Growth of gallium nitride thin films by liquid-target pulsed laser deposition

Gallium nitride (GaN) thin films have been grown on several kinds of substrates (fused silica, single crystal silicon, and sapphire) by liquid-target pulsed laser deposition at substrate temperature of 600 °C and in the presence of ammonia $(NH3)$ gas. X-ray diffraction, scanning electron microscopy, tunneling electron microscopy, Rutherford backscattering (RBS), and UV/VIS spectrometry were used to characterize the as-grown GaN films. It is shown that $c$-axis oriented GaN films can be formed if a thin zinc oxide (ZnO) buffer layer was first grown on the virgin substrate. Without the buffer layer the GaN films were found to be polycrystalline with randomly oriented grains. Once the ZnO buffer layer was used, the GaN films immediately grew in a columnar structure with its $c$-axis normal to the surface of the film. RBS data have shown that the obtained films are stoichiometric GaN. It was also found that the surface morphology and optical transparency of the GaN films were greatly improved by the ZnO buffer layer.