Research on the integration of GaN devices

Abstract

III-nitride semiconductors have emerged as indispensable materials for a wide range of applications, including photonic devices and electrical devices. With wide bandgaps, light emitted from III-nitride semiconductors covers the wavelength range from ultraviolet to infrared, making them ideal materials for photonic devices, including light-emitting diodes (LEDs) and laser diodes (LD). Attribute to the high saturation electron drift speed and high critical breakdown field strength, electrical devices also have been developed on the GaN platform. However, most GaN devices require other components to work. It is desirable to integrate these devices and components into a single platform. Monolithic integration of devices on a signal chip can enhance their functionality and offers many advantages including compactness and robustness. Many successful instances of the monolithic integration of GaN devices have been reported during the past decades; however, there still exist several challenges, especially for the monolithic integration of photonic and electrical devices. GaN devices with significantly different functions will require optimized wafer structures to achieve optimal performance. Although selective-area epitaxy can be implemented to overcome this problem, it complicates the fabrication and design process and significantly increases the costs. More importantly, as the functionality of the system becomes increasingly complex, the integration tasks also become even more challenging. The heterogeneous integration of separately manufactured components provides a practical alternative that allows the use of optimized chips to build optimized platforms. This thesis investigated the electrical-isolation between monolithically integrated devices as a necessary consideration when fabricating GaN devices. Non-isolated devices that monolithically integrated LED and photodetector (PD) were fabricated and compared with isolated devices. Ground loop signals were observed in the non-isolated devices. Experiments and simulations were conducted to understand the origins of the ground loop signal. Two heterogeneous integration systems were demonstrated. The first system is the heterogeneous integration of monolithic GaN LED- PD with circuitry for intensity stabilization. The stabilization circuit, consisting of a transimpedance amplifier (TIA), proportional-integral controller, and a low-dropout regulator, was heterogeneously integrated with a monolithically integrated GaN LED-PD chip as a system-in-package, making use of bare dies for the majority of components. Using this approach, the entire system was shrunk down to 4 mm × 5 mm, which is comparable to the size of the LED-PD chip itself. Besides, the stability was further improved to 0.01% on average over one-hour periods. The second system is the heterogeneous integration of an on-chip communication system. A monolithic PIC chip integrating GaN-on-Si LEDs, PDs and waveguides was heterogeneously integrated with a TIA and capacitors. Moreover, the size of the system was remarkably reduced. A pseudorandom binary sequence-3 signal was successfully transmitted across this integrated system at the data rate of 280 Mbit/s with a clear opening eye diagram and with a small delay time of 3.54 ns.