Group III-Nitride Based VCSEL for Applications at the Wavelength of 400nm
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Group III-Nitride Based VCSEL for Applications at the Wavelength of 400nm
M. Linnik and A. Christou Department of Materials and Nuclear Engineering and Materials Research Science and Engineering Center, University of Maryland, College Park MD 20742
ABSTRACT Short wavelength Vertical Cavity Surface Emitting Laser based on the group III nitrides, GaN, AlN, InN, and their ternary alloys is reported. Optical properties such as band gap and index of refraction of the nitride binary compound were calculated based on the fitting of the experimental data and the first principle calculations. The ternary alloy optical properties were determined in the same manner but based on the binary compound data. The active region containing InGaN strained multiple quantum wells is formed between two Distributed Bragg Reflectors. GaInN/AlN material systems are shown to be the most suitable for highly reflective Bragg mirrors with minimized number of layers. LiGaO2 substrate is proposed for GaN growth due to its small lattice mismatch with GaN (about 0.9%) and its ability to provide a good thermal matching between the two materials. We report VCSEL calculations on threshold current and emission spectra.
INTRODUCTION Group III-Nitride heterostructures are very promising for optoelectronic, high power and high temperature electronic devices due to their large band gap energies, large heterojunction discontinuities, and high saturation velocities [1]. It has been established that GaInN/GaN multiple quantum wells and AlGaN cladding layers can be successfully fabricated for diodes emitting in the blue and violet region of the spectrum [2]. However, the development of the suitable materials is still underway for highly reflective Bragg mirrors for Vertical Cavity Surface Emitting Lasers (VCSEL) in the blue-green spectral region. A number of groups have reported the fabrication of AlGaN/GaN Distributed Bragg Reflectors (DBR) with the employment of 30 – 40 quarter wavelength periods and with peak reflectance in the near ultraviolet to blue-green region of the spectrum [3, 4, 5]. As a wide band gap semiconductor material system, AlGaN/GaN heterostructure demonstrates a small index of refraction contrast, as well as a high defect density and poor electrical conductivity. [6] Higher index of refraction contrast is found in AlN/GaN system [3, 7, 8] which reduces the number of periods required for the highly reflective DBR to approximately half. The large lattice mismatch between AlN and GaN ( ~2.5%) results in the poor morphology of the DBR microstructure as reported by Fritz et al. [8]. Another approach to the high reflectivity mirrors employs low loss SiO2/HfO2 or ZrO2/SiO2 dielectric stacks that are flip-chip bonded to Si, SiC or sapphire substrates [3, 9, 10]. The major concern in the realization of a VCSEL with dielectric mirrors is the need to separate the AlGaN/GaN/InGaN heterostucture from the sapphire or SiC substrate [6]. In this letter, we are reporting a design of a VCSEL structure built on LiGaO2 substrate for operation at the wavele
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