Selective Area Doping of GaN by Epitaxial Layer Overgrowth and Its Power Electronic Applications
GaN has wide bandgap, high critical electric field, and high electron saturation velocity, making it an ideal candidate for power switching electronics. However, the development of GaN-based power electronics is still hampered by the high cost of GaN substrate and selective area doping capabilities (especially for p-type) to produce laterally patterned p-n junctions. Epitaxial lateral overgrowth (ELO) is known to render low-dislocation-density GaN in the overgrown regions (wings) on inexpensive foreign substrates. Furthermore, the combination of ELO (prior to the coalescence stage) and in-situ doping process produces the half-core-shell doping profile which has been used in the optoelectronics such as microrod LEDs. However, the application of ELO-GaN has not been explored in the power applications mostly because such doping profile is not configured to withstand high reverse blocking voltage unless the modified structures and methods could be adopted. In this dissertation, a holistic approach was employed to study the innovative measures either in the material growth or device processing stage to tailor the half-core-shell doping profile produced by the ELO of GaN into the desired selective-area doping profiles for power switching electronics featuring the building block of laterally patterned p-n junctions. For the device processing innovation, the concept of true-lateral device architecture was proposed which consists of fully lateral aligned p-n junctions. The general advantages of such device architecture were comprehensively discussed from various aspects. In addition, the low-dislocation density GaN in the wing regions of ELO was fully utilized as an ideal drift layer of a power device. As a result, both power diodes (Schottky barrier diode and p-n junction diode) and power bipolar transistors (gated lateral power bipolar junction transistor and insulated gate bipolar transistor) with the true-lateral device architecture were experimentally demonstrated either with superior performance or for the first time, highlighted by the record high critical electric field in a GaN p-n junction and the record high current gain of a power bipolar transistor. Alternatively, for the material growth innovation, the hybrid epitaxy-enabled substrate transfer approach was demonstrated to produce the GaN substrate with repeating laterally patterned p-n junctions suitable for a number of advanced electronic devices such as a planar-gate vertical MOSFET. In addition, the selective area doping profiles of the GaN substrate product also rendered a number of state-of-the-art characterization techniques which provided valuable information to study the incorporation and diffusion of dopants (especially for acceptors) in GaN. With these innovative measures and a deeper understanding of selective area doping of GaN, the potential of epitaxial lateral overgrowth to simultaneously realize the low threading dislocation density and the selective-area doping profile (lateral patterned p-n junctions) was initially and finally unleashed to yield unprecedented opportunities in the power electronic applications. This may spawn a revival of interest into ELO-GaN for power electronic applications.