Recently Zinc Oxide has received a renewed attention for the realization of intersubband devices such as quantum cascade lasers (QCLs). Indeed this material is predicted to be able to tackle the main limitation of current terahertz (THz) QCLs: the limited operation temperature. We report the observation of electronic coupling within ZnO/(Zn, Mg)O asymmetric quantum wells (QWs), first step towards this goal. Samples were grown by molecular beam epitaxy (MBE) with surfaces down to 0.4 nm. X-ray reflectivity (XRR) was used for thickness measurements checking and for the investigation of the interface quality. Atomic resolution scanning transmission electron microscopy (STEM) images reveals that we are able to grow 2 monolayers (MLs) thin (Zn, Mg)O barriers in a reproducible way while keeping abrupt interfaces. Room temperature (RT) photoluminescence (PL) spectra show that QWs are still coupled when separated by a 1.0 nm thick barrier. On the contrary, a 4.0 nm thick barrier allows no coupling. Doped samples were investigated by absorption experiment. Absorption spectra were successfully fitted by a theoretical model, proving a clear electronic coupling in our heterostructures. This demonstration allows us to seriously envision the realization of ZnO based intersubband devices.
Intersubband absorption at normal incidence is forbidden by the selection rules and requires oblique incidence operation or texturing of the surface of intersubband-based devices such as focal plane arrays, adding additional processing steps to their fabrication and therefore increasing complexity and costs. Here we demonstrate normal-incidence, polarization sensitive intersubband absorption by wurtzite ZnO/MgZnO quantum wells grown on an m-plane orientation. When grown in this non-polar plane, the ZnO/MgZnO quantum wells spontaneously assemble forming a V-groove profile in the direction perpendicular to the c-axis, i.e. along the a-direction. A stack of quantum wells featuring this morphology acts as a metamaterial that allows for intersubband absorption at normal incidence whenever the electric field of the light is polarized in the direction perpendicular to the c axis. This phenomenon occurs because when the electric field is perpendicular to the c-axis it is no longer contained in the plane of the quantum wells therefore allowing for a small intersubband absorption. On the contrary, if the electric field is parallel to the c-axis, the usual normal-incidence conditions are recovered and no absorption is observed.
Although ZnO and its related heterostructures are really attractive for their potential application in optoelectronics, their developments have been limited by the p-type doping issue. Here, we will show why ZnO properties are also very attractive for unipolar structures, only dealing with electrons, and how the material quality has been improved to reach these devices requirements.
First, the benefit of homoepitaxy through material quality improvement is presented. We will show that molecular beam epitaxy allows getting defect density, surface roughness, and residual doping, comparable to the state-of-the-art of GaAs. Moreover, (Zn,Mg)O alloy could be used to fabricate heterostructures with very good optical and transport properties.
In the second part, we will give a brief overview of the main transport results, especially bidimensional electron gas, reported in the literature. Few examples of possible applications will also be addressed. Then, we will focus on the potentialities of nonpolar ZnO heterostructures for unipolar devices based on intersubband transitions. Once the advantages of using ZnO for TeraHertz quantum cascade laser discussed, we will show that the structural properties of the ZnO/(Zn,Mg)O heterostructures fulfill the requirements of these devices operation. Moreover, we will finish with absorption measurements clearly showing intersubband transitions in agreement with the light polarization selection rule. The strong influence of physical parameters, like doping level, on the energy of these kind of transitions will also be discussed.
This work was funded by EU commission under the H2020 FET-OPEN program; project “ZOTERAC” FET-OPEN 6655107.
The development of Zinc Oxide (ZnO)-based applications have been strongly limited due to the lack of reproducible p-type doping. Here we present novel opportunities in the field of unipolar oxide wide band gap semiconductors. First we have developed the growth of nonpolar ZnO/ZnMgO multiple quantum wells (MQWs) and then we demonstrate that the structural and optical properties of the MQWs are reaching the required level for intersubband devices in terms of defects, surface and interface roughness and doping. We will show and discuss the most recent results as, for instance, intersubband transitions which have been observed in such structures.
This "Zoterac" project has received funding from the European Union’s Horizon 2020 research and
innovation programme under grant agreement No 665107