AlN epitaxial thin films were grown on both vicinal (0001)-oriented native single crystal AlN substrates and AlN templates grown on vicinal (0001)-oriented sapphire to develop a surface kinetic ...framework for the control of surface morphology. A Burton, Cabrera, and Frank (BCF) theory-based model is formulated and utilized to understand the dependence of the surface kinetics on the vapor supersaturation, σ, and substrate misorientation angle, α. The surface energy of the Al-polar surface of AlN was experimentally determined using BCF theory to be 149±8meV/Å2. The critical misorientation angle for the onset of step-bunching was determined to be ~0.25° for a growth rate of 500nm/h and temperature of 1250°C. Transitioning from a surface with 2D nuclei to one with bilayer steps required a decrease in σ or an increase in α, whereas the suppression of step-bunching required an increase in σ or a decrease in α.
•A kinetic framework for controlling the surface during AlN growth is developed.•The Al-polar surface energy of AlN is experimentally determined using BCF theory.•The transitions between morphologies is controlled by the vapor supersaturation.
Metal–polar, Al-rich AlGaN films were grown on both single crystalline AlN and sapphire substrates. The role of surface morphology and surface kinetics on AlGaN composition is presented. With the ...reduced dislocation density of the films grown on AlN substrates, atomically smooth bilayer stepped surfaces are achieved with RMS roughness of less than 50pm for a 5×5µm2 AFM scan area. By controlling the surface supersaturation through adjusting the growth rate, a transition from 2D nucleation to step flow was observed. The critical misorientation angle for step-bunching in nominal Al0.70Ga0.30N grown with a growth rate of 600nm/h on AlN substrates was found to be 0.4°. The composition of bilayer stepped AlGaN was strongly dependent on substrate misorientation angle, where a compositional variation by a factor of two for a change in misorientation angle from 0.05 to 0.40° was observed; this is explained by the different surface diffusion lengths of Ga and Al. Step-bunching resulted in strong compositional inhomogeneity as observed by photoluminescence and scanning transmission electron microscopy studies.
•A kinetic framework for controlling the surface during AlGaN growth on Al-polar surfaces by MOCVD is developed.•The role of surface morphology and surface kinetics on AlGaN composition is presented.•Apparent phase separation of AlGaN was determined not to be an intrinsic property of AlGaN.
•GaN substrates grown by the ammonothermal method are characterized by X-ray topography.•Threading screw, edge and mixed dislocations are imaged.•Dislocations characterized by correlation with ray ...tracing simulated images.
Ammonothermal growth of bulk gallium nitride (GaN) crystals is considered the most suitable method to meet the demand for high quality bulk substrates for power electronics. A non-destructive evaluation of defect content in state-of-the-art ammonothermal substrates has been carried out by synchrotron X-ray topography. Using a monochromatic beam in grazing incidence geometry, high resolution X-ray topographs reveal the various dislocation types present. Ray-tracing simulations that were modified to take both surface relaxation and absorption effects into account allowed improved correlation with observed dislocation contrast so that the Burgers vectors of the dislocations could be determined. The images show the very high quality of the ammonothermal GaN substrate wafers which contain low densities of threading dislocations (TDs) but are free of basal plane dislocations (BPDs). Threading mixed dislocations (TMDs) were found to be dominant among the TDs, and the overall TD density (TDD) of a 1-inch wafer was found to be as low as 5.16 × 103 cm−2.
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