Semiconductor-metal Hybrid Structures Research Articles

HYBRID METAL-SEMICONDUCTOR STRUCTURES BASED ON InP AND GaAs NANOTEMPLATES FOR ELECTRONIC AND PHOTONIC APPLICATIONS

In this scientific work are presented the results that contribute to solving an important scientific problem related to obtaining of porous templates with controlled morphology and design by replacing acidic and alkaline electrolytes, the use of which presents a danger for the environment, with neutral electrolyte (NaCl) as well as obtaining of the metal-semiconductor hybrid structures using pulsed electrodeposition that offers additional possibilities to control the localized deposition in certain portions of the porous template and allows the controlled fabrication of nanodots, nanowires, nanotubes and perforated metal nanomembranes. Mechanisms of pore propagation in InP and GaAs semiconductor substrates and electrochemical deposition of metals in the produced porous templates are identified and discussed, which allowed to control the direction of pore growth, including those parallel to the substrate surface as well as localized Au deposition.

Pt cocatalyst morphology on semiconductor nanorod photocatalysts enhances charge trapping and water reduction.

In photocatalysis, metal-semiconductor hybrid structures have been proposed for ideal photocatalytic systems. In this study, we investigate the effect of morphology and surface nature of Pt cocatalysts on photocatalytic hydrogen evolution activity in Pt-tipped CdSe nanorods. Three distinct morphologies of Pt cocatalysts were synthesized and employed as visible light photocatalysts. The rough tips exhibit the highest activity, followed by the round and cubic tips. Kinetic investigations using transient absorption spectroscopy reveal that the cubic tips exhibit lower charge-separated states feasible for reacting with water and water reduction rates due to their defectless surface facets. In contrast, the rough tips show a similar charge-separation value but a two-fold higher surface reaction rate than the round tips, resulting in a significant enhancement of hydrogen evolution. These findings highlight the importance of rational design on metal cocatalysts in addition to the main semiconductor bodies for maximizing photocatalytic activities.

Symmetry breaking in magnetoresistive devices

Detecting weak magnetic fields is paramount in areas such as scanning magnetometers and manipulation of magnetic nanoparticles, thus rendering it crucial to increase the weak-field sensitivity for developing next-generation magnetic sensors. The current approaches for high-sensitivity sensors, such as superconducting quantum interference devices, are complex and expensive. By contrast, magnetoresistive sensors and particularly extraordinary magnetoresistive sensors offer a simple operation at room temperature but, to date, at inferior sensitivity. To overcome these challenges, we induce device symmetry breaking to enhance the weak magnetic field sensitivity in semiconductor-metal hybrid structures exhibiting extraordinary magnetoresistance. Retaining the device mirror symmetry yields symmetric magnetoresistance curves with $R(B)=R(\ensuremath{-}B)$, which results in inferior detection of weak magnetic fields as ${[dR/dB]}_{B\ensuremath{\rightarrow}0}=0$. Using finite element modeling, we study the change in device behavior as the symmetry is broken by varying the device geometry by spatially varying the constituent material properties or both. We show that symmetry breaking has three important implications: First, breaking the mirror symmetry causes an asymmetric sensor response with $R(B)\ensuremath{\ne}R(\ensuremath{-}B)$, benefiting from a largely enhanced sensitivity to weak magnetic fields and detection of the magnetic field direction. Second, an interplay with the Hall effect causes a large negative magnetoresistance exceeding 79% at $B=1$ T and room temperature without magnetic constituents or explicit optimization of this property. Third, the asymmetric geometries can be used as a key ingredient toward designing on-demand magnetoresistive characteristics such as linearity at low magnetic fields, step functions, and magnetic switchlike behavior. These implications pave the way for asymmetric topology optimization of magnetoresistive devices with unparalleled performance.

Enhanced Output Performance of a Flexible Piezoelectric Nanogenerator Realized by Lithium-Doped Zinc Oxide Nanowires Decorated on MXene.

A flexible piezoelectric composite is composed of a polymer matrix and piezoelectric ceramic fillers to achieve good mechanical flexibility and processability. The overall piezoelectric performance of a composite is largely determined by the piezoelectric filler inside. Thus, different dispersion methods and additives that can promote the dispersion of piezoelectric ceramics and optimal composite structures have been actively investigated. However, relatively few attempts have been made to develop a filler that can effectively contribute to the performance enhancement of piezoelectric devices. In the present work, we introduce the fabrication and performance of the composite piezoelectric devices composed of Li-doped ZnO nanowires (Li: ZnO NWs) grown on the surface of MXene (Ti3C2) via the hydrothermal process. Through this approach, a semiconductor-metal hybrid structure is formed, increasing the overall permittivity. Moreover, the Ti3C2 layer can serve as a local ground in the composite so that the ferroelectric phase-transformed Li: ZnO NWs grown on its surface can be more effectively polarized during the poling process. In addition, the NW-covered surface of Ti3C2 prevents the aggregation of metallic Ti3C2 particles, promoting a more uniform electric field distribution during the poling process. As a result, the output performance of the piezoelectric nanogenerator (PENG) fabricated with a Li: ZnO NW/Ti3C2 composite was greatly improved compared to that of the devices fabricated with Li: ZnO NWs without the Ti3C2 platform. Specifically, the Li: ZnO NW/Ti3C2 composite piezoelectric nanogenerator (PENG) demonstrated a twofold higher output power density (∼9 μW/cm2) compared with the values obtained from the PENG devices based on Li: ZnO NWs. The approach introduced in this work can be easily adopted for an effective ferroelectric filler design to improve the output performance of the piezoelectric composite.

Chemoselective Surface Trap-Mediated Metal Growth on Semiconductor Nanocrystals

We present a highly chemoselective deposition of precious metals on semiconductor nanoheterostructures with a strong preference for cadmium and zinc telluride over the lighter chalcogenides. The selectivity is explained by p-type surface traps on the tellurides, compared to n-type defects of the homologous sulfides and selenides, and can be turned off by passivating the particle surface. The results give insight into the nature and role of surface defects for semiconductor nanocrystals. The fast formation of many, small metal seeds leads to aggregation of the particles into star-shaped or branched superstructures, leaving the rest of the semiconductor surface exposed. It provides a preparative route toward complex, yet well-defined semiconductor-metal hybrid structures with potential application in photocatalysis.

Magnetic sensors and geometrical magnetoresistance: A review

Magnetic sensing devices are of the extremely significant kind of detectors, that are used several important and useful applications. Geometrical extraordinary magnetoresistance (EMR) is the geometrical kind of magnetoresistance associated with the non-magnetic semiconductor-metal hybrid structure and influenced by geometrical shape. As a result of Lorentz force, the current path change from metal (in absence of magnetic field) to semiconductor (under the subjection of the magnetic field) in semiconductor-metal hybrid structure is the key of EMR phenomena, i.e. once the metal is placed in a semiconductor, it works as a short circuit with the majority of applied current moving through metallic inhomogeneity and the almost whole resistance of semiconductor-metal hybrid structure drops to value smaller than that of homogeneous semiconductor in absence of magnetic field, in other hands, applying of magnetic field alters the current route to be around the metallic inhomogeneity where it works as an open circuit and the whole resistance turns into a quite high magnitude relies on the geometrical form of a device. The variables govern these phenomena are metal and semiconductor conductivity, semiconductor charge carriers mobility, and device geometry. Within this review, EMR phenomena history, variables governed it, materials, and applications of EMR devices are overviewed.

Interfacial facet engineering on the Schottky barrier between plasmonic Au and TiO2 in boosting the photocatalytic CO2 reduction under ultraviolet and visible light irradiation

Hybrid photocatalytic nanostructures composed of plasmonic metal and wide-band-gap semiconductor components have been widely developed, in which metal not only acts as a cocatalyst to trap the photogenerated electrons from semiconductor for improved charge separation and provide highly active sites for accelerated reaction kinetics, but also serves as a light-harvesting antennae to extend the light absorption region based on the injection of plasmonic hot electrons into the semiconductor. In both circumstances, rational design of metal/semiconductor interface is highly desirable to smooth the migration of electrons and promote the separation of carriers. Herein, based on the deposition of Au on TiO2 nanocrystals with different exposed facets, it is found that the formation of Au/TiO2(101) interface lowers the height of Schottky barrier in comparison with Au/TiO2(001) interface, enhancing the transfer of conduction band (CB) electrons from TiO2 to Au cocatalysts under ultraviolet light irradiation and promoting the hot electron injection from plasmonic Au into the CB of TiO2 with the excitation of Au by visible light. The more efficient interfacial charge transfer and separation enable more electrons participating in the conversion of CO2 to CO and CH4. As a result, at both excitation wavelengths, the Au-TiO2 sample with exclusive Au/TiO2(101) interfaces significantly ameliorates the photocatalytic activities in CO and CH4 production compared to other samples containing Au/TiO2(001) interfaces. The interfacial facet engineered Schottky barrier may open a new window to rationally designing metal–semiconductor hybrid structures for photocatalysis.

Carbon Monoxide Oxidation Promoted by Surface Polarization Charges in a CuO/Ag Hybrid Catalyst

Composite structures have been widely utilized to improve material performance. Here we report a semiconductor-metal hybrid structure (CuO/Ag) for CO oxidation that possesses very promising activity. Our first-principles calculations demonstrate that the significant improvement in this system’s catalytic performance mainly comes from the polarized charge injection that results from the Schottky barrier formed at the CuO/Ag interface due to the work function differential there. Moreover, we propose a synergistic mechanism underlying the recovery process of this catalyst, which could significantly promote the recovery of oxygen vacancy created via the M-vK mechanism. These findings provide a new strategy for designing high performance heterogeneous catalysts.

Selective deposition of gold particles onto silicon at the nanoscale controlled by a femtosecond laser through galvanic displacement.

Control of the deposition location and morphology of metals on semiconductors is of considerable importance for the fabrication of metal–semiconductor hybrid structures. For this purpose, selective nanoscale deposition of gold on silicon was successfully achieved by a two-step method in this paper. The first preparation step comprises the fabrication of ripples with a femtosecond laser. The second preparation step is to immerse the samples in a mixed aqueous solution of hydrofluoric acid (HF) and chloroauric acid (HAuCl4). The periodically ablated ripple structures on silicon surfaces fabricated by the femtosecond laser changed the physical and chemical properties of silicon and then controlled the nucleation positions of gold nanoparticles. Gold particles tend to grow in raised positions of the ripples and no substantial growth was observed in the recesses of the ablated ripple structures. Similar phenomena were observed on the modified ripple structures; this led to the formation of periodically distributed gold sub-micron wires. Above all, this paper proposes a new mask-free method of selective metal electroless deposition that can be realized without complicated experimental equipment and tedious experimental operations.

Graphene “bridge” in transferring hot electrons from plasmonic Ag nanocubes to TiO2 nanosheets for enhanced visible light photocatalytic hydrogen evolution

The integration of plasmonic metal with wide-bandgap semiconductor is a promising approach to utilize the visible light without compromise of the redox ability of photogenerated charge carriers. However, a larger work function of metal than that of semiconductor is indispensable to enable the injection of hot electrons from plasmonic metal to semiconductor. In this paper, we demonstrated that reduced graphene oxide (rGO) nanosheets as conductive “bridge” can breakthrough the restriction and transfer hot electrons from Ag of smaller work function to TiO2 of larger work function. In the design, both of the Ag nanocubes and TiO2 nanosheets are co-deposited on the surface of rGO nanosheets to form Ag-rGO-TiO2 structure, which was characterized by XRD, TEM, Raman and XPS spectra. On one hand, the Ag-rGO interface facilitates the transfer of hot electrons from Ag to rGO through conductor–conductor contact. On the other hand, the new formed Schottky junction on the rGO-TiO2 interface further pumps the transferred electrons to the surface of TiO2 for photocatalytic reduction reaction resulted from the larger work function of rGO than that of TiO2. Enabled by this unique design, the hydrogen production activity achieved under visible light irradiation is dramatically enhanced in comparison with that of Ag-TiO2 counterpart with the direct contact between the same Ag nanocubes and TiO2 nanosheets. This work represents a step toward the rational interfacial design of plasmonic metal-semiconductor hybrid structures for broad-spectrum photocatalysis.

Electroless deposition of gold on silicon and its potential applications: review

Metal–semiconductor hybrid structures are important in a number of technologies such as sensors, catalysis, integrated circuits and more broadly in nano- and microsystem technologies. Electroless deposition, in this case, galvanic displacement process, is a simple way to develop such metal–semiconductor hybrid systems. In this review, we will first briefly discuss on the electroless deposition of gold nanoparticles and then look into the details of galvanic displacement of gold on silicon. From the fundamental aspects of galvanic displacement of gold on silicon, we will see how the experimental parameters such as deposition time, silicon surface, influence of light/laser during deposition, etc. affect the gold morphology on silicon. We will then explore the potential applications of such hybrid structures, especially its sensoric and catalytic properties.

Integration of Multiple Plasmonic and Co-Catalyst Nanostructures on TiO2 Nanosheets for Visible-Near-Infrared Photocatalytic Hydrogen Evolution.

Utilization of visible and near-infrared light has always been the pursuit of photocatalysis research. In this article, an approach is developed to integrate dual plasmonic nanostructures with TiO2 semiconductor nanosheets for photocatalytic hydrogen production in visible and near-infrared spectral regions. Specifically, the Au nanocubes and nanocages used in this work can harvest visible and near-infrared light, respectively, and generate and inject hot electrons into TiO2 . Meanwhile, Pd nanocubes that can trap the energetic electrons from TiO2 and efficiently participate in the hydrogen evolution reaction are employed as co-catalysts for improved catalytic activity. Enabled by this unique integration design, the hydrogen production rate achieved is dramatically higher than those of its counterpart structures. This work represents a step toward the rational design of semiconductor-metal hybrid structures for broad-spectrum photocatalysis.

Designing p‐Type Semiconductor–Metal Hybrid Structures for Improved Photocatalysis

AbstractA practical strategy is proposed to facilitate the migration of holes in semiconductor (the low rate of which limits photocatalytic efficiency) by taking advantage of the Schottky barrier between p‐type semiconductor and metal. A high work function is found to serve as an important selection rule for building such desirable Schottky junction between semiconductor surface facets and metal. The intrinsic charge spatial distribution has to be taken into account when selecting the facets, as it results in accumulation of photoexcited electrons and holes on certain semiconductor facets. Importantly, the facets have a high work function, the same characteristic required for the formation of Schottky junction in a p‐type semiconductor–metal hybrid structure. As a result, the semiconductor crystals in the hybrid design may be better enclosed by single facets with high work function, so as to synergize the two effects: Schottky barrier versus charge spatial separation.

Designing p‐Type Semiconductor–Metal Hybrid Structures for Improved Photocatalysis

A practical strategy is proposed to facilitate the migration of holes in semiconductor (the low rate of which limits photocatalytic efficiency) by taking advantage of the Schottky barrier between p-type semiconductor and metal. A high work function is found to serve as an important selection rule for building such desirable Schottky junction between semiconductor surface facets and metal. The intrinsic charge spatial distribution has to be taken into account when selecting the facets, as it results in accumulation of photoexcited electrons and holes on certain semiconductor facets. Importantly, the facets have a high work function, the same characteristic required for the formation of Schottky junction in a p-type semiconductor-metal hybrid structure. As a result, the semiconductor crystals in the hybrid design may be better enclosed by single facets with high work function, so as to synergize the two effects: Schottky barrier versus charge spatial separation.

Extraordinary magnetoresistance in two and three dimensions: Geometrical optimization

The extraordinary magnetoresistance (EMR) in metal-semiconductor hybrid structures was first demonstrated using a van der Pauw configuration for a circular semiconductor wafer with a concentric metallic inclusion in it. This effect depends on the orbital motion of carriers in an external magnetic field, and the remarkably high magnetoresistance response observed suggests that the geometry of the metallic inclusion can be optimized to further significantly enhance the EMR. Here, we consider the theory and simulations to achieve this goal by comparing both two-dimensional (2D) and three-dimensional (3D) structures in an external magnetic field to evaluate the EMR in them. New results for 3D structures are presented to show the feasibility of such modeling. Examples of structures that are compatible with present day technological capabilities are given together with their expected responses in terms of EMR. For a 10 μm 2D square structure with a square metallic inclusion, we find an MR up to 107 percent for an applied magnetic field of 1 T. In 3D, for a 10 μm cube with a 5 μm centered metallic inclusion, we obtain an MR of ∼104 percent, which is comparable with the 2D structure of equivalent dimensions. The results presented here for specific geometries are scalable to smaller dimensions down to the onset of ballistic effects in the transport. The present calculations open up the possibility of 3D magnetic field sensors capable of determining the magnitude and also direction of the magnetic field once a full characterization of the sensor response is performed.

Couplings in GaAs/AlGaAs/Metal Photonic Waveguides with Metallic Variations

ABSTRACTTo have better light-emitting performance, semiconductor-metal periodic photonic waveguides can generate stable wavelengths. This work constructs a multi-parameter model to compute the backward-wave mode-coupling coefficients, which are important to the analysis and performance of photonic devices. For such a semiconductor-metal hybrid structure, a proper photonic technique needs to be utilized to solve this computational complexity.Numerical results demonstrate how the materials of metal gratings, the corrugation amplitudes of metal gratings, and the metallic aluminum mole fraction can affect the coupling coefficients. Further physical interpretation and discussion can support and explain the above results. The results can help engineers decide the values of parameters used in fabrication. Future work and applications will be proposed.

Modeling Silicon-Based Periodic Waveguides for Optical Interconnects

ABSTRACTTo assist the precision and stability of wavelength at 1550 nm and 1300 nm in planar optical waveguides, hybrid semiconductor-metal corrugated gratings with nanometer period are integrated into silicon-based optical interconnects. This work utilizes multi-parametric optical waveguide models to compute the mode-coupling coefficients in the silicon photonic devices. For such a semiconductor-metal hybrid structure, a proper photonic technique needs to be utilized to solve this computational complexity. The optical method and the photonic method are used to compute coupling coefficients. Both methods have close numerical values shown in figures. Numerical results demonstrate how the normalized corrugation amplitudes of metal gratings can affect the coupling coefficients. Further physical interpretation and discussion can support and explain the above results. The modeling results can help engineers decide the values of parameters used in the design and fabrication of optical waveguides.

Transport measurements and analytical modeling of extraordinary electrical conductance in Ti-GaAs metal-semiconductor hybrid structures

We present a comprehensive study of a phenomenon, extraordinary electroconductance (EEC), in microscopic metal-semiconductor hybrid (MSH) structures at room temperature. Our artificially designed MSH structure shows highly efficient external electric field sensing properties not exhibited by bare semiconductor structures. The microscopic device is fabricated from a GaAs epitaxial layer with a Ti/Au shunt subject to an external electric field and gives a maximum 5.2% EEC effect corresponding to an external electric field resolution of 3.05 V/cm at a bias field of 2.5 kV/cm. Moreover, the study reveals a strong dependence of the transport properties on the geometry of the MSH. An analytical two-layer model is developed which provides good agreement with the experimentally observed data. We propose that scaled down nanoscopic EEC sensor arrays can be used as an imaging technique for the charge distribution on a single cell surface in real time.

Extraordinary electroconductance in metal-semiconductor hybrid structures.

We report the phenomenon of extraordinary electroconductance in microscopic metal-semiconductor hybrid structures fabricated from GaAs epitaxial layer and a Ti thin film shunt. Four-lead Van der Pauw structures show a gain of 5.2% in electroconductance under +2.5 kVcm with zero shunt bias. The increase in the sample conductance results from the thermionic field emission of electrons and the geometrical amplification. A model provides good agreement with the experimental data and clearly demonstrates the geometry dependence of the field effect in extraordinary electroconductance (EEC). The differences between EEC devices and field effect transistors, such as junction field effect transistor (FET) and Schottky barrier gate FET, are discussed.

Theory of the surface plasmon polariton–exciton interaction in multi‐layer systems

AbstractA thorough theoretical understanding of the interaction of excitons and surface plasmon polaritons (SPP) in metal–semiconductor hybrid structures is of fundamental importance for the design of SPP‐based devices. A theory to describe SPP–exciton interaction in a periodic multilayer film system is presented and demonstrated. In a hybrid system of gold nanowires on top of a GaAs/AlGaAs quantum well (QW), our calculations show the formation of coupled SPP–exciton modes. These are predicted to be observable in reflection spectra and by near‐field scanning optical microscopy. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)