With this work we seek to cover a key space in the study of new materials for IR plasmonics: their integration on functional substrates. We show that Cd(Zn)O, a promising semiconductor oxide characterized by its low losses, can be grown in a polycrystalline form on a GaAs substrate. Despite this polycrystallinity, its plasmonic response can be described based on the physics of a homogeneous thin layer. With this study, we validate Cd(Zn)O for future integrated plasmonic systems on GaAsbased photonic devices.
We show here that Cd(Zn)O can be deposited on GaAs by MOCVD forming nanoparticles with a hemispherical shape. These nanoparticles maintain the key characteristics from a CdO film: very high plasma frequency and very low losses, hence retaining the strong plasmonic character. As a result of this, when illuminated with infrared light, two localized surface plasmon (LSP) modes are excited at 2.7 and 5.3 microns, and the electric field is heavily amplified in the underlying GaAs substrate. Moreover, their hemispherical geometry allows them to partially change the orientation of the field, creating a component perpendicular to the surface. We prove the coupling between the CdO LSPs and the intersubband transitions from a multiple QW structure, where the absorption is largely enhanced for p-polarized electric fields, and it is observed even under normal incidence conditions.
In this work we propose the use of self-assembled CdZnO nanoparticles as a route to improve power absorption in midinfrared GaAs-based quantum well infrared photodetectors (QWIPs). We experimentally demonstrate low temperature growth of CdZnO nanoparticles on GaAs and characterize their plasmonic response in the mid-IR. Computational analysis of the plasmonic resonances coupled to intersubband transitions in GaAs quantum wells show that intersubband absorption at normal incidence, forbidden by quantum selection rules, can be obtained. Gains in the quantum well power absorption as high as 5.5 are also reported.
Transparent Conductive Oxides (TCOs) have been described as a promising alternative to metals for IR plasmonics. However, low propagation of surface plasmons is a major shortcoming limiting the potential of these materials in waveguiding-related applications. As a proof of concept, we propose using polar plasmonic substrates as a method to overcome this issue. In this study, we demonstrate the existence of hybrid phonon-plasmon surface waves in an air-CdZnO-sapphire system and characterize their properties, including an improved propagation distance when compared to a plasmonic-only equivalent system.
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.
We report on the first demonstration of quantum cascade detectors based on ZnO/ZnMgO quantum wells grown by molecular beam epitaxy on an m-plane ZnO substrate. The sample is processed in the form of square mesas with special attention paid to the passivation of the side facets. Photocurrent spectroscopy reveals a resonance at 2.8 μm wavelength slightly blue-shifted with respect to the intersubband absorption peak at 3 μm wavelength. The photocurrent persists up to room temperature. The peak responsivity amounts to 0.15 mA/W under irradiation at Brewster’s angle of incidence of the top surface of the mesas.
In this work we show the potential of the ZnO/ZnMgO material system for intersubband (ISB)-based devices. This family of alloys presents a unique set of properties that makes it highly attractive for THz emission as well as strong coupling regimes: it has a very large longitudinal optical phonon energy of 72 meV, it can be doped up to ~1021 cm-3, it is very ionic with a large difference between the static and high frequency dielectric constants, and it can be grown homoepitaxially on native substrates with low defect densities. The films analyzed here are grown by molecular beam epitaxy (MBE) on a non-polar orientation, the m-plane, with varying QW thicknesses and 30% Mg concentrations in the barrier, and are examined with polarization-dependent IR absorption spectroscopy. The QW band structure and the intersubband transitions energies are modeled considering many body effects, which are key to predict correctly the measured values.
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
Quantum cascade (QC) lasers opens new prospects for powerful sources operating at THz frequencies. Up to now the best THz QC lasers are based on intersubband emission in GaAs/AlGaAs quantum well (QW) heterostructures. The maximum operating temperature is 200 K, which is too low for wide-spread applications. This is due to the rather low LO-phonon energy (36 meV) of GaAs-based materials. Indeed, thermal activation allows non-radiative path through electron-phonon interaction which destroys the population inversion. Wide band gap materials such as ZnO have been predicted to provide much higher operating temperatures because of the high value of their LO-phonon energy. However, despite some observations of intersubband absorption in c-plane ZnO/ZnMgO quantum wells, little is known on the fundamental parameters such as the conduction band offset in such heterostructures. In addition the internal field inherent to c-plane grown heterostuctures is an handicap for the design of QC lasers and detectors.
In this talk, we will review a systematic investigation of ZnO/ZnMgO QW heterostructures with various Mg content and QW thicknesses grown by plasma molecular beam epitaxy on low-defect m-plane ZnO substrates. We will show that most samples exhibit TM-polarized intersubband absorption at room temperature linked either to bound-to-quasi bound inter-miniband absorption or to bound-to bound intersubband absorption depending on the Mg content of the barrier material. This systematic study allows for the first time to estimate the conduction band offset of ZnO/ZnMgO heterostructures, opening prospects for the design of QC devices operating at THz frequencies.
This was supported by the European Union’s Horizon 2020 research and
innovation programme under grant agreement #665107.
ZnO has great potential for devices in the mid IR and the THz range through the use of intersubband (ISB) transitions in multiple quantum wells (MQWs), although exploiting these transitions requires great control of the epitaxial layers as well as of the physics involved. In this work we present an analysis of non-polar ZnO grown homoepitaxially by molecular beam epitaxy on m-plane ZnO substrates as an ISB optical absorber. The MQWs were characterized under a 45°-bevelled multi-pass waveguide configuration allowing the observation at room temperature of an ISB transition in the 4-6 μm region for p-polarized incident light.
AlGaN/GaN field effect transistors (FETs) have shown tremendous advances in performance and reliability over recent years. They are unique in that they operate under the presence of a high density of defects, imperfect surfaces and interfaces. We review key challenges related to defects in these transistors, and recent novel characterization techniques and approaches to study the impact of these imperfections on the device thermal characteristics and reliability, as basis for developing devices with an increased safe operating area (SOA). This includes the development of a nanometer resolution junction temperature analysis using SiC solid immersion lenses, results on hot electron effects and on the role of dislocations and point defects for device reliability. In addition techniques such as dynamic transconductance to access traps near the channel are presented. The approaches shown take advantage of the complementary nature of electrical, optical and microstructural device analysis, combined with thermal and electrical device simulations.
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