thin film resistance increases because the probability of an electron scattering at the grain boundary with higher potential increases [36]. The columnar distance of direction parallel to the deposition plane is larger than that of the direction perpendicular to the deposition plane. Hence, the sheet resistance parallel to the deposition plane is much larger. In the worst situation (for the films deposited at larger flux incident angle and fast rotation), the voids even isolate the ATO nanocolumns from each other and obstruct the available conduction paths considerably. This is the reason why the resistance increases sharply from a = 708 to 858. For ATO thin films deposited on alkaline glasses, the alkaline impurities such as Na+, K+ ions can penetrate into the thin films and increase the resistivity, which has been confirmed in ATO thin films [37]. Hence, the SiO2 or TiO2 buffer layers are always used to resist the alkaline ion. The compositional analysis of ATO thin films by EDX also show that some Na+ penetrate into the thin films. In the present work, the OAD ATO thin film deposited at a = 808 with SiO2 buffer layer was prepared (sample g) and the compositional analysis by EDX show that none alkaline ions can be detected.
Fig. 5. The sheet resistance vs. deposition angles, the square is the resistance along deposition plane(x-direction), the circle is the resistance perpendicular the deposition plane(y-direction); the insert (left-up) is the sheet resistance for samples e, g and h.; the insert (right-down) is the coordinates for oblique deposition thin films.
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X. Xiao et al. / Applied Surface Science 256 (2010) 1636–1640
Simultaneously, the sheet resistance is inevitably decreasing, while the electrical anisotropy is not affected (see the insert in Fig. 5), which means that SiO2 buffer layers is helpful to resist alkaline ions and decrease resistance. However, for OAD ATO thin films, the porosity is the main reason to increase sheet resistance. Therefore, to balance the anisotropic structure and resistance, the sandwich structure ATO thin films are prepared (sample h). With the SiO2 buffer layer and 08 ATO thin films, the sheet resistance of OAD ATO thin films deposited at a = 808 is almost the same as OAD ATO thin films deposited at a = 408 (see the insert in Fig. 5). Compared with the sheet resistance of samples e, g and h, it can be found that SiO2 buffer layer and 08 ATO thin films are effective to decrease the resistance of OAD ATO thin films. Simultaneously, the transmittance of sample h is also high enough in visible range (see the insert in Fig. 3), which means that the sandwich structural thin film is suitable as the anisotropic transparent conductive thin film. 4. Conclusion In summary, the ATO thin films with tilted columns are prepared by oblique angle deposition. Due to anisotropic microstructure, the optical and electrical anisotropy are obtained. The birefringence of optical anisotropy is up to 0.035 at a = 708, which is much larger than some crystals, such as quartz and MgF2, etc. Due to the porosity, the sheet resistance is much larger than the normal deposited ATO thin films and is increasing with deposition angle. The sheet resistance change from 900 V/& to 3500 V/& for deposition angle from a = 408 to a = 858, which means that the resistance is tunable by changing the deposition angle. Simultaneously, the anisotropic electrical property shows that the sheet resistance parallel to the deposition plane is larger than that perpendicular to the deposition plane due to the tilted columnar structure. Compared with a single layer of OAD ATO thin films, double layers thin films (SiO2 buffer layer plus OAD ATO thin films) and sandwich structural thin film (SiO2 buffer layer plus normal ATO thin films and OAD ATO thin films), thin films with sandwich structure have superior electrical property without loss of anisotropy and transparency. It is expected that OAD ATO thin films can be used as transparent conductive thin films in solar cell, anti-foggy windows and multifunctional carrier in liquid crystal display. Acknowledgements This work was funded by the National Natural Science Foundation of China (No. 60778026), Shanghai Rising-Star Program (No. 07QB14006).
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Optical and electrical properties of SnO2:Sb thin films deposited by oblique angle deposition Xiudi Xiao a,b,*, Guoping Dong a,b, Jianda Shao a, Hongbo He a, Zhengxiu Fan a a b
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. BOX 800-211, Shanghai 201800, PR China Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 May 2009 Received in revised form 23 September 2009 Accepted 23 September 2009 Available online 2 October 2009
The antimony doped tin oxide (SnO2:Sb) (ATO) thin films were prepared by oblique angle electron beam evaporation technique. X-ray diffraction, field emission scanning electron microscopy, UV–vis–NIR spectrophotometer and four-point probe resistor were employed to characterize the structure, morphology, optical and electrical properties. The results show that oblique angle deposition ATO thin films with tilted columns structure are anisotropic. The in-plane birefringence of optical anisotropy is up to 0.035 at a = 708, which means that it is suitable as wave plate and polarizer. The electrical anisotropy of sheet resistance shows that the sheet resistance parallel to the deposition plane is larger than that perpendicular to the deposition plane and it can be changed from 900 V/& to 3500 V/& for deposition angle from 408 to 858, which means that the sheet resistance can be effectively tuned by changing the deposition angle. Additionally, the sandwich structure of SiO2 buffer layer plus normal ATO films and oblique angle deposition ATO films can reduce the resistance, which can balance the optical and electrical anisotropy. It is suggested that oblique angle deposition ATO thin films can be used as transparent conductive thin films in solar cell, anti-foggy windows and multifunctional carrier in liquid crystal display. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Thin films Physical vapor deposition Anisotropy Optical spectroscopy
1. Introduction Transparent conductive thin films attract unceasing attention due to its extensive application in flat panel displays [1], solar cells [2], anti-foggy windows [3], etc. A variety of transparent conductive materials, such as In2O3:Sn (ITO), SnO2:Sb (ATO), SnO2:F (FTO), CdOx, CuS, ZnO:Al (AZO), etc. are investigated. Sndoped In2O3 (ITO) thin film, which is one of the most mature transparent conductive thin films, has been commercially used due to its low resistivity and high transparency [4]. However, ITO contains the rare and expensive indium element, thus it is necessary to develop new transparent conductive materials, which are composed of abundant and low-cost elements [5,6]. SnO2 is one of the traditional, low-cost and multifunctional materials, which can be used as gas sensors, oxide catalyst, and transparent conductor because of its oxygen vacancy and dual valence [7]. Recently, SnO2 matrix doped materials attract great attention for their potential application as one of the ITO substitutions. SnO2:Sb (ATO) is one of the popular transparent conductive materials. A
number of methods such as chemical vapor deposition [8], physical vapor deposition [9], spray pyrolysis [10] and sol–gel spin coating [11], have been used to prepare ATO thin films. Oblique angle deposition (OAD) is a convenient method to fabricate thin films with a variety of nanostructures and unique optical properties [12– 15]. Recently, transparent conductive thin film prepared by OAD technique is confirmed to improve the optical, electrical and light emitting properties of liquid crystal display (LCD) and light emitting diode (LED), which shows greatly potential application in photoelectrical field [16–18]. However, to our best knowledge, as one of important transparent conductive thin films, the research on ATO thin films prepared by OAD technique has not been reported. Hence, the systematical and fundamental research on the optical and electrical properties of OAD ATO thin films is significant and meaningful. In the present paper, we prepare the ATO thin films by OAD technique and investigate the optical and electrical properties of ATO thin films deposited at different deposition parameters. 2. Experimental 2.1. Preparation
* Corresponding author at: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. BOX 800-211, Shanghai 201800, PR China. Tel.: +86 21 69918476; fax: +86 21 69918476. E-mail address: [email protected] (X. Xiao). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.09.084
ATO thin films were prepared by oblique angle electron beam (EB) evaporation technique with base pressure of 1.7 103 Pa. The substrate tilted angle a was measured as the direction of the
X. Xiao et al. / Applied Surface Science 256 (2010) 1636–1640
incident flux with respect to the substrate normal. Two motors were used to control the tilted angle a and rotation speed of the substrate. BK7 glass (Ø 30 mm 3 mm) and n-Si (1 0 0) substrates were ultrasonically cleaned in acetone and ethanol before introduced into the vacuum system. The substrates were not heated during deposition. ATO bulk with 5 at% Sb2O3 was evaporated from an EB source located 27 cm from the substrate with an O2 pressure of 2.7 102 Pa. The deposition rate monitored by the fixed quartz crystal oscillator was 0.3 nm/s. In our experiment, ATO thin films were prepared at different deposition angles and the sample was labeled according to the deposition angle as follows: (a) a = 408, (b) a = 608, (c) a = 708, (d) a = 758, (e) a = 808, (f) a = 858. For comparison, we also prepared the samples g–i. Sample g is dual layers with one layer SiO2 and one layer tilted ATO thin films deposited at a = 808, sample h is the sandwich structure thin films with SiO2, ATO deposited at a = 08 and ATO deposited at a = 808, sample i is the zigzag structure ATO thin film deposited at a = 858. The nominal thickness was controlled by the quartz crystal sensor. After deposition, all of the films were annealed in the muffle with atmosphere at 400 8C for 4 h. 2.2. Characterization Crystal structure of films was characterized by X-ray diffraction (XRD) using a Rigaku D/MAX-2550 with Cu Ka (l = 1.5418 A´˚ ). The surface and cross-sectional microstructure were observed by field emission scanning electron microscopy (FE-SEM) in Hitachi S-4700
1637
Fig. 1. The XRD patterns of ATO thin films for samples e and h.
microscope. For cross-sectional observation, thin films were cleaved along the deposited plane. The composition of thin films was analyzed by energy-dispersive X-ray (EDX) spectroscopy equipped on SEM. The UV–vis–NIR spectra of thin films were measured by Lambda 900 spectrophotometer. For the polarization measurement, the polarizer was introduced into the light path. The incident light was two orthogonal polarization lights normal to the substrate surface plane. The wavelength range was 300–1200 nm.
Fig. 2. The top-view (left) and cross-section (right) morphologic images for samples e, f and i.
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The sheet resistance was measured by the four-point probe resistor. 3. Results and discussion 3.1. The structure and morphology of ATO thin films The XRD patterns of ATO thin films for samples e and h are shown in Fig. 1. It can be found that there is no evident diffraction peak, indicating that the ATO thin films are amorphous. Due to oblique angle deposition, the crystallization is decreasing with increasing of deposition angles [19], which indicate that all of the OAD ATO thin films are all amorphous. This may be due to the limited adatom diffusion of room temperature deposition and the shadowing effect, which inhibit the diffusion of deposited atoms even in the post-annealing process. The typical surface and cross-sectional microstructure of samples e, f and i are shown in Fig. 2. Through the images, it can be seen that thin films are porous with isolated columns. The intercolumnar pore is open and increases with the increase of flux incident angle. The tilted sample exhibits slanted columns (see Fig. 2e and f), while rotates the substrate 1808 during deposition, the zigzag structure is obtained (see Fig. 2i), which is consistent with the results obtained by Motohiro and Taga [20]. The microstructures are formed due to the self-shadowing effect and limited atom mobility during the oblique deposition [21]. Column angle b defined as the angle between substrate normal and the long axis of slanted column. The measured column angle b for samples e, f and i are 458, 478 and 458, respectively, which are smaller than those predicted by tangent rule tan b = 1/2tan a (718, 808, 808) [22] or cosine rule b = a-arcsin[(1 cos a)/2] (558, 588, 588) [23]. The deviation from the empirical equation may result from the difference of material, deposition parameters (deposition pressure, rate and temperature) and flux vapor angular distribution. The columnar angle difference of tilted column and zigzag structure deposited at a = 858 may be caused by the angle error of the controlled motor. 3.2. The optical properties of ATO thin films The composition of ATO thin films was measured by EDX and shown in Table 1 (not including C and Na, C is from conductive layer and Na migrates from substrate). It can be found that the content of oxygen is increasing with the increase of deposition angle, while the content of tin and antimony shows no significant changes, which is consistent with the results shown on ITO thin films [24]. This can be attributed to growth rate and porosity of thin film. With the increase of deposition angle, the deposition rate on crystal oscillator is constant while the sticking coefficient and growth rate of thin film on the substrate is decreasing [25]. Gas phase oxygen introduced into the vacuum chamber during deposition can more fully react with thin film at lower growth rate and higher deposition angle. Simultaneously, due to the shadowing effect, the porosity and specific surface area is increasing with the increase of deposition angle, which also improves gas phase oxygen react with thin film. The transmittance spectra of ATO thin films are shown in Fig. 3. The OAD ATO thin films show high transmittance in the visible range. The transmittance of ATO thin films increases gradually as Table 1 The compositional analysis of ATO thin films. Sample No.
Oxygen (wt%)
Tin (wt%)
Antimony (wt%)
a d f
17 19 23
60 61 62
8 7 7
Fig. 3. The transmission spectra of samples deposited at different deposition angles; the insert is the transmission spectra for samples e, g and h.
the flux incident angle increases. Simultaneously, with the increase of flux incident angle, the ultraviolet cut-off wavelength shifts to short wavelength. There may be two factors attributed to the blue shift. One factor of the blue shift maybe result from the Burstein– Moss effect due to the increase of oxygen content in the ATO thin film [24,26]. As the deposition angle increases, ATO thin films may become degenerate semiconductors due to the higher oxygen content so that the Fermi level shifts to higher position from the bottom of the conduction band. Another factor may result from the increase of columnar angle. As the deposition angle increases, the obliquity of columnar structure ATO thin films is increasing and the thin films is thinner due to shadowing effect. Hence, the blue shift probably results from the quantum confinement effect caused by the exciton quantization in the ATO thin films [27,28]. Based on the transmittance spectra of natural light, the average refractive indices (nav) of ATO thin films were calculated according to Swanepoel’s method [29]. The dispersive curves were then fitted by the Cauchy dispersion equation. The effective refractive index (neff = nav) at l = 600 nm are compiled in Table 2. At l = 600 nm, the effective refractive index changes from 1.96 to 1.45, which is far less than that of the bulk materials (n = 2.05). The refractive index of porous material is determined by the porosity and the refractive index of dense material. The decrease of refractive index in OAD ATO thin films can be ascribed to the increasing porosity as shown in SEM images (see Fig. 2). Base on the Bruggeman effective medium approximation [18], pA
eA e eB e þ pB ¼0 eA þ 2e eB þ 2e
(1)
where e, eA, eB are the dielectric functions of effective medium, material A and material B, respectively. pA and pB represent the packing density of material A (air) and B (ATO bulk), respectively. The relationship between packing density and effective refractive index of ATO thin films can be obtained, as shown in Table 2. The packing density of ATO thin films decreases with the increase of flux incident angle, which result in the decrease of effective refractive index. By adjusting the flux incident angle, the effective index, packing density and ultraviolet cut-off wavelength of ATO thin films can be engineered in a continuous range. It is known that ATO thin film is a direct band gap semiconductor. The relationship between photon energy (hn) and absorption coefficient (a), which can be evaluated from the transmission spectra, is given by [30] 2
ðahnÞ ¼ Aðhn Eg Þ
(2)
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Table 2 The average refractive index, in-plane birefringence at l = 600 nm and the band gap of ATO thin films at different deposition angles. Sample No.
Average refractive index, nav
Packing Density, r
In-plane birefringence, Dn = ns np
Eg (eV)
a b c d e f
1.96 1.80 1.62 1.59 1.48 1.45
0.916 0.769 0.61 0.582 0.488 0.46
0.005 0.024 0.035 0.019 0.018 0.012
3.86 3.93 3.95 3.97 3.98 4.01
where A is a constant and Eg is the direct band gap energy. The linear relationship visualized by these plots indicated a direct transition. Thus the direct band gaps Eg were obtained by extrapolating the linear portion of the curve to the axis of abscissa. According to Eq. (2), the Eg for ATO thin films are obtained in Fig. 4. For convenience, the Eg are compiled and shown in Table 2, it can be found that the band gap is increasing from 3.86 to 4.01 eV with the increase of deposition angle from 408 to 858, which means that the band gap can be tuned by controlling deposition angle. 3.3. The optical and electrical anisotropy of ATO thin films Through the SEM images, it can be seen that the columns incline towards the direction of the incident flux. The higher the flux angle is, the greater the column inclination is. It is well known that the anisotropic structure will introduce the anisotropic dependence into the optical, electrical, magnetic and thermal properties of thin films [31]. The highly orientated nanostructure of the slanted columns indicates that OAD ATO thin films are anisotropic. According to transmittance spectra of two orthogonal polarized lights, the refractive indices along the directions parallel (np) and perpendicular (ns) to the deposition plane are extracted using the envelope method. The linear birefringence Dn = ns np [32,33] at l = 600 nm are obtained in Table 2. It can be found that birefringence is firstly increasing and then decreasing with the increase of deposition angle, which is ascribed to the competition between anisotropic structure and packing density with the increasing of deposition angle [34]. The maximum of birefringence is up to 0.035 at a = 708. The result is consistent with previous results about ZrO2, TiO2 and Ta2O5 thin films fabricated by OAD technique [20]. The maximum birefringence of OAD ATO thin films is higher than that of the common bulk materials, such as quartz (Dn = 0.009) and MgF2 (Dn = 0.012) [35]. It is suggested that OAD technique may offer an effective method to obtain large birefringence, and it seems particularly flexible to create designed devices such as retardation plate and polarizer.
Fig. 4. The band gap for samples deposited at different deposition angles.
Considering the conductive and anisotropic properties, the sheet resistances measured along directions parallel (x-direction) and perpendicular (y-direction) (see the insert in Fig. 5 at the rightdown) to deposition plane are shown in Fig. 5. Due to the oblique angle deposition, the thin films are porous, so the sheet resistance of OAD ATO thin films is larger than that of normal ATO thin films. The sheet resistance is increasing with deposition angle due to the increasing porosity. Because of the anisotropic column structure, the sheet resistance parallel to the deposition plane is larger than that perpendicular to the deposition plane. The similar results have been reported for oblique angle deposited ITO thin films [16]. As the flux incident angle becomes larger, more voids are introduced into the thin films. The grain boundary potential also increases. The thin film resistance increases because the probability of an electron scattering at the grain boundary with higher potential increases [36]. The columnar distance of direction parallel to the deposition plane is larger than that of the direction perpendicular to the deposition plane. Hence, the sheet resistance parallel to the deposition plane is much larger. In the worst situation (for the films deposited at larger flux incident angle and fast rotation), the voids even isolate the ATO nanocolumns from each other and obstruct the available conduction paths considerably. This is the reason why the resistance increases sharply from a = 708 to 858. For ATO thin films deposited on alkaline glasses, the alkaline impurities such as Na+, K+ ions can penetrate into the thin films and increase the resistivity, which has been confirmed in ATO thin films [37]. Hence, the SiO2 or TiO2 buffer layers are always used to resist the alkaline ion. The compositional analysis of ATO thin films by EDX also show that some Na+ penetrate into the thin films. In the present work, the OAD ATO thin film deposited at a = 808 with SiO2 buffer layer was prepared (sample g) and the compositional analysis by EDX show that none alkaline ions can be detected.
Fig. 5. The sheet resistance vs. deposition angles, the square is the resistance along deposition plane(x-direction), the circle is the resistance perpendicular the deposition plane(y-direction); the insert (left-up) is the sheet resistance for samples e, g and h.; the insert (right-down) is the coordinates for oblique deposition thin films.
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Simultaneously, the sheet resistance is inevitably decreasing, while the electrical anisotropy is not affected (see the insert in Fig. 5), which means that SiO2 buffer layers is helpful to resist alkaline ions and decrease resistance. However, for OAD ATO thin films, the porosity is the main reason to increase sheet resistance. Therefore, to balance the anisotropic structure and resistance, the sandwich structure ATO thin films are prepared (sample h). With the SiO2 buffer layer and 08 ATO thin films, the sheet resistance of OAD ATO thin films deposited at a = 808 is almost the same as OAD ATO thin films deposited at a = 408 (see the insert in Fig. 5). Compared with the sheet resistance of samples e, g and h, it can be found that SiO2 buffer layer and 08 ATO thin films are effective to decrease the resistance of OAD ATO thin films. Simultaneously, the transmittance of sample h is also high enough in visible range (see the insert in Fig. 3), which means that the sandwich structural thin film is suitable as the anisotropic transparent conductive thin film. 4. Conclusion In summary, the ATO thin films with tilted columns are prepared by oblique angle deposition. Due to anisotropic microstructure, the optical and electrical anisotropy are obtained. The birefringence of optical anisotropy is up to 0.035 at a = 708, which is much larger than some crystals, such as quartz and MgF2, etc. Due to the porosity, the sheet resistance is much larger than the normal deposited ATO thin films and is increasing with deposition angle. The sheet resistance change from 900 V/& to 3500 V/& for deposition angle from a = 408 to a = 858, which means that the resistance is tunable by changing the deposition angle. Simultaneously, the anisotropic electrical property shows that the sheet resistance parallel to the deposition plane is larger than that perpendicular to the deposition plane due to the tilted columnar structure. Compared with a single layer of OAD ATO thin films, double layers thin films (SiO2 buffer layer plus OAD ATO thin films) and sandwich structural thin film (SiO2 buffer layer plus normal ATO thin films and OAD ATO thin films), thin films with sandwich structure have superior electrical property without loss of anisotropy and transparency. It is expected that OAD ATO thin films can be used as transparent conductive thin films in solar cell, anti-foggy windows and multifunctional carrier in liquid crystal display. Acknowledgements This work was funded by the National Natural Science Foundation of China (No. 60778026), Shanghai Rising-Star Program (No. 07QB14006).
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