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Strain relaxation in InGaN/GaN micro-pillars evidenced by high resolution cathodoluminescence hyperspectral imaging

Xie, E. Y. and Chen, Z. Z. and Edwards, P. R. and Gong, Z. and Liu, N. Y. and Tao, Y. B. and Zhang, Y. F. and Chen, Y. J. and Watson, I. M. and Gu, E. and Martin, R. W. and Zhang, G. Y. and Dawson, M. D. (2012) Strain relaxation in InGaN/GaN micro-pillars evidenced by high resolution cathodoluminescence hyperspectral imaging. Journal of Applied Physics, 112 (1).

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Abstract

A size-dependent strain relaxation and its effects on the optical properties of InGaN/GaN multiple quantum wells (QWs) in micro-pillars have been investigated through a combination of high spatial resolution cathodoluminescence (CL) hyperspectral imaging and numerical modeling. The pillars have diameters (d) ranging from 2 to 150 μm and were fabricated from a III-nitride light-emitting diode (LED) structure optimized for yellow-green emission at ∼560 nm. The CL mapping enables us to investigate strain relaxation in these pillars on a sub-micron scale and to confirm for the first time that a narrow (≤2 μm) edge blue-shift occurs even for the large InGaN/GaN pillars (d > 10 μm). The observed maximum blue-shift at the pillar edge exceeds 7 nm with respect to the pillar centre for the pillars with diameters in the 2–16 μm range. For the smallest pillar (d = 2 μm), the total blue-shift at the edge is 17.5 nm including an 8.2 nm “global” blue-shift at the pillar centre in comparison with the unetched wafer. By using a finite element method with a boundary condition taking account of a strained GaN buffer layer which was neglected in previous simulation works, the strain distribution in the QWs of these pillars was simulated as a function of pillar diameter. The blue-shift in the QWs emission wavelength was then calculated from the strain-dependent changes in piezoelectric field, and the consequent modification of transition energy in the QWs. The simulation and experimental results agree well, confirming the necessity for considering the strained buffer layer in the strain simulation. These results provide not only significant insights into the mechanism of strain relaxation in these micro-pillars but also practical guidance for design of micro/nano LEDs.

Item type: Article
ID code: 40400
Keywords: cathodoluminescence, light emitting diodes, spectral line shift, quantum well devices, Physics, Physics and Astronomy(all)
Subjects: Science > Physics
Department: Faculty of Science > Physics
Faculty of Science > Institute of Photonics
Technology and Innovation Centre > Photonics
Technology and Innovation Centre > Bionanotechnology
Depositing user: Pure Administrator
Date Deposited: 11 Jul 2012 15:04
Last modified: 26 Mar 2015 20:19
URI: http://strathprints.strath.ac.uk/id/eprint/40400

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