|About this Abstract
||2010 Electronic Materials Conference
||TMS 2010 Electronic Materials Conference
||AA6, Observation of a Strong Polarization Induced Quantum-Confined Stark Effect in MgXZn1-XO/ZnO Quantum Wells
||Matthias Brandt, Holger von Wenckstern, Marko Stölzel, Alexander Müller, Gabriele Benndorf, Martin Lange, Jan Zippel, Jörg Lenzner, Christof P Dietrich, Michael Lorenz, Marius Grundmann
|On-Site Speaker (Planned)
The quantum-confined Stark effect (QCSE) has been observed for samples grown by molecular beam epitaxy (MBE) containing a Mg<sub>x</sub>Zn<sub>1-x</sub>O/ZnO quantum well (QW)<sup></sup>. For the occurrence of the QCSE a difference in the electric polarization at the heterointerfaces, a significantly large QW width and abrupt interfaces are required. Quantum confinement effects in Mg<sub>x</sub>Zn<sub>1-x</sub>O/ZnO QWs grown by pulsed laser deposition (PLD) were previously observed<sup></sup>. However, even for high QW widths no significant influence of the QCSE on the 2K luminescence was detected in these experiments. Similar results are reported by numerous authors for PLD grown Mg<sub>x</sub>Zn<sub>1-x</sub>O/ZnO QWs<sup></sup>. In the experimental study reported here, the laser fluence in the PLD process was controlled in order to modify the energy of the particles in the laser plasma. This had a tremendous influence on the sharpness of the heterointerfaces. For an adequate choice of the laser fluence a systematic redshift of the QW luminescence with increasing well width of up to 230 meV below the emission of the free exciton in bulk ZnO was observed. Additionally, due to the decreased overlap of the electron and hole wave-functions the transition probability is reduced. An increase in the exciton lifetime up to 0.3ms is observed. Values previously reported for MBE grown samples<sup></sup>, showing very high quality QW interfaces, are quantitatively different, with the longer decay time for a given red-shift in the PLD QWs.
Mg<sub>x</sub>Zn<sub>1-x</sub>O/ZnO heterostructures were grown by pulsed laser deposition (PLD) on a-plane sapphire and ZnO substrates. Mg<sub>x</sub>Zn<sub>1-x</sub>O grows pseudomorphically on ZnO substrates, leading to perfectly organized vicinal surfaces<sup></sup>. Pendellösung-oscillations were observed in high resolution X-ray diffraction, confirming the sharpness of the interfaces. A systematic linear dependence of the c-axis lattice constant on the Mg content in the barrier layer is seen for the samples grown on ZnO. The relation was explained by classical elastic theory, as well as similar data obtained on films grown by MBE<sup></sup>. In order to cover a large number of different thicknesses, wedge-shaped QWs were grown by leaving off substrate rotation during growth. Crystallizing in the wurtzite structure, both ZnO and Mg<sub>x</sub>Zn<sub>1-x</sub>O possess a spontaneous polarization. Upon the formation of a heterostructure an additional piezoelectric polarization component is introduced, increasing or diminishing the polarization. The difference in the spontaneous polarization in adjacent ZnO and Mg<sub>x</sub>Zn<sub>1-x</sub>O layers and its dependence on the Mg content x can be estimated from the thickness dependence of the transition energy and the exciton lifetime in the QW.