Micro-Raman imaging and micro-photoluminescence measurements of strain in ZnMgSe/ZnSe microdiscs
Introduction
Semiconductor microdiscs (MDs) were proposed as lasing devices in the early 1990s [1], [2]. Compared to other optical cavities, MDs are easy to fabricate and yield strong confinement due to internal total reflection of the confined whispering gallery modes (WGM) at the disc boundary. Since its first introduction (in 1992 [1]), many efforts were made to improve the Purcell factor in MD lasers to reduce the laser thresholds. In this context experimental implementations of MD lasers were realised in layer structures, which were nearly lattice matched to the substrate to minimize structural damage due to strain relaxation in the freestanding region of the disc. Raman spectroscopy (RS) and photoluminescence (PL) are suitable techniques for non-destructive characterization of the structural and electronic properties in such semiconductor MD heterostructures. Low-threshold lasing was recently observed in fluorine-doped ZnMgSe/ZnSe MDs on donor-bound exciton transitions [3]. Here, we present detailed investigations of the strain distribution along the MDs performed by μ-PL and μ-RS.
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Experimental details
The ZnMgSe/ZnSe/ZnMgSe multilayer structures were grown by molecular beam epitaxy on GaAs-(0 0 1) substrates. For optimal interface properties, a 20 nm buffer of undoped ZnSe was first deposited on the substrate, followed by 30 nm ZnMgSe layer with a magnesium content of about 8%. Photolithography was then performed to define features of different circular dimensions (i.e. 3–10 μm diameter). Samples were etched by reactive-ion etching (RIE) in an Oxford Plasmalab 80 Plus etch system. Methane (CH4),
Results and discussions
Considering the volume ratio of ZnMgSe vs. ZnSe in our discs, the ZnSe QW is likely to be tensile strained on ZnMgSe in the periphery of the freestanding material. As a result of the tensile strain, the relative positions of the heavy-hole free exciton (FX-hh) and light-hole free exciton (FX-lh) energies are reversed. Fig. 1 shows the 5 K PL spectrum of an unstructured ZnMgSe/ZnSe QW sample (blue) for reference and the μPL spectra of the 4.5 μm diameter MD, measured at different positions from
Conclusions
We have demonstrated that dry and wet etching techniques can be used to fabricate defect-free ZnMgSe/ZnSe MDs on GaAs posts. The structural properties of these ZnSe microstructures were investigated by μ-PL and μ-RS experiments. μ-PL spectra, measured at different positions of our MDs, reveal a clear shift to lower energies and the appearance of sharp WGMs when moving the excitation from the center to the edge of the disc, indicating the release of lattice strain. μ-Raman measurements show that
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