The optical properties of nonpolar m-plane ZnO/Zn1-xMgxO (x = 0.1, 0.2) multiple quantum wells grown by pulsed laser deposition
Ning-An Chang1*, Hou-Ren Chen1, Chih-Ya Tsai2, Wen-Feng Hsieh1
1Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan
2Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan
* presenting author:張寧安,
In order to prevent the quantum confined Stark effect (QCSE) and enhance the exciton binding energy, the nonpolar ZnO/(Zn,Mg)O multiple quantum wells (MQWs) structures are highly demanded. However, there are no papers report on growth of the m-ZnO/(Zn,Mg)O QWs on m-sapphire substrate. There are two methods can enhance the binding energy of exciton: (1) Decreasing the well with of QWs and (2) increasing the Mg content. 5-pairs ZnO/Zn0.9Mg0.1O and 3-pairs ZnO/Zn0.8Mg0.2O MQWs were fabricated on the m-sapphire substrate with m-ZnO and m-Zn0.9Mg0.1O buffer layers, respectively by pulsed laser deposition. The 5-pairs ZnO/Zn0.9Mg0.1O MQWs have fixed barrier thickness of 55 nm and three different well widths of 4, 8 and 16 nm, respectively. The PL spectra measured at 13K show progressively blue shifted two dominant peaks at 3.326 and 3.370 eV for 16-nm QWs, 3.342 and 3.382 eV for 8 nm, and 3.395 and 3.429 eV for 4 nm. They are attributed to BSFs emission1 and near-band edge (NBE) emission from QWs, respectively. Comparing with the bulk ZnO excitonic peak (~3.365 eV), the blue-shifted NBE emission of MQWs even with 16-nm QWs at 3.370 eV reveals the quantum confinement effect with absence of QCSE that is also confirmed by the unshifted PL peaks with the increasing pumping power density in all samples due to non-polar quantum structures.
The integrated NBE PL intensities and exciton energies as a function of inverse temperature2 are plotted. The larger exciton binding energy of 63, 67 and 91 meV than the bulk value are obtained for 16, 8 and 4 nm, respectively. In addition, the 3-pairs ZnO/Zn0.8Mg0.2O MQWs with well-width of-nm have been grown on m-ZnO or m-Zn0.9Mg0.1O buffer layer. The ZnO/Zn0.8Mg0.2O MQWs on m-ZnO buffer layer reveals a stronger and broader BSF (FWHM of BSF ~82 meV, D0X ~40 meV) emission than on Zn0.9Mg0.1O buffer layer (FWHM of BSF ~71 meV, D0X ~40.5 meV). It can be due the larger lattice mismatched between m-ZnO buffer and Zn0.8Mg0.2O barrier than m-Zn0.9Mg0.1O buffer and Zn0.8Mg0.2O barrier. The ZnO/Zn0.8Mg0.2O MQWs has a larger blue shift than ZnO/Zn0.9Mg0.1O and show two dominant peaks at 3.4701 and 3.5232 eV, which are attributed to BSFs and NBE emission from QWs, respectively. By fitting the integrated NBE PL intensities and exciton energies as a function of inverse temperature, the 101 meV of binding energy can be obtained. Based on the experimental data, for a given well width of 4 nm, the increase of the Mg composition results in a ~11% enhancement of the exciton binding energy. These results clearly show the exciton confinement in nonpolar ZnO/(Zn,MgO) MQWs can be tuned independently by varying the barrier composition or well thickness.

This work is partially supported by Ministry of Science and Technology of Taiwan under grants NSC 102-2112-M-009-016-MY3, and MOST 103-2221-E-009-106-MY3
1. M. Schirra, R. Schneider, A. Reiser, G. M. Prinz, M. Feneberg, J. Biskupek, U. Kaiser, C. E. Krill, K. Thonke, and R. Sauer, Phys. Rev. B 77 , 125215 (2008).
2. W. T. Hsu, K. F. Lin, and W. F. Hsieh, Appl. Phys. Lett. 91 , 181913 (2007).

Keywords: pulsed laser deposition, non-polar quantum wells, quantum-confined Stark effect , near-band edge emission, exciton binding energy