Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part I. Impact of the nitridation chemistry on material characteristics

The impact of the nitridation temperature on sapphire/GaN interface modifications and the structural, chemical, and optical properties of GaN epitaxial thin films with N plasma radicals is investigated. Based on ex situ spectroscopic ellipsometry and x-ray photoelectron spectroscopy analysis, it is found that the sapphire nitridation chemistry, specifically AlN versus oxynitride (NO) production, depends on the surface temperature. Nitridation at $200 °C$ produces a very thin AlN layer with 90% coverage, while high temperature nitridation leads to a 70% coverage of AlN layer containing NO. These initial stages of growth significantly impact the characteristics of the layers following the nitridation step, specifically the low temperature buffer, annealed buffer, and the GaN epitaxial layer. The annealed buffer on a $200 °C$ nitridation provides a homogeneous GaN thin layer covering most of the sapphire surface. This homogeneous GaN layer after annealing produces a superior template for subsequent growth, resulting in improved structural and optical properties of GaN epitaxial films. On the other hand, the annealed buffer grown on sapphire nitrided at temperatures lower or higher than $200 °C,$ has islands of GaN nuclei revealing the sapphire substrate, and ultimately, resulting in degraded GaN epitaxial film quality as demonstrated by photoluminescence and x-ray diffraction measurements. The results can be traced back to the chemistry of the nitridation process.