Growth and Photovoltaic Properties of High-Quality GaAs Nanowires Prepared by the Two-Source CVD Method
© Wang et al. 2016
Received: 2 March 2016
Accepted: 5 April 2016
Published: 12 April 2016
Growing high-quality and low-cost GaAs nanowires (NWs) as well as fabricating high-performance NW solar cells by facile means is an important development towards the cost-effective next-generation photovoltaics. In this work, highly crystalline, dense, and long GaAs NWs are successfully synthesized using a two-source method on non-crystalline SiO2 substrates by a simple solid-source chemical vapor deposition method. The high V/III ratio and precursor concentration enabled by this two-source configuration can significantly benefit the NW growth and suppress the crystal defect formation as compared with the conventional one-source system. Since less NW crystal defects would contribute fewer electrons being trapped by the surface oxides, the p-type conductivity is then greatly enhanced as revealed by the electrical characterization of fabricated NW devices. Furthermore, the individual single NW and high-density NW parallel arrays achieved by contact printing can be effectively fabricated into Schottky barrier solar cells simply by employing asymmetric Ni-Al contacts, along with an open circuit voltage of ~0.3 V. All these results indicate the technological promise of these high-quality two-source grown GaAs NWs, especially for the realization of facile Schottky solar cells utilizing the asymmetric Ni-Al contact.
KeywordsGaAs Chemical vapor deposition Two-source Contact printing Nanowire parallel arrays Schottky solar cells
PACS73.21.Hb 78.56.-a 81.05.Ea
Due to the direct, suitable bandgap (1.42 eV) and its superior high carrier mobility (8000 cm2/Vs for electrons, 400 cm2/Vs for holes), GaAs materials possess a maximum theoretical single-junction photon-to-electricity conversion efficiency of ~30 %; therefore, their nanowire (NW) materials are widely adopted as fundamental building blocks for next-generation electronics and photovoltaics [1–5]. For example, Colombo et al. demonstrated a single GaAs NW radial p-i-n solar cell with an efficiency of 4.5 % , and Holm et al. later realized a similar single GaAs NW solar cell with an efficiency of 10.2 % after surface passivation by GaAsP shells  under air mass (AM) 1.5G illumination. Until recently, Krogstrup and his group fabricated the vertically aligned single GaAs NW solar cell with an efficiency over the Shockley-Queisser limit of ~40 % due to the light concentration effect of the NWs with diameters comparable with the incident photon wavelength . Although the illustrated efficiency is promising, the associated fabrication and material cost require a significant reduction in order to meet the requirements of third-generation cost-effective photovoltaics.
Generally, most of the GaAs NWs reported in the literature are grown by molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD) methods via the vapor-liquid-solid (VLS) and/or vapor-solid-solid (VSS) mechanism [1, 9, 10]. The equipment employed in these technologies is typically complicated, and the required single-crystalline growth substrates are expensive, accounting for the substantial cost of NW synthesis. Meanwhile, most of the reported solar cells are always configured into p-i-n junctions, which necessitate the finely controlled growth conditions and device fabrication for reliable high-quality junction formation and metallization [6–8, 11, 12]; therefore, it is highly desired to develop low-cost alternative methods to grow crystalline GaAs NWs in the large scale, as well as to develop a facile process to fabricate solar cell devices in the simple structure. For example, Dhaka et al. prepared high-quality GaAs NWs on glass substrates, but neither solar cell devices nor the corresponding electrical property was fabricated and characterized, respectively . Even though Yang et al. reported employing a cost-effective Schottky contact for the large-scale preparation of carbon nanotube (CNT) solar cells, the obtained efficiency is not acceptable with only 0.11 % at an illumination of 90 kW/cm2 due to the small bandgap of CNTs .
In the previous reports, we have achieved long and high-quality GaAs NWs on a non-crystalline SiO2 substrate by a two-step CVD method where a high-temperature nucleation step is needed to obtain the higher supersaturated Ga concentration in Au seeds before the NW growth at a relatively lower temperature. The growth procedure is somewhat complicated, and special care should be taken to control temperature profiles especially in the heating and cooling steps [15, 16]. We also fabricated Schottky-contacted GaAs NW solar cells with Au catalyst tips . Although the efficiency is demonstrated with ~2.8 % for the single NW device, large-scale integration of these GaAs NWs with well-aligned Au-Ga catalysts is difficult and has not succeeded till now. In this study, we prepare GaAs NWs by a facile two-source method in the one-step growth platform. The NWs can be grown longer with high crystallinity as compared with those grown by the one-source method. The long NWs can then facilitate the contact printing for the fabrication of high-density NW arrays and the subsequent fabrication of Schottky barrier solar cells with Ni-Al asymmetric contacts, which are promising for the next-generation, high-efficiency, low-cost solar cells.
GaAs NW Synthesis and Characterization
Surface morphologies of the grown GaAs NWs are examined by scanning electron microscopy (SEM, FEI/Philips XL30). Crystal structures are determined by collecting X-ray diffraction (XRD) patterns on a Philips powder diffractometer using Cu Kα radiation (λ = 1.5406 Å) and by high-resolution transmission electron microscopy (HRTEM, JEOL 2100F). For the HRTEM analysis, the GaAs NWs are first suspended in absolute ethanol by ultrasonication and then drop-casted onto a copper grid for the corresponding characterization.
GaAs NW Device Fabrication and Measurement
The GaAs NW arrays are fabricated by contact printing on SiO2/Si substrates (50-nm-thick thermally grown oxide) as reported previously [20, 21]. Typically, a pre-patterned SiO2/Si substrate is coated with photoresist to serve as the receiver, while the donor NW chip is flipped onto the receiver and slid at a rate of 20 mm/min with a pressure of 20 g/cm2. After the photoresist is removed, the GaAs NW arrays are then left on the patterned region, where conventional photolithography is utilized to define the electrodes and the 70-nm-thick Ni film is thermally deposited as the contact electrodes followed by a lift-off process in order to realize the NW field-effect transistors (FETs).
The single GaAs NW solar cell is also fabricated by photolithography. Firstly, NWs are dispersed in absolute ethanol by ultrasonication, and the dispersion is then drop-casted onto the SiO2/Si substrates. Next, two successive photolithographies are carried out to define the first Ni layer and the second Al layer by the careful alignment with the controllable channel length. Similarly, the GaAs NW array solar cells are fabricated by contact printing and the two consecutive photolithographic steps in order to define the Ni-Al contact.
Electrical performances of the NW array FETs and solar cells are characterized with a standard electrical probe station and an Agilent 4155C semiconductor analyzer. For the solar cell testing, the illumination is provided by using a solar simulator (Newport 96000) with an intensity of AM 1.5G (100 mW/cm2).
Results and Discussion
To shed light on the advantage and characteristics of this two-source growth, we would first focus on the operation of our CVD system. During the NW growth, the GaAs source powder is decomposed and evaporated into Ga and As2 species to serve as the gas phase precursors. Due to the relatively higher vapor pressure of As2 than Ga, more As2 species get evaporated from the powder surface as compared with the ones of Ga. This way, a high V/III ratio resulted, in which this ratio would decrease eventually in the prolonged growth since the As2 species would get depleted faster in the source [24, 25]. In this context, the additional crucible carrying the source powder, located at a relatively lower temperature (i.e., ~20 °C) due to the temperature gradient in the furnace, will not only give rise to the initial higher V/III ratio but also help to sustain a constant magnitude of this ratio. Notably, this stable V/III ratio is found to play a key role in most of the III-V NW growth [26–29]. For example, the higher growth rates and better crystal quality of GaAs NWs have been readily obtained by providing the higher V/III ratio along with the enhanced precursor amount in the MOCVD system . By solely tuning the V/III ratio, cubic zinc-blende and hexagonal wurtzite phases can be finely tailored for most of the III-V NW growth . All these have evidently revealed that this two-source method is exceedingly beneficial for the facilitation of a high V/III ratio required for the high-quality GaAs NW growth environment.
In conclusion, high-quality, long, and dense GaAs NWs are synthesized by a facile two-source chemical vapor deposition method. The grown NWs are found to have better crystallinity as well as retain the same cubic zinc-blende phase and diameter distribution, via the vapor-liquid-solid growth mechanism as compared with those grown by the conventional one-source technique. Specifically, fewer crystal defects and longer NW length resulted, probably attributable to the higher V/III ratio and precursor concentration enabled by this two-source configuration, which is in good agreement with the literature. When fabricated into NW parallel array FET devices, the enhanced NW crystal quality together with the higher NW print density can contribute improved performance to their p-type conductivity. At the same time, the asymmetric Ni-Al Schottky contact works well for both single NW and NW parallel photovoltaic devices. All these results indicate the successful synthesis of high-quality GaAs NWs by tailoring the V/III ratio in the two-source growth method, and importantly the potential applications in the area of facile NW Schottky solar cells, illustrating their promising potency for next-generation photovoltaics.
This research was financially supported by the National Natural Science Foundation of China (61504151), the General Research Fund of the Research Grants Council of Hong Kong SAR, China (CityU 11204614), the State Key Laboratory of Multiphase Complex Systems (MPCS-2015-A-04), the CAS-CSIRO project of the Bureau of International Co-operation of Chinese Academy of Sciences (122111KYSB20150064), and the Science Technology and Innovation Committee of Shenzhen Municipality (JCYJ20140419115507588), and was supported by a grant from the Shenzhen Research Institute, City University of Hong Kong.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
- LaPierre RR, Chia ACE, Gibson SJ, Haapamaki CM, Boulanger J, Yee R, Kuyanov P, Zhang J, Tajik N, Jewell N, Rahman KMA (2013) III-V nanowire photovoltaics: review of design for high efficiency. Phys Status Solidi (RRL) 7:815–830View ArticleGoogle Scholar
- Mariani G, Wong PS, Katzenmeyer AM, Léonard F, Shapiro J, Huffaker DL (2011) Patterned radial GaAs nanopillar solar cells. Nano Lett 11:2490–2494View ArticleGoogle Scholar
- Yoon J, Jo S, Chun IS, Jung I, Kim HS, Meitl M, Menard E, Li XL, Coleman JJ, Paik U, Rogers JA (2010) GaAs photovoltaics and optoelectronics using releasable multilayer epitaxial assemblies. Nature 465:329–334View ArticleGoogle Scholar
- Mariani G, Scofield AC, Hung CH, Huffaker DL (2013) GaAs nanopillar-array solar cells employing in situ surface passivation. Nat Commun 4:1497View ArticleGoogle Scholar
- Chao JJ, Shiu SC, Lin CF (2012) GaAs nanowire/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hybrid solar cells with incorporating electron blocking poly(3-hexylthiophene) layer. Sol Energy Mater Sol Cells 105:40–45View ArticleGoogle Scholar
- Colombo C, Heiss M, Gratzel M, Morral AFI (2009) Gallium arsenide p-i-n radial structures for photovoltaic applications. Appl Phys Lett 94:173108View ArticleGoogle Scholar
- Holm JV, Jorgensen HI, Krogstrup P, Nygard J, Liu HY, Aagesen M (2013) Surface-passivated GaAsP single-nanowire solar cells exceeding 10 % efficiency grown on silicon. Nat Commun 4:1498View ArticleGoogle Scholar
- Krogstrup P, Jorgensen HI, Heiss M, Demichel O, Holm JV, Aagesen M, Nygard J, Morral AFI (2013) Single-nanowire solar cells beyond the Shockley-Queisser limit. Nat Photonics 7:306–310View ArticleGoogle Scholar
- Borgstrom MT, Immink G, Ketelaars B, Algra R, Bakkers EPAM (2007) Synergetic nanowire growth. Nat Nanotechnol 2:541–544View ArticleGoogle Scholar
- Persson AI, Larsson MW, Stenstrom S, Ohlsson BJ, Samuelson L, Wallenberg LR (2004) Solid-phase diffusion mechanism for GaAs nanowire growth. Nat Mater 3:677–681View ArticleGoogle Scholar
- Kempa TJ, Cahoon JF, Kim SK, Day RW, Bell DC, Park HG, Lieber CM (2012) Coaxial multishell nanowires with high-quality electronic interfaces and tunable optical cavities for ultrathin photovoltaics. Proc Natl Acad Sci U S A 109:1407–1412View ArticleGoogle Scholar
- Li X, Yaohui Z, Wang C: Broadband enhancement of coaxial heterogeneous gallium arsenide single-nanowire solar cells. Prog Photovolt: Res Appl 2014, accepted:DOI: 10.1002/pip.2480.Google Scholar
- Dhaka V, Haggren T, Jussila H, Jiang H, Kauppinen E, Huhtio T, Sopanen M, Lipsanen H (2012) High quality GaAs nanowires grown on glass substrates. Nano Lett 12:1912–1918View ArticleGoogle Scholar
- Yang LJ, Wang S, Zeng QS, Zhang ZY, Pei T, Li Y, Peng LM (2011) Efficient photovoltage multiplication in carbon nanotubes. Nat Photonics 5:673–677View ArticleGoogle Scholar
- Han N, Hou JJ, Wang FY, Yip S, Yen YT, Yang ZX, Dong GF, Hung T, Chueh YL, Ho JC (2013) GaAs nanowires: from manipulation of defect formation to controllable electronic transport properties. ACS Nano 7:9138–9146View ArticleGoogle Scholar
- Hou JJ, Han N, Wang F, Xiu F, Yip S, Hui AT, Hung T, Ho JC (2012) Synthesis and characterizations of ternary InGaAs nanowires by a two-step growth method for high-performance electronic devices. ACS Nano 6:3624–3630View ArticleGoogle Scholar
- Han N, Wang F, Yip S, Hou JJ, Xiu F, Shi X, Hui AT, Hung T, Ho JC (2012) GaAs nanowire Schottky barrier photovoltaics utilizing Au-Ga alloy catalytic tips. Appl Phys Lett 101:013105View ArticleGoogle Scholar
- Han N, Wang FY, Hui AT, Hou JJ, Shan GC, Xiu F, Hung TF, Ho JC (2011) Facile synthesis and growth mechanism of Ni-catalyzed GaAs nanowires on non-crystalline substrates. Nanotechnology 22:285607View ArticleGoogle Scholar
- Han N, Wang F, Hou JJ, Yip S, Lin H, Fang M, Xiu F, Shi X, Hung T, Ho JC (2012) Manipulated growth of GaAs nanowires: controllable crystal quality and growth orientations via a supersaturation-controlled engineering process. Cryst Growth Des 12:6243–6249View ArticleGoogle Scholar
- Fan ZY, Ho JC, Takahashi T, Yerushalmi R, Takei K, Ford AC, Chueh YL, Javey A (2009) Toward the development of printable nanowire electronics and sensors. Adv Mater 21:3730–3743View ArticleGoogle Scholar
- Han N, Wang FY, Hou JJ, Yip SP, Lin H, Xiu F, Fang M, Yang ZX, Shi XL, Dong GF et al (2013) Tunable electronic transport properties of metal-cluster-decorated III-V nanowire transistors. Adv Mater 25:4445–4451View ArticleGoogle Scholar
- Algra RE, Vonk V, Wermeille D, Szweryn WJ, Verheijen MA, van Enckevort WJP, Bode AAC, Noorduin WL, Tancini E, de Jong AEF et al (2011) Formation of wurtzite InP nanowires explained by liquid-ordering. Nano Lett 11:44–48View ArticleGoogle Scholar
- Wacaser BA, Dick KA, Johansson J, Borgstrom MT, Deppert K, Samuelson L (2009) Preferential interface nucleation: an expansion of the VLS growth mechanism for nanowires. Adv Mater 21:153–165View ArticleGoogle Scholar
- Foxon CT, Harvey JA, Joyce BA (1973) The evaporation of GaAs under equilibrium and non-equilibrium conditions using a modulated beam technique. J Phys Chem Solids 34:1693–1701View ArticleGoogle Scholar
- Goldstein B, Szostak DJ, Ban VS (1976) Langmuir evaporation from (100), (111a), and (111b) faces of GaAs. Surf Sci 57:733–740View ArticleGoogle Scholar
- Plante MC, LaPierre RR (2008) Control of GaAs nanowire morphology and crystal structure. Nanotechnology 19:495603View ArticleGoogle Scholar
- Joyce HJ, Gao Q, Tan HH, Jagadish C, Kim Y, Fickenscher MA, Perera S, Hoang TB, Smith LM, Jackson HE et al (2009) Unexpected benefits of rapid growth rate for III-V nanowires. Nano Lett 9:695–701View ArticleGoogle Scholar
- Joyce HJ, Wong-Leung J, Gao Q, Tan HH, Jagadish C (2010) Phase perfection in zinc blende and wurtzite III−V nanowires using basic growth parameters. Nano Lett 10:908–915View ArticleGoogle Scholar
- Joyce HJ, Gao Q, Tan HH, Jagadish C, Kim Y, Fickenscher MA, Perera S, Hoang TB, Smith LM, Jackson HE et al (2008) High purity GaAs nanowires free of planar defects: growth and characterization. Adv Funct Mater 18:3794–3800View ArticleGoogle Scholar
- Han N, Wang F, Hou JJ, Xiu F, Yip S, Hui AT, Hung T, Ho JC (2012) Controllable p-n switching behaviors of GaAs nanowires via an interface effect. ACS Nano 6:4428–4433View ArticleGoogle Scholar
- Walukiewicz W, Lagowski J, Jastrzebski L, Gatos HC (1979) Minority-carrier mobility in p-type GaAs. J Appl Phys 50:5040–5042View ArticleGoogle Scholar
- Parkinson P, Joyce HJ, Gao Q, Tan HH, Zhang X, Zou J, Jagadish C, Herz LM, Johnston MB (2009) Carrier lifetime and mobility enhancement in nearly defect-free core-shell nanowires measured using time-resolved terahertz spectroscopy. Nano Lett 9:3349–3353View ArticleGoogle Scholar
- Lide DR (2010) CRC handbook of chemistry and physics, 90th edn. CRC press, Boca RatonGoogle Scholar
- Mead CA (1966) Metal–semiconductor surface barriers. Solid State Electron 9:1023–1033View ArticleGoogle Scholar
- del Alamo JA (2011) Nanometre-scale electronics with III-V compound semiconductors. Nature 479:317–323View ArticleGoogle Scholar
- Han N, Yang Z, Wang F, Dong G, Yip S, Liang X, Hung TF, Chen Y, Ho JC (2015) High performance GaAs nanowire solar cells for flexible and transparent photovoltaics. ACS Appl Mater Interfaces 7:20454–20459View ArticleGoogle Scholar
- Javey A, Nam S, Friedman RS, Yan H, Lieber CM (2007) Layer-by-layer assembly of nanowires for three-dimensional, multifunctional electronics. Nano Lett 7:773–777View ArticleGoogle Scholar