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Illumination & Displays

UV-transparent glass electrodes for high-efficiency nitride-based LEDs

Aluminum gallium nitride LEDs with glass electrodes can deliver high luminous efficiency in the UV region owing to low contact resistance and high transmittance.
26 June 2017, SPIE Newsroom. DOI: 10.1117/2.1201704.006869

Nitride-based UV LEDs are promising replacements for conventional UV lamps1 because of their higher energy efficiency, longer lifetime, and greater reliability. However, the external quantum efficiency of UV LEDs is currently much lower than that of visible LEDs. This difference is mainly due to the light absorption that occurs in the p-type gallium nitride (p-GaN) contact layer and the metal electrode layers. In deep-UV LEDs, absorption becomes an even greater problem.2

One possible solution to this fundamental issue is to obtain a direct ohmic contact to p-type aluminum gallium nitride (p-AlGaN). This can be achieved using UV-transparent conductive electrodes (TCEs), thus avoiding absorption and increasing device efficiency. Prior to our work, no solution had been found to overcoming the trade-off between high electrical conductivity and high optical transmittance. Indeed, these properties have generally been considered mutually exclusive. In recent years, some groups have reported the use of metal nanowires, metal nanomeshes, graphene, carbon nanotubes, metal oxides, and conductive polymers as replacements for conventional indium tin oxide (ITO),3, 4 but these efforts are still under way.

We have proposed a universal method for producing TCEs using wide bandgap (WB) materials such as silicon oxides and nitrides.5 Glass-based TCEs (G-TCEs) enable effective current injection from a metal to a WB semiconductor (e.g., p-type AlGaN under bias) via conducting filaments (CFs) that are formed by the electrical breakdown (EBD) that occurs in the G-TCE. In these devices, high transmittance is maintained even in the deep-UV region (i.e., more than 95% at a wavelength of 280nm). To achieve this, we developed a G-TCE using aluminum nitride (AlN) as a unique solution and implemented the resultant electrode within an LED structure. We also demonstrated its feasibility for use in such a device and its superiority in terms of both blue and near-UV LEDs, particularly those with a p-AlGaN top layer, compared with a conventional ITO layer.6

Figure 1(a) shows a schematic of our lateral-type AlGaN LEDs with AlN-based G-TCEs. We designed a tripod-shaped p-metal chromium/nickel (Cr/Ni) pad, with 55 metal dots beneath it, to enable the formation of CFs during the EBD process. This pad also enables us to easily observe the current-spreading effect under low-current operation. Figure 1(a) shows that current can be injected via CFs that form in the AlN TCE and can then spread out via thin indium tin oxide (ITO) buffer layers.


Figure 1. (a) Schematic view of a lateral-type aluminum gallium nitride—(Al)GaN—based LED with aluminum nitride (AlN)-based glass transparent conducting electrodes (G-TCEs), after electrical breakdown (EBD). This magnified image shows that current can be injected via conductive filaments (CFs), which are formed in the AlN layer after EBD, and can subsequently spread through the device via thin indium-tin-oxide (ITO) buffer layers. (b) Current-voltage characteristics measured for the AlN-based G-TCE, before (red) and after (blue) EBD. The inset shows conductive atomic force microscopy images taken for the AlN top layer before (left) and after (right) EBD at 1V with a compliance current of 10nA.

We measured the current–voltage (I–V) curves of our devices (i.e., AlN-based G-TCEs deposited on LED wafers) before and after EBD. These experiments were performed in air and at room temperature. We swept direct-current (DC) voltage from 0 to 6V (with a ramp rate of 0.1V per second) using a two-point-probe contact between the metal pad and the ITO buffer layer. This led to the formation of CFs in the G-TCE. Initially, the TCE maintained a high resistance state (HRS), but we observed a steep increase in the current level at ∼4V (i.e., the voltage at which EBD occurs). To prevent any damage to the device, we imposed a compliance current of 10mA. In the second DC voltage sweep, we found that the current level increased linearly, reaching a maximum compliance current (10mA) at below 0.5V. This abrupt transition (from an HRS to a low-resistance state) occurs as a result of the formation of CFs in the G-TCE, which causes the current level to increase from a few pA to ∼100mA at 1V. We also obtained conductive atomic force microscopy images at 1V from the AlN-based G-TCE before and after the EBD, and after removal of the Cr/Ni pad, as shown in the inset of Figure 1(b). Our results indicate that the CFs are formed stably in the AlN-based G-TCE after the EBD process.

Our investigations determined that both the lower forward voltage and reverse leakage current were reduced (by 3.7 and 30.2%, respectively) for the LEDs with AlN-based G-TCEs when compared with those based on reference ITOs: see Figure 2(a). We also observed a higher light output power (by 8.6%), as shown in Figure 2(b). Finally, we measured the light emission images for LEDs with AlN-based G-TCEs, 100nm-thick ITOs, and 10nm-thick ITOs at 20mA and 50mA: see Figure 2(c). Of these, we observed the brightest light emission in LEDs with AlN-based G-TCEs.


Figure 2. The electrical and performance characteristics of AlGaN LEDs with AlN-based G-TCEs after EBD (red), and AlGaN LEDs based on 100nm- and 10nm-thick ITO layers (blue and green, respectively). All are deposited on p-AlGaN-terminated 365nm wafers. (a) Typical light output power–current–voltage characteristics. (b) Electroluminescence (EL) spectra versus wavelength and reverse leakage currents (inset). (c) Microscopic light-emission photographs, measured at different currents (20 and 50mA). The linear color scale for intensity distribution is shown to the right. a.u.: Arbitrary units.

In summary, we have proposed a new concept of G-TCEs, based on an AlN-based thin film, which achieve high transmittance and low contact resistance. We have also successfully demonstrated its validity at the device level by applying the G-TCEs in nitride-based LEDs that operate from the visible to the near-UV range (365–450nm). Our G-TCE concept is a unique approach capable of making direct ohmic contact with p-AlGaN while maintaining high optical transmittance in even the UV region. We believe that this technology has wide application in organic/inorganic WB semiconductors, and will thus enable performance breakthroughs in a wide range of optoelectronic devices (e.g., deep-UV LEDs, solid-state lighting, displays, and solar cells). Our recent work on the application of this approach for the development of deep-UV LEDs will be reported shortly.

The authors acknowledge financial support from the National Research Foundation of Korea grant, funded by the Korean government (2016R1A3B1908249).


Tae Geun Kim, Tae Ho Lee
School of Electrical Engineering
Korea University
Seoul, Republic of Korea

Tae Geun Kim is a professor of electrical engineering. His research is in the field of display and solid-state lighting devices such as transparent electrodes, oxide thin-film transistors, and LEDs.


References:
1. A. Khan, K. Balakrishnan, T. Katona, Ultraviolet light-emitting diodes based on group three nitrides, Nat. Photon. 2, p. 77-84, 2008. doi:10.1038/nphoton.2007.293
2. Y. Muramoto, M. Kimura, S. Nouda, Development and future of ultraviolet light-emitting diodes: UV-LED will replace the UV lamp, Semicond. Sci. Technol. 29, p. 084004, 2014. doi:10.1088/0268-1242/29/8/084004
3. G. Zhao, W. Wang, T.-S. Bae, S.-G. Lee, C. Mun, S. Lee, H. Yu, G.-H. Lee, M. Song, J. Yun, Stable ultrathin partially oxidized copper film electrode for highly efficient flexible solar cells, Nat. Commun. 6, p. 8830, 2015. doi:10.1038/ncomms9830
4. Y. Ahn, Y. Jeong, D. Lee, Y. Lee, Copper nanowire-graphene core-shell nanostructure for highly stable transparent conducting electrodes, ACS Nano 9, p. 3125-3133, 2015. doi:10.1021/acsnano.5b00053
5. H. D. Kim, H. M. An, K. H. Kim, S. J. Kim, C. S. Kim, J. Cho, E. F. Schubert, T. G. Kim, A universal method of producing transparent electrodes using wide-bandgap materials, Adv. Funct. Mater. 24, p. 1575-1581, 2014. doi:10.1002/adfm.201301697
6. T. H. Lee, K. H. Kim, B. R. Lee, J. H. Park, E. F. Schubert, T. G. Kim, Glass-based transparent conductive electrode: its application to visible-to-ultraviolet light-emitting diodes, ACS Appl. Mater. Interfaces 8, p. 35668-35677, 2016. doi:10.1021/acsami.6b12767