GaN nanowires (NWs) can be synthesized in high structural quality even on foreign substrates, in contrast to conventional heteroepitaxial GaN films. This may result in cost-efficient implementations for opto-electronic applications. Most of these applications require a large number of NWs, i.e., NW ensembles. However, NW ensembles always consist of a large number of individual NWs, which exhibit different properties due to different lengths, diameters, and shapes because of random nucleation processes and local fluctuations in the growth parameters. Consequently, the response from the NW ensemble is composed of an average of the responses from the individual NWs within the ensemble.
Figure 1 Time-integrated PL spectra for three individual GaN NWs labeled NW1, NW2, and NW3 recorded at 5 K for pulsed excitation. The dotted lines in (a) at 3.471 and 3.478 eV indicate the energy of the donor bound and free exciton line, respectively, in unstrained GaN. The dashed line denotes the energy of a donor bound surface exciton. (b) PL transients of the three different individual GaN NWs labeled NW1, NW2, and NW3 and corresponding single exponential fits (black lines) convoluted with the system response function (SRF, gray line) at 5 K.
Figure 2 Time-integrated PL spectra for an ensemble of about 1500 GaN NWs and (b) PL transient (red line) of the donor bound exciton line of the GaN NW ensemble recorded at 5 K. The dotted lines correspond to single exponential functions with decay times of 170 and 600 ps. The black line is a fit with the model mentioned in the text.
For a better understanding of the properties of the NW ensemble, detailed investigations of individual NWs are therefore necessary. In a previous study, we analyzed statistically several hundred photoluminescence (PL) spectra from different free-standing individual NWs. The analysis included the intensity, the energy, and the origin of the PL lines, demonstrating the individual character of each NW. In order to obtain information on their recombination dynamics, the measurement of the decay time of individual NWs using time-resolved photoluminescence (TRPL) spectroscopy is required.
Figure 1(a) shows representative spectra of three individual NWs. The corresponding PL transients in Figue 1(b) integrated over the whole PL line exhibit single exponential decays with decay times between 98 ps (NW1) and 349 ps (NW3).
The spectrum of an ensemble with about 1500 NWs recorded at low temperatures shown in Fig. 2(a) is dominated by the donor bound exciton. The corresponding PL transient in Fig. 2(b) is clearly nonexponential with decay times between 170 and 600 ps. Therefore, we conclude that the nonexponential decay is due to a superposition of the different single exponential decay of each individual NW of the ensemble.
We applied a model taking nonradiative surface recombination into account by introducing an effective surface recombination velocity S. The fits to our data reveal that nonradiative surface recombination essentially dominates the PL transients of these ultra-thin NWs.