III-V nitrides can be employed in optical devices ranging from red to ultra-violet wavelengths and have thus been widely studied for wide bandgap semiconductor device applications. Classic examples of these materials include Aluminum nitride, Gallium Nitride and Indium Nitride. However, in spite of the extensive research on these materials, obtaining high quality single crystalline structures on the common practical substrates of silicon and sapphire is still a challenge. This shortcoming can be attributed to the large lattice misfit that is experienced between the materials and the substrate. Furthermore, there has been another drawback regarding the high-quality material as it has been difficult to obtain a suitable substrate of close lattice constant and thermal coefficient that matches that of the nitrides. Sapphire has been widely used as a substrate, despite its structural incompatibility, simply because of its high-temperature stability, hexagonal symmetry, and easy cleaning procedure. Unfortunately, III-IV nitrides have a rather large lattice misfit over 12% with sapphire, which cannot be accommodated via conventional lattice matching concepts, thereby presenting a platform for further studies aimed at reducing this misfit strain.
Adele Moatti (Ph.D. candidate) and Professor Jagdish Narayan from the North Carolina State University, North Carolina thoroughly investigated and exploited the large lattice misfit strain experienced when matching III-IV nitrides with sapphire substrate, in order to relax the buffer layer and film completely through domain matching epitaxy paradigm. With domain matching epitaxy, they have solved the problem of lack of active slip system in the aforementioned orientation as a viable and alternative solution. Their work is currently published in the research journal, Acta Materialia.
The research method employed commenced with the growth of Epitaxial titanium nitride/aluminum nitride and Titanium nitride heterostructures. Next, High power Lambda Physic KrF excimer laser was employed to ablate the target material for deposition. The targets to be utilized were rotated during the deposition in order to provide uniformity and control the stoichiometry. The two scholars then cleaned and degreased the c-sapphire substrates. Lastly, X-Ray Diffraction scans, Transmission electron microscopy diffraction pattern and imaging, and Transmission Kikuchi Diffraction were employed to characterize the samples processed.
Adele Moatti and Jagdish Narayan observed that the domain matching epitaxy paradigm was able to predict that at the large misfit strain at the Titanium Nitride/Aluminum Nitride interface would relax through 8/9 and 9/10 alternating domains with a frequency factor of 0.4. Moreover, there researchers here were able to establish the epitaxial relationship between layers. Additionally, the two scholars noted that the recorded in-plane strain of 0.22% showed that only thermal strain was unrelaxed in the TIN thin films.
Adele Moatti-Jagdish Narayan study successfully developed TiN/AlN/c-sapphire epitaxial heterostructures and compared them with TiN/c-sapphire epitaxial heterostructures, needed for Gallium Nitride-based light emitting diodes and lasers. It was reported that the introduction of the buffer layer decreased the critical thickness beyond which dislocations could grow in Gallium Nitride thin films due to higher misfit strain compared to sapphire, which also improved the quality of potential Gallium Nitride thin films. Altogether, large misfit systems were relaxed by using the Titanium Nitride on different substrates and buffer layer, better than in small misfit systems.
A. Moatti, J. Narayan. High-quality TiN/AlN thin film heterostructures on c-sapphire. Acta Materialia volume 145 (2018) page 134-141