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Indium tin oxide coatings properties as a function of the deposition atmosphere
Thin Solid Films 520 (2012) 4041–4045
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Indium tin oxide coatings properties as a function of the deposition atmosphere Mario Tului a,⁎, Alessandra Bellucci a, Stefania Bellini a, Augusto Albolino b, Giuseppe Migliozzi c a b c
Centro Sviluppo Materiali S.p.A., Via di Castel Romano, 100, 00128 Rome Italy Alenia Aermacchi S.p.A., Viale dell'Aeronautica, snc, 80038 Pomigliano d'Arco, Naples, Italy Alenia Aermacchi S.p.A., Corso Marche 41, 10146 Torino Italy
a r t i c l e
i n f o
Article history: Received 14 May 2009 Received in revised form 27 January 2012 Accepted 1 February 2012 Available online 8 February 2012 Keywords: Indium oxide Deposition process Physical vapour deposition Optical properties Conductivity Structural properties
a b s t r a c t Indium tin oxide coatings were deposited by magnetron sputtering physical vapour deposition under different atmospheres. Microstructural, electrical, and optical properties were measured, finding a correlation among properties and process parameters. Texture analyses carried out by X-ray diffraction showed that films microstructure depended by the oxygen content in the deposition vessel: high values of the oxygen content (e.g., 5%) caused the film to grow along the b111> orientation; under pure Ar, the grains grew along the b100> orientation. Intermediate values of the oxygen content caused the growth of two families of grains, respectively oriented along the b111> and the b100> directions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The relevance of TCO (transparent conductive oxide) films is constantly increasing in the last years [1,2]. Their applications span from photovoltaic cell up to thermal glasses for energy saving buildings [3–5]. Among the various possible compositions for TCO, the most important is the one based on indium oxide doped with tin, which is known with the acronym ITO (indium tin oxide) [6,7]. Several methods have been proposed to deposit ITO films. One of the most used on industrial bases is the PVD (physical vapour deposition) sputtering [8,9]. ITO films deposited by PVD are now commercially available, but the effort for its optimisation is still in progress. In particular, it is very important to correlate process parameters, films microstructure, and electrical and optical properties [9–20]. Many researchers have reported a change of the preferred orientation of ITO films with varied process conditions [16–28], which has a significant effect on the variation of film optical and electrical characteristics. In this frame, the present work aims at investigating the effect of the presence of oxygen in the deposition vessel, one of the parameters which have the highest influence on the functional properties of the films, devoting a particular effort to elucidate the mechanism of texture development. Published papers about this topic mainly report qualitatively evaluated X-ray patterns and only very seldom texture analyses ⁎ Corresponding author. Tel.: + 39 06 5055 742; fax: + 39 06 5055 488. E-mail addresses: [email protected] (M. Tului), [email protected] (A. Bellucci), [email protected] (S. Bellini), [email protected] (A. Albolino), [email protected] (G. Migliozzi). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.02.002
based on polar figures are present. The aim of the present work is to give a contribution to this lacking field. 2. Experimental details Coatings were deposited onto glass substrates by means of a magnetron sputtering PVD (physical vapour deposition) equipment, produced by Microcoat (Italy). A In2O3 + 10%SnO2 target, supplied by Cerac (USA), was used as a raw material. The target had a surface of 450 × 150 mm. All the deposition runs were carried out using 1500 W of sputtering power, at a temperature of 623 K; sputtering time was 900 s. The pressure during deposition was 0.7 Pa. The composition of the atmosphere of each deposition run was a mixture of argon and oxygen, the latter varying from 0 up to 5%. In particular, deposition runs were carried out with oxygen percentages of 0.0, 0.6, 1.5, 3.0, and 5.0. In the following, the samples will be identified with a label depending from the oxygen percentage used during their deposition: Ox0.0, for samples deposited under pure Ar, Ox0.6 for sample deposited with a mixture of argon and 0.6% of oxygen, and so on. From the coated substrates, for each deposition cycle, samples for the following characterisations were extracted: i) XRD (X-ray diffraction) was carried out, using an Italstructure APD 2000 (I) equipment, to determine phase content; a Cu Kα X-ray source, with a wavelength of 1.541 , was used; ii) the same samples were characterised by means of a pole figure goniometer, installed on a X-ray diffractometer Siemens D5000 (D), to determine eventual preferred crystallographic
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M. Tului et al. / Thin Solid Films 520 (2012) 4041–4045
orientations; defocusing correction was carried out using the XRD pole figures of an ITO powder; thickness correction was carried out using the following equation [21,29]:
Icorrected ¼
1− expð−2Aρ t= sinϑÞ I 1− expð−2Aρ t= sinϑ cosχ Þ measured
ð1Þ
where I is the intensity; A and ρ, the mass absorption coefficient and the density of the ITO, respectively; t, the sample thickness; ϑ, the measurement angle; and χ, the sample tilting angle; coated surfaces and fracture surfaces were observed by means of a Leica Cambridge S 360 SEM (scanning electron microscope), using an operating voltage of 20 kV; some samples were cut, embedded in resin, and polished, to have their cross sections observed by SEM to determine thickness; on 11 × 11 mm samples, surface resistivity was measured by means of Van der Pauw method; on the same samples, Hall effect characterisation was carried out by means of a Bio-Rad HL5500 (USA), in order to determine sign, concentration, and mobility of the charge carriers; optical transmittance was measured in the wavelength range 0.2–3.0 μm by means of a spectrophotometer Varian CARY 5E (USA).
iii)
iv) v) vi)
vii)
Fig. 2 shows the (222) and (400) pole figures of the various samples. The Ox5.0 sample presents a strong preferential orientation, with the (222) direction aligned almost perpendicular to the sample surface; the maximum of the peak is misaligned by about 5°. The (400) polar figure shows a fibre structure, centred around the (222) peak; the radius of this circle is about 54.7°, i.e., the angle between the b111> and b100> directions. A secondary peak can be noted, around χ 35° and φ 180°. Also the Ox3.0 and Ox1.5 samples are oriented along the b222> direction, but with a higher misalignment. The pole figures of the Ox0.6 sample reveal the presence of both the orientations (222) and (400). Finally, the pole figures of the Ox0.0 sample show that the grains are aligned with the (400) direction perpendicular to the sample surface, with the (222) pole figure showing a fibre structure around the (400) peak, that is the opposite configuration of the Ox5.0 sample.
75 50 25 0 50 25 75
75 50 25 0 50 25 75
Ox0.0
3. Results 3.1. X-ray diffraction Fig. 1 shows the XRD patterns of the samples Ox0.0, Ox0.6, Ox1.5 and Ox5.0, respectively. The XRD patterns can be fully indexed as In2O3, by comparison with the standard pattern reported in the card no. 06-0416 of the ICDD (the International Centre for Diffraction Data) database. In all the XRD patterns shown in Fig. 1, the ratios between the intensities of the peaks are not the ones expected for reference polycrystalline powder, suggesting the presence of preferred orientations in the samples. In particular, according to the ICDD card the ratio between the intensity of the (222) reflection I222 and the intensity of the (400) peak I400 should be I222/I400 = 3.33, whilst in the case of the samples Ox1.5 and Ox5.0, the (222) reflections are dominant (I222/I400 = 15). In the XRD pattern of the sample Ox0.6, vice versa, the I222/I400 ratio value is about 1, and in the pattern of the Ox0.0 sample, the (400) reflection is the dominant one. To clarify the grains orientation of the samples, texture measurements (pole figures) were carried out. In the pole figures which will be shown in the following, the sample was scanned by in-plane rotation (φ) around the plane normal to the different azimuthal angles (χ).
Ox0.6
Ox1.5
Ox3.0 (211)
(222)
(400)(411)
(332) (431) (521)(440)
(611) (622)(631)
(721)
Intensity (arbitrary unit)
O2-5.0% O2-1.5%
O2-0.6%
Ox5.0
O2-0.0% 20
30
40
50
60
70
2 (degrees) Fig. 1. XRD patterns of the samples Ox0.0, Ox0.6, Ox1.5 and Ox5.0.
Fig. 2. Pole figures, referred to the reflections (222) and (400), of the sample Ox0.0, Ox0.6, Ox1.5, Ox3.0, and Ox5.0.
M. Tului et al. / Thin Solid Films 520 (2012) 4041–4045
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to the fact that the reduction is obtained eliminating part of the interstitial oxygen atoms, which produces a less deformed lattice.
(a)
3.4. Optical properties Fig. 5a shows the transmittance as a function of wavelength for the various samples, in the region 0.2–3.0 μm. From all the graphs, the contribution of the substrate was subtracted. As in the case of the resistivity, the high values of transmittance in the visible region (more than 80%), compared with the results obtained by other authors [5,6,8,10–13,17,19,20,23,28], confirmed the good quality of the films. Fig. 5b shows a detail of the previous mentioned graphs in the ultraviolet region, where a difference of the absorption edge between the various samples can be noted. In particular, the higher the oxygen content in the deposition vessel, the higher the absorption edge in terms of wavelength. Since the oxygen content is inversely proportional to the carrier concentration in the film, as shown by the data reported in Table 1, the absorption edge shift is coherent with the well known Burstein–Moss effect [30]. Transmittance spectra can be used to determine the values of the optical band gap Eog of the various samples, by means of the following relationships [7]:
1 µm
(b)
α¼
1 µm
1 1 ln d T
ð2Þ
2
ðahvÞ ¼ hv−Eog Fig. 3. Top (a) and fracture surface (b) of the sample Ox3.0, observed by SEM.
3.2. Microstructure Fig. 3 shows SEM images of top and fracture surface of the sample Ox3.0. The coating structure appears to be constituted by columnar grains, as long as the entire coating thickness; grain dimensions, in the direction parallel to the substrate surface, spanned from a minimum of 15–20 nm up to 100 nm. The grains are not perpendicular to the substrate surface, but they appear to be tilted of about 10° with respect to the normal direction. Similar structures were observed also on the other samples. 3.3. Electromagnetic properties Table 1 reports the values of thickness, surface resistance, resistivity, electrical charge carrier concentration, and mobility of the various samples. Regarding the sign of the carrier, Hall effect measurement showed that all the samples were n-type semiconductors. Resistivity values as low as 0.14 mΩ cm were obtained, in agreement with the better results found in the literature [7,8,10–12,15,19,20,24]. The data on carrier concentration and mobility are reported in Fig. 4 as a function of oxygen percentage in the deposition atmosphere. Both the curves drastically increase as the oxygen percentage becomes lower than 1.5%. The increase of carrier concentration is a consequence of the partial reduction of the ITO, due to the low oxygen partial pressure during the deposition. The increase of mobility is likely due
ð3Þ
where α is the absorption coefficient; d, the thickness of the film; T, the transmittance; h,the Planck constant; and ν, the frequency. The optical band gap is obtained by extrapolating the linear region of the (αhν)2 versus hν curve, up to zero absorption, as shown in Fig. 6. The values of Eog for films obtained under different oxygen partial pressure are reported in the last column of Table 1. 4. Discussion According to the observations of several authors [14,16–18,20– 22,24,25], ITO films deposited by sputtering are composed by columnar grains, which can present the plane (100), or, alternatively, the plane (111), parallel to the substrate plane. The former situation occurs when the film is deposited at high temperature (i.e., higher than 250–300 °C) and with a reduced oxygen content in the deposition atmosphere. The latter orientation, vice versa, is observed in coatings deposited at room temperature or under an oxygen containing atmosphere. Moreover, the texture selection occurs at the very beginning phase of the film growth [22]. Those phenomena have been explained by the quoted authors in the following way: the (100) texture seems to be thermodynamically preferred, but, since its planar density is lower than the (111) one, the adatoms need to diffuse for longer distances to nucleate it. As a consequence, when the adatoms energy, in the nucleation stage, is low, (111) oriented crystallites will be generated; vice versa, when the adatoms have enough energy to diffuse, (100) grains will be nucleated. The quoted literature attributes a similar effect to the oxygen content in the deposition vessel: when an ITO coating is deposited
Table 1 Thickness, surface resistance, resistivity, carrier concentration, mobility, and optical band gap values measured on the realised samples. Oxygen percentage (%) 0.0 0.6 1.5 3.0 5.0
Thickness (nm)
Surface resistance (Ω/□)
Resistivity (mΩ cm)
Carrier concentration (cm− 3)
Mobility (cm2/Vs)
Optical band gap (eV)
803 760 725 707 610
1.7 4.1 14.2 19.1 87.5
0.14 0.31 1.03 1.35 5.34
1.50 × 1021 8.11 × 1020 3.85 × 1020 3.08 × 1020 1.23 × 1020
29.4 24.5 15.7 15.0 9.5
3.97 3.96 3.86 3.85 3.72
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M. Tului et al. / Thin Solid Films 520 (2012) 4041–4045
25
40
16
12
30
Mobility
20
8
4
10
0
0 0
1
2
3
O2-5.0%
20
Mobility (cm2/Vs)
Carrier concentration (1020cm-3)
Carrier concentration
4
O2-3.0% O2-1.5%
15
O2-0.6% 10
O2-0.0%
5 0 3.5
3.7
3.9
4.1
Fig. 6. Absorption coefficient α as a function of the impinging photons energy.
5
Oxigen content (%)
under an oxygen containing atmosphere, oxygen atoms saturate the film, occupying all the reticular sites in the crystallisation nuclei, and hindering, as a consequence, the diffusion of the adatoms. The described mechanisms can explain the XRD patterns and the pole figures reported in Figs. 1 and 2, respectively. When the coatings were deposited under pure Ar (sample Ox0.0), they resulted to be oriented along the b400> direction. Vice versa, when the oxygen content in the deposition atmosphere was high (samples Ox3.0 and Ox5.0), the preferred orientation was along the b222> direction. The samples Ox0.6 and Ox1.5 are in an intermediate situation, since their pole figures clearly show the presence of two families of grains, as can be observed, e.g., in Fig. 7. It shows the sections of the Ox1.5 sample pole figures obtained keeping the φ angle equal to 40°, i.e., the angle where the maximum of both the two polar figures is present (the y axes report the corrected intensities, which coincide with the measured values at χ = 0). In Fig. 7 the (222) pole figure peaks are indicated with solid lines and labelled with capital letters, whilst dashed lines and lowercase letters are used for (400) peaks.
1.0
(a) Transmittance
0.8 0.6
O2-5.0% O2-3.0%
0.4
O2-1.5% O2-0.6%
0.2
O2-0.0% 0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Wavelength (µm)
The two main peaks of the two pole figures, i.e., C and b for (222) and (400), respectively, can not be generated by the same grains, because in the cubic lattice, which is the case of ITO, the angle between the b111> and b100> directions is 54.7°. In fact, secondary peaks (e.g., B and a) which form such an angle with the main peaks, can be observed in both the polar figures. Repeating the same analyses on all the polar figures, similar conclusions can be achieved, i.e., two main families of grains are present in the coatings: one with the b111> axis, another one with the b100> axis, roughly perpendicular with the substrate. Both the intensity and the misalignment of the main peaks observed in the pole figures depend by the deposition process parameters: the higher the oxygen content in the deposition atmosphere, the higher the intensity of the main peak in the (222) pole figure, and the lower its distance from the centre (i.e., the misalignment of the b111> axis with the substrate normal direction for the former family of grains). The reverse happens for the main peak in the (400) pole figures. Moreover, the angular distance between the maxima of the two pole figures for each sample, reported in Fig. 8, appears to be constant and it is about 40°. To explain all the observation reported above, the following mechanism can be hypothesised: the nucleation of crystallites orientated along the b111> or along the b100> depends by the adatoms energy and by the oxygen concentration, and local variations of such parameters can occur on the substrate during the initial stage of the deposition, causing in some points the nucleation of (111) crystallites (i.e., crystallites with the b111> direction roughly perpendicular to the substrate surface), in other points, the nucleation of (100) crystallites. In specific process conditions, one orientation will be preferred, not excluding that in some points the conditions to obtain the nucleation of the other orientation can be obtained. Increasing the oxygen concentration in the deposition vessel, the (111) crystallites number will increase too, whilst the (100) crystallites numbers will decrease. In the extreme conditions, i.e., without any oxygen in the deposition vessel, or, vice versa, with a very high oxygen content, crystallites oriented along the b111> direction or along the b100> direction,
1.0
(b)
4000
Corrected counts (222)
Transmittance
0.8 O2-5.0% 0.6 0.4
O2-3.0% O2-1.5% O2-0.6%
0.2 0.0 0.25
O2-0.0%
0.30
0.35
0.40
0.45
Wavelength (µm) Fig. 5. Transmittance as a function of wavelength in the region 0.2–3.0 μm (a), and in the ultraviolet region (b).
A B a b
C
D
70.5°
1000
(222) (400)
54.7°
3000
c
800
54.7°
600
70.5°
2000
400
90° 1000 0 -75
200 0 -50
-25
0
25
50
75
Fig. 7. Sections of the Ox1.5 sample pole figures for φ = 40°.
Corrected counts (400)
Fig. 4. Carrier concentration and mobility as a function of the oxygen percentage in the deposition atmosphere.
M. Tului et al. / Thin Solid Films 520 (2012) 4041–4045
Deviation from the origin (degrees)
40
30 222 20
400
10
0 0
1
2
3
4
5
4045
and the resistivity decreased. Intermediate values of the oxygen content caused the growth of two families of grains, respectively oriented along the b111> and the b100> directions. Such grain families were not exactly aligned with the normal direction to the substrate plane, but they presented a misalignment which depended by the process conditions, i.e., by the oxygen content in the deposition atmosphere: the higher the oxygen content, the lower the misalignment of the (111) grains; the reverse is for the (100) grains. Moreover, the angle formed by the average orientations of the two families of grains seems to be independent by the deposition parameters. The reason of this phenomenon is not yet clear.
Oxygen content (%) Fig. 8. Angular distance from the origin of the (222) and (400) pole figures main peaks, as a function of the oxygen percentage in the deposition atmosphere.
respectively, will prevail. Under a low oxygen content atmosphere, nuclei belonging to both the orientations will be present. Only a few of all the nuclei will grow, at the expense of the surrounding nuclei, up to becoming the columnar grains observed in the coatings. When only one type of crystallites is present, i.e., crystallites oriented along the b111> or the b100> direction, grains well aligned with the normal to the substrate surface seem to be preferred. When both the types of nuclei are present, obtained results suggest the presence of a selection mechanism which promotes the nuclei which orientation presents a specific angle with the orientation of the surrounding nuclei of different type. In the case of the Ox5.0 sample, each nucleus had an elevated probability to be in touch with (111) oriented nuclei: as a consequence, the selected grains were the (111) nuclei well aligned with the substrate plane normal, and the very few ones, among the few (100) oriented grains, which formed the right angle with the surrounding (111) grains. In the case of the Ox0.6 and Ox1.5 samples, vice versa, each nucleus had an elevated probability to be in touch with at least another nucleus oriented in a different way; as a consequence, couples of (111) and (100) grains, forming the right angle between their orientation, will be selected for the growth. Regarding the observed preferential angle of about 40° between the orientations of the two families of grains, the authors speculate that such an angle could correspond to a low energy grain boundary. Further investigations are needed to confirm such a hypothesis, or to find another mechanism which better describes the observed phenomenon. 5. Conclusions Several ITO films were deposited by PVD sputtering under different oxygen partial pressure. The presence of oxygen influenced the growth direction of the crystal grains which constituted the films, and determined its conductivity: at high oxygen content, the films grew along the b111> direction and the resistivity was high; at low oxygen content, the grains grew also along other orientations, e.g., the b100>,
Acknowledgements Authors would like to thank Dr. Stefano Martelli, from CSM, for the useful discussions; Ms. Cristina Bernini, from INFN (I), for SEM characterisations; Prof. Antonio Serra, from Lecce University (I), for the Hall effect measurements; and Dr. Mario Giustini, from the Rome University “la Sapienza” (I), for transmittance characterisations.
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Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Indium tin oxide coatings properties as a function of the deposition atmosphere Mario Tului a,⁎, Alessandra Bellucci a, Stefania Bellini a, Augusto Albolino b, Giuseppe Migliozzi c a b c
Centro Sviluppo Materiali S.p.A., Via di Castel Romano, 100, 00128 Rome Italy Alenia Aermacchi S.p.A., Viale dell'Aeronautica, snc, 80038 Pomigliano d'Arco, Naples, Italy Alenia Aermacchi S.p.A., Corso Marche 41, 10146 Torino Italy
a r t i c l e
i n f o
Article history: Received 14 May 2009 Received in revised form 27 January 2012 Accepted 1 February 2012 Available online 8 February 2012 Keywords: Indium oxide Deposition process Physical vapour deposition Optical properties Conductivity Structural properties
a b s t r a c t Indium tin oxide coatings were deposited by magnetron sputtering physical vapour deposition under different atmospheres. Microstructural, electrical, and optical properties were measured, finding a correlation among properties and process parameters. Texture analyses carried out by X-ray diffraction showed that films microstructure depended by the oxygen content in the deposition vessel: high values of the oxygen content (e.g., 5%) caused the film to grow along the b111> orientation; under pure Ar, the grains grew along the b100> orientation. Intermediate values of the oxygen content caused the growth of two families of grains, respectively oriented along the b111> and the b100> directions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The relevance of TCO (transparent conductive oxide) films is constantly increasing in the last years [1,2]. Their applications span from photovoltaic cell up to thermal glasses for energy saving buildings [3–5]. Among the various possible compositions for TCO, the most important is the one based on indium oxide doped with tin, which is known with the acronym ITO (indium tin oxide) [6,7]. Several methods have been proposed to deposit ITO films. One of the most used on industrial bases is the PVD (physical vapour deposition) sputtering [8,9]. ITO films deposited by PVD are now commercially available, but the effort for its optimisation is still in progress. In particular, it is very important to correlate process parameters, films microstructure, and electrical and optical properties [9–20]. Many researchers have reported a change of the preferred orientation of ITO films with varied process conditions [16–28], which has a significant effect on the variation of film optical and electrical characteristics. In this frame, the present work aims at investigating the effect of the presence of oxygen in the deposition vessel, one of the parameters which have the highest influence on the functional properties of the films, devoting a particular effort to elucidate the mechanism of texture development. Published papers about this topic mainly report qualitatively evaluated X-ray patterns and only very seldom texture analyses ⁎ Corresponding author. Tel.: + 39 06 5055 742; fax: + 39 06 5055 488. E-mail addresses: [email protected] (M. Tului), [email protected] (A. Bellucci), [email protected] (S. Bellini), [email protected] (A. Albolino), [email protected] (G. Migliozzi). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.02.002
based on polar figures are present. The aim of the present work is to give a contribution to this lacking field. 2. Experimental details Coatings were deposited onto glass substrates by means of a magnetron sputtering PVD (physical vapour deposition) equipment, produced by Microcoat (Italy). A In2O3 + 10%SnO2 target, supplied by Cerac (USA), was used as a raw material. The target had a surface of 450 × 150 mm. All the deposition runs were carried out using 1500 W of sputtering power, at a temperature of 623 K; sputtering time was 900 s. The pressure during deposition was 0.7 Pa. The composition of the atmosphere of each deposition run was a mixture of argon and oxygen, the latter varying from 0 up to 5%. In particular, deposition runs were carried out with oxygen percentages of 0.0, 0.6, 1.5, 3.0, and 5.0. In the following, the samples will be identified with a label depending from the oxygen percentage used during their deposition: Ox0.0, for samples deposited under pure Ar, Ox0.6 for sample deposited with a mixture of argon and 0.6% of oxygen, and so on. From the coated substrates, for each deposition cycle, samples for the following characterisations were extracted: i) XRD (X-ray diffraction) was carried out, using an Italstructure APD 2000 (I) equipment, to determine phase content; a Cu Kα X-ray source, with a wavelength of 1.541 , was used; ii) the same samples were characterised by means of a pole figure goniometer, installed on a X-ray diffractometer Siemens D5000 (D), to determine eventual preferred crystallographic
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M. Tului et al. / Thin Solid Films 520 (2012) 4041–4045
orientations; defocusing correction was carried out using the XRD pole figures of an ITO powder; thickness correction was carried out using the following equation [21,29]:
Icorrected ¼
1− expð−2Aρ t= sinϑÞ I 1− expð−2Aρ t= sinϑ cosχ Þ measured
ð1Þ
where I is the intensity; A and ρ, the mass absorption coefficient and the density of the ITO, respectively; t, the sample thickness; ϑ, the measurement angle; and χ, the sample tilting angle; coated surfaces and fracture surfaces were observed by means of a Leica Cambridge S 360 SEM (scanning electron microscope), using an operating voltage of 20 kV; some samples were cut, embedded in resin, and polished, to have their cross sections observed by SEM to determine thickness; on 11 × 11 mm samples, surface resistivity was measured by means of Van der Pauw method; on the same samples, Hall effect characterisation was carried out by means of a Bio-Rad HL5500 (USA), in order to determine sign, concentration, and mobility of the charge carriers; optical transmittance was measured in the wavelength range 0.2–3.0 μm by means of a spectrophotometer Varian CARY 5E (USA).
iii)
iv) v) vi)
vii)
Fig. 2 shows the (222) and (400) pole figures of the various samples. The Ox5.0 sample presents a strong preferential orientation, with the (222) direction aligned almost perpendicular to the sample surface; the maximum of the peak is misaligned by about 5°. The (400) polar figure shows a fibre structure, centred around the (222) peak; the radius of this circle is about 54.7°, i.e., the angle between the b111> and b100> directions. A secondary peak can be noted, around χ 35° and φ 180°. Also the Ox3.0 and Ox1.5 samples are oriented along the b222> direction, but with a higher misalignment. The pole figures of the Ox0.6 sample reveal the presence of both the orientations (222) and (400). Finally, the pole figures of the Ox0.0 sample show that the grains are aligned with the (400) direction perpendicular to the sample surface, with the (222) pole figure showing a fibre structure around the (400) peak, that is the opposite configuration of the Ox5.0 sample.
75 50 25 0 50 25 75
75 50 25 0 50 25 75
Ox0.0
3. Results 3.1. X-ray diffraction Fig. 1 shows the XRD patterns of the samples Ox0.0, Ox0.6, Ox1.5 and Ox5.0, respectively. The XRD patterns can be fully indexed as In2O3, by comparison with the standard pattern reported in the card no. 06-0416 of the ICDD (the International Centre for Diffraction Data) database. In all the XRD patterns shown in Fig. 1, the ratios between the intensities of the peaks are not the ones expected for reference polycrystalline powder, suggesting the presence of preferred orientations in the samples. In particular, according to the ICDD card the ratio between the intensity of the (222) reflection I222 and the intensity of the (400) peak I400 should be I222/I400 = 3.33, whilst in the case of the samples Ox1.5 and Ox5.0, the (222) reflections are dominant (I222/I400 = 15). In the XRD pattern of the sample Ox0.6, vice versa, the I222/I400 ratio value is about 1, and in the pattern of the Ox0.0 sample, the (400) reflection is the dominant one. To clarify the grains orientation of the samples, texture measurements (pole figures) were carried out. In the pole figures which will be shown in the following, the sample was scanned by in-plane rotation (φ) around the plane normal to the different azimuthal angles (χ).
Ox0.6
Ox1.5
Ox3.0 (211)
(222)
(400)(411)
(332) (431) (521)(440)
(611) (622)(631)
(721)
Intensity (arbitrary unit)
O2-5.0% O2-1.5%
O2-0.6%
Ox5.0
O2-0.0% 20
30
40
50
60
70
2 (degrees) Fig. 1. XRD patterns of the samples Ox0.0, Ox0.6, Ox1.5 and Ox5.0.
Fig. 2. Pole figures, referred to the reflections (222) and (400), of the sample Ox0.0, Ox0.6, Ox1.5, Ox3.0, and Ox5.0.
M. Tului et al. / Thin Solid Films 520 (2012) 4041–4045
4043
to the fact that the reduction is obtained eliminating part of the interstitial oxygen atoms, which produces a less deformed lattice.
(a)
3.4. Optical properties Fig. 5a shows the transmittance as a function of wavelength for the various samples, in the region 0.2–3.0 μm. From all the graphs, the contribution of the substrate was subtracted. As in the case of the resistivity, the high values of transmittance in the visible region (more than 80%), compared with the results obtained by other authors [5,6,8,10–13,17,19,20,23,28], confirmed the good quality of the films. Fig. 5b shows a detail of the previous mentioned graphs in the ultraviolet region, where a difference of the absorption edge between the various samples can be noted. In particular, the higher the oxygen content in the deposition vessel, the higher the absorption edge in terms of wavelength. Since the oxygen content is inversely proportional to the carrier concentration in the film, as shown by the data reported in Table 1, the absorption edge shift is coherent with the well known Burstein–Moss effect [30]. Transmittance spectra can be used to determine the values of the optical band gap Eog of the various samples, by means of the following relationships [7]:
1 µm
(b)
α¼
1 µm
1 1 ln d T
ð2Þ
2
ðahvÞ ¼ hv−Eog Fig. 3. Top (a) and fracture surface (b) of the sample Ox3.0, observed by SEM.
3.2. Microstructure Fig. 3 shows SEM images of top and fracture surface of the sample Ox3.0. The coating structure appears to be constituted by columnar grains, as long as the entire coating thickness; grain dimensions, in the direction parallel to the substrate surface, spanned from a minimum of 15–20 nm up to 100 nm. The grains are not perpendicular to the substrate surface, but they appear to be tilted of about 10° with respect to the normal direction. Similar structures were observed also on the other samples. 3.3. Electromagnetic properties Table 1 reports the values of thickness, surface resistance, resistivity, electrical charge carrier concentration, and mobility of the various samples. Regarding the sign of the carrier, Hall effect measurement showed that all the samples were n-type semiconductors. Resistivity values as low as 0.14 mΩ cm were obtained, in agreement with the better results found in the literature [7,8,10–12,15,19,20,24]. The data on carrier concentration and mobility are reported in Fig. 4 as a function of oxygen percentage in the deposition atmosphere. Both the curves drastically increase as the oxygen percentage becomes lower than 1.5%. The increase of carrier concentration is a consequence of the partial reduction of the ITO, due to the low oxygen partial pressure during the deposition. The increase of mobility is likely due
ð3Þ
where α is the absorption coefficient; d, the thickness of the film; T, the transmittance; h,the Planck constant; and ν, the frequency. The optical band gap is obtained by extrapolating the linear region of the (αhν)2 versus hν curve, up to zero absorption, as shown in Fig. 6. The values of Eog for films obtained under different oxygen partial pressure are reported in the last column of Table 1. 4. Discussion According to the observations of several authors [14,16–18,20– 22,24,25], ITO films deposited by sputtering are composed by columnar grains, which can present the plane (100), or, alternatively, the plane (111), parallel to the substrate plane. The former situation occurs when the film is deposited at high temperature (i.e., higher than 250–300 °C) and with a reduced oxygen content in the deposition atmosphere. The latter orientation, vice versa, is observed in coatings deposited at room temperature or under an oxygen containing atmosphere. Moreover, the texture selection occurs at the very beginning phase of the film growth [22]. Those phenomena have been explained by the quoted authors in the following way: the (100) texture seems to be thermodynamically preferred, but, since its planar density is lower than the (111) one, the adatoms need to diffuse for longer distances to nucleate it. As a consequence, when the adatoms energy, in the nucleation stage, is low, (111) oriented crystallites will be generated; vice versa, when the adatoms have enough energy to diffuse, (100) grains will be nucleated. The quoted literature attributes a similar effect to the oxygen content in the deposition vessel: when an ITO coating is deposited
Table 1 Thickness, surface resistance, resistivity, carrier concentration, mobility, and optical band gap values measured on the realised samples. Oxygen percentage (%) 0.0 0.6 1.5 3.0 5.0
Thickness (nm)
Surface resistance (Ω/□)
Resistivity (mΩ cm)
Carrier concentration (cm− 3)
Mobility (cm2/Vs)
Optical band gap (eV)
803 760 725 707 610
1.7 4.1 14.2 19.1 87.5
0.14 0.31 1.03 1.35 5.34
1.50 × 1021 8.11 × 1020 3.85 × 1020 3.08 × 1020 1.23 × 1020
29.4 24.5 15.7 15.0 9.5
3.97 3.96 3.86 3.85 3.72
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25
40
16
12
30
Mobility
20
8
4
10
0
0 0
1
2
3
O2-5.0%
20
Mobility (cm2/Vs)
Carrier concentration (1020cm-3)
Carrier concentration
4
O2-3.0% O2-1.5%
15
O2-0.6% 10
O2-0.0%
5 0 3.5
3.7
3.9
4.1
Fig. 6. Absorption coefficient α as a function of the impinging photons energy.
5
Oxigen content (%)
under an oxygen containing atmosphere, oxygen atoms saturate the film, occupying all the reticular sites in the crystallisation nuclei, and hindering, as a consequence, the diffusion of the adatoms. The described mechanisms can explain the XRD patterns and the pole figures reported in Figs. 1 and 2, respectively. When the coatings were deposited under pure Ar (sample Ox0.0), they resulted to be oriented along the b400> direction. Vice versa, when the oxygen content in the deposition atmosphere was high (samples Ox3.0 and Ox5.0), the preferred orientation was along the b222> direction. The samples Ox0.6 and Ox1.5 are in an intermediate situation, since their pole figures clearly show the presence of two families of grains, as can be observed, e.g., in Fig. 7. It shows the sections of the Ox1.5 sample pole figures obtained keeping the φ angle equal to 40°, i.e., the angle where the maximum of both the two polar figures is present (the y axes report the corrected intensities, which coincide with the measured values at χ = 0). In Fig. 7 the (222) pole figure peaks are indicated with solid lines and labelled with capital letters, whilst dashed lines and lowercase letters are used for (400) peaks.
1.0
(a) Transmittance
0.8 0.6
O2-5.0% O2-3.0%
0.4
O2-1.5% O2-0.6%
0.2
O2-0.0% 0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Wavelength (µm)
The two main peaks of the two pole figures, i.e., C and b for (222) and (400), respectively, can not be generated by the same grains, because in the cubic lattice, which is the case of ITO, the angle between the b111> and b100> directions is 54.7°. In fact, secondary peaks (e.g., B and a) which form such an angle with the main peaks, can be observed in both the polar figures. Repeating the same analyses on all the polar figures, similar conclusions can be achieved, i.e., two main families of grains are present in the coatings: one with the b111> axis, another one with the b100> axis, roughly perpendicular with the substrate. Both the intensity and the misalignment of the main peaks observed in the pole figures depend by the deposition process parameters: the higher the oxygen content in the deposition atmosphere, the higher the intensity of the main peak in the (222) pole figure, and the lower its distance from the centre (i.e., the misalignment of the b111> axis with the substrate normal direction for the former family of grains). The reverse happens for the main peak in the (400) pole figures. Moreover, the angular distance between the maxima of the two pole figures for each sample, reported in Fig. 8, appears to be constant and it is about 40°. To explain all the observation reported above, the following mechanism can be hypothesised: the nucleation of crystallites orientated along the b111> or along the b100> depends by the adatoms energy and by the oxygen concentration, and local variations of such parameters can occur on the substrate during the initial stage of the deposition, causing in some points the nucleation of (111) crystallites (i.e., crystallites with the b111> direction roughly perpendicular to the substrate surface), in other points, the nucleation of (100) crystallites. In specific process conditions, one orientation will be preferred, not excluding that in some points the conditions to obtain the nucleation of the other orientation can be obtained. Increasing the oxygen concentration in the deposition vessel, the (111) crystallites number will increase too, whilst the (100) crystallites numbers will decrease. In the extreme conditions, i.e., without any oxygen in the deposition vessel, or, vice versa, with a very high oxygen content, crystallites oriented along the b111> direction or along the b100> direction,
1.0
(b)
4000
Corrected counts (222)
Transmittance
0.8 O2-5.0% 0.6 0.4
O2-3.0% O2-1.5% O2-0.6%
0.2 0.0 0.25
O2-0.0%
0.30
0.35
0.40
0.45
Wavelength (µm) Fig. 5. Transmittance as a function of wavelength in the region 0.2–3.0 μm (a), and in the ultraviolet region (b).
A B a b
C
D
70.5°
1000
(222) (400)
54.7°
3000
c
800
54.7°
600
70.5°
2000
400
90° 1000 0 -75
200 0 -50
-25
0
25
50
75
Fig. 7. Sections of the Ox1.5 sample pole figures for φ = 40°.
Corrected counts (400)
Fig. 4. Carrier concentration and mobility as a function of the oxygen percentage in the deposition atmosphere.
M. Tului et al. / Thin Solid Films 520 (2012) 4041–4045
Deviation from the origin (degrees)
40
30 222 20
400
10
0 0
1
2
3
4
5
4045
and the resistivity decreased. Intermediate values of the oxygen content caused the growth of two families of grains, respectively oriented along the b111> and the b100> directions. Such grain families were not exactly aligned with the normal direction to the substrate plane, but they presented a misalignment which depended by the process conditions, i.e., by the oxygen content in the deposition atmosphere: the higher the oxygen content, the lower the misalignment of the (111) grains; the reverse is for the (100) grains. Moreover, the angle formed by the average orientations of the two families of grains seems to be independent by the deposition parameters. The reason of this phenomenon is not yet clear.
Oxygen content (%) Fig. 8. Angular distance from the origin of the (222) and (400) pole figures main peaks, as a function of the oxygen percentage in the deposition atmosphere.
respectively, will prevail. Under a low oxygen content atmosphere, nuclei belonging to both the orientations will be present. Only a few of all the nuclei will grow, at the expense of the surrounding nuclei, up to becoming the columnar grains observed in the coatings. When only one type of crystallites is present, i.e., crystallites oriented along the b111> or the b100> direction, grains well aligned with the normal to the substrate surface seem to be preferred. When both the types of nuclei are present, obtained results suggest the presence of a selection mechanism which promotes the nuclei which orientation presents a specific angle with the orientation of the surrounding nuclei of different type. In the case of the Ox5.0 sample, each nucleus had an elevated probability to be in touch with (111) oriented nuclei: as a consequence, the selected grains were the (111) nuclei well aligned with the substrate plane normal, and the very few ones, among the few (100) oriented grains, which formed the right angle with the surrounding (111) grains. In the case of the Ox0.6 and Ox1.5 samples, vice versa, each nucleus had an elevated probability to be in touch with at least another nucleus oriented in a different way; as a consequence, couples of (111) and (100) grains, forming the right angle between their orientation, will be selected for the growth. Regarding the observed preferential angle of about 40° between the orientations of the two families of grains, the authors speculate that such an angle could correspond to a low energy grain boundary. Further investigations are needed to confirm such a hypothesis, or to find another mechanism which better describes the observed phenomenon. 5. Conclusions Several ITO films were deposited by PVD sputtering under different oxygen partial pressure. The presence of oxygen influenced the growth direction of the crystal grains which constituted the films, and determined its conductivity: at high oxygen content, the films grew along the b111> direction and the resistivity was high; at low oxygen content, the grains grew also along other orientations, e.g., the b100>,
Acknowledgements Authors would like to thank Dr. Stefano Martelli, from CSM, for the useful discussions; Ms. Cristina Bernini, from INFN (I), for SEM characterisations; Prof. Antonio Serra, from Lecce University (I), for the Hall effect measurements; and Dr. Mario Giustini, from the Rome University “la Sapienza” (I), for transmittance characterisations.
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