Raman spectroscopy based measurements of carrier concentration in n-type GaN nanowires grown by plasma-assisted molecular beam epitaxy
Lawrence H. Robins, Elizabeth Horneber, Norman A. Sanford, Kristine A. Bertness, John B. Schlager
The carrier concentration in ensembles of n-type GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy was determined by curve-fitting analysis of Raman spectra, based on modeling of the carrier concentration dependence of the longitudinal phonon plasmon (LPP+) frequency. NWs from 12 growth runs were examined. For these growth runs, the carrier concentration determined from modeling of the Raman results ranged from less than 7×1015 cm3 to (2.59 0.22)×1017 cm3. The frequency of a surface optical (SO) mode of the NWs, at 29.5 cm1 below the LPP+ mode, shows a similar dependence on carrier concentration to the LPP+ mode. Simultaneous excitation of the NWs by a 3.81 eV (325 nm) UV laser, with intensity at the sample of 0.7 W/cm2, induced an increase in the average carrier concentration measured by Raman (as compared to measurements without UV excitation) of only (4 5)×1015 cm3. This result implies that surface depletion does not have a significant effect on the Raman carrier concentration measurements (even in the absence of UV excitation). Additional sample preparation was done to examine the contribution of GaN material with a faceted non-nanowire morphology, referred to as the matrix layer, that is present in some samples. Subtraction of the matrix layer contribution produced a decrease of (9 7)×1015 cm3 in the measured carrier concentration for run B982, and had an insignificant effect on the measured carrier concentration for run B738. The effect of immersion of undoped NWs (from run C024) and doped NWs (from run C144) in high dielectric constant oil was examined. The LPP+ frequencies in the undoped and doped NWs show downshifts of similar magnitude with oil immersion. This result implies that the LPP+ mode has bulk plasmon rather than surface plasmon character.