Materials Letters 149 (2015) 43–46
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Effect of the annealing process on electrical and optical properties of SnO2/Ag/SnO2 nanometric multilayer ﬁlm Lingyun Liu a,b, Shanshan Ma a, Hao Wu a, Bo Zhu a, Huimin Yang a, Jingjing Tang a, Xiangyang Zhao a,n a b
Patent Examination Cooperation Center of the Patent Ofﬁce, SIPO, Chengdu, Sichuan 610213, China College of Materials Science and Engineering, Sichuan University Chengdu 610064, China
art ic l e i nf o
a b s t r a c t
Article history: Received 29 December 2014 Accepted 18 February 2015 Available online 26 February 2015
The SnO2/Ag/SnO2 nano-multilayer structure has been designed and fabricated on quartz glass by magnetron sputtering and then annealed in air. The effect of annealing temperature on the structural, optical, and electrical properties of the SAS ﬁlms was investigated. The SEM images and XRD patterns show that crystallinity of the samples was improved as a result of annealing. High-quality transparent conductive ﬁlms with sheet resistance of 4.4 Ω/sq and maximum transmittance of 91% at 500 nm wavelength were obtained with an annealing temperature of 200 1C. The ﬁgure of merit of the SAS ﬁlms annealed at 200 1C reached a maximum of 3.39 10 2 Ω 1. It is observed the allowed direct band gap decreases with increasing substrate temperature. These multilayer ﬁlms can be used as transparent conductive electrodes in optoelectronic devices. & 2015 Elsevier B.V. All rights reserved.
Keywords: SnO2/Ag/SnO2 Nano-multilayer Energy band structure Optical Electrical properties
1. Introduction Low resistivity and high transmittance of transparent conductive oxides (TCOs) ﬁlms have received extensive attention because of their technological applications in optoelectronic devices [1–3]. The most common materials used for transparent conductive electrodes are doped metal oxides based on In2O3, ZnO, or SnO2. However, their application are limited by indium rarity, poor mechanical ﬂexibility or high cost . Recent investigations show that dielectric/metal/dielectric (D/M/D) multilayer ﬁlms are suitable candidates for transparent conductive electrodes. Many sandwich structures have been recommended such as ITO/Ag/ITO, ZnO/ Cu/ZnO, IZO/Ag/IZO, AZO/Mo/AZO, AZO/Ni/AZO, ZnS/Ag/ZnS and so on [5–7]. In this work, SnO2 was chosen to be used as a dielectric layer because of its mechanically hard, low cost, high chemical and thermal stabilities. Silver was chosen to be used as metal layer, due to its low resistivity. The SnO2/Ag/SnO2 tri-layers structure transparent conductive ﬁlm was fabricated by magnetron sputtering. Various parameters have signiﬁcant inﬂuence on characteristics of TCO ﬁlms. Conducting the annealing process under a suitable temperature has a major role in decreasing the intrinsic stress, increasing mobility of charge carriers, and improving homogeneity of the ﬁlm [8–9]. The microstructures
and composition of as-deposited ﬁlms would be rearranged during the annealing procedures. In our work, the inﬂuence of annealing temperature on the structural, optical, and electrical properties of the SnO2/Ag/SnO2 multilayer ﬁlms was investigated.
2. Experimental procedures Thin dielectric layer of SnO2 were deposited by RF magnetron sputtering at 50 W power; Ag layer was deposited by DC magnetron sputtering at 120 W power. Both top and bottom SnO2 layers were approximately 25 nm thick, while the thickness of silver layer was 10 nm. The samples were annealed in air at 120 1C, 200 1C, 300 1C and 400 1C for an hour, respectively. The phase structure of the ﬁlms was identiﬁed by using grazing incidence X-ray diffraction (XRD) (DX-1000, Dandong, China). The surface morphology was performed using scanning electron microscopy (SEM) (SU8010, HITACHI, Japan). The resistivity was characterized by a four point probe method. The optical transmittance was obtained from a UV–vis spectrometer (UV-2100, Shimadzu, Japan).
3. Results and discussion n
Corresponding author. Tel.: þ 86 28 62967613. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.matlet.2015.02.093 0167-577X/& 2015 Elsevier B.V. All rights reserved.
Fig. 1 shows the schematic diagram of the energy band structure of SnO2/Ag/SnO2 nano-multilayer ﬁlm. The metal and
L. Liu et al. / Materials Letters 149 (2015) 43–46
the semiconductor have different positions of Fermi levels relative to the vacuum level. Ag has a work function of Wm ¼4.4 eV  and SnO2 has a work function of Ws ¼5.2 eV . When Ag layer is in contact with SnO2 layers, charges ﬂow from Ag to SnO2 in such a way that the Fermi level is constant throughout the structure in thermodynamic equilibrium. The electron ﬂowing results in an accumulation-type contact between the Ag and SnO2 layers due to band bending at the contact. A Schottky contact barrier forms at the interface with a barrier height of qVD ¼Ws Wm ¼0.8 eV. Based on Schottky theory, we expect high carrier concentrations in the multilayer SnO2/Ag/SnO2 ﬁlm. Fig. 2 shows the XRD patterns of the as-deposited SAS ﬁlms and after annealing, respectively. The assigned peaks are related to the crystalline SnO2 with tetragonal structure (JCPDS no.41-1445) and Ag with cubic structure (JCPDS no.65-2871). It can be seen that before heat treatment, the SnO2 ﬁlms are amorphous. However after annealing, X-ray diffraction pattern shows diffraction peak at 2θ ¼ 26.6341 which correspond to SnO2 (110). Since the grain boundary may absorb the visible light and reduce the carrier
Fig. 1. Schematic band structure diagram of SnO2/Ag/SnO2 nano-multilayer structure.
mobility, optical transmittance and sheet resistance depend strongly on the crystallized state of the ﬁlm. The diffraction peaks at 2θ ¼ 38.3701, 44.1161, 64.3061, and 77.7651 are related to Ag (111), Ag (200), Ag (220), and Ag (311), respectively. The peaks' intensity of Ag are enhanced by increasing the annealing temperature, which can be understood based on the fact that the thermal energy during annealing may enable atoms to diffuse and stack, As a result, a more perfect crystal structure was obtained. After annealing at 400 1C, the peak of Ag2O (003) and Ag2O (113) appeared, indicating the silver layer is oxidized at 400 1C. Fig. 3(a–d) exhibits the SEM images of the SnO2/Ag/SnO2 multilayer ﬁlms for as-deposited and annealed in air. It is found that the average grain size of the multilayer ﬁlms increases with the increasing of annealing temperature. After annealeing at 200 1C for an hour, the surface morphology of the multilayer ﬁlms transforms to compact and smooth, which indicates the ﬁlms have better crystallization. When annealing temperature increases to 400 1C, the size of surface grains becomes non-uniform. A crosssectional SEM image of the SnO2/Ag/SnO2 multilayer ﬁlms annealed at 200 1C is shown in Fig. 3(e). Fig. 4(A) shows optical transmittance spectra for SnO2/Ag/SnO2 multilayer ﬁlms as a function of annealing temperature. The maximum optical transmittance of the as-deposited ﬁlms is about 88% in the visible region. After annealing in air at 200 1C for an hour, the SAS multilayer shows a maximum transmittance of 91% at 500 nm wavelength due to decreased optical absorption at the grain boundary induced by crystal growth. This result agrees with the surface morphology of the ﬁlms. While the ﬁlms annealed at 300 1C and 400 1C had lower optical transmission values of 86% and 80%, respectively. It is supposed that the SnO2 ﬁlms decrease the reﬂectance from the Ag surface and promote high optical transmission in the visible region. However, diffusion of Ag atoms into the SnO2 ﬁlms at high annealing temperature may decrease the antireﬂection effect of the SnO2 ﬁlms, resulting in more scattering of the incident light and reduction of the transmittance. Sahu D.R.  also reported a similar result for ZnO/Cu/ZnO ﬁlms annealed in an oxygen atmosphere. The absorption coefﬁcient can be obtained from Lambert's formula . Also, the absorption coefﬁcient for the direct allowed transition can be described as a function of photon energy  ðahvÞ 2 ¼ Aðhv Eg Þ Fig. 4(B) exhibits the variation of the band gap energy of the SAS multilayer system. It is found that the band gap energies of the multilayer ﬁlms decreased from 3.94 eV to 3.76 eV for as prepared and annealed at 400 1C. This decrease in band gap can be associated with increasing grain size of the ﬁlms upon annealing and decrease of the fraction of the amorphous SnO2 phase as conﬁrmed from the XRD measurements. Also, annealing above the glass transition temperature causes thin ﬁlms to crystallize with the formation of dangling bonds around the surfaces of the crystallites. These dangling bonds are responsible for the formation of defects that cause the optical band gap to decrease. The variation in sheet resistance and ﬁgure of merit of the SnO2/Ag/SnO2 ﬁlms as a function of annealing temperature are displayed in Fig. 5. The measured resistance of the multilayer structure, Rs, can be expressed as a function of the resistance of an individual layer coupled in parallel, as 1 1 1 1 ¼ þ þ Rs RSnO2 RAg RSnO2
Fig. 2. XRD patterns of the SAS ﬁlms: (a) as-deposited, (b) annealed at 120 1C, (c) annealed at 200 1C, (d) annealed at 300 1C, and (e) annealed at 400 1C.
As the resistivity of Ag is much lower than SnO2, the total resistance mainly depends on RAg, thus the SnO2/Ag/SnO2 multilayer ﬁlm has a lower resistance than SnO2 ﬁlm. It is observed that the sheet resistance is decreased slightly from 5.0 Ω/sq for as-deposited to 4.4 Ω/sq for annealing at 200 1C. The reduction of
L. Liu et al. / Materials Letters 149 (2015) 43–46
Fig. 3. SEM images of SnO2/Ag/SnO2 multilayer ﬁlms at different annealing temperatures: (a) as-deposited, (b) 120 1C, (c) 200 1C, (d) 400 1C, and (e) cross-sectional SEM image of the multilayer ﬁlm annealed at 200 1C.
Fig. 4. (A) Optical transmittance spectra for SAS multilayer ﬁlms as a function of annealing temperature; (B) band-gap (Eg) estimation of SAS multilayer ﬁlms from Tauc relation: (a) as-deposited, (b) annealed at 120 1C, (c) annealed at 200 1C, (d) annealed at 300 1C, and (e) annealed at 400 1C.
resistivity may be due to grain growth in both dielectric and metal layers which results in reduction of grain boundary's carriers scattering and increasing the electron mobility. However, the sheet resistance increases when annealing temperature is up to 300 1C. That is because oxygen atoms diffuse through Ag layers at higher temperature and decrease the Ag purity. The ﬁgure of merit (FTC) is an important factor that represents the relationship between sheet resistance and optical transmittance. A ﬁgure of merit, FTC, (as deﬁned by Haacke ) was estimated for each of the SAS multilayer ﬁlms. Since a higher FTC
denotes a higher quality SAS multilayer ﬁlm, it can be concluded that the SAS multilayer ﬁlm annealed at 200 1C is a suitable structure for transparent conductive electrode.
4. Conclusions The SnO2/Ag/SnO2 nano-multilayer structure has been designed and fabricated by magnetron sputtering and then annealed in air. The effect of annealing temperature on the structural, optical, and
L. Liu et al. / Materials Letters 149 (2015) 43–46
transmittance: 82%; ﬁgure of merit: 1.27 10 2 Ω 1) , FTO (sheet resistance: 5.2 Ω/sq; transmittance: 78.5%; ﬁgure of merit: 1.7 10 2 Ω 1)  and AZO (sheet resistance: 29 Ω/sq; transmittance: 93%; ﬁgure of merit: 1.8 10 2 Ω 1) , these nanomultilayer ﬁlms show improved electrical and optical properties, indicating their potential application in transparent conductive electrodes of optoelectronic devices.
Fig. 5. Variation in sheet resistance and ﬁgure of merit of SnO2/Ag/SnO2 ﬁlms as a function of annealing temperature.
electrical properties of the SAS ﬁlms was investigated. The SEM images and XRD patterns show that the crystallinity of the samples was improved as a result of annealing. High-quality transparent conductive ﬁlms with sheet resistance of 4.4 Ω/sq and maximum transmittance of 91% at 500 nm wavelength were obtained with an annealing temperature of 200 1C. The ﬁgure of merit of the SAS ﬁlms annealed at 200 1C reached a maximum of 3.39 10 2 Ω 1. It is observed the allowed direct band gap decreases with increasing substrate temperature. Finally, our results show that annealing temperature has an important role in controlling the electrical, optical and structural properties of the nano-multilayer ﬁlms. Compared with the conventional TCOs, such as ITO (sheet resistance: 63 Ω/sq;
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