Nanowire Arrays in Multicrystalline Silicon Thin Films on Glass: A

Letter pubs.acs.org/NanoLett

Nanowire Arrays in Multicrystalline Silicon Thin Films on Glass: A Promising Material for Research and Applications in Nanotechnology Sebastian W. Schmitt,†,* Florian Schechtel,† Daniel Amkreutz,‡ Muhammad Bashouti,† Sanjay K. Srivastava,†,∥ Björn Hoffmann,† Christel Dieker,§ Erdmann Spiecker,§ Bernd Rech,‡ and Silke H. Christiansen†,⊥ †

Max Planck Institute for the Science of Light, Günther-Scharowsky-Straße 1, 91058 Erlangen, Germany Helmholtz-Zentrum Berlin für Materialien und Energie, Kekuléstrasse 5, 12489 Berlin, Germany ∥ Parent affiliation: CSIR-National Physical Laboratory, New Delhi-110012, India § Friedrich Alexander Universität Erlangen Nürnberg, Cauerstraße 6, 91058 Erlangen, Germany ⊥ Institute of Photonic Technology, Albert-Einstein-Straße 9, 07702 Jena, Germany ‡

ABSTRACT: Silicon nanowires (SiNW) were formed on large grained, electron-beam crystallized silicon (Si) thin films of only ∼6 μm thickness on glass using nanosphere lithography (NSL) in combination with reactive ion etching (RIE). Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) studies revealed outstanding structural properties of this nanomaterial. It could be shown that SiNWs with entirely predetermined shapes including lengths, diameters and spacings and straight side walls form independently of their crystalline orientation and arrange in ordered arrays on glass. Furthermore, for the first time grain boundaries could be observed in individual, straightly etched SiNWs. After heat treatment an electronic grade surface quality of the SiNWs could be shown by X-ray photoelectron spectroscopy (XPS). Integrating sphere measurements show that SiNW patterning of the multicrystalline Si (mc-Si) starting thin film on glass substantially increases absorption and reduces reflection, as being desired for an application in thin film photovoltaics (PV). The multicrystalline SiNWs directly mark a starting point for research not only in PV but also in other areas like nanoelectronics, surface functionalization, and nanomechanics. KEYWORDS: Silicon nanowires, multicrystalline, silicon thin film on glass, e-beam crystallization, photovoltaics, absorption, reactive ion etching, EBSD, TEM, XPS

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controlled etching of SiNW arrays is not accessible. Here, we will show the formation of SiNWs in high quality mc-Si thin films (thickness: 6 μm) that were realized by e-beam recrystallization of amorphous silicon (aSi) layers on glass substrates.27 We could prove that NSL patterning and subsequent RIE of NWs in mc-Si films on glass results in NWs perpendicular to the film surface, irrespectively of grain orientations. Not even grain boundaries disturbed the controlled, homogeneous formation of the SiNWs and single NW consisting of different grains were observed. The mc-Si films show a good crystalline quality with relatively large grains (up to a few mm2), low thermo-mechanical stresses and a low level of residual impurities.27 XPS studies revealed an electronic grade surface quality of the RIE etched mc-SiNW after cleaning. Because of the aforementioned structural and chemical bulk and surface properties, the fabricated mc-SiNW are interesting for fundamental research and applications in nano thermo- and

iNWs are active building blocks in various research areas of modern nanotechnology. Individual nanowires (NW) exhibit resonant optical absorption/reflection1,2 and scale and functionalization dependent electrical properties3,4 as well as crystalline orientation and size related mechanical properties5,6 while ensembles of NWs constitute large surface areas and an overall increased absorption of visible light.7,8 These properties were found to be beneficial for sensor applications,9−11 (opto-) electronic12−14 and electromechanical devices,15,16 and solar cells.17−22 SiNWs can be realized using bottom up growth processes (e.g., the thoroughly studied so-called vapor−liquid− solid (VLS) growth or modifications of which, e.g., the vaporsolid−solid (VSS) growth, which are all relying on supply of the Si growth species from the gas phase23) or top down wet and dry etching processes, which are usually realized in Si wafers.7,24,25 There are only very few studies on the top down etching of mc-SiNWs, probably due to a limited availability of high quality, large grained mc-Si starting layers and since wet chemical etching is strongly anisotropic26 and yields SiNWs of different directions in grains of different orientations22 so that a © 2012 American Chemical Society

Received: April 15, 2012 Revised: July 8, 2012 Published: July 23, 2012 4050

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electro-mechanics and single NW electronics. Since mc-SiNW from different grains show the same morphology but different side facets with different surface energies, dangling bond structures and surface reconstructions they represent an ideal material for extensive surface functionalization studies. Furthermore, top down SiNWs on alternative substrates such as, e.g., glass have only been realized in one single study, where uncontrolled wet chemical etching was applied to pattern a thin, laser crystallized, mc-Si film on glass.22 All other top down SiNW etching studies were carried out on wafers, despite the well accepted fact that an increased light absorption7,28,29 and strain relaxation30 in thin films and a higher material defect tolerance due to radial charge separation31 are the main reasons for the implementation of NWs in advanced (vertical) PV concepts. The absorption efficiency of solar light was substantially improved by NW patterning of the mc-Si films used for this study, a fact which justifies the nanopatterning of the mc-Si thin films on glass and their potential application in PV devices. Fabrication. The high structural and compositional quality mc-Si films on glass were realized by sweeping a tightly focused electron beam (with 0.8 mm line width and 80 mm line length) with a constant scanning speed of 6 mm/sec and an energy density of 935 mJ/mm2 over a hydrogen free a-Si film, as deposited by e-beam evaporation of Si targets at a rate of 4 nm/ sec onto Corning Eagle XG glass.32 Thereby, the a-Si liquefies and recrystallizes with large grains. The process requires a diffusion barrier to the Si film to prevent diffusion of impurities from the underlying glass substrate. For that purpose, 200 nm SiOx was deposited by plasma enhanced physical vapor deposition (PECVD) followed by 50 nm SiCx by magnetron sputtering that serves as a wetting agent for the liquid Si.27 The resulting mc-Si thin films on glass were patterned using NSL. This process relies on self-assembly of monolayers of e.g. polystyrene nanosphere (PSS) of different diameters. The PSS monolayers assemble with high periodicity (hexagonally densely packed) on the mc-Si film using the Langmuir− Blodgett technique (see, e.g., ref 33) so that areas of several cm2 could be achieved. The PSS size was reduced using an inductively coupled O2 plasma (40 W/15 mTorr/285 Vdc) and the NSL patterned mc-Si films were subsequently exposed to RIE (inductively coupled plasma at 600W, optimized Bosch process for deep RIE with sequential exposure to SF6/C4F8/O2 mixture for etching and C4F8 for passivation). Figure 1 shows sizes and aspect ratios of SiNWs that were realized by RIE. For various etching studies, PSS of 1000 and 500 nm initial diameter di were used which after O2 plasma reduction and RIE of 5 min led to hexagonally ordered arrays of SiNWs with diameters from about 350 to 900 nm. RIE time variations between 2 and 14 min were used to increase the SiNW length linearly from 0.5 to 5.3 μm. The good control of the parameters di (initial PSS diameter) and dr (reduced PSS diameter) shows that via this simple and reproducible lithography technique a large number of NW array geometries (compare inset Figure 1) on mc-Si thin films on glass can be realized (in the following text, all samples will be named according to the diameters used in Figure 1). Structure. The structure of the synthesized SiNW arrays was investigated by EBSD in more detail, to reveal the interplay between the etching process with the underlying crystal structure of the mc-Si thin films. In Figure 2, a secondary electron (SE) micrograph of a thin film, partially covered by SiNWs is shown. The adjacent EBSD image gives three-

Figure 1. Diameters and lengths of etched SiNWs in mc-Si thin films on glass using NSL patterning with PSS. Different initial PSS diameters di (500 and 1000 nm) for the NSL patterning result in two different lattice constants di of the self-assembled colloidal etching masks (upper inset). O2 plasma etching reduces the PSS initial diameters di resulting in structures of variable diameter dr (left y-axis/ lower inset, scale bar 1 μm). SiNW lengths linearly increased with RIE time (right y-axis).

Figure 2. Left: SEM and EBSD images of a mc-Si thin film on glass with a surface area partially covered with SiNWs. The EBSD image shows a variety of different grains. Along trajectories A/B the grain misorientation (a measure for dislocation densities) was measured (cf. text). EBSD detail: SiNWs in five different grains with different crystal orientations. SE detail (45° tilt): Grain boundary at the transition of SiNWs and unpatterned mc-Si film. RIE permits SiNW formation perpendicular to the surface irrespectively of the grain orientation (wire axis directions are indicated by arrows; scale bars left, 50 μm; right, 3 μm).

dimensional information on crystal orientations34 of the probed spot. Different colors indicate grains of different orientation. Relatively large grains (e.g., the green one) can be observed with an area of up to several mm2. EBSD measurements, e.g., along a line A of ∼500 μm length (Figure 2) show a misorientation of less than 2°, which is a measure for a low density of extended lattice defects such as dislocations and their networks.35 EBSD measurements along a line B of again ∼500 μm length indicate intersections with various grains. The grain boundaries that are met are mostly simple twin boundaries (Σ3 4051

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in DSC notation36) and rarely high angle (and thus high energy) grain-boundaries are found. The SE image detail in Figure 2 shows SiNWs, etched into adjacent grains of different orientations. Obviously, irrespective of the grain orientation the dry RIE process results in aligned SiNWs with the same length, diameter and spacings that all reside perpendicular to the glass substrate. (The SiNW surface in the middle of the image exactly follows the topography of the initial thin film, i.e. including a surface step at a grain boundary. The occurrence of these surface steps is related to details of the Si-melt mediated crystallization of the Si film27). The EBSD image detail in Figure 2 shows SiNWs in five adjacent differently oriented grains with undisturbed shapes. Figure 3 shows the SEM cross section of a mc-Si film on a glass substrate with short SiNWs at the film surface. As can be Figure 4. XPS spectra of a sample directly after etching (as is) and after subsequent cleaning procedure (first annealing/second annealing and HF as detailed in the text). The small figures on the right show high resolution details of the overview spectra on the left side. All spectra are normalized with respect to the initial F1s peak.

observed at 532.02 ± 0.02 eV (O1s) and 285.2 ± 0.02 eV (C1s) were assigned to adventitiously adsorbed hydrocarbons showing O bonded to C (286.69 ± 0.02 eV). It is reasonable to assume that these hydrocarbons could result from the solvents in the wet chemical processing used for the removal of PSS spheres. In addition, carbonaceous materials present in the laboratory environment can be considered as the second source of hydrocarbon surface contamination.37 To remove the F from the SiNW surfaces, the samples were annealed twice for 30 min at 500 °C in an argon−oxygen atmosphere (99% Ar, 1% O2). After the first annealing step, the F1s intensity was reduced to 3.47% and after the second step the F1s peak may no longer be observed (See Figure 4). However, the intensity of the silicon oxide peak (SiOx) increased significantly and it could be argued that the oxide and hydrocarbons eliminated or suppressed the F1s signal. Thus, we removed the SiOx by immersing the sample in 5% hydrofluoric acid (HF) solution. The hydrogen terminated SiNWs did not exhibit SiOx in the high resolution Si2p XPS scans and an oxide-free surface was obtained as well (cf. Figure 4 SiOx detail). The absence of F1s signals in the XPS data confirmed that the F was totally removed from the NW surfaces after annealing and they are not covered by F-containing molecules any longer (cf. Figure 4), neither physically nor chemically. Moreover, the C1s emission shows no fluorinated carbon (Figure 4 C−F detail). The physically adsorbed F contaminants can obviously be removed by annealing the sample in vacuum at 500 °C for 1 h while preserving the terminating Si−H bonds (after HF dipping treatment). Optics. The optical properties of the mc-Si thin films on glass before and after SiNW etching were investigated by UV− vis-NIR photospectrometry in an integrating sphere (Varian Cary 5000). The SiNW formation in the thin mc-Si films indicates an increased light absorption,7 as compared to the two-dimensional mc-Si film (cf. Figure 5). In such a measurement, the black line shows the absorption of a 6 μm unpatterned mc-Si thin film on 1.1 mm glass, compared to the same film with a surface pattern with SiNWs of ∼759 nm diameter and a length of ∼2.2 μm. The optical measurements in Figure 5 show that the SiNW surface etching increased the absorption of visible light by 20−30% which can be attributed to the light trapping and enhanced light scattering in the SiNW

Figure 3. Lower lef t: SEM cross section of the 716 nm sample (Figure 1) showing the glass substrate and mc-Si thin film with SiNWs. Upper lef t: SE detail indicating the region of the adjacent EBSD scan. It shows single NW that are divided by twin grain boundaries. Right: TEM cross sections of the sample showing SiNW orientation regardless of intrinsic grain distribution in the thin film (all scale bars 1 μm).

seen from the SE and EBSD image details, even grain boundaries in the mc-Si film do not affect the etching regularity of the SiNWs. In the EBSD image essentially SiNWs with a vertical twin boundary are visible. TEM investigations in crosssection geometry confirm these findings (Figure 3, right side). The TEM bright-field image in the upper part provides evidence of neighboring SiNWs on different grains. The bright field image in the lower part shows SiNWs that are divided by inclined microtwins without apparent effect on the wire shape. Chemistry. The composition of the mc-Si thin films has been studied and reported in ref 27. The surface chemistry of the SiNWs may, however, be modified by the fluorine chemistry of the etching process. X-ray photoelectron spectrometry (XPS) spectra of freshly prepared SiNW surfaces show atoms such as Si, carbon (C), oxygen (O) and fluorine (F), where the F results from the etching process itself (Figure 4). Occasionally, after the RIE process a considerable signal of F is observed: F1s at 684 eV, Auger K1L23L23 at 830 eV and K1L1L23 at 857 eV, all lines indicating a surface contamination with F throughout the dry etching process (the F1s peak directly after etching was normalized to unity). Fluorinated carbons (i.e. C−F bonds) were observed with 5−7 eV shift from the C1s core emission (cf. detail in Figure 4). O and C 4052

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in part be explained by the coupling of optical modes to SiNWs of different diameter and are further a result of refractive scattering on the well aligned wire arrays (c.f. inset to the right in Figure 5).1,28 The strong resonances in reflection explain peaks in the absorption spectra as well (compare A759 nm and R759 nm in Figure 5). The characterization of all samples by photo spectrometry shows that the SiNW structuring of thin mc-Si films on glass in general enhances absorption and reduces reflection of solar light so that such patterning can be optimized for PV applications. The material shows strongly resonant reflection/absorption which permits a control of optical properties by patterning geometries so that innovative device developments in PV and opto-electronics can be carried out. Conclusions. We have successfully demonstrated large area fabrication of mc-SiNW arrays with controlled dimensions and patterns on mc-Si thin films on glass. Large area NSL combined with a fluorine chemistry based dry etching (RIE) process was applied. SiNWs form perpendicular to the mc-Si thin film surface, independently of the grain orientations in the multicrystalline Si starting layer and interestingly, even for the case of a grain boundary running through a NW. These multicrystalline SiNWs are interesting for fundamental research in nanoelectronics and nanomechanics. XPS studies revealed that the as-etched SiNW surfaces contain fluorine impurities that can easily be removed by moderate temperature annealing in vacuum. The optical properties of mc-SiNW arrays are dominated by a strong resonant reflectivity as compared to the unpatterned mc-Si films and a strongly enhanced overall absorption of visible and near-infrared light which makes this material a strong candidate for applications in opto-electronics and PV.

Figure 5. Graph: Absorption spectrum Amc‑Si of a 6 μm mc-Si thin film on glass compared to the absorption spectrum A759 nm of the 759 nm sample in Figure 1 (% absorption was calculated as %A = 100% − %R − %T, with R representing reflection and T representing transmission). The sample with SiNWs shows a significantly increased absorption A over the whole measured spectrum. The comparison between the diffuse spectral reflection of the 759 nm and the 704 nm sample (Figure 1) shows the strong dependence of resonant reflection on the SiNW diameter. The reflection R of the unpatterned sample shows no resonances at all. The AM1.5 spectrum (NREL/y-axis on the right) is highlighted in yellow for direct comparison. Lef t inset: Backside illuminated photograph of the 759 nm sample. The increased absorption A in the center part of the samples that is structured with SiNWs is clearly visible due to a dark appearance. Right inset: Resonant reflection of the front side illuminated 759 nm sample (scale bars: 2 mm).

arrays.38 The SiNW dimensions which are presented here are not at all optimized for PV applications and longer SiNWs do even further increase the absorption of solar light.7 In the infrared part of the spectrum (>780 nm) an absorption enhancement of up to 50% can be reached. Since the Si bandgap is located at 1.12 eV, all absorption above 1100 nm and an absorption enhancement above ∼780 nm is not related to Si bulk properties. This can already be concluded from the measurement of the unstructured mc-Si film (Amc‑Si) on glass in Figure 5. The phenomenon of infrared (IR) absorption in SiNW has already been discussed in literature. Tsakalakos et al.38 report surface states of SiNW in combination with light trapping in SiNW arrays to be responsible for the Si absorption enhancement in the IR. Since our absorption measurements over the whole spectral range showed similar results for as etched samples with surface molecules (compare Figure 4) and the samples after cleaning, we attribute the sub-bandgap absorption to stuctural defects (roughness, nanocrystals) on the SiNW surface for which IRabsorption and luminecence has already been reported in literature.39,40 The left inset in Figure 5 qualitatively shows the clear enhancement of absorption in the structured part of the mc-Si film, which is the dark area in the center of the sample. Three diffuse reflection spectra in Figure 5, R759 nm, R704 nm, Rmc‑Si show the differences that occur for no SiNW patterning (Rmc‑Si) and patterning with SiNWs of identical length of ∼2.2 μm but different NW diameters of 704 nm and 759 nm at a spacing of 1 μm in both cases. A strongly resonant reflection of visible light occurs in the SiNW samples as discernible from peaking curves in Figure 5 compared to the smooth reflection spectrum of the unpatterned mc-Si thin film. These resonances strongly depend on SiNW diameters (cf. R759 nm and R704 nm in comparison) and on the angle of reflection. The resonances can

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Keith Thomas (MPL Erlangen) for assisting photospectrometry measurements, Anke Haas (IISB Erlangen) for assisting RIE and Paul Plocica (HZB Berlin) for e-beam crystallisation. S.W.S., B.H., and S.C. acknowledge partial financial support by the FP7 Projects RodSol (FP7-NMP227497) and Fiblys (FP7-NMP-214042). B.R. and D.A. acknowledge financial support by the BMU project 0325200. S.K.S. thanks the Department of Science and Technology, Government of India for BOYSCAST fellowship (Award No. SR/BY/P-03/10).

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