We present ZnO/ZnMgO heterostructures grown on Si (111) by molecular beam epitaxy method.with single and double quantum wells. The samples were grown at temperature about 550°C and 940°C. For structures with one well no specific buffer layers were deposited on silicon except a 2 nm layer of Zn. ZnMgO capping layers of the same composition as the ZnMgO barrier were deposited. The samples with single quantum well were grown as follows: first the  ~100 nm ZnMgO layer, then ZnO quantum well (QW) (about 3 nm) and finally ~25 nm ZnMgO cap layer. For structures with the double quantum wells, first a very thick (400 nm) low temperature buffer (450^oC) was grown and then about 250 nm ZnMgO barrier (in high temperature), then two ZnO QWs (3 nm and 6 nm thick) and 15 nm ZnMgO cap layer. During the growth the RF power of oxygen plasma was fixed at 350 W. For the characterization of these samples, X-ray diffraction, AFM, SEM microscopy, photoluminescence and cathodoluminescence were used.

The photoluminescence for a series of ZnO/ZnMgO single quantum wells of different widths were carried out within a temperature range of 4-300K. In the ZnO wells with thicknesses smaller than 3 nm a blue shift of excitonic emission in comparison to bulk ZnO was observed, which demonstrates a quantum confinement. We also show that excitonic emission from QWs is dominated by localized excitons. The characteristic energy of PL decay of the dominant excitonic lines agrees well with the values of localization energies of excitons to donor centers.

In the CL spectrum two major peaks are observed: the higher energy peak from the ZnMgO barrier/capping layers and the other at the lower energy side, originating from the confined excitons in the ZnO wells. At room temperature, the emission coming from the QW dominates.

On the SEM images we can see a uniform sample surface however the uniformly distributed arrays of hillocks are observed. A typical cross-section SEM image of ZnMgO films with the thick buffer reveals that nanocolumns were grown. The XRD spectra suggest that ZnMgO films grow with the c-axis preferred orientation. No other phases of ZnO and ZnMgO were found.

Acknowledgement: Work supported in part within European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08).

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