Structural and morphological evolution of GaN grown by metalorganic chemical vapor deposition on SiC substrates using an AlN initial layer

Abstract

The morphological and structural evolution is presented for GaN grown by metalorganic chemical vapor deposition on 25 nm thick or 150 nm thick AlN initial layers on (0 0 0 1) 4 H and 6 H SiC substrates. The 25 nm thick AlN on SiC was a rough, partially coalesced film, whereas the 150 nm thick AlN on SiC was smooth and was characterized by a step-terrace structure. Both the 25 and 150 nm thick AlN layers on SiC were nearly free of elastic strains. For both AlN initial layers, the GaN films grew by a coarse islanding mechanism. Measurement of the GaN (0 0 0 2) interplanar spacing shows that these islands were largely strain relaxed throughout the growth process. Plan view transmission electron microscopy (TEM) showed a well developed misfit dislocation network at the GaN island/AlN interface. Cross sectional TEM revealed that these islands are free of threading dislocations (TDs) prior to coalescence. A simple, low-angle grain boundary model based on island misorientation at coalescence provides reasonable agreement with TD densities measured using plan view TEM. An electron mobility value as high as 818 cm 2/V s has been measured at room temperature confirming a high degree of material quality.