Stress evolution during growth of GaN (0001)/Al2O3(0001) by reactive dc magnetron sputter epitaxy

We study the real time stress evolution, by in situ curvature measurements, during magnetron sputter epitaxy of GaN (0 0 0 1) epilayers at different growth temperatures, directly on Al2O3(0 0 0 1) substrates. The epilayers are grown by sputtering from a liquid Ga target in a mixed N2/Ar discharge. For 600 °C, a tensile biaxial stress evolution is observed, while for 700 °C and 800 °C, compressive stress evolutions are observed. Structural characterization by cross-sectional transmission electron microscopy, and atomic force microscopy, revealed that films grew at 700 °C and 800 °C in a layer-by-layer mode while a growth temperature of 600 °C led to an island growth mode. High resolution x-ray diffraction data showed that edge and screw threading dislocation densities decreased with increasing growth temperature, with a total density of 5.5 × 1010 cm−2 at 800 °C. The observed stress evolution and growth modes are explained by a high surface mobility during magnetron sputter epitaxy at 700–800 °C. Other possible reasons for the different stress evolutions are also discussed. (paper)