Characteristics of polarization emission in a -plane GaN-based multiple quantum wells structures
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Characteristics of polarization emission in a-plane GaN-based multiple quantum wells structures Chiao-Yun Chang1, Huei-Min Huang1, Tien-Chang Lu1, Hao-Chung Kuo1, Shing-Chung Wang1, and Chih Ming Lai2 1 Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 30050, Taiwan 2 Department of Electronic Engineering, Ming Chuan University, 250 Zhong Shan N. Rd., Sec. 5, Taipei 111, Taiwan ABSTRACT We studied the polarization-dependent photoluminescence (PL) of a-plane GaN /AlGaN multiple quantum wells (MQWs) grown on r-plane sapphire substrate with the various well width from 1.5 to 7.33 nm. To clarify the reasons of light emission polarization properties, we applied the 6×6 k p model to simulate the E-k dispersion relation and the wave functions to obtain the polarization optical transitions. According to the experimental result, the PL emission peak position exhibits a red-shifted with increasing well width, due to the reduction of the quantum confinement effect. The polarization ratio of a-plane GaN/AlGaN MQWs increased from 0.236 to 0.274 with increasing the quantum well width. This phenomenon is believed to be that y-polarized light emission gradually dominates the PL spectrum and thus enhances the polarization ratio. INTRODUCTION The GaN-based semiconductor materials is a material of wide band gap, which versatile applications in UV to visible optoelectronic devices. The GaN-based quantum wells (QWs) grown on the c-plane sapphire has found unique application in deep ultraviolet optoelectronic devices[1, 2] . However, due to the large spontaneous and piezoelectric polarization fields along the c-direction, the band structure of GaN-based MQWs is in a triangular shape would cause the electron wave functions separate from the hole wave functions spatially in the well to decrease the recombination efficiency[3]. Therefore, the Growth of nonpolar GaN-based MQWs have been availably excluded the quantum confined Stark effect (QCSE)[4] to enhance internal quantum efficiency (IQE) and the peak shift of light emission. Due to the asymmetry of the WZ GaN crystal and anisotropic biaxial strains in the nonpolar a-plane or m-plane orientation, the nonpolar structure has exhibited the polarized light emission[5-7]. In this study, we used the photoluminescence (PL) measurement to investigate the emission properties on a series of a-plane GaN/AlGaN MQWs with various well thicknesses. The optical
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polarization properties were analyzed according to the polarization dependent PL measurement. This analysis can be used to determine the degree of polarization (DOP) of the emission. Then, the 6×6 k᧪p method was constructed for the a-plane GaN/AlGaN MQWs structures to simulate the optical emission properties by calculating the dispersion relationships of the energy level and the corresponding wave functions. The issues of valence-band states mixing and polarized emissions were also discussed.
EXPERIMENT AND THEORY The structures of a-plane GaN/Al0.17Ga0.83N QWs
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