Abrupt Heterostructures Research Articles

High thermoelectric performance of PNP abrupt heterostructures by independent regulation of the electrical conductivity and Seebeck coefficient

Improving thermoelectric conversion efficiency is a hotspot in the current field of thermoelectricity. In this work, a novel scheme for the PI-Bi0.5Sb1.5Te3/N-Bi2Te2.97Se0.03/PII-Bi0.5Sb1.5Te3 abrupt heterostructure vertical to temperature gradient is presented, resulting in theoretical results of high output power and conversion efficiency. Under the effect of temperature gradient, the hole concentrations in the PNP heterostructure change linearly and the build-in voltages also vary. As a result, the PNP heterostructure can possess the amplifying characteristic of the bipolar transistor, namely, the forward conduction occurs at the cold end in PI-N junction and the reverse bias occurs at the hot end in N-PII junction. The calculation results show that the optimal output power can reach 48.38 mW and conversion efficiency can reach 14.56% at ΔT = 50 K. This work offers a new idea and method for the realization and application of high-performance thermoelectric materials.

Structural and optical properties of nonpolar m- and a-plane GaN/AlGaN heterostructures for narrow-linewidth mid-infrared intersubband transitions

Mid-infrared intersubband transitions are investigated in nonpolar m-plane and a-plane GaN/AlGaN multi-quantum well heterostructures. Nominally identical heterostructures were grown by ammonia molecular-beam epitaxy on free-standing m-plane and a-plane GaN substrates. A total of 12 well- and barrier-doped samples with intersubband transition energies in the range of 220–320 meV (wavelength range 3.8–5.6 μm) were grown. The intersubband absorption lines of the m-plane samples were 10–40% narrower than those of the a-plane samples, and a very narrow intersubband absorption linewidth of 38 meV (full width at half maximum) at a transition energy of approximately 250 meV (5 μm wavelength) was observed in an m-plane sample. Narrower intersubband absorption linewidths of m-plane samples can be explained by more abrupt heterostructure interfaces revealed by structural characterization, which is attributed to a higher stability of the m-plane compared to the a-plane. No significant difference in the intersubband absorption linewidth was observed between the barrier- and well-doped samples.

Interfacial micro-structure and properties of TiO2/SnO2 heterostructures with rutile phase: A DFT calculation investigation

Composite photocatalyst with heterostructure is often recognized as an effective strategy to achieve efficient charge separation and increase carrier concentration. In this study, the (1 1 0) and (1 0 1) surfaces of TiO2 and SnO2 were chosen to construct the abrupt and graded TiO2/SnO2 hetero-structures. The interface properties and band offsets of four heterostructures are calculated and analyzed in detail. It was found that the graded heterostructures are more stable and easier to preparation than the abrupt heterostructures, the charge depletion and accumulation occur at both ends of the transition layer. The interfacial electric field caused by this effect allows the photo-generated electron-hole pairs to be easily separated and transferred in the opposite direction at the interface, which is important for improving the photocatalytic activity. In addition, the intrinsic mechanism of the four hetero-structures is discussed according to the analysis of the band offset, which belongs to the type II heterostructures. For TiO2/SnO2-(1 1 0) heterostructure, the photo-generated electrons transfer from the conduction band of TiO2 to the conduction band of SnO2, while the photo-generated holes transfer from the SnO2 to the valence band of TiO2. This phenomenon leads to the accumulation of the photo-generated electrons in the conduction band of TiO2, accumulation of the photo-generated holes in the valence band of SnO2. In short, the graded heterostructure has more favorable effects to enhance the photocatalytic activity than that of the abrupt heterostructure. The internal electric field at the TiO2/SnO2 interface acts as a selector for photo-generated carriers, which may lead to higher photocatalytic quantum efficiency and carrier concentration under the same external conditions. These intrinsic photocatalytic mechanisms and the interpretation of the interfacial microscopic properties of heterostructure can be used as a basis for the design of the sensing devices and composite photocatalysts with heterostructures.

Numerical Investigation of Vertical Cavity Lasers With High-Contrast Gratings Using the Fourier Modal Method

We show the strength of the Fourier modal method (FMM) for numerically investigating the optical properties of vertical cavities including subwavelength gratings. Three different techniques for determining the resonance frequency and Q-factor of a cavity mode are compared. Based on that, the Fabry-Perot approach has been chosen due to its numerical efficiency. The computational uncertainty in determining the resonance frequency and Q-factor is investigated, showing that the uncertainty in the Q-factor calculation can be a few orders of magnitude larger than that in the resonance frequency calculation. Moreover, a method for reducing 3D simulations to lower-dimensional simulations is suggested, and is shown to enable approximate and fast simulations of certain device parameters. Numerical calculation of the cavity dispersion, which is an important characteristic of vertical cavities, is illustrated. By employing the implemented FMM, it is shown that adiabatic heterostructures designs are advantageous compared to abrupt heterostructures for minimizing the cavity scattering loss.

Controllable growth of layered selenide and telluride heterostructures and superlattices using molecular beam epitaxy

Abstract

Trap behaviors in AlGaN/GaN based polarization-doped field effect transistors by frequency-dependent conductance–voltage characterizations

AlGaN/GaN polarization-doped field-effect transistors (PolFETs) were fabricated on the graded AlGaN/GaN heterojunctions grown by metal organic chemical vapor deposition, and the trap behaviors were first systemically evaluated by frequency-dependent conductance–voltage characterizations. It was found that there was no obvious effective traps accumulation in the graded AlGaN/GaN heterojunctions of the PolFETs due to the absence of abrupt heterostructure as in the traditional HFETs. The exactly exponential dependence of the trap state time constants on the bias voltage in PolFETs was observed, which was a typical characteristic for interface traps, indicating a multi-heterointerface feature of the PolFETs.

Encoding abrupt and uniform dopant profiles in vapor-liquid-solid nanowires by suppressing the reservoir effect of the liquid catalyst.

Semiconductor nanowires (NWs) are often synthesized by the vapor-liquid-solid (VLS) mechanism, a process in which a liquid droplet-supplied with precursors in the vapor phase-catalyzes the growth of a solid, crystalline NW. By changing the supply of precursors, the NW composition can be altered as it grows to create axial heterostructures, which are applicable to a range of technologies. The abruptness of the heterojunction is mediated by the liquid catalyst, which can act as a reservoir of material and impose a lower limit on the junction width. Here, we demonstrate that this "reservoir effect" is not a fundamental limitation and can be suppressed by selection of specific VLS reaction conditions. For Au-catalyzed Si NWs doped with P, we evaluate dopant profiles under a variety of synthetic conditions using a combination of elemental imaging with energy-dispersive X-ray spectroscopy and dopant-dependent wet-chemical etching. We observe a diameter-dependent reservoir effect under most conditions. However, at sufficiently slow NW growth rates (≤250 nm/min) and low reactor pressures (≤40 Torr), the dopant profiles are diameter independent and radially uniform with abrupt, sub-10 nm axial transitions. A kinetic model of NW doping, including the microscopic processes of (1) P incorporation into the liquid catalyst, (2) P evaporation from the catalyst, and (3) P crystallization in the Si NW, quantitatively explains the results and shows that suppression of the reservoir effect can be achieved when P evaporation is much faster than P crystallization. We expect similar reaction conditions can be developed for other NW systems and will facilitate the development of NW-based technologies that require uniform and abrupt heterostructures.

Abrupt Lateral-Source Heterostructures with Relaxed/Strained Layers for Ballistic Complementary Metal Oxide Semiconductor Transistors Fabricated by Local O+Ion-Induced Relaxation Technique of Strained Substrates

Abrupt Lateral-Source Heterostructures with Relaxed/Strained Layers for Ballistic Complementary Metal Oxide Semiconductor Transistors Fabricated by Local O⁺Ion-Induced Relaxation Technique of Strained Substrates

Abrupt Lateral-Source Heterostructures with Relaxed/Strained Layers for Ballistic Complementary Metal Oxide Semiconductor Transistors Fabricated by Local O+ Ion-Induced Relaxation Technique of Strained Substrates

We have experimentally studied an abrupt lateral-relaxed/strained layer heterojunction for ballistic complementary metal oxide semiconductor (CMOS) transistors, which is fabricated by a local O+ ion-induced relaxation technique for strained semiconductors on a buried oxide layer. We have demonstrated that strained substrates in various conditions are suddenly relaxed at a critical recoil energy of O+ ions at the strained semiconductor/buried oxide layer interface. Moreover, after O+ ion implantation into strained substrates with a SiO2 mask as well as post-annealing processes, we have successfully formed lateral relaxed/strained Si layers with an abrupt strain distribution at the mask edge, according to Raman spectroscopy analysis of implanted strained substrates. In addition, strained Si layers even under the 50-nm length stripe SiO2 mask region can still keep over 60% of the strain value in strained Si layers with a large area.

Diameter Dependent Growth Rate and Interfacial Abruptness in Vapor–Liquid–Solid Si/Si1−xGex Heterostructure Nanowires

A strong diameter dependence is observed in the interfacial abruptness and growth rates in Si/Si 1- x Ge x axial heterostructure nanowires grown via Au-mediated low pressure CVD using silane and germane precursors. The growth of these nanowires has similarities to that of heterostructure thin films with similar compositional interfacial broadening, which increases with and is on the order with diameter. This broadening may reveal a fundamental challenge to fabrication of abrupt heterostructures via VLS growth.

Least-action principle for envelope functions in abrupt heterostructures

We apply the envelope function approach to abrupt heterostructures starting with the least action principle for the microscopic wave function. The interface is treated nonperturbatively, and our approach is applicable to mismatched heterostructure. We obtain the interface connection rules for the multiband envelope function and the short-range interface terms which consist of two physically distinct contributions. The first one depends only on the structure of the interface, and the second one is completely determined by the bulk parameters. We discover new structure inversion asymmetry terms and new magnetic energy terms important in spintronic applications.

Semiconductor nanowires for 0D and 1D physics and applications

During the last 5 years the potential for applications of semiconductor nanowires has grown rapidly via the development of methods for catalytically induced nanowire growth using the, so-called vapor–liquid–solid (VLS) growth mode. The VLS method offers a high degree of control of parameters such as position, diameter, length and composition, including the realization of atomically abrupt heterostructure interfaces inside a nanowire. In this review, we summarize the progress and the standing of our research from the point of view of controlled growth, structural and electronic properties and in terms of different families of devices which have been possible to realize.

Growth and characterization of GaAs and InAs nano-whiskers and InAs/GaAs heterostructures

Semiconducting InAs and GaAs nano-whiskers have been grown using a chemical beam epitaxy approach in combination with size-selected catalytic Au aerosol particles. The characterization of InAs and GaAs whiskers shows high crystalline quality as seen by transmission electron microscopy. Gate-dependent transport measurements suggests a diffusive electronic transport mechanism. We have also combined these two material systems by growing a very abrupt heterostructure interface within the whiskers, allowing the growth of highly mismatched structures without misfit dislocations.

Connection Rules versus Differential Equations for Envelope Functions in Abrupt Heterostructures

Connection Rules versus Differential Equations for Envelope Functions in Abrupt Heterostructures

The energy spectra of narrow-gap semiconductor heterostructures: spin splitting in the case of asymmetry

Using the two-band model, the energy spectrum is explicitly obtained as a function of the spin projection and transverse momentum for two abrupt semiconductor heterostructures of type II: (i) a double heterojunction and (ii) a system of two quantum wells, one in the valence band and one in the conduction band. The transverse motion of the carriers and the effective repulsion of the electron-like and hole-like energy levels give rise to effects which cannot be properly described in the framework of the single-band theory. The two special cases form the basis for a discussion of the properties of more general classes of heterostructures of any type. The spin splitting of the energy spectrum is found to be a characteristic property of asymmetric heterostructures. Degeneracy occurs in the case of spatially symmetric heterostructures and for a class of heterostructures which are spatially asymmetric but symmetric with respect to energy. The possibility of a new type of transition is discussed.

Envelope-function formalism for electrons in abrupt heterostructures with material-dependent basis functions.

An envelope-function model is derived for electrons in abrupt semiconductor heterostructures. It uses material-dependent basis functions that diagonalize the bulk zone-center Hamiltonian in each unit cell of the crystal. The initial formalism is exactly equivalent to the one-electron Schr\odinger equation; approximations suitable for abrupt junctions are then developed. The abrupt change in microscopic potential at an ideal interface is shown to introduce no new interband coupling; all such coupling arises from the kinetic energy, specifically from a momentumlike matrix element containing the gradient of the basis functions with respect to changes in material composition. This generates interface effects not included in conventional envelope-function theories, such as zone-center coupling between heavy and light holes. An effective-mass equation is derived for the conduction band of a layered zinc-blende structure; it exhibits both envelope discontinuities and \ensuremath{\delta}-function potentials, in agreement with the transfer matrices derived from other microscopic theories. \textcopyright{} 1996 The American Physical Society.

Interfacial recombination of current carriers: influence on magnetoconcentration effect in semiconductor heterostructure

This paper investigates theoretically the magnetoconcentration effect in an abrupt heterostructure on the basis of non-degenerate semiconductors with an intrinsic conductivity in the case when a recombination of non-equilibrium current carriers occurs at the heterostructure interface. The influence of the interfacial recombination velocity on the current-voltage characteristics of the heterostructure is analysed.

Integrated heterostructure devices composed of II–VI materials with Hg-based contact layers

Integrated heterostructure devices (IHDs) composed of II–VI materials in epitaxial multilayered structures for light-emitting diode and laser diode applications are described. These IHDs combine a light emission multilayer structure with an abrupt or graded heterostructure which includes Hg-based materials for improved ohmic contact to the upper p-type layer of the light emitting structure.

Band diagram of a HgTe-CdTe semimetal-semiconductor abrupt heterostructure

The energy-band diagram and interface potentials at a HgTe-CdTe abrupt heterostructure are presented. An analytic approximation is formulated and compared to an exact (numeric) calculation of the interface potentials. The error introduced by the analytic approximation is negligible for a wide range of doping levels of the CdTe. The analysis predicts that HgTe forms ohmic contacts to p-type CdTe and rectifying junctions to n-type CdTe. Interface charges above 1012 cm−2 modify the interface potentials. Positive interface charge imposes depletion in p-type CdTe resulting in potential barriers that may degrade the ohmic properties of the contacts. The methodology presented in this study may be extended to additional semimetal-semiconductor heterostructures.

Integrated Heterostructure Devices (IHDS): A New Approach for the Fabrication of High-Efficiency Blue/Green Light Emitters Based on II-VI Materials

ABSTRACTIntegrated heterostructure devices (IHDs) comprised of II-VI materials in multi-layered structures for light emitting diode (LED) and laser diode (LD) applications are described. These IHDs combine a light emission multilayer structure (wide band gap II-VI layers) with an abrupt or graded heterostructure (comprised of narrow band gap II-VI layers) for improved ohmic contact to the upper p-type layer of the light emitting structure.