Variation of passivation behavior induced by sputtered energetic particles and thermal annealing for ITO/SiOx/Si system
Gao Ming1, Du Hui-Wei1, Yang Jie1, Zhao Lei1, Xu Jing2, Ma Zhong-Quan1, 2, †
SHU-SOEN’s Research and Development Laboratory, Department of Physics, Shanghai University, Shanghai 200444, China
Instrumental Analysis and Research Center, Shanghai University, Shanghai 200444, China

 

† Corresponding author. E-mail: zqma@shu.edu.cn

Project supported by the National Natural Science Foundation of China (Grant Nos. 61274067, 60876045, and 61674099) and the Research and Development Foundation of SHU-SOENs PV Joint Laboratory, China (Grant No. SS-E0700601).

Abstract

The damage on the atomic bonding and electronic state in a SiOx(1.4–2.3 nm)/c-Si(150 m) interface has been investigated. This occurred in the process of depositing indium tin oxide (ITO) film onto the silicon substrate by magnetron sputtering. We observe that this damage is caused by energetic particles produced in the plasma (atoms, ions, and UV light). The passivation quality and the variation on interface states of the SiOx/c-Si system were mainly studied by using effective minority carrier lifetime ( ) measurement as a potential evaluation. The results showed that the samples’ was reduced by more than 90% after ITO formation, declined from 107 s to 5 s. Following vacuum annealing at 200 C, the can be restored to 30 s. The components of Si to O bonding states at the SiOx/c-Si interface were analyzed by x-ray photoelectron spectroscopy (XPS) coupled with depth profiling. The amorphous phase of the SiOx layer and the “atomistic interleaving structure” at the SiOx/c-Si interface was observed by a transmission electron microscope (TEM). The chemical configuration of the Si–O fraction within the intermediate region is the main reason for inducing the variation of Si dangling bonds (or interface states) and effective minority carrier lifetime. After an appropriate annealing, the reduction of the Si dangling bonds between SiOx and near the c-Si surface is helpful to improve the passivation effect.

1. Introduction

Indium tin oxide (ITO) film has been widely used as transparent conductive oxide (TCO) in the efficient silicon-based heterojunction solar cells, owing to its “semiconductor-assisted role”[1] and “window function”.[2] Usually, the ITO film has been made by the sol–gel method,[3] chemical vapor deposition,[4] magnetron sputtering,[5] and so on. The radio frequency (RF) magnetron sputtering is the most common method to deposit ITO film on a substrate because of its advantages in reproducibility, uniformity and film properties.[2,5,6] However, during the ITO deposition process, the energetic particle beam in the plasma will cause damage on the atomic bonding and electronic state of the substrate.[612] For ITO/SiOx/n-Si (SIS) devices, if the SiOx is formed prior to ITO film, the sputtering would damage the electronic structure of the SiOx layer and SiOx/c-Si interface, increasing the interface recombination rate and decreasing the diffusion length of carriers, and leading to a loss in the open circuit voltage and the short circuit current of the device, respectively. This negatively affects the power conversion efficiency of the device. The researchers usually focus on the passivation effect before sputtering of ITO or the overall performance of those kinds of devices, and ignore sputtering damage in the efficient silicon-based heterojunction solar cells, especially for the HIT (Heterojunction Intrinsic Thin Films) and SIS (Semiconductor-Insulator-Semiconductor) device.

In this report, three techniques have been applied for the evaluation of the intermediate region modified by energetic beams during ITO sputtering. Firstly, effective minority carrier lifetime ( is a physical parameter to assess the interface recombination rate caused by sputtering degradation. The is usually measured by a microwave photoconductivity decay (μ-PCD) method in the solar cell production line, evaluating the surface passivation effect of semiconductor materials.[13,14] Secondly, according to the surface model of silicon oxide reported by Seah et al.,[15] typically non-stoichiometric silicon oxides of Si2O, SiO, and Si2O3 exist in the SiOx/c-Si interface whether formed or induced by other interactions, corresponding to the charged electronic states of Si , Si , and Si , respectively. The fixed positive charges and interface states in the vicinity of SiOx/c-Si stem from sub-oxide SiOx, inducing the carriers’ recombination and affecting samples’ . Thus, the chemical composition and phase structure of the SiOx/c-Si interface are critical for the performance of an optoelectronic device and are required to be figured out by powerful characterization methods, such as either x-ray photoelectron spectroscopy (XPS) with depth profiling or transmission electron microscope (TEM) for the microstructure confirmation. In addition, the post-annealing of a semiconductor is an effective method to lessen the interface states and the charge traps of the materials and devices.[16] On the other hand, thermal annealing in a high vacuum chamber can avoid impurity contaminants. Therefore, we first measured the τ of the samples with different treatments using μ-PCD, and studied the origin of the sputtered damage at the SiOx/c-Si interface. Afterwards, we investigated the variation of under the variable temperature annealing conditions, and discussed the electronic structures and chemical bonding of SiOx/c-Si in the interfacial region corresponding to the different samples with a specific .

2. Experimental in details
2.1. Preparation of samples

Solar grade n-type Czochralski silicon [(100), phosphor doped, chemical polished, (1.5–3.0) cm, (150±3) m, 3.0 cm 3.0 cm] was cleaned by a standard RCA method.[17] Silicon wafers were first dipped into 0.08-mol/L iodine, which is considered as the best passivation to silicon.[13,14] Passivated samples were sequentially sonicated for 10 min in absolute ethanol, deionized water, removing iodine. An ultrathin ( nm) SiOx layer was formed by thermally oxidizing in an oxidation furnace for 10 min at 700 C. The flow ratio of high purity nitrogen and oxygen was 4 L/min: 1 L/min. It was verified that the thin SiOx layer was involved into four compositions: lots of SiO2, very few of Si2O, SiO, and Si2O (Si 2p peak in the XPS spectrum as shown in Fig. S1 in Appendix A). Subsequently, ITO films of about (80±3) nm were deposited by the radio frequency magnetron sputtering system. The ITO target used for sputtering purposes was a sintered ceramic mixture disk of 10-wt% SnO2 and 90-wt% In2O3. The base pressure was 3 10 Pa for the sputtering system. The substrate temperature of 250 C, a power of 100 W, an argon partial pressure of 1.0 Pa and an argon gas flow rate of 40 sccm were set during the ITO film deposition, respectively. The carrier concentration, electronic mobility, and resistivity of ITO film were cm , ( cm V S , cm, respectively, which were measured by Hall Effect measurement. The average transmittance of visible spectrum was more than 85% and the optical band gap was deduced to be 3.60 eV (as shown in Fig. S2 in Appendix A), which was measured by a UV-VIS spectrophotometer.

To compare the passivation effectiveness, sputtering damage and annealing effect, the samples were divided into two groups. (i) The first set of samples, which were successively cleaned by immersion in 37% concentrated hydrochloric acid, deionized water and 5% hydrofluoric acid, removing both the ITO film and the silicon oxide layer. Then, samples are secondary passivated by 0.08-mol/L iodine. For the comparison, ITO films were directly deposited on two-sides of the unoxidized silicon substrate, with the same process of magnetron sputtering and treatment conditions. (ii) The second set of samples, which were annealed at the temperatures of 100 C–700 C (step length of 100 C) for 30 min in the transfer sample chamber of the magnetron sputtering apparatus. The working pressure was maintained at 1 10 Pa. By contrast, ITO films are directly deposited on two-sides of seven unoxidized silicon wafers, with the same process of magnetron sputtering and post-annealing.

2.2. Characterization

The was measured using a μ-PCD instrument (Semilab WCT-2000) with two-dimensional image. A 200-ns laser pulsed at 904 nm was used for the gain of photon-generated carriers. The number of photons in one pulse was 1.20 1019. Each sample was scanned three times at 1 mm 1 mm resolution. Silicon wafer treated with iodine was encapsulated in a plastic bag to prevent contamination prior to be contacted by the prober. The electronic states and the chemical bonding of ITO/SiOx/c-Si samples were examined by XPS with depth profiling (Thermo Fisher Scientific ESCA-250Xi). The sample was etched by Ar ion beam with 1 keV, and the etching step was 20 s. A monochromatic Al Kα excitation source eV was used for the XPS measurements. The pass energy of 30.00 eV, scan step of 0.10 eV and test area of 500 m 500 m were set for testing. The lattice array morphology of ITO film, SiOx layer and SiOx/c-Si interface were analyzed by a TEM (JEOL JEM-2010 F) operated at 200 kV.

In the measurements, an xy two-dimensional coordinate system was taken for the fixed-points to monitor the changes of before and after depositing ITO film or annealing, so that the lateral non-uniformity of the sample could be avoided. The measuring principle was designed as: at the fixed-point beside the central region, each of was adopted to the average value, and the consistency of seven measurements in τ before and after sputtering of ITO film was guaranteed. The average value of was taken from three-time measurements and a standard deviation was derived.

3. Results and discussion
3.1. Effect of sputtering ITO on effective minority carrier lifetime

Generally, for the high pure silicon wafer, the minority carrier lifetime ( is substantially determined by both the bulk minority carrier lifetime ( and the surface minority carrier lifetime ( as shown in the following equation[13,18]

where is the diffusion time for minority carriers from the interior of the wafer to the surface, decided by the wafer thickness ( and the diffusion coefficient of minority carriers ( ; is determined by wafer thickness ( and surface recombination velocity ( . When the silicon surface is passivated by 0.08-mol/L iodine which is considered as the best passivation to silicon, S is very low, and the τ primarily indicates the bulk characteristics of the wafer. When the silicon surface is not passivated, S is very high, so the will be dominant by the surface feature of the wafer.[13,14]

Figure 1 shows the variation of minority carrier lifetime 2D distribution of the one silicon wafer under the different states. As shown in Figs. 1(b) and (c), for those passivated samples undergoing furnace oxidation, the ITO sputtering process leads to a loss in of over 90%, dropping to about 5 s. For the collected data in Figs. 1(a),1(d), and 1(f), the is basically maintained steady when it is once, twice and thrice passivated by 0.08-mol/L iodine, and S is very low, indicating that the remains unchanged before and after sputtering, and it is larger than 280 s. By contrast, the surface passivation validity of the sample where the ITO film is directly deposited on two-sides of unoxidized silicon is also poor as shown in Fig. 1(e). As a result, the structural damage should be localized at the SiOx/c-Si interface, most likely in the form of the deep defects and the Si dangling bonds (or interface states).[19,20]

Fig. 1. (color online) Minority carrier lifetime profiles of the identical silicon wafer under the different conditions: (a) a bare silicon is passivated by 0.08-mol/L iodine; (b) 700- C/10-min thermal oxide passivation; (c) after magnetron sputtering deposition of ITO film; (d) second passivation with 0.08-mol/L iodine; (e) double-sided deposition of ITO film by sputtering on a non-oxidized silicon wafer; (f) third passivation with 0.08-mol/L iodine (unit: s).

In order to make clear what the new electronic states are within the intermediate region of ITO/c-Si, after sputtering deposition of ITO thin film, the chemical components in the region are determined by XPS in elemental inner core–electron peaks such as In 3d, Sn 3d, Si 2p, and O 1s, with depth profiling. The relative atomic percent of In, Sn, and Si across the interface of the sample (here ITO film is deposited on silicon after 700- C/10-min thermal oxidation process) is shown in Fig. 2(a) by an area integral estimation. The depth distribution of the chemical components is divided into three parts with etching times: the front ITO film, the intermediate SiOx layer, and Si substrate subsequently. At the etching time of 720 s, the measured Si 2p spectra as shown in Fig. 2(c) is dashed dots, while the Si 2p , Si 2p , SiOx, and SiO2 are the deconvolution, fitted by the likely Si-correlated compositions in the region. According to the surface model of silicon oxide proposed by Seah et al.,[15] the Si dangling bond (or interface state) within the SiOx and SiOx/c-Si interface is induced by sub-oxide SiOx, which increases interface recombination and decreases samples’ . However, for the samples without furnace oxidation in advance, just the ITO film being directly deposited on two-sides of silicon substrate, the relative atomic percent of In, Sn, and Si across the interface are given in Fig. 2(b). The depth distribution of chemical components is similar to the former case except a right shift of 40 s. The sub-oxide SiOx is also observed in Fig. 2(d), corresponding to the SiOx/c-Si interface. The different finding is that the existence of a broad SiOx layer at the interface region between ITO and c-Si is also revealed by XPS signal, and SiOx just come into being in the process of sputtering deposition of ITO film.[21] On the other side, it is reasonable for the right shift of the middle position of the interface region taking place in Fig. 2(b) and a little decrease of the intensity of Si2p peaks in Fig. 2(d), because of the different processing for the two kinds of samples. Therefore, the Si dangling bond appeared in both kinds of samples is identically linked to the sputtering process.

Fig. 2. (color online) The XPS depth profiling of ITO/SiOx/n-Si structure. (a) ITO film is deposited on silicon after 700 C/10 min thermal oxidation treatment, relative atomic percent distribution of In, Sn, Si, and SiOx as a function of etching time; (b) ITO film is directly deposited on two-sides of unoxidized silicon, relative atomic percent distribution of In, Sn, Si, and SiOx as a function of etching time; (c) Si 2p spectra of etching time at 720 s corresponds to Fig. 2(a); (d) Si 2p spectra of etching time at 760 s corresponds to Fig. 2(b).

Figures 3(a) and 3(b) show the cross-section TEM images of the sample, corresponding to Figs. 2(a) and 2(b), respectively. It is apparent that an ultrathin SiOx layer presents between the ITO layer and Si substrates in all samples. The thicknesses of SiOx layers in Figs. 3(a) and 3(b) are separately found to be about 2.3 nm and 1.4 nm. Furthermore, we observed the amorphous phase of the SiOx layer and “atomistic interleaving structure” at the SiOx/c-Si interface, which may relate to the intermediate oxidation state of SiOx and most of the Si dangling bonds.

Fig. 3. Cross-sectional TEM micrograph of ITO/SiOx/n-Si structure. (a) ITO film is deposited on silicon after 700- C/10-min thermal oxidation treatment; (b) ITO film is directly deposited on two-sides of unoxidized silicon.

We now turn to the origin of the sputtering damage. During the sputtering deposition of ITO film, the average kinetic energy of energetic particles (ions, atoms, molecules, and atomic clusters) is about 10 eV.[22] Oxygen atoms (O , oxygen ions (O , indium atom (In , indium ions (In , tin atoms (Sn , tin ion (Sn , and reflected argon atoms (Ar , argon ions (Ar (energies up to 150 eV) primarily bombarded the substrate in the processing of magnetron sputtering.[6,9] The bond energies of Si–Si, Si(Si)–Si, Si(Si)2–Si, Si–O, and Si(O)–O are 3.21, 4.19, 4.56, 8.29, and 4.71 eV, respectively, which are less than that of energetic particles.[23] Therefore, the energetic particles will break the above-mentioned atomic bonds and alter the electron state of the SiOx layer as well as the SiOx/c-Si interface, increasing the Si dangling bonds and the interface recombination velocity, and reducing the . In addition, ultraviolet (UV) radiation from glow discharge and particle collision, reach up to 7.8 eV (or higher), which can pass through the SiOx layer, reaching to the SiOx/c-Si interface and the interior of c-Si.[6] UV photons can excite electrons, injecting from the conduction band of Si into the conduction band of SiO2, activating the deep defects at the SiOx/c-Si interface and generating additional interface states (or Si dangling bonds). The required energy for this effect is only 3.1 eV, which is less than the UV light energy.[24] Therefore, UV irradiation can damage the SiOx/c-Si interface, resulting in the attenuation of . In summary, the Si dangling bonds induced by sputtering damage are consistent with above experiment results. The Si dangling bonds are created from particle bombardment and UV light irradiation.

3.2. Effect of vacuum annealing on effective minority carrier lifetime

Figure 4 shows the variation of dependent on the vacuum annealing temperatures, thermal oxidation passivation, and sputtering deposition of ITO on the polished silicon wafer. It is found that a large decrease of for the samples after a sputtering deposition of ITO films occurs, which means a deterioration of the normally thermal oxidation passivation. The of c-Si after thermal oxidation at 700 C for 10 min is constantly kept at 107 s, while the τ is decreased to 5 s upon subsequent sputtering deposition of ITO films. The likely interpretation is the worsening of the saturation of surface dangling bonds on c-Si, as the may not be changed during the sputtering deposition of ITO films. However, the damage of SiOx/c-Si interface states is partially repaired after vacuum annealing under 100 C–400 C for 30 min, improving the of the samples. The increases at first and then decreases with the temperature. Herein the is recovered to nearly 30 s at 200 C for 30 min, after that it is declined with the temperature until to about 3 s after 500 C. These changes of the can be verified by a two-dimensional mapping measurement as shown in Fig. 5. Figures 5(a), 5(b), and 5(c) are corresponding to the thermal oxidation, sputtering deposition of ITO, and vacuum annealing at 200 C, respectively. It should be mentioned that all measurements of the for the different situation are taken at those fixed points according to the former established xy frame of axes on the c-Si wafer.

Fig. 4. (color online) The varying curves of samples’ τ with the temperature, the samples including normal thermal oxidation of silicon, then deposition of ITO film on silicon by magnetron sputtering and a vacuum annealing for 30 min, respectively (“ ” represents the standard deviation).
Fig. 5. (color online) Minority carrier lifetime profiles of the samples before and after 200- C vacuum annealing, and the corresponding ITO deposition by sputtering: (a) 700- C/10-min thermal oxide passivation; (b) ITO sputtering treatment; (c) 200- C vacuum annealing (in unit s).

Based on the definition and description of efficient minority carrier lifetime in Subsection 3.1, the lowering of the τ either in the sputtering process of ITO films or in the vacuum annealing at higher temperatures must be correlated to the somehow variation of the electronic states on the surface of c-Si or inside the SiOx or the interfacial region of SiOx/c-Si, because the is very sensitive to the changes of the chemical components on the top several layers of SiOx/c-Si. Thus, it is necessary to extract the microstructure information within the interfacial region between ITO and c-Si so as to gain essential cause to induce the lessening after the sputtering deposition of ITO film on Si. So far, the best choice and combination for the purpose of this characterization is to apply XPS for stoichiometry and TEM for phase structure determination from the surface, down to the interface and entering the bulk to the ITO/SiOx/Si system. In Fig. 3 we have learned the physical phase and the geometrical structure of the interfacial materials through TEM. We are sure that the ITO film and SiOx are polycrystalline and amorphous phases, respectively. In the following sections, we will investigate the chemical configuration and electronic states of the ITO/SiOx/Si system, especially for the SiO and SiOx/Si intermediate matter.

From the point of view of the application of photovoltaic device, the last step of low temperature processing at about 200 C is always used for the windows or electrode fabrication. The process is equivalent to the low temperature annealing treatment. Therefore, in the photoemission spectroscopy studies, we pay main attention on the annealing effect of the evolution associated with the new electronic structures and chemical states of the SiOx/c-Si for those samples after ITO deposition by sputtering method and subsequent annealing. There are two kinds of samples selected for XPS with depth profiling. One is for the annealing at 200 C, of about 30 s. Another is for the annealing at 600 C, of about 3 s. They are two extreme conditions for the comparison, in order to reveal the change of the fraction of S–O bonds in the intermediate region. All the Si 2p electrons emission peaks are collected at the middle position of the depth profile of chemical components as shown in Fig. 2(a), which can well manifest the variation of the electronic structures and chemical states in the SiOx/c-Si interface.

In detail, figure 6 shows XPS spectra for the vacuum annealing samples at 600 C ( is about 3 s) and 200 C ( is about 30 s), respectively. The typical binding energies for the different Si–O bonds stoichiometric amorphous SiOx ( ) are listed in Table 1. Those XPS-Si 2p peak spectra with subtraction of the background noise in 3 and 30 s statuses are shown in Fig. 6(a). It is apparent that there is a little difference for the peak shape of Si 2p, corresponding to SiOx (the range of binding energy is 100 eV–102 eV) of the two samples, with this high resolution analysis ( eV). In addition to the illustration, a set of normalized and partially enlarged spectra, corresponding to the SiOx component, is obviously observed in Fig. 6(b), and an intensity increase of sub-dioxide SiOx after 600- C vacuum annealing has been obtained. To understand these variations on the electronic structures where they are closely correlated to the SiOx fraction in the intermediate region of ITO-Si, the deconvolution of XPS spectra for 3 and 30 s in are separately shown in Figs. 6(c) and 6(d). The Si 2p (SiO peak of XPS spectra for 3 s is stronger than that for 30 s; oppositely, the Si 2p (SiO peak of XPS spectra for 3 s is weaker than that for 30 s. This contrast implies an increase of SiOx and a decrease of SiO after 600- C vacuum annealing. Secondarily, the distinct deconvolution of XPS spectra for 3 s and 30 s in are shown in Figs. 6(e) and 6(f), respectively. Several sub-dioxides such as Si2O3, SiO, and Si2O exist in the SiOx/c-Si interface for both samples. However, the distort Si–O bonding states in the SiOx/c-Si interface have more of a fraction after 600- C vacuum annealing. According to the gaseous unstable SiO forming principle proposed by Yanjun Wang et al.,[25] under low oxygen partial pressure (below about 10 Pa during annealing) and high temperature (500 C–700 C) conditions, two chemical reactions occurred in the SiOx/c-Si interface as follows:

where SiO (gaseous unstable substances) is difficult to be oxidized into SiO2 phase under low oxygen partial pressure, then SiO escapes from SiO2, which will result in an oxygen vacancy in the SiOx/c-Si interfacial region, so that the Si dangling bonds (or interface state density) was promoted and the severe attenuation in was induced.[25]

Fig. 6. (color online) Si 2p electron emission in XPS spectra for the samples at 600 C (τ is about 3 s) and 200 C (τ is about 30 s) vacuum annealing, respectively: (a) the subtraction of background; (b) normalized and enlarged; (c) curve fitting of Si 2p peak for 3 s in τ ; (d) curve fitting of Si 2p peak for 30 s in τ ; (e) secondary curve fitting of Si 2p peak for 3 s in τ ; (f) secondary curve fitting of Si 2p peak for 30 s in τ .
Table 1.

Binding energies for amorphous SiOx ( ).[15,26] (peak error eV).

.

From the literatures, the dissociation energies of Sn–O and In–O are separately 5.47 eV and 3.59 eV in the crystallized ITO.[26] On the other hand, the unintentional oxide behavior of Si in the ITO/c-Si has been reported to grow when the sample is heat treated at high temperatures (heated at 785 C for 33 min),[27] but the oxygen atoms (or oxygen ions) come from the decomposition of ITO materials. He et al. found that the sub-dioxides oxidize into SiO2 until annealing temperature exceeds 1000 K at enough oxygen ambient.[28] However, as shown in Fig. 6(d), more SiO2 and less SiOx exist in the SiOx/c-Si interface of our sample for 30 s, indicating that additional oxygen atoms (or oxygen ions) involved in the reaction processes as Eqs. (2) and (3) under low temperature annealing (100 C–400 C). We did a comparison experiment to explore the source of oxygen atoms (or oxygen ions). The ITO films were directly deposited on two-faces of seven unoxidized silicon, and the samples were treated with the same processing of magnetron sputtering and post-annealing conditions. The unintentional oxide layer (seen in Fig. 3(b)) has a poor passivation effect on the Si layer (as shown in Fig. 1(e)). If these samples’ τ are not improved by low-temperature annealing, the additional oxygen atoms (or oxygen ions) are not derived from the ITO film. Figure 7 shows the varying curves of τ with the temperature before and after vacuum annealing for 30 min (double-sided deposition of ITO by sputtering on a non-oxidized silicon wafer). The samples’ are unchanged before and after low temperature annealing. By comparison with low temperature parts of the annealing curve in Fig. 4, they have different trends, indicating that oxygen atoms (or oxygen ions) are derived from the SiOx/c-Si interface rather than from the ITO film (refer to the samples with thermally grown SiO . Nevertheless, the vacuum annealing under 500 C–700 C leads to a loss in . It is also verified that the occurrence of chemical reaction is like Eqs. (2) and (3). The degree of attenuation in becomes weak at 700 C, attributing to a small amount of oxygen atoms from ITO film diffused into the SiOx/c-Si interface, which is consistent basically with the previous report.[27]

Fig. 7. (color online) The varying curves of samples’ τ with the temperature before and after vacuum annealing for 30 min (double-sided deposition of ITO by sputtering on a non-oxidized silicon wafer) (“ ” represents the standard deviation).

To summarize, on the one hand, the electronic structure of ultrathin silicon oxide passivation was inevitably deteriorated by energetic particles bombardment and UV light radiation during the sputtering deposition of ITO film on c-Si. Thus, the interface recombination rate of photovoltaic devices has to be increased, which is particularly prominent in the SIS and HIT devices. On the other hand, low-temperature annealing can improve samples’ τ . The following factors are implied by the above discussion: the energetic particle breaks the Si–O bond of the SiOx/c-Si interface through impact ionization, which damages the lattice structure of the oxide layer and generates defects (Si dangling bonds). Some oxygen atoms (or oxygen ions) will obtain certain energy and diffuse to the SiOx/c-Si interface. These oxygen atoms (or oxygen ions) will relax if their energy is less than the reaction activation energy (2 eV) of an oxygen atom in the crystalline silicon.[29] Moreover, excessive silicon ions of the SiOx/c-Si interface have come into being in the intentional oxidation process.[16] The relaxation oxygen atoms (or oxygen ions) will react with excessive silicon ions under low-temperature annealing for 30 min, partially repairing the SiOx/c-Si interface damage and reducing the interface states. Therefore, the passivation quality of the SiOx layer and samples’ are improved. In consequence, from the formation of heterojuntion structure, establishment of built-in electric field, depositing of TCO films and preparation of external electrodes to the post-annealing process, which make it possible to prepare efficient silicon-based thin-film heterojunction solar cells in low temperature and achieve a good negative temperature coefficient ( %/ .[30]

4. Conclusion

In summary, sputtering damage occurs in the process of fabricating ITO/SiOx/c-Si photovoltaic devices, leading to a loss in of over 90%, decreasing to about 5 s. Microwave photoconductivity decay experiments indicated that the sample’s remains unchanged before and after sputtering, and both greater than 280 s, indicating that sputtering mainly degrades the electronic passivation quality of the SiOx/c-Si interface. The further Si dangling bonds are created by high energy particles (O , Ar , In , Sn bombardment and UV radiation. These defects are verified by XPS and TEM. However, these Si dangling bonds are partially repaired under low-temperature vacuum annealing, improving the passivation effect of the SiOx/c-Si interface. Wherein is restored up to nearly 30 s at the annealing temperature of 200 C for 30 min, which is the typical temperature in the processing of most efficient crystalline silicon heterojunction solar cells. Nevertheless, the can be attenuated to about 3 s after vacuum annealing at 500 C–700 C for 30 min. The gaseous unstable SiO might be generated in the SiOx/c-Si interface under this annealing condition, which damages the SiOx/c-Si interface and increases unsaturated bonds of SiOx, resulting in the attenuation of . As a result, the structural sputter-induced damage is partially recovered under low-temperature vacuum annealing, which is helpful for the construction and the performance improvement of the SIS photovoltaic devices.

Acknowledgments

Financial supports were given in the footnote on the first page. Some of the measurements were conducted by the Instrumental Analysis and Research Center of Shanghai University.

Appendix A: Supplementary information
A1. The compositions of ultra-thin SiOx layer

A thin SiOx layer was produced by thermally oxidizing in an oxidation furnace for 10 min at 700 C. The flow ratio of high purity nitrogen and oxygen was 4 L/min:1 L/min. The thin SiOx layer was made up of four compositions: plenty of SiO2, very few of Si2O, SiO, and Si2O3 (as shown in Fig. S1).

Fig. S1. (color online) The Si 2p XPS spectra of ultra-thin SiOx layer (thermal oxidation for 10 min at 700 C).
A2. The optical characteristics of ITO film

The optical characteristics of ITO film were measured by UV-VIS spectrophotometer. The average transmittance of the visible spectrum is more than 85% and the optical band gap was extrapolated to be 3.60 eV (as shown in Fig. S2).

Fig. S2. (color online) The transmittance spectra of ITO film (80 nm) on glass.
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