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Samsung Electro-Mechanics Co., Suwon 442-743 ... work functions, such as Ni [2,3], Pt [4], Pd [5], Ru, Co. -899- ..... [13] G. S. Marlow and M. B. Das, Sol.
Journal of the Korean Physical Society, Vol. 49, No. 3, September 2006, pp. 899∼902

Characteristics of Hydrogen Storage Alloy p-GaN Ohmic Contacts for InGaN LEDs Seung Wan Chae Department of Electronic Engineering, Korea University, Seoul 136-713 and Samsung Electro-Mechanics Co., Suwon 442-743

Joon Seop Kwak Department of Materials Science and Metallurgical Engineering, Sunchon National University, Chonnam 540-742

Suk Kil Yoon and Mi Yang Kim Samsung Electro-Mechanics Co., Suwon 442-743

June O Song Georgia Institute of Technology, Atlanta, U.S.A.

Tae Yeon Seong Department of Material Science Engineering, Korea University, Seoul 136-713

Tae Geun Kim∗ Department of Electronic Engineering, Korea University, Seoul 136-713 (Received 29 November 2005) We report the electrical and the optical properties of ZnNi/Au electrodes and compare them to those of Ni/Au electrodes. From the experiment and a systematic analysis using fabricated InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with ZnNi/Au (5 nm/5 nm) electrodes, the forward voltage with ZnNi/Au was compared with the values of 3.45 V and 22 mW, respectively for Ni/Au under the same conditions. We attribute this to both the reduced barrier height of p-type GaN and the enhanced mobility of Mg-doped GaN, due to hydrogen absorption by ZnNi PACS numbers: 73.40.Cg, 73.40.Jn, 73.40.Kp, 78.66.Fd Keywords: GaN, Nitride, LED, p-GaN ohmic, Hydrogen storage material, Schottky barrier, ZnNi

I. INTRODUCTION

luminous efficiency of a GaN-based white LED is much lower than that of a fluorescent light lamp. One of the primary reasons for the low efficiency in conventional InGaN LEDs is the low light extraction efficiency due to the poor transparency of a broadcontact electrode on p-GaN. If the external quantum efficiency is to be enhanced, transparent p-type ohmic contacts with low contact resistances must be developed. Many researchers [2–9] have reported various development schemes for p-GaN ohmic electrodes with low resistance and high transmittance. However, because of the high contact resistance due to the wide band gap energy and the low p-doping concentration of GaN, many technical barriers still exist. They must be overcome by the development of p-type electrodes. Metals with large work functions, such as Ni [2,3], Pt [4], Pd [5], Ru, Co

GaN and its related materials have been extensively researched in various applications for future illumination systems and for high density storage recording systems. In particular, GaN-based white-light-emitting diodes (white-LEDs) have recently drawn a great deal of attention because of their wide spectrum of applications to back-lighting units for liquid-crystal displays, car interiors, and solid-state lightings [1]. However, despite of these possible applications, white-LEDs have many weaknesses which should be addressed before it can be substituted for conventional fluorescent lamps. The most critical problem is the output power. For example, the ∗ E-mail:

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Journal of the Korean Physical Society, Vol. 49, No. 3, September 2006

[6], and Ir [7], have been used for p-GaN ohmic contacts; however, these materials are very absorptive at short wavelengths (i.e, 460 nm) of light, leading to low transmittance. To solve above problems, transparent conducting oxides (TCOs), such as ITO, ZnO, etc., have been investigated as alternatives to p-GaN ohmic metal. Kim et al. [8] and Margalith et al. [9] reported that ITO would be effective in increasing the light output of LEDs due to its higher transmittance; however, significantly large voltage would be incurred penalty because of the high specific contact resistance, in the range of ∼10−1 Ω cm2 of ITO. Therefore, these materials are poor alternatives for a p-GaN ohmic metal because of their contact resistance being higher than those of conventional metals with large work functions. In this paper, we report the electrical and the optical properties of ZnNi/Au electrodes and compare them with those of Ni/Au electrodes. p-GaN contacts schemes based ZnNi known as hydrogen storage material were derived from the characteristics of hydrogen storage material which could adsorb hydrogen in p-GaN surface, so we thought that the mobility of Mg-doped GaN could be enhanced by breaking the Mg-H complex [10], and that the hydrogen desorption in p-type GaN could lower the ionization energy of the Mg acceptor. Lowering the ionization energy of Mg acceptor reduces the Schottky barrier height between p-GaN and the electrode [11,12]. From the experiment and systematic analysis, we confirmed that the ZnNi/Au electrode outperformed the Ni/Au electrode.

II. EXPERIMENTS p-GaN layers (1.2 µm-thick, 3 × 1017 cm−3 ) were grown by using metal organic chemical vapor deposition (MOCVD) of Aixtron Co. These layers were ultrasonically degreased by using trichloroethylene, acetone, methanol, and DI water for 5 min at each step and were then blown dry by using N2 gas. Prior to lithography, the GaN samples were treated with a diluted sulfur-oxide (H2 SO4 ) solution for 5 min to eliminate native oxide on the GaN surface and were then rinsed in DI water for 20 min. Transfer length method (TLM) patterns, defined by using a standard photolithographic technique, were used to measure the specific contact resistance. The pattern size was fixed to 150 × 500 µm2 , while the spacing was varied from 5 to 25 µm. Prior to metal deposition, all the samples were treated in a diluted HCl solution for 1 min, and ZnNi/Au and Ni/Au layers in 5 nm/5 nm were deposited by using electron beam evaporation. To constitute the ohmic contact materials, we rapid-thermally annealed these samples at temperatures of 400 ∼ 600 ◦ C for 1 min in air. Current-voltage (I-V) characteristics were measured

Fig. 1. (a) I-V curve and (b) specific contact resistance of ZnNi/Au and Ni/Au electrodes at various RTA temperatures.

using a parameter analyzer (HP 4155A), and the transmittance was evaluated by using a spectro-photometer and the beam analyzer (NST BL-200). In addition, InGaN/GaN multiple-quantum-well (MQW) LEDs were fabricated and examined in a LED tester from Opto Company, Japan. To analyze the ohmic contact mechanism made by using a hydrogen storage material, we determined the chemical properties of the samples by using X-ray photoelectron spectroscopy (XPS). The XPS measurement system (Ulvac-PHI Quantera SXM) consisted of a spherical analyzer and a monochromatic Al Ka X-ray source.

III. RESULTS AND DISCUSSION First, to understand the roles of the ZnNi/Au electrodes, we prepared four ZnNi/Au samples: - asdeposited, annealed at 400 ◦ C, annealed at 500 ◦ C, and annealed at 600 ◦ C and Ni/Au annealed at 500 ◦ C for reference. The annealing time in air was fixed at 1 min. The I-V characteristics and the specific contact resistance of ZnNi/Au at various RTA temperatures are shown in Fig. 1. In the Fig. 1, a nonlinear curve is shown for the as-deposited sample; however, with increasing annealing temperature to 600 ◦ C, the curve becomes linear

Characteristics of Hydrogen Storage Alloy p-GaN Ohmic· · · – Seung Wan Chae et al.

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Fig. 2. Light transmittance curves for the ZnNi/Au and the Ni/Au electrodes after annealing at 500 ◦ C for 60 sec.

and the specific contact resistance decreases. The specific contact resistance was determined from plots of the measured resistance as a function of the spacing between the TLM pads. The least-square method was used to fit a straight line to the experimental data [13]. Typical specific contact resistances of ZnNi/Au electrodes was varied from 6.36 × 10−2 Ω cm2 to 2.26 × 10−4 Ω cm2 for annealing from 400 ◦ C to 600 ◦ C for 60 sec while that of the Ni/Au electrode after annealing at 500 ◦ C was 4.87 × 10−4 Ω cm2 . Fig. 2 shows a comparison of the magnitudes of the optical light transmittances between ZnNi/Au and Ni/Au electrodes. The light transmittance of ZnNi/Au electrode was measured to be higher than 80 % and that of the Ni/Au was 76 % at a wavelength of 460 nm. The ZnNi/Au electrode shows more transparent characteristics in the range of wavelengths from 350 nm to 600 nm than the Ni/Au electrode did for the same thicknesses of electrodes and the same annealing conditions. We fabricated three InGaN/GaN blue-LEDs with different types of p-type ohmic contacts: - i.e., ZnNi/Au (5 nm/5 nm) films and a Ni/Au (5 nm/5 nm) film. All samples were annealed at a temperature of 500 ◦ C for 60 sec in air. After annealing, we fabricated the n-ohmic and the bonding electrodes with Cr/Au. Fig. 3(a) shows the I-V characteristics and the light-output power versus current (L-I) curve for InGaN/GaN LEDs. By comparison of the slopes among the three InGaN/GaN blue LEDs, the lowest forward-bias voltage was ∼3.45 V at 20 mA from the InGaN/GaN LED with ZnNi/Au electrodes. For reference, the forward-bias voltage of the InGaN/GaN LEDs with Ni/Au was measured to be 3.55 V at 20 mA. These results are consistent with those for the specific contact resistance described in Fig. 1. Fig. 3 (b) shows the characteristics of the light-output power versus current (L-I) curve for the three InGaN/GaN LEDs and the LED chip shaped through these fabrication. The output power increases with the injection current for all samples up to 100 mA. By comparison, InGaN/GaN

Fig. 3. (a) Typical I-V curve and (b) Light-output power versus current curve comparison between ZnNi/Au and Ni/Au electrodes after the InGaN/GaN blue LEDs fabrication process. All samples were annealed at a temperature of 500 ◦ C.

LEDs with ZnNi/Au electrodes recorded higher output power up to 100 mA than InGaN/GaN LEDs with Ni/Au electrodes did. The output power at 20 mA from the InGaN/GaN LEDs with ZnNi/Au electrodes was 22 mW, as compared to 18 mW from the InGaN/GaN LEDs with Ni/Au electrodes, at the same annealing temperature of 500 ◦ C. To understand the effect of the p-GaN ohmic contact mechanism for hydrogen storage materials, we used XPS analysis the binding energy of Ga 2p in the ZnNi/Au and the Ni/Au electrodes after annealing. As Fig. 4 shows, the binding energy of Ga 2p was decreased by about 0.5 eV in Ni/Au electrodes during the annealing process while that of Ga 2p was decreased by about 1.6 eV in ZnNi/Au electrodes during the annealing process. The difference of Ga 2p binding energies between the ZnNi/Au and the Ni/Au electrodes during the annealing process is thought to be due to the ionization energy difference resulting from the Mg-H complex desorption caused by the hydrogen storage properties [9]. These results are consistent with those for ZnNi/Au with

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V and 18 mW, respectively, for the Ni/Au electrode. We attribute this improvement to both reduced barrier height of p-type GaN and the enhanced mobility of Mgdoped GaN, due to hydrogen desorption caused by ZnNi.

ACKNOWLEDGMENTS

Fig. 4. Comparison of Ga 2p XPS data for (a) ZnNi/Au electrode and (b) Ni/Au electrode after 500 ◦ C annealing for 60 sec.

This work was financially supported by Samsung Electro-Mechanics Co. and partly supported by MOST/ KOSEF through the Quantum Photonic Science Research Center.

REFERENCES the lower specific contact resistance and forward voltage than for Ni/Au, as described in Figs. 1 and 3. This indicates that a gallium oxide layer would be made upon GaN and Ga vacancy would be formed below the contact by means of in-diffusing of Zn and out-diffusing of Ga. Therefore, it is possible to decrease the Schottky barrier height of the p-GaN/electrode interface and to enhance the optical properties [14].

IV. CONCLUSIONS We investigated the effect of the p-GaN ohmic contact due to a hydrogen storage material, such as ZnNi, on the electrical and the optical properties of InGaN/GaN LEDs in order to make highly transparent and low resistance ohmic contacts to p-type GaN (p = 3 × 1017 cm−3 ). The specific contact resistances of the ZnNi/Au electrodes were as low as 2.26 × 10−4 Ω cm2 after 600 ◦ C annealing and those of the Ni/Au electrode were 4.87 × 10−4 Ω cm 2 after 500 ◦ C annealing. The light transmittance of ZnNi/Au electrode was measured to be higher than 80 % and that of the Ni/Au was 76 % at a wavelength of 460 nm. That is, by applying ZnNi/Au films at a fixed annealing temperature of 600 ◦ C,compared to Ni/Au film, the specific contact resistance could be decreased by 2.6 × 10−4 Ω cm2 ,and the transmittance of the p-electrode could be increased by ∼5 %. Regarding the InGaN/GaN LED chip’s performance, the forward voltage of the ZnNi/Au electrode at 20 mA was 3.45 V, and the output power was 22 mW, compared to 3.55

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