MicroLED displays fabricated from compound semiconductor materials are a disruptive new technology offering advantages in efficiency, brightness, resolution and power consumption over incumbent LCD and OLED approaches. Today, such displays are assembled from arrays of micron sized emitters, made from traditional LED materials such as GaN (blue/green) and red (GaAs). And whilst an all GaN RBG solution remains the goal of the industry, this affording many advantages in display manufacture, producing high efficiency emitters at longer wavelength remains a challenge in GaN, so instead, device manufacturers use indium gallium phosphide (InGaP) to fabricate red microLEDs. In this work we present on a new 200 mm wafer platforms form for red microLED epitaxy where active structures are deposited on to CMOS fab compatible germanium substrates. Germanium is an excellent substrate starting material for epitaxial growth given it is lattice matching to GaAs, very high levels of crystal quality as well as mechanical strength and flatness. Here we compare the quality and performance of microLED devices grown on both GaAs and Ge and discuss the fab adoption strategies which are creating opportunities for the insertion of Ge based microLEDs into RBG displays We also discuss how these results create a path for large-scale microLED production on 300 mm platforms, integrated with CMOS backplanes to deliver the low cost, scaled approach which will be essential for the mass adoption of compound semiconductor based microLED displays.
Growth of IIIN microLEDS on Si(100) substrates would enable direct integration with Si based CMOS. An oxide buffer is used to translate the crystal symmetry so that GaN(0001) can be grown on Si(100) in a wurtzite form. InGaN quantum dots (QDs) are presented as a novel solution to obtaining a red LED from the III-N material system. An engineered bottom contact is demonstrated by the addition of an epitaxial metal layer below the QDs.
Sensing in the critical 1100-1400nm spectral region is well served by VCSELs and photodetector utilizing dilute nitride (DN) materials grown on GaAs substrates. For CMOS compatibility the growth of DN materials on 200mm Ge wafers has also been demonstrated. The DN VCSEL structure can also be grown using MOCVD for the DBR which gives the ideal mix of performance and high-volume throughput. The successful growth of equivalent performance DN materials by MOCVD has been achieved for photodetector, work on VCSEL is ongoing.
Long Wavelength VCSELs and edge emitters have been grown by Molecular Beam Epitaxy (MBE) on GaAs substrates for applications in 3D sensing and LIDAR. Adding small amounts of nitrogen to the InGaAs QW material allows longer emission wavelengths to be achieved, but these alloys are notoriously difficult to achieve good optical material quality. Careful control of the layer structure and growth conditions of the dilute nitride active region has resulted in state of the art device characteristics at these wavelengths, which will be presented. Further optimization has been carried out to demonstrate appropriate device reliability under standard test conditions.
Long wavelength quantum dot in plane lasers have been scaled to both high volume and the larger 200mm GaAs substrates. Scaling to 300mm on a silicon platform has also been achieved. All wafers are grown on production-ready MBE platforms. Other material systems such as InGaAs and dilute nitride have been similarly scaled often by going back to first principle of process design in conjunction with substrate engineering. The main drivers for this transition to high volume are the need to integrate compound semiconductors with silicon foundries for not only data communications but other applications like automotive sensing and health care.
Wafer bow/warp in high performance 940nm VCSEL epitaxial wafers has been eliminated through the use of 150 mm Ge substrates, replacing conventional GaAs substrates. Ge is a drop-in replacement for GaAs for this application and has additional benefits in that it is zero EPD and mechanically more robust. High performance 940nm VCSELs have been fabricated on Ge and compared directly with those grown on GaAs with the same structure, with no discernible difference in device performance between the two approaches. Use of Ge also provides an immediate route to 200 mm VCSEL growths as Ge is readily available at that diameter.
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