Double-crystal X-ray topography of freestanding HVPE grown n-type GaN

High-resolution X-ray topography (HRXT) and rocking curve measurements were performed on unintentionally doped, freestanding HVPE grown n-type Gallium Nitride (GaN). Based on the rocking curve widths, the dislocation density is estimated to be in the range of 105-107/cm2, and the lower limit of average crystallite sizes to be 340-500 nm normal to the surface of the film. The lateral dimensions of crystallites and cavities were obtained from HRXT images, and are estimated to be in 200-500 nm, and 0.5-400 μm ranges, respectively. Although the GaN films are freestanding, they are warped with a radius of curvature of about 0.5 m, as determined from topographic measurements. The warpage is attributed to thermal mismatch between GaN and the substrate during growth.