Effects of strain on the characteristics of InGaN–GaN multiple quantum-dot blue light emitting diodes

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Physics Letters A 355 (2006) 118–121

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Effects of strain on the characteristics of InGaN–GaN

multiple quantum-dot blue light emitting diodes

Liang-Wen Ji

a

_{, Te-Hua Fang}

b,∗

_{, Teen-Hang Meen}

c a_{Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan} b_{Institute of Mechanical and Electro-Mechanical Engineering, National Formosa University, Yunlin 632, Taiwan}

c_{Department of Electronic Engineering, National Formosa University, Yunlin 632, Taiwan}

Received 13 January 2006; accepted 13 February 2006 Available online 20 February 2006

Communicated by A.R. Bishop

Abstract

In this study, Raman spectra were measured in the backscattering geometry at temperatures from 100 K to 298 K. Samples with the InGaN self-assembled quantum dot (SAQD) structures of high strain show a strong compressive stress in InGaN epilayer by Raman measurement. Furthermore, we have applied the dots-in-a-well (DWELL) structure to nitride-based light-emitting diodes (LEDs). It was found that EL peak variation of the LED with DWELL structure is more sensitive to the amount of injection current, as compared with the MQW LEDs.

Keywords: Raman; GaN; Light-emitting diodes (LEDs); Quantum dots (QDs)

1. Introduction

Heteroepitaxial growth of highly strained material systems has been quite attractive as it offers the possibility of produc-ing low-dimensional carrier confinement nanostructures, such as quantum wells (QWs) and quantum dots (QDs) [1]. They present the utmost challenge to semiconductor technology, making possible fascinating novel devices. III-nitride semicon-ductor materials have a wurtzite crystal structure and a direct energy band gap. We could also achieve nitride-based het-eroepitaxial growth easily. At room temperature, the band gap energy of AlInGaN varies from 0.7 to 6.2 eV depends on its composition. Therefore, III-nitride semiconductors are particu-larly useful for light-emitting diodes (LEDs) and laser diodes (LDs) in this wavelength region[2–5]. Typical high-brightness LEDs have a multiple quantum well (MQW) active region. The MQW LED is a kind of heterojunction device, in which elec-trons and holes are confined in the well layers. Thus, one can achieve high quantum efficiency from the MQW LEDs since

* _{Corresponding author. Tel.: +886 5631 5395; fax: +886 5631 5397.}

E-mail address:fang.tehua@msa.hinet.net(T.-H. Fang).

carrier can recombine easily in the confined well layers. Al-though high brightness InGaN–GaN MQW LEDs are already commercially available, it can be theoretically predicted that the realization of LEDs with QDs in the active layer would im-prove the performance of LEDs.

Recently, it has been shown that nitride nanostructures can be self-assembled using the strain-induced Stranski–Krastanov (SK) growth mode without any substrate patterning process

[6–8]. It has also been shown that nitride nanostructures can be self-assembled using growth interruption during the metal-organic chemical vapor deposition (MOCVD) growth[9,10]. Although the size fluctuations of self-assembled QDs could result in inhomogeneous optical and electrical characteristics, the self-assembly of strain-induced islands provides the means for creating zero-dimensional quantum structures without hav-ing to overcome the current limitations of lithography. These self-assembled QDs could also be used to study novel device physics[11–13].

In this Letter, we report the InGaN self-assembled quantum dot (SAQD) structures of high strain show a strong compres-sive stress in InGaN epilayer by Raman measurement. Blue LEDs with multiple InGaN dots-in-a-well (DWELL) structures have been successfully fabricated, i.e., InGaN–GaN multiple 0375-9601/$ – see front matter © 2006 Elsevier B.V. All rights reserved.

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L.-W. Ji et al. / Physics Letters A 355 (2006) 118–121 121

dium composition fluctuation due to InGaN phase separation results in the formation of indium-rich clusters, which acts as QDs-like. Therefore, high luminescence efficiency could be expected if the density of QDs is much higher than that of dislocations. However, DWELL structures different from the phase-separation induced QDs were shown in this work, we employed the micro-Raman technique to prove that they were strain-induced SAQDs indeed. Such a nanostructure will de-grade the performance of blue LEDs. In summary, one can see the InGaN–GaN SAQD structures show a strong compressive stress in InGaN epilayer by Raman spectra. A higher biaxial strain exists in the SAQD structure will result in stronger QCSE of the fabricated QD LEDs. It agrees with that EL position of the MQD LED is more sensitive to the amount of injection cur-rent, as compared to conventional MQW LEDs.

Acknowledgements

The authors would like to thank professor Qi-Kun Xue at State Key Laboratory for Surface Physics, Institute of Physics (Beijing), Chinese Academy of Sciences, for many illuminating discussions. This work was financially supported by the Na-tional Science Council of Taiwan (Project No. NSC 94-2215-E-150-009).

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