Quantum-well Structures Research Articles

One-Dimensional Perovskite-like Cu(I)-Halides with Ideal Bandgap Based on Quantum-Well Structure.

Low-dimensional halide perovskites with quantum-well structures are promising materials for electronics and optoelectronics because of their excellent optoelectronic properties. This work concerns two novel, lead-free, one-dimensional organic-inorganic hybrid perovskite-like Cu(I) halides, (MV)Cu2X4 (MV = methyl viologen; X = Br, I), for optoelectronic applications. Both Cu(I) halides exhibited good stability under ambient conditions. The optical bandgaps of (MV)Cu2Br4 and (MV)Cu2I4 are 1.4 and 1.5 eV, respectively, which are in the ideal bandgap range for solar cells. (MV)Cu2Br4 possessed a characteristic quantum-well structure in which [CuBr4]n3n- chains with a nanowire-like structure were rolled up and isolated by tightly packed organic cations. Thanks to quantum confinement in the unique structure, the optical bandgap of (MV)Cu2Br4 fell in the ideal bandgap range for solar cells and was superior to that of (MV)Cu2I4. The good photoresponse properties of these Cu(I) halides suggest their great potential for application as light-harvesting materials in solar cells.

Weak antilocalization effect and multi-channel transport in SnTe quantum well

Magnetoresistance measurements were performed on a 30 nm-thick SnTe quantum well (QW) grown by molecular beam epitaxy on the BaF2 substrate in the temperature range of 1.9–50 K. The weak antilocalization (WAL) effect was observed at low temperatures and low magnetic fields as a result of the strong spin–orbit coupling present in the QW. Using the Hikami–Larkin–Nagaoka equation, we analyzed the experimental data and found that the WAL effect is not purely 2D but composed of 2D and 3D channels that exist within the QW structure. The spin–orbit and phase coherence mechanisms are also extracted, and a general view of the transport properties of the QW is also provided.

Theoretical studies on in-plane polarization characteristics of (11$$\bar{2}$$0) nonpolar InGaN/GaN quantum-well structures grown on InGaN substrates

Theoretical studies on in-plane polarization characteristics of (11$$\bar{2}$$0) nonpolar InGaN/GaN quantum-well structures grown on InGaN substrates

Vertical hole transport through unipolar InGaN quantum wells and double heterostructures

We report unipolar hole transport through unintentionally doped (UID) c-plane Ga-polar InGaN heterostructures to investigate the impact of alloy disorder on vertical transport. Simulations and experimental investigations were conducted on unipolar InGaN double heterostructures (DHs) and quantum well (QW) test structures by varying thicknesses and the number of QWs. The structures were simulated using one-dimensional (1D) and three-dimensional (3D) algorithms, incorporating the effects of random alloy disorder in the 3D model using the newly developed Localization Landscape theory. The electrical polarization discontinuity between GaN and InGaN results in a significant barrier for vertical carrier transport. Band-diagram and current density-voltage simulations indicate asymmetric polarization barriers to the vertical hole transport for the InGaN DHs. For the QW structures, however, the simulations indicate a symmetric barrier to the hole transport in forward and reverse bias. In the case of the InGaN DH layers, the 3D simulation results indicate a smaller barrier to hole transport compared to 1D simulations. For QW simulations, the barriers were found to be the same in both 1D and 3D simulations. The simulation results are experimentally verified using unipolar p-type vertical transport structures, enabled by n-to-p tunnel junctions to facilitate the current spreading within the bulk material for the mesa structures grown by ammonia-molecular beam epitaxy. The results indicate that increasing the UID ${\mathrm{In}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{N}$ DH layer thickness from 15 to 30 nm increases the forward bias voltage drop (\ensuremath{\sim}2 V at $500\phantom{\rule{0.16em}{0ex}}\mathrm{A}/\mathrm{c}{\mathrm{m}}^{2}$) more than the reverse bias voltage drop (\ensuremath{\sim}0.2 V at $500\phantom{\rule{0.16em}{0ex}}\mathrm{A}/\mathrm{c}{\mathrm{m}}^{2}$). For the QW structures, increasing the number of QWs from one to three increases the voltage penalty similarly in forward and reverse directions (\ensuremath{\sim}0.25 V per QW at $500\phantom{\rule{0.16em}{0ex}}\mathrm{A}/\mathrm{c}{\mathrm{m}}^{2}$). The results are beneficial in understanding the impact of alloy disorder on the transport properties of the III-nitride heterostructures.

Spin-polarized charge transport through a single barrier in HgTe/CdTe heterostructure interface

We present a theoretically investigation about the electron transport through a single-barrier structures in HgTe/CdTe quantum wells (QWs) with inverted band structure. For HgTe/CdTe QWs structure, the transmission probabilities show great dependence on the Rashba spin–orbit interaction (RSOI), the incident angle, the incident Fermi energy and the electric modulation potential. By tuning the modulation amplitude with gate voltage or the strength of RSOI modulation, the efficient spin-polarized PZ=1 is achieved, which is formed due to the spin-split in band spectrum induced by the RSOI modulation. This electric mechanism provides us a way to manipulate the spin polarization electrically and convenient for experimental verification.

Two-dimensional hole gases in SiGeSn alloys

Two-dimensional hole gases are demonstrated in modulation doped Si x Ge1−x−y Sn y quantum wells (QWs), which are embedded in Si0.2Ge0.8 barrier layers. The modulation doped QW structures are fabricated with molecular beam epitaxy on a thin (100 nm) virtual SiGe substrate on a (001) oriented Si substrate. The virtual substrate (VS) concept utilizes the Si diffusion into an as- grown thin, strain relaxed Ge layer during a following annealing step. The lateral lattice spacing of the SiGe-VS could be varied by the annealing temperature in the range between 830 °C and 860 °C. Half-hour anneal at 848 °C results in nearly strain free growth for the following Si0.2Ge0.8 barrier layer. Boron doping above an undoped 10 nm spacer on top of the 15 nm QW provides a reservoir for hole transfer from the barrier to the well. Electrical conductivity, sheet hole density ps and mobility are measured as function of temperature. In all investigated Si x Ge1−x−y Sn y channels the Hall measurements show the typical freeze out of holes outside the QW. Alloy scattering dominates the low-temperature mobility by adding Sn or Si to the Ge reference well. A linear relationship for the charge transfer from the modulation doping into the undoped Si x Ge1−x−y Sn y channel as function of the lattice mismatch between the channel material and the matrix material could be found at low-temperatures (8 K). An analytical model for this charge transfer confirms the nearly linear relationship by considering the triangular shape of the potential in modulation doped QW structures.

Topological Dirac states in asymmetric Pb1−xSnxTe quantum wells

The electronic structure of lead-salt (IV–VI semiconductor) topological quantum wells (T-QWs) is investigated with analytical solutions of the effective 4x4 Dimmock k ⋅ p model, which gives an accurate description of the bands around the fundamental energy gap. Specific results for three-layer Pb 1 − x Sn x Te nanostructures with varying Sn composition are presented and the main differences between topological and normal (N) QWs highlighted. A series of new features are found in the spectrum of T-QWs, in particular in asymmetric QWs where large (Rashba spin–orbit) splittings are obtained for the topological Dirac states inside the gap. • The band structure of PbSnTe quantum wells from trivial to topological is analyzed. • The Rashba effect for topological Dirac states is analytically discussed. • The spin textures of longitudinal and oblique valley states are obtained.

Strong coupling between a topological insulator and a III-V heterostructure at terahertz frequency

We theoretically probe the emergence of strong coupling in a system consisting of a topological insulator (TI) and a III-V heterostructure using a numerical approach based on the scattering matrix formalism. Specifically, we investigate the interactions between terahertz excitations in a structure composed of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and GaAs materials. We find that the interaction between the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ layer and AlGaAs/GaAs quantum wells with intersubband transitions (ISBTs) in the terahertz frequency regime creates new hybrid modes, namely Dirac plasmon-phonon-ISBT polaritons. The formation of these hybrid modes results in anticrossings (spectral mode splitting) whose magnitude is an indication of the strength of the coupling. By varying the structural parameters of the constituent materials, our numerical calculations reveal that the magnitude of splitting depends strongly on the doping level and the scattering rate in the AlGaAs/GaAs quantum wells, as well as on the thickness of the GaAs spacer layer that separates the quantum-well structure from the TI layer. Our results reveal the material and device parameters required to obtain experimentally observable signatures of strong coupling. Our model includes the contribution of an extra two-dimensional hole gas (2DHG) that is predicted to arise at the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$/GaAs interface, based on density functional theory (DFT) calculations that explicitly account for details of the atomic terminations at the interface. The presence of this massive 2DHG at the TI/III-V interface shifts the dispersion of the Dirac plasmon-ISBT polaritons to higher frequencies. The damping rate at this interface, in contrast, compensates the effect of the 2DHG. Finally, we observe that the phonon resonances in the TI layer are crucial to the coupling between the THz excitations in the TI and III-V materials.

Electron spectra in double quantum wells of different shapes

We suggest a method for calculating electronic spectra in ordered and disordered semiconductor structures (superlattices) forming double quantum wells (QWs). In our method, we represent the solution of Schrödinger equation for QW potential with the help of the solution of the corresponding diffusion equation. This is because the diffusion is the mechanism, which is primarily responsible for amorphization (disordering) of the QW structure, leading to so-called interface mixing. We show that the electron spectrum in such a structure depends on the shape of the QW, which, in turn, corresponds to an ordered or disordered structure. Namely, in a disordered substance, QW typically has smooth edges, while in ordered one it has an abrupt, rectangular shape. The present results are relevant for the heterostructures like GaAs/AlGaAs, GaN/AlGaN, HgCdTe/CdTe, ZnSe/ZnMnSe, Si/SiGe, etc, which may be used in high-end electronics, flexible electronics, spintronics, optoelectronics, and energy harvesting applications.

Electromechanical behaviors in piezotronic quantum wells based on a quantum-corrected phenomenological theory

Piezotronic devices have attracted a great deal of attention due to their potential applications in self-powered tactile sensing, nano-device memory, human-electronic interface, etc. As the size of piezotronic devices shrinks, some interesting quantum effects begin to appear. In this paper, we establish a theory oriented to the engineering application of piezoelectric semiconductors, called quantum-corrected phenomenological (QCP) theory, by coupling the density-gradient theory and the linear piezoelectricity theory through Gauss's law. For numerical verification, we specifically studied the electromechanical behaviors in GaN/AlGaN heterostructure quantum wells (QWs) with both infinite and finite barrier height. The results of electron density, electric potential, and quantum potential are provided, and their dependence on the doping density, the applied stress, and the Al mole fraction is investigated. Some interesting quantum effects are revealed, and their influencing mechanisms are well investigated from a macroscopic perspective. Not only do the conclusions drawn in this paper enrich the fundamental understanding of the piezotronic effect in a QW structure, but also the proposed QCP theory can serve as a valuable tool for future device engineering.

Review on Organic–Inorganic Two-Dimensional Perovskite-Based Optoelectronic Devices

Organic–inorganic two-dimensional (2D) perovskite is an optoelectronic material, with quantum-well structure and improved moisture stabilities, which has been widely used in various optoelectronic devices recently. In this review, the structure and properties of organic–inorganic 2D perovskite materials are first briefly introduced. After that, according to the different photoelectron coupling mechanisms, the recent progress of typical 2D perovskite-based optoelectronic devices is described in detail: including photodetectors, light-emitting diodes, solar cells, and lasers, as well as some optoelectronic devices (e.g., optical memory and optical synapses). We analyzed the influence of structure, manufacturing process, and material selection on device performance and showed promising progress in different applications. Subsequently, we proposed the possible breakthrough development direction of 2D perovskite-based optoelectronic devices in the coming years. This work points out the way for future progress of 2D perovskite-based optoelectronic devices, which is conducive to further improving device performance and inspiring designs of high-performance organic–inorganic 2D perovskite-based optoelectronic devices.

Unprecedented Self-Powered Visible-Infrared Dual-Modal Photodetection Induced by a Bulk Photovoltaic Effect in a Polar Perovskite.

Visible-infrared dual-modal light harvesting is crucial for various optoelectronic devices, particularly for solar cells and photodetectors. Hybrid metal-halide perovskites are recently emerging for visible-infrared dual-modal photodetection owing to their prominent multiphoton absorption and carrier transport performances. However, they work relying on an applied external power source or complicated heterostructures. It is still a difficult task to realize visible-infrared dual-modal self-powered photoresponse induced by a bulk photovoltaic effect (BPVE) in a single material. In this work, we constructed a polar multilayered perovskite, (Br-BA)2(EA)2Pb3Br10 (BEP; EA+ = ethylammonium, and Br-BA+ = 4-brombutylammonium). Notably, the polar feature endows BEP with a BPVE. In addition, BEP presents a distinctive two-photon activity arising from the layered quantum-well structure. Benefitting from these striking characteristics, self-powered visible-infrared dual-modal photodetection is realized, and a direct self-powered detection of 800 nm light with a photocurrent of 2.1 nA cm-2 is achieved. This work will inspire the design of desired photoelectric materials with a BPVE for high-performance self-powered visible-infrared dual-modal photodetection.

Piezo-phototronics in quantum well structures

Quantum well (QW) structures are formed in nanometer-thickness-scale semiconductors with different bandgaps in sandwiched configurations and can offer a wide variety of advantages as active layers for optoelectronic devices, e.g., laser diodes, light emit diodes, photodetectors, and solar cells. Due to the non-centrosymmetric crystal structure, the third-generation semiconductor, such as ZnO, AlN, GaN, and InN, can generate a piezopotential within the crystal by applying an external or internal strain and lead to an effective modulation of the optoelectronic device performance, which is also called piezo-phototronics. With reducing the feature size of materials into several tens of nanometers (e.g., forming QW structures), the multiway coupling effects of quantum physics and piezo-phototronics (coupling with piezoelectricity, photoexcitation, and semiconductor properties) make this research topic more attractive and open a new window for fabricating advanced intelligent optoelectronic devices. This Perspective reviews the recent advances of piezo-phototronics in QW structures, including the fundamental theories and device performance enhancements, and aims to offer a summary and outlook for future research directions and practical applications of piezo-phototronic QW devices.

Reduction of optical transition energy in composite GaInAsBi quantum wells

Light emitting diode structures of quaternary GaInAsBi bismide quantum wells with AlInAs barriers were grown on InP substrates by molecular-beam epitaxy. Intermediate additional layers of GaInAs were inserted between the bismide quantum wells and AlInAs barriers to reduce the electron size-quantization energy and, therefore, the energy of optical transitions. It has been demonstrated both theoretically and experimentally that the redshift of emission wavelength in the composite GaInAs1−xBix (x∼0.06) quantum wells with the additional intermediate GaInAs layers can be as large as ∼0.4μm.

Donor-acceptor-donor type organic spacer for regulating the quantum wells of Dion-Jacobson 2D perovskites

Two-dimensional (2D) metal halide perovskite system has attracted significant attentions due its tunable optoelectronic characteristics and outstanding environmental durability. However, quantum wells (QWs) of 2D perovskite severely restrain the charge transfer between organic/inorganic layers, resulting in the significantly lower photovoltaic performance of 2D perovskite solar cells (PSCs) in contrast to their 3D analogues. To intrinsically refine the QW structures of 2D perovskite for less quantum confinement, we innovatively design the D-A-D (D: donor; A: acceptor) type organic cation as 2D spacer. The push-pull effects between D, A unit endows the organic spacer intramolecular charge transfer characteristic, which effectively lower its bandgap. The resulting 2D perovskite demonstrates flattened energy landscape of the QWs, which suppresses the charge transfer barrier and elongates the carrier diffusion lengths in 2D perovskite film. As a result, optoelectronic properties of this new 2D perovskite resemble those of the 3D counterparts, delivering an impressive PCE of 18.6% for 2D (n = 4) PSCs. This is one of the highest reported efficiency for DJ-type 2D perovskite (n ≤ 4) to date. By rationally selecting other D and A unit, QW structures of 2D perovskites can be further adjusted, which opens huge possibilities for developing a new system of 2D perovskites.

Resonant tunneling driven metal-insulator transition in double quantum-well structures of strongly correlated oxide

The metal-insulator transition (MIT), a fascinating phenomenon occurring in some strongly correlated materials, is of central interest in modern condensed-matter physics. Controlling the MIT by external stimuli is a key technological goal for applications in future electronic devices. However, the standard control by means of the field effect, which works extremely well for semiconductor transistors, faces severe difficulties when applied to the MIT. Hence, a radically different approach is needed. Here, we report an MIT induced by resonant tunneling (RT) in double quantum well (QW) structures of strongly correlated oxides. In our structures, two layers of the strongly correlated conductive oxide SrVO3 (SVO) sandwich a barrier layer of the band insulator SrTiO3. The top QW is a marginal Mott-insulating SVO layer, while the bottom QW is a metallic SVO layer. Angle-resolved photoemission spectroscopy experiments reveal that the top QW layer becomes metallized when the thickness of the tunneling barrier layer is reduced. An analysis based on band structure calculations indicates that RT between the quantized states of the double QW induces the MIT. Our work opens avenues for realizing the Mott-transistor based on the wave-function engineering of strongly correlated electrons.

Electron mobility in asymmetric GaN/AlGaN quantum well transistor structure: effect of alloy disorder scattering

We analyze the electron mobility μ of GaN/AlGaN based quantum well (QW) transistor structure. We consider the potential profile V(z) by including the potential due topolarization (V p ) and Hartree potential (V H ) owing to surface electron density N s . The low temperature mobility is governed by the alloy disorder (ad-) and interface roughness (ir-) scatterings. As N s increases, μ increases. However, for larger N s (N s > 0.6 × 1013 cm−2), there is a deviation showing decreasing trend of μ. We show that the ad- scattering plays a vital role in governing μ. An increase in N s causes narrowing of the polarization induced channel potential through V H and hence facilitates the larger extension of the subband wave function into the surface barrier. Accordingly, the ad-scattering increases, thereby reducing μ. we show that with an increase in well width there is a substantial rise in μ in quantum well (QW) structures while almost no change in μ in double heterostructures (DH). Enhancement of height of the barriers leads to different results, i.e., for the back barrier, there is a reduction in μ in both QW and DH structures, while for the surface barrier, there is a rise in μ. The fascinating trends of our results of μ in different GaN/AlGaN structures elucidate the importance of ad-scattering on low temperature μ.

Ultrahigh Power Factor and Ultralow Thermal Conductivity at Room Temperature in PbSe/SnSe Superlattice: Role of Quantum-Well Effect.

Reduced dimension is one of the effective strategies to modulate thermoelectric properties. In this work, n-type PbSe/SnSe superlattices with quantum-well (QW) structure are fabricated by pulsed laser deposition. Here, it is demonstrated that the PbSe/SnSe multiple QW (MQW) shows a high power factor of ≈25.7 µW cm-1 K-2 at 300 K, four times larger than that of PbSe single layers. In addition, thermal conductivity falls below 0.32± 0.06W m-1 K-1 due to the phonon scattering at interface when the PbSe well thickness is confined within the scale of phonon mean free path (1.8nm). Featured with ultrahigh power factor and ultralow thermal conductivity, ZT at room temperature is significantly increased from 0.14 for PbSe single layer to 1.6 for PbSe/SnSe MQW.

Spacer Engineering of Diammonium‐Based 2D Perovskites toward Efficient and Stable 2D/3D Heterostructure Perovskite Solar Cells

AbstractPerovskite solar cells (PSCs) based on 2D/3D heterostructures show great potential to combine the advantages of the high efficiency of 3D perovskites and the high stability of 2D perovskites. However, an in‐depth understanding of the organic‐spacer effects on the 2D quantum well (QW) structures and electronic properties at the 2D/3D interfaces is yet to be fully achieved, especially in the case of 2D perovskites based on diammonium spacers/ligands. Here, a series of diammonium spacers is considered for the construct ion 2D/3D perovskite heterostructures. It is found that the chemical structure and concentration of the spacers can dramatically affect the characteristics of the 2D capping layers, including their phase purity and orientation. Density functional theory calculations indicate that the spacer modifications can induce shifts in the energy‐level alignments at the 2D/3D interfaces and therefore influence the charge‐transfer characteristics. The strong intermolecular interactions between the 2,2‐(ethylenedioxy)bis(ethylammonium) (EDBE) cations and inorganic [PbI6]4− slabs facilitate a controlled deposition of a phase‐pure QW structure (n = 1) with a horizontal orientation, which leads to better surface passivation and carrier extraction. These benefits endow the EDBE‐based 2D/3D devices with a high power conversion efficiency of 22.6% and remarkable environmental stability, highlighting the promise of spacer‐chemistry design for high‐performance 2D/3D PSCs.

Observation of elastic heterogeneity and phase evolution in 2D layered perovskites using coherent acoustic phonons

Abstract Two-dimensional (2D) organic–inorganic perovskites have shown interesting optical properties due to the natural quantum-well structures. The repetition of soft organic and hard inorganic intercalations also renders 2D perovskites rich phonon dynamics. Here, we investigated the coherent acoustic phonons in (PEA)2PbI4 perovskite films by time-resolved Brillouin spectroscopy. The coherent acoustic phonons were launched indirectly in perovskite films by exciting Au nanoplates which were used as optoacoustic transducers. A longitudinal sound velocity ν = 1937 ± 31 m/s, and an elastic modulus E = 9.84 GPa along the cross-plane direction of perovskites were obtained from analysis of the Brillouin oscillation frequency. Following a bead-spring model, we calculated a spring constant k ≈ 1.709 N m−1 for PEA cations which is comparably small for perovskites. We also demonstrated that coherent acoustic phonons are sensitive to differentiate structural heterogeneity and monitor dynamic phase evolution in perovskite films. Domains of PbI2-rich and PbI2-poor phases were identified. Under light stimulus, PbI2-poor phases were gradually disappearing and PbI2-rich phases became crystallized. The observations of structural and elastic heterogeneity and dynamic phase evolution using coherent acoustic phonons provide a toolbox for submicroscale elastic characterization of perovskites.