Photoluminescence and strain relaxation of CdZnSe/ZnSe quantum wires

Different II-VI semiconductors (ZnSe, CdMgTe, CdZnTe and CdTE) were etched by a reactive ion etching (RIE) technique with a methane/hydrogen gas mixture. Holographically defined lateral periodic surface corrugations and MnTe/CdTe superlattice wires were investigated by x-ray reciprocal space mapping. Biaxially strained quantum well heterostructures (Cd0.2Zn0.8Se/ZnSe) were laterally patterned to quantum wire and dot structures using electron beam lithography. Thus different lateral widths on an area of 40 μm x 40 μm were defined ranging from 1 μm down to 60 nm and 40 nm for dots and wires, respectively. The wires were oriented along the [110] direction on the (001) growth plane. Additionally, on the same sample 2D reference mesa were defined with a size of 40 μm x 40 μm. A systematic spatially resolved photoluminescence (PL) study was performed on these structures as well as on the reference mesa. Down to the narrowest widths photoluminescence was observed, which exhibited first a red shift and finally below 110 nm and 80 nm for the quantum dots and quantum wires a subsequent blue shift was observed, respectively. Parallel to the red shift the full width half maximum increases by 100 % from 7.5 meV to 15 meV and diminishes for the structure widths which show a blue shift. Due to the lateral patterning process the structures undergo a partial elastic relaxation, which effectively distorts the initial biaxially strained status. Detailed calculations of this spatially inhomogeneous strain status were performed as a function of the wire width, by a solution of the Airy stress equation based on a Fourier series method. Whereas for wide wires of 1 μm only a part twice the quantum well thickness at the borders is strongly relaxed, for wires below 200 nm the whole quantum wire relaxes more and more with decreasing width. For comparison with the experimental PL spectra which reflect the change of the electronic properties after the patterning process, the shift of the band edge energies in the nanostructures was calculated for the inhomogeneous strain field with the aid of the Pikus and Bir k.p Hamiltonian. These calculations yield a spatial dependence of the energy gap. It turns out that the observed red shift in the PL spectra for wire widths below 200 nm to about 80 nm can be explained quantitatively as being caused by the elastic wire relaxation. I.e. for the 80 nm wide wire a red shift of 8 meV is observed experimentally, a value in agreement with the average calculated energy gap for this particular wire structure. (author)