Monolithic Integration of a-Indium Gallium Zinc Oxide TFT with Inorganic GaN Based Nano-LEDs Array

The demand for advanced self-emissive high-resolution displays, such as augmented and virtual reality (AR/VR) devices has increased over the past few years. To meet this demand, each pixel should be in micron size (<3 µm) and needs to be controlled individually to gain higher brightness, contrast, and energy efficiency. Inorganic GaN-based micro light-emitting diode (µ-LED) is considered the most emerged display technology for its higher brightness, efficiency, lifetime, and lower power consumption. Individual pixel units can be controlled by a switching unit consisting of a metal-oxide-semiconductor field-effect transistor (MOSFET) or thin film transistor (TFT). As display technology is moving towards a lightweight plastic-like substrate, a low-temperature back-end-of-line (BEOL) compatible fabrication process is required. Amorphous-Indium Gallium Zinc Oxide (a-IGZO) TFT has drawn the attention of the research community owing to its uniformity, higher mobility, lower subthreshold swing, stability, and low temperature fabrication process. Besides, it has a wider band gap energy (≈3.5 eV) with good optical transparency which is suitable for transparent displays. Integration technology like the mass transfer method is a popular and efficient choice for low pixel-per-inch (PPI) larger display devices. In this method, each µ-LED is transferred from its growth substrate to the target display substrate. However, it is a time-consuming process and inefficient for industrial-scale manufacturing of higher PPI devices like AR/VR. Monolithic integration is required for this purpose where TFTs are fabricated on top of the µ-LEDs directly [1, 2]. This technology has a higher yield/cost ratio compared to the mass transfer method. In this work, we have monolithically integrated a-IGZO TFT with both GaN-based planar µ-LEDs and an array of nano-sized LEDs. We have fabricated blue planar µ-LEDs and an array of nano-sized LEDs (array size 85 × 85) by patterning through photolithography and electron beam lithography (EBL) respectively using a top-down approach. The µ-LEDs are passivated and planarized using dielectric deposition. After completing the fabrication of the µ-LEDs, the bottom gate a-IGZO TFT is fabricated on top of the µ-LEDs where the source of the TFT is in direct contact with the p-GaN of the µ-LEDs. The metal gate, source, and drain electrodes, gate dielectric, and active-channel layer a-IGZO are deposited and annealed at optimum conditions. All the fabrication processes were under a lower thermal budget (<300 °C). The monolithic integrated planar µ-LED with TFT shows proper LED and TFT behavior individually. We have achieved enhancement mode TFT with mobility of ≈8 cm 2 /V-s, an on-off ratio of 10 6 , and a subthreshold swing of 180 mV/dec which is suitable for a higher refresh rate (120-240 Hz) display and minimal power consumption. Additionally, we successfully drove the µ-LEDs with the TFT, resulting in blue light emission. Around 8 nm blue shift is observed (from 440.50 nm to 448.76 nm) with increasing applied gate voltage from 4 V to 12 V which corresponds to the injection current of 22 mA/cm 2 to 911 mA/cm 2 . The integrated nano-sized LED array with TFT showed similar electrical characteristics to planar µ-LED. However, no optical output current is observed while driving the array of nanowires through the TFT. The total output current is divided among 7225 parallelly connected nanowires, which significantly reduces the current per LED and prevents illumination. To address this issue, further work with a decreased nanowires array is in progress. Acknowledgments: This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through Alliance Grant Programs. References: [1] Yang, Junghoon, et al. "Active-matrix micro-light-emitting diode displays driven by monolithically integrated dual-gate oxide thin-film transistors." Journal of Materials Chemistry C 10.26 (2022): 9699-9706. [2] Durnan, Oliver, et al. "An active‐matrix microLED display based on monolithic integration with IGZO backplane." Journal of the Society for Information Display (2024). Figure 1

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