Crack formation and development in AlGaN/GaN structures

In the absence of the use of prevention methods, AlGaN layers on GaN are known to form an array of cracks if a critical thickness is exceeded. In this study growth of AlGaN-GaN structures was carried out by metalorganic vapour phase epitaxy using sapphire substrates. Under optical and atomic force microscopy two distinct crack populations have been obseved. In thin, highly strained films an initial high density population of microcracks are found propagating from threading dislocations. This crack array extends as the thickness increases, before contracting with the onset of large cracks whick extend over many mm across the wafer. The formation of the microcrack array is very dependent on the stain in the epilayer and does not follow the classic critical thickness dependence with lattice mismatch. Avoiding stresses high enough to prevent the microcracking phenomenon may be critical in the use of highly tensile strained layers in nitride devices.