Comparative study of amorphous indium tin oxide prepared by pulsed-DC and unbalanced RF magnetron sputtering at low power and low temperature conditions for heterojunction silicon wafer solar cell applications

Vacuum 119 (2015) 68e76

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Comparative study of amorphous indium tin oxide prepared by pulsed-DC and unbalanced RF magnetron sputtering at low power and low temperature conditions for heterojunction silicon wafer solar cell applications Mei Huang*, Ziv Hameiri 1, Armin G. Aberle, Thomas Mueller Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, Block E3A, #06-01, Singapore 117574, Singapore

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 March 2015 Received in revised form 20 April 2015 Accepted 22 April 2015 Available online 29 April 2015

Comparative study of indium tin oxide (ITO) was performed between films prepared by pulsed direct current (PDC) and radio frequency (RF) magnetron sputtering methods at low power and low temperature conditions. The best achieved film resistivity is in the range of 3e6
104 U cm with the highest electron carrier mobility of ~60 cm2/V. The properties of the surface layer and the bulk material were analysed in details by means of X-ray photoelectron spectroscopy (XPS). Although there is a little difference in elemental compositions, it is revealed that the ITO's properties are directly related to the chemical bonding conditions. Owing to the fast film growth rate of PDC sputtering method, second phase Sn3O4 is formed which impedes the optical performance of the ITO films. Our observations suggest that the darkening of ITO is most likely due to the existence of Sn3O4 second phases that trapped inside of the film bulk. Besides material properties, we have demonstrated that the PDC sputtering method introduces more particle bombardment damage, while for RF sputtering the damage is mainly contributed by ultraviolet radiation emission. The major sputtering induced damage can be recovered by 15 min thermal annealing at 200  C in air. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Magnetron sputtering Indium tin oxide XPS Sputter damage Surface passivation Heterojunction silicon wafer solar cells

1. Introduction Indium tin oxide (ITO) is a heavily doped, wide-gap (bandgap in the range of 3.7 eV) n-type semiconductor which is extensively used for photovoltaic applications due to its high electrical conductivity and optical transparency [1,2]. ITO films can be obtained by various growth techniques including magnetron sputtering [3], evaporation [4], pulsed laser deposition [5], chemical vapour deposition [6] and solegel methods [7]. Among the deposition techniques, magnetron sputtering is the most widely used deposition method to date for its high deposition rate, good reproducibility and capability of large area deposition [1,8e10]. Heterojunction (HET) silicon wafer solar cells are one of the highest efficiency silicon solar cells that have been put into mass

* Corresponding author. Tel.: þ65 86441246; fax: þ65 67751943. E-mail address: [email protected] (M. Huang). 1 Current address: School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia. http://dx.doi.org/10.1016/j.vacuum.2015.04.032 0042-207X/© 2015 Elsevier Ltd. All rights reserved.

production [11e13]. Due to the low lateral carrier conduction ability of amorphous silicon (a-Si), thin (~80 nm) ITO layers are normally used in HET cells to form the electrodes on both sides and also function as anti-reflection coating (ARC) on the front side. To avoid high energy particle bombardment and thermal damage to the underlying amorphous layers [14e16], ITO films are normally deposited at low power (0.5e2 W/cm2) and low temperature conditions (below 250  C) which normally result in ITO in the amorphous nature. In the last decades, significant amount of research has been done on ITO films. The influences of deposition parameters were extensively studied. The electrical and optical properties of ITO are reported to be greatly depending on the sputtering conditions [2,9,17]. Although these studies provide some insights on how to achieve high quality ITO films, they fail to give a more general guidance as the results, in most cases, are system dependent. In order to better understand the mechanisms responsible for the properties of ITO films, researchers have investigated the element compositions and microstructures of the ITO films by using techniques such as X-ray photoelectron spectroscopy (XPS).

M. Huang et al. / Vacuum 119 (2015) 68e76

Investigation of ITO blackening/darkening [18,19], oxygen influence on the chemical composition [20,21] and the conduction mechanism of the ITO films [22,23] have been discussed by previous researchers, however, the detailed mechanism has not yet been fully revealed. In the present work, we report on the optimal low-damage sputtering conditions of amorphous ITO films by two types of sputtering methods, namely pulsed direct current (PDC) sputtering and radio frequency (RF) sputtering. The comparison of ITO films was based on three representative conditions which were preoptimized by each system. XPS measurements monitor the changes of the local atomic and molecular bonding as a result of varying deposition parameters. The reason to cause ITO darkening, the chemical bonding of the composing elements and its influence on the optoelectrical properties of the ITO films are investigated and discussed in details. Besides the basic material properties, the sputter induced damages were also investigated. The damages due to the particle bombardment and ultraviolet (UV) radiation were quantified based on the passivation quality provided by the intrinsic a-Si layer before and after the sputtering process. 2. Experiment Altogether six samples are included in this study which are labelled A to F. Samples A to C are prepared by the PDC sputtering system while Samples D to F are deposited by the RF sputtering system. The film thickness for all the prepared samples was kept at 75 nm. Detailed deposition conditions are shown in Table 1. ITO films prepared by PDC sputtering were deposited in a commercial inline sputtering system (Line 540, FHR Anlagenbau) from a ceramic ITO target (90 wt.% In2O3 and 10 wt.% SnO2). ITO is sputtered dynamically in this system to improve the coating uniformity (<±5%), where the substrate moves in between of predefined positions. A mixture of argon with a small amount of oxygen is normally used as the sputtering gas, otherwise high resistive and opaque ITO films are obtained [24]. In this system, a power of 800 W (corresponding to power density of 1.6 W/cm2) was applied at three conditions: room temperature (25  C) sputtering in pure argon ambient (Sample A), room temperature sputtering with optimal mixture of oxygen (Sample B), and the same condition at elevated temperature of 200  C (Sample C). RF sputtering was performed in an unbalanced magnetron sputtering system (UBM system, NanoFilm) which uses a ceramic ITO target (90 wt.% In2O3 and 10 wt.% SnO2). Substrate rotation is enabled in this system in order to achieve high coating uniformity (<±5%). Our initial investigation shows that a very small amount of hydrogen in the sputtering gas improves the film quality, while excess hydrogen degrades the film transparency and conductivity. Different from the PDC sputtering, in the RF system, oxygen was found to degrade the conductivity of ITO films (similar effect is observed for H2, O2 and Ar mixture, which is different from the findings of Harding et al. [25]). In this system, RF power of 100 W, corresponding to power density of 1.2 W/cm2, was investigated at three conditions: room temperature sputtering in pure argon ambient (Sample D), room temperature sputtering with optimal

69

hydrogen partial pressure (Sample E), and the same condition at elevated temperature of 200  C (Sample F). Glass plates (125 mm
125 mm) were used as substrate which were cleaned by isopropyl alcohol (IPA), acetone and deionized (DI) water in sequence in an ultrasonic bath each for 10 min. The substrates were then dried in nitrogen environment at 80  C for 30 min. After the deposition, the thickness of the film was obtained by a stylus profiler (Dektak 150, Veeco). The electrical characterisations of the ITO were based on a four point probe (4PP, RM3-AR, Jandel) and Hall effect measurement (Bio-Rad HL5500, Accent) using Van der Pauw method. The transmission of the sample was obtained by a haze meter (NDH 5000, Nippon) which provides information on average visible transmission of the sample in a certain spectrum (380e780 nm). The film crystallinity was studied by X-ray diffraction (XRD, D8 venture, Bruker) measurement. The composition and chemical bonding states of composing elements within the film were analysed by an XPS tool (Escalab 220i-XL, VG Thermo) with a monochromatized Al Ka (1486.6 eV) X-ray source. Besides the investigation of ITO properties, the sputter damage was studied using symmetrical a-Si double-side-coated lifetime samples. The a-Si films (24 nm on either side) were deposited on 5 inch 140 mm n-type Czochralski silicon wafers (3 U cm resistivity, planar surface) using a commercial plasma-enhanced chemical vapour deposition (PECVD) deposition reactor (Singular-HET, Singulus Technologies). Before ITO deposition, the lifetime samples were first annealed in a muffin furnace in air ambient for 15 min at 200  C to activate the passivation quality by hydrogen assisted relaxation [26,27]. The effective lifetime of the minority carriers was measured by a photoconductance-based lifetime tester (WCT120, Sinton Instruments) [28,29]. The lifetime measurements were carried out on all samples sequentially after the thermal annealing as well as double-side ITO coating. The measurements were done under transient illumination (using transient analyse mode, for samples with effective lifetime > 200 ms) or quasi-steady-state illumination (using the generalized analyse mode for effective lifetime < 200 ms). The sample was then dipped in a hydrochloric (HCl) acid bath (approx. 0.5% by volume) for a duration of 30e60 s until the ITO coating was fully etched away. Afterwards, the sample was dried with a nitrogen gun followed by a second thermal annealing to recover the lifetime. The effective lifetime change between the two thermal annealing steps is associated with the sputter damage caused by the ITO coating. It is well known that electrons, neutrals and other charged particles (such as oxygen radicals) bombardment, as well as plasma luminescence, damages the passivation quality of the a-Si layer [16,30]. In order to separate the damage that is caused by plasma luminescence, a reference sample was included in each group which was covered by a 1 mm thick quartz glass plate through ITO sputtering. In doing so, no particle reaches the a-Si surface during ITO deposition. Therefore the lifetime drop of the reference sample can be contributed merely to the plasma luminescence damage. Besides the quantitative analysis by the lifetime tester, the sputter damage to a-Si coating was also investigated by a transmission electron microscope (TEM, Jeol 2010f).

Table 1 Deposition conditions of ITO samples prepared by both PDC and RF sputtering. Discharge mode PDC

RF

Sample ID A B C D E F

Power density (W/cm2) 1.6 1.6 1.6 1.2 1.2 1.2

O2 or H2 partial pressure (%) 0 O2/Ar ¼ 2.4 O2/Ar ¼ 2.4 0 H2/Ar ¼ 0.24 H2/Ar ¼ 0.24

Pressure (mbar) 4.2 4.2 4.2 6.7 6.7 6.7









3

10 103 103 103 103 103

Temperature ( C) 25 25 200 25 25 200

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3. Results and discussion 3.1. Material properties ITO films prepared by both sputtering systems are presented in amorphous state at all deposition conditions. The crystallinity difference between the sputtered ITO films by both systems is shown to be very minor. The optoelectrical properties of the prepared samples were listed in Table 2. All ITO films are colourless except Sample A, which is darkened with a poor visible transmission (Tvisible) of 64.2%. The film resistivity of Sample A is also one magnitude higher than the other samples. When introducing a small amount of oxygen (Sample B), the film sheet resistance drops to 108.3 U/, while the visible transmission significantly increases to 77.3%. The carrier mobility (mcarrier) of Sample B is also improved from 9.1 cm2/V to 45.8 cm2/V. In addition to the oxygen, the electrical properties improve further with substrate heating (Sample C). The RF sputtered film has quite good quality when deposited in pure argon ambient (Sample D), with both a high visible transmission of 82.5% and a high carrier mobility of 48.4 cm2/V. The resistivity and carrier mobility improves when a small amount of hydrogen is added into the sputtering gas (Sample E). Sputtering with thermal heating is shown to increase carrier concentration and to slightly reduce carrier mobility with an overall improved conductivity (Sample F). XPS analysis was used to examine the composition and chemical bonding states of ITO films. The position of sp3 carbon 1s peak (existence is due to surface contamination) is taken as a standard (binding energy ¼ 285 eV) to compensate for any charge-induced shifts. Photoelectron peaks for In, Sn, O were recorded in the binding energy range from 0 eV to 1300 eV. Table 3 shows the binding energies of the O 1s peaks and the atomic ratio with respect to In atoms determined from the area intensity of the O 1s and In 3d5/2 peaks on the surfaces of the ITO and within the films (after 1 min argon-ion bombardment sputtering until the intensity of C 1s peak becomes zero, around 2.5 nm into the film). The O 1s scan spectra is shown to have three individual components in the surface layer defined as O I, O II, O III at binding energy of 529.9 ± 0.3 eV, 531.4 ± 0.3 eV, and 532.3 ± 0.2 eV, while only two chemical states (O I and O II) are noticeable in the film, as shown in Fig. 1. The multiple fitting peaks exhibit the existence of oxygen in different environments in ITO. The exact peak position may vary depending on the deposition conditions and film properties. For instance, Ishida et al. attributed the peak at 530.8 eV to the lattice oxygen in the crystalline ITO and the one at 532.4 eV to oxygen atoms in amorphous phase [31]. Chuang et al. reported their best fitting of O 1s peak using five individual peaks [32,33], in which they define peaks at 529.87 eV (lattice oxygen in amorphous ITO phase), 530.41 eV (lattice oxygen in a crystalline ITO phase), 531.42 eV (oxygen deficient regions of the In2O3 matrix), 532.56 eV (oxygen in the In(OH)3 or InOOH species), and 533.64 eV (oxygen in binding with organic impurities). Fan et al. attributes the peak at 529.9 eV to In2O3 lattice oxygen, in which the In ions are fully bonded with neighbouring O2 ions, while the one at 531.6 eV is

considered to be OeIn bonding state in oxygen-deficient region [19]. The O I peak is the main oxygen peak of In2O3 films and ITO films with different film crystallinities (unpublished work), therefore the O I peak is assigned to lattice oxygen of In2O3 where In atoms (or substitutional Sn) are neighbouring with six O2 with their full complement [18,34,35]. The intensity of the O I peak represents the oxygen atoms in fully oxidized and stoichiometric surrounding. The O II peak is considered to be the O2 ions in oxygen deficient In2O3-x region, same as reported by Fan et al. [19] and Chuang et al. [32,33]. Therefore the intensity changes of the O II components are most probably related to changes in the concentration of oxygen vacancies. The O III peak can be assigned to chemisorbed oxygen such as hydroxyl group or water molecules on the ITO surface since this peak can't be detected after surface cleaning, this agrees well with the conclusions of Chuang et al. [32,33]. In addition to the vanishing of O III peak after surface cleaning, the drop of O II peak is also clearly notable which indicates that less O2 ions in In2O3x regions are presented in the film bulk. Interestingly, it is found that Sample A has a low O II intensity, which indicates that there are only a small amount of oxygen vacancies contributing free electrons. Therefore, the excess free electron carriers observed in Sample A may be contributed by cation interstitials [36]. These cation interstitials, namely In interstitials and Sn interstitials, are shallow donors and attribute probable source of conductivity. The interstitial atoms also contribute to the scattering defects, resulting in low carrier mobility as observed for Sample A. Compared to Sample B, Sample C has a smaller O II intensity and a higher concentration of free electrons (same for Samples E and F). The increased number of free electrons in Sample C (same for Sample F) could be originated from the increased number of substitutional Sn atoms when ITO is deposited in a heated environment. By studying the percentage atomic ratio (At.) between the O 1s and In 3d5/2 peaks (represented as Otot/In), for all our ITO samples, the Otot/In ratios are smaller than the stoichiometric value of 1.70. A relatively higher Otot/In ratio is achieved at the surface layer which is due to the chemisorbed oxygen on ITO surface. This observation agrees well with that reported by other researchers [22,37]. When comparing the oxygen peaks and the Otot/In ratios between PDC and RF sputtered films, not too many differences can be identified, even for the darkened sample. It is therefore suggested that the poor optical transparency of Sample A is not originated from oxygen deficiencies. Besides oxygen, it is found that the In/Sn ratios on ITO surfaces are smaller than the stoichiometric value of 9.8, and those in the ITO films are higher than the stoichiometric value. This observation confirms the conclusion reported by other researchers that Sn tends to segregate to grain boundaries including the top surface of ITO films [38e41]. The PDC sputtered films are shown to have smaller In/Sn ratios than those of the RF sputtered films, indicating a severer tin segregation. Based on the previous discussions, the darkening of Sample A is not originated from oxygen deficiency. The other possibilities, as reported in the literature, include the existence of In/Sn metallic

Table 2 Optoelectrical properties of ITO films prepared by both PDC and RF sputtering. Sample ID A B C D E F

Sheet resistance (U/,) 415.2 108.3 69.6 67.9 62.3 49.5

Resistivity (U cm) 3.11 8.12 5.22 5.09 4.67 3.71









3

10 104 104 104 104 104

Tvisible (%) 64.2 77.3 76.8 82.5 81.9 81.8

Ncarrier (cm3) 2.31 1.59 2.25 2.63 2.47 3.23









20

10 1020 1020 1020 1020 1020

mcarrier (cm2/Vs) 9.1 45.8 51.2 48.4 56.9 53.7

M. Huang et al. / Vacuum 119 (2015) 68e76

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Table 3 Positions and relative strengths of the resolved peaks (O I, O II, O III) obtained according to the quantification data from the O 1s, In 3d5/2 and Sn 3d5/2 XPS peaks. Sample ID

Observed region

OI

O II

O III

Positions (eV) A B C D E F

Surface Film Surface Film Surface Film Surface Film Surface Film Surface Film

530.0 530.1 529.9 529.6 529.8 529.8 530.0 529.9 529.9 529.9 529.9 529.9

OI

O II

O III

Relative strengths (%) 531.4 531.4 531.4 531.1 531.4 531.1 531.5 531.3 531.6 531.2 531.6 531.2

532.1 e 532.3 e 532.2 e 532.3 e 532.5 e 532.5 e

atoms (due to the decompositions of oxygen [42]) or oxygen deficient Sn3O4 second phase [19]. To identify the source of the darkening effect, the fitting of both In 3d5/2 and Sn 3d5/2 spectral lines were applied. For both the In 3d5/2 and Sn 3d5/2 peaks, asymmetry shapes are presented (shown below in Fig. 2 and Fig. 3), indicating the existence of multiple components. This is rarely reported in literature, as most researchers found that the In 3d5/2 peaks and Sn 3d5/2 peaks are less sensitive to the changes in valance states [19,34]. The best fitting of the In 3d5/2 peaks and Sn 3d5/2 peaks is achieved by using two adjacent peaks, of which the details are listed in Table 4. There is little signal for the existence of metal phase In and Sn at binding energies of 443.6 eV and 484.5 eV as reported by other researchers [20,43e45]. Therefore the metal phase In or Sn is not the key factor to result in the opaque film. We assign the In 444.5 eV peak (In I) to In3þ bonding in lattice In2O3.

47.9 92.6 51.9 86.1 52.0 89.8 53.2 85.9 49.7 90.0 52.9 91.4

35.0 7.4 31.0 13.9 29.5 10.2 31.8 14.1 39.3 10.0 32.7 8.6

17.1 e 17.1 e 18.5 e 15.0 e 11.0 e 14.4 e

Otot/In

In/Sn

At. (%)

At. (%)

1.56 0.93 1.48 1.02 1.58 0.96 1.54 0.97 1.63 1.00 1.52 0.94

8.79 14.31 8.59 11.29 8.24 12.25 9.90 15.90 10.00 13.05 9.13 13.83

The In 445.4 eV peak (In II) is rarely reported [46] and it is possibly the intermediated oxidation states of In atoms such as Inþ or In2þ bonding. The Sn 486.3 eV peak (Sn I) is assigned to Sn4þ bonding in either SnO2 or oxygen deficient Sn3O4-like structures. The Sn3O4like structure could either be Sn3O4 second phase or substitutional Sn4þ atoms in oxygen deficient In2O3-x regions of a similar structure [19]. The Sn 487.2 eV peak (Sn II) represents the Sn2þ bonding in the form of SnO [19,34]. As shown in Fig. 3, only the Sn I peak is observable in the surface layer of ITO films deposited by PDC sputtering. The most obvious difference between Sample A and the other samples is the existence of only Sn I peak in both the surface and the bulk of the coating film. Although only Sample A is severely darkened, it is also noticed that the visible transmissions of Samples B and C are relatively lower than those of the RF sputtered ITO films, as shown

Fig. 1. XPS spectra in the O1s region for ITO films prepared at different deposition conditions by both PDC and RF sputtering methods (solid lines), the fitting curves are presented by dashed lines.

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Table 4 Positions and relative strengths of the resolved peaks (In I & In II and Sn I & Sn II) obtained according to the quantification data from the In 3d5/2 and Sn 3d5/2 XPS peaks. Sample ID

A B C D E F

Positions/Relative strength

eV % eV % eV % eV % eV % eV %

In film

In surface

Sn film

Sn surface

In I

In II

In I

In II

Sn I

Sn II

Sn I

Sn II

444.5 83.5 444.5 85.0 444.4 83.3 444.5 80.9 444.4 84.7 444.4 86.0

445.5 16.5 445.6 15.0 445.5 16.7 445.4 19.1 445.5 15.3 445.4 14.0

444.6 83.3 444.3 80.5 444.3 70.8 444.5 85.5 444.4 77.7 444.4 76.7

445.6 16.7 445.4 19.5 445.1 29.2 445.7 14.5 445.2 22.3 445.3 23.3

486.4 100 486.5 100 486.4 100 486.3 77.5 486.2 71.2 486.2 69.1

e e e e e e 487.4 22.5 487.2 28.8 487.3 30.9

486.4 100 486.5 61.5 486.1 75.9 486.1 72.7 486.1 81.0 486.2 85.3

e e 487.4 38.5 486.8 24.1 487.1 27.3 486.9 19.0 487.1 14.7

in Table 2. Since we have excluded the other possibilities, therefore the darkening of ITO is most probably related to the presence of abnormal Sn4þ bonding states. According to Fan et al. [19], in the presence of the oxygen vacancies, the Sn-rich regions could nucleate a Sn3O4-like phase. At high deposition temperature and low deposition rates, the atomic mobilities are high enough relative to film growth therefore any second phase will only be ejected to the surface layer. The film should exhibit a Sn-rich surface layer but remain clear. While with a high degree of oxygen deficiency and a low deposition temperature, the rate of film growth may be faster than the rate at which a second phase forms and migrates in the host structure, in this case, the films would have an Sn-rich second phase dispersed through the bulk and be darkened. Therefore the darkening of ITO is most probably related to the existence of Sn3O4 second phase. When Sn3O4 second phase only

exist on the surface, the sample remains clear with only a small impact of visible transmission, such as Samples B and C. When Sn3O4 second phase is trapped inside of the film bulk, the film is darkened as observed for Sample A. In addition to the influence on the film transmission, it is suspected the Sn3O4 second phase could also degrade the electrical properties. The reason for the observed darkening effect may be associated with the different deposition rates, since the PDC sputtering deposits films with a much faster speed, there is a high possibility that some second phase Sn3O4 is trapped and darkens the ITO film when there is no additional oxygen supply. 3.2. Sputter damage investigation Representative samples with similar lifetime (800 ms at an excess carrier concentration of 1
1015 cm-3 after the first thermal

Fig. 2. XPS spectra in the In 3d5/2 region for ITO films prepared at different deposition conditions by both PDC and RF sputtering methods (solid lines), the fitting curves are presented by dashed lines.

M. Huang et al. / Vacuum 119 (2015) 68e76

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Fig. 3. XPS spectra in the Sn 3d5/2 region for ITO films prepared at different deposition conditions by both PDC and RF sputtering methods (solid lines), the fitting curves are presented by dashed lines.

activation) were chosen for the sputter damage investigation. The lifetime curves are presented in Fig. 4 (for PDC) and Fig. 5 (for RF). The stated numbers in both figures are the effective lifetime at an excess carrier concentration of 1
1015 cm-3. A significant drop of the effective lifetime (to the range of 100 ms at 1
1015 cm3) of the test samples was observed after double-side ITO coating by both sputtering methods. The lifetime recovers to ~65% of its original value after the second thermal annealing. The reference samples (for both sputtering methods) are shown to have different degrees of plasma luminescence induced damage. The differences of sputter damage may be attributed to the differences of the target voltage and current density. For both systems, the target voltage and current density are auto-optimized by the system and they vary for different powers. At the same power density, PDC sputtering tool has higher target voltage (>200 V) and smaller current density than those of the RF

sputtering tool. The high target voltage (240 V) used in the PDC system accelerates the electrons or ionic particles towards the coating surface which dislocate the atoms through the collision with the atoms on the surface of the passivation layer which then induce Si atoms displacement and generate defects along the ion penetration path in the film [47]. This part of ion bombardment damage as shown in Fig. 4 can only be partially recovered through annealing. However, for RF sputtering, the target voltage is much lower (112 V). Hence relatively low energetic ions are reaching to the coating substrate and only minor bombardment damage is formed, which is easy to be annihilated during the follow up annealing process [32], as shown in Fig. 5. Although the detrimental role of ion bombardment to the surface passivation is still under investigation, it is commonly accepted that ion induced defects are typically limited to the transfer of ion kinetic energy to the silicon network [47].

Fig. 4. Sputter damage investigation of PDC sputtered ITO (a) on test lifetime samples and (b) on a reference sample shielded with quartz glass.

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Fig. 5. Sputter damage investigation of RF sputtered ITO (a) on test lifetime samples and (b) on a reference sample shielding with quartz glass.

Besides the damage caused by ion bombardment, light-induced defects are also reported to degrade the passivation quality of the aSi layer. According to the literature [30,47], the damage caused by plasma luminescence is due to the creation or activation of deep defects (most likely the Si dangling bond) at the interface of a-Si and silicon substrate by ultraviolet (UV) radiation. In our investigation, the plasma luminescence damage is more markedly presented for samples prepared by unbalanced RF sputtering. It is found that the lifetime of the reference sample drops to half of the original value after ITO deposition [on shielding glass e see Fig. 5(b)] and it is not fully recovered by annealing at 200  C for 15 min. On the contrary, the plasma luminescence damage on PDC sputtered samples is very minor [Fig. 4(b)], and the damage can be almost fully recovered via thermal annealing. The UV light generation within plasma region is due to the ionization process. Dense plasma will be formed in the target region due to the increased ionization efficiency. Different from the other sputtering methods using normal magnetron (such as the PDC sputtering in this study), the unbalanced magnetron normally implies a high flux (>2 mA/cm2) with relatively low energy ions (low target voltage) to form condensed films with low ion bombardment. Due to the extended magnetic field lines, the secondary electrons are no longer confined to the target region, but flow out towards to substrate, resulting in an extended plasma region to the coating substrate [48]. The high energy photons will thus have higher probability to interact with the passivation layer to create increased number of dangling bonds hence reduces the passivation quality. In addition to the quantitative analysis of lifetime change, the sputter damage to the amorphous layer was also viewed under high resolution TEM, as shown in Fig. 6, after ITO sputtering by both PDC and RF sputtering methods, epitaxial grown silicon regions are

presented near the interface of a-Si layer and Si wafer substrate. For PDC sputtering method, the epitaxial grown silicon is well extended to the regions very near to the ITO/a-Si interface, whilst for RF sputtering the epitaxial exists only in the region closer to the a-Si/Si interface. The areal difference of the epitaxial layer may imply that the particle bombardment is the main cause to the reform large volume of epitaxial silicon layer (small epitaxial growth region is observed before ITO coating, as shown in Fig. 6(c)). Besides the epitaxial regions, diverse features of amorphous layer are presented. The amorphous layer after ITO deposition by the RF method is more distorted and shown to have larger features, the reason is currently unknown. This may be due to the increased dangling bonds created due to the plasma luminescence, or because of hydrogen out-diffusion between a-Si and hydrogenated ITO. 4. Conclusion In this study, we have compared the properties of ITO films prepared by two sputtering methods at different deposition conditions. By studying the ITO film properties, chemical composition and microstructure of the prepared ITO films, the discharge mode induced changes were presented and analysed. The donors of free electrons in ITO film can be cation interstitials, oxygen vacancies as well as substitutional Sn atoms. The dominant type of donors changes when different deposition conditions are applied. Oxygen vacancies are the dominant electron donors in amorphous ITO. A small portion of substitutional Sn is activated when deposited is done in heated ambient. The ITO film prepared in pure argon ambient by PDC sputtering method is found to be darkened. It is demonstrated that the darkening is not due to the presence of metal phase In/Sn atoms but rather the existence of the Sn3O4 second phase in the bulk of the ITO film. The presence of Sn3O4

Fig. 6. TEM images of sputter damage due to ITO coating by (a) PDC and (b) RF sputtering methods, and (c) a reference a-Si sample with no ITO coating (free from sputter damage).

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second phase could impede the conductivity and optical transparency of the ITO film which is not preferred. It is suspected that the formation of Sn3O4 may be associated with the high film growth rate by PDC sputtering method. It has shown that both sputtering methods introduce similar degree of sputter damage. It is demonstrated by us that ion bombardment tends to cause amorphous to crystallize, while UV luminescence will cause the distortion of amorphous layer by probably creating of dangling bonds. The sputtering induced damage can be partially annealed after thermal annealing at 200  C for 15 min. Acknowledgements The Solar Energy Research Institute of Singapore (SERIS) is supported by the National University of Singapore (NUS) and Singapore's National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). This research is supported by the National Research Foundation, Prime Minister's Office, Singapore under its Clean Energy Research Programme (CERP Award No. NRF2010EWT-CERP001-022). The authors would like to thank Jia GE for providing the lifetime samples, Muzhi TANG for the help of TEM images, Xia YAN and Fang Jeng LIM for the help with XPS analysis. The authors would also like to thank the Institute of Materials Research and Engineering (IMRE) for providing access to the UBM RF sputtering tool. Special thanks to Lay Ting ONG (IMRE) for the help with the XPS measurements, Chan Choy CHUM (IMRE) for fruitful discussions regarding the sputtering processes, and Geok Kheng TAN (Department of Chemistry, NUS) for the help with the XRD measurements. References [1] Manavizadeh N, Boroumand FA, Asl-Soleimani E, Raissi F, Bagherzadeh S, Khodayari A, et al. Influence of substrates on the structural and morphological properties of RF sputtered ITO thin films for photovoltaic application. Thin Solid Films 2009;517:2324e7. [2] Lippens P, Büchel M, Chiu D, Szepesi C. Indium-tin-oxide coatings for applications in photovoltaics and displays deposited using rotary ceramic targets: recent insights regarding process stability and doping level. Thin Solid Films 2013;532:94e7.
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