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.
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.
Remarkable progress made in vertical cavity surface emitting lasers (VCSELs) emitting at 850 and 980 nm has led them to find an increasing number of applications in high speed data communications as well as in potential space satellite systems. However, little has been reported on reliability and failure modes of InGaAs VCSELs emitting at ~980 nm although it is crucial to understand failure modes and underlying degradation mechanisms in developing these VCSELs that exceed lifetime requirements for space missions. The active layer of commercial VCSELs that we studied consisted of two or three InGaAs quantum wells. The laser structures were fabricated into deep mesas followed by a steam oxidation process to form oxide-apertures for current and optical confinements. Our multi- mode VCSELs showed a laser threshold of ~ 0.5 mA at RT. Failures were generated via accelerated life-testing of VCSELs. For the present study, we report on failure mode analysis of degraded oxide-VCSELs using various techniques. We employed nondestructive techniques including electroluminescence (EL), 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 to study VCSELs that showed different degradation behaviors. Especially, we employed FIB systems to locally remove a portion of top-DBR mirrors of degraded VCSELs, which made it possible for our subsequent EBIC and OBIC techniques to locate damaged areas that were generated as a result of degradation processes and also for our HR-TEM technique to prepare TEM cross sections from damaged areas. Our nondestructive and destructive physical analysis results are reported including defect and structural analysis results from pre-aged VCSELs as well as from degraded VCSELs life-tested under different test conditions.
Finite difference time domain (FDTD) simulations are used to find the electric field intensity at the center of a cluster
of plasmonic nanoparticles irradiated by a planewave source. We use an iterative optimization algorithm to maximize the
electric field intensity. The resulting optimized configurations are found to be non-symmetric and non-intuitive, and
cannot be obtained by analytical calculation methods. Experimentally, we investigate a novel technique using angle
evaporation to produce plasmonic nanostructures with gap sizes of 1-2 nm. We evaluate the plasmonic activity of these
nanoparticles both experimentally using surface enhanced Raman spectroscopy (SERS) measurements and theoretically
using FDTD simulations. These simulations predict an electric field intensity enhancement of 82,400 at the center of the
nanoparticle dimer, and an electromagnetic SERS enhancement factor of 109-1010.
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