Letter pubs.acs.org/NanoLett
III−V Nanowire Synthesis by Use of Electrodeposited Gold Particles Reza Jafari Jam,† Magnus Heurlin,† Vishal Jain,†,‡ Anders Kvennefors,† Mariusz Graczyk,† Ivan Maximov,† Magnus T. Borgström,† Håkan Pettersson,†,‡ and Lars Samuelson*,† †
Division of Solid State Physics/Nanometer Structure Consortium, Lund University, Box 118, SE-221 00, Lund, Sweden School of Information Science, Computer and Electrical Engineering, Halmstad University, Box 823, S-301 18, Halmstad, Sweden
‡
S Supporting Information *
ABSTRACT: Semiconductor nanowires are great candidates for building novel electronic devices. Considering the cost of fabricating such devices, substrate reuse and gold consumption are the main concerns. Here we report on implementation of high throughput gold electrodeposition for selective deposition of metal seed particles in arrays defined by lithography for nanowire synthesis. By use of this method, a reduction in gold consumption by a factor of at least 300 was achieved, as compared to conventional thermal evaporation for the same pattern. Because this method also facilitates substrate reuse, a significantly reduced cost of the final device is expected. We investigate the morphology, crystallography, and optical properties of InP and GaAs nanowires grown from electrodeposited gold seed particles and compare them with the properties of nanowires grown from seed particles defined by thermal evaporation of gold. We find that nanowire synthesis, as well as the material properties of the grown nanowires are comparable and quite independent of the gold deposition technique. On the basis of these results, electrodeposition is proposed as a key technology for large-scale fabrication of nanowire-based devices. KEYWORDS: Gold electrodeposition, nanoimprint lithography, crystal phase, nanowires
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and additional material is lost in the resist removal process. Additionally, vacuum condition is required in the evaporation process in order to obtain a high material purity, which results in a low throughput. Here we report on the use of gold electrodeposition for selective gold particle definition. Electrodeposition has successfully been used to produce nanoscale patterns in combination with lithography32,33 and in addition been shown to have several applications in semiconductor industry.34−37 Electrodeposition provides several key advantages as compared to conventional methods of creating seed particles for NW growth: (i) it is 100% selective to conductive surfaces that drastically reduces the material consumption because a nonconductive surface, like photoresist, will not attract any metal; (ii) it provides a high throughput because it neither relies on vacuum conditions, nor requires specific time demanding preparation steps; (iii) in contrast to a typical lift-off process, the thickness of electrodeposited gold is not limited by the undercut in the resist, enabling taller features to be deposited for a given resist height; (iv) because no undercut is required, seed particles can be defined with small pitch; (v) the possibility of using inorganic masks in combination with a ripoff technique38 makes it feasible to reuse the substrate, which would drastically reduce the cost of the end product; and (vi) it
anowires (NWs) are emerging as a competitive technology in electronic and photonic applications.1−8 Epitaxial NWs have to date been fabricated by a number of techniques, including vapor−liquid−solid (VLS),9−11 selective area,12,13 self-catalyst,14,15 and oxide assisted16,17 growth. The most widely used NW growth mode, VLS, is based on the use of a catalytic metal seed particle,18 primarily gold. NW-based heterostructures19 and quantum dots20 were demonstrated that could be used to facilitate fabrication of solar cells,21,22 light emitting diodes,23,24 and transistors25,26 as examples of III−V NW structures utilizing gold metal seed particles. However, the requirement of metal seed particles in the growth process is becoming a critical issue with respect to commercialization. For instance, as the fundamental efficiency limit of NW-based solar cells is approached, the main goal is to reduce the fabrication cost. Because III−V substrates and gold are expensive materials, a reduced consumption is desirable to cut the cost of the final device. The required gold seed particles for NW synthesis can be deposited by, for example, thermal evaporation,27 aerosol,28 and colloid29 particle methods. These methods provide different advantages and drawbacks that have been thoroughly reviewed.30 It is challenging to order aerosol or colloid particles in a pattern,31 which makes these techniques less suitable for device fabrication with requirements on high reproducibility and uniformity. Thermal evaporation of a thin film is the conventional method for patterned seed particle definition. However, during deposition on the substrate, material is lost by simultaneous deposition on the evaporation chamber sidewalls, © XXXX American Chemical Society
Received: August 20, 2014 Revised: November 17, 2014
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provides the possibility for local and selective deposition of catalytic seed particles also in complex nanoscale geometries, for example, high aspect ratio features, for which nonselective thermal evaporation is unable to controllably deposit metal at the semiconductor surface. In this study, the pattern of gold seed particles for NW growth was defined by electrodeposition using nonconductive organic photoresist, as well as inorganic SiNx masks. Subsequently, InP and GaAs NWs were grown from these particles, as well as from gold particles deposited by standard thermal evaporation and lift-off. Because the NWs were grown under identical conditions from catalysts defined by both types of gold deposition methods, a direct comparison of their properties could be performed. The grown NWs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy. The seed particle array was defined by nanoimprint lithography (NIL), using an Intermediate Polymer Stamp in a Simultaneous Thermal and UV-process (IPS- STU).39 The IPS, which consists of an array of 100 nm tall pillars with a diameter of 160 nm and a pitch of 400 nm was made as a replica from a 2.5 in. master Ni stamp. To reveal the substrate surface the remaining resist residues were removed by reactive ion etching in oxygen. This process results in surface oxidation; but electrodeposition requires a conductive surface and any oxide would lead to process failure. In order to remove this surface oxide, the sample was etched with buffered oxide etch 1:10 (BOE) for 30 s. The SiNx mask was etched in an inductively coupled plasma reactive ion etching process (see Supporting Information for detailed information) which leads to the formation of polymers on the substrate surface. In order to remove these polymers, the substrate was oxygen plasma ashed for 2 min. A 24K pure gold solution40 with 99.9% purity was used for direct gold electrodeposition. According to the supplier, 1 quart (about 950 mL) of this solution contains enough gold to cover around 8000 cm2 of area to a thickness of around 250 nm. The process is reliable as long as the gold content of the solution is above 80% of the initial value, while the pH and temperature should be kept within 5% of their initial values.40 To compensate the gold depletion, the bath can be replenished by a commercial replenisher. The pH value can be adjusted by slowly adding 10% phosphoric acid to the solution. The low pH of around 5.8 of this cyanide-based solution does not significantly affect the resist during electrodeposition. We carefully verified that the resist was intact after gold deposition, being aware that cyanide-based solutions can affect the resist profile.41 For electrodeposition, the substrate, which acts as the cathode, was mounted on a printed circuit board while platinum-plated titanium was used as the anode electrode. Both electrodes were submerged in a beaker filled with 30 mL of gold solution. In order to increase the throughput, the solution temperature was kept at 35 °C rather than its nominal temperature of 60 °C. At lower temperatures, more carbon contamination in the deposited material is expected.42 A Keithley 2400 source meter was used as the current source to feed the required current for 10 s. After the electrodeposition process, the samples were rinsed in deionized water and dried with a nitrogen gun. Figure 1 shows samples at different fabrication steps from pattern definition and electrodeposition to a final structure with gold particles in an array on the substrate. Figure 1a,b shows
Figure 1. SEM images of samples during different processing steps. (a) InP substrate with resist mask after NIL and dry etching (b) sample in (a) after deposition of 33 nm thick gold particles and resist striping (c) array of electrodeposited gold particles defined by SiNx mask on GaAs, and (d) cross-sectional view of gold seed particles on a Si substrate transformed into hemisphere-shaped seed particles after annealing.
SEM images of an InP sample with patterned resist mask and the resulting electrodeposited gold seed particles after removal of the resist, respectively. Figure 1c shows a corresponding SEM image of a GaAs sample with patterned inorganic SiNx mask and electrodeposited gold seed particles. The resist layers on this mask were removed during the oxygen plasma ashing. In order to simulate the profile of the gold particles during growth, a Si sample electrodeposited with gold particles was annealed at the eutectic temperature of gold and Si using rapid thermal processing. Figure 1d shows a SEM image of the sample after annealing, revealing the expected change in shape of the gold particles into a hemisphere. The density of the array on all substrates is 6.25 μm−2, and SEM inspection shows that the hole diameter is around 175 nm that results in a conductive area of 0.054 cm2 on a 6 mm × 6 mm sample. For a gold thickness of 25 nm to be deposited on n-type InP, GaAs and Si substrates, the required current density is 5.5, 5.9, and 5.6 mA/cm2, respectively. The current density levels used in this study were about 2.5 times higher than the nominal values to compensate for the lower deposition rate at 35 °C compared to 60 °C. We could reproducibly perform at least 10 deposition cycles on our 6 mm × 6 mm samples without replenishing the gold solution. We expect the process to be reproducible as long as the solution is kept within the specifications mentioned before. By calculating how much gold is actually deposited, we estimate that 30 mL of the gold solution would be enough to cover 20 2 in. wafers in a reproducible process. The deposition rate and quality of the deposited gold particles are strongly dependent on doping level of the substrate. In this work, substrates had doping levels between 1018 cm−3 and 1019 cm−3 (resistivities between 2 × 10−3 and 5 × 10−3 Ω-cm). Lower doped (higher resistivity) substrates need higher current densities.37 Considering that the area of the deposited material is almost 10 times less than if gold was deposited on the entire substrate, which would be the case if evaporation was used, the electrodeposition process is more efficient with respect to material consumption. Moreover, there is a substantial waste of gold by deposition on the chamber sidewalls in conventional evaporation. As an example, in order to deposit a thickness of 272 nm on a 4 in. wafer using B
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with gold. The recorded spectra for individual InP NWs grown from electrodeposited gold particles exhibit peaks ranging from 1.4 to 1.45 eV; for different laser intensity, see Figure 3a. The
a Pfeiffer Classic 500 evaporator, 4.2 g of gold is required, out of which approximately half can be recovered. In contrast, the electrodeposition process only consumes 6.4 mg of gold, which means a factor of 300 times less material as compared to thermal evaporation after chamber cleaning and recovery. In order to compare InP NW growth from particles defined with electrodeposition and from thermal evaporation, corresponding samples were prepared with identical processing steps except for the gold deposition step. Both samples had the same gold thickness of 33 nm (the corresponding current density for electrodeposition is 7.4 mA/cm2) and particle diameter of 175 nm. The porosity of the electrodeposited gold particles is higher as compared to evaporated particles, possibly resulting in a smaller final volume. It might be possible to deposit more uniform particles by use of pulsed electrodeposition.43 The NWs were grown using metal organic vapor phase epitaxy; see Supporting Information for details about the growth conditions. Subsequent SEM inspection revealed InP NWs with a length of 1.5 μm and a diameter of 140 nm; see Figure 2a. Analysis of top view images of the grown NWs shows
Figure 3. (a) Excitation-dependent PL spectra from InP NWs grown from evaporated (EV) and from electrodeposited (EP) gold particles, respectively. The gold particles were defined using organic resist masks, (b) PL spectra from InP NWs grown from electrodeposited gold particles defined by a SiNx mask (red trace) and resist mask (blue trace), respectively. (c) Excitation-dependent PL spectra from GaAs NWs. Percentages given in the figures indicate relative laser excitation intensity.
peaks are attributed to different interband transitions in type-II heterostructures resulting from alternating layers of both WZ and ZB crystal phases in the NWs.46,47 NWs grown from evaporated gold particles additionally display transitions from 1.42 to 1.49 eV, which indicates a higher WZ content in these samples. Interestingly, a red shift is observed in the spectra from NWs grown with electrodeposited as compared to evaporated seed particles, which can be attributed to different numbers of WZ layers sandwiched between thicker segments of ZB along the NWs.48 TEM inspection confirms that both crystal phases are formed in InP NWs (Figure 4). The
Figure 2. SEM images of NWs grown from electrodeposited gold particles. (a) InP NWs grown from particles defined by resist mask, (b) InP NWs grown on a substrate with SiNx mask, (c) GaAs NWs grown on a substrate with SiNx mask, and (d) large area image demonstrating a high growth yield of GaAs NWs.
a growth yield of 99.5% for the electrodeposited samples, while the evaporated samples had a yield of 99.7% as compared to the pattern. All of the missing NWs were observed to be due to merged gold particles. The NWs grown on the samples with SiNx mask similarly had a length of 1.5 μm and a diameter of 140 nm, see Figure 2b. The growth yield of these samples was 99.6 and 99.9% for electrodeposited and evaporated gold particles, respectively. Here we could observe more kinked NWs, which are due to different optimal nucleation conditions when using an inorganic mask during NW synthesis because the mask does not catalyze the pyrolysis of the precursors.44,45 GaAs NWs were grown from 25 nm thick gold particles defined by a SiNx mask. Figure 2c shows a SEM image of 1.2 μm long GaAs NWs with a diameter of 100 nm. The growth yield in this case was 99.8%. Figure 2d shows GaAs NWs on a large area of the substrate. The optical properties of the grown NWs were investigated by PL characterization. To avoid any contribution of substrate luminescence to the signal, individual NWs were mechanically transferred from the as-grown substrate to a Si substrate coated
Figure 4. HRTEM of InP NWs grown from (a) electrodeposited and (b) evaporated gold particles, respectively.
difference in luminescence observed between the NWs can thus be attributed to a different content of WZ versus ZB. From the TEM inspection, we could not establish any general trend indicating that NWs grown from electrodeposited gold particles would contain more or less of one crystal phase compared to those grown from evaporated gold particles. As shown in Figure 3b, the PL from NWs grown from electrodeposited gold particles depends on the specific mask used. We observed that a SiNx mask results in a blue-shifted PL signal whereas the organic resist mask results in more spectral features. These differences suggest that the local properties of the surface exposed to the gas precursors are important factors for the C
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Industrial and Technological Development, the Swedish Foundation for Strategic Research and the Swedish Energy Agency.
growth of NWs. For example, it is well-known that the effective V/III ratio, which depends on the surface conditions,44,45 affects the formation of ZB vs WZ phases in III−V NWs.49 The GaAs NW PL spectra showed sharp peaks at 1.52 and 1.525 eV, respectively, as seen in Figure 3c. From the PL spectra it can be concluded that the GaAs NWs also have a mixed crystal structure. In contrast to the PL from InP NWs, a small amount, if any, of band filling is observed with increasing laser intensity. This can be explained by the high surface recombination velocity of GaAs NWs which leads to efficient nonradiative recombination.50 The results show that while the overall growth results are similar for electrodeposited and evaporated seed particles, the local growth conditions for NWs grown from electrodeposited particles may slightly differ from those of evaporated particles. One contributing factor that might influence the growth conditions is incorporation of impurities into the gold particles. The electrodeposited gold particles were therefore inspected by use of electron dispersive X-ray spectroscopy (EDS) in TEM to reveal the presence of any impurities. Results show that the deposited material is not contaminated with impurities in concentrations exceeding the sensitivity of the EDS system (about 1%). The fact that the diameter of the NWs is the same for both deposition methods furthermore indicates that the porosity of the deposited particle has no significant effect on the final volume of the seed particles. However, it can affect the wetting angle and hence the crystal properties, for example, relative contribution of WZ and ZB phases, of the NWs. From these considerations, it seems plausible that any differences resulting from the deposition methods can in fact be circumvented by proper optimization of the growth parameters. In conclusion, gold seed particles for NW growth were patterned by use of large area NIL and electrodeposition. Both organic resist and inorganic SiNx masks were used for the selective gold electrodeposition. Gold electrodeposition provides a higher throughput than common thermal evaporation and reduces the gold consumption by a factor of at least 300. Other potential advantages with electrodeposition include a reuse of substrates and selective deposition of catalytic seed particles in high aspect ratio geometries for which traditional evaporation is inapplicable. The NWs grown from electrodeposited particles have good surface morphology with excellent growth yield comparable to those grown from thermally evaporated gold particles. PL measurements and TEM characterization could not reveal any differences in the crystal quality of the grown NWs, suggesting electrodeposition as a potential key technology for more industrially viable and yet less expensive, large-scale fabrication of NW-based devices.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Authors would like to thank Dr. Mikael Björk and Daniel Jacobsson for valuable discussions.
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ASSOCIATED CONTENT
S Supporting Information *
Details of the lithographical steps and growth conditions for InP and GaAs NWs. This material is available free of charge via the Internet at http://pubs.acs.org.
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The authors acknowledge financial support from the Nanometer Structure Consortium nmC@LU, Halmstad University, the Swedish Research Council, the Swedish National Board for D
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