Probing strain in wurtzite InP-InAs core-shell nanowires with Raman spectroscopy

We used Raman spectroscopy to investigate phonon modes of single InP-InAs core-shell nanowire with a pure wurtzite phase to reveal the embedded strained state. Raman spectra show that the InP core exhibits tensile strain and the InAs shell exhibits compressive strain due to the radial heterostructure growth. Using different polarization excitation configurations, we identified distinct A(1) (TO) and E-1 (TO) modes, atomic vibrations along and perpendicular to the [0001] direction, respectively. We also identified E-2h phonon modes in wurtzite InP and InAs, lattice vibration along with the [0001] direction, a fingerprint to distinguish a wurtzite phase from a zinc-blende phase. The Raman spectra changes reveal the resonance effects of InP and InAs between the E-0 + Delta(0) energy level and the E-1 gap by varying excitation energy, which indicates Gamma(7c)-Gamma(6v) transition consists of a split-off valence band and a band-gap transition along [0001] directions of the Brillouin zone. The polar patterns show the Raman A(1) (TO) mode of InP only predominates in the x(yy)(x) over bar over bar configuration, indicating the InP A(1) (TO) phonon is sensitive to excitation polarization, suggesting the resonant Raman spectroscopy with specific polarization offers a direct way for characterizing wurtzite InP. Our findings deepen the understanding of strain in wurtzite-phase core-shell nanowire, providing a significant reference for engineering strain in core-shell nanowire to control optical and electric properties.