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High Luminescence In Cockpit

Periodic Reporting for period 2 - HiLICo (High Luminescence In Cockpit)

Reporting period: 2018-12-01 to 2020-05-31

In many application fields, there is a growing market demand for high quality information displays. This is particular true in commercial avionics where high luminance displays able to provide readable information in a very bright environment are expected. Nowadays, the existing technologies still do not allow the manufacturing of displays exhibiting the desired brightness combined with a very low power consumption and a very compact volume. In this context, the HiLICo project aims at developing a new generation of monochrome and full-color emissive GaN micro-displays with 1920 x 1200 pixel resolution (WUXGA), very high brightness (over 1MCd/cm²) and good form factor capabilities that will enable the design of ground breaking compact see-through systems for next generation Avionics applications.

With this respect and for a perfect visual experience under any environment, challenging microdisplay specifications have been defined. With brightness targets of 100,000 cd/m² for full color RGB, and 1 Million cd/m² for monochrome green display respectively, the challenge turns out to be particularly demanding.
Behind these brightness figures, the challenge ahead is to get a GaN/InGaN epilayer technology able to provide a particularly high electron-to-photon conversion efficiency at low voltage (<5.5V) and low current injection (<10µA per pixel). This also implies to design and manufacture a particularly performing active matrix able to drive the microdisplay at high speed for large data transfer and to deliver enough electric power for high pixel luminance.

To achieve this very ambitious goal, the HiLICo project addresses numerous technical topics. It includes: (i) the development of high-quality GaN based LED epilayers, (ii) the design and fabrication of a dedicated CMOS active matrix to control each individual pixel, (iii) the coupling of the LED structure to the CMOS matrix followed by high precision LED pixelisation (8-10µm pixel pitch), (IV) the transfer on blue emitting devices of dedicated light conversion layers to manufacture bi- or full-colour display demonstrators and finally (V) the design and manufacturing of an electronics able to drive efficiently such high luminance micro-displays.
The developments carried out so far demonstrated good progress. Nevertheless, their implementation took much more time than expected, in particular due to the very high number of technological steps involved in the display manufacturing process. That is why the project duration will be extended by 1.5 year. At the end of this second project period and after 3 years of development, several key results and advances must be definitely highlighted:
• In the first project period, fully optimized 4’’ blue LED epilayers grown at NOVAGAN and providing single 80 µm2-micro-LEDs with external quantum efficiency (EQE) exceeding 9% at 2.8V were demonstrated. For green emission, a new epilayer growth technology has been recently implemented showing EQE approaching 6%, values outperforming by a factor of 3 the previous NOVAGAN state of the art. Similarly, the optimization of growth conditions has noticeably reduced the emission wavelength variation over the whole wafer surface, from ±30 to ±6 nm complying with the variation claimed for the application.
• The final architecture of the CMOS micro-display driving circuit is now finalized and manufacturing of a first batch of several CMOS backplanes successfully done. Chips of CMOS integrated circuits able to control a 1640 x 1033 micro-LED array with a 9.5 µm-pixel pitch and to deliver up to 3W electric power (Vmax = 5.5 V, Imax = 20 µA/pixel) are now available for further micro-display processing . In parallel, the principal architecture of the driving board (mother board) that will receive the display package (daughter board) has been fixed and the various boards necessary to drive the final microdisplays ordered.
• Static green displays with size representative of future microdisplays (1700×1150 pixels) have been processed on passive integrated circuits developed at wafer level. Several operational static displays have been thus successfully produced. They definitely qualify the complete micro-LED array manufacturing workflow which will be implemented for the manufacturing of final active microdisplays. Green luminance as high as 1.4 Mcd/m2 was measured at a maximum forward voltage of 5.5V and injected current of 20µA/pixel. Some static displays have been delivered to the topic manager THALES for further evaluation on a dedicated optical bench representative of the optical combiner to be used in the Augmented Reality headset.
• Assessment of two light conversion strategies have been continued with new characterisation results to help dimensioning the best integration scenario for colour generation into the future RGB microdisplays. Theoretical investigations [1] conducted at CEA showed that the viability of light conversion strategy requires conversion efficiencies of at least 25-30%
Blue-to-red conversion:
2D-conversion layers based on AlInGaP Multi-Quantum Wells (MQWs) have been previously considered. While efficient blue light absorption and blue-to-red down conversion were demonstrated, a strong light guiding effect was observed, requiring complex light extraction structures to be implemented. As an easier approach for integration, resin layers developed by NEXDOT and containing red-emitting nanoplatelets (NPL) have been also evaluated. This route provides conversion layers with strong blue light absorption, conversion efficiency in line with application request (30%), good photostability and integration ability adapted to a pixel pitch of 9.5 µm.
Blue-to-green conversion:
A previous growth study of 2D-conversion layers based on a large number of InGaN/GaN MQWs (x30) was initially investigated. Their blue light absorption was estimated to be too low (1.07% per QW) for efficient conversion. That is why, novel 2D-conversion layers based on InGaN/In GaN MQWs (x10) have been recently developed, that exhibit a blue light absorption two times higher and effective blue-to-green conversion. The processability of this solution for microdisplay will be further evaluated as well as its conversion efficiency.
NPL-based conversion approach is also under investigation for blue-to-green conversion. It offers same ease of integration than red NPL-based resin, potential conversion efficiency up to 30%, but 3 times less blue absorption. Photo-stability is still an issue that will require further chemical engineering (on-going).

[1]E. Quesnel et al., « Dimensioning a full color LED µdisplay for augmented reality headset in a very bright environment”, J Soc Inf Display. 2020;1–14.
Based on all these results, the manufacturing of the various micro-display demonstrators (single-, bi- or RGB colour micro-displays) will be carried out in the last project period (until end of 2021). Among the remaining key issues, the full functioning of the CMOS matrix once inter-connected with the blue or green microLED arrays will be carefully monitored. The second issue addresses the integration of colour conversion layers into blue micro-LED arrays enabling the manufacturing of bi- or full RGB- colour displays demonstrating at least 100,000 cd/m2.