Quantum-confined Structures Research Articles

Scalable photonic sources using two-dimensional lead halide perovskite superlattices

Miniaturized photonic sources based on semiconducting two-dimensional (2D) materials offer new technological opportunities beyond the modern III-V platforms. For example, the quantum-confined 2D electronic structure aligns the exciton transition dipole moment parallel to the surface plane, thereby outcoupling more light to air which gives rise to high-efficiency quantum optics and electroluminescent devices. It requires scalable materials and processes to create the decoupled multi-quantum-well superlattices, in which individual 2D material layers are isolated by atomically thin quantum barriers. Here, we report decoupled multi-quantum-well superlattices comprised of the colloidal quantum wells of lead halide perovskites, with unprecedentedly ultrathin quantum barriers that screen interlayer interactions within the range of 6.5 Å. Crystallographic and 2D k-space spectroscopic analysis reveals that the transition dipole moment orientation of bright excitons in the superlattices is predominantly in-plane and independent of stacking layer and quantum barrier thickness, confirming interlayer decoupling.

Electroluminescence of a Polymer Film with a Polymer/Polymer Interface

The effect of a two-dimensional quantum-confined structure formed at a polymer/polymer interface on the radiative recombination of excitons (i.e., electroluminescence) is investigated. It is shown that the implementation of such a structure in a polymer film leads to an increase in the emission intensity by more than two orders of magnitude and a twofold decrease in the voltage threshold for double injection. It is established that the polymer/polymer interface serves as the front of exciton recombination, and its position inside the film affects the electroluminescence intensity. The position of the two-dimensional structure within the polymer film corresponding to the highest intensity of radiative recombination is a function of the ratio of electron and hole mobilities.

Exciton Polarization and Renormalization Effect for Optical Modulation in Monolayer Semiconductors

The ideal quantum confinement structure of monolayer semiconductors offers prominent optical modulation capabilities that are mediated by enhanced many-body interactions. Herein, we establish an electrolyte-gating method for tuning the luminescence properties that are in transition metal dichalcogenide (TMDC) monolayers. We fabricate electric double-layer capacitors on TMDC/graphite heterostructures to investigate electric-field- and carrier-density-dependent photoluminescence. The exciton peak energy initially shows a slight quadratic red shift of ∼1 meV without carrier accumulations, which is caused by the quantum-confined Stark effect. In contrast, the exciton resonance exhibits a larger red shift up to 10 meV with the accumulated carrier density above 1013 cm-2. These results indicate that the optical transitions can be largely modulated by the carrier density control in S- and Se-based TMDCs, as triggered by the doping-induced band gap renormalization effect. To further inspire this modulation capability, we also apply our method to electrolyte-based TMDC light-emitting devices. Biasing solely in electrolyte-induced p-i-n junctions yields pronounced red shifts up to 40 meV for exciton and trion electroluminescence. Consequently, our approach reveals that the doping effects in the high-carrier-density regimes are potentially significant for efficient optical modulation in monolayer semiconductors.

Spontaneous Radiation of a Two-Level System Confined in a Reflective Spherical Shell Quantum Dot

Using a first-order time-dependent perturbation theory, we calculate the spontaneous emission rate of a two-level system trapped between perfectly reflecting concentric spheres. The emitter is represented by a two-level monopole coupled to a Hermitian massless scalar field satisfying Dirichlet boundary conditions in such quantum-confined low-dimensional structure. We obtained the appropriate Green’s function evaluated in worldline of the atom which incorporates contributions from an infinite set of variable image charges. We provide an analytical expression for the decay rate to investigate the radiation process of the trapped atomic system. We perform a broad analysis of the dependence of the decay rate for different relations between the radii of spheres and the emitted radiation energy. We unveil regimes of strong suppression of the spontaneous emission rate as well as the development of irregular oscillations as a function of the quantum of emitted energy.

Size-Dependent Asymmetric Auger Interactions in Plasma-Produced n- and p-Type-Doped Silicon Nanocrystals

Nonradiative Auger recombination (AR) tends to dominate carrier dynamics in charged, quantum-confined structures. Consequently, it complicates the practical realization of many semiconductor nanocrystal (NC)-based devices such as light-emitting diodes, photovoltaic cells, and single-photon emitters, in which charged exciton states often occur. To this end, extensive experimental studies on direct band gap NCs have investigated the trion components (both positive and negative) that construct the total AR rate. However, such an analysis has remained elusive for indirect band gap Si NCs. In this study, we investigate AR decay of non-thermal plasma-produced n- and p-type-doped Si NCs. We expand the study over a large NC size range (DNC ≈ 3–8 nm), in which n- and p-type doping is achieved by either a substitutional or surface doping effect, respectively. First, we monitor the AR of charge-neutral multiexcitons by measuring the biexciton lifetime (τXX) as a function of the NC size and doping configuration. We s...

Effect of Ga droplet deposition rate on morphology of concentric quantum double rings

For the fabrication of particular nanostructures, Stranski-Krastanov (SK) growth mode driven by strain is most widely used. Meanwhile, another technique that is used to form the complex nanostructures is the droplet epitaxy technique, which is based on the deposition of group III element nanoscale droplets onto substrate and followed by the reaction with group V element for crystallization into III-V compound nanostructures. Droplet epitaxy technique is simple and flexible, and it does not require additional complicated processing and has potential to develop various quantum nanostructures. It, unlike standard MBE growth, exploits the sequential supply of group-III and group-V elements to form quantum nanostructures. Quantum rings are a special class of quantum-confinement structure that can be fabricated by the droplet epitaxy technique and have attracted wide attention due to the Aharonov-Bohm effect, which is specific to the topology of a ring. In this paper, GaAs/GaAs (001) concentric quantum double rings (CQDRs) are prepared by droplet epitaxy technique at different Ga droplet deposition rates in monolayer per second (ML/S). The 2 μm × 2 μm atomic force microscope images are obtained to show the morphologies of CQDRs. We study the effects of Ga droplet deposition rates (0.09 ML/s, 0.154 ML/s, 0.25 ML/s, 0.43 ML/s) on CQDRs. The results show that with the increase of Ga droplet deposition rate, the density of CQDRs increases and the radius of inner ring and the radius of outer ring decrease. According to the nucleation theory, through the relationship between the maximum cluster density and the Ga droplet deposition rate, the critical number of atom nucleations is found to be 5, which suggests that the stable Ga atom crystal nucleus should contain at least 5 Ga atoms in the process of forming Ga droplet, and a nucleation state transformation diagram is drawn in order to obtain an insight into the process of forming Ga droplet according to the nucleation theory and fitting results. The research results could be instructive for preparing the GaAs concentric quantum double rings that the density can be controlled by droplet epitaxy.

Probing molecule-like isolated octahedra via phase stabilization of zero-dimensional cesium lead halide nanocrystals

Zero-dimensional (0D) inorganic perovskites have recently emerged as an interesting class of material owing to their intrinsic Pb2+ emission, polaron formation, and large exciton binding energy. They have a unique quantum-confined structure, originating from the complete isolation of octahedra exhibiting single-molecule behavior. Herein, we probe the optical behavior of single-molecule-like isolated octahedra in 0D Cesium lead halide (Cs4PbX6, X = Cl, Br/Cl, Br) nanocrystals through isovalent manganese doping at lead sites. The incorporation of manganese induced phase stabilization of 0D Cs4PbX6 over CsPbX3 by lowering the symmetry of PbX6 via enhanced octahedral distortion. This approach enables the synthesis of CsPbX3 free Cs4PbX6 nanocrystals. A high photoluminescence quantum yield for manganese emission was obtained in colloidal (29%) and solid (21%, powder) forms. These performances can be attributed to structure-induced confinement effects, which enhance the energy transfer from localized host exciton states to Mn2+ dopant within the isolated octahedra.

InGaAs-Based Well–Island Composite Quantum-Confined Structure with Superwide and Uniform Gain Distribution for Great Enhancement of Semiconductor Laser Performance

In the development of semiconductor lasers, it has been a dream all along to simultaneously obtain extremely wide and uniform gain distribution, because such a gain configuration can greatly enhance semiconductor laser performance. Hence, it has also been a huge challenge to realize this dream so far. In this paper, we are reporting a special InGaAs-based well–island composite quantum-confined structure, with which the best results to date in achieving both superwide and very uniform gain and power distributions are obtained. The spectral flatness of the output power can reach 0.1 dB, and the gain bandwidth is broadened to 6-fold broader than the fwhm (full width at half-maximum) of the standard gain spectrum from a classic InGaAs quantum well under the same carrier density. The formation of the well–island composite quantum-confined structure is associated with the indium-rich island effect in the material growth. The great significance of this work lies in that it is making the above dream come true, si...

Efficient ab initio analysis of quantum confinement and band structure effects in ultra-scaled Si1−xGex gate-all-around and fin field-effect transistors for sub-10 nm technology nodes

This paper analyzes the sub-10 nm ultra-scaled Si1−xGex fin field-effect transistor (FinFET) and gate-all-around transistor (GAAFET), based on an improved method by combining Slater partial occupation and ab initio density functional theory (DFT). Compared to the existing generalized gradient approximation (GGA) DFT, which is computationally efficient but inaccurate, and the established hybrid functional DFT, which is accurate but computationally demanding, the proposed method achieves both hybrid functional-like high accuracy and GGA-like high computational efficiency concurrently. By using this improved methodology, batch simulations are performed to investigate the size- and composition-dependent quantum confinement and band structure effects in the Si1−xGex FinFET and GAAFET. It is shown that the quantum confinement in ultra-scaled FinFET and GAAFET significantly increases the bandgap of Si1−xGex channel in the sub-10 nm scale. It is quantitatively demonstrated that, due to its two-dimensional quantum confinement, the ultra-scaled GAAFET has larger confinement-induced bandgap increase, lower direct source-drain quantum tunneling, lower off-state transmission and low-bias conductance, and smaller drain-induced barrier lowering, compared to the FinFET which is confined only in one-dimension. These differences jointly indicate that the ultra-scaled GAAFET could offer better device performance than the ultra-scaled FinFET. It is quantitatively shown that, by reducing the Ge composition of the Si1−xGex channel in ultra-scaled FinFET and GAAFET, the bandgap and the threshold gate voltage can be significantly increased; and the transmission coefficients, the low-bias conductance, and the source-drain current can be significantly reduced. These trends can be utilized to optimize device performance by tuning Ge composition in different technology nodes.

Connecting Composition-Driven Faceting with Facet-Driven Composition Modulation in GaAs-AlGaAs Core-Shell Nanowires.

Ternary III-V alloys of tunable bandgap are a foundation for engineering advanced optoelectronic devices based on quantum-confined structures including quantum wells, nanowires, and dots. In this context, core-shell nanowires provide useful geometric degrees of freedom in heterostructure design, but alloy segregation is frequently observed in epitaxial shells even in the absence of interface strain. High-resolution scanning transmission electron microscopy and laser-assisted atom probe tomography were used to investigate the driving forces of segregation in nonplanar GaAs-AlGaAs core-shell nanowires. Growth-temperature-dependent studies of Al-rich regions growing on radial {112} nanofacets suggest that facet-dependent bonding preferences drive the enrichment, rather than kinetically limited diffusion. Observations of the distinct interface faceting when pure AlAs is grown on GaAs confirm the preferential bonding of Al on {112} facets over {110} facets, explaining the decomposition behavior. Furthermore, three-dimensional composition profiles generated by atom probe tomography reveal the presence of Al-rich nanorings perpendicular to the growth direction; correlated electron microscopy shows that short zincblende insertions in a nanowire segment with predominantly wurtzite structure are enriched in Al, demonstrating that crystal phase engineering can be used to modulate composition. The findings suggest strategies to limit alloy decomposition and promote new geometries of quantum confined structures.

Probing Quantum Confinement and Electronic Structure at Polar Oxide Interfaces.

Polar discontinuities occurring at interfaces between two materials constitute both a challenge and an opportunity in the study and application of a variety of devices. In order to cure the large electric field occurring in such structures, a reconfiguration of the charge landscape sets in at the interface via chemical modifications, adsorbates, or charge transfer. In the latter case, one may expect a local electronic doping of one material: one example is the two‐dimensional electron liquid (2DEL) appearing in SrTiO3 once covered by a polar LaAlO3 layer. Here, it is shown that tuning the formal polarization of a (La,Al)1− x(Sr,Ti)xO3 (LASTO:x) overlayer modifies the quantum confinement of the 2DEL in SrTiO3 and its electronic band structure. The analysis of the behavior in magnetic field of superconducting field‐effect devices reveals, in agreement with ab initio calculations and self‐consistent Poisson–Schrödinger modeling, that quantum confinement and energy splitting between electronic bands of different symmetries strongly depend on the interface total charge densities. These results strongly support the polar discontinuity mechanisms with a full charge transfer to explain the origin of the 2DEL at the celebrated LaAlO3/SrTiO3 interface and demonstrate an effective tool for tailoring the electronic structure at oxide interfaces.

Tuning Confinement in Colloidal Silicon Nanocrystals with Saturated Surface Ligands.

The optical properties of silicon nanocrystals (Si NCs) are a subject of intense study and continued debate. In particular, Si NC photoluminescence (PL) properties are known to depend strongly on the surface chemistry, resulting in electron-hole recombination pathways derived from the Si NC band-edge, surface-state defects, or combined NC-conjugated ligand hybrid states. In this Letter, we perform a comparison of three different saturated surface functional groups-alkyls, amides, and alkoxides-on nonthermal plasma-synthesized Si NCs. We find a systematic and size-dependent high-energy (blue) shift in the PL spectrum of Si NCs with amide and alkoxy functionalization relative to alkyl. Time-resolved photoluminescence and transient absorption spectroscopies reveal no change in the excited-state dynamics between Si NCs functionalized with alkyl, amide, or alkoxide ligands, showing for the first time that saturated ligands-not only surface-derived charge-transfer states or hybridization between NC and low-lying ligand orbitals-are responsible for tuning the Si NC optical properties. To explain these PL shifts we propose that the atom bound to the Si NC surface strongly interacts with the Si NC electronic wave function and modulates the Si NC quantum confinement. These results reveal a potentially broadly applicable correlation between the optoelectronic properties of Si NCs and related quantum-confined structures based on the interaction between NC surfaces and the ligand binding group.

Dynamic phase response and amplitude-phase coupling of self-assembled semiconductor quantum dots

The optical excitation of semiconductor gain media introduces both gain and refractive index changes, commonly referred to as amplitude-phase coupling. Quantum-confined structures with an energetically well separated carrier reservoir usually exhibit a decreased amplitude-phase coupling compared to bulk materials. However, its magnitude and definition is still controversially discussed. We investigate the fundamental processes influencing the amplitude-phase coupling in semiconductor quantum-dot media using a coupled-carrier rate-equation model. We are able to analyze the dependence on the electronic structure and suggest routes towards an optimization of the dynamic phase response of the gain material.

Attojoule Optoelectronics for Low-Energy Information Processing and Communications

Optics offers unique opportunities for reducing energy in information processing and communications while resolving the problem of interconnect bandwidth density inside machines. Such energy dissipation overall is now at environmentally significant levels; the source of that dissipation is progressively shifting from logic operations to interconnect energies. Without the prospect of substantial reduction in energy per bit communicated, we cannot continue the exponential growth of our use of information. The physics of optics and optoelectronics fundamentally addresses both interconnect energy and bandwidth density, and optics may be the only scalable solution to such problems. Here we summarize the corresponding background, status, opportunities, and research directions for optoelectronic technology and novel optics, including sub-femtojoule devices in waveguide and novel 2D array optical systems. We compare different approaches to low-energy optoelectronic output devices and their scaling, including lasers, modulators and LEDs, optical confinement approaches (such as resonators) to enhance effects, and the benefits of different material choices, including 2D materials and other quantum-confined structures. Beyond the elimination of line charging by the use optical connections, the next major interconnect dissipations are in the electronic circuits for receiver amplifiers, timing recovery and multiplexing. We can address these through the integration of photodetectors to reduce or eliminate receiver circuit energies, free-space optics to eliminate the need for timing and multiplexing circuits (while solving bandwidth density problems), and using optics generally to save power by running large synchronous systems. One target concept is interconnects from ~ 1 cm to ~ 10 m that have the same energy (~ 10fJ/bit) and simplicity as local electrical wires on chip.

Engineering carrier lifetimes in type-II In(Ga)Sb/InAs mid-IR emitters

Type-II In(Ga)Sb quantum-confined structures in InAs matrices offer a potential material system for wavelength flexible, high-efficiency, surface-emitting mid-infrared sources. In this work, the authors investigate the carrier dynamics in this material system and demonstrate a number of techniques for engineering carrier lifetimes in such emitters. Samples are grown by molecular beam epitaxy and optically characterized using temperature dependent Fourier transform infrared spectroscopy and mid-infrared time-resolved photoluminescence. The authors investigate both In(Ga)Sb quantum wells and quantum dots, and demonstrate significant improvements in isolated quantum well emitter carrier lifetimes by controlling quantization in the conduction band, or alternatively, by the formation of InGaSb quantum dot structures in InAs matrices. The authors correlate the engineered improvement in carrier lifetime with the emitters temperature performance of our emitters.

Optical properties of hybrid quantum-confined structures with high absorbance

The methods of photoluminescence and photoconductivity spectroscopy and the spectroscopy of photocurrent of a p–i–n structure are used to study samples with hybrid quantum-confined medium “quantum well–dots” (QWD) structures grown on GaAs substrates. The significant contribution of QWD states, which extends the GaAs absorption range to 1075 nm, is found in the photoconductivity and photocurrent spectra. The absorption and luminescence of the quantum-confined structures possess characteristic features of quantum wells. Analysis of the photocurrent and photoconductivity spectra demonstrate that the excitation of carriers from localized QWD states has a combined nature: at temperatures lower than 100 K and an electric-field strength of below 40 kV/cm, excitation is possible via tunneling to the matrix, while at higher temperatures thermal activation coming into play. Also, lateral photoconductivity is observed in the layers of quantum-confined structures.

Morphology in Porous Silicon Prepared from Si-Nanowires Grown By Electroless Etching

Porous silicon nanowires (SiNWs) shown remarkable advantages due to the chemical and physical properties [1]. These properties cause special interest in optoelectronics and sensing applications [2]. Porous silicon nanowires were prepared in an electrolytic solution concentration after grown these nanostructures in an electroless etching solution [3]. Changes in the morphology of porous SiNWs may be the result of increasing the concentration of the oxidizer (C2H6O) and etching time when these nanoparticles are etched in an electrolytic solution. SiNW arrays grown by electroless etching is shown in Figure 1a. Further changes are achieved in the diameter and length when the prepared electroless SiNW arrays are etched in an anodic solution to reduce its quantum confinement to obtain porous silicon nanowires, as shown in Figure 1b. Silicon nanowires were grown by electroless etching in an etching solution concentrations of hydrofluoric Acid (HF) and silver nitrate (AgNO3) at room temperature [4]. The prepared nanowires were then etched in an electrolytic solution containing HF and ethanol to make porous Si quantum confinement structures. SEM images were obtained for morphology analysis. Figure 1

(Invited) Multiscale Modeling of Stress-Mediated Compositional Patterning in SiGe Substrates

The ability to scalably fabricate periodic, large-area assemblies of Ge quantum dots (QDs) on Si or SiGe substrates with high degree of spatial and size uniformity is expected to impact numerous technologies, including nano-/micro-electronics, optoelectronics, nanosensor arrays, and high-density patterned media for data storage. One approach to achieve this goal is to harness the Stranski-Krastonov (SK) growth mechanism [1,2]. Here, a lattice mismatched species is heteroepitaxially deposited on a substrate (e.g., Ge on Si or InAs on GaAs), creating strain that increases with film thickness, leading to the spontaneous formation of a distribution of islands. The size and shape of such islands has attracted much attention in regards to their potential use as arrays of QDs, but island evolution during SK growth is inherently stochastic, leading to broad distributions of island size with random locations. As a result, much effort has been directed towards substrate patterning in order to spatially direct island formation and narrow the size distribution [2-4]. While these approaches have proven successful in principle, they are generally process-intensive and will be difficult to scale to large-area applications. Here, we describe a process in which SiGe substrates are compositionally patterned over large areas using spatially-modulated elastic fields applied by a nano-indenter array [5]. The indenter array, which is fabricated by interferometric lithography and dry etching, is pressed against a Si0.8Ge0.2 wafer in a custom-made mechanical press. The entire assembly is then annealed at high temperatures, during which the larger Ge atoms are selectively driven away from areas of compressive stress via the process of interdiffusion. Compositional analysis of the substrates demonstrates that this approach leads to a transfer of the indenter array pattern into the near-surface compositional distribution. In particular, the process leads to sharp compositional gradients that separate well-delineated regions in the near-surface region of the SiGe substrate, suggesting the possible use of this approach to simply and robustly create structures with quantum confinement properties. Employing this approach in practice to create useful configurations, however, requires a systematic assessment of the processing parameter space. For example, the final compositional distribution in the substrate depends on the indenter tip shapes and sizes, the indenter array geometric pattern, the annealing temperature-time history, as well as the initial substrate composition and thickness. In order to fulfill this task, we have developed a multiscale computer model based on the lattice kinetic Monte Carlo (LKMC) method to simulate the ‘stress transfer’ process. The LKMC simulation is propagated using rates for atomic diffusion that depend explicitly on local values of stress, composition, and temperature [6,7]. The dependence of atomic diffusion on composition is regressed to experimental data while the stress dependence is described using the theory of activation volumes [8]. The model is used to investigate the dynamics of the compositional patterning process, as well as the impact of various parameters on the compositional configurations that result. We find that certain indenter configurations produce compositional patterns that may be favorable for engineering lateral arrays quantum-confined structures. Some comparisons also are made to preliminary experimental demonstrations. [1] F. M. Ross, R. M. Tromp and M. C. Reuter, Science, 1999, 286, 1931. [2] O. G. Schmidt et al., Appl. Phys. Lett., 2000, 77, 4139. [3] F. M. Ross, M. Kammler, M. C. Reuter and R. Hull, Philos. Mag., 2004, 84, 2687. [4] J. J. Zhang et al., Appl. Phys. Lett., 2007, 91, 173115. [5] S. Ghosh, D. Kaiser, J. Bonilla, T. Sinno and S. M. Han, Appl. Phys. Lett., 2015, 107, 072106. [6] P. Castrillo, R. Pinacho, M. Jaraiz and J. E. Rubio, J. Appl. Phys., 2011, 109, 103502. [7] D. Kaiser, S. Ghosh, S. M. Han, and T Sinno, to appear in Mol. Sys. Des. & Eng. (2016). [8] M. J. Aziz, Applied Physics Letters 70, 2810 (1997).

Spontaneous formation of three-dimensionally ordered Bi-rich nanostructures within GaAs1−xBix/GaAs quantum wells

In this work, we report on the spontaneous formation of ordered arrays of nanometer-sized Bi-rich structures due to lateral composition modulations in Ga(As,Bi)/GaAs quantum wells grown by molecular beam epitaxy. The overall microstructure and chemical distribution is investigated using transmission electron microscopy. The information is complemented by synchrotron x-ray grazing incidence diffraction, which provides insight into the in-plane arrangement. Due to the vertical inheritance of the lateral modulation, the Bi-rich nanostructures eventually shape into a three-dimensional assembly. Whereas the Bi-rich nanostructures are created via two-dimensional phase separation at the growing surface, our results suggest that the process is assisted by Bi segregation which is demonstrated to be strong and more complex than expected, implying both lateral and vertical (surface segregation) mass transport. As demonstrated here, the inherent thermodynamic miscibility gap of Ga(As,Bi) alloys can be exploited to create highly uniform Bi-rich units embedded in a quantum confinement structure.

ChemInform Abstract: Semiconductor and Carbon‐Based Fluorescent Nanodots: The Need for Consistency

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