EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Semiconductor nanowires (NW) have received much attention for their unique properties providing exceptional flexibility in device engineering. In particular, gallium phosphide NWs demonstrate effective wave-guiding properties and broadband nonlinear frequency conversion, and find applications in green to amber optical range optoelectronics and photocatalytic devices.

NW elastic strain limit is exceedingly large which opens paths for the investigation of fundamental phenomena appearing in highly-stressed semiconductors as well as for strain engineering in nanoscale devices. Thus, elastic deformation shifts the working wavelength of light emitting structures and induces direct-to-pseudodirect bandgap transitions. Raman spectroscopy is a powerful technique to monitor the strain value as well as electric field distribution inside a NW.

Here, strained gallium phosphide NWs were investigated via micro-Raman mapping. Bending deformations of magnitude 5% and higher were created in individual horizontal NWs lying on nickel substrate using atomic force microscope probe as nanomanipulator. Strain level was estimated from NW curvature. Bent geometry was chosen as it allows studying the effects of both tensed and compressed strains. Micro-Raman mapping with 532 nm excitation laser was conducted in parallel and perpendicular polarization with respect to the NW growth axis. Transverse optical (TO) and longitudinal optical (LO) modes were analyzed. The Raman spectra in strained regions showed significant broadening which is consistent with deformation potential theory. LO mode shifting and splitting was observed experimentally for the first time. TO mode revealed considerable splitting in perpendicular polarization that is connected with specific internal electric field distribution over the NW radial cross-section confirmed by numerical calculations in COMSOL Multiphysics.

Thus, polarized Raman spectra of individual NW experiencing 5% elastic strain were obtained and analyzed for the first time.