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Semiconductors
Semi-Insulating GaAs and InP Semiconductor Wafer Quality Uniformity
Problem
• Performance and reliability of optical and electrical devices, such as laser diodes and metal-semiconductor field-effect transistor MESFETs, are impacted by imperfections including, dislocations and point defects, and vary across semi-insulating as-grown and processed 2-inch GaAs and InP:Fe wafers.
Technique & Results
• Use of Photoluminescence PL as a simple but particularly sensitive and non-destructive technique to detect defect densities as low as 1011 cm-3.
• Build high-spatial resolution scanners to detect integrated near-band-edge
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• Anti-correlation between
• Comparison between emission and transmission PL topographs allowed surface defects to be distinguished from volume defects.
Outcome
• Assessing uniformity and quality of as-grown and processed wafers by recording PL maps led to a dramatic improvement of device functionality and processing yield. (World’s-first).
Selected References
→ Epperlein: “Photoluminescence topography studies in
Quality Measure of Epitaxially-Grown Semiconductor Material
Problem
• The quality of epitaxially grown compound semiconductor layers and structures including quantum heterostructures and interfaces sensitively determine the functionality of optical and electrical devices.
Technique & Results
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• Luminescence intensities of radiative recombinations – exciton (free and bound), band-to-band, band-to-donor, band-to-acceptor and donor-acceptor pair transitions – were used as suitable quantitative quality indicators.
Outcome
• Optimized semiconductor layers grown by MBE or MOVPE led to device products with excellent performance and reliability.
Quantum Well Laser Performance and Reliability
Problem
• Performance and reliability of InGaAs/AlGaAs quantum well laser diodes impacted by laser wafer growth.
Technique & Results
• Resolution – Characterization of quality of grown structure, i.e. active quantum well, cladding layers and interfaces by low-temperature (2K)
• PL spectra clearly showed features that impurities (carbon) in growth chamber residual atmosphere are being deposited at high concentrations before the growth of the quantum well layer.
• These non-intentional impurities/doping close to the active quantum well impact functionality of laser sensitively.
Outcome
• Remedial actions by growing so-called thin pre-layers before the actual quantum well layer improved the functionality of the laser significantly. Pre-layers getter the impurities floating on growth surface and keep them away from the quantum well. (World’s-first).
Selected References
→ Epperlein, Meier: “Impurity trapping in nominally undoped GaAs/AlGaAs quantum wells” in “Defect Control in Semiconductors”, ed. Sumino, Elsevier Science Publisher, 1223-1228, (1990). ISBN 0-444-88429-7.
Local Temperatures along Laser Diode Cavities
Problem
• Hot spots in the laser cavity – close to laser facets or in the bulk of the cavity – affect performance and reliability of laser diodes.
Technique & Results
• Measurement of the local cavity temperature along the cavity from the spectral shift of the Electroluminescence EL emitted through a lateral window.
• Fast decay of temperature from mirror into the cavity over a length of 6 µm (1/e point)
• Bathtub-like distribution of temperature along cavity – mirrors are ‘hot’ and cavity is ‘cold’.
Outcome
• Effective reduction of mirror temperatures by a factor of 3 by depositing a heat spreader metal layer onto the top of the laser structure and lined up with the mirror edge. Cooling effect of this configuration is more efficient than that of mounting the laser with junction-side down onto a heat-sink. Laser diodes with such heat spreaders possessed much improved performance and reliability. (World’s-first).
Selected References
Laser Diode Mirror Temperatures
Problem
• Laser degradation processes, such as catastrophic optical mirror damage COMD, depend sensitively on local operating temperatures on the laser facet surfaces.
Technique & Results
• Pioneered Raman spectroscopy to measure temperatures on laser mirror facets with sub-micrometer spatial resolution. (World’s-first).
• Temperatures dependent sensitively on optical power emission, laser structure, number of active quantum wells, materials, mirror design, mirror passivation and reflectivity coating, laser chip mounting.
Outcome
• Optimization of all these parameters led to devices with significantly increased performance (e.g. higher kink-free power and thermal roll-over point) and reliability (e.g. increased COMD power level and life time).
Selected References
Root Cause Analysis and Impact of Increased Laser Mirror Temperatures
Problem
• Catastrophic optical mirror damage (COMD) is a well-known major degradation mechanism in laser diodes. Exceedingly high local temperatures are responsible for causing a so-called thermal runaway, which finally leads to COMD, i.e. physical damage of the laser mirror. The details of the actual root causes for the high temperatures are usually not known.
Technique & Results
• Pioneered Raman microprobe spectroscopy to detect sensitively structural and compositional disorder in the crystal lattice of the laser mirror material.
• Established a (world’s-first) triple correlation between the strength of structural / compositional disorder, laser facet temperature, and laser COMD power level. That is, the higher the lattice disorder, the higher the temperatures and the lower the COMD power.
Outcome
• Careful and special preparation of laser facets including passivation treatment and reflectivity coating led to more robust laser diodes with substantially higher COMD power levels.
Selected References
→ Epperlein: “Correlations between disorder, heating and failure of semiconductor lasers” in “Conference on Lasers and Electro-Optics / CLEO“, OSA 11, 158-159, (1993).
Unstable Laser Mirror Coating Material
Problem
• Fast recrystallization of amorphous silicon in ion-beam or electron-beam deposited high-reflectivity coating stacks of lasers under high optical power. Change of emitted power at back facet, movement of optical mode (“kink”) and shrinkage of lateral near-field pattern with increasing recrystallization.
Technique
• Real-time observation of recrystallization dynamics in Raman spectra recorded with high spatial resolution on laser mirrors. (World’s-first).
Outcome
• Detrimental recrystallization effects leading to instable laser operation can be avoided by depositing amorphous silicon in coating stacks with a chemical vapour deposition technique using silane.
Selected References
→ Epperlein, Gasser: “Silicon recrystallization effects in mirror coatings of high-power 980-nm InGaAs/AlGaAs lasers” in “Int. Symp. Compound Semicond.”, Inst. Phys. Conf. Ser. 141, 537-542, (1994).
→ Epperlein: “Raman spectroscopy of semiconductor lasers” in “Conference on Lasers and Electro-Optics / CLEO“, OSA 9, 108-109, (1996).
Highly Localized Hot Spots on Laser Diode Mirror Facets
Problem
• High temperatures on laser mirrors sensitively reduce the laser functionality, in particular, reliability by triggering degradation mechanisms, such as COMD. Of great importance for remedying this negative effect would be, therefore, to know the temperature distribution across mirror surfaces.
Technique
• Pioneered thermoreflectance, a fast powerful alternative technique to Raman spectroscopy, for laser diode thermal metrology. Generated in the early 90’s a world’s-first temperature map of a laser diode mirror by raster-scanning a probe laser beam across the mirror surface of the laser under power.
Outcome
• Temperature maps show a striking, well-defined, localized hot spot within the near-field pattern NF. Temperatures decay rapidly into the adjacent layers with a strength dependent on the layer material thermal conductivity values. Effective passivation and non-absorbing mirror technologies along with tailoring the NF pattern can lead to much reduced temperatures in the hot spot and its negative effects.
Selected References
→ Epperlein: “Reflectance modulation – a novel approach to laser mirror characterization“ in “Int. Symp. Compound Semicond.”, Inst. Phys. Conf. Ser. 112, 633-638, (1990).
→ Epperlein, Martin: “Reflectance modulation studies on laser diode mirrors” in “Int. Symp. Compound Semicond.”, Inst. Phys. Conf. Ser. 120, 353-358, (1991).
Temperature–Monitored Laser Degradation Dynamics
Problem
• Laser degradation is usually accompanied by an increase in temperature. Monitoring the temperature with time could reveal essential information on the development of a specific degradation mechanism and hence deliver details on possible corrective actions. Hitherto this information was not accessible.
Technique & Results
• Pioneered thermoreflectance in world’s-first experiments to monitor in real time the temperature in various degradation modes, such as, dark-line defects, time to COMD and critical temperature at COMD, i.e. thermal runaway.
Outcome
• Knowing, for example, the critical temperature to COMD, which is 415 K for the AlGaAs system and practically independent of surface treatment and coating, allowed to prepare the mirror surfaces such that never this critical temperature was exceeded even under the operation of the highest practical power levels. In general, the information obtained from these temperature-monitored laser degradation studies has significantly contributed to a better understanding of the underlying mechanisms and thus has led to the development of more reliable devices.
Selected References
High-Power Laser Diode Optimization with Bent-Waveguide Non-Absorbing Etched Mirrors
Problem
• To assess and optimize the effectiveness of a non-absorbing mirror
Technique & Results
• Optimization was achieved by measuring laser mirror temperature and catastrophic optical mirror damage COMD level for a set of laser diodes with different
• Data show a minimum in mirror temperatures and maximum in COMD levels over a
Outcome
• Lasers with bent-waveguide NAMs resulted in a 5x improvement of the COMD power levels compared with lasers with conventional mirrors, and hence in a dramatically improved functionality.
Selected References
Mechanical Stress in Laser Diodes
Problem
• Stress fields in laser diode structures may (i) trigger the formation of detrimental defect centres leading to reduced laser lifetime and (ii) also impact, for example, the optical mode and threshold current.
Technique & Results
• Pioneered Raman spectroscopy to measure mechanical stress on laser diode mirror facets with sub-micrometer spatial resolution. (World’s-first).
• Ridge waveguide lasers showed a characteristic camel hump-like stress profile with high compressive stress levels close to the ridge slopes and reduced compressive stress or even low tensile stress fields towards the ridge center. Independent photoluminescence spectroscopy measurements and stress modelling confirmed the findings.
• Origin of these stress fields were the dielectric ridge embedding layer and p-metal film on top of the ridge. Controlled measurements using a three-point laser bar bending technique confirmed the impact of stress on performance and reliability.
Outcome
• Careful selection of ridge profile, materials and processing conditions led to reduced stress levels resulting in more reliable laser diodes with improved performance parameters.
Selected References
→ Epperlein, Fried, Jakubowicz: “Stress-iduced defects in GaAs quantum well lasers“ in “Int. Symp. Compound Semicond.”, Inst. Phys. Conf. Ser. 112, 567-572, (1990).
→ Epperlein, Hunziker, Daetwyler, Deutsch, Dietrich, Webb: “Mechanical stress in AlGaAs ridge lasers: its measurement and effect on the optical near field“. Inst. Phys. Conf. Ser. 141, 483-488, (1994).
Stress-Induced Defects in Ridge Waveguide Laser Diodes
Problem
• It is of vital importance to understand and reveal the fundamental degradation processes in laser diodes, such as rapid degradation by large dislocation network growth, gradual degradation due to formation of lattice defects and fast catastrophic optical mirror damage at high optical power densities. In this context, it is essential to know, among others, the relationship between local stress and local susceptibility to degradation processes.
Technique & Results
• Pioneered a procedure to detect “weak spots” on laser mirrors by exposing these mirrors to electron irradiation in a scanning electron microscope to activate the formation of defects. The distribution of these activated defects was imaged in-situ by spatially resolved electron-beam induced current EBIC measurements as a function of time. (World’s-first).
• Results showed that the crossover from compressive to tensile stress and the strongest spatial EBIC signal changes are located close to the lower ridge edges which are most susceptible to degradation processes.
• This can be explained by taking into account a possible migration and separation of point defects in stress fields with strong gradients, i.e. interstitials move to dilated areas and vacancies to compressed areas.
Outcome
• Also here, an optimization of ridge profile, ridge embedding layer and p-metal contact layer and processing conditions led to reduced stress levels with softer gradients resulting in higher quality laser diodes.
Selected References
→ Epperlein, Fried, Jakubowicz: “Stress-iduced defects in GaAs quantum well lasers“ in “Int. Symp. Compound Semicond.”, Inst. Phys. Conf. Ser. 112, 567-572, (1990).
Deep Level Defects Impact Semiconductor Device Functionality
Problem
• Deep traps in semiconductors can pose well-known problems regarding the performance and reliability of optical and electrical semiconductor devices. To be in control of these parameters, it is therefore essential to determine types and concentrations of detrimental traps which can be introduced, in principle, at all significant stages of the device manufacturing process.
Technique & Results
• Deep-Level-Transient-Spectroscopy (DLTS) was employed at all development stages to measure deep traps in laser structures and metal-semiconductor field-effect transistors (MESFETs).
• Examples include, active and barrier layers of quantum well lasers where in one case deep electron traps were accumulated in the top AlGaAs barrier layer within 15nm close to the interface of an MBE-grown GaAs/AlGaAs single quantum well laser structure. The origin of these traps was an insufficiently high growth temperature for AlGaAs ramped up from a temperature usually to be lower for the GaAs growth.
Outcome
• Systematic DLTS measurements on trap species, concentrations and in-depth profiles on as-manufactured as well as failed devices enabled the design, growth and processing of optical and electrical devices with state-of-the-art performance and reliability parameters.
Selected References
Laser Diode Emission Characteristics
Problem
• Vertical far-field angle too big leading to dramatically reduced effective fiber-coupled power levels. Vertical far-field angles were in stark contradiction to epitaxy growth conditions in particular selected material compositions.
Technique & Results
• Approach - Independent measurement of composition.
• Use of sophisticated material analysis technique –
• Results showed composition (aluminium mole fraction in AlGaAs) was wrong.
• Correction down in composition for optimized laser growth.
Outcome
• Significant improvement of fibre-coupled powers due to reduced far field angles (corrected material composition of laser cladding layers).
Laser Diode Reliability
Problem
• Reliability predominantly governed by laser facet failures (catastrophic optical mirror damage COMD).
Technique & Results
• Resolution approach - Root-causing by measuring temperatures and material instabilities on laser facets.
• Pioneered application of Raman spectroscopy in laser characterization (world’s-first results).
• Pioneered Thermo-Reflectance as powerful and fast alternative technique to Raman for temperature measurements (world’s-first results).
• Laser facets can develop high temperatures and hence show material instabilities dependent on laser structure, material and optical output power.
Outcome
• Significantly higher laser reliability, i.e. reduced COMD events by strongly reduced laser heating through appropriate passivation of laser facets, laser design (heat spreader) and die mounting on heat sink (junction down).
Selected References
Reliability Qualification Testing
Problem
• One of the most critical requirements in reliability work is to know early in the life of a product how that product will perform at some time in the future. This means that reliability information must be obtainable in a short time, and that it must be predictive.
Technique
• Custom-designed accelerated testing methods were applied to predict the reliability of various devices in a relatively short time. For example, a multi-cell lifetest, i.e. a matrix of several lifetests covering a range of temperatures and drive currents was performed on a large population of laser diodes from different wafers. Stress test conditions were carefully selected on the basis of expected failure mechanisms, that is, (i) not to generate additional degradation mechanisms but to (ii) generate failures so that the underlying life distribution (lognormal, exponential or Weibull) can be revealed and the actual lifetime at normal operating conditions calculated, taking into account the effective acceleration factor.
Outcome
• Accelerated life tests proved to be highly useful delivering the required reliability prediction in an acceptable short time. For example, lifetests on pump lasers for submarine optical communications applications delivered extremely low failure rates, i.e. <1% and < 100 FITs for wear-out and sudden failures, respectively, both at 25˚ C and during 27-year system lifetime. These data were in remarkable agreement with data collected on field failures.
Resonant Tunnelling Diode – Essential Part of a Novel Optical Modulator
Problem
• Resonant tunnelling diode (RTD) performance was not sufficient for proper operation of a novel, low-drive voltage, optical indium phosphide modulator with wide margins (Essient Photonics Ltd.)
Technique
• New design – careful selection of optimized material compositions, layer thicknesses and molecular-beam-epitaxy growth conditions, such as, substrate temperature and growth rate.
Outcome
• Redesigned RTD devices showed excellent performance, that is, state-of-the-art peak current / valley current ratios >20 which led to an improved modulator operation.
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