**5. Sonochemical passivation of Si and SiGe**

A cogent resource for the burgeoning field of the surface passivation coating utilizing hydrocarbon chains [69], which can reduce the density of surface states and increase the recombination lifetime of the majority carriers. Different organic solvents can be used in practical ways for these purposes such as chloroform (CHCl3) and dichloromethane (CH2Cl2). For example, silicone polymers were grown on the Si surfaces with gaseous CHCl3 and Cu catalyst [70], brominated aromatic moieties were successfully prepared from KBr/H2O2 in sonochemically treated chloroform [71]. Recent work in applying ultrasound to chemical reactions demonstrates the promise of the sonochemical approach, yet the bromination of aromatic compounds is not achieved with simple mechanical stirring replacing sonication.

This topic is covered in more detail below for two types of samples. Type A sample is a GexSi1-x alloy layer, 100 nm thick, grown on a p-doped Cz-Si wafer. Type B sample is obtained by coating sample A with a 10-nm thick a-Si layer (see **Figure 6**). The Ge content *x* in the GexSi1-x layers is about 30 at.%.

In order to give an illustrating example for the differences in the effective lifetime *τeff* , quasi-steady-state measurements (QSSPC) [72] done in a-Si/GexSi1-x/Si sample are plotted in **Figure 7**. Here, an effective minority carrier lifetime *τeff* is obtained from the given by

$$J\_{ph} = \frac{\Delta neW}{\tau\_{\text{eff}}},\tag{5}$$

complementary ultrasonic techniques that employ reactant solutions, make a significant contribution to developing a detailed picture of ultrasonic processing. With respect to the reactant solutions that can be used, recent investigations report that SPV signal in Si can be significantly enhanced, by almost an order of magnitude, due to ultrasonic treatments in dichloromethane. Similar effect in CH2Cl2 can be observed for GexSi1-x surfaces exhibiting a 50% increase in the SPV

*Current–voltage curve (a) and variation of the effective lifetime τeff with excess carrier density* Δ*n (b) for sample B before (open circles) and after sonication in chloroform (closed circles). Reproduced with permission from Nadtochiy A, Korotchenkov O, Schlosser V. Physica Status Solidi (a). 2019;216:1900154. Copyright*

The operating frequency range of sonochemical apparatus is typically up to dozens of kHz. A general working principle, which follows from the above guidelines, relies upon a specific assumption that the size of the cavitation bubble is inversely related to the frequency of ultrasound. Therefore, because the bubble size drops with increasing the ultrasonic frequency and the bubble implosions become less violent, the energy released by each imploding cavitation bubble decreases with the ultrasonic frequency. However, the number of the imploding events increases due to increased number of sound waves passing through the liquid at a higher frequency [60]. One may compare the data obtained in GexSi1-x with lower- and higherfrequency sonochemical processing in dichloromethane at about 25 kHz and 400 kHz, respectively. Etching in HF makes initial single-exponential decay nearly double-exponential. Sonication at 25 kHz slightly slows down the tail component of the decay while the higher-frequency processing at 400 kHz turns the SPV decay

amplitude [74].

**147**

**Figure 7.**

back into nearly single-exponential form.

*2019, Wiley-VCH Verlag GmbH & Co. KGaA.*

*Ultrasonic Processing of Si and SiGe for Photovoltaic Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.96939*

where *Jph* is the photogenerated current density and Δ*n* is the excess carrier density. At low values of Δ*n* ranging from about 10<sup>12</sup> to 1013 cm�<sup>3</sup> , **Figure 7(b)** shows≈ 4 times increase in the lifetime due to ultrasonic processing in chloroform (closed circles compared with open circles).

Regarding the role of amorphous Si coating layer (in sample B) and ultrasonic processing on the photovoltaic behavior, one can distinguish several maps of the surface-distributed SPV amplitudes *U*<sup>0</sup> and decay times *τ* shown in **Figures 8** and **9**.

It is seen in **Figure 8** that both the U(0) and τ distributions narrow but shift to smaller values once the GexSi1-x/Si structure (sample A) has been coated with a-Si layer (sample B). Faster decays in a-Si/GexSi1-x/Si can be accounted for by an increased number of fast recombination canters in sample B due to the deposited amorphous Si layer, which typically reduces the amplitude of the SPV response.

It is also seen in **Figure 8** that the sonication in chloroform allows for improved SPV performance. Indeed, the SPV decays are spread over much longer time scales to enlarge the SPV amplitude *U*<sup>0</sup> up to about 50% can be realized in samples, as observed in appropriate distributions, which are marked by B-sono and A-sono in **Figure 8**.

This effect is not pronounced in the samples sonochemically processed in distilled water; see **Figure 9**. Although the distributions of *U*<sup>0</sup> narrow, the SPV amplitude is itself quenched in the samples treated in distilled water (upper distributions in **Figure 9**). However, similar sonication-affected SPV decay dynamics is observed both in chloroform and distilled water (lower distributions in **Figures 8** and **9**).

The examples in water demonstrate that sonication provides a convenient tool to achieve surface cleaning, as reported previously [7, 73]. In this case, the assumption is based on the fact that (i) the cavitating bubbles are capable of locally removing the surface oxide layer affecting the dangling bonds on the bare Si surface, and (ii) the oxygen and hydrogen, decomposed in water by the presence of local strain fields and elevated temperatures inside a cavitating bubble, can micro-precipitate the Si wafer thus changing the recombination rate. These insights, combined with

#### **Figure 6.**

*(a) Cross-sectional scanning electron microscope (SEM) image of a GexSi1-x on Si layer covered with a 10 nm thick a-Si (sample B). (b) and (c) distributions of Si and Ge atoms near the interface mapped using a scanning auger microscopy technique. Reproduced with permission from Nadtochiy A, Korotchenkov O, Schlosser V. Physica Status Solidi (a). 2019;216:1900154. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.*

*Ultrasonic Processing of Si and SiGe for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.96939*

#### **Figure 7.**

In order to give an illustrating example for the differences in the effective lifetime *τeff* , quasi-steady-state measurements (QSSPC) [72] done in a-Si/GexSi1-x/Si sample are plotted in **Figure 7**. Here, an effective minority carrier lifetime *τeff* is

> *Jph* <sup>¼</sup> <sup>Δ</sup>*neW τeff*

where *Jph* is the photogenerated current density and Δ*n* is the excess carrier

shows≈ 4 times increase in the lifetime due to ultrasonic processing in chloroform

Regarding the role of amorphous Si coating layer (in sample B) and ultrasonic processing on the photovoltaic behavior, one can distinguish several maps of the surface-distributed SPV amplitudes *U*<sup>0</sup> and decay times *τ* shown in **Figures 8** and **9**. It is seen in **Figure 8** that both the U(0) and τ distributions narrow but shift to smaller values once the GexSi1-x/Si structure (sample A) has been coated with a-Si layer (sample B). Faster decays in a-Si/GexSi1-x/Si can be accounted for by an increased number of fast recombination canters in sample B due to the deposited amorphous Si layer, which typically reduces the amplitude of the SPV response. It is also seen in **Figure 8** that the sonication in chloroform allows for improved SPV performance. Indeed, the SPV decays are spread over much longer time scales to enlarge the SPV amplitude *U*<sup>0</sup> up to about 50% can be realized in samples, as observed in appropriate distributions, which are marked by B-sono and A-sono in **Figure 8**. This effect is not pronounced in the samples sonochemically processed in distilled water; see **Figure 9**. Although the distributions of *U*<sup>0</sup> narrow, the SPV amplitude is itself quenched in the samples treated in distilled water (upper distributions in **Figure 9**). However, similar sonication-affected SPV decay dynamics is observed both in chloroform and distilled water (lower distributions in **Figures 8** and **9**).

The examples in water demonstrate that sonication provides a convenient tool to achieve surface cleaning, as reported previously [7, 73]. In this case, the assumption is based on the fact that (i) the cavitating bubbles are capable of locally removing the surface oxide layer affecting the dangling bonds on the bare Si surface, and (ii) the oxygen and hydrogen, decomposed in water by the presence of local strain fields and elevated temperatures inside a cavitating bubble, can micro-precipitate the Si wafer thus changing the recombination rate. These insights, combined with

*(a) Cross-sectional scanning electron microscope (SEM) image of a GexSi1-x on Si layer covered with a 10 nm thick a-Si (sample B). (b) and (c) distributions of Si and Ge atoms near the interface mapped using a scanning auger microscopy technique. Reproduced with permission from Nadtochiy A, Korotchenkov O, Schlosser V. Physica Status Solidi (a). 2019;216:1900154. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.*

density. At low values of Δ*n* ranging from about 10<sup>12</sup> to 1013 cm�<sup>3</sup>

(closed circles compared with open circles).

*Solar Cells - Theory, Materials and Recent Advances*

, (5)

, **Figure 7(b)**

obtained from the given by

**Figure 6.**

**146**

*Current–voltage curve (a) and variation of the effective lifetime τeff with excess carrier density* Δ*n (b) for sample B before (open circles) and after sonication in chloroform (closed circles). Reproduced with permission from Nadtochiy A, Korotchenkov O, Schlosser V. Physica Status Solidi (a). 2019;216:1900154. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.*

complementary ultrasonic techniques that employ reactant solutions, make a significant contribution to developing a detailed picture of ultrasonic processing.

With respect to the reactant solutions that can be used, recent investigations report that SPV signal in Si can be significantly enhanced, by almost an order of magnitude, due to ultrasonic treatments in dichloromethane. Similar effect in CH2Cl2 can be observed for GexSi1-x surfaces exhibiting a 50% increase in the SPV amplitude [74].

The operating frequency range of sonochemical apparatus is typically up to dozens of kHz. A general working principle, which follows from the above guidelines, relies upon a specific assumption that the size of the cavitation bubble is inversely related to the frequency of ultrasound. Therefore, because the bubble size drops with increasing the ultrasonic frequency and the bubble implosions become less violent, the energy released by each imploding cavitation bubble decreases with the ultrasonic frequency. However, the number of the imploding events increases due to increased number of sound waves passing through the liquid at a higher frequency [60].

One may compare the data obtained in GexSi1-x with lower- and higherfrequency sonochemical processing in dichloromethane at about 25 kHz and 400 kHz, respectively. Etching in HF makes initial single-exponential decay nearly double-exponential. Sonication at 25 kHz slightly slows down the tail component of the decay while the higher-frequency processing at 400 kHz turns the SPV decay back into nearly single-exponential form.

#### **Figure 8.**

*Probability of occurrences of particular values of the SPV amplitude U*<sup>0</sup> *and decay time τ in samples a (GexSi1-x/Si) and B (a-Si/GexSi1-x/Si), which are measured by surface mappings of the SPV decays. The distributions marked by "sono" are taken after sonochemical treatment in chloroform during 1 min. Reproduced with permission from Nadtochiy A, Korotchenkov O, Schlosser V. Physica Status Solidi (a). 2019;216:1900154. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.*

The likely mechanism that has come to describe the observations is based on that both chloroform and dichloromethane can act as carbon sources. Being decomposed into hydrocarbon species due to extreme conditions in the solvents and at the etchant/ solid interfaces, the sonicated reactants seem to saturate the dangling bonds revealed on the surface of Si and GexSi1-x alloys and hence to passivate the surface [14].

This is in accord with previous reports on the thermal decomposition of chloroform, which results in by-products of CCl2, C2Cl4, Cl, H and HCl. When they react with metal (M) atoms, the reactions pathways are [75, 76].

$$\text{CHCl}\_3(+\text{M}) \to \text{CCl}\_2 + \text{HCl} \,(+\text{M}), \tag{6}$$

CCl2 þ M ! CCl þ Cl þ M, (8) CCl2 þ CCl2 þ M ! C2Cl4 þ M, (9) HCl þ M ! H þ Cl þ M, (10)

The first reaction step given by Eq. (6) is the decomposition of CHCl3, which is

*Probability of occurrences of particular values of the SPV amplitude U*<sup>0</sup> *and decay time τ in samples a (GexSi1 x/Si) and B (a-Si/GexSi1-x/Si), which are measured by surface mappings of the SPV decays. The distributions marked by "sono" are taken after sonochemical treatment in distilled water during 1 min. Reproduced with permission from Nadtochiy A, Korotchenkov O, Schlosser V. Physica Status Solidi (a). 2019;216:1900154.*

The usual analysis approach for high temperatures achieved during the sonica-

followed by secondary decomposition reactions in Eqs. (7)–(10).

*Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.*

*Ultrasonic Processing of Si and SiGe for Photovoltaic Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.96939*

**Figure 9.**

**149**

tion process involves steps of radical formation, e.g., C2 radicals:

$$\text{CCl}\_2 + \text{CCl}\_2 \rightarrow \text{C}\_2\text{Cl}\_2 + \text{Cl} + \text{Cl},\tag{7}$$

*Ultrasonic Processing of Si and SiGe for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.96939*

#### **Figure 9.**

The likely mechanism that has come to describe the observations is based on that both chloroform and dichloromethane can act as carbon sources. Being decomposed into hydrocarbon species due to extreme conditions in the solvents and at the etchant/ solid interfaces, the sonicated reactants seem to saturate the dangling bonds revealed on the surface of Si and GexSi1-x alloys and hence to passivate the surface [14].

*Probability of occurrences of particular values of the SPV amplitude U*<sup>0</sup> *and decay time τ in samples a (GexSi1-x/Si) and B (a-Si/GexSi1-x/Si), which are measured by surface mappings of the SPV decays. The distributions marked by "sono" are taken after sonochemical treatment in chloroform during 1 min. Reproduced*

*with permission from Nadtochiy A, Korotchenkov O, Schlosser V. Physica Status Solidi (a).*

*2019;216:1900154. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.*

*Solar Cells - Theory, Materials and Recent Advances*

This is in accord with previous reports on the thermal decomposition of chloroform, which results in by-products of CCl2, C2Cl4, Cl, H and HCl. When they react

> CHCl3ð Þ! þM CCl2 þ HCl ð Þ þM , (6) CCl2 þ CCl2 ! C2Cl2 þ Cl þ Cl, (7)

with metal (M) atoms, the reactions pathways are [75, 76].

**Figure 8.**

**148**

*Probability of occurrences of particular values of the SPV amplitude U*<sup>0</sup> *and decay time τ in samples a (GexSi1 x/Si) and B (a-Si/GexSi1-x/Si), which are measured by surface mappings of the SPV decays. The distributions marked by "sono" are taken after sonochemical treatment in distilled water during 1 min. Reproduced with permission from Nadtochiy A, Korotchenkov O, Schlosser V. Physica Status Solidi (a). 2019;216:1900154. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.*

$$\text{CCl}\_2 + \text{M} \rightarrow \text{CCl} + \text{Cl} + \text{M},\tag{8}$$

$$\text{CCl}\_2 + \text{CCl}\_2 + \text{M} \rightarrow \text{C}\_2\text{Cl}\_4 + \text{M},\tag{9}$$

$$\text{HCl} + \text{M} \rightarrow \text{H} + \text{Cl} + \text{M}, \tag{10}$$

The first reaction step given by Eq. (6) is the decomposition of CHCl3, which is followed by secondary decomposition reactions in Eqs. (7)–(10).

The usual analysis approach for high temperatures achieved during the sonication process involves steps of radical formation, e.g., C2 radicals:

$$\text{CCl} + \text{CCl} \rightarrow \text{C}\_2 + \text{Cl} + \text{Cl},\tag{11}$$

$$\text{C}\_2\text{Cl}\_2 \rightarrow \text{C}\_2 + \text{Cl} + \text{Cl},\tag{12}$$

removal of H from the coating layers due to ultrasonic processing, supporting the

*FTIR spectra of samples GexSi1-x/Si (curve 1) and a-Si/GexSi1-x/Si (3), taken before ultrasonic processing and the ones obtained after the treatment in chloroform – Spectra 2 and 4, respectively. Reproduced with permission from Nadtochiy A, Korotchenkov O, Schlosser V. Physica Status Solidi (a). 2019;216:1900154. Copyright*

Two techniques related to ultrasonic cleaning of Si wafers and sonochemical modification of Si, SiGe and a-Si/SiGe surfaces in hydrocarbon solutions of chloro-

In spite of our lack of knowledge of the exact sonication mechanisms even in distilled water, this research field can be considered to be among potential candidates to develop a new class of environmental friendly cleaning steps in siliconbased technologies. Some progress has recently been made in understanding a unique potential capability of sonicated water in Si wafer cleaning processes. The underlying mechanisms related to the fundamental properties of cavitation and bubble implosion events, the role of a thin interphase layer between the bubble and the surface placed in the sonicated liquid can offer new far-reaching implications

It is demonstrated that organic particle contaminants are effectively removed during the kHz-frequency sonication of crystalline Si wafers in distilled water over the first 40–60 min. When ultrasonically processing the wafers for treatment times

bonds at the air/oxide and oxide/wafer interface can be activated. That affects barriers of the free carrier migration at the interfaces, as revealed by the current– voltage curves, and acts as recombination centers, accelerating the surface photovoltage decays. A healing of the bonds may occur at longer cleaning times (from 60 to 120 min) with a partial recovery of the interfaces and a consequent reversing of the observed changes. The potential of using distilled water in environmental friendly and non-toxic ultrasonic cleaning step in crystalline Si wafer

, the dangling

form (CHCl3) and dichloromethane (CH2Cl2) are outlined.

*Ultrasonic Processing of Si and SiGe for Photovoltaic Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.96939*

and importance for heterogeneous liquid–solid systems.

less than ≈60 min at the peak acoustic intensity of about 400 W/cm<sup>2</sup>

pictorial view given in **Figure 10**.

*2019, Wiley-VCH Verlag GmbH & Co. KGaA.*

**6. Conclusions**

**Figure 11.**

preparation is addressed.

**151**

Therefore, employing chloroform CHCl3 or dichloromethane CH2Cl2, one gets the Si-H and C-Cl bonds that react yielding C-H species. This, in turn, resembles the chlorination/alkylation process that forms Si–alkyl converting Si–H into Si–CnH2n+1 (*n*≥ 1). The alkyl chains on Si surfaces are known to provide low surface recombination velocities [77] thus featuring effective Si surface passivation [78].

In the model presented in **Figure 10**, presumed chemical reactions for the sonochemical surface passivation are made available. The above analysis assumes that the Si–H bond on the surface breaks up at high local temperatures and pressures inside the cavitation bubble. This produces highly reactive Si and Ge dangling bonds, as shown in **Figure 10(a)**. Being short-lived, they quickly react with the sonicated chloroform molecules. Next, molecular hydrocarbon and chlorine atoms cover the a-Si or GexSi1-x surface, as shown in **Figure 10(b)**. The wavy arrow illustrates that Si atoms can be released from the surface due to carbon atoms decomposed from chloroform (or dichloromethane). Finally, these carbon atoms at the surface create Si–C bonds and dangling carbon bonds being then saturated by the atoms of H and Cl. Some of them can meet activated carbon-containing molecules to form Si–C bonds.

In order to obtain the signatures of the chemical constituents, Fourier transformed-infrared (FTIR) spectroscopy is usually applied. FTIR transmittance spectra are shown in **Figure 11**. Among prominent infrared absorption peaks related to the Si–Si, Si–O, Ge–Ge, Ge–O and Ge–O–Si vibration modes, there are resolved bulk-like Si–H and Si–H2 stretching modes at about 2000 and 2090 cm�<sup>1</sup> , respectively, as well as a weak shoulder near 1880 cm�<sup>1</sup> related to Ge–H vibrations [79, 80]. These results indicate that Si-H hydrides are present in the deposited a-Si and GexSi1-x films.

One also finds a spectral feature at about 670 cm�<sup>1</sup> (arrow in the left-hand panel of **Figure 11**). This obviously strengthens in the hydrogenated a-Si film (spectrum 3). To account for this enlargement, one has to assume that this feature is related to the hydrogen complexes. In clear accord, the wagging modes near 640 cm�<sup>1</sup> can be due to three bonding units of Si–Hn (n = 1, 2, 3) [81]. It is seen in spectrum 4 of **Figure 11** that the sonication quenches the 670 cm�<sup>1</sup> , which is indicative of the

#### **Figure 10.**

*How to passivate SiGe surface using chloroform reactants H, Cl, C, CCl2, HCl released in Eqs. (6)–(12). These remove Si atoms on the surface and saturate the dangling bonds. Reproduced with permission from Nadtochiy A, Korotchenkov O, Schlosser V. Physica Status Solidi (a). 2019;216:1900154. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.*

*Ultrasonic Processing of Si and SiGe for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.96939*

#### **Figure 11.**

CCl þ CCl ! C2 þ Cl þ Cl, (11) C2Cl2 ! C2 þ Cl þ Cl, (12)

, respec-

, which is indicative of the

Therefore, employing chloroform CHCl3 or dichloromethane CH2Cl2, one gets the Si-H and C-Cl bonds that react yielding C-H species. This, in turn, resembles the chlorination/alkylation process that forms Si–alkyl converting Si–H into Si–CnH2n+1 (*n*≥ 1). The alkyl chains on Si surfaces are known to provide low surface recombi-

nation velocities [77] thus featuring effective Si surface passivation [78].

*Solar Cells - Theory, Materials and Recent Advances*

cules to form Si–C bonds.

and GexSi1-x films.

**Figure 10.**

**150**

*Wiley-VCH Verlag GmbH & Co. KGaA.*

In the model presented in **Figure 10**, presumed chemical reactions for the sonochemical surface passivation are made available. The above analysis assumes that the Si–H bond on the surface breaks up at high local temperatures and pressures inside the cavitation bubble. This produces highly reactive Si and Ge dangling bonds, as shown in **Figure 10(a)**. Being short-lived, they quickly react with the sonicated chloroform molecules. Next, molecular hydrocarbon and chlorine atoms cover the a-Si or GexSi1-x surface, as shown in **Figure 10(b)**. The wavy arrow illustrates that Si atoms can be released from the surface due to carbon atoms decomposed from chloroform (or dichloromethane). Finally, these carbon atoms at the surface create Si–C bonds and dangling carbon bonds being then saturated by the atoms of H and Cl. Some of them can meet activated carbon-containing mole-

In order to obtain the signatures of the chemical constituents, Fourier transformed-infrared (FTIR) spectroscopy is usually applied. FTIR transmittance spectra are shown in **Figure 11**. Among prominent infrared absorption peaks related to the Si–Si, Si–O, Ge–Ge, Ge–O and Ge–O–Si vibration modes, there are resolved

bulk-like Si–H and Si–H2 stretching modes at about 2000 and 2090 cm�<sup>1</sup>

**Figure 11** that the sonication quenches the 670 cm�<sup>1</sup>

tively, as well as a weak shoulder near 1880 cm�<sup>1</sup> related to Ge–H vibrations [79, 80]. These results indicate that Si-H hydrides are present in the deposited a-Si

*How to passivate SiGe surface using chloroform reactants H, Cl, C, CCl2, HCl released in Eqs. (6)–(12). These remove Si atoms on the surface and saturate the dangling bonds. Reproduced with permission from Nadtochiy A, Korotchenkov O, Schlosser V. Physica Status Solidi (a). 2019;216:1900154. Copyright 2019,*

One also finds a spectral feature at about 670 cm�<sup>1</sup> (arrow in the left-hand panel of **Figure 11**). This obviously strengthens in the hydrogenated a-Si film (spectrum 3). To account for this enlargement, one has to assume that this feature is related to the hydrogen complexes. In clear accord, the wagging modes near 640 cm�<sup>1</sup> can be due to three bonding units of Si–Hn (n = 1, 2, 3) [81]. It is seen in spectrum 4 of

*FTIR spectra of samples GexSi1-x/Si (curve 1) and a-Si/GexSi1-x/Si (3), taken before ultrasonic processing and the ones obtained after the treatment in chloroform – Spectra 2 and 4, respectively. Reproduced with permission from Nadtochiy A, Korotchenkov O, Schlosser V. Physica Status Solidi (a). 2019;216:1900154. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.*

removal of H from the coating layers due to ultrasonic processing, supporting the pictorial view given in **Figure 10**.
