Near ultraviolet InGaN/AlGaN-based lightemitting diodes with highly reflective tin-doped indium oxide/Al-based reflectors Chang-Hoon Choi,1 Jaecheon Han,1 Jae-Seong Park,2 and Tae-Yeon Seong1,2,* 2

1 Department of Nanophotonics, Korea University, Seoul, 136-713, South Korea Department of Materials Science and Engineering, Korea University, Seoul, 136-713, South Korea * [email protected]

Abstract: The enhanced light output power of a InGaN/AlGaN-based lightemitting diodes (LEDs) using three different types of highly reflective Sndoped indium oxide (ITO)/Al-based p-type reflectors, namely, ITO/Al, Cudoped indium oxide (CIO)/s-ITO(sputtered)/Al, and Ag nano-dots(nAg)/CIO/s-ITO/Al, is presented. The ITO/Al-based reflectors exhibit lower reflectance (76 - 84% at 365 nm) than Al only reflector (91.1%). However, unlike Al only n-type contact, the ITO/Al-based contacts to p-GaN show good ohmic characteristics. Near-UV (365 nm) InGaN/AlGaN-based LEDs with ITO/Al, CIO/s-ITO/Al, and n-Ag/CIO/s-ITO/Al reflectors exhibit forward-bias voltages of 3.55, 3.48, and 3.34 V at 20 mA, respectively. The LEDs with the ITO/Al and CIO/s-ITO/Al reflectors exhibit 9.5% and 13.5% higher light output power (at 20 mA), respectively, than the LEDs with the n-Ag/CIO/s-ITO/Al reflector. The improved performance of near UV LEDs is attributed to the high reflectance and low contact resistivity of the ITO/Al-based reflectors, which are better than those of conventional Albased reflectors. ©2013 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (230.4040) Mirrors.

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#197615 - $15.00 USD Received 12 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026774 | OPTICS EXPRESS 26774

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1. Introduction AlGaN-based ultraviolet (UV) light-emitting diodes (LEDs) are of technological importance because of their potential applications in water purification and solid-state lighting [1–3]. However, UV LEDs usually show very low external quantum efficiency (EQE) and high forward voltage due to poor light extraction efficiency and high contact resistance. Thus, to enhance the EQE, light extraction should be increased significantly. For GaN-based flip-chip LEDs and vertical geometry LEDs, metal-based reflectors, such as Ag- and Al-based schemes, have been widely used so as to increase the light extraction [4–12]. Unlike Al reflector, however, the reflectance of Ag contact drops rapidly in the UV region. On the other hand, Al has a small work function and so serves as an electrode for n-type GaN [13]. Thus, to form Al-based ohmic contacts to p-GaN, a diffusion barrier was employed to hamper the indiffusion of Al into p-GaN. For instance, Song et al. [9] used an Ag/tin-doped indium oxide (ITO) barrier layer to form high-quality p-type Al-based ohmic reflectors for flip-chip LEDs. The Ag/ITO/Al contacts exhibited a specific contact resistance of 8.7 × 10−5 Ωcm2 and a reflectance of 85% at 460 nm, which are much better than those of oxidized Ni/Au schemes. LEDs fabricated with the annealed Ag/ITO/Al reflector gave forward-bias voltages of 3.29– 3.37 V at 20 mA. Furthermore, Lee et al. [11], investigating the effect of ITO surface conditions on the performance of ITO/Al ohmic reflectors, reported that the schemes produced a reflectance of 85% at 460 nm and a specific contact resistance of 2.03 × 10−3 Ω·cm2 after CF4 plasma-treatment. They attributed the improved electrical properties to the increase in the work function of ITO films through the formation of Al-F bonding. LEDs fabricated with the surface-treated reflectors showed 72% higher light output power at 60 mA than LEDs with untreated ITO/Al contacts. Recently, Takehara et al. [12], investigating a high-reflectivity electrode for 350 nm UV LEDs, reported that Al contact combined with an sputtered ITO interlayer exhibited high reflectance in the UV region and good contact characteristics. The UV LEDs fabricated with the ITO/Al reflector showed an operating voltage (5.2 V at 100 mA) similar to that (4.7 V) of a conventional Ni/Au contact and a high light output power comparable to that with Al only contacts. These results indicate that the development of optimized electrodes with low contact resistance and high reflectance in the UV region is crucial to the fabrication of high-external quantum efficiency UV LEDs. In this study, to reduce the forward bias voltage and to increase the output power of near UV InGaN/AlGaN-based LEDs, we investigated the addition effects of a Cu-doped indium oxide (CIO) layer, Ag nano-dots, and an ITO layer on the electrical and optical properties of Al

#197615 - $15.00 USD Received 12 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026774 | OPTICS EXPRESS 26775

only reflector. Near UV (365 nm) LEDs were fabricated with three different types of ITO/Albased reflectors and their performance was compared. 2. Experimental A metalorganic chemical vapor deposition system was used to grow near UV (365 nm) InGaN/AlGaN multiple quantum-well (MQW) LED structures on (0001) sapphire substrates. The LED structures consisted of a 2-nm-thick p-GaN:Mg layer, a 0.1-μm-thick p-AlGaN:Mg (na = 5 × 1017 cm−3) layer, a 20-nm-thick AlGaN electron blocking layer, a 100-nm-thick active layer, and a 200-nm-thick spreading layer, a 2.0-μm-thick n-AlGaN:Si (nd = 5 × 1018 cm−3) layer, and a 2.0-μm-thick undoped GaN layer on a sapphire substrate. Prior to metal deposition, all of the samples were treated with a diluted HCl (HCl: DI water = 1: 1) solution for 1 min, rinsed in DI water, and blown dry in a N2 stream. For LEDs fabricated with three different ITO-based ohmic contacts, mesa-shaped structures were defined by the standard photolithography and etched by inductively coupled plasma (ICP) etcher (OERIKON). For the first set, a 5-nm-thick Cu-doped indium oxide (CIO) layer was first e-beam evaporated on p-GaN using an In2O3 target containing 10 at% Cu, followed by the sputtering of a 10-nmthick ITO layer, which was annealed at 500°C for 1 min in air to form ohmic contact. Then, a 200-nm-thick Al layer was deposited by e-beam. The sample was referred to herein as a “CIO/s-ITO/Al reflector”. For the second, a 1-nm-thick Ag layer was first e-beam evaporated, followed by annealing at 500°C for 1 min in air to form Ag nano-dots. Then, CIO (5nm) and ITO (10 nm) layers were consecutively deposited by e-beam and sputtering, respectively, after which the sample was annealed at 500°C. Then, a 200-nm-thick Al reflector was deposited by e-beam. The sample was referred to herein as a “n-Ag/CIO/s-ITO/Al reflector”. For the third, a ITO (10 nm) layer was first deposited by e-beam, (followed by annealing at 500°C), after which a 200-nm-thick Al layer was deposited. The sample was referred to herein as an “ITO/Al reflector”. For n-type ohmic contacts, Cr/Ni/Au (25/25/50 nm) layers were used. The schematic diagrams of LEDs are shown in Fig. 1. Circular transfer length method (CTLM) patterns were defined by the standard photolithographic technique for measuring specific contact resistance. The outer radius of the CTLM patterns was fixed at 200 μm and the gap spacing between the outer and inner radii was varied from 5 to 40 μm. Current-voltage (I–V) measurements were carried out by a high-current source-measuring unit (Keithley 238). X-ray photoelectron spectroscopy (XPS, Sigma Probe model) was performed using an Al Kα X-ray source (1486.6 eV) in an UHV system in order to characterize the surface characteristics and to understand the improvement in the electrical properties. The optical outputs of UV-LED chips (500 × 250 μm2) were examined by means of a Newport dual channel powermeter.

Fig. 1. Schematic diagram of LED structures fabricated with (a) CIO/ITO/Al and (b) Ag nanodot/CIO/ITO/Al reflectors.

3. Results and discussion Figure 2 exhibits the reflectance of Al contacts with different ITO-based multilayers. The Al only reflector exhibits the highest reflectance across the whole wavelength region of 300 to 500 nm. The reflectance becomes significantly reduced when additional layers are introduced. #197615 - $15.00 USD Received 12 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026774 | OPTICS EXPRESS 26776

For example, the Al only, n-Ag/CIO/s-ITO/Al, CIO/s-ITO/Al, s-ITO/Al, and ITO/Al samples show a reflectance of 91.1, 76, 80.5, 84.3, and 77.5% at 365 nm, respectively. The Ag/CIO/sITO/Al reflector shows the lowest reflectance due to the Ag nano-dots (the inset in Fig. 2). The atomic force microscopy (AFM) result shows that the nano-dots is varied from 5 to 20 nm in size. All of the layers were e-beam-evaporated except for s-ITO that was sputterdeposited. Although the s-ITO/Al sample exhibits higher reflectance than the e-beamevaporated ITO/Al sample, it is non-ohmic due to plasma damage during sputtering (as shown in the inset of Fig. 3). Thus, e-beam-evaporated ITO (referred to herein as “ITO”) was used in this study. It is noted that the reflectances (at 365 nm) of our ITO/Al-based contacts are higher

Fig. 2. The reflectance of Al contacts with different ITO-based multilayers. All of the layers were e-beam evaporated except for s-ITO that was sputter-deposited. The inset shows an AFM image of Ag nano-dots formed on p-GaN.

than those of the previously reported Al-based contacts [9,11,14], but comparable with the sputtered ITO (10 nm)/Al contacts [12].

Fig. 3. The typical I-V characteristics of near-UV (365 nm) InGaN/AlGaN MQW LEDs fabricated with Al contacts with different ITO-based multilayers. The inset shows the currentvoltage characteristics of different ITO-based contacts.

Figure 3 exhibits the typical I-V characteristics of near-UV (365 nm) InGaN/AlGaN MQW LEDs fabricated with Al reflectors with different ITO-based layers. The LEDs with the n-Ag/CIO/s-ITO/Al reflector shows a forward-bias voltage of 3.34 V at an injection current of 20 mA, which is lower than those (3.48 and 3.55 V) of the LEDs with the CIO/s-ITO/Al and ITO/Al reflector, respectively. The series resistances of the LEDs with the n-Ag/CIO/sITO/Al, CIO/s-ITO/Al and ITO/Al reflectors were 11.8, 16.9, and 17.4 Ω, respectively. Note

#197615 - $15.00 USD Received 12 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026774 | OPTICS EXPRESS 26777

that the forward voltages are proportional to the specific contact resistances (the inset of Fig. 3). The specific contact resistances of the Ag/CIO/s-ITO/Al, CIO/s-ITO/Al, s-ITO/Al, and ITO/Al samples sample were measured to be 8.7 × 10−3, 6.3 × 10−2, 17.4, and 7.0 × 10−2 Ωcm2, respectively. In particular the n-Ag/CIO/s-ITO/Al contact showed better electrical properties than Ni/Au-based Al reflectors [14]. Figure 4 shows the light output-current (L-I) characteristics of near UV LEDs fabricated with Al reflectors with different ITO-based ohmic contacts. The measurements show that the LEDs fabricated with the CIO/s-ITO/Al and ITO/Al reflectors yield 13.5 and 9.5% higher light output power (at 20 mA), respectively, than the LEDs with the n-Ag/CIO/s-ITO/Al reflector. A comparison shows that the output power is higher in the LEDs with the CIO/sITO/Al and ITO/Al reflectors, but the forward bias-voltage is lower in the LEDs with the Ag nano-dots combined reflector. Figure 5 displays the XPS Ga 2p core level spectra obtained from CIO/s-ITO/Al and nAg/CIO/s-ITO/Al contacts after annealing at 500°C. XPS core-level peak fittings were carried out with a Shirley-type background and Lorentzian–Doniac–Sunsic curves convoluted with a Gaussian profile. The Ga 2p core levels for both the samples consist of Ga-N and Ga-O bonds. It is worth noting that after annealing, the Ga 2p core level for the n-Ag/CIO/s-ITO/Al contact shifted to the lower binding-energy side by 0.13 eV compared to that of the CIO/sITO/Al contact. This denotes that the surface Fermi level shifts toward the valence band edge [15,16], resulting in a reduction in the band-bending of p-GaN, namely, a lowering of the Schottky barrier height (SBH). The Ga 2p peak shift indicates a change of the band-bending since the

Fig. 4. The light output-current (L-I) characteristics of near-UV LEDs fabricated with Al reflectors with different ITO-based multilayers.

N 1s core level spectra exhibit a similar shift behavior [17]. The normalized N/Ga atomic ratio for both the samples was obtained from the ratio of the integral intensity of the XPS N 1s peak to that of the Ga 2p peak (Ga-N bond) with reference to that of the as-deposited sample. The normalized N/Ga ratio was 1.08 ± 0.03. This shows that the p-GaN surface of the n-Ag/CIO/s-ITO/Al sample becomes Ga-deficient, compared with that of the CIO/s-ITO/Al contact, namely, the generation of Ga vacancies at the surface region. The n-Ag/CIO/s-ITO/Al and CIO/s-ITO/Al reflectors exhibited better electrical properties than the ITO/Al reflectors after annealing (Fig. 3). The improvement could be understood by the formation of inhomogeneous Schottky barriers at the contact scheme/GaN interface due to the breaking-up of the CIO layer [18]. It is assumed that there are only two uniform contacts at metal-semiconductor (MS) interface with inhomogeneous Schottky barriers [19] implies that the presence of the CIO nano-dots and the difference of the SBHs between CIO/GaN and ITO/GaN could increase the electric field at the MS interface. It was reported that the increase in the electric field causes a lowering of SBHs, leading to reduction in the specific contact

#197615 - $15.00 USD Received 12 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026774 | OPTICS EXPRESS 26778

resistance [20–22]. Assuming the same sheet resistance of p-GaN for the two samples, the lower forward voltage can be attributed to the lower SBH, namely, the lower contact resistivity. Furthermore, the best electrical behavior of the n-Ag/CIO/s-ITO/Al reflector could be explained by an additional effect, namely, reduction in the SBH caused by the shift of the surface Fermi level to the valence-band edge due to the increase in the acceptor-like Ga vacancies, as shown by the XPS results (Fig. 5) [6].

Fig. 5. The XPS Ga 2p core level spectra obtained from (a) CIO/s-ITO/Al and (b) n-Ag/CIO/sITO/Al contacts, both of which were annealed at 500°C before the deposition of an Al layer.

4. Summary and conclusion We demonstrated the enhanced performance of near UV (365 nm) InGaN/AlGaN-based LEDs using highly reflective ITO/Al-based p-type reflectors. Unlike an Al only n-type contact, the ITO/Al-based multilayer contacts to p-GaN exhibited good ohmic behavior. Among the ITO/Al-based reflectors, the Ag nano-dot combined contacts showed the lowest contact resistivity, but the lowest reflectance (76%) at 365 nm. Near-UV LEDs with the ITO/Al and CIO/s-ITO/Al reflectors yielded a higher light output power (at 20 mA) than the LEDs with the n-Ag/CIO/s-ITO/Al reflector. Considering the fact that high-reflectance p-type ohmic contacts in the UV region are difficult to achieve, the ITO/Al-based multilayer reflectors with high reflectance or low forward bias voltages have the ability to serve as an important reflector for the fabrication of high-performance UV LEDs. Acknowledgments This work was supported by the industrial technology development program funded by the Ministry of Knowledge Economy (MKE), Korea and the industrial strategic technology development program, 10041878, Development of WPE 75% LED device process and standard evaluation technology funded by the MKE, Korea.

#197615 - $15.00 USD Received 12 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026774 | OPTICS EXPRESS 26779

Al-based reflectors.

The enhanced light output power of a InGaN/AlGaN-based light-emitting diodes (LEDs) using three different types of highly reflective Sn-doped indium o...
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