Candidate: Mohsen Asad
Title: GaN micro-LED integration with thin-film transistor for flexible displays
Date: April 2, 2020
Time: 8:30 AM
Place: REMOTE PARTICIPATION
Supervisor(s): Wong, William
Abstract:
This Ph.D. dissertation is providing the enabling technologies for developing the next-generation flexible micro-LED displays. As one of the most important building-blocks, the developed mass-transfer processes is presented for a wide range of micro-LED dimensions from 90×90 µm2 to nanowire (NW). A “paste-and-cut” approach is developed to integrate micro-LEDs directly from sapphire substrates onto flexible thin-film transistors (TFTs) at low temperatures (150°C). After demonstrating the feasibility of driving micro-LEDs with conventional a:Si-H TFTs, an efficient pixel circuit is implemented by using a “double-transfer” process. Compared to the conventional pixel circuit, by employing the proposed structure, 2.4 times higher brightness was obtained at the same driving voltage.
It was also found that the performance of the micro-LEDs on flexible platforms might be affected by the poor thermal conductivity of the plastics. A flexible substrate coated with copper pads thicker than 600 nm was found to be effective in extracting the generated heat from the micro-LEDs and maintain the device efficiency. As another requirement, a flexible display color characteristics should remain unaffected under mechanical bending. Theoretical simulations showed micro-LEDs with diameters smaller than 20 µm will not suffer from the stress-induced degradation. The simulation result was validated experimentally by testing micro-LEDs under different mechanical bending conditions. The mass-transfer process was developed further to integrate GaN NWs onto flexible platforms. It was experimentally demonstrated that the GaN-based “dot-in-nanowire” light-emitting diodes, cylindrical devices that mimics a quantum dot within a nanowire (NW) structure, are an emerging technology that allows the LED positions to change by mechanically manipulating the flexible platform. The simulated and experimental findings show that substrate bending provides a means to mechanically manipulate NW LED structures that enhance their light output compared to planar device counterparts.
Finally, a 440% improvement in the topside directional electroluminescence was achieved after tuning the micro-LED optical cavity length from 6.5 µm to 1.5 µm and applying a novel self-aligned sidewall gated reflective electrode. The unbalanced electron and hole density caused fewer defect-assisted non-radiative recombination at the sidewall dangling bonds. While both self-aligned gate electrode and cathode are connected together, without the requirement to an additional electrode and complex driving circuit, this technique can be used in high-resolution micro-LED displays.