UC Berkeley

Flexible and wearable electronics are envisioned as a future platform of electronics integrated into a variety of emerging technologies from sensing and monitoring to human-inspired applications. Among many fascinated flexible applications, displays are aggressively developing field of research and they are widely connected with transistor platform. Currently, the needs of people are being particular about for mobile phones, portable devices, and even televisions; they require new and more advanced displays. Therefore, the future displays should have high performances, such as low power consumption, ultra-high resolution, high frame rate and robust flexible platform. High-mobility thin film transistors will be a key enabling technology for achieving such performance by reducing the transistor size in the display’s active area. Also, they helps to increase aperture ratio, brightness, and frame rates and to decrease bus-line load power consumption.

In this regard, 2D transition metal dichalcogenides have attracted much interest owing to their finite band gap values, rich excitonic dynamics, and even valley polarization (valleytronics) associated with the broken inversion symmetry. Among these 2D materials, molybdenum disulfide (MoS2) has been considered a channel material for high speed and/or flexible devices and a component material to improve the performance of conventional silicon devices. Furthermore, these layered semiconductors are emerging alternatives to silicon-based electronics. Despite their potentials in electronics and optoelectronics, reliable and stable processing methods are needed for successful transition to practical applications, especially for the flexible/wearable electronics used flexible materials with a low thermal budget (< 200 °C). Typically, used flexible materials cannot be applied conventional high thermal processes, which affect the entire panel including unwanted areas where the high thermal process should be excluded.

For the process having the critical limitations of the flexible/wearable applications, the many features of laser are exactly appropriate; the laser is spatially local selective, air-stable, and widely tunable by varying the duration and intensity of laser irradiation. Moreover, laser process can be digitally controlled by the computer with programmable software. In this dissertation, the proposed approaches (laser annealing, direct laser writing, laser welding, and interference lithography) represent a powerful means to realize high performance flexible devices based 2D materials, especially for MoS2.

First, I demonstrate that mechanically flexible and optically transparent (more than 81% transmittance in visible wavelength) multilayered MoS2 thin-film transistors (TFTs) in which the source/drain electrodes are selectively annealed using picosecond laser achieve the enhancement of device performance without plastic deformation, such as boosted mobility, increased output resistance, and decreased subthreshold swing. Numerical thermal simulation for the temperature distribution, transmission electron microscopy (TEM) analysis, current-voltage measurements, and contact-free mobility extracted from the Y-function method (YFM) enable understanding of the compatibility and the effects of pulsed laser annealing process; the enhanced performance originated not only from a decrease in the Schottky barrier effect at the contact, but also an improvement of the channel interface.

Second, through laser direct writing (LDW) method, arbitrary fine patterns are produced for the electrodes as well as defining the channel of TFTs. Also, LDW with ink-jet printing shows the possibility and the potential for the future of flexible/wearable electronics as more versatile process. In addition, interference lithography using phase shift mask allows making periodic nano-scale features, readily.

Furthermore, laser process is also applied to the welding for fabricating mechanically robust and electrically attractive 2D-like random networks. The laser welding is compatible with cost-effective solution process and good for the flexible applications. After the laser welding, the sheet resistance of 2D-like networks created by silver nanowires is significantly improved. Note that the best improvement is around 55 times without any degradation of the electrical performance during cycling bending test.

These various outcomes from the experiments indicate that the site selective laser process can open up opportunities to fabrication of the electrical devices on flexible platforms.