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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - HiLICo (High Luminescence In Cockpit)

Teaser

Summary

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/cmÂ²) 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/mÂ² for full color RGB, and 1 Million cd/mÂ² 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 (<6V) and low current injection (<10ÂµA 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, enough high pixel luminance while keeping the operating device temperature low enough to remain compatible with a see-through visualisation system.

To achieve this very ambitious goal, the HiLICo project addresses numerous technical topics including (i) the development of high-quality GaN based LED epilayers, (ii) the design and fabrication of a CMOS active matrix able to control each individual pixel, (iii) the coupling of the LED structure to the CMOS matrix followed by high precision LED pixelisation (8-10Âµm pixel pitch), (IV) the transfer on blue emitting devices of dedicated light conversion layers (quantum dots or 2D Multi-Quantum Wells) to manufacture single-, bi- or full-colour display demonstrators and finally (V) the design and manufacturing of an electronics able to drive efficiently such high luminance micro-display. First demonstrators will be qualified for future commercialisation.

Work performed

At project mid-term, the development of the main technological building blocks mentioned above have globally well progressed with several key results and advances:
â€¢ Fully optimized 4â€™â€™ blue LED epilayers were grown at NOVAGAN providing single micro-LEDs with external quantum efficiency (EQE) exceeding 9% at 2.8V. Despite a possible LED size impact on EQE, this performance proves to be in line with the 10% required at the micro-display level to reach 1Mcd/m2 brightness. Similarly, the optimization of growth conditions has noticeably reduced the emission wavelength variation over the whole wafer surface, from Â±12 to Â±4 nm complying with the variation claimed for the application.
â€¢ The architecture of the CMOS backplane circuit and the related driving electronics turned out to be far more complex than anticipated, for two reasons. Firstly, initial studies on uniformity of a GaN matrix made clear that a pixel to pixel correction of homogeneity in a wide current range is indispensable, which significantly increases complexity (and therefore size) of the pixel circuit. Secondly, the requirement from the end user in terms of pixel size as well as of very large range of brightness control, together with the first point made it impossible to go for a standard active matrix architecture. In the end, a suitable architecture was found that relies on the combination of an analog current control for homogeneity compensation combined with a digital PWM-type control for the video. Due to size limitations, in particular, time sequential gray-level scheme based on a patent of CEA-LETI is used. As a consequence, the complexity of the driving electronics has substantially increased with the demand for several frame memories and fast & complex video processing. The final architecture and appropriate CMOS technology of the driving circuit of 9.5 Âµm-pixel pitch LED arrays are now chosen, and the IC design step has been started. The tape out of the circuit is foreseen for end of May 2019. In parallel, the principal architecture of the driving board (mother board) that will receive the display package (daughter board) has been fixed. With respect to the original plan, these tasks are delayed by 7 months.
â€¢ Successful GaN-epilayer transfer on 8â€™â€™ CMOS-like silicon wafer and further processing of 8Âµm pixel/10Âµm pitch LED array have been demonstrated.
â€¢ First assessment of two light conversion strategies have been done paving the way to efficient blue to green and blue to red innovative light conversion solutions:
o Blue-to-green 2D-conversion layers based on a large number of InGaN/GaN Multi-Quantum Wells (30 x MQW) have been successfully grown on sapphire substrates. Their blue light absorption, prerequisite for an efficient conversion, was estimated to be relatively low (1.07% per QW). An additional light trapping strategy (resonating optical cavities on top of the MQW layer) is under study.
o In the same manner, blue-to-red 2D-conversion layers based on AlInGaP MQWs grown on GaAs have been successfully transferred on sapphire for opto-electronic characterisation. While blue-to-red down conversion was effectively shown, a strong light guiding effect was observed, which necessitates dedicated light extraction structures to be implemented.
o As an alternative approach to 2D conversion layers, photo-patterned resin layers containing 2D-nanoplatelets (NPL) as light converters is also considered. Synthesis of more absorbing and more stable core-shell NPLs has been successfully achieved at NEXDOT. First photo-patterning of red NPL/resist composite was demonstrated on the CEA technological platform with pixel pitch resolution down to 10Âµm. However, the composite formulation still needs careful adjustment to get a better control on the localization of the conversion layer.

Final results

On the basis of these first results, the next project period will focus on several specific technology building blocks necessary for the realisation of the various micro-display demonstrators (single-, bi- or RGB colour micro-displays). Among the key issues, the first one deals with the complete design and fabrication of the active CMOS matrix on the basis of project specifications. The second issue concerns the manufacturing of a green micro-LED array demonstrating 1 Mcd/cm2 in powering conditions compatible with a see-through visualisation system. Finally, the third 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.