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Far‐ultraviolet‐C (far‐UVC) light emitting diodes (LED) emitting at an emission wavelength of 226 nm with different n‐AlGaN contact layers, quantum well barriers, and quantum well numbers are compared regarding their emission power, operation voltage, and lifetime. Electroluminescence measurements show higher emission power but also an increased operation voltage with increasing Al mole fraction in the n‐AlGaN contact layer. Furthermore, it is found that both the mean emission power and the device lifetime decrease with increasing Al mole fraction (82–89%) of the quantum well barriers and therefore with increasing barrier height. Finally, 226 nm LEDs with 6 and 9 quantum wells are compared. It is observed that the sample with 9 quantum wells shows an around 30% lower mean emission power but, in contrast, the L70 lifetime of these LEDs is higher by a factor of around five. Based on these optimizations, 226 nm LEDs with a maximum external quantum efficiency of 0.28% (wall plug efficiency of 0.18%) as well as an emission power of 2.1 mW and an operation voltage of 9.6 V at 200 mA are realized. [ABSTRACT FROM AUTHOR]

The development of processes for epitaxial growth of vertical gallium nitride (GaN) drift layers enabling 1.2 kV breakdown voltage on low‐cost sapphire substrates is presented in comparison to GaN bulk substrates. The targeted blocking capability demands drift layers with a thickness of 10 μm and low but controllable n‐type doping. Using a growth rate of 2.5 μm h−1 the concentration of unintentionally incorporated carbon is sufficiently low to adjust the n‐type carrier concentration to ≈1 × 1016 cm−3 for all types of substrates. To assess GaN drift region properties in terms of forward bias conductivity and reverse bias blocking strength, a quasi‐vertical p‐n‐diode structure is utilized. Bow reduction of GaN‐on‐sapphire structures is achieved using a stealth laser scribing process. Breakdown voltages higher than 1600 V and a specific on‐state resistance as low as 0.7 mΩ cm2 are obtained with diodes fabricated on GaN substrates. Similar structures grown on sapphire show breakdown voltages of about 1300 V due to higher levels of current leakage. Comparing different types of substrates, a direct correlation between dislocation density in the drift layer with the leakage current in p‐n diodes is deduced. [ABSTRACT FROM AUTHOR]

This paper reports on a monolithically integrated gallium arsenide (GaAs)-based photonic integrated circuit platform for wavelengths around 1064 nm. Enabled by spatially selective quantum well removal and two-step epitaxial growth, it supports on-chip gain as well as passive waveguides. In addition, shallow- and deep-etched waveguides are realized. The former result in waveguide losses of less than 2 dB/cm, while the latter enable compact integrated waveguide components. To demonstrate the performance of the platform, racetrack ring resonators based on deep-etched Euler bends and shallow-etched directional couplers are realized, achieving high intrinsic quality factors of 2.6 × 105 and 3.2 × 105 for the fundamental TE and TM mode, respectively. To demonstrate the use of these resonators, ring-resonator-coupled lasers are fabricated, resulting in one-sided output powers of up to 14 mW and single-mode operation with 40 dB side-mode suppression. The successful integration of ring resonators on a GaAs-based active/passive photonic integrated circuit platform paves the way for the realization of fully monolithic, widely tunable, and narrow linewidth ring-resonator-coupled laser sources. [ABSTRACT FROM AUTHOR]

Widespread commercial adoption of telecom‐band quantum key distribution (QKD) will require fully integrated, room‐temperature transmitters. Implementing highly efficient spontaneous parametric down‐conversion (SPDC) on a platform that offers co‐integration of the pump laser has been an outstanding challenge. Here, using such a platform based on AlGaAs‐on‐insulator waveguides, telecom‐band SPDC (and SHG, second harmonic generation) is reported with exceedingly large efficiencies of 26 GHz generated pairs per mW over a 7 THz bandwidth, which would saturate the usable photon‐flux for a 70‐channel wavelength‐multiplexed QKD system at merely 1.7 mW of pump laser power. [ABSTRACT FROM AUTHOR]

Recovery of epitaxial AlN films on sapphire at high temperatures is now an established process to produce pseudo-substrates with high crystalline perfection, which can be used to grow epitaxial structures for UV-light-emitting devices. To elucidate the elementary mechanisms taking place during the thermal treatment of MOVPE-grown films, we studied as-grown and annealed samples combining transmission electron microscopy techniques and secondary ion mass spectrometry (SIMS). By using SIMS, we find a temperature-dependent increase in the overall oxygen content of the films, which cannot be explained quantitatively with either simple bulk or pure pipe-diffusion from the sapphire substrate. Instead, we propose a lateral outdiffusion from the dislocation cores to explain qualitatively and quantitatively the presence of observed oxygen concentration plateaus. Based on the formation enthalpy of various atomic defects and complexes found in literature, we conclude that the di-oxygen/aluminum vacancy complex (VAl–2ON) is the dominant point defect controlling the annealing process. The formation of this defect at high temperatures promotes a dislocation core climb process, which causes the annihilation/fusion of the threading dislocation segments. [ABSTRACT FROM AUTHOR]

AlN layers annealed at high temperatures offer low threading dislocation densities of mid 108 cm−2 and are therefore increasingly used as base layers in ultraviolet (UV) light emitting diode (LED) heterostructure growth. These LEDs, just like those grown on conventional metalorganic vapor phase epitaxy (MOVPE) AlN templates, often suffer from long-wavelength parasitic luminescence. In this work, luminescence properties of far-UVC LED heterostructures grown on MOVPE-AlN/sapphire templates and high-temperature annealed AlN/sapphire templates are compared. To investigate the origin of parasitic emission with high spatial resolution, cross section scanning transmission electron microscopy was combined with cathodoluminescence measurements. As a result, the main origin of the parasitic luminescence band centered at 3.5 eV (354 nm) for the heterostructure grown on annealed AlN is assigned to point defects related to oxygen in the AlN template layer. The defect band centered at 3.0 eV (413 nm) for the heterostructure grown on MOVPE-AlN was found to be related to self-compensating VAl-Si point defect complexes in the n-AlGaN layer and oxygen incorporation close to the AlN/sapphire interface. The results also suggest that the type of AlN template determines the kind of parasitic luminescence from the n-AlGaN layer. [ABSTRACT FROM AUTHOR]

Heavily n-doped GaN/Al0.18Ga0.82N short-period superlattices with and without SiN protection layers are studied in spectrally and temporally resolved photoluminescence (PL) experiments. The room-temperature PL from a protected sample displays a nonexponential decay with an initial decay time of 150 ps for low excitation levels and an exponential decay with a time constant of ∼300 ps for higher excitation. The PL decays are governed by nonradiative carrier relaxation into deep defect states which are partially saturated at high excitation densities. PL measurements at low sample temperature reveal a marked influence of carrier cooling on the PL kinetics in a time range up to 50 ps and a significant radiative decay component. SiN coatings are shown to provide long-term stabilization of surface morphology while increasing nonradiative carrier relaxation rates. [ABSTRACT FROM AUTHOR]