The Growth and Micro-Raman Characterization of GaN Nanowires

Micro-Raman spectroscopy was used to study strain in GaN nanowires grown by plasma-assisted molecular beam epitaxy on Si(111) substrates. The growth comprised of a substrate deoxidation, exposure to a nitrogen flux and self-organized growth of nanowires under nitrogen-rich conditions, without catalyst. Two types of nanowires (NWs), hereafter labeled as A and B, grown at two different Ga source temperature (T-Ga similar to 820 degrees C and T-Ga similar to 800 degrees C) were processed. High resolution scanning electron microscopy images of sample A show that the NWs are partly coalesced whereas the images of sample B an ensemble of separated NWs. Raman spectra exhibit Si related signal at similar to 521 cm(-1) and E-2 (high) GaN mode at about 568 cm(-1). The temperature of the samples was estimated using Stokes'/anti-Stokes' intensity ratio of the Si phonon mode and E-2 (high) related signal. Small red-shift of the E-2 (high) mode for both samples with increasing laser power was observed and it is attributed to the local increase of temperature upon laser illumination. For the coalesced NWs slight strain is observed whereas the ensemble of separated NWs are strain-free.