Structure and thermophysical properties of GaN films deposited by reactive sputtering using a metal Ga target

Abstract

The influence of dislocation density and impurities on the thermal conductivity of epitaxial GaN thin films on c-plane sapphire substrates was studied. GaN thin films with nominal thicknesses of 100, 300, and 1500 nm were fabricated by reactive direct current magnetron sputtering using a Ga metal target and a mixture gas of Ar and N2. A 300-nm-thick GaN film was also fabricated using a mixture gas of Ar–N2–H2. For all the 300- and 1500-nm-thick films, epitaxial growth was confirmed from the sixfold symmetry spots in the pole figure and selected area electron diffraction patterns. Rocking curves of GaN(0002) of these films showed highly oriented growth along the c-axis. The dislocation density deduced from the rocking curves of GaN(101¯0) ranged from 1011 to 1012 cm−2. In the cases where a mixture gas of Ar–N2 was used, films included O and H impurities on the order of 1022 atoms cm−3 in a layer of approximately 50–100 nm thickness near the substrate, and a low-impurity region with impurities on the order of 1021 atoms cm−3 existed above the high-impurity region. The addition of H2 to the sputtering gas led to a reduction in the impurity concentration to a level on the order of 1021 atoms cm−3; it also prevented the formation of the high-impurity region near the substrate. The thermal conductivity of GaN thin films on c-plane sapphire substrates was measured by the pulsed-light-heating thermoreflectance method. The thermal conductivity of the low-impurity region in the 300- and 1500-nm-thick films ranged from 14 to 18 W m−1 K−1. The dislocation density and the oxygen impurities in our films were attributed to inhibitory factors of the thermal conductivity. The breakdown of the bonding network caused by the formations of Ga-NH2 and Ga-OH was not a negligible inhibitory factor of heat conduction.