Myriam Oudart

By-emitter degradation analysis of high-power laser bars

The study of degradation process in high-power laser diodes, in particular, high-power laser bars, has become increasingly important as the output power of these devices continues to rise. We present a “by-emitter” degradation analysis technique, which examines degradation processes at both the bar and emitter levels. This technique focuses on understanding the dynamic mechanisms by which packaging-induced strain and operating conditions lead to the formation of defects and subsequent emitter and bar degradations. In the example presented, we examine a highly compressively strained bar, where thermally induced current runaway is found to be an important factor in the bar degradation and eventual device failure.

A physical model for the rapid degradation of semiconductor laser diodes

The degradation of AlGaAs based high power laser bars (808 nm) is modeled in terms of the thermal stress gradient induced by the overheating produced at a facet defect by self-absorption and nonradiative recombination. Using a thermomechanical model, the local heating at the defect is shown to induce local stress above the yield strength necessary for plastic deformation. Cathodoluminescence images of the facets show the formation of large facet defects. The role of the packaging stress is also elucidated. The power density dissipation and the local temperature necessary to achieve the plastic deformation are in good agreement with the experimental values reported for laser degradation.

Thermomechanical model for the plastic deformation in high power laser diodes during operation

A thermomechanical model for the mechanism of rapid degradation of AlGaAs based high power laser bars (808 nm) is presented. Thermal stresses induced in the device by local heating around a facet defect by nonradiative recombination and self-absorption of photons are calculated, as well as the conditions for the beginning of plastic deformation, when these thermal stresses overcome the yield strength. The values of the power density and of the local temperature at which the yield limit is surmounted are in agreement with the threshold values for the degradation of Al based lasers given in the literature. The present model can also elucidate the role played by the packaging stress, being able to explain how this stress reduces the optical power density threshold for failure of these lasers.

Cation intermixing at quantum well/barriers interfaces in aged AlGaAs-based high-power laser diodes bars

We report an analysis of quantum well (QW) degradation in high-power AlGaAs-based laser bars emitting at 808nm. Using low-temperature spectrally resolved cathodoluminescence (LT-SRCL) we evidenced a redshift of the AlGaAs QW luminescence peak in the less degraded regions and a blueshift in the heavily degraded parts. This blueshift is interpreted as an experimental evidence of cation intermixing between the QW and the barriers. A degradation scenario is proposed where locally higher defects concentration at QW interfaces triggers QW degradation assisted by recombination enhanced defect reactions (REDR) leading to cation intermixing as a final product of the degradation.

Mechanical strain and defect distributions in GaAs-based diode lasers monitored during operation

We monitor the mechanical strain and the defect concentration in AlGaAs–GaAs-based high-power diode laser arrays. This allows studying the interplay between these extrinsic parameters in dependence on device operation. There are two parameters, which contribute to the spread of the mechanical strain, the local position at the device, and, the device operation time that substantially enhances the strain. For midgap levels as well as shallower defect levels, which are due to physically different defects, very different creation scenarios are observed. The concentration of shallow defects and band-tail states is strongly correlated with compressive strain in their vicinity, no matter how the strain is created. For midgap levels, there is no direct correlation; however, an increase by a factor of 3 after 1500h of operation time is observed. The knowledge on defect creation scenarios is extensible to other GaAs-based devices.

Role of the thermal boundary resistance of the quantum well interfaces on the degradation of high power laser diodes

The influence of the quantum well (QW) interfaces with the barrier layers on the rapid degradation of AlGaAs based high power laser bars (808 nm) is investigated. Thermal stresses induced in the device by the local heating produced by nonradiative recombination areas at the facet mirror are calculated by means of a thermomechanical model. Results show that the laser power density threshold necessary to achieve the plastic deformation, leading to the generation of dislocations and to the failure of these devices, is reduced as the quality of the QW interfaces worsens in terms of thermal boundary resistance.

Gradual degradation of red-emitting high-power diode laser bars

The authors analyze early stages of gradual degradation in highly reliable 650nm emitting high-power diode laser arrays with continuous wave emission powers of 2.5W (facet load of 4mW∕μm). In all cases the edges of the metallized emitter stripes are identified as the starting points of gradual degradation. The magnitude of the observed degradation signatures, however, is highly correlated with the bar-specific packaging-induced strain at each emitter. We find a bar-specific effect, namely, the presence of packaging-induced strain, to be the driving force of gradual degradation. Our findings point to the significance of proper strain management in advanced device structures.

Relaxation of packaging-induced strains in AlGaAs-based high-power diode laser arrays

We monitor the mechanical strain evolution with typical use in the quantum well of AlGaAs∕GaAs-based high-power diode laser arrays (cm bars) by spectroscopic means. We show experimentally that pristine devices are essentially uniaxially compressed along the 110-direction with a strain maximum of −0.16% at the center of the device. At the device edges, almost no packaging-induced strain is detectable. After 500h of continuous wave operation at a current of I=80A, the strain is reduced by 50%. Furthermore, we observe the growth of a localized region of compressive strain, of hydrostatic symmetry, in one emitter of a particular cm-bar. A compression of about −0.017% is observed, and is most likely caused by point defect accumulation. Our results demonstrate information about absolute strain values and, at least in part, about strain symmetry as well can be obtained by spectroscopic means even within packaged complex optoelectronic devices.

Spatially resolved and temperature dependent thermal tuning rates of high-power diode laser arrays

Thermal tuning properties of passively cooled 808nm emitting high-power diode laser bars are analyzed. Data from standard devices packaged on Cu heat sinks and identical devices mounted on expansion-matched Cu–W heat sinks are compared. For a standard device, we find up to one-fifth of the thermal tuning rate of −(0.56±0.04)meVK−1 to be caused by pressure tuning driven by the relaxation of compressive packaging-induced stress for increasing temperatures. For devices packaged on expansion-matched heat sinks the observed tuning rate of −(0.46±0.01)meVK−1 represents almost the genuine thermal tuning rate of the semiconductor device structure. Thus this technology potentially leads to improved device properties.

Screening of high-power diode laser bars by optical scanning

High-power diode laser bar arrays (808 nm) with very uniform emission properties are inspected by the laser-beam-induced current technique (LBIC). Setting the excitation energy to 50 meV below the lasing energy, we observe distinctive signatures within the LBIC scans at certain locations on the devices. After 1000 h of high-power operation, we observe degradation at exactly those positions that previously showed a characteristic LBIC signature. A cathodoluminescence analysis reveals that the quantum wells of these anomalous device sections suffer from the existence of spots with reduced luminescence efficiency. Additionally, a concomitant 2.3 meV redshift of the quantum-well cathodoluminescence spectrum is also observed. Our measurements demonstrate the efficiency of the LBIC approach as a screening tool as well as its capacity for predicting device failure well before any degradation of the emission properties is observed.