Deep level centers in InGaN/GaN heterostructure grown on sapphire and free-standing GaN

Deep level transient spectroscopy has been used to characterize the deep levels in InGaN/GaN grown on sapphire substrate as well as on free-standing GaN. The deep levels at E c - E t ∼ 0.17-0.23 eV and E c - E t ∼ 0.58-0.62 eV have been detected in our samples which are present in GaN samples reported by others. These two deep levels have been attributed by us to threading dislocations as they exhibit logarithmic capture kinetic behavior and are found to be substantially reduced in its trap concentration (∼ from 1014 to 1012 cm-2) in GaN grown on free-standing GaN template. Other than the two deep levels, an additional level at E c - E t ∼ 0.40-0.42 eV has been identified in both samples, which is believed to be related to In segregation. AFM image shows region of pits formation in InGaN epilayer for sample grown on u-GaN using sapphire substrate while the latter gives a much smoother morphology. From the X-ray diffraction space mapping, the mosaicity of the sample structure for both samples were studied. Dislocations do not play a significant role in the structural properties of InGaN grown on free-standing GaN since the FWHM based on the Δ ω is relatively small (± 0.15o) in the case of InGaN/GaN on free-standing GaN substrate as compared to that on sapphire (± 0.35o). The wider spread in Δω-2θ value for InGaN layer on free-standing GaN also suggested the effect of compositional pulling with increasing InGaN layer thickness