Studies of surface optical phonons based on GaN and ZnO nanostructures prepared with “top down” method

Abstract

Surface vibration modes in small solids have received great interests in the past decades. Currently, various nanostructures and nanometerials with huge surface-to-volume ratios offer an excellent platform to study surface vibrations and great chance to verify some theoretical modifications on the classical phonon theories for large crystals. Originated from the Lyddane-Sachs-Teller relation which describes the relationship between the transverse optical (TO) and longitudinal optical (LO) modes in crystals, surface optical (SO) phonon mode was predicated with a frequency lying in between the TO and LO modes. The observations of SO phonons have been reported in semiconductor nanostructures such as nanowires and nanospheres etc. However, the SO phonons discussed in the previous reports fall in two different categories: one model considers individual nanostructures as an isolated object and SO phonon frequency is related to its geometric features such as shape and size while the other believes that phonon frequency is highly associated with the nanostructures’ density or filling factor rather than its geometry. So far most of the reported experimental studies were based on nanostructures fabricated by “bottom up” approach. In these samples concentration and uniformity of nanostructures could not be accurately controlled. Meanwhile, the signals recorded in most of the previous studies were actually from many nanostructures. In the present study, we conduct a systemic study of SO phonons based on GaN nanostructures fabricated by focused ion beam (FIB) milling. The focused beam of high-density ions bombards the sample with high precision according to the preset digital mask which offers great flexibility in structural dimension and density. GaN nanostrips and nanopillars with various dimensions were fabricated and optically characterized by a WITec scanning confocal Raman microscopy. Meanwhile, FIB fabricated ZnO nanostructures were also investigated for a comparative study. Preliminary Raman results for the SO phonons show a good consistency in these two systems and detailed systematic results will be presented.