Letter pubs.acs.org/NanoLett
Electromechanical Properties and Spontaneous Response of the Current in InAsP Nanowires Jong Hoon Lee,† Min Wook Pin,†,‡ Su Ji Choi,†,§ Min Hyeok Jo,∥ Jae Cheol Shin,∥ Seong-Gu Hong,† Seung Mi Lee,† Boklae Cho,† Sang Jung Ahn,†,‡ Nam Woong Song,† Seong-Hoon Yi,§ and Young Heon Kim*,†,‡ †
Korea Research Institute of Standards and Science, 267 Gajeong-Ro, Yuseong-Gu, Daejeon 34113, Republic of Korea University of Science and Technology, 217 Gajeong-Ro, Yuseong-Gu, Daejeon 34113, Republic of Korea § Department of Materials Science and Metallurgical Engineering, Kyungpook National University, Daegu 41566, Republic of Korea ∥ Department of Physics, Yeungnam University, Gyeongsan 38541, Republic of Korea ‡
S Supporting Information *
ABSTRACT: The electromechanical properties of ternary InAsP nanowires (NWs) were investigated by applying a uniaxial tensile strain in a transmission electron microscope (TEM). The electromechanical properties in our examined InAsP NWs were governed by the piezoresistive effect. We found that the electronic transport of the InAsP NWs is dominated by space-chargelimited transport, with a I ∞ V2 relation. Upon increasing the tensile strain, the electrical current in the NWs increases linearly, and the piezoresistance gradually decreases nonlinearly. By analyzing the space-charge-limited I−V curves, we show that the electromechanical response is due to a mobility that increases with strain. Finally, we use dynamical measurements to establish an upper limit on the time scale for the electromechanical response. KEYWORDS: Indium arsenic phosphide (InAsP) nanowire, piezoresistance, transmission electron microscopy (TEM)
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properties of Si.2 In addition, the combination of onedimensional (1D) nanostructures of semiconducting materials that show sensitive piezoresistive effects with nanoelectromechanical systems (NEMS) devices enables the creation of novel devices. Since these devices were first created, the mechanical and electromechanical properties of semiconducting 1D nanostructures have been the subject of much attention among various research groups.1,8−13 Many studies on the electromechanical properties of semiconducting materials have focused on Si-based materials; Si has shown anomalous piezoresistance behaviors under tensile strain depending on the crystal direction.14 More recently, the electromechanical properties of InAs, which is one type of compound semiconductor, has been investigated under tensile and compressive
everal studies have reported on the various effects of strain on semiconducting materials, such as changes in electronic structure.1−3 The change of resistance under mechanical stress is referred to as the “piezoresistive effect”. The change of resistance within metals under strain is dependent upon change of dimension; resistance under tensile strain increases due to increased length and decreased cross-section area.2,4 The resistance of semiconducting materials sometimes decreases under tensile strain; however, specifically, the transport properties of semiconducting materials are more severely affected by strain than are metals, sometimes in the range of 50−100 times.2 Change in electronic structures under compressive and tensile strain conditions was observed within semiconducting silicon (Si), indium arsenide (InAs), and zinc oxide (ZnO).5−7 Various sensing devices, such as pressure sensors, accelerometers, cantilever force sensors, inertial sensors, and strain gauges, have been fabricated based on the electromechanical © XXXX American Chemical Society
Received: May 28, 2016 Revised: September 6, 2016
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Nano Letters strain conditions.12,13 Also, a tunable InAsxP1−x system, from InAs to InP, is of potential interest for both electrical (e.g., majority-carrier devices, such as field-effect transistors) and optoelectronic devices (e.g., emitters and detectors for the 1.3− 3.0 μm). In addition, the control of physical properties by strain may contribute to the realization of InAsP-based devices. Although a deep understanding of the electromechanical properties of nanometer-sized semiconducting materials is essential for exact applications, studies thus far have been conducted only on limited systems and physical properties. In addition, no studies of the electromechanical properties of more complex systems, such as ternary and quaternary compounds, have yet been reported in the literature. In this paper we report on the electromechanical properties of ternary InAsP nanowires (NWs), which are one type of 1D nanostructure. The piezoresistive effect on InAsP NWs was studied by measuring their transport properties in relation to tensile strain. A device composed of a single InAsP NW was fabricated to investigate the electromechanical properties. All of the measurements and observations were conducted in a transmission electron microscope (TEM). InAsP NWs were grown on a Si (111) substrate by using a metal−organic chemical vapor deposition (MOCVD) system without a metal catalyst. The Si wafer was cleaned with buffer oxide etch and deionized (DI) water. The wafer was then immediately dipped in poly-L-lysine solution and rinsed in DI water. The Si wafer was loaded into the MOCVD reactor. The source materials were trimethylindium (TMIn), arsine (AsH3), and phosphine (PH3). The molar flows (mol/min) of TMIn, AsH3, and PH3 were 2 × 10−5, 2.2 × 10−4, and 4.5 × 10−3, respectively, and the growth temperature was 630 °C. Shin et al. reported in detail on the growth of InAsP nanowire via MOCVD.15 After the growth of the InAsP nanowire, the InAsP NWs were separated from the Si substrate and dispersed in methanol solution by ultrasonic waves. The methanol solution which included the InAsP NWs was dropped onto a porous filter paper. After drying of the methanol, a single InAsP NW was transferred to an electrical push-to-pull (EPTP) device (Hysitron, Inc.) for the measurement of its electrical and mechanical properties (Supporting Information S1). For the transfer of the single NW, a homemade transfer system, composed of an optical microscope and a manipulator with a sharp metal probe, was used in our experiment. The electrodes on the EPTP device were also modified with nickel (Ni) metal to improve the contact property at the interface between the NW and each electrode. After the single InAsP NW was transferred to EPTP device, platinum (Pt) metal was carefully deposited by using a focused ion beam (FIB) system to form a stable contact between the NW and the electrodes. Specifically, for the deposition of Pt metal (i-Pt), ion-beam-induced deposition (IBID) with gallium (Ga) ions was adopted, because the contact between the InAsP NW and the electrodes showed more ohmic-like behavior compared with the deposition of Pt metal (e-Pt) by electron-beam-induced deposition (EBID). Although more detailed study is required, we believe that the bombardment of heavy Ga ions improves the contact properties between NWs and electrodes. In our experiment, the deposition conditions of the Pt metal by IBID were carefully determined after preliminary experiments to exclude side effects, such as the formation of a sheath layer on the NWs and damage of the NWs. The spreading of remnant Ga ions was limited around the Pt-metal layer; most of the
middle region of the InAsP NWs was free from Ga ions because of the diffusion limit due to the low deposition current. The microstructural characterization and in situ measurement of the electrical and mechanical properties were conducted with an FEI F30 transmission electron microscope operating at 300 kV. The displacement for a tensile test on the InAsP NWs was governed by following the designed sequences. The EPTP device was pulled for 1 s at the rate of 5 nm/s at each step, and each strained state was then maintained for 10 s for electromechanical measurement. A fast cyclic displacement test (10 nm/sec) was also conducted to investigate the response rate and repeatability (Supporting Information S2). Although the displacement control mode was used for all of the experiments, the exact displacement to determine the actual strain was extracted from the TEM image frame data for an accurate analysis. The electrical properties were measured with a Keithley 2602B source-meter; the measurement was controlled by a program constructed based on the labVIEW program. Figure 1a and b shows tilted and top-view scanning electron microscope (SEM) images. As seen in Figure 1a, the nanowires
Figure 1. Tilted (a) and top-view (b) SEM images of the InAsP NWs vertically grown on a Si (111) substrate. (c) Representative HRTEM micrograph of an InAsP NW. The inset shows the diffractogram by fast-Fourier transform (FFT). The white arrows indicate planar defects, stacking faults, and microtwins. The stacking sequence is expressed with the red, yellow, and green circles, which correspond to the A, B, and C layers, respectively. (d) The SAED pattern taken along the [1̅10] direction of a ZB structure in an InAsP nanowire. The white arrow indicates the streak lines caused by planar defects.
are vertically aligned on the Si (111) substrate with lengths in the range of 7−13 μm. The cross-sectional morphology of InAsP NWs is hexagonal with the long diameter in the range of 50−150 nm. The high-resolution transmission electron microscopy (HRTEM) image in Figure 1c shows a representative atomic arrangement of an InAsP NW. Various hexagonal polytypes (ZB (3C), WZ (2H), 4H) of InAsP are observed in the HRTEM micrograph from the InAsP NW; the existence of many stacking faults and microtwins, indicated by arrows in Figure 1c, is related to the polytype structures. The red, yellow, and green circles respectively indicate A, B, and C layers in the hexagonal polytypes and are related to the stacking B
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Nano Letters sequences. The stacking sequences of ABCA and/or ACBA are ZB segments, whereas ABA, ACA, or BCB stackings are WZ segments.16 The 4H segments are also composed of stacking sequences of ABCBA and/or ACBCA. The InAsP NW consists of many segments of 3C and partial 2H and 4H segments (Figure 1c). The diffractogram obtained by fast Fourier transform (FFT) in the inset of Figure 1c and the selected-area electron diffraction (SAED) pattern in Figure 1d are indexed along the [1̅10] direction of a ZB structure as the beam direction. Analyses of the diffraction patterns confirm that the growth direction of the NW is in the [111] direction of a ZB structure. The streak lines observed across those spots from the (111) plane of a ZB structure, indicated by the white arrow in Figure 1d, are related to the existence of hexagonal polytypes and planar defects, such as stacking faults and microtwins (Figure 1c). To verify the composition of the ternary InAsP NWs, energydispersive X-ray spectroscopy (EDS) analysis was conducted by TEM before electromechanical measurement. A representative EDS spectrum taken at the center area of the NW, indicated by the cross in the inset in Figure 2a, is shown in Figure 2a. Three strong peaks observed at 1.28, 2.01, and 3.29 keV originated from As-Lα, P-Kα, and In-Lα, respectively, which indicates that the NW is composed of In, As, and P. The relative atomic ratio of As and P atoms, averaged with the data from 20 points, is 0.81:0.19, with a standard deviation of ±3.20 at %. Highresolution X-ray diffraction (HRXRD) analysis was also used to
confirm the microstructural properties and composition information on the InAsP NWs. Figure 2b shows the result of a 2θ−ω scan near the {111} planes of the Si; the range of 2θ was 24°−30°. Two peaks were detected at 25.56° and 28.44° in Figure 2b, which correspond to the reflections of the {111} planes of InAsP and the {111} planes of Si substrate, respectively. From Bragg’s law, λ = 2d(sin θB), by referencing the second peak at 28.44° from Si, the first peak at 25.56° corresponds to 3.482 Å in a spacing, which means that the interplanar spacing of the {111} planes of a ZB structure is 3.482 Å and the lattice constant of the ZB structure is 6.031 Å. Because the lattice parameters of InAs and InP are known to be 6.0583 and 5.8687 Å, respectively, the relative composition of As and P can be approximated by applying Vegard’s law, which is the empirical rule that the lattice parameter of a solid solution (InAsP in our experiment) of two constituents, InAs and InP, is approximately equal to a rule of mixtures of the two constituents’ lattice parameters at the same temperature: aInAsP = xaInAs + (1 − x)aInP
(1)
where aInAsP is the lattice parameter of InAsP, aInAs and aInP are the lattice parameters of InAs and InP, respectively, and x is the molar fraction of InAs in InAsP. By substituting the lattice parameters 6.031, 6.0583, and 5.8687 Å for InAsP, InAs, and InP, respectively, the molar fraction of InP is approximated to 0.144. The value of the molar fraction of P in InAsP from the X-ray diffraction (XRD) analysis may show a minor discrepancy compared with those from the EDS analyses, because the XRD analysis shows the global information on various InAsP nanostructures on a Si substrate. We inferred that the slight difference in the relative composition between As and P is reflected in the broad fullwidth-half-maximum (fwhm) value of the XRD peak from the {111} planes of InAsP. In actuality, the difference in the relative composition of P to As depends on the shape and growth position on the substrate that was identified from the EDS analyses (Supporting Information S3). The existence of hexagonal polytypes may be another reason for the wide fwhm of the XRD peak as well. The correct determination of the exact geometric shape and dimensions of nanomaterials is important to correctly extract their physical properties. A top-view SEM image of the EPTP device with an InAsP NW is shown in Figure 3. The Pt metal layers deposited by EBID and IBID are observed on the device. In our experiment, the exact length for the evaluation of the electromechanical properties was determined by measuring the distance between two Pt electrodes via IBID (i-Pt) in a topview SEM image because a TEM image would only show the gap of the EPTP device. As seen in Figure 3a, the distance between two i-Pt electrodes is 5770 nm, which was used to calculate the exact values of the physical properties. Bright-anddark contrasts vertically aligned along the longitudinal direction of the InAsP NW are observed in the bright-field (BF) TEM image (Figure 3b), which are considered to originate from planar defects as discussed regarding Figure 1.16 For the evaluation of mechanical properties, the diameters of the InAsP NWs were carefully derived by using a line profile of an image contrast, as shown in Figure 3b. The {110} planes of a ZB structure of InAsP NWs were contacted on the surface of the EPTP device, because the side facets of the hexagonal shape of InAsP NWs are composed of the {110} planes of a ZB structure (Figure 3b). The diameter of the InAsP in Figure 3b is approximated to 110 nm, which is consistent with that from
Figure 2. (a) EDS spectrum in the range of 0−40 keV. The inset shows the high-angle annular dark field (HAADF) STEM image of the InAsP NW, while the cross mark, x, indicates the position at which the EDS spectrum was taken. *Cu−Kα was derived from the sample support. (b) HR-XRD spectrum (ω−2θ scan) along the [111] direction through the (111) Bragg peak of the InAsP NWs. C
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property was investigated by four-point measurement, and the ohmic-like behavior at room temperature was confirmed (Supporting Information S4). In Figure 4b, all of the I−V curves show a symmetric shape under negative and positive bias states, regardless of tensile conditions. In our measurements, the I−V curves were free from dependence on the sweep direction of the applied voltage, which means that the I−V characteristics were free from any hysteresis characteristics. This characteristic, freedom from hysteresis, was observed in all of the I−V curves measured in our experiment, which indicates that the relative current change originated from the piezoresistive effect. We thus deduced that the increase of the ΔI/I0 with tensile strain was mainly attributable to decreased resistance in the InAsP NWs due to the piezoresistive effect. Although the piezoelectric field effect may be considered one of the origins of the increase in current, if it is working, the I−V curves must show an asymmetric shape at negative and positive applied voltages and hysteresis characteristics. In addition, many planar defects along the longitudinal direction of the NWs observed in our InAsP NWs, including stacking faults and microtwins (Figure 1a), must have counteracted the piezoelectric field effect by forming the opposite field at the boundaries.17 Li et al. recently explained the symmetric I−V shape of InAs NWs with an increase in the tensile strain by adopting the piezoresistive effect.13 The I−V curves in the highvoltage region in our study showed a nonlinear increase, which indicates that carrier transport through the NW was governed by the space-charge limited current. This phenomenon is discussed in the following paragraphs.1,18,19 Figure 4c shows the relative current change, ΔI/I0, depending on tensile strain; ΔI/ I0 linearly increases with increased tensile strain. The resistivity of the InAsP NW, ρ, is calculated by the following equation:
Figure 3. (a) SEM image of the E-PTP device with the InAsP NW fixed by Pt layers. The e-Pt and i-Pt indicate the Pt layers deposited via EBID and IBID methods, respectively. (b) Multibeam BFTEM micrograph of an InAsP NW. The bottom image shows the line profile of the electron-signal intensity along the red arrow in panel b.
the top-view SEM images, since the cross-sectional area, A, of the InAsP NW in Figure 3b was calculated to be approximately 7859.2 nm2. To study the piezoresistive effect in the InAsP NW, the current−voltage (I−V) characteristics were investigated by using two-point probe measurement. The I−V characteristics were swept in the range of −1.5 to 1.5 V under varying strain conditions. The BFTEM frame data corresponding to the change in tensile strain is shown in Figure 4d. The linear characteristics of the I−V curve in the low-voltage region indicate the ohmic-like behaviors of the contacts between the NW and the electrodes, as shown in Figure 4a. The contact
ρ = R × A /L
(2)
Figure 4. Current (I)−voltage (V) curves of the InAsP NW under various tensile strains in the low-voltage region (from −0.3 to 0.3 V) (a) and the wide-sweep voltage region (from −1.5 to 1.5 V) (b). The black arrows indicate the increase of current at the same voltage. (c) The relative current change, ΔI/I0, as a function of strain on the InAsP nanowire. (d) BF TEM images corresponding to the tensile strain conditions: the exact strain was calculated from the frame data. D
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10.7 × 10−20 Pa−2, −0.7 × 10−30 Pa−3, and 19.0 × 10−40 Pa−4. The obtained first-order piezoresistance coefficient of the InAsP NW is comparable to or slightly higher than those of recently reported InAs and Si NWs.13,21,22 These results were consistently observed in our experiments (Supporting Information S5). The piezoresistance coefficients could then be used to design InAsP nanodevices based on strain engineering. If the effective carrier concentration is low, the current of the NWs is dominantly determined by mobility rather than by carrier concentration.18 In particular, because the NWs in our experiment have a very large length/radius ratio (which reached 52), the current flow can be explained by the space-chargelimited current model, which shows the mobility-dominant transport if the effective carrier concentration is low. Specifically, the nonlinear increase in the high-voltage region in the current in each I−V curve, bent in a parabola-like shape, is related to the space-charge-limited current; the current is increased by following an I ∞ V2 relation.18 The mobility of the space-charge-limited current, μSCL, is expressed as19
where R is the resistance. At a tensile strain of 0%, the resistivity of the NW is calculated to be approximately 1.54 × 10−1 Ω·cm from the linear increase of the current with increasing applied voltage (Figure 4a). This value is comparable to the result reported for an InAs NW with a mixed-crystal structure (zincblende + wurtzite), ∼10−1−10−2 Ω·cm.20 From the analysis of the I−V curves, the change of relative resistance, ΔR/R0, was also derived under varying strain conditions, and the results are shown in Figure 5a. The resistance nonlinearly decreases with
μSCL =
IL V 2πεd
(5)
where εd is the dielectric constant of InAsP. For eq 5, the dielectric constant of the InAsP NW was calculated by assuming that there is a linear relationship between those of InAs and InP; the dielectric constant, εd,InAs = 15.15 for InAs and εd,InP = 12.5 for InP, was used to calculate for the InAsP nanowire.23 The molar fraction of 0.144 for the InP was used for the calculation, which was arrived at via EDS, HRTEM, and HRXRD analyses. Figure 6a shows the behavior of the mobility
Figure 5. Relative resistance change (a) and relative resistivity change (b) of the InAsP NW under various tensile strain conditions. The red line shows the result fitted with a polynomial.
increased tensile strain. As described above, the decrease in resistance can be explained by the piezoresistive effect and can be modeled with a series expansion:14,21 Δρ = π1σ + π2σ 2 + π3σ 3 + ... ρ
(3)
where π(i) are the effective piezoresistance coefficients, and σ is stress. As it is a nonlinear piezoresistive effect, second and fourth-order polynomial fits are required to describe the piezoresistive behavior. The plot in Figure 5b shows the relation between the stress and the relative resistivity change in the InAsP NW. The stress, σ, for the fitting of eq 3 can be found by applying the elastic stress−strain relationship, σ = Eε with the terms of the strain, ε, and the Young’s modulus, E (eq 6). The Young’s modulus of 96.7 GPa, which was sought from the stress−strain curve in Figure 7b, was adopted for the stress calculation. To extract the effective piezoresistance coefficients, π(i), the graph in Figure 5b was fitted with a polynomial by neglecting the geometric effect. The best fit, represented by the red line in Figure 5b, is a fourth-order polynomial and the coefficients from π(1) to π(4) are, in turn, −5.0 × 10−10 Pa−1,
Figure 6. (a) Effective electron mobility versus tensile strain for the InAsP NW. (b) I/V versus V plot extracted by the linear fitting of the I−V curves for the InAsP NW. E
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Nano Letters in relation to tensile strain. The mobility was sought from the slopes of I/V−V curves (Figure 6b). The mobility increased with an increase in tensile strain. It increased from 62 cm2/V·s to 353 cm2/V·s in the range of 0−3.22% with a given strain. This increase in mobility causes an increase in the relative current changes. The effect of the mobility origin on the piezoresistive effect has been demonstrated and emphasized in single crystalline silicon (Si) systems.1,24−26 The enhancement of mobility on strained InAsP NWs may result from the modification of electronic structures by the reduced dimensions and increased surface-to-volume ratio; the surface effect may contribute significantly to the transport properties because the diameter of the InAsP NWs is approximately 100 nm. In order to verify our intuition, we performed the density functional theory (DFT) simulations27 for analyzing the change of the carrier effective mass according to the tensile strain from the electronic structure in quantum level. The effective mass of electron me* of the WZ phase of InAs0.75P0.25 in the [0001] direction was calculated as 0.0250 (/m0) without strain. The me* was 0.0233 (/m0) under 1% and it became 0.0211 (/m0) under 3% of the tensile strain along the [0001] direction. This can be interpreted that the electron in InAs0.75P0.25 moves faster as the tensile strain increases, at least in the range of up to 3%. Therefore, our theoretical calculations are in good agreement with our experimental observations, finding the origin from the electronic structure changes induced by the strain (details for DFT calculation are described in Supporting Information S6). Moreover, the strain on the InAsP NWs may modify the band structure at the surface; discrete surface states are induced by the strain, which would result in different quantization in the space charge layers, especially in the charge accumulation layers existing on the {110} facets surrounding the NWs.28−32 In addition, we deduce that the reduction of scattering by various mechanisms, especially, by phonon scattering and surfaceroughness scattering due the existence of planar defects and the consequent atomic-scale steps on the surface, plays a role in the improvement of the mobility of InAsP NWs. Figure 7a shows the variation of the load with time, measured by a fast cyclic displacement test. The exact displacement change is also shown in Figure 7a, extracted by analysis of the TEM frame data. It is clear that a linear correlation exists between the displacement and the load, although the measured displacement has a small amount of discrepancy with the intended displacement as a control variant. The linear coefficient between the load and the displacement is related to the Young’s modulus of the InAsP NW; the Young’s modulus can be calculated as E = σ /ε
Figure 7. (a) Changes of the load (black line) and the exact displacement (red line) with time during the fast cyclic displacement test for the InAsP NW. (b) Stress−strain curve for the InAsP NW. The curve corresponds to the black dashed square in panel a. The slope indicates the Young’s modulus of the NW.
characteristic of the InAsP NW under repeated loading and unloading conditions. The dynamic and spontaneous response of the current with the change of strain on the InAsP NW is shown in Figure 8a. Cyclic change of strain was allowed in the InAsP NW (Supporting Information S2 and Supporting Video 1). The exact strain, calculated by measuring the displacement from the time-depending frame analysis of the video, is indicated by red circles in Figure 8a; a locally weighted scatterplot smoothing (LOESS) was conducted on the strain curve. The black squares indicate the corresponding current on the strain. The current change, ΔI, clearly matched with the strain change, Δε. The current reacted in an agile manner to the change of strain; the interval between two current measurements was 0.1 s, while the change rate of the strain was 0.168%/s, 9.67 nm/s. The current linearly increased with the increase in strain, and the value of the linear coefficient between the current and the strain was the same during all processes of the cyclic test. These results indicate that the change of current, depending on the strain, is repeatable and reversible; the NW was free from any variation in microstructural properties. We deduced that stress relaxation did not work into the InAsP NW during measurement. This also indicates that the contact between the NW and the electrodes was safe and stable. In addition, the NW was free from fatigue failure at the strain of 3.22%. The electromechanical properties of individual InAsP NWs were investigated under a uniaxial tensile strain by transmission electron microscopy (TEM). Many planar defects were observed in the InAsP NWs, and a linear increase of the
(6)
From eq 6, the Young’s modulus is the slope in the σ−ε plot when a linear relation exists between a stress and a strain. From the analysis of the specified part in Figure 7a (indicated by a dashed rectangle), the stress, σ, was plotted as a function of the strain, ε (Figure 7b). The slope of the plot in Figure 7b is the Young’s modulus of the InAsP NW, the value of which is 96.7 GPa. The measured Young’s modulus of the InAsP NWs is quite consistent with those of InAs and InP along the [111] direction of a ZB structure because these are known to be around 97 and 113 GPa, respectively.33−35 This linear relationship between stress and strain was repeatedly observed during measurement, as shown in Figure 7a. Also, InAsP NWs remain elastic until fracture (Supporting Information Figure S7). We provide Supporting Video 1 to show the elastic F
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describes the results of EDS analyses, the four-point measurement, and the mechanical test, which indicate reproducible transport and mechanical properties (PDF) Video of in situ electromechanical tests of an InAsP NW (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail address:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the R&D Convergence Program of NST (National Research Council of Science & Technology) of Republic of Korea. Work in part supported by Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) (grant number: 20110030233, 2014M3A7B6020163, and 2016M3A7B4025406).
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Figure 8. (a) Changes of the current and the exact strain with time during the fast cyclic displacement test for the InAsP NW. We observed a dynamic and spontaneous response of the current, corresponding to the change of the exact strain. The bottom images show the BFTEM micrographs corresponding to the change in tensile strain. (b) Change of the current as a function of the strain value in the dashed square in panel a.
relative current, ΔI/I0, with increased tensile strain was observed in the InAsP NWs. We determined that the origin of the specific electromechanical properties was due to the piezoresistive effect by considering the microstructural and transport properties of the material. The nonlinear transport behavior at the high-voltage region of the InAsP NWs was explained by the introduction of a space-charge-limited (SCL) current: specifically, the increase of mobility. The current that passed through an InAsP NW was sensitively affected by strain status. From the cyclic test, the reliability and repeatability of the electromechanical properties of InAsP NWs were confirmed. We believe that our study of these electromechanical properties will facilitate the exact application and exploration of novel phenomena within InAsP nanowires in the near future.
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REFERENCES
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S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b02155. Modified EPTP device, a procedure for a fast cyclic displacement test, and details for DFT calculation. It also G
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