Lithography-Free Fabrication of Crystalline Silicon Nanowires Using

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Litho-Free Fabrication of Crystalline Silicon Nanowires Using Amorphous Silicon Substrate for Wide-Angle Energy Absorption Applications Sara Magdi, Joumana El Rifai, and Mohamed Swillam ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00598 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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ACS Applied Nano Materials

Litho-Free Fabrication of Crystalline Silicon Nanowires Using Amorphous Silicon Substrate for Wide-Angle Energy Absorption Applications Sara Magdi1, Joumana El-Rifai2, Mohamed A. Swillam1,2* 1

Nanotechnology Program, American University in Cairo, AUC Avenue New Cairo 11835, Cairo, Egypt;

2

Department of Physics, American University in Cairo, AUC Avenue New Cairo 11835, Cairo, Egypt.

*E-mail: [email protected]

We report a one-step fabrication technique of silicon nanowires using KrF excimer laser. Nanowires (NWs) are fabricated by re-distributing the silicon mass within the sample without etching any of the deposited amorphous silicon (a-Si). Melting and re-solidification of a-Si after multiple pulses laser irradiation induced the formation of NWs with lengths more than triple the thickness of the deposited film achieving a longer light path length. This resulted in a broadband absorption enhancement with reflection less than 5% for angle of incidences up to 60o. The effect of changing each laser parameter such as energy density, exposure time, and frequency on the morphology and optical properties of the NWs are systemically analyzed and compared. Keywords: Thin films, amorphous silicon solar cells, KrF excimer laser, nanowires, broadband absorption, wide-angle absorption.

The unique one-dimensional structure of silicon nanowires (NWs) made it one of the most promising configurations to realize highly efficient solar cells with low cost1. The decoupling of the absorption depth and minority carrier diffusion length could solve the trade-off between obtaining highly absorbing solar cells with decent electrical properties2–5. There are many technologies presented in literature for the fabrication of crystalline silicon NWs with supressed reflection such as wet chemical etching, VLS growth, lithography and dry etching6–16. However, lithographic techniques are expensive and requires masks, photoresists and multiple steps for the formation of NWs. Chemical etching techniques such as metal assisted chemical etching (MACE) are lower in cost and it could be done in single or multiple steps. However, this technique always requires a catalyst and/or HF and/or H2O2. In addition, the deposition of crystalline silicon requires very high energy, temperature and cost. Moreover, to obtain NWs, etching in the deposited material is commonly pursued which results in wasting some of the silicon material9,16. On the other hand, deposition of amorphous silicon (a-Si) is relatively inexpensive and does not require high temperature or energy (i.e. typical deposition temperatures between 100oC and 250oC). It is a direct band gap material, which makes the excitation of electrons upon photons absorption easier compared to indirect band gap materials such as crystalline silicon. However, amorphous silicon has higher refractive index (4.32) and thus higher surface reflection. In addition, the random crystal structure of a-Si limits the diffusion length to only ~300 nm. These limitations significantly affect the absorption of this material. Driven by the promising optical and electrical properties of NWs, it is believed that NWs fabricated from low cost a-Si materials using an easily scalable method and with enhanced optical properties would be a strong candidate in the photovoltaic market. 1

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Amorphous silicon NWs have been fabricated previously using lithography and reactive ion etching17–19. Although well controllable NWs with enhanced absorption were reported, these techniques are expensive and require multiple fabrication steps. In this work, we report a faster, easier and more cost effective technique for fabricating silicon NWs from low-cost a-Si films and using only one step process. Moreover, the fabrication process does not require any masks, clean room, catalysts, or special gases and it results in the formation of NWs in few seconds without etching any of the deposited material. A more detailed comparison between the optical properties of the proposed NWs and other types of NWs is provided in the supporting information. Developing of surface structures on molten and re-solidified silicon after nanosecond laser irradiation pulses have been previously reported20–27. Multiple pulse excimer laser was used to fabricate an array of cones and columns in the micro-scale using SF6- and O2- rich atmospheres to achieve deep etching28–32. Excimer laser has been also used to induce nanostructures on silicon surfaces21,22. Random pores and cones are formed upon laser irradiation with laser energy below the ablation threshold. The developing of single conical nano-tips has also been realized upon single pulse irradiation using excimer laser33–35. Here, we report the formation of NWs using a KrF excimer laser. Different energies, exposure times, and frequencies are examined in order to study their effect on the morphology and optical properties of the fabricated NWs to achieve the highest absorption. NWs are formed by re-distributing the mass within the sample and none of the deposited material is etched resulting in NWs with length larger than the thickness of the deposited a-Si. In addition, these NWs showed an increased light absorption over the entire visible wavelength range.

Less

than

5%

reflection

was

measured

for

angle

of

incidences

up

to

60o.

Results and Discussion Plasma enhanced chemical vapor deposition (PECVD) is used for depositing amorphous silicon thin films. Silicon NWs are fabricated by laser irradiation using a KrF nanosecond excimer laser. Figure 1 illustrates the fabrication process using the excimer laser. First, the frequency (f = 10 Hz) and exposure time (T = 40 s) are kept constant and the laser energy density (E) was varied between 130 mJ/cm2, 175 mJ/cm2, 215 mJ/cm2, 260 mJ/cm2, 300 mJ/cm2, and 350 mJ/cm2. Figure 2 (a-d) shows the SEM of the obtained NWs at different energy densities. At a small energy density, 130 mJ/cm2, small NWs were obtained with the average length (~190 nm) almost equal to the average diameter. Increasing the energy density to 175 mJ/cm2 caused an increase in the average length (~720 nm) and the diameter (~320 nm). It could be seen in Figure 2(b) for 175 mJ/cm2 that very small NWs are still present in the sample while the length of others reached 720 nm indicating that some of the NWs grow with a rate higher than others.

Figure 1: Sketch of the fabrication process of silicon NWs. Amorphous silicon was first deposited using PECVD on glass substrates and NWs are formed after laser irradiation.

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Figure 2: SEM images of silicon NWs formed with f=10 Hz, T=40 s and E equals (a) 130 mJ/cm2, (b) 175 mJ/cm2, (c) 260 mJ/cm2, and (d) 300 mJ/cm2 and silicon NWs formed with E=215 mJ/cm2, f=10 Hz and T equals (e) 20 s and (f) 60 s.

Further increase in energy to 215 mJ/cm2 and 260 mJ/cm2 caused a further increase in both the diameter and the length. Figure 2(c) shows the NWs formed with 260 mJ/cm2 and it could be seen that a more uniform distribution in the NWs length is obtained with an average of 1.6 µm. This length is larger than the thickness of the deposited a-Si layer (i.e. ~850 nm) indicating that the NWs are formed by re-distributing the mass within the sample and not by etching the silicon material. Any additional increase in energy caused the NWs to have irregular shapes because the deposited material was already consumed and the added energy caused the remaining residues of the material to re-deposit on the base of the NWs only forming these irregular shapes as shown in Figure 2(d) for energy density equals 300 mJ/cm2. More SEM figures for E = 215 and 350 mJ/cm2 and larger areas SEM are shown in supporting information in Figure S1. Next, the effect of changing the laser exposure time (t) and frequency (f) is studied. A fixed energy density of 215 mJ/cm2 was set while the exposure time is changed from 20 seconds to 70 seconds. Similar to increasing the energy, increasing the time formed longer NWs as shown in Figure 2 (e and f) for 20 and 60 seconds, respectively. More SEM figures for the other exposure times are shown in Figure S2 in supporting information. While the surface of the NWs fabricated at high energy (i.e. 300 mJ/cm2) looks rough and will lead to large surface recombination, the NWs fabricated at lower energies are acceptably smooth. Thus, we give most attention to the NWs fabricated at moderate energies to eliminate the high surface recombination drawback. Even when the laser exposure time is increased to 60 seconds, the NWs smoothness are not jeopardized as shown in the higher magnification SEM in figure S3. Figure 3 (a and b) shows the calculated length and diameter histograms and Gaussian distributions for the samples fabricated with different energy densities. These results are obtained by calculating the mean and standard deviation from the histogram of each SEM image and this data is used to plot the Gaussian curves (the standard deviation values are shown in Figure S4). These curves show the spread in the length and diameter under different energy densities which demonstrates the inherent randomness in the size of the obtained NWs.

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Figure 3: The histograms and the Gaussian distribution of (a) the length and (b) diameter for the samples with different E. (c) Average size for NWs formed using excimer laser with E, f=10 Hz and T=40 s. (d) Average size for NWs formed with E=215 mJ/cm2 (Green and purple lines are for T=40 s and different f, blue and pink lines are for f= 10 Hz and different T). The error bar in (a) shows the variation of the obtained size between different samples fabricated under the same conditions.

To summarize the effect of changing the energy density on the morphology of the NWs, the mean length and diameter of each sample is plotted in Figure 3 (c). Changing the energy density from 130 mJ/cm2 to 260 mJ/cm2 increased the average length from ~200 nm to ~1.7 µm. A rapid increase in length was observed at lower energy densities (i.e. from 130 mJ/cm2 to 215 mJ/cm2) followed by an additional slight increase from 215 mJ/cm2 to 260 mJ/cm2. On the other hand, only a small increase in diameter from ~120 nm to ~530 nm is observed with increasing the energy. A higher energy density will increase the melt depth inside the silicon layer34. Thus, more material is involved in the mass transfer causing longer NWs to form. Similarly, as the sample is exposed for more time to the laser energy, more material is transferred forming longer NWs. However, the effect of increasing the time on the length of the NWs was not as strong as increasing the energy as could be seen in the blue square line in Figure 3 (d). In addition, it had almost no effect in changing the NW diameter [pink circle line in Figure 3 (d)]. Figure 3 (c) also shows an error bar that demonstrates the variation in the size obtained for different samples under the same conditions which proves the repeatability of the proposed fabrication process. Increasing the frequency had a significant effect on increasing the length of the NWs. Figure 4(a, b and c) show the NWs fabricated with 20, 30 and 40 Hz, respectively. It could be seen that as the frequency increases, the length of the NW increases and the diameter almost remains unchanged as shown in Figure 3 (d). At 40 Hz, the longest NWs reported in this work are obtained with a 2.7 µm length and an approximate 500 nm diameter. This length is more than triple the thickness of the originally deposited a-Si film. A large area image of these NWs is shown in Figure 4(d) and large area images of the NWs formed at other frequencies are shown in Figure S5. To ensure that the NWs has not been oxidized during their formation, energy dispersive X-ray spectroscopy (EDX) analysis is made. Figure 4 (e) shows the silicon and oxygen content and Figure 4 (f) shows the mapping for the two elements. It could be seen from the EDX mapping that only the tips of the NWs are partially oxidized during the process maintaining a silicon weight % of 89.5% compared to only 10.5% oxygen weight %.

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Figure 4: SEM of NWs fabricated with E= 215 mJ/cm2, T= 30 s and f equals (a) 20 Hz, (b) 30 Hz, and (c,d) 40 Hz. (e,f) EDX mapping showing the silicon content (green) and oxygen content (red).

The process of forming the NWs is realized due to the lower thermal conductivity of silica relative to silicon. Thus, the heat absorbed due to laser irradiation is dissipated laterally causing the material to melt in a lateral direction21. After melting, the material cools again causing the solidified liquid silicon to push the rest of the material upward. This is enhanced by the fact that liquid silicon has higher density (2.52 g/cm3) than solid silicon (1.1 g/cm3) and the presence of both liquid and solid silicon in the same area causes the re-solidified material to occupy a larger volume22,33. Since this larger volume can only expand upward pushing the remaining liquid silicon upward, NWs are formed on top of the silicon surface. They are formed in an array due to the excitation of either capillary waves or surface plasmon polaritons waves 31,36,37. Thus, the incident light intensity of the remaining laser pulses is affected by these waves and becomes periodically modulated causing these arrays to form. The excitation of surface plasmon polaritons waves is attributed to the large number of electrons excited in the conduction band during sample exposure to laser irradiation. Controlling the laser parameters controls the shape of the obtained nanostructures. The lower the energy, exposure time, or frequency, the smaller the portion of the a-Si involved in the melting and re-solidifying process forming a nanoparticle-like shapes as shown in Figure 2 (a). When the energy, time, or frequency increases, more material melts and re-solidifies and the large volume occupied by the re-solidified material continues pushing upward forming longer NWs. It could be seen that the dominant change in the NW shape is a longitudinal growth upon increasing the energy density, time, or frequency. The radial growth of the NWs could be controlled by changing the energy density as the only increase in diameter is observed while increasing the energy density. Next, the optical properties of the fabricated NWs are analyzed. The absorption of the bare a-Si is shown in Figure 5 (the black solid line in a-c). The peaks observed in the absorption is due to Fabry Perot resonances in the thin film. A significant part of the solar spectrum is lost due the refractive index mismatch and due to light transmission at this small thickness. Forming the NWs caused a significant increase in absorption in the whole visible and near infrared range for all energy densities used as shown in Figure 5(a). As E increases, the absorption increases due to obtaining longer NWs which increases the light path length. Changing the exposure time had a small effect on increasing the absorption and almost no change is observed between the T= 50 to 70 s especially in the long wavelength range. Alternatively, increasing the frequency caused an enhancement in the 5



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absorption along the whole wavelength range. At 40 Hz, the absorption reached more than 90% between λ=400nm and λ=600nm compared to less than 40% for bare a-Si. An enhancement factor is then calculated for each wavelength, ( % A λ A λ/ A λ 100, where A λ and A λ are the absorption in the silicon before and after NWs formation, respectively. Figure 5 (f) shows the enhancement factor calculated for the samples with the highest E, T, and f. An enhancement of more than 100% is obtained throughout most of the wavelength range. Samples with E=260mJ/cm2 and f=40 Hz have an enhancement of up to 350% at λ=740 nm. In addition, the NWs fabricated at f=40 Hz had the highest enhancement factor in the short wavelength range. Maintaining the high absorption properties at different angle of incidences is crucial for next generation solar cells. The flat a-Si films have more than 30% reflection in most of the measured wavelength range and increases to more than 50% for larger angles of incidences as shown in Figure 5 (d). This means that a large portion of the incident solar spectrum is lost due to reflection. The dips in reflection observed in Figure 5 (d) correspond to the Fabry Perot resonance peaks found in the absorption in Figure 5 (a-c). On the other hand, less than 3% reflection is measured for the NWs fabricated at f=40 Hz across the whole measured wavelength range for angles up to 50o. For 60o, the reflection slightly increases at the long wavelength range and was still less than 4%. This extremely low reflection is a major reason behind the enhanced absorption inside the silicon. Having a metal back reflector or plasmonic gratings at the back of the solar cell will decrease the transmission as well and scatter back the remaining light38. This will further increase the absorption achieving higher overall efficiency.

Figure 5: Absorption of NWs obtained with (a) different E, f=10 Hz and T=40 s, (b) different T, E=215 mJ/cm2, and f=10 Hz, and (c) different f, E=215 mJ/cm2, and T=30 s. Reflection at multiple angles for (d) bare a-Si, and (e) fabricated NWs with E=215 mJ/cm2, f=40 Hz, and T=30 s. (f) Enhancement factor calculated for the NWs fabricated with E=260 mJ/cm2, f=10 Hz, T=40 s (pink square line), the NWs fabricated with E=215 mJ/cm2, f=10 Hz, T=70 s (cyan circle line) and the NWs fabricated with E=215mJ/cm2, f=40 Hz, T=30 s (purple line).

This increased absorption is attributed to multiple reasons. It could be seen in Figure S6 that as E increases, the thickness of the silicon under layer decreases due to consuming more silicon in forming the NWs. At small E (130 mJ/cm2), the remaining silicon thickness is only reduced from 850 nm to ~830 nm. Thus, it is believed that the enhanced absorption in this case is due to the better light coupling to the silicon under layer after the addition of NWs. However, for larger E, the array of longer NWs offer multiple light scattering between them resulting in trapping the light12. In 6


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addition, the lengths of the NWs are larger than the original thickness of the a-Si. It was reported previously that guided modes cause absorption enhancement particularly for NWs with lengths more than 1 µm and these modes are supported by the tapered part of the NWs as in the case of the ones reported here1,11,39. Thus, it is believed that these guided modes along with the increased light path length leads to higher absorption. Therefore, the highest absorption reported here is for the longest NWs fabricated at 40 Hz. Moreover, the NWs offer an impedance matching between air and silicon which significantly decreases the reflection. This effect is enhanced because of the better refractive index grading offered by the small tip on top of the NWs1,11. The larger diameter obtained for larger E is expected to contribute in the increased absorption because of the larger volume occupied by the wires40. In addition, it has been proven that larger diameters leads to the excitation of multiple higher modes compared to exciting only the fundamental mode in smaller diameters leading to broader absorption17. Furthermore, the randomness in diameters offers multiple optical scattering that causes the anti-reflective properties to be broadband. Since NWs with a specific diameter are generally able to confine light at a specific wavelength, the range of diameters offered within the same sample in this work is the main reason for the broadband absorption enhancement17,18,41. Another advantage of NWs fabricated using this method is that they are not agglomerated, which will make them suitable for radial junction formation for the solar cell device. Doped NWs could be fabricated by adding a dopant during a-Si deposition and form the NWs using the same technique. Afterwards, p-doped silicon will be deposited on it using PECVD to obtain a core-shell structure which will form the p-n junction. Their non-agglomeration will also make them suitable for hybrid solar cells by giving a clear interface between the silicon and the polymer by spin-coating a p-type polymer on the n-doped SiNWs16,42–44. Quantum confinement and self-equilibrium strain effects, which generally affect the absorption properties of the material, are mainly dominant in NWs with small diameters (i.e. less than 10 nm) 4,45–47. Since NWs fabricated here have diameters more than 100 nm, it is not believed that they offer quantum confinement or bandgap change. On the other hand, the formation of NWs using this method affects the crystallinity of the film. Although the deposited film was amorphous, Raman spectroscopy show that the laser induced NWs have crystalline properties. The Raman measurements in Figure 6 shows the Raman peaks for the as-deposited amorphous silicon (i.e. the reference) and for the same samples after laser exposure with different energies. It could be seen that the as deposited silicon has a typical a-Si peak at ~480 cm-1 (magnified in the inset picture in Figure 6). However, the Raman spectrum for the laser irradiated samples is characterized by a sharp peak around ~520 cm-1 corresponding to a typical c-Si peak36,37. Thus, this change is believed to affect the opto-electronic properties of the device because of the enhanced material quality and carrier lifetime inside crystalline materials compared to amorphous materials.

Figure 6: Raman measurements for the reference as-deposited a-Si film (black dashed line), SiNWs fabricated at E=175 mJ/cm2 (pink circle line) and at E=220 mJ/cm2 (navy square line). The inset is a magnification for the reference a-Si Raman measurement.

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Conclusion In summary, we reported the fabrication of silicon NWs from an amorphous silicon thin film. It could be seen that increased attention has been directed to the fabrication of NWs due to their enhanced light absorption. However, they are mostly fabricated using expensive multiple-steps techniques which demotivate their utilization on large scale productions. The method reported here is easily scalable, fast, affordable, and does not require any special conditions such as special gases, masks, clean room environment, vacuum, or any catalyst. Excimer laser is used to re-distribute the material within the sample forming NWs with lengths equal to triple the thickness of the deposited film. The fabricated NWs showed broadband absorption enhancement of up to 350%. In addition, less than 4% reflection is measured for angle of incidences up to 60o. These indispensable optical properties along with the easy fabrication and the low cost material used (i.e. a-Si) will help in making NWs based solar cells more competitive in the photovoltaic market. Materials and Methods Amorphous silicon deposition. Oxford instruments plasma enhanced chemical vapor deposition (PECVD) is used to deposit the a-Si films on glass slides from a SiH4 gas source along with He gas to enhance the uniformity of the deposited films with a 1:1 ratio48,49. The thickness of the deposited layer was ~850 nm as shown in Figure S7. Fabrication of silicon NWs. The samples were mounted on a motorized computer controlled stage and irradiated with multiple pulses using a KrF excimer laser for nanowires formation. Characterization. The morphology was characterized using a Zeiss Leo field emission scanning electron microscopy (FESEM). Thirty seconds Au sputtering was performed prior to SEM imaging to make the surface conductive. All images shown in the paper is cross section and were taken with a 20o tilt. The optical properties were measured using a Perkin-Elmer Lambda 950 (UV-VIS NIR) spectrophotometer. A ProRaman-L analyzer was used for the Raman measurements. More details for the materials and methods could be found in the supporting information. Supporting Information Available: More SEM figures and more detailed materials and methods section could be found in the supporting information. References (1)

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Plasma Enhanced Chemical Vapour Deposition of Helium Diluted Silane. ICONIP ’02. Proc. 9th Int. Conf. Neural Inf. Process. Comput. Intell. E-Age (IEEE Cat. No.02EX575) 2002, 300–303. TOC:

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