Cathodoluminescence characteristics of light emission properties of axial ZnO/Zn1-xMgxO multiple quantum wells on vertical ZnO microrods

We have fabricated axial ZnO/ZnMgO multiple quantum well (MQW) heterostructures consisting of five ZnO wells with different thicknesses separated by thin ZnMgO barriers on the top of ZnO microrods and investigated the optical... [ view full abstract ]

We have fabricated axial ZnO/ZnMgO multiple quantum well (MQW) heterostructures consisting of five ZnO wells with different thicknesses separated by thin ZnMgO barriers on the top of ZnO microrods and investigated the optical properties of such structures by spatially and spectrally resolved cathodoluminescence (CL) spectroscopy and imaging.

ZnO microrods have been prepared by the microwave-assisted hydrothermal method and then overgrown by plasma-assisted molecular beam epitaxy technique which has been employed for control of well widths and compositions of the axial MQW heterostructures.

To study local optical properties, CL spectrum line scans have been recorded for a number of the axial MQW heterostructures. Example of results is seen in Fig. 1(a).The orientation of the line scan is illustrated in Fig. 1(b). The CL line scan shows the spatial distribution of the emission bands with respect to the microrod position. It is visible that the ZnO QWs with different thicknesses and the ZnMgO layers are located on the top end of the microrod. The spectra measured at the bottom of the microrod, where no QW and barrier are present, show only the ZnO emission band at 3.35 eV related to recombination of the donor bound exciton. Relative intensity of the emission bands depends on position in the microrod. 

The CL-intensity image of individual MQW heterostructure (Fig. 1(c)) composed of two images taken for the two spectral features (QW and barrier emission) confirms that the ZnMgO barrier is only grown on the top end of the microrods and that nearly no lateral growth of the material along the microrod axis takes place under our growth conditions. Emission at 3.36 eV is localized between the ZnMgO layers and most likely is related to recombination in QW.

In conclusion, the presence of the axial heterostructure grown on the top of ZnO microrods is confirmed by low-temperature CL. Luminescence lines are observed at the spectral positions expected for the QW and barrier emissions from the top part of the microrods.