Future space satellite systems will use high-speed 25 Gbps 850-nm multi-mode (MM) vertical cavity surface emitting lasers (VCSELs) for communications and 795-nm single-mode (SM) VCSELs for atomic clocks applications. The main advantage of deploying these VCSELs in space satellites over 850- or 795-nm edge emitting lasers is the absence of COMD (catastrophic optical mirror damage). In particular, VCSEL-based atomic clocks have a potential to significantly improve timekeeping accuracy and navigation positioning errors. Space satellite systems require stringent reliability of these VCSELs, but they are significantly lacking reliability data. We assess suitability of these VCSELs for high reliability applications through the physics of failure investigation to study reliability, failure modes, and degradation mechanisms. Also, this work is part of our efforts to understand the physical origin of degradation in oxide confined VCSELs under high current density operation. For the present study, we investigated reliability and failure modes of two state-of-the-art VCSEL types – 25 Gbps 850- nm MM oxide confined VCSELs and 795-nm SM oxide confined VCSELs. Accelerated life-tests of both VCSEL types were performed under varying stress conditions to model wear-out failures for reliability assessment. These life-tests were performed under ACC (automatic current control) mode. We also studied VCSELs that were exposed to ESD (electrostatic discharge). We employed optical beam induced current (OBIC), photocurrent spectroscopy, and electron beam induced current (EBIC) techniques for failure mode analysis (FMA). FMA was performed on life-tested VCSEL failures as well as on ESD tested VCSEL failures. Lastly, we employed plasma focused ion beam (PFIB) for removal of portions of top-DBR mirrors for EBIC and for slice-and-view techniques.
High-power broad-area lasers are critical components for space satellite communications systems. Broad-area lasers with InGaAs-AlGaAs strained Quantum Well (QW) active regions are currently used in space satellite systems. These QW lasers have shown excellent power and efficiency characteristics, but these lasers are still susceptible to COD (catastrophic optical damage) leading to catastrophic and sudden degradation. Thus, their long-term reliability in space environments is a major concern. Furthermore, our group has shown that these lasers predominantly degrade by a new failure mode due to Catastrophic Optical Bulk Damage (COBD). The 3-D confinement of carriers in InAs-GaAs Quantum Dot (QD) active region has a potential to suppress nonradiative recombination of carriers at growth or radiation induced defect sites. This feature makes the QD lasers attractive for space applications. For the present study, we employed time-resolved analysis techniques including time-resolved electroluminescence (TR-EL) and time-resolved photoluminescence (TR-PL) techniques to investigate degradation in high-power broad-area lasers. We studied broad-area lasers with two different types of active regions – strained InGaAs-AlGaAs single QW layer and ten stacks of InAs-GaAs QD layers. TR-EL techniques were employed for time-resolved analysis of degradation processes in QW and QD lasers to study the sequence of critical events including the formation and propagation of dark line defects in ⪅110⪆, ⪅11̅0⪆, and ⪅100⪆ directions during accelerated life-tests. TR-PL techniques were employed to measure carrier lifetimes in QW laser wafer. Lastly, we report our understanding on degradation mechanisms in broad-area lasers with QW and QD active regions.
Remarkable progress made in performance characteristics and reliability of high-speed (> 10 GHz) 850-nm multi-mode (MM) oxide-confined vertical cavity surface emitting lasers (VCSELs) during the last decade has led them to find applications in space satellite systems. The main advantage of deploying high-speed VCSELs in space satellites over directly modulated 850-nm edge emitting lasers is the absence of COMD (catastrophic optical mirror damage). In recent years, leading VCSEL manufacturers introduced higher speed (~ 20 GHz or 25 Gbps) VCSELs with encouraging characteristics. However, little has been reported on reliability and failure modes of these state-of-the-art VCSELs although it is crucial to understand failure modes and degradation mechanisms in these VCSELs through physics of failure investigation and subsequently develop VCSELs that exceed lifetime requirements for space satellite systems. For the present study, we performed short-term and long-term accelerated life-tests on 25 Gbps oxide-confined MM VCSELs to study reliability of these devices. Our goal is to extract credible reliability model parameters (thermal activation energy and current exponent factor) from these life-tests to determine suitability of these lasers for future space satellite systems. We also performed failure mode analysis on VCSELs at different stages of degradation using various techniques. We employed nondestructive techniques including optical beam induced current (OBIC) and electron beam induced current (EBIC) techniques as well as destructive techniques including focused ion beam (FIB) and high-resolution TEM techniques. Our detailed reliability and failure mode analysis results are reported along with our understanding on the physical origin of degradation in high-speed VCSELs with strained InGaAs quantum wells.
High-power single-mode (SM) and multi-mode (MM) InGaAs-AlGaAs strained QW lasers are critical components for space satellite systems. Both SM and MM QW lasers have shown excellent output power and efficiency characteristics, but these lasers are susceptible to COD. In addition, our group has shown that these lasers predominantly degrade by a new failure mode due to catastrophic optical bulk damage (COBD) leading to catastrophic and sudden degradation, which is a major concern for space applications. In recent years, InAs-GaAs quantum dot (QD) lasers have received much attention as an alternative to QW lasers for Si Photonics because 3-D confinement of carriers in QD lasers reduces the chance of nonradiative recombination of carriers at growth or radiation induced defect sites. This feature also makes the QD lasers attractive for space applications, but their failure modes and mechanisms are still unknown. For the present study, we investigated high-power broad-area lasers with strained InGaAs-AlGaAs QW and InAs-GaAs QD active regions. We performed short-term and long-term accelerated life-tests and failure mode analysis using various destructive and nondestructive techniques. We employed electroluminescence (EL) and time-resolved electroluminescence (TR-EL) techniques to study degradation processes in QW and QD window lasers. This configuration allows for observation of critical events including self-focusing of filaments, formation of dark spot and dark line defects (DLDs), and propagation of DLDs in real time during life-tests. We also employed focused ion beam (FIB) techniques to prepare TEM cross sections and plan-view TEM specimens of degraded QW and QD lasers for structural and defect analysis using a high-resolution TEM. Finally, we report our physics of failure investigation results on failure mechanisms in high-power broad-area lasers.
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