Improving the luminescence of ingan and

Improving the Luminescence of InGaN–GaN Blue LEDs Through Selective Ring-Region Activation of the Mg-Doped GaN Layer Ray-Ming Lin, Jen-Chih Li, Yi-Lun Chou, Kuo-Hsing Chen, Yung-Hsiang L

Improving the Luminescence of InGaN–GaN Blue LEDs Through Selective Ring-Region Activation

of the Mg-Doped GaN Layer Ray-Ming Lin, Jen-Chih Li, Yi-Lun Chou, Kuo-Hsing Chen, Yung-Hsiang Lin, Yuan-Chieh Lu, Meng-Chyi Wu,

Hung Hung, and Wei-Chi Lai

Abstract—In this study, we used the selective ring-region

acti-vation technique to restrain the surface leakage current and to

monitor the luminescence characteristics of InGaN–GaN multiple

quantum-well blue light-emitting diodes (LEDs) To access the

current blocking region after forming a periphery high-resistance

ring-region of the Mg-doped GaN layer and to reduce the degree

of carrier trapping by the surface recombination centers, we

deposited a titanium film onto the Mg-doped GaN epitaxial layer

to form a high-resistance current blocking region To characterize

their luminescence performance, we prepared LEDs incorporating

titanium films of various widths of the highly resistive current

blocking layer The hole concentration in the Mg-doped GaN

epi-taxial layer decreased from3:45 2 1017cm03to3:31 2 1016cm03

after capping with a 250-nm-thick layer of titanium and annealing

at 700C under a nitrogen atmosphere for 30 min Furthermore,

the luminescence characteristics could be improved by varying the

width of the highly resistive region of the current blocking area;

in our best result, the relative electroluminescence intensity was

30% (20 mA) and 50% (100 mA) higher than that of the as-grown

blue LEDs.

Index Terms—InGaN–GaN, light-emitting diodes (LEDs),

selec-tive activation.

I INTRODUCTION

semicon-ductors currently play important roles in many

optoelec-tronic devices The bandgap energy of GaN is 3.4 eV at room

temperature (RT); it forms a number of alloys having bandgaps

ranging from 0.7 eV [with indium nitride (InN)] to 6.2 eV [with

aluminum nitride (AlN)] Because of this wide range of bandgaps

and its excellent electronic, optical, and thermal properties, GaN

is becoming increasingly more attractive for use in a variety

of applications High-brightness light-emitting diodes (LEDs)

based on group III nitrides are of great interest for use in such

Manuscript received November 10, 2006; revised March 17, 2007 This work

was supported by the National Science Council of the Republic of China under

Contract NSC 94-2215-E-182-004.

R.-M Lin, J.-C Li, K.-H Chen, Y.-H Lin, and Y.-C Lu are with the

Depart-ment of Electronic Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan

333, Taiwan, R.O.C (e-mail: rmlin@mail.cgu.edu.tw).

Y.-L Chou and M.-C Wu are with the Institute of Electronics Engineering

and Department of Electrical Engineering, National Tsing-Hua University,

Hsinchu 300, Taiwan, R.O.C.

H Hung is with the Institute of Microelectronics and Department of Electrical

Engineering, National Cheng Kung University, Tainan 701, Taiwan, R.O.C.

W.-C Lai is with the Institute of Electro-Optical Science and Engineering,

National Cheng Kung University, Tainan 701, Taiwan, R.O.C.

Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2007.898870

systems as outdoor full-color displays, traffic lights, indicators, indoor illuminations, and liquid crystal display (LCD) back-lights To reduce power consumption, these applications will require LEDs exhibiting high light output power and low forward voltage Because of the limited internal quantum efficiency and wastage through internal reflection of large amounts of their gen-erated light, improvements in the quantum and light extraction efficiencies of LEDs are presently issues that must be addressed

in the search for LEDs exhibiting higher performance and bright-ness A few reports have described the selective activation of Mg-doped GaN layers by capped Ti–Au [1] and Ni [2] films as

an approach toward obtaining a current blocking layer under the p-electrode metal of LEDs and, consequently, enhancing the performance of the devices [3] To date, however, only a few reports have discussed methods to improve the external quantum efficiency by restraining the surface leakage current of InGaN–GaN blue LEDs In this letter, we describe the use of the selective ring-region activation technique to restrain the surface leakage current and to monitor the luminescence characteristics

of InGaN–GaN multiple quantum-well (MQW) blue LEDs

II EXPERIMENT The InGaN–GaN MQW blue LED wafers were grown on a c-plane sapphire substrate using metal–organic chemical vapor deposition The LED structure consisted of a 30-nm-thick low-temperature GaN buffer layer, a 2- m-thick lightly doped n-type GaN layer, a 2- m-thick highly doped n-type GaN layer,

0.3- m-thick Mg-doped GaN layer

After growth, the current blocking area was capped with a 250-nm-thick titanium film to form the high-resistance area The sample was then treated in a quartz furnace for selective activa-tion of the Mg-doped GaN under a nitrogen atmosphere The an-nealing temperature and time were 700 C and 30 min, respec-tively After thermal annealing, the titanium film was removed using HF solution The hole concentration of the Mg-doped GaN epitaxial layer, as determined through Hall measurement,

cap-ping with the 250-nm-thick layer of titanium Next, a nickel film was used as the etching mask layer in an inductively coupled plasma (ICP) dry etching process The ICP process was used to etch through the Mg-doped p-type GaN and InGaN–GaN MQW

to the n-type GaN layer The nickel film was then removed A

Ni (5 nm)/Au(10 nm) transparent contact layer (TCL) was de-posited on the p-type layer using an electron-beam evaporator; the sample was then thermally annealed at 500 C under an 1041-1135/$25.00 © 2007 IEEE

Fig 1 Top-view illustration of the InGaN–GaN MQW blue LED chip formed

using the selective ring-region activation technique The descriptor d refers to

the width of the highly resistive region of the current blocking area; it is better

defined as the spacing from the edge of the selective activation area to the edge

of the LED chip.

Fig 2 Luminescence images of conventional fully activated as-grown LEDs

and selective-ring-region activated LEDs operated at 20 mA.

oxygen atmosphere for 5 min The Ti and Al metals used for

the p-bonding pad and n-electrode were also deposited using

the electron-beam evaporator Fig 1 provides a schematic

illus-tration of the InGaN–GaN blue MQW LED chips prepared by

using the selective ring-region activation technique

III RESULTS ANDDISCUSSION

Fig 2 displays luminescence images, recorded at 20 mA, of

a conventional fully activated as-grown LED (Sample A) and of

the selective region-activated LEDs (Samples B-H) possessing

different highly resistive regions The value of in Samples B-H

ranged from 10 to 70 m, respectively, with a step interval of

10 m We observed light extraction from the entire mesa of

the LED chip in Sample A, whereas it was centered in the

se-lectively activated areas adjacent to the highly resistive regions

in Samples B-H Note that the TCL was present in all of the

LED samples

Fig 3 presents the forward current–voltage ( – )

character-istics of the LEDs The forward voltages of Samples A-H

oper-ated at 20 mA were 3.24, 3.34, 3.37, 3.40, 3.42, 3.54, 3.60, and

3.73 V, respectively: i.e., the forward voltage operated at 20 mA

increased slightly upon increasing the width from 10 to 70 m

The slight increase in forward voltage can be attributed to the

reduction in the total of the p-GaN neutral region and the

non-activation selective ring-region of the Mg-doped GaN layer as a

result of the presence of a high-resistance region which is used

as a current blocking region [4]–[6] The series resistances of the

samples are determined according to the slopes of

versus plots, not shown here It is indeed seen that the series

Fig 3 Forward I–V characteristics of LEDs plotted as the function of the space d.

Fig 4 Reverse I–V characteristics of LEDs plotted as a function of the space d.

resistances, i.e., , are proportional to the inverse of the aper-ture area

The reverse – characteristics of the LEDs are also shown

in Fig 4 The reverse leakage currents of the LEDs at 7 V

general, the diode leakage current includes the junction leakage current and the surface leakage current Under reverse bias, the injection of minority carriers and the thermal generation of elec-tron hole pairs in the space charge region primarily determine the current through the junction Both the Shockley equation and space charge layer generation are proper to the junction cross-sectional area So the surface leakage current of the LEDs are clearly retrained after forming a periphery high-resistance re-gion by using the selective ring-rere-gion activation technique Fur-ther mechanistic investigations of the surface leakage current in these LEDs are presently in progress The maximum breakdown voltage occurred when the width was 70 m and the reverse breakdown voltage decreased upon reducing the spacing

In order to clarify the increase of light emission by the selec-tive ring-region activation technique, electroluminescence (EL) relative intensity-injection current characteristics are measured,

as shown in Fig 5 The greatest EL intensity of the LEDs ( m) increased by 50% over that of the as-grown blue LEDs

at the injection currents ranging from 0 to 100 mA

Fig 6 displays the relative EL intensities as a function of the space of Samples A-H operated at 20 mA We observed that the EL intensity increased upon increasing the width from 10

Fig 5 EL relative intensities of the InGaN–GaN MQW blue LEDs presented

as a function of the injection current when operated at room temperature.

Fig 6 EL relative intensities of the InGaN–GaN MQW blue LEDs presented

as a function of the space d when operated at 20 mA.

to 40 m, but then it decreased upon increasing from 50 to

70 m We observed the increase in the relative EL intensity for

of the as-grown bar-chip LEDs (averaged at 3.67 mW, 465 nm;

derived by integrate sphere)

Because of the presence of the high-resistance region of the

current blocking region, we observed a current blocking area

under the p-electrode metal and a periphery high-resistance

ring-region of the Mg-doped GaN layer [4], [5] Thus, the

current was dominated injected into the central-region of the

p-GaN through the TCL; then, the parasitic optical absorption

can be reduced due to the reduction in light absorption at the

thick p-pad electrode, which reduced the number of carriers

trapping by the surface recombination centers But as the

-space is larger than 50 m, the more series resistance causes

a higher voltage drop as the current passes through Therefore,

the effective p-n junction voltage decreases from the edge to

the center Because the amount of carrier injection depends on

the voltage at a given position, most of the injection occurs near

the highly resistive edge (not the central-region of the p-GaN)

This phenomenon, current crowding, will be discussed in detail

elsewhere As discussed above, reducing the aperture sizes will

increase not only resistance in the device but also the ohmic

thermal effect The current crowding and the considerable

heating effect leads to a deterioration in the LED’s external

quantum efficiency As a result, it indicates that the samples

prepared by using the selective ring-region technique can be

optimized to improve the external quantum efficiency of the

as-grown conventional LED [6]

The typical reliability test of the variation in relative bright-ness as a function of the width is not shown here For each value of , we tested five samples and normalized the bright-ness variation to its initial reading During the reliability test, the driving current was 20 mA; the samples were analyzed at room temperature Despite of the heating effect due to the diode junction area decreasing and the current density increasing upon proceeding from Samples A to H, the brightness decayed by only 15%–20% after 500 h in the burn-in test The results of these reliability tests followed the same trend as did the bright-ness decays of Samples A-H This result indicates that the se-lective ring-region technique can be used to control the surface state damage of LEDs; because it does not affect the results of the reliability tests, such LEDs are reliable for use in commer-cial applications

IV CONCLUSION

In this study, we demonstrated a method to increase the ex-ternal quantum efficiency of InGaN–GaN blue LEDs through selective ring-region activation of the Mg-doped GaN layer As

a result, the hole concentration of the Mg-doped GaN epitaxial

after capping with a 250-nm-thick layer of titanium and an-nealing at 700 C under a nitrogen atmosphere for 30 min The experimental result reveals that an efficient current blocking layer was produced under the p-electrode metal and a periphery high-resistance ring-region of the Mg-doped GaN layer The EL intensity of the selective ring-region activated LEDs was found

to be greatly increased, compared to that of a as-grown LED due

to the increase in current injection into the active layer of the LED structure and a reduced number of carriers trapped by the surface recombination centers Furthermore, we found that the

EL intensity could be improved by varying a periphery spacing width of the high-resistance ring-regions; the greatest EL inten-sity of the blue LEDs increased by 50% over that of the as-grown blue LEDs at the injection currents ranging from 0 to 100 mA

ACKNOWLEDGMENT The authors would like to thank Dr Y.-C Lin and Prof S.-J Chang for their technical assistance

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