Sapphire surface preparation and gallium nitride nucleation by hydride vapor phase epitaxy

The nucleation and initial growth of gallium nitride (GaN) films on sapphire substrates using hydride vapor phase epitaxy (HVPE) technique depends on many factors including the chemical treatment of sapphire surface, nitridation, and the specific growth conditions. Liquid and gas phase treatments of the sapphire surface were systematically studied as a function of temperature and time. Phosphoric acid (H₃PO₄) etches sapphire preferentially at defect sites and resulted in pits formation on the surface, while etching in sulfuric acid (H₂SO₄) can produce a smooth, pit-free surface. Air-annealing the sapphire at 1400°C produces an atomically smooth surface consisting of a terrace-and-step structure.; The mechanism of sapphire nitridation within the HVPE environment was elucidated. During nitridation, nitrogen is incorporated into the sapphire surface. The sapphire nitridation mechanism can be modeled as a diffusion couple of aluminum nitride (AlN) and aluminum oxide (Al₂O ₃), where N3− and O2− inter-diffuse in the ‘rigid’ Al3+ framework. Nitrogen diffuses into sapphire and substitutes for oxygen to bond with aluminum. The replaced oxygen diffuses out to the surface. The overall nitridation rate is controlled by the diffusion of oxygen.; Sapphire surface treatments of air-annealing and liquid-based etchings have different effects on nitridation and HVPE GaN nucleation. Upon nitridation, the air-annealed sapphire has ∼1.5 times higher nitrogen content compared to liquid-based etchings. Nevertheless, the air-annealed sapphire yields the lowest density of GaN islands. Sapphire nitridation, which yields a thin AlN layer, results in the growth of higher GaN island densities with a smaller mosaic spread. Sapphire surface, which is etched in H₂SO₄ and then nitridated, produces a high density GaN islands resulting in improved-quality of thick GaN films.; The nucleation and initial growth kinetics of GaN on sapphire grown by HVPE were investigated. As the growth temperature increases, the island density decreases due to adatom desorption and higher surface diffusion rates. A simple atomistic nucleation theory was used to model the island density data. The activation energy for adatom surface diffusion on the nitridated sapphire was estimated to be ∼2.3 eV. At high temperatures (≥1050°C), different surface processes, including adatom desorption and island mobility are taking place.