Journal of Alloys and Compounds 767 (2018) 642e650 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom Melting point of Sn as the optimal growth temperature in realizing the favored transparent conducting properties of In2O3:Sn ﬁlms Laxmikanta Karmakar, Debajyoti Das* Nano-Science Group, Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata, 700 032, India a r t i c l e i n f o a b s t r a c t Article history: Received 11 May 2018 Received in revised form 7 July 2018 Accepted 11 July 2018 At substrate temperature (TS) close to melting point of Sn (TSn) rapid incorporation of metallic dopants in signiﬁcant amount introduces sharp rise in mobility (m) and concentration (ne ) of charge carriers, leading to substantial reduction in resistivity (r); simultaneous sharp widening in optical gap (Eg ) results in optimum Figure-of-Merit (F). The Eg vs. ne 2/3 plot demonstrates two distinct regimes of TS across TSn, leading to higher reduced effective mass of charge carriers, m*vc by 0.072m0 owing to rapid incorporation of Sn4þ at substitutional site of In3þ in In2O3 matrix. Consequently, the self-energy due to electronimpurity scattering rises and/or the material self-converts to an alloy-like ensemble. On further increase in TS, F reduces due to enhanced optical absorption by metallic dopants and dopant induced defects, as noted by enhanced Urbach-energy (EU ). Rather than any arbitrary TS, TSn has been demonstrated as an optimal growth temperature for ITO ﬁlms grown by RF magnetron sputtering. © 2018 Elsevier B.V. All rights reserved. Keywords: ITO Magnetron sputtering Melting point of Sn Burstein-Moss effect Urbach energy 1. Introduction Transparent conducting oxides (TCOs) belong to a special class of semiconductor materials that can simultaneously be both optically transparent and electrically conducting. Those are used mostly as transparent electrodes in photovoltaic devices [1e10], liquid crystal displays (LCDs) [11,12], light emitting diodes (LEDs) [13,14], thin ﬁlm transistors [15,16] and in many other applications. TCOs are generally based on metal oxide semiconductors such as In2O3 [17e19], SnO2 [20e26] and ZnO [27e30], and sometimes doped by metals and halogens [31e36]. An exclusive TCO material obtained from In2O3 on Sn doping is known as ITO (tin doped indium oxide, In2O3:Sn) [37e39]. It is yellowish to grey in bulk form, while transparent and colourless in thin layers, with a wide band gap (~3.9 eV). ITO is a highly degenerate n-type semiconductor owing to the oxygen vacancies as well as substitutional Sn dopants. Compared to other TCO ﬁlms, e.g., SnO2 and ZnO, the ITO ﬁlms are widely used because of their simultaneous low resistivity (<103 U cm), high transmittance (~90%) in the visible region, high reﬂectance in infrared region, and long-term physical stability due to high substrate adherence, good hardness, and chemical inertness. Thin ﬁlms of TCO are mostly deposited using electron beam * Corresponding author. E-mail address: [email protected] (D. Das). https://doi.org/10.1016/j.jallcom.2018.07.130 0925-8388/© 2018 Elsevier B.V. All rights reserved. evaporation [22,24] ion-assisted deposition , pulsed laser deposition , sol-gel spin-coating technique , thermal evaporation  and DC and RF magnetron sputtering [28,30,44,45]. Among these processes RF magnetron sputtering technique is more versatile and widely used commercially by virtue of the involved advantages of producing high quality thin ﬁlms with superior crystalline quality due to very low contamination and controllable deposition parameter [46e48]. In course of further development of the ITO thin ﬁlms, two issues are of concern: (i) the optical transparency and electrical conductivity of the ﬁlms hold a trade-off relation which needs to overcome, and (ii) although a higher substrate temperature generally facilitates the ordered crystalline orientation, a low temperature growth of the ﬁlms is always preferred in device structures in order to make various inexpensive substrates usable and/or to keep other component layers of the device unaffected. Keeping all the necessary requirements in mind the ITO ﬁlms in the present investigation have been optimized at a temperature close to the melting point of Sn, in order to utilize its best favorable contribution to the growth of properly doped ﬁlms having a balanced combination of relevant optical, electrical and nano-structural properties. The melting point of metallic Sn (TSn) as a dopant to the In2O3 network in ITO ﬁlms has been demonstrated to play a key role in the optimization of the TCO properties, which is the novel approach involved in the present investigation. L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650 2. Experimental details The tin doped indium oxide (ITO) ﬁlms were prepared using an indigenously developed RF magnetron sputtering system, as schematically shown in Fig. 1. The stainless steel sputtering chamber was connected to diffusion pump and rotary pump assembly (Make: Hind High Vacuum Co. India) providing a base vacuum ~9 10e7 Torr. A 3-inch diameter circular planar Torus® magnetron sputter source (Make: KJ Lesker, USA) was used with 99.999% purity ITO (90% In2O3, 10% SnO2 by weight) sputtering target (Make: Vin Karola, USA) with a Cu backing plate connected by Indium-alloy solder bonding. High purity Ar (supplied by Matheson, USA), in regulated ﬂow through a mass ﬂow controller (Bronskhorst, Netherland), was used as the sputtering gas. Ar plasma was produced by 13.56 MHz RF power from a regulated power source (Comdel: CX-600S, USA) and the gas pressure in the plasma during sputtering was measured by Baratron Guage (MKS, USA). During deposition of the ﬁlms the substrates were coupled through a metal mask to a grounded stainless steel holder which was heated from the back by a coil heater placed in close vicinity. The temperature of the heater was controlled and monitored by a K-type thermocouple directly inserted into it and the exact temperature of the substrate, placed on the substrate holder, was calibrated at different gas pressures. At actual operating conditions, however, the substrate holder was rotated at 10 rpm by using a rotary driver, in order to maintain excellent uniformity of the ﬁlm growth, and the growth temperature was considered from the calibration chart. Samples were deposited on Corning® Eagle2000™ glass substrates, properly cleaned by standard procedure using Extran solution with distilled water (1:50) and subsequent ultra-sonication with acetone, alcohol, DI water and dried in a stream of hot air. The substrates were placed on the holder with surface placed parallel to and 6 cm away from the target. Before the actual growth of the ﬁlms the target was cleaned by pre-sputtering for 5 min, keeping the substrate covered by shutters mechanically controlled from outside. The thickness of the ﬁlms was regulated Fig. 1. Schematic diagram of the RF sputtering system used in the present study. 643 using a quartz crystal monitor (Sycon Thickness Monitor, Model STM-100/MF). During deposition uninterrupted chilled water cooling of the magnetron target holder was maintained to prevent the target from breaking. The optical transmission of the ﬁlms prepared on glass substrates was measured using a Varian Cary 5000 double beam spectrophotometer. The X-ray diffraction analysis was carried out using a conventional Cu-K X-ray radiation (~1.5418 Å) source and Bragg diffraction setup (Seifert 3000P). The surface morphology of the ﬁlms was studied by Veeco dI CP II (Model: 0100) atomic force microscope (AFM). Room-temperature electrical resistivity of the ﬁlms was measured by four-probe method using Keithley 2400 source meter. The Hall mobility and carrier concentration were measured in Hall measurement setup using Van der Pauw conﬁguration with 0.1 T magnetic ﬁeld. A JEOLJSM2010 transmission electron microscope operating at 200 kV was used for obtaining high-resolution micrographs (HR-TEM) from 30 nm thick samples deposited on carbon coated copper microscope grids, supplied by Paciﬁc Grid-Tech, USA. 3. Results and discussion A set of ﬁlms were prepared by varying the substrate temperature from 50 C to 350 C at an applied RF power of 50 W, maintaining the gas pressure in the plasma ﬁxed at 20 mTorr arising from 3.3 sccm Ar gas ﬂow. Samples of 250 nm thickness were prepared on glass substrates. The samples were used in 1.5 1.5 cm2 square shape for general characterization and in conﬁguration for Hall measurements in Van der Pauw technique. Especially for transmission electron microscopic measurements samples of ~30 nm thickness were prepared on carbon coated Cu micro-grids. Fig. 2(a) shows the XRD patterns of the ITO thin ﬁlms deposited on glass at different substrate temperatures (TS) varying from 50 C to 350 C with steps of 50 C. No signiﬁcant XRD peak appeared up to TS ¼ 150 C, demonstrating the amorphous-like structure. At TS ¼ 200 C, for the ﬁrst time some crystalline peaks appeared corresponding to the <222> and the <411> planes at 2q ¼ 30.54 and 37.48 , respectively, the <411> peak being the dominant one. On further increase in TS to 250 C the <222> peak increased signiﬁcantly, the <411> peak reduced in intensity and in addition, a number of peaks e.g., <211>, <400>, <440> and <622> appeared. On continued increase in TS to 300 and 350 C the <411> peak became gradually insigniﬁcant, the most dominant <222> peak systematically lost its prominence and the <400> peak gained in intensity, while the other three peaks remained virtually unchanged. It was carefully noted that during increase in TS the <222> peak signiﬁcantly increased within 200e250 C and gradually reduced thereafter. This observation indicated some speciﬁc chemical process in the materials growth occurring within a temperature zone between 200 and 250 C, which drew special interest on critical investigation in the region. Accordingly, three more samples were prepared at that temperature zone. Fig. 2(b) demonstrates a comparative study among ﬁve samples prepared at a close variation of growth temperature. It was identiﬁed that the <222> peak position shifted systematically towards a higher magnitude of 2q from 30.54 to 30.72 at TS ¼ 230 C with corresponding increase in peak intensity and sharpness (peak width reduced to a minimum of D(2q) ¼ 0.38 ); however, all such changes occurred along the opposite direction when TS was further increased to 240 and 250 C. The <400> peak, however, increased gradually with increasing TS while the <411> peak became undetectable at 220 C and that again re-appeared at TS ¼ 250 C. Since In2O3 has a cubic bixbyite structure it possesses the <111> lowest energy plane . For the present set of samples, in particular, <222> plane demonstrates the highest intensity peaks 644 L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650 Fig. 2. (aeb) XRD patterns of the ITO thin ﬁlms deposited at different substrate temperature (TS). (c) Variation of the intensity ratio of <222> and <400> XRD peaks, I<222>/I<400>, on changes in TS, demonstrating its highest magnitude at around 230 C. in mostly crystalline structures at TS above 200 C. Continued increase in crystallization along the <222> orientation appeared as the regular consequence of elevated substrate temperature up to 230 C; however, the decreasing intensity of <222> peak above 230 C could be correlated to the consequence of rapid incorporation of Sn in the In2O3 matrix , the melting point of Sn (TSn) being ~231.9 C. On the other hand, the gradually increasing intensity of the <400> peak might have arisen due to the elevated oxygen vacancy in the In2O3 matrix, consequent to the increased TS [43,51]. None of the characteristic peaks of Sn, SnO, or SnO2 appeared, indicating complete miscibility of In and Sn atoms in the In2O3 lattice . Sn being tetravalent, each Sn (IV) atom substitutionally replaced In (III) atom and thereby, donated free electrons for pursuing elevated electrical conductivity in the carrier transport process. So, the ITO retained the cubic In2O3 structure up to the solid solubility limit of the SnO2 in In2O3 . So, the development of <222> peak was inﬂuenced by two competing processes: elevated substrate temperature and the enhanced dopant incorporation; while the <400> orientation in the material, arising out of the created oxygen vacancy, was a consequence of only the increasing substrate temperature. Accordingly, the intensity ratio, I<222>/I<400> versus TS plot in Fig. 2(c) demonstrates that the critical inﬂuence of the dopant incorporation started occurring at TS above 230 C, at the vicinity of TSn (~231.9 C). At TS > 230 C, increasing dopant (Snþ 4 ) incorporation into the In2O3 matrix superseded the temperature effect in controlling the crystalline structure of In2O3:Sn (ITO) ﬁlms by introducing dopant induced defects, leading to a sharp reduction in I<222>/I<400> at elevated temperatures. Similar effect was pronounced from the position of the <222> crystalline peak in Fig. 2(b), the continued shift of which towards increasing 2q magnitude at elevated TS reversed back spontaneously to lower 2q after critical inﬂuence of the dopant incorporation which started occurring at TS > 230 C, corresponding to the melting point of Sn. The average grain size of the ITO ﬁlms was estimated from the XRD patterns, using the Debye-Scherrer's formula , D¼ 0:89 l b cosq (1) where l (1.5418 A) is the wavelength of X-ray beam, b is the FWHM in radian at diffraction angleq. The grain size of the crystalline In2O3:Sn ﬁlms was found to vary only from 19 to 21 nm over the span of TS varying from 200 to 350 C. Fig. 3(a) presents the transmission electron micrograph of the ITO ﬁlm prepared at TS ¼ 230 C and demonstrates its signiﬁcantly crystalline structure, as was identiﬁed from the XRD studies. Each crystallite in the micrograph was identiﬁed by the individual sharp boundary. An average grain size of ~15 nm was estimated from the histogram shown at the inset in Fig. 3(a), although the XRD estimate identiﬁed little larger size. The high resolution micrograph in Fig. 3(b) clearly identiﬁed the prominent presence of <211>, <222>, <400>, <440> and <622> crystallographic planes of In2O3, as observed in the XRD pattern. Virtually identical crystallographic features were obtained from the TEM studies for the ﬁlm prepared at TS ¼ 250 C. However, elemental analysis by energy dispersive spectroscopy (EDS) identiﬁed prominent changes in the elemental composition in the In2O3:Sn ﬁlms when those were grown at 230 C and at a relatively higher TS ¼ 250 C. Fig. 4(a) and (b) present the EDS spectra of the ﬁlms prepared at L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650 645 Fig. 3. (a) TEM micrograph of the ITO ﬁlm prepared at TS ¼ 230 C. Histogram at the inset shows the average grain size ~15 nm. (b) HRTEM micrograph demonstrating the distinct co-existence of <211>, <222>, <400>, <440> and <622> crystallographic planes. Fig. 4. (a) & (b) EDS spectra of the In2O3:Sn ﬁlms prepared at TS ¼ 230 C and 250 C. (c) & (d) Elemental analysis on relative content of In, O and Sn shown by bar diagram. TS ¼ 230 and 250 C, respectively, and the corresponding distribution of estimated elements In, O and Sn in at.% in pie charts. The bar diagrams in Fig. 4(c) and (d) identify the relative contents of the individual elemental components and particularly, the changes in the ratios of O:In and Sn:In. It has been demonstrated that the Sn content in the In2O3:Sn matrix increased from 1.5 at.% to 3.7 at.% for an elevation in growth temperature from 230 to 250 C [22,24]. The typical surface morphology of the ITO ﬁlm prepared at TS ¼ 230 C is shown in Fig. 5, demonstrating an average roughness of ~1.13 nm. Low resistivity with high transparency, particularly over the visible light region, is a desired property in applications as transparent electrodes in optoelectronic devices. The resistivity, carrier concentration and Hall mobility of ITO thin ﬁlms prepared at different substrate temperatures are shown in Fig. 6. The resistivity (r) decreased from 1.42 10¡2 U cm to 1.28 10¡3 U cm with increasing substrate temperature from 50 to 350 C. Although there 646 L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650 providing higher concentration of charge carriers. For boundary scattering, the mean free path of the free carrier is described by the relation , 1 1 h 3ne 3 m ¼ 2:05258 1015 ne 3m 2e p = Fig. 5. AFM micrograph the ITO ﬁlm prepared at TS ¼ 230 C, demonstrating typical surface morphology with an average roughness of ~1.13 nm. Fig. 6. Variations of the electrical resistivity (rÞ, carrier concentration (ne ) and Hall mobility (m) of the ITO thin ﬁlms prepared by RF magnetron sputtering at different substrate temperature, exhibiting signiﬁcant changes in magnitude within a narrow temperature span, 200 < TS ( C) < 250. The dashed vertical line corresponds to the temperature (231.9 C), melting point of Sn. was a gross decrease in r at the initial rise in TS, the most signiﬁcant and sharp reduction in r was evident in the region 200 < TS ( C) < 250, followed by continued reduction further. On the contrary, the carrier concentration (ne ) of the ﬁlms increased monotonically from 3.21 1019 cm¡3 to 1.48 1020 cm¡3 with enhanced substrate temperature from 50 to 350 C, as obtained from Hall measurement. The Hall mobility (m) of the charge carriers was obtained from the relation: m¼ 1 rne e = l¼ (3) where, h is the plank constant, e is the electronic charge, ne is the carrier concentration and m is the mobility. Using this formula the mean free path of the charge carriers in ITO samples prepared at different substrate temperatures was estimated and plotted in Fig. 7. The mean free path of the charge carriers increased signiﬁcantly from 1.2 to 3.0 nm within a short span of TS from 200 to 250 C, close to TSn. Improved crystallinity implies, in general, relatively smaller volume of grain boundaries and the subsequently reduced grain boundary scattering of charge carriers . The mean free path of the charge carriers is much shorter than the estimated grain size which implies that the scattering due to grain boundary is not dominant in the present case . However, at TS close to the vicinity of TSn rapid incorporation of metallic dopants in signiﬁcant amount introduces sharp rise in mobility of the charge carriers, leading to a signiﬁcant reduction in resistivity. During deposition of the In2O3 network growth orientation occurs spontaneously via the lowest energy plane along <111> or its parallel along <222> direction of the cubic bixbyite structure. Up to TS ¼ 230 C, controlled incorporation of Sn as the dopant promotes sharp lowering in the electrical resistivity, simultaneous to increasing intensity of the <222> orientational growth. However at TS above a critical temperature close to the melting point of Sn (TSn ~231.9 C) uncontrolled incorporation of Sn into the In2O3 network obstructs the spontaneous crystalline growth along <222> orientation and simultaneously, systematic promotion of the electrical transport on increasing temperature gets obstructed due to a constrained carrier enhancement and mobility escalation. The optical transmission spectra of ITO thin ﬁlms prepared at different substrate temperatures are shown in Fig. 8. From the nature of variation of the transmission spectra two distinct groups of samples prepared at TS below and above 230 C, were identiﬁed. All the ITO ﬁlms demonstrated above 80% transmission over the wide wavelength range 400 nme800 nm (visible range). However, the natures of the onset of optical transmission at the lower wavelength region were clearly separated into two different categories, along with slight differences in individual slopes. Sharp reduction of the optical transmission at shorter wavelength occurred due to sharp absorption at the band edges and the slope (2) where e is the electronic charge (1.6 10¡19 C). The Hall mobility maintained a steady magnitude ~13.77 V¡1cm2s¡1 for increasing TS from 50 to 200 C. However, it was carefully noted that the carrier mobility increased very rapidly from 13.78 to ~30.28 V¡1cm2s¡1 within a very small span of TS from 200 to 250 C beyond which, however, m attained a shallow saturation at an average magnitude of ~32.5 V¡1cm2s¡1 above 300 C, as shown in Fig. 6. Spontaneous reduction in the resistivity of the ﬁlms with increasing TS could be correlated to the mutually additive effects of (i) growth temperature induced transformation of the network from amorphous to crystalline structure and (ii) enhanced incorporation of Sn4þ as dopants at the substitutional site for In3þ, Fig. 7. Variations of the estimated mean free path of charge carriers with substrate temperature (TS) for ITO ﬁlms prepared by RF magnetron sputtering. L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650 647 the Burstein-Moss effect that deals with the shift of Fermi level caused by the increased carrier concentration of the conduction band electrons. According to Burstein-Moss effect, the band gap changes can be accounted by Hamberg et al.  as: Eg ¼ Eg0 þ ZS þ DEgBM (6) where Eg0 is the band gap of undoped semiconductor, ZS represent self-energy due to electron-electron and electron-impurity scattering. Further, DEgBM , the shift in Eg due to Burstein-Moss effect, is given by, determined the sharpness of the band edges. From transmission spectra the optical absorption coefﬁcient (a) was obtained using Lambert's formula , a¼ 1 T ln where T is the transmittance and t is the thickness of samples. Neglecting the reﬂection losses and scattering effects, the variation of absorption coefﬁcient follows the Tauc's relation, aE ¼ A E Eg where ne is the carrier concentration and m*vc is the reduced effective mass which is given as, 1 1 1 ¼ þ mvc mv mc n (5) where E is the photon energy, Eg is the optical band gap, A is the constant and n is the exponent which can be taken as 0.5, 2, 1.5 and 3 for direct allowed, indirect allowed, direct forbidden and indirect forbidden electronic transitions, respectively. Since ITO is a direct band gap material, n has been taken as 0.5 and Eg can be determined from the Tauc's plot as shown in Fig. 9(a). The extrapolation of linear part of ðaEÞ2 vs E curve intersecting the energy axis at a ¼ 0 gives the Tauc's band gap, Eg . Fig. 9(b) shows the variation of Tauc's optical band gap of the ITO ﬁlms at different substrate temperatures. Eg grossly increased from 3.61 eV at TS ¼ 50 C to 4.03 eV at TS ¼ 350 C. However, a signiﬁcant and sharp rise in Eg from 3.73 to 3.99 eV was noted for increasing TS from 200 to 250 C . The sharp increase in the optical band gap can be explained by (8) where m*v and m*c are the effective electron mass in valence band and conduction band respectively. Finally, the optical band gap Eg is given by: (4) t (7) Eg ¼ Eg0 þ ZS þ Z2 2 2 3 3p ne 2mvc = Fig. 8. Transmission spectra of the ITO ﬁlms prepared by RF magnetron sputtering at different TS. Z2 2 2 3 3p ne 2mvc = DEgBM ¼ (9) Fig. 10 shows the variation of Eg as a function of ne 2/3 and demonstrates two distinct linear segments with slightly different slopes, with a transition at ne (carrier concentration) corresponding to TS ~230 C and above. From the slope of individual segments, m*vc , the reduced effective mass of the charge carriers (electron) was estimated for two different temperature regimes. The reduced effective mass of the charge carriers, m*vc , increased by: [m*vc ]HT e [m*vc ]LT ¼ (0.274e0.202) m0 ¼ 0.072m0 due to an abrupt incorporation of Sn4þ at TS 230 C, around TSn . Further, as a consequence of such impulsive effect, the apparently constant factor ðEg0 þ ZSÞ in Eq. (9) for two distinct linear segments with slightly different slopes increased by 0.15 eV across TSn. This signiﬁes either the increase of the self-energy (ZS) mostly due to increased electron-impurity scattering, and/or increase of Eg0 itself by the self-conversion of the material from doped to an alloy-like ensemble . Presence of a high concentration of impurity or defect states in the ﬁlms perturbed the band structure, resulting in a prolonged tail extending into the energy gap. Such effect was pronounced by the Fig. 9. (a) Tauc's plot demonstrating the direct band gap of ITO thin ﬁlms prepared by RF magnetron sputtering at different TS, and (b) sharp widening in the optical band gap (Eg ) occurring at around TS ~230 C. 648 L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650 For application of TCO ﬁlms in solar cells, not only high transparency over the visible region is a desired property but also high conductivity is essential; furthermore, these two properties maintain a trade-off relation. Hence, the Figure of merit, deﬁned by Haacke , plays an important role in determining the quality of the ITO ﬁlms. Figure of merit is given by, F¼ Fig. 10. Optical band gap (Eg ) exhibiting linear relationship with (2/3)-rd power of carrier concentration (ne ) for ITO thin ﬁlms prepared at different TS. absorption coefﬁcient tail, directly below the fundamental absorption edge. The experimental data were ﬁtted to Urbach's relation , a ¼ ao Exp hn EU (10) where ao is a characteristic parameter of the material, and EU is called Urbach absorption energy, which is normally denoted as an indicator of the structural defects. The relation between natural logarithm of the absorption coefﬁcient and photon energy has been depicted in Fig. 11(a). The magnitude of the Urbach energy EU was estimated from the slope of the linear extension of the decaying absorption tail towards lower energy, and its changes with the variations of the growth temperature of the ITO ﬁlms are shown in Fig. 11(b). The EU systematically reduced during initial increase in TS up to 230 C corresponding to the improving crystalline structure attained in the network. However, at TS > 230 C,EU increased indeed and attained a virtual saturation at higher temperatures. Thus, signiﬁcant dopant incorporation into the ﬁlm structure plausibly occurred at and above TSn and the subsequent defect formation at elevated temperatures superseded the temperature effect on defect elimination and changed the ultimate nature of variation of EU at higher TS [62,63]. T 10 RS (11) in which T is the transmittance at speciﬁc wavelength, normally taken at l ¼ 550 nm where the intensity of solar spectrum is maximum [22,64] and RS is the sheet resistance. Fig. 12 shows the variation of F with the substrate temperature. The Figure of Merit of the ITO ﬁlms increased very rapidly at TS approaching TSn, attained the highest magnitude at TS ¼ 240 C and then reduced less promptly on further increase in TS. At higher TS increased dopant incorporation although increases the carrier concentration marginally, enhanced optical absorption by the dopants leads to the lowering in the Figure of Merit of the ITO ﬁlms. In order to make a comparison in the variation of the Figure of Merit of the ITO ﬁlms as a function of the applied substrate temperature few results have been taken from the literature of already published data on closely similar deposition conditions following mostly identical growth mechanism via magnetron sputtering [47,65,66] and also pulsed laser deposition . Fig. 13 demonstrates that in terms of the magnitude of Figure of Merit, the present result does not deserve to be the superior one mainly as because the present results are not the parametrically optimized data. However, the objective of the present experiment was to study the effect of substrate temperature on the characteristic changes on the optoelectronic properties of In2O3:Sn ﬁlms at the close vicinity of the melting point of Sn. In this context it is apparently clear that the available data on systematic analysis in two cases [47,66] identify rather very close Figure of Merit values at temperatures far across the melting point of Sn. The present analysis, however, identiﬁes sharp changes in the magnitudes of Figure of Merit across the close vicinity of the melting point of Sn, which is the novelty of the present work that has not been carried out earlier. 4. Conclusions Retaining a cubic bixbyite structure, the In2O3:Sn ﬁlms in mostly crystalline phase at TS above 200 C possess the lowest energy Fig. 11. (a) Absorption co-efﬁcient spectra of ITO ﬁlms prepared at different TS. (b) Variation of the Urbach energy (EU ) with TS exhibiting signiﬁcant increase in magnitude at TS 230 C. L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650 Fig. 12. Variation of the ﬁgure of merit (F) of ITO ﬁlms demonstrating a sharp increase at TS within 230e240 C. 649 matrix. As a consequence of such impulsive effect, either the selfenergy (ZSÞ due to electron-impurity scattering rises and/or Eg0 itself increases by self-conversion of the material from a doped to an alloy-like ensemble. The Figure of Merit of the ITO ﬁlms increases very rapidly corresponding to the sharp increase in carrier mobility at TS approaching TSn. At higher TS additional dopant incorporation although increases the carrier concentration marginally, enhanced optical absorption by the dopants leads to the lowering in the Figure of Merit of the ITO ﬁlms. Thus, signiﬁcant dopant incorporation into the In2O3 matrix at growth temperature close to TSn leads to the substantial changes in most of the optoelectronic properties of ITO ﬁlms as a TCO material. Thereby, the melting point of Sn (TSn), rather than any arbitrary substrate temperature, has been identiﬁed as an optimal temperature for growing ITO thin ﬁlms, suitable for device fabrication; although opportunities for further improvement remains open by controlling other conventional parametric variations, as usual. Acknowledgement The work has been done under projects funded by Department of Science and Technology (Nano-Mission Program) and Council of Scientiﬁc and Industrial Research, Government of India. One of the authors (LK) acknowledges CSIR, GoI, for providing the Senior Research Fellowship. References Fig. 13. Variations in Figure of Merit of In2O3:Sn ﬁlms as a function of substrate temperature across the close vicinity of the melting point of Sn. orientation along the <222> crystallographic plane, which normally advances at elevated growth temperature; simultaneously, enhanced oxygen vacancy builds up the <400> orientation. On further increase in TS > 230 C, enhanced dopant (Snþ 4 ) incorporation into the In2O3 matrix supersedes the temperature effect on reducing the structural defects of the ITO ﬁlms by introducing dopant induced defects (as noted by the enhanced Urbach energy EU ). This leads to a sharp reduction in I<222>/I<400> at elevated temperatures. On increasing TS, mutually additive effects of (i) growth temperature induced transformation of the network from amorphous to crystalline structure and (ii) enhanced incorporation of Sn4þ as dopants at the substitutional site for In3þ providing higher concentration of charge carriers, progressively reduce the resistivity of the ITO ﬁlms. However, at TS close to the vicinity of the melting point of Sn (TSn) rapid incorporation of metallic dopants in signiﬁcant amounts introduces sharp rise in the mobility of charge carriers, leading to a sudden substantial reduction in resistivity. The optical gap of the ITO ﬁlms increases, in general, with increasing substrate temperature. Whereas, Eg increases sharply due to the critical inﬂuence of the dopant incorporation in abundance at growth temperature around the melting point of Sn. Similarities in the nature of changes in electrical and optical properties are noteworthy. Across this temperature zone, Eg vs. ne 2/3 plot demonstrates two distinct linear segments with slightly different slopes, demonstrating an enhancement of the reduced effective mass of charge carriers, m*vc by 0.072m0 owing to the abrupt incorporation of Sn4þ at the substitutional site of In3þ in the In2O3  D. Das, P. Mondal, Correlation between the physical parameters of the ienc-Si absorber layer grown by 27.12 MHz plasma with the nc-Si solar cell parameters, Appl. Surf. Sci. 416 (2017) 980e987.  R. Saive, M. Boccard, T. Saenz, S. Yalamanchili, C.R. Bukowsky, P. Jahelka, Z.J. Yu, J. Shi, Z. Holmanb, H.A. Atwater, Silicon heterojunction solar cells with effectively transparent front contacts, Sustain. Energy Fuel. 1 (2017) 593e598.  N. Kim, H.D. Um, I. Choi, K.H. Kim, K. Seo, 18.4%-Efﬁcient heterojunction Si solar cells using optimized ITO/top electrode, ACS Appl. Mater. Interfaces 8 (2016) 11412e11417.  S. Albrecht, M. Saliba, J.P.C. Baena, F. Lang, L. Kegelmann, M. Mews, L. Steier, A. Abate, J. Rappich, L. Korte, R. Schlatmann, M.K. Nazeeruddin, A. Hagfeldt, M. Gr€ atzeld, B. Rech, Monolithic perovskite/silicon-heterojunction tandem solar cells processed at low temperature, Energy Environ. Sci. 9 (2016) 81e88.  F. Feldmann, M. Bivour, C. Reichel, M. Hermle, S.W. Glunz, Passivated rear contacts for high-efﬁciency n-type Si solar cells providing high interface passivation quality and excellent transport characteristics, Sol. Energy Mater. Sol. Cells 120 (2014) 270e274.  A.K. Barua, A. Sarker, A.K. Bandyopadhyay, D. Das, S. Ray, Improvement in the conversion efﬁciency of single and double junction a-Si solar cells by using high quality p-SiO:H window layer and seed layer/thin n-mc-Si:H bilayer, in: Proc. of 28th IEEE PV Specialists Conf, vols. 17e22, Anchorage Alaska, USA, 2000, pp. 829e832. September.  O. Amiri, M. Salavati-Niasari, N. Mir, F. Beshkar, M. Saadat, F. Ansari, Plasmonic enhancement of dye-sensitized solar cells by using Au-decorated Ag dendrites as a morphology-engineered, Renew. Energy 125 (2018) 590e598.  S. Kuk, Z. Wang, S. Fu, T. Zhang, Y.Y. Yu, J.M. Choi, J.H. Jeong, D.J. Hwang, Nanosecond laser scribing of CIGS thin ﬁlm solar cell based on ITO bottom contact, Appl. Phys. Lett. 112 (2018), 134102.  O. Amiri, N. Mir, F. Ansari, M. Salavati-Niasari, Design and fabrication of a high performance inorganic tandem solar cell with 11.5% conversion efﬁciency, Electrochim. Acta 252 (2017) 315e321.  J.K. Choi, M.L. Jin, C.J. An, D.W. Kim, H.T. Jung, High-Performance of PEDOT/PSS free organic solar cells on an air-plasma-treated ITO substrate, ACS Appl. Mater. Interfaces 6 (2014) 11047e11053.  Y.C. Su, C.C. Chiou, V. Marinova, S.H. Lin, N. Bozhinov, B. Blagoev, T. Babeva, K.Y. Hsu, D.Z. Dimitrov, Atomic layer deposition prepared Al-doped ZnO for liquid crystal displays applications, Opt. Quant. Electron. 50 (2018) 205.  R.A. Afre, N. Sharma, M. Sharon, M. Sharon, Transparent conducting oxide ﬁlms for various applications: a review, Rev. Adv. Mater. Sci. 53 (2018) 79e89.  W. Tu, Z. Chen, Y. Zhuo, Z. Li, X. Ma, G. Wang, Performance optimization of AlGaN-based LEDs by use of ultraviolet-transparent indium tin oxide: effect of in situ contact treatment, Appl. Phys. Exp. 11 (2018), 052101.  T. Passow, M. Kunzer, A. Pfeuffer, M. Binder, J. Wagner, Ultraviolet laser ablation as technique for defect repair of GaN-based light-emitting diodes, Appl. Phys. A 124 (2018) 257.  X. Zhang, B. Wang, X. Sun, H. Zheng, S. Li, P. Zhang, W. Zhang, Highly transparent and conductive W-Doped ZnO/Cu/W-doped ZnO multilayer source/ drain electrodes for metal-oxide thin-ﬁlm transistors, IEEE Electron. Device 650 L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650 Lett. 39 (2018) 967e970.  T. Varma, C. Periasamy, D. Boolchandani, Performance evaluation of bottom gate ZnO based thin ﬁlm transistors with different W/L ratios for UV sensing, Superlattice. Microst. 114 (2018) 284e295.  M. Gebhard, M. Hellwig, A. Kroll, D. Rogalla, M. Winter, B. Mallick, A. Ludwig, M. Wiesing, A.D. Wieck, G. Grundmeierd, A. Devi, New amidinate complexes of indium (III): promising CVD precursors for transparent and conductive In2O3 thin ﬁlms, Dalton Trans. 46 (2017) 10220e10231.  X. Tang, C. Hseih, F. Oua, S.T. Ho, Ohmic contact of indium oxide as transparent electrode to n-type indium phosphide, RSC Adv. 5 (2015) 22685e22691.  D.B. Buchholz, Q. Ma, D. Alducin, A. Ponce, M.J. Yacaman, R. Khanal, J.E. Medvedeva, R.P.H. Chang, The structure and properties of amorphous indium oxide, Chem. Mater. 26 (2014) 5401e5411.  H. Yu, H.I. Yeom, J.W. Lee, K. Lee, D. Hwang, J. Yun, J. Ryu, J. Lee, S. Bae, S.K. Kim, J. Jang, Superfast room-temperature activation of SnO2 thin ﬁlms via atmospheric plasma oxidation and their application in planar perovskite photovoltaics, Adv. Mater. 30 (2018), 1704825.  J.C. Li, H.L. Yuan, Morphology and electric properties of tin oxide composite thin ﬁlms prepared by sol-gel method, Cryst. Res. Technol. 52 (2017), 1700183.  D. Das, R. Banerjee, Properties of electron-beam-evaporated tin oxide ﬁlms, Thin Solid Films 147 (1987) 321e331.  A. Herklotz, S.F. Rus, T.Z. Ward, Continuously controlled optical band gap in oxide semiconductor thin ﬁlms, Nano Lett. 16 (2016) 1782e1786.  R. Banerjee, D. Das, Properties of tin oxide ﬁlms prepared by reactive electron beam evaporation, Thin Solid Films 149 (1987) 291e301.  A.M. Ganose, D.O. Scanlon, Band gap and work function tailoring of SnO2 for improved transparent conducting ability in photovoltaics, J. Mater. Chem. C 4 (2016) 1467e1475.  V. Fauzia, M.N. Yusnidar, L.H. Lalasari, A. Subhan, A.A. Umar, High ﬁgure of merit transparent conducting Sb-doped SnO2 thin ﬁlms prepared via ultrasonic spray pyrolysis, J. Alloys Compd. 720 (2017) 79e85.  V. Pawar, P.K. Jha, S.K. Panda, P.A. Jha, P. Singh, Band-gap engineering in ZnO thin ﬁlms: a combined experimental and theoretical study, Phys. Rev. Appl. 9 (2018), 054001.  D. Das, P. Mondal, Transparent and conducting intrinsic ZnO thin ﬁlms prepared at high growth-rate with c-axis orientation and pyramidal surface texture, RSC Adv. 4 (2014) 35735e35743.  P.Y. Dave, K.H. Patel, K.V. Chauhan, A.K. Chawla, S.K. Rawal, Examination of zinc oxide ﬁlms prepared by magnetron sputtering, Procedia Technol. 23 (2016) 328e335.  P. Mondal, D. Das, Photoluminescence phenomena prevailing in c-axis oriented intrinsic ZnO thin ﬁlms prepared by RF magnetron sputtering, Appl. Surf. Sci. 286 (2013) 397e404.  D.B. Potter, M.J. Powell, I.P. Parkin, C.J. Carmalt, Aluminium/gallium, indium/ gallium, and aluminium/indium co-doped ZnO thin ﬁlms deposited via aerosol assisted CVD, J. Mater. Chem. C 6 (2018) 588e597.  D. Das, P. Mondal, Low temperature grown ZnO:Ga ﬁlms with predominant caxis orientation in wurtzite structure demonstrating high conductance, transmittance and photoluminescence, RSC Adv. 6 (2016) 6144e6153.  Z.Y. Banyamin, P.J. Kelly, G. West, J. Boardman, Electrical and optical properties of ﬂuorine doped tin oxide thin ﬁlms prepared by magnetron sputtering, Coatings 4 (2014) 732e746.  D. Das, P. Mondal, The growth of ZnO:Ga:Cu as new TCO ﬁlm of advanced electrical, optical and structural quality, Physica E 91 (2017) 1e7.  X. Huo, S. Jiang, P. Liu, M. Shen, S. Qiu, M.Y. Li, Molybdenum and tungsten doped SnO2 transparent conductive thin ﬁlms with broadband high transmittance between the visible and near infrared regions, CrystEngComm 19 (2017) 4413e4423.  Y.J. Choi, K.M. Kang, H.S. Lee, H.H. Park, Non-laminated growth of chlorinedoped zinc oxide ﬁlms by atomic layer deposition at low temperatures, J. Mater. Chem. C 3 (2015) 8336e8343.  H. Taha, Z.T. Jiang, C.Y. Yin, D.J. Henry, X. Zhao, G. Trotter, A. Amri, Novel approach for fabricating transparent and conducting SWCNTs/ITO thin ﬁlms for optoelectronic applications, J. Phys. Chem. C 122 (2018) 3014e3027.  R. Banerjee, D. Das, S. Ray, A.K. Batabyal, A.K. Barua, Characterization of tin doped indium oxide ﬁlms prepared by electron beam evaporation, Sol. Energy Mater. 13 (1986) 11e23.  N.W. Pu, W.S. Liu, H.M. Cheng, H.C. Hu, W.T. Hsieh, H.W. Yu, S.C. Liang, Investigation of the optoelectronic properties of Ti-doped indium tin oxide thin ﬁlm, Materials 8 (2015) 6471e6481.  M.S. Farhan, E. Zalnezhad, A.R. Bushroa, A.A.D. Sarhan, Electrical and optical properties of indium-tin oxide (ITO) ﬁlms by ion-assisted deposition (IAD) at room temperature, Int. J. Precis. Eng. Manuf. 14 (2013) 1465e1469.  I.A. Petukhov, A.N. Shatokhin, F.N. Putilin, M.N. Rumyantseva, V.F. Kozlovskii, A.M. Gaskov, D.A. Zuev, A.A. Lotin, O.A. Novodvorsky, A.D. Khramova, Pulsed                          laser deposition of conductive indium tin oxide thin ﬁlms, Inorg. Mater. 48 (2012) 1020e1025. T.O.L. Sunde, E. Garskaite, B. Otter, H.E. Fossheim, R. Sæterli, R. Holmestad, M.A. Einarsrud, T. Grande, Transparent and conducting ITO thin ﬁlms by spin coating of an aqueous precursor solution, J. Mater. Chem. 22 (2012) 15740e15749. M.K.M. Ali, K. Ibrahim, M.Z. Pakhuruddin, M.G. Faraj, Optical and electrical properties of indium tin oxide (ITO) thin ﬁlms prepared by thermal evaporation method on polyethylene terephthalate (PET) substrate, Adv. Mater. Res. 545 (2012) 393e398. S.J. Lee, Y. Kim, J.Y. Hwang, J.H. Lee, S. Jung, H. Park, S. Cho, S. Nahm, W.S. Yang, H. Kim, S.H. Han, Flexible indiumetin oxide crystal on plastic substrates supported by graphene monolayer, Sci. Rep. 7 (2017) 3131. O. Tuna, Y. Selamet, G. Aygun, L. Ozyuzer, High quality ITO thin ﬁlms grown by dc and RF sputtering without oxygen, J. Phys. D Appl. Phys. 43 (2010), 055402. P. Mondal, D. Das, Further improvements in conducting and transparent properties of ZnO:Ga ﬁlms with perpetual c-axis orientation: materials optimization and application in silicon solar cells, Appl. Surf. Sci. 411 (2017) 315e320. V.A. Dao, H. Choi, J. Heo, H. Park, K. Yoon, Y. Lee, Y. Kim, N. Lakshminarayan, J. Yi, rf-Magnetron sputtered ITO thin ﬁlms for improved heterojunction solar cell applications, Curr. Appl. Phys. 10 (2010) S506eS509. P. Mondal, D. Das, Effect of hydrogen in controlling the structural orientation of ZnO:Ga:H as transparent conducting oxide ﬁlms suitable for applications in stacked layer devices, Phys. Chem. Chem. Phys. 18 (2016) 20450e20458. L. Wei, C. Shuying, Photoelectric properties of ITO thin ﬁlms deposited by DC magnetron sputtering, J. Semiconduct. 32 (2011), 013002. H. Kim, C.M. Gilmore, A. Pique, J.S. Horwitz, H. Mattoussi, H. Murata, Z.H. Kafaﬁ, D.B. Chrisey, Electrical, optical, and structural properties of indiumetineoxide thin ﬁlms for organic light-emitting devices, J. Appl. Phys. 86 (1999) 6451e6461. C.G. Choi, K. No, W.J. Lee, H.G. Kim, S.O. Jungb, W.J. Leeb, W.S. Kim, S.J. Kim, C. Yoon, Effects of oxygen partial pressure on the microstructure and electrical properties of indium tin oxide by d.c. magnetron sputtering, Thin Solid Films 258 (1995) 274e278. G. Ramanathan, K.R. Murali, Characteristics of Tin doped Indium oxide ﬁlms, Int. J. Chem. Tech. Res. 8 (2015) 260e264. Y. Ohya, T. Ito, M. Kaneko, T. Ban, Y. Takahashi, Solid solubility of SnO2 in In2O3, J. Ceram. Soc. Jpn. 108 (2000) 803e806. A.L. Patterson, The scherrer formula for x-ray particle size determination, Phys. Rev. 56 (1939) 978. L.J. Meng, M.P. Santos, Structure effect on electrical properties of ITO ﬁlms prepared by RF reactive magnetron sputtering, Thin Solid Films 289 (1996) 65e69. T. Karasawa, Y. Miyata, Electrical and optical properties of indium tin oxide thin ﬁlms deposited on unheated substrates by d.c. reactive sputtering, Thin Solid Films 223 (1993) 135e139. F.K. Mugwang, P.K. Karimi, W.K. Njoroge, O. Omayio, Characterization of aluminum doped zinc oxide (Azo) thin ﬁlms prepared by reactive thermal evaporation for solar cell applications, J. Fund. Renew. Energy Appl. 5 (2015) 170. I. Hamberg, C.G. Granqvist, K.F. Berggren, B.E. Sernelius, L. Engstrom, Band-gap widening in heavily Sn-daped In2O3, Phys. Rev. B 30 (1984) 3240e3249. L.M. Wong, S.Y. Chiam, J.Q. Huang, S.J. Wanga, W.K. Chim, J.S. Pan, Examining the transparency of gallium-doped zinc oxide for photovoltaic applications, Sol. Energy Mater. Sol. Cells 95 (2011) 2400e2406. P. Banerjee, W.J. Lee, K.R. Bae, S.B. Lee, G.W. Rubloff, Structural, electrical, and optical properties of atomic layer deposition Al-doped ZnO ﬁlms, J. Appl. Phys. 108 (2010), 043504. N.M. Khusayfan, M.M. El-Nahass, Study of structure and electro-optical characteristics of indium tin oxide thin ﬁlms, Adv. Condens. Matter Phys. 2013 (2013). Article ID 408182. G. Wisz, I. Virt, P. Sagan, P. Potera, R. Yavorskyi, Structural, optical and electrical properties of zinc oxide layers produced by pulsed laser deposition method, Nano. Res. Lett. 12 (2017) 253. N.A. Bakr, S.A. Salman, M.N. Ali, Effect of ﬂuorine doping on structural and optical properties of SnO2 thin ﬁlms prepared by chemical spray pyrolysis method, Adv. Mater. 5 (2016) 23e30. G. Haacke, New ﬁgure of merit for transparent conductors, J. Appl. Phys. 47 (1976) 4086e4089. F.E. Akkad, M. Maraﬁ, A. Punnoose, G. Prabu, Effect of substrate temperature on the structural, electrical and optical properties of ITO ﬁlms prepared by RF magnetron sputtering, Phys. State Solidi (A) 177 (2000) 445e452. V.S. Reddy, K. Das, A. Dhar, S.K. Ray, The effect of substrate temperature on the properties of ITO thin ﬁlms for OLED applications, Semicond. Sci. Technol. 21 (2006) 1747e1752.