Bandgap Tuning In M-plane Zn1-xMgxO Thin Film By Varying Oxygen Ambient In Pulsed Laser Deposition
Yu-Fan Wu1*, Cheng-An Chien1, 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:YU-FAN WU,
The ZnO built-in electric field along the c-orientation makes the energy band tilting. This phenomenon is called the quantum confinement Stark effect (QCSE). The QCSE leads the electric field to separate the electrons and holes present in the quantum wells (QWs) after excitation and to reduce the overlap of their wave functions, in turn reducing the exciton binding energy and red shift of the optical emission. Some groups choose non-polar planes to avoid the QCSE. Based on ZnO/ZnMgO finite QWs, the quantized ground state of the finite well shows increasing with reducing the well width and with increasing the potential barrier V0.We choose to tune barrier height by varying the Mg concentration to enhance the quantum confinement. However, the larger Mg dopant would make the phase separation of different crystal structures of ZnMgO (hexagonal) and MgO (rock-salt structure). To keep lattice match in ZnO/Zn1-xMgxO quantum wells, we maintain the structure of Zn1-xMgxO as the hexagonal.
In this report, we have successfully grown ZnMgO films with various Mg dopants by controlling O2 ambient in pulsed laser deposition from 8% and 15% Mg contents ZnMgO targets. From the photoluminescence (PL) spectrum measurement, we obtained the largest band gap of 3.783eV, corresponding to ~22% Mg content, grown under high vacuum (10-7 torr).The alloy fluctuation by Mg dopant in ZnO causes spectral width broadening of near-band-edge emission. Determined from the temperature dependent PL, the coupling strengths of exciton-phonon with the E2low, A1 (LO) and E1 (LO) modes show increasing as increasing Mg content, and the exciton binding energy decreases with increasing Mg dopant caused by the disorders. Furthermore, we didn’t observe extra domain and phase separation in the X-ray diffraction even for x reaching 0.22. The lattice vibration study from Raman measurement reveals that peak shift of E2high mode with x shows m-direction tensile strain (b constant increases) and c-direction compress strain (c constant decreases).

Keywords: Pulsed laser deposition, Non-polar quantum wells, Quantum-confined Stark effect, Mg concentration