Electrical and Optical Properties of Au-Catalyzed GaAs Nanowires Grown on Si (111) Substrate by Molecular Beam Epitaxy
© The Author(s). 2017
Received: 1 March 2017
Accepted: 7 April 2017
Published: 21 April 2017
In this study, defect-free zinc blende GaAs nanowires on Si (111) by molecular beam epitaxy (MBE) growth are systematically studied through Au-assisted vapor-liquid-solid (VLS) method. The morphology, density, and crystal structure of GaAs nanowires were investigated as a function of substrate temperature, growth time, and As/Ga flux ratio during MBE growth, as well as the thickness, annealing time, and annealing temperature of Au film using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), cathodoluminescence (CL), and Raman spectroscopy. When the As/Ga flux ratio is fixed at 25 and the growth temperature at 540 °C, the GaAs nanowires exhibit a defect-free zinc blende structure with uniform and straight morphology. According to the characteristics of GaAs nanowires grown under varied conditions, a growth mechanism for defect-free zinc blende GaAs nanowires via Au-assisted vapor-liquid-solid (VLS) method is proposed. Finally, doping by Si and Be of nanowires is investigated. The results of doping lead to GaAs nanowires processing n-type and p-type semiconductor properties and reduced electrical resistivity. This study of defect-free zinc blende GaAs nanowire growth should be of assistance in further growth and applications studies of complex III-V group nanostructures.
KeywordsNanowire Molecular beam epitaxy (MBE) GaAs Vapor-liquid-solid (VLS)
Semiconductors are expected to shrink in scale to sub 10 nm. In this regard, the semiconductor industry is being severely challenged to produce semiconductor materials of suitable mobility (boosted processing speed) and architecture (for reduced power leakage) on a nanometric scale . There is an urgent need for the development of semiconductor materials that can address this problem. Since unique electrical and optical properties are required, III-V group semiconductors have been proposed as candidates to replace Si as a high-speed device material [2–5]. One such material is gallium arsenide (GaAs). It is representative of III-V group semiconductor materials and possesses a direct bandgap of 1.424 eV [6, 7]. III-V group semiconductor nanowire arrays of high volume to surface ratios are potential materials whose bandgap can be tuned for the efficient transfer of solar energy to electric energy [8–10]. The literature presents various examples of the growth of binary and ternary III-V group semiconductor nanowires with controlled bandgaps on various substrates. Among these studies, single-crystal GaAs is used as a homogeneous substrate for growing GaAs semiconductor nanowires [11, 12]. However, single-crystal GaAs substrate is expensive and difficult to integrate into the present Si-based industry. On this basis, Si substrates are more desirable in support of III-V group semiconductor nanowire growth via vapor-liquid-solid (VLS) mechanism or deposit of a GaAs film as a buffer layer [13–15]. VLS as a growth method is popular as it facilitates reduced growth temperatures. Lower growth temperatures inhibit strain and defects causing lattice mismatching. Furthermore, small lattice mismatch also helps zinc blende GaAs to overcome strain issue as grown on Si substrate. Si is diamond structure and its lattice constant is 0.5431 nm, on the other hand, GaAs is zinc blende structure and its lattice constant is 0.5653 nm. The small lattice mismatch, ~4%, leads to GaAs nanowires could epitaxial growth on Si substrate . There are many studies point that diameters, density, and quality of nanowires/nanorods are influenced by growth parameters, such as thickness and annealing temperature via Au-catalyzed VLS growth mechanism [16–19]. Over the past several years, the synthesis of GaAs nanowires (NWs) on Si has predominantly been investigated using metal organic vapor phase epitaxy via a Au-catalyzed VLS mechanism [20–24]. On the other hand, few studies have employed molecular beam epitaxy (MBE) technique. In this systematic study, GaAs nanowires are grown under an MBE system on Si (111) via Au-catalyzed VLS mechanism. The investigation includes thickness and annealing conditions of the Au film, Ga to As flux ratio, growth temperature, dopant (Si and Bi) effects on electrical properties, and optical properties.
Samples were grown on Au film-coated Si (111) substrates in a Varian Modular GEN-II MBE system. Before using an e-gun to deposit thickness-varied Au film (0.6, 1, 1.5, and 2.0 nm), the silicon native oxide on the Si (111) substrate is removed by 1 wt% HF then rinsed with DI water and dried with N2 gas. In this work, all MBE system sources for growing nanowires are solid elements (including Ga, Be, Si, and As) and their flux is controlled using a temperature controller and shutter under 1 × 10−10 Torr. Au film is annealed to form nanoparticles on the Si (111) substrate then Ga and As are provided to grow GaAs nanowires. For in situ Si- and Be-doped GaAs nanowires, the concentrations of Si and Be are tuned by controlling the solid elements temperature. Doping temperatures of Si and Be are 1250~1400 °C and 1000~1150 °C, respectively. The corresponding concentrations and carrier species of Si- and Be-doped GaAs samples are calibrated by Hall measurement to confirm. To prepare samples for Hall measurement, (111) Si substrate without Au film is used to deposit GaAs films with different dopant source temperatures. SEM images were obtained using a HITACHI-S4700 field-emission SEM, operated at 5–15 kV accelerating voltage. The TEM samples were prepared by drop-casting nanostructures from toluene dilute dispersions onto 200-mesh carbon-coated copper grids (Electron Microscope Sciences). Energy dispersive spectrometry was conducted using a 200-kV accelerating voltage on a JEOL JEM-2100F. X-ray diffraction (XRD) was performed with a Rigaku Ultima IV X-ray diffractometer using Cu Kα radiation (λ = 1.54 Å) with of 1°/min scan rate. A cathodoluminescence (CL) detector was attached to SEM, and Raman spectrum was measured through Horiba, HR 800 by 633 nm laser and electrical properties measured by Keithly-590 for I-V curves.
Results and Discussion
The results of Si-doped and Be-doped Au-catalyzed GaAs nanowires electrical measurements, including electrical resistivity that determined by single Au-catalyzed GaAs nanowire devices, reference layer doping concentration, and carrier species are determined by Hall measurements
Resistivity (Ω m)
Reference layer doping (cm−3)
Si-doped 1250 °C
7.89 × 10−2
2.75 × 1019
Si-doped 1300 °C
5.98 × 10−2
3.95 × 1019
Si-doped 1350 °C
2.69 × 10−3
8.92 × 1019
Si-doped 1400 °C
4.31 × 10−1
2.51 × 1018
Be-doped 1000 °C
4.30 × 10−3
1.12 × 1018
Be-doped 1150 °C
4.39 × 10−2
5.92 × 1016
Au-catalyzed GaAs nanowires were grown using an MBE system. Diameter and length density were controlled for thickness, annealing time, annealing temperature of the Au film, Ga/As ratio, and growth temperature. A growth model for Au-catalyzed GaAs nanowires was elucidated based on the analysis of SEM, XRD, and TEM results. Electrical properties of Au-catalyzed GaAs nanowires can be adjusted by controlling concentrations of Be and Si dopants through source temperatures. The carrier species and concentrations in Au-catalyzed GaAs nanowires are calibrated through Hall measurement at room temperature. High Si dopant source temperatures affect substitution sites; further, different concentrations of Si-doped and Be-doped Au-catalyzed GaAs nanowires are fabricated as single devices to measure their electrical resistivity and compare these results using Hall measurement.
Molecular beam epitaxy
Scanning electron microscopy
Transmission electron microscopy
Selected area electron diffraction
Energy dispersive spectrometer
The research was supported by the Ministry of Science and Technology through Grant Nos. MOST 103-2218-E-011-007-MY3, MOST 105 - 2221 - E - 492 - 030 - MY2 and MOST 104-2112-M-007-011-MY3.
CYW, YCH, and ZJK performed the experiments, analyzed the results, and wrote the manuscript. CYW, YCH, and ZJK participated in the sample fabrication and characterizations. CYW, YWS, and JHH contributed to the data interpretation, manuscript writing, and supervised the research. All authors read and approved the final version of the manuscript.
The authors declare that they have no competing interests.
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