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p s scurrent topics in solid state physics

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caphys. stat. sol. (c) 5, No. 6, 2195–2197 (2008) / DOI 10.1002/pssc.200778538

Estimation of internal quantum efficiency in InGaN-based light emitting diodes using electroluminescence decay times

Shinji Saito*,1, Tetsuo Narita2, Kotaro Zaima1, Koichi Tachibana1, Hajime Nago1, Gen-ichi Hatakoshi1, and Shinya Nunoue1

1 Corporate R&D Center, Toshiba Corporation 1, Komukai-Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan 2 Department of Electrical Engineering and Computer Science, Graduate School of Engineering, Nagoya University, Chikusa-ku,

Nagoya 464-8603, Japan

Received 7 September 2007, accepted 25 December 2007

Published online 10 April 2008

PACS 78.60.Fi, 85.60.Bt, 85.60.Jb

* Corresponding author: e-mail sh.saito@toshiba.co.jp, Phone: +81 44 549 2138, Fax: +81 44 520 1501

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction InGaN-based light-emitting diodes (LEDs) have wide application in solid-state lighting and display. High quantum efficiency is important for many applications, not only at normal injection current density but also at high injection current density. The decrease of the electroluminescence efficiency was reported by many authors [1, 2]. The decrease in efficiency is slightly due to increased heating, but is in fact dominated by decreasing another efficiency as current density is raised. This can be easily confirmed by comparison of external quantum effi-ciency dependence on current measured in DC and pulse mode [2, 3]. The models of quenching of external quantum efficiency with increasing the current have been proposed. The carrier localization models [2] have been reported for InGaN-based LED. The current distribution change model [4] has been reported for the InGaAlP-based LED. The ef-ficiency of LEDs is defined as the external quantum effi-ciency, which is the product of injection efficiency, inter-nal quantum efficiency, and extraction efficiency. Al-though the internal quantum efficiency could be analyti-cally determined, few works have been reported on the de-termination method [5, 6].

In this work, a method is presented for estimating the internal quantum efficiency based on data of electrolumi-nescence decay times measured as a function of current in the pulse injection.

2 Theoretical considerations and experimental The external quantum efficiency (ηex) of light emitting di-odes is expressed as a product of three factors as shown in Fig. 1.

intex inj extrη η η η= ¥ ¥ (1)

where ηint is the internal quantum efficiency, and ηinj is the injection efficiency; ηextr is the light extraction efficiency. We estimated the internal quantum efficiency of InGaN-based LEDs using the electroluminescence decay time. The current continuity equation for carriers is given by

21dndivJ Bn An

dt q= - - (2)

where n, J, B and A denote the carrier density, the current density, the coefficient of bimolecular recombination and

We investigate the internal quantum efficiency of InGaN-

based Light Emitting Diodes (LEDs) from the semiconductor

rate equation of pulse current injection. A method is pre-

sented for estimating the internal quantum efficiency based

on data of electroluminescence decay times measured as a

function of current in the pulse injection. For the screening of

built-in electric field, the pulse current was injected with bias

voltage. For blue LED, the internal quantum efficiency was

measured to be about 70% in high injection condition. It is

found that the internal quantum efficiency increase with in-

jection current, whereas the external quantum efficiency de-

creases at high injection.

2196 S. Saito et al.: Internal quantum efficiency in InGaN-based light emitting diodes

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

ph

ysic

ap s sstat

us

solid

i c

the coefficient of nonradiative recombination of carriers, respectively.

Figure 1 The schematic diagram of the carrier flow in the active

layer. Each arrow indicates carriers and photons.

The pulse current injects in the active area. The current continuity equation for carriers is given by Eq. (3).

2

0 0 0

1( ) ( )

d nJ B n n A n n

dt qd

∆∆ ∆= - + - + (3)

where ∆n, d and n0 are the carrier density of pulse current, the width of active area and the carrier density in the equilibrium. The time-dependent carrier density ∆n is given by integrating Eq. (3) .

1

exp( ) 1EL

nC t

∆τ

=

-

(4)

where C is the factor that is not dependent on time. The τEL is defined as

τ0

12

EL

Bn A∫ +

In this study, the time decays of the electroluminescence were measured on the pulse current injection with the bias voltage. The emission peak wavelength of the LED was 460 nm. With the amplitude of pulse current set constant, the base current density J is changed as shown in Fig. 2. The time decays of the electroluminescence depended on the carrier density, if the pulse amplitude was high. Since there is built-in electric field in pn-junction and quantum well, the bias voltage was necessary to flatten the built-in electric field.

3 Results and discussion In Fig. 3, the time decay of electroluminescence in the pulse injection is plotted. The decay is observed to be non-single exponential. The τEL is obtained by the fitting of Eq. (4).

Figure 2 The schematic diagram of the pulse current injection.

Figure 4 shows the τEL as a function of the current density of samples. The other expression for τEL is

τ

21 4

EL

BA J

q= + (5)

By linear fitting of the (1/τEL)2 vs J, the coefficients of A and B in the Eq. (5) are determined. The internal quantum efficiency is defined as

τη

τ

2

0

int 2

0 0

1 ( )( )

1 ( )

EL

EL

Bn A JJ

An Bn A J

-= =

+ +

(6)

In Fig. 5, the internal quantum efficiency of LED is plotted as a function of current density. The internal quantum effi-ciency increased with respect to the current density to satu-rate at 70%. The external quantum efficiency decreases at high injection. This indicates that the injection efficiency or the extraction efficiency was decreased with current in-jection. Rozhansky et al. [7] showed by simulations that the current blocking AlGaN layer is inefficient at high current density due to piezoelectric field of GaN/AlGaN interface.

Although the measured devices have the current blocking AlGaN layer, poor confinement of the electrons in the ac-tive region is responsible for the decrease of injection effi-ciency in high injection condition. Y. Takahashi et al. [8] indicated the electron capture process by the radiative re-combination centres in the InGaN MQW. They found that by adding the electron reservoir layer, the EL spectral in-tensity is significantly enhanced. For the simple model pre-sented here, the poor confinement of the electrons in the active region is responsible for the quenching of external quantum efficiency with increasing the current. G. Hatako-shi [4] proposed that the current spreading layer and cur-rent blocking layer have significant effects on extraction efficiency, since the current distribution change with cur-rent injection reduced the extraction efficiency.

Time

0n

n∆

cu

rren

t

ηinj: injection efficiency

ηint: internal efficiency

ηext: extraction efficiency

phys. stat. sol. (c) 5, No. 6 (2008) 2197

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Contributed

Article

Figure 3 The time decay of electroluminescence. The decay pro-

file is fitted by Eq. (4) function (solid line) and single exponential

function (dashed line).

Figure 4 The square of 1/τEL as a function of the current density

for bias voltage. The solid line is fit to Eq. (5).

Figure 5 The estimated internal quantum efficiency (left axis)

and the measured external quantum efficiency (right axis) plotted

with respect to the current density.

4 Conclusion We have proposed a method of esti-mating internal efficiency and have measured the internal quantum efficiency dependent on the current density. It is found that the internal quantum efficiency increases with injection current, whereas the external quantum efficiency decreases at high injection. This indicates that the injection efficiency was decreased with current injection. We showed that the quenching of external quantum ef-ficiency was caused the decrease of injection efficiency and/or extraction efficiency.

References

[1] T. Mukai, M. Yamada, and S. Nakamura, Jpn. J. Appl. Phys.

38, 3976 (1999).

[2] M. R. Krames, J. Bhat, D. Collins, N. F. Gardner, W. Gotz, C.

H. Lowery, M. Ludowise, P. S. Martin, G. Mueller, R. Muel-

ler-Mach, S. Rudaz, D. A. Steigerwald, S. A. Stockman, and J.

J. Wierer, phys. stat. sol. (a) 192, 237 (2002).

[3] M. Baeumler, M. Kunzer, R. Schmidt, S. Lui, W. Plestchen,

P. Schlotter, K. Kohler, U. Kaufmann, and J. Wager, phys.

stat. sol. (a) 204 (4), 1018 (2007).

[4] G. Hatakoshi, Proceedings of the IEEE/LEOS 3rd Interna-

tional Conference on Numerical Simulation of Semiconductor

Optoelectronic Devices (2003), p. 21.

[5] Y. Kawakami, K. Omae, A. Kaneta, K. Okamoto, Y. Naru-

kawa, T. Mukai, and S. Fujita, J. Phys.: Condens. Matter 13,

6993 (2001).

[6] M. S. Minsky, S. Watanabe, and N. Yamada, J. Appl. Phys.

91, 5176 (2002).

[7] I. V. Rozhansky and D. A. Zakheim, phys. stat. sol. (c) 3(6),

2160 (2006).

[8] Y. Takahashi, A. Satake, K. Fujiwara, J. K. Shue, U. Jahn, H.

Kostial, and H. T. Grahn, Physica E 21, 876 (2004).

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