Epitaxial GaN Layers: Low Temperature Growth Using Laser Molecular Beam Epitaxy Technique and Characterizations
Generally, the GaN growth by conventional techniques like metal organic chemical vapor deposition (MOCVD) and plasma assisted- molecular beam epitaxy (PA-MBE) techniques employ a higher growth temperature (900–1000 °C in MOCVD and >720 °C in PA-MBE), in which, the probability of forming unwanted alloys or compounds with the substrate at interface will be quite high. To minimize the formation of undesirable interfacial compounds, a low temperature growth is favorable. Here, we have explored the possibility of low temperature growth of GaN layers on sapphire (0001) substrates using an ultra-high vacuum laser assisted molecular beam epitaxy (LMBE) system under different growth conditions. GaN epitaxial layers have been grown by laser ablating liquid Ga metal and polycrystalline solid GaN targets in the presence of active nitrogen environment supplied by radio frequency nitrogen plasma source. The structural and optical properties of the epitaxial GaN layers were characterized using reflection high energy electron diffraction, high resolution x-ray diffraction, atomic force microscopy, Raman spectroscopy, Rutherford Backscattering Spectroscopy, secondary ion mass spectroscopy and photoluminescence spectroscopy. The low temperature LMBE grown GaN layers showed high crystalline structures with a screw dislocation density in the range of 107 cm−2 as calculated from the x-ray rocking measurements along (0002) plane, which is the lowest value obtained so far using LMBE technique. A strong near band-edge photoluminescence emission has been obtained for the grown GaN layers at room temperature with a relatively weak yellow band emission. Our results indicate that the LMBE technique is capable of growing high quality III-nitride crystalline films at a relatively lower growth temperature compared to the conventional growth techniques.