Role of Hydrogen in the Preparation of Amorphous Silicon Nanowires

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C: Physical Processes in Nanomaterials and Nanostructures

Role of Hydrogen in the Preparation of Amorphous Silicon Nanowires by Metal Assisted Chemical Etching Sergio Pinilla, Rocio Barrio, Nieves González, rafael Perez Casero, Francisco M. Marquez-Linares, José María Sanz, and Carmen Morant J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05332 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 8, 2018

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The Journal of Physical Chemistry

Role of Hydrogen in the Preparation of Amorphous Silicon Nanowires by Metal Assisted Chemical Etching Sergio Pinilla1, Rocio Barrio2, Nieves González2, Rafael Pérez Casero1, Francisco Márquez3, José M. Sanz1, Carmen Morant1* 1

Department of Applied Physics and Nicolás Cabrera Institute, Universidad Autónoma

de Madrid, 28049 Madrid, Spain 2

3

Photovoltaic Solar Energy Unit, CIEMAT, 28040 Madrid, Spain School of Natural Sciences and Technology, Universidad del Turabo, Gurabo,

PR00778, USA

Abstract: Hydrogenated amorphous silicon (a-Si:H) has recently proved to be a suitable base material for the synthesis of silicon nanowires (SiNWs) by metal assisted chemical etching (MACE). The etching procedure on this material shows an extraordinary sensitivity to slight compositional changes and, although dopant influence on the process has been previously addressed, little is known on the role of hydrogen. In this paper, we have studied the behavior of the MACE on a-Si:H films with different hydrogen contents and bonds configurations. As-grown films were studied by Raman, Fourier-Transform Infrared spectroscopy and Ion Beam analysis to obtain a complete description of the material composition. Additionally, these results were further correlated with the morphology and characteristics of the obtained SiNWs, showing that

1 Environment ACS Paragon Plus

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the material stability under the MACE is reduced as the bond configuration is shifted from monohydrides to polyhydrides. The effect of bond configuration has an extraordinary importance regarding the material application, as it is intrinsically related to the content of hydrogen, which simultaneously controls the optical properties of the material. This study proves that bond configuration also affects to the nanostructuration, which should be considered in future devices based on this material.

Introduction In the last years, semi-conductive one dimensional structures have been intensively investigated because of their promising properties and applications1.

Specifically,

silicon nanowires (SiNWs) have shown extraordinary performance in solar cells2,3, energy storage devices4,5, chemical sensors6,7, field effect transistors8 and other devices9. Among the synthesis routes for the SiNWs production, the Metal Assisted Chemical Etching (MACE) stands out for its high controllability, quality and scalability10. Briefly explained, the MACE method consists on the localized etching of silicon where certain catalyst particles were previously deposited. In general, this method requires a crystalline Si (c-Si) substrate to produce the SiNWs11, what makes difficult its application when the SiNWs have to be grown on non-silicon substrates. In order to expand the use of this method for further practical applications, a solution would be the SiNWs growth over non-crystalline Si layers, which could be deposited over any substrate. This approach was recently explored by Douani et al.12, demonstrating that despite the structural differences between amorphous and crystalline silicon, the MACE method can produce SiNWs on certain hydrogenated amorphous Si (a-Si:H) layers. The study also revealed that the process is very sensitive to dopants, imperfections and other slight compositional changes. However, there is no report available focusing on the role of hydrogen in the MACE. The required quantity, the


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preferred incorporation mechanism and its influence in the etching are still unknown parameters in this novel process. Herein, we report a comprehensive study on the influence of the hydrogen in the SiNWs formation by MACE onto a-Si:H substrates. Layers with compositional differences were systematically studied by Scanning Electron Microscopy (SEM), FourierTransform Infrared spectroscopy (FTIR), Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Rutherford Backscattering Spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA), stablishing a correlation between the material parameters and their effect on the SiNWs formation. Additionally, as a proof of concept, the optimized MACE procedure developed along the work, was applied for the growth of SiNWs on non-silicon substrates, using commercial Cu foils.

Experimental Hydrogenated amorphous silicon (a-Si:H) growth The growth of hydrogenated amorphous silicon (a-Si:H) was performed by two techniques: Plasma Enhanced Chemical Vapor Deposition (PECVD) and Radio Frequency Magnetron Sputtering (RF-MS). The growth mechanisms of both techniques are quite similar due to the presence of reactive plasma but differs in certain points that can affect the composition, morphology and quality of the films. PECVD a-Si:H films are mainly grown by an absorption-reaction mechanism of SiH3 molecules

13

. These

species are generated in the plasma by the decomposition of the SiH4 precursor, being 14

the most abundant molecules in the plasma and the main silicon source

. The SiH3

molecules are incorporated into the growing film through the dangling bonds of the surface, which are produced by hydrogen abstraction and desorption

15

. Finally, the

hydrogen concentration in the films is determined by the equilibrium between the



hydrogen abstraction and incorporation, being this equilibrium dependent of the temperature. This process is further illustrated in scheme 1A. On the other hand, RF-MS adds to the previously discussed mechanism, a growth procedure based on the impact of fast particles on the surface

16

. Si and SiH particles

impact and adhere to the surface forming the film, while hydrogen atoms are implanted in the film. The process is represented in scheme 1B, being the growth mechanism in RF-MS a combination of scheme 1A and 1B processes.

A)

Absorption - Reaction mechanisms

SiH3

Sticking to dangling bonds

H Abstraction SiH4 (H ) H2 2

H Abstraction (SiH4)

Hydrogen

c-Si substrate

Silicon

B)

Momentum driven mechanisms Si / SiH

H H2 Abstraction (H2)

H Implantation

Hydrogen Silicon

c-Si subst rate

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Scheme 1 Diagram of the surface processes involved in the growth of the a-Si:H films. A) Absorption-reaction mechanisms usually present in a-Si:H reactive plasma depositions. B) Growth procedure based on fast particles impacts, present in sputtering depositions. Adapted from references 13 and 16.

The differences between these techniques are notorious, and usually produce layers with different hydrogen bonding configurations (monohydrides, SiH, and polyhydrides,


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SiHx) which is represented by the microstructural parameter (Rfactor)

17–20

. Specifically,

the Rfactor indicates the ratio of the hydrogen incorporated in the form of polyhydrides with respect to the total hydrogen content ( = [ ]⁄([ ] + [ ])), which is considerably important for some applications

17

and it tends to be higher in RF-MS

samples 21. The growth of the a-Si:H films was performed on boron p-doped monocrystalline Si wafers (100) of 300 µm thickness and resistivity in the range of 1-3 Ωcm. Before the deposition, the Si substrates were cleaned with HF 5% for 2 minutes. The copper foil used as substrate for the a-Si:H was 9 µm thick and purity 99.99% (MTI corporation), the standard for the Lithium Ion Batteries anodes. The deposition of the a-Si:H films by RF-MS was accomplished using an intrinsic silicon target (Purity 99.999%, 3 mm thickness, supplied by Goodfellow). The substrate temperature was maintained at ca. 200º C. The plasma was established with Ar and H2 (both Premier Plus 99.9992%, Carburos Metálicos) in the H2/Ar proportion of 1:3 in volume with a flow rate of 3 sccm and a chamber pressure of 9 mTorr. Under those conditions, the growth rate was of 0.8 Å/s. On the other hand, the a-Si:H films deposited via PECVD were grown in a system operated at 13.56 MHz and a power of 1W. The chamber pressure was maintained at 400 mTorr during the deposition with an inlet SiH4 (ultraplus 99.999%, Praxair) flow of 20 sccm. To obtain samples with different hydrogen contents, the temperature of the substrate was varied between 80 ºC and 266 ºC. MACE process The MACE process used in this work entailed an Ag catalyst and consisted of two consecutive baths, AgNO3/HF and H2O2/HF. The HF concentration of the first bath was 4.8 M, while AgNO3 concentration ranged from 0.01 M to 0.001 M to investigate its



influence on the growth of the a-SiNWs. The second bath consisted of HF and H2O2, at concentrations of 4.8 M and 0.2 M, respectively. The immersion time for the first bath was fixed to 90 seconds, while for the second bath it ranged from 30 seconds to 3 minutes, depending on the SiNWs desired length. Once the MACE procedure was completed, the samples were rinsed with deionized (DI) water and dried in a desiccator overnight. Characterization Techniques The infrared absorption spectra of the a-Si: H films were obtained by a Perkin-Elmer Lambda 100 FT-IR spectrophotometer. The energy gaps and the thicknesses of the films were evaluated from spectral transmittance and reflectance measurements. Spectra were acquired from 300 to 1250 nm, using a UV/Visible/NIR Perkin-Elmer Lambda 1050 spectrophotometer. Optical gaps were calculated using the method described by Tauc et al.22 for indirect optical transitions. RBS and ERDA were carried out in a Tandem Cockcroft Walton accelerator at the Centre for Microanalysis of Materials (CMAM) at Universidad Autónoma de Madrid (UAM). Both techniques were simultaneously performed with helium ions at an incident energy of 2 MeV, and a grazing incidence of 15º. The ERDA detector was at 30º relative to the incidence direction, while the RBS detector was at 170º. As absorber for the ERDA, an 8.5 µm Mylar film was used. Field Emission Scanning Electron Microscopy (FESEM) images were obtained with a Philips XL30 S-FEG. XPS spectra were acquired using the Mg Kα radiation from a PHI 04-548 X-ray source and a PHI 15-255GAR energy analyzer with a pass energy of 50 eV. The binding energies were corrected using the C 1s transition at 284.8 eV as reference.


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Raman spectra were recorded on a Renishaw spectrometer (In Via, UK) with an excitation wavelength of 514 nm produced by an Ar+ laser. The 500–3500 cm−1 spectral range was evaluated using 20 scans for each measurement. A sharp line of c-Si at 520 cm−1 was taken as the reference in the Raman shift scale.

Results and discussion The morphology of the a-Si:H films deposited on c-Si by RF-MS and PECVD (at a substrate temperature of ca. 200 ºC), was characterized by FESEM, figure 1. As can be seen there, the samples grown by PECVD show a compact appearance, figure 1A, while the samples grown by RF-MS are less dense, revealing a rough microstructure, figure 1B.

Figure 1 FESEM images of a-Si:H /c-Si by PECVD, A), and RF-MS, B).

After deposition, the films were subjected to the MACE process described in the experimental section. Due to the fact that the two step MACE had never been applied before in a-Si:H films, the first stage of this work was the optimization of the etching parameters. One of the most sensitive parameters of the MACE is the amount of catalyst deposited on the Si surface. It has been shown to significantly influence the reactivity of the etching12, severely affecting the SiNW growth. For this reason, four different AgNO3 concentrations were used in both RF-MS and PECVD samples (0.01 M, 0.005 M, 0.0025 M and 0.001 M).



For the a-Si:H films grown by RF-MS, the four concentrations resulted on the immediate dilution of the amorphous silicon layer, only producing SiNWs in the c-Si substrate, figure 2 A-D.

Figure 2 FESEM images of a-Si:H/c-Si after MACE procedure. The used deposition technique (PECVD or RF-MS), as well as the AgNO3 concentration, are displayed in each image.

In the case of the a-Si:H layers grown by PECVD, we found a similar result as in RFMS samples at high AgNO3 molarity (i.e. 0.01 M), figure 2E. Reversely, at lower concentrations (0.005 M, 0.0025 M and 0.001 M), figure 2F, 2G and 2H, the MACE successfully produced SiNWs from the a-Si:H (a-SiNWs). In these images, it is also noticeable that the a-SiNWs obtained with the 0.0025 M concentration, showed a much denser growth than that obtained with the 0.005 M concentration. On the other hand, when comparing the results from concentrations 0.0025 M and 0.001 M in cross sectional FESEM images, is not possible to appreciate great differences in the nanowire morphology beyond a slight increase of diameter. However, in top-view FESEM images, figure S1, it can be seen that while 0.0025 M sample show well defined aSiNWs, the 0.001 M sample consists in isolated pits all over the surface, with a poorly defined nanowire structure. For this reason, the AgNO3 concentration of 0.0025 M was


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established as the optimal, producing a-SiNWs with an average diameter of 80 ± 30 nm, and a size distribution shown in figure S2. The above results corroborates that the etching reactivity increases with the AgNO3 concentration12. However, in our case, the higher reactivity not only increases the etching rate, but it leads to a very aggressive secondary etching23 that is much faster in the a-Si:H layer than in the c-Si, and can even remove it. Furthermore, it is also observed that the a-Si:H films deposited by PECVD and RF-MS behave differently when subjected to MACE, suggesting that compositional and structural differences prevent the growth of a-SiNWs in the RF-MS films. In order to investigate the differences between the films and their impact on the MACE process, the as-deposited samples were characterized by FTIR. Two samples, one synthesized by RF-MS and the other by PECVD, were prepared with the same thickness (250 nm). Figure 3 shows the respective FTIR spectra of these samples, as labeled. The “rocking and wagging” band (630 cm-1), the “bending” SiH2 band (ca. 875 cm-1), the “stretching” SiOx band (1070 cm-1) and the “stretching” bands for the silicon monohydrides and polyhydrides (2000 cm-1 and 2100 cm-1, respectively) are indicated by a dashed line and labeled. PECVD sample shows intense and well defined “rocking and wagging” and “stretching” bands at 630 cm-1 and 2000 cm-1 respectively. The “rocking and wagging” band, contains all possible Si-H bondings, being directly related to the total amount of the hydrogen in the samples24. The analysis of this band (through the BCC method24 with the corrected proportional factors of Langford et al.25) in the PECVD sample, revealed an hydrogen content (cH) of 12 %. On the other hand, the “stretching” band shows slight changes in their central frequency depending on the nature of the Si-H bond. If the bond corresponds to a monohydride (Si-H), the central frequency appears at



ca. 2000 cm-1 and if it comes from a polyhydride (Si-Hx), it is located at ca. 2100 cm-1 25

. This shifting of frequencies allows the identification of bonding configuration by the

analysis of the “stretching” band in both components. Applying this analysis to the PECVD sample, a very low Rfactor (defined as I2100/ (I2100+I2000)26) close to 0.1 is obtained. For the RF-MS sample, the “rocking and wagging” band is very weak, which would normally indicate a low hydrogen content. On the other hand, the stretching band is slightly more visible, although only the component centered at 2100 cm-1 is appreciable. This suggests that hydrogen is incorporated mainly in the form of polyhydrides, forming microvoids and anomalous microstructures27. Usually, a high contribution of the 2100 cm-1 component is associated with a very high H2 incorporation25, which clearly contradicts the observations made from the “rocking and wagging band”. In fact, when trying to calculate the cH of this spectrum, the mismatch between both bands does not allow obtaining a reliable value of the cH of the sample.

A

Bending SiH2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Stretching SiH

Rocking and Wagging SiH

Ι) PECVD

Stretching SiOX

500

1000

ΙΙ) RF-MS with H2

Stretching SiHx

2000

2500

Frequency (cm-1)

Figure 3. FTIR spectra of a-Si:H/c-Si samples by PECVD and RF-MS. Dashed lines indicate the different bands: rocking and wagging, SiH2 bending, SiOx stretching, and the stretching for the monohydrides (SiH) and polyhydrides (SiHx).


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In order to further clarify the information of the hydrogen content of the films, Ion Beam Analysis (IBA) (e.g. RBS and ERDA), was performed on the PECVD and RFMS samples. Results are displayed in figure 4, where, in the ERDA spectra, figure 4A, a carbon calibration sample containing a known amount of hydrogen (32%) is also included. Interestingly, the ERDA spectrum of the RF-MS sample, figure 4A, shows a stronger hydrogen signal than the obtained in the PECVD sample, indicating a higher amount of hydrogen incorporated into the film. Additionally, the flatness of the signals in both samples indicates a homogenous distribution of hydrogen along the thicknesses. 1,2K

5K Reference sample RF-MS PECVD

A 4K

Yield (a.u.)

Yield (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,8K

0,4K

RF-MS PECVD

B O

3K Si 2K 1K Si from film

Si from substrate 0,0K 200

400

600

800

0K 300

Energy (KeV)

600

900

1200

Energy (KeV)

Figure 4. ERDA (A) and RBS (B) spectra of RF-MS (red) and PECVD (green) a-Si:H/c-Si samples. A reference sample with 32% hydrogen is also included in the ERDA measurements.

The RBS spectra of the a-Si:H films grown by PECVD and RF-MS, figure 4B, also show substantial differences in their signals. The Si signal from the a-Si:H films (comprised between 850 KeV and 1100 KeV) is considerably higher in the PECVD sample than in the RF-MS. This indicates a higher atomic percentage of silicon in the aSi:H film grown by PECVD. A similar observation can also be done in the interface between the RF-MS deposited film and its c-Si substrate (ca. 850 KeV), where the sharp



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increase of the signal indicates a sudden change in composition to a richer Si compound (pure c-Si from substrate). On its own, the incorporated hydrogen obtained by ERDA would not be able to explain the difference in the Si content of the films. Therefore, the presence of another element in the film grown by RF-MS should be considered. In RBS spectra, at ca. 740 KeV, we can find a substantial increase of the signal only observable in the RF-MS sample. The energy at which the signal appears indicates that it comes from the recoil of the oxygen atoms present in the deposited film. This can be further proved by the length of the oxygen signal (from ca. 740 KeV to 470 KeV), which is coincident with the thickness of the a-Si:H film deposited by RF-MS. On the contrary, in the PECVD sample, no oxygen was detected in the resolution limit of the technique (roughly 5%). From these observations, we can conclude that Si, O and H are present in RF-MS film; however, in the PECVD film only Si and H were detected. By adjusting the RBS and ERDA spectra using the SIMNRA software, the atomic composition of the films was obtained (see Table 1). Table 1 Atomic composition of RF-MS and PECVD a-Si:H samples obtained by IBA techniques.

Silicon

Oxygen

Hydrogen

RF-MS

52%

31%

17%

PECVD

90%

0%

10%

The data presented in Table 1 show that although the RF-MS film possesses a higher percentage of hydrogen, the oxide content is significant. It is important to point out that the presence of oxygen in the film does not come from the film growth, since the XPS


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measurements made just after the deposition of the film, figure S3, shows a very small oxygen contribution. The composition of the a-Si:H film deposited by RF-MS as determined by RBS/ERDA along with the analysis made by FTIR, suggest that most of the hydrogen is incorporated in the form of polyhydrides (SiHx). These polyhydrides show a very poor passivation against oxidation, which results ineffective even under the action of a mild oxidizer as the air, producing, as revealed by RBS, an oxygen incorporation throughout the volume of the film. Thus, when the sample is introduced in a much more reactive media, as the HF/H2O2 bath, an intense and anisotropic secondary etching is activated even in the absence of the catalytic action of the Ag nanoparticles (AgNPs)10,23. This etching affects the side-walls of the nanowires leading to the removal of the whole aSi:H material. The above discussed not only explains the results of figure 2 for the RF-MS samples, but also the fact that the secondary etching is intensified by a higher polyhydride content, perfectly explains the differences between RF-MS and PECVD samples under the MACE. To verify if the proposed model is general and applicable to all a-Si:H films rich in silicon polyhydrides, we produced a set of PECVD samples with different hydrogen percentages. Through this methodology, we can observe how small variations in the film composition influence the growth of a-SiNW by MACE. Five a-si:H films were deposited by PECVD in a c-Si substrate. The deposition was carried out at different temperatures ranging from 80 to 270 ºC so that the hydrogen concentration in the films could be varied between 9% and 23% (see table S1). The optical band gap and refractive index were also measured and displayed in table S1, showing good agreement with already published results.17,27.



Figure 5 shows the FTIR spectra of the as-deposited films as a function of the growth temperature and hydrogen content (determined from the “rocking and waging” band at 630 cm-1). As can be seen there, as the hydrogen content increases the polyhydride component of the stretching band becomes stronger, being in the sample with cH of 23% the predominant feature. Additionally, the increase of this component is accompanied by the appearance of a significant band of SiH2 bending at 890 cm-1. These results unambiguously indicate that the higher incorporation of hydrogen is associated with a predominant SiHX bonding configuration in the a-Si:H films, agreeing with previous researches25. Rocking Bending Si-H2

A Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500

750

Stretching

266 ºC

9%

236 ºC

11%

198 ºC

13%

140 ºC

19%

80 ºC

23%

1000

2000

2250

2500

-1

Frequency (cm ) Figure 5 FTIR spectra of a-Si:H samples grown by PECVD with increasing hydrogen content, from 9% to 23%, and the corresponding growth temperature.

To quantify the evolution of the polyhydride component, the stretching band was analyzed in terms of its two components (SiH and SiHx bending at 2000 and 2100 cm-1, respectively), figure 6 A to E. Then, the normalized intensity of both contributions was plotted as a function of the hydrogen content, figure 6 F. Surprisingly, in this figure, the polyhydride proportion of the incorporated hydrogen (Rfactor), follows a linear trend with the raw hydrogen content. This result further confirms the increase of the polyhydride


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proportion as the cH increase, and also provide us a simple way to obtain the Rfactor directly from the hydrogen content (under our reported conditions).

2100

cH = 13%

1900

2000

2100

1900

Frequency (cm-1)

Frequency (cm-1)

2000

2100

Frequency (cm-1) 1,0

E Intensity (a.u.)

cH = 19%

1,0

F

cH = 23% Si-H proportion

D

0,8

0,8

0,6

0,6

0,4

0,4

0,2

0,2

Slope 0,042 ± 0,004 1900

2000

2100

Frequency (cm-1)

1900

2000

2100

0,0

8%

12%

Frequency (cm-1)

16%

20%

24%

Si-HX proportion (R-factor)

2000

C

cH = 11%

Intensity (a.u.)

Intensity (a.u.)

Si-H Si-HX

1900

B

cH = 9%

Intensity (a.u.)

A

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,0

cH

Figure 6 Fitting results of the stretching FTIR band for different cH, (A to E). Graph F shows the evolution of the Si-H and Si-HX proportion as a function of the cH.

The characterized films were employed for the growth of a-SiNWs by MACE, using the conditions previously established as optimal (AgNO3 at a concentration of 0.0025 M). For each film, etching times ranging from 15 to 60 seconds were used, obtaining the etching rate by analyzing the cross-sectional FESEM images, figure 7A. The etching rate follows a linear growth with the hydrogen content, a behavior surprisingly coincident with the trend shown by the Rfactor. This behavior suggests that the rate depends on the content of polyhydrides rather than on the raw concentration of hydrogen. The above-mentioned matches well with our previous observations from the RF-MS samples, where the lower material stability under MACE was ascribed to a higher polyhydride content.



Figure 7 MACE etching rate of PECVD a-Si:H samples with different hydrogen content (A) and cross-sectional FESEM images of a-SiNWs after an etching process of 30 seconds (B and C) and 50 seconds (D and E). In purple, the a-Si:H.

Regarding the morphologies of the a-SiNWs obtained through the MACE process from substrates with different cH, it cannot be found substantial differences for short etching times, figure 7B and C. However, when the reaction time is longer and the etching front gets closer to the interface between the c-Si and the a-Si:H, some divergences between the samples with low and high cH (or analogously Rfactor) begin to arise. Figure 7D shows a sample with low cH after 50 seconds of MACE, where almost the entire thickness of the a-Si:H film (approximately 700 nm) has been transformed into aSiNWs. On the other hand, figure 7E shows a sample with high cH that has been subjected to the same MACE process. This sample shows significantly shorter nanowires despite of having a higher etching rate. Moreover, the interface between the c-Si and the a-Si:H has been widely surpassed and the etching has penetrated the c-Si substrate. The explanation behind these observations is similar to that discussed for the a-Si:H deposited by RF-MS. The higher polyhydride content of the sample makes it more vulnerable to the secondary etching, which etches away the a-SiNWs. In the


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present case, the stability of the material is enough to produce a-SiNWs with low etching times, but when the time increases the secondary etching begin to shorten the tips of the SiNWs. This effect is illustrated in scheme 2, where low and high cH (or Rfactor) samples are compared.

Scheme 2 Diagram of etching effect on samples with low and high hydrogen content.

All afore discussed further demonstrates our model, since we have been able to observe how the reactivity of the material gets higher, both in term of etching rate and strength of the secondary etching, as the polyhydride content increases. To conclude the study of the effect of the MACE process on a-Si:H samples, Raman spectra of a-Si:H films were acquired before and after the etching process (see figure 8). Before etching, figure 8 and figure S4, the a-Si:H films show vibrational bands with an asymmetric transverse optical structure (wTO)24 centered at 490 cm-1, slightly shifted from the wTO position of the c-Si at 521 cm-1. Additionally, the spectra of the aSi:H films show a doublet structure from the stretching band (wS)24 that, similarly to FTIR, one component corresponds to monohydrides and the other to polyhydrides. As the polyhydride content increases, the band moves from 2000 cm-1 (w1S) to 2100 cm-1 (w2S), figure S4, reproducing the observations made by FTIR in figure 5.



a-Si:H with 9% hydrogen a-SiNWs with 9% hydrogen wTO

Intensity (a.u.)

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w2TO

wS1

x2

500

1000

2000

Frequency (cm-1) Figure 8 Raman of the starting a-Si:H film (black), and from a-SiNWs obtained by MACE (red). The intensity of the a-Si:H signal has been magnified (x2) in order to compare the features with the spectra of the a-SiNWs sample.

As can be seen in figure 8, after the MACE, the spectrum of the a-SiNWs shows a much higher intensity than the base material due to the SERS effect (Surface-Enhanced Raman Spectroscopy) produced by the AgNPs deposited during the process28. Furthermore, the wTO of the a-SiNWs shows a shifting towards higher frequency positions, reaching 515 cm-1 and acquiring the same central Raman frequency and shape as the crystalline SiNWs29,30. These features are usually ascribed to quantum confinement effects in Si29, being quite notorious to find them in amorphous structures such as the a-SiNWs. This result can be considerably relevant for potential applications in optics and photonics. Finally, figure 8 also shows that after the MACE, the a-SiNWs wS bands retain their positions, implying that the etching does not change the hydrogen content of the a-Si:H or the bond configuration of the nanowires surface.


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In order to further expand the applicability of the herein discussed procedure of aSiNWs growth, the synthesis was performed on non-silicon substrates. Specifically, Cu foils were used as substrates for the deposition of a-Si:H via PECVD. Afterwards, the MACE procedure was applied in the conditions stablished as optimal (AgNO3 concentration of 0.0025 M). Results are shown in figure S5, were it can be seen that a-SiNWs perfectly grow on the pre-deposited a-Si:H on top of the Cu film (figures S5A, B and C), without any substantial difference with their counterpart in c-Si (figure 7). Moreover, the a-SiNW film show a very good adherence with the Cu substrate and a considerably high flexibility (figure S5D), being possible to bend it without damage. This result is a first step towards the use of this material in practical applications, as we have successfully overcome the main limitation of its growth procedure. We have not only revealed the influence of hydrogen in the nanostructuration of a-Si:H but we have also demonstrated that this type of nanowires can be synthetized over substrates different form the c-Si. Further steps of our work will be devoted to the use of these nanowires into the anodes of Lithium Ion Batteries.

Conclusions The influence of the hydrogen on the growth of a-SiNWs by MACE on a-Si:H films has been studied. Throughout this work, we have observed that the way in which hydrogen is incorporated into the amorphous silicon matrix has proven to play a decisive role on whether the etching process successfully produces nanowires or not. Through complete material characterization using FESEM, FTIR, RBS, ERDA, XPS and Raman, we have confirmed that, when the hydrogen is bonded to silicon forming silicon polyhydrides, the reactivity of the silicon dilution by MACE increases dramatically. This sharp



increase of the a-Si:H reactivity under the etching, activates a secondary etching process that does not require the catalytic action of the AgNPs of the MACE and, therefore, produces an anisotropic removal of silicon. As the polyhydride content augments, the stability of the material is reduced and the etching rate gets higher. With the reduction of the stability, the secondary etching begins to act on the side-walls of the as-produced a-SiNWs and shorten them from the tips until the a-Si:H is fully consumed. Additionally, we further confirmed that the MACE etching does not produce any chemical modification of the base material, and that the a-SiNWs display similar quantum confinement effects as the c-SiNWs. Finally, it was demonstrated that the described growth procedure can be extended to non-silicon substrates, allowing the use of the a-SiNWs in practical applications like lithium ion batteries or other thin film devices. Corresponding Author *E-mail: [email protected]. Phone: +34 914974924. Fax: ++34 914973969

Acknowledgments We acknowledge the following funding support: MINECO research Project ENE201457977-C2-1-R and ENE2016-78933-C4-3-R, Fundación Iberdrola España, through its Call for Research on Energy and the Environment Grants, the U.S. Department of Energy, through the Massie Chair Project at Universidad del Turabo, and the U.S. Department of Defence under Grant W911NF-14-1-0046. The technical assistance of I. Poveda from “Servicio Interdepartamental de Investigacion, SIdI” at UAM, and S. Mazo from the “Instituto de Ceramica y Vidrio”, ICV, is gratefully acknowledged. Also, we specially thank J.L. Balenzategui and E. Elizalde for the fruitful discussions and ideas. 20 Environment ACS Paragon Plus

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Supporting Information. Figure S1, Top-view FESEM images of a-Si:H PECVD films after the MACE procedure with 0.0025 M (A) and 0.001 M (B) AgNO3 concentrations; Figure S2, Histogram and size distribution of a-SiNWs; Figure S3, XPS spectrum of a-Si:H RF-MS; Table S1, growth parameters and optical properties of PECVD samples with different hydrogen contents; Figure S4, Raman spectra of the asdeposited a-Si:H films with different H proportion, and a c-Si substrate as reference; Figure S5, FESEM images of a-Si:H deposited on Cu before (A) and after (B) the SiNW growth. C and D are pictures of the samples showing high homogeneity and flexibility.

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Role of Hydrogen in the Preparation of Amorphous Silicon Nanowires

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