A major challenge in vertical power devices in GaN and other large bandgap materials is the high defect density that compromises the performance, reliability and yield. We are developing selective area growth approaches that have successfully reduced the densities of these defects on scalable and cheap substrates to a comparable level of native substrates. With these new approaches to material growth, we are able to demonstrate devices whose performance correlates well with the improved material quality. Our choice of material for studying these growth approaches is GaN that while have been extensively studied for decades suffer from large defect densities on large-scale commercially viable Si substrates. Recently, bulk GaN crystal growth techniques such as Na-flux, Hydride Vapor Phase Epitaxy and ammonothermal methods have been developed and, homoepitaxial vertical GaN devices have made it possible to achieve thick drift layers and low dislocation densities but at significant cost. Issues of reliability and uniformity over large areas remain challenging for market adoption of these bulk GaN technologies. For GaN growth on Si, since the GaN cannot be typically grown thicker than 3-4 µm on Si, the dislocation density at the surface cannot be lowered below 108 cm-2 due to lattice and thermal mismatches. In contrast, we are able to grow over 20 µm thick GaN on Si [Tanaka et al., Adv. Mat. 29, 1702557, 2017], and report here systematic studies on GaN Schottky barrier diodes with different thicknesses from 5 µm to 20 µm of unintentionally doped GaN on Si substrates and a newly commercialized QST substrate (Qromis Inc.) and reference GaN substrates. We observed a number of dislocations generated at GaN/Si interface were annihilated with thickness and decreased from 1.89x107 cm-2 in 5 µm thick GaN to 3.39x106 cm-2 in 20 µm thick GaN at the surface. The improvement of the material quality in thick GaN lowered the Schottky diode leakage current 2 orders of magnitude compared to thin GaN and made it possible to fabricate a vertical trench gate field effect transistor(FET) on Si with comparable leakage current as devices on GaN substrate. Similarly, GaN Schottky diodes on QST substrate successfully resulted in comparable leakage current with devices on GaN substrate and showed breakdown voltage of 400 V without edge termination. Vertical devices with optimized edge termination will be reported in the talk.