Studies on Optical and Material Properties of InGaN Thin Films with Different Silicon Doping Conditions

In this research, the optical properties and material nano-structures of an undoped and a Si-doped InGaN thin films beyond the critical layer thickness are studied. First, we systematically conduct the optical and nano-material analyses on the as-grown samples. From the results of photoluminescence (PL) measurements, we find that Si-doping results in stronger localized state emission. Cathodoluminescence (CL) and high-resolution transmission electron microscopy results show that Si-doping can alter the nano-structures of the In-rich clusters. Second, we study the thermal annealing effects on the properties of the InGaN thin films. From the results of PL and CL measurements, it is found that the localized state emission is enhanced due to the increased density of In-rich clusters near the surface during thermal annealing process. However, the integrated PL intensity is suppressed due to the formation of thermally induced nonradiative defects. In the third part of this research, we compare the depth-dependent optical and nano-material properties by etching the surfaces of the InGaN thin films. In either sample, the emission peaks corresponding to the localized states in both PL and CL measurements become dominant with increasing depth, implying stronger clustering in deep layers. By using various acceleration electron voltage in CL measurements, it is found that the trends of CL spectral peak shift are opposite between the two samples (red-shift in undoped sample, and blue-shift in Si-doped sample) This result is consistent with that from measurement of various etching depths. The difference between the two samples is attributed to the better strain relaxation in deeper layers of the Si-doped sample.