O. Reentilä’s research while affiliated with Ferdinand-Braun-Institut and other places

... AlN (aluminum nitride) has direct wide bandgap (6.2 eV), high resistance (10 11 -10 13 X cm), high piezoelectric coefficient (d 33 = 5.56), high hardness (12 GPa for the (0001) plane), and good thermal conductivity (285 W / mK) [1][2][3][4][5]. Since AlN has unique properties, it has many different application areas, for example, ultraviolet photodetectors, lightemitting diodes (LED), quantum cascade lasers (QCL), radio frequency filters, micro-electro-mechanical systems, and solar cells [6][7][8][9][10][11][12][13][14][15]. Moreover, AIN is used as a buffer layer for GaN (the most studied extensively III-nitride semiconductor) growth on Si. ...

Reference:

High-quality AlN growth: a detailed study on ammonia flow

... It was reported that the pits forming at the ends of the threading dislocations on the AlN surface supplied the nucleation sites for the growth of GaN; thus, the larger the number of dislocations, the greater the density of the GaN nuclei, and denser nuclei lead to a faster coalescence of the GaN layer. [17,18] Moreover, a GaN layer which coalesces faster preserves compressive strain rather than relaxing it, through forming the misfit dislocations. [18] Therefore, the phenomenon of an increased compressive strain can be explained by the faster coalescence of the GaN channel, caused by a higher density of dislocations in the AlN buffer. ...

Reference:

The Effect of AlN Buffer Layer on AlGaN/GaN/AlN Double‐Heterostructure HEMT