Highly Efficient Deep-UV Light-Emitting Diodes Using AlN-Based

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Highly efficient deep-UV light-emitting diodes using AlN-based, deep-UV transparent glass electrodes Tae Ho Lee, Byeong Ryong Lee, Kyung Rock Son, Hee Woong Shin, and Tae Geun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13624 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Applied Materials & Interfaces

Highly efficient deep-UV light-emitting diodes using AlN-based, deep-UV transparent glass electrodes Tae Ho Lee,† Byeong Ryong Lee,† Kyung Rock Son,† Hee Woong Shin,†,‡ and Tae Geun Kim*,† †

‡

School of Electrical Engineering, Korea University, Seoul, 136-701, Republic of Korea

LED R&D Center, LED Division, LG Innotek Co., Ltd., Paju, 413-901, Republic of Korea

Keywords: deep-UV light-emitting diode, direct Ohmic contact, glass electrode, pulsed electrical breakdown, transparent conductive electrode

ABSTRACT Many studies have set out to develop electrodes that are both highly conductive and transparent across a wide spectral region, from visible to deep-UV (DUV). However, few solutions have been proposed because these two properties are mutually exclusive. In this paper, an AlN-based glass electrode film with a conducting filament formed by the application of an AC pulse is proposed as a solution, which exhibits a high transmittance in the DUV region (over 95.6% at 280 nm) and a low contact resistance with a p-Al0.4Ga0.6N layer (ρc = 3.2 × 10-2 Ω·cm2). The Ohmic conduction mechanism at the interface between the AlN film and pAl0.4Ga0.6N layers is fully examined using various analytical tools. This AlN film is finally applied to a 280-nm top-emitting LED, to verify the validity of the method, which exhibits very

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stable operations with a forward voltage of 7.7 V at 20 mA, a light output power of 7.49 mW at 100 mA, and, most importantly, a record-high external quantum efficiency of 2.8% after packaging.


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1. INTRODUCTION The development of a highly transparent and conductive electrode (TCE), with a broad spectral range, is the ultimate goal of researchers working on optoelectronic devices such as lightemitting diodes (LED),1-3 organic LEDs (OLED),4-6 and solar cells.7-9 To date, indium-doped tin oxide (ITO) has been the most widely used TCE in the visible region because of its high electrical conductivity and high optical transmittance (around 90% at 450 nm).10 However, this advantage is abruptly lost in the UV region because the absorption edge of ITO is near 365 nm.10 Another drawback of ITO is the high cost of indium, the supply of which is limited. For these reasons, there have been considerable efforts to find a replacement for ITO, including graphenes,11,12 metal nanowires,2,4,13,14 thin metals,15 metal oxides,16,17 topological insulators,18-22 and conducting polymers.23,24 However, none of these efforts have been able to identify a fundamental solution to the trade-off between electrical conductivity and optical transmittance, particularly for device applications in the deep-UV (DUV) region. Among those devices that require the use of TCEs, AlGaN-based DUV LEDs,25-35 operating at a wavelength of 280 nm or less, pose the most challenging task, in that it is necessary to attain a high external quantum efficiency (EQE). The DUV LED is an eco-friendly, high-value optical source that has been adopted for a wide range of applications such as sterilization, purification, and biochemistry. Currently, however, the EQE of such a DUV LED is quite low (1.3% or under for a top-emitting LED28,29 and 3–20% for a flip-chip LED at ~280 nm30-35) because of the light absorption in the p-GaN contact layer and the non-availability of a DUV-transparent electrode. Additionally, there is currently no proven means of effectively injecting a current into p-AlGaN, either. To overcome these problems, it would be necessary to establish a direct Ohmic contact with a p-AlGaN contact layer using DUV-transparent electrodes. However, this would be quite


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difficult to realize when using conventional Ohmic methods because of the large work function difference between the metal and p-AlGaN contact layers, as well as the non-availability of ptype dopants in AlGaN and DUV-transparent electrodes. For this reason, most reports addressing the development of a DUV LED have related to costly flip-chip designs, rather than the simple and cheap top-emitting types. A conventional DUV LED uses a p-GaN contact layer for Ohmic contact; hence, most of the UV light generated in AlGaN-based multiple quantum wells is absorbed by the p-GaN contact layer as well as the top electrode,28,29 thus greatly reducing the light-extraction efficiency. Recently, we reported on a method for producing a UV-transparent glass electrode (GE) using wide-bandgap (WB) materials such as SiO2, AlN, Al2O3, and Si3N4.36 This method involves forming conductive filaments (CFs) in the WB thin film through an electrical breakdown (EBD) process, so that carriers can be injected and spread from the metal to a p-AlGaN contact layer via CFs in the WB film. We also demonstrated the validity of this method at the device level, for (Al)GaN-based, lateral-type visible and near-UV LEDs.37 Using this approach, we improved the performance of LEDs by incorporating a thin ITO buffer layer (< 10 nm) between the AlN GE and p-AlGaN, which spread the current over the p-AlGaN layer and prevented multiple quantum well (MQW) damage during the EBD, without any light absorption loss in the near-UV region. However, this buffer layer cannot be incorporated into a DUV LED application, because of the large degree of absorption, even for an ITO buffer layer that is 10-nm thick (> 36% at 280 nm). Therefore, there is a need for another means of forming CFs in a WB thin film without incurring damage or current-spread problems. To overcome these problems, we used an alternating current (AC) pulse bias instead of a direct current (DC) bias during the EBD. Pulses of only a few


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nanoseconds were used to reduce the damage during the EBD as well as enhance the current spread by making use of the pulsation effect, without any buffer layers. In the present study, a highly conductive and transparent AlN thin film, fabricated by AC-pulsebased EBD (PEBD) processes, is introduced, and applied to a 280-nm top-emitting DUV LED with a p-Al0.4Ga0.6N contact layer. In addition, the Ohmic and conduction mechanism in these devices is explained through various analyses such as conductive atomic force microscopy (CAFM), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) analyses.

2. RESULTS AND DISCUSSION Figure 1a shows a cross-sectional STEM image of an AlGaN-based DUV LED grown by metal-organic chemical vapor deposition on a sapphire substrate. A typical LED structure consists of a 3.7-µm undoped AlN buffer layer grown on sapphire, followed by a 1.8-µm Sidoped n-Al0.5Ga0.5N layer, an AlGaN-based 90-nm 5-pair undoped MQW region, a 50-nm 5-pair Mg-doped AlGaN/AlGaN multiple quantum barrier electron blocking layer, a 30-nm Mg-doped Al-graded p-AlGaN layer, a 50-nm Mg-doped p-Al0.4Ga0.6N contact layer, a 15-nm AlN GE layer, and a 15-nm Ni metal pad with a 2-nm Cr layer for adhesion in the PEBD process. Here, the composition of Al and Ga atoms in the p-AlxGa1-xN contact layer is confirmed to be 40 and 60% from the Energy-dispersive X-ray spectroscopy (EDS) spectra measured at the p-AlGaN surface, as shown in Figure 1b. Figure 1c shows a typical photoluminescence (PL) spectrum measured for an AlGaN-based DUV LED wafer using a KrF excimer laser (λ = 248 nm). A dominant emission peak is observed at approximately 278 nm.


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To fabricate AlGaN-based lateral-type, top-emitting DUV LED, standard photolithography and inductively coupled plasma reactive-ion etching were used to form n-AlGaN terminated isolated fan-shaped mesa structures. Then, 15-nm-thick AlN layer was deposited on the p-Al0.4Ga0.6N layers for GE by using the RF magnetron sputtering system in ambient Ar–N2 gas at a base pressure of 2 × 10-7 Torr and a working pressure of 3 × 10-3 Torr, and sample was annealed by rapid thermal process in N2 gas environment at 600 °C for 30 s for recrystallization. Next, 2/15nm-thick Cr/Ni PEBD pads were deposited by e-beam evaporation, and PEBD process was performed between the Cr/Ni PEBD pads and n-AlGaN layers. Finally, Cr/Ni/Au (2/50/150 nm) layers were deposited by e-beam evaporation for p- and n-electrodes. The size of LED chip was 390 µm × 390 µm. Details on the fabrication process of lateral-type AlGaN-based DUV LEDs with AlN GEs are presented in Supplementary Figure S1. Figure 2a is a schematic illustration of a lateral-type AlGaN-based DUV LED with AlN GE. To uniformly form CFs in an AlN film, 55 metal dots (Cr/Ni) were deposited along the p-metal pad, and a 500-ns AC square (pulse) wave was applied between the Cr/Ni dot and n-AlGaN surface (left). Then, the metal dot area was covered by a tripod-shaped p-metal pad (Ni/Au) to observe the current spread during the operation of the LED (right). The magnified figure on the right schematically illustrates that the current can be injected through the CFs formed in the AlN GE after the PEBD. Next, to determine the optimal bias conditions for the PEBD process in the AlN GE, we investigated the change in the electrical state of the film before and after the AC-biased pulse application. Here, the AC pulse for the PEBD process was created by using a pulse pattern generator (81110A, Agilent). Then, the PEBD process in the time domain (transient AC pulse bias and corresponding transient sample current as a function of time) was measured using a 1GHz 4-channel digital oscilloscope (DS6104, Rigol) with a shunt resistance of 1 Ω. In Figure 2b,


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an applied voltage pulse (black line) and its current transient response (red line) were measured for a pulse magnitude of 7 V, using a two-point auto-probing contact system between a metal dot (Cr/Ni) and the n-AlGaN surface to form the CFs in the AlN GE (Supplementary Movie S1). Prior to the application of the 7 V AC pulse (before the PEBD), hardly any current flowed in Region 1 of Figure 2b (region from -2 to -1 μs with a 2 V amplitude). In other words, initially, the AlN GE is in the high-resistance state (HRS). However, the current exhibited a steep increase when the 7 V AC pulse was applied, as in Region 2 of Figure 2b (region from 0–500 ns with a 7 V amplitude). The PEBD switching time, τPEBD, defined as instant to switch the AlN film from the HRS to the low-resistance state (LRS), is found to be 100 ns. After the PEBD process, the current flows well, even with a 2 V read voltage, as in Region 3 of Figure 2b (1.5–2.5 μs with a 2 V amplitude). To examine the long-term stability at the LRS of AlN GE, we also measured the retention properties at 2 V in terms of the read delay at room temperature, as shown in Supplementary Figure S2. Based on these experiments, we first measured the specific contact resistances (ρc) of the AlN GE after the PEBD deposited on 280-nm DUV LEDs with p-Al0.4Ga0.6N contact layers. Figure 2c shows the current–voltage (I–V) curves measured for different transmission line method (TLM) contact spacings. Quasi-Ohmic behavior was observed with a relatively low ρc of 2.6 × 10-2 Ω·cm2 owing to the changes in the composition and crystal structure of the pAl0.4Ga0.6N/AlN/Cr/Ni layers, as well as the formation of CFs in the AlN GE after the PEBD. On the other hand, a 10-nm ITO exhibited non-Ohmic behavior, as shown in Supplementary Figure S3, probably due the increase in the sheet resistance of the thin ITO and the large Schottky barrier height (SBH) between the reference ITO and highly resistive p-Al0.4Ga0.6N layers. We also measured the ρc of Ni/Au (50/150 nm), typically used as p-electrodes for DUV LEDs, on p-


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Al0.4Ga0.6N for comparison, which was 3.3 × 10-2 Ω·cm2. This was similar to that of the AlN GE (Supplementary Figure S3). The ρc was calculated from a plot for total resistance versus pad spacing (Supplementary Figure S4). We also measured the optical transmittance as a function of wavelength for the AlN GE before and after the PEBD process, as well as that for the reference 10-nm ITO deposited on a quartz substrate, as shown in Figure 2d. Before the PEBD, the 15-nm AlN GEs exhibited no losses in transmittance (< 1% loss at 280 nm) due to the large bandgap energy (EAlN = 6.05 eV; around 205 nm). However, after the completion of the PEBD, the AlN GEs exhibited transmittance losses in the order of 4% (TAlN, after PEBD = 95.6% at 280 nm), which may be related to the CFs formed in the AlN layer. However, the transmittance after the PBED is still sufficiently high to allow its use as a TCE for an AlGaN-based DUV LED. Meanwhile, the transmittance of the reference 10-nm ITO was a maximum of 64% at 280 nm. The ρC and transmittance values of each sample are summarized in Table 1. To understand the ohmic conduction mechanism, we investigated the p-AlGaN/AlN GE/metal structure before and after the PEBD process by using C-AFM, high-angle annular dark-field (HAADF) STEM, XPS, and XRD spectroscopy, as shown in Figures 3 and 4. First, we captured C-AFM images at 3 V on top of the AlN GE before and after the PEBD, as shown in Figures 3a and 3b, respectively, after removing the Cr/Ni pad (Supplementary Figure S5). Compared to the image acquired before the PEBD (Figure 3a), we found that a current flows in the AlN GE, implying that the CFs are uniformly formed after the PEBD (Figure 3b). In addition, to identify the origin of the CFs in the AlN film, we captured HAADF-STEM images before and after the PEBD. Compared to the STEM image captured before the PEBD, shown in Figure 3c, we found some composition changes related to the formation of CFs on the p-Al0.4Ga0.6N surface after the


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PEBD as shown in Figure 3d. We also analyzed the edge electron energy loss spectroscopy (EELS) using STEM, from which Ni-L (red), Al-K (blue), and N-K (green) edge spectra for the local region corresponding to the HAADF-STEM images, labeled C1 and D1, were measured as part of the qualitative and quantitative analyses of CFs in the AlN GE before and after the PEBD, as shown in Figures 3e, 3g–i and Figures 3f, 3j–l, respectively. Relative to the EELS images before the PEBD, the Ni atoms had diffused towards the p-Al0.4Ga0.6N layer after the PEBD (see Figures 3g and j). We also found that N vacancies were formed simultaneously after the PEBD (Figure 3i and 3l). Again, in general, the origins of the CFs are well known to be N vacancies or conductive bridges composed of metal ions.37 We then analyzed the interface between the p-Al0.4Ga0.6N and AlN layers before and after the PEBD, using XPS and XRD, as shown in Figure 4 and Supplementary Figure S6, to explain the Ohmic mechanism at the AlN GE/p-Al0.4Ga0.6N contact layers. Figure 4a shows an XPS analysis of the Ni 2p core-level spectra obtained at the p-Al0.4Ga0.6N surface before and after the PEBD. In comparison, the AlN GEs after the PEBD exhibit a clear Ni 2p core-level spectrum,38 whereas no peaks are observed for the sample before the PEBD in the same binding energy region. This result indicates that the Ni atoms diffused into the AlN GE as well as the p-Al0.4Ga0.6N layers. Similar results were obtained for the XRD spectra measured at the p-Al0.4Ga0.6N surface (Supplementary Figure S6). Only a peak related to the p-AlGaN layer was detected before the PEBD,39 whereas an additional peak related to the Ni3N film was detected along with p-AlGaNrelated peaks after the PEBD.40 Ni3N is well known to improve the Ohmic behavior of p-AlGaN by increasing the effective carrier concentration of the p-AlGaN surface.40 Both the XPS and XRD results are in good agreement with the EELS results that support the in-diffusion of Ni atoms toward the p-Al0.4Ga0.6N surface. Figure 4b shows an XPS analysis of the Ga 2p core-level


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spectra for the same sample before and after the PEBD and the annealed 10-nm ITO layer (after removal). This Ga 2p core level predominantly consists of Ga-N bonds near 1116 eV,37 and shifts towards the low-energy side by around 0.625 eV after the PEBD and 0.125 eV for the annealed ITO, compared to the peak of the sample before the PEBD process. This result indicates that the PEBD shifts the surface Fermi level of p-Al0.4Ga0.6N towards the valence band edge and, as a result, the band bending is reduced. This blue shift also indicates a lowering of the SBH.41,42 We also measured the UPS for the AlN film after PEBD process and the annealed 10nm ITO layer as shown in Supplementary Figure S7, from which the work function was calculated to be 5.04 eV for the AlN film after the PEBD and 4.56 eV for the annealed ITO. This result implies the SBH can be lowered at the interface of p-AlGaN/AlN layers against pAlGaN/ITO layers, which is consistent with the XPS results. Based on these analyses of the p-AlGaN/AlN GE/metal structure, we can conclude that the reduced contact resistance is caused by the Ga out-diffusion (Ga vacancies) and Ni in-diffusion (the formation of an NiN film lowering the SBH between the p-Al0.4Ga0.6N and AlN layers through the PEBD process). Finally, we fabricated three lateral-type top-emitting DUV LEDs with p-Al0.4Ga0.6N contact layers using 15-nm AlN GEs, 10-nm reference ITOs, and Ni/Au (50/150-nm) electrodes including the packaging. Details of the packaging process are shown in Supplementary Figure S8. The light output power–current–voltage (L–I–V) curves of the three devices were plotted up to 100 mA, as shown in Figure 5a. The LEDs with the 10-nm ITOs exhibited forward voltages of 13.2 V whereas those with AlN GEs exhibited forward voltages of 7.7 V at 20 mA, a reduction of 42%. This improvement may be a result of the reduced contact resistance. Meanwhile, those LEDs fabricated with Ni/Au contacts, which exhibited a similar ρc value to that of the AlN GE


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with CFs, exhibited similar L–I–V characteristics to those of an LED with the AlN GE at a low current injection. However, an EBD occurred at around 30 mA, probably due to the current crowding that occurs in the absence of TCEs. The output power of the DUV LED with the AlN GE was 7.49 mW, which is approximately 416% higher than the 2.78 mW of the LED with the 10-nm reference ITO, at 100 mA. Then, we measured the reverse leakage currents during a voltage sweep from 0 to −10 V, to evaluate the reliability of the device operation as shown in Figure 5b. The leakage current levels at a reverse voltage of −10 V were found to be approximately −400 and −600 nA for those DUV LEDs with the AlN GEs and the 10-nm reference ITO, respectively. These values are sufficiently low to ensure the reliable operation of the LEDs. Figure 5c compares the electroluminescence (EL) spectra measured for the three DUV LEDs with the AlN GEs, 10-nm reference ITO, and Ni/Au contacts. A single peak emission at 280.5 nm was observed for the DUV LEDs with the AlN GEs at 100 mA. The EL intensity of the LED with the AlN GE increased by approximately 524% relative to that of the LED with the 10-nm reference ITO. On the other hand, the EL intensity of the LED with the Ni/Au contacts was observed up to 30 mA, at which point the intensity was slightly lower than that of the LED with the AlN GE. Meanwhile, we investigated the sample-to-sample uniformity in terms of the EL spectra of 50 different LED samples with AlN GEs to determine the reproducibility (Supplementary Figure S9). We found that all the samples shown in this figure operate very stably and with good uniformity. Figure 5d shows the EQE as a function of the current measured for the DUV LEDs with the AlN GEs and the 10-nm reference ITOs. The maximum EQE of an LED with the AlN GE was found to be 2.8% at 5 mA, which is much higher than the 0.54% of the reference LED with the 10-nm ITO. Here, the EQE was calculated from the LED photoelectricity detecting system (Supplementary Figure S10). The EQE is enhanced by


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approximately 420% in this comparison. Meanwhile, a record-high EQE of 2.8% has been reported from lateral-type top-emitting DUV LEDs, which is comparable to the values possible with flip-chip LEDs with ordinary reflectors.28-34 These improvements in the performance of the DUV LEDs with AlN GEs could be attributed to the enhanced light extraction efficiency given by the much higher optical transmittance of the AlN GE and the improved ohmic behavior attained through the use of the CFs in AlN GE. Not mentioned earlier, the dependence of the AlN film thickness on the performance of LEDs has been investigated, from which the thickness of the AlN film was determined to be 15 nm because PEBD voltages simply increased without any gain in device performances as the thickness increased (Supplementary Figure S11). Finally, we performed a three-dimensional finite-difference time-domain (FDTD) simulation (Supplementary Figure S12) to evaluate the light extraction efficiency in relation to optical transmittance of the TCE. In this simulation, the transmittance of the15-nm AlN GE was calculated to be 97.9% at 280 nm, much higher than that (62.7%) of the 10-nm ITO, which is in good agreement with the experiment result presented in Figure 2d. Then, the relative light extraction efficiencies of the DUV LEDs with AlN GEs and 10-nm ITOs were calculated to be 2.91 and 1.08, respectively, from the contour plot for polar projection of far-field radiation intensity, by solving Maxwell's equations. These results indicate that much higher light extraction efficiency can be achieved in the DUV LED by simply using AlN GEs, mainly due to the noticeably increased transmittance of the AlN layer. Details on the AlGaN-based DUV LED performance (e.g., forward voltage (VF), reverse leakage current (IL), light output power (PO), EQE, specific contact resistance (ρC), and transmittance) are summarized in Table 1.

3. CONCLUSION


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In the present study, we examined CF-based AlN GEs with ultrahigh transmittance and a low contact resistance on the p-Al0.4Ga0.6N layers using a damage-free PEBD process. By adopting an AC-biased PEBD process using a 100-ns square wave instead of a DC bias, electrical damage that might occur in the MQW active layers of a DUV LED could be avoided, leading to highly uniform and reliable device operation. The optical transmittance of the AlN GE after the PEBD process was as high as 95.6% at 280 nm, whereas that of the 10-nm-thick ITO was 64% at 280 nm. To understand the conduction mechanism related to the CFs in the AlN GE, we conducted C-AFM, HAADF-STEM, and EELS analyses. In addition, we analyzed the XPS and XRD spectra to explain the Ohmic mechanism of the AlN GE on the p-Al0.4Ga0.6N. Finally, we applied these AlN GEs to AlGaN-based lateral-type, top-emitting DUV LEDs, and achieved a significant improvement in the device performance relative to that of the reference LEDs with the 10-nm ITO. These damage-free PEBD methods are expected to lead to significant progress in the areas of damage-sensitive organic-based optoelectronic devices such as organic LEDs and organic photovoltaics.

4. EXPERIMENTAL SECTION Electrical and optical characterization. Current–voltage characteristics of the contacts were measured for TLM patterns with a spacing of 5–25 µm using a Keithley 4200 semiconductor parameter analyzer. Optical transmittances of the AlN GE and 10-nm reference ITO films deposited on quartz substrates were measured as a function of the wavelength by using a Lambda 35 UV/VIS spectrometer. Next, optical and electrical properties including L-I-V curves, EL


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intensities, and EQE of the 280 nm top-emitting DUV LEDs were measured by LED measurement system (CAS140CT-152, Instrument Systems with ISP-500; integrating sphere). CAFM measurement and sample preparation. Current mapping on the top of the AlN layer after PEBD was obtained by a Cr/Pt−Ir coated Si probe (Park systems, CONTSCPt) under contact mode, and a sample bias of 3.0 V after removing the Cr/Ni layers using the nichrome etchant (Sigma-Aldrich) (Supplementary Figure S5).

HAADF-STEM and EELS Analyses. The Cs-corrected STEM and EELS analyses were performed using a field emission TEM with a Cs corrector (Titan G2 80-300, FEI) at an accelerating voltage of 300 kV.

XRD and XPS analyses and sample preparation. Samples for XRD and XPS analysis were measured of the top surface of p-Al0.4Ga0.6N layers after removing the AlN layers using the Sodium hydroxide (NAOH). XRD θ-2θ scan was studies with the automated multipurpose X-ray diffractometer system (SmartLab, Rigaku) using Cu Kα radiation. The voltage and current were 45 kV and 200 mA, respectively. The 2θ scan range was 30−100° at the scan rate of 0.01° and the scanning speed of 1°/min. XPS measurement was performed using a 24.1 W monochromatized Al Kα radiation source (X-tool, ULVAC-PHI) with the takeoff angle of 45°.

SUPPORTING INFORMATION Details on the fabrication of lateral-type DUV LEDs with AlN GEs using PEBD process (Figure S1); Retention measurement of the LRS current at 2 V for the AlN glass electrode after the PEBD for long-term stability (Figure S2); Ohmic behavior of Ni/Au (50/150 nm) metal


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contacts and 10-nm ITO layers deposited on a p-Al0.4Ga0.6N contact layer (Figure S3); Ohmic behaviors of GE and Ni/Au on p-Al0.4Ga0.6N AlGaN contact layers (Figure S4); Measurement set-up for C-AFM (Figure S5); XRD analysis before and after PEBD process (Figure S6); UPS spectra of 15-nm AlN GE and 10-nm ITO (Figure S7); Packaging information (Figure S8); Electroluminescence spectrum of AlGaN-based lateral-type DUV LEDs (Figure S9); Equation used for calculating the EQE of DUV LEDs (Figure S10); Dependence of the AlN layer thickness on the electrical and optical properties of DUV LEDs (Figure S11); 3D FDTD simulation for AlGaN-based lateral-type DUV LEDs with AlN GEs and 10-nm ITO (Figure S12); 5-speed (5x) PEBD process of 55 metal dots (Cr/Ni) using auto-probing contact system (Movie S1) are described in supporting information.

AUTHOR INFORMATION Corresponding Author *Corresponding author: E-mail: [email protected], Phone: +82-2-3290-3255, Fax: +82-2-9245119

ACKNOWLEDGEMENTS This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean government (No. 2016R1A3B1908249).


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Figure Captions Figure 1. (a) Typical cross-sectional HAADF-STEM images of 280 nm AlGaN-based DUV LED structure on the Sapphire substrate with glass electrode and Cr/Ni PEBD metal dots. Scale bar: 50 nm. (b) EDS spectra measured at the p-AlGaN surface, to confirm the atomic percentage


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ratio of Al and Ga atoms in the p-AlxGa1-xN contact layer. (c) Photoluminescence spectrum of a 280 nm AlGaN-based DUV LED wafer measured using a 245 nm KrF excimer laser.

Figure 2. (a) Schematic illustrations of lateral-type AlGaN-based DUV LED with AlN GE. The 55 metal dots consisting of Cr/Ni (2/15 nm) were uniformly deposited to form CFs without fieldinduced damage using the PEBD process, under a Ni/Au p-metal pad. The magnified image on the right illustrates that the current can be injected through the CFs formed in the AlN GE after the PEBD process. (b) Applied voltage pulse (black solid line) and current transient response pulse (red solid line) in the AlN GE, as recorded by oscilloscope. (c) Typical I−V curves measured for different TLM pattern spacings (5, 10, 15, 20, and 25 µm) of AlN GEs after PEBD process. (d) Transmittance spectra measured for 15-nm AlN electrodes before (green line) and after (orange line) PEBD process, and 10-nm reference ITO layers (blue line) deposited on quartz substrates.

Figure 3. Conduction mechanism in proposed AlN GE. C-AFM images obtained for AlN layer (a) before and (b) after PEBD process at 3 V with a compliance current of 10 nA. The current is distinguished by the color scale bar displayed on the right side of the CAFM images. HAADFSTEM images of the AlGaN-based DUV LED using a p-Al0.4Ga0.6N contact layer with AlN/Ni(Cr) metal obtained (c) before and (d) after the PEBD process. Scale bar: 40 nm. Combined Ni-L, Al-K, and N-K EELS edge mapping images extracted from C1 and D1 (inset of (c), (d)) (e) before and (f) after PEBD process, respectively. Corresponding positions of C1 and D1 in HAADF-STEM image are indicated by solid orange lines in (c), (d), respectively.


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Segregated Ni-L, Al-K, and N-K EELS edge mapping images were extracted from C1 and D1; (g), (h), (i) before and (j), (k), (l) after PEBD process, respectively.

Figure 4. Ohmic mechanism at the interface between p-Al0.4Ga0.6N and AlN layers before and after PEBD process. XPS analyses for (a) Ni 2p core level and (b) Ga 2p core level measured at the p-Al0.4Ga0.6N surface before (blue line)/after (red line) PEBD process and annealed 10-nm ITO layers after removal (green line).

Figure 5. Electrical and optical characteristics of AlGaN-based lateral-type, top-emitting DUV LEDs. (a) Typical light-output power (solid line)−current−voltage (open symbols) characteristics, (b) reverse leakage current versus reverse voltage, and (c) electroluminescence spectra versus wavelength of 280-nm LEDs with AlN GEs after PEBD process (red), 10-nm reference ITO (navy), and typical Ni/Au (50/150 nm) contact without TCEs (green). Inset: light emission photograph measured at 100 mA for 280-nm LEDs with AlN GEs after PEBD process. (d) External quantum efficiency versus injection current measured for 280-nm LEDs with AlN GEs after PEBD process (red) and 10-nm reference ITO layers (navy). Inset: schematic illustration of packaged sample using Al reflector cup and actual photograph of sample after packaging.


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Figure 1

Figure 2


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Figure 3

Figure 4


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Figure 5


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Table 1. Performance parameters for the fabricated lateral-type DUV LEDs. Forward Voltage (VF), Reverse Leakage Current (IL), Light Output Power (PO), EQE, Specific Contact Resistance (ρC), and Transmittance Electrodes

10-nm-thick ITO

VF at 20 mA (V)

13.2

IL at -10 V (nA)

1.45 at 100 mA

-400

7.49 at 100 mA (416% ↑, ITO)

(42% ↓, ITO) (33% ↓)

2.78 at 30 mA (15% ↑, Ni/Au)

.

2.42 at 30 mA

(7% ↓, Ni/Au)

transmittance ρC (Ω cm2)

(%, max)

-600

7.7 CF-AlN GE

EQE PO (mW)

at 280 nm (%)

0.54

Non-Ohmic

64

2.81 (420% ↑)

2.6 × 10-2

95.6

.

3.3 × 10-2

.

Ni/Au 8.3 (without TCE)

Table 1


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Table of Contents graphic


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Highly Efficient Deep-UV Light-Emitting Diodes Using AlN-Based

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