Strain studies on Cd xZn 1− xSe/ZnSe quantum wires by micro-resonance Raman spectroscopy

Abstract

Cd,Zn, _.Se/ZnSe quantum wires with different Cd-content x and lateral widths down to 13 nm were fabricated by electron beam lithography and wet chemical etching. In order to study the wirewidth dependence of the LO phonon splitting between the barrier and the wire regions, micro-resonance Raman spectroscopy has been performed. Reducing the wire width down to about 30 nm, we found an increasing splitting between the ZnSe LO phonon of the barrier layers and the ZnSe-like LO phonon of the Cd,Zn,_,Se wire regions. The size of the splitting is a function of the Cd-content. This behavior is due to the strain relaxation of the deeply etched, biaxially strained wires. The experimental data are characterized by three different size regimes: for very wide wires (i.e. L, > 100 nm), the wire data are comparable to the 2D reference. An increase of the ZnSe-like LO-phonon energy compared to the unstrained Cd,Zni _,Se bulk material is observed in the biaxially strained Cd,Zn, _,Se layers. Reduction of the wire size down to about 30 nm leads to a size-dependent strain relaxation. This is indicated by a reduction of the LO-phonon energy of the ZnSe-like LO phonon of the Cd,Zni _,Se layer. Reducing the lateral sizes of the wires further down below 30 nm, the splitting between the ZnSe LO phonon of the barrier and the ZnSe-like phonon of the Cd,Zni_,Se remains nearly constant, indicating that no significant further strain release occurs in narrow wires. The experimental data are in good agreement with the photoluminescence measurements showing an increasing red-shift of the photoluminescence energy with decreasing wire width for wire widths between 100 and 30 nm. In order to discuss the Raman data more quantitatively, detailed calculations of the lateral strain distribution in the wires and the strain-induced change of the LO-phonon energy were performed. Good agreement between experiment and theory is obtained. As a result, the importance of partial strain relaxation for the discussion of the vibronic properties of the patterned semiconductor systems is confirmed.