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Improvement in the Light Output Power of GaN-Based Light Emitting Diodes

by One-Step Current Blocking Design

Chun-Fu Tsai, Yan-Kuin Su1;2, and Chun-Liang Lin3

Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, No. 1 University Road, Tainan 701, Taiwan, R.O.C.1Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, R.O.C.2Department of Electrical Engineering, Kun-Shan University, Yung-Kang City, No. 949, Da-Wan Road, Tainan 710, Taiwan, R.O.C.3Department of Electronic Engineering, Kun-Shan University, Yung-Kang City, No. 949, Da-Wan Road, Tainan 710, Taiwan, R.O.C.

Received April 22, 2010; revised May 24, 2010; accepted June 28, 2010; published online January 20, 2011

This paper presented the InGaN/GaN light-emitting diodes (LEDs) using one-step design of indiumtin-oxide (ITO) layer as the current blocking

layer (CBL) structure was fabricated successfully and the optoelectronic properties were also measured. The ITO CBL LEDs exhibit higher light

output power (16.3% at 20 mA) compared with that of the reference LEDs without CBL. As for the usual current blocking process, the one-step

design of ITO CBL as the current blocking structure was demonstrated in our experiment and proved to be an effective, feasible and inexpensive

way, with fewer steps and less cost, to improve the LEDs performance. # 2011 The Japan Society of Applied Physics

1. Introduction

GaN-based wide bandgap semiconductors have recentlyattracted considerable interests, in terms of applications for

optoelectronic devices, which operate in the blue, green, and

ultraviolet (UV) wavelength regions.1) A general configura-

tion of GaN-based light-emitting diodes (LEDs) is that both

n- and p-type pads are formed on the same side of the LED

chip due to the insulating sapphire substrates. In such a

structure, the p-electrode is located in the middle of the light

path, some loss of light is inevitable as a result of photon

absorption near the p-pad.2) Therefore, in order to increase

the light output of GaN-based LEDs, the issue of light

absorption at p-pad will be important.

Recently, several current blocking (CB) methods that

could enhance the light output of LEDs have been

demonstrated as follows: using CF4+O2 gases plasma-

treatment to p-GaN under p-pad for current blocking;3)

forming and varying the number and size of blocking-

holes in the shortest current path between p- and n-pads;4)

inserting an insulator in p-GaP layer under p-pad for the

current blocking application of AlGaInP-based material

system;5) localized Ti deposition associated with indium

zinc-oxide (IZO) was proposed to serve as a Schottky

current blocking layer for the vertical-structure GaN-based

LEDs;6) the activation of Mg-doped p-GaN using a Ni film

as the catalyst to form the selective high resistivity region

(SHRR) as the purpose of current blocking,7)

etc. All thesesolutions have one significant purpose in common, which

is reducing current density under p-pad and improving the

performance of LEDs by current blocking structure.

In this article, we report on the fabrication and

characterization of InGaN/GaN multiple-quantum well

(MQW) LEDs with a one-step current blocking design

under the p-pad by means of standard photolithography and

wet chemical etching. The results show that the performance

of a LED chip with the current blocking design is

considerably enhanced compared to that of the conventional

LED. In our experiment, through the one-step selective

area current blocking design without additional steps to

form current blocking structure as above mentioned in the

literature, we could define a Schottky contact region under

the p-pad, which the Schottky contact region is just the

interface of exposed p-GaN and p-pad Cr/Pt/Au. In this

way, we can avoid spontaneous emission beneath the opaque

p-pad and to enhance an additional current injection into

the effective active layers of the LED, thereby significantly

increasing the light output power.

2. Experimental Procedure

The samples were grown by metalorganic vapor chemical

deposition (MOCVD) system. The LED structure consisted

of a GaN nucleation layer, a Si-doped n-GaN layer, InGaN/

GaN MQWs, a Mg-doped p-AlGaN was grown as theelectron blocking layer (EBL), and then a Mg-doped p-GaN

layer. After defining mesa by standard photolithography and

inductively coupled plasma (ICP) dry etching, the indium

tin-oxide (ITO) layer was deposited by e-beam evaporation

served as the transparent conduction layer (TCL), forming

ohmic contact to p-GaN. We define the ITO covered region

by photolithographic patterning and wet chemical etching,

also define the current blocking region to expose p-GaN at

the same time without additional step to fabricate the current

blocking layer (CBL) structure. We can see the comparison

of CBL process flowchart in Fig. 1. Finally, the metal layer

composed of Cr/Pt/Au was simultaneously deposited onto

n-GaN and ITO, served as the n- and p-pads by e-beam

evaporation and the chip fabrication was finished. Note,

the Cr/Pt/Au can form ohmic contact to n-GaN and ITO

but we get Schottky contact between Cr/Pt/Au and p-GaN,

Fig. 1. Process flowchart of (a) the conventional CBL process with four steps,

and (b) the one-step ITO CBL design with three steps in this experiment.

E-mail address: chunfutsai@hotmail.com

Japanese Journal of Applied Physics 50 (2011) 01AD05

01AD05-1 # 2011 The Japan Society of Applied Physics

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DOI: 10.1143/JJAP.50.01AD05

http://dx.doi.org/10.1143/JJAP.50.01AD05http://dx.doi.org/10.1143/JJAP.50.01AD05http://dx.doi.org/10.1143/JJAP.50.01AD05http://dx.doi.org/10.1143/JJAP.50.01AD05

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that is the key point of our current blocking design in this

experiment.

The cross-sectional view of the reference and ITO CBL

LED structures were shown in Fig. 2. From the figure we

can see the current blocking region, where the p-pad Cr/Pt/

Au was directly onto the exposed p-GaN to form the

Schottky contact. In Fig. 3, it shows the image of fabricated

ITO CBL chip, labeled with exposed p-GaN and defined

ITO region as the CBL design, just as we can see in the

Fig. 2. Conventional LED chips with the same size(330 330 m2) were also fabricated on the same wafer

as reference. The optical and electrical properties of the

LEDs were both measured with the form of TO-can using

an optics LED characterization system with a calibrated

integrated sphere detector.

3. Results and Discussion

Figure 4 is the room temperature electroluminescence (EL)

spectra of the reference and ITO CBL LEDs at dc 20 mA

injection. It was found that EL peak of these two LEDs both

occurred at $455 nm with nearly the same full-width at half-

magnitude (FWHM) about 18.5 nm. It was also found that

the EL intensity of the ITO CBL LED was larger than that

of the reference LED without CBL. Such an observation can

be attributed to the better light extraction efficiency for the

LED with ITO CBL design.

In the beam profile measurement of Fig. 5, we can see the

ITO CBL LED has better current spreading. The injection

current blocked by the Schottky contact under the p-pad

would be forced to spread outward, hence the current density

in the active region beneath the p-pad can be effectively

reduced, thus increasing the light output power of LED at

the same time. Figure 6 shows the LIV curve of these two

fabricated LEDs. At 20 mA, as compared to reference LED

(12.7 mW), it was found the light output power of ITO CBL

LED (14.77 mW) was significantly improved by 16.3%.

As for the IV characteristic, the higher forward voltage

Fig. 2. (Color online) Schematic cross-sectional view of the epitaxial layers and GaN LED fabricated (a) without, as the reference and (b) with the one-step ITO

CBL design.

Fig. 3. CCD image of the ITO CBL chip with size 330 330m2

, labeledwith defined p-GaN and ITO region.

400 420 440 460 480 500 520

ELi

ntensity(arb.un

it)

Wavelength (nm)

@20mA

Reference LEDITO CBL LED

Fig. 4. (Color online) Room temperature EL spectra of the reference and ITO

CBL LEDs at 20 mA dc injection current.

Fig. 5. (Color online) Beam profile measurements of the (a) reference and (b)

ITO CBL LEDs at 20mA dc current injection.

C.-F. Tsai et al.Jpn. J. Appl. Phys. 50 (2011) 01AD05

01AD05-2 # 2011 The Japan Society of Applied Physics

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(increased about 0.1 V) of ITO CBL LED can be attributed

to the current-blocking design, with less contact areabetween ITO and p-GaN than that of reference LED, so

the serial resistance and forward voltage of ITO CBL LED

will be inevitably higher.

4. Conclusions

In conclusion, this paper presented the InGaN/GaN LEDs

using one-step design of ITO layer as the current blocking

structure was fabricated successfully and the optoelectronic

properties were also measured. The ITO CBL LEDs exhibit

higher light output power (16.3% at 20 mA) compared with

that of the reference LEDs without CBL. As for the usual

current blocking process, the one-step design of ITO CBL

as the current blocking structure was demonstrated in our

experiment and proved to be an effective, feasible and

inexpensive way, with fewer steps and less cost, to improve

the LEDs performance.

Acknowledgement

Funding from the Advanced Optoelectronic TechnologyCenter, National Cheng Kung University, under projects

from the Ministry of Education and the National Science

Council (NSC 96-2221-E-006-079-MY3) of Taiwan are

gratefully acknowledged. This work was partially supported

by TDPA Lamp Development of White Light-Emitting

Diode for Local Lighting program and in part by National

Science Council of the Republic of China (R.O.C.) in

Taiwan under Contract Nos. TDPA 97-EC-17-A-07-S1-105

and NSC 97-2623-E-168-001-IT.

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Fig. 6. (Color online) LIV characteristics of the reference and ITO CBL

LEDs.

C.-F. Tsai et al.Jpn. J. Appl. Phys. 50 (2011) 01AD05

01AD05-3 # 2011 The Japan Society of Applied Physics

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