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Effect of balanced and unbalanced magnetron sputtering processes on the properties of SnO2 thin films
Current Applied Physics 19 (2019) 697–703
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effect of balanced and unbalanced magnetron sputtering processes on the properties of SnO2 thin films
T
Gauri Shanker, P. Prathap, K.M.K. Srivatsa, Preetam Singh∗ CSIR- National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi, 110012, India
A R T I C LE I N FO
A B S T R A C T
Keywords: SnO2 thin film Electrical properties Spectroscopic ellipsometer Photoluminescence Unbalanced magnetron sputtering
A comparative study has been carried on the role of balanced magnetron (BM) and unbalanced magnetron (UBM) sputtering processes on the properties of SnO2 thin films. The oxygen partial pressure, substrate temperature and deposition pressure were kept 20%, 700 °C and 30 mTorr, respectively and the applied RF power varied in the range of 150–250 W. It is observed that the UBM deposition causes significant effect on the structural, electrical and optical properties of SnO2 thin films than BM as evidenced by X-ray diffraction, C-V, Spectroscopic Ellipsometer and Photoluminescence measurements. The value of band gap (Eg) of the films deposited at 150 W in UBM is found as Eg = 3.83 eV which is much higher than the value of Eg = 3.69 eV as observed in BM sputtering indicating that UBM sputtering results in good crystalline quality. Further, the C-V measurements of SnO2 thin films deposited using UBM at high power 250 W show hysteresis with large flat band shift indicating that these thin films can be used for the fabrication of memory device. The observed results have been attributed to different mechanisms which exist simultaneously under unbalanced magnetron sputtering due to ion bombardment of growing SnO2 thin film by energetic Ar+ ions.
1. Introduction Tin dioxide (SnO2), also called as stannic oxide, is one of the most studied and extensively useful transition metal oxide. It has received significant attention due to its potential utility in various practical applications such as conductive electrodes and transparent coatings, heterojunction solar cells, optoelectronic devices, gas sensors, heat reflectors, flat display devices, photovoltaic cells, dye-based solar cells, and thin film transistors, etc [1–7]. SnO2 is an n-type wide band gap semiconductor oxide and is highly transparent in the visible region due to its high direct band gap (Eg) in the range of 3.6–3.9 eV at room temperature [8,9]. Since the properties of thin film materials are strongly dependent on the conditions of thin film preparation and deposition techniques, efforts have been put by several workers for the deposition of SnO2 thin films using different deposition techniques viz DC/RF magnetron sputtering, thermal evaporation, sol-gel, spray pyrolysis, chemical vapor deposition, hydrothermal process, etc [10–16]. For the deposition of stoichiometric thin films of metal oxides, magnetron sputtering technique has been recognized as one of the promising versatile technique due to its various advantages such as good adhesion to substrates, high density and homogeneity of deposited thin films. Thin films deposition by magnetron sputtering can be performed ∗
under two configurations; conventional balanced magnetron (BM) and unbalanced magnetron (UBM). Under UBM the plasma gets extended towards the substrate and the energetic Ar+ ions keep bombarding the growing film, there by several mechanisms come into picture. As the plasma power increases, the ions in the plasma gain high energy and actively participate in film growth through sputtering, atomic mixing, densification, enhanced migration of adatoms and field-enhanced diffusion via charging and restructuring [17,18]. The ionic bombardment of the energetic ions on the growing film leads to the creation of various metastable states/structures due to thermal spike [17]. The thin film properties depend mainly on the precursors used and the energies of the impinging ions. It is to be noted that the energy of the ions in the plasma can be varied either by increasing the electrical power or by applying the substrate bias. In our earlier studies [17–21] we have observed such structural changes in the case of Diamond like carbon (DLC) and Titania (TiO2) thin films deposited by Plasma enhanced chemical vapor deposition (PECVD) process in which the energetic ions are created by applied bias. Though several workers have reported on the deposition of thin films of various materials using (BM and UBM) sputtering processes [22–24], to the best of our knowledge there is no such report available for SnO2 thin films. In the present study, we have investigated the effect of balanced and unbalanced RF magnetron sputtering processes on the deposition of
Corresponding author. E-mail address: [email protected] (P. Singh).
https://doi.org/10.1016/j.cap.2019.03.016 Received 30 January 2019; Received in revised form 10 March 2019; Accepted 19 March 2019 Available online 21 March 2019 1567-1739/ © 2019 Korean Physical Society. Published by Elsevier B.V. All rights reserved.
Current Applied Physics 19 (2019) 697–703
G. Shanker, et al.
Fig. 1. (a) Magnets array, (b) Balanced magnetron sputtering and (c) unbalanced magnetron sputtering conditions.
(below 10−2 mbar) but also enhanced excitation and ionization rates. This dipole is created by using rows of permanent magnets in the order of S-N-S or N-S-N with the inner/centre magnet in opposite polarization with respect to other two rows [25,26]. Normally, the outer magnets are fixed on soft iron rings as shown the magnets array in Fig. 1 (a). We have used ALNICO permanent bar magnets on the rings and cylindrical magnets at the centre. If all the magnets have same strength then all field lines from outer magnets will pass through the centre magnet and the resulting magnetic field will be such that the plasma electrons cannot escape easily from the magnetic trap, and the plasma will be confined close to the target, and this condition of magnetic arrangement is called balanced magnetron as shown in Fig. 1 (b). On the other hand, if only the centre magnets are week, all the magnetic field lines will not be caught up by centre magnet and the shape of the resultant
SnO2 thin films and its properties. It has been observed that the UBM enables to deposit SnO2 thin films with improved structural, optical and electrical properties. Very interestingly, the C-V measurements on the SnO2 thin films deposited under UBM have showed suitability of SnO2 films for the fabrication of electron trapping and de-trapping memory devices.
2. Experimental details 2.1. Balanced and unbalanced magnetron device Basically, the magnetron device consists of dipole configuration to trap the electrons emitted at the cathode in a sputtering system [25,26]. The dipole enables not only the operation of discharge at low pressures 698
Current Applied Physics 19 (2019) 697–703
G. Shanker, et al.
magnetic field will change allowing some of the electrons from plasma to escape easily towards the substrate, and this condition of magnetron arrangement is called unbalanced magnetron and is shown in Fig. 1 (c). In this situation the gas ionization occurs near the substrate and the energetic gas ions (normally Ar gas ions) will bombard the surface of the substrate causing ion assisted deposition of the growing films. 2.2. Deposition of SnO2 thin films Tin oxide (SnO2) thin films have been deposited on p-type and a resistivity of 5–10 Ωcm (boron doped) Si (100) substrate, using SnO2 sputtering target in an in-house designed and fabricated downstream RF magnetron sputtering system. The diameter and the thickness of sputtering target were 6 inch and 3 mm, respectively. The thin films were deposited at different RF powers varying in the range from 150 W to 250 W, using unbalanced magnetron and conventional balanced magnetron sputtering processes. The Si substrates were first degreased with boiled trichloroethylene (TCE), then followed by acetone, methanol and de-ionized water cleaning. After that the substrates were dipped in 10% hydrofluoric acid (HF) for 10 s to remove the native SiO2 layer, followed by rinsing in de-ionized water. The deposition pressure, deposition time, substrate temperature and the distance from target to substrate were kept fixed at 30 mTorr, 60 min, 700 °C and 5 cm, respectively. A turbo-based pumping system, backed by roots and rotary pumps, was used to achieve a base pressure about 2 × 10−6 Torr. Premixed Ar (99.999% purity) + O2 (99.999% purity) gases in the ratio of 8:2 were used during sputtering process. The deposition pressure in the vacuum chamber was measured by a compact process ion gauge (Pfeiffer) and the gas flow rates were accurately controlled by mass flow controllers (Aalborg, model GFC-17). 2.3. Characterization techniques To carry out electrical characterizations (C-V measurements) of SnO2 thin films, MOS structures have been fabricated. The SnO2/p-Si devices were metalized (Al contacts) for front and bottom electrodes. Al contacts were formed in the form of circular dots of 2 mm in diameter and 100 nm thickness on the surface of SnO2 by vacuum thermal evaporation at the pressure of 1 × 10−6 Torr. On the back side of Si substrate Al film of about 250 nm was deposited by sputtering process using high-purity Al (99.999%) target, followed by sintering at 580 °C in the inert ambient for 5 min to form ohmic contacts. The C-V measurements were carried out at room temperature and at 1MHz frequency. The structural properties of SnO2 thin films were characterized by Rigaku X-ray diffractometer with Cu-Kα1 radiation (λ = 1.54 Å) in θ-2θ geometry. The optical properties of SnO2 thin films were studied by Spectroscopic ellipsometer (J.A. Woollam, model: VASE32) and room temperature photoluminescence (RT-PL) with Xenon Lamp (at 310 nm) as an excitation source. The thickness of SnO2 thin films were also measured using a Stylus profiler (Ambios, model: XP-200). 3. Results and discussions Fig. 2(a) and (b) show the XRD patterns of as deposited SnO2 thin films on Si substrate at different applied RF powers (150 W–250 W) using UBM and BM sputtering processes, respectively. These XRD patterns reveal the formation of tetragonal rutile structure of SnO2 [JCPDS file No. 77-0452] with number of well-defined diffraction peaks. It can be seen that under UBM, at low sputtering power of 150 W, the films tend to have random orientation with intense (110), (101), (211) peaks and a very low intensity (200) and (111) peaks. As the power increased to 200 W the films showed a dominant (101) orientation in comparison to other peaks, indicating that it is possible to obtain highly oriented SnO2 thin films simply by tailoring the applied RF power. Further increase in the RF power the polycrystalline nature of SnO2 is increased.
Fig. 2. X-ray diffraction pattern of SnO2 thin films deposited at different RF powers using (a) unbalanced and (b) balanced magnetron sputtering.
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Fig. 3. (a-f). C-V hysteresis curves at 1 MHz frequency of SnO2 thin films deposited at different RF powers using unbalanced and balanced magnetron sputtering.
deposited SnO2 thin films at different applied RF powers using unbalanced and balanced magnetron sputtering processes. It can be seen that for the thin films deposited using BM sputtering the C-V curve shows very small hysteresis for low RF power (150 W) which is reduced to negligible at high RF power (250 W). On the other hand, for the films deposited using UBM sputtering the hysteresis increased with increasing the RF power from 150 to 250 W. In this case there is a counter clockwise shift of C-V curve with a huge flat band shift of about 3 V for the films deposited at 250 W, indicating that it can act as electron trapping and de-trapping memory device. The overall observed C-V behavior in both UBM and BM sputtering can be attributed to the formation/curing of different traps in the growing films during the physical bombardment of growing film by energetic Ar+ ions due to
Similar behavior is observed with increasing the RF power from 150 to 250 W in BM, however, no sign of single oriention has been observed. It is also observed that the intensity of XRD peaks in UBM is high in comparison to the intensity of peaks in BM i.e. the UBM results in improved crystalline quality compared to conventional BM sputtering process for the entire range of applied RF powers. This indicates that the crystallographic orientation and crystalline quality of the deposited SnO2 thin films can be controlled by applied RF power and the magnetron configurations (UBM and BM) have significant influence on the structural properties. This observation is attributed to the re-crystallization process which occurs during physical bombardment by energetic Ar+ ions using UBM sputtering process. Fig. 3(a-f) shows the hysteresis curves of C-V at 1 MHz for the as 700
Current Applied Physics 19 (2019) 697–703
G. Shanker, et al.
Fig. 5. Dispersion behavior of refractive index (n) of SnO2 thin films deposited at different RF powers using (a) unbalanced and (b) balanced magnetron sputtering. Inset shows the extinction coefficients (k), respectively.
Fig. 4. Experimental and model fit ellipsometric parameters (Ψ & Δ) of SnO2 thin films deposited at 150 W using unbalanced magnetron sputtering. The dashed lines represent the model-fit data .
496.8 ± 0.5 nm in UBM and from 255.1 ± 0.3 nm to 537 ± 0.6 nm in BM sputtering process with increasing the value of RF power from 150 W to 250 W. It has been observed that there is not much change in the value of thickness in both UBM and BM at 150 W while at 250 W the value of thickness is lower (∼40 nm) in UBM than BM sputtering. This decrease in thickness is attributed to the back-sputtering of the growing film due to bombarding energetic Ar+ ions in UBM process. These extracted values of thickness from ellipsometric data are in good agreement with the thicknesses measured by the Stylus profiler. Fig. 5 (a & b) represents the dispersion behavior of refractive index (n) as a function of wavelength in the range of 300–1000 nm as obtained from the corresponding ellipsometric data of SnO2 thin films deposited at different RF powers from 150 to 250 W using UBM and BM sputtering, respectively. The value of n at 600 nm wavelength increases from 1.94 to 1.96 with an increase in RF power from 150 W to 200 W using UBM sputtering and there is no considerable change in the value of n (=1.96) by further increasing the RF power to 250 W. Similarly there is no considerable change in the value of n (vary in the range of 1.98 to 1.96) in BM sputtering with an increase in the RF power from 150 W to 250 W. The measured values of obtained n of SnO2 thin films are in good agreement with reported values in the literature [9,27]. The extinction coefficient (k) of SnO2 thin films deposited by UBM and BM sputtering at different RF powers is shown in the inset of Fig. 5 (a & b).
simultaneous occurrence of various mechanisms as mentioned above. It is to be mentioned here that it is the resultant energy that the growing film experienced matters in the formation/curing of different traps in the growing films. The optical properties of SnO2 thin films were studied by Spectroscopic ellipsometer (SE) in the wavelength range of 300–1000 nm. SE measurements provide the data related to the ellipsometric parameters; angle (Ψ) and phase (Δ) with respect to wavelength or photon energy. It is a non-destructive model fitting based technique, which minimizes the difference between experimental and calculated fitting values as a function of wavelength. Fig. 4 (a & b) represent the SE data for the ellipsometric parameters (Ψ & Δ) of the SnO2 thin films deposited at 150 W using UBM sputtering for the incident angles 55°, 65° and 75°, respectively. The experimental data has been fitted using Lorentz model for the air/SnO2/Si layer which takes into account the film thickness and contribution of the substrate. The dashed lines in the figures represent the model-fit data and it can be seen that all the features present in the experimental spectra are reproduced by the model fit. The fitting parameters within the parametric dispersion model yields thickness of SnO2 thin films deposited at different sputtering powers using UBM and BM sputtering processes. The value of thickness was increased from 256.6 ± 0.4 nm to 701
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Extinction coefficient is the imaginary part of the complex index of refraction, which also relates to the absorption of light. It defines how strongly a substance absorbs light at a given wavelength and is directly related to the absorption coefficient (α) as follow [28]:
k=
αλ 4π
It is clear from the graphs that all the SnO2 thin films show absorption in UV region, but in the visible and IR range the values of k are almost zero. These results indicate that the SnO2 thin films are highly transparent in the visible region. According to inter-band absorption theory, optical band gap of the thin films can be calculated using the following Tauc relation [29,30];
αhv = A (hv − Eg )n where A, Eg, hν and n are the probability parameter for the transition, band gap of the material, incident photon energy and the transition coefficient (2 for indirect and 1/2 for direct band gap), respectively. The absorption coefficient α is extracted from the ellipsometric data after model fitting. Here, the direct band gap of the SnO2 thin films was evaluated by extrapolating the straight line part of the curves (αhv )2 = 0 as shown in Fig. 6 (a & b). We have observed the decrease in value of band gap from 3.83 eV to 3.68 eV with an increase in RF power from 150 W to 200 W in UBM sputtering and there is not much change in the
Fig. 7. Photoluminescence spectra of SnO2 thin films deposited at different RF powers using (a) unbalanced and (b) balanced magnetron sputtering.
value of Eg (3.69 eV) by further increasing the RF power to 250 W. On the other hand in BM sputtering the value of Eg was found to increase from 3.69 eV to 3.72 eV with increasing the RF power from 150 to 250 W. This is attributed to the difference in the growth processes in both UBM and BM sputtering, that causes changes the film density and crystalline quality of the deposited thin films resulting change in the optical properties. The optical properties are in agreement with the XRD results (explained above). All the obtained values of band gap (Eg) of SnO2 thin films are in good agreement with the reported results (3.6–3.9 eV) in the literature [8,9]. Fig. 7 (a & b) shows room temperature photoluminescence (RT-PL) spectra recorded in the range of 350–700 nm for the as deposited SnO2 thin films using UBM and BM sputtering, respectively. The excitation wavelength was 310 nm. Several workers have observed the emission peaks as we observed in the present work [27–34]. For all the deposited SnO2 thin films high intensity emission peak was observed at about 430 nm, which corresponds to SnO and SnO2. Normally, in SnO2 thin films point defects like oxygen vacancies and Sn interstitials exist which are donor type defects, and present at grain boundaries. These defects act as radiative centers in luminescence processes which are responsible for defect level emissions. There exists three types of oxygen vacancies Voo, V+o and V++o in oxide materials for the SnO2 thin films, the broad peak emission at about 590 nm is attributed to V+o, which
Fig. 6. Energy band gap plots of SnO2 thin films deposited at different RF powers using (a) unbalanced and (b) balanced magnetron sputtering. 702
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combines with a hole of the valence band forming V++o and is responsible for the band edge emission peak [27–34]. The emission peak at about 430 nm is attributed to the Sn interstitials while the peak at about 485 nm attributed to the oxygen vacancies Voo. It is to be noted that the intensity of broad peak at about 590 nm decreased significantly as RF power is increased. Further, for the thin films deposited at 250 W using UBM sputtering the PL recording the emission peak at 430 nm was broadened considerably, which shows that the energetic ion bombardment at high RF power caused formation of some additional type of oxygen vacancies resulting the observed broadness (that is, due to superimposition of the additional emission peak). The formation of the additional oxygen vacancies is supported by C-V measurements which have been reflected through huge flat band shift.
W. Shams-Kolahi, Mater. Res. Bull. 40 (2005) 1303. [2] C. Can, I. Grcia, A. Gtz, L. Fonseca, E. Lora-Tamayo, M.C. Horrillo, I. Sayago, J.I. Robla, J. Rodrigo, J. Gutirrez, Sens. Actuators B 65 (2000) 244. [3] P. Nelli, G. Faglia, G. Sberveglieri, E. Cereda, G. Gabetta, A. Dieguez, A. RomanoRodriguez, J.R. Morante, Thin Solid Films 371 (2000) 249. [4] M. Mwamburi, E. Wackelgard, A. Roos, Thin Solid Films 374 (2000) 1. [5] S.J. Laverty, P.D. Maguire, J. Vac. Sci. Technol. 19 (2001) 1. [6] L.T. Yin, J.C. Chou, W.Y. Chung, T.P. Sun, S.K. Hsiung, Sens. Actuators B 71 (2000) 106. [7] S. Ferrere, A. Zaban, B.A. Gsegg, J. Phys. Chem. B 101 (1997) 4490. [8] Y. Caglar, M. Caglar, S. Ilican, F. Yakuphanoglu, Microelectron. Eng. 86 (2009) 2072. [9] K. Ravikumar, S. Agilan, N. Muthukumarasamy, M. Raja, R. Lakshmanan, R. Ganesh, Silicon 10 (2018) 1591. [10] M.A. Gubbins, V. Casey, S.B. Newcomb, Thin Solid Films 405 (2002) 270. [11] S.M. Ingole, S.T. Navale, Y.H. Navale, D.K. Bandgar, F.J. Stadler, R.S. Mane, N.S. Ramgir, S.K. Gupta, D.K. Aswal, V.B. Patil, J. Colloid Interface Sci. 493 (2017) 162. [12] W. Luo, J. Deng, Q. Fu, D. Zhou, Y. Hu, S. Gong, Z. Zheng, Sens. Actuators B 217 (2015) 19. [13] C. Sankar, V. Ponnuswamy, M. Manickam, R. Mariappan, R. Suresh, Appl. Surf. Sci. 349 (2015) 931. [14] O. Takeya, O. Takayoshi, L.S. Dong, F. Shizuo, Phys. Status Solidi 8 (2011) 540. [15] N. Talebian, F. Jafarinezhad, Ceram. Int. 39 (2013) 8311. [16] Z. Yuan, D. Li, M. Wang, P. Chen, D. Gong, P. Cheng, D. Yang, Appl. Phys. Lett. 92 (2008) 121908. [17] K.M.K. Srivatsa, D. Chhikara, M.S. Kumar, J. Mater. Sci. Technol. 27 (2011) 696. [18] S. Kumar, C.M.S. Rauthan, K.M.K. Srivatsa, P.N. Dixit, R. Bhattacharya, Appl. Surf. Sci. 82 (2001) 326. [19] S. Kumar, C.M.S. Rauthan, P.N. Dixit, K.M.K. Srivatsa, M.Y. Khan, R. Bhattacharya, Vacuum 63 (2001) 433. [20] K.M.K. Srivatsa, M. Bera, A. Basu, Thin Solid Films 516 (2008) 7443. [21] K.M.K. Srivatsa, M. Bera, A. Basu, Mater. Lett. 62 (2008) 3527. [22] J.J. Olaya, S.E. Rodil, S. Muhl, E. Sanchez, Thin Solid Films 516 (2008) 8319. [23] E.C. Romero, M.G. Botero, W. Tillmann, F.B. Osorio, G.B. Gaitán, Bull. Mater. Sci. 41 (2018) 97. [24] G.J. Choi, H. Jung, D.H. Kim, Y. Sohn, J.S. Gwag, Catal. Sci. Technol. 8 (2018) 89. [25] B. Window, N. Savvides, J. Vac. Sci. Technol. A 4 (1986) 196. [26] http://www.gencoa.com/resources/documents/comparison-balanced-unbalancedarray-designs.pdf. [27] K.S. Shamala, L.C.S. Murthy, K.N. Rao, Bull. Mater. Sci. 27 (2004) 295. [28] L. Chaturvedi, S. Howladera, D. Chhikaraa, P. Singh, S. Bagga, K.M.K. Srivatsa, Indian J. Pure Appl. Phys. 55 (2017) 630. [29] J. Tauc (Ed.), Amorphous and Liquid Semiconductor, Plenum Press, New York, 1974, p. 159. [30] P. Singh, K.M.K. Srivatsa, S. Das, Adv. Mater. Lett. 6 (2015) 371. [31] H. Sefardjella, B. Boudjema, A. Kabir, G. Schmerber, Curr. Appl. Phys. 13 (2013) 1971. [32] N. Dharmaraj, C.H. Kim, K.W. Kim, H.Y. Kim, E.K. Suh, Spectrochim. Acta Part A 64 (2006) 136. [33] F. Gu, S.F. Wang, M.K. Lu, G.J. Zhou, D. Xu, D.R. Yuan, J. Phys. Chem. B 108 (2004) 8119. [34] J.X. Zhou, M.S. Zhang, J.M. Hong, Z. Yin, Solid State Commun. 138 (2006) 242.
4. Conclusions SnO2 thin films have been deposited by sputtering process under balanced and unbalanced magnetrons using SnO2 target with varying applied RF electrical power. It has been observed that using UBM it is possible to obtain highly crystalline SnO2 thin films with dominant (101) orientation just by tailoring the applied RF power. Also, the UBM results in SnO2 thin films with good optical properties as revealed by ellipsometric and PL measurements. Further, for the thin films deposited using UBM at 250 W RF power the C-V curve showed very large counter clockwise shift of about 3 V (with electron retention), clearly indicating the suitability for a practical memory device. The observed results clearly suggest the unbalanced magnetron of sputtering process enables to deposit high quality SnO2 thin films with respect to its structural, optical and electrical properties with added advantage of providing SnO2 thin films suitable for fabrication of memory devices. Acknowledgements The authors are grateful to the Director, CSIR-National Physical Laboratory, for his continuous encouragement and support during this work. Authors would like to fully acknowledge the help of Dr. V.P.S. Awana and Dr. D. Haranath for XRD and PL measurements, respectively. The financial support provided by Department of Science & Technology (DST) through SERB, New Delhi, India with reference No: SB/EMEQ-040/2014 is gratefully acknowledged. References [1] A.M. Gheidari, E.A. Soleimani, M. Mansorhoseini, S. Mohajerzadeh, N. Madani,
703
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effect of balanced and unbalanced magnetron sputtering processes on the properties of SnO2 thin films
T
Gauri Shanker, P. Prathap, K.M.K. Srivatsa, Preetam Singh∗ CSIR- National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi, 110012, India
A R T I C LE I N FO
A B S T R A C T
Keywords: SnO2 thin film Electrical properties Spectroscopic ellipsometer Photoluminescence Unbalanced magnetron sputtering
A comparative study has been carried on the role of balanced magnetron (BM) and unbalanced magnetron (UBM) sputtering processes on the properties of SnO2 thin films. The oxygen partial pressure, substrate temperature and deposition pressure were kept 20%, 700 °C and 30 mTorr, respectively and the applied RF power varied in the range of 150–250 W. It is observed that the UBM deposition causes significant effect on the structural, electrical and optical properties of SnO2 thin films than BM as evidenced by X-ray diffraction, C-V, Spectroscopic Ellipsometer and Photoluminescence measurements. The value of band gap (Eg) of the films deposited at 150 W in UBM is found as Eg = 3.83 eV which is much higher than the value of Eg = 3.69 eV as observed in BM sputtering indicating that UBM sputtering results in good crystalline quality. Further, the C-V measurements of SnO2 thin films deposited using UBM at high power 250 W show hysteresis with large flat band shift indicating that these thin films can be used for the fabrication of memory device. The observed results have been attributed to different mechanisms which exist simultaneously under unbalanced magnetron sputtering due to ion bombardment of growing SnO2 thin film by energetic Ar+ ions.
1. Introduction Tin dioxide (SnO2), also called as stannic oxide, is one of the most studied and extensively useful transition metal oxide. It has received significant attention due to its potential utility in various practical applications such as conductive electrodes and transparent coatings, heterojunction solar cells, optoelectronic devices, gas sensors, heat reflectors, flat display devices, photovoltaic cells, dye-based solar cells, and thin film transistors, etc [1–7]. SnO2 is an n-type wide band gap semiconductor oxide and is highly transparent in the visible region due to its high direct band gap (Eg) in the range of 3.6–3.9 eV at room temperature [8,9]. Since the properties of thin film materials are strongly dependent on the conditions of thin film preparation and deposition techniques, efforts have been put by several workers for the deposition of SnO2 thin films using different deposition techniques viz DC/RF magnetron sputtering, thermal evaporation, sol-gel, spray pyrolysis, chemical vapor deposition, hydrothermal process, etc [10–16]. For the deposition of stoichiometric thin films of metal oxides, magnetron sputtering technique has been recognized as one of the promising versatile technique due to its various advantages such as good adhesion to substrates, high density and homogeneity of deposited thin films. Thin films deposition by magnetron sputtering can be performed ∗
under two configurations; conventional balanced magnetron (BM) and unbalanced magnetron (UBM). Under UBM the plasma gets extended towards the substrate and the energetic Ar+ ions keep bombarding the growing film, there by several mechanisms come into picture. As the plasma power increases, the ions in the plasma gain high energy and actively participate in film growth through sputtering, atomic mixing, densification, enhanced migration of adatoms and field-enhanced diffusion via charging and restructuring [17,18]. The ionic bombardment of the energetic ions on the growing film leads to the creation of various metastable states/structures due to thermal spike [17]. The thin film properties depend mainly on the precursors used and the energies of the impinging ions. It is to be noted that the energy of the ions in the plasma can be varied either by increasing the electrical power or by applying the substrate bias. In our earlier studies [17–21] we have observed such structural changes in the case of Diamond like carbon (DLC) and Titania (TiO2) thin films deposited by Plasma enhanced chemical vapor deposition (PECVD) process in which the energetic ions are created by applied bias. Though several workers have reported on the deposition of thin films of various materials using (BM and UBM) sputtering processes [22–24], to the best of our knowledge there is no such report available for SnO2 thin films. In the present study, we have investigated the effect of balanced and unbalanced RF magnetron sputtering processes on the deposition of
Corresponding author. E-mail address: [email protected] (P. Singh).
https://doi.org/10.1016/j.cap.2019.03.016 Received 30 January 2019; Received in revised form 10 March 2019; Accepted 19 March 2019 Available online 21 March 2019 1567-1739/ © 2019 Korean Physical Society. Published by Elsevier B.V. All rights reserved.
Current Applied Physics 19 (2019) 697–703
G. Shanker, et al.
Fig. 1. (a) Magnets array, (b) Balanced magnetron sputtering and (c) unbalanced magnetron sputtering conditions.
(below 10−2 mbar) but also enhanced excitation and ionization rates. This dipole is created by using rows of permanent magnets in the order of S-N-S or N-S-N with the inner/centre magnet in opposite polarization with respect to other two rows [25,26]. Normally, the outer magnets are fixed on soft iron rings as shown the magnets array in Fig. 1 (a). We have used ALNICO permanent bar magnets on the rings and cylindrical magnets at the centre. If all the magnets have same strength then all field lines from outer magnets will pass through the centre magnet and the resulting magnetic field will be such that the plasma electrons cannot escape easily from the magnetic trap, and the plasma will be confined close to the target, and this condition of magnetic arrangement is called balanced magnetron as shown in Fig. 1 (b). On the other hand, if only the centre magnets are week, all the magnetic field lines will not be caught up by centre magnet and the shape of the resultant
SnO2 thin films and its properties. It has been observed that the UBM enables to deposit SnO2 thin films with improved structural, optical and electrical properties. Very interestingly, the C-V measurements on the SnO2 thin films deposited under UBM have showed suitability of SnO2 films for the fabrication of electron trapping and de-trapping memory devices.
2. Experimental details 2.1. Balanced and unbalanced magnetron device Basically, the magnetron device consists of dipole configuration to trap the electrons emitted at the cathode in a sputtering system [25,26]. The dipole enables not only the operation of discharge at low pressures 698
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magnetic field will change allowing some of the electrons from plasma to escape easily towards the substrate, and this condition of magnetron arrangement is called unbalanced magnetron and is shown in Fig. 1 (c). In this situation the gas ionization occurs near the substrate and the energetic gas ions (normally Ar gas ions) will bombard the surface of the substrate causing ion assisted deposition of the growing films. 2.2. Deposition of SnO2 thin films Tin oxide (SnO2) thin films have been deposited on p-type and a resistivity of 5–10 Ωcm (boron doped) Si (100) substrate, using SnO2 sputtering target in an in-house designed and fabricated downstream RF magnetron sputtering system. The diameter and the thickness of sputtering target were 6 inch and 3 mm, respectively. The thin films were deposited at different RF powers varying in the range from 150 W to 250 W, using unbalanced magnetron and conventional balanced magnetron sputtering processes. The Si substrates were first degreased with boiled trichloroethylene (TCE), then followed by acetone, methanol and de-ionized water cleaning. After that the substrates were dipped in 10% hydrofluoric acid (HF) for 10 s to remove the native SiO2 layer, followed by rinsing in de-ionized water. The deposition pressure, deposition time, substrate temperature and the distance from target to substrate were kept fixed at 30 mTorr, 60 min, 700 °C and 5 cm, respectively. A turbo-based pumping system, backed by roots and rotary pumps, was used to achieve a base pressure about 2 × 10−6 Torr. Premixed Ar (99.999% purity) + O2 (99.999% purity) gases in the ratio of 8:2 were used during sputtering process. The deposition pressure in the vacuum chamber was measured by a compact process ion gauge (Pfeiffer) and the gas flow rates were accurately controlled by mass flow controllers (Aalborg, model GFC-17). 2.3. Characterization techniques To carry out electrical characterizations (C-V measurements) of SnO2 thin films, MOS structures have been fabricated. The SnO2/p-Si devices were metalized (Al contacts) for front and bottom electrodes. Al contacts were formed in the form of circular dots of 2 mm in diameter and 100 nm thickness on the surface of SnO2 by vacuum thermal evaporation at the pressure of 1 × 10−6 Torr. On the back side of Si substrate Al film of about 250 nm was deposited by sputtering process using high-purity Al (99.999%) target, followed by sintering at 580 °C in the inert ambient for 5 min to form ohmic contacts. The C-V measurements were carried out at room temperature and at 1MHz frequency. The structural properties of SnO2 thin films were characterized by Rigaku X-ray diffractometer with Cu-Kα1 radiation (λ = 1.54 Å) in θ-2θ geometry. The optical properties of SnO2 thin films were studied by Spectroscopic ellipsometer (J.A. Woollam, model: VASE32) and room temperature photoluminescence (RT-PL) with Xenon Lamp (at 310 nm) as an excitation source. The thickness of SnO2 thin films were also measured using a Stylus profiler (Ambios, model: XP-200). 3. Results and discussions Fig. 2(a) and (b) show the XRD patterns of as deposited SnO2 thin films on Si substrate at different applied RF powers (150 W–250 W) using UBM and BM sputtering processes, respectively. These XRD patterns reveal the formation of tetragonal rutile structure of SnO2 [JCPDS file No. 77-0452] with number of well-defined diffraction peaks. It can be seen that under UBM, at low sputtering power of 150 W, the films tend to have random orientation with intense (110), (101), (211) peaks and a very low intensity (200) and (111) peaks. As the power increased to 200 W the films showed a dominant (101) orientation in comparison to other peaks, indicating that it is possible to obtain highly oriented SnO2 thin films simply by tailoring the applied RF power. Further increase in the RF power the polycrystalline nature of SnO2 is increased.
Fig. 2. X-ray diffraction pattern of SnO2 thin films deposited at different RF powers using (a) unbalanced and (b) balanced magnetron sputtering.
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Fig. 3. (a-f). C-V hysteresis curves at 1 MHz frequency of SnO2 thin films deposited at different RF powers using unbalanced and balanced magnetron sputtering.
deposited SnO2 thin films at different applied RF powers using unbalanced and balanced magnetron sputtering processes. It can be seen that for the thin films deposited using BM sputtering the C-V curve shows very small hysteresis for low RF power (150 W) which is reduced to negligible at high RF power (250 W). On the other hand, for the films deposited using UBM sputtering the hysteresis increased with increasing the RF power from 150 to 250 W. In this case there is a counter clockwise shift of C-V curve with a huge flat band shift of about 3 V for the films deposited at 250 W, indicating that it can act as electron trapping and de-trapping memory device. The overall observed C-V behavior in both UBM and BM sputtering can be attributed to the formation/curing of different traps in the growing films during the physical bombardment of growing film by energetic Ar+ ions due to
Similar behavior is observed with increasing the RF power from 150 to 250 W in BM, however, no sign of single oriention has been observed. It is also observed that the intensity of XRD peaks in UBM is high in comparison to the intensity of peaks in BM i.e. the UBM results in improved crystalline quality compared to conventional BM sputtering process for the entire range of applied RF powers. This indicates that the crystallographic orientation and crystalline quality of the deposited SnO2 thin films can be controlled by applied RF power and the magnetron configurations (UBM and BM) have significant influence on the structural properties. This observation is attributed to the re-crystallization process which occurs during physical bombardment by energetic Ar+ ions using UBM sputtering process. Fig. 3(a-f) shows the hysteresis curves of C-V at 1 MHz for the as 700
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Fig. 5. Dispersion behavior of refractive index (n) of SnO2 thin films deposited at different RF powers using (a) unbalanced and (b) balanced magnetron sputtering. Inset shows the extinction coefficients (k), respectively.
Fig. 4. Experimental and model fit ellipsometric parameters (Ψ & Δ) of SnO2 thin films deposited at 150 W using unbalanced magnetron sputtering. The dashed lines represent the model-fit data .
496.8 ± 0.5 nm in UBM and from 255.1 ± 0.3 nm to 537 ± 0.6 nm in BM sputtering process with increasing the value of RF power from 150 W to 250 W. It has been observed that there is not much change in the value of thickness in both UBM and BM at 150 W while at 250 W the value of thickness is lower (∼40 nm) in UBM than BM sputtering. This decrease in thickness is attributed to the back-sputtering of the growing film due to bombarding energetic Ar+ ions in UBM process. These extracted values of thickness from ellipsometric data are in good agreement with the thicknesses measured by the Stylus profiler. Fig. 5 (a & b) represents the dispersion behavior of refractive index (n) as a function of wavelength in the range of 300–1000 nm as obtained from the corresponding ellipsometric data of SnO2 thin films deposited at different RF powers from 150 to 250 W using UBM and BM sputtering, respectively. The value of n at 600 nm wavelength increases from 1.94 to 1.96 with an increase in RF power from 150 W to 200 W using UBM sputtering and there is no considerable change in the value of n (=1.96) by further increasing the RF power to 250 W. Similarly there is no considerable change in the value of n (vary in the range of 1.98 to 1.96) in BM sputtering with an increase in the RF power from 150 W to 250 W. The measured values of obtained n of SnO2 thin films are in good agreement with reported values in the literature [9,27]. The extinction coefficient (k) of SnO2 thin films deposited by UBM and BM sputtering at different RF powers is shown in the inset of Fig. 5 (a & b).
simultaneous occurrence of various mechanisms as mentioned above. It is to be mentioned here that it is the resultant energy that the growing film experienced matters in the formation/curing of different traps in the growing films. The optical properties of SnO2 thin films were studied by Spectroscopic ellipsometer (SE) in the wavelength range of 300–1000 nm. SE measurements provide the data related to the ellipsometric parameters; angle (Ψ) and phase (Δ) with respect to wavelength or photon energy. It is a non-destructive model fitting based technique, which minimizes the difference between experimental and calculated fitting values as a function of wavelength. Fig. 4 (a & b) represent the SE data for the ellipsometric parameters (Ψ & Δ) of the SnO2 thin films deposited at 150 W using UBM sputtering for the incident angles 55°, 65° and 75°, respectively. The experimental data has been fitted using Lorentz model for the air/SnO2/Si layer which takes into account the film thickness and contribution of the substrate. The dashed lines in the figures represent the model-fit data and it can be seen that all the features present in the experimental spectra are reproduced by the model fit. The fitting parameters within the parametric dispersion model yields thickness of SnO2 thin films deposited at different sputtering powers using UBM and BM sputtering processes. The value of thickness was increased from 256.6 ± 0.4 nm to 701
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Extinction coefficient is the imaginary part of the complex index of refraction, which also relates to the absorption of light. It defines how strongly a substance absorbs light at a given wavelength and is directly related to the absorption coefficient (α) as follow [28]:
k=
αλ 4π
It is clear from the graphs that all the SnO2 thin films show absorption in UV region, but in the visible and IR range the values of k are almost zero. These results indicate that the SnO2 thin films are highly transparent in the visible region. According to inter-band absorption theory, optical band gap of the thin films can be calculated using the following Tauc relation [29,30];
αhv = A (hv − Eg )n where A, Eg, hν and n are the probability parameter for the transition, band gap of the material, incident photon energy and the transition coefficient (2 for indirect and 1/2 for direct band gap), respectively. The absorption coefficient α is extracted from the ellipsometric data after model fitting. Here, the direct band gap of the SnO2 thin films was evaluated by extrapolating the straight line part of the curves (αhv )2 = 0 as shown in Fig. 6 (a & b). We have observed the decrease in value of band gap from 3.83 eV to 3.68 eV with an increase in RF power from 150 W to 200 W in UBM sputtering and there is not much change in the
Fig. 7. Photoluminescence spectra of SnO2 thin films deposited at different RF powers using (a) unbalanced and (b) balanced magnetron sputtering.
value of Eg (3.69 eV) by further increasing the RF power to 250 W. On the other hand in BM sputtering the value of Eg was found to increase from 3.69 eV to 3.72 eV with increasing the RF power from 150 to 250 W. This is attributed to the difference in the growth processes in both UBM and BM sputtering, that causes changes the film density and crystalline quality of the deposited thin films resulting change in the optical properties. The optical properties are in agreement with the XRD results (explained above). All the obtained values of band gap (Eg) of SnO2 thin films are in good agreement with the reported results (3.6–3.9 eV) in the literature [8,9]. Fig. 7 (a & b) shows room temperature photoluminescence (RT-PL) spectra recorded in the range of 350–700 nm for the as deposited SnO2 thin films using UBM and BM sputtering, respectively. The excitation wavelength was 310 nm. Several workers have observed the emission peaks as we observed in the present work [27–34]. For all the deposited SnO2 thin films high intensity emission peak was observed at about 430 nm, which corresponds to SnO and SnO2. Normally, in SnO2 thin films point defects like oxygen vacancies and Sn interstitials exist which are donor type defects, and present at grain boundaries. These defects act as radiative centers in luminescence processes which are responsible for defect level emissions. There exists three types of oxygen vacancies Voo, V+o and V++o in oxide materials for the SnO2 thin films, the broad peak emission at about 590 nm is attributed to V+o, which
Fig. 6. Energy band gap plots of SnO2 thin films deposited at different RF powers using (a) unbalanced and (b) balanced magnetron sputtering. 702
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combines with a hole of the valence band forming V++o and is responsible for the band edge emission peak [27–34]. The emission peak at about 430 nm is attributed to the Sn interstitials while the peak at about 485 nm attributed to the oxygen vacancies Voo. It is to be noted that the intensity of broad peak at about 590 nm decreased significantly as RF power is increased. Further, for the thin films deposited at 250 W using UBM sputtering the PL recording the emission peak at 430 nm was broadened considerably, which shows that the energetic ion bombardment at high RF power caused formation of some additional type of oxygen vacancies resulting the observed broadness (that is, due to superimposition of the additional emission peak). The formation of the additional oxygen vacancies is supported by C-V measurements which have been reflected through huge flat band shift.
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4. Conclusions SnO2 thin films have been deposited by sputtering process under balanced and unbalanced magnetrons using SnO2 target with varying applied RF electrical power. It has been observed that using UBM it is possible to obtain highly crystalline SnO2 thin films with dominant (101) orientation just by tailoring the applied RF power. Also, the UBM results in SnO2 thin films with good optical properties as revealed by ellipsometric and PL measurements. Further, for the thin films deposited using UBM at 250 W RF power the C-V curve showed very large counter clockwise shift of about 3 V (with electron retention), clearly indicating the suitability for a practical memory device. The observed results clearly suggest the unbalanced magnetron of sputtering process enables to deposit high quality SnO2 thin films with respect to its structural, optical and electrical properties with added advantage of providing SnO2 thin films suitable for fabrication of memory devices. Acknowledgements The authors are grateful to the Director, CSIR-National Physical Laboratory, for his continuous encouragement and support during this work. Authors would like to fully acknowledge the help of Dr. V.P.S. Awana and Dr. D. Haranath for XRD and PL measurements, respectively. The financial support provided by Department of Science & Technology (DST) through SERB, New Delhi, India with reference No: SB/EMEQ-040/2014 is gratefully acknowledged. References [1] A.M. Gheidari, E.A. Soleimani, M. Mansorhoseini, S. Mohajerzadeh, N. Madani,
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