Structural studies of nonpolar (10-10) ZnO/ZnMgO monolithic multiquantum well structures

Introduction Superlattices (SLs) of ZnO/ZnMgO are interesting for future applications in advanced optoelectronic devices such as polaritonic lasers. In this work nonpolar ZnO/ZnMgO multiquantum well structures were grown on... [ view full abstract ]

Introduction

Superlattices (SLs) of ZnO/ZnMgO are interesting for future applications in advanced optoelectronic devices such as polaritonic lasers. In this work nonpolar ZnO/ZnMgO multiquantum well structures were grown on (10-10) m-plane ZnO substrates by molecular beam epitaxy. The structures were composed of 10-20 pairs of ZnO and ZnMgO layers and the thicknesses of the individual layers varied from 10 nm to 50 nm. The Mg content in ZnMgO layers was ~30-70%, depending on the growth conditions.

Methods

The structures were characterized using a scanning electron microscope (SEM) in secondary electrons mode, cathodoluminescence (SEM-CL), photoluminescence (PL) and X-ray diffraction (XRD). Atomic Force Microscope (AFM) was used to monitor surface morphology. Rutherford backscattering and channeling (RBS/C) of 2 MeV He+ ions  were applied to determine the composition and quality of the structures. Channeling angular dependence of the backscattering yield was measured to obtain information about strain in the layers. For thick, ~50 nm layers, the RBS/C gives a unique opportunity not only to control the composition and thickness of the layers, but also to determine strain at each individual ZnO/ZnMgO and ZnMgO/ZnO interfaces.

Results and discussion

XRD measurements revealed satellite lines up to the 7th order related to the existence of SLs, thus confirming their very good crystalline quality and coherent growth. The figure below shows typical AFM and SEM images for a SL grown on m-ZnO. The stripes seen in the AFM image are along the c-xis. The SEM image of the structure cross section was taken along the c-axis. SEM imaging revealed that ZnO/ZnMgO interfaces are well defined. Low temperature SEM-CL maps taken in panchromatic and monochromatic modes show the spatial distribution of the total luminescence intensity and individual lines. It has been found that even for a 50 nm/50 nm SL the luminescence comes predominantly from ZnO layers and the maximum intensities are observed near the interfaces. Analysis of experimental RBS spectra and numerical simulations reavealed some gradient of Mg across the structures. It suggests a slight instability of the Mg flux during the long time growth process.

Acknowledgements: The work supported by the NCN project DEC-2014/15/B/ST3/04105.