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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - PhotoHeatEffect (Heat Transport and its Effects on the Performance of Nanostructured, Photonic Materials)

Teaser

Summary

During the last decades tremendous research efforts aimed to understand the photonic properties of semiconductor nanostructures for first real-world applications. However, the high level of sophistication for such optical analyses is often contradicted by limited knowledge about the interconnected thermal phenomena. Commonly, either the photonic or thermal properties are studied, whereas their interrelation remains obscured, preventing any mutual optimizations - a key objective of the PhotoHeatEffect project. Evidently, such optimizations exhibit a far-reaching societal impact, which is in-line with the climate protection aims of the European Union regarding the energy efficiency, longevity, and sustainability of future devices.

Generally, as soon as photonic or electrical devices are operated close to their utmost limits (high currents, light output, repetition rates, etc.) excessive heat is generated - a very common observation. Exactly this heat generation limits the achievable device performance and lowers the overall device efficiency. Generally, one always strives to achieve maximum performance [e.g., for lasers build into car headlights, light-emitting diodes (LEDs) for illumination and display technology, etc.] to ensure cost efficiency but also sustainability. Clearly, an optimization of, e.g., photonic and thermal material properties - or even just a well-balanced trade-off - can yield more energy efficient devices (less heat generation yields a lower power consumption) but can also contribute to the longevity of devices (heating of structures generates structural defects that ultimately limit the device lifetime). Both key points - energy efficiency and longevity - are directly connected to the CO2 footprint of all photonic and electrical devices based on semiconductor nanostructures. For such structures any reduction in energy consumption and CO2 footprint will always be of high importance to realize the global aim of a CO2 neutral society that manages to stop climate change - a promising and socially most relevant aspect that motivated the PhotoHeatEffect project and all subsequent research.

Work performed

The following bullet points give a brief overview for the work conducted during the PhotoHeatEffect project, the corresponding results and their exploitation and dissemination.

- Development of a unique optical setup that combines (time-resolved) ÂµPhotoluminescence (ÂµPL) with 2-laser Raman thermometry (2LRT), comprising mapping capabilities for both functions:

It was demonstrated that 2LRT can be integrated into a modern ÂµPL setup, which allows for mappings, polarization resolved studies, time-resolved, and even photon correlation analyses. Throughout the PhotoHeatEffect project the corresponding setup was designed and built at the host institution. In addition, all required software for the data acquisition and analysis was developed, which sets the basis for a fruitful use of the setup beyond the time-frame of the PhotoHeatEffect project. The employed thermal technique was carefully calibrated by recording temperature-dependent Raman shifts of the most relevant Raman modes for state-of-the-art bulk III-nitride material. This unique spectroscopic setup formed and will form the basis for numerous publications.

- Processing of III-nitride membrane structures and nanobeam lasers along with the development of a quantum dot (QD) positioning approach especially suited for III-nitrides:

InxGa1-xN/GaN QW structures (x = 15%) were grown on silicon, which allows fabricating underetched membrane structures. Such freestanding, two-dimensional III-nitride structures are ideal for analysing the in-plane heat transport, which can directly be monitored by the aforementioned conjunction of ÂµPL and 2LRT spectroscopy. These III-nitride membranes were etched into one-dimensional nanobeam structures, which form the basis for nanolasers. Furthermore, the positioning of GaN/AlN QDs was addressed in order to establish the basis for future three-dimensional thermal crystals. Consequently, the entire set of growth and processing efforts forms the basis for publications.

- In-detail characterization of the optical properties of all employed optical media, which mainly comprises InGaN/GaN QW structures:

InxGa1-xN samples were studied by PL, ÂµPL, and time-resolved ÂµPL in order to, e.g., determine the exciton diffusion lengths and the internal quantum efficiency depending on the indium content x. Knowledge of such parameters proved important for detangling the different contributions to thermal transport evoked by thermal phonons, excitons, exciton-polaritons, and free carriers. The characterization results of the PhotoHeatEffect project have been published in peer-reviewed journals. Here, a publication in Physical Review X (Callsen et al., PRX 9, 031030, 2019) represents a dissemination highlight of the PhotoHeatEffect project, which will be followed by further future publications.

- Two-dimensional 2LRT mappings of heat distributions across nanostructures were obtained with a spatial resolution of down to 500 nm under operating conditions of the selected photonic membranes (optical injection):

The developed experimental techniques were showcased for dedicated III-nitride membrane structures that form the basis for nanolasers. As a result, highly spatially resolved temperature distributions could be recorded for varying ambient temperature, injection levels, and sample geometries. Clear signatures of different phonon propagation regimes were attested by the employed 2LRT spectroscopy, which will be the topic of an upcoming publication. Furthermore, the direct link in between local heating and the detrimental effects on the ÂµPL signal was established. Clearly, the entire set of these measurements will in the future lead to novel device designs that consider both, the optical and thermal prerequisites of a photonic membrane structure like a III-nitride nanobeam laser.

Final results

The numerous results of the PhotoHeatEffect project provide a strong motivation for future research. After having established the characterization tools and having understood the employed optically media, it will in the future be possible to detangle all contributions to the intricate problem of heat transport in III-nitride photonic membranes. As a result, the best trade-off in between thermal and optical properties will be approached, which can benefit future device generations. Generally, most achievements of the PhotoHeatEffect project are beyond state-of-the-art and will in the future not only be applicable to photonic structures but also to electronic structures like diodes and transistors. The present achievements comprise the development of a world-wide unique optical setup that merges the worlds of photonic and thermal characterization and has consequently already let to the direct observation of different thermal phonon propagation regimes in III-nitride membranes.

The former research of the PhotoHeatEffect developed the seed for future studies that will enable more energy-efficient devices with drastically improved longevity. Both aspects will consequently lower the energy consumption and the overall CO2 footprint of related devices, supporting the efforts of the European Union to develop an increasingly CO2-neutral society.