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Fabrication of protective over layer for enhanced thermal stability of zinc oxide based TCO films
Applied Surface Science 287 (2013) 323–328
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of protective over layer for enhanced thermal stability of zinc oxide based TCO films K. Ravichandran ∗ , P. Ravikumar, B. Sakthivel P.G & Research Department of Physics, A.V.V.M. Sri Pushpam College (Autonomous), Poondi, 613 503 Thanjavur, Tamil Nadu, India
a r t i c l e
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Article history: Received 18 May 2013 Received in revised form 23 August 2013 Accepted 26 September 2013 Available online 7 October 2013 Keywords: Double layered films ZnO SnO2 Thermal stability Opto-electrical properties Structural studies
a b s t r a c t To prevent the loss of oxygen vacancies in aluminium doped zinc oxide (AZO) thin films at high temperature process, and to enhance the thermal stability a protective tin oxide (TO) over layer has been realized. To investigate the protective nature of doped tin oxide layer, fluorine doped tin oxide (FTO) and antimony doped tin oxide (ATO) layers have also been coated on AZO layer. Then, to confirm its stability of opto-electrical properties under high temperature process, structural, optical and electrical studies of AZO single layer, TO/AZO, FTO/AZO and ATO/AZO double layered films were carried out before and after annealing and the results are reported. The XRD results showed that the crystalline nature of double layered films remains unchanged, even after the heat treatment. The UV results depicted that, in all the double layer films the transmission spectra remain unchanged or changed negligibly after annealing, indicating the thermal stability of double layered films. The photoluminescence results also strongly supported the improvement in the thermal stability of double layered films. The electrical studies suggested that the double layered films exhibited better electrical resistivity with bare AZO films. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Transparent conducting oxides (TCOs) gain much attention in the field of materials research for their excellent optical transparency, electrical conductivity and industrial process compatibility [1–4]. They have become essential components in basic structure of many devices like thin film solar cells, flat panel liquid crystal displays, light emitting diodes, gas sensors and they have also acted as anti bacterial agents [5–12]. In emerging new generation photovoltaics, TCOs have become one of the most significant part that determines the efficiency of the system [3]. Hence, well known TCO materials are often subjected to numerous investigations to improve their characteristics to make them suitable for aforesaid applications. A large number of TCOs such as indium oxide (In2 O3 ), tin oxide (SnO2 ), cadmium oxide (CdO), titanium dioxide (TiO2 ) and zinc oxide (ZnO) have been widely investigated for conventional transparent electrode applications [13,14]. Among all the TCOs, ZnO is proposed as one of the best future candidates for the optoelectronic applications, since, the inexpensive zinc oxide has a direct wide band gap of 3.37 eV [15] and a large exciton binding energy of 60 meV [16,17], much larger than many other semiconductor materials. In spite of all these good qualities, the electrical
∗ Corresponding author. Tel.: +91 4362 278602; fax: +91 4374 239328; mobile: +91 9443524180. E-mail address: [email protected] (K. Ravichandran). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.09.150
and optical properties of the doped zinc oxide film get altered during post deposition heat treatments (>250 ◦ C). Generally, photovoltaic devices are exposed to very high temperatures during device fabrication and hence it is very much essential to improve the thermal stability of ZnO film [3,18]. Another well known TCO material SnO2 has a wide band gap of 3.8 eV [19], better optical transparency, electrical conductivity and thermal stability than ZnO. Most importantly, when it is exposed to temperature higher than 350 ◦ C, its properties are generally not affected undesirably, rather they are even improved to a reasonable extent [3,18]. Generally, ZnO and SnO2 films were doped with cationic or anionic elements to increase their carrier concentration and to tune their optical properties [20]. Hence, in order to produce a low cost and thermally stable TCO a double layered structure which consists of tin oxide (TO)/fluorine doped tin oxide (FTO)/antimony doped tin oxide (ATO) as a top layer on aluminium doped zinc oxide (AZO) layer has been realized. Even though various deposition techniques are available to fabricate ZnO and SnO2 films viz. pulsed laser deposition, sputtering, chemical vapour deposition, sol–gel and spray pyrolysis [13,14,21–25], etc., a simplified spray technique has been employed in the present work considering the cost of fabrication [23–26]. Several researchers have reported improved TCO characteristics of bi-layer films involving doped and undoped ZnO and SnO2 films. Vaezi [27] prepared SnO2 /ZnO double layer films using sol–gel technique and two stage chemical deposition technique. Montero et al. [3] studied the properties of ZnO:Al/SnO2 :Sb bi-layer using DC magnetron sputtering. Anandhi et al. [18] prepared SnO2 :F/ZnO:F
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Table 1 Deposition parameters of single layer AZO, bilayered TO/AZO, FTO/AZO and ATO/AZO films. Deposition parameters
AZO layer
TO layer
FTO layer
ATO layer
Host precursor Dopant precursor Spray interval Substrate temperature Solvent
Zinc acetate dihydrate Aluminium chloride (3 at.%) 10 s 340 ± 5 ◦ C Water:methanol:acetic acid (7:2:1)
Tin II chloride Nil 5s 340 ± 5 ◦ C Water and few drops of HCl
Tin II chloride Ammonium fluoride (20 at.%) 5s 340 ± 5 ◦ C Water and few drops of HCl
Tin II chloride Antimony chloride (2.5 at.%) 5s 340 ± 5 ◦ C Water and few drops of HCl
films employing simplified spray pyrolysis technique. To the best of our knowledge this is the first report comparing structural, optical and electrical properties before and after annealing of T0/FTO/ATO top layer over AZO films employing simplified spray pyrolysis technique.
2. Experimental details The single layer AZO and double layered TO/AZO, FTO/AZO and ATO/AZO films have been fabricated using simplified spray pyrolysis technique. For the double layered films, AZO layer was deposited first onto the glass substrate as bottom layer and the TO, FTO and ATO layer are deposited separately as the top layer (over layer). The deposition parameters employed in this study are presented in Table 1. Borosilicate glasses were used as substrates. The starting solution was stirred continuously for 1 h to get a clear and homogenous solution. The solution was sprayed intermittently onto preheated ultrasonically cleaned glass substrates at certain intervals. Distance between nozzle and substrate and the spray angle with respect to the plane of the substrate were kept as 30 cm and 45◦ , respectively. Details of the simplified spray system have been described
elsewhere [28]. Using chromel alumel thermo couple, temperature was maintained at 350 ± 5 ◦ C. For each double layered system, two sets of samples with two different thicknesses of the over layer namely 50 nm and 100 nm were prepared and their structural, optical and electrical properties were studied before and after thermal annealing at 400 ◦ C for 1 h. X-ray diffractometer (PANalytical – PW 340/60 X’pert PRO), UV-vis-NIR double beam spectrophotometer (LAMBDA-35, range of 300–1100 nm), Spectro Fluorometer (450 W, Jobin YvonFLUOROLOG-FL3-11) and four-point probe were employed to obtain X-ray diffraction (XRD) patterns, transmission spectra, photoluminescence (PL) spectra and sheet resistance, respectively.
3. Results and discussion 3.1. Structural studies The XRD patterns of as deposited and annealed AZO films are shown in Fig. 1(a). In the case of as deposited films, three peaks corresponding to (0 0 2), (1 0 1) and (1 0 3) are observed, of which (0 0 2) is found to be predominant. According to the JCPDS card no. 36-1451, the observed peaks clearly showed that the films
Fig. 1. XRD patterns of as-deposited and annealed (a) AZO, (b) TO/AZO, (c) FTO/AZO and (d) ATO/AZO thin films.
K. Ravichandran et al. / Applied Surface Science 287 (2013) 323–328
Fig. 2. Variation in the crystallite size values of as-deposited and annealed AZO, TO/AZO, FTO/AZO and ATO/AZO thin films
have hexagonal structure of ZnO. After annealing, all the above mentioned three peaks disappeared and a single strong peak corresponding to the (1 0 0) plane appeared. Literature showed that doping of some elements like boron and tin changes the microstructure of ZnO from (1 0 0) to (0 0 2) orientation [29,30]. As it is well known that, ZnO is thermally unstable, these types of change over from (1 0 0) to (0 0 2) or (0 0 2) to (1 0 0) orientations are reasonable to expect during the annealing process.
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This result clearly showed that, the annealing treatment causes a drastic change in the orientation of the crystal structure in bare AZO film. Santana et al. [31] reported a similar result for asdeposited and annealed undoped ZnO films. Several researchers [32,33] reported that the post heat treatments strongly influence the orientation of the doped ZnO films. Some researchers [18,29,30] suggested that, since the (0 0 2) plane has low surface energy (due to the densely packed structure); majority of the stable zinc oxide films possesses (0 0 2) as preferential orientation plane. When it losses the stability it gets reoriented in favour of other planes like (1 0 0) as observed in our case. Hence, there is a need to make the ZnO film stable in terms of its transparent conducting characteristics even after the post deposition heat treatment by keeping the structure oriented along (0 0 2) itself. It is interesting to found that in the case of all double layered films (Fig. 1(b)–(d)) with TO, FTO and ATO as over layers on AZO base layer, the result is different. In TO/AZO films, when the thickness of the TO layer is 50 nm, (1 0 0) peak emerge slightly after annealing. But, when the TO layer has higher thickness, it tends to prevent the reorientation in favour of (1 0 0) plane. In other words, the TO over layer with higher thickness improved the stability of the double layer system. Similar, change was observed in the case of FTO/AZO and ATO/AZO films also. From these results, we can clearly understand that the undoped as well as doped TO over layers act as a protective layer to keep the structure of ZnO remains unchanged. Fig. 2 shows variation in crystallite sizes of the as deposited and annealed bare AZO films as well as TO/AZO, FTO/AZO and ATO/AZO bilayered films. The crystallite size (D) is calculated using the Scherrer’s formula, D = 0.94/ˇcos where, is the wavelength of the
Fig. 3. Transmission spectra of as-deposited and annealed (a) AZO, (b) TO/AZO, (c) FTO/AZO and (d) ATO/AZO thin films.
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Fig. 4. Plot of dT/d vs. avg of as-deposited and annealed single layered and double layered films.
applied X-ray (0.154056 nm for Cu-K␣ , ˇ is the full-width at halfmaximum and is the Bragg’s angle [34–36]. In the case of AZO films crystallite size decreases appreciably due to annealing, whereas in all the double layered films, the change is negligible. This stability in crystallite size itself confirms the thermal stability of bilayer films. 3.2. Optical studies The optical spectra of the as-deposited and annealed AZO films are shown in Fig. 3(a). Though the band edge of the as-deposited and annealed AZO films almost overlapped on each other and even the pattern of oscillations is similar, the percentage of transmission increases remarkably after annealing. For instance, in the wavelength range of 450–600 nm, the increase in average transmittance is about 20%. This result indicates the thermal instability of the AZO films in terms of optical characteristics also. But, when any one of the layers like TO, FTO and ATO (Fig. 3(b)–(d)) over layer is deposited on the AZO film, the scenario is different. In all the three cases, the transmission spectrum remains unchanged or changed only negligibly after annealing, indicating clearly the thermal stability of the double layer system. From these observations, we can strongly say that all the three over layers (TO, FTO and ATO) act as protective layer to make the system unaltered by heat treatment. The average transmittance of single layered as well as double layered films were around 80% and all the films show sharp absorption band edge. The optical band gap (Eg ) of all the samples is estimated from the plot (Fig. 4) drawn for the first derivative of
the transmittance with respect to the wavelength (dT/d) against the average wavelength (avg ) [37]. The Eg value remains almost the same in all the double layered films, which is another evidence for the thermal stability of bilayered films. The Eg value of doped TO double layered films slightly increases when compared with undoped films, which may be due to the increase in carrier concentration according to Moss Burstein effect [38]. Generally, the amount of free carrier concentration in the material determines the position of absorption edge. The Eg value of the double layer films lies around 3.20 eV, which is very close to that of zinc oxide, suitable for photovoltaic and opto-electronic applications [39]. 3.3. Photoluminescence studies Fig. 5(a) shows the PL spectra of as-deposited and annealed AZO films. Three peaks were found at wavelength 398, 467 and 529 nm. The peak at 398 nm corresponds to the ultra-violet near band edge transition of ZnO material [40], which is responsible for crystalline quality. The intensity of the 398 nm peak decreased appreciably in the case of annealed AZO single layered film, which shows the major change in the crystalline nature of the film, when subjected to post deposition high temperature treatment [41]. This variation in PL peak intensity is corroborated well with XRD results. Also, the peak around 529 nm, corresponds to green emission originated from the electron transition from the deep-level donor of the ionized oxygen vacancies to the valance band is found missing in the case of annealed AZO film [42,43]. This is another evidence for above mentioned observation. The peak at 467 nm associated with
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Fig. 5. PL spectra of as-deposited and annealed (a) AZO, (b) TO/AZO, (c) FTO/AZO and (d) ATO/AZO thin films.
the blue emission originated from the electron transition from the shallow donor level of oxygen vacancies to the valence band and electron transition from the shallow donor level of zinc interstitials to the valence band [42,43]. Fig. 5(b)–(d) shows the PL spectra of as-deposited and annealed TO/AZO, FTO/AZO and ATO/AZO double layered films, respectively. In the double layered system, except the introduction of the peak at 344 nm, which is attributed to band to band transition of SnO2 [44], the intensity of all other peaks were not affected appreciably after annealing when compared with that of bare AZO film. This manifests the thermal stability of the double layer system due to the presence of tin oxide based over layers. But the intensity of these peaks increases, with increase in over layer thicknesses. The peak at 398 nm is found to be broadened in the case of doped double layered films, when compared with undoped double layered film, which may be due to the slight deformity in the crystalline structure, caused by dopant incorporation, which in turn increase the carrier concentration [45].
in the sheet resistance as seen from Fig. 6. It is found that annealing does not affect Rsh much. From Fig. 6 it can be inferred that the double layered films exhibit better thermal stability in terms of electrical sheet resistance than that of single layer AZO film. Generally, when AZO films are exposed to high temperature process, sheet resistance value decreases to a small amount due to the agglomeration of grains which causes an increase in mobility and
3.4. Electrical studies Fig. 6 shows the electrical sheet resistance values of single layered ZnO and double layered TO/AZO, FTO/AZO and ATO/AZO films measured before and after annealing. In the case of as-deposited films, the sheet resistance is maximum (53.97 /) for the bare AZO film, but the protective overlayer induced a drastical reduction
Fig. 6. Graphical representation of variation in sheet resistance of as-deposited and annealed single layered and double layered thin films.
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inturn reduces the sheet resistance. But, at high temperature process, due to the loss of oxygen vacancies, there is also a decrease in carrier concentration observed causing an increase in resistivity [46–48]. In the present study, as the later is predominant over the former, there is a net increase in sheet resistance after thermal annealing. The loss of oxygen vacancies in bare AZO film due to annealing is clearly evinced by the suppresion of PL peak at 529 nm (Fig. 5(a)). Moreover, from XRD studies (Fig. 1(a)) it is observed that while annealing the bare AZO film, the change in predominancy of peak from (0 0 2) to (1 0 0) occurs due to degradation of crystal structure [17,30], which inturn increases the resistivity. Kumar et al. [30] clearly showed that doped ZnO has stable microstructure when the preferential orientation is along (0 0 2), whereas it is unstable when the preferential growth is in favour of (1 0 0). In addition, they found that, resistivity increased with the decreases in the intensity of (0 0 2) peak. Moreover, according to Rahman et al. [49], there is always an increase in resistivity with the increase of the intensity of (1 0 0) peak. Both these changes are observed simultaneously in the present findings, it is clear that this degradation of crystallinity may be the reason for further reduction of sheet resistance in the bare AZO film after annealing. In the present study, the resistivity of double layered films is reduced appreciably as observed in Fig. 6, which may be due to the presence of protective layers like TO, FTO and ATO layers on AZO layer that prevented the loss of oxygen vacancies in AZO layer and also maintained the crystalline structure without much change. These findings are confirmed well from the XRD studies, where the occurance of (1 0 0) peak is almost restricted to a great extent in the double layered film. Thus the electrical results also become a strong evidence to confirm the thermal stability of the double layered films. When compared with the undoped TO over layered films, doped TO over layered films have lesser resistivity, which may be due to increase in carrier concentration caused by dopant incorporation. This observation well corroborates with PL findings [Section 3.3]. 4. Conclusion A novel double layered TCO system involving AZO as base layer and three different seperate over layers viz. tin oxide (TO), fluorine doped tin oxide (FTO) and antimony doped tin oxide (ATO) was realized and their thermal stability with respect to their structural, optical and electrical properties were investigated and reported. The structural studies revealed that the peak corresponding to (1 0 0) plane emerged predominantly in the case of annealed bare AZO films, which is an indicator of instability of AZO was desirably suppressed by the TO, FTO and ATO over layers. The electrical studies showed that the sheet resistance of the double layered system remains unchanged even after annealing which was not true in the case of bare AZO films, where the sheet resistance got increased after annealing due to the loss of oxygen vacancies. The optical studies also clearly showed the thermal stability of the double layered system. Hence, we can convincingly conclude that these double layered systems may be very much useful for solar cell devices as they required thermally stable low cost TCOs for cost effective photovoltaic technology. Among the double layered films, FTO/AZO film appeared to be better with fine crystalline structure and lower sheet resistance. Acknowledgement The financial support by the University Grants Commission, New Delhi through the Major Research project (F.No 40-28/2011(SR)) is gratefully acknowledged.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of protective over layer for enhanced thermal stability of zinc oxide based TCO films K. Ravichandran ∗ , P. Ravikumar, B. Sakthivel P.G & Research Department of Physics, A.V.V.M. Sri Pushpam College (Autonomous), Poondi, 613 503 Thanjavur, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 18 May 2013 Received in revised form 23 August 2013 Accepted 26 September 2013 Available online 7 October 2013 Keywords: Double layered films ZnO SnO2 Thermal stability Opto-electrical properties Structural studies
a b s t r a c t To prevent the loss of oxygen vacancies in aluminium doped zinc oxide (AZO) thin films at high temperature process, and to enhance the thermal stability a protective tin oxide (TO) over layer has been realized. To investigate the protective nature of doped tin oxide layer, fluorine doped tin oxide (FTO) and antimony doped tin oxide (ATO) layers have also been coated on AZO layer. Then, to confirm its stability of opto-electrical properties under high temperature process, structural, optical and electrical studies of AZO single layer, TO/AZO, FTO/AZO and ATO/AZO double layered films were carried out before and after annealing and the results are reported. The XRD results showed that the crystalline nature of double layered films remains unchanged, even after the heat treatment. The UV results depicted that, in all the double layer films the transmission spectra remain unchanged or changed negligibly after annealing, indicating the thermal stability of double layered films. The photoluminescence results also strongly supported the improvement in the thermal stability of double layered films. The electrical studies suggested that the double layered films exhibited better electrical resistivity with bare AZO films. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Transparent conducting oxides (TCOs) gain much attention in the field of materials research for their excellent optical transparency, electrical conductivity and industrial process compatibility [1–4]. They have become essential components in basic structure of many devices like thin film solar cells, flat panel liquid crystal displays, light emitting diodes, gas sensors and they have also acted as anti bacterial agents [5–12]. In emerging new generation photovoltaics, TCOs have become one of the most significant part that determines the efficiency of the system [3]. Hence, well known TCO materials are often subjected to numerous investigations to improve their characteristics to make them suitable for aforesaid applications. A large number of TCOs such as indium oxide (In2 O3 ), tin oxide (SnO2 ), cadmium oxide (CdO), titanium dioxide (TiO2 ) and zinc oxide (ZnO) have been widely investigated for conventional transparent electrode applications [13,14]. Among all the TCOs, ZnO is proposed as one of the best future candidates for the optoelectronic applications, since, the inexpensive zinc oxide has a direct wide band gap of 3.37 eV [15] and a large exciton binding energy of 60 meV [16,17], much larger than many other semiconductor materials. In spite of all these good qualities, the electrical
∗ Corresponding author. Tel.: +91 4362 278602; fax: +91 4374 239328; mobile: +91 9443524180. E-mail address: [email protected] (K. Ravichandran). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.09.150
and optical properties of the doped zinc oxide film get altered during post deposition heat treatments (>250 ◦ C). Generally, photovoltaic devices are exposed to very high temperatures during device fabrication and hence it is very much essential to improve the thermal stability of ZnO film [3,18]. Another well known TCO material SnO2 has a wide band gap of 3.8 eV [19], better optical transparency, electrical conductivity and thermal stability than ZnO. Most importantly, when it is exposed to temperature higher than 350 ◦ C, its properties are generally not affected undesirably, rather they are even improved to a reasonable extent [3,18]. Generally, ZnO and SnO2 films were doped with cationic or anionic elements to increase their carrier concentration and to tune their optical properties [20]. Hence, in order to produce a low cost and thermally stable TCO a double layered structure which consists of tin oxide (TO)/fluorine doped tin oxide (FTO)/antimony doped tin oxide (ATO) as a top layer on aluminium doped zinc oxide (AZO) layer has been realized. Even though various deposition techniques are available to fabricate ZnO and SnO2 films viz. pulsed laser deposition, sputtering, chemical vapour deposition, sol–gel and spray pyrolysis [13,14,21–25], etc., a simplified spray technique has been employed in the present work considering the cost of fabrication [23–26]. Several researchers have reported improved TCO characteristics of bi-layer films involving doped and undoped ZnO and SnO2 films. Vaezi [27] prepared SnO2 /ZnO double layer films using sol–gel technique and two stage chemical deposition technique. Montero et al. [3] studied the properties of ZnO:Al/SnO2 :Sb bi-layer using DC magnetron sputtering. Anandhi et al. [18] prepared SnO2 :F/ZnO:F
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Table 1 Deposition parameters of single layer AZO, bilayered TO/AZO, FTO/AZO and ATO/AZO films. Deposition parameters
AZO layer
TO layer
FTO layer
ATO layer
Host precursor Dopant precursor Spray interval Substrate temperature Solvent
Zinc acetate dihydrate Aluminium chloride (3 at.%) 10 s 340 ± 5 ◦ C Water:methanol:acetic acid (7:2:1)
Tin II chloride Nil 5s 340 ± 5 ◦ C Water and few drops of HCl
Tin II chloride Ammonium fluoride (20 at.%) 5s 340 ± 5 ◦ C Water and few drops of HCl
Tin II chloride Antimony chloride (2.5 at.%) 5s 340 ± 5 ◦ C Water and few drops of HCl
films employing simplified spray pyrolysis technique. To the best of our knowledge this is the first report comparing structural, optical and electrical properties before and after annealing of T0/FTO/ATO top layer over AZO films employing simplified spray pyrolysis technique.
2. Experimental details The single layer AZO and double layered TO/AZO, FTO/AZO and ATO/AZO films have been fabricated using simplified spray pyrolysis technique. For the double layered films, AZO layer was deposited first onto the glass substrate as bottom layer and the TO, FTO and ATO layer are deposited separately as the top layer (over layer). The deposition parameters employed in this study are presented in Table 1. Borosilicate glasses were used as substrates. The starting solution was stirred continuously for 1 h to get a clear and homogenous solution. The solution was sprayed intermittently onto preheated ultrasonically cleaned glass substrates at certain intervals. Distance between nozzle and substrate and the spray angle with respect to the plane of the substrate were kept as 30 cm and 45◦ , respectively. Details of the simplified spray system have been described
elsewhere [28]. Using chromel alumel thermo couple, temperature was maintained at 350 ± 5 ◦ C. For each double layered system, two sets of samples with two different thicknesses of the over layer namely 50 nm and 100 nm were prepared and their structural, optical and electrical properties were studied before and after thermal annealing at 400 ◦ C for 1 h. X-ray diffractometer (PANalytical – PW 340/60 X’pert PRO), UV-vis-NIR double beam spectrophotometer (LAMBDA-35, range of 300–1100 nm), Spectro Fluorometer (450 W, Jobin YvonFLUOROLOG-FL3-11) and four-point probe were employed to obtain X-ray diffraction (XRD) patterns, transmission spectra, photoluminescence (PL) spectra and sheet resistance, respectively.
3. Results and discussion 3.1. Structural studies The XRD patterns of as deposited and annealed AZO films are shown in Fig. 1(a). In the case of as deposited films, three peaks corresponding to (0 0 2), (1 0 1) and (1 0 3) are observed, of which (0 0 2) is found to be predominant. According to the JCPDS card no. 36-1451, the observed peaks clearly showed that the films
Fig. 1. XRD patterns of as-deposited and annealed (a) AZO, (b) TO/AZO, (c) FTO/AZO and (d) ATO/AZO thin films.
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Fig. 2. Variation in the crystallite size values of as-deposited and annealed AZO, TO/AZO, FTO/AZO and ATO/AZO thin films
have hexagonal structure of ZnO. After annealing, all the above mentioned three peaks disappeared and a single strong peak corresponding to the (1 0 0) plane appeared. Literature showed that doping of some elements like boron and tin changes the microstructure of ZnO from (1 0 0) to (0 0 2) orientation [29,30]. As it is well known that, ZnO is thermally unstable, these types of change over from (1 0 0) to (0 0 2) or (0 0 2) to (1 0 0) orientations are reasonable to expect during the annealing process.
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This result clearly showed that, the annealing treatment causes a drastic change in the orientation of the crystal structure in bare AZO film. Santana et al. [31] reported a similar result for asdeposited and annealed undoped ZnO films. Several researchers [32,33] reported that the post heat treatments strongly influence the orientation of the doped ZnO films. Some researchers [18,29,30] suggested that, since the (0 0 2) plane has low surface energy (due to the densely packed structure); majority of the stable zinc oxide films possesses (0 0 2) as preferential orientation plane. When it losses the stability it gets reoriented in favour of other planes like (1 0 0) as observed in our case. Hence, there is a need to make the ZnO film stable in terms of its transparent conducting characteristics even after the post deposition heat treatment by keeping the structure oriented along (0 0 2) itself. It is interesting to found that in the case of all double layered films (Fig. 1(b)–(d)) with TO, FTO and ATO as over layers on AZO base layer, the result is different. In TO/AZO films, when the thickness of the TO layer is 50 nm, (1 0 0) peak emerge slightly after annealing. But, when the TO layer has higher thickness, it tends to prevent the reorientation in favour of (1 0 0) plane. In other words, the TO over layer with higher thickness improved the stability of the double layer system. Similar, change was observed in the case of FTO/AZO and ATO/AZO films also. From these results, we can clearly understand that the undoped as well as doped TO over layers act as a protective layer to keep the structure of ZnO remains unchanged. Fig. 2 shows variation in crystallite sizes of the as deposited and annealed bare AZO films as well as TO/AZO, FTO/AZO and ATO/AZO bilayered films. The crystallite size (D) is calculated using the Scherrer’s formula, D = 0.94/ˇcos where, is the wavelength of the
Fig. 3. Transmission spectra of as-deposited and annealed (a) AZO, (b) TO/AZO, (c) FTO/AZO and (d) ATO/AZO thin films.
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Fig. 4. Plot of dT/d vs. avg of as-deposited and annealed single layered and double layered films.
applied X-ray (0.154056 nm for Cu-K␣ , ˇ is the full-width at halfmaximum and is the Bragg’s angle [34–36]. In the case of AZO films crystallite size decreases appreciably due to annealing, whereas in all the double layered films, the change is negligible. This stability in crystallite size itself confirms the thermal stability of bilayer films. 3.2. Optical studies The optical spectra of the as-deposited and annealed AZO films are shown in Fig. 3(a). Though the band edge of the as-deposited and annealed AZO films almost overlapped on each other and even the pattern of oscillations is similar, the percentage of transmission increases remarkably after annealing. For instance, in the wavelength range of 450–600 nm, the increase in average transmittance is about 20%. This result indicates the thermal instability of the AZO films in terms of optical characteristics also. But, when any one of the layers like TO, FTO and ATO (Fig. 3(b)–(d)) over layer is deposited on the AZO film, the scenario is different. In all the three cases, the transmission spectrum remains unchanged or changed only negligibly after annealing, indicating clearly the thermal stability of the double layer system. From these observations, we can strongly say that all the three over layers (TO, FTO and ATO) act as protective layer to make the system unaltered by heat treatment. The average transmittance of single layered as well as double layered films were around 80% and all the films show sharp absorption band edge. The optical band gap (Eg ) of all the samples is estimated from the plot (Fig. 4) drawn for the first derivative of
the transmittance with respect to the wavelength (dT/d) against the average wavelength (avg ) [37]. The Eg value remains almost the same in all the double layered films, which is another evidence for the thermal stability of bilayered films. The Eg value of doped TO double layered films slightly increases when compared with undoped films, which may be due to the increase in carrier concentration according to Moss Burstein effect [38]. Generally, the amount of free carrier concentration in the material determines the position of absorption edge. The Eg value of the double layer films lies around 3.20 eV, which is very close to that of zinc oxide, suitable for photovoltaic and opto-electronic applications [39]. 3.3. Photoluminescence studies Fig. 5(a) shows the PL spectra of as-deposited and annealed AZO films. Three peaks were found at wavelength 398, 467 and 529 nm. The peak at 398 nm corresponds to the ultra-violet near band edge transition of ZnO material [40], which is responsible for crystalline quality. The intensity of the 398 nm peak decreased appreciably in the case of annealed AZO single layered film, which shows the major change in the crystalline nature of the film, when subjected to post deposition high temperature treatment [41]. This variation in PL peak intensity is corroborated well with XRD results. Also, the peak around 529 nm, corresponds to green emission originated from the electron transition from the deep-level donor of the ionized oxygen vacancies to the valance band is found missing in the case of annealed AZO film [42,43]. This is another evidence for above mentioned observation. The peak at 467 nm associated with
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Fig. 5. PL spectra of as-deposited and annealed (a) AZO, (b) TO/AZO, (c) FTO/AZO and (d) ATO/AZO thin films.
the blue emission originated from the electron transition from the shallow donor level of oxygen vacancies to the valence band and electron transition from the shallow donor level of zinc interstitials to the valence band [42,43]. Fig. 5(b)–(d) shows the PL spectra of as-deposited and annealed TO/AZO, FTO/AZO and ATO/AZO double layered films, respectively. In the double layered system, except the introduction of the peak at 344 nm, which is attributed to band to band transition of SnO2 [44], the intensity of all other peaks were not affected appreciably after annealing when compared with that of bare AZO film. This manifests the thermal stability of the double layer system due to the presence of tin oxide based over layers. But the intensity of these peaks increases, with increase in over layer thicknesses. The peak at 398 nm is found to be broadened in the case of doped double layered films, when compared with undoped double layered film, which may be due to the slight deformity in the crystalline structure, caused by dopant incorporation, which in turn increase the carrier concentration [45].
in the sheet resistance as seen from Fig. 6. It is found that annealing does not affect Rsh much. From Fig. 6 it can be inferred that the double layered films exhibit better thermal stability in terms of electrical sheet resistance than that of single layer AZO film. Generally, when AZO films are exposed to high temperature process, sheet resistance value decreases to a small amount due to the agglomeration of grains which causes an increase in mobility and
3.4. Electrical studies Fig. 6 shows the electrical sheet resistance values of single layered ZnO and double layered TO/AZO, FTO/AZO and ATO/AZO films measured before and after annealing. In the case of as-deposited films, the sheet resistance is maximum (53.97 /) for the bare AZO film, but the protective overlayer induced a drastical reduction
Fig. 6. Graphical representation of variation in sheet resistance of as-deposited and annealed single layered and double layered thin films.
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inturn reduces the sheet resistance. But, at high temperature process, due to the loss of oxygen vacancies, there is also a decrease in carrier concentration observed causing an increase in resistivity [46–48]. In the present study, as the later is predominant over the former, there is a net increase in sheet resistance after thermal annealing. The loss of oxygen vacancies in bare AZO film due to annealing is clearly evinced by the suppresion of PL peak at 529 nm (Fig. 5(a)). Moreover, from XRD studies (Fig. 1(a)) it is observed that while annealing the bare AZO film, the change in predominancy of peak from (0 0 2) to (1 0 0) occurs due to degradation of crystal structure [17,30], which inturn increases the resistivity. Kumar et al. [30] clearly showed that doped ZnO has stable microstructure when the preferential orientation is along (0 0 2), whereas it is unstable when the preferential growth is in favour of (1 0 0). In addition, they found that, resistivity increased with the decreases in the intensity of (0 0 2) peak. Moreover, according to Rahman et al. [49], there is always an increase in resistivity with the increase of the intensity of (1 0 0) peak. Both these changes are observed simultaneously in the present findings, it is clear that this degradation of crystallinity may be the reason for further reduction of sheet resistance in the bare AZO film after annealing. In the present study, the resistivity of double layered films is reduced appreciably as observed in Fig. 6, which may be due to the presence of protective layers like TO, FTO and ATO layers on AZO layer that prevented the loss of oxygen vacancies in AZO layer and also maintained the crystalline structure without much change. These findings are confirmed well from the XRD studies, where the occurance of (1 0 0) peak is almost restricted to a great extent in the double layered film. Thus the electrical results also become a strong evidence to confirm the thermal stability of the double layered films. When compared with the undoped TO over layered films, doped TO over layered films have lesser resistivity, which may be due to increase in carrier concentration caused by dopant incorporation. This observation well corroborates with PL findings [Section 3.3]. 4. Conclusion A novel double layered TCO system involving AZO as base layer and three different seperate over layers viz. tin oxide (TO), fluorine doped tin oxide (FTO) and antimony doped tin oxide (ATO) was realized and their thermal stability with respect to their structural, optical and electrical properties were investigated and reported. The structural studies revealed that the peak corresponding to (1 0 0) plane emerged predominantly in the case of annealed bare AZO films, which is an indicator of instability of AZO was desirably suppressed by the TO, FTO and ATO over layers. The electrical studies showed that the sheet resistance of the double layered system remains unchanged even after annealing which was not true in the case of bare AZO films, where the sheet resistance got increased after annealing due to the loss of oxygen vacancies. The optical studies also clearly showed the thermal stability of the double layered system. Hence, we can convincingly conclude that these double layered systems may be very much useful for solar cell devices as they required thermally stable low cost TCOs for cost effective photovoltaic technology. Among the double layered films, FTO/AZO film appeared to be better with fine crystalline structure and lower sheet resistance. Acknowledgement The financial support by the University Grants Commission, New Delhi through the Major Research project (F.No 40-28/2011(SR)) is gratefully acknowledged.
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