Characterization of optical properties and thermo-optic effect for non-polar AlGaN thin films using spectroscopic ellipsometry

Abstract

The non-polar a-plane AlGaN thin films with various Al compositions were grown successfully on the r-plane semi-polar sapphire substrates using the continuous growth, two-way pulsed-flow, and three-way pulsed-flow growth methods, respectively with metal-organic chemical vapor deposition. The optical properties and thermo-optic effect of these films were studied extensively for the first time with angle- and temperature-dependent spectroscopic ellipsometry (SE) under both isotropic and anisotropic fitting modes. The SE fitting results for the energy band-gap, the layer thickness, and the surface roughness of the non-polar GaN, AlGaN, and AlN thin films were found to be comparable or even consistent quite well with the characterization results of high-resolution x-ray diffraction, ultraviolet-visible absorption spectroscopy, and atomic force microscopy. It was revealed that the non-polar AlGaN has higher refractive index than its polar counterpart with the same Al composition, and the non-polar AlGaN with the lowest surface roughness could be achieved with the three-way pulsed-flow growth method. Moreover, it was demonstrated that the anisotropy for the non-polar AlGaN thin film increased with increasing the Al composition. These characterization results should be useful for the fabrication of the non-polar AlGaN-based high-temperature power and ultraviolet-polarized optoelectronic devices utilizing thermo-optical effect and optical anisotropy.