# Micro LED performance - a cathodoluminescence study

Névine Rochat, Palmerina González-Izquierdo, Davide Zoccarato, Fabian Rol, Julia Simon, Patrick Le Maitre, Marion Volpert, Matthew Charles, Matthieu Lafossas, Simona Torrengo, et al.

► To cite this version:

Névine Rochat, Palmerina González-Izquierdo, Davide Zoccarato, Fabian Rol, Julia Simon, et al. Micro LED performance - a cathodoluminescence study. BIO Web of Conferences, 2024, 129, pp.32006. 10.1051/bioconf/202412932006. hal-04753103

HAL Id: hal-04753103

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Submitted on 25 Oct 2024

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# Micro LED performance - a cathodoluminescence study

Mrs Névine Rochat¹, Palmerina González-Izquierdo¹, Davide Zoccarato¹, Fabian Rol¹, Julia Simon¹, Patrick Le Maitre¹, Marion Volpert¹, Matthew Charles¹, Matthieu Lafossas¹, Simona Torrengo¹, Noemie Bonnet², Toon Coenen², Lukasz Borowik¹

¹Univ. Grenoble Alpes, CEA, Leti, Grenoble, France, ²DELMIC B.V, Delft, The Netherlands

## Background & aims

Gallium Nitride (GaN) microLEDs have received a lot of attention in the field of visible light microdisplays due to their interesting properties, such as high luminosity, wide wavelength range and robustness. However, their miniaturization poses a challenge due to the decrease of LED performance as the perimeter-to-surface ratio increases, a phenomenon that has not yet been fully understood. Studying carrier dynamics in these devices is crucial for understanding how recombination mechanisms affect their performance. The lateral diffusion length and carrier lifetime are therefore a major subject of interest for understanding and improving the characteristics of pixelated mesa LEDs.

In this work, we show the impact of Light Extraction Efficiency (LEE) and Internal Quantum Efficiency (IQE) independently as a function of mesa size and shape. Furthermore, lifetime measurements provide insights about the origin of IQE decrease near the mesa edge, showing the importance of diffusion coefficient and lifetime measurement techniques.

## Methods

Multiple quantum well (MQW) LED structures (nGaN/5x(2.5nm InGaN/8nm GaN)/AlGaN EBL/pGaN) were patterned into 2.5, 5, 7.5 and 10 µm square and round mesas. These structures were analyzed using steady-state hyperspectral cathodoluminescence (CL) measurements at 10K and room temperature. The impact of mesa size and shape on LEE versus IQE has been explored. Additionally, Time Resolved Cathodoluminescence (TRCL) was performed with a streak camera at room temperature across a mesa to determine QW carrier lifetime and wavelength variation of the emitted light with the distance to the mesa edge.

## Results

The maximum CL intensity at the center of the MQW mesas exhibits a non-monotonic trend with increasing mesa size, showing an optimal emission for the intermediate sized mesa of 7.5 µm, for both circular and square geometries. This behavior is explained by a competition between IQE (which decreases with mesa size reduction) and LEE (which increases with size reduction, with different trends for square and circular geometries), separated using the low and room temperature CL measurements. These results highlight the difficulty in properly interpreting CL intensity maps in terms of the impact of non-radiative recombination from the sides on LED mesa efficiency. However, the importance of IQE cartography at the nano scale to explore the impact of the mesa side remains significant. As already demonstrated in numerous papers, IQE decreases near the mesa wall. This decrease is mainly attributed to non-radiative recombination at the mesa edges and strongly affects smaller mesa performances. TRCL lifetime measurements across a mesa indeed show that carrier lifetime decreases near the mesa side. Furthermore, we observed a CL intensity reduction up to 3.5 µm from the mesa edge, which determines the optimal mesa size for emission. Thus, the diffusion length of carriers in the QW is one of the key parameters to minimize the impact of sidewall damage on LED efficiency. This means that a smaller diffusion length will reduce the impact of non-radiative recombination on the sidewalls on the LED efficiency.

## Conclusion

This study investigates the influence of mesa size and geometry on external quantum efficiency, highlighting the significant impact of non-radiative recombination at the mesa
edge. Moreover, a competition between IQE and LEE is observed, elucidated through
measurements at both low (10 K) and room temperature. This competition reveals an
optimal LED size for two distinct mesa geometries, offering valuable insights for the design of
microLED arrays. Furthermore, our findings provide clear indications for enhancing LED
characteristics, such as mitigating surface recombination at sidewalls to reduce non-radiative
recombination, or minimizing diffusion length to mitigate sidewall non-radiative impacts on
small mesa sizes. Since diffusion length is intricately linked to the square root of the diffusion
coefficient and radiative lifetime, characterizing the diffusion coefficient becomes critical.
Both steady-state and time-resolved cathodoluminescence techniques are proposed to
address these parameters effectively at the mesa scale.

This work, carried out on the Platform for Nanocharacterisation (PFNC), was supported by the "Recherche Technologique de Base" and "France 2030 - ANR-22-PEEL-0014" programs of the French National Research Agency (ANR). Manufacture of the samples was fund in the frame of the Important Project of Common European Interest (IPCEI) for microelectronics and as part of Nano 2022IPCEI.

## Graphic:

<figure><img src="image_2.png" /></figure>

Integrated MQW CL intensity maps at 300K, 10K and I(300K)/I(10K) for four square and circular mesas with different widths (2.5, 5, 7.5, and 10 µm). Mesas of the same geometry are shown using the same color scale that covers the intensity range of the mesa showing the highest intensity value within the series. Right: Integrated MQW CL profile at 300K, 10K and I(300K)/I(10K), normalized to the highest value for each shape, and averaged in the vertical direction over the area marked as red rectangles in the maps, for four different sized square/circular mesas.

## Keywords:

cathodoluminescence, quantum well, microLED
**Reference:**

P. González-Izquierdo et al, Influence of Shape and Size on GaN/InGaN µLED Light Emission: A Competition between Sidewall Defects and Light Extraction Efficiency. ACS Photonics 2023, 10, 4031–4037

S. Finot et al, Surface Recombinations in III-Nitride Micro-LEDs Probed by Photon-Correlation Cathodoluminescence. ACS Photonics 2022, 9 (1), 173–178