Large-scale and uniform preparation of pure-phase wurtzite GaAs NWs on non-crystalline substrates
© Han et al.; licensee Springer. 2012
Received: 4 September 2012
Accepted: 7 October 2012
Published: 21 November 2012
One of the challenges to prepare high-performance and uniform III-V semiconductor nanowires (NWs) is to control the crystal structure in large-scale. A mixed crystal phase is usually observed due to the small surface energy difference between the cubic zincblende (ZB) and hexagonal wurtzite (WZ) structures, especially on non-crystalline substrates. Here, utilizing Au film as thin as 0.1 nm as the catalyst, we successfully demonstrate the large-scale synthesis of pure-phase WZ GaAs NWs on amorphous SiO2/Si substrates. The obtained NWs are smooth, uniform with a high aspect ratio, and have a narrow diameter distribution of 9.5 ± 1.4 nm. The WZ structure is verified by crystallographic investigations, and the corresponding electronic bandgap is also determined to be approximately 1.62 eV by the reflectance measurement. The formation mechanism of WZ NWs is mainly attributed to the ultra-small NW diameter and the very narrow diameter distribution associated, where the WZ phase is more thermodynamically stable compared to the ZB structure. After configured as NW field-effect-transistors, a high ION/IOFF ratio of 104 − 105 is obtained, operating in the enhancement device mode. The preparation technology and good uniform performance here have illustrated a great promise for the large-scale synthesis of pure phase NWs for electronic and optical applications.
KeywordsGaAs nanowires Wurtzite phase Non-crystalline substrates P-type semiconductors 61.46.Km 73.63.Nm 78.40.Fy
Due to the outstanding chemical and physical properties, III-V compound semiconductor nanowire (NW) materials such as InAs and GaAs are considered to be one of the most promising candidates for next-generation electronics and photonics[1–4]. Typically, they are synthesized via metal-catalyzed (e.g., Au, Ni) vapor–liquid-solid (VLS), and/or vapor-solid-solid (VSS) processes in metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) systems. During the growth, the temperature, V/III ratio, catalyst dimension, and other processing parameters can be precisely varied, aiming to control the crystal quality and growth orientation of NWs[5–9], in which the corresponding electrical and optical properties can then be tailored for various technological applications.
Since the cubic zincblende (ZB) crystal structure is more energy-stable than the hexagonal wurtzite (WZ) one in most of the bulk III-V semiconductors, the WZ structure is commonly observed as crystal defects (stacking faults and twin planes) in NWs due to the surface energy change in the nanometer scale[10, 11]. Notably, pure WZ-structured NWs are only observed with small diameters (approximately 10 nm). These crystal defects are detrimental to electronic and optical properties of NWs; for example, the mixed phase InAs NWs have a resistivity up to 2 orders of magnitude higher than that of single phase NWs, and the electron mobility is found to decrease significantly in highly defective InAs and InP segments[14, 15]. In general, the pure phase NWs can be prepared by controlling the basic growth parameters in MOCVD or MBE system, in which the low growth temperature and high V/III ratio favor the ZB structure, while the reverse condition is preferred for the WZ structure[5–7]. Even though the WZ structure dominates in the growth of thin NWs, the thick WZ ones can still be achieved by the lateral growth of firstly prepared thin WZ NWs with the tapering morphology and low defect density at a high V/III ratio of 200. At the same time, single-crystalline wafers such as GaAs(111)B and InAs(111)B are usually used as substrates for the epitaxial growth of NWs. Nowadays, non-crystalline substrates are highly preferred as to lower the preparation cost and to ease the subsequent NW integration by assembling NW parallel arrays[16–18]. However, since Au/GaAs(111)B interface favors the WZ stacking while Au/GaAs(111)A interface favors the ZB structure, it is understood that it is even more difficult to achieve pure phase NWs on non-crystalline substrates, without the underlying lattice, which guides the formation of NWs. As a result, controlling the crystal structure of III-V NWs for the pure-phase and uniform property in the large scale is still a challenging topic.
Herein, in this work, pure-phase WZ GaAs NWs are successfully prepared on non-crystalline SiO2/Si substrates in the large scale by just controlling the NW diameter in the order of approximately 10 nm. XRD patterns show the first observation of pure WZ phase NWs grown on amorphous substrates in the literature, while the electronic bandgap determined by the reflectance spectra fits well with the relatively larger bandgap of WZ phase as compared to the one of ZB structure. Excellent electronic properties are also revealed after configuring the NW as a field-effect-transistor (FET).
Synthesis of GaAs NWs
A dual-zone horizontal tube furnace, one zone for the solid source (upstream) and one zone for the sample (downstream), was used as the reactor for the synthesis of GaAs NWs, as reported previously. At first, thermal evaporation was carried out with 99.995% pure Au to deposit a 0.1-nm thick Au film on SiO2/Si substrates (50 nm thermally grown) under a vacuum of approximately 1 × 10−6 Torr. The processed substrate was then placed in the middle of the downstream zone with a tilt angle of approximately 20° and thermally annealed at 800°C for 10 min in a hydrogen environment to obtain Au nanoclusters as the catalysts. The solid source, GaAs powders (approximately 1.0 g), placed within a boron nitride crucible, was positioned in the upstream zone with a distance of 10 cm away from the sample. During the NW growth, the source was heated to the required source temperature (900°C), while the substrate was cooled to the preset growth temperature (580°C to 620°C). Hydrogen (99.9995% purity, 100 sccm) was used as the carrier gas to transport the thermally vaporized solid GaAs source to the downstream, and the pressure was maintained at approximately 0.5 Torr for the entire duration of the growth (1 h). After the growth, the source and substrate heater were stopped together and cooled down to room temperature under the hydrogen flow. In this case, the NWs were grown chemically intrinsic without any intentional dopants.
Characterization of GaAs NWs
Surface morphologies of the grown GaAs NWs were examined with a scanning electron microscope (SEM, FEI Company, Oregon, USA/Philips XL30, Philips Electronics, Amsterdam, The Netherlands) and transmission electron microscope (TEM, Philips CM-20). Crystal structures were determined by collecting X-ray diffraction (XRD) patterns on a Philips powder diffractometer using Cu Kα radiation (λ = 1.5406 Å), imaging with a high resolution TEM (JEOL 2100F, JEOL Co., Ltd., Tokyo, Japan), and selected area electron diffraction (SAED, Philips CM-20). Elemental mappings were performed using an energy dispersive X-ray detector attached to the JEOL 2100F to measure the chemical composition of grown NWs. For the elemental mapping and TEM, the GaAs NWs were first suspended in the ethanol solution by ultrasonication and drop-casted onto the grid for the corresponding characterization. The reflectance spectrum was measured with a Lambda 750 spectrophotometer (PerkinElmer Inc., MA, USA) at room temperature.
The GaAs NWFETs were fabricated by drop-casting the NW suspension onto highly doped p-type Si substrates with a 50-nm thermally grown gate oxide. Photolithography was utilized to define the source and drain regions, and 50-nm thick Ni film was thermally deposited as the contact electrodes followed by a lift-off process. Electrical performance of fabricated back-gated FETs was characterized with a standard electrical probe station and Agilent 4155C semiconductor analyzer (Agilent Technologies, CA, USA).
Results and discussion
In this case, all the crystallographic studies have verified the WZ structure of GaAs NWs grown by 0.1-nm thick Au catalyst film deposited on non-crystalline substrates. Without the guidance of underlying crystalline lattice, the obtained pure WZ phase is mainly attributed to the ultra-small NW diameter and the very narrow distribution associated as shown in Figure 1c. It is commonly believed that the WZ phase is more preferred in small diameter III-V NWs due to the thermodynamically lower energy surfaces. Although the ZB phase is favored in the bulk material, when the materials are scaled down to the nanometer, there exists a transition regime where the dominant ZB phase is replaced by the WZ structure. NWs with the diameter of approximately 10 to 25 nm for GaAs and approximately 50 to 60 nm for InAs are reported as the critical diameters for this transition in the literature[11, 12, 23–25]. Consequently, as most of our NWs are grown in the diameter <14 nm (mean = 9.5 nm), well below the critical diameter, it is predictable that all NWs existed in the WZ phase on amorphous substrates.
In summary, the high aspect ratio, smooth, large-scale, and uniform WZ GaAs NWs are prepared on non-crystalline SiO2/Si substrate, utilizing 0.1-nm thick Au film as the catalyst. The WZ structure is verified by specific characteristics in the XRD pattern, SAED pattern, HRTEM, and FFT. The electronic bandgap is also determined to be approximately 1.62 eV by the reflectance measurement. Notably, the WZ GaAs NWs all exhibit p-type semiconducting behavior with a high ION/IOFF ratio of 104 to 105, as revealed from the electrical characterization in fabricated back-gated NWFETs. All of these have demonstrated the successful control of pure WZ NWs grown on non-crystalline substrates, which present the potency of large-scale preparation for various high performance technological applications.
This research was financially supported by the City University of Hong Kong (project no.: 7002751).
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