Room temperature-synthesized vertically aligned InSb nanowires: electrical transport and field emission characteristics
© Kuo et al.; licensee Springer. 2013
Received: 29 December 2012
Accepted: 31 January 2013
Published: 11 February 2013
Vertically aligned single-crystal InSb nanowires were synthesized via the electrochemical method at room temperature. The characteristics of Fourier transform infrared spectrum revealed that in the syntheses of InSb nanowires, energy bandgap shifts towards the short wavelength with the occurrence of an electron accumulation layer. The current–voltage curve, based on the metal–semiconductor–metal model, showed a high electron carrier concentration of 2.0 × 1017 cm−3 and a high electron mobility of 446.42 cm2 V−1 s−1. Additionally, the high carrier concentration of the InSb semiconductor with the surface accumulation layer induced a downward band bending effect that reduces the electron tunneling barrier. Consequently, the InSb nanowires exhibit significant field emission properties with an extremely low turn-on field of 1.84 V μm−1 and an estimative threshold field of 3.36 V μm−1.
KeywordsInSb nanowires Electrical transport Field emission Electron accumulation layer Electrochemical method
Group III-V semiconductor nanowires, i.e., InAs, InP, GaAs, GaP, and InSb, have attracted substantial scientific and technological interests in nanoelectronic devices due to their high electronic transfer characteristic with low leakage currents. Meanwhile, the existence of an electron accumulation layer occurs near the material surface that causes high surface sensitivity and electric conductivity . Among the III-V group, indium antimony (InSb) bulk (Eg = 0.17 eV, at 300 K) is a promising III-V direct-bandgap semiconductor material with zinc-blende (FCC) structure. Due to its narrow bandgap, InSb is extensively used in the fabrication of infrared optical detectors, infrared homing missile guidance systems, and infrared astronomy [2–4]. Next, a significant advantage of InSb is that it has extremely high electron mobility (electron mobility of 77,000 cm2 V−1 s−1) that resulted from the natural small effective mass (m* = 0.013 me) and the ballistic length (up to 0.7 μm at 300 K), which are higher than those of any known semiconductor [5, 6]. Hence, there is significant interest in InSb for the fundamental investigation of its nanostructure for potential application as nanoelectronic devices.
Interestingly, owing to their high surface-to-volume ratio and quantum confinement effect, one-dimensional (1-D) semiconductive nanostructures exhibit unique optical, electronic, and transport properties, which are widely applied in photoconductors , electron field emitters , and dye-sensitized solar cells . In the middle of these various application fields, 1-D electron field emission has attracted wide attention recently due to the sufficient high current density obtained from small electrical field. It is because a cone nanostructure (usually several hundred nanometers) is able to greatly amplify the electrical field within an extremely tiny region of the tips. Nanostructures have consequently served as the proper candidates for electron field emitters .
Up to now, different thermal synthesis methods have been used to produce InSb nanowires, i.e., chemical beam epitaxy , chemical vapor deposition , and pulsed laser deposition . However, the fast and simple synthesis of stoichiometric InSb nanostructures is also of priority concern. The different partial vapor pressures of In and Sb make it difficult to form the InSb compound. In particular, the low bonding energy of InSb causes the tendency of In and Sb to dissociate over 400°C. Additionally, the In-rich and Sb-rich regions derive from the large different melting points of In and Sb elements. Therefore, synthesizing InSb nanowires via thermal synthesis method is a challenging task since the growth of stoichiometric InSb nanowires requires precisely critical temperature control [6, 12, 14]. To address this concern, this work has utilized the electrochemical method at room temperature to fabricate single-crystal InSb nanowires with an anodic aluminum oxide (AAO) template. The synthesized process was a simple, fast, low-temperature (avoids the phase dissociation at a high temperature), and straightforward process for fabricating large-area, highly ordered, aligned InSb nanowires. Furthermore, the as-prepared InSb nanowires are expected to possess the electron accumulation layer on the surface. Importantly, the electron accumulation layer significantly affects the optical, transport, and field emission characteristics.
These as-prepared nanowires were examined using a field emission scanning electron microscope (FESEM; HITACHI S-4800, operated at 10 kV, Chiyoda-ku, Japan), a desktop X-ray diffractometer (Bruker, D2 Phaser, Madison, WI, USA), a high-resolution transmission electron microscope (HRTEM; JEOL JEM-3000 F, operated at 300 kV, Akishima-shi, Japan) with an energy-dispersive X-ray spectrometer (EDX), and an X-ray photoelectron spectroscopy system (XPS, PerkinElmer model PHI600 system, Waltham, MA, USA). The optical properties were then examined from a Fourier transform infrared spectrometer (Bruker, Verpex 70 V). For the transport measurement, the synthesized InSb nanowires were dispersed onto a SiO2/Si substrate with pre-patterned Pt/Ti electrodes through a photolithograph, then through e-beam evaporation and the lift-off process, respectively. Subsequently, the focused ion beam was used to deposit Pt, which connects wires between Pt/Ti electrodes. Finally, the current–voltage (I-V) measurements were carried out using the Keithley 237 (Cleveland, OH, USA). The field emission current density versus applied field (J-E) measurements were performed in a vacuum chamber with a base pressure of about 6 × 10−6 Torr at room temperature. The inter-electrode gap (distance) between the anode and the cathode (InSb nanowires) was controlled using a preci-sion screw meter. The Keithley 237 high-voltage source-measurement unit was used to provide the sweeping electric field to record the corresponding emission currents.
Results and discussion
where n is the electron carrier concentration, k is the Boltzmann constant, and T is the absolute temperature. The me* and mh* are the effective masses of electron and hole, respectively. Given that me* = 0.014 m0 and mh* = 0.43 m0, the electron carrier concentration could be calculated from Equation 1. According to the calculation, the electron carrier concentration was 3.94 × 1017 cm−3, which is more than the intrinsic carrier concentration of InSb . Therefore, the enlargement of energy bandgap and high electron density characteristics verified that the synthesized InSb nanowires are degenerate semiconductors, of which the Fermi level is located above the conduction band minimum . Based on the theoretical calculation using Equation 1, during the crystal growth process, the high carrier concentration can be ascribed to the formation of Sb vacancies in InSb nanowires.
where E00 is an important parameter in tunneling theory, Nd is the electron concentration, εs and ε0 are the relative permittivity of the semiconducting nanowire and free space, respectively. As is estimated, the electron carrier concentration was 2.0 × 1017 cm−3, which is close to the estimative value of the BM effect. At the large bias, differentiating the I-V curve can obtain the total resistance associated with the nanowire. The resistivity ρ of 0.07 Ω cm was obtained from the I-V curve at large bias. Furthermore, according to σ = nqμ, the corresponding electron mobility μ of the InSb nanowire was estimated to be 446.42 cm2 V−1 s−1. The value is three times higher than that of reported n-type InSb nanowires . However, the value is much smaller than those of the bulk and thin films. The reason of decay is attributed to the enhanced surface roughness scattering [13, 35, 36]. The nanowire surface becomes rough due to the presence of surface defects. Moreover, surface roughness scattering becomes strong and further limits the movement of electrons due to the decrease of nanowire diameter. It is still higher than that of known oxide semiconductor nanowires [33, 37, 38]. This implies that it has high potential for application in high-speed nanoelectronic devices.
The F-N emission behavior can be observed by plotting the ln(J/E2) versus 1/E curve, shown in the inset of Figure 5b. The linear curve implies that the field emission behavior of nanowires follows the F-N theory. Based on the F-N theory, the field enhancement factor β of InSb nanowires can be calculated. According to the work function of InSb (4.57 eV) , the field enhancement factor β is regarded as 20,300. Generally, the field emission performance is usually associated with the crystal geometry, the dimension of the material, emission height, crystal structure, conductivity, work function, and the density of nanostructures . In this work, the excellent turn-on field (Eon) of InSb nanowires can be attributed as follows: The high carrier concentration of the InSb nanowires with the Fermi level is located above the conduction band minimum, significantly reducing the effective electron tunneling barrier. Figure 5c illustrates the band diagram of degenerate InSb nanowires. The large density of states in the InSb conduction band (i.e., surface accumulation layer) causes a downward band bending near the surface region that eventually leads to lower the electron tunneling barriers. Additionally, the Fermi level is located above the conduction band minimum that can also improve the efficiency of tunneling at a low electric field. Next, the vertically aligned nanowires also play an important role. The high aspect ratio of the nanowires at applied electric field easily makes the electrons to accumulate on the surface and enhance significant field emission property. However, the density of nanowires must be moderate [46, 47]. Previous works reported that the electrostatic screening effect increased the turn-on field and decreased the overall emission current density of densely packed grown nanowires [48, 49]. This is because the applied electric field will overlap with that of the others. Consequently, the effective electric field of densely packed nanowires will be lowered compared to the stand-alone nanowires. Here, there is a reduced screening effect in the vertically aligned InSb nanowires due to a sufficient spacing between the emitters; meanwhile, there is the nanodimension structure with high aspect ratio. Therefore, the electron accumulation that occurs in the conduction band and sufficient spacing in aligned nanostructures can simultaneously enhance field emission property.
Single-crystalline InSb nanowires can be successfully synthesized via the electrochemical method at room temperature. The I-V curve of the InSb nanowires based on the M-S-M model shows low resistivity ρ of 0.07 Ω cm owing to the existence of Sb vacancies. Meanwhile, InSb nanowires have a high electron concentration of 2.0 × 1017 cm−3 and a high electron mobility of 446.42 cm2 V−1 s−1. Also, the energy bandgap increases from 0.17 to 0.208 eV due to the filling up of low-energy states in the conduction band by excess electrons. Thus, the enlargement of energy bandgap and high electron concentration reveal that the InSb nanowires are degenerate semiconductors with the Fermi level located above the conduction band minimum. The accumulation layer occurs at the surface of InSb nanowires. The surface accumulation layer in the InSb conduction band causes a downward band bending near the surface region that eventually leads to lowering of the electron tunneling barriers. Moreover, a sufficient spacing between the InSb nanoemitter can significantly reduce the screening effect. Consequently, the vertically aligned InSb nanowires exhibit an extremely low turn-on field of 1.84 V μm−1 and an estimative threshold field at 3.36 V μm−1 when the current density was 1 μA cm−2 and 0.1 mA cm−2, respectively. The outstanding characteristics of InSb nanowires are highly promising for use in nanoelectronics, especially in the front area of flat panel displays and high-speed-response field-effect transistors.
The authors thank the financial supports from the National Science Council, Taiwan, under grant nos. NSC-99-2221-E-007-069-MY3 and NSC-100-2628-E-035-006-MY2.
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