Martin Hempel

In modern electronic modules, the packing density of components is increasing. This means that the points at which different assembly and connection technologies are used are moving closer and closer together. This significantly increases the probability of mutual interference. The negative effects of later process steps in particular can be critical here. In the present work, a typical case for power electronics production is considered, consisting of thick-wire bonding on the power semiconductor and subsequent ultrasonic welding of load terminals on the same substrate. The effect on the quality of the bond connections was investigated in setups with different load geometries, materials and load intensities. It was found that both wire damage due to cyclic mechanical alternating loading and a change in the interface between substrate and wire occurred. In the case of harder wire materials, complete breakage of the wire connections was observed after just 24 welds on terminals in typical geometries. However, the interface between the wire and the substrate was also damaged and lost 10% of its strength in this case. The investigations show how such effects can be minimized by the choice of wire material and geometry. This is another important building block in holistic process development, taking into account the interactions between different interconnection processes.

Among the limitations known from semiconductor lasers, catastrophic optical damage (COD) is perhaps the most spectacular power‐limiting mechanism. Here, absorption and temperature build up in a positive feedback loop that eventually leads to material destruction. Thus, this is truly an ultimate mechanism, and its continued suppression is a manifestation of progress in device design and manufacturing. After an overview of the current state of knowledge, new investigations of COD using artificially micrometer‐sized starting points created within the active zone in the cavity of 450 nm GaN semiconductor lasers are reported on. Defect growth mechanisms and characteristics are studied during 800 ns current pulses. The defect growth follows the highest light intensity. Secondary defect patterns are studied: complete destruction of the active zone and generation of a point defect cloud at least ≈10 μm into the remaining surrounding material. Extremely large angles (>90°) of damage growth are traced back to the material properties and the aging scenario. The results are compared with former experiments with GaAs‐based lasers.

Damage patterns caused by Catastrophic Optical Damage (COD) are analyzed in GaN-based high-power diode lasers. We find the process to result in material loss including the formation of an empty channel. This is consistent with the observation of ejections of hot material out of the front facet of the device during COD. In the immediate vicinity of the empty channel, the laser structure seems to be absolutely undisturbed. These results are compared with earlier results from comparable experiments obtained with GaAs-based devices. While the COD process in both material systems follows similar scenarios and root causes, the damage pattern differs substantially. The completely empty channel at the position of the optical mode is in striking contrast to the results of earlier studies in GaAs-based devices, degraded under almost identical conditions. There molten, phase segregated, and both recrystallized and amorphous materials with well-pronounced melting fronts are observed. In the GaN-based devices we observe average defect front propagation velocities along the laser axis of 110 m/s. This is faster by a factor of 4-5 than what has been observed in GaAs-based devices under comparable experimental conditions.

Terahertz (THz) quantum-cascade lasers (QCLs) are powerful radiation sources for high-resolution and high-sensitivity spectroscopy with a discrete spectrum between 2 and 5 THz as well as a continuous coverage of several GHz. However, for many applications, a radiation source with a continuous coverage of a substantially larger frequency range is required. We employed a multi-mode THz QCL operated with a fast ramped injection current, which leads to a collective tuning of equally-spaced Fabry-Pérot laser modes exceeding their separation. A continuous coverage over 72 GHz at about 4.7 THz was achieved. We demonstrate that the QCL is superior to conventional sources used in Fourier transform infrared spectroscopy in terms of the signal-to-noise ratio as well as the dynamic range by one to two orders of magnitude. Our results pave the way for versatile THz spectroscopic systems with unprecedented resolution and sensitivity across a wide frequency range.

Damage patterns caused by the Catastrophic Optical Damage (COD) are analyzed in GaN‐based high‐power diode lasers. In contrast to standard failure analysis, the devices are intentionally degraded under well‐defined conditions. We find that defect growth during COD is fed by the optical mode, that is, laser energy. This process involves melting and even vaporization of quantum‐well and waveguide materials. Average defect propagation velocities along the laser axis of 110 m s⁻¹ are observed. COD results in material loss including the formation of an empty channel. This is well consistent with material loss due to ejections of hot material out of the front facet of the device. The laser structure in the immediate vicinity of the empty channel seems to be absolutely undisturbed and no transition regions or remaining material are observed. These results are compared with earlier findings from comparable experiments obtained with GaAs‐based devices.

The dynamical tuning due to rear facet illumination of single-mode, terahertz (THz) quantum-cascade lasers (QCLs) which employ distributed feedback gratings are compared to the tuning of single-mode QCLs based on two-section cavities. The THz QCLs under investigation emit in the range of 3 to 4.7 THz. The tuning is achieved by illuminating the rear facet of the QCL with a fiber-coupled light source emitting at 777 nm. Tuning ranges of 5.0 and 11.9 GHz under continuous-wave and pulsed operation, respectively, are demonstrated for a single-mode, two-section cavity QCL emitting at about 3.1 THz, which exhibits a side-mode suppression ratio better than −25 dB.

Shortwave infrared emission from 450 nm InGaN diode lasers is analyzed, and its physical origin is located by SWIR imaging of operating devices. Emission spectra taken in the 900-1700 nm range reveal three main contributions located at 900-1130 nm, 1130-1350 nm, and beyond 1350 nm. In concert with photoluminescence measurements at the substrate, these emission bands are identified as, first, genuine deep-level electroluminescence from the active region and deep-level defect-related emission from the substrate that is pumped by spontaneous 450 nm primary emission, second, pure deep-level defect emission, and third, Planck’s black-body radiation from the entire heated device and an additional deep-level defect contribution.

GaAs-based terahertz quantum-cascade lasers (QCLs) are continuously tuned in their emission frequency by illuminating the rear facet with a near-infrared, high-power diode laser. For QCLs emitting around 3.1 THz, the maximum tuning range amounts to 2.8 GHz for continuous-wave operation at a heat sink temperature of 55 K, while in pulsed mode 9.1 and 8.0 GHz are achieved at 35 and 55 K, respectively.

Surface morphology changes and transient reflectance changes at diode laser facets are monitored during the catastrophic optical damage (COD) process in a single pulse operation. Time-resolved micro-reflectance spectroscopy with a streak-camera (time resolution ~20 ns) allows us to observe the creation sequence of up to four distinct degradation seed points at a device facet within <300 ns. The shape of the COD seeds is created within <30–40 ns. Creation of non-planar facet areas by local melting represents the main mechanism behind the observed reflectivity changes. Subsequently the surface temperature decreases within the pulse which caused the COD.