Characterisation of III-nitrides in the scanning electron microscope

This thesis presents research on the characterisation of group III-nitrides using scanning electron microscope (SEM) techniques. In particular structural and morphological properties were investigated by electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). ECCI reveals threading dislocations (TDs), sub-grains, atomic steps and step bunches in the material under study, while EBSD provides quantitative data on sub-grain misorientation. ECCI was also correlated with atomic force microscopy (AFM) to reveal and identify TDs in an epitaxially laterally overgrown GaN film, where the pattern for overgrowth comprised of stripes parallel to the GaN [1100] direction. ECCI revealed both vertically threading and inclined dislocations. Each dislocation was identified, using complementary information provided by AFM and ECCI, as either a-type or c/(a+c)-type dislocations. To extend ECCI to the study of insulating samples, ECCI micrographs were acquired in a variable pressure scanning electron microscope (VP-SEM) at pressures ranging from 0.3 mbar to 0.8 mbar using gaseous secondary electron detectors (GSEDs). ECCI micrographs are produced by detecting a gas-amplified secondary electron signal, indicating that the intensity of the detected secondary electrons is modulated as a result of diffraction of the incident electron beam. The influence of pressure, detector bias, sample tilt and working distance on the quality of ECCI micrographs were investigated. Aluminum nitride (AlN) samples grown on nano-patterned (nano-pillars) sapphire substrate (nPSS) were investigated. The miscut of the substrate and the size of the nano-pillars were shown to influence the surface morphology and TD density. The analysis of these AlN/nPSS samples showed a dependence of the density of step bunches with the density of TDs, the higher the density of step bunches, the lower the TD density. EBSD was used to measure the misorientations of sub-grains in the AlN/nPSS and the misorientation was observed to increase as the density of TDs increased.This thesis presents research on the characterisation of group III-nitrides using scanning electron microscope (SEM) techniques. In particular structural and morphological properties were investigated by electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). ECCI reveals threading dislocations (TDs), sub-grains, atomic steps and step bunches in the material under study, while EBSD provides quantitative data on sub-grain misorientation. ECCI was also correlated with atomic force microscopy (AFM) to reveal and identify TDs in an epitaxially laterally overgrown GaN film, where the pattern for overgrowth comprised of stripes parallel to the GaN [1100] direction. ECCI revealed both vertically threading and inclined dislocations. Each dislocation was identified, using complementary information provided by AFM and ECCI, as either a-type or c/(a+c)-type dislocations. To extend ECCI to the study of insulating samples, ECCI micrographs were acquired in a variable pressure scanning electron microscope (VP-SEM) at pressures ranging from 0.3 mbar to 0.8 mbar using gaseous secondary electron detectors (GSEDs). ECCI micrographs are produced by detecting a gas-amplified secondary electron signal, indicating that the intensity of the detected secondary electrons is modulated as a result of diffraction of the incident electron beam. The influence of pressure, detector bias, sample tilt and working distance on the quality of ECCI micrographs were investigated. Aluminum nitride (AlN) samples grown on nano-patterned (nano-pillars) sapphire substrate (nPSS) were investigated. The miscut of the substrate and the size of the nano-pillars were shown to influence the surface morphology and TD density. The analysis of these AlN/nPSS samples showed a dependence of the density of step bunches with the density of TDs, the higher the density of step bunches, the lower the TD density. EBSD was used to measure the misorientations of sub-grains in the AlN/nPSS and the misorientation was observed to increase as the density of TDs increased