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.
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.
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