Crystal Face-Dependent Nanopiezotronics of an Obliquely Aligned

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Letter pubs.acs.org/NanoLett

Crystal Face-Dependent Nanopiezotronics of an Obliquely Aligned InN Nanorod Array Nai-Jen Ku,† Jun-Han Huang,† Chao-Hung Wang,† Hsin-Chiao Fang,† and Chuan-Pu Liu*,†,‡,§,∥ †

Department of Materials Science and Engineering, ‡Center for Micro/Nano Science and Technology, §Research Center for Energy Technology and Strategy, and ∥Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan S Supporting Information *

ABSTRACT: This paper proposes an obliquely aligned InN nanorod array to maximize nanorod deformation in the application of nanopiezotronics. The surface-dependent piezotronic I−V characteristics of the InN nanorod array with exposed polar (0002) and semipolar (1̅102) planes were studied by conductive atomic force microscopy. The effects of the piezopotential, created in the InN under straining, and the surface quantum states on the transport behavior of charge carriers in different crystal planes of the InN nanorod were investigated. The crystal plane-dependent electron density in the electron surface accumulation layer and the strain-dependent piezopotential distribution modulate the interfacial contact of the Schottky characteristics for the (0002) plane and the quasi-ohmic behavior for the (1̅102) plane. Regarding the piezotronic properties under applied forces, the Schottky barrier height increases in conjunction with the deflection force with high current density at large biases because of tunneling. The strain-induced piezopotential can thus tune the transport process of the charge carriers inside the InN nanorod over a larger range than in ZnO. The quantized surface electron accumulation layer is demonstrated to modulate the piezopotentialdependent carrier transport at the metal/InN interfaces and become an important factor in the design of InN-based piezotronic devices and nanogenerators. KEYWORDS: InN nanorods, molecular beam epitaxy, glancing angle deposition, surface accumulation, piezotronic effect, surface quantum state

W

nanodevices based on ZnO micro/nanowires have been realized.3 To develop a higher output power nanogenerator, materials with higher piezoelectric coefficients such as ZnS,16 CdS,17 GaN,18,19 InN,20 and mechanisms for larger deformations are being investigated, but the research is only in its infancy. Among all the promising piezoelectric nanomaterials that have been studied, a single-InN-nanowire nanogenerator is distinct with its largest output voltage of up to 1 V,20 which is more than 10 times higher than that of conventional ZnO nanowires. Therefore, InN could be a candidate for high-output power nanogenerator devices. Moreover, the current nanogenerators incorporating vertically aligned nanowires function mainly by shear forces under which the total deformation is restricted. To provide the maximum deformation under a

ith natural resources being depleted at an ever fast rate, it is not surprising that research into alternative energy has become more attractive. Recently, based on the piezoelectric and semiconductor properties, a new field of piezotronics1,2 has been created as basic building blocks for fabricating innovative devices such as nanogenerators,3−6 piezopotential-gated field effect transistors,7,8 piezoelectric diodes,9 piezoelectric logic nanodevices,10,11 piezoelectric chemical sensors,12 and piezophototronic devices,13−15 which uses the effects of piezoelectric potential created in the crystal for controlling or tuning the charge carrier transport characteristics to fabricate mechanical electronic devices. The fundamental principle of the piezotronic devices is to control the carrier transport by creating a piezopotential within the semiconductor through the application of a strain.2 Therefore, the polarity-dependent piezopotential distribution inside piezoelectric semiconductor materials becomes an important factor for the design of nanopiezotronic devices. Since 2005, when ZnO nanomaterials matured, various unique piezotronic © 2011 American Chemical Society

Received: August 11, 2011 Revised: December 16, 2011 Published: December 30, 2011 562

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maintained at 480 without using any catalysts, and the substrate temperature was held at 400 °C. The incident molecular beam subtended an angle of approximately 60° relative to the substrate surface. The morphology and crystallography of the InN NRs were characterized by field emission scanning electron microscopy (FE-SEM, JEOL JSM-7000F) and highresolution transmission electron microscopy (HR-TEM, JEOL JEM-2100F). The force-dependent nanopiezotronic measurements on individual NRs were performed by conductive atomic force microscopy (C-AFM, Seiko SPA400) in contact mode at room temperature. The freshly cut Pt/Ir coated tip of tetrahedral shape with a cone angle of 25° was used for the C-AFM. A silver electrode was used to guarantee contact with the sample and the C-AFM stage with the tip grounded. (For more information, please see Supporting Information, Figure 1S). Figure 1a shows a cross-sectional SEM image of an obliquely aligned InN NR array, exhibiting a fairly uniform height of

simple normal force for the largest piezoelectric potential, we propose an InN nanorod (NR) array aligned obliquely with deformation along the direction of the piezoelectric field. InN is well-known as having the lowest electron effective mass for applications in high-speed electronic devices. In contrast to ZnO, InN uniquely possesses an inversion layer of electrons at the surface, resulting in downward surface band bending, which affects the optical properties greatly.21−23 According to first principle calculations, the microscopic origin of the surface electron accumulation is the donor type surface states caused by In−In bonding.24 The surface electron accumulation layer is universal and has been observed for cplane,25 a-plane,25 and m-plane InN26 within a distance typically of the order of 10 nm. The influence of the quantization of the electron accumulation layer because of the nanoscale at the surface or interface on the electron transport of InN nanostructures has been recently reported.27−30 This quantized surface electron accumulation layer (QSEAL) causes an interesting resistance anomaly in which the InN nanowire resistance increases with the crosssectional area when the wire radius is small.27 A three-region model31 has been developed to characterize the contribution of InN conductivity from the bulk electron density, quantized surface electron accumulation layer, and defect donors. Increasing InN conductivity will further lessen internal consumption by the inner resistances of the devices, such as the output voltage of a nanogenerator. However, the piezopotential is also reduced because of the screen-effect caused by the excess electrons, but the excess electrons cannot deplete the piezopotential completely.32 Therefore, InN NR with a conductivity below an optimum value generated the largest output voltage among the nitride nanogenerators.33 Although the degree of surface band bending in InN is crystal plane-dependent, few facet-dependent electrical transport properties have been explored.28,30 These unique physical properties of InN differ drastically from ZnO and are expected to increase the efficiency of nanopiezotronic devices immensely. This study investigates the contribution of the QSEAL at the NR surface, together with the nanopiezotronic effect on the electron transport of an orientation-controlled InN NR. This obliquely aligned nanostructure provides a unique geometry with two-faceted surfaces: a common c-(0002) plane on top and six r-(1̅102) side planes. The local transport properties are studied on the formation of the Schottky barrier (SB) relative to the surface morphology and piezotronic properties of InN nanostructures. The degree of surface band bending with the QSEAL for the c-plane and r-plane in relation to the I−V character and the piezotronics are also studied. The distribution of the carrier density across a NR has a profound influence on the output voltage of a nanogenerator.33 The higher electron density inside the NR ensures a reduced inner resistance but the piezopotential is also reduced by screening the piezoelectric field. Nevertheless, the overall effect of combining a high piezoelectric effect with the electron distribution in this study might help improve the efficiency of the InN nanogenerator. The orientational control of the obliquely aligned epitaxial InN NR array was achieved by glancing angle deposition with a plasma-assisted molecular beam epitaxy (MBE) system. Si(111) was employed as the substrate, which was precoated with a ZnO buffer layer composed of a faceted ZnO nanopillar array approximately 100 nm in height using RF magnetron sputtering. During growth, the indium/nitrogen ratio was

Figure 1. Morphology of obliquely aligned InN NRs. (a) A crosssection SEM image of an obliquely aligned InN NR array; (b) TEM image of a single InN NR; (c) corresponding nanobeam electron diffraction pattern of the single InN NR; and (d) high-resolution TEM image of the InN NR at the marked region in (b). The lattice fringe spacing of 0.564 nm is consistent with the d0001 spacing of the hexagonal InN structure.

approximately 1 μm and an average diameter of approximately 82 nm. The InN NRs preferentially subtend an angle of ∼58° to the Si substrate. Figure 1b shows a TEM image of a single oblique InN NR. The corresponding nanobeam electron diffraction pattern in Figure 1c taken along the [112̅0] zone axis indicates that the NR has a wurtzite structure grown along the [11̅01] direction. The InN NR in this study was verified to have In polarity by TEM convergent-beam electron diffraction (not shown). Figure 1d is the HRTEM image of the boxed region in Figure 1b. The InN NR is composed of a top (0002) plane and six {1̅102} sidewalls. Figure 1c,d shows that the InN NRs are single-crystalline and free of dislocations. The presence of a QSEAL is confirmed by the coupled plasmon-LO-phonon mode21,26 at ∼435 cm−1 in the μ-Raman spectrum (Supporting Information, Figure 2S) with the electron concentration of the order of 1019∼1020 cm−3 close to the NR surface. 563

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Figure 2. (a) I−V curves measured at (0002) vs (1̅102) plane fitting with thermionic emission-diffusion model. (b) Schematic energy band diagram of metal to (0002) and (1̅102) plane before contact. φtip and φInN are the work function of the Pt/Ir tip and InN, respectively; χsInN is the electron affinity at an InN surface; and u0 is the potential of the surface band bending relative to the Fermi level. (c) Schematic energy band diagram of Schottky contact at (0002) and (1̅102) plane. (d) Conductance vs bias curve measured at (0002) and (1̅102) plane.

Figure 2a shows the I−V characteristics measured by the probe of the C-AFM held stationary at a spot on the (0002) and (1̅102) plane of an InN NR. The nonlinear and asymmetric characteristics are typical of the Schottky barrier height (SBH) between the metal electrode and InN. Wener et al.27 showed that the resistance of the shell and core of an InN NR are equal at a critical diameter ∼55 ± 15 nm. Therefore, the electron conduction in the InN NRs is dominated by the core rather than the surface electron accumulation layer for their diameters are larger than the critical diameter. For any given voltage, the current is lower for the (0002) plane than the (1̅102) plane, indicating the involvement of different SBH. From the I−V characteristics, the SBH is changed from demonstrating rectifying behavior for the (0002) plane to quasi-ohmic characteristics for the (1̅102) plane at V > 0.3 V or V < −0.2 V because of the increased surface electron density.34 This behavior is analogous to turning an SB into an ohmic contact through a highly doped surface layer. For the Schottky contact formed at the interface between the metal and InN shown in Figure 2c, a certain quantity of electrons could accumulate in a region near the surface because of the downward surface band bending and could tunnel through the SBH due to the tipinduced high electrical field under an appropriate bias voltage.34,35 The electron conduction through the Schottky diodes for the (0002) and the (1̅102) plane is confirmed to be caused by the classic thermionic emission-diffusion (TED) mechanism, as evidenced by the perfect match with the logarithmic plot of the current with V1/4, as shown in Supporting Information Figure 3S. Assuming the temperature (T) and the donor concentration (ND+) are the same between these two planes over the entire

InN NR and neglecting the different contact area of the SB, the ratio of the current density between these two planes can be expressed by

I (0002) I ( 1̅ 102)

⎛ (0002) ⎞ ⎜ −Δφ ⎟ s ⎜ ( 1̅ 102) ⎟ A** + ΔA** ⎟ ≈ exp⎜ A** kT ⎜ ⎟ ⎜ ⎟ ⎝ ⎠

(1)

where I(0002) and I(1̅102) are the current through the InN NR for the (0002) and (1̅102) plane, respectively, ΔA** is the change in the effective Richardson constant and Δφs is the change in SBH between the (0002) and (1̅102) planes. Since A** is only a function of stress dependent effective mass, the change in the effective Richardson constant ΔA** ≪ A**, φs can thus be deduced as2,36,37

Δφs

⎛ I (0002)| ⎞ V⎟ ≈ − kT ln⎜⎜ ( 1̅ 102) ⎟ ( 1̅ 102) I | ⎝ V⎠ (0002)

(2)

By calculating the SBH difference between the two planes from eq 2, the SBH for the (11̅ 02) plane is determined to be lower than that of the (0002) plane by roughly 58.94 ± 1.33 meV. The SB formation between an InN NR and a Pt/Ir tip can be visualized in Figure 2b. The actual barrier height not only relies on the difference between the work function (eφm) of the metal tip and electron affinity (eχ) of the InN NR, but also on the image force, electrical field penetration, and the existence of the QSEAL layer at the surface.34 Figure 2c shows the SBH formed 564

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Figure 3. (a) Surface morphology and (b) probe current (the InN NR-to-tip bias is 0.5 V) images for the obliquely aligned InN NR array. (c) Probe current images obtained along the scan direction [11̅00] (left) and [1̅100] (right). (d) Dual cross section of InN NRs with the topography and current signals superimposed. An increased current signal is observed on one side of an InN NR (1̅102) plane.

by the angle resolved photoemission spectrum results.39,40 A further increase in surface state density would lead to a deeper potential well at the surface, increasing the number and depth of the subbands at which electrons are bound.39 For the (1̅102) plane with higher surface electron density, three subband minima are observed in Figure 2d, V1(1̅102) = 172 mV, V2 (1̅102) = 274 mV, and V3(1̅102) = 435 mV, which is consistent with the fact that the potential well for the (11̅ 02) plane is deeper than for the (0002) plane. More quantitative comparisons of electron subband minima and surface electron density for both planes are made and discussed in Supporting Information and Figure 4S. Quantum effects become more apparent as the width of the QSEAL layer is reduced from the (0002) to (11̅ 02) plane. The C-AFM topographic and current images41,42 of opposite scanning directions shown in Figure 3a−c are expected to provide deeper insights into the localized electrical properties. The substrate bias was maintained at 0.5 V relative to the probe (ground). Although the contact area differs between the top and side surfaces, highly conductive paths are formed at the (1̅102) plane over the (0002) plane, irrespective of the scanning directions, as shown in Figure 3c, revealing an asymmetric current distribution around the InN NR top surface. Figure 3d shows the dual cross sections of the height and current profiles along the line marked in Figure 3a,b. Further examination of Figure 3d shows that the tip-moving current values measured from the (1̅102) planes of individual InN NRs exceed the current limit of the C-AFM as the tip scanning along the [1̅100] direction and are significantly higher

at both interfaces for the (0002) and (1̅102) planes. It has been reported that the degree of surface band bending for nonpolar plane is larger than for the polar c-(0002) plane by approximately 0.11 eV.25 Therefore, based on the crystallographic similarities, it is reasonable to assume that the 2D surface electron density accumulating at the (1̅102) plane is higher than at the (0002) plane, as indicated in Figure 2(b). As the Fermi level lines up for the formation of the Schottky contact, the different conduction/valence band edges (u0(0002) and u0(1̅102)) of InN bend upward to yield different SBH values between the (0002) and (1̅102) surface, leading to the emergence of a potential well near the surface. The degree of surface band bending agrees with the SBH value calculated above using eq 2. The minimum energy of the quantized two-dimensional electron subbands of the QSEAL layer for the (0002) and (1̅102) plane can be observed in the plot of conductance (dI/ dV) versus voltage shown in Figure 2d.38,39 The electrons in a given subband Fn are calculated to have the same bound-state wave function. Therefore, the conductance variation is assumed to arise from the step discontinuity of the density of state function at the subband minimum of the surface accumulation layer.38,39 The quantized energy levels determined by the second derivatives d2I/dV2 for the (0002) plane are V1(0002) = 97 mV and V2(0002) = 304 mV. Therefore, the energy difference of (0002) = 207 mV, which agrees these two subband minima is V1,2 well with that of the two confined subband minima (k∥ = 0), which are 0.80 and 0.51 eV below the Fermi level, as measured 565

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Figure 4. (a) Typical I−V characteristics of an InN NR at different tip deflection forces. (b) Schematic diagram showing the mechanism of the charge depletion region created in the compressive strain region. (c) The derived change in SBH at a tip−substrate bias of 1.0 V based on the TED model and the ideality factor in eq 3 as a function of tip deflection force.

be a combinatory effect of the controlled piezopotential and the SBH change associated with the (0002) plane and the (1̅102) plane. The SB formation at the metal−semiconductor interfaces represents the fundamental mechanism of the nanogenerators. The nanopiezotronic effect of the oblique InN NR is studied by applying a deflection (normal) force to the AFM Pt/Ir tip on a NR. Figure 4a shows a set of I−V curves with various deflection forces ranging from ∼0 to 4 nN, which exhibit the asymmetric characteristics of ideal Schottky diodes with different SBH confirmed by perfect fits with TED model, as shown in Supporting Information Figure 5S. The current reduction effect with increasing tip deflection forces is a combination of the piezopotential gating effect that reduces NR conductance and the force-dependent SBH change. When an oblique NR is pressed downward by a normal force, a strain field is created with the exposed side plane stretched and the other side plane compressed. Therefore, a piezoelectric potential is created inside the NR with a positive and negative piezopotential for the stretched and compressed side, respectively.2,36 When the deflection force was increased, the current for a given bias under both positive and negative bias dropped significantly. These asymmetric I−V curves are attributed to the different SBH formed between both metal/InN interfaces at the two ends of a NR. The strain distribution within an oblique InN NR in this case is by no means homogeneous because of the complex shape and different degree of deformation involved at different heights and will not be further analyzed. When an oblique NR is depressed, an asymmetric strain distribution

than the tip-stationary current value of 70 nA shown in Figure 2b. Conversely, once the tip scanning direction was reversed to the [11̅00] shown in Figure 3c, the sidewall current signals decreased to 50−70 nA. Nevertheless, no current is measured at the (0002) surfaces for both scanning directions. During scanning, the tip exerted a deflection force of 1 nN, which creates a transverse strain field and, thus, a piezopotential.20 The current increase and decrease at the (1̅102) plane are clearly caused by the inversion of the piezopotential-induced barrier height change. Nevertheless, high current regions always correspond to the side surfaces, irrespective of scanning direction, confirming the lower barrier height between the Pt/Ir tip and the InN (1̅102) plane for the higher surface electron concentration compared to the (0002) plane. Conversely, current suppression to zero at the (0002) plane is caused by a combined gating effect with a piezopotentialinduced barrier height increase. For the gating effect, free electrons trapped at the tensile bending side of the NR for the positive piezopotential that form a charge depletion zone around the compressive bending side of the NR. Consequently, when the Pt/Ir tip was swept across a NR, the width of the conducting channel was reduced under the influence of the shear forces. Hence, the piezoelectric field established across the NR restrains the current signal measured at the (0002) plane. This effect would dominate the current suppression phenomenon, as confirmed by reversing the tip scan direction. For the other effect, Figure 4 shows that the tip deflection force increases in conjunction with the piezopotential-induced barrier height. Therefore, the modulation of the carrier transport could 566

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these accumulated electrons further tunnel through the SBH with increasing tip deflection force. When the depletion force is increased under a larger external force, tunneling becomes more significant as η increases as a result of the higher built in piezoelectric field, indicating a strong tunneling component of the overall charge transport characteristics. From our results, the surface-dependent and piezopotentialdependent electron transport properties have been investigated with the influence of the QSEAL. The SBH is formed at the interface between the Pt/Ir tip and InN and can be described by the classic TED model. Thus, the ability to systematically tune the SBH through the external normal force demonstrates one of the largest achievements for the application of nanogenerators. The influence of the surface electron concentration on SBH is evident since a higher surface electron concentration at the (1̅102) plane results in lower SBH than the (0002) plane by approximately 59 meV. At larger bias voltages, electrons accumulated at the surfaces have been observed to tunnel through the SBH. The SBH can be tuned by adjusting the tip deflection force. In addition, the fraction of electron tunneling increases in conjunction with deflection forces. With the largest piezoelectric constant, the InN nanostructure used in nanopiezotronics can produce a significantly wider range of threshold voltages and transport characteristics that are superior to ZnO.

across its diameter results in an asymmetric potential distribution with a positive potential (V+) produced at the positive strain (ε > 0) side of the NR and a compressed surface with negative piezopotential (V−) caused by the piezoelectric effect, as shown in Figure 4b. Because of the charges induced by the piezoelectric effect, two possible effects (a carrier trapping effect and the creation of a charge depletion zone)7,9 account for the reduction of current in the NR. A number of free electrons flowed and were trapped at the positive side surface7 from which a negative potential was created, which repelled other electrons to form a charge depletion zone. Therefore, the width of the conducting channel and the effective electron density in the InN NR are reduced, whereas the depletion region grows in conjunction with an increase in tip deflection forces. If the piezopotential suffices in largeness, the piezoelectric field created across the bent NR serves as the gate that controls the electric current flowing through the InN NR, which decreases NR conductance. As deflection force increased, the depletion zone became larger while the conduction channel became narrower, resulting in the current gradually decreasing inside the NR.7 As the bias voltage increased, the current increased abruptly from a high-resistance state to a lowresistance state at the threshold point (Vth) caused by the electron tunneling through the SB. The threshold voltage shifted from 0.63 (−0.56) to 2.73 (−1.58) V at positive (negative) bias and increasing the tip deflection force. In addition, the change in effective SBH produced at the interface between the Pt/Ir tip and the (0002) plane is caused by the nonuniform strain distribution inside the InN volume,2,36 and can be derived by eq 2 with the results shown in Figure 4c. The change in SBH, Δφs, exhibits an approximately linear relationship with the magnitude of the deflection force for the high-resistance region below Vth, which is consistent with the behavior of ZnO NRs.2,36 The actual piezopotential distribution is not fixed because of the nonuniform distribution of the strain. Thermionic-field emission was suggested as the mechanism responsible for the excess tunneling currents through SB observed both in the forward and reverse current of the SB in highly doped semiconductors.37 In the low-resistance region, the tunneling current can be further facilitated by thermionic emission, and the total current density consisting of both thermionic emission and tunneling can be expressed as37

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information on the experimental details and equilibrium circuit for the nanopiezotronic measurements, extensive material characterization, and the validation of theoretical models are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

(3)

ACKNOWLEDGMENTS This work was supported by the National Science Council under Grant NSC- 98-2221-E-006-079-MY3 and the Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan. The NSC Core Facilities Laboratory for Nano-Science and Nano-Technology in the Kaohsiung-Pingtung area are accessed to equipment and technical support.

where η is the ideal factor related to the slope of the log−linear plot, which increases from 10 for ∼0 nN to 14 for 4 nN by linear fitting with the experimental data in Figure 4a, as shown in Figure 4c for the low-resistance region. If there is little or no tunneling current, η is extremely close to unity at low doping concentration. However, η can depart substantially from unity when the electron concentration is increased37 at the InN NR surfaces, confirming that the electron transport through the SB is dominated by thermionic emission and the electron tunneling effect for the high-resistance and low-resistance region, respectively. The strong tunneling behavior with increasing tip deflection forces could be inferred from the influence of the asymmetric distribution of the piezopotential. As shown in Figure 4b, the positive piezopotential created at the tensile side of the NR results in adsorption of free electrons this surface, which widens the width of the QSEAL. Therefore,

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⎡ ⎛ qV ⎞ ⎤ J = J0 ⎢exp⎜ ⎟ − 1⎥ ⎣ ⎝ ηkT ⎠ ⎦

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REFERENCES

dx.doi.org/10.1021/nl202782q | Nano Lett. 2012, 12, 562−568

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dx.doi.org/10.1021/nl202782q | Nano Lett. 2012, 12, 562−568