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AlN/ITO-Based Hybrid Electrodes with Conducting Filaments: Its Application to Ultraviolet Light-Emitting Diodes Kyeong Heon Kim, Tae Ho Lee, and Tae Geun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06362 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 10, 2017

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ACS Applied Materials & Interfaces

AlN/ITO-Based Hybrid Electrodes with Conducting Filaments: Its Application to Ultraviolet LightEmitting Diodes Kyeong Heon Kim†, Tae Ho Lee†, and Tae Geun Kim*

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

Keywords: AlN rod array, conducting filament, transparent conductive electrode, electrical breakdown ultraviolet light-emitting diode †

These authors contributed equally to this work.

*Corresponding author: E-mail: [email protected], Phone: +82-2-3290-3255, Fax: +82-2-924-5119

ABSTRACT A hybrid-type transparent conductive electrode (H-TCE) structure comprising an AlN rod array with conducting filaments (CFs) and indium tin oxide (ITO) films is proposed to improve both current injection and distribution as well as optical transmittance in the UV region. These CFs, generated in UV-transparent AlN rod areas using an electric field, can be used as conducting paths for carrier injection from a metal to a semiconductor such as p(Al)GaN, which allows perfect ohmic behavior with high transmittance (> 95% at 365 nm) to be obtained. In addition, conduction across AlN rods and ohmic conduction mechanisms are investigated by analyzing AlN rods and AlN rod/p-AlGaN film interfaces. We apply these H-TCEs to three near UV light-emitting diodes (LEDs) (385 nm LEDs with p-GaN and p-AlGaN terminated surfaces and 365 nm LED with p-AlGaN terminated surface). We confirm that the light power outputs increase by 66%, 79%, and 103%, whereas the forward voltages reduce by 5.6%, 10.2%, and 8.6% for 385 nm p-GaN terminated, 385 nm p-AlGaN terminated, and 365 nm p-AlGaN terminated LEDs with H-TCEs, respectively, compared to LEDs with reference ITOs.

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1. INTRODUCTION

The use of ultraviolet (UV) light-emitting diodes (LEDs) is rapidly growing in the areas of air/water sterilization, phototherapy, automotive industry, UV, or general lighting. However, the external quantum efficiency (EQE) of UV LEDs is still much lower than that of blue LEDs, limiting its high-power application.1-6 Development of UVtransparent conductive electrodes (TCE) is one of the critical problems that needs to be solved to enhance the EQE of AlGaN-based, cost-effective, top-emitting UV LEDs. There have been considerable efforts to develop such TCEs using various materials and/or structures such as conductive polymers,7 metal nanowires,8 and carbon materials.9-11 However, these efforts have not solved the problem due to an existing trade-off between optical transmittance and electrical conductivity.12-15 This challenge becomes more serious for deeper UV LEDs. In addition to low transmittance, deeper UV LEDs require higher Al contents in p-AlGaN contact layers,16-17 making it difficult to obtain ohmic contacts because of large work-function difference (WFD) with metal18-21 and impurity doping problems. We have reported a method to form direct ohmic contacts on p-AlGaN using wide-bandgap (WB) TCE materials.22 Using this method, we can achieve current injection via a conducting path known as a conducting filament (CF), which is formed in a WB thin film between metal and p-AlGaN contact layers. CFs originate from O or N vacancies aligned in WB thin films under electric fields.23-27 Greiner at el. reported that the energy levels between two materials can be aligned by manipulating the content or distribution of O vacancies in the WB film during the deposition.28-30 By analogy, we tried to control the activation energies (or effective work functions) of O or N vacancies in WB thin films using an electrical breakdown (EBD) process, which could reduce WFD between a metal and semiconductor.22 Using this method, we developed a direct ohmic contact to a semiconductor and successfully applied it to both lateral- and vertical-type (Al)GaN LEDs.31-34 However, in lateral-type LEDs, particularly AlGaN-based UV LEDs, both current injection and distribution effect in the p-AlGaN contact layer were limited due to its high resistivity and large WFD. Hence, we introduced a thin indium tin oxide (ITO) buffer layer between AlN TCE and p-(Al)GaN layers for both effective current distribution and damage prevention during the EBD process.34 This method was nicely working for blue and near-UV (NUV) LEDs, but no longer effective below 365 nm owing to increasing light absorption via ITO buffer layers. Therefore, we need to develop another methods that can form ohmic contact directly on the p-AlGaN layer.


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In this study, we proposed a hybrid-type TCE (H-TCE) based on AlN rods and current spreading layer (CSL) such as ITO to enhance the current injection and distribution efficiency. Here, the injection is enhanced by direct ohmic contact on the p-AlGaN layer through the CFs formed in the AlN rods while the distribution is enhanced through the CSL in the surrounding of the AlN rods. Then, this H-TCE was applied to three lateral-type NUV LEDs with different contact layers (i.e., p-GaN terminated 385 nm wafer, p-Al0.05GaN terminated 385 nm wafer, and pAl0.1GaN terminated 365 nm wafer) and compared their performance directly to that of reference LEDs with ITO electrodes. In addition, we investigated the conduction mechanism across AlN rods and ohmic conduction mechanism at the interface between AlN rod and p-(Al)GaN using conductive atomic force microscopy (C-AFM), electron energy-loss spectroscopy (EELS) based on scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS).

2. RESULTS AND DISCUSSION

Figure 1a shows a schematic overview of a lateral-type (Al)GaN LED with a proposed H-TCE. A tripod-shaped pmetal pad (Cr/Ni/Au) was used to observe the current distribution effect under low current operation, as illustrated in an H-TCE cross-section on the right side of figure 1a. The proposed H-TCE is composed of AlN rod arrays with CFs and surrounding ITO films for current distribution at the p-(Al)GaN surface. Using this H-TCE structure, we can reduce the area for EBD to decrease the risk of the electrical damage to multi-quantum-well (MQW) active layers while maintaining the current distribution via surrounding ITO films. Figure 1b shows the STEM image for 365 nm AlGaN LED with a proposed H-TCE. Detailed information on the epitaxial wafers used in this study is provided in Figure S1 (Supporting Information). In the AlN rod region, a 10-nm-thick ITO layer was deposited on an 8-nm-thick AlN rod in the form of AlN/ITO to reduce the damage that may be caused by probe tips during the EBD process, thereby lowering EBD voltage (VEBD) (Figure 1b). The EBD process was performed using an automatic probing system not only to improve the process stability but also to reduce the process time.

In this study, we prepared four different types of TCEs including ITO (reference 1), AlN film with CFs (reference 2), type 1 (H-TCE consisting of AlN rod arrays with CFs formed along the p-metal pad line and surrounding ITO film), and type 2 (proposed H-TCE consisting of AlN rod arrays with CFs formed over the entire surface and surrounding ITO film) and applied them to three (Al)GaN-based NUV LEDs to validate our method at the device


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level, as shown in Figure S2a (Supporting Information). All TCEs were fabricated on a single 2-inch epitaxial wafer using metalorganic chemical vapor deposition under the same growth conditions. The size of the LED chip was 390 µm × 390 µm, but the effective emission area was 280 µm × 280 µm, as shown in Figure S2b (Supporting Information). Details of the manufacturing process for three (Al)GaN LEDs with H-TCEs are presented in Figure S3 (Supporting Information).

First, we measured current–voltage (I–V) characteristics of the AlN/ITO rod film, which was deposited on three (Al)GaN LED wafers, before and after EBD (Figure S4, Supporting Information) to observe its electrical state. During the EBD process, the direct current (DC) voltage was swept under 25 V using a two-point probe contact between the AlN/ITO rod and p-(Al)GaN layer to form the CFs in the AlN/ITO film. As an example, for the p-GaN terminated 385 nm wafer, the AlN/ITO rod film was initially in a high-resistance state (HRS). However, the current level was abruptly increased at ~15 V (VEBD). To prevent any damage that might occur in this process, a 10 mA compliance current was imposed. Then, the current linearly increased until it reached a maximum compliance current at ~2 V, as shown in Figure S4a (Supporting Information). This abrupt transition from the HRS to a lowresistance state (LRS) is a result of CFs formation in the AIN/ITO rod. The current level after EBD increased from a few picoamperes to ~6 mA at 1 V. In addition, we measured and extrapolated the variation of the LRS currents at 1 V as a function of time to estimate the long-haul stability of the LRS (Figure S4b, Supporting Information). It was confirmed that the LRS could be maintained up to ten years, which is sufficient for device applications. The same I– V curve and the retention properties of the LRS at 1 V were also measured for the p-AlGaN terminated 385 and 365 nm wafers (Figure S4c–f, Supporting Information). Then, we measured typical I–V curves of the AlN/ITO film before and after EBD (Figure 2) and the reference ITO (Figure S5, Supporting Information) on 385 nm LEDs with p-GaN and p-AlGaN terminated surfaces and 365 nm LED with p-AlGaN terminated surface, for different transmission line model (TLM) spacings. Nearly no current flow was observed for all samples before EBD due to insulating properties of AlN; however, all samples exhibited excellent ohmic properties after EBD in the range of ±1 V (Figure 2a–c). In addition, we plotted the I–V characteristics for the reference ITOs for comparison, as shown in Figure S5a–c (Supporting Information). Next, the specific contact resistance (ρC) of the AlN/ITO film after EBD and reference ITO on p-(Al)GaN was calculated using the TLM plot (Figure S6, Supporting Information), and it was found that the ρC value of the AlN/ITO film after EBD is much lower than that of the reference ITO at each


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wavelength. Table 1 shows the ρC values calculated for each sample. The thickness of the reference ITO was determined for anti-reflective coatings at 385 and 365 nm using the following equation:

d=λ/4n

(1)

Here, n is the refractive index of ITO (~2.0). Using equation (1), ITO thicknesses (d) at target wavelengths (λ) of 385 and 365 nm were calculated to be 48 and 45 nm, respectively. However, we chose the optimal thickness of ITO to be 50 and 40 nm at each wavelength by considering the sheet resistances and transmittance in the NUV region. The sheet resistances of the 50-nm and 40-nm-thick ITO films were in the range of 250–300 Ω/□, while the transmittances of such films were 82.3% and 82.7% at 385 nm and 365 nm, respectively, as shown in Figure 2d. On the other hand, the transmittance of the AlN/ITO (8/10 nm) film was 95% or higher at 365 nm. The transmittance was measured for the samples deposited on the quartz substrates in the wavelength range between 200 nm and 700 nm using a Lambda 35 UV/VIS Spectrometer. The AlN thin film was deposited using a radio-frequency (RF) magnetron sputtering system and the ITO was deposited using an electron-beam (E-beam) evaporator.

H-TCEs consisting of AlN/ITO rod arrays and surrounding ITO CSL were used in conduction studies. To understand both the conduction mechanism across AlN rods and ohmic conduction mechanism in p-(Al)GaN LEDs with the H-TCEs, we analyzed one of the LED samples (p-AlGaN terminated 365 nm LED) before and after EBD, particularly at the interface between p-AlGaN and AlN/ITO rod, using the C-AFM, EELS of the STEM,35 and XPS analyses. First, we examined the rupture and formation of CFs in the AlN/ITO TCE before and after EBD using CAFM and STEM to explain the conduction mechanism in the TCE film. The C-AFM images were obtained at 1 V at the AlN/ITO surface before and after EBD, respectively, as shown in Figures 3a and b. According to the images, peak-shaped CFs are formed with some intervals as a result of EBD. In addition, CFs are only formed in the AlN/ITO rod film, not in the ITO CSL. During the C-AFM measurement, a compliance current of 10 nA was imposed to prevent any damage due to a nano-sized conductive cantilever. Interestingly, CFs were created with an interval of ~15 µm, which is similar to the periodicity of AlN rod array in the H-TCEs that were fabricated using photomasks (Figure S2, Supporting Information).

In addition, we used STEM to obtain cross-sectional images of the AlN/ITO rod films before and after EBD and observe changes in the elemental composition and electronic structure (Figure 3c). The bright field (BF) STEM


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image obtained before EBD shows uniform lattice structures of top ITO layers and uniform distributions in AlN layers, whereas BF STEM image obtained after EBD shows nonuniform composition distributions in AlN layers along the dotted orange line. Here, Al atoms are dark and N atoms are bright, as shown in the image after EBD in Figure 3c.

We also observed atomic composition using the EELS mapping and analysis (Figures 3d, e). The EELS is an analytical technique that measures the change in kinetic energy of electrons after they have interacted with a specimen. Note that CFs are known to originate from either O or N vacancies aligned in a branchlike shape under the applied bias.36-40

Spatially resolved EELS experiments were carried out in a STEM mode not only to measure the atomic profile of N and Al atoms in AlN/ITO film but also to detect EBD-induced changes. We analyzed the AlN/ITO rod film after EBD in the local region indicated by a dotted red square in Figure 3c. Figure 3d shows STEM–EELS mapping spectra with Al–K and N–L edges, respectively. The images of Al- and N-rich areas show that Al is uniformly distributed throughout the sample, whereas N is non-uniformly distributed in the AlN layer, in accordance with the fact that N vacancies are the origin of CFs. This indicates that N atoms of Al-rich area (conducting path) are moved to the sides of AlN layers (N-rich area) after EBD. In addition, a presence of distinct Al L-shell and N K-shell peaks in the EELS spectra clearly shows the difference between Al- and N-rich areas (Figure 3e). As pointed by black arrows, the EELS spectrum of N-rich area (red line in Figure 3e) corresponds to the right side of the dotted red square in Figure 3c, whereas that of Al-rich area (green line) corresponds to its left side. In particular, the spectrum for Al-rich area has only an Al peak (near 300 eV) because of lost N atoms (N vacancies). In contrast, both Al peak and N peak (near 408 eV) are present in the spectrum for N-rich area (Figure 3e).

This result indicates that N vacancies were formed in the AlN film during the EBD processing. In particular, the ITO lattice above Al-rich areas is clearly changed (area between yellow dotted lines in Figure 3c). To understand the process mechanism at the interface between AlN/ITO and p-AlGaN layers and in the AlN/ITO film, we investigated changes in relative atomic concentrations of Ga and N by using XPS analysis before and after EBD, respectively. (Figures 4a, b, and Figure S7, Supporting Information). Ga atoms in p-GaN diffuse toward the AlN film, creating Ga vacancies (serving as acceptors) in p-AlGaN contact layers after EBD (Figure 4a). Moreover, we observed a change in N content in the AlN film caused by EBD, which proves that CFs are formed via N vacancies as a result of EBD.


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Based on the XPS depth profile analyses, N concentration in the AlN thin film was reduced by ~2% after EBD. Figure 4b shows the XPS spectra measured for Ga 2p core levels of the same sample (365 nm LED) before and after EBD. The XPS core-level peaks were fitted with a Shirley-type background and Lorentzian–Doniac–Sunsic curves convoluted with a Gaussian profile. The Ga 2p core levels originate from Ga−N and Ga−O bonds. As a result of EBD process, the Ga 2p core level of the sample shifts toward the lower binding energy by 0.5 eV. Therefore, EBD shifted the surface Fermi level toward the valence band edge, which, in turn, reduced the band bending of p-AlGaN. This blueshift points to a decrease of the Schottky barrier height (SBH).24,41 Based on these analyses, we conclude that both the increase in the number of Ga vacancies in p-AlGaN contact layers and the decrease in the SBH between the AlN/ITO rods and p-AlGaN via Ga out-diffusion after EBD might result in the decreased contact resistance of the proposed H-TCE on p-AlGaN.

Finally, we compared the performances of the three LEDs with type 1 and type 2 H-TCEs and reference ITOs (40 and 50 nm) in each set of three LEDs: p-GaN terminated 385 nm LED, p-AlGaN terminated 385 nm LED, and pAlGaN terminated 365 nm LED (Figures 5 and S8, Supporting Information). The LEDs with AlN film TCEs were excluded from this comparison, because they did not operate properly owing to insufficient current distribution over the surface, as shown in Figure S8a (Supporting Information). Figures 5a–c show light output power–current– voltage (L–I–V) curves, electroluminescence (EL) spectra, and images of light emission under low and high current injection (Figure S8a, Supporting Information) for p-GaN terminated 385 nm lateral-type LEDs with type 1 and 2 H-TCEs and reference ITOs (50 nm). The I–V curves (Figure 5a) show lower forward voltages by 2.8% and 5.6% for the LEDs with H-TCEs of type 1 and 2, respectively, as compared to that of the LED with 50-nm-thick reference ITO. In addition, we observed higher light output powers by 28% and 66% for the LEDs with H-TCEs of type 1 and 2, respectively, as compared to that of the reference devices. These improvements are attributed to the enhanced current injection and distribution via ITO CSL as well as higher transmittance (98.2% at 385 nm) via proposed HTCEs. In order to investigate the current injection and distribution effect, we captured light-emission images for the three samples and observed brighter images for the LED with type 2 H-TCE at 5 mA and 10 mA compared to those for other LEDs used in this study.

Figures 5d–f show the L–I–V curves, EL spectra, and images of light emission under low and high current injection (Figure S8a, Supporting Information) obtained for the same three p-AlGaN terminated LEDs. In Figure 5d,


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we observe lower forward voltages by 4.1% and 10.2% and higher light output powers by 50% and 79% for the LEDs with H-TCEs of type 1 and 2, respectively, as compared to those of the LEDs with 50-nm-thick reference ITOs. These results are similar to those obtained for the p-GaN terminated 385 nm LEDs. In addition, we obtained brighter light emission images for the LED with type 2 H-TCE at 5 mA and 10 mA as compared to the images of all other LEDs in this study. Higher performance gains for p-AlGaN terminated rather than p-GaN terminated 385 nm LEDs may be due to a lower light absorption via p-AlGaN contact layers.

Figures 5g–i show the L–I–V curves, EL spectra, and images of light emission under low and high current injection (Figure S8a, Supporting Information) obtained for the p-AlGaN terminated 365 nm LEDs with type 1 and 2 H-TCEs and reference ITO (40 nm). We observe lower forward voltages by 3.4% and 8.6% and higher light output powers by 59% and 103% for the LEDs with H-TCEs of type 1 and 2, respectively, as compared to those of the LED with 40-nm-thick reference ITO. Brighter light emission images were obtained for the LED with type 2 H-TCE at both 25 mA and 50 mA among the images for all the samples used in this study. Detailed performance characteristics (i.e., forward voltage (VF), light-output power (PO), ρC, and transmittance) of each LED are summarized in Table 1. Interestingly, we observe that the AlN/ITO rod region is much brighter than that of the surrounding ITO region after EBD for both 385 and 365 nm LEDs at both 5 mA and 25 mA (Figures S8b, c, Supporting Information). This may be the result of a lower contact resistance (or better current injection and distribution) and higher transmittance in the UV region of the proposed H-TCE on p-AlGaN as compared to those of the conventional ITO. This local current injection feature (using CFs formed in highly transparent AlN rod arrays) can be utilized in some other applications such as micro-LEDs that need to improve the fill factor.

3. CONCLUSION

In summary, we proposed a H-TCE comprising AlN/ITO rod arrays for current injection and surrounding ITO films for current distribution that can provide a high UV transmittance and low contact resistance on p-(Al)GaN and successfully demonstrated its performance at the device level by applying it to (Al)GaN-based 385 and 365 nm NUV LEDs. By filling the surroundings of the AlN/ITO rod array with 60-nm-thick ITO CSL layers, we were able to enhance the uniform light emission. As a result, ρC of the AlN/ITO rods was found to be lower than that of the reference ITO for all samples. To understand these observations, we investigated the ohmic conduction mechanisms in these devices using C-AFM, STEM, and XPS and concluded that the improvement in the contact properties might


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be the result of both the effective current injection into the p-(Al)GaN layer via CFs across the AlN/ITO rods and increase in a number of Ga vacancies (serving as acceptors) in p-(Al)GaN contact layers during the EBD process. The optical transmittance of the AlN/ITO rod film was also much higher (97.9–98.1%) than that of the ITO (78.8– 86.0%) in a wavelength range between 365 nm and 385 nm. Finally, we applied these H-TCEs to three (Al)GaNbased NUV LEDs with different contact layers and confirmed that all LEDs with H-TCEs exhibited a much higher light output power and lower forward voltages than those of LEDs with ITOs. In particular, p-AlGaN terminated UV LED exhibited better optical properties than p-GaN terminated one due to a reduced light absorption via p-AlGaN contact layers.

4. EXPERIMENTAL SECTION

Fabrication of (Al)GaN-based lateral-type LEDs. We prepared three (Al)GaN LED wafers (p-GaN terminated 385 nm wafer, p-Al0.05GaN terminated 385 nm wafer, and p-Al0.1GaN terminated 365 nm wafer), grown by metalorganic chemical vapor deposition on c-plane sapphire substrates, as shown in Figure S1a (Supporting Information) and fabricated three NUV LEDs with different types of TCEs (type 1, an H-TCE with AlN/ITO rods only below the p-metal electrode; type 2, an H-TCE with AlN/ITO rods over the entire TCE areas; reference ITOs: 40 nm for 365 nm LED, 50 nm for 385 nm LED) on the same wafers. The space between AlN/ITO rods was filled with 60-nmthick ITO films for both samples. For LED fabrication, we used a standard photolithography and an inductively coupled plasma reactive-ion etching to form isolated mesa structures in all LED samples. Then, to remove any organic residues from the substrates, we cleaned the samples with a piranha solution (1:1 mixture of H2SO4:H2O2) for 10 min. Next, the samples were dipped in 30:1 buffered oxide etchants for 10 min to get rid of the surface oxide. Then, we deposited 8-nm-thick AlN-rod films with isolated mesa structures on p-(Al)GaN layers using RFsputtering in an Ar-N2 gas atmosphere at a base pressure of ~2 × 10-7 Torr, a working pressure of ~5 × 10-3 Torr, and a very low power of 5 W. A gas of N2 and Ar with a 1:1 ratio was supplied at about 5 sccm during the deposition. Next, 10-nm-thick ITO layers on 8-nm-thick AlN rods with isolated masa structures were deposited using E-beam evaporation. Then, EBD processing was performed to form the CFs in the AlN/ITO rods by applying voltages between AlN/ITO rods and p-(Al)GaN layers. Next, 40- and 50-nm-thick ITO overlays were deposited by E-beam evaporation, and subsequently annealed using rapid thermal annealing at 650 °C for 60 s. Finally, Cr/Ni/Au


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multilayer metallization was deposited using E-beam evaporation to form p- and n-type electrodes, respectively. The chip size was 390 µm × 390 µm (Figure S2b, Supporting Information). For comparison, we also fabricated (Al)GaN LEDs with 40- and 50-nm-thick ITOs and Cr/Ni/Au electrodes using the same wafer in the same fabrication conditions.

Electrical and optical characterization. First, ohmic contact properties of the proposed H-TCE deposited on p(Al)GaN layers were investigated using a TLM method. To fabricate the TLM patterns, 8-nm-thick AlN and 10-nmthick ITO layers were deposited with spacing varied from 5 µm to 25 µm, which was followed by the EBD processing between AlN/ITO and p-(Al)GaN layers. Then, Cr/Ni/Au multilayer metallization was deposited as ptype electrode. For comparison, we also fabricated TLM patterns consisting of 40- and 50-nm-thick ITO layers and Cr/Ni/Au electrodes using the same wafer and fabrication conditions. We used a Keithley 4200 semiconductor parameter analyzer to measure the I–V curves in each TLM pattern. Next, to investigate the optical transmittance of the proposed H-TCE, we prepared 8-nm-thick AlN and 10-nm-thick ITO layers deposited on the quartz substrates using the E-beam evaporation and RF sputtering, respectively. The processing was performed under the same conditions as those used for LED fabrication. For comparison, we fabricated 40- and 50-nm-thick ITO layers that were deposited on the quartz substrates using E-beam evaporation in the same conditions. Then, we measured the optical transmittance of the samples in the wavelength range of 90–1100 nm using the Lambda 35 UV/VIS spectrometer. Finally, we measured the performance of each LED (i.e., VF, PO, ρC, and transmittance), using a LED measurement setup (PLATO, EtaMax Co., Ltd.). In particular, we measured the light output power and EL intensity from the top side of the LED using a Si photodiode connected to the optical power meter. Then, we used a photoemission microscope to capture light emission images from the surfaces of the LED chips. In addition, light emission images of the LED chip surface were displayed in the monitor using a charge-coupled device camera and processed using an image converter program.

Photo MASK design. The Cr photomasks were designed using computer-aided design program to fabricate two types of H-TCEs, as shown in Figure S2b (Supporting Information). The diameter of a single dot was 10 µm and the gap between dots was 5 µm. 55 dots were located along the metal electrode line, and 215 dots were located over the entire surface. The total LED chip size was 390 µm × 390 µm, but the size of the TCE region (effective emission area) was 280 µm × 280 µm.


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ACKNOWLEDGEMENTS

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (No. 2016R1A3B1908249). T.G.K. and K.H.K. conceived and designed the experiment. K.H.K. and T.H.L. prepared the samples and conducted the experiment. T.G.K. and K.H.K. analyzed the data and wrote the manuscript. T.G.K. supervised the experiments and contributed to manuscript preparation. All authors discussed the research progress and reviewed the manuscript.

SUPPORTING INFORMATION

Detailed three epilayer structures of (Al)GaN LEDs (Figure S1); Top view of the p-AlGaN terminated 365 nm LED with four different types of TCEs and two types of Cr photomasks (Figure S2); Schematic illustration of the fabrication process of lateral-type (Al)GaN LEDs with proposed H-TCEs (Figure S3); I-V curves and current retention properties (Figure S4); I-V curves using TLM patterns of p-type contact layer (Figure S5); Total resistance vs. pad spacing plots of p-type contact layer (Figure S6); XPS data before and after EBD (Figure S7); Lightemission images of three (Al)GaN LEDs with four different types of TCEs (Figure S8); Photographs of real emission images at 100 mA (Figure S9) are described in supporting information.

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Figure Captions Figure 1. (a) Schematic illustration of (Al)GaN-based lateral LED with proposed H-TCE consisting of AlN/ITO rods and surrounding ITO films. Tripod-shaped metal pad (Cr/Ni/Au), under which AlN/ITO rods with CFs are located, is used to observe the current distribution effect on p-(Al)GaN surface. A magnified view on the right shows H-TCE cross-section. (b) Cross-sectional STEM image of the p-AlGaN terminated 365 nm LED with 8-nmthick AlN/10-nm- thick ITO rod film.

Figure 2. Ohmic behavior between p-(Al)GaN and AlN/ITO rod films and optical transmittance of TCEs used in this study. Typical I–V curves measured before and after EBD for different TLM patterns of AlN/ITO films deposited on (a) p-GaN terminated 385 nm wafer, (b) p-AlGaN terminated 385 nm wafer, and on (c) p-AlGaN terminated 365 nm wafer, respectively. (d) Transmittance spectra measured for 8/10-nm-thick AlN/ITO film, 40-nmthick-ITO, and 50-nm-thick ITO films deposited on quartz substrates.

Figure 3. Conduction mechanism across AlN/ITO rod film. C-AFM images (a) before and (b) after EBD measured at 1 V with a 10 nA compliance current. The current level is distinguished by color scale bars displayed next to CAFM images. (c) STEM images before and after EBD. (d) Al-L and N-K edge EELS mapping images of the AlN layer after EBD corresponding to red dotted square in (c). (e) EELS spectra of the AlN layer after EBD corresponding to left (Al-rich) and right (N-rich) sides of the red dotted square in (c).

Figure 4. Ohmic conduction mechanism between p-AlGaN and AlN/ITO rod film. (a) XPS depth profiles of N and Ga before (solid line) and after (circles) EBD. (b) XPS spectra measured for Ga 2p core levels of p-GaN surface before (green circles) and after (red circles) EBD.


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Figure 5. Electrical and optical performance of the proposed UV LEDs. Typical L–I–V characteristics (a, d, g), EL intensity vs. wavelength (b, e, h), and microscopic light emission photographs (c, f, i) obtained for p-(Al)GaN LEDs with type 1 H-TCEs (green line), type 2 H-TCEs (red line), and reference ITOs (blue line) deposited on p-GaN terminated 385 nm LED, p-AlGaN terminated 385 nm LED, and p-AlGaN terminated 365 nm LED, respectively.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5.


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Table 1. Performance parameters for the fabricated LEDs. Wafer

Sample

Type 2 p -GaN terminated 385 nm LED

p -AlGaN terminated 385 nm LED

p -AlGaN terminated 365 nm LED

Type 1

VF at 20 mA [V]

PO at 200 mA [arb. Units]

3.4 (5.6 %↓)

0.053 (66 %↑)

3.5 (2.8 %↓)

0.041 (28 %↑)

50-nm-thick ITO

3.6

0.032

Type 2

4.4 (10.2 %↓)

0.025 (79 %↑)

Type 1

4.7 (4.1 %↓)

0.021 (50 %↑)

50-nm-thick ITO

4.9

0.014

Type 2

5.3 (8.6 %↓)

0.0069 (103 %↑)

Type 1

5.6 (3.4 %↓)

0.0054 (59 %↑)

40-nm-thick ITO

5.8

0.0034

࣋C [Ω·cm2]

Transmittance [%]

7.1 ☓ 10-4

98.1 at 385 nm

1.6 ☓ 10-3

82.3 at 385 nm

8.2 ☓ 10-4

98.1 at 385 nm

2.8 ☓ 10-3

82.3 at 385 nm

1.4 ☓ 10-3

97.9 at 365 nm

4.8 ☓ 10-3

82.7 at 365 nm

Table 1


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Graphic for manuscript


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ITO-Based Hybrid Electrodes with Conducting ... - ACS Publications

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