Design and Fabrication of a VCSEL With Graded Bragg Mirror Interfaces for Operation at 850nm
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Design and Fabrication of a VCSEL With Graded Bragg Mirror Interfaces for Operation at 850nm Chichang Zhang, Aris Christou Dept. of Materials Science and Engineering, Univ. of Maryland at College Park College Park, Maryland, 20742 USA ABSTRACT Based on the Alx Ga1-x As/GaAs system and with graded Distributed Bragg mirrors (DBRs), a VCSEL for operation at 850-860nm is reported. The graded transition bands inside the DBRs were designed in order to achieve the tradeoff between the number of DBR layers and the low threshold current of the laser. The structure optimized had 39 pairs in the n-DBR stack and a low threshold current of 1.6mA was achieved. The device was fabricated with MBE growth, oxide confinement and RIE for MESA definition. The experimental results and physical characterization of the device are also reported to fully understand the VCSEL performance. The threshold current and other parameters predicted by the simulation are in good agreement with experimental results. INTRODUCTION The VCSEL1,2,3,4,5 (Vertical Cavity Surface Emitting Laser) has rapidly become a prominent device for photonics. This device has an extensive range of potential applications such as high-speed data communications, optical switching, and printing. This device, however, must continuously be modified in order to further improve performance. The selective wet oxidation technique6,7, of Alx Ga1-x As is currently used to produce oxide layers for VCSELs, which demonstrate improved performance such as high power conversion efficiency, low threshold current, and low threshold voltage. We report on the utilization of graded interface Bragg mirrors as one technique to lower series resistance, improve thermal management and obtain low threshold current operation. EXPERIMENTAL DETAILS The investigated VCSEL structure is designed for emission at 850nm and consists of 1λ thick cavity with 4 periods of 100Å Al0.2Ga0.8As/ 120Å GaAs quantum wells. There is one aperture layer on top and bottom of each side of the quantum wells which is used for aperture formation by oxidation confinement of the beam. The aperture layers, which are the layers that have highest Al concentration, consist of 568Å Al0.98Ga0.02As. The DBR consisted of Al0.92Ga0.08As/Al0.12Ga0.88As multilayers. The DBRs consisted of 19 and 39 periods of AlGaAs/GaAs. Each period of the unoxidized DBR consists of 433Å Al0.92Ga0.08As/354Å Al0.12Ga0.88As. The transition band between these two layers is 260Å thick. Detailed information of the structure is shown in Fig 1. The contact layers were p-GaAs and n-GaAs. The entire structure was grown by molecular beam epitaxy. VCSEL structures with steeper transition layers (thinner layer C in Fig 1) could achieve higher reflectivity with the same number of layers in the DBR. However the heterojunction
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resistances (the resistance between layer B and D) would also increase. The electrical properties, especially threshold current and threshold voltage, are greatly reduced if proper transition interfaces and lyaers are achieved. Ho
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