Similarities in the Optical Properties of Hexagonal and Cubic InGaN Quantum Wells
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Similarities in the Optical Properties of Hexagonal and Cubic InGaN Quantum Wells S. F. Chichibu1,2,a), M. Sugiyama1, T. Onuma1, T. Kuroda3, A. Tackeuchi3, T. Sota4, T. Kitamura5,b), H. Nakanishi6, Y. Ishida5, H. Okumura5,6, S. Keller7, U. K. Mishra7, S. P. DenBaars7, and S. Nakamura2,7 1
Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan 2 Nakamura Inhomogeneous Crystal Project, ERATO, Japan Science and Technology Corporation, 2-4-6 Fujimi, Chiyoda-ku, Tokyo 102-0071, Japan 3 Department of Applied Physics, Waseda University, 3-4-1 Ohkubo, Shinjuku, Tokyo 169-8555, Japan 4 Department of Electrical, Electronics, and Computer Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku, Tokyo 169-8555, Japan 5 Power Electronics Research Center 2, National Institute of Advance Industrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan 6 Department of Electrical Engineering, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japan 7 Department of Materials Engineering and Department of Electronics and Computer Engineering, University of California, Santa Barbara, CA93106 a) Contacting author, [email protected] b) On leave from Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japan
ABSTRACT Optical properties of fully-strained wurtzite and zincblende InxGa1-xN/GaN multiple quantum well (MQW) structures were compared to discuss the origin of exciton localization. In contrast to the hexagonal InGaN MQWs, the photoluminescence (PL) peak energy of cubic InGaN MQWs showed a moderate blueshift with decreasing well thickness, L, and low-temperature PL decay time of the cubic MQWs did not depend strongly on L. The results imply that the wavefunction overlap in cubic InGaN MQWs was not reduced compared to the hexagonal ones, since they do not suffer from the electric field normal to the QW plane due either to spontaneous or piezoelectric polarization. Both MQWs exhibited a large and composition-dependent bandgap bowing, and time-resolved PL (TR-PL) signals showed a stretched-exponential decay even at room temperature. The exciton localization is considered to be an intrinsic property of InGaN.
INTRODUCTION InxGa1-xN quantum-wells (QWs) are attracting attention because they serve as an active region [1-3] of UV to visible light-emitting-diodes (LEDs) and purple laser diodes (LDs). Since they exhibit an efficient emission with external quantum efficiency up to 20 % at 470 nm in spite of the large threading dislocation (TD) density up to 1010 cm-2 [4], optical properties of InGaN QWs have been investigated intensively. Spontaneous emission efficiency, η, of hexagonal (h-) InxGa1-xN QW LEDs has been recognized to be controlled by a balance [5] between the exciton
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localization [6] and wavefunction separation [5,7] due to quantum-confined Stark effect (QCSE) [8], which results from an electric field, F, normal to the QW plane owing to spontaneous [7] and piezoelectric [9] polarizations. The interna
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