Barrier thickness dependence of the built-in electric field in pseudomorphic ZnO/Zn_0.55Mg_0.45O multi-quantum wells

The electric polarization of (Zn, Mg)O alloys, known as stable wide-bandgap semiconductors, is an intrinsic property that has a significant impact on the properties and performance of polar ZnO-based optoelectronic devices, such in the case of the quantum-confined Stark effect. Because Wurtzite (WZ)-ZnMgO exhibits a high potential for epitaxial growth with tunable bandgap energy, we fabricated high-quality pseudomorphic ZnMgO thin films on O-polar ZnO substrates and achieved a maximum Mg solubility of 45%. In addition, the high-quality texture and accurate design of ZnO/ZnMgO multi-quantum wells (MQWs) was confirmed. The electric-polarization-induced built-in electric field (E_in) of polar ZnO/ZnMgO MQWs was examined by using a combination of cryogenic photoluminescence measurements and self-consistent Poisson–Schrödinger method. An electric field of 1.9 MV/cm was obtained for a barrier thickness of 13.6 nm. The barrier-thickness-dependent E_in was fitted with an electrostatic model, and a maximum electric field (E_in^max) of 2.4 MV/cm was obtained for a 3.4-nm-thick quantum well. This value was perfectly reproduced by using the crystal parameters of the pseudomorphic ZnMgO epilayer. These results are expected to enhance our understanding of WZ-ZnMgO crystallography and provide a method for the systematic design of polar-ZnO-based optoelectronic devices.