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Investigation on electrical properties of indium tin oxide thin films by effective control of crystallographic orientation
Journal of Alloys and Compounds 786 (2019) 177e182
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Investigation on electrical properties of indium tin oxide thin films by effective control of crystallographic orientation Yi Zhuo a, 1, Zimin Chen a, 1, Zeqi Li a, Guangshuo Cai a, Xuejin Ma a, Yanli Pei a, Gang Wang a, b, * a b
State Key Laboratory of Optoelectronics Materials and Technologies, Sun Yat-sen University, Guangzhou 510006, China Foshan Institute of Sun Yat-sen University, Foshan 528225, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2018 Received in revised form 22 December 2018 Accepted 21 January 2019 Available online 22 January 2019
Although indium tin oxide (ITO) film has been widely used in optoelectronic devices, it remains difficult to systemically determine the structural properties' influence on electrical properties for ITO. In this work, ITO films are grown by metal organic chemical vapor deposition (MOCVD). By introducing nucleation layers, the structural properties could be controlled effectively so that the dependence of electrical properties on crystallographic orientation and O2 partial pressure could be investigated separately. It is found that (1) for the samples grown with the same doping level, the decrease in carrier concentration with increasing oxygen flow rate is purely attributed to the increasing O2 partial pressure rather than different crystallographic orientation; (2) electron mobility of the ITO films is strongly affected by crystallographic orientation and it decreases with (100)-grain density. Analysis by using carrier's transport theory reveals that the averaged barrier height of grain boundary scattering increases with the (100)-grain density. © 2019 Elsevier B.V. All rights reserved.
Keywords: Indium tin oxide (ITO) Two-step growth method Crystallographic orientation Electrical properties
1. Introduction Transparent conductive oxides, especially indium tin oxide (ITO), are widely used in many applications, such as touch panel displays, solar cells, gas sensors, and light emitting diodes [1e5]. ITO films have been deposited by various methods including thermal evaporation [6], sputtering [7,8], sol gel [9], spray pyrolysis [10], and pulse laser deposition (PLD) [11]. Both the crystallographic orientation and the electrical properties of ITO films are highly sensitive to the growth parameters. For most deposition methods, the ITO film is grown by one-step method, where same conditions are employed for both nucleation and film-thickening stage. This brings difficulties in understanding how the electrical properties of ITO films are affected by the growth conditions because the crystallographic orientation may also have influence on the electrical properties. Hence, when studying the growth parameter dependence of the electrical property, the contribution of crystallographic orientation is usually neglected [12,13]. A few studies have
* Corresponding author. State Key Laboratory of Optoelectronics Materials and Technologies, Sun Yat-sen University, HEMC, Guangzhou 510006, China. E-mail address: [email protected] (G. Wang). 1 These authors contributed equally. https://doi.org/10.1016/j.jallcom.2019.01.232 0925-8388/© 2019 Elsevier B.V. All rights reserved.
focused on the relationship between the electrical properties and crystallographic orientation. P. Thilakan et al. suggested that (100) oriented ITO films had better conductivity and mobility [14]. However, E. Terzini et al. suggested the (100) oriented films had poor tin doping and reduced carrier mobility [15]. One reason for the contradiction is the lack of deposition methods that can control the crystallographic orientation without changing the growth conditions of the main body of the ITO film. Recently, metal organic chemical vapor deposition (MOCVD) has been demonstrated to be an effective method to fabricate high quality ITO thin films [2,16,17]. Comparing with other deposition techniques, MOCVD has advantages in the precise control of growth conditions, which are suitable for the growth of multi-layer material [18,19]. Growing ITO with two-step method is a feasible strategy to fabricate ITO films with pre-determined orientation, which makes it possible to study the relationship between the growth conditions, the crystallographic orientation, and the electrical property of ITO films in a stricter way. In this paper, ITO films were grown by MOCVD under different oxygen (O2) conditions. First, the influence of O2 conditions on the structural, electrical, and optical properties of as-grown ITO thin films are investigated. Two-step method was employed to distinct the effects of O2 flow rate and crystallographic orientation on
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electrical properties. Then the relationships between electrical properties and crystallographic orientation of the ITO film are discussed in detail. 2. Experimental details Four groups of ITO thin films, namely A, B, C, and D, were grown on double-polished c-plane sapphire in a home-made MOCVD system. Commercially available trimethylindium (TMIn), tetrakis (dimethylamine) tin (TDMASn) and O2 were used as precursors. The bubbler of TMIn was maintained at 25 C, while that of TDMASn was kept at 5 C for group A, B, and D, and 25 C for group C. The organometallic sources were introduced into the reactor chamber using high-purity argon as carrier gas. O2 was injected into the chamber using a separated delivery line. All samples were grown at 530 C under a pressure of 9 Torr. The O2 flow rate was fixed at various values between 500 and 3000 sccm corresponding to O2 partial pressure ranging from 0.28 to 1.73 Torr. The molar ratio between O2 and TMIn was calculated to be higher than 1000 ensuring oxygen-rich condition. Group A, B, and C were ITO films grown with variable O2 flow rates at three Sn/In ratios. Group D is a verifying experiment where two-step growth method was employed. Detailed growth parameters are listed in Table 1. The structural property was examined by high-resolution X-ray diffraction (XRD) in Bruker D8 Discover. The surface morphology was investigated on a Hitachi S-4800 scanning electron microscope (SEM). The room temperature Hall effect measurement was carried out in Accent HL5500 system with van der Pauw geometry. The ultravioletevisible transmittance spectrums were measured by a Shimadzu UV-2250 spectrometer. 3. Results and discussions 3.1. Influence of O2 flow rate on film properties Fig. 1 shows the XRD spectra (2q scans) for group A samples. The observed peaks located at 30.43 ± 0.02 and 35.37 ± 0.02 can be indexed to the (222) and (400) planes of the body-centered cubic (bcc) ITO (space group Ia3), respectively. The existence of both (111) and (100) orientations indicate the polycrystalline nature of the asgrown ITO films. Using the intensity ratio between (400) and (222) peaks (I400/I222) as a measurement of crystallographic orientation, the as-grown ITO films are determined to be dominated by (111) orientation since the I400/I222 are over an order of magnitude lower than that of the randomly oriented powder sample (0.3) [20,21]. The preference of (111) orientation often observed in ITO films deposited above 500 C [22,23]. The inset in Fig. 1 depicts the value
Table 1 Growth conditions of ITO.
A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 D1 D2 D3 D4
TMIn (sccm)
TDMASn (sccm)
O2 (sccm)
175 175 175 175 140 140 140 140 115 115 115 175 175 175 175
160 160 160 160 240 240 240 240 60 60 60 140 140 140 140
500 700 1000 3000 900 1500 2000 2600 3000 2000 1000 1000 500 500/1000 1000/500
Fig. 1. XRD patterns (q-2q) of ITO films grown on c-plane sapphire with different O2 flow rates (A1-A4). The inset shows I400/I222 as a function of O2 flow rates.
of I400/I222 as a function of O2 flow rate. As O2 flow rate increases, I400/I222 first increases from 0.003 to 0.03 as the O2 flow rate increases from 500 to 1000 sccm, then decreases to 0.005 as the O2 flow rate further increases to 3000 sccm. For ITO films deposited by other techniques, high oxygen partial pressure is reported to be favorable to the growth of (111) grains [23,24]. This tendency could be observed in our experiment when the O2 flow rate is larger than 1000 sccm. However, the tendency changes when the O2 flow rate is lower than 1000 sccm. The result could be understood considering the sapphire substrate and the high growth temperature (530 C) we employed. Compared with (111) grains, the nucleation of (100) ITO grains on c-plane sapphire possesses higher interfacial energy due to the mismatch of crystal symmetry [22]. At the temperature of 530 C, desorption process takes place during the nucleation stage of ITO. The nuclei with (100) orientation are less stable due to the higher interfacial energy and thus easier to re-evaporate. Moreover, the decrease of oxygen partial pressure could enhance the desorption of adatoms and nuclei. As a result, the (100) grain density decreases as the oxygen flow rate decreases from 1000 to 500 sccm. Fig. 2(a)-(d) shows the SEM morphologies for samples A1-A4. The thickness was determined to be 90 nm by cross-sectional SEM for all the four samples. The O2-independent growth rate indicates all samples were grown under an oxygen-rich growth condition, in which excess oxygen is supplied and the growth rate is limited by the TMIn flow rate. The as-grown ITO films show similar columnar polycrystalline structures where two kinds of grains were observed. Based on the results of XRD measurement, the sparsely distributed pyramid-like grains should be index as (100)-grain while the triangle plate-like grains are (111)-oriented [19]. The surface morphologies of both (111)- and (100)-grains are schematically shown in Fig. 2(f). Previous reports have demonstrated that the crystal morphology of bcc In2O3 is intrinsically determined by the sequence of surface energy (g) for low-index crystallographic facets, i.e. g{111} < g{110} < g{100} [25]. The observed grains are bounded by {111} facets, indicating that the growth was carried out under quasi-equilibrium conditions where adatoms processed enough energy for inter-facet diffusion [26]. Fig. 2(e) depicts the (100)-grain density as a function of O2 flow rate. The (100)-grain density increases over an order of magnitude as the O2 flow rate increases from 500 to 1000 sccm. Further increase in O2 flow rate results in the decline of (100)-grain density. The findings from SEM reveal that, at 530 C, moderate O2 flow rate facilitates the growth of (100)-grains, whereas (111)-grains prefer to grow at low and high O2 flow rate regimes, which is in consistent
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Fig. 4. Electrical properties of the MOCVD-grown ITO films (A1-A4) as a function of O2 flow rate.
Fig. 2. SEM morphologies of the ITO films (A1-A4) grown on c-plane sapphire with different O2 flow rates: (a) 500, (b) 700, (c) 1000, (d) 3000; (e) (100)-grain density as a function of O2 flow rate; (f) Crystallographic model of In2O3 octahedron and the morphologies of (100)- and (111)-oriented grains. The white scale bars are equal to 50 nm.
with the XRD measurement of Fig. 1. The transmittance spectra and deduced optical bandgap are shown in Fig. 3. The optical bandgap is obtained by extrapolating the linear segment of (ahn)2 ¼ hn-Eg, where hn is the photon energy, a is the absorption coefficient, and Eg is the optical bandgap. All samples exhibit high transparency in visible and near-ultraviolet region with optical bandgaps of about 4.3 eV. In Fig. 4, we plot
Fig. 3. Transmittance spectra of ITO films (A1-A4). The inset depicts the optical bandgap energy for the ITO films as a function of O2 flow rate.
the dependence of room temperature electrical resistivity (r), carrier concentration (N), and mobility (m) on O2 flow rate. The hall mobility firstly decreases with the increasing O2 flow rate and reaches an inflection point at O2 ¼ 1000 sccm with a hall mobility of 32.7 cm2 V1s1. The highest hall mobility is 36.9 cm2 V1s1 at O2 ¼ 500 sccm. In contrast, the carrier concentration decreases monotonically from to 8.9 1020 to 7.2 1020 cm3 when the O2 flow rate increases from 500 to 3000 sccm. As a result, by reducing O2 flow rate from 1000 down to 500 sccm the resistivity decreases from 2.60 104 to 1.90 104 U cm.
3.2. Relationship between structural and electrical properties Two additional experiment group, B and C, were grown with different Sn/In molar ratios. The tin doping levels for the three groups are listed in Table 1. Fig. 5(b and c) show the XRD profiles for samples of group B and C, respectively. The crystal orientations of (111) and (100) are both detected by XRD. Higher doping level of Sn may reduce the surface energy of (100) plane, which facilitates the nucleation of (100)-grains, and result in higher I400/I222 [25]. The highest I400/I222 ratio reaches the order of 101 which is close to that for the randomly oriented sample. Fig. 6 shows the carrier concentration as a function of O2 flow rate for samples of group A, B, and C. For all the three groups of samples, carrier concentration is observed to decrease with increasing O2 flow rate. Similar results have been observed for ITO films deposited by sputtering [8]. It is widely accepted that the carrier concentration is affected by the concentration of interstitial 00 oxygen O2 or their complexes with Sn, such as (2SnIn Oi ), which i works as compensation impurity [27]. The incorporation of O2 i into ITO thin film has been reported to be affected by two factors: the oxygen partial pressure and the crystallographic orientation [12,28]. The results in Fig. 6 seem to indicate that the carrier concentration is determined by oxygen partial pressure. However, it should be noticed that the crystallographic orientation changes with oxygen flow rate at the same time (see Fig. 1), which may also affect the carrier concentration [15,28].
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Fig. 5. XRD patterns for samples of group (a) B, (b) C, and (c) D. The (400) diffraction peaks are shown in the 10 (a)~(b) and 30 (c) magnified views.
Fig. 6. Carrier concentration as a function of oxygen flow rate for group A, B, and C samples.
For the deposition of ITO films, the crystallographic orientation is mainly determined by the nucleation stage of ITO grains on the substrate [24]. Generally, ITO films are grown by one-step growth method, where the same conditions are employed for both nucleation and film-thickening stages. For this reason, the crystallographic orientation could not be controlled without changing the conditions for film-thickening stage, which brings difficulties to distinguish the effect of O2 partial pressure and crystallographic orientation. In order to clarify how O2 partial pressure and crystallographic orientation affect the electrical properties of the ITO films, four samples (D1-D4) with a thickness of 90 nm have been prepared. The growth rate is calculated to be 1.5 nm/min. Sample D1 and D2 were grown by conventional one-step method with O2 flow rates of 1000 and 500 sccm, respectively. Sample D3 and D4 were grown by two-step methods in which ~15 nm-thick nucleation layers were introduced, followed by the growth of the main layers. The schematic diagram is shown in Fig. 7(a). For example, for sample D3, the growth condition of the nucleation layer is the same as that of sample D2, while the growth condition of the main layer is identical with that of sample D1. Fig. 7(b)-(e) show the SEM morphologies for samples D1-D4, respectively. The (100)-grain density are summarized in Table 2. Regardless of the differences in the main layer, samples with the same nucleation layer show similar XRD profiles, morphologies and (100)-grain densities. Fig. 5(c) shows the XRD patterns for samples of group D. The I400/I222 ratios are observed to be ~0.035 and ~0.005 for samples nucleate at O2 ¼ 1000 sccm and O2 ¼ 500 sccm, respectively. The XRD experiments agree well with the SEM observation. By employing two-step method, the crystallography
Fig. 7. (a) Schematic diagrams of sample D1-D4; The SEM morphologies of (b) D1, (c) D2, (d) D3 and (e) D4. The scale bar is 500 nm.
orientation of the ITO film is effectively controlled. The results of Hall-effect measurement are also summarized in Table 2. Although the nucleation layers and (100)-grain densities are different, sample D2 and D4 possess similar carrier concentrations which are higher than that of sample D1 and D3. It should be noticed that the nucleation layer is so thin that the carrier concentration is mainly determined by the main layer. The main layers in sample D2 and D4 were grown at the same O2 flow rate which is lower than that of D1 and D3. These findings indicate that the variation in carrier concentration is mainly determined by the direct effect of varying O2 partial pressure rather than the difference in crystallographic orientation. On the other hand, similar values of mobility are observed for samples grown with the same nucleation layer. Despite the difference in carrier concentration, samples with lower (100)-grain density show relatively higher mobility, indicating the grain orientation dependence of carrier mobility. To further understand the effect of crystallographic orientation, the structural and electrical information for the four groups of samples are summarized in Fig. 8. Fig. 8(a) shows the hall mobility as a function of (100)-grain density. Negative correlation between the hall mobility and (100)-grain density has been observed. For ntype polycrystalline semiconductor, the scattering mechanisms dominating electron mobility include ionized impurity scattering,
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Table 2 Summaries of (100)-grain density, resistivity, mobility, carrier concentration and deduced barrier height for sample D1-D4.
D1 D2 D3 D4
(100)-grain density (/mm2)
Resistivity (104 U cm)
Mobility (cm2V1s1)
Carrier concentration (1020 cm3)
EB (meV)
87 18 15 100
2.71 ± 0.02 2.10 ± 0.02 2.28 ± 0.02 2.30 ± 0.05
32.1 ± 0.2 38.7 ± 0.5 38.9 ± 0.4 34.4 ± 0.6
7.17 ± 0.04 7.65 ± 0.15 7.04 ± 0.12 7.88 ± 0.32
20 15 15 18
Fig. 8. Mobility as a function of (a) (100)-grain density and (b) carrier concentration and (c) grain boundary barrier height as a function of (100)-grain density for group A-D samples. The solid black line is a calculated dependence of mg(n) for single crystal In2O3 at 300 K [29].
polar optical phonon scattering, acoustic-phonon scattering, and grain boundary scattering [30]. The scatterings by ionized impurities and phonons have already been intensively investigated in previous research [29,31]. On the other hand, grain boundary scattering is caused by the space charge region formed around the grain boundaries, which is an important scattering mechanism for polycrystalline ITO. Adopting the approach suggested by Kazmerski et al., the mobility can be expressed by Refs. [32,33]:
mpoly ¼ mg ðT; nÞ$exp
EB kB T
(1)
where T is the temperature, n the carrier concentration, kB the Boltzmann constant, mg(T, n) the carrier mobility of bulk In2O3 as a function of temperature and carrier concentration and EB the averaged grain boundary barrier height. For degenerated ITO, the effect of grain boundary scattering has been observed even for carrier concentrations up to ~1021 cm3 and is attributed to the aggregation of defects at the grain boundary such as Sn segregation [33]. Fig. 8(b) shows the hall mobility data as a function of carrier concentration. There is no clear relation between the carrier concentration and the mobility. The solid line in Fig. 8(b) corresponds to the calculated mobility for bulk ITO mg(n) at 300 K, where scatterings by polar optical phonons, acoustic deformation potential phonons, and ionized impurities were considered [29]. For the textured polycrystalline ITO thin films investigated in this work, high density of grain boundary exists in the films. Therefore, the difference between the hall mobility and the calculated is mainly attributed to grain boundary scattering. With the mg(n) shown in Fig. 8(b), grain boundary barrier height can be deduced according to Eq. (1). The averaged barrier height EB changes significantly from 14 to 34 meV and shows a trend to increase with (100)-grain density as could be seen in Fig. 8(c). 4. Conclusion In conclusion, ITO thin films were grown by MOCVD with different O2 flow rates. The as-grown ITOs presented
polycrystalline structure with both (111)- and (100)-oriented grains. The density of (100)-grains reached maximum at medium O2 partial pressure. By introducing nucleation layers, we realized effective control of the crystallographic orientation of ITO thin films, which allowed us to individually investigate the effect of crystallographic orientation and O2 condition. Excluding the effect of crystallographic orientation, the variation of carrier concentration's dependence on O2 flow rate was attributed to the direct effect of the varied O2 flow rate (Fig. 6). On the other hand, the carrier mobility of the ITO film was found to be considerably affected by the crystallographic orientation [Fig. 8(a)]. Analysis on carrier's transport reveals that the averaged barrier height of grain boundary increases with the (100)-grain density. This work also offers a new way to control the morphology and electrical properties of ITO films. Acknowledgements This work was supported by the Science and Technology Project of Guangdong Province, China [grant numbers 2015B010132008, 2016B090918106, 2016B010129002 and 2017B090911002], the Science and Technology Planning Project of Guangzhou, China [grant numbers 201607020036 and 201804020051] and the Fundamental Research Funds for the Central Universities [grant number 20177612031650012]. References [1] J. Bai, M. Athanasiou, T. Wang, Influence of the ITO current spreading layer on efficiencies of InGaN-based solar cells, Sol. Energy Mater. Sol. Cells 145 (2016) 226e230. [2] Z. Chen, Y. Zhuo, W. Tu, X. Ma, Y. Pei, C. Wang, G. Wang, Highly ultraviolet transparent textured indium tin oxide thin films and the application in light emitting diodes, Appl. Phys. Lett. 110 (2017) 242101. [3] W. Zheng, F. Huang, R. Zheng, H. Wu, Low-dimensional structure vacuumultraviolet-sensitive (lambda < 200 nm) photodetector with fast-response speed based on high-quality AlN micro/nanowire, Adv. Mater. 27 (2015) 3921e3927. [4] V.S. Vaishnav, S.G. Patel, J.N. Panchal, Development of ITO thin film sensor for detection of benzene, Sens. Actuators B Chem. 206 (2015) 381e388. [5] W. Zheng, R. Lin, Y. Zhu, Z. Zhang, X. Ji, F. Huang, Vacuum ultraviolet photodetection in two-dimensional oxides, ACS Appl. Mater. Interfaces (2018)
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Investigation on electrical properties of indium tin oxide thin films by effective control of crystallographic orientation Yi Zhuo a, 1, Zimin Chen a, 1, Zeqi Li a, Guangshuo Cai a, Xuejin Ma a, Yanli Pei a, Gang Wang a, b, * a b
State Key Laboratory of Optoelectronics Materials and Technologies, Sun Yat-sen University, Guangzhou 510006, China Foshan Institute of Sun Yat-sen University, Foshan 528225, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2018 Received in revised form 22 December 2018 Accepted 21 January 2019 Available online 22 January 2019
Although indium tin oxide (ITO) film has been widely used in optoelectronic devices, it remains difficult to systemically determine the structural properties' influence on electrical properties for ITO. In this work, ITO films are grown by metal organic chemical vapor deposition (MOCVD). By introducing nucleation layers, the structural properties could be controlled effectively so that the dependence of electrical properties on crystallographic orientation and O2 partial pressure could be investigated separately. It is found that (1) for the samples grown with the same doping level, the decrease in carrier concentration with increasing oxygen flow rate is purely attributed to the increasing O2 partial pressure rather than different crystallographic orientation; (2) electron mobility of the ITO films is strongly affected by crystallographic orientation and it decreases with (100)-grain density. Analysis by using carrier's transport theory reveals that the averaged barrier height of grain boundary scattering increases with the (100)-grain density. © 2019 Elsevier B.V. All rights reserved.
Keywords: Indium tin oxide (ITO) Two-step growth method Crystallographic orientation Electrical properties
1. Introduction Transparent conductive oxides, especially indium tin oxide (ITO), are widely used in many applications, such as touch panel displays, solar cells, gas sensors, and light emitting diodes [1e5]. ITO films have been deposited by various methods including thermal evaporation [6], sputtering [7,8], sol gel [9], spray pyrolysis [10], and pulse laser deposition (PLD) [11]. Both the crystallographic orientation and the electrical properties of ITO films are highly sensitive to the growth parameters. For most deposition methods, the ITO film is grown by one-step method, where same conditions are employed for both nucleation and film-thickening stage. This brings difficulties in understanding how the electrical properties of ITO films are affected by the growth conditions because the crystallographic orientation may also have influence on the electrical properties. Hence, when studying the growth parameter dependence of the electrical property, the contribution of crystallographic orientation is usually neglected [12,13]. A few studies have
* Corresponding author. State Key Laboratory of Optoelectronics Materials and Technologies, Sun Yat-sen University, HEMC, Guangzhou 510006, China. E-mail address: [email protected] (G. Wang). 1 These authors contributed equally. https://doi.org/10.1016/j.jallcom.2019.01.232 0925-8388/© 2019 Elsevier B.V. All rights reserved.
focused on the relationship between the electrical properties and crystallographic orientation. P. Thilakan et al. suggested that (100) oriented ITO films had better conductivity and mobility [14]. However, E. Terzini et al. suggested the (100) oriented films had poor tin doping and reduced carrier mobility [15]. One reason for the contradiction is the lack of deposition methods that can control the crystallographic orientation without changing the growth conditions of the main body of the ITO film. Recently, metal organic chemical vapor deposition (MOCVD) has been demonstrated to be an effective method to fabricate high quality ITO thin films [2,16,17]. Comparing with other deposition techniques, MOCVD has advantages in the precise control of growth conditions, which are suitable for the growth of multi-layer material [18,19]. Growing ITO with two-step method is a feasible strategy to fabricate ITO films with pre-determined orientation, which makes it possible to study the relationship between the growth conditions, the crystallographic orientation, and the electrical property of ITO films in a stricter way. In this paper, ITO films were grown by MOCVD under different oxygen (O2) conditions. First, the influence of O2 conditions on the structural, electrical, and optical properties of as-grown ITO thin films are investigated. Two-step method was employed to distinct the effects of O2 flow rate and crystallographic orientation on
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electrical properties. Then the relationships between electrical properties and crystallographic orientation of the ITO film are discussed in detail. 2. Experimental details Four groups of ITO thin films, namely A, B, C, and D, were grown on double-polished c-plane sapphire in a home-made MOCVD system. Commercially available trimethylindium (TMIn), tetrakis (dimethylamine) tin (TDMASn) and O2 were used as precursors. The bubbler of TMIn was maintained at 25 C, while that of TDMASn was kept at 5 C for group A, B, and D, and 25 C for group C. The organometallic sources were introduced into the reactor chamber using high-purity argon as carrier gas. O2 was injected into the chamber using a separated delivery line. All samples were grown at 530 C under a pressure of 9 Torr. The O2 flow rate was fixed at various values between 500 and 3000 sccm corresponding to O2 partial pressure ranging from 0.28 to 1.73 Torr. The molar ratio between O2 and TMIn was calculated to be higher than 1000 ensuring oxygen-rich condition. Group A, B, and C were ITO films grown with variable O2 flow rates at three Sn/In ratios. Group D is a verifying experiment where two-step growth method was employed. Detailed growth parameters are listed in Table 1. The structural property was examined by high-resolution X-ray diffraction (XRD) in Bruker D8 Discover. The surface morphology was investigated on a Hitachi S-4800 scanning electron microscope (SEM). The room temperature Hall effect measurement was carried out in Accent HL5500 system with van der Pauw geometry. The ultravioletevisible transmittance spectrums were measured by a Shimadzu UV-2250 spectrometer. 3. Results and discussions 3.1. Influence of O2 flow rate on film properties Fig. 1 shows the XRD spectra (2q scans) for group A samples. The observed peaks located at 30.43 ± 0.02 and 35.37 ± 0.02 can be indexed to the (222) and (400) planes of the body-centered cubic (bcc) ITO (space group Ia3), respectively. The existence of both (111) and (100) orientations indicate the polycrystalline nature of the asgrown ITO films. Using the intensity ratio between (400) and (222) peaks (I400/I222) as a measurement of crystallographic orientation, the as-grown ITO films are determined to be dominated by (111) orientation since the I400/I222 are over an order of magnitude lower than that of the randomly oriented powder sample (0.3) [20,21]. The preference of (111) orientation often observed in ITO films deposited above 500 C [22,23]. The inset in Fig. 1 depicts the value
Table 1 Growth conditions of ITO.
A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 D1 D2 D3 D4
TMIn (sccm)
TDMASn (sccm)
O2 (sccm)
175 175 175 175 140 140 140 140 115 115 115 175 175 175 175
160 160 160 160 240 240 240 240 60 60 60 140 140 140 140
500 700 1000 3000 900 1500 2000 2600 3000 2000 1000 1000 500 500/1000 1000/500
Fig. 1. XRD patterns (q-2q) of ITO films grown on c-plane sapphire with different O2 flow rates (A1-A4). The inset shows I400/I222 as a function of O2 flow rates.
of I400/I222 as a function of O2 flow rate. As O2 flow rate increases, I400/I222 first increases from 0.003 to 0.03 as the O2 flow rate increases from 500 to 1000 sccm, then decreases to 0.005 as the O2 flow rate further increases to 3000 sccm. For ITO films deposited by other techniques, high oxygen partial pressure is reported to be favorable to the growth of (111) grains [23,24]. This tendency could be observed in our experiment when the O2 flow rate is larger than 1000 sccm. However, the tendency changes when the O2 flow rate is lower than 1000 sccm. The result could be understood considering the sapphire substrate and the high growth temperature (530 C) we employed. Compared with (111) grains, the nucleation of (100) ITO grains on c-plane sapphire possesses higher interfacial energy due to the mismatch of crystal symmetry [22]. At the temperature of 530 C, desorption process takes place during the nucleation stage of ITO. The nuclei with (100) orientation are less stable due to the higher interfacial energy and thus easier to re-evaporate. Moreover, the decrease of oxygen partial pressure could enhance the desorption of adatoms and nuclei. As a result, the (100) grain density decreases as the oxygen flow rate decreases from 1000 to 500 sccm. Fig. 2(a)-(d) shows the SEM morphologies for samples A1-A4. The thickness was determined to be 90 nm by cross-sectional SEM for all the four samples. The O2-independent growth rate indicates all samples were grown under an oxygen-rich growth condition, in which excess oxygen is supplied and the growth rate is limited by the TMIn flow rate. The as-grown ITO films show similar columnar polycrystalline structures where two kinds of grains were observed. Based on the results of XRD measurement, the sparsely distributed pyramid-like grains should be index as (100)-grain while the triangle plate-like grains are (111)-oriented [19]. The surface morphologies of both (111)- and (100)-grains are schematically shown in Fig. 2(f). Previous reports have demonstrated that the crystal morphology of bcc In2O3 is intrinsically determined by the sequence of surface energy (g) for low-index crystallographic facets, i.e. g{111} < g{110} < g{100} [25]. The observed grains are bounded by {111} facets, indicating that the growth was carried out under quasi-equilibrium conditions where adatoms processed enough energy for inter-facet diffusion [26]. Fig. 2(e) depicts the (100)-grain density as a function of O2 flow rate. The (100)-grain density increases over an order of magnitude as the O2 flow rate increases from 500 to 1000 sccm. Further increase in O2 flow rate results in the decline of (100)-grain density. The findings from SEM reveal that, at 530 C, moderate O2 flow rate facilitates the growth of (100)-grains, whereas (111)-grains prefer to grow at low and high O2 flow rate regimes, which is in consistent
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Fig. 4. Electrical properties of the MOCVD-grown ITO films (A1-A4) as a function of O2 flow rate.
Fig. 2. SEM morphologies of the ITO films (A1-A4) grown on c-plane sapphire with different O2 flow rates: (a) 500, (b) 700, (c) 1000, (d) 3000; (e) (100)-grain density as a function of O2 flow rate; (f) Crystallographic model of In2O3 octahedron and the morphologies of (100)- and (111)-oriented grains. The white scale bars are equal to 50 nm.
with the XRD measurement of Fig. 1. The transmittance spectra and deduced optical bandgap are shown in Fig. 3. The optical bandgap is obtained by extrapolating the linear segment of (ahn)2 ¼ hn-Eg, where hn is the photon energy, a is the absorption coefficient, and Eg is the optical bandgap. All samples exhibit high transparency in visible and near-ultraviolet region with optical bandgaps of about 4.3 eV. In Fig. 4, we plot
Fig. 3. Transmittance spectra of ITO films (A1-A4). The inset depicts the optical bandgap energy for the ITO films as a function of O2 flow rate.
the dependence of room temperature electrical resistivity (r), carrier concentration (N), and mobility (m) on O2 flow rate. The hall mobility firstly decreases with the increasing O2 flow rate and reaches an inflection point at O2 ¼ 1000 sccm with a hall mobility of 32.7 cm2 V1s1. The highest hall mobility is 36.9 cm2 V1s1 at O2 ¼ 500 sccm. In contrast, the carrier concentration decreases monotonically from to 8.9 1020 to 7.2 1020 cm3 when the O2 flow rate increases from 500 to 3000 sccm. As a result, by reducing O2 flow rate from 1000 down to 500 sccm the resistivity decreases from 2.60 104 to 1.90 104 U cm.
3.2. Relationship between structural and electrical properties Two additional experiment group, B and C, were grown with different Sn/In molar ratios. The tin doping levels for the three groups are listed in Table 1. Fig. 5(b and c) show the XRD profiles for samples of group B and C, respectively. The crystal orientations of (111) and (100) are both detected by XRD. Higher doping level of Sn may reduce the surface energy of (100) plane, which facilitates the nucleation of (100)-grains, and result in higher I400/I222 [25]. The highest I400/I222 ratio reaches the order of 101 which is close to that for the randomly oriented sample. Fig. 6 shows the carrier concentration as a function of O2 flow rate for samples of group A, B, and C. For all the three groups of samples, carrier concentration is observed to decrease with increasing O2 flow rate. Similar results have been observed for ITO films deposited by sputtering [8]. It is widely accepted that the carrier concentration is affected by the concentration of interstitial 00 oxygen O2 or their complexes with Sn, such as (2SnIn Oi ), which i works as compensation impurity [27]. The incorporation of O2 i into ITO thin film has been reported to be affected by two factors: the oxygen partial pressure and the crystallographic orientation [12,28]. The results in Fig. 6 seem to indicate that the carrier concentration is determined by oxygen partial pressure. However, it should be noticed that the crystallographic orientation changes with oxygen flow rate at the same time (see Fig. 1), which may also affect the carrier concentration [15,28].
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Fig. 5. XRD patterns for samples of group (a) B, (b) C, and (c) D. The (400) diffraction peaks are shown in the 10 (a)~(b) and 30 (c) magnified views.
Fig. 6. Carrier concentration as a function of oxygen flow rate for group A, B, and C samples.
For the deposition of ITO films, the crystallographic orientation is mainly determined by the nucleation stage of ITO grains on the substrate [24]. Generally, ITO films are grown by one-step growth method, where the same conditions are employed for both nucleation and film-thickening stages. For this reason, the crystallographic orientation could not be controlled without changing the conditions for film-thickening stage, which brings difficulties to distinguish the effect of O2 partial pressure and crystallographic orientation. In order to clarify how O2 partial pressure and crystallographic orientation affect the electrical properties of the ITO films, four samples (D1-D4) with a thickness of 90 nm have been prepared. The growth rate is calculated to be 1.5 nm/min. Sample D1 and D2 were grown by conventional one-step method with O2 flow rates of 1000 and 500 sccm, respectively. Sample D3 and D4 were grown by two-step methods in which ~15 nm-thick nucleation layers were introduced, followed by the growth of the main layers. The schematic diagram is shown in Fig. 7(a). For example, for sample D3, the growth condition of the nucleation layer is the same as that of sample D2, while the growth condition of the main layer is identical with that of sample D1. Fig. 7(b)-(e) show the SEM morphologies for samples D1-D4, respectively. The (100)-grain density are summarized in Table 2. Regardless of the differences in the main layer, samples with the same nucleation layer show similar XRD profiles, morphologies and (100)-grain densities. Fig. 5(c) shows the XRD patterns for samples of group D. The I400/I222 ratios are observed to be ~0.035 and ~0.005 for samples nucleate at O2 ¼ 1000 sccm and O2 ¼ 500 sccm, respectively. The XRD experiments agree well with the SEM observation. By employing two-step method, the crystallography
Fig. 7. (a) Schematic diagrams of sample D1-D4; The SEM morphologies of (b) D1, (c) D2, (d) D3 and (e) D4. The scale bar is 500 nm.
orientation of the ITO film is effectively controlled. The results of Hall-effect measurement are also summarized in Table 2. Although the nucleation layers and (100)-grain densities are different, sample D2 and D4 possess similar carrier concentrations which are higher than that of sample D1 and D3. It should be noticed that the nucleation layer is so thin that the carrier concentration is mainly determined by the main layer. The main layers in sample D2 and D4 were grown at the same O2 flow rate which is lower than that of D1 and D3. These findings indicate that the variation in carrier concentration is mainly determined by the direct effect of varying O2 partial pressure rather than the difference in crystallographic orientation. On the other hand, similar values of mobility are observed for samples grown with the same nucleation layer. Despite the difference in carrier concentration, samples with lower (100)-grain density show relatively higher mobility, indicating the grain orientation dependence of carrier mobility. To further understand the effect of crystallographic orientation, the structural and electrical information for the four groups of samples are summarized in Fig. 8. Fig. 8(a) shows the hall mobility as a function of (100)-grain density. Negative correlation between the hall mobility and (100)-grain density has been observed. For ntype polycrystalline semiconductor, the scattering mechanisms dominating electron mobility include ionized impurity scattering,
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Table 2 Summaries of (100)-grain density, resistivity, mobility, carrier concentration and deduced barrier height for sample D1-D4.
D1 D2 D3 D4
(100)-grain density (/mm2)
Resistivity (104 U cm)
Mobility (cm2V1s1)
Carrier concentration (1020 cm3)
EB (meV)
87 18 15 100
2.71 ± 0.02 2.10 ± 0.02 2.28 ± 0.02 2.30 ± 0.05
32.1 ± 0.2 38.7 ± 0.5 38.9 ± 0.4 34.4 ± 0.6
7.17 ± 0.04 7.65 ± 0.15 7.04 ± 0.12 7.88 ± 0.32
20 15 15 18
Fig. 8. Mobility as a function of (a) (100)-grain density and (b) carrier concentration and (c) grain boundary barrier height as a function of (100)-grain density for group A-D samples. The solid black line is a calculated dependence of mg(n) for single crystal In2O3 at 300 K [29].
polar optical phonon scattering, acoustic-phonon scattering, and grain boundary scattering [30]. The scatterings by ionized impurities and phonons have already been intensively investigated in previous research [29,31]. On the other hand, grain boundary scattering is caused by the space charge region formed around the grain boundaries, which is an important scattering mechanism for polycrystalline ITO. Adopting the approach suggested by Kazmerski et al., the mobility can be expressed by Refs. [32,33]:
mpoly ¼ mg ðT; nÞ$exp
EB kB T
(1)
where T is the temperature, n the carrier concentration, kB the Boltzmann constant, mg(T, n) the carrier mobility of bulk In2O3 as a function of temperature and carrier concentration and EB the averaged grain boundary barrier height. For degenerated ITO, the effect of grain boundary scattering has been observed even for carrier concentrations up to ~1021 cm3 and is attributed to the aggregation of defects at the grain boundary such as Sn segregation [33]. Fig. 8(b) shows the hall mobility data as a function of carrier concentration. There is no clear relation between the carrier concentration and the mobility. The solid line in Fig. 8(b) corresponds to the calculated mobility for bulk ITO mg(n) at 300 K, where scatterings by polar optical phonons, acoustic deformation potential phonons, and ionized impurities were considered [29]. For the textured polycrystalline ITO thin films investigated in this work, high density of grain boundary exists in the films. Therefore, the difference between the hall mobility and the calculated is mainly attributed to grain boundary scattering. With the mg(n) shown in Fig. 8(b), grain boundary barrier height can be deduced according to Eq. (1). The averaged barrier height EB changes significantly from 14 to 34 meV and shows a trend to increase with (100)-grain density as could be seen in Fig. 8(c). 4. Conclusion In conclusion, ITO thin films were grown by MOCVD with different O2 flow rates. The as-grown ITOs presented
polycrystalline structure with both (111)- and (100)-oriented grains. The density of (100)-grains reached maximum at medium O2 partial pressure. By introducing nucleation layers, we realized effective control of the crystallographic orientation of ITO thin films, which allowed us to individually investigate the effect of crystallographic orientation and O2 condition. Excluding the effect of crystallographic orientation, the variation of carrier concentration's dependence on O2 flow rate was attributed to the direct effect of the varied O2 flow rate (Fig. 6). On the other hand, the carrier mobility of the ITO film was found to be considerably affected by the crystallographic orientation [Fig. 8(a)]. Analysis on carrier's transport reveals that the averaged barrier height of grain boundary increases with the (100)-grain density. This work also offers a new way to control the morphology and electrical properties of ITO films. Acknowledgements This work was supported by the Science and Technology Project of Guangdong Province, China [grant numbers 2015B010132008, 2016B090918106, 2016B010129002 and 2017B090911002], the Science and Technology Planning Project of Guangzhou, China [grant numbers 201607020036 and 201804020051] and the Fundamental Research Funds for the Central Universities [grant number 20177612031650012]. References [1] J. Bai, M. Athanasiou, T. Wang, Influence of the ITO current spreading layer on efficiencies of InGaN-based solar cells, Sol. Energy Mater. Sol. Cells 145 (2016) 226e230. [2] Z. Chen, Y. Zhuo, W. Tu, X. Ma, Y. Pei, C. Wang, G. Wang, Highly ultraviolet transparent textured indium tin oxide thin films and the application in light emitting diodes, Appl. Phys. Lett. 110 (2017) 242101. [3] W. Zheng, F. Huang, R. Zheng, H. Wu, Low-dimensional structure vacuumultraviolet-sensitive (lambda < 200 nm) photodetector with fast-response speed based on high-quality AlN micro/nanowire, Adv. Mater. 27 (2015) 3921e3927. [4] V.S. Vaishnav, S.G. Patel, J.N. Panchal, Development of ITO thin film sensor for detection of benzene, Sens. Actuators B Chem. 206 (2015) 381e388. [5] W. Zheng, R. Lin, Y. Zhu, Z. Zhang, X. Ji, F. Huang, Vacuum ultraviolet photodetection in two-dimensional oxides, ACS Appl. Mater. Interfaces (2018)
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