n-IGZO Heterojunction

Mar 9, 2017 - ... p-type Si nanowires (p-Si NWs) and n-type indium gallium zinc oxide (n-IGZO) were fabricated with the different top electrode materi...
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Enhanced Light Extraction from p‑Si Nanowires/n-IGZO Heterojunction LED by Using Oxide−Metal−Oxide Structured Transparent Electrodes Yun Cheol Kim, Su Jeong Lee, and Jae-Min Myoung* Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea S Supporting Information *

ABSTRACT: Heterojunction light-emitting diodes (LEDs) comprising p-type Si nanowires (p-Si NWs) and n-type indium gallium zinc oxide (n-IGZO) were fabricated with the different top electrode materials: Al, indium zinc oxide (IZO), and IZO/ Ag/IZO oxide−metal−oxide (OMO) multilayer. All the LEDs exhibited typical rectifying behaviors of the p−n junction. Moreover, broad light-emission spectra in the visible range were observed because of the quantum confinement effect (QCE) of the Si NW and Si nanocrystals/nonstoichiometric Si oxide (SiOx) (x < 2) interfaces. In comparison to the LEDs with Al and single IZO electrode, the LED with the OMO multilayer electrode exhibited an enhanced optical performance because the OMO multilayer had an excellent transmittance of 87.7% in the visible range with a low sheet resistance of 5.65 Ω/sq. Furthermore, by investigating the transmittance spectra of the single IZO and OMO multilayer electrodes as a function of the light incidence angle, the OMO multilayer electrode is confirmed to be more suitable for white light emission from p-Si NWs/n-IGZO heterojunction LED. near-zero refractive index.6 However, it is difficult to obtain a high visible transmittance while maintaining the electrical properties because reducing the metal thickness results in a discontinuous film that increases the resistance of the metal films. To secure both the electrical and optical properties, the most commonly used electrode materials are transparent conducting oxides (TCOs). In particular, indium tin oxide (ITO) film has been extensively used in numerous industrial applications owing to its outstanding electrical and optical properties as a transparent electrode (sheet resistance 90%). However, a high-quality ITO film is obtained at a fairly high temperature (>300 °C) during or after deposition for activating the Sn dopants.7 When it is prepared at low temperatures, the electrical properties are degraded because of the carrier scattering phenomenon between the crystal and amorphous grains.8,9

1. INTRODUCTION Light-emitting diodes (LEDs) have been increasingly investigated in the last few decades. They have been widely employed in various optoelectronic applications owing to their high color quality, high energy efficiency, fast response time, and low manufacturing cost. In LEDs, the external quantum efficiency (EQE); i.e., luminous efficiency is modulated by the internal quantum efficiency (IQE) and light extraction efficiency (LEE). Here, the IQE depends on the quality of semiconductor materials and carrier injection efficiency, whereas the LEE is related to the light emission outside the LEDs.1,2 Although LEDs have the same IQE, the LEE considerably influences the EQE because of internal reflection at the interface between the electrodes and air, which causes transmission loss of photons.3−5 To achieve a high EQE, a bottom or a top electrode (depending on the LED emitting direction) should not only have a high transmittance with low resistivity but also have the proper work function. In general, metal is a common electrode material because of its low resistivity. Moreover, noble metals, such as Au, Ag, and Cu, can transmit light when they are sufficiently thin with a © XXXX American Chemical Society

Received: January 21, 2017 Revised: March 8, 2017 Published: March 9, 2017 A

DOI: 10.1021/acs.jpcc.7b00674 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Schematic fabrication process of the heterojunction LED comprising p-Si NWs and n-IGZO along with the corresponding SEM images: (a) Si NWs produced by the MCE method and (b) thermal oxidation of Si NWs. (c) Exposed top-end regions of the Si NWs array after RIE process. (d) Formation of the p−n heterojunction by deposition of n-IGZO layer on the exposed top p-Si NWs. (e) Final device structure with the bottom Au electrode and different top electrode materials (Al, single IZO, and OMO multilayer).

Then, these were rinsed with deionized (DI) water and dried by N2 gas. The Si NWs produced by this method have an average length of 6.5 μm with diameters in the range of 20−300 nm. For the oxidation of the Si NWs, rapid thermal annealing (RTA) was carried out in an O2 ambient condition with a flow rate of 5 L/min at 750 °C for 10 min. Microstructural analyses of the synthesized porous Si NWs were conducted by a scanning electron microscopy (SEM, S-5000, Hitachi) and a transmission electron microscopy (TEM, JSM-2100F, JEOL). Optical properties were measured using a microphotoluminescence (μ-PL) spectroscopy with a He−Cd laser (λ = 325 nm) at room temperature. Formation of p-Si NWs/n-IGZO Heterojunction. An ntype amorphous-IGZO (a-IGZO) layer was deposited on the pSi NWs by using an ultrahigh vacuum (UHV) sputter with an IGZO (4N-purity, InO3:Ga2O3:ZnO = 1:1:1 mol %) target. During the deposition, a direct-current (DC) power of 150 W, an Ar/O2 mixture gas ratio of 40/10 sccm, and a working pressure of 5 mTorr were maintained for 5 h at room temperature. After the deposition, the thickness of a-IGZO layer was measured to be 1.3 μm. Before forming the p−n junction, the top-end region of the oxidized Si NW surfaces was etched by a reactive-ion-etching (RIE) process in CF4 ambient condition at a radio frequency (RF) power of 100 W and a working pressure of 3 mTorr for 10 s. Finally, p-Si NWs/nIGZO heterojunction was annealed in air at 300 °C for 1 h. Deposition of Electrode Materials. The 100 nm thick Au electrode was deposited on the p-type Si substrate by an ebeam evaporator after cutting out a part of Si NWs with a blade. Then, three different electrodes of Al, single indium−zinc-oxide (IZO), and OMO multilayer with the same thickness (∼92 nm) were deposited by using a shadow mask (300 μm in diameter) on the top of the p-Si NWs/n-IGZO structure. The Al electrode was deposited by e-beam evaporation. In the UHV sputter, an IZO layer was deposited by using an IZO (4Npurity, In2O3:ZnO = 9:1 wt %) target as a transparent electrode at an RF power of 100 W, a working pressure of 10 mTorr, and an Ar flow rate of 50 sccm. In-situ sputtering was employed to fabricate the OMO multilayer structure with the IZO and Ag (5 N purity) targets. The 40 nm thick bottom IZO layer was first

For developing alternative transparent electrodes with better electrical and optical properties as well as considering process temperature, the oxide−metal−oxide (OMO) multilayer has been investigated. Despite the amorphous phase of the oxide layer, the conductivity and transmittance of the OMO multilayer can surpass those of the TCOs in the visible range.10−12 In the OMO multilayer structure, the resistivity is attributed to the conductive ultrathin metal film inserted between the two oxide layers. Moreover, the symmetrically formed two oxide layers cause an antireflection effect at the interfaces between the metal and oxide layers, resulting in a high transmittance. Hence, using the OMO multilayer electrode for LEDs, which yields low resistivity and a high visible transmittance, is very promising. In this study, as a light source, a heterojunction was formed between the p-type Si nanowires (p-Si NWs) and n-type indium gallium zinc oxide (n-IGZO). Using the heterojunction structures, a comparative study was performed by employing metal, single TCO, and OMO multilayer as the top electrodes. The LEDs with the different electrode materials showed typical rectifying behaviors of the p−n junction. The pristine white light emission was confirmed by using a metal electrode. In particular, compared to the LED with a single TCO electrode, using the OMO multilayer electrode enhanced light extraction without changing color of white light from the p-Si NWs/nIGZO LED.

2. EXPERIMENTAL SECTION Fabrication of Si NWs. Vertically aligned p-Si NWs were fabricated from boron (B)-doped p-type Si wafer (100) with a resistivity of 1−10 Ω·cm by a metal catalyst-assisted etching (MCE) method. The Si substrates were cleaned by a conventional piranha solution (H2SO4:H2O2 = 2:1) and HF sequentially to remove organic residues and native oxide. The substrates were immersed into dilute solutions containing 0.02 M silver nitrate (AgNO3) and 5 M HF for 1 min at room temperature to form Ag dendrite on the surface of Si wafer. Subsequently, the substrates were immersed into an electroless etching solution consisting of HF and H2O2 molar concentrations of 5.0 and 0.2 M, respectively, at 60 °C for 2.5 min. B

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Figure 2. (a) HRTEM image of a Si NW showing single crystallinity along the ⟨100⟩ direction with the inset of the SAED pattern. (b) TEM image showing the surface morphology of the as-prepared Si NW and (c) HRTEM image of the Si NCs in the porous Si NW surface. (d) TEM image showing the surface morphology of the Si NW after the thermal oxidation process and (e) HRTEM image of the Si NCs covered by SiOx. μ-PL spectra with deconvoluted subpeaks corresponding to the various emission sources obtained from the porous Si NW: (f) before and (g) after the thermal oxidation.

the designed emission structure in this study, the electrons and holes recombine at the interface between the p-Si NWs and nIGZO layer. However, the existence of an oxide layer at the top-end region of the Si NWs disturbs the charge flow. Hence, the top-end oxide layer of the Si NWs needed to be removed using a RIE process (Figure 1c). Thereafter, a 1.3 μm thick aIGZO layer was deposited onto the exposed p-Si NWs to form the p−n heterojunction. A schematic structure of the p−n junction with the corresponding side and 45°-tilted SEM images shows that the morphology of the a-IGZO does not seem to be continuous, but a match-head shape is observed (Figure 1d). This is because the nucleation of the a-IGZO starts from the top-end region of the Si NWs during the sputtering process, and the Si NWs were sufficiently long and dense for aIGZO not to penetrate into the bundle of the Si NWs (details about the a-IGZO film prepared on the rigid substrates are shown in Figure S1 and Table S1, Supporting Information). In the last step, to evaluate the performances of the LEDs, such as electrical and optical properties, model devices were prepared with the top electrodes of Al, single indium zinc oxide (IZO), and OMO multilayer electrodes for Ohmic contact with the nIGZO layer (Figure 1e). Here, a Au electrode was employed for the Ohmic contact with the p-Si NWs.13 The top Al electrode with a near-zero transmittance was applied to confirm pristine light emission from the p-Si NWs/ n-IGZO LED because the emitted light cannot pass a thick metal electrode and can be observed only at the edge of the electrode. For comparison, a single IZO layer as a TCO material was employed because it can be prepared with a good surface smoothness without requiring an annealing process.14,15 This single IZO electrode exhibited a visible transmittance of 85.1% at a thickness of 92 nm, which is identical to the total

deposited, and subsequently, a 12 nm thick Ag layer was deposited on the bottom IZO layer by using a dc sputter system at a power of 20 W, a working pressure of 10 mTorr, and an Ar flow rate of 50 sccm. Finally, the 40 nm thick top IZO layer was deposited on the Ag layer. Transmittance of each electrode was measured by using an ultraviolet−visible spectrophotometer (UV−vis−NIR, V-670, JASCO). Thickness and refractive index of each electrode were confirmed by a spectroscopic ellipsometer (alpha-SE, J. A. Woollam). The electrical properties of each electrode were examined by Hall measurement system (HMS-3000, Ecopia) and a four-point probe measurement system (CMT-SR1000N, AiT). The work functions of the single IZO layer and OMO multilayer were analyzed an ultraviolet photoelectron spectrometer (UPS, AC2, Riken Co. Ltd.). Device Characterization. The electrical characteristics of the devices were analyzed by a semiconductor parameter analyzer system (Agilent B1500A, Agilent Technologies). The light-emission properties of the devices were evaluated by an electroluminescence (EL) spectroscopy and a cooled chargedcoupled device (CCD, DV401A-BV, Andor) camera.

3. RESULTS AND DISCUSSION Figure 1 illustrates the fabrication process of the p-Si NWs/nIGZO heterojunction with the corresponding SEM images. First, vertically aligned Si NWs were fabricated by a MCE method. The side and 45°-tilted view SEM images revealed that an average length of vertically aligned wire is approximately 6.5 μm (Figure 1a). In the next step, the Si NWs were thermally oxidized to make the porous surface flat because the asprepared porous surface of the Si NWs could interrupt hole transport and increase the resistance. (Figure 1b). Considering C

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Figure 3. (a) J−V characteristics of the electrode/a-IGZO/electrode structures revealing Ohmic contacts between the a-IGZO and different electrodes (Al, single IZO, and OMO multilayer electrodes) and (b) J−V characteristics of the p-Si NWs/n-IGZO structures showing a typical p−n junction diode.

Table 1. Electrical Properties and Work Function with Different Electrodes electrode

N (cm−3)

Al IZO OMO

1.57 × 10 4.87 × 1020 1.37 × 1022 23

μ [cm2/(V·s)]

ρ (Ω·cm) −6

6.84 × 10 6.47 × 10−4 2.02 × 10−5

5.8 19.8 22.6

Rsh (Ω/sq)

ϕ (eV)

0.84 190 5.65

4.06−4.26 4.86 4.56

substituted by oxygen. Subsequently, the porous surface with the Si NCs is completely covered by nonstoichiometric Si oxide (SiOx) (x < 2) because of the formation of the Si−O−Si bonds, as shown in Figure 2e. Therefore, it is expected that the surface of the Si NWs is passivated from the outside. The optical properties of the as-prepared and oxidized Si NWs were investigated by using a μ-PL at room temperature. Figure 2f shows the μ-PL spectrum of the Si NWs array before the thermal oxidation (i.e., as-prepared). A broad emission peak centered at 654 nm corresponding to a photon energy of 1.9 eV is observed and it is much higher than the band gap energy of the bulk Si (1.1 eV). This spectrum is deconvoluted into four Gaussian subpeaks centered at 587, 642, 696, and 734 nm and they correspond to yellow (Y), orange (O), red (R), and dark red (DR) colors, respectively. Moreover, an additional peak approximately at 760 nm is observed in the DR color region. In this porous structure of the Si NWs, the wide emission spectra having various colors from Y to DR are caused by the quantum confinement effect (QCE) because of the broad size distribution of the NCs.24,25 However, after the thermal oxidation of the Si NWs, the μ-PL spectrum exhibits additional deconvoluted Gaussian subpeaks centered at 462 and 534 nm, as shown in Figure 2g. These wavelengths correspond to the blue (B) and green (G) emissions, respectively, which are presumed to be generated from the additional recombination centers because the B and G emissions are only observed after the oxidation process. When the Si NWs are thermally oxidized, SiOx is formed on the porous surface with the Si NCs. Also, during the thermal oxidation process, the SiOx depends on the excess Si concentration which determines the size of the Si NCs in SiOx as well as the stoichiometric ratio of SiOx (x = [O]/ [Si]). Consequently, the energy of the emitted photon shifts to the shorter wavelength side of the visible spectrum, resulting in the B and G emissions.26−29 To evaluate the performance of the LEDs with the different top electrodes, contact properties between a-IGZO layer and different electrodes were investigated by using an electrode− semiconductor−electrode (ESE) structure, as shown in Figure 3a. The current density−voltage (J−V) characteristics indicate

thickness of the OMO multilayer electrode. Moreover, to fabricate the OMO multilayer electrode, Ag was used as an intermetal layer owing to its lowest refractive index with a good conductivity, and the IZO was employed as the upper and lower layers of the Ag.16,17 The optimizing process of the OMO multilayer electrode was described in our previous work.10 It is worth noting that the thickness of the IZO-Ag-IZO OMO multilayer was optimized to be 40−12−40 nm, which exhibited an outstanding transmittance of 87.7% in the visible range (380−750 nm) (details about the transmittance spectra of all electrodes as a function of wavelength at an incident angle of 0° are shown in Figure S2, Supporting Information). Figure S2 also shows the difference in optical property between the OMO multilayer and the single IZO layer. The lower transmittance of the OMO multilayer than the single IZO layer at shorter wavelengths (below 450 nm) is due to absorption of light and interband electronic transitions at the interface between Ag and IZO layers.18 At longer wavelengths (above 700 nm), the transmittance of the OMO multilayer decreases because of high reflectance of the Ag layer. Typically, thin metal films transmit visible light and reflect over the near-infrared wavelengths.19,20 Figure 2a shows a high-resolution (HR) TEM image of a single crystalline Si NW with the corresponding selected area electron diffraction (SAED) pattern, which is oriented along the ⟨100⟩ direction. The Si NWs fabricated by the MCE process exhibit a rough and porous surface, as shown in Figure 2b. Figure 2c shows a HRTEM image of the Si NW surface, where many Si nanocrystals (NCs) are observed. On the basis of our previous results,21,22 many defects exist in this porous surface, and the defects act as carrier traps and degrade the electrical properties of the Si NW. In other words, such defects produce a strong positive charge toward the outer surface of the Si NW because of the dangling bonds.23 Thereafter, this positively charged surface induces a depletion region for the inner Si NW, which reduces the effective conducting area where carriers can flow. Hence, to prevent this effect occurring because of the surface defects, the Si NWs were thermally oxidized as shown in Figure 2d. During the oxidation process, the dangling bonds and weak Si−Si bonds are gradually D

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Figure 4. (a) Comparison of the EL intensity measured at a forward bias of 15 V from LEDs with Al, single IZO, and OMO multilayer electrodes. Normalized EL spectra with the deconvoluted subpeaks of LEDs with different electrodes: (b) Al, (c) single IZO, and (d) OMO multilayer electrodes. The insets show the bright OM image, EL microscopy, and OM images under the dark condition for each electrode.

Figure 3b shows the J−V curves of the p-Si NWs/n-IGZO LEDs. The current densities of the LEDs with all electrodes increased exponentially as the applied voltage increased under a forward bias. However, they remained almost zero under a reverse bias. Hence, these rectifying characteristics of the LEDs indicate the formation of a typical p−n junction. Moreover, the current densities of the LED were observed to be 9.96, 8.33, and 9.50 A·cm−2 with the Al, single IZO, and OMO multilayer electrodes, respectively, at a forward bias of 10 V. The current densities increased with decreasing in ρ and ϕ of the electrode material. To investigate the light-emitting performance of the LEDs with the different electrodes, the EL intensity of the LEDs was measured at a forward bias of 15 V, as shown in Figure 4a. The broad light-emitting spectra were observed in the visible range between 400 and 800 nm centered at 585, 636, and 587 nm from the LEDs with the Al, single IZO, and OMO multilayer electrodes, respectively. In general, the higher current density of the p−n junction promotes the recombination between the electrons and holes; hence, a higher EL intensity can be expected. However, the LED with the Al electrode, which showed the highest current density in the J−V curves, exhibited the lowest EL intensity, because light is reflected beneath the metal electrode. For the transparent electrodes, i.e., the single IZO layer and OMO multilayer, the LED with the OMO multilayer electrode exhibited higher EL intensity than the LED with the single IZO electrode because the OMO multilayer electrode had the high transmittance of 87.7% and the low sheet resistance of 5.65 Ω/sq. Moreover, in comparison to the LED with the Al electrode, the LED with the single IZO electrode exhibited a red-shifted EL spectrum, whereas the

the feasibility of three different electrodes for Ohmic contacts with the a-IGZO layer. The Al electrode shows an outstanding Ohmic contact with the highest current level, whereas the single IZO electrode shows the lowest current level. However, the OMO multilayer electrode exhibits a higher current level than the single IZO electrode does. The current levels of the ESE structures fabricated with the different electrodes are mainly attributed to the work function and resistivity of each electrode. Table 1 lists the carrier concentration (N), carrier mobility (μ), resistivity (ρ), sheet resistance (Rsh), and work function (ϕ) of each electrode prepared on a glass substrate. As expected, the Al electrode showed the lowest resistivity and sheet resistance because of its metallic nature. However, when the single IZO layer and OMO multilayer were considered as electrode materials, the OMO multilayer exhibited a lower resistivity and sheet resistance because of the higher carrier concentration than those of the single IZO layer. In the OMO multilayer structure, better conductivity is mainly obtained because of an ultrathin metal (Ag) layer which has a high electron concentration. Hence, the OMO multilayer showed a remarkably reduced resistivity compared to that of the single IZO layer.10,30 Moreover, the work function of an Al film is found to be 4.06−4.26 eV in the literature.31 In addition, the work functions of the single IZO layer and OMO multilayer were analyzed by using an UPS, which yielded 4.86 and 4.56 eV, respectively (details about photoelectron spectroscopy results are shown in Figure S3, Supporting Information). Therefore, it is concluded that the lower work function of the OMO multilayer compared to that of the single IZO electrode provides better contact with the n-type a-IGZO layer.32,33 E

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similar area percentages. However, the LED with the single IZO electrode emitted a reddish light because of the decreased B and G areas with the increased R and DR areas compared to those of the LED with the OMO multilayer electrode. Hence, it is necessary to investigate the reason for different light emissions from the LEDs with the single IZO and OMO multilayer electrodes. Figure 5a shows the conceptual diagram of the optical paths in the LEDs with the single IZO electrodes. When the light generated at the recombination center of the LED transfers from one medium (a-IGZO) to air via the electrode (single IZO), loss of photons occurs because the reflected light at each boundary returns to the original medium.34 Moreover, if light in a medium with a higher refractive index (IZO, n = 2.0) strikes a boundary with a medium of lower refractive index (air, n = 1.0) at a specific angle of incidence, the reflection angle becomes 90°. This particular angle of incidence is defined as the critical angle. For an incidence angle greater than the critical angle, light will undergo total internal reflection and will not be transmitted. In the LED with the OMO multilayer electrode, an additional phenomenon should be considered, i.e., the constructive and destructive interferences, as shown in Figure 5b. The constructive interference occurs when the reflected lights at the boundaries are in phase, which increases the reflectance and decreases the transmittance of lights, as shown in the left schematic image of Figure 5b. However, the reflected lights have an opposite phase with nearly equal amplitudes, which induces destructive interference, as shown in the right

LED with the OMO multilayer electrode showed a similar EL spectrum. For more systematic analyses, the normalized EL peaks of the LEDs with the different electrodes were deconvoluted into six subpeaks, which were consistent with the μ-PL spectra representing B, G, Y, O, R, and DR colors, as shown in Figure 4b−d. Table 2 shows the summary of the area percentages Table 2. Area Percentage under the Subpeaks after Deconvolution of EL Spectra for LEDs with Different Electrodes area (%) electrode

blue

green

yellow

orange

red

dark red

Al IZO OMO

11.9 7.4 13.8

26.1 16.5 23.4

22.4 24.4 20.4

19.2 21.9 22.7

8.6 15.2 9.3

11.8 14.6 10.4

under the deconvoluted subpeaks from the LEDs. Moreover, the insets show the bright optical microscopy (OM) image, EL microscopy, and OM images under the dark condition for each electrode. Because Al is not transparent, the LED with the Al electrode showed a ring-like white light at the edge of the electrode as shown in the inset of Figure 4b. However, the LEDs with transparent electrodes, single IZO layer and OMO multilayer, exhibited a bright light from the whole electrode area, as shown in the insets of Figure 4c,d. The LEDs with the Al and OMO multilayer electrodes emitted white light with the

Figure 5. Conceptual diagram for (a) optical paths in LED with a single IZO electrode and (b) constructive and destructive interference of lights in LED with the OMO multilayer electrode. Transmittance spectra as a function of light incidence angle in the visible range for (c) the single IZO layer and (d) the OMO multilayer on an a-IGZO layer. F

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multilayer electrode with the optimized thicknesses, the reflection of light was effectively suppressed, resulting in a higher transmittance in the visible range. Hence, it is believed that successful matching of transmittance with the OMO multilayer electrode enhanced the light extraction from the p-Si NW/n-IGZO heterojunction. Therefore, it is expected that this outstanding visible transmittance of the OMO multilayer electrode with low resistivity can extend the application areas of optoelectronic devices.

schematic image of Figure 5b. So, high transmittance of the OMO multilayer can be obtained due to the destructive interference. In our previous results, it was confirmed that, in the OMO multilayer structure, the wavelength range exhibiting higher transmittance due to the destructive interference is controlled by changing the thicknesses of the IZO layers on both sides of the Ag layer. Also, we have verified that the enhanced transmittance of the OMO multilayer resulted from the destructive interference at the specific thickness of the IZO layer.35 Here, the 40 nm thick top and bottom IZO layers in the OMO multilayer effectively suppressed the reflection and brought the antireflection effect at the boundary between the Ag and IZO, resulting in the highest transmittance in the visible region. Therefore, the OMO multilayer electrode with optimized thicknesses can show a higher transmittance than that of the single IZO electrode in the visible range.36,37 As indicated previously, the generated light loses intensity because of the differences in the light incidence angle and refractive index. Hence, as shown in Figure 5c,d, the transmittance spectra of the single IZO layer and OMO multilayer on the a-IGZO layer were measured in the visible range as a function of the light incidence angle. Because the LEDs with the single IZO and OMO multilayer electrodes had identical boundaries between the IZO and air, both the single IZO and OMO multilayer electrodes had the same critical angle of approximately 76°. It is noticeable that the single IZO layer showed a low transmittance in the range between 400 and 500 nm and a high transmittance in the range between 700 and 800 nm. On the contrary, the OMO multilayer exhibited a high transmittance in the visible range between 450 and 750 nm. This different dependence of the transmittance spectra with the single IZO and OMO multilayer electrodes is the main reason for the different light emissions from the LEDs with those electrodes (the insets of Figure 4c,d). Furthermore, compared with the Al electrode performance, the performance of the LED with the OMO multilayer electrode is notably superior without changing color of white light from the p-Si NWs/n-IGZO heterojunction (Figure 4a). Consequently, on the basis of these results combined with the electrical, optical, and light-emitting properties, the OMO multilayer electrode was determined as the appropriate transparent electrode for the LEDs in this research.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00674. Visible transmittance spectra of an a-IGZO film on a glass substrate, transmittance spectra of Al, single IZO, and OMO multilayer electrodes as a function of wavelength, photoelectron spectroscopy results of single IZO and OMO multilayer on glass substrates, electrical properties and work function of a-IGZO film prepared on a glass substrate (PDF)

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AUTHOR INFORMATION

Corresponding Author

*J.-M. Myoung. E-mail: [email protected]. ORCID

Jae-Min Myoung: 0000-0002-9895-4915 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the IT R&D program of MOTIE/ KEIT (No. 10042414, Development of 8th Generation (2200 × 2500 mm2) Major Equipment for Transparent Flexible Display in Large-Area) and by the LG Display Academic Industrial Cooperation Program.

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REFERENCES

(1) Windisch, R.; Heremans, P.; Knobloch, A.; Kiesel, P.; Döhler, G. H.; Dutta, B.; Borghs, G. Light-Emitting Diodes with 31% External Quantum Efficiency by Outcoupling of Lateral Waveguide Modes. Appl. Phys. Lett. 1999, 74, 2256−2258. (2) Zhao, H.; Liu, G.; Zhang, J.; Arif, R. A.; Tansu, N. Analysis of Internal Quantum Efficiency and Current Injection Efficiency in IIINitride Light-Emitting Diodes. J. Disp. Technol. 2013, 9, 212−225. (3) Horng, R.-H.; Huang, S.-H.; Yang, C.-C.; Wuu, D.-S. Efficiency Improvement of GaN-Based LEDs with ITO Texturing Window Layers Using Natural Lithography. IEEE J. Sel. Top. Quantum Electron. 2006, 12, 1196−1201. (4) Huh, C.; Lee, K.-S.; Kang, E.-J.; Park, S.-J. Improved LightOutput and Electrical Performance of InGaN-Based Light-Emitting Diode by Microroughening of the p-GaN Surface. J. Appl. Phys. 2003, 93, 9383−9385. (5) Ee, Y.-K.; Arif, R. A.; Tansub, N.; Kumnorkaew, P.; Gilchrist, J. F. Enhancement of Light Extraction Efficiency of InGaN Quantum Wells Light Emitting Diodes Using SiO2/Polystyrene Microlens Arrays. Appl. Phys. Lett. 2007, 91, 221107. (6) O’Connor, B.; Haughn, C.; An, K.-H.; Pipe, K. P.; Shtein, M. Transparent and Conductive Electrodes Based on Unpatterned, Thin Metal Films. Appl. Phys. Lett. 2008, 93, 223304. (7) Chu, C.-H.; Wu, H.-W.; Huang, J.-L. AZO/Au/AZO Tri-Layer Thin Films for the Very Low Resistivity Transparent Electrode Applications. Mater. Sci. Eng., B 2014, 186, 117−121.

4. CONCLUSIONS The p-Si NW/n-IGZO heterojunction LEDs were fabricated with the different electrode materials, and their electrical and optical performances were evaluated. The Si NWs produced a μ-PL spectrum having various colors: yellow, orange, red, and dark red. Moreover, the surface oxidation of the Si NWs contributed to the formation of additional recombination centers of blue and green at the Si NCs/SiOx interface. Combination of the above colors resulted in a white light, and this production of pristine white light was confirmed by using the LED with the Al electrode. In addition, the LED with the OMO multilayer electrode exhibited higher EL intensity than the LED with the single IZO electrode because the OMO multilayer electrode showed a low sheet resistance of 5.65 Ω/sq as well as a high transmittance of 87.7%. Moreover, the LED with the OMO multilayer electrode showed a similar EL spectrum as that with the Al electrode, but the LED with the single IZO electrode exhibited a red-shifted EL spectrum. By investigating the transmittance spectra of the single IZO and OMO multilayer electrodes, it was found that, in the OMO G

DOI: 10.1021/acs.jpcc.7b00674 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C (8) Choi, H.-J.; Yoon, S.-G.; Lee, J.-H.; Lee, J.-Y. Crystallized IndiumTin Oxide (ITO) Thin Films Brown at Low Temperature onto Flexible Polymer Substrates. ECS J. Solid State Sci. Technol. 2012, 1, Q106−Q109. (9) Khusayfan, N. M.; El-Nahass, M. M. Study of Structure and Electro-Optical Characteristics of Indium Tin Oxide Thin Films. Adv. Condens. Matter Phys. 2013, 2013, 408182. (10) Kim, Y. C.; Lee, S. J.; Jung, H.; Park, B.-E.; Kim, H.; Lee, W.; Myoung, J.-M. Optimization and Device Application Potential of Oxide−Metal−Oxide Transparent Electrode Structure. RSC Adv. 2015, 5, 65094−65099. (11) Kim, H.-K.; Lim, J.-W. Flexible IZO/Ag/IZO/Ag Multilayer Electrode Grown on a Polyethylene Terephthalate Substrate Using Roll-to-Roll Sputtering. Nanoscale Res. Lett. 2012, 7, 67. (12) Xue, Z.; Liu, X.; Zhang, N.; Chen, H.; Zheng, X.; Wang, H.; Guo, X. High-Performance NiO/Ag/NiO Transparent Electrodes for Flexible Organic Photovoltaic Cells. ACS Appl. Mater. Interfaces 2014, 6, 16403−16408. (13) Lee, S. H.; Lee, T. I.; Ham, M.-H.; Lee, S. J.; Park, J. H.; Kim, Y. C.; Biswas, P.; Myoung, J.-M. Direct Transfer Printing with Metal Oxide Layers for Fabricating Flexible Nanowire Devices. Adv. Funct. Mater. 2015, 25, 6921−6926. (14) Lim, W.; Wang, Y. L.; Ren, F.; Norton, D. P.; Kravchenko, I. I.; Zavada, J. M.; Pearton, S. J. Indium Zinc Oxide Thin Films Deposited by Sputtering at Room Temperature. Appl. Surf. Sci. 2008, 254, 2878− 2881. (15) Wang, Y.-L.; Ren, F.; Lim, W.; Norton, D. P.; Pearton, S. J.; Kravchenko, I. I.; Zavada, J. M. Room Temperature Deposited Indium Zinc Oxide Thin Film Transistors. Appl. Phys. Lett. 2007, 90, 232103. (16) Hong, K.; Son, J. H.; Kim, S.; Koo, B. H.; Lee, J.-L. Design Rules for Highly Transparent Electrodes Using Dielectric Constant Matching of Metal Oxide with Ag Film in Optoelectronic Devices. Chem. Commun. 2012, 48, 10606−10608. (17) Melpignano, P.; Cioarec, C.; Clergereaux, R.; Gherardi, N.; Villeneuve, C.; Datas, L. E-Beam Deposited Ultra-Smooth Silver Thin Film on Glass with Different Nucleation Layers: An Optimization Study for OLED Micro-Cavity Application. Org. Electron. 2010, 11, 1111−1119. (18) Lee, S. Y. Low Emissivity Property of Amorphous Oxide Multilayer (SIZO/Ag/SIZO) Structure. Trans. Electr. Electron. Mater. 2017, 18, 13−15. (19) Behera, G.; Ramakrishna, S. A. Enhanced Broadband Transmission through Structured Plasmonic Thin Films for Transparent Electrodes. J. Nanophotonics 2014, 8, 083889. (20) Gong, J.; Dai, R.; Wang, Z.; Zhang, Z. Thickness Dispersion of Surface Plasmon of Ag Nano-thin Films: Determination by Ellipsometry Iterated with Transmittance Method. Sci. Rep. 2015, 5, 9279. (21) Kim, D. H.; Lee, S. J.; Lee, S. H.; Myoung, J.-M. Electrical Properties of Flexible Multi-Channel Si Nanowire Field-Effect Transistors Depending on the Number of Si Nanowires. Chem. Commun. 2016, 52, 6938−6941. (22) Lee, S. H.; Lee, T. I.; Moon, K.-J.; Myoung, J. M. A Route for Modulating the Diameter of Cylindrical Silicon Nanowires by Using Thermal Self-Ordering Silver Nanoparticles. ACS Appl. Mater. Interfaces 2013, 5, 11777−11782. (23) Seo, K.-I.; Sharma, S.; Yasseri, A. A.; Stewart, D. R.; Kamins, T. I. Surface Charge Density of Unpassivated and Passivated MetalCatalyzed Silicon Nanowires. Electrochem. Solid-State Lett. 2006, 9, G69−G72. (24) Moon, K.-J.; Lee, T. I.; Lee, W.; Myoung, J.-M. White Light Emission from Heterojunction Diodes Based on Surface-Oxidized Porous Si Nanowire Arrays and Amorphous In-Ga-Zn-O Capping. Nanoscale 2014, 6, 3611−3617. (25) Fauchet, P. M.; Tsybeskov, L.; Peng, C.; Duttagupta, S. P.; Behren, J. V.; Kostoulas, Y.; Vandyshev, J. M. V.; Hirschman, K. D. Light-Emitting Porous Silicon: Materials Science, Properties, and Device Applications. IEEE J. Sel. Top. Quantum Electron. 1995, 1, 1126−1139.

(26) Belyakov, V. A.; Burdov, V. A.; Lockwood, R.; Meldrum, A. Silicon Nanocrystals: Fundamental Theory and Implications for Stimulated Emission. Adv. Opt. Technol. 2008, 2008, 279502. (27) Liu, L.; Sham, T.-K. The Effect of Thermal Oxidation on the Luminescence Properties of Nanostructured Silicon. Small 2012, 8, 2371−2380. (28) Moreno, J. A.; Garrido, B.; Pellegrino, P.; Garcia, C.; Arbiol, J.; Morante, J. R.; Marie, P.; Gourbilleau, F.; Rizk, R. Size Dependence of Refractive Index of Si Nanoclusters Embedded in SiO2. J. Appl. Phys. 2005, 98, 013523. (29) Korsunska, N.; Baran, M.; Khomenkova, L.; Yukhymchuk, V.; Goldstein, Y.; Savir, E.; Jedrzejewski, J. Mechanism of Photoexcitation of Oxide-Related Emission Bands in Si−SiO2 Systems. Mater. Sci. Eng., C 2003, 23, 691−696. (30) Choi, K.-H.; Choi, Y.-Y.; Jeong, J.-A.; Kim, H.-K.; Jeon, S. Highly Transparent and Conductive Al-doped ZnO/Ag/Al-doped ZnO Multilayer Source/Drain Electrodes for Transparent Oxide Thin Film Transistors. Electrochem. Solid-State Lett. 2011, 14, H152−H155. (31) Ka, J.; Cho, E. N.; Lee, M.-J.; Myoung, J.-M.; Yun, I. Electrode Metal Penetration of Amorphous Indium Gallium Zinc Oxide Semiconductor Thin Film Transistors. Curr. Appl. Phys. 2015, 15, 675−678. (32) Wang, W.; Song, M.; Bae, T.-S.; Park, Y. H.; Kang, Y.-C.; Lee, S.-G.; Kim, S.-Y.; Kim, D. H.; Lee, S.; Min, G.; et al. Transparent Ultrathin Oxygen-Doped Silver Electrodes for Flexible Organic Solar Cells. Adv. Funct. Mater. 2014, 24, 1551−1561. (33) No, Y.-S.; Yang, J.-D.; Park, D.-H.; Kim, T.-W.; Choi, J.-W.; Choi, W.-K. Improved Electrical Properties of Indium Gallium Zinc Oxide Thin-Film Transistors by AZO/Ag/AZO Multilayer Electrode. J. Sens. Sci. Technol. 2013, 22, 105−110. (34) Allen, S. C.; Steckl, A. J. A Nearly Ideal Phosphor-Converted White Light-Emitting Diode. Appl. Phys. Lett. 2008, 92, 143309. (35) Kim, Y. C.; Lee, S. J.; Oh, I.-K.; Seo, S.; Kim, H.; Myoung, J.-M. Bending Stability of Flexible Amorphous IGZO Thin Film Transistors with Transparent IZO/Ag/IZO Oxide−Metal−Oxide Electrodes. J. Alloys Compd. 2016, 688, 1108−1114. (36) Shimada, Y.; Ishino, J.; Shirasaki, S.; Irie, T.; Kikuchi, A. Effect of MgZnO-Bilayer/BA-CH3 Combination Interlayer on Emission Characteristics of MoO3/F8BT/ZnO Hybrid Light Emitting Diodes Fabricated on ZnO/Ag/ZnO Transparent Cathode. Displays 2013, 34, 437−441. (37) Kang, H.; Jung, S.; Jeong, S.; Kim, G.; Lee, K. Polymer-Metal Hybrid Transparent Electrodes for Flexible Electronics. Nat. Commun. 2015, 6, 6503.

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DOI: 10.1021/acs.jpcc.7b00674 J. Phys. Chem. C XXXX, XXX, XXX−XXX