Low-dimensional Heterostructures Research Articles

Optical field dissipation in heterostructures for nanophotovoltaic devices

In heterostructures for nanophotovoltaic (NPV) devices, a number of layers are concatenated in a multilayer configuration. In the analysis of a multilayer configuration, it is commonly assumed that the intensity of the optical field has an exponential decrease along the direction of propagation inside the structure. Effects such as reflections and interference are neglected. These neglected effects become especially important ones once the layer dimension reaches several nanometers. At this width regimen, quantum effects are present since layers are thin compared with the penetration depth and the wavelength of the incident light. Quantum effects influence photon absorption and affect the optical field dissipation that controls electron-hole pairs generation. Hence, dissipation of the optical field inside an NPV device is an important aspect to consider in studying and determining performance properties. We employed the one-dimensional optical transfer matrix theory and the quantum well theory to analyze the optical field dissipation in the active layer of heterostructures for NPV devices. Illumination of 100 mW·cm−2 air mass 1.5 global (AM 1.5G) standard was considered for the analysis. The study was extended to low-dimensional heterostructures of the binary compound CdS/CdSe/CdS, the ternary compound Ga0.9Al0.1As/GaAs/Ga0.9Al0.1As, and the quaternary compound In0.85Ga0.15As0.30P0.70/In0.7Ga0.3As0.6P0.4/In0.85Ga0.15As0.30P0.70.

Ab Initio-Based Bond Order Potential to Investigate Low Thermal Conductivity of Stanene Nanostructures.

We introduce a bond order potential (BOP) for stanene based on an ab initio derived training data set. The potential is optimized to accurately describe the energetics, as well as thermal and mechanical properties of a free-standing sheet, and used to study diverse nanostructures of stanene, including tubes and ribbons. As a representative case study, using the potential, we perform molecular dynamics simulations to study stanene's structure and temperature-dependent thermal conductivity. We find that the structure of stanene is highly rippled, far in excess of other 2-D materials (e.g., graphene), owing to its low in-plane stiffness (stanene: ∼ 25 N/m; graphene: ∼ 480 N/m). The extent of stanene's rippling also shows stronger temperature dependence compared to that in graphene. Furthermore, we find that stanene based nanostructures have significantly lower thermal conductivity compared to graphene based structures owing to their softness (i.e., low phonon group velocities) and high anharmonic response. Our newly developed BOP will facilitate the exploration of stanene based low dimensional heterostructures for thermoelectric and thermal management applications.

Long-range magnetic interactions and proximity effects in an amorphous exchange-spring magnet.

Low-dimensional magnetic heterostructures are a key element of spintronics, where magnetic interactions between different materials often define the functionality of devices. Although some interlayer exchange coupling mechanisms are by now well established, the possibility of direct exchange coupling via proximity-induced magnetization through non-magnetic layers is typically ignored due to the presumed short range of such proximity effects. Here we show that magnetic order can be induced throughout a 40-nm-thick amorphous paramagnetic layer through proximity to ferromagnets, mediating both exchange-spring magnet behaviour and exchange bias. Furthermore, Monte Carlo simulations show that nearest-neighbour magnetic interactions fall short in describing the observed effects and long-range magnetic interactions are needed to capture the extent of the induced magnetization. The results highlight the importance of considering the range of interactions in low-dimensional heterostructures and how magnetic proximity effects can be used to obtain new functionality.

System-size dependent band alignment in lateral two-dimensional heterostructures

The electronic band alignment in semiconductor heterostructures is a key factor for their use in electronic applications. The alignment problem has been intensively studied for bulk systems but is less well understood for low-dimensional heterostructures. In this work we investigate the alignment in two-dimensional lateral heterostructures. First-principles calculations are used to show that the electronic band offset depends crucially on the width and thickness of the heterostructure slab. The particular heterostructures under study consist of thin hydrogenated and fluorinated diamond slabs which are laterally joined together. Two different limits for the band offset are observed. For infinitely wide heterostructures the vacuum potential above the two materials is aligned leading to a large step potential within the heterostructure. For infinitely thick heterostructure slabs, on the other hand, there is no potential step in the heterostructure bulk, but a large potential step in the vacuum region above the heterojunction is observed. The band alignment in finite systems depends on the particular dimensions of the system. These observations are shown to result from an interface dipole at the heterojunction that tends to align the band structures.

Indirect-to-direct band gap transition in relaxed and strained Ge1−x−ySixSny ternary alloys

Sn-containing group IV semiconductors create the possibility to independently control strain and band gap thus providing a wealth of opportunities to develop an entirely new class of low dimensional systems, heterostructures, and silicon-compatible electronic and optoelectronic devices. With this perspective, this work presents a detailed investigation of the band structure of strained and relaxed Ge1−x−ySixSny ternary alloys using a semi-empirical second nearest neighbors tight binding method. This method is based on an accurate evaluation of the deformation potential constants of Ge, Si, and α-Sn using a stochastic Monte-Carlo approach as well as a gradient based optimization method. Moreover, a new and efficient differential evolution approach is also developed to accurately reproduce the experimental effective masses and band gaps. Based on this, we elucidated the influence of lattice disorder, strain, and composition on Ge1−x−ySixSny band gap energy and directness. For 0 ≤ x ≤ 0.4 and 0 ≤ y ≤ 0.2, we found that tensile strain lowers the critical content of Sn needed to achieve a direct band gap semiconductor with the corresponding band gap energies below 0.76 eV. This upper limit decreases to 0.43 eV for direct gap, fully relaxed ternary alloys. The obtained transition to direct band gap is given by y > 0.605 × x + 0.077 and y > 1.364 × x + 0.107 for epitaxially strained and fully relaxed alloys, respectively. The effects of strain, at a fixed composition, on band gap directness were also investigated and discussed.

Recent Progress in Semiconductor Properties Engineering by Ultrasonication

This work presents review of the recent patents and applied researches in the field of ultrasonic processing of the semiconductor materials and devices. The ultrasound demonstrates selective and, simultaneously, complex character of the effect on semiconductors. In contrast to the thermal or light energy, uniformly absorbed over semiconductor volume, the acoustic wave energy is mainly absorbed by the crystal lattice defects. The peculiarities of this interaction results in practical applications of the ultrasonication of semiconductors for electronic properties design. It was shown that US processing has found technological niche as supporting operation during ion implantation process and growth of semiconductors. The use of an inhomogeneous stress and piezoelectric harmonic potential produced by surface acoustic waves in low dimensional heterostructures to improves the efficiency of available optoelectronic and nanoelectronic devices as well as create new ones. The phenomenon of acoustic cavitation that underlies at the basis of such technological processes as cleaning and sonochemical synthesis is discussed separately. Keywords: Acoustics cavitation, gettering effect, semiconductor devices, sonochemistry, ultrasound.

Diffusion-Controlled Growth of Molecular Heterostructures: Fabrication of Two-, One-, and Zero-Dimensional C60 Nanostructures on Pentacene Substrates

A variety of low dimensional C60 structures has been grown on supporting pentacene multilayers. By choice of substrate temperature during growth the effective diffusion length of evaporated fullerenes and their nucleation at terraces or step edges can be precisely controlled. AFM and SEM measurements show that this enables the fabrication of either 2D adlayers or solely 1D chains decorating substrate steps, while at elevated growth temperature continuous wetting of step edges is prohibited and instead the formation of separated C60 clusters pinned at the pentacene step edges occurs. Remarkably, all structures remain thermally stable at room temperature once they are formed. In addition the various fullerene structures have been overgrown by an additional pentacene capping layer. Utilizing the different probe depth of XRD and NEXAFS, we found that no contiguous pentacene film is formed on the 2D C60 structure, whereas an encapsulation of the 1D and 0D structures with uniformly upright oriented pentacene is achieved, hence allowing the fabrication of low dimensional buried organic heterostructures.

Dependence of Spin-orbit Parameters in AlxGa1−xN/GaN Quantum Wells on the Al Composition of the Barrier

In this paper, we obtain considerable spin-orbit (SO) parameters in AlxGa1−xN/GaN quantum wells (QWs) with sheet carrier concentration Ns = 120 × 1011/cm2. With increasing Al content (x) of the barrier, the SO parameters increase as a whole, and the two major contributions are found to be the decrease of the expansion region of the envelope functions and the increase of the polarized electric field in the well. Compared with the Rashba parameters for the first two subbands, the intersubband SO parameter is a bit smaller and varies more slowly with x. The results indicate the SO parameters, especially the Rashba parameters can be engineered by the Al composition of the barrier, which may be helpful to the spin manipulation of III-nitride low-dimensional heterostructures.

Optical bistability in low-dimensional semiconductor heterostructures under cw pump laser and infrared pulse signals

We have investigated theoretically the optical bistability (OB) and optical multistability in quantum-well (QW) semiconductor heterostructures. The effect of a cw pump laser field and a control infrared pulse signal on OB is discussed. It is found that the OB threshold can be controlled by varying the strength of applied fields. It is also shown that OB can be switched to optical multistability or vice versa just using an appropriate relative phase of applied fields. This investigation may open up some new possibilities for applications in all-optical switching in quantum-well structures.

Novel Understanding of Electron States Architecture and Its Dimensionality in Semiconductors

Some important insights into the electron-states-architecture (ESA) and its dimensionality (from 3 to 0) in a semiconductor (or generally crystalline) material are obtained. The self-consistency of the set of density of states (DOS) expressions with different dimensionalities is remediated through the clarification and rearrangement of the wave-function boundary conditions for working out the eigenvalues in the wave vector space. The actually too roughly observed and theoretically unpredicted critical points for the dimensionality transitions referring to the integer ones are revealed upon an unusual assumption of the intrinsic energy-level dispersion (ELD). The ELD based quantitative physical model had been established on an immediate instinct at the very beginning and has been properly modified afterwards. The uncertainty regarding the relationship between the de Broglie wavelength of electrons and the dimensionality transitions, seeming somewhat mysterious before, is consequentially eliminated. The effect of the material dimensions on the ELD width is also predicted and has been included in the model. The continuous evolution of the ESA dimensionality is convincingly and comprehensively interpreted and thus the area of the fractional ESA dimensionalities is opened. Another new assumption of the spatial extension shrinkage (SES) closely related to the ELD has also been made and thus the understanding of the behavior of an electron or, in a general sense, a particle has become more comprehensive. This work would manifest itself a new basis for further development of nanoheterostructures (or low dimensional heterostructures including the quantum wells, quantum wires, quantum dots and especially the hetero-dimensional structures). Expected should also be the possible inventions of some novel electronic and optoelectronic devices. More basically, it leads to a new quantum mechanical picture, the essential modifications of Schrödinger equation and Newtonian equation that give rise to a full cosmic-scope picture, and a super-low-speed relativity assumption.

Calculating the Energy Spectrum of Complex Low-Dimensional Heterostructures in the Electric Field

An algorithm for solving the steady-state Schrödinger equation for a complex piecewise-constant potential in the presence of the E-field is developed and implemented. The algorithm is based on the consecutive matching of solutions given by the Airy functions at the band boundaries with the matrix rank increasing by no more than two orders, which enables the characteristic solution to be obtained in the convenient form for search of the roots. The algorithm developed allows valid solutions to be obtained for the electric field magnitudes larger than the ground-state energy level, that is, when the perturbation method is not suitable.

Identification of an Intrinsic Source of Doping Inhomogeneity in Vapor–Liquid–Solid-Grown Nanowires

The vapor-liquid-solid (VLS) process of semiconductor nanowire growth is an attractive approach to low-dimensional materials and heterostructures because it provides a mechanism to modulate, in situ, nanowire composition and doping, but the ultimate limits on doping control are ultimately dictated by the growth process itself. Under widely used conditions for the chemical vapor deposition growth of Si and Ge nanowires from a Au catalyst droplet, we find that dopants incorporated from the liquid are not uniformly distributed. Specifically, atom probe tomographic analysis revealed up to 100-fold enhancements in dopant concentration near the VLS trijunction in both B-doped Si and P-doped Ge nanowires. We hypothesize that radial and azimuthal inhomogeneities arise from a faceted liquid-solid interface present during nanowire growth, and we present a simple model to account for the distribution. As the same segregation behavior was observed in two distinct semiconductors with different dopants, the observed inhomogeneity is likely to be present in other VLS grown nanowires.

Intense laser field effects on the linear and nonlinear intersubband optical properties of a semi-parabolic quantum well

By using the compact-density matrix approach, the effect of a nonresonant intense laser field on the linear and nonlinear optical absorptions based on intersubband transitions and the refractive index changes in an asymmetric semiconductor quantum well have been presented. Our results show that the peak position of the absorption coefficient is sensitive to intense laser field, the absorption maximum shifts towards lower energies for increasing intense laser field value. Also we observe as the intense laser field strength increases, the total refractive index change has been increased in magnitude and also shifted towards lower energies. The results indicate that linear and nonlinear optical properties of the low dimensional semiconductor heterostructures can be adjusted in a desired energy range by using intense laser field.

Optical knobs from slow- to fast-light with gain in low-dimensional semiconductor heterostructures

The light pulse propagation through semiconductor quantum-well heterostructures under realistic experimental conditions is studied analytically with the Schrödinger equations. It is shown that slow light and superluminal propagation with gain can be observed by varying the relative phase and the strength of the applied fields. Such investigation may open up the possibility to control the light propagation and may lead to potential applications such as high-fidelity optical delay lines, optical buffers and optical communication in quantum wells solid materials.

(Invited) Ion-Cut from Smart to Smarter: Ulrich Gösele's Impact on Science and Technology of Ultrathin Layer Transfer

Ulrich Gösele made numerous contributions to the science and technology of semiconductor low dimensional systems, heterostructures and nanoscale structures. This paper will be based upon the personal experience of the author who worked directly with Ulrich Gösele on several fundamental and engineering subjects. In particular, the impact of Ulrich Gösele on the basic understanding and applications of the ion-cut process will be addressed.

ChemInform Abstract: Ferromagnetism and Heterostructures of III-V Magnetic Semiconductors

Abstract Ferromagnetism in III–V based magnetic semiconductors introduced a new degree of freedom associated with the magnetic cooperative phenomena to the low-dimensional heterostructures. This article reviews the preparation and the structure, magnetic, and transport properties of ferromagnetic III–V semiconductor (Ga,Mn)As, which can be grown pseudomorphically on GaAs, and heterostructures related to the ferromagnetic semiconductor.

Spin-orbit coupling induced by effective mass gradient

The existence of a spin-orbit coupling (SOC) induced by the gradient of the effective mass in low-dimensional heterostructures is revealed. In structurally asymmetric quasi-two-dimensional semiconductor heterostructures the presence of a mass gradient across the interfaces results in a SOC which competes with the SOC created by the electric field in the valence band. However, in graded quantum wells subjected to an external electric field, the mass-gradient-induced SOC can be finite even when the electric field in the valence band vanishes.

Theoretical investigation of infrared generation mechanism by quantum coherence in low-dimensional semiconductor heterostructures

We present an infrared generation mechanism without population inversion between subbands in quantum well and quantum dot lasers. The infrared generation scheme is based on the resonant nonlinear mixing of the two optical laser fields. These two optical fields come from two interband transitions in the same active region and serve as the coherent drive for infrared field. This mechanism of frequency down conversion should work efficiently at room temperature with injection current pumping, not relying on any ad hoc assumptions of long-lived coherence in the semiconductor active medium. Under optimized waveguide and cavity parameters, the intrinsic down-conversion efficiency can reach the limiting quantum value corresponding to one infrared photon generated by one optical photon. Because the proposed infrared generation is parametric, the proposed scheme without population inversion is especially promising for long-wavelength infrared operation.

Transference Numbers for In‐Plane Carrier Conduction in Thin Film Nanostructured Gadolinia‐Doped Ceria Under Varying Oxygen Partial Pressure

We demonstrate a modified Hebb–Wagner approach to quantitatively estimate transference numbers for carrier conduction in thin film oxide conductors using blocking electrodes in an in‐plane geometry. We report ionic transference numbers, ti, for gadolinia‐doped ceria (GDC) thin films, a model mixed ion–electron conductor, at 973K and oxygen partial pressure ranging from 0.21 atm down to approximately 10−22 atm. Our results indicate that GDC reaches the electrolytic regime (ti=0.5) at an oxygen partial pressure of 5 × 10−19 atm at 973K. This approach may be useful for understanding carrier transport mechanisms in low‐dimensional oxide heterostructures with specific relevance to nanostructured energy materials.

Dynamic propagation study of phase-controlled infrared-light pulses in low-dimensional semiconductor heterostructures

We study the propagation effects of the phase-controlled infrared-light pulses in semiconductor quantum-well (QW) heterostructures under realistic experimental conditions. By numerically solving the coupled Bloch–Maxwell equations for electrons and fields simultaneously on numerical grids in time and space, we show that the propagation dynamics can be dramatically modified by varying the relative phase of the applied fields. This opens up the possibility to study optimal control of light propagation in the QW solid materials.