As micro-LEDs are further miniaturized for applications in high-resolution displays, size is becoming increasingly important for overall efficiency. To achieve high internal quantum efficiency, non-radiative recombination processes at the sidewall must be well understood and adequately mitigated. For this purpose, blue InGaN/GaN micro-LEDs were defined under two different dry etching conditions by changing the plasma power and Ar/Cl2 ratio, resulting in LEDs that were either stronger physically etched or stronger chemically etched. The LEDs were investigated using monochromatic cathodoluminescence imaging and time-resolved cathodoluminescence to determine how the etching conditions affect the detrimental influence of the sidewall on the quantum efficiency. A significant reduction of cathodoluminescence intensity and lifetime of the stronger physically etched structure from the bulk material toward the sidewall is observed, which is caused by an increase in the non-radiative recombination activity. Conversely, the stronger chemical etching conditions do not lead to a considerable degradation of the luminescence properties near the sidewall, which is attributed to the passivation of near-surface point defects with hydrogen during the etching process. This is corroborated by the intentional depassivation of such defects by low-energy electron beam irradiation or thermal annealing in an inert gas atmosphere leading to a similar efficiency deterioration, as well as by the recovery of efficiency by hydrogen plasma treatment. Overall, the results demonstrate that hydrogen can very effectively restore the efficiency of micro-LEDs at the sidewall. Therefore, the possible influence of hydrogen on the point defects at the sidewall should be considered in future studies investigating sidewall treatments.

Micro-LEDs generally suffer a loss of efficiency when they are reduced in size, as the sidewall has an increasingly greater impact on overall performance and promotes non-radiative surface recombination. However, as small micro-LEDs are in high demand due to promising applications in high-resolution displays, the influence of the sidewall must be well understood and mitigated by improvements in fabrication, e.g., directly during the etching process or by surface treatment. To improve the sidewall quality, blue InGaN/GaN micro-LEDs were defined under two different dry etching conditions by changing the plasma power and Ar/Cl2 ratio, and their sidewalls were treated either by plasma-enhanced chemical vapor deposition of SiOx or KOH wet chemical etching. The influence of the different surface conditions is analyzed by cathodoluminescence experiments to investigate how the distance between the excitation area and the sidewall affects the luminescence behavior. The distance dependence is significantly different for the investigated etching conditions, which is attributed to different degrees of surface passivation by hydrogen incorporated during dry etching. In addition, strong differences are observed for excitation close to or far from the sidewall, which cannot be explained only by differences in surface recombination, but can be ascribed to lateral confinement of charge carriers by the sidewall restricting spreading by diffusion. Lastly, the results strongly suggest that KOH etching not only removes the surface damaged by the etching process, but also improves the sidewall quality most likely due to the interaction of the etching by-product NH3 with GaN.

The progressive miniaturization of superconducting quantum interference devices (SQUIDs) used, e.g. for magnetic imaging on the nanoscale or for the detection of the magnetic states of individual magnetic nanoparticles causes increasing problems in realizing a proper flux-bias scheme for reading out the device. To overcome the problem, a multi-terminal, multi-junction layout has been proposed and realized recently for the SQUID-on-tip configuration, which uses constriction-type Josephson junctions (JJ). This geometry is also interesting for SQUIDs based on overdamped superconductor—normal metal—superconductor (SNS) JJ. We fabricated four-terminal, four-junction SQUIDs based on a trilayer Nb/HfTi/Nb process and study their static and dynamic transport properties in close comparison with numerical simulations based on the resistively and capacitively shunted junction model. Simulations and measurements are in very good agreement. However, there are large differences to the transport properties of conventional two-junction SQUIDs, including unusual phase-locked and chaotic dynamic states which we describe in detail. We further extract the current-phase relation of our SNS junctions, which turns out to be purely sinusoidal within the experimental error bars.

We have developed a cryogenic characterization platform for ultrafast photodiodes, whose time domain responses are extracted by electro-optic sampling using femtosecond laser pulses in a pump-probe configuration. The excitation of the photodiodes with the pump beam and the electro-optic sampling crystals with the probe beam are realized in a fully fiber-coupled manner. This allows us to use the characterization platform at different temperatures, ranging from cryogenic to room temperature. As an application example, we characterize the time-domain response of commercial p-i-n photodiodes with a nominal bandwidth of 20 GHz and 60 GHz at temperatures of 4 K and 300 K and in a large parameter range of photocurrent and reverse bias. For these photodiodes, we detect frequency components up to approximately 250 GHz, while the theoretical bandwidth of our sampling method exceeds 1 THz. Our measurements demonstrate a significant excitation power and temperature dependence of the photodiodes’ ultrafast time responses, reflecting, most likely, changes in carrier mobilities and electric field screening. Since our system is an ideal tool to characterize and optimize the response of fast photodiodes at cryogenic temperatures, it has a direct impact on applications in superconducting quantum technology such as the enhancement of optical links to superconducting qubits and quantum-accurate waveform generators.

We present a fabrication technology for nanoscale superconducting quantum interference devices (SQUIDs) with overdamped superconductor-normal metal-superconductor (SNS) trilayer Nb/HfTi/Nb Josephson junctions. A combination of electron-beam lithography with chemical-mechanical polishing and magnetron sputtering on thermally oxidized Si wafers is used to produce direct current SQUIDs with 100-nm-lateral dimensions for Nb lines and junctions. We extended the process from originally two to three independent Nb layers. This extension offers the possibility to realize superconducting vias to all Nb layers without the HfTi barrier, and hence to increase the density and complexity of circuit structures. We present results on the yield of this process and measurements of SQUID characteristics.