Resonant micro-Raman investigations of the ZnSe–LO splitting in II–VI semiconductor quantum wires

Resonant micro-Raman spectroscopy was performed on ZnSe/Cd0.2Zn0.8Se quantum wires. The LO phonon frequencies observed for the wires were investigated as a function of the temperature and the wire width. ZnSe/Cd0.2Zn0.8Se quantum wells grown by molecular beam epitaxy were patterned by electron beam lithography and wet chemical etching to obtain quantum wires with widths down to the sub-100 nm region. The sample under investigation consisted of different areas each containing wires with well defined widths. Micro-Raman spectroscopy enabled us to perform measurements on a particular area of the sample. By this method Raman spectra could be obtained as a function of the wire width. The sample temperature was chosen to be between room temperature and 9 K. A splitting of the ZnSe and the ZnSe-like LO phonons of the ZnSe/ Cd0.2Zn0.8Se quantum wires was observed. The size of this splitting was found to depend only on the wire width and not on the temperature. This behavior is due to the strain relaxation, which occurs at the edge zone of each quantum wire. The increasing influence of the edge zone for decreasing wire width is the reason for the wire width dependence of the splitting between the ZnSe and the ZnSe-like LO phonon. As a result, the importance of partial strain relaxation for the discussion of the vibronic properties of the patterned semiconductor systems was confirmed.