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We report on fabrication and optical characterization of GaAs nanopillar (NP) arrays, obtained using a combination of low-cost mask generation by self-assembled silica particles (nanosphere lithography) and dry etching. Tapered structures (conical and frustum NP arrays) are fabricated by appropriate optimization of process parameters. Significant suppression of surface reflectance is observed for both geometries over a broad wavelength range. Simulations, based on finite difference time domain (FDTD) method, show good agreement with reflectivity measurements and serve as a guideline for design of NPs and understanding their interaction with light. A combination of wet chemical etching and sulfur–based passivation of GaAs NPs, results in more than one order of magnitude enhancement in PL intensity and recovery of PL line-width, which is very promising for photovoltaic applications.
©2012 Optical Society of America
1. Introduction
Semiconductor nanostructures and their potential applications in photonics, have been researched extensively in the past few years [1,2]. Arrays of nanopillars (NPs)/ nanowires (NWs) have interesting optical properties such as low-reflectivity and enhanced absorption and emission compared to the flat substrate [3]. These optical properties, which depend on the pillars size, material, spatial separation and shape, are very attractive for photovoltaic (PV) cells [4], light emitting diodes (LEDs) [5] and detectors [6]. Additionally, the waveguiding of light in semiconductor NPs/NWs, where the refractive index contrast with respect to the surrounding (e.g. air) can be very high, allows light confinement at smaller dimensions. Both the waveguiding of light and large surface to volume ratio of NPs can be very attractive for many other applications including lasing [7], nonlinear optics [8,9] and bio-sensing [9].
Solar cells based on NPs can benefit both from light trapping due to the pillars’ geometry, and the effective carrier collection due to enhanced surface to volume ratio at junction area, for example in radial junctions [4]. Optimized geometry and size of NPs for maximum absorption of sunlight should be carefully considered [10,11]. Material saving and cost effective fabrication processes are additional needs; requiring appropriate choice of semiconductor materials and NP array fabrication methods. Both growth techniques [12] and top-down approaches [13] have been used for NP fabrication. Nanosphere lithography (NSL), where colloidal nanoparticles act as etch masks, is an inexpensive method for fabricating uniform NP arrays over large areas. An appropriate anisotropic etching technique can be then used to etch the desired material(s). The particle size and etching conditions including chemistry can be optimized to vary the size, shape, and geometry of NPs. Among semiconductors, GaAs is potentially a very good candidate for NP solar cells because of the direct band gap, 1.43eV (868 nm) at 300K, is well matched to the solar spectrum [14]. GaAs also has a very high electron mobility (8500 cm2V−1 s−1 at 300K) and high resistance to radiation and thermal effects, which makes it useful for space applications [15]. However, GaAs has a very high surface recombination velocity (105-106 cm/s). For efficient device performance, the fabricated GaAs NPs require effective surface passivation to minimize carrier loss by non-radiative surface recombination [16].
Here, we report on fabrication and optical properties of GaAs NP arrays, obtained using the combination of NSL (using silica particles) and dry etching (Cl2/H2/CH4 chemistry). By choosing the right process parameters, tapered structures (conical and frustum NP arrays) have been fabricated. Both geometries show dramatic suppression of reflectance, due to the graded change of refractive index. Simulations based on 3D finite difference time domain (FDTD) method, have been used to obtain deeper understanding of light interaction with NPs. A strongly suppressed reflectance is obtained, for a broad wavelength range, which is advantageous for solar cell applications. The photoluminescence (PL) spectra of GaAs NPs, measured at room temperature, show more than one order of magnitude enhancement after wet-chemical passivation, using combination of citric acid and sulfur-based passivation, compared to as-etched NPs which indicates efficacy of the passivation process.
2. Fabrication of GaAs nanopillars
GaAs NPs’ fabrication process is schematically illustrated in Figs. 1(a) –1(c). Colloidal silica nanospheres in aqueous suspension (from Sigma Aldrich) with 500 nm diameter [± 5%; quality grade: 1σ], are dispersed on GaAs substrate. Prior to dispersion of silica particles, GaAs samples are cleaned in standard organic solvents, rinsed in DI water, blow-dried under nitrogen flow and subsequently treated by oxygen plasma to increase the wettability of the surface. Oxygen plasma treatment is done for 10 min at 1 kW with an O2 flow of 500 sccm. In order to obtain monolayers of silica particles a spin coating method is used. The spin coating is done in two steps, initially with a spin speed of 500 rpm for 10 s and then at 2500 rpm for 30 s. This procedure results in monolayer patches (typically a few mm2) with close packed particles. The above procedure for dispersion of the silica particles is reproducible in terms of surface coverage. Recently, other methods for obtaining highly-ordered 2-D patterns at wafer-scale, such as rapid convective deposition method for silica micro-/nano-particle monolayer arrays [17,18] and diblock-copolymer lithography methods [19,20] have been reported. Both these methods had been employed in the fabrication of light-emitting diodes [18] and quantum dot layers [20]. These methods can enable realization of NPs/NWs arrays with wafer scale uniformity.
Fig. 1 (a)-(c): Schematic illustration of the GaAs nanopillar (NP) fabrication process steps. (a) Dispersed SiO2 colloidal particles on GaAs surface (b) Size reduction (by RIE) of silica colloidal particles dispersed on GaAs surface. (b) Anisotropic etching to produce GaAs NP by ICP-RIE (Cl2 /H2 /CH4 chemistry) using silica particles as masks. (d) Representative SEM images (top view) of hexagonal close-packed array of silica particles after dispersion and (e) after size reduction. (f) Cross sectional SEM image of the fabricated GaAs NP array. (g) Cross sectional SEM image of different GaAs NP geometries. Scale bars represent 200 nm.
Prior to transferring the pattern into the GaAs substrate the silica particles are size reduced, Fig. 1(b), by reactive ion etching (RIE) operating at 40 mT with a CHF3 flow of 20 sccm and Ar flow of 10 sccm. This recipe has an etch rate of: 20 nm/min. Using different etching time, depending on the final size needed, we could obtain different sizes of particles in a non-close packed lattice. Thus, this size reduction step also allows varying the pillars’ diameters and the spatial separation. In our experiments, we used 5 min etching time for silica particles with initial diameter of 500 nm, which results in the spheres with the diameter of: 400 nm. In the regions with monolayer coverage, the density is: 4.6 × 108 particles/cm2. Figures 1(d) and 1(e) show the top view SEM images of the dispersed silica particles before and after size reduction, respectively. Using the silica particles as etch-masks, the pattern is transferred into GaAs by inductively coupled plasma reactive ion etching (ICP-RIE). As noted earlier, tapered NPs have the advantage of gradual change of refractive index from air to the semiconductor, and therefore could offer reduced reflectance. Using an optimized chemistry for anisotropic etching of GaAs using Cl2/H2/CH4 chemistry and with gas flows of 14, 10, and 5.5 sccm respectively, NPs with tapered profiles, Fig. 1(f), have been attained. The ICP and radio frequency (rf) powers were 1000 and 60 W, respectively and the operating pressure was 4 mT. Cl2/H2/CH4 chemistry has the advantage that a wide range of III-V materials can be etched and smooth etched surfaces could be achieved. While the chlorine radical acts as the main etchant, methane provides smoothness of the sidewalls and hydrogen is used primarily to avoid polymerization of hydrocarbons that can result in polymer deposition on the etched structures and chamber walls. Using optimized etch parameters mentioned above, we obtained ~7:1 selectivity of GaAs over SiO2 and an etch rate of ~150 nm/ min. Figure 1(g) shows a cross-sectional SEM view of the etched GaAs NP array with frustum shape with the remnant SiO2 particle on top(i) and after the removal of the silica particle with HF(ii). The conical structures can be obtained by effectively utilizing erosion of the SiO2 particle during ICP etching, Fig. 1(g-iii). If desired, by applying a post process step, using a wet chemical etchant (crystal orientation dependent), the shape and lateral size of the pillars can be modified. For the reflectance measurements and simulations, discussed in the next section, two representative NP array samples (conical and frustum geometries) were chosen. The period in both the samples is 500 nm and the NP base diameter is: 450 nm. The top diameter and height of the frustum are 100 nm and 825 nm, respectively. In the case of the conical sample, the NP height is 790 nm.
3. Reflectance measurements and simulation
For most of the semiconductors, which have high refractive index in comparison to air (~3.5 compared to 1), the surface reflectance is about 30% - 40% in the visible and near infrared region. In this context, NP geometry can be very useful in suppressing the reflectance and consequently increasing the absorptivity. NPs can cause concentration of the incident light at a microscopic level [21], which leads to an enhanced absorption in the NP geometry compared to an equal thickness (amount of material) of a flat substrate. Enhanced absorption would increase the generation of electron hole pairs and correspondingly an increase in short circuit current and efficiency of a solar cell device [22]. Therefore it is important to have a quantitative evaluation of the reflectance from the NPs.
We use a Lambda 950 (Perkin Elmer) spectrophotometer with an integrating sphere for the total reflectance measurements of the fabricated GaAs NPs. The integrating sphere collects all the reflected light (both specular and diffuse reflectance). The beam spot size is about 2 mm in diameter; a photomultiplier detector is used for the visible spectrum and a PbS detector for the NIR range. Figure 2 shows the measured total reflectance of the frustum GaAs NP array sample at 8° angle of incidence; for comparison, the data for a GaAs bare substrate is also included. In case of GaAs NPs array, the appreciable reduction of more than a factor of 3 for below band gap (in energy) light and a factor of 7 for the above band gap, in the total reflectance can be clearly observed. Thus, suppression of reflectance is obtained for a broad wavelength range, which is advantageous for solar cell applications. It has also been shown that NPs with tapered profiles provide omnidirectional [3] (from incidence angles of 0 to above 60°) suppression of reflectance, which is superior to conventional anti-reflection coatings. The suppression of reflectance below the band gap suggests that more light is getting into the substrate (although it would not be absorbed by NPs). This property of the NPs can be relevant for example in multi-junction solar cells and for efficient light extraction in LEDs. In the reflectance spectra (Fig. 2) of both the NPs and the bare substrate, the band edge of GaAs (at around 870 nm) is clearly seen, which is made visible due to the reflection of below band gap light from the back surface of the substrate. As mentioned before, the uniform coverage of silica particles is few mm2, meaning that the obtained total reflectance data is affected by reflectance from areas without pillars or with non-uniform coverage of NPs. Hence, the actual total reflectance reduction is expected to be even more than what is shown on Fig. 2 in case of having a smaller spot size or a larger array.
Fig. 2 Total reflectance (specular + diffuse) of the frustum GaAs NP array and that of the bare GaAs substrate measured using an integrating sphere.
To have a better understanding of light behavior in NPs with different geometries, and to avoid the bare substrate contribution to the measured data, two sets of samples with different geometries (frustum and conical as explained in the fabrication section) are chosen. To obtain a smaller beam spot size, the reflectance spectra are measured in a backscattering geometry with an optical probe (Filmetrics F40) attached to a Zeiss optical microscope [11]. An objective with 5 × magnification and numerical aperture (NA) of 0.13 is used. With this configuration, we can obtain a spot size area of: 100 μm × 100 μm. The background and the baseline calibrations are performed with a Si reference substrate. The measured reflectancespectra are plotted in Fig. 3(a) . Both geometries show very low measured reflectance (less than 2%).We also performed a set of simulations using Lumerical FDTD Solutions software on the reflectance spectra of NPs, Fig. 3(b). The simulations could provide a guideline for an optimized design for NP based solar cells. The simulation and measured data show good agreement in terms of the position of dips and peaks. The absolute value of the simulated reflectance data is slightly higher, which can be attributed to the fact that the measured reflected light is collected with an objective with the NA of 0.13 and 7.5° half-angle, and thus does not include all the reflected light (diffuse reflectance). It should be noted that the fabricated NPs do not have the exact same diameters and heights. The slight discrepancies between the simulation and the measured data can be explained by the differences between individual NPs in the array. The diffuse reflectance is caused by the irregularities in the actual fabricated sample compared to the perfect array assumed in the modeling. One source for the differences in the size of the pillars is inherited from the initial size variance of silica masks. As a consequence, the spatial separation and etch-depths are also affected. The conical NP array also shows less pronounced dips and peaks, which can be due to an improved refractive index matching between air and GaAs NPs. In case of the frustum NP array, a pronounced dip can be observed in Fig. 3(a), around the wavelength of 550 nm, which corresponds to the coupling of light to the NP waveguide.
Fig. 3 Dependence of the measured reflectance spectra (a) and 3D FDTD simulated (b) reflectance of two different geometries of GaAs NP array samples (conical and frustum), with typical period of 500 nm, on the wavelength of light; the dashed black lines show the measured and simulated data for the frustum NP array and the solid lines that of the conical NP array. The measurements were carried out in backscattering geometry with a 5 × objective and NA of 0.13.
4. Photoluminescence and surface passivation
While surface states are a major concern for most III-V semiconductor devices, GaAs is known to be specially a difficult material in this regard. In the case of NPs, due to the large surface area to volume ratio the density of dangling bonds, which act as carrier traps and nonradiative recombination centers, is very high. In addition, the process steps in NP fabrication can also introduce surface defects. In general, the interface and surface states due to stoichiometric imbalance (either As-rich or Ga-rich) and crystal lattice defects, result in surface Fermi level pinning within the band gap of GaAs [23]. Existence of carrier traps on the NPs’ surfaces would be a limiting factor for device performance, since they are a major source of carrier loss. Hence, an effective passivation method to reduce the surface recombination rate by passivating the surface states is critical. For example, in a solar cell device, any non-radiative recombination hinders the carrier density buildup, which limits the open-circuit voltage and consequently the efficiency [24]. Therefore, combining proper optical design for enhanced optical absorption (and reciprocally enhanced external emission [24]), in our case tapered NPs, with the improved NP material optical quality by reducing nonradiative surface recombination is necessary to obtain high performance solar cells. Passivation of GaAs NWs/NPs can be realized using a suitable passivating chemical [16] or by applying a core-shell structure, where the high band gap material (e.g. AlInP) [25] acts as a shell. Here we show that employing a two-step post-processing treatment, namely chemical etching by citric acid to remove the near surface layer followed by a sulfur-based passivation step dramatically decreases surface recombination in GaAs NPs and results in a 12 fold increase of PL intensity.
GaAs NPs are fabricated on one micron thick epitaxial grown n-GaAs with a doping concentration of 1016 cm−3 on (100) n+-GaAs substrate. After the fabrication of GaAs NPs, the samples were dipped in the diluted HF to remove the remnant silica particles from the top of pillars. Subsequently they are rinsed with DI water and etched with a citric acid and hydrogen peroxide solution (C6H8O7: H2O2: H2O = 25: 1: 75) for 20 sec. The etch solution has a very slow etch rate (~15.3 nm/min) and is used to remove the few nanometers of the NP surface layer damaged by ion bombardment during the dry-etch process. The passivation effect of sulfur-based solutions are known for GaAs [26,27] and other III-V semiconductors [28,29]. Here, we use a passivation solution based on organic polysulfide by dissolving sulfur in oleylamine (OA) [29]. Sulfur is dissolved, with the concentration of 1.5%, in commercially available OA (technical grade, ≥ 70% (GC) from Sigma Aldrich) at 90 °C. The samples then are immersed in the solution at 92°C for 2 hrs and subsequently rinsed thoroughly with isopropanol.
The passivation effect can be qualified by photoluminescence spectroscopy, by comparing the PL-yield (intensity) and line-width before and after passivation. Figure 4 shows comparison of the measured PL spectra of as-etched, sulfur-passivated (S), wet-etched using citric acid and combination of wet-etched (citric acid) and S-passivated NPs. The PL spectra were measured at room temperature, using a μ-PL set-up in a backscattering geometry. A 50 × objective of NA = 0.45 was used for the excitation and detection. An Ar + laser (514 nm), with an effective power of 0.25 mW at the sample, was used for excitation in all the measurements. It can clearly be seen from Fig. 4 that sulfur passivation considerably enhances PL from NPs. Wet etching using citric acid alone has no significant enhancement on PL, but when wet-etching is followed by sulfur passivation, the best result is obtained (i.e. highest PL intensity and narrowest linewidth). Compared with as-etched GaAs NP arrays, wet citric acid-etched and then passivated NPs demonstrated ~12 times higher PL. This strongly suggests considerable reduction of surface states on the surfaces of GaAs NPs; since it removes damaged layers induced by ICP-RIE step first, by wet-etching, and then passivates GaAs NPs. Even though, sulfur passivation without any wet etching resulted in ~5 times higher PL,indicating a reduction in surface states, a preceding step to remove the damage layer makes it more effective. To have an additional qualification of the passivation effect, the normalized PL intensity of treated NP samples along with the epi-layer bare sample, as reference, is plotted in Fig. 5 . It is evident that as-etched NPs result in much broader spectrum (due to more exposed surfaces and hence Fermi-level pinning) compared to the bare epi sample. As it can be seen, the full width at half maximum (FWHM) of the PL spectrum of the passivated NPs (citric acid and sulfur passivation) is close to the epi-grown reference sample. Recovery of the PL line-width and increased PL yield (12 fold enhancement achieved) are both indeed very important criteria in terms of material optical quality and indicate the effective surface passivation of GaAs NPs, attributed to the formation of sulfur bonds at the surface of GaAs NPs.
Fig. 4 Comparison of room temperature PL intensity of as-etched GaAs NP array sample and the ones after different types of post-treatments: citric acid (red line), sulfur passivation (S) (blue line), both citric acid and sulfur passivation (Citric + S) (green line). The passivation effect of combined citric acid and sulfur treatments can be noted by the PL yield which is about 10 times.
Fig. 5 Comparison of normalized room temperature PL spectra of as-etched GaAs NP array sample and the ones after different types of post-treatments. The bare epi grown sample PL spectra is also plotted as the reference. The linewidths of the treated samples are very close to the epi-grown reference sample, which indicate the passivation effect.
5. Conclusion
We have demonstrated that GaAs NPs with tapered geometries and smooth sidewalls, suitable for enhanced absorption (and efficient light extraction), could be fabricated using a simple and cost-effective method. Nanopillars were fabricated using the combination of nanosphere lithography (using silica particles) and dry etching. By optimizing the etch parameters, conical and frustum GaAs NP arrays with a period of 500 nm and typical length of 800 nm are obtained. The geometry and shape of NPs result in appreciable reduction of the total reflectance for a broad wavelength range, which is advantageous for solar cell applications. The measured reflectance spectra were in good agreement with FDTD simulations. A 12 fold enhancement in PL intensity and almost complete recovery of PL line-width were observed by applying a combination of chemical etching and sulfur passivation on GaAs NPs, which make them a promising candidate for future solar cell applications. Further improvements can be expected upon regrowth of the GaAs NPs, e.g. by a suitable high-band gap material such as GaInP or AlGaAs.
Acknowledgments
The work was performed within the Linné center for advanced optics and photonics (ADOPT), and was supported by the Swedish Research Council (VR), European Union FP7 Network of Excellence Nanophotonics4Energy (N4E) and Nordic Innovation Center project NANORDSUN.
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