Observation of Intersubband Absorption in ZnO Coupled Quantum Wells

Abstract

Intersubband transitions in ZnO material systems are predicted to be promising candidates for infrared and terahertz (THz) optoelectronic devices due to their unusual material properties. In particular, the temperature performance of THz quantum cascade lasers is postulated to be significantly enhanced using ZnO material systems due to their large optical phonon energy. Taking a step forward toward that goal, intersubband transitions in ZnO/Mg x Zn 1−x O asymmetric coupled quantum wells are observed on a nonpolar m plane ZnO substrate. Two absorption peaks are observed in the energy range from approximately 250 meV to approximately 410 meV at room temperature, unambiguously demonstrating the interwell coupling in the asymmetric coupled quantum wells. A theoretical model taking into account the interaction between intersubband transitions shows reasonable overall agreement with the experimental results, thus proving the strong coupling nature of the investigated system. As the building block of complex quantum structures based on intersubband transitions, the results presented show great potential applications of ZnO/Mg x Zn 1−x O material systems in infrared and THz optoelectronics and physics.

Terahertz Intersubband Electroluminescence from Nonpolar m-Plane ZnO Quantum Cascade Structures

“…Otherwise a huge energy shift appears between the measured and the targeted intersubband transition energies. We set a fine calibration procedure to tackle this very strict requirement . It involves a specifically designed calibration sample and a combination of the scanning electron microscopy and X-ray reflectivity experiments for thickness determination.…”

“…The development of commercially available ZnO substrates and/or the excellent ZnO/ZnMgO heterostructure growth control have enabled the realization of UV LEDs, high mobility two-dimensional electron gases and integrated devices . More recently, the intersubband absorption has been observed on nonpolar ZnO substrates, and more complex physical phenomena have also been investigated. , This work represents a new step toward the realization of ZnO-based THz QCLs. Indeed, we report for the first time THz intersubband electroluminescence from ZnO/Zn 1– x Mg x O quantum cascade structures based on a four-well scheme.…”

Intersubband Transitions in Nonpolar ZnO/BeMgZnO Quantum Wells: Effects of Physical Dimension, Concentration and Donor Level

“…The distortion of the wave functions by the field reduces their overlaps and therefore yields a smaller oscillator strength (Feezell et al, 2013;Monavarian et al, 2018;Yıldırım, 2021). To eliminate these effects, quantum well structures grown in a nonpolar orientation have been suggested (Meng et al, 2019).…”

“…The ISBT energies of the conduction band electrons in the nonpolar ZnO/BexMgyZn1-x-yO quantum wells are calculated by applying the effective mass and envelope function approximations. Within these approximations (Harrison and Valavanis, 2016), the Hamiltonian of an electron can be expressed as: where 𝑉(𝑧) is the conduction band offset, which is given by 67.5% of the difference between the band gaps of the well and barrier layers (Meng et al, 2019), 𝜑(𝑧) is the electrostatic potential due to the free and bound charges, 𝑉 𝑠 (𝑧) is the strain-induced shift in the conduction band edge (Harrison and Valavanis, 2016), and 𝑉 𝑥𝑐 is the exchange-correlation energy (Helm, 1999).…”

Exception of terahertz quantum cascade laser based on new material systems

“…ACQWS waveguide structure schematic diagram, in which the thickness of t QW1 , t B1 , t QW2 , and t B2 are different in different samples; (b) the sub-band absorption spectrum of the sample obtained from the experiment (blue dotted line) and the relative spectrum calculation result (red solid line), reprinted with permission from ref [44]…”