Quantum Confinement Of Carriers Research Articles

Quantum confinement of carriers in the type-I quantum wells structure

Abstract Quantum confinement is recognized to be an inherent property in low-dimensional structures. Traditionally, it is believed that the carriers trapped within the well cannot escape due to the discrete energy levels. However, our previous research has revealed efficient carrier escape in low-dimensional structures, contradicting this conventional understanding. In this study, we review the energy band structure of quantum wells along the growth direction considering it as a superposition of the bulk material dispersion and quantization energy dispersion resulting from the quantum confinement across the whole Brillouin zone. By accounting for all wave vectors, we obtain a certain distribution of carrier energy at each quantized energy level, giving rise to the energy subbands. These results enable carriers to escape from the well under the influence of an electric field. Additionally, we have compiled a comprehensive summary of various energy band scenarios in quantum well structures relevant to carrier transport. Such a new interpretation holds significant value in deepening our comprehension of low-dimensional energy bands, discovering new physical phenomena, and designing novel devices with superior performance.

Effect of uniaxial compressive strain on the thermoelectric properties of two-dimensional HfNF

Two-dimensional transition metal nitride halides have shown promise in thermoelectric applications due to their low dimensionality, excellent electron transfer properties, and quantum confinement of carriers. This study focuses on investigating the impact of uniaxial compressive strain on the stability, electronic and thermoelectric properties of monolayer HfNF through first-principles calculations. The research findings reveal that the semiconductor properties of monolayer HfNF remain unchanged under various strain conditions. Furthermore, the thermoelectric properties of monolayer HfNF materials are examined using Slack model and the Boltzmann transport theory under different strain conditions. The findings indicate that applying uniaxial compressive strains at temperatures of 500 K, 700 K, and 900 K increase the Seebeck coefficients of n-type and p-type HfNF, resulting in an enhanced power factor for the material. Specifically, the power factor of p-type HfNF under uniaxial compressive strain increased by 83%, with the ZT value reaching 2.01 at 900 K, which is approximately 40% higher than the ZT value without strain. These results suggest that strain can be utilized as a modulation method to enhance the thermoelectric properties of materials. Moreover, the study suggests that two-dimensional HfNF holds great promise for thermoelectric applications when subjected to uniaxial compressive strain.

Linewidth narrowing in self-injection-locked on-chip lasers

Stable laser emission with narrow linewidth is of critical importance in many applications, including coherent communications, LIDAR, and remote sensing. In this work, the physics underlying spectral narrowing of self-injection-locked on-chip lasers to Hz-level lasing linewidth is investigated using a composite-cavity structure. Heterogeneously integrated III–V/SiN lasers operating with quantum-dot and quantum-well active regions are analyzed with a focus on the effects of carrier quantum confinement. The intrinsic differences are associated with gain saturation and carrier-induced refractive index, which are directly connected with 0- and 2-dimensional carrier densities of states. Results from parametric studies are presented for tradeoffs involved with tailoring the linewidth, output power, and injection current for different device configurations. Though both quantum-well and quantum-dot devices show similar linewidth-narrowing capabilities, the former emits at a higher optical power in the self-injection-locked state, while the latter is more energy-efficient. Lastly, a multi-objective optimization analysis is provided to optimize the operation and design parameters. For the quantum-well laser, minimizing the number of quantum-well layers is found to decrease the threshold current without significantly reducing the output power. For the quantum-dot laser, increasing the quantum-dot layers or density in each layer increases the output power without significantly increasing the threshold current. These findings serve to guide more detailed parametric studies to produce timely results for engineering design.

Study on the Quantum Confinement of Photo-Generated Carriers in Quantum Wells

According to the classical theory, multi-quantum wells (MQWs) have quantum confinement effect on photo-generated carriers, which limits its application in light-to-electric devices. However, relevant experiments showed that a large proportion of photo-generated carriers can escape from MQWs sandwiched in the p-type layer and n-type layer (PIN structure), but not in the NIN structure. In order to study this phenomenon carefully, we applied positive and negative bias to an NIN structure respectively to simulate the PIN structure. By analyzing the photoluminescence (PL) spectra, we observed a weak escape behavior of photon-generated carriers among QWs in NIN structure. The experiment results indicate that strong electric field could drive carriers to escape from QWs rather than relaxation and recombination, while in NIN structure the inhomogeneous distribution of the electric field intensity reduces the carrier transport efficiency. The further study will give new ideas to design and produce photoelectric devices.

In Situ Exfoliation Method of Large-Area 2D Materials.

2D materials provide a rich platform to study novel physical phenomena arising from quantum confinement of charge carriers. Many of these phenomena are discovered by surface sensitive techniques, such as photoemission spectroscopy, that work in ultra-high vacuum (UHV). Success in experimental studies of 2D materials, however, inherently relies on producing adsorbate-free, large-area, high-quality samples. The method that yields 2D materials of highest quality is mechanical exfoliation from bulk-grown samples. However, as this technique is traditionally performed in a dedicated environment, the transfer of samples into vacuum requires surface cleaning that might diminish the quality of the samples. In this article, a simple method for in situ exfoliation directly in UHV is reported, which yields large-area, single-layered films. Multiple metallic and semiconducting transition metal dichalcogenides are exfoliated in situ onto Au, Ag, and Ge. The exfoliated flakes are found to be of sub-millimeter size with excellent crystallinity and purity, as supported by angle-resolved photoemission spectroscopy, atomic force microscopy, and low-energy electron diffraction. The approach is well-suited for air-sensitive 2D materials, enabling the study of a new suite of electronic properties. In addition, the exfoliation of surface alloys and the possibility of controlling the substrate-2D material twist angle is demonstrated.

An Insight into nd10 Metal Cyanide-based Coordination Polymers Through ab-initio Calculations: Electronic Properties and Optical Response.

Semiconductors are essential for modern life since they are the basis of many current technologies that are related to better living standards. Some of them, characterized by the periodic assembling of metal cyanides with filled d-shell (nd10 ) constitute an interesting series of cyanide-based coordination polymers with physical properties such like anomalous anisotropic thermal expansion and quantum confinement effects related to the polymer's width that can be exploited for technological applications. Herein, the electronic structure of nd10 metal cyanide-based systems were studied both experimentally and through Density Functional Theory. The band gap found for one-dimensional (1D) -M-C≡N- (M=Cu, Ag, Au) and tetrahedral M-(C≡N)2 (M=Zn, Cd, Hg) systems can be attributed to Laporte-allowed π π* (Metal to Ligand Charge Transfer mechanism) combined with metal center (d s,p) electronic transitions. Aurophilic bonding was found on the AuCN structure, and a new forbidden electronic transition associated to its band gap is reported. Computed effective and reduced masses from carriers revealed that carrier mobility and quantum confinement effects are greater in 1D systems.

Electronic, Thermodynamic Stability, and Band Alignment Behavior of the CoVSi/NaCl Heterojunction

We report the band discontinuity of the CoVSi/NaCl heterointerface. First principle calculations based on density functional theory using GGA, GGA + U, and GGA + mbJ approximations were applied to study the structural, electronic, and band alignment properties. Structural and thermodynamic stability studies indicate that this semiconductor - dielectric heterojunction can be synthesized experimentally in thermodynamic equilibrium conditions. The valence and conduction band offset values (VBO and CBO) were 0.74 and 3.02 eV, respectively. Also, the effective electron affinity parameter (χ e) for both CoVSi and NaCl were calculated as ∼1.51 and ∼0.769 eV, respectively, using Anderson’s law. The study of the electronic structure expresses the occurrence of half-metallic ferromagnetic behavior with a narrow band gap of about 0.09 eV. In this heterojunction, electrons and holes were confined to the CoVSi layers, and conduction band minimum and valence band minimum were replaced in the CoVSi layers. This restriction, applied to load carriers on one side of the interface, significantly increases the light-material interaction in light-emission programs. Therefore, this heterojunction can be recommended for light-emitting applications and thin atomic layer materials with quantum confinement of charge carriers.

Can armchair nanotubes host organic color centers?

We use time-dependent density functional theory to investigate the possibility of hosting organic color centers in (6, 6) armchair single-walled carbon nanotubes, which are known to be metallic. Our calculations show that in short segments of (6, 6) nanotubes nm in length there is a dipole-allowed singlet transition related to the quantum confinement of charge carriers in the smaller segments. The introduction of defects to the surface of (6, 6) nanotubes results in new dipole-allowed excited states. Some of these states are redshifted from the native confinement state of the defect-free (6, 6) segments; this is similar behavior to what is observed with defects to exciton transitions in semiconducting carbon nanotubes. This result suggests the possibility of electrically wiring organic color centers directly through armchair carbon nanotube hosts.

Effect of as flux rate during growth interruption on the performances of InAs/InGaAsP/InP quantum dots and their lasers grown by metal-organic chemical vapor deposition

We report the effect of As flux rate (FR) during growth interruption (GI) after quantum dot (QD) deposition on the performances of InAs/InGaAsP/InP QDs and their lasers, grown by metal–organic chemical vapor deposition. The active region consists of seven layers of self-assembled InAs QDs separated by InGaAsP lattice-matched to InP. Room-temperature photoluminescence (PL) measurements show that both the PL peak intensity and emission wavelength of the active region increase with As FR. Temperature-dependent PLs reveal that the activation energies of the QDs from the ground state to higher energy level or barrier become greater with the increase of As FR, indicating less carrier thermal escape from the QDs to their barriers. These above observations can be attributed to the promoted migration and suppressed desorption of surface In atoms, leading to enlarged QDs with deepened energy levels and thus enhanced carrier quantum confinement effect. Moreover, a 6-μm-wide and 3-mm-long ridge-waveguide QD laser with the greatest As FR shows a lowest threshold current of 477 mA under pulse mode at room temperature, corresponding to a threshold current density of 379 A/cm2 per QD layer. Meanwhile, the laser exhibits the highest characteristic temperatures T0 and T1 of 331 K and 775 K, respectively, in the temperature range between 20 °C and 60 °C.

Gate-controlled quantum dots in monolayer WSe2

Quantum confinement and manipulation of charge carriers are critical for achieving devices practical for quantum technologies. The interplay between electron spin and valley, as well as the possibility to address their quantum states electrically and optically, makes two-dimensional (2D) transition metal dichalcogenides an emerging platform for the development of quantum devices. In this work, we fabricate devices based on heterostructures of layered 2D materials, in which we realize gate-controlled tungsten diselenide (WSe2) hole quantum dots. We discuss the observed mesoscopic transport features related to the emergence of quantum dots in the WSe2 device channel, and we compare them to a theoretical model.

Physical Parameter Variation Analysis on the Performance Characteristics of Nano DG-MOSFETs

DG-MOSFETs are the most widely explored device architectures for nano-scale CMOS circuit design in sub-50 nm due to the improved subthreshold slope and the reduced leakage power compared to bulk MOSFETs. In thin-film (tsi by tsi- induced quantum confinement along with the confinement caused by a very high electric field at the interface. Therefore, quantum confinement effects on the device characteristics are also quite important and it needs to be incorporated along with short channel effects for nano-scale circuit design. In this paper, we analyzed a DG-MOSFET structure at the 20 nm technology node incorporating quantum confinement effects and various short channel effects. The effect of physical parameter variations on performance characteristics of the device such as threshold voltage, subthreshold slope, ION - IOFF ratio, DIBL, etc. has been investigated and plotted through extensive TCAD simulations. The physical parameters considered in this paper are operating temperature (Top), channel doping concentration (Nc), gate oxide thickness (tox) and Silicon film thickness (tsi). It was observed that quantum confinement of charge carriers significantly affected the performance characteristics (mostly the subthreshold characteristics) of the device and therefore, it cannot be ignored in the subthreshold region-based circuit design like in many previous research works. The ATLASTM device simulator has been used in this paper to perform simulation and parameter extraction. The TCAD analysis presented in the manuscript can be incorporated for device modeling and device matching. It can be used to illustrate exact device behavior and for proper device control.

Temporally modulated energy shuffling in highly interconnected nanosystems

Abstract Advances in lighting and quantum computing will require new degrees of control over the emission of photons, where localized defects and the quantum confinement of carriers can be utilized. In this contribution, recent developments in the controlled redistribution of energy in rare earth (RE)–doped nanosystems, such as quantum dots or within bulk insulating and semiconducting hosts, will be reviewed. In their trivalent form, RE ions are particularly useful dopants because they retain much of their atomic nature regardless of their environment; however, in systems such as GaN and Si, the electronic states of the RE ions couple strongly to those of the host material by forming nanocomplexes. This coupling facilities fast energy transfer (ET) (<100 ps) and a carrier-mediate energy exchange between the host and the various states of the RE ions, which is mediated by the presence of carriers. A model has been developed using a set of rate equations, which takes into consideration the various ET pathways and the lifetimes of each state within the nanocomplex, which can be used to predict the nature of the emitted photons given an excitation condition. This model will be used to elucidate recent experimental observations in Eu-doped GaN.

Thin-film InAs/GaAs quantum dot solar cell with planar and pyramidal back reflectors.

Quantum dot solar cells are promising for next-generation photovoltaics owing to their potential for improved device efficiency related to bandgap tailoring and quantum confinement of charge carriers. Yet implementing effective photon management to increase the absorptivity of the quantum dots is instrumental. To this end, the performance of thin-film InAs/GaAs quantum dot solar cells with planar and structured back reflectors is reported. The experimental thin-film solar cells with planar reflectors exhibited a bandgap-voltage offset of 0.3 V with an open circuit voltage of 0.884 V, which is one of the highest values reported for quantum dot solar cells grown by molecular beam epitaxy to our knowledge. Using measured external quantum efficiency and current-voltage characteristics, we parametrize a simulation model that was used to design an advanced reflector with diffractive pyramidal gratings revealing a 12-fold increase of the photocurrent generation in the quantum dot layers.

Emerging Opportunities for Electrostatic Control in Atomically Thin Devices.

Electrostatic control of charge carrier concentration underlies the field-effect transistor (FET), which is among the most ubiquitous devices in the modern world. As transistors and related electronic devices have been miniaturized to the nanometer scale, electrostatics have become increasingly important, leading to progressively sophisticated device geometries such as the finFET. With the advent of atomically thin materials in which dielectric screening lengths are greater than device physical dimensions, qualitatively different opportunities emerge for electrostatic control. In this Review, recent demonstrations of unconventional electrostatic modulation in atomically thin materials and devices are discussed. By combining low dielectric screening with the other characteristics of atomically thin materials such as relaxed requirements for lattice matching, quantum confinement of charge carriers, and mechanical flexibility, high degrees of electrostatic spatial inhomogeneity can be achieved, which enables a diverse range of gate-tunable properties that are useful in logic, memory, neuromorphic, and optoelectronic technologies.

Effect of Ag2S-BSA nanoparticle size on 3T3 fibroblast cell line cytotoxicity

Silver sulfide nanoparticles, smaller than 5 nm, are of great interest in the biomedical field due to their improved optical properties from the quantum confinement of charge carriers. The effect of silver sulfide nanoparticles (6.4 ± 2.2, 13 ± 4.9, and 33 ± 12.5 nm) coated with BSA (bovine serum albumin) on cytotoxicity, cell proliferation (MTT assay), and cell morphology was determined in vitro using 3T3 fibroblast cell line as assay model. The synthesized Ag2S-BSA nanoparticles were characterized by X-ray diffraction, infrared spectroscopy, and thermogravimetric analysis. Transmission electron microscopy and dynamic light scattering were used for particle size measurement. The MTT results showed a tendency to decrease cell viability with time exposure rather than concentration, in all particle’s groups. A greater affectation was observed in cells incubated with smaller nanoparticles, and this is associated with a higher speed of cell uptake due to a higher content of adsorbed BSA. It was found that the largest nanoparticles (33 ± 12.5 nm) at 72 h and at higher concentration presents the same cell viability as observed with the smallest attributing it to the agglomeration of particles formed by the low stability of the sample. Regarding cell morphology, fluorescence microscopy showed good cell growth in all the nanoparticle groups, observing a slighter increase in the cell population of the sample of intermediate particle size (13 ± 4.9 nm).

Study the Mechanisms of Enhanced Phonon Bottleneck Effect for the Absorber of Hot Carrier Solar Cell in III-V Multiple Quantum Wells

The most vital key to realize hot carrier solar cell is reducing carrier relaxation time to nanoseconds by phonon bottleneck effect often observed in nanostructure. However, the mechanisms underlying this are still not well understood. In this paper, we systematically investigated the mechanisms of phonon interfacial mismatch and carrier quantum confinement over phonon bottleneck effect in InN/InxGa(1-x)N multiple quantum wells (MQWs). Highly promising hot carrier lifetimes due to enhanced phonon bottleneck effect were observed in these MQWs, where the longest hot carrier lifetime is 3.2±0.12 ns. It was found the quantum confinement of carriers could play more important role in the reduction of carrier cooling rate, while the optical phonon confinement is more likely to dominate the initial carrier temperature. This study clarifies two of the most important mechanisms of phonon bottleneck effect and directs a promising application of III-V MQWs on the absorber of hot carrier solar cell.

Long carrier lifetime in faceted PbS quantum dot superlattice fabricated by sedimentation method

We fabricated colloidal quantum dot (QD) superlattice films and investigated their primal optical properties. The films were prepared by depositing faceted PbS QDs on pyramidal-microhole-array template and flat substrate in solution. The red shift in the quantum state emission of QDs was observed in photoluminescence spectra after film formation, which suggested the weakened quantum confinement of carriers in intermediate bands. Emission decay curves at the excited states in the QD superlattice film were double exponential. The longer lifetime was several tens of nanoseconds and attributed to the carrier delocalization in the intermediate bands. The emission lifetime of the QD film prepared on the template was found to be more than twice as long as that on the flat substrate, which suggested that the template helped to form large area QD superlattice.

Quasicubic model for metal halide perovskite nanocrystals.

We present an analysis of quantum confinement of carriers and excitons, and exciton fine structure, in metal halide perovskite (MHP) nanocrystals (NCs). Starting with coupled-band k · P theory, we derive a nonparabolic effective mass model for the exciton energies in MHP NCs valid for the full size range from the strong to the weak confinement limits. We illustrate the application of the model to CsPbBr3 NCs and compare the theory against published absorption data, finding excellent agreement. We then apply the theory of electron-hole exchange, including both short- and long-range exchange interactions, to develop a model for the exciton fine structure. We develop an analytical quasicubic model for the effect of tetragonal and orthorhombic lattice distortions on the exchange-related exciton fine structure in CsPbBr3, as well as some hybrid organic MHPs of recent interest, including formamidinium lead bromide (FAPbBr3) and methylammonium lead iodide (MAPbI3). Testing the predictions of the quasicubic model using hybrid density functional theory (DFT) calculations, we find qualitative agreement in tetragonal MHPs but significant disagreement in the orthorhombic modifications. Moreover, the quasicubic model fails to correctly describe the exciton oscillator strength and with it the long-range exchange corrections in these systems. Introducing the effect of NC shape anisotropy and possible Rashba terms into the model, we illustrate the calculation of the exciton fine structure in CsPbBr3 NCs based on the results of the DFT calculations and examine the effect of Rashba terms and shape anisotropy on the calculated fine structure.

Enhancement of photoluminescence of GaAsBi quantum wells by parabolic design of AlGaAs barriers

Influence of barrier material and structure on carrier quantum confinement in GaAsBi quantum wells (QWs) is studied comprehensively. Single- and multi-QW structures were grown using solid-state molecular beam epitaxy with conventional rectangular, step-like and parabolically graded AlGaAs barrier designs. It was discovered that room temperature photoluminescence is increased by more than 50 times in the GaAsBi QWs with parabolically graded barriers (PGBs) if compared to standard rectangular and step-like structures. The enhancement of photoluminescence was reproducible within the range of growth parameters. The carrier localization and increase of trapping efficiency in GaAsBi QWs is responsible for observed enhancement in radiative properties of PGB structures. The random potential field fluctuations for carriers were increased up to 44 meV due to the blurred well-barrier interface causing the conditions for Bi content and/or well width variations. Due to the impact of self-organizing effects on the reproducibility of optical properties, the GaAsBi QWs with AlGaAs PGBs open the window for fabrication of 1.0–1.55 μm wavelength emission lasers based on GaAsBi quantum structures.

Strained SI/SIGE/SI Nano-Channel Hoi Mosfet

Strained Si technology has headed in the development of single or dual channel strained silicon MOSFETs devices. Comprehending the need of advancement in recent technologies with miniaturized features, developing a novel MOSFET on ultrathin double strained Si with strained SiGe sandwiched in between and forming a tri-channel MOSFET has been the crux of this present research. Incorporation of quantum carrier confinement effect on the ultrathin dual strained Si layers in the channel has been implemented to counterbalance the threshold voltage roll-off induced by the strained layers. A comparison of the conventional strained silicon on relaxed silicon-germanium with double strained silicon channel MOSFET has been perceived leading to eloquent drain current enhancement of ~49% with a small reduction in the threshold voltage caused by the additional bottom strained Si layer. Further, 100nm and 50nm channel length have been compared and a superior device characteristic for the reduced device dimension is attained as the prominence of velocity overshoot is more in short channel device approaching to quasi-ballistic transport in the channel region