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We present fabrication and optical characterization of Si microstructure-ZnO nanowire (NWs) hierarchical structures for light management. Random and periodic hierarchical structures constituting Si micro pillar or micro pyramid arrays with overgrown ZnO NWs have been fabricated. Inexpensive colloidal lithography in combination with dry and wet chemical etching is used to fabricate Si microstructures, and ZnO NWs are grown by hydrothermal synthesis. The periodic Si micro pyramid-ZnO NWs hierarchical structure shows broadband antireflection with average reflectance as low as 2.5% in the 300-1000 nm wavelength range. A tenfold enhancement in Raman intensity is observed in this structure compared to planar Si sample. These hierarchical structures with enriched optical properties and high surface to volume ratio are promising for photovoltaic (PV) and sensor applications.
©2013 Optical Society of America
1. Introduction
In recent years there has been extensive research interest in hybrid material systems and devices in optics, electronics and optoelectronics [1–7]. The hybrid material system where two or more materials are combined in different compositional and geometrical forms can exploit the better of both worlds for improved performance and added functionalities. Silicon and Zinc Oxide (ZnO) hybrid material system is an important combination that has found many applications in photovoltaic devices [2,6].
The hybrid Si-ZnO aggregate has been utilized in different compositional and geometrical combinations including hierarchical 2D and 3D structures. In particular, 3D hierarchical structured Si with ZnO nanowires (NWs) overgrowth has the advantage of large surface to volume ratio and refractive index modulation compared to simple structures. These properties, in different contexts, result in superior antireflection, higher absorption [6], higher photo detection sensitivity [7], enhanced field emission [8] etc. The refractive index modulation providing an effective index gradient between two media is advantageous for controlling light propagation. Recently, hybrid ZnO NWs and micro pyramid Si solar cell with very low weighted average reflectance of 3.2% has been reported [9] where chemically etched random Si micro pyramids have been utilized. Si-ZnO hybrid structures have been reported in several applications including antireflective coatings (ARCs) [6,9,10], solar water splitting and H2 generation [11], and sensing [12].
From a practical point of view, comparison of different architectures of hierarchical structures, including random and periodic ones, is essential. Periodic structures can be easy to model and modify with respect to height, period and shape for better optical performance. Also it may be relatively easy to fabricate electrical contacts on periodic and uniform structures compared to random structures. On the other hand, random structures may have the advantage of ease of fabrication if they can be generated without use of lithography patterning steps, for example by utilizing self-masking effects occurring during the etching of the structures.
In this work, we investigate the antireflective performance of different hierarchical Si-ZnO structures. We present periodic Si micro pyramid/ZnO NWs hierarchical arrays with enhanced antireflective ability, together with a comparison of random structures. We also compare Si micro pillar-ZnO hierarchical structures with ZnO NWs on planar Si. We have used inexpensive colloidal lithography technique and inductively coupled plasma (ICP) dry etching for Si micro pillar fabrication. A wet chemical etching process is additionally used to fabricate the Si micro pyramid arrays from the Si micro pillar arrays. The ZnO NWs are grown on the Si microstructures by solution based hydrothermal process for different durations. We have performed total reflectance measurements and Raman spectroscopy to evaluate the optical properties of the fabricated structures. We have achieved broadband antireflection with average reflectance as low as ~2.5% with the periodic Si micro pyramid/ ZnO NWs hierarchical arrays as compared to ~3.4% with random structures for the wavelength range of 300-1000 nm. Raman spectroscopy shows a tenfold enhancement in the intensity with periodic Si micro pyramid/ZnO NWs structures compared to the planar Si substrate. A two fold increase in Raman scattering intensity is observed for all the Si/ZnO NW hierarchical structures compared to their Si counterparts without ZnO NWs.
2. Fabrication of Si/ZnO NW hierarchical structures
To realize the Si microstructure–ZnO NW hierarchical structures hexagonal arrays of Si micro pillars or pyramids are first fabricated. Subsequently, ZnO NWs are overgrown on the different types of structured Si samples. The detailed procedures are described in the following subsections.
2.1 Silicon microstructure fabrication
For periodic and quasi-periodic structuring of materials, an etch mask is generally required to get the desired structure. In micro-fabrication technology the preferred method of pattering is optical lithography with a photo mask. But, in our work we have used colloidal lithography [13,14] to get the required hexagonal lattice pattern, as it is more convenient and less time consuming.
For anisotropic etching of Si there exist many industry standard wet and dry etching methods [15]. Reactive ion etching (RIE) and inductively coupled plasma reactive ion etching (ICP-RIE) with different gas chemistries are well established dry etching processes [16]. Wet chemical etching of Si is possible with different chemicals including KOH and TMAH (tetraethyl ammonium hydroxide) [17]. In this work, both dry and wet chemical processes have been used to fabricate the desired Si microstructures.
2.1.1 Fabrication of Si micro pillar arrays
Fabrication of Si micro pillars arrays is schematically illustrated in Figs. 1(a)-1(d), and includes:1) pattern generation on Si substrate by self–assembly of silica (SiO2) spheres as etch masks (colloidal lithography technique), 2) size reduction of silica spheres with RIE, and 3) ICP-RIE etching of Si to obtain micro pillar arrays. To get high aspect ratio pillars a pseudo Bosch process is used, where etching and sidewall passivation occur simultaneously resulting in better side wall profiles.
Fig. 1 (a)-(d): Schematics of Si micro pillar fabrication process steps. (a) SiO2 colloidal particles dispersed on a Si substrate. (b) Size reduction (by RIE) of as-dispersed SiO2 colloidal particles. (c) Anisotropic ICP-RIE etching of Si to produce Si micro pillars. (d) Si micro pillars after removal of SiO2 particles in HF. (e) Representative SEM images (top view) of hexagonal close-packed array of SiO2 particles after dispersion; image corresponds to process step shown on Fig. 1(a). (f) Tilted (30 degree) SEM image of Si micro pillar arrays; image corresponds to process step shown on Fig. 1(d).
An aqueous solution of silica particles (2 or 3 microns in diameter) is spin coated on a Si wafer which forms a hexagonal close packed monolayer of silica particles, as shown in the scanning electron microscope (SEM) image (Fig. 1(e)). Before dispersion of silica particles by spin coating, the Si sample is first cleaned with standard organic solvents and blow-dried under nitrogen flow, and then treated with oxygen plasma at a power of 1 kW and O2 flow of 500 sccm. for 8 min. to increase the wettability of the Si surface. The spin coating is done at 1500 rpm. for 30 s which results in patches of closely packed monolayer of silica particles. These patches are typically few mm2 to a maximum of 1 cm2 in area, which is sufficient for the experiments described here. However, uniform monolayer coverage of particles on a wafer scale is desirable for practical applications, and can be achieved using techniques such as rapid convective deposition [18] and diblock-copolymer lithography methods [19].
The dispersed SiO2 particles are used as etch masks to fabricate Si micro-pillar arrays with a period of 2 and 3 microns and with pillar diameters in the range of 1.5 to 2.5 microns. The SiO2 particles are size reduced in a RIE chamber with Ar/CHF3 chemistry. This size reduction step of the mask particle is used to vary the Si pillar diameter and to open areas around particles thereby varying the spacing between the Si pillars. Here, we comment that the geometry of the etched pillars is predominantly determined by the particle size and the open space between the particles. For ICP-RIE etching of Si microstructures in the pseudo Bosch method we have used SF6 gas for etching and C4F8 for simultaneous surface passivation. This process is described in detail in reference [20]. The following process parameters are used to fabricate the Si micro pillars: Gas flows - 35 sccm. SF6 and 90 sccm. C4F8; ICP power of 600 W; RF platen power of 10W; pressure 10 mT; and etch time in the range 10 to 20 minutes. A near vertical etch profile and an etch rate of ~170 nm/min are obtained with this process. Figure 1(f) shows a representative SEM image of the fabricated Si pillar arrays having slightly tapered pillar profiles. Such tapered structures have the advantage of a graded refractive index profile which can reduce surface reflection of light.
2.1.2 Fabrication of pyramidal Si microstructure arrays
The Si micro pyramidal arrays are fabricated from Si micro pillars by wet chemical etching with KOH (Figs. 2(a) to 2(d)). Wet chemical etching of Si by KOH is an anisotropic process, as the etch rate strongly depends on the crystallographic orientations of Si. These differences in the etch rates are due to the bonding energy of Si atoms which is different in the different crystallographic planes. For example, the etch rate is higher for (100) compared to (111) planes. As a consequence, pyramid-like structures bounded by (111) planes appear when the Si substrate with (100) orientation is etch by KOH. The size and shape of the etched structures depend on the degree of anisotropy of the etching solution, which in turn depends on temperature, concentration of the etchant, and doping and defects in the Si wafer [21].
Fig. 2 (a)-(d): Schematic sketches of Si micro pyramid fabrication process steps. (a) ICP-RIE etched Si micro pillar arrays (b) and (c) Different stage of KOH etching of Si pillar to form Si micro pyramid (d) As etched micro pyramid arrays. (e) Representative SEM cross section images of Si micro pillar arrays and (f) corresponding Si micro pyramid arrays.
An aqueous solution of KOH (30%) is heated up to 70 °C and the samples are kept immersed in it for different durations. The areas between pillars (including the sidewalls) have been bombarded with ions in the dry etching process, and atoms in these areas have weaker bindings. Thus, during the wet etching process these areas are the first places where Si will be removed. But {111} act as etch stop planes and only {100} planes will be etched. As a result, pyramidal structures are formed beneath the pillars, and have the same periodicity as the starting micro pillar array (Fig. 2(c)). The cross section SEM image of the as-fabricated Si micro-pillar is shown in Fig. 2(e), and the generated micro pyramid arrays after KOH etching of the micro-pillar arrayed Si in Fig. 2(f). These results clearly show that the micro-pyramid arrays have the same periodicity and base dimensions of the micro-pillar arrays.
For comparison, planar Si samples were also etched alongside the Si pillar array samples to get random pyramidal structures. Thus the etch-conditions are the same for both samples. In the case of etching of planar Si KOH, random pyramid structures with different dimensions are produced which is commonly attributed to masking effect caused by the generated hydrogen bubbles [21].
2.2 ZnO nanowire growth on Si
ZnO NWs are grown on the fabricated Si microstructures by using a hydrothermal technique [22]. This is an aqueous solution based process and has advantages, over other processes, such as low process temperatures, simple equipment, high growth rates, and low-cost. This method consists of two steps, first is the deposition of a ZnO seed layer and second is the hydrothermal growth of ZnO nanowires.
For preparation of the ZnO seed layer, zinc acetate dihydrate [Zn(CH3COO)2.H2O] is dissolved in dry ethanol (100mM). In order to make Zinc acetate soluble in ethanol, few droplets of diethanolamine (DEA) were added and stirred for around 30 minutes in order to obtain a homogeneous and transparent solution. The amount of DEA was optimized to have the right viscosity, while completely dissolving zinc acetate. The different Si samples, described in the previous section, were cleaned by acetone, isopropanol and rinsed in DI-water and dried by blowing nitrogen. ZnO (seed) solution is then spin coated on the Si samples at a rotation speed of 3000 rpm and dried on a hot plate at 80 °C. This procedure is repeated three times to ensure uniform ZnO film coverage. Finally, the samples are annealed in an oven under ambient conditions at 400 °C to form a homogeneous seed layer of ZnO crystals.ZnO NWs are grown on four different sets of Si samples coated with a ZnO seed layer, namely, planar Si substrates, Si micro pillar arrays, Si micro pyramid arrays and Si with random micro pyramid structures. For ZnO NWs growth, a solution of 50 mM zinc nitrate and 50 mM hexamethylenetetramine (HMT) was dissolved in 30 ml of DI water. The samples are placed horizontally with the seeded surface facing down in the glassware so that the ZnO particles, which are produced in the solution, do not settle on the sample surface while the reactants in the solution can have access to the seeded surface. The prepared solution for ZnO NW growth is added to the glassware and refluxed at 95 °C in an oil bath for one hour. Finally, the samples with ZnO NWs are removed from the solution, rinsed in water and dried under nitrogen flow. Several samples are prepared for different reaction durations and the morphology is investigated by SEM. Figure 3(a) shows the top view of the Si pillar array of 3 micrometer period. Figure 3(b)-3(d) show the ZnO NWs on Si micro pillar arrays grown for different durations: 30 minutes (Fig. 3(b)), 50 minutes (Fig. 3(c)) and 70 minutes (Fig. 3(d)). It is evident that with increase in growth time, the ZnO NWs grow longer thereby filling up the space between the Si pillars. The optimum ZnO growth time for the Si micro pillar arrays with different periods and pillar diameters is found from total reflection data as described in section 3.1. Also, the cross sectional SEM image (Fig. 3(e)) reveals conformal growth of the NWs on the side walls as well. ZnO NW growth time on planar Si substrates (Fig. 3(f)) is varied from 30 min to 120 min in 30 min increment steps. By measuring the lengths and the diameters of the grown ZnO NWs, it is found that as the growth time is increased from 30 min to 120 min the average diameter increases from ~38 nm to ~105 nm while the average length increases from ~300 nm to ~1μm. The hexagonal shape of the grown ZnO NWs (Fig. 3(f)) is due to its hexagonal close-packed crystal structure with the most common growth direction with wurtzite crystal structure being (0 0 1) [23]. Finally, the ZnO NWs are grown on the periodic and random Si micro pyramid structures with same hydrothermal process described above. A SEM top view of the periodic Si micro pyramid arrays is shown in Fig. 4(a). Figures 4(b) and 4(c) show SEM top views of the periodic and random Si micro pyramid arrays decorated with ZnO NWs, respectively; whereas cross sectional SEM image (Fig. 4(d)) confirms that the ZnO nanowires have covered the Si surface homogeneously.
Fig. 3 SEM images showing the growth of ZnO NWs on Si samples: (a) Si micro pillar arrays before ZnO NWs growth and (b) after 30 minutes (c) after 50 minutes (d) after 70 minutes of ZnO NW growth (magnified). (e) Representative cross sectional SEM image of a hierarchical Si micro pillar-ZnO NWs structure. (f) SEM top view of as-grown ZnO NWs on a planar Si substrate.
Fig. 4 SEM top views of (a) periodic Si micro pyramid arrays without ZnO NWs, (b) hierarchical ZnO NWs on periodic Si micro pyramid arrays, and (c) hierarchical ZnO NWs on random Si micro pyramids. (d) Cross sectional SEM showing conformal ZnO NWs growth on the Si micro pyramid structure.
3. Optical characterization of Si/ZnO NWs structures
Total reflectivity and Raman measurements have been performed on the fabricated Si/ZnO NWs structures to investigate their applicability as antireflection coating and for enhancement of Raman intensity, respectively.
3.1 Total reflectivity
Total reflectivity of the nanostructures is measured using a spectrophotometer (Perkin Elmer LAMBDA 950) equipped with a 150 mm integrating sphere. The measurements were done in the wavelength range 250–1400 nm which covers UV to near infrared (NIR) regions. The integrating sphere collects both specular and diffuse part of the reflected light. The spot size of light beam is about 2 mm in diameter. On the detection part, a photomultiplier detector is used for the visible spectrum and a PbS detector for the NIR range. The results obtained from four different sets of samples, namely, planar Si, Si micro pillars arrays, Si micro-pyramid arrays, and Si random micro pyramids with and without ZnO NWs are presented and discussed in the following.
To investigate the antireflective property of ZnO NWs grown on planar Si total reflection measurement were carried out. Three different Si samples with ZnO NWs grown for 30 min, 60 min and 120 min have been measured (Fig. 5(a)). Compared to bare Si all samples show a reduction in total reflectivity. It is observed that in wavelength range of 400 to 1000 nm the average reflectance is around 12% for all three samples. At around 375 nm, the abrupt drop in reflection is due to band edge absorption by ZnO NWs; whereas above Si band edge (1100 nm) reflection increases substantially due to back reflection from the Si substrate. The suppressed reflection compared to planar Si can be attributed to the effective (intermediate) refractive index (RI) of the ZnO (RI ~2.0) NWs layer in between air (RI ~1.0) and Si (RI ~3.5).
Fig. 5 Total reflectance spectra of different structures with and without ZnO NWs: (a) planar Si with ZnO NWs grown for different durations; (b) and (c) - Si micro pillar arrays of 2 and 3 micrometer period, respectively, with ZnO NWs grown for different durations; and (d) Planar Si, Si micro pyramid arrays and random Si micro pyramid structure.
The increase in growth time, i.e. the dimensions, of NWs affects the interference peaks in the reflectance spectra. These peaks are due to constructive and destructive interferences between the reflected light from the air /ZnO NWs (top) interface and ZnO NWs /Si (bottom) interface and their positions depends on NWs’ size. As the ZnO NWs are very densely grown (Fig. 3(f)), they can be considered as a layer with an effective refractive index for the considered wavelength range. The positions of the resonance peaks depend on the effective index and length (thickness) of the ZnO NWs layer between air and Si. Hence any changes in the NWs diameter that changes the effective index seen by the light in the ZnO NWs layer shifts the resonance peak in the spectra. On the other hand, the increase in ZnO NWs height increases the thickness of ZnO NWs layer which shifts the peak positions towards longer wavelength. From Fig. 5(a) we can see that, as the growth time increases from 1 to 2 hours the resonance peaks shift towards longer wavelengths because of the increase in the ZnO NWs length. On the other hand for growth time of 30 minutes ZnO NWs are very short (~250 nm) and the ZnO NWs layer is not sufficiently thick to show any significant resonance behavior. Similar observations of resonance fringes in ZnO NWs grown on planar Si have been reported previously [24].
With the knowledge of the antireflective behavior of ZnO NWs on planar Si we further explored the optical properties of more complex, Si micro pillars/ZnO NWs hierarchical structures (Figs. 3(b) to 3(e)) where the refractive index modulation is in all three directions. Here, it is useful to take into consideration the effect of the Si micro pillar diameter, period and ZnO NWs dimension and/or growth time to optimize the hierarchical structure for minimum possible reflection in a wide wavelength range. In this case we have investigated the total reflectivity of two sets of structures, namely, (i) Si pillar arrays of 2 µm period and (ii) Si pillar arrays of 3 µm periods, both without and with 30, 50 and 70 minutes duration of ZnO NW overgrowth. The corresponding reflection spectra for 2 and 3 µm period Si micro pillar/ZnO NWs hierarchical structures are shown in Figs. 5(b) and 5(c), respectively. It can be seen from the data that there is a decrease in reflection in the hierarchical structures compared to Si micro pillar arrays without ZnO NWs. This reduction in reflection can be attributed to the three dimensional (3D) branched structure that provide better optical impedance matching between air and Si so that more light is scattered downwards to Si leading to decrease in reflection. Also for 2 µm structure the ZnO NWs growth time for minimum reflection is found to be 50 minutes whereas for 3 µm one it is 70 minutes. The possible explanation for this can be that the inter-pillar spacing for the 2 µm period pillar array is less than in the 3 µm period pillar array, and hence for optimum refractive index contrast the 2µm period pillar array requires shorter ZnO NWs compared to the 3 µm period pillar array.
Finally, we have analyzed the Si micro-pyramid/ZnO NWs hierarchical structures and compared the random and periodic structures for their antireflective capability. The corresponding reflection spectra are shown in Fig. 5(d). As expected, all micro-pyramidal structures have much lower reflectance than planar Si. As evident from the graph the periodic micro-pyramid structures, both with and without ZnO NWs have better antireflective performance then their random counterparts. In the wavelength range of 300-1000 nm the average reflectance drops down to ~2.5% in periodic Si micro pyramid/ZnO NW hierarchical structures compared to ~3.4% for its random counterpart. As explained in ref.9, such low reflectance attributes to combine effect of effective index gradient due to ZnO NWs on Si and multiple scattering of light within the Si micro-pyramidal structures. On the other hand the further lowering in reflectance in periodic case compared to random one is possibly due to directional scattering of incident light into guided mode towards the structure [25] and better refractive index gradient. Detailed analysis of such structures require full 3D electromagnetic simulations, however, for appropriate wavelengths it is possible by analytical methods, such as effective medium theory (EMT) and gradient refractive index theory (GRIN) [26].
From the above results it is evident that the best possible antireflection performance is achieved with the periodic Si micro-pyramid/ZnO NWs hierarchical structures. Although this structure needs more processing steps than the other structures, from practical device point of view it might lead to overall performance improvement in photovoltaic applications.
3.2 Raman spectroscopy
Another important aspect of our investigation is the study of enhanced Raman scattering from the hierarchical structures. For comparison, four different samples, viz. planar Si, ZnO NWs on planar Si, periodic Si micro pyramid arrays with and without ZnO NWs have been studied. For optical excitation, 514 nm line of Ar + laser has been used with 50X objective of NA = 0.45. The incident laser power was set to 5 mW with spot size of 2-3 μm, in order to have a good intensity in collected spectra. The measured Raman spectra are shown in Fig. 6. The Raman intensity peak occurs between the wave numbers 520-521 cm−1 for all four structures which is characteristics of crystalline Si. As seen from Fig. (6), the Raman intensity is enhanced by a factor of 10 with the Si micro pyramid/ZnO NW hierarchical structure compared to planar Si. For a given Si sample/structure type, approximately two fold enhancement in the Raman intensity is observed with ZnO NWs compared to that without ZnO NWs. This increment is mainly due to the excitation light trapping or anti-reflective behavior of such structures leading to enhanced Raman intensity. This enhancement in Raman intensity can be useful for sensing application with optical interrogation methods.
4. Conclusion
We have demonstrated the fabrication of Si/ZnO hierarchical structures and evaluated their optical properties. We have realized the Si/ZnO structures using a combination of inexpensive colloidal lithography, dry and wet chemical etching processes together with hydrothermal ZnO NWs synthesis. We have measured the total reflectivity and compared the different hierarchical structures for their antireflection performance. The periodic Si micro pyramid-ZnO NWs hierarchical arrays showed the lowest average reflectivity of ~2.5% in the 300-1000 nm wavelength range. Raman measurements demonstrate 10 fold enhancement in intensity in these structures compared to planar Si. These Si microstructure-ZnO NW hierarchical structures can improve the performance and versatility of photovoltaic devices and optical sensors.
Acknowledgments
The work was performed within the Linné center for advanced optics and photonics (ADOPT), and was supported by the Swedish Research Council (VR). Partial supports from the European Union FP7 Network of Excellence Nanophotonics4Energy (N4E) and Nordic Innovation Center project NANORDSUN are also acknowledged.
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