High sensitivity of middle-wavelength infrared photodetectors based on an individual InSb nanowire
© Kuo et al.; licensee Springer. 2013
Received: 2 April 2013
Accepted: 11 July 2013
Published: 18 July 2013
Single-crystal indium antimony (InSb) nanowire was fabricated into middle-infrared photodetectors based on a metal–semiconductor-metal (M-S-M) structure. The InSb nanowires were synthesized using an electrochemical method at room temperature. The characteristics of the FET reveal an electron concentration of 3.6 × 1017 cm−3 and an electron mobility of 215.25 cm2 V−1 s−1. The photodetectors exhibit good photoconductive performance, excellent stability, reproducibility, superior responsivity (8.4 × 104 A W−1), and quantum efficiency (1.96 × 106%). These superior properties are attributed to the high surface-to-volume ratio and single-crystal 1D nanostructure of photodetectors that significantly reduce the scattering, trapping, and the transit time between the electrodes during the transport process. Furthermore, the M-S-M structure can effectively enhance space charge effect by the formation of the Schottky contacts, which significantly assists with the electron injection and photocurrent gain.
One-dimensional (1D) nanostructure materials have received considerable attention because of their importance in potential applications in electronics and photoelectric nanodevices . With high surface-to-volume ratio and Debye lengths comparable to nanomaterial's diameters, the electronic and photoelectric properties of 1D nanostructure are strongly affected by the surface effect via chemisorption (oxygen adsorption) and native surface defects [2–4]. Thus, 1D nanostructure exhibits a superior sensitivity to light and chemical molecules compared to the thin film and bulk. Due to these properties, electronic devices fabricated using 1D nanostructure have been extensively adapted in photodetectors , gas sensors , and dye-sensitized solar cells , respectively. Of these application fields, photodetectors or switches based on semiconductor materials have been the focus of considerable attention in recent years because of their high sensitivity and high quantum efficiency. Furthermore, the different energy band gaps imply that photodetectors can be applied flexibly on various wavelengths. To date, photodetectors based on 1D semiconductor nanostructures, such as SnO2 nanowires , ZnO nanowires , ZnSe nanobelts , CdS nanoribbons , and CuO nanowires , have been reported. These 1D nanostructure photodetectors exhibit outstanding performance; however, the detection range that has been investigated so far falls primarily between the infrared and ultraviolet region. In fact, 1D nanostructure photodetectors of the mid- to long-wavelength infrared (IR) region have seldom been reported because only a few other materials can be used in this region.
Indium antimony (InSb), one of the III-V compounds with a face-centered cubic structure of the zincblende type, is a useful material for producing mid- to long-wavelength IR photodetectors because of the smallest band gap (Eg = 0.17 eV, at 300 K). In addition, owing to the small effective mass (m*e = 0.014 mo) and the ballistic length (up to 0.7 μm at 300 K), InSb has an extremely high carrier mobility (i.e., electron mobility of 77,000 cm2V-1s-1) . Therefore, InSb is a highly promising material for device applications involving high-speed-response electronic nanodevices, optical communication devices, and optical detectors [13, 14]. Owing to the aforementioned unique characteristics, now, many groups use different synthesis methods to produce InSb nanowires, i.e., chemical beam epitaxy , chemical vapor deposition , and pulsed laser deposition (PLD) . Meanwhile, the electrical transport characteristics are also widely investigated [18, 19]. However, only few groups study on the IR detectors, particularly on the mid- to long-wavelength region [20, 21]. This work shows that InSb nanowires can be successfully synthesized at room temperature by applying electrochemical method with an anodic aluminum oxide (AAO) template. The synthesizing process was simple, fast, and straightforward in fabricating large-area InSb nanowires at low temperature compared to other thermal reactive processes. Moreover, individual InSb nanowires based on a metal–semiconductor-metal (M-S-M) structure were fabricated into the photodetectors. It shows high sensitivity, good stability, reproducibility, and response speed after illumination with middle-infrared (M-IR; 5.5 μm) light. Furthermore, a systematic study of the photoresponse was performed, which revealed a clear dependence of the photocurrent, carrier lifetime, and quantum efficiency on the light intensity, defect, and M-S-M structure.
InSb nanowires were synthesized using the electrochemical method. A gold (Au) film coated on an AAO (Whatman®, GE Healthcare, Maidstone, UK) membrane was used as a conductive layer to grow the nanowires. The pore diameter of the AAO membrane was approximately 200 nm. The electrolyte consisted of 0.15 M InCl3, 0.1 M SbCl3, 0.36 M C6H8O7·H2O, and 0.17 M KCl. The solvent of the electrolyte was distilled water. A typical three-electrode electrochemical cell was used during the InSb electrodeposition. The Au film on the AAO membrane was regarded as the working electrode. A platinum wire and an Ag/AgCl electrode were subsequently applied as the counter electrode and the reference electrode, respectively. The deposition time was controlled at 40 min in conditions of a deposition potential of −1.5 V, in contrast to the Ag/AgCl reference electrode at room temperature. Following the deposition, the sample was removed from the AAO membrane with a 5 wt % NaOH solution and then washed five times with distilled water.
The as-prepared nanowires were examined using field emission scanning electron microscope (FESEM; operated at 10 kV; HITACHI S-4800, Chiyoda-ku, Japan), a desktop X-ray diffractometer (D2 Phaser, Bruker, Madison, WI, USA), a high-resolution transmission electron microscope (HRTEM; operated at 200 kV, JEM-2100F, JEOL Ltd., Tokyo, Japan) with energy-dispersive X-ray spectroscope (EDX), and an X-ray photoelectron spectroscope system (PHI600 system, PerkinElmer, Waltham, MA, USA). Furthermore, the transport property was evaluated using the InSb nanowires further fabricated into a field-effect transistor (FET). The synthesized InSb nanowires were dispersed uniformly in ethanol and dropped on a SiO2/p-Si substrate. The Si substrate was applied as a back-gate. After drying out the suspension, the Ti/Cu (20/120 nm) electrodes were deposited on the two ends of the nanowire through photolithograph, e-beam evaporation, and lift-off processes. Additionally, the InSb nanowire-based M-S-M structure photodetectors were fabricated through a microfabrication process and focused ion beam (FIB) technique. Here, the pattern of Ti/Au (20/120 nm) electrode was fabricated using standard lithographic methods on a SiO2/Si substrate. The synthesized InSb nanowires were transferred onto a SiO2/Si substrate with pre-patterned Ti/Au electrodes. Subsequently, the FIB instrument (Dual-Beam Helios 600i, FEI, Shanghai, China) was used to deposit Pt, which connects the wires between the Ti/Au electrodes. Finally, The Pt-InSb-Pt (M-S-M) photodetector structure of back-to-back Schottky contacts was obtained. To evaluate the M-S-M photodetectors, a M-IR light at a 5.5-μm wavelength was used as an excitation light source. The transport and photosensitivity properties were analyzed using the semiconductor characterization system (4200-SCS, Keithley Instruments Inc., Cleveland, OH, USA) at room temperature.
Results and discussion
The negative Gibbs energy (Δ G) implies that the formation reaction is spontaneous. Equation (1) demonstrates the feasibility of applying the electrochemical method to synthesize the InSb nanowires at room temperature.
This work finds that Rλ and QE decrease with increasing light intensity. The reductions of Rλ and QE are strong manifestations of a hole trap at a relatively high light intensity. Under illumination, the photogenerated holes were trapped by the oxygen ions, and the electrons contributed to the photocurrent. However, the saturation of the electron is trap at high light intensity, reducing the number of available hole traps because of the increasing recombination of photogenerated electron–hole pairs [38, 42]. Furthermore, the onset of electron–hole pair recombination at a high light intensity might also contribute to the shortening of the carrier lifetime.
In this work, the high QE for the InSb nanowires is ascribed to the high surface-to-volume ratio and superior crystallinity of the InSb nanowires and the M-S-M structure. The high surface-to-volume ratio can significantly increase the number of hole-trap states and prolong the carrier lifetime. In the dark, oxygen molecules are adsorbed on the nanowire surface and capture free electrons (O2(g) + e− → O2−(ad)), and thus, the depletion layer forms near the surface, which reduces the density and mobility of the carrier. When illuminated (hν → e− + h+), electron–hole pairs are generated; the holes migrate to the surface and discharge the adsorbed oxygen ions through an electron–hole recombination (h+ + O2−(ad) →O2(g)). Unpaired electrons become major carriers that contribute to the photocurrent. This hole-trapping process significantly separates the electron–hole pairs and largely increases the carrier lifetime. [3, 4, 47] Meanwhile, the superior crystallinity of InSb nanowires can reduce the scattering and carrier trapping during the transport process between two electrodes, and the photocurrent rapidly reaches a steady state in both the response and the recovery stages .
This work demonstrated the feasibility of synthesizing single-crystal InSb nanowires using the electrochemical method at room temperature. Characteristic FET devices based on InSb nanowires have n-type conductivity because of the Sb vacancies. Meanwhile, InSb nanowires have an electron concentration of 3.6× 1017 cm−3 and an electron mobility of 215.25 cm2 V−1 s−1. Individual InSb nanowire was fabricated for M-IR photodetectors based on the M-S-M structure. A power-law dependence of the photocurrent on the light intensity was observed, which suggests the existence of defect states that are consistent with an n-type conductivity mechanism in the InSb nanowires. Moreover, the photodetectors exhibit good photoconductive performance, good stability and reproducibility, superior responsivity (8.4 × 104 A W−1), and quantum efficiency (1.96 × 106%). These unique properties are attributed to the high surface-to-volume ratio and superior crystallinity of InSb nanowires. In addition, the M-S-M structure can further enhance Ne (or ΔI) and the electron transport speed, significantly increasing the sensitivity of the photodetectors. The superior photoelectric properties of InSb nanowires are highly promising for application in high-sensitivity and high-speed nanoscale optical communication devices and photodetectors.
CHK and WCC are PhD students at National Tsing Hua University. SJL holds a professor position at National Tsing Hua University. JMW holds an associate professor position at National Tsing Hua University.
The authors thank Mr. Guo-Kai Hsu for the helpful SEM analyses, Mr. Hsin-I Lin for the helpful FIB experiment, and the financial supports from the National Science Council, Taiwan, under grant numbers NSC-99-2221-E-007-069-MY3 and NSC-100-2628-E-035-006-MY2.
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