Intersubband transition in the polar ZnO/BexMgyZn1-x-yO multiple quantum well structures with wide bandgap barrier

We studied experimentally and theoretically intersubband transitions (ISBTs) in Zn-polar ZnO/BexMgyZn1-x-yO multiple quantum wells (MQWs) with polarization field tunable via tailoring strain in MQWs afforded by opposite signs of a lattice parameter change in wide-bandgap BexMgyZn1-x-yO when ZnO is alloyed with BeO and MgO. As shown by Sentaurus Synopsys simulations, the presence of polarization field in the polar ZnO-based MQW heterostructures results in non-zero probability for 1-to-2 and 1-to-3 energy-level transitions in the QW conduction band due to shift of the wavefunctions and their overlap. ZnO MQWs with QWs width in the range of 2-3nm separated by wide-bandgap (up to 4.37 eV) BexMgyZn1- x-yO (x ~ 0.075 and y = 0.26, 0.31 and 0.42) barriers with width of 3 and 10nm were grown by oxygen plasma-assisted molecular beam epitaxy on c-plane sapphire with resulting lattice mismatch up to 0.5%. These conditions provide compressive strain to the barriers in the MQWs. Polarization-resolved Fourier transform infrared spectroscopy revealed absorption peaks in the mid-wave infrared spectral range. These results indicate that ZnO/BexMgyZn1-x-yO MQWs can be successfully implemented for ISBT-based infrared photodetectors operating at high temperatures.