Vertical GaN-on-GaN devices have received extensive interest to expand GaN power device capability utilizing the benefits of freestanding GaN substrates. However, to obtain the projected high-voltage performance with low on-resistance, it is essential to have high-quality thick drift layers with low-background doping and low compensating acceptors. This is often challenging to achieve with conventional growth methods since thick epilayer growth also involves increased incorporation of unintentional impurities and degraded surface morphology. Here, the Ammonia-Molecular beam epitaxy (NH3 MBE) method offers exciting advantages of ultra-low impurity GaN film growth, facilitated by its ultra-high vacuum growth environment. In this chapter, we demonstrated optimizing NH3 MBE growth condition that can enable low-background doping in thick drift layer with a reasonably high growth rate. Moreover, by using Indium as surfactants, smooth surface morphology was achieved in the thick GaN layers, even at growth rates of ~1 μm/hr. The fabricated NH3 MBE GaN-on-GaN field-plate p-n diodes exhibited kilovolt range breakdown voltage with punch-through field profile. The diodes also showed excellent forward transport properties with low specific on-resistance (0.28 mΩ-cm2) and low ideality factor (1.36). This reveals that the NH3 MBE can be a promising growth technique for future high-performance vertical GaN power devices.