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Degradation behavior and thermal properties of red (650 nm) high-power diode single emitters and laser bars Jens W. Tomm,a Tran Quoc Tien,a Mathias Ziegler,a Fritz Weik,a Bernd Sumpf,b Martin Zorn,b Ute Zeimer,b and Götz Erbertb* a

Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Str. 2A, 12489 Berlin, Germany ; b Ferdinand-Braun-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Str. 4, 12489 Berlin, Germany ABSTRACT

The degradation behavior of broad-area laser diodes and bars emitting at 650 nm under constant power operation is investigated. In addition to the increase in operation current the temperature of the laser facets was monitored using Raman spectroscopy. The formation of defects was studied using photocurrent spectroscopy while cathodoluminescence provided insight into the position of extended non-radiative defects at different stages of degradation. Although the facet does not show any visible alteration even for failed devices, its immediate vicinity appears to be the starting point of the observed gradual degradation effects. At the same time the local facet temperature is increased. The observed aging behavior is compared to the known degradation scenarios for devices emitting at 808 nm. In both cases there is a clear correlation between packaging-induced strain and observed degradation effects as demonstrated by the results obtained for bars. For the red devices a correlation between optical load and facet temperature exists which proves that here facet heating is indeed caused by re-absorption processes. Furthermore, the gradual degradation process is not accompanied by the creation of dark bands along 100 directions as observed earlier for 808 nm devices. The observed gradual degradation of the 650 nm devices is primarily accompanied by the formation of deep-level point defects, followed by the creation of macroscopic areas of reduced luminescence intensity. Packaging induced strains become important when gradual bar degradation is monitored at early stages. Keywords: 650 nm, high-power diode laser, cm-bar, degradation, thermal analysis

1. INTRODUCTION Continuous wave red-emitting InGaAlP laser diodes found a number of important commercial bulk applications such as in color printers, bar-code readers, and for writing and readout of optical discs. More powerful devices with emission powers exceeding the 100 mW-level can serve, e.g., in laser displays, as direct pump sources for Cr:LiSAF tunable, and or femtosecond lasers, and in medical applications such as photodynamic therapy. These high-power applications also require a sufficient long-term reliability of at least >1000 h. High power 650 nm broad area (BA) diode lasers for this wavelength region were reported by Orsila et al.1 and Toikkanen et al.2 For those devices the authors report maximum output powers of 2.1 W at 15°C for a diode laser with 150 µm stripe width and 1 mm length.1 Devices with 100 µm stripe width reached a slightly lower maximum output power of 2 W. The latter diode lasers had a maximal wall-plug efficiency of 37%.1 For the 100 µm stripe width lasers, the authors report reliable operation over 1000 h at 250 mW. Commercially available diode lasers from OSRAM3 claim reliable operation at 500 mW and 20°C over 4000 h. Recently, Sumpf et al.4 reported 6300 h reliable output power of 500 mW at 15°C (facet load: 5 mW/µm) and 1170 h at 600 mW (facet load 6 mW/µm). Both lifetime and facet load are still far below that, what is achieved for near-infrared devices. Thus there is still need for improving insight into the particular sources of degradation, establishing special degradation scenarios for red-emitting devices, etc. This topic represents the main goal of this study, which follows reliability investigations that have been performed for high-power BA devices. *

Further author information: (Send correspondence to J.W.T.) J.W.T.: E-mail: [email protected], Telephone: +49 (0)30 6392 1453 B.S.: E-mail: [email protected], Telephone: +49 (0)30 6392 2659

High-Power Diode Laser Technology and Applications V, edited by Mark S. Zediker, Proc. of SPIE Vol. 6456, 645606, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.696552

Proc. of SPIE Vol. 6456 645606-1

Ropers et al.5 investigated aging properties of red-emitting BA-lasers. They found that even in very early stages of degradation a variety of deep-level-defects become subsequently created by regular operation. It has been shown, that these deep-level-defects directly interact with photons, which are confined within the laser waveguide. Bou Sanayeh et al.6 reported work on BA devices, which were affected by catastrophical failure events. After removing the substrate, networks of reduced quantum well (QW) photoluminescence along the laser axis (110), which start in the close vicinity of the facet are detected. This is very similar, to what has been reported for 808 nm emitting devices, which suffered catastrophic degradation.7 Very similar observations regarding the formation of a network of reduced luminescence were made by Tien et al.8 In this report, we address the degradation behavior of BA laser diodes and bars emitting at 650 nm. After having described the behavior of the BA devices, the question is addressed, how the aging processes in bars differ from the ones in single BA devices. It is shown, that also for this type of bars, packaging-induced stress is a primary source of gradual degradation.

2. EXPERIMENTAL Compared to laser diodes in the near infrared region, the design of the structures for red-emitting devices is more challenging. The effective height of the barriers for electrons and holes is significantly smaller compared to the longer wavelength devices. This leads to higher temperature sensitivity and smaller wall-plug efficiencies. The transparency current density and the series resistance are larger. Together with the quality of the crystal and the facet this leads to relatively short lifetimes at small facet load. In the investigated structure an InGaP double QW was used as active layer embedded in AlGaInP waveguide layers. The n-cladding layer is formed by AlInP; the p-cladding layer is formed by an AlGaAs layer. This allows for carbon doping and the use of the less sophisticated AlGaAs process. For the BA laser structure, the threshold current for an uncoated 100 µm x 1 mm devices is Ith = 550 mA, the differential efficiency ηD = 0.76, the characteristic temperature of the threshold current T0 = 64 K, the transparency current density is jTR = 370 A/cm2, the modal gain Γg0 = 34 cm-1, the internal efficiency ηi = 0.86, the internal losses αi = 2.2 cm-1. BA lasers with stripe widths of 60 µm were processed. Low-mesa structures were fabricated in the stripe region. Outside the stripe, an insulator layer was sputtered. The p-side contact was formed by evaporating a Ti–Pt–Au contact. After wafer thinning and n-metallization of the wafer, this contact system was alloyed into the GaAs by a rapid thermal annealing process. Then, the wafer was cleaved to obtain a laser length of 750 µm. The facet coating was performed including a facet passivation as described by Ressel et al.9 For the characterization in CW regime, the facets of the devices were AR and HR coated (20% and 94%, respectively). Red-emitting cm-bars are prepared from the same material. They consist of 19 emitter sections with a pitch of 500 µm. The width of the BA section is reduced to 30 µm. All devices, BA lasers and bars, are mounted p-side (epi-side) down on Cu-W submounts and soldered with AuSn (p-side). The n-side contact has been performed by wire bonding. For facet temperature measurements, we employ a DILOR x-y-micro-Raman (µR) spectrometer equipped with a microscope and a liquid-N2-cooled CCD-camera. The 514.5 nm line of an Ar+-ion laser serves as excitation (excitation power ~850 µW, spot ∅1/e ~1 µm). The temperature calibration is done by measuring temperature dependent spectra at the non-operating device. Photocurrent spectra are detected with a BRUKER IFS 66v spectrometer. The laser beam induced current (LBIC) measurements are performed with an excitation wavelength of 633 nm, i.e. resonant to the lasing transition in the QW. For all techniques, the heatsinks, which the devices are fixed to, are stabilized to 25 °C. More details on the analytical methodology applied here, is collected in Ref. 7.

3. RESULTS 3.1. Analysis of aging properties of BA lasers For the discussion, we exemplarily chose 3 devices called BAA, BAB, and BAC. While device BAA was kept as a reference, devices BAB and BAC were aged at 400 mW constant power; see Fig. 1. Note that the setup, which serves for constant-power-aging, stops operation if the current suddenly increases by more than 20 percent above the average. Thus this aging regime not automatically leads to catastrophically degraded devices. Device BAB showed an averaged


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Figure 1. Degradation behavior of devices BAB and BAC for a constant emission power of 400 mW at T=15°C.

degradation rate of 8×10-5 h-1 over 500 h. Device BAC showed a similar rate of 11×10-5 h-1 but failed after 432 h, resulting in a sudden current increase by about 20%; see Fig. 1. A subsequent L-I-V-curve measurement revealed a threshold current increase to 0.26 A, and a slope efficiency reduction to 0.7 W/A. Figure 2 shows photocurrent spectra of the three devices. Obviously, the defect-related structures, see 1.0-1.8 eV range, are differently pronounced. The pristine reference device BAA exhibits almost no below-bandgap photocurrent, whereas the degraded one BAC shows distinct shoulders. In addition to the defect-related changes also the QWphotocurrent shows systematic changes following the degradation status of the particular device; see Fig. 2 (right). The creation of defects increases the number of recombination channels for non-equilibrium carries thus resulting in a reduced non-equilibrium carrier lifetime τ. Since the QW photocurrent monitors the steady-state non-equilibrium carrier population it can be considered a measure for τ. The defects, which cause the τ-reduction must not be physically the same as those, which are detected by the below-bandgap photocurrent; see the shoulders produced by defect-to-bandtransitions in Fig. 2 (left). Degradation is expected to produces physically different defects within devices and the two photocurrent contributions shown in Fig. 2 allow for visualizing a few of them. 150 Photocurrent signal (a.u.)

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Figure 2. Photocurrent spectra of all BA devices. The defect-region energetically below the devices lasing energy (left) shows the increase of the defect bands from device BAA (pristine) to device BAB (aged) and device BAC (degraded); see arrow. The QWregion (right) shows the opposite behavior; see arrow. The emission energy of the laser is indicated by a gray arrow, whereas the two other arrows indicate the tendencies related to gradual degradation.


BAA

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Figure 3. Electroluminescence pattern (left column) and optical microscope images (right column) of the front facets. In the microscope images, the epilayers appear slightly darker than the substrate (bright, top), whereas the heatsink (bottom) is substantially darker. The 60 µm wide BA-region serves as the ruler.

Figure 3 provides the results of standard inspection of the 3 BA devices. Visual inspection delivers absolutely no indication of any degradation. Scattered dark spots at the substrate above the active region, see at devices BAB and BAC, are no degradation signatures, but a known effect observed for many types of high-power diode lasers, even if operated, as in our case, under clean-room conditions. Sometimes this effect is called “particle collection phenomenon”. Electroluminescence pattern (left) show a slightly darker signature for the aged device BAB. This is consistent with the assumption of point-defect creation caused by the gradual aging; compare photocurrent results. Obviously in device BAC, severe degradation took place (see electroluminescence pattern), however, the facet still remains unaffected. Thus we decided to proceed with analysis of the status of the facets. This was accomplished by µR-measurements of the facet temperatures, which have been done after all other characterization was finished. The time for measuring a complete dependence is on the order of 10 h and after the µR-experiment, we observe for device BAB substantial power degradation. Thus, this particular experiment might suffer from interference by degradation and from now on the status of device BAB should be considered rather ‘degraded’ than ‘aged’. The µR data are presented in Fig. 4 (left column). A clear facet temperature increase is detected for the aged/degraded (BAB) and degraded (BAC) devices in contrast to the pristine one. After having completed these nominally non-destructive measurements, we remove n-contact and substrate; see Ref. 10. Figure 4 (right column) presents panchromatic cathodoluminescence maps of the active region plane. While the pristine device BAA does not show any specific signatures, we see areas with reduced luminescence within the emitter stripe for devices BAB and BAC starting at the device facet (left) and extending along the 110-direction of the cavity for ~ 310 and ~ 550 µm, respectively. Our findings point to a region in the immediate vicinity of the laser facet, which is actually not visibly altered by degradation, as the starting point of the observed effects. There are three distinct differences compared to earlier findings for 808-nm emitting devices:


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Figure 4. Front facet temperatures determined by µR versus operation current taken at the waveguide in the center of the emitter stripes of devices BAA, BAB, and BAC, respectively (left column). The full lines are guides to the eye. Cathodoluminescence images of the active region planes with the emitter stripe in center (right column). The images are taken after removing the substrate, i.e. after all other analytical work was finished. The devices are kept at T=80 K. The front facets are located at the left side of the images.

First we find a correlation between optical load and facet temperature, second, gradual degradation is not accompanied by the creation of dark bands along the 100 direction as found in InAlGaAs 808 nm emitting devices7 and third, there is absolutely no visible sign of facet degradation. The first finding points to the absorption of laser light as a major source for facet heating. Degradation is primarily accompanied by the creation of deep-level point defects eventually followed by the creation of macroscopic areas of reduced luminescence along the laser axis. It should be noted that our findings are very similar to those reported by Bou Sanayeh,6 who claims to have analyzed devices, which experienced catastrophic degradation.

3.2. Analysis of aging properties of cm-bars After having dealt with BA devices, it seems to be interesting to learn, up to what degree this knowledge will help us to understand the degradation behavior of bars. The BA devices, of which a bar consists, experience basically the same as single BA devices usually do. In addition the thermal surroundings as well as packaging induced strains, which are more relevant in such extended devices as bars are, are substantially modified. In section 3.1 we were able to show that gradual degradation is accompanied by point defect creation. Thus it seemed to be appropriate to search for the sources, i.e. the starting points, of this effect. This has been done by LBIC-analysis of the bars. Here we employ the effect, which is illustrated by Fig. 2 (right). The QW photocurrent is correlated to τ, and τ is a measure for the material quality. In our experiment, probing of τ takes place by LBIC excitation by 633 nm photons, i.e. 17 nm (51 meV) above the onset of the QW absorption (or laser emission), i.e. well below the barrier. Thus resonant QW excitation is ensured.


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Figure 5. LBIC scans across the complete cm-bar RA before operation (full line) and after an operation time of 200 h (dashed line) at 2.5 W constant power operation (left). Ratio of the LBIC signals obtained after 200 h by the one obtained for the pristine device at one particular emitter (right). The region of the emitter stripe is marked by the hatched area. This ratio is indicative for the operationinduced reduction of the carrier lifetime in the QW; i.e. a measure for gradual degradation.

Figure 5 presents these data for a bar before and after 200 h of operation (left). During the constant power aging, the current increase for this particular device amounts to about 0.5 percent within the 200 h only. Nevertheless, LBIC data reveal a signature, namely a decrease of the QW photocurrent in the close vicinity of the emitters. For one particular emitter, this is presented on an expanded scale; see Fig. 5 (right). Thus the τ-reduction takes place at the edges of the emitter stripes and becomes extended into the surrounding of the emitters. The active region, i.e. the region underneath the metallization seems to be still less affected at this early stage. Now we tried to correlate the strength of the degradation signature, i.e. the amount of LBIC-signal reduction, with other parameters, that are acting along the bar in a non-uniform way, such as averaged temperature during operation and packaging-induced stress. Figure 6 (left) represents the spatial distribution of the packaging-induced strain as determined from photocurrent spectra.7 Although expansion-matched Cu-W material has been used, a slight deviation of the thermal expansion coefficient of the heat spreader material from the one of the GaAs is the most likely source of the observed bow-like strain distribution. 0.98 LBIC signal (%)

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Figure 6. Photon energy of the lasing transition versus local position determined by photocurrent spectroscopy (left). The photon energy shift is a measure for the packaging-induced strain. The right part shows the LBIC signal reduction as a measure for gradual degradation versus photon energy of the lasing transition at the corresponding local position. Thus increased packaging-induced strain results in enhanced gradual degradation.


The right part of Fig. 6 shows the correlation between packaging-induced strain (represented by the photon energy at the abscissa) and the LBIC-signal ratio (after to before operation as a measure for τ); see ordinate. There is clear indication that in very early stages of degradation, also in red-emitting devices the packaging-induced strain acts as the driving force for creating non-uniformity within the bar. Analysis of aging processes in red-emitting bars will have to take into account these starting points of gradual degradation.

4. SUMMARY Thermal properties and the degradation behavior of high-power BA diode lasers emitting at 650 nm are analyzed. Facet temperature is measured by micro-Raman spectroscopy. Although no visible facet alteration is observed, power degradation is found to be accompanied by increased temperatures at the facets. The immediate vicinity of them also turns out to be the starting point for the creation of defect networks within the quantum well seen in cathodoluminescence images. The observed behavior is compared to that known for near-infrared emitting devices. In addition, we analyze gradual degradation of bars, made of the same material. It turns out that at very early stages of gradual degradation, the packaging-induced strain acts as a major driving force for creating non-uniformity in the bar. Thus starting points for severe degradation mechanisms are created.

ACKNOWLEDGMENTS The authors would like to thank Tobias Grunske, Sandy Schwirzke-Schaaf, and Petra Wochatz for expert technical assistance. This study was funded by the European Union within the project WWW.BRIGHT.EU, contract No. 511 722.

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