Design of a 364 nm Electrically Pumped Multi-Quantum Well Continuous Wave Nitride Vertical Cavity Surface Emitting Laser
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Design of a 364 nm Electrically Pumped Multi-Quantum Well Continuous Wave Nitride Vertical Cavity Surface Emitting Laser Shelia C. Luke1 , Abhishek Motayed2 , and Aris Christou Department of Materials Science and Engineering University of Maryland College Park College Park, Maryland 20742 U.S.A 2 Department of Electrical Engineering Howard University Washington D.C. U.S.A ABSTRACT Short-wavelength vertical cavity surface emitting lasers (VCSELS) are typically important for high-density optical memory and optical imaging systems. An AlGaN/GaN multiquantum well (MQW) VCSEL was designed for operation at wavelength of 364 nm. The design process consisted of careful selection of materials and optimization of the parameters for distributed Bragg reflector (DBR) stacks, active region, and calculation of the threshold current density, external quantum efficiency, and threshold carrier concentration. InGaN/AlGaN material system was selected for constructing DBR stacks in order to minimize power dissipation and to achieve high reflectivity. A reflectivity of 99% was calculated for 33 pairs in the bottom DBR stack and 97% was calculated for 30 pairs in the top DBR stack. An external quantum efficiency of 85% has been achieved through parametric optimization. Results were comparable to that of the similar structures based on the review of recent literatures. INTRODUCTION The goal of the present investigation was to design an AlGaN/GaN MQW Vertical Cavity Surface Emitting Laser operating at 364 nm. Commercial realization of a short-wavelength VCSEL can indeed revolutionize today’s communication technology. While for some semiconductor systems the edge-emitting configuration is ideal, it is believed that for nitrides, the most suitable configuration is the surface-emitting lasers. The nitride semiconductor, GaN and its alloys (AlGaN, InN) is the most prominent material system for the realization of shortwavelength lasers due to the band structure and the width of their direct band gap. DESIGN CONSIDERATIONS Gallium nitride and related compounds have large bandgap energies and are attractive for light emitting devices operating in the blue to ultraviolet spectral region [1]. It has been shown that the quantum well (QW) structure as the active layer is the desired method for realizing the GaN-based laser. A single homogeneous gain medium replaced by multi quantum well structure increases the over all gain of the laser structure. In this particular design, a MQW GaN/ Ga0.8Al0.2N (×10) was used. This structure provided a bandgap offset, ∆Eg of 0.6 eV and a difference in the refractive index, ∆n of 0.09. For better carrier and optical confinement, it is essential to have these figures as large as possible. A quantum well laser emits light with a wavelength that corresponds to the bandgap of the material that forms the well. In this case, GaN
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forms the well and has a bandgap of 3.40 eV, which translates to the lasing wavelength of 365 nm. This wavelength may change due to the strain and other quantum effects. In ord
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