U. Zeimer

Novel passivation process for the mirror facets of Al-free active-region high-power semiconductor diode lasers

A novel process for the passivation of mirror facets of Al-free active-region high-power semiconductor diode lasers is presented. Designed for technological simplicity and minimum damage generated within the facet region, it combines laser bar cleaving in air with a two-step process consisting of 1) removal of thermodynamically unstable species and 2) facet sealing with a passivation layer. Impurity removal is achieved by irradiation with beams of atomic hydrogen, while zinc selenide is used as the passivating medium. The effectiveness of the process is demonstrated by operation of 808-nm GaAsP-active ridge-waveguide diode lasers at record optical powers of 500 mW for several thousand hours limited only by bulk degradation.

12 W continuous-wave diode lasers at 1120 nm with InGaAs quantum wells

Highly strained InGaAs quantum wells were grown by metalorganic vapor-phase epitaxy. By lowering the growth temperature to 530 °C, a maximum photoluminescence wavelength of 1192 nm was achieved. High-power diode lasers with a maximum lasing wavelength of 1175 nm were fabricated. A continuous-wave output power of 12 W at a heat-sink temperature of 25 °C was obtained at a lasing wavelength of 1120 nm.

MOVPE growth of highly strained InGaAs/GaAs quantum wells

The indium incorporation into strained InGaAs quantum wells grown on GaAs substrate by metalorganic vapourphase epitaxy is found to be reduced in comparison to relaxed layers. Additionally, the indium uptake into strained QWs is limited to approximately 30% InAs at 650°C. Excessive trimethyl indium supply in the vapour phase leads to a drop of the In-content of the QW and to a reduced total In-content in the whole structure. Only a small amount of the excess indium is incorporated into InAs-rich clusters observed as dark-spot defects and into a graded interfacial layer. A model for this behaviour based on the enhanced In-reevaporation from In-rich areas is presented.

20W continuous wave reliable operation of 980nm broad-area single emitter diode lasers with an aperture of 96μm

High power broad area diode lasers provide the optical energy for all high performance solid state and fiber laser systems. The maximum achievable power density from such systems is limited at source by the performance of the diode lasers. A crucial metric is the reliable continuous wave optical output power from a single broad area laser diode, typically for stripe widths in the 90-100 μm range, which is especially important for users relying on fibered multi-mode pumps. We present the results of a study investigating the reliable power limits of such 980nm sources. We find that 96μm stripe single emitters lasers at 20°C operate under continuous wave power of 20W per emitter for over 4000 hours (to date) without failure, with 60μm stripe devices operating reliably at 10W per stripe. Maximum power testing under 10Hz, 200μs QCW drive conditions shows that 96μm stripes reach 30W and 60μm stripes 21W per emitter, significantly above the reliable operation point. Results are also presented on step-stress-studies, where the current is step-wise increased until failure is observed, in order to clarify the remaining reliability limits. Finally, we detail the barriers to increased peak power and discuss how these can be overcome.

Catastrophic optical mirror damage in diode lasers monitored during single-pulse operation

Catastrophic optical mirror damage (COMD) is analyzed for 808 nm emitting diode lasers in single-pulse operation in order to separate facet degradation from subsequent degradation processes. During each pulse, nearfield and thermal images are monitored. A temporal resolution better than 7 μs is achieved. The thermal runaway process is unambiguously related to the occurrence of a “thermal flash.” A one-by-one correlation between nearfield, thermal flash, thermal runaway, and structural damage is observed. The single-pulse excitation technique allows for controlling the propagation of the structural damage into the cavity. We propose this technique for the analysis of early stages of COMD.

Mechanism of ohmic contact formation in AlGaN/GaN high electron mobility transistors

We tested various ohmic contact metallization schemes on AlGaN/GaN HEMTs to achieve low contact resistance, good surface morphology and proper line edge definition. Mo, Ni and Pt intermediate layers in these schemes replaced the intermediate Ti layer interposed between Al and Au in Ti/Al/Ti/Au contacts. We recorded almost similar values of the contact resistance lying in the range of 0.3–0.5 Ω mm on all the metallization stacks, but a significant difference was observed in their surface morphology and line edge definition. The formation of particular intermetallic compounds due to different intermediate layers was found to be responsible for this difference. Ti/Al/Ti/Au/WSiN contacts possessed excellent surface morphology. Mo based contact not only delivered good surface morphology and low contact resistance but also proper line edge definition. We found that the formation of the Al–Mo phase and GaMo3 compound was the key factor responsible for its remarkable performance.

Defect evolution during catastrophic optical damage of diode lasers

We present an analysis of the catastrophic optical damage effect that is artificially provoked in 808 nm emitting broad area diode lasers by single current pulses. The kinetics of the sudden degradation process, monitored with a nanosecond temporal resolution, is linked to the damage pattern observed. This involves in situ tracing of emission power and hot-spot motion within the cavity as well as the verification of the resulting defects by defect spectroscopy and cathodoluminescence mapping. A complementary model is presented which explains the shape of the observed defect pattern. The combination of unidirectional energy transfer to defects by laser light within the laser cavity, spatially isotropic defect growth, and the presence of shadowing effects explain the complex damage pattern observed in the gain material, including effects of defect branching. The study is made with standard industrial devices making the findings directly applicable for device testing and performance improvements.

Electrical properties and microstructure of vanadium-based contacts on ICP plasma etched n-type AlGaN:Si and GaN:Si surfaces

Light-emitting diodes emitting in the UV-B spectral range usually contain an n-type AlxGa1?xN contact layer with x of about 0.4 and require a low specific contact resistivity in the range of 10?6?? cm2?to operate at their optimal capacities. We compare the Ti/Al/Mo/Au metal system commonly used on GaN with the V/Al//V/Au metal system on n-Al0.4Ga0.6N:Si and GaN:Si surfaces. On Al0.4Ga0.6N, the lowest specific contact resistivity, (2.3 ? 0.2)??10?6?? cm2, was reached with V/Al/V/Au whereas for Ti/Al/Mo/Au the lowest resistivity was (1.7 ? 0.7)??10?3?? cm2. Independently of the metal system, lower contact resistivities are obtained on GaN where Ti/Al/Mo/Au is still performing at least one order of magnitude better than V/Al/V/Au. The contact resistivity of V/Al/V/Au on Al0.4Ga0.6N was found to be sensitive to surface treatments (wet chemical etching or sputtering) applied prior to metal deposition, to the thickness of the first vanadium layer and to that of the aluminum layer above. Structural and compositional investigations showed that a low contact resistivity is accompanied by the formation of an interfacial layer between AlGaN and V/Al/V/Au on the nm-scale, which contains aluminum, in the form of aluminum nitride, and separately vanadium as metal or metal nitride.

Thermal properties and degradation behavior of red-emitting high-power diode lasers

The thermal properties and the degradation behavior of high-power broad-area diode lasers emitting at 650nm are analyzed. Imaging thermography is applied to assess the bulk temperature while the 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.

How does external feedback cause AlGaAs-based diode lasers to degrade?

The effect of external feedback on the degradation of 808 nm emitting AlGaAs-based high-power broad-area diode lasers is studied. For this purpose, early stages of gradual degradation are induced by accelerated aging at high power levels. While the quantum well that actually experiences the highest total optical load remains unaffected, severe impact by point defects is observed on the cladding layers and the waveguide. Extended defects such as dislocations, however, are not observed in such early stages of degradation, which are accompanied by gradual power loss of a few percent only.