Challenges in AlN crystal growth and prospects of the AlN-based technology
AlN boule expansion was pursued in an inductively-heated reactor, capable of temperatures in excess of 2400°C. Sintered AlN powder was used as a source material and growth was performed in a nitrogen atmosphere. Detailed finite element analysis was used to design radial and axial gradients to follow the desired boule expansion; realistic expectations and limitations of this technique will be discussed.
Despite the excellent FWHM of x-ray rocking curves and low dislocation density, which were typically below 20 arcsec and 103 cm-2, respectively, formation of low-angle grain boundaries during crystal expansion and control of point defects (intrinsic and extrinsic) seem to be the remaining issues.
AlN and AlGaN films with varying compositions and doping levels were deposited on single crystal AlN substrates. A hydroxide passivation layer that could be removed in situ prior to MOCVD growth, was crucial for achieving homoepitaxial growth. TEM studies confirmed epitaxial relationship and absence of interfacial oxide or defects; AlxGa1-xN films with x ≥ 0.7 grew peseudomorphically on AlN substrates. When doped with Si, these films showed several orders of magnitude higher conductivity as compared to those grown on sapphire substrates. UV LED structures and Schottky diodes were fabricated on these materials and turn-on voltages below 5 V and breakdown fields greater than 10 MV/cm were achieved, respectively.