Native aluminum nitride (AlN) single crystal substrates are promising for use in epitaxial growth of III-nitride materials for fabrication of reliable deep UV light emitting and high power RF devices. Furthermore, the single crystals can be cut into different orientations, such as non-polar or pre-determined and specific misorientations, which will open a door for growing device structures with specially desired properties.
We have found several novel patterns on different orientation AlN substrates using Atomic Force Microscope (AFM). Aluminum nitride substrates with chemical-mechanical-polishing (CMP) finished surfaces, for a number of different misorientations with respect to the main hexagonal axis, that is, c-, a- and b-axis, show well differentiated structures. The AFM images reveal atomic level closed-loop rings. We believe these closed-loop rings are indicative of edge-dislocations. These structures are mostly seen on the near and on-axis aluminum polar surfaces. Atomic steps are not visible on the non-polar bare surfaces and for larger deviations from the polar axis (i.e., larger than 6 degrees from the c-axis). It is also noticed that even though atomic level steps are not always visible on the bare substrate for non-polar and large misorientations from the polar aluminum face, they can be observed after homoepitaxial and heteroepitaxial growth on these orientations. The research results indicate that both substrate orientation and surface treatment play an important role in forming different kinds of surface morphologies.