Horizontal transfer of aligned Si nanowire arrays and their photoconductive performance
© Zhang et al.; licensee Springer. 2014
Received: 6 October 2014
Accepted: 25 November 2014
Published: 9 December 2014
An easy and low-cost method to transfer large-scale horizontally aligned Si nanowires onto a substrate is reported. Si nanowires prepared by metal-assisted chemical etching were assembled and anchored to fabricate multiwire photoconductive devices with standard Si technology. Scanning electron microscopy images showed highly aligned and successfully anchored Si nanowires. Current-voltage tests showed an approximately twofold change in conductivity between the devices in dark and under laser irradiation. Fully reversible light switching ON/OFF response was also achieved with an ION/IOFF ratio of 230. Dynamic response measurement showed a fast switching feature with response and recovery times of 10.96 and 19.26 ms, respectively.
Physical properties of one-dimensional (1D) materials are quite different from those of bulk materials because of their distinct features such as high surface-to-volume ratio and quantum confinement effect [1–3]. Therefore, 1D materials, especially 1D semiconductor materials, have drawn much attention during the past decades [4–6]. Silicon nanowires (Si NWs), as a fundamental material in microelectronics, are one of the most attractive 1D semiconductor materials [7–11]. Si NWs have been considered in various potential applications such as in optoelectronics [3, 12], electronic devices , and energy conversion and storage [14–16]. Bottom-up [17, 18] and top-down techniques [11, 19, 20] are the most common synthesis methods for Si NWs. However, NWs prepared with these techniques are mostly vertically aligned; thus, studying their electron transport features and applications in a variety of devices is a key experimental challenge [21–23].
The main challenge is generally to transfer vertical NWs to a defined position laterally and anchor them with metal electrodes. The most common transfer method mainly involves two steps [3, 22]. In the first step, NWs are separated from the growth substrate and dispersed in a volatile solvent. A drop of this solvent is then casted onto the target substrate. In the second step, lithography is used to define the electrode windows, followed by metal deposition and lift-off techniques. However, in this case, NWs are randomly arranged on the substrate. Finding a device with NWs that bridge metal contacts at both ends is time consuming. Moreover, fabricating a device with multiple aligned NWs using the aforementioned method is quite difficult. Several new methods and specific equipment of assembly of nanowires are reported recently [23–27]. Lieber et al. developed a nanocombing technique that yields arrays with >98.5% of the NWs aligned to within ±1° of the combing direction . Javey et al. used a special print assembled apparatus to print NWs aligned to a receiver substrate [24, 25]. Yu et al. used a blown-bubble thin film from a solvent containing Si NWs and then stamped a substrate onto the bubble to transfer Si NWs . They transferred uniformly aligned and controlled density NWs onto wafers with a diameter of at least 200 mm. Vertical transfer of Si NW array on glass was also demonstrated by other researchers . In the current study, a simple approach to assemble large-scale and highly aligned Si NW arrays horizontally onto a target substrate surface is proposed. Moreover, multiwire photoconductive devices were fabricated with the assembled NWs. The photoresponse measurements showed a rapid switching property (10.96 and 19.26 ms for the response and recovery times, respectively) and a high ION/IOFF ratio (230). The NW assembly and device fabrication process were easily implemented and cost-effective, i.e., without specific equipment or installation. The proposed method could be a potential candidate for developing large-scale multiwire devices on a flexible substrate.
Metal-assisted chemical etching (MACE) method  was used to fabricate Si NWs. P-type Si(100) wafers (resistivity ρ < 0.01 Ω · cm) were first cleaned before etching. The wafers were rinsed for several times with deionized (DI) water and then dipped into boiling piranha solution [H2SO4 (95% to 98%) and H2O2 (30%) at a volume ratio of 3:1] for 3 min to remove metallic and organic residues. The wafers were immersed in diluted hydrofluoric acid solution [DI water:HF (10:1 by volume ratio), 40%] for 30 s to remove native oxide. The as-cleaned samples were etched in MACE solution [0.04 M AgNO3 and HF (40%) at a volume ratio of 3:1]. Etching was performed in a water bath at 40°C for 2.5 h with an etching rate of 20 μm/h. All chemicals were of analytical reagent grade and purchased from Sinopharm Chemical Regent Co., Ltd, Beijing, China. In the MACE process, Ag ions were reduced to an Ag dendritic film on a Si wafer, which catalyzed the etching of Si to finally form vertical Si NWs . Ag dendrites formed in the MACE procedure were removed in diluted nitric acid (5 M), rinsed with DI water, and dried naturally.
Results and discussion
To confirm that aligned Si NW array devices were successfully fabricated, the photoconductive response of the devices was measured using SUSS MicroTec Test Systems (SÜSS MicroTec AG, Garching, Germany) and Agilent B1500A Semiconductor Device Analyzer (Agilent Technologies, Santa Clara, USA). An 808-nm laser diode with a power density of approximately 0.1 W/mm2 was used. Current-voltage (I-V) characteristics were examined with voltage scanning from -1 to 1 V. Light switching ON/OFF response of the device was measured under a fixed voltage of 1 V. Laser was chopped by an optical chopper (Stanford Research SR540 Optical Chopper, Stanford Research Systems, Sunnyvale, USA) at 3 Hz. All measurements were performed at room temperature.
where t 0 and t are the initial and final recovery times, and τ is the characteristic time constant, that is, lifetime. Idark is the dark current, and ΔI is the current amplitude. Accosting to this equation, the lifetime of carriers was extracted from the recovery curve in Figure 4c, and it is determined to be 7.05 ms.
A simple technique for horizontal transfer of aligned Si NW arrays onto a defined substrate has been demonstrated. Multiwire photoconductive devices were fabricated and tested. The fabricated devices exhibited a twofold change in conductivity between light and dark states. The devices also showed a fully reversible light ON/OFF switching response. High response time (10.96 ms) and recovery time (19.26 ms) were also achieved. The proposed technique provides a facile and cost-effective way to study properties of NWs and planar multiwire device applications.
This work was supported in part by the National Thousand Talents Program of China, the bilateral collaboration project between the Chinese Academy of Sciences and Japan Society for the Promotion of Science (Grant no GJHZ1316), the National Natural Science Foundation of China (Grant nos. 61176013 and 61177038), the Beijing Natural Science Foundation (Grant no. 2142031), the Beijing Municipal Science and Technology Commission project (Grant no. Z141100003814002), the Major State Basic Research Development Program of China (Grant nos. 2013CB632103 and 2011CBA00608), and the National High-Technology Research and Development Program of China (Grant nos. 2012AA012202 and 2011AA010302).
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