ZnO-based quantum wells have attracted much attention in the last few years due to their opportunity of combining band gap engineering, along with large excitonic binding energies. Indeed theoretical works suggest that the 60meV-binding energy of excitons in ZnO, could be further doubled in quantum wells (QWs). Studies on ZnO have mainly focused on films grown in (0001) orientation. In this configuration, the wurtzite ZnO layers exhibit built-in electric fields along the growth direction. Non-polar surfaces are thus of a particular interest since the c-axis lies within the growth plane. As a result it is expected that quantum well structures can be grown without any screening of the exciton binding energies.

In this communication, we report on the growth, structural and photoluminescence properties of a-oriented (non-polar) (Zn,Mg)O/ZnO quantum wells (QW). The heterostructures are grown by plasma assisted Molecular Beam Epitaxy (MBE) on sapphire and a-plane ZnO substrate. A series of QWs with different widths and different Mg content demonstrates the absence of Quantum Confined Stark Effect [1]. The strain relaxation process exhibits a strongly anisotropic behavior when a sapphire substrate is used [2]. The anisotropy of the lattice parameters gives rise to an unusual in-plane strain state on ZnO QWs: tensile strain along [1-100], compressive strain along [0001]. Due to this strain state, the excitonic transitions are strongly blue-shifted, and the quantum-well confined exciton energies measured by photoluminescence excitation are very satisfactorily explained by the theory that takes this into account [3]. Finally, we show the drastic improvement of the structural and optical properties when the QWs are grown on non-polar ZnO substrates.

[1] J.-M. Chauveau et al., Semicond. Sci. Tech. 23 (3), 035005 (2008).
[2] J.-M. Chauveau et al., J. Appl. Phys. 104, 073535 (2008).
[3] J.-M. Chauveau et al., Appl. Phys. Lett. 23 (3), 035005 (2008).

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1. MBE growth of ZnO layers and their physical properties