Role of low-temperature (200 degrees C) nitridation in the growth of GaN by plasma-assisted molecular-beam epitaxy

The effects of sapphire nitridation temperature on the properties of the GaN epitaxial layers grown by rf plasma-assisted molecular-beam epitaxy were investigated. It is found that a nitridation temperature as low as 200 degreesC acts as a singular point for producing a homogeneous AIN layer on the sapphire substrate. The optical and structural characteristics of GaN epitaxial layers were dramatically improved by exploiting a 200 degreesC nitridation temperature. Systematic characterization of the nitrided layer, using spectroscopic ellipsometry and x-ray photoelectron spectroscopy, reveals that the sapphire nitridation chemistry, specifically, AIN vs NO production, depends on the surface temperature. A temperature at 200 degreesC produces a smooth, uniform AIN layer with 6 Angstrom thickness that is close to the critical thickness of AIN on sapphire substrate. This homogeneous AIN acts as diffusion barrier for oxygen during high-temperature growth, as confirmed with secondary ion mass spectrometry measurement. In contrast, a high-temperature nitridation produces a nonhomogenous AIN layer embedded with NO which provides a source for oxygen diffusion into the subsequent GaN epitaxial layer and induces the degraded GaN epitaxial layer. Therefore, improved GaN quality can be traced back to the chemistry of the nitridation.