Double-Quantum-Well AlGaN/GaN Field Effect Transistors with Top and Back Gates: Electrical and Noise Characteristics

AlGaN/GaN fin-shaped and large-area grating gate transistors with two layers of two-dimensional electron gas and a back gate were fabricated and studied experimentally. The back gate allowed reducing the subthreshold leakage current, improving the subthreshold slope and adjusting the threshold voltage. At a certain back gate voltage, transistors operated as normally-off devices. Grating gate transistors with a high gate area demonstrated little subthreshold leakage current, which could be further reduced by the back gate. The low frequency noise measurements indicated identical noise properties and the same trap density responsible for noise when the transistors were controlled by either top or back gates. This result was explained by the tunneling of electrons to the traps in AlGaN as the main noise mechanism. The trap density extracted from the noise measurements was similar or less than that reported in the majority of publications on regular AlGaN/GaN transistors.

Photon Statistics of Incoherent Cathodoluminescence with Continuous and Pulsed Electron Beams

Photon bunching in incoherent cathodoluminescence (CL) spectroscopy originates from the fact that a single high-energy electron can generate multiple photons when interacting with a material, thus, revealing key properties of electron-matter excitation. Contrary to previous works based on Monte Carlo modeling, here we present a fully analytical model describing the amplitude and shape of the second order autocorrelation function (g ⁽²⁾(τ)) for continuous and pulsed electron beams. Moreover, we extend the analysis of photon bunching to ultrashort electron pulses, in which up to 500 electrons per pulse excite the sample within a few picoseconds. We obtain a simple equation relating the bunching strength (g ⁽²⁾(0)) to the electron beam current, emitter decay lifetime, pulse duration, in the case of pulsed electron beams, and electron excitation efficiency (γ), defined as the probability that an electron creates at least one interaction with the emitter. The analytical model shows good agreement with the experimental data obtained on InGaN/GaN quantum wells using continuous, ns-pulsed (using beam blanker) and ultrashort ps-pulsed (using photoemission) electron beams. We extract excitation efficiencies of 0.13 and 0.05 for 10 and 8 keV electron beams, respectively, and we observe that nonlinear effects play no compelling role, even after excitation with ultrashort and dense electron cascades in the quantum wells.

The drag of photons by electric current in quantum wells

The flow of electric current in quantum well breaks the space inversion symmetry, which leads to the dependence of the radiation transmission on the relative orientation of current and photon wave vector, this phenomenon can be named current drag of photons. We have developed a microscopic theory of such an effect for intersubband transitions in quantum wells taking into account both depolarization and exchange-correlation effects. It is shown that the effect of the current drag of photons originates from the asymmetry of intersubband optical transitions due to the redistribution of electrons in momentum space. We show that the presence of dc electric current leads to the shift of intersubband resonance position and affects both transmission coefficient and absorbance in quantum wells.

Stability of electron-hole liquid in quantum wells

Density functional theory is used to calculate the energy of electron-hole liquid and the equilibrium density of electron-hole pairs in quantum wells. Nonlinear Kohn-Sham equations for electrons and holes are solved numerically. The influence of the depth and width of the quantum well, the ratio of the hole and electron masses, and the spin splitting of the hole band on the properties of electron-hole liquid is studied. The critical temperature of electron-hole liquid in quantum wells is estimated. Good agreement between the calculations and experimental results is obtained.

Surface Nanostructuring during Selective Area Epitaxy of Heterostructures with InGaAs QWs in the Ultra-Wide Windows

Selective area epitaxy (SAE) is widely used in photonic integrated circuits, but there is little information on the use of this technique for the growth of heterostructures in ultra-wide windows. Samples of heterostructures with InGaAs quantum wells (QWs) on GaAs (100) substrates with a pattern of alternating stripes (100-μm-wide SiO₂ mask/100-μm-wide window) were grown using metalorganic chemical vapour deposition (MOCVD). It was found that due to a local change in the growth rate of InGaAs QW in the window, the photoluminescence (PL) spectra measured from the edge to the center of the window exhibited maximum blueshifts of 14 and 19 meV at temperatures of 80 K and 300 K, respectively. Using atomic force microscopy, we have demonstrated that the surface morphologies of structures grown using standard epitaxy or SAE under identical MOCVD growth conditions correspond to a step flow growth with a step height of ~1.5 ML or a step bunching growth mode, respectively. In the structures grown with the use of SAE, a strong variation in the surface morphology in an ultra-wide window from its center to the edge was revealed, which is explained by a change in the local misorientation of the layer due to a local change in the growth rate over the width of the window.

Quantifying Exciton Heterogeneities in Mixed-Phase Organometal Halide Multiple Quantum Wells via Stark Spectroscopy Studies

Solution-processable two-dimensional (2D) organic-inorganic hybrid perovskite (OIHP) quantum wells naturally self-assemble through weak van der Waals forces. In this study, we investigate the structural and optoelectronic properties of 2D-layered butylammonium (C₄H₉NH₃⁺, BA⁺) methylammonium (CH₃NH₃⁺, MA) lead iodide, (BA)₂(MA)_n-1Pb_nI_3n+1 quantum wells with varying n from 1 to 4. Through conventional structural characterization, (BA)₂(MA)_n-1Pb_nI_3n+1 thin films showcase high-quality phase (n) purity. However, while investigating the optoelectronic properties, it is clear that these van der Waals heterostructures consist of multiple quantum well thicknesses coexisting within a single thin film. We utilized electroabsorption spectroscopy and Liptay theory to develop an analytical tool capable of deconvoluting the excitonic features that arise from different quantum well thicknesses (n) in (BA)₂(MA)_n-1Pb_nI_3n+1 thin films. To obtain a quantitative assessment of exciton heterogeneities within a thin film comprising multiple quantum well structures, exciton resonances quantified by absorption spectroscopy were modeled as Gaussian features to yield various theory-generated electroabsorption spectra, which were then fit to our experimental electroabsorption features. In addition to identifying the quantum well heterostructures present within a thin film, this novel analytical tool provides powerful insights into the exact exciton composition and can be utilized to analyze the optoelectronic properties of many other mixed-phase quantum well heterostructures beyond those formed by OIHPs. Our findings may help in designing more efficient and reproducible light-emitting diodes based on 2D mixed-phase metal-organic multiple quantum wells.

Molecular Beam Epitaxy of Layered Group III Metal Chalcogenides on GaAs(001) Substrates

Development of molecular beam epitaxy (MBE) of two-dimensional (2D) layered materials is an inevitable step in realizing novel devices based on 2D materials and heterostructures. However, due to existence of numerous polytypes and occurrence of additional phases, the synthesis of 2D films remains a difficult task. This paper reports on MBE growth of GaSe, InSe, and GaTe layers and related heterostructures on GaAs(001) substrates by using a Se valve cracking cell and group III metal effusion cells. The sophisticated self-consistent analysis of X-ray diffraction, transmission electron microscopy, and Raman spectroscopy data was used to establish the correlation between growth conditions, formed polytypes and additional phases, surface morphology and crystalline structure of the III–VI 2D layers. The photoluminescence and Raman spectra of the grown films are discussed in detail to confirm or correct the structural findings. The requirement of a high growth temperature for the fabrication of optically active 2D layers was confirmed for all materials. However, this also facilitated the strong diffusion of group III metals in III–VI and III–VI/II–VI heterostructures. In particular, the strong In diffusion into the underlying ZnSe layers was observed in ZnSe/InSe/ZnSe quantum well structures, and the Ga diffusion into the top InSe layer grown at ~450 °C was confirmed by the Raman data in the InSe/GaSe heterostructures. The results on fabrication of the GaSe/GaTe quantum well structures are presented as well, although the choice of optimum growth temperatures to make them optically active is still a challenge.

Dimensional Tailoring of Ultrahigh Vacuum Annealing-Assisted Quantum Wells for the Efficiency Enhancement of Perovskite Light-Emitting Diodes

Quasi-two-dimensional (Q-2D) perovskites featured with multidimensional quantum wells (QWs) have been the main candidates for optoelectronic applications. However, excessive low-dimensional perovskites are unfavorable to the device efficiency due to the phonon-exciton interaction and the inclusion of insulating large organic cations. Herein, the formation of low-dimensional QWs is suppressed by removing the organic cation 1-naphthylmethylamine iodide (NMAI) through ultrahigh vacuum (UHV) annealing. Perovskite light-emitting diode (PLED) devices based on films annealed with optimized UHV conditions show a higher external quantum efficiency (EQE) of 13.0% and wall-plug efficiency of 11.1% compared to otherwise identical devices with films annealed in a glovebox.

Role of Underlayer for Efficient Core-Shell InGaN QWs Grown on m-plane GaN Wire Sidewalls

Different types of buffer layers such as InGaN underlayer (UL) and InGaN/GaN superlattices are now well-known to significantly improve the efficiency of c-plane InGaN/GaN-based light-emitting diodes (LEDs). The present work investigates the role of two different kinds of pregrowth layers (low In-content InGaN UL and GaN UL namely "GaN spacer") on the emission of the core-shell m-plane InGaN/GaN single quantum well (QW) grown around Si-doped c̅-GaN microwires obtained by silane-assisted metal organic vapor phase epitaxy. According to photo- and cathodoluminescence measurements performed at room temperature, an improved efficiency of light emission at 435 nm with internal quantum efficiency >15% has been achieved by adding a GaN spacer prior to the growth of QW. As revealed by scanning transmission electron microscopy, an ultrathin residual layer containing Si located at the wire sidewall surfaces favors the formation of high density of extended defects nucleated at the first InGaN QW. This contaminated residual incorporation is buried by the growth of the GaN spacer and avoids the structural defect formation, therefore explaining the improved optical efficiency. No further improvement is observed by adding the InGaN UL to the structure, which is confirmed by comparable values of the effective carrier lifetime estimated from time-resolved experiments. Contrary to the case of planar c-plane QW where the improved efficiency is attributed to a strong decrease of point defects, the addition of an InGaN UL seems to have no influence in the case of radial m-plane QW.

Real Space Imaging of Topological Edge States in InAs/GaSb and InAs/InxGa1-xSb Quantum Wells

Structure dependent differential tunneling conductance, dI/dV, profiles obtained using scanning tunneling microscopy on both (110)-cleaved surfaces and (001)-growth surfaces in InAs/GaSb and InAs/In_xGa_1-xSb quantum wells (QWs), which are platforms of two-dimensional topological insulator (2D-TI), clearly demonstrated the edge states formed on the 2D-TI surfaces. The results were confirmed by kp-based electronic structure calculations, which demonstrated that the edge states extended to the 10 nm range from cleaved surfaces generated in the appropriately designed InAs/(In)GaSb QW systems.

Compositional Control in 2D Perovskites with Alternating Cations in the Interlayer Space for Photovoltaics with Efficiency over 18

2D perovskites stabilized by alternating cations in the interlayer space (ACI) represent a very new entry as highly efficient semiconductors for solar cells approaching 15% power conversion efficiency (PCE). However, further improvements will require understanding of the nature of the films, e.g., the thickness distribution and charge-transfer characteristics of ACI quantum wells (QWs), which are currently unknown. Here, efficient control of the film quality of ACI 2D perovskite (GA)(MA)_n Pb_n I_{3 n +1} (〈n〉 = 3) QWs via incorporation of methylammonium chloride as an additive is demonstrated. The morphological and optoelectronic characterizations unambiguously demonstrate that the additive enables a larger grain size, a smoother surface, and a gradient distribution of QW thickness, which lead to enhanced photocurrent transport/extraction through efficient charge transfer between low-n and high-n QWs and suppressed nonradiative charge recombination. Therefore, the additive-treated ACI perovskite film delivers a champion PCE of 18.48%, far higher than the pristine one (15.79%) due to significant improvements in open-circuit voltage and fill factor. This PCE also stands as the highest value for all reported 2D perovskite solar cells based on the ACI, Ruddlesden-Popper, and Dion-Jacobson families. These findings establish the fundamental guidelines for the compositional control of 2D perovskites for efficient photovoltaics.

Indium Incorporation into InGaN Quantum Wells Grown on GaN Narrow Stripes

InGaN quantum wells were grown using metalorganic chemical vapor phase epitaxy (vertical and horizontal types of reactors) on stripes made on GaN substrate. The stripe width was 5, 10, 20, 50, and 100 µm and their height was 4 and 1 µm. InGaN wells grown on stripes made in the direction perpendicular to the off-cut had a rough morphology and, therefore, this azimuth of stripes was not further explored. InGaN wells grown on the stripes made in the direction parallel to the GaN substrate off-cut had a step-flow-like morphology. For these samples (grown at low temperatures), we found out that the InGaN growth rate was higher for the narrower stripes. The higher growth rate induces a higher indium incorporation and a longer wavelength emission in photoluminescence measurements. This phenomenon is very clear for the 4 µm high stripes and less pronounced for the shallower 1 µm high stripes. The dependence of the emission wavelength on the stripe width paves a way to multicolor emitters.

Spontaneous Self-Assembly of Cesium Lead Halide Perovskite Nanoplatelets into Cuboid Crystals with High Intensity Blue Emission

Colloidal all-inorganic perovskite nanocrystals have gained significant attention as a promising material for both fundamental and applied research due to their excellent emission properties. However, reported photoluminescence quantum yields (PL QYs) of blue-emitting perovskite nanocrystals are rather low, mostly due to the fact that the high energy excitons for such wide bandgap materials are easily captured by interband traps, and then decay nonradiatively. In this work, it is demonstrated how to tackle this issue, performing self-assembly of 2D perovskite nanoplatelets into larger size (≈50 nm × 50 nm × 20 nm) cuboid crystals. In these structures, 2D nanoplatelets being isolated from each other within the cuboidal scaffold by organic ligands constitute multiple quantum wells, where exciton localization on potential disorder sites helps them to bypass nonradiative channels present in other platelets. As a result, the cuboid crystals show an extremely high PL QY of 91% of the emission band centered at 480 nm. Moreover, using the same synthetic method, mixed-anion CsPb(Br/Cl)3 cuboid crystals with blue emission peaks ranging from 452 to 470 nm, and still high PL QYs in the range of 72-83% are produced.

Measurement of Diffusion and Segregation in Semiconductor Quantum Dots and Quantum Wells by Transmission Electron Microscopy: A Guide

Strategies are discussed to distinguish interdiffusion and segregation and to measure key parameters such as diffusivities and segregation lengths in semiconductor quantum dots and quantum wells by electron microscopy methods. Spectroscopic methods are usually necessary when the materials systems are complex while imaging methods may suffice for binary or simple ternary compounds where atomic intermixing is restricted to one type of sub-lattice. The emphasis on methodology should assist microscopists in evaluating and quantifying signals from electron micrographs and related spectroscopic data. Examples presented include CdS/ZnS core/shell particles and SiGe, InGaAs and InGaN quantum wells.

Nitride-Based Microarray Biochips: A New Route of Plasmonic Imaging

The desire to improve human lives has led to striking development in biosensing technologies. While the ongoing research efforts are mostly dedicated to enhancing speed and sensitivity of the sensor, a third consideration that has become increasingly important is compactness, which is strongly desired in emergency situations and personal health management. Surface plasmon resonance imaging (SPRi) is one of the few techniques that can potentially fulfill all the three goals, considering its multiplexed assay capability. However, miniaturizing SPRi biosensors remains elusive as it entails complicated optical gears. Here, we significantly slim the architecture of SPRi devices by visualizing the varied local density of states around analytes. The unusual detection scheme is realized by building a gain-assisted SPRi with InGaN quantum wells (QWs), where the QW-plasmon coupling efficiency hinges on localized refractive index variation. This new modality abolishes the prism, the polarizer, and the beam-tracking components in the most used Kretschmann configuration without compromising the performances.

Tuning Lasing Emission toward Long Wavelengths in GaAs-(In,Al)GaAs Core-Multishell Nanowires

Semiconductor nanowire (NW) lasers are attractive as integrated on-chip coherent light sources with strong potential for applications in optical communication and sensing. Realizing lasers from individual bulk-type NWs with emission tunable from the near-infrared to the telecommunications spectral region is, however, challenging and requires low-dimensional active gain regions with an adjustable band gap and quantum confinement. Here, we demonstrate lasing from GaAs-(InGaAs/AlGaAs) core-shell NWs with multiple InGaAs quantum wells (QW) and lasing wavelengths tunable from ∼0.8 to ∼1.1 μm. Our investigation emphasizes particularly the critical interplay between QW design, growth kinetics, and the control of InGaAs composition in the active region needed for effective tuning of the lasing wavelength. A low shell growth temperature and GaAs interlayers at the QW/barrier interfaces enable In molar fractions up to ∼25% without plastic strain relaxation or alloy intermixing in the QWs. Correlated scanning transmission electron microscopy, atom probe tomography, and confocal PL spectroscopy analyses illustrate the high sensitivity of the optically pumped lasing characteristics on microscopic properties, providing useful guidelines for other III-V-based NW laser systems.

Quantum Confinement in Oxide Heterostructures: Room-Temperature Intersubband Absorption in SrTiO3/LaAlO3 Multiple Quantum Wells

The Si-compatibility of perovskite heterostructures offers the intriguing possibility of producing oxide-based quantum well (QW) optoelectronic devices for use in Si photonics. While the SrTiO₃/LaAlO₃ (STO/LAO) system has been studied extensively in the hopes of using the interfacial two-dimensional electron gas in Si-integrated electronics, the potential to exploit its giant 2.4 eV conduction band offset in oxide-based QW optoelectronic devices has so far been largely ignored. Here, we demonstrate room-temperature intersubband absorption in STO/LAO QW heterostructures at energies on the order of hundreds of meV, including at energies approaching the critically important telecom wavelength of 1.55 μm. We demonstrate the ability to control the absorption energy by changing the width of the STO well layers by a single unit cell and present theory showing good agreement with experiment. A detailed structural and chemical analysis of the samples via scanning transmission electron microscopy and electron energy loss spectroscopy is presented. This work represents an important proof-of-concept for the use of transition metal oxide QWs in Si-compatible optoelectronic devices.

The microstructure, local indium composition and photoluminescence in green-emitting InGaN/GaN quantum wells

In this work, we analyse the microstructure and local chemical composition of green-emitting In_x Ga_1-x N/GaN quantum well (QW) heterostructures in correlation with their emission properties. Two samples of high structural quality grown by metalorganic vapour phase epitaxy (MOVPE) with a nominal composition of x = 0.15 and 0.18 indium are discussed. The local indium composition is quantitatively evaluated by comparing scanning transmission electron microscopy (STEM) images to simulations and the local indium concentration is extracted from intensity measurements. The calculations point out that the measured indium fluctuations may be correlated to the large width and intensity decrease of the PL emission peak.

Hot-Carrier-Mediated Photon Upconversion in Metal-Decorated Quantum Wells

Manipulating the frequency of electromagnetic waves forms the core of many modern technologies, ranging from imaging and spectroscopy to radio and optical communication. The process of converting photons from higher to lower energy is easily accomplished and technologically widespread. However, upconversion, which is the process of converting lower-energy photons into higher-energy photons, is still a growing field of study with nascent applications and burgeoning interest. Here, we experimentally demonstrate a new photon upconversion technique mediated by hot carriers in plasmonic nanostructures. Hot holes and hot electrons generated via plasmon decay in illuminated metal nanoparticles are injected into an adjacent semiconductor quantum well where they radiatively recombine to emit higher-energy photons. Using GaN/InGaN quantum wells decorated with gold and silver nanoparticles, we show photon upconversion from 2.4 to 2.8 eV. The process scales linearly with illumination power and enables both geometry- and polarization-based tunability. The conversion of plasmonic losses into upconverted optical emission has the potential to impact bioimaging, on-chip wavelength conversion, and high-efficiency photovoltaics.

Strain Driven Shape Evolution of Stacked (In,Ga)N Quantum Disks Embedded in GaN Nanowires

The fabrication of nanowires with axial multiquantum wells or disks presenting a homogeneous size and shape distribution along the whole stack is still an unresolved challenge, despite being essential for narrowing their light emission bandwidth. In this work we demonstrate that the commonly observed change in the shape of the disks along the stacking direction proceeds in a systematic, predictable way. High- resolution transmission electron microscopy of stacked (In,Ga)N quantum discs embedded in GaN nanowires with diameters of ∼40 nm and lengths of ∼700 nm and finite element method calculations show that, contrary to what is normally assumed, this change is not related to the radial growth of the nanowires, which is shown to be negligible, but to the strain relaxation of the whole active region. A simple model is proposed to account for the experimental observations. The model assumes that each disk reaches an equilibrium shape that minimizes the overall energy of the system, given by the sum of the surface and strain energies of the disk itself and the barrier below. The strain state of the barrier is affected by the presence of the disk buried directly below in a way that depends on its shape. This gives rise to a cumulative process, which makes the aspect ratio of each quantum disk to be smaller compared to the disk grown just before, in qualitative agreement with the experimental observations. The obtained results imply that strain relaxation is an important factor to bear in mind for the design of multiquantum disks with controlled shape along the stacking direction in any lattice mismatched nanowire system.

Infrared imaging: a potential powerful tool for neuroimaging and neurodiagnostics

Infrared (IR) imaging is used to detect the subtle changes in temperature needed to accurately detect and monitor disease. Technological advances have made IR a highly sensitive and reliable detection tool with strong potential in medical and neurophotonics applications. An overview of IR imaging specifically investigating quantum well IR detectors developed at Jet Propulsion Laboratory for a noninvasive, nonradiating imaging tool is provided, which could be applied for neuroscience and neurosurgery where it involves sensitive cellular temperature change.

A study of the optical and polarisation properties of InGaN/GaN multiple quantum wells grown on a-plane and m-plane GaN substrates

We report on a comparative study of the low temperature emission and polarisation properties of InGaN/GaN quantum wells grown on nonpolar ([Formula: see text]) a-plane and ([Formula: see text]) m-plane free-standing bulk GaN substrates where the In content varied from 0.14 to 0.28 in the m-plane series and 0.08 to 0.21 for the a-plane series. The low temperature photoluminescence spectra from both sets of samples are broad with full width at half maximum height increasing from 81 to 330 meV as the In fraction increases. Photoluminescence excitation spectroscopy indicates that the recombination mainly involves strongly localised carriers. At 10 K the degree of linear polarisation of the a-plane samples is much smaller than of the m-plane counterparts and also varies across the spectrum. From polarisation-resolved photoluminescence excitation spectroscopy we measured the energy splitting between the lowest valence sub-bands to lie in the range of 23-54 meV for the a- and m-plane samples in which we could observe distinct exciton features. Thus the thermal occupation of a higher valence sub-band cannot be responsible for the reduction of the degree of linear polarisation at 10 K. Time-resolved spectroscopy indicates that in a-plane samples there is an extra emission component which is at least partly responsible for the reduction in the degree of linear polarisation.

Sub-Picosecond Auger-Mediated Hole-Trapping Dynamics in Colloidal CdSe/CdS Core/Shell Nanoplatelets

Quasi-two-dimensional colloidal nanoplatelets (NPLs) have recently emerged as a class of semiconductor nanomaterials whose atomically precise monodisperse thicknesses give rise to narrow absorption and emission spectra. However, the sub-picosecond carrier dynamics of NPLs at the band edge remain largely unknown, despite their importance in determining the optoelectronic properties of these materials. Here, we use a combination of femtosecond transient absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate the early time carrier dynamics of CdSe/CdS core/shell NPLs. Band-selective probing reveals sub-picosecond Auger-mediated trapping of holes with an effective second-order rate constant of 3.5 ± 1.0 cm²/s. Concomitant spectral blue shifts that are indicative of Auger hole heating are found to occur on the same time scale as the sub-picosecond trapping dynamics, whereas spectral red shifts that emerge at low excitation densities furnish an electron-cooling time scale of 0.84 ± 0.09 ps. Finally, nonadiabatic molecular dynamics simulations relate the observed sub-picosecond Auger-mediated hole-trapping dynamics to a shallow trap state that originates from the incomplete passivation of dangling bonds on the NPL surface.

Distance dependence of the energy transfer rate from a single semiconductor nanostructure to graphene

The near-field Coulomb interaction between a nanoemitter and a graphene monolayer results in strong Förster-type resonant energy transfer and subsequent fluorescence quenching. Here, we investigate the distance dependence of the energy transfer rate from individual, (i) zero-dimensional CdSe/CdS nanocrystals and (ii) two-dimensional CdSe/CdS/ZnS nanoplatelets to a graphene monolayer. For increasing distances d, the energy transfer rate from individual nanocrystals to graphene decays as 1/d(4). In contrast, the distance dependence of the energy transfer rate from a two-dimensional nanoplatelet to graphene deviates from a simple power law but is well described by a theoretical model, which considers a thermal distribution of free excitons in a two-dimensional quantum well. Our results show that accurate distance measurements can be performed at the single particle level using graphene-based molecular rulers and that energy transfer allows probing dimensionality effects at the nanoscale.

Understanding quantum confinement of charge carriers in layered 2D hybrid perovskites

Layered hybrid organic perovskites (HOPs) structures are a class of low-cost two-dimensional materials that exhibit outstanding optical properties, related to dielectric and quantum confinement effects. Whereas modeling and understanding of quantum confinement are well developed for conventional semiconductors, such knowledge is still lacking for 2D HOPs. In this work, concepts of effective mass and quantum well are carefully investigated and their applicability to 2D HOPs is discussed. For ultrathin layers, the effective-mass model fails. Absence of superlattice coupling and importance of non-parabolicity effects prevents the use of simple empirical models based on effective masses and envelope function approximations. An alternative method is suggested in which 2D HOPs are treated as composite materials, and a first-principles approach to the calculation of band offsets is introduced. These findings might also be relevant for other classes of layered 2D functional materials.