Optical properties of GaN nanowhiskers produced by photoelectrochemical etching

We investigated the luminescence properties of GaN nanowhiskers produced by photoelectrochemical (PEC) etching using a combination of photoluminescence (PL) and cathodoluminescence (CL) spectroscopy in order to obtain information regarding the spatial dependence of light emission. It was shown previously that the density of GaN nanowhiskers after PEC etching is well correlated with the threading dislocation density [1,2]. However there is little data regarding the spatial distribution of luminescence from etched samples. For PEC etching, the GaN sample was patterned with Ti contacts and immersed in a 0.002 M KOH solution while illuminated with a mercury arc lamp. Nanowhiskers similar to those in Ref. 1 were obtained after the PEC etch. Fig. 1 shows a comparison of the low temperature (~ 4.2 K) PL and CL spectra taken from the sample before and after etching. We observed peaks attributed to free and bound excitons along with their LO-phonon replicas. The main donor-bound exciton peak was red-shifted by 9 meV after etching due to strain relaxation. Figs. 2 and 3 show a comparison of the SEM and CL images taken at l = 356.8 nm (3.475 eV). The inverse correlation between the bright regions (tops of the bundled nanowhiskers) in Fig. 2 and the dark spots in Fig. 3 indicated that the nanowhiskers are non-radiative. Transmission electron microscopy data from Ref. 1 showed that threading dislocations propagate through the core of the nanowhiskers. Therefore, our observations support the conclusion that threading dislocations are non-radiative centers.