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Metal-oxides hold promise as superior plasmonic materials in the mid-infrared compared to metals, although their integration over established material technologies still remains challenging. We demonstrate localized surface plasmons in self-assembled, hemispherical CdZnO metal-oxide nanoparticles on GaAs, as a route to enhance the absorption in mid-infrared photodetectors. In this system, two localized surface plasmon modes are identified at 5.3 and 2.7 μm, which yield an enhancement of the light intensity in the underlying GaAs. In the case of the long-wavelength mode the enhancement is as large as 100 near the interface, and persists at depths down to 50 nm. We show numerically that both modes can be coupled to infrared intersubband transitions in GaAs-based multiple quantum wells, yielding an absorbed power gain as high as 5.5, and allowing light absorption at normal incidence. Experimentally, we demonstrate this coupling in a nanoparticle/multiple quantum well structure, where under p-polarization the intersubband absorption is enhanced by a factor of 2.5 and is still observed under s-polarization, forbidden by the usual absorption selection rules. Thus, the integration of CdZnO on GaAs can help improve the figures of merit of quantum well infrared photodetectors, concept that can be extended to other midinfrared detector technologies.

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.

A method based on infrared reflectance spectroscopy is presented by which the Mg content in ZnO/(Zn,Mg)O multiple quantum wells with very thin barriers can be determined. The method relies on the observation of interface phonon-polaritons which appear as sharp dips in the p-polarized reflectance spectra at oblique incidence near the longitudinal optical (LO)-phonon frequencies of both QW and barrier materials. By fitting the reflectance spectra to a dielectric function model, the LO phonon frequency of the (Zn,Mg)O barrier layers can be determined. The LO phonon frequency depends on the Mg content. Comparing to Mg content calibration via cathodoluminescence, a linear relationship between the reflectance dip frequency and the Mg content is obtained. The presented method serves as a rapid means to determine the Mg content on final structures with very thin (Zn,Mg)O layers-such as device structures-where alternative, destructive methods cannot be used., Applied Surface Science, 567, ISSN:0169-4332, ISSN:1873-5584

Semiconductor material systems based on GaAs or InP are excellent candidates for optoelectronics in the mid-IR to terahertz spectral range. Consequently, they have readily been used for devices like THz quantum cascade lasers (QCLs) in recent years. But besides significant progress in recent years, THz-QCLs are still limited to operate well below room temperature only [1] . One main driving mechanism for such (temperature-) limitations, is based on the relatively low LO-phonon energy in the primary used GaAs material system of E LO,GaAs ~ 36 meV. This yields, especially at high temperatures approaching room-temperature (E kT, room temp. ~ 26 meV), a very strong thermally activated non-radiative scattering process, that is competing with the wanted optical transition. To overcome these material limitations, we follow a disruptive approach: we changed and investigated another material system that has a much larger LO-phonon energy: ZnO (E LO, ZnO ~ 72 meV), which is promising above room temperature lasing operation [2] . In the following, we present our results for realizing, i.e. designing and fabricating [3] MESA structures with light outcoupling gratings into MBE-grown ZnO/ZnMgO THz QCL structures. In addition, we present the first observation of THz intersubband electroluminescence from the ZnO material system [4] .

The ZnO-based heterostructures are predicted to be promising candidates for optoelectronic devices in the infrared and terahertz (THz) spectral domains owing to their intrinsic material properties. Specifically, the large ZnO LO-phonon energy reduces the thermally activated LO-phonon scattering, which is predicted to greatly improve the temperature performance of THz quantum cascade lasers. However, to date, no experimental observation of intersubband emission from ZnO optoelectronic devices has been reported. Here, we report the observation of THz intersubband electroluminescence from ZnO/MgxZn1–xO quantum cascade structures grown on a nonpolar m-plane ZnO substrate up to room temperature. The electroluminescence peak shows a line width of ∼20 meV at a center frequency of ∼8.5 THz at 110 K, which is not accessible for GaAs-based quantum cascade structures because of the reststrahlen band absorption from 8 to 9 THz. This result is an important step toward the realization of ZnO-based THz quantum cascade lasers. ISSN:2330-4022

This paper proposes a methodology for assessing the concentrator solar cell reliability in a real application for a given location provided the results from accelerated life tests. We have applied this methodology for the evaluation of warranty times of commercial triple junction solar cells operating inside real concentrator modules in Golden (Colorado, USA), Madrid (Spain) and Tucson (Arizona, USA) for the period 2012–2015. Warranty times in Golden and Madrid, namely, 68 and 31 years, respectively, for the analysed period, indicate the robustness of commercial triple junction solar cells. Nevertheless, the warranty time of 15 years for Tucson suggests the need of improvement in the heat extraction of the solar cell within the concentrator module. Therefore, the influence of the location on the reliability of concentrator solar cells is huge, and it has no sense to supply general reliability values for a given concentrator product. The influence of these warranty times for the three locations on the levelised cost of electricity has been analysed. Cost of €c10–12/kWh can be achieved nowadays, while after 1 GWp cumulative installed power, a dramatic reduction to levels of €c2–3/kWh is achievable. Copyright © 2017 John Wiley & Sons, Ltd.

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 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/MgxZn1−xO 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/MgxZn1−xO material systems in infrared and THz optoelectronics and physics. ISSN:2331-7019

The development of new nanophotonic devices requires the understanding and modulation of the propagating surface plasmon and phonon modes arising in plasmonic and polar dielectric materials, respectively. Here we explore the CdZnO alloy as a plasmonic material, with a tunable plasma frequency and reduced losses compared to pure CdO. By means of attenuated total reflectance, we experimentally observe the hybridization of the surface plasmon polariton (SPP) with the surface phonon polariton (SPhP) in the air-CdZnO-sapphire three-layer system. We show how through the precise control of the CdZnO thickness, the resonance frequencies of the hybrid surface plasmon-phonon polariton (SPPP) are tuned in the mid-infrared, and the nature of the hybrid mode turns from a plasmon-like behavior in the thicker films to a phonon-like behavior in the thinnest films. The presence of sapphire phonons not only allows the hybrid mode to be formed, but also improves its characteristics with respect to the bare SPP. The reduced damping of the phonon oscillators allows to reduce the losses of the hybrid mode, enhancing the propagation length above 500 microns, one order of magnitude larger than that of typical SPPs, clearing the path for its application on emerging devices such as plasmonic waveguides., Supporting Information available

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.

Unexpected light propagation effects, such as negative refraction, have been reported in artificial media. Leveraging on the intersubband resonances in heterostructured semiconductors, we show that all possible optical regimes, ranging from classical dieletric and metal to hyperbolic metamaterial types 1 and 2, can be achieved. As a demonstration, we prove that the negative refraction effect can occur at a designed frequency by controlling the electronic quantum confinement.

International audience; This paper reports on the demonstration of quantum cascade detectors (QCDs) based on ZnO/ ZnMgO quantum wells (QWs) grown by molecular beam epitaxy on an m-plane ZnO substrate. The TM-polarized intersubband absorption is peaked at a 3 lm wavelength. The sample has been processed in the form of square mesas with sizes ranging from 10_10 lm2 up to 100_100 lm2. The I-V characteristics reveal that 86% of the 260 devices are operational and that the surface leakage current is negligible at room temperature, which is not the case at 77 K. The photocurrent spectroscopy of 100_100 lm2 QCDs reveals a photocurrent resonance at a 2.8 lm wavelength, i.e., slightly blue-shifted with respect to the intersubband absorption peak. The photocurrent persists up to room temperature. The calibrated peak responsivity amounts to 0.15mA/W under irradiation at Brewster’s angle of incidence. This value allows us to estimate the transfer efficiency (1.15%) of the photoexcited electrons into the active QW of the next period. Published by AIP Publishing.

Optoelectronic devices based on intersubband transitions (ISBT) typically require surface texturing after growth, to overcome the selection rules for polarization and operate under normally incident light. The authors demonstrate that $m$-plane ZnO/(Mg,Zn)O multiple quantum wells (QWs) self-assemble in a geometry that allows absorption at normal incidence, depending on the polarization state of the light with respect to the $c$ axis. These multiple QWs behave as a self-assembled metamaterial, a building block for polarization-sensitive optoelectronic devices.

In a previous work the warranty time of commercial lattice-matched GaInP/Ga(In)As/Ge triple junction concentrator solar cells was evaluated under real climatic conditions. The solar cells had a size of 7x7 mm operating with an efficiency of 35% at 820×. For these particular solar cells the warranty time for three locations, Golden (CO-USA), Madrid (Spain), and Tucson AZUSA), exhibits a 4 to 1 ratio, which affects the LCOE (Levelized Cost of Electricity) in an important way. In this work, we go a step further evaluating the influence of concentration and solar cell size on the warranty for a specific thermal design.

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.

We have developed a method to grow and characterize the state of the art non-polar ZnO/(Zn,Mg)O multi-quantum wells on m-plane ZnO substrates as a prerequisite for applications based on intersubband transitions. The epilayer interfaces exhibit a low roughness, and the layer thickness remains constant within one monolayer in these heterostructures. The optical properties have been studied in the UV and IR domains by means of photoluminescence and absorption experiments, respectively. In the UV, the photoluminescence is very well described by an excitonic transition, with the clear effect of quantum confinement as a function of the well thickness in the absence of the internal field. In the IR, the intersubband transitions can be precisely modeled if a large depolarization shift is taken into account. Overall, we demonstrate a very good control in the design and fabrication of ZnO quantum wells (QWs) for intersubband transitions. Our result gives a clear understanding of the ISBTs in ZnO QWs.

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.

Transparent conducting oxides (TCOs) are widely used in applications from solar cells to light emitting diodes. Here, we show that the metal organic chemical vapor deposition (MOCVD)-grown, rock-salt CdZnO ternary, has excellent potential as a TCO. To assess this compound, we use a combination of infrared reflectance and ultraviolet-visible absorption spectroscopies, together with Hall effect, to determine its optical and electrical transport characteristics. It is found that the incorporation of Zn produces an increment of the electron concentration and mobility, yielding lower resistivities than those of CdO, with a minimum of 1.96 × 10 − 4 Ω · cm for a Zn content of 10%. Moreover, due to a combination of increasing bandgap energy and band filling, the absorption band-edge energy increases from 2.58 eV in CdO up to 2.89 eV for 15% Zn, yielding a largely enhanced transparency in the visible. The low required growth temperatures, together with the scalability of CVD deposition, provide an excellent ground for the integration of this TCO ternary alloy on foreign devices.

In this contribution, we report on negative refraction effect occurring in layered semiconductors. The origin of this effect is attributed to the presence of intersubband plasmon resonance induced by the electronic confinement of the electrons in thick quantum wells. Relying on the effective medium theory, we analyse both the in plane and the out of plane effective optical properties of highly doped ZnO/ZnMgO semiconductor material. We theoretically show and experimentally demonstrate that thicknesses and doping levels of each layer can be carefully chosen to feature strong intersubband transitions, leading to type 1 and type 2 hyperbolic response. Understanding the optical properties of intersubband materials in the frame of hyperbolic materials would shed new light on which physical mechanisms are controlling the radiative decay of intersubband plasmon excitations. This approach could be further utilized to designing efficient mid-IR sources.

The electrical and optical properties of m-plane Ga-doped ZnMgO alloys are analyzed by capacitance-voltage profiling, Hall effect, and IR reflectance and UV-VIS absorption spectroscopies, spanning more than four orders of magnitude in electron concentration. Mg contents up to 50% are achieved while maintaining the wurtzite phase, with a measured band-edge energy of 4.41 eV. Despite the deterioration of the electrical properties with Mg incorporation, high electron concentrations ( ∼ 4.4 × 10 19 c m − 3 ) are measured for Mg contents up to 35%, revealing doping efficiencies close to 100%. The potential physical origin for the drop of the electron concentration and mobility with Mg is analyzed and correlated to the absorption measurements. In ZnO:Ga epilayers band filling is observed, although upon alloying with Mg, bandgap shrinkage and Urbach tailing become evident. It is deduced that the observed carrier compensation arises from the presence of acceptor traps, such as VZn and VZn-GaZn complexes, whereas impurity scattering and electron trapping at extended defects are the mechanisms affecting the mobility.

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.