Fabrication of Dual-Type Nanowire Arrays on a Single Substrate

Department of Micro- and Nanosciences, Micronova, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland ... In this Letter, we propose a novel met...
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Fabrication of Dual-Type Nanowire Arrays on a Single Substrate Joona-Pekko P. Kakko,*,† Tuomas Haggrén,† Veer Dhaka,† Teppo Huhtio,† Antti Peltonen,‡ Hua Jiang,¶ Esko Kauppinen,¶ and Harri Lipsanen† †

Department of Micro- and Nanosciences, Micronova, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland Aalto−NanoFab, Micronova, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland ¶ Department of Applied Physics and Nanomicroscopy Center, Aalto University, P.O. Box 15100, FI-00076 Aalto, Finland ‡

S Supporting Information *

ABSTRACT: A novel method for fabricating dual-type nanowire (NW) arrays is presented. Two growth steps, selective-area epitaxy (SAE) in the first step and vapor− liquid−solid (VLS) in the second step, are used to grow two types of NWs on the same GaAs substrate. Different precursors can be used for the growth steps, resulting in sophisticated compositional control, as demonstrated for sideby-side grown GaAs and InP NWs. It was found that parasitic growth occurs on the NWs already present on the substrate during the second growth step and that the SAE NWs shadow the growth of the VLS NWs. Optical reflectance measurements revealed the dual-type array having improved light trapping properties compared to single-type arrays. Dual-type NW arrays could be practical for thermoelectric generation, photovoltaics and sensing where composition control of side-by-side NWs and complex configurations are beneficial. KEYWORDS: dual-type, III−V, nanowire, vapor−liquid−solid, selective-area epitaxy, metallo-organic vapor phase epitaxy

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deposition of growth seeds in between to achieve compositional difference between the NWs.9 This method, however, does not allow the position control of the second-grown NWs (they tend to grow on top of the first-grown ones), and it is limited to the vapor−liquid−solid (VLS) NW growth mechanism. In this Letter, we propose a novel method that combines SAE of NWs10 and VLS-growth to implement different types of NWs side-by-side into a single NW array on a single substrate, what we name a dual-type NW array. The proposed method allows sophisticated degree of control for the composition and position. We demonstrate the growth of GaAs and InP NWs on the same substrate and show that dual-type NW arrays allow increased light trapping. GaAs SAE and GaAs VLS NWs were fabricated on the same GaAs substrate by metallo-organic vapor phase epitaxy (MOVPE) using two subsequent growth steps. The complete process is presented schematically in Figure 1. The process began by depositing Au nanoparticles (NPs) on a GaAs (111)B substrate with either dispersing NPs from a colloidal solution (60 nm Au NPs from BBI solutions) or by evaporating Au with the help of a electron beam lithography (EBL) defined lift-off mask. In the case of evaporation, a poly(methyl methacrylate) (PMMA, 950 000 g/mol, 2% in anisole from Microchem) and copolymer (MMA, 11% in ethyl lactate from Microchem) resist

ompound semiconductor nanowires (NWs), that is, GaAs, InP, GaN and others, are being explored for several applications such as light emitting diodes,1,2 solar cells,3 photoelectrochemical cells,4 and thermoelectric generators.5 For most of these applications, one goal is to implement different material compositions or NW geometries to a single device to increase efficiency and performance. The current mindset is affixed to perfect the NW device in a single growth run or with additional processing and regrowth on top of the already present NWs.6,7 Little consideration has been given to architectures with different kind of NWs on the same substrate side-by-side. Compositional contrast in side-by-side NWs could be beneficial for NW solar cells and photodetectors by introducing additional band gaps to widen the spectral absorption range. Similarly with several band gaps, white light NW light emitting diode arrays could be fabricated. Also, side-by-side n- and ptype NWs could be beneficial to NW thermoelectric generators and a NW array with additional selectivities to different ions and molecules could be useful in sensing applications. The common case to observe different kinds of NWs on the same substrate is the different crystal phases, growth directions, or morphology the NWs have obtained during the growth.8 The growth-induced differences within a NW ensemble are random or distributed statistically. In order to achieve controlled contrast between the NWs, especially chemical contrast, fabrication needs to be split into two or more growth steps with processing in between. One earlier method describes the use of a two-step growth process with an additional © XXXX American Chemical Society

Received: November 10, 2014 Revised: February 5, 2015

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DOI: 10.1021/nl504308x Nano Lett. XXXX, XXX, XXX−XXX

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SiO2 layer was patterned with 50 nm holes in a triangular lattice with EBL and reactive-ion etching (RIE). The hole lattice is intentionally slightly offset from the Au disc lattice in order not to have a hole and a disc overlap (Figure 1b). In the first MOVPE growth step, SAE of GaAs NWs was carried out from the patterned holes. GaAs NWs were grown at 750 °C for 5 min with trimethylgallium (TMGa) and tertiarybutylarsine (TBAs) flows of 0.81 μmol/min and 210.1 μmol/min, respectively. Prior to growth, thermal cleaning was performed at 750 °C for 5 min under TBAs protection. The NPs are left intact underneath the SiO2 layer during the SAE growth as seen in the SEM image of Figure 1c. After the first growth step, the sample was removed from the reactor and immersed in buffered hydrofluoric (BHF) acid for 30 s to etch the SiO2 layer. After rinsing with deionized water, the drying was done in isopropanol to decrease the damage to the NWs from the surface tension between the drying water droplets and NWs. For the second growth step, the growth suppressing SiO2 layer was no more present and the governing growth mode was VLS via the deposited Au NPs or discs. However, before the second growth could be performed, the evaporated Au discs or dispersed Au NPs had to be annealed in order to form liquid Au droplets needed for the VLS growth. The annealing was performed at 650 °C for 5 min under TBAs protection. For the VLS growth of GaAs NWs, the TMGa and TBAs flows were 4 μmol/min and 11.7 μmol/min, respectively. Growth was performed at 470 °C for 2 min. A Thomas Swan system was used for the MOVPE growth, operated in atmospheric pressure with H2 as the carrier gas, and total gas flow of 5 slm. A Vistec EPBG5000pES was used for the EBL and an Edwards E306A system for the Au evaporation. An Oxford Instruments Plasmalab 80Plus system was used for the PECVD SiO2 deposition and for the RIE. A Zeiss Supra 40 scanning electron microscope (SEM) and JEOL JEM-2200FS transmission electron microscope (TEM) operated at 200 kV in scanning mode (STEM) were used for the characterization of the NW arrays. Detailed description of all the fabricated samples can found tabulated in Table S1 in the Supporting Information. By using several different masking layers in the

Figure 1. Scheme for the fabrication of a dual-type NW array and the SEM images for the corresponding steps. (a) GaAs substrate with deposited Au NPs. (b) Deposited SiO2 is patterned with holes using EBL and RIE. (c) First NW growth step using SAE. (d) The SiO2 is removed with 30s BHF etching. (e) Annealing of the NPs and second NW growth step via VLS-method. 20° tilted SEM image of dual-type GaAs/GaAs NW array. The longer NWs are grown with SAE and the shorter ones via VLS. Inset shows a plan view SEM image of the same array.

stack was used as the lift-off mask. Holes, 120 nm in diameter, in a triangular lattice were patterned to the resist stack with EBL and the residual resist was removed with acetone after electron beam evaporation of 5 nm Au. Several 30 μm by 30 μm arrays were patterned with varying pitch from 300 nm upward with a 50 nm step. Next, the Au NPs or discs were covered with approximately 40 nm thick SiO2 layer with plasma-enhanced chemical vapor deposition (PECVD). The SiO2 layer suppresses epitaxial growth on the substrate and VLS growth from the NPs. The

Figure 2. Average NW lengths (a) and diameters (b) of the GaAs NW arrays as a function of the pitch. The blue upside-down triangles are the firstgrown SAE NWs, red upright triangles the second-grown VLS NWs, both in the GaAs/GaAs dual-type array, magenta diamonds are the reference sample with VLS-only NWs. The standard deviation of each array is shown with the error bars. B

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could be explained by the combination of the decreased catalytic decomposition of TMGa due to reduced Au catalyst areal coverage at higher NW spacings and the efficient blocking of the TMGa intermediate decomposition products in the gas phase by the SAE-grown NWs. The blocked material can potentially contribute to the parasitic growth on the SAE-grown NWs. At the 300 nm pitch, where no apparent difference is seen in the lengths of the VLS-grown NWs and the VLS-only NWs, the VLS growth is limited by material competition and the Au catalytic action is increased due to increased areal coverage. These effects could possibly be the cause why the shadowing effect is not clearly observed there. Yet, the shadowing effect is observed for the 300 nm pitch in the diameters of the second-grown NWs in Figure 2b. Independent of the array pitch, the diameters of the second-grown NWs remain smaller than the diameters of the VLS-only reference sample. A comprehensive study is needed to reveal more details on the nature of the shadowing effect and parasitic growth. So far we have demonstrated, that NW arrays with two distinct types of NW, SAE and VLS with different lengths and diameters, can be fabricated with the proposed method. Furthermore, compositional control, such as doping contrast or chemical contrast, between the first- and second-grown NWs can be easily implemented by introducing different precursors in the growth steps. Here, one sample with SAE-grown GaAs NWs and VLS-grown InP NWs on GaAs (111)B was fabricated to demonstrate this capability and to study the effects of the parasitic growth with spectroscopic methods. The SAE GaAs NWs were grown with similar growth parameters as for the GaAs/GaAs dual-type array (see Table S1 in the Supporting Information). The InP NWs were grown in the second growth step using trimethylindium (TMIn) and tertiarybutylphosphine (TBP) precursors. The TMIn and TBP flows were 5.6 μmol/min and 1100 μmol/min, respectively, the growth temperature was 430 °C, and growth time was 1 min. Figure 3a shows a SEM image of the sample. Again, clearly two types of NWs are present on the substrate. The short ones are GaAs NWs and the longer ones, with the particles at the tips, are InP NWs. Colloidal Au was used for the NP dispersion; hence, the InP NW positioning is random. Energy-dispersive X-ray (EDX) spectroscopy was performed on a cross-section sample of the GaAs/InP dual-type array. A FEI Helios Nanolab 600 dual-beam focused ion beam (FIB) system was used for the preparation of the cross section sample and STEM was used for the characterization. Figure 3b shows a false-color dark field (DF) STEM image of the cross section of the NW array. The NWs are covered with a PECVD-deposited SiN and FIB-deposited metallic Pt for protection from the Gabeam used in the FIB. Some of the NWs have bent during the SiN and Pt covering. The short (blue) NWs are from the first SAE growth step and the longer ones are from the second VLS growth step (red). An Au particle is even visible at the tip of one of the longer NWs. EDX was performed along the cyan line marked in Figure 3c. The EDX signals shown are the counts for the atomic As (green) and In (red). Along the EDX line scan, the NW on the left is a GaAs NW grown with SAE and the other two are InP NWs grown via VLS. Some In is present in the GaAs NW, due to parasitic growth that occurred during the second growth step. The InP NWs seem to be free of As, which is an indication that these NWs were grown in growth conditions free of TBAs, as is the case in the second growth step. The high As signal at the edges

process, additional complexity or NW types could be implemented into a single array. Figure 1e shows a 20° tilted SEM image of the GaAs/GaAs dual-type NW array grown with the described method. Clearly two types of NWs were grown side-by-side on the same substrate. (More SEM images are in Figures S1 and S2 in the Supporting Information.) The VLS NWs are shorter, are slightly tapered, and have Au droplets at their tips, whereas the SAE NWs are longer, do not have droplets at their tip, and exhibit a hexagonal morphology and vertical sidewalls. The pitch of the dual-type NW array is defined in this Letter as the pitch of an individual NW lattice. A 600 nm pitch dual-type array has both SAE and VLS-grown NWs with the 600 nm pitch. Parasitic growth is visible at the roots of the NWs and on the sidewalls and on the top of the first-grown SAE NWs. The parasitic growth takes place during the second growth step as the growth suppressing SiO2 layer is not present in it and the already present SAE-grown NWs offer additional surface for the precursor materials to attach. The parasitic growth is observed by comparing the heights, diameters, and morphology of the SAE-grown NWs before and after the second growth step (not shown here). The parasitic axial growth on the first-grown NWs is approximately 250 nm during the 2 min second growth step for the 600 nm pitch array. To avoid the parasitic growth on the first-grown NWs’ sidewalls, a growth inhibiting layer, such as SiO2, could be deposited on them and etched back with RIE to reveal the Au NPs. Also, radial growth suppressing HCl in situ etching could be used to inhibit the parasitic growth.11 The SAE NWs already present on the substrate, before the second growth step, affect the growth of the VLS NWs. This is observed from the plot of NW lengths versus array pitch in Figure 2a. The figure plots the average lengths of NWs for the first-grown SAE NWs (blue ▼)), second-grown VLS NWs (red ▲), both in the GaAs/GaAs dual-type NW array, and for a reference sample (magenta ◇), which only had VLS NWs grown from the Au droplets. The growth parameters for the VLS growth were identical in the reference and in the dual-type NW array. Average lengths for the first-grown NWs at 400 and 450 nm pitch could not be measured as the EBL alignment in the hole patterning step was slightly off and the holes overlapped with the Au discs (see figure S1b in the Supporting Information). Only VLS-grown NWs were observed from these two arrays. The heights and diameters of the NWs were measured from 20° tilted SEM images. A sample of 20 was used to measure the averages and standard deviations from each array. Up to the pitch of 350 nm, the lengths of the VLS-grown NWs in the GaAs/GaAs dual-type NW array and the VLS-only reference array are roughly identical until the first-grown SAE NWs start to shadow the growth of the VLS NWs. The VLS NWs get shorter as the pitch increases and simultaneously the first-grown SAE NWs become longer. Even though saturation is reached for the SAE NWs after the pitch of 650 nm, the average lengths of the second-grown VLS NWs continue to decrease. It has been observed elsewhere12 that the average length of VLS-grown NWs begins to decrease as the NW spacing increases beyond the gas phase diffusion length of the TMGa intermediate decomposition products. Here, the behavior is observed already at very close spacing, whereas in the VLS-only reference the turning point is not observed within the measured range. The cause for the decreased turning point, or the onset of the shadowing effect, in the GaAs/GaAs dual-type array C

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optics yield a collecting spot size of approximately 20 μm in diameter. The reflected light was coupled to a collecting fiber using the optics and led to a spectrometer (Avantes AvaSpec 2048 × 14). A plain Si wafer was used as the reference for the reflectance measurements. The NW lengths and diameters in the single-type samples were targeted to match the dimensions of the individual NW types in the dual-type array (see growth parameters from table S1 in the Supporting Information). The lengths and diameters of the NWs are tabulated in Table 1. As shown in Figure 4, Table 1. Average Diameters, d, and Lengths, l, and Their Standard Deviation for the NW Arrays in the Reflectance Measurements

d (nm) l (μm) d (nm) l (μm)

dual-type array

single-type

First Grown SAE 107.8 ± 7.7 2.3 ± 0.2 Second Grown VLS 73.1 ± 4.1 1.1 ± 0.2

SAE Control 106.1 ± 6.5 1.9 ± 0.1 VLS Control 68.2 ± 3.0 1.1 ± 0.2

lower reflectance was measured from the GaAs/GaAs dual-type array (thick blue line) compared to the single-type reference arrays, VLS-control (thick red), and SAE-control (dashed blue). From the measurements, we observe that when two NW arrays are combined into a single dual-type array, the reflectance is lower, giving dual-type array enhanced light trapping properties compared to a single-type array. Light coupling in NWs is dominated by the NW diameter.13,14 However, the larger diameter first-grown NWs in the dual-type array are not the sole contributor for the low reflectance. Only when they are combined with the secondgrown NWs, a decrease in the reflectance is seen. One possible cause for the enhanced light-trapping is the additional step in the effective refractive index of the NW array due to different heights of the first and second-grown NWs.15 From top to bottom, there are initially only the longer first-grown NWs and around halfway down the both types of NWs are present. The step decreases the contrast of refractive indices between surrounding air and the substrate, thus increasing refraction into the dual-type array. Furthermore, the NW density in the GaAs/GaAs dual-type array (∼6.4 NW/μm2) is double compared to the single-type NW arrays (∼3.2 NW/μm2) with the same NW pitch in Figure 4. However, even when a single-type array with similar NW density is compared to the dual-type array, the overall reflectance is still lower from the dual-type array (see Figure S3 in the Supporting Information). In conclusion, a method to fabricate dual-type NW arrays was demonstrated. Two types of NWs were grown on the same GaAs substrate by first growing NWs with SAE and subsequently with VLS. Growth of GaAs and InP NWs sideby-side on the same substrate was demonstrated and the composition of the NWs was verified by EDX spectroscopy. It was discovered that the first-grown SAE NWs shadow the growth of the VLS NWs and parasitic growth occurs on the SAE NWs, as observed from SEM images and EDX measurements. Methods to inhibit the parasitic growth are needed to decrease its effect. The dual-type NW array offers increased light trapping compared to single-type arrays, suggesting that the additional step in the effective refractive index increases refraction into the NW array. The increased

Figure 3. (a) SEM image of the GaAs (short ones) and InP (longer ones with the particles at the tips) NWs in the GaAs/InP dual-type NW array. (b) Cross section DF STEM image of the same sample with false coloring. The blue tinted NWs are GaAs and the red tinted InP. (c) Magnification of the dashed area in (b) with an EDX line scan along the cyan line. The EDX counts for atomic As and atomic In are shown in green and red, respectively. The NWs are covered with SiN and Pt for protection during the cross section sample preparation.

of the line scan is due to sputtered As from the GaAs substrate during the fabrication of the cross section sample. Optical properties of the 600 nm pitch GaAs/GaAs dual-type NW array were studied with reflectance measurements and compared to single-type GaAs NW arrays grown with VLS and SAE, also with the 600 nm pitch. The reflectance was measured with broadband light from deuterium and halogen lamps coupled to an optical fiber and focused to the sample at normal incidence with optics and a 40× (0.6 NA) objective. The used D

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Figure 4. Reflectance from GaAs NW arrays with the 600 nm pitch. The blue thick line is the reflectance from the GaAs/GaAs dual-type NW array. The red thick line is the reflectance from a VLS control sample, and the reflectance from a SAE control sample is shown by the thin dashed blue line. The lengths and diameters of the NWs in the arrays are tabulated in Table 1 (6) Kempa, T. J.; Tian, B.; Kim, D. R.; Hu, J.; Zheng, X.; Lieber, C. M. Nano Lett. 2008, 8, 3456−3460 DOI: 10.1021/nl8023438. (7) Kandala, A.; Betti, T.; Fontcuberta i Morral, A. Phys. Status Solidi A 2009, 206, 173−178 DOI: 10.1002/pssa.200723672. (8) Fonseka, H. A.; Caroff, P.; Wong-Leung, J.; Ameruddin, A. S.; Tan, H. H.; Jagadish, C. ACS Nano 2014, 8, 6945−6954 DOI: 10.1021/nn5017428. (9) Dick, K. A.; Deppert, K.; Larsson, M. W.; Martensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Nat. Mater. 2004, 3, 380−384 DOI: 10.1038/nmat1133. (10) Ikejiri, K.; Noborisaka, J.; Hara, S.; Motohisa, J.; Fukui, T. J. Cryst. Growth 2007, 298, 616−619 DOI: 10.1016/j.jcrysgro.2006.10.179. (11) Berg, A.; Lehmann, S.; Vainorius, N.; Gustafsson, A.; Pistol, M.E.; Wallenberg, L. R.; Samuelson, L.; Borgström, M. T. J. Cryst. Growth 2014, 386, 47−51 DOI: 10.1016/j.jcrysgro.2013.09.043. (12) Borgstrom, M. T.; Immink, G.; Ketelaars, B.; Algra, R.; Bakkers, E. P. Nat. Nanotechnol. 2007, 2, 541−544 DOI: 10.1038/ nnano.2007.263. (13) Wu, P. M.; Anttu, N.; Xu, H. Q.; Samuelson, L.; Pistol, M.-E. Nano Lett. 2012, 12, 1990−1995 DOI: 10.1021/nl204552v. (14) Seo, K.; Wober, M.; Steinvurzel, P.; Schonbrun, E.; Dan, Y.; Ellenbogen, T.; Crozier, K. B. Nano Lett. 2011, 11, 1851−1856 DOI: 10.1021/nl200201b. (15) Zhu, J.; Yu, Z.; Burkhard, G. F.; Hsu, C.-M.; Connor, S. T.; Xu, Y.; Wang, Q.; McGehee, M.; Fan, S.; Cui, Y. Nano Lett. 2009, 9, 279− 282 DOI: 10.1021/nl802886y.

light trapping can be beneficial in photovoltaic and photodetection applications. The described method offers sophisticated compositional control of side-by-side NWs and enables the fabrication of complex architectures with controlled variations of NW diameter, length, and composition within one NW array.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the samples, including MOVPE growth details, SEM images of GaAs/GaAs dual-type arrays, and reflectance measurements from similar NW density GaAs/GaAs dualtype and single-type GaAs arrays. This material is available free of charges via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: joonapekko.kakko@iki.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Moppi project of Aalto Energy Efficiency Program, the Datis/ENPI project and the Aalto University ELEC doctoral school. Majority of the work was performed in the Micronova clean room facilities and the use of Aalto University Nanomicroscopy Center (Aalto-NMC) premises is acknowledged.

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REFERENCES

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