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.
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.
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.
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.
The unavoidable presence of the wetting layer (WL) in Stranski-Krastanov quantum dots (QD) has typically a negative impact on the performance of QD solar cells. In this work, a simple method to engineer the WL of InAs/GaAs QD solar cells is investigated. In particular, we show that covering the QDs at high GaAs capping rates reduces In-Ga intermixing and, therefore, In redistribution from the QDs to the WL. This results not only in larger QDs, but also in thinner WLs, with larger quantum confinement energies and reduced potential barriers for electrons and holes. Carrier trapping by the WLs and subsequent recombination is therefore reduced, resulting in larger photocurrent values of the QD solar cells under short circuit conditions.
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.
Dilute nitride GaAsSbN is an ideal candidate to form the 1-1.15 eV lattice-matched sub-cell that would significantly enhance the performance of 3- and 4-junction solar cells. However, growth problems inherent to this quaternary alloy lead typically to a poor crystal quality that limits its applicability. Better compositional control and crystal quality have been recently reported by growing the material as a GaAsSb/GaAsN superlattice, because of the spatial separation of Sb and N that avoid miscibility problems. Moreover, these structures provide bandgap tunability trough period thickness. Here we study the performance of lattice-matched 1.15 eV GaAsSb/GaAsN type-II superlattice p-i-n junction solar cells with different period thickness and compare them with the bulk and GaAsSbN/GaAs type-I superlattice counterparts. We demonstrate carrier lifetime tunability through the period thickness in the type-II structures. However, the long carrier lifetimes achievable with periods thicker than 12 nm are incompatible with a high carrier extraction efficiency under short-circuit conditions. Only superlattices with thinner periods and short carrier lifetimes show good solar cell performance. Quantum kinetic calculations based on the non-equilibrium Green’s function (NEGF) formalism predict a change in transport regime from direct tunneling extraction to sequential tunneling with sizable thermionic emission components when passing from 6 nm to 12 nm period length, which for low carrier lifetime results in a decrease of extraction efficiency by more than 30%.
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.
Detecting the UV part of the spectrum is fundamental for a wide range of applications where ZnMgO has the potential to play a central role. The shortest achievable wavelength is a function of the Mg content in the films, which in turn is dependent on the growth technique. Moreover, increasing Mg contents lead to an electrical compensation of the films, which directly affects the responsivity of the photodetectors. In addition, the metal-semiconductor interface and the presence of grain boundaries have a direct impact on the responsivity through different gain mechanisms. In this work, we review the development of ZnMgO UV Schottky photodiodes using molecular beam epitaxy and spray pyrolysis, and we analyze and compare the physical mechanisms underlying the photodetector behavior.
In this manuscript we carry out a comparative analysis of p-i-n junction solar cells based on 10 stacks of InAs/GaAs quantum dots (QDs) capped with GaAs(Sb)(N) capping layers (CLs). The application of such CLs allows to significantly extend the photoresponse beyond 1.3 μm. Moreover, a strong photocurrent from the CLs is observed so that the devices work as QD-quantum well solar cells. The GaAsSb CL leads to the best results, providing a strong sub-band-gap contribution, which is higher than that in a sample containing standard GaAs-capped QDs, despite giving rise to the highest accumulated strain. The use of a GaAsN CL reduces the photocurrent originating from GaAs, pointing to electron retrapping and hindered extraction and/or the introduction of point defects as possible reasons for this. Nevertheless, the addition of N helps to balance the accumulated strain, necessary to stack a higher number of QD layers. In addition, the possibility to independently tune the hole and electron confinements by the simultaneous presence of Sb and N in the CL is also confirmed for 10 stacked QD layers. This not only allows to further extend the QD ground state and, therefore, the photoresponse, but also offers the possibility to design an optimized structure facilitating carrier extraction from the QDs. Nevertheless, carrier losses seem to be stronger under the simultaneous presence of N and Sb in the CL.
We review in this paper the application of ZnO/(Zn,Mg)O quantum wells to the photodetection of the polarization state of UV light. This photodetection is achieved by using the natural anisotropy that exists in non-polar ZnO/(Zn,Mg)O quantum wells, which separates the excitonic absorption from the three valence bands to the conduction band depending on the incident light polarization. The device structures covered here consist of Schottky photodiodes on a- and m-plane orientations, grown by molecular beam epitaxy on ZnO or sapphire substrates, and are analyzed as a function of the incident light polarization.
The optical and structural properties of InAs/GaAs quantum dots (QD) are strongly modified through the use of a thin (~
5 nm) GaAsSb(N) capping layer. In the case of GaAsSb-capped QDs, cross-sectional scanning tunnelling microscopy
measurements show that the QD height can be controllably tuned through the Sb content up to ~ 14 % Sb. The increased
QD height (together with the reduced strain) gives rise to a strong red shift and a large enhancement of the
photoluminescence (PL) characteristics. This is due to improved carrier confinement and reduced sensitivity of the
excitonic bandgap to QD size fluctuations within the ensemble. Moreover, the PL degradation with temperature is
strongly reduced in the presence of Sb. Despite this, emission in the 1.5 μm region with these structures is only achieved
for high Sb contents and a type-II band alignment that degrades the PL. Adding small amounts of N to the GaAsSb
capping layer allows to progressively reduce the QD-barrier conduction band offset. This different strategy to red shift
the PL allows reaching 1.5 μm with moderate Sb contents, keeping therefore a type-I alignment. Nevertheless, the PL
emission is progressively degraded when the N content in the capping layer is increased.
We present in this work the effects of the rapid thermal annealing on the optical properties of InGaAsN single quantum wells grown on two different kind of misoriented GaAs (111)B substrates: 1° toward [-211] and 2° toward [2-1-1]. An increase of more than one order of magnitude of the photoluminescence emission is shown, as well as a shift towards higher energies of peak emission of the quantum well. This blueshift was found to be greater for the samples grown on the 2° misoriented substrates than for the first misorientation. These samples were grown by molecular beam epitaxy simultaneously, to assure tha same growth conditions for both samples. Different annealing temperatures were used to find the optimum optical properties for the InGaAsN quantum wells on GaAs (111)B. A comparison of the electrical characteristics of p-i-n diodes processed using as grown and annealed samples is presented. Finally, the application of the RTA optimization to InGaAsN laser devices grown on GaAs (111)B is presented.
The influence of carrier localization on the opto-electronic properties of GaInNAs/GaAs quantum well (QW) light emitting diodes (LED) and laser diodes (LD) grown by molecular beam epitaxy is studied. The external quantum efficiency of the LEDs at low temperature is found to be strongly affected by emission from localized states, and its evolution with the injected current is modified compared to the typical one of a QW LED. The light-current characteristics of GaInNAs LDs are measured for different temperatures between 15 and 295 K, and an anomalous behaviour of the threshold current with temperature is obtained comparing to a reference InGaAs laser. In particular, a negative or infinite T0 is obtained at very low temperatures, followed by a region of very small T0. In addition, if the temperature is further increased, a change to a higher T0 is obtained at a temperature which is in the range of the typical delocalization temperatures in GaInNAs QWs. All these features are attributed to the influence of carrier localization. The temperature induced changes in the relative carrier population of the localized states and the band edge states change the lineshape of the gain spectrum and its peak value, and consequently the threshold current of GaInNAs QW lasers.
Lasing emission is demonstrated at room temperature in the entire spectral region from 1.29 to 1.52 microns using GaInNAs/GaAs quantum well (QW) laser diodes (LD) grown by molecular beam epitaxy on GaAs substrates. The separate confinement heterostructures (SCH) is made up by AlGaAs cladding layers, a GaInNAs-based QW and GaAs barriers. To achieve lasing emission from 1.29 to 1.52 microns the In in the QW content is maintained at 40%, while the N content is varied from 1.3 to 3.3%. With this structure, the threshold current density (Jth) and external differential quantum efficiency (hd) at 1.29 microns are 685 A/cm2 and 45 %, respectively. Increasing the wavelength to reach 1.5 micron emission degrades these figures to Jth=2890 A/cm2 and hd=23% at 1.49 microns, and to Jth=4060 A/cm2 and hd=16% at 1.52 microns, which still represent a very large improvement with respect to previous reports of LDs based on the quaternary. Even though adding N to the structure decreases the internal quantum efficiency (hi), from 75% to 50%, this figure does not change with increasing wavelengths up to 1.44 microns. The differential modal gain also degrades as a result of adding N to the QW, but like the case of hi, does not change significantly with increasing wavelength. Thus, achieving long wavelength emission up to 1.55 micron emission starts to become viable, even with simple LD structures.
A lot of progress have been recently realized concerning the laser performances at 1.3 μm. However, extending the emission of (Ga,In)(N,As) lasers above 1.3 μm with good performances is still challenging, since it is reported that the threshold current density significantly increases. In order to extend the lasing wavelength above 1.3 μm, while keeping good laser characteristics, we have optimized the growth of (Ga,In)(N,As)/GaAs quantum wells (QWs) grown by molecular beam epitaxy in view of realizing laser structures. During the growth of a laser structure the QW is "self"-annealed due to the growth of the upper AlGaAs cladding layer at high temperature. It is important to know the effect of this self-annealing on the QW optical properties. For that purpose, we have realized in situ thermal annealing on QWs grown at different temperatures and with different nitrogen composition. Separate confinement hetero-structure laser diodes with a single In0.4Ga0.6As1-xNx (x=0.015, 0.021 and 0.033)/GaAs QW have been grown, combining a low growth temperature and a high in situ annealing temperature. The broad area devices have a room temperature threshold current density of 1500 A/cm2 and emit around 1.34 μm just above threshold. Furthermore, increasing the nitrogen composition extends the lasing operation up to 1.44 μm with a threshold of 1755 A/cm2 and even to 1.52μm with a 4060A/cm2 threshold.
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