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In this work, we report on the realization of vertical (100) β-Ga2O3 FinFET devices for the use in power electronics applications. The experiments are carried out on structures consisting of highly conducting (100) β-Ga2O3 substrates with a doping concentration N D of 3 × 1018 cm−3, and epitaxially grown layers with N D of 5 × 1016 cm−3 for the drift and channel region. The fabricated FinFET devices feature enhancement-mode properties with a threshold voltage of +4.2 V and on/off-current ratio of 105. Moreover, breakdown measurements of these devices reveal an average breakdown strength of 2.7 MV cm−1. Additional device simulation indicates the presence of electric field peaks near the gate edge outside the active device as high as 7 and 5 MV cm−1 in the Al2O3 gate oxide and β-Ga2O3 semiconductor, respectively.

Among the work done to improve laser performance by optimizing the efficiency of vertical laser structures and thermal management, the spectral quality of the emitted light is a key criterion for the usability of the device. Especially the exact adjustment of the emission wavelength, a small linewidth and a preferably small dependence of the wavelength on temperature changes are important for many applications. These requirements are met by implementing surface gratings into the devices. In the first part of the paper the theoretical tools which are used to simulate and optimize grating reflectivity are presented and discussed. 2D and 3D simulation models confirm possible high reflectivities (more than 90 %) of surface Bragg gratings, if the duty cycle is high ( 9 @ λ = 630 nm) are required. Electron beam lithography allows better design flexibility and larger etch depth tolerances, but requires a more complex etch mask preparation. With both techniques gratings up to a depth of 1.7 μm can be etched, which is sufficient to reach a high reflectivity for typical AlGaAs based laser structures. A single grating can be thermally tuned up to 7.5 nm. As an example for high power applications a 6 mm long distributed feedback broad laser with a stripe width of 30 μm as part of a spectral beam combining setup is presented. This device achieved a maximum output power of 6 W and a spectral width of 0.3 nm.

Single longitudinal mode continuous-wave operation of distributed-feedback (DFB) laser diodes based on GaN is demonstrated using laterally coupled 10th-order surface Bragg gratings. The gratings consist of V-shaped grooves alongside a 1.5 µm wide p-contact stripe fabricated by using electron-beam lithography and plasma etching. By varying the period of the Bragg grating, the lasing wavelength could be adjusted between 404.8 and 408.5 nm. The feasibility of this device concept was confirmed by mode-hop-free operation up to an optical output power of 90 mW, a low temperature sensitivity of the lasing wavelength, and a Gaussian lateral far-field distribution.

Single longitudinal mode continuous-wave operation of distributed-feedback (DFB) laser diodes based on GaN is demonstrated using laterally coupled 10th-order surface Bragg gratings. The gratings consist of V-shaped grooves alongside a 1.5 µm wide p-contact stripe fabricated by using electron-beam lithography and plasma etching. By varying the period of the Bragg grating, the lasing wavelength could be adjusted between 404.8 and 408.5 nm. The feasibility of this device concept was confirmed by mode-hop-free operation up to an optical output power of 90 mW, a low temperature sensitivity of the lasing wavelength, and a Gaussian lateral far-field distribution.

Distributed Bragg reflector tapered diode lasers (DBR-TPL) emitting at 1154 nm are ideal light sources to be implemented into medical devices and hand-held tools for treatment in dermatology and ophthalmology at 577 nm due to their high spectral radiance enabling second harmonic generation from near infrared to yellow. In this work, we present DBR-TPLs which are able to emit more than 10 W in continuous-wave operation with a narrow spectral emission at 1154 nm and a very good beam quality providing excellent spectral radiance. The investigated DBRTPLs are based on three different epitaxial structures with varying vertical far field angles of 35°, 26°, and 17°. To optimize the coupling efficiency into non-linear crystals we studied DBR-TPL with a vertical far field angle of approx. 17° based on an asymmetrical super large optical cavity epitaxial structure. At a pump current of 18 A these devices are able to emit more than 9 W at 25°C and nearly 11 W at 10°C. The spectral emission is very narrow (ΔλFWHM = 18 pm) and single mode over the entire current range. While the beam quality factor M2 according to the 1/e2-level remains 1.1, the M2 according to second order moments deteriorates when the laser is pumped with higher currents. Therefore, the power content in the central lobe increases somewhat less rapidly than the total power.

A design is presented for a beam splitter suitable for ultrashort pulses in the mid-infrared and terahertz spectral range consisting of a structured metal layer on a diamond substrate. Both the theory and experiment show that this beam splitter does not distort the temporal pulse shape.

Tapering of vapour-liquid-solid (VLS) grown nanowires (NWs) is a widespread phenomenon resulting from dynamics of the liquid droplet during growth anddirect vapour-solid (VS) growth on the sidewall. To investigate both effects in ahighly controlled way, we developed a novel two-step growth approach for the selective area growth (SAG) of GaAs nanowires (NWs) by molecular beam epitaxy. In this growth approach optimum growth parameters are provided for thenucleation of NWs in a first step and for the shape variation during elongationin a second step, allowing NWs with a thin diameter (45 nm) and an untapered morphology to be realized with high vertical yield. We quantify the flux dependenceof radial VS growth and build a model that takes into account diffusion on theNW sidewalls to explain the observed VS growth rates. As our model is consistent with axial VLS growth we can combine it with an existing model for the diameter variation due to the droplet dynamics at the NW top. Thereby, we achieve fullunderstanding of the diameter of NWs over their entire length and the evolutionof the diameter and tapering during growth. We conclude that only the combinationof droplet dynamics and VS growth results in an untapered morphology. This result enables NW shape engineering and has important implications for doping of NWs.

The selective area growth of Ga-assisted GaAs nanowires (NWs) with a high vertical yield on Si(111) substrates is still challenging. Here, we explore different surface preparations and their impact on NW growth by molecular beam epitaxy. We show that boiling the substrate in ultrapure water leads to a significant improvement in the vertical yield of NWs (realizing 80%) grown on substrates patterned by electron-beam lithography (EBL). Tentatively, we attribute this improvement to a reduction in atomic roughness of the substrate in the mask opening. On this basis, we transfer our growth results to substrates processed by a technique that enables the efficient patterning of large arrays, nano imprint lithography (NIL). In order to obtain hole sizes below 50 nm, we combine the conventional NIL process with an indirect pattern transfer (NIL-IPT) technique. Thereby, we achieve smaller hole sizes than previously reported for conventional NIL and growth results that are comparable to those achieved on EBL patterned substrates.

We report on the simultaneous definition and fabrication of Bragg gratings and ridge waveguides using wafer stepper lithography and reactive ion etching, respectively. Single-longitudinal mode emission from two-section ridge-waveguide distributed feedback lasers with sixth and seventh order gratings will be reported. This technology enables a cheap fabrication of wavelength-stabilized lasers and a simple variation of the parameters of the gratings on the wafer using optical lithography.

In contrast to Distributed Feedback DFB) lasers for wavelengths of about 1060nm for α-DFB lasers the fabrication of Bragg gratings using conventional wafer stepper lithography is possible, due to the necessary larger grating period. The use of a wafer stepper enables us to study especially the influence of slant angle of the fabricated gratings and stripe width on optical properties of the fabricated lasers on the same wafer independent from fluctuations of the vertical structure. Best beam quality was achieved for 15° and 13.5° tilted gratings with a small period of 594nm and 658nm respectively. For optimised structures a nearly diffraction limited beam with an output power of more than 1W, a lateral far field divergence angle of θ= 0.3°, a beam quality factor M2 = 1.1 and 3.2 at 2.1W respectively, and a line width of 5.8 pm with 28dB side mode suppression rate was achieved.

We present an integrated pulse source at 1064 nm and discuss design, fabrication and measurement results of the 3.5 mm long ridge-waveguide distributed Bragg reflector laser. The optical pulses with a peak power of 0.9 W, a temporal duration of 12.4 ps and a repetition frequency of 13 GHz are generated by passive mode-locking and are well suited for seeding master oscillator power amplifier systems for efficient green light generation with nonlinear frequency doubling in single pass configuration.