Elsevier

Microelectronics Journal

Volume 40, Issue 2, February 2009, Pages 221-223
Microelectronics Journal

Micro-Raman imaging and micro-photoluminescence measurements of strain in ZnMgSe/ZnSe microdiscs

https://doi.org/10.1016/j.mejo.2008.07.056Get rights and content

Abstract

Semiconductor microdiscs are promising for applications in photonics and quantum-information processing, such as efficient solid-state-based single-photon emitters. Strain in the multilayer structure of those devices has an important influence on their optical properties. We present measurements of the strain distribution in ZnMgSe/ZnSe microdiscs by means of micro-photoluminescence and micro-Raman imaging. Photoluminescence measurements of microdiscs reveal substantially broadened emission lines with a shift to lower energy at the undercut part of microdiscs, indicating local relaxation in this area. The distribution of the strain in the microdiscs is obtained from an imaging micro-Raman analysis, revealing that the freestanding part of the microdiscs is free of defects.

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.

Section snippets

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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