Solution-Grown ZnO Films toward Transparent and Smart Dual-Color

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Solution-Grown ZnO Films towards Transparent and Smart Dual-Color Light-Emitting Diode Xiaohu Huang, Li Zhang, Shijie Wang, Dongzhi Chi, and Soo-Jin Chua ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03868 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on May 30, 2016

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ACS Applied Materials & Interfaces

Solution-Grown ZnO Films towards Transparent and Smart Dual-Color Light-Emitting Diode Xiaohu Huang,*,† Li Zhang,‡ Shijie Wang,† Dongzhi Chi,† and Soo Jin Chua*,‡,§ †

Institute of Materials Research and Engineering, Agency for Science, Technology and Research, Singapore 138634, Singapore ‡

§

Singapore-MIT Alliance for Research and Technology, Singapore 138602, Singapore

Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore

KEYWORDS: ZnO, solution growth, defects, photoluminescence, electroluminescence, smart light-emitting diode

ABSTRACT: An individual light-emitting diode (LED) capable of emitting different colors of light under different bias conditions not only allows for compact device integration, but also extends the functionality of LED beyond traditional illumination and display. Herein, we report a color-switchable LED based on solution-grown n-type ZnO on p-GaN/n-GaN hetero-junction. The LED emits red light with a peak centered at ~ 692 nm and a full width at half maximum of ~ 90 nm under forward bias, while it emits green light under reverse bias. These two lighting colors can be switched repeatedly by reversing the bias polarity. The bias-polarity-switched dual-

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color LED enables independent control over the lighting color and brightness of each emission with two-terminal operation. The results offer a promising strategy towards transparent, miniaturized and smart LEDs, which hold great potential in optoelectronics and optical communication.

1. INTRODUCTION Although light-emitting diodes (LEDs) have achieved remarkable progress over the past few decades,1-3 exploration of new materials for low-cost and environmentally-friendly LEDs has never stopped.4,5 ZnO is a wide bandgap semiconductor with a direct bandgap of 3.37 eV at room temperature and an exciton binding energy of 60 meV.6 Besides, it is earth-abundant and less toxic than many other semiconductors. Therefore, it is regarded as one of the most promising materials for near-ultraviolet (UV) LEDs and lasers.6 Although ZnO p-n homojunction LEDs and laser diodes have been demonstrated in the literature,7-9 challenges in achieving stable and reliable p-type ZnO have led to the studies of p-n hetero-junction LEDs with n-type ZnO grown on other p-type materials such as p-GaN,10,11 p-Si,12 and p-type organic materials.13 Among them, ZnO/GaN LEDs offer the merits of small lattice mismatch, transparency in visible light range, and thermal stability. The emission spectra of ZnO/GaN LEDs mainly cover UV and blue light range because of the wide bandgap of both ZnO and GaN.10,11 Although radiative recombination at the deep level defects in solution-grown ZnO could lead to emission in the orange light range,14-16 the instability of the defects-correlated emission in ZnO needs to be addressed. Besides, further extending the emission spectra of ZnO/GaN LEDs to red light range is desired, as red light is widely used in full-color display, signaling and biomedical applications.


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Independent control over the color and brightness of the emission from an individual LED lighting unit is of great importance. So far, the most common practice to construct a pixel in fullcolor display is laterally arranging two (or more) independently addressable units emitting light of different colors, however such a route leads to a big pixel pitch. Although it is a significant improvement via vertically stacking multiple LEDs to act as one pixel towards compact pixel design,17,18 additional electrodes are needed to individually address each LED unit, and sometimes tedious wafer transfer and wafer bonding processes are required.17 In contrast, dynamic tuning the emission color of an individual LED offers an attractive alternative towards ultra-high-definition full-color display with simplified LEDs chip manufacture, and it also facilitates innovative applications such as optogenetics, where miniaturized LEDs with different emission colors are implanted for in vivo illumination of biological objects.19 One strategy to tune the emission color of an individual LED is based on the shift of emission spectrum under different magnitude of bias voltage,20 in which only two electrodes are needed so that the structure of the LED can be quite simple. However, the voltage-driven spectrum shift is accompanied by dramatic variation in device brightness,18,20 which limits its applications. Recently, bias-polarity-controlled color switching has been demonstrated in both organic and inorganic LEDs with well-designed interfaces,18,21 and it appears to be an appealing strategy to switch the lighting color of an individual LED without influencing on the brightness. Extending such a concept to other LEDs and understanding the lighting mechanism will have significant impact on miniaturized optoelectronic devices. Herein, we report a red and green dual-color LED based on solution-grown n-type ZnO film grown on top of p-GaN/n-GaN hetero-junction. The emission color of the LED is controlled by the bias polarity, and the brightness of each emission is controlled by the magnitude of the bias,


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thus independent control over the color and brightness of the LED can be achieved with twoterminal operation. Visualizing the spatial distribution of the emission across a well-defined micro-LED under both bias polarities helps us to shed light on the lighting mechanism. 2. EXPERIMENTAL DETAILS Materials synthesis: A buffer layer of un-doped GaN with a thickness of ~ 4 µm was grown on single-side-polished c-plane sapphire substrates, followed by Mg-doped GaN thin film with a thickness of ~ 200 nm grown by metal organic chemical vapor deposition (MOCVD, EMCORE D125). ZnO films with a thickness of ~ 1 µm was grown on top of the fabricated GaN substrate in an aqueous solution comprising of 0.03 M zinc acetate and sodium citrate.22 Sodium citrate was added to promote the lateral coalescence of the grains.23 The pH value of the growth solution was adjusted to about 10 by adding ammonia. The growth was conducted in water-bath at 90 oC for 5 hours. To improve the properties of the as-grown ZnO films, the samples were annealed in vacuum at 425 oC for 30 min using an ULVAC rapid thermal annealing system. To study the electrical properties of the ZnO films, ZnO films were grown separately on c-plane sapphire substrates and annealed under the same condition. LEDs fabrication: AZ5214 photo resist with a thickness of around 1.3 µm was spin-coated on top of the annealed ZnO layer and baked at 140 oC for 90 seconds. The photo resist was patterned through photolithography using a mask, and then etched in developer so as to expose the ZnO layer selectively. The exposed ZnO layer was etched away by dilute HCl solution (15%) for 1 min, so that the underlying p-GaN layer can be exposed, onto which a semi-transparent thin layer of Ni/Au was evaporated using Denton e-beam evaporator to make ohmic contact on the pGaN.10 The ZnO film was deliberately over-etched during the etching process, and a lateral gap measured to be ~ 13 µm was produced between the p-contact and ZnO. The remaining


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photoresist was etched away by acetone. A second photolithography process with mask aligner and liftoff was carried out, and was followed by evaporation of circular Ti/Al/Ni/Au multilayer ohmic contacts onto the remaining ZnO layer.8 Materials and LEDs characterizations: Morphology of the samples were characterized by scanning electron microscope (SEM, Hitachi S4100), elemental analysis were conducted with energy dispersive x-ray spectroscopy (EDS) attached to the SEM and time-of-flight secondary ion mass spectrometry (SIMS, TOF-SIMS IV). X-ray diffraction (XRD) patterns were obtained on Rigaku SmartLab with Cu kα radiation (λ=1.54Å). UV-visible absorption was measured on SHIMADZU UV-3101PC UV-VIS-NIR scanning spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were performed using VG ESCALAB 220i-XL instrument equipped with a monochromatic Al Kα (1486.7 eV) X-ray source. Photoluminescence (PL) spectra were recorded using a micro-PL system (Renishaw Ramanscope 2000) with a He-Cd laser (λ = 325 nm) as excitation source, electroluminescence (EL) spectra were recorded on the same micro-PL setup except a long-focus lens was used to accommodate the probes. During the EL measurement, the LEDs were biased by a Keithley 6340 SourceMeter in the absence of laser excitation. Optical images of the LED were captured by camera (Moticam 5.0 MP) attached to the ocular of the micro-PL setup. Hall measurements were conducted by Van der Pauw method under 0.3 T magnetic field and ambient conditions. All measurements were carried out at room temperature. 3. RESULTS AND DISCUSSION Both ZnO and GaN have the same crystalline structure of hexagonal wurtzite, and their lattice mismatch is 1.9% along the a-axis and 0.5% along the c-axis (JCPDF no. 89-1397 and JCPDF


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Figure 1. (a) SEM image of ZnO film grown on GaN. (b) EDS spectra of the ZnO layer and the underneath GaN. (c) SIMS spectra of ZnO film grown on GaN. (d) XRD pattern of ZnO grown on GaN, the peak labeled by “M” comes from the metal contacts on the LED structure, and the inset is the fine-scanned XRD result of the overlapped ZnO and GaN (0004) peak. (e) UV-visible absorption spectra of GaN substrate before and after growing ZnO film on top of it. (f) Plots of (αhν)2 versus hν of the data shown in (e), where α is the absorption coefficient, and hν is the photon energy. no. 76-0703), respectively. Therefore, ZnO can be grown on GaN without a seed layer. The intrinsic growth direction of ZnO along its c-axis is disturbed by sodium citrate via preferential adsorption of citrate groups on the polar surface of ZnO,23 which facilitates lateral growth of ZnO thus formation of continuous ZnO film on top of p-GaN, as manifested by the contour of quasi-hexagonal grains shown in Figure 1a. Mg is barely detectable from the p-GaN film by EDS


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(Figure 1b), indicating the upper limit of the Mg doping concentration is 0.5 ~ 1.5 at.%. EDS and SIMS spectra from ZnO film (Figure 1b and Figure 1c) suggest a small amount of Na impurities with an upper limit of 0.5 ~ 1.5 at.% are present in ZnO as a result of adding sodium citrate into the growth solution. The lattice parameters of ZnO and GaN are so close that their XRD peaks overlap with each other (Figure 1d), and the difference can only be distinguished by a small shoulder at the small angle side of the asymmetric fine-scanned XRD peak in the inset of Figure 1d,24 which corresponds to ZnO due to the slightly larger lattice constant of ZnO than GaN. The ZnO/GaN heterostructure shows low absorption in the visible light range (Figure 1e) because of the large bandgap of both ZnO and GaN. Based on the optical absorption edge, the optical bandgap of ZnO and GaN are deduced to be 3.38 eV and 3.41 eV, respectively (Figure 1f), which are in good agreement with the literature.6,25 The undoped n-GaN at the bottom layer has a thickness of ~ 4 µm and a carrier concentration of 1016 cm-3. The p-GaN thin film has a thickness of 200 nm and a carrier concentration of 1017 cm-3. The n-ZnO film has a thickness of ~ 1 µm and a carrier concentration of 1016 cm-3. After annealing the samples in vacuum at 425 oC, the carrier concentration of ZnO increases to 1018 cm-3. The dramatic increase of carrier concentration in solution-grown ZnO after annealing can be attributed to annealing-induced defects evolution. Figure 2a-2c shows the PL spectra of ZnO film and the GaN substrate. As shown in Figure 2a, the PL spectrum of solution-grown ZnO film exhibits two emission bands: one is a near-band-edge (NBE) emission band around 399 nm, and the other is a broad defects emission band ranging from 480 nm to 700 nm. The defects emission band can be roughly deconvoluted into three emission bands: P1 at 517 nm, P2 at 547 nm, and P3 at 625 nm. P1 and P2 can be tentatively ascribed to recombination at zinc vacancies (VZn) and/or oxygen vacancies (VO),26,27 and P3 can be tentatively ascribed to oxygen interstitials


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(Oi).28,29 The intensity of the defects emission bands reduces to one third of the initial intensity after annealing, which suggests that many of the defects may be removed by annealing. The NBE emission from the annealed ZnO film is at 385 nm, which is consistent with the earlier results on ZnO.6 The slight difference between the peak position of the NBE emission from the as-grown ZnO film and the annealed one is probably caused by the intrinsic shallow defects in the as-grown ZnO such as zinc interstitials (Zni), thus the blueshift of the NBE emission from ZnO film after annealing infers that Zni could also be removed after annealing. In a word, PL results indicate the annealing process removes/passivates most of the intrinsic defects in ZnO, no matter they are donor-like defects (such as VO and Zni) or acceptor-like defects (such as VZn and Oi). XPS results reveal more information on the surface of the film. As shown in Figure 2d, the O 1s XPS peak of ZnO film exhibits two components: the one at 530 eV is attributed to O2- in wurtzite ZnO, the other at 531.1 eV is most likely associated with O2- in the oxygen deficient regions in ZnO matrix.30 The ratio of these two components changes little after annealing, indicating the oxygen vacancies on the surface of ZnO film is more likely to be passivated rather than removed after annealing. As shown in Figure 2e, the Zn 2p3/2 XPS peak of ZnO film shifts by ~ 0.25 eV towards the higher binding energy side after annealing the film, indicating a slight shift of the Fermi levels towards the conduction band minimum.31 This shift is in agreement with the increase of carrier concentration in ZnO film after annealing. As suggested by both the calculation and experimental results in the literature,22,32,33 the vacuum annealing process activates hydrogen impurities. The hydrogen impurities not only passivate the acceptors (such as VZn),34 but also act as shallow donors in both interstitial site (Hi) or substitutional oxygen site (HO).22 Therefore, the simultaneous improvement in both the carrier concentration and the NBE emission of solution-grown ZnO via annealing is mainly attributed to annealing-induced removal


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of intrinsic defects and activation of the hydrogen donors (especially HO).22,32 As can be seen from Figure 2c, PL spectrum of the p-GaN/n-GaN exhibits a NBE emission at around 366 nm and a broad defects emission covering green and yellow light, which is consistent with the previous results.25 The Mg-acceptors-related emission around 442 nm10,25 is difficult to be distinguished from the PL spectrum of p-GaN/n-GaN, which can be ascribed to two reasons: one is the possible overlap with the tail of NBE emission, the other is the annealing-induced hydrogen passivation of the Mg acceptors35 within the penetration depth of the He-Cd 325 nm laser in GaN (~ 100 nm).29

Figure 2. PL spectra of (a) as-grown ZnO film, (b) annealed ZnO film, and (c) p-GaN/n-GaN substrate after it is annealed together with the above ZnO film. All the PL spectra are measured


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under the same condition. (d) XPS spectra of O 1s from (1) as-grown ZnO film and (2) annealed ZnO film. (d) XPS spectra of Zn 2p3/2 from (1) as-grown ZnO film and (2) annealed ZnO film.

Figure 3. (a) Schematic illustration of the side view and top view of the LED structure. (b) SEM image of the micro-LEDs array, the inset is an enlarged image of an individual LED. The scale bar is 300 µm. (c) Optical microscope image of an individual LED. (d) Optical microscope image of the red-light-emitting LED under forward bias. (e) EL spectra of the LED under forward bias with different injection currents. (f) Optical microscope image of the same LED as in (c). (g) Optical microscope image of the green-light-emitting LED under reverse bias. (h) EL spectra of the LED under reverse bias with different injection currents. Direct growth of ZnO films onto p-GaN/n-GaN substrates facilitates the fabrication of nZnO/p-GaN/n-GaN LEDs. The structure of the LEDs is schematically illustrated in Figure 3a. The top-view morphology of the LEDs is shown in Figure 3b, in which each square with one corner missing is an individual LED with a lateral dimension of 300 µm x 300 µm. The morphology of a single LED under optical microscope is shown in Figure 3c. Distinctively, the


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LED emits red light under forward bias, and the red light mainly comes from the region along the contour of the device (Figure 3d). Dim blue and green light appear in the area close to the pcontact (Figure 3d), however they can be hardly seen in the EL spectra in Figure 3e. Under forward bias with an injection current of 1.5 mA, the EL spectrum mainly displays one peak at 692 nm with a full width at half maximum (FWHM) of ~ 90 nm. The emission peak shifts to ~ 672 nm, and broadens to a FWHM of ~ 100 nm with an increase of the injection current to 3 mA (Figure 3e). The possibility of the red light originating from incandescence can be excluded, as the LED emits green light along its contour when reversing the bias (Figure 3g). The EL spectra under reverse bias are composed of several emission bands and dominated by green emission, as shown in Figure 3h. Interestingly, the emission color of the LED can be switched between red and green by simply alternating the bias polarity (Video S1 in Supporting Information). As shown in Figure 4a, the I-V curve of the LED shows an almost symmetric behavior under forward and reverse bias (Figure S1 in Supporting Information). The symmetric I-V curve is due to the n-p-n structure of the LED, which enables it emitting light under both forward bias and reverse bias. Consistent with the literature,36 the threshold voltage producing a current of 0.01 mA is about 3.1 V for forward bias and -3.7 V for reverse bias, which are quite close to the bandgap of ZnO and GaN.6,25 The current increases exponentially with bias voltage in higher voltage range under both forward and reverse bias. Detectable EL signal by our measurement system requires an operation voltage of at least 10 V, which is quite large yet still comparable to the previous results on ZnO/GaN LEDs.10,15 The high working voltage is mainly due to the high series resistance originating from p-GaN, whose sheet resistance is as large as tens of thousands ohm/sq. The incorporation of Na impurities inside the ZnO layer may also contribute to the high series resistance,37 as more Na impurities are detected by SIMS deep inside the ZnO film (Figure


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1c). As shown in Figure 4b, the LED turns brighter while maintains its original lighting color with increasing the injection current, as corroborated by the corresponding EL spectra of the

Figure 4. (a) Typical semi-log I-V curve of the LED. The dashed lines are guide for the eyes. (b) Optical microscope images of the green-light-emitting LED under reverse bias (left column) and red-light-emitting LED under forward bias (left column) with different injection currents, all the images are captured from the same LED. (c) EL spectra of the green-light-emitting LED under different injection currents.


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green-light-emitting LED shown in Figure 4c. The spectra show a predominant emission around 557 nm and a weak emission band around 442 nm, which are similar to the spectra in Figure 3h. The absence of the near UV emission in Figure 4c could be due to variation of defects concentration in p-GaN. The spectra of the red-light-emitting LED are the same with those in Figure 3e (not shown here). Apparently, one important advantage of the bias-polarity-switched strategy is the independent control over the lighting color and brightness. The lighting mechanism and the corresponding band diagram of the n-ZnO/p-GaN/n-GaN LED under forward bias are illustrated in Figure 5a and Figure 5b, respectively. Under forward bias, electrons drift towards p-GaN from n-ZnO, while holes drift towards n-ZnO from p-GaN, thus carrier recombination occurs mainly at the p-GaN/n-ZnO interface. There are several possible origins of the red EL. Firstly, radiative recombination at deep level defects in GaN can lead to a red luminescence.25,38 However the corresponding peak position does not shift with either measurement temperature or excitation density,25,38 which contradicts the blueshift of the red EL with injection current in Figure 3e (also Figure S2 in Supporting Information). Secondly, a red EL ranging from 660 nm to 710 nm was reported in ZnO p-n homo-junction LED, and it was ascribed to radiative recombination at deep level acceptor centers in ZnO.8 The incorporation of Na impurities could form acceptor centers in ZnO close to the ZnO/GaN interface. But PL results suggest most of the defects are removed/passivated after the annealing treatment, deep level defects in ZnO might neither be the main contributor of the red EL. Thirdly, considering the valence-band offset of type II band alignment between n-ZnO and pGaN can be as large as ~ 1 eV,39 if the electrons at the shallow donor states close to the conduction band of ZnO (e.g. hydrogen donors) recombine with the holes at the Mg acceptor states of GaN at the ZnO/GaN interface, then a red EL is produced. The blueshift and broadening


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of the red EL with increasing the injection current (Figure 3e) is caused by two reasons: One is the band-filling effect in ZnO and GaN. Briefly, the lowest unoccupied states near the conduction band of ZnO lift up with an increase of the injection current due to electron accumulation.8 Such an effect is supposed to be more profound in p-GaN, as both shallow and deep acceptors related to Mg dopants in p-GaN are reported.40 The other is the reduced band offset under elevated forward bias across the n-ZnO/p-GaN interface. The diffusion length of minority carriers in p-GaN is only a few hundred nanometers,41 which is much smaller than the gap between the p-contact and ZnO (~ 13 µm). Therefore, only holes can drift through the gap between the p-contact and ZnO. When these holes travel across the gap, a larger number of electrons have already been waiting for them on the edge of the LED mesa due to the better conductivity of ZnO. Therefore, the electron-hole recombination is limited by the holes reaching the ZnO, thus preferentially occurs along the outmost edge of ZnO (nearest to the p-contact), leading to the appearance of EL mainly along the contour of the LED mesa. Decreasing the gap between the p-contact and ZnO to a few hundred nanometers should improve the emission homogeneity across the whole LED mesa. The appearance of dim light along the contour of the p-contact under high injection current (Figure 3d) indicates a small amount of holes could also be injected from p-GaN to the underneath n-GaN due to the thin thickness of the p-GaN layer. The absence of the NBE emission in the EL spectra is due to its weak intensity as well as the self-absorption.8 The lighting mechanism and the corresponding band diagram of the n-ZnO/p-GaN/n-GaN LED under reverse bias are illustrated in Figure 5c and Figure 5d, respectively. Firstly, the electrons injected to p-GaN under reverse bias can recombine with the holes in p-GaN, which explains the strong EL at the vicinity of the probe on p-GaN in Figure 3g. Similarly, the holes


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injected to n-ZnO under reverse bias can also recombine with the electrons in n-ZnO. Secondly, the thin thickness of the p-GaN layer (~ 200 nm) and its low carrier concentration make it possible that the electrons could tunnel through the p-type GaN downwards into the undoped GaN, from there the electrons drift across the gap between p-contact and ZnO, then the electrons are injected back to p-type GaN under an “effective” positive bias applied through the above ZnO layer. The electrons travel a short distance if they recombine along the outmost edge of the LED mesa, thus the edge emission can be still obvious (Figure 3g). However, as the electron mobility in n-GaN can be quite high (hundreds of cm2V-1s-1),42 the radiative recombination is not limited to the edge of the LED mesa, as corroborated by the appearance of green light within the contour of the LED under reverse bias (Figure 4b). The non-homogeneity of the emission across the LED mesa may be caused by the non-uniform distribution of recombination centers as well as non-uniform current spreading across the ZnO layer, which needs to be improved. The diverse origin of the EL under reverse bias is consistent with the complex EL spectra in Figure 3h, which include many emission bands: The sharp UV emission at 370 nm can be ascribed to NBE emission in GaN,25 the emission band around 397 nm is in line with the NBE emission in ZnO,11 the emission band around 442 nm is likely to be the related to Mg-acceptors,10,25 and the broad emission band dominated by green emission but also extends to yellow emission is likely to be associated with deep level defects in GaN.25 The presence of the Mg-acceptors-related emission in the EL spectra suggests that carrier recombination occurs in the p-GaN layer close to the pGaN/n-GaN interface, as the hydrogen passivation of Mg-acceptors in p-GaN is only limited to the upper 100 nm layer of the p-GaN thin film. The slight redshift of the NBE peak position in the EL spectra in Figure 3h compared to the PL results in Figure 2 is caused by the heating effect of current injection. The lighting color of the LED under reverse bias may vary slightly from one


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device (bluish green in Figure 3h) to another (yellowish green in Figure 4b), which depends on the defects concentration in p-GaN, as well as the fluctuation of the focus distances (Figure S3 in Supporting Information).

Figure 5. (a) Schematic illustrations of the dominant lighting mechanism and (b) the corresponding band diagram of the red-light-emitting LED under forward bias, in which the carrier recombination mainly occurs at the p-GaN/n-ZnO interface. (c) Schematic illustrations of the dominant lighting mechanism and (d) the corresponding band diagram of the green-lightemitting LED under reverse bias, in which the carrier recombination mainly occurs at the pGaN/n-GaN interface. Traditional inorganic red LEDs are made of narrow bandgap semiconductors,1 while our red and green dual-color LEDs are constructed by wide bandgap semiconductors. Wide bandgap semiconductors tend to show low absorption in visible light range (Figure 1e), facilitating transparent LEDs display. It is noteworthy that the intensity of the red light from the LEDs retains 40% of the initial value after storing the LEDs in ambient conditions for as long as 30 months without packaging (Figure S2 in Supporting Information). The relatively good stability of the red EL agrees with the proposed mechanism that involves the recombination of the


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electrons from the HO donors in n-ZnO with the holes from the Mg acceptors in p-GaN at the ZnO/GaN interface, as both the HO donors in ZnO32 and Mg acceptors in GaN are thermodynamically stable. ZnO/GaN LEDs are generally regarded as a promising solid state lighting source in the near-UV range,8,10 herein we demonstrate for the first time that red and green dual-color LED can be realized by chemical-solution-grown ZnO film on p-GaN/n-GaN. Improved electrical and optical properties of annealed ZnO22 lead to stable performance of the LEDs. Switching the emission color of an individual LED via reversing the bias polarity enables two-terminal operation. In contrast to the dramatic change of emission intensity through voltagedriven spectrum shift,20 the bias-polarity-switched strategy allows for tuning the color and brightness of each emission independently (Figure 4b), which facilitates compact LEDs integration. Besides, our dual-color LED is capable of distinguishing both the magnitude and polarity of the bias, which could find application in bias indicator and optical communication. Although the external quantum efficiency of LED based on a single ZnO microwire on p-GaN substrate is a few percent,36 the external quantum efficiency of simple hetero-junction LED based on ZnO film grown on p-GaN are a few magnitude lower,16,43 thus more complicated structures are needed to improve the efficiency. The emission efficiency and homogeneity of our LEDs could be improved by increasing the conductivity of the GaN and ZnO films, smoothing the interface between p-GaN and n-ZnO, optimizing the device geometries, etc. More detailed XPS characterizations,44 especially operando XPS,45 could give valuable insight on the lighting mechanism. All of these warrant future investigations. 4. CONCLUSION In conclusion, the electrical and optical properties of solution-grown ZnO films are improved by annealing, and ascribed to removal of intrinsic defects and activation of hydrogen donors. The


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films are used to fabricate red and green dual-color n-ZnO/p-GaN/n-GaN LEDs with independent control over the emission color and brightness under two-terminal operation. The emission color of the LEDs is controlled by the bias polarity, and the intensity of each emission is modulated by adjusting the magnitude of the bias. Visualization of the spatial distribution of the luminescence across the well-defined micro-LED helps us to understand the lighting mechanism. The bias-polarity-switched dual-color LEDs offer a promising solution towards smart LEDs with miniaturized size and transparent structure, which hold great potential in optoelectronics and biomedical applications.

ASSOCIATED CONTENT Supporting Information Video showing the repeated switching of the lighting color between red and green by reversing the bias polarity of the LED; I-V curve of the LED in linear scale; EL results of one LED after being kept in ambient conditions for 30 months; Optical microscope images of the dual-color LED under different focus distances. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] NOTES The authors declare no competing financial interest.


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ACKNOWLEDGMENTS The authors appreciate the support from Agency for Science, Technology and Research in Singapore, as well as Singapore-MIT Alliance for Research and Technology. REFERENCES (1)

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Figure 1. (a) SEM image of ZnO film grown on GaN. (b) EDS spectra of the ZnO layer and the underneath GaN. (c) SIMS spectra of ZnO film grown on GaN. (d) XRD pattern of ZnO grown on GaN, the peak labeled by “M” comes from the metal contacts on the LED structure, and the inset is the fine-scanned XRD result of the overlapped ZnO and GaN (0004) peak. (e) UV-visible absorption spectra of GaN substrate before and after growing ZnO film on top of it. (f) Plots of (αhν)2 versus hν of the data shown in (e), where α is the absorption coefficient, and hννis the photon energy. 80x102mm (300 x 300 DPI)


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Figure 2. PL spectra of (a) as-grown ZnO film, (b) annealed ZnO film, and (c) p-GaN/n-GaN substrate after it is annealed together with the above ZnO film. All the PL spectra are measured under the same condition. (d) XPS spectra of O 1s from (1) as-grown ZnO film and (2) annealed ZnO film. (d) XPS spectra of Zn 2p3/2 from (1) as-grown ZnO film and (2) annealed ZnO film. 130x114mm (300 x 300 DPI)



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Figure 3. (a) Schematic illustration of the side view and top view of the LED structure. (b) SEM image of the LEDs, the inset is an enlarged image of an individual LED. (c) Optical microscope image of an individual LED. (d) Optical microscope image of the red-light-emitting LED under forward bias. (e) EL spectra of the LED under forward bias with different injection currents. (f) Optical microscope image of the same LED as in (c). (g) Optical microscope image of the green-light-emitting LED under reverse bias. (h) EL spectra of the LED under reverse bias with different injection currents. 119x90mm (300 x 300 DPI)


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Figure 4. (a) Typical semi-log I-V curve of the LED. The dashed lines are guide for the eyes. (b) Optical microscope images of the green-light-emitting LED under reverse bias (left column) and red-light-emitting LED under forward bias (left column) with different injection currents, all the images are captured from the same LED. (c) EL spectra of the green-light-emitting LED under different injection currents. 80x144mm (300 x 300 DPI)



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Figure 5. (a) Schematic illustrations of the dominant lighting mechanism and (b) the corresponding band diagram of the red-light-emitting LED under forward bias, in which the carrier recombination mainly occurs at the p-GaN/n-ZnO interface. (c) Schematic illustrations of the dominant lighting mechanism and (d) the corresponding band diagram of the green-light-emitting LED under reverse bias, in which the carrier recombination mainly occurs at the p-GaN/n-GaN interface. 80x65mm (300 x 300 DPI)


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TOC Graphic 45x35mm (300 x 300 DPI)


Solution-Grown ZnO Films toward Transparent and Smart Dual-Color

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