Enhancement of Trap-Assisted Green Electroluminescence Efficiency

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Enhancement of Trap-Assisted Green Electroluminescence Efficiency in ZnO/SiO2/Si Nanowire Light-Emitting Diodes on Bendable Substrates by Piezophototronic Effect Kwangeun Kim,† Youngin Jeon, Kyoungah Cho, and Sangsig Kim* Department of Electrical Engineering, Korea University, Seoul 136-701, Republic of Korea S Supporting Information *

ABSTRACT: The trap-assisted green electroluminescence (EL) efficiency of a light-emitting diode (LED) consisting of a ZnO nanowire (NW), a SiO2 layer, and a Si NW on a bendable substrate is enhanced by piezophototronic effect. The green EL originates from radiative recombination through deep-level defects such as interstitial zinc, interstitial oxygen, oxygen antisite, and zinc vacancy in the component ZnO NW. The efficiency of the trap-assisted green EL is enhanced by a piezophototronic factor of 2.79 under a strain of 0.006%. The piezoelectric field built up inside the component ZnO NW improves the recombination rate of the electron−hole pairs thereby enhancing the efficiency of the trap-assisted green EL. KEYWORDS: piezophototronics, piezoelectronics, ZnO, nanowire, trap-assisted electroluminescence, finite difference time domain, finite element method

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room temperature.14,15 These features enable the bendable oxide NWs to be utilized as effective optoelectronic material in piezophototronic research that necessitates optical and mechanical coupling. ZnO NWs exhibit a near band-edge emission in the ultraviolet (UV) region and a broad deep-level emission in the visible region. The broad green emission (GE) band centered at around 520 nm originates from surface traps in the ZnO NWs.16−18 An understanding of the charge-carrier trapping mechanism in the trap levels is required to optimize the efficiency of light-emission in the visible range. However, so far, tuning of light-emission in the ZnO NWs has mainly focused on the UV emission, because of a large band gap and difficulty in realization of visible light. For a ZnO NW on a bent substrate, tensile and compressive strains are formed at the outer and inner surfaces, respectively.19 The strains produce positively and negatively charged surfaces, due to the piezoelectric effect. An electric field is then built up within the NW, shifting electrons and holes to

INTRODUCTION Of late, the piezophototronic effect has been extensively used for improving the performance of optoelectronic devices. The three-way coupling among piezoelectricity, photoexcitation, and semiconductor properties of noncentral symmetric crystals regulates charge-carrier transport, generation, and recombination at the junction interface because of the strain-induced piezoelectric potential (piezopotential).1−12 The piezopotential is created by charge polarization due to external strain on the wurtzite or zinc-blende structure semiconductors, such as ZnO, InN, ZnS, CdS, CdSe, and GaN, that are piezoelectric and semiconducting. Various applications of this phenomenon such as photovoltaics, photodetectors, light-emitting diodes (LEDs), and logic computations have been successfully pioneered by Wang’s group.1−7,9−12 Among the various piezoelectric semiconductors, ZnO nanowires (NWs) have been increasingly used in optoelectronics owing to significant exciton binding energy (60 meV), direct wide band gap (3.24 eV), and a large refractive index as well as inner-piezopotential properties.1−7,10,11,13 This significant binding energy (i.e., 2.4 times larger than the effective thermal energy (25 meV)), in particular, makes the role of the phonons ineffectual, giving rise to efficient light-emission at © XXXX American Chemical Society

Received: November 16, 2015 Accepted: January 12, 2016

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DOI: 10.1021/acsami.5b11053 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematics of an n-ZnO/SiO2/p-Si NW LED constructed on a bendable substrate. SEM images of NWs: (b) an as-grown ZnO NW (scale bar = 30 nm), (c) an as-prepared Si NW before detaching from the mother Si substrate (scale bar = 500 nm), and (d) crossed NWs and a SiO2 layer between these NWs on a bendable substrate (scale bar = 1.1 μm). Optical images of the LEDs: (e) downwardly and (f) upwardly bent substrates (scale bar = 1 mm).

Scheme 1. Process Steps for n-ZnO NW/SiO2/p-Si NW LEDs: (a) Definition of Si Active Regions, (b) Si Trench Etching, (c) Crystallographic Wet Etching, (d) Imprinting onto Bendable Substrate, (e) Transfer of Si NWs, (f) SiO2 Sputtering, (g) Metal Deposition, (h)-a ZnO NW Alignment by Dielectrophoresis, (h)-b Growth of ZnO NWs, (i) Metal Deposition and Annealing, (j) Downward Bending, and (k) Upward Bending

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RESULTS AND DISCUSSION The LED under study consists of a hexagonal n-type ZnO NW, a triangular p-type Si NW, and a SiO2 dielectric film in-between these NWs on a bendable substrate, as shown in Figure 1a (for applications of ZnO/SiO2/Si heterostructures, see section A in Supporting Information).20−22 Note that Ni/Au and Al/Au metals are used as ohmic contacts in the Si and ZnO NWs, respectively (see section C in Supporting Information). The ZnO NW is perpendicularly crossed on the Si NW, and the nanoscale contact point of these crossed NWs emits the green light (see Experimental Section and Scheme 1). In an n-ZnO/ SiO2/p-Si NW LED, the SiO2 layer is required for the confinement of the electric field and tunneling of the charge carriers.23−26SEM images shown in Figure 1b−d manifest the hexagonal cross section of the ZnO NW, the inverted-triangular

the outer and inner surfaces, respectively. Meanwhile, the carriers trapped at the surface defects participate in the electron−hole pair (EHP) recombination, contributing to the enhancement of GE efficiency. In this study, an LED is constructed on a bendable substrate by crossing a ZnO NW and a Si NW, with an intervening thinlayered SiO2. The trap-assisted green electroluminescence (EL) is examined by varying the strain applied to the ZnO NW. Modulation mechanism of the trap-assisted EL efficiency on the strain is explained using the piezophototronic effect. Further, the structural features for the light intensity of the LED are examined by finite difference time domain (FDTD) simulation, and the strain-induced pieozopotential on the LED is analyzed by finite element method (FEM). B

DOI: 10.1021/acsami.5b11053 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces cross section of the Si NW before detachment from the mother Si substrate, and the crossed NWs with the thin layer of SiO2, respectively. The diameter and length of the ZnO NW are 90 nm and 10 μm, and those of the Si NW are 250 nm and 1 mm, respectively. Optical images captured in Figure 1e,f display the LEDs on downwardly and upwardly bent substrates, respectively. When the substrate is bent, the ZnO NW experiences an applied strain, but the Si NW does not because it is positioned perpendicular to the direction of bending. To understand the influence of the ZnO/SiO2/Si NW LED geometry (i.e., the junction interface formed with the triangular-shaped Si NW) on light extraction, light intensities are patterned by means of 2D FDTD simulation for two different LEDs constructed using triangular and rectangularshaped Si NWs, as shown in Figure 2. The dimensions of these

Figure 3. Injection current versus junction voltage (I−V) characteristics of the ZnO/SiO2/Si NW LED in a log−log scale plot. Four different I−V regions are presented in the plot. A turn-on voltage of 1.1 V is acquired by linear fitting between 2 and 3 V. The inset shows a ln(I) versus V plot. The ideality factor (η) is estimated from the slope of the fitting curve.

dielectric constant (8.5 εo),28 ND is the donor concentration (5.2 × 1017 cm−3), and NA is the acceptor concentration (1.0 × 1019 cm−3) (see Experimental Section). With these values, α is calculated as 1.29 V−1. The recombination−tunneling behavior supports the existence of deep-levels at the junction interface.27 As the operating bias voltage increases from 1.7 to 4.6 V (region II), the I−V curve follows the relation I ∝ V6, similar to the trap-limited conduction governing the transport of charges in amorphous dielectric films as well as in quantum dot and organic LEDs.29−32 In this region, the interfacial trap sites are filled so that the injected charge carriers can move freely (i.e., turn-on). Above the turn-on voltage (4.6−8.3 V, region III), the I−V curve follows the relation I ∝ V4.5. The formation of the conduction regions II and III may be attributed to the existence of unavoidable large trap densities stemming from mechanical interfacial contacts between the component ZnO and SiO2/Si NWs formed by van der Waals forces.29 For bias voltages higher than 8.3 V (region IV), space charge limited current (SCLC) conduction occurs, following the relation I ∝ V2. The SCLC is typically caused by single-carrier injection in wide band gap semiconductors.27 Larger bias voltages allow electrons to tunnel through the dielectric SiO2 barrier, making the depletion region in the ZnO NW wider, and causing current saturation. For bias voltages higher than 10 V, this behavior disappears due to tunneling of the holes in the Si NW into the deep-levels of the ZnO NW, leading to EHP recombination causing EL. The diode ideality factor (η) is estimated to be ∼13.4 from the slope of the ln(I) versus V plot in the range 2−3 V, as shown in the inset of Figure 3, by the equation

Figure 2. FDTD simulation of the LEDs: schematic diagrams and light intensity patterns of the LEDs with a triangular-shaped Si NW (a, b), and with a rectangular-shaped Si NW (c, d), respectively.

two LEDs are the same as that of the LED fabricated in this study (Figure 2a,c) (refer to the Experimental Section and the SEM images in Figure 1b−d). The refractive indices (n) of Si (4.20), SiO2 (1.46), ZnO (2.04), PES (1.65), and air (1) determined at a wavelength of 519 nm are used for the simulation. The light intensity of the LED with a triangularshaped Si NW (Figure 2b) shows greater top light extraction as well as higher light intensity in the top ZnO layer, compared to the light intensity of the LED with a rectangular-shaped Si NW (Figure 2d). An injection current versus junction voltage (I−V) curve of the ZnO/SiO2/Si NW LED is plotted on a log−log scale in Figure 3. Four different regions appear on the I−V curve according to the charge-carrier transport mechanism. For forward bias voltages lower than 1.7 V (region I), the I−V characteristics can be described by the recombination-tunneling model, and the forward current can be represented by the relation I ∝ exp(αV). The constant α is given by27 α=

mh*εs ND 8π · 3h NA (NA + ND)

η=

−2

where h is the Planck’s constant (6.626 × 10 m kg s ), mh* is the effective hole mass (0.69 mo), εs is the semiconductor 2

(2)

where e is the electron charge, kB is Boltzmann’s constant, and T is the temperature. This equation is derived from the driftdiffusion model based on the Sah−Noyce−Shockley theory.33 The significant deviation of η from the ideal case (η = 1) indicates that the I−V characteristics are not only limited by the thermionic emission model, but are also influenced by the SCLC conduction, trap-assisted tunneling, series resistance, and junction interface states.34,35 These factors can induce inhomogeneity in the barrier height at the junction interface, resulting in a large ideality factor. The EL spectrum of the ZnO/SiO2/Si NW LED at a forward bias voltage of 15 V is fitted using five Gaussian curves by the

(1) −34

e δV · kBT δ(ln I )

C

DOI: 10.1021/acsami.5b11053 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) EL spectrum of the LED at a forward bias voltage of 15 V, fitted using five Gaussian curves with peak wavelengths of 416, 477, 519, 559, and 643 nm. The chromaticity coordinates (i.e., CIE (x, y)) of the three principal Gaussian peaks (P1, P2, and P3) are calculated and plotted within the greenish area of the CIE 1931 chromaticity diagram, as shown in the insets of part a. (b) Energy band diagram of the trap-levels in the ZnO NW. (c) EL spectra of the LEDs with and without the SiO2 layer and PL spectrum of an as-grown ZnO NW. (d) The integrated light intensity versus injection current (L−I) characteristics of the LED, fitted using the power law L ∝ IP.

weakened in the spectrum of the ZnO/SiO2/Si NW LED. By the intervening SiO2 layer, the energy levels of the defective layers on the surface of the ZnO NW can be tightly pinned to the interface energy states of the SiO2 layer. Both of the electrons in the band-to-band transition and in the falling of energy from the CB to the deep-levels can be arrested at these energy levels. The trapped electrons then recombine with the holes injected from of the Si NW, leading to emitting photons. The integrated light intensity versus injection current (L−I) curve of the ZnO/SiO2/Si NW LED is plotted in Figure 4d. The L−I curve is fitted using the power law,38 L ∝ IP, where P represents the effect of the defects on light-emission. The curve exhibits superlinearity with a P of 2.9 at relatively lower current values and sublinearity with a P of 0.84 at relatively higher current values. The superlinearity is ascribed to the existence of nonradiative centers in the space charge region as a shunt path to the current. Because the ZnO/SiO2/Si NW LED has a relatively high density of defects, the value of P (2.9) in the superlinear zone is higher than those previously reported by other research groups using ZnO/Si (1.8), AlGaN/GaN (2.4), and ZnO/GaN (1.2) LEDs.28,38,39 When the nonradiative centers are saturated at a bias current of around 22 μA, most of charge carriers injected into the space charge region can lead to photon emission with a saturation trend. The saturation tendency of the EL intensity can be represented by40

peak deconvolution method, as depicted in Figure 4a. The centers of five different Gaussian curves are at wavelengths of 416, 477, 519, 559, and 643 nm. Energies converted from these wavelengths correspond to the transition energies (inside the band gap) of the five different trap-levels in the ZnO NW. They are 2.98 eV for interstitial zinc (Zni), 2.6 eV for interstitial oxygen (O″i), 2.39 eV for oxygen antisite (OZn), 2.22 eV for interstitial oxygen (Oi), and 1.93 eV for unoccupied zinc vacancy (VZn * ).17,20,34,36 This correspondence suggests that the green light-emission of the LED originates from radiative recombination caused by the deep-level defects of the component ZnO NW. Figure 4b shows the energy band diagram of the trap-levels in the ZnO NW, referred to as the valence band maximum (VBM). In addition, the principal Gaussian peaks marked as P1, P2, and P3 in Figure 4a result in calculated chromaticity coordinates of (0.067, 0.832), (0.366, 0.631), and (0.103, 0.103), respectively. These coordinates are plotted within the green and greenish azure regions of the Commission Internationale de l’Eclairage (CIE) 1931 chromaticity diagram,37 as illustrated in the inset of Figure 4a. In order to confirm the effect of the SiO2 layer that intervened in the junction interface between the ZnO and Si NWs on the green light-emission, the PL spectra of an asgrown ZnO NW and the EL spectra of the LEDs composed of ZnO/Si NWs with and without the SiO2 layer are plotted in Figure 4c. The PL spectrum consists of a UV emission peak at a wavelength of 389 nm and broad emission bands at around 520 nm. The UV peak originates from the band-to-band transition of the electrons in the ZnO NW, and the greenish bands are attributed to the fall of electrons to lower energy levels, from the bottom of the conduction band (CB) to the deep-levels on the surface of the ZnO NW. Compared to the EL spectra of the LED without the SiO2 layer, the greenish emission bands at around 520 nm are enhanced, while the UV emission is

L=

No 1 jc σcap

+ τrad

(3)

where No is the number of localized states, jc is the carrier injection flux, σcap is the effective cross section for carrier capture, and τrad is the lifetime of the radiative recombination. According to eq 3, L saturates to the value of No/τrad when the injection current approaches infinity (j c → ∞). The degeneration of the sublinear curve (P = 0.84 < 1) at relatively D

DOI: 10.1021/acsami.5b11053 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) Energy band diagram of the LED with the trap energy levels (Et) inside the ZnO NW in thermal equilibrium. (b) Electron tunneling into the valence band (VB) of the Si NW at a turn-on voltage of 1.1 V. The electrons from the conduction band (CB) of the ZnO NW fall into the trap-states and then pass through the SiO2 barrier into the Si NW. (c) Trap-assisted green EL by hole tunneling from the VB of the Si NW into the trap-levels of the ZnO NW at a bias around 10 V, yielding a recombination of electron−hole pairs (EHPs).

Figure 6. Forces applied to the ZnO NW/SiO2/Si NW LED on a bendable substrate: (a) downwardly and (b) upwardly bent states.

VB of the Si NW to the ZnO NW. At a bias voltage of around 10 V, holes accumulated between the SiO2 barrier and the VB of the Si NW are directly injected into the energy levels of the defects in the ZnO NW, yielding EHPs with the energy of 2.38 eV (Figure 5c). Similar to electron tunneling, the holes in the VB of the Si NW first fall into the trap energy levels and then tunnel into the deep-levels of the ZnO NW (see section B in the Supporting Information). When the bias voltage is sufficiently high, nonradiative auger-recombination or selfheating can occur, leading to quantum efficiency droop.42,43 In order to investigate the piezophototronic effect on the efficiency of the trap-assisted emission from the ZnO/SiO2/Si NW LED, the various forces applied on the suspended NW LED structure under downwardly and upwardly bent states are analyzed, as shown in Figure 6.44,45 On the basis of the geometry of the custom-built bending machine,46 the external force (Fx) is perpendicularly applied to the ends of the substrate. The external force at one end (Fx/2) induces the internal bending momentum (M) which decides the bending

higher currents is possibly caused by the limitation of quantum efficiency.28 Figure 5 illustrates the schematics of the band diagram depicting charge-carrier transports in the ZnO/SiO2/Si NW LED under an applied forward bias. The heterojunction exhibits a type II staggered band alignment at thermal equilibrium, as determined by the Anderson model (Figure 5a). The diode begins conducting first by electron tunneling, corresponding to a turn-on voltage of 1.1 V. According to the analysis of the carrier paths in tunneling by Riben et al.,41 electrons in the CB of the component ZnO NW first fall into the available trap-states and then tunnel into the valence band (VB) of the component Si NW (Figure 5b). This electron injection does not give rise to EL emission because of the indirect band gap of the Si NW. As the forward bias increases, the VB of the Si NW moves toward lower energy levels and becomes aligned with the energy states of the deep-levels of the ZnO NW. Owing to the large potential barrier of the holes (3.88 eV), a larger bias is necessary for hole injection from the E

DOI: 10.1021/acsami.5b11053 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. Coordinate bending system and estimation of the strain in the downwardly bent ZnO NW.

Figure 8. (a) Modulation of relative light intensity (Ls/Lo) and relative injection current (Is/Io) of the LED as a function of the applied strain in the range −0.012−(0.006)% at a bias of 15 V by the piezoelectric effect. (b) Enhanced efficiency of the trap-assisted green EL from the LED by the piezophototronic factor (β). Inset shows the images of the green light-emission captured by a cooled CCD under applied strain. (c) Localized ⎯→ piezoelectric field ( E P ) built up in the cross section of the strained ZnO NW. (d) Piezopotential distribution (φ(V)) on the LED under negative strain, analyzed by a finite-element method (FEM).

ends, and the strain on the substrate is calculated from the formula εsub = t/2rsub × 100(%) and

direction (i.e., downwardly or upwardly). Then, these two forces are transferred to the end of the ZnO NW through the electrode metal which has no deformation by the substrate bending (see section C in the Supporting Information). The force vector (FNW) which is the sum of the forces applied at each end of the ZnO NW is located at the middle of the suspended NW, deforming the shape of the LED as downward or upward bends (see section D in the Supporting Information). The strain is applied to the component ZnO NW by bending the plastic substrate downwardly or upwardly along the longitudinal direction (c-axis). As the component Si NW is positioned perpendicular to the oxide NW, it does not experience any applied strain. For the calculation of the applied strain in the ZnO NW, the coordinate bending system is drawn on the basis of beam theory,44,45 as shown in Figure 7. The substrate is bent by perpendicular forces applied at the two

rsub = L /(2π ΔL /L − π 2t 2/12L2 ), where t and L are the thickness and length of the substrate, respectively. In the case of the downwardly bent state, the stress applied on the substrate area can be transferred to the ZnO NW suspended on the same area through the electrodes. With Young’s modulus values of the PES substrate (Esub = 2.84 GPa) and the ZnO NW (ENW = 129 GPa), the applied strain in the outer side of ZnO NW at the junction interface is calculated to be 0.006% (tensile strain). Considering the net force rate (see section D in the Supporting Information), under the upwardly bent state, the applied strain in the inner side of ZnO NW at the junction interface is to be −0.012% (compressive strain). The relative light intensity (Ls/Lo) and relative injection current (Is/Io) are plotted as a function of the applied strain in F

DOI: 10.1021/acsami.5b11053 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 9. Modulation of the efficiency in trap-assisted ZnO NW/SiO2/Si NW LED by piezophototronic effect. (a) Enhanced efficiency under positive strain and (b) reduced efficiency under negative strain. The red line represents the band diagram considering the piezophototronic effect.

the range −0.012−0.006% at a bias voltage of 15 V, as shown in Figure 8a. Ls and Is are the light and current intensities under the strain, respectively, and Lo and Io are the light and current intensities without any strain, respectively. The relative injection current decreases while the relative light intensity increases continuously as the strain changes from negative to positive. Two possible assumptions can be proposed here to account for the decrease in the injection current for both the positive and negative strains. The first assumption is the reduction of the ZnO NW’s conductance by the carrier trapping effect and the creation of a charge depletion zone.19 The second assumption is the trapping of the induced electrons into the deep-levels in the vicinity of the interface between the ZnO NW and SiO2/Si NW (see section E in the Supporting Information). Figure 8b shows the enhanced efficiency of the trap-assisted GE from the LED by the piezophototronic factor (i.e., conversion efficiency), derived from the ratio of the relative light intensity to the relative injection current. The relative light intensity, the relative injection current, and the relative external efficiency exhibit a natural logarithmic relationship with the piezopotential change (ΔΨ) and the strain function (f(ε)), represented by10 ⎛ η ⎞ Δψ ⎛L ⎞ ⎛I ⎞ + f ( ε) ln⎜ s ⎟ = ln⎜ s ⎟ + ln⎜⎜ s ⎟⎟ = kT ⎝ Lo ⎠ ⎝ Io ⎠ ⎝ ηo ⎠

piezophototronic effect and thereby accurately correlate the enhancement factor with the external strain. When the component ZnO NW is downwardly bent along the longitudinal direction (c-axis), the tensile and compressive strains occur at the outer and inner sides of the NW, respectively, due to the piezoelectric effect.19 Further, positive (or negative) static ionic charges are induced on the surface of the oxide NW under compressive (or tensile) strain and thus a ⎯→ localized piezoelectric field, E P = ϵ/d, where ϵ is the piezopotential generated by the applied strain and d is the piezoelectric coefficient, is built up across the cross section of the oxide NW, as shown in Figure 8c. The piezopotential distribution (φ(V)) on the LED under the downwardly bent state is shown in Figure 8d, analyzed by a finite-element method (FEM) with the aspect ratio (a/l) of ZnO NW. Here the aspect ratio is 0.1, where a is the diameter and l is the length of the ZnO NW. The efficiency enhancement of the trap-assisted lightemission ZnO NW/SiO2/Si NW LED by the piezophototronic effect can be explained by the mechanism in Figure 9. The red line represents the band diagram considering the piezophototronic effect under downwardly (Figure 9a) and upwardly (Figure 9b) bent states. In the band diagram, the formation of the dip can be explained by the coupling of Poisson and piezoelectric constitutive equations.1−5,10,11 The efficiency is limited by majority electron flow to the junction interface. When the ZnO NW is applied with a positive strain, the potential drop is generated at the junction interface along the caxis of ZnO NW, and the opposite potential is generated at the junction interface when the ZnO NW is applied with a negative strain. In the case of the generation of positive piezocharges at the junction interface by the positive strain, free electrons are induced toward the positively ionized region. However, the induced electrons are not strongly bound to the positive piezocharges with the low piezopotential (due to the low aspect ratio of the ZnO NW). Instead, they can be trapped and accumulated into the defect levels, not contributing to the charge-carrier injection, but enhancing the EHP recombination rate due to the increased carrier density in the trap-levels (i.e., enhanced efficiency). In the case of the generation of negative piezocharges by the negative strain, the depletion width in ZnO NW becomes wider, so the whole conduction becomes smaller, resulting in a reduction of the injection current and light intensity (i.e., reduced efficiency).

(4)

where ηs and ηo are the external efficiencies with and without the strain, respectively (see section F in the Supporting Information). The piezophototronic factor (β) in Figure 8b is defined as9 ⎧ ⎛ q2ρ W 2 ⎞⎫b − 1 ⎪ piezo piezo ⎟⎪ β = ⎨exp⎜⎜ ⎟⎬ ⎪ ⎝ 2εskT ⎠⎪ ⎩ ⎭

(5)

where ρpiezo is the density of the polarization charges, Wpiezo is the distribution width of the piezoelectric charges, εs is the permittivity of the material, and b is the power law parameter. With an applied strain of 0.006% at a bias voltage of 15 V, β is calculated as 2.79. This value is slightly different from the β values (2.30 and 4.25) reported by Wang’s group.10,11 This discrepancy results from the differences in the geometry, the emission origins, and the materials of the LEDs used in this study and those used by Wang’s group. Our study suggests that it is important to control emission by means of the G

DOI: 10.1021/acsami.5b11053 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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charge-coupled device (CCD; Scion 1612C), and the electroluminescence spectra were acquired and analyzed using a spectrophotometer (Ocean Optics USB2000plus). The photoluminescence (PL) spectrum of the as-grown ZnO NWs was obtained with a 325 nm wavelength excitation source from a He−Cd laser. The physical properties of the NWs were observed by scanning electron microscopy (SEM; Hitachi S-4300). The strain was applied to the c-axis of the ZnO NW by means of a custom-built bending machine.46 Numerical modeling of the LED was developed using FDTD (Optiwave OptiFDTD) and FEM (Simulia Abaqus 6.14SE) software. All the measurements were made in ambient air at room temperature.

CONCLUSION In summary, the EL efficiency is enhanced by a piezophototronic factor of 2.79 under an applied strain of 0.006% in the longitudinal direction of the component ZnO NW by the piezophototronic effect. The trap-assisted green EL of a ZnO/ SiO2/Si NW LED constructed on a bendable substrate originates from the radiative recombination caused by the deep-level defects in the component ZnO NW. By the positive piezo charges generated by the strain-induced piezopotential, the free carriers are induced and captured into the defect levels, enhancing the EHP recombination rate.

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ASSOCIATED CONTENT

* Supporting Information S

EXPERIMENTAL SECTION

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11053. Reported applications of ZnO/SiO2/Si heterostructures, considerations for the enhancement of EHP recombination rate, analysis of the applied strain based on sandwich beam theory, net forces on the suspended ZnO NW under strain, mechanism of decrease in injection current under strain, and validity of eq 4 under strain (PDF)

Growth of ZnO NWs. ZnO NWs were synthesized by thermal evaporation from ball-milled ZnO powders on a Au-deposited SiO2/Si substrate.47 The thermal evaporation was performed at 1380 °C for 3 h with an argon flow rate of 500 standard cubic centimeters per minute (sccm) under a constant furnace chamber pressure of 0.5 atm. The ZnO NWs grown on the SiO2/Si substrates were then dispersed in methanol solution in order to manipulate the NWs individually. Doping Concentration (ND) of ZnO NWs. The doping concentration (ND) of the ZnO NWs is 5.2 × 1017 cm−3. First we obtained carrier concentration (ne) of the omega-shaped-gate ZnO NW field effect transistor from the formula ne = Qtot/eπr2nwLg where Qtot = CVth.47 The radius (rnw) of the ZnO NW was 45 nm, and the gate length (Lg) of the field effect transistor was 3 μm. The capacitance (C) is C = Comg/2πrgLg, and the oxide capacitance (Comg) is Comg = 2πεrεoLg/ln(rg/rnw) where the outer radius of the Al2O3 coated ZnO NW (rg) is 62 nm and the dielectric constant of Al2O3 (εr) is 8.4. The carrier concentration (ne) was estimated to be 5.2 × 1017 cm−3. Then, the donor concentration is obtained from the formula

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-2-3290-3245. Fax: +82-2-3290-3894. Present Address †

Department of Electrical and Computer Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States.

ne = (ND − NA )/2 + ((ND − NA )/2)2 + n i2 , and the intrinsic carrier concentration (ni) of ZnO NW is 106 cm−3. We assume that the acceptor concentration (NA) is zero since the ZnO NWs are grown from the nondoped ZnO power. As a result, the doping concentration of the ZnO NWs is calculated to be 5.2 × 1017 cm−3. Formation of Si NWs. Triangular-shaped Si NWs were formed by etching a single crystalline Si wafer with a boron doping concentration (NA) of 1 × 1019 cm−3. A crystallographic wet etching using a 20 wt % potassium hydroxide (KOH) solution at 40 °C was processed to obtain inverted-triangular-shaped Si NWs. These shapes were achieved on the basis of the difference in the etching ratio between the (111)to (110)-oriented surfaces of the Si wafer.48 Construction of ZnO NW/SiO2/Si NW LEDs. A resin (Q-sys, NIR Q1) was laminated on a bendable poly(ether sulfone) (PES) substrate by a spin-coating process (500 rpm for 5 s, and 10 000 rpm for 55 s).48 The thickness of the resin layer is 50 nm. As-prepared Si NWs were then transferred onto the bendable PES substrate using an imprinting method. For detachment of the Si NWs from the mother Si substrate, the resin was hardened by UV light, and the Si substrate was removed. After the transfer of the Si NWs, the precoated resin layer was removed by ethanol solution. A SiO2 dielectric layer with a thickness of 5 nm was deposited on the Si NWs using RF sputtering. Then, Ni/Au (230/20 nm) electrodes were deposited at the ends of a Si NW chosen from the transferred NWs on the substrate, and Al (230 nm) electrodes with a separation of 5 μm were deposited perpendicular to the Ni/Au electrodes. The ZnO NWs were perpendicularly aligned to the Si NW by the dielectrophoresis method.49 The ZnO NW-dispersed solution was dropped into the separation of the Al electrodes with an alternating current (ac) bias of amplitude 10 V at 1 kHz. Finally, Al/Au (70/20 nm) was deposited at the ends of the aligned ZnO NW again and annealed at 100 °C for 3 min. Whole process steps are represented in Scheme 1. Characterization. The electrical characteristics were examined by a semiconductor parameter analyzer (Agilent 4155C), and the ac electric fields were generated by a function generator (Instek SFG2104). The emission images of the LED were captured by a color

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the Midcareer Researcher Program (No. NRF-2013R1A2A1A03070750), a grant funded by the Korean Government (MSIP) (No. NRF2015R1A5A7037674), and the Brain Korea 21 Plus Project in 2015 through the National Research Foundation of Korea (NRF)

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