ohmic contacts for high power leds

phys. stat. sol. (a) 201, No. 12, 2831–2836 (2004) / DOI 10.1002/pssa.200405111

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Ohmic contacts for high power LEDs

Ho Won Jang, Jong Kyu Kim, Soo Young Kim, Hak Ki Yu, and Jong-Lam Lee*

Department of Materials Science and Engineering, POSTECH, Pohang, Kyungbuk 790-784, Korea

Received 22 March 2004, revised 17 June 2004, accepted 8 July 2004 Published online 15 September 2004

PACS 73.40.Cg; 73.40.Ns, 78.66.–w, 85.60.Jb

We report novel metallization schemes of transparent p-ohmic contacts for conventional LEDs and reflec-tive p-ohmic contacts for flip-chip and vertical-structure LEDs. Thermally stable and low-resistivity Ru/Ni/ITO ohmic contacts on p-type GaN resulted in the low contact resistivity of 2 × 10–4 Ω cm2 and the high transmittance of 92% at 470 nm wavelength. The light output power of the LED with the Ru/Ni/ITO p-contact was increased by 50% compared to the LED of a Ni/Au transparent p-contact. Using a newly developed Ni/Ag/Ru/Ni/Au reflective p-ohmic contact, the low contact resistivity of 5 × 10–5 Ω cm2 could be achieved. The light reflectance of the contact was as high as 90% at 470 nm wavelength.


1 Introduction

GaN-based light-emitting diodes (LEDs) have been of great interest because of their application to white LEDs, which might replace conventional light bulbs. GaN-based LEDs are commonly fabricated on insulating sapphire substrates. For these devices, all contacts must be made from the top side of the de-vice. Thus, a transparent p-ohmic contact is required to spread current from the p-pad electrode across the junction area due to the low conductivity of Mg-doped p-type GaN [1]. Because the film thickness of the transparent p-ohmic contact is typically below 150 Å, the resistivity of the contact is three to five orders higher in magnitude compared to its corresponding bulk material [2]. This causes current crowd-ing near the p-pad electrode. The high current density in the localized area of the device could lead to a decrease in light extraction efficiency and a significant degradation in reliability characteristics. There-fore, improving transparent p-ohmic contacts is a key for realizing high-power LEDs. High transmit-tance, low contact resistivity, good thermal stability, and high conductivity are requirements of transpar-ent p-contacts for high-power LEDs. Recently, flip-chip and vertical-structure designs have been exploited in GaN-based LEDs to improve the light extraction efficiency and thermal management of the devices [3, 4]. In these configurations, highly reflective p-ohmic contacts are essential for increasing the light extraction efficiency because the emitted light from the active region is reflected-up from the p-ohmic contact. The contact resistivity of 10–5 Ω cm2 was demonstrated for reflective p-ohmic contact, but the reflectance is still lower than 80%. Therefore, p-ohmic contacts with the contact resistivity of 10–5 Ω cm2 and the reflectance as high as 90% should be developed. In this study, novel metallization schemes are reported for transparent p-ohmic contacts of conven-tional LEDs and reflective p-ohmic contacts of flip-chip and vertical-structure LEDs.

2 Experiment

InGaN/GaN LED structures used in this study were grown by metalorganic chemical-vapor deposition (MOCVD) on c-face sapphire substrates. The LED structures were annealed at 750 °C for 10 min in the

* Corresponding author: e-mail: [email protected], Phone: +82 54 279 2152, Fax: +82 54 279 5242

2832 H. W. Jang et al.: Ohmic contacts for high-power LEDs


growth chamber for the activation of Mg dopants. Net hole concentration was determined to be 3 × 1017 cm–3 by Hall measurements. For the measurement of specific contact resistivity using the trans-mission line method (TLM), an active region was defined by Cl2/BCl3 inductively coupled plasma etch-ing, followed by dipping the samples into boiling aqua resia solution of HCl:HNO3 (3:1) solution to remove surface oxides formed during MOCVD and/or the thermal annealing [5]. TLM test structure was patterned using the photolithographic technique, followed by the deposition of contact metals. Various contact metals including Ru, Ni, Ir, Pt, Ag, and Au were deposited by electron beam evaporation under a pressure of 4 × 10–7 Torr, followed by removing metals deposited on photoresist. The samples were then annealed at temperatures from 300 °C to 800 °C for 2 min. Two types of samples were prepared. One was the samples with TLM structures annealed in O2 ambient. The other was the samples annealed in N2 ambient for comparison. Indium tin oxide (ITO) overlayer on Ru/Ni contacts was deposited using rf magnetron sputtering. Current–voltage (I–V) characteristics of the contacts were examined by the four-point-probe measurements. Light transmittance and reflectance of contact metal layers were measured by a photospectrometer. Back-side polished sapphire substrate and 96% reflectance Ag mirror were used as reference for the transmittance and reflectance, respectively.

For the fabrication of LEDs, the surface of p-type GaN layer was partially etched to the n-type GaN using Cl2/BCl3 inductively coupled plasma etching. Transparent Ru/Ni/ITO contacts were deposited on the p-type GaN layer, followed by the deposition of Ti/Al contacts on the n-type GaN layer. And then, rapid thermal annealing was carried out at 500 °C for 1 min in O2 ambient. For pad electrodes, Cr/Au metals were deposited on both transparent p- and n-ohmic contacts. The optical and electrical character-istics of fabricated LEDs were measured under bare-chip geometry at room temperature dc operation.

3 Results and discussion

3.1 Transparent p-ohmic contacts

Figure 1 shows I–V curves for the Ru/Ni contacts on p-type GaN. I–V curves were measured between the TLM pads with the interspacing of 5 µm. The as-deposited contact showed non-ohmic behavior. As the contact was annealed at 500 °C, I–V curves were improved in both annealed samples. However, the improvement was more pronounced in the sample annealed in O2 ambient than that in N2 ambient, indi-cating the formation of a good ohmic contact on p-type GaN. Specific contact resistivity was calculated from the least-squares fit of measured resistances as a function of TLM spacing. The contact resistivity was determined to be 4.5 × 10–5 Ω cm2 for the contact annealed in O2 and 4.4 × 10–3 Ω cm2 for the an-nealed in N2. Figure 2 depth profiles of secondary ion mass spectroscopy for the Ru/Ni contacts. In the contact an-nealed in N2, Ni indiffused into the Ru layer, and both Ga and N atoms were simultaneously outdiffused.

As-deposited

Annealed in O2

Annealed in N2

-2 0 2-0.50

0

0.50

-1 1

-0.25

0.25

Cur

rent

(mA

)

Voltage (V)

Fig. 1 I–V characteristics of Ru/Ni (50/50 Å) contacts on p-type GaN. Annealing was carried out at 500 °C for 1 min.

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Sputtering time (s)

(a)L

og

inte

nsi

ty(a

.u.)

0 200 400 600 800 1000

Ni

RuGa

N

O

0 200 400 600 800 1000 1200

Ni

Ru

Ga

N

O

Annealed in O2Annealed in N2 (b)

Fig. 2 SIMS depth profiles for Ru/Ni contacts on p-type GaN annealed at 500 °C for 5 min.

In the contact annealed in O2, oxygen concentration at the surface was remarkably high and its depth profile coincided with both Ni and Ru ones. This indicates the transformation of Ni and Ru metal layers into NiO and RuO2 oxide layers, respectively. The Ni indiffusion into Ru was also observed, but its amount was much smaller than that in the annealed in N2. Intermixing between Ru and GaN was signifi-cant, but there was no outdiffusion of Ga and N atoms to the surface in the oxidized Ru/Ni contact. This indicates that the oxidation annealing promoted interfacial reactions and simultaneously NiO layer acted as a diffusion barrier for the outdiffusion of released Ga and N atoms from decomposed GaN during annealing at 500 °C [6, 7]. Therefore, the contact degradation with N outdiffusion was suppressed in the oxidized Ru/Ni contact, resulting in the lower contact resistivity.

ITO thin film has several advantages for transparent p-ohmic contacts. It is highly transparent to visi-ble light and has low resistivity of ~10–4 Ω cm. In addition, the reflective index of ITO (nITO) at 470 nm is about 2.0, which is between nGaN (= 2.45) and nair (= 1.0). This means that the ITO film of the 4/nλ thickness (~600 Å) on p-type GaN acts as an antireflection coating layer [8]. Therefore, it was expected that an ITO overlayer on the transparent Ru/Ni contact could lead to the improvement of the current spreading and an increase in the light extraction efficiency [9, 10].

Ni/Au

Ru/Ni/ITO

-2 -1 0 1 2-0.5

0

0.5

Cu

rren

t(m

A)

Voltage (V)

Ni/Au

Ru/Ni/ITO

-2 -1 0 1 2-0.5

0

0.5

Cu

rren

t(m

A)

Voltage (V) 380 400 420 440 460 480 500

60

70

80

90

100

Tra

nsm

itta

nce

(%)

Wavelength(nm)

Ni/AuRu/Ni/ITO

Fig. 3 I–V characteristics of Ru/Ni/ ITO and Ni/Au contacts annealed at 500 °C for 1 min.

Fig. 4 Light transmittance of Ru/Ni/ITO and Ni/Au contacts annealed at 500 °C for 1 min.



Fig. 5 Light emission images of functioning LED chips at the injection current of 5 mA; (a) LED with the Ni/Au p-ohmic contact (b) LED with the Ru/Ni/ITO p-ohmic contact.

I–V characteristics of Ru/Ni/ITO contacts were very similar to those of Ni/Au contacts, as shown in Fig. 3. After annealing at 500 °C for 1 min in O2, the contact resistivity was determined to be 2.0 × 10–4 Ω cm2. However, the light transmittance of the Ru/Ni/ITO contact was much better than that of the Ni/Au contact, as shown in Fig. 4. It is noteworthy that the transmittance is as high as ~92% at 470 nm. This value is the lowest among the previously reported works on the transparent p-ohmic con-tacts [11–13]. Figure 5 shows light emission images of functioning LED chips at the injection current of 5 mA. For the LED with the Ni/Au transparent, non-uniform light emission was observed due to current crowing near the p-pad electrode. However, the LED with the Ru/Ni/ITO contact showed uniform light emission and higher brightness, which was attributed to the lower current spreading resistance and the higher light extraction efficiency. As a result, the light output power of the LED with the Ru/Ni/ITO p-ohmic contact was increased by ~50% compared to that with the Ni/Au p-ohmic contact.

3.2 Reflective p-ohmic contacts

Figure 6 shows contact resistivities of Ni/Ag/Ru/Ni/Au and Ni/Au contacts as a function of annealing temperature. Minimum contact resistivities of the contacts were obtained after annealing at 500 °C, determined to be ~5 × 10–5 for Ω cm2 Ni/Ag/Ru/Ni/Au contacts and 1 × 10–4 Ω cm2 for the Ni/Au con-tact. At annealing temperatures higher than 500 oC, the contacts showed degradation in contact resis-tivity. The Ni/Au contact exhibited rectifying behavior after annealing at 700 °C. However, the

Co

nta

ctre

sist

ivit

y( Ω

cm2 )

Temperature (oC)300 400 500 600 700 800

10-5

10-4

10-3

10-2

10-1

100

101

102

Ni/Ag/Ru/Ni/Au

Ni/Au

Fig. 6 Specific contact resistivities of Ni/Ag/Ru/Ni/ Au and Ni/Au contacts as a function of annealing temperature.

phys. stat. sol. (a) 201, No. 12 (2004) / www.pss-a.com 2835


Lig

ht

refl

ecta

nce

(%)

Wavelength (nm)400 420 440 460 480 500 520 540

30

40

50

60

70

80

90

100

Ni/Ag/Ru/Ni/Au

Ni/Au

Ni/Ag/Ru/Ni/Au contacts still showed ohmic behavior after annealing at 800 °C. This implies that the thermal stability of the Ni/Ag/Ru/Ni/Au contacts is superior to that of the Ni/Au contact.

In order to estimate the feasibility of Ni/Ag/Ru/Ni/Au contacts to LEDs, optical reflectance was meas-ured. Figure 7 shows light reflectance spectra of Ni/Ag/Ru/Ni/Au and Ni/Au contacts annealed at 500 °C for 2 min. The reflectance at a wavelength of 470 nm was higher than 90% for the Ni/Ag/Ru/Ni/Au con-tacts, whereas it was as low as 46% for the Ni/Au contact. Because the Ru layer could prevent intermix-ing of the Ag layer with other metals during the annealing, the Ag layer could act as an effective reflec-tion mirror. It is noted that the light reflectance is the highest value among the previously reported works on the reflective p-ohmic contacts [14, 15].

4 Conclusions

Metallization schemes of ohmic contacts on p-type GaN for high-power LEDs were developed. The Ru/Ni contact on p-type GaN resulted in the low contact resistivity of 4.5 × 10–5 Ω cm2 after annealing at 500 °C in O2 ambient. Using an ITO overlayer on the oxidized Ru/Ni contact, the light transmittance of 92% at 470 nm could be achieved. The light output power of the LED with the Ru/Ni/ITO p-ohmic con-tact was 1.5 times higher than that of the Ni/Au p-ohmic contact due to the uniform current spreading and the improved extraction efficiency. Newly developed Ni/Ag/Ru/Ni/Au reflective p-ohmic contacts for flip-chip and vertical-structure LEDs led to low contact resistivities of ~5 × 10–5 Ω cm2. The light reflectance of the contacts was as high as 90% at 470 nm, which is 2 times higher than that of the con-ventional Ni/Au contact.

Acknowledgements This work was financially supported in part by research program for “National Research Laboratory” sponsored by the Korea Institute of Science and Technology Evaluation and Planning (KISTEP) and in part by Korea Science and Engineering Foundation through the Quantum-functional Semiconductor Research Center at Dongguk University in 2004.

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Fig. 7 Light reflectance spectra of Ni/Ag/Ru/Ni/Au and Ni/Au contacts annealed at 500 °C for 2 min.



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ohmic contacts for high power leds

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