Effects of Asymmetric Local Joule Heating on Silicon Nanowire-Based Devices Formed by Dielectrophoresis Alignment Across Pt Electrodes
© The Author(s). 2018
Received: 15 November 2017
Accepted: 25 December 2017
Published: 16 January 2018
We demonstrate the fabrication and characterization of silicon nanowire-based devices in metal-nanowire-metal configuration using direct current dielectrophoresis. The current-voltage characteristics of the devices were found rectifying, and their direction of rectification could be determined by voltage sweep direction due to the asymmetric Joule heating effect that occurred in the electrical measurement process. The photosensing properties of the rectifying devices were investigated. It reveals that when the rectifying device was in reverse-biased mode, the excellent photoresponse was achieved due to the strong built-in electric field at the junction interface. It is expected that rectifying silicon nanowire-based devices through this novel and facile method can be potentially applied to other applications such as logic gates and sensors.
One-dimensional (1D) semiconductor nanowires (NWs) have attracted much attention due to their high surface-to-volume ratio, quantum confinement effect, and high crystal quality. With the tunable electrical and optical properties, Si NWs have been successfully incorporated in solar cells , light-emitting diodes , and photodetectors .
Several fabrication techniques have been reported for Si NWs, and these can be divided in two categories: bottom-up and top-down methods. In the bottom-up methods, atoms and molecules can be used as building blocks for the nanostructures utilizing vapor-liquid-solid (VLS) technique , molecular beam epitaxy (MBE) , or laser ablation . The top-down methods including deep reactive-ion etching (DRIE) [7, 8] and metal-assisted chemical etching (MACE) [9, 10] have been introduced for nanostructures by downscaling bulk materials. Recently, a facile and high-throughput method for large-area Si NW arrays of the same dimensions has been proposed by combing MACE with nanosphere lithography (NSL) [11, 12].
Dielectrophoresis (DEP) is one of the commonly used methods applied to align NWs such as metal , metal oxides [14–19], Si [20–22], silicide , and III–V semiconductor  NWs for integrated devices, which were usually in metal-semiconductor-metal structures. In DEP process, the dielectric NWs are exerted by DEP forces through induced dipoles when the NWs are usually subjected to a nonuniform alternating current (AC) electric field, and therefore can precisely align across electrodes. The devices fabricated by DEP method have been investigated extensively for their electrical properties and used for many applications such as logic gates  and sensors [14, 16–19]. However, these devices with rectifying current-voltage (I-V) characteristics would be possibly formed in the DEP alignment. Harnack et al.  proposed that the factors for the rectifying behavior in the ZnO NW-based device can be attributed to dipole moment in ZnO nanocrystals with wurtzite structure or the different Schottky barrier heights on both ends of the aligned NW. Wang et al.  further identified that the origin of the rectifying behavior in this case could be the asymmetrical ZnO NW/Au contacts, which were generated with a different degree of annealing at the two sides in the DEP alignment.
In order to apply Si NWs on integrated devices, it is essential to understand the role of NW/metal contacts and its effect on electrical properties. Here, we demonstrate the fabrication of Si NW-based devices by direct current (DC) DEP and systematically investigate the contacts of homogeneous single-crystalized Si NWs with Pt electrodes. After an investigation of the electrical properties in these devices, we found that their I-V characteristics showed rectifying behavior and unique photosensing properties.
For the Si NWs fabrication method, MACE combined with NSL, reported elsewhere [11, 12], an n-type Si (100) with resistivity ranging from 1 to 10 Ω cm was cut into 1 × 1 cm2 pieces. The substrates were cleaned using the standard Radio Corporation of America (RCA) procedures and made hydrophilic after immersing in boiling Piranha solution, a mixture of H2O2 with H2SO4 in a ratio of 1:3, for 10 min. A close-packed monolayer of polystyrene (PS) spheres with an average diameter of 220 nm was formed on the substrates by a modified dip-coating method  and subsequently reduced sphere size by O2 plasma. A 20-nm-thick sputtered Ag thin film was deposited on the patterned substrates. The samples were etched by a mixture solution of HF, H2O2, and deionized water (HF = 5 M and H2O2 = 0.176 M) at 25 °C for 15 min. A large-area ordered Si NW arrays were obtained after removing the residual PS spheres and Ag thin film by tetrahydrofuran (THF) and HNO3 solution, respectively. The as-synthesized products were characterized by field emission scanning electron microscope (FESEM, JEOL, JSM-6700F) and high-solution transmission electron microscope (HRTEM, JEOL, JEM-2100F).
The electrical transport properties of Si NW-based devices were conducted by the probe station using system source meter (Keithley 2612A). A broadband white light with the intensity of 825 mW/cm2 from an arc Hg-Xe lamp was vertically shown on the devices, and the corresponding photoresponse characteristics were recorded.
Results and Discussion
When the I-V curves of the device were measured in reducing atmosphere (H2/Ar), the rectifying property was not obtained by sweeping in the large voltage range (from − 3 to 3 V) as shown in Additional file 2: Figure S2(a). The I-V curve is symmetrical and near linear, which indicates just a small barrier at the interface between the nanowire and two electrodes. However, the Pt and n-Si can theoretically form a Schottky barrier at the Pt/n-type Si contact as the work function of Pt (~ 6.1 eV) is larger than n-type Si (~ 4.15 eV). In this study, the nanowires just adsorb on the electrodes by the DEP aligning method. Thus, the change of barrier height may be due to the gas adsorption on the Si surface. After sweeping in the large voltage range, the slope of I-V curve increased as shown in Additional file 2: Figure S2(b), which indicates that a large voltage range sweeping measurement in reducing gas can reduce the resistance on both NW-electrode contacts. However, air containing O2 and H2O is an oxidative atmosphere. In air, the oxidation rate of Si is higher at high temperature compared with that at low temperature. Thus, we can infer that for the large voltage range sweeping measurement in air, the increase of barrier height at the anode region is due to the formation of a thin oxidized SiO x layer at the interface, which exhibits electron trapping sites.
In terms of the photoresponse performance, the discrepancy of these above results can be explained as follows. When the device is in forward-biased mode, the depletion region width decreases and enhances the current flow that leads to the lower sensitivity to white light. However, the device in reverse-biased mode, by contrast, has the larger depletion region where the strong built-in electric field exists. The photogenerated electrons and holes can be separated efficiently and reduce the electron-hole recombination rates under the white light illumination, thus resulting in an abrupt increase in free carrier density. Therefore, rectifying devices have a high response rate property. However, in previous studies [27, 28], rectifying devices with one Ohmic contact electrode and the other Schottky contact electrode were fabricated by selecting various electrode materials. In this study, an easy manufacturing process was used. The rectifying behavior of the NW devices formed by dielectrophoresis alignment was obtained just by asymmetric Joule heating in the electrical measurement process.
In summary, the Si NW-based devices were fabricated by aligning the single-crystalized Si NWs across the Pt electrodes using DC-DEP method. The rectifying I-V characteristics of these devices can be obtained, and the direction of rectification can be determined by the voltage sweep direction. This phenomenon can be associated with the asymmetric Joule heating effects produced in the electrical measurement process. The high speed and high photoresponse can be achieved for the rectifying devices in reverse-biased mode due to the efficient electron-hole separation by strong built-in electric field in the depletion region. This rectifying Si NW-based device can potentially be used for photodetectors and other applications such as logic gates or sensors.
This work was supported financially by funding from the Republic of China Ministry of Science and Technology Grants 105-2221-E-005-024-.
HHH and HFH were involved in the design, data analyzing, and manuscript writing. CLL helped HFH in the manuscript writing. WCT and CHL provided the data of measurements of FESEM. LZH provided the data of measurements of FETEM. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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