Spatially resolved Raman spectroscopy study of uniform and tapered InAs micro-nano wires: Correlation of strain and polytypism

The asymmetric peak at 212 - 218 cm-1 occurring in InAs micro-nano wires is investigated using spatially resolved Raman spectroscopy (SRRS) of uniform, bent and long tapered MNWs grown on a Si (001) substrate. It is attributed to superposition of E2h phonon (wurtzite : WZ) and TO phonon (zinc blende : ZB) of InAs. Polarized and wavelength dependent SRRS establishes the presence of WZ and ZB phases in these MNWs. However, formation of WZ phase for larger diameter InAs MNWs is not commensurate with existing growth mapping studies, which needs to be understood further. Study of several of these MNWs suggest that the fraction of WZ to ZB in a MNW is decided not only by diameter, but also by local growth seeding/conditions leading to either tapered or uniform MNWs formation, although, external growth conditions are same. Variation of these frequencies that from bulk value are correlated to residual stress generated in ZB and WZ phases due to presence of WZ and ZB phases, respectively. Consistently, temperature dependent Raman data shows that there is a measurable contribution of stress to dw/dT, a positive for ZB and negative for WZ phonons, due to difference in their thermal expansions. Further, effective thermal expansion coefficient of WZ InAs in presence of ZB phase is calculated to vary in the range 10 - 19 x 10-6/K from base to tip of a MNW at 80 K, which is not possible to determine otherwise.
Comment: 36 pages, 11 figures