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Abstract
The structure and optical properties of a hydrogenated amorphous silicon (a-Si:H) film for a short-wave near-infrared bandpass filter (BPF), fabricated using the rotary table type layer-by-layer (LbL) sputtering method, were evaluated. Although the extinction coefficient k tended to increase as the film thickness per cycle increased, the bonded hydrogen composition increased owing to an increase in the hydrogen partial pressure of the reactive plasma area, thereby decreasing k to 1 × 10−3 or less. Meanwhile, the occurrence of (Si–H2)n bonds was suppressed. Consequently, in contrast to reactive sputtering, it was found that the a-Si:H achieved via LbL sputtering has a high refractive index, low absorption characteristics, and high thermal durability suitable for BPFs, due to the chemical annealing effect.
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1. Introduction
Research on hydrogenated amorphous silicon (a-Si:H) began in the 1970s, focusing on electronic device applications such as solar cells and thin film transistors [1]. Nevertheless, its range of applications has expanded over the years owing to its low cost and high compatibility with many devices. It has attracted attention as an optical material for interference filters, waveguides, and nonlinear optical devices that require high refractive indices and low absorption characteristics in the near-infrared wavelength band [2–4]. When a-Si:H with a refractive index of about 3.5 is used for the narrow bandpass filter (BPF), the number of laminated layers can be reduced to less than half that of a general high-refractive film (Ta2O5, TiO2, etc.) with a refractive index of 2.0–2.5 [5,6]. In addition, since Si is the base material, it is highly compatible with devices and has a wide optical band gap of 1.3 to 1.9 eV (>1.1 eV) compared to c-Si. Hence, the fundamental absorption edge wavelength decreases to 650–950 nm (<1100 nm) [3]. Therefore, it has the potential to be used as a transparent material in the wavelength region longer than the fundamental absorption edge, including short-wave near-infrared (SW-NIR) bands. However, since a-Si:H has an amorphous crystal structure, it contains dangling bonds in the film, which increases the optical absorption; therefore, defect termination by hydrogen is important. Many studies in the field of solar cells have shown that defects in a-Si:H are generated when the bond of silicon–hydrogen is broken by heat or light irradiation [6–9], and therefore, stable hydrogen termination is required. One of the known countermeasures is a chemical annealing process through a layer-by-layer (LbL) method, in which chemical vapor deposition (CVD) film formation of 1-to-3-nm and hydrogen plasma treatment are repeated [9–12]. It has been reported by Shimizu et al. that a-Si:H formed by this method suppresses the formation of Si–H2 bonds, which are inferior in stability, improves photodegradation durability, and exhibits features such as low-temperature crystallization [9–12]. This process has been studied and verified using the CVD method, and hence, it is necessary to use a hazard gas such as SiH4. On the contrary, in the production of thin films for general optical applications, physical vapor deposition (PVD) methods such as the sputtering method, which uses gas for which safety management is easy, are used. The establishment of PVD film formation technology is considered to be important for the widespread use of a-Si:H as an optical film. In this study, we will verify the chemical annealing effect in a-Si:H using the LbL process of the sputtering method for optical applications. The improvement in the optical quality owing to the chemical annealing effect of a-Si:H when using the sputtering method, which can be safely managed, opens more possibilities for widespread use of a-Si:H in optical applications. In our research so far, we have been studying the application of a-Si:H reactive sputtering films with high refractive indices and low absorption characteristics for SW-NIR optical filters [13–15]. It has been found that when the amount of hydrogen added to achieve low absorption levels is increased, the Si–H2 bonds increase and a film containing voids is formed, thereby decreasing the refractive index and thermal durability of the a-Si:H film. Dalal et al. proposed a model in which the excess hydrogen generated on the film surface during CVD is vacuum desorbed owing to chemical annealing [11], which may also be effective in the sputtering method. To verify this effect, we used CCS2110 manufactured by Shibaura Mechatronics Co. Ltd., whose structure consists of processing chambers that are separated into a sputtering film formation area and a reactive gas plasma irradiation area and are placed on the upper side of a rotary table holding the substrates [16–18]. By continuously rotating the table while simultaneously performing sputter discharge and hydrogen plasma discharge processes, the LbL process can be performed, which achieves high speed thin film formation and hydrogenation, with a several nm thickness on the substrate. This process is one of the film forming methods suitable for mass production of optical multilayer films capable of stably forming an oxide and nitride films at high speed and low temperature [19,20]. This is associated with the fact that oxide and nitride films can be formed by the metal mode target discharge with a high sputtering yield, instead of the reactive mode target discharge with a low sputtering yield. In addition, it is possible to suppress the generation of high electrical resistance reactants on the target surface and nearby areas, which causes arc discharge, during deposition of the oxide and nitride film. These features are also advantageous in a-Si:H films, which exhibit high electrical resistivity of approximately 1 × 108 to 1 × 1010 Ωcm. Furthermore, the optical multilayer filters achieved using a-Si:H exhibit a laminated structure with oxide or nitride films [5,6]. It is therefore important to prepare a-Si:H through an LbL sputtering process, which is suitable for the stable mass production of oxide and nitride films. Their characteristics must also be evaluated so that a-Si:H may be practically developed as an optical multilayer film. In this study, we investigated the relationship between the a-Si:H film thickness per cycle of the rotary-table-type LbL sputtering method, as well as the hydrogen plasma irradiation conditions and optical characteristics. Adequate conditions for achieving high refractive index and low absorption characteristics were thereby determined. In addition, the bonded hydrogen composition, bond configurations, film structure, and thermal durability of the fabricated a-Si:H films were investigated, and the effect of chemical annealing was verified via comparison with an a-Si:H film deposited using a general hydrogen-reactive direct current (DC) sputtering method.
2. Experimental method
For the deposition of a-Si:H, we used an LbL sputtering system, which repeatedly performs DC magnetron sputtering with a boron-doped single-crystal Si target and plasma treatment with an inductively coupled plasma (ICP) discharge of hydrogen. This was achieved in conjunction with the high-speed horizontal rotation of the table upon which the substrates were fixed (CCS2110, Shibaura) [17,18]. Figure 1 shows a schematic diagram of this system. The inside of the chamber was evacuated to 3 × 10−4 Pa or less with a turbo molecular pump. The substrates were fixed via a load lock chamber on the rotary table (diameter of approximately 2 m), and they were rotated at approximately 120 rpm. The magnetron sputtering system and ICP plasma source were separated by a separation wall on the upper side of the table and arranged as sputtering deposition and hydrogenation areas, respectively. A single crystal Si target with a resistivity of about 10 mΩcm by 0.2 at% boron doping was used because it is necessary to reduce the electrical resistance for performing stable DC sputtering discharge with high power. In this LbL process, the generation of arc discharge is suppressed owing to low reactive gas pressure in the sputtering area. Hence, stable discharge is possible using DC power instead of pulsed DC power. The ICP discharge used radio frequency (RF) power (13.56 MHz). Ar-gas-based magnetron discharges and hydrogen-gas-based ICP discharges were continuously generated in the sputtering film formation and hydrogenation areas, respectively. Owing to the separation wall, the contamination of the reactive gas into the sputtering film formation area was reduced to less than 10%. The sputtering film formation and hydrogen plasma irradiation were repeated over cycles of 0.5 s as the substrate passed through the processing at a rotation speed of 120 rpm. The process gas pressure in the sputtering film formation area was approximately 0.3 Pa, the sputtering DC power was 4 to 7 kW, and a maximum of three sputtering areas were used. In the hydrogenation area, a process gas mixed with Ar and H2 was used with a total pressure of approximately 1 Pa, hydrogen partial pressure ratio of 7.5–17.5%, and ICP discharge power of 5 kW RF. The hydrogen plasma was irradiated onto the substrate with a power density of 1.96 W/cm2 and an irradiation time of approximately 40 ms. Under these conditions, the film thickness per cycle varied between 0.15 and 0.75 nm, including the monatomic layer (0.23 nm) that is expected to be highly reactive. A film with a total thickness of 1 µm was deposited to determine processing conditions that achieve a high refractive index and low absorption. No heater was used to raise the temperature of the substrate; however, the temperature was 80–90 °C during deposition because of the heat caused by hydrogen plasma irradiation. To evaluate the optical characteristics of the product, the extinction coefficient k and refractive index Nf at a wavelength of 940 nm were evaluated from the transmittance and reflectance in the visible-near infrared light region [13–15,21]. The optical bandgap Eg was determined from a Tauc plot [22]. In addition, the Urbach energy (Eu), which is the range of the density of states of the band tail, was obtained from the slope of the exponential absorption change in the low-energy wavelength region and used to identify changes in structural disorder [23,24]. The thermal durability was evaluated via the differences in Eg, Eu, Nf, and k values obtained before and after the post-annealing process. Post-annealing was carried out under atmospheric conditions at 300 °C for 5 h. To evaluate the film structure, surface morphologies were observed using atomic force microscopy (AFM) measurements, the oxygen composition in the films were determined using X-ray photoelectron spectroscopy measurements, and the bonded hydrogen composition CH and bond configurations were determined using Fourier transform infrared (FTIR) spectroscopy measurements. The CH was quantitatively calculated from the transmission spectrum with a wave number in the range of 1800 to 2200 cm−1 [25]. The absorption peak with a wave number of approximately 2000 cm−1 originates from the Si–H bond, whereas that at approximately 2090 cm−1 originates from the Si–H2 bond, which is related to voids and defects. Considering this, the FTIR spectrums were waveform separated. The area ratio of Si–H2/Si–H was used to verify the existence of voids and defects attributed to the silicon-hydrogen bond configuration [26]. In addition, (Si–H2)n bonds consisting of chained Si–H2 structures are more strongly related to voids and defects. The absorption peak of these bonds was observed to be approximately 845 cm−1, and the area ratio was evaluated using an absorption peak of approximately 2090 cm−1 originating from the Si–H2 bonds. (Si–H2)n/Si–H2 was used as a guideline to determine the existence of more influential voids and defects attributed to the clustered silicon-hydrogen bond configuration [27].
Figure 2 shows a schematic of the bond configurations between silicon and hydrogen as explained above. For comparison, a-Si:H was also prepared by reactive DC sputtering using a small-sized batch-type offset rotational sputtering system (CFS-4ES, Shibaura) [13–15]. The processing conditions were a base pressure of 8 × 10−4 Pa before film deposition, process pressure of 0.5 Pa, hydrogen partial pressure ratio of 3–50%, sputtering DC power of 400 W, and deposition rate of 0.3 nm/s. Our previous studies have shown that the voids in the film may be attributed to Si–H2 bonds, while the defects may be attributed to dangling bonds and structural disorder [14]. These voids and defects may be reduced by increasing the substrate temperature, resulting in a high refractive index and low absorption characteristics. The obtained a-Si:H was suitable for use in a BPF [14]. Therefore, the film structure, optical characteristics, and thermal durability of a-Si:H formed at 200 °C for use in a BPF and the one formed at room temperature (RT) (which was 100 °C or less after deposition) without heating were compared with that formed via the LbL sputtering. From these results, the chemical annealing effect of LbL sputtering was investigated. Therefore, the BPF performances were simulated using a-Si:H optical characteristics achieved via LbL sputtering and reactive sputtering.
3. Experimental results
3.1 Relationship between the film thickness per cycle and optical characteristics
Figure 3(a) shows the relationship among the film thickness per cycle, refractive index Nf, and extinction coefficient k. The hydrogen partial pressure ratio in the ICP discharge area is 10% (partial pressure = 0.1 Pa). Nf increased from 2.9 to 3.76 as the film thickness per cycle increased. In contrast, when the film thickness per cycle was between 0.15 to 0.3 nm/cycle, k tended to decrease with an increase in the thickness, and when it was 0.3 nm/cycle or greater, k tended to increase. Generally, defects in a-Si:H are reduced owing to the promotion of hydrogenation, resulting in a decrease in k [27–30]. Therefore, in the LbL process, k increased with an increase in the film thickness per cycle. However, k decreased as the thickness increased at 0.22 nm/cycle or less. The cause of this trend is considered to be oxidation. Figure 3(b) shows the relationship between the film thickness per cycle and the oxygen composition inside of the a-Si:H film. The oxygen composition was at 1 to 3 at%, under thick film conditions of 0.38 to 0.62 nm/cycle, and it tended to increase to 5 to 10 at% under thin film conditions of 0.22 nm/cycle or less. This occurred due to the film oxidation achieved via the residual oxygen in the chamber during the rotational movement after the thin film was formed during each round. Additionally, Nf tended to decrease sharply under thin-film conditions, which is also attributed to the influence of film oxidation. In a previous study of a-Si:H conducted using the CVD method, it was reported that defects were generated in the band gap owing to the influence of residual gas, and the optical absorption increased [31]. In the general reactive sputtering process, silicon is mainly hydrogenated on the target surface and continuously deposited with no time interval. Therefore, oxidation has only a small influence on this process. In the case of a-Si:H deposited using the reactive sputtering system described in Section 2, the oxygen content was 1 to 3 at% at a film deposition rate of 0.1 to 0.3 nm/s. In this LbL process, the influence of oxidation during the rotational movement of the substrate is considered to be suppressed when the film thickness is 0.4 nm/cycle or more.
Fig. 3. (a) Thickness per cycle dependence of Nf and k for a-Si:H deposited via the LbL method. (b) Thickness per cycle dependence of the oxygen contents of a-Si:H deposited via the LbL method.
3.2 Relationship between the hydrogen partial pressure and optical characteristics
Figure 4(a) shows the relationship among the partial pressure of hydrogen in the process gas of the ICP discharge, the refractive index Nf, and the extinction coefficient k. The film thickness was constant at 0.6 nm/cycle. As the hydrogen partial pressure increased, Nf decreased by approximately 5%, and k decreased significantly to 1/10 or less of its initial value. The decrease in k occurred because of the reduced structural disorder and defects owing to hydrogenation [13,15,29,30]. The decrease in Nf is considered to have occurred due to the decrease in the film density with an increase in the silicon-hydrogen bond in the film [27].
Fig. 4. (a) H2 partial pressure in the hydrogeneration zone dependence of Nf and k for a-Si:H deposited via the LbL method. (b) Dependence of Nf and k on the bonded hydrogen content CH of a-Si:H deposited via the LbL method and reactive sputtering at 200 °C and RT.
3.3 Relationship between the bonded hydrogen composition, bond configuration, and optical characteristics
Figure 4(b) shows the bonded hydrogen composition CH dependencies of the refractive index Nf and extinction coefficient k for a-Si:H using LbL and reactive sputtering. a-Si:H films with 0.3 nm/cycle or less during LbL sputtering, which are greatly affected by oxidation, were excluded from this graph. The CH of LbL sputtering is the measured value of a-Si:H with variable hydrogen partial pressure in the ICP discharge area shown in Fig. 4(a). The CH of reactive sputtering is the measured value of a-Si:H with variable hydrogen partial pressure in the sputtering chamber. Nf and k tended to decrease as CH increased for all conditions. A larger decrease in k as CH increased was observed for LbL sputtering compared to reactive sputtering at 200 °C, and the latter method exhibited a larger decrease than that observed for reactive sputtering at RT. Therefore, when LbL sputtering was used, a-Si:H with low CH (i.e., high Nf) and low k values compared to reactive sputtering was obtained. This was achieved because dangling bonds were reduced owing to the promotion of structural relaxation. Moreover, the surplus hydrogen was evacuated during the time interval after hydrogen plasma irradiation.
Table 1 summarizes the relationship between CH, Si–H2 bond ratio (Si–H2/Si–H), (Si–H2)n bond ratio (Si–H2)n/Si–H2, Nf, and k under low-absorption conditions during each film deposition process.
Table 1. Relationship between the deposition conditions, silicon-hydrogen bond states, and optical constants.
The FTIR absorption spectrums of each bond are shown in Figs. 5 and 6. Generally, the Si–H2 bond ratio increases when the film deposition temperature is low [14]. In the case of reactive sputtering at RT, the Si–H2 bond ratio was highest during each process, which resulted in low Nf and high k values. Alternatively, although LbL sputtering was conducted at RT, the Si–H2 bond ratio was relatively low; the Si–H2 bond ratio was 1/4 or less of that of reactive sputtering at RT and equal to or less than that of reactive sputtering at 200 °C. In particular, the (Si–H2)n bond ratio, which is greatly affected by voids and defects, was suppressed to 1/4 or less of that of reactive sputtering conducted at 200 °C and RT.
Fig. 5. FTIR absorption spectrums of (Si-H) and (Si-H2) stretching mode for a-Si:H achieved using (a) LbL sputtering at RT, (b) reactive sputtering at 200 °C, and (c) reactive sputtering at RT.
Fig. 6. FTIR absorption spectrums of (Si-H2)n bending mode for a-Si:H achieved using (a) LbL sputtering at RT, (b) reactive sputtering at 200 °C, and (c) reactive sputtering at RT.
3.4 Surface morphology
Figure 7 shows the AFM surface morphology obtained under conditions of low absorption during each film deposition process. In Fig. 7(a), the surface of a-Si:H formed by LbL sputtering shows that the grains uniformly grew to a diameter of approximately 80 nm as compared to the reactively sputtered films shown in Figs. 7(b) and 7(c). The values of arithmetic mean roughness (Ra) of the film surface were approximately 1.2, 1.6, and 2.1 nm in the case of LbL sputtering (Fig. 7(a)), reactive sputtering at 200°C (Fig. 7(b)), and reactive sputtering at RT (Fig. 7(c)), respectively. The Ra value of LbL sputtering was the smallest, which reflects the low amount of hydrogen composition and clustered silicon-hydrogen bond configuration.
Fig. 7. AFM morphology images of a-Si:H achieved using (a) LbL sputtering at RT, (b) reactive sputtering at 200 °C, and (c) reactive sputtering at RT.
3.5 Thermal durability of the band structure and optical characteristics
Table 2 summarizes a comparison of the optical bandgaps Eg, Urbach energies Eu, refractive indices Nf, and extinction coefficients k before and after post-annealing under low absorption conditions for each film deposition process. When the deposition process was LbL sputtering, reactive sputtering at 200 °C, and reactive sputtering at RT, the CH values corresponded to 18, 31.2, and 47 at%, respectively. Moreover, Eg was 1.67, 1.86, and 1.96 eV, respectively. Previous studies have reported that the Eg of a-Si:H has a positive correlation with CH because the bonding energy of Si–H is larger than that of Si–Si [32]. When LbL sputtering was used, the CH value was the smallest in each film deposition process, and therefore Eg was the smallest. Eu represents the width of the density of states of the band tail and increases in the presence of large structural disorders and defects. When reactive sputtering at 200 °C was used, Eu was the smallest at 68.2 eV because structural relaxation was promoted via the heated deposition process. Comparatively, Eu was as large as 80 meV when LbL sputtering was used. However, this value was smaller than that noted of the reactive sputtering process conducted at RT, suggesting the promotion of structural relaxation owing to the effect of chemical annealing. Furthermore, higher quality a-Si:H films with less structural disorder can be obtained by including a heating step during the LbL sputtering film deposition process. Considering the changes observed after annealing, the Eg of the LbL sputtering films increased by 0.08 eV, and Eu decreased by approximately 16 meV. In addition, Nf slightly decreased by 0.01, and k decreased to 1/3 (or less) of its former value. These trends suggest that annealing promoted structural relaxation and reduced defects. When the number of Si-H2 bonds and clustered Si-H2 bonds before annealing are low, the thermal desorption of hydrogen is reduced, and hence the increases in dangling bonds and structural disorder owing to the release of hydrogen are considered low. Therefore, the effects of structural relaxation are dominant. Additionally, the k value is thought to decrease because of a decrease in the number of dangling bonds due to an increased number of Si–Si bonds and a decrease in the trap levels of the band tail. Alternatively, the Eg value of the reactive sputtering film at 200 °C decreased by 0.2 eV and Eu increased by approximately 7 meV. Moreover, Nf increased by 0.07, and k increased by approximately 3 times its initial value. These results suggest that the bonded hydrogen was thermally desorbed via annealing, the film structure was disordered, and the number of defects increased. When reactive sputtering at RT was used, Nf increased by 0.07, and k decreased to approximately 40% of its initial value. Moreover, Eu decreased by 10.8 mV, and Eg decreased by 0.13 eV. These changes are attributed to the thermal desorption of hydrogen in the film and the structural relaxation of the film due to annealing. The decrease in k and Eu indicates that defects in the film were reduced compared to before annealing owing to the promotion of structural relaxation. However, after annealing, the values of Nf and k were 3.44 and 2.5 × 10-3, respectively, and therefore, low Nf and high k values were obtained compared to other processes. Considering the above findings, it was found that the a-Si:H film obtained via LbL sputtering has a high refractive index, low absorption, and high thermal durability compared to those obtained via reactive sputtering. This is because the excess hydrogen is simultaneously evacuated during film formation when structural relaxation is promoted via chemical annealing, resulting in an a-Si:H film with few defects, a low CH value, and few Si–H2 and (Si–H2)n bonds.
Table 2. Relationship between deposition conditions and Eg, Eu, Nf, and k before and after annealing.
Figure 8 shows the wavelength dispersion characteristics of Nf and k of a-Si:H achieved using LbL sputtering and reactive sputtering at 200 °C, which had a low k value of 1 × 10-3 or less. The Tauc plots calculated from the absorption characteristics are shown as inset diagrams. The characteristics of each process are compared before and after annealing. Based on the data in Fig. 8, the BPF characteristics using a-Si:H by LbL sputtering and reactive sputtering at 200 °C were simulated, and the changes in filter characteristics before and after annealing were predicted. The filter was a Fabry-Perot type single cavity structure with a-Si:H and SiO2 laminated and a narrow band BPF with a wavelength of 940 nm. The layer structure is 8 layers of “Glass substrate | (HLH) 2 L (HLHL) | Air” [13–15]. Here, the H layer (high refractive index layer) is an a-Si:H film, and the L layer (low refractive index layer) is a SiO2 film. The optical film thickness (refractive index Nf × film thickness) of each layer was 1/4 × λ (wavelength 940 nm). Nf of SiO2 was 1.47, k was 1 × 10-6, and it was assumed to be constant before and after annealing. The constituent film thickness was determined by the Nf values of a-Si:H and SiO2 before annealing, and the BPF characteristics were simulated assuming that Nf and k of a-Si:H after annealing change according to Fig. 8. Concise Macleod (Thin Film Center) was used for the simulation, and the backside reflection of the glass was not calculated.
Fig. 8. Wave length vs k and Nf of a-Si:H achieved using (a) LbL sputtering at RT, (b) reactive sputtering at 200 °C, before and after annealing. Insets show Tauc plots.
Figures 9(a) and 9(b) show the spectrums of BPF transmission and absorption (100 − transmittance + reflectance) using LbL sputtering and reactive sputtering at 200 °C, respectively. Before annealing, both LbL sputtering and reactive sputtering at 200 °C had a peak wavelength of 940 nm, a peak transmittance of about 96%, and an absorption rate of about 0.5%. After annealing, in the case of LbL sputtering, the peak wavelength was shifted to the short wave side by 2 nm, and the absorption rate was halved. Contrarily, in the case of reactive sputtering at 200 °C, the peak wavelength shifted to the longer wavelength side by 7 nm, and the absorption rate increased about 3 times. It is considered that the thermal durability of a-Si:H of LbL sputtering is reflected in the stability of BPF, and the change in BPF characteristics after annealing was reduced to 1/3 or less.
Fig. 9. The transmittance and absorbance spectrums of the BPF using a-Si:H achieved using (a) LbL sputtering at RT, (b) reactive sputtering at 200 °C (broken lines are annealed data). The spectrums were calculated based on the characteristics of a-Si:H.
In general, when a-Si:H/SiO2 BPF is annealed, the peak shifts to the longer wavelength side due to the desorption of hydrogen [13,6]. However, the trend of LbL sputtering is different. This is because the optical properties (Nf and k) of a-Si:H show no increase due to thermal desorption of hydrogen; instead, there is a peak shift to the shorter wavelength side of the BPF due to a slight decrease in the refractive index (Δ = 0.01). The refractive index is considered to be affected by residual stress relaxation due to annealing. Therefore, there is a possibility that it can be further stabilized by adding a heating process during film formation.
4. Summary
In this study, a-Si:H films were prepared using rotary-table-type LbL sputtering for SW-NIR BPFs, and the film structure and optical characteristics were evaluated. It was found that the influence of oxidation during the sputtering process was suppressed at an increased film thickness per cycle of 0.4 nm/cycle or greater. Although the extinction coefficient k tended to increase as the film thickness per cycle increased, the bonded hydrogen composition CH increased by increasing the hydrogen partial pressure of the reactive plasma irradiation area, while k decreased to 1 × 10−3 or less.
Consequently, it was found that the a-Si:H film obtained via LbL sputtering had a high refractive index, low absorption, and high thermal durability compared to those achieved via reactive sputtering. The obtained characteristics were associated to the chemical annealing process, which promoted structural relaxation and removed excess hydrogen on the silicon surface during film formation, resulting in the suppression of Si–H2 bonds, especially (Si–H2)n bonds. In future studies, based on the above results, we will prepare laminated BPFs utilizing a-Si:H films fabricated via LbL sputtering and evaluate their optical characteristics and durability.
Acknowledgements
The results described in this paper were obtained owing to the film preparation in Shibaura Co., Ltd and the evaluation in National Institute of Technology, Tokyo College. The authors would like to thank A. Fujita of Shibaura Co., Ltd. for helping our collaborate. The authors would also like to appreciate all members involved in this research, and the editors to give us the opportunity to introduce our research activities. Moreover, the authors would like to thank Optica Publishing Group Editing Services for proofreading the article.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not currently publicly available but may be obtained from the authors upon reasonable request.
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