Semiconductor Heterointerface Research Articles

Real-space observation of a two-dimensional electron gas at semiconductor heterointerfaces.

Mobile charge carriers are essential components in high-performance, nano-engineered semiconductor devices. Employing charge carriers confined to heterointerfaces, the so-called two-dimensional electron gas, is essential for improving device performance. The real-space visualization of a two-dimensional electron gas at the nanometre scale is desirable. However, it is challenging to accomplish by means of electron microscopy due to an unavoidable strong diffraction contrast formation at the heterointerfaces. We performed direct, nanoscale electric field imaging across a GaN-based semiconductor heterointerface using differential phase contrast scanning transmission electron microscopy by suppressing diffraction contrasts. For both nearly the lattice-matched GaN/Al0.81In0.19N interface and pseudomorphic GaN/Al0.88In0.12N interface, the extracted quantitative electric field profiles show excellent agreement with profiles predicted using Poisson simulation. Furthermore, we used the electric field profiles to quantify the density and distribution of the two-dimensional electron gas across the heterointerfaces with nanometre precision. This study is expected to guide the real-space characterization of local charge carrier density and distribution in semiconductor devices.

Anomalous photoluminescence enhancement and resonance charge transfer in type-II 2D lateral heterostructures

Type-II band alignment can realize the efficient charge transfer and separation at the semiconductor heterointerface, which results in photoluminescence (PL) quenching. Recently, several researches demonstrated great enhancement of localized PL at the interface of type-II two-dimensional (2D) heterostructure. However, the dominant physical mechanism of this enhanced PL emission has not been well understood. In this work, we symmetrically study the exciton dynamics of type-II lateral heterostructures of monolayer MoS2 and WS2 at room temperatures. The strong PL enhancement along the one-dimensional (1D) heterointerface is associated with the trion emission of the WS2 shell, while a dramatic PL quenching of neutral exciton is observed on the MoS2 core. The enhanced quantum yield of WS2 trion emission can be explained by charge-transfer-enhanced photoexcited carrier dynamics, which is facilitated by resonance hole transfer from MoS2 side to WS2 side. This work sheds light on the 1D exciton photophysics in lateral heterostructures, which has the potential to lead to new concepts and applications of optoelectronic device.

NiS/MoS2 Mott-Schottky heterojunction-induced local charge redistribution for high-efficiency urea-assisted energy-saving hydrogen production

Urea-assisted water electrolysis possesses the prospective prospect for high-efficiency hydrogen production by replacing oxygen evolution reaction (OER) with thermodynamically more favorable urea oxidation reaction (UOR). Modulating the electronic structure of electrocatalysts through constructing metal–semiconductor heterointerface represents an effective strategy to promote the electrochemical performances. Herein, we construct a Mott-Schottky bifunctional electrocatalyst by in-situ growth of NiS/MoS2 hetero-nanoflowers on the conductive carbon cloth (CC) substrate (abbreviated as NiS/MoS2@CC hereafter) for both hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Thanks to the Mott-Schottky effect, the self-driven charge transfer occurs across the NiS/MoS2 heterointerfaces, which results in the built-in electric field, the accelerated charge transfer rate, and the modified chemisorption free energies for reaction intermediates, ultimately expediting the dissociation of water and urea molecules. Consequently, the as-fabricated NiS/MoS2@CC electrode only requires an overpotential of 87 mV for hydrogen evolution reaction (HER) in 1.0 M KOH and a potential of 1.36 V for UOR in 1.0 M KOH solution with 0.5 M urea to attain a current density of 10 mA cm−2, respectively. Moreover, when served as the free-standing anode and cathode simultaneously, the NiS/MoS2@CC-assembled urea electrolyzer requires a cell voltage of 1.46 V at 10 mA cm−2, which is 200 mV smaller than that of the pure water splitting counterpart. This study may deepen the understanding of electronic modulation via Mott-Schottky establishment.

Charge density waves and Fermi level pinning in monolayer and bilayer SnSe2

Materials with reduced dimensionality often exhibit exceptional properties that are different from their bulk counterparts. Here, we report the emergence of a commensurate $2\ifmmode\times\else\texttimes\fi{}2$ charge density wave (CDW) in monolayer and bilayer ${\mathrm{SnSe}}_{2}$ films by scanning tunneling microscopy. The visualized spatial modulation of the CDW phase becomes prominent near the Fermi level, which is pinned inside the semiconductor band gap of ${\mathrm{SnSe}}_{2}$. We show that both CDW and Fermi level pinning are intimately correlated with band bending and virtual induced gap states at the semiconductor heterointerface. Through interface engineering, the electron-density-dependent phase diagram is established in ${\mathrm{SnSe}}_{2}$. Fermi surface nesting between symmetry inequivalent electron pockets is revealed to drive the CDW formation and to provide an alternative CDW mechanism that might work in other compounds.

Semiconductor Heterostructure SrTiO3/CeO2 Electrolyte Membrane Fuel Cells

Semiconductor heterostructure SrTiO3/CeO2 (STO/CeO2) membrane was used as the electrolyte material of low-temperature solid oxide fuel cells. This STO/CeO2 membrane demonstrated a high ionic conductivity of 0.24 S cm−1 under fuel cell conditions at 550°C, and also its fuel cell delivered a maximum power output of 745.6 mW cm−2 at the same temperature. Moreover, the high electronic conductivity of CeO2 is blocked by the STO/CeO2 semiconductor heterojunction, thus resulting in the open circuit voltage increasing from 0.90 to 1.09 V at 550°C. The semiconductor heterointerface is found to play an important role in inhibiting the electron transfer between CeO2 particles. Thus, this work makes it feasibility of using semiconductor materials as the electrolyte by constructing heterojunctions to suppress the electronic conductivity.

X-ray STM: Nanoscale elemental analysis & Observation of atomic track.

Scanning tunneling microscopy (STM) combined with brilliant X-rays from synchrotron radiation (SR) can provide various possibilities of original and important applications, such as the elemental analysis on solid surfaces at an atomic scale. The principle of the elemental analysis is based on the inner-shell excitation of an element-specific energy level "under STM observation". A key to obtain an atomic locality is to extract the element-specific modulation of the local tunneling current (not emission that can damage the spatial resolution), which is derived from the inner-shell excitation [1]. On this purpose, we developed a special SR-STM system and smart tip. To surmount a tiny core-excitation efficiency by hard X-rays, we focused two-dimensionally an incident beam having the highest photon density at the SPring-8.After successes in the elemental analyses by SR-STM [1,2] on a semiconductor hetero-interface (Ge on Si) and metal-semiconductor interface (Cu on Ge), we succeeded in obtaining the elemental contrast between Co nano-islands and Au substrate. The results on the metallic substrate suggest the generality of the method and give some important implications on the principle of contrast. For all cases of three samples, the spatial resolution of the analysis was estimated to be ∼1 nm or less, and it is worth noting that the measured surface domains had a deposition thickness of less than one atomic layer (Fig. 1, left and center).jmicro;63/suppl_1/i14-a/DFU045F1F1DFU045F1Fig. 1.(left) Topographic image and (center) beam-induced tip current image of Ge(111)-Cu (-2V, 0.2 nA). (right) X-ray- induced atomic motion tracks on Ge(111) that were newly imaged by the Xray-STM. On the other hand, we found that the "X-ray induced atomic motion" can be observed directly with atomic scale using the SR-STM system effectively under the incident photon density of ∼2 x10(15) photon/sec/mm(2) [3]. SR-STM visualized successfully the track of the atomic motion (Fig. 1, right), which enabled the further analysis on the mechanism of the atomic motion. It is worth comparing our results with past conventional thermal STM observations on the same surface [4], where the atomic motion was found to occur in the 2-dimensional domain. However, our results show the atomic track having a local chain distribution [3].The above mentioned results will allow us to investigate the chemical analysis and control of the local reaction with the spatial resolution of STM, giving hope of wide applications.

GaAs/AlGaAs Nanowire Photodetector

We demonstrate an efficient core-shell GaAs/AlGaAs nanowire photodetector operating at room temperature. The design of this nanoscale detector is based on a type-I heterostructure combined with a metal-semiconductor-metal (MSM) radial architecture, in which built-in electric fields at the semiconductor heterointerface and at the metal/semiconductor Schottky contact promote photogenerated charge separation, enhancing photosensitivity. The spectral photoconductive response shows that the nanowire supports resonant optical modes in the near-infrared region, which lead to large photocurrent density in agreement with the predictions of electromagnetic and transport computational models. The single nanowire photodetector shows a remarkable peak photoresponsivity of 0.57 A/W, comparable to large-area planar GaAs photodetectors on the market, and a high detectivity of 7.2 × 10(10) cm·Hz(1/2)/W at λ = 855 nm. This is promising for the design of a new generation of highly sensitive single nanowire photodetectors by controlling the optical mode confinement, bandgap, density of states, and electrode engineering.

Room-temperature spin injection and spin loss across a GaNAs/GaAs interface

Recently discovered effect of spin-filtering and spin amplification in GaNAs enables us to reliably obtain detailed information on the degree of spin loss during optical spin injection across a semiconductor heterointerface at room temperature. Spin polarization of electrons injected from GaAs into GaNAs is found to be less than half of what is generated in GaNAs by optical orientation. We show that the observed reduced spin injection efficiency is not only due to spin relaxation in GaAs, but more importantly due to spin loss across the interface due to structural inversion asymmetry and probably also interfacial point defects.

Nanoscale elemental identification by synchrotron‐radiation‐based scanning tunneling microscopy

AbstractScanning tunneling microscopy (STM) combined with synchrotron radiation (SR) enabled elemental analysis on solid surfaces at an anometer scale. The principle of analysis is based on the inner‐shell excitation of a specific energy level under STM observation. After first demonstrative results on a semiconductor heterointerface (Ge nanoisland on an Si(111) 7 × 7 surface), a second trial of nanoscale elemental analysis by the SR‐STM system was accomplished for Cu nanodomains on a Ge(111) 2 × 8 surface. A key to achieve successful results is to effectively increase the signal to noise (S/N) ratio and to extract the element‐specific modulation of the tunneling current, which is derived from the inner‐shell excitation. An essential point for signal detection with a high S/N ratio is to construct a total system including lock‐in amplifier with an optical chopper for high accuracy, and a real‐time control system to check the incident‐beam condition by observing the beam‐induced current profiles. Using these techniques, it has been shown that the SR‐STM system provides new possibilities forn anoscale surface analysis. Copyright © 2008 John Wiley & Sons, Ltd.

First Principle Study on the Domain Matching Epitaxy Growth of Semiconductor Hetero-Interface

In conventional lattice-matching epitaxy with lattice misfit of less than 7–8%, the thin films grow pseudomorphically (i.e. film take lattice constant of the substrate up to some ‘‘critical thickness’’ of one to several monolayers). Above this misfit, it was surmised that the film will grow textured or largely polycrystalline. The recent discovery of Domain Matching Epitaxy Growth method had proposed the new growth technology using Pulsed laser deposition (PLD), by which the epitaxial growth of hetero-thin films with very large lattice mismatch is possible by matching of domains where an integer multiples of major lattice planes match across the interface. Misfit dislocation generated along the interface (i.e. critical thickness is less than one-monolayer). Based on all of these exciting technology applications, there is an urgent need to study the material properties, structure correlations and defect microstructure for this thin film growth mode. In this work, we performed an ab-initio study of the structural properties of that pseudomorphic-like growth of semiconductor hetero-interface using the well-known density functional-based tightbinding (DFTB) simulation tool. We examine the simulation study of domain matching growth of GaN(0001)/Si(111) - a Hex-on-Cub problem. [doi:10.2320/matertrans.MB200818]

XPS and UPS Characterization of the TiO2/ZnPcGly Heterointerface: Alignment of Energy Levels

The electronic structure and the highest occupied molecular orbitals (HOMO)/the lowest unoccupied molecular orbitals (LUMO) alignment at the molecular semiconductor heterointerface of nanostructured TiO2/ZnPcGly dye sensitizer were characterized by X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). The HOMO level of the dye ZnPcGly was determined to be located at 1.62 eV below the Fermi edge, and the corresponding LUMO level was estimated to be 0.10 eV above the conduction band of TiO2 based on the HOMO/LUMO gap (1.82 eV) of ZnPcGly determined by optical absorption measurements. This energy level matching between the orbitals of the dye and the bands of TiO2 can enable efficient electron transfer from photoexcited ZnPcGly to TiO2, which is very important in photoinduced charge-transfer reactions and for applications in dye-sensitized solar cells.

Breakdown of rotational symmetry at semiconductor interfaces: a microscopic description of valence subband mixing

The recently discovered in-plane optical anisotropy of [001]-grown quantum wells offers a new theoretical and experimental insight into the electronic properties of semiconductor interfaces. We first discuss the coupling of X and Y valence bands due to the breakdown of roto-inversion symmetry at a semiconductor hetero-interface, with special attention to its dependence on effective parameters such as the valence band offset. The intracell localization of Bloch functions is explained from simple theoretical arguments and evaluated numerically from a pseudo-potential microscopic model. The role of envelope functions is then considered, and we discuss the specific case of no-common atom interfaces. Experimental results and applications to interface characterization are presented. These calculations give a microscopic justification, and establish the limits of the heuristic “HBF” model.

Utilising Zn segregation at InP/InGaAs interface for as-deposited ohmic contact formation for photonic and electronic device applications

An approach based on Zn segregation at the semiconductor heterointerface in Zn diffusion is proposed for beneficial use in photonic and electronic device fabrication. It is shown that Zn segregation at the semiconductor heterointerface can lead to a substantial increase in surface hole concentration, making it possible to fabricate as-deposited ohmic contacts for improved product uniformity and yield.

Breakdown of Rotational Symmetry at Semiconductor Interfaces: a Microscopic Description of Valence Subband Mixing

The recently discovered in-plane optical anisotropy of [001]-grown quantum wells offers a new theoretical and experimental insight into the electronic properties of semiconductor interfaces. We first discuss the coupling of X and Y valence bands due to the breakdown of roto-inversion symmetry at a semiconductor hetero-interface, with special attention to its dependence on effective parameters such as the valence band offset. The intracell localization of Bloch functions is explained from simple theoretical arguments and evaluated numerically from a pseudo-potential microscopic model. The role of envelope functions is then considered, and we discuss the specific case of no-common atom interfaces. Experimental results and applications to interface characterization are presented. These calculations give a microscopic justification, and establish the limits of the heuristic “H BF” model.

In-plane optical anisotropy of quantum well structures: From fundamental considerations to interface characterization and optoelectronic engineering

The recently discovered in-plane optical anisotropy of (001)-grown quantum wells offers a new theoretical and experimental insight into the electronic properties of semiconductor interfaces. We first discuss the coupling of X and Y valence bands due to the breakdown of rotoinversion symmetry at a semiconductor heterointerface, with special attention to its dependence on effective parameters such as the valence band offset. The intracell localization of Bloch functions is explained from simple theoretical arguments and evaluated numerically from a pseudopotential microscopic model. The role of envelope functions is then considered, and we discuss the specific case of no-common atom interfaces. Experimental results and applications to interface characterization are presented, and the potential of the “quantum confined Pockels effect” for device applications is finally discussed.

Coherent transfer of spin through a semiconductor heterointerface.

Spin transport between two semiconductors of widely different band gaps is time resolved by two-color pump-probe optical spectroscopy. Electron spin coherence is created in a GaAs substrate and subsequently appears in an adjacent ZnSe epilayer at temperatures ranging from 5 to 300 K. The data show that spin information can be protected by transport to regions of low spin decoherence, and regional boundaries used to control the resulting spin coherent phase.

Crystal interface and high-resolution electron microscopy—the best partner

Several contributions of HRTEM on the interface science are reviewed in chronological order. The first contribution of HRTEM is the observation of gold (113)S°11 boundary, giving experimental proof of the CSL model. An observation of the asymmetric (112)S°3 boundary follows. A SiC grain boundary is effectively assessed not by the density of CSL point but the number of dangling bonds in the boundary. A ZnO/Pd interface provides an example that a misfit dislocation does not necessarily accommodate the lattice mismatch. Segregated interface shows characteristic HRTEM image contrast, suggesting change in atomic bonding. An atomic height step in the semiconductor hetero interface is observed by the Chemical Lattice Image technique. In the diamond grain boundary a dangling bond may not elevate the boundary energy, being contradictory of the least dangling bond rule. Super–high resolution of the HVHRTEM enable us to determine atomic species in the grain boundary. Combined use of HRTEM and EELSE allows us to discuss the correlation between atomic structure and nature of the corresponding interface. It is not exaggeration to say that modern interface science does not exist witout HRTEM. On the other hand, many complicated interfaces found by HRTEM remained as unaswered questions. An innovative structural model is requested to appear on the scene.

Donor level in a quasi-two dimensional heterojunction system

A variational method is used to investigate the ground state of an electron bound to a donor impurity near a single semiconductor heterointerface by considering the influence of a realistic heterojunction potential and the screened Coulombic impurity potential. The screening effect is considered within the random phase approximation. A one subband model is adopted to describe the behavior of the conduction electrons in the potential. The impurity state binding energy is obtained numerically for the GaAs/Al x Ga 1− x As (0.2< x<0.4) system. It is found that the screening effect dramatically decreases the impurity state binding energy which is influenced obviously by the barrier potential and the conduction band bending. The binding energy obtained versus Al concentration, electron density, and impurity position is given and discussed.

Determination of frontier orbital alignment and band bending at an organic semiconductor heterointerface by combined x-ray and ultraviolet photoemission measurements

The alignment of the highest occupied molecular orbitals (HOMO) at the tris (8-hydroxy quinoline) aluminum (Alq3)/N,N′-di-(3-methylphenyl)-N,N′diphenyl-4,4′-diaminobiphenyl (TPD) heterojunction, used in organic light-emitting diodes (OLED), was determined by growing a TPD layer in several steps on a thick Alq3 substrate layer. After each growth step the sample was characterized in situ by x-ray and ultraviolet photoemission spectroscopy. The offset of the HOMO maxima at the interface was determined to be −0.13 eV from Alq3 to TPD. By including the known HOMO–lowest occupied molecular orbital (LUMO) gaps for both molecules into the evaluation, the offset of the LUMO minima was determined to be −0.33 eV from Alq3 to TPD. These values are consistent with previous assumptions that this interface represents a higher barrier for electron injection from Alq3 to TPD than for hole injection from TPD to Alq3.

Anomalous mobility and photo-Hall effect in ZnSe-GaAs heterostructures

The Hall and photo-Hall effect are studied experimentally in pure, unintentionally doped n-ZnSe epilayers grown by molecular-beam epitaxy on semi-insulating GaAs substrates. An anomalous depression of the dark values of the room-temperature and peak mobility in the n-ZnSe layer is observed in a substantial fraction of the samples studied, while normal values for the mobility are recovered under illumination with above-band-gap light. This is accompanied by an increase in the effective areal electron density. These results point to space-charge scattering causing the dark mobility reduction. The photo-Hall effect in ZnSe-GaAs heterostructures is discussed in terms of simple models considering both misfit dislocations at the heterostructure interface and extended defects in the bulk of the ZnSe epilayer. Additional photo-Hall experiments have been performed for longer-wavelength laser lines. In this case, the areal electron concentration decreases strongly under intense illumination, while a large room-temperature mobility enhancement is found. This net negative photoconductivity effect suggests a depletion of mobile electrons from the ZnSe epilayer, accompanied by the creation of a low-density but highly mobile conducting layer in the GaAs near the semiconductor heterointerface.