Long-wavelength vertical-cavity lasers based on InP/GaInAsp bragg reflectors
Abstract: Vertical-cavity surface-emitting lasers (VCSELs) operatingat long wavelength (1.3-1.55µm) are of great interest asinexpensive, high-performance light sources for opticalcommunication systems. The vertical geometry and the smalldimensions of the laser cavity are advantageous for on-chiptesting, packaging, effective fibre coupling and potentiallow-cost fabrication. Operation at long wavelength isfavourable for applications in fibre-optical communication dueto the superior transmission characteristics in standard silicafibres. However, in contrast to GaAs-based VCSELs operating atwavelengths below 1µm, which have seen a tremendousdevelopment in recent years, the progress for thelong-wavelength devices has been much slower. This is mainlydue to intrinsic material problems in the GaInAsP alloy system.The limited refractive index contrast between InP and GaInAsPis of specific importance, impeding the fabrication of anall-epitaxial device structure with two monolithicallyintegrated semiconductor distributed Bragg reflectors (DBRs) asit is common for GaAs-based VCSELs. Instead, the mostsuccessful designs have relied on the combination of anInP-based active region with DBRs from different materialsystems. To date, the best performing VCSELs for longwavelength employ two GaAs-based mirrors, requiring twowafer-fusion steps and three substrates; hence leavingquestions for a strategy toward reliability, full wafer scaleprocessing and cost management. For this reason, there is astrong interest in monolithic devices, i.e., devices that canbe grown entirely on InP-substrate in a more inexpensiveprocess. However, only very recently there are designs reportedin literature operating continuous-wave at room temperatureemploying a monolithically integrated mirror.In this thesis, long-wavelength VCSELs based on integratedInP/GaInAsP Bragg reflectors grown by metal-organicvapour-phase epitaxy are presented and analysed. The firstchapter gives an introduction to long-wavelength VCSELs,including a review of the state-of-the-art devices as well as adiscussion of material and design related issues. The secondchapter of the thesis goes into detail with DBR design, devicedesign, fabrication technology as well as the characterisationand analysis of the lasers. Two structures based on anInP/GaInAsP bottom mirror are presented: a semi-insulatingInP:Fe regrown laser structure and a single-wafer-fusedstructure. The regrown laser employs a dielectric top mirrorand operates continuous-wave up to -9°C and pulsed up to45°C.The single-wafer-fused laser makes use of a GaAs/AlGaAs top mirror and shows an improved performance. This wasthe first device based on an integrated InP/GaInAsP mirrorreported to operate continuous-wave near room temperature (at17°C), and it operates pulsed up to temperatures as highas 101°C. The advantages and drawbacks of both designs arediscussed. A third VCSEL structure discussed in this thesis isbased on a so-called air-gap mirror. Herein, every second layerin an InP/GaInAs stack has been removed, resulting in a veryhigh-index contrast Bragg reflector. This VCSEL, which isoptically pumped, was the first of its kind ever reported forlong wavelength.
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