Metal-semiconductor Interface Research Articles

In Situ Transmission Electron Microscopy Analysis of Aluminum-Germanium Nanowire Solid-State Reaction.

To fully exploit the potential of semiconducting nanowires for devices, high quality electrical contacts are of paramount importance. This work presents a detailed in situ transmission electron microscopy (TEM) study of a very promising type of NW contact where aluminum metal enters the germanium semiconducting nanowire to form an extremely abrupt and clean axial metal–semiconductor interface. We study this solid-state reaction between the aluminum contact and germanium nanowire in situ in the TEM using two different local heating methods. Following the reaction interface of the intrusion of Al in the Ge nanowire shows that at temperatures between 250 and 330 °C the position of the interface as a function of time is well fitted by a square root function, indicating that the reaction rate is limited by a diffusion process. Combining both chemical analysis and electron diffraction we find that the Ge of the nanowire core is completely exchanged by the entering Al atoms that form a monocrystalline nanowire with the usual face-centered cubic structure of Al, where the nanowire dimensions are inherited from the initial Ge nanowire. Model-based chemical mapping by energy dispersive X-ray spectroscopy (EDX) characterization reveals the three-dimensional chemical cross-section of the transformed nanowire with an Al core, surrounded by a thin pure Ge (∼2 nm), Al2O3 (∼3 nm), and Ge containing Al2O3 (∼1 nm) layer, respectively. The presence of Ge containing shells around the Al core indicates that Ge diffuses back into the metal reservoir by surface diffusion, which was confirmed by the detection of Ge atoms in the Al metal line by EDX analysis. Fitting a diffusion equation to the kinetic data allows the extraction of the diffusion coefficient at two different temperatures, which shows a good agreement with diffusion coefficients from literature for self-diffusion of Al.

The electronic properties of Au and Pt metal contacts on quasi-one-dimensional layered TiS3(001)

The interfaces of layered trichalcogenide TiS3(001), with metals Au and Pt, were examined using X-ray photoemission spectroscopy. In spite of the fact that both Au and Pt are large work function metals, no evidence of Schottky barrier formation was found with this n-type semiconductor. Two- and four-terminal field-effect transistor measurements performed on exfoliated few-nm-thick TiS3 crystals using pure Au contacts indicate that Au forms an Ohmic contact on TiS3(001), with negligible contact resistance. The absence of appreciable Schottky barrier formation is attributed to strong interactions with sulfur at the metal-semiconductor interface.

Metal/Semiconductor Nanocomposites for Photocatalysis: Fundamentals, Structures, Applications and Properties.

Due to the capability of utilizing light energy to drive chemical reactions, photocatalysis has been widely accepted as a green technology to help us address the increasingly severe environment and energy issues facing human society. To date, a large amount of research has been devoted to enhancing the properties of photocatalysts. As reported, coupling semiconductors with metals is one of the most effective methods to achieve high-performance photocatalysts. The excellent properties of metal/semiconductor (M/S) nanocomposite photocatalysts originate in two aspects: (a) improved charge separation at the metal-semiconductor interface; and (b) increased absorption of visible light due to the surface plasmon resonance of metals. So far, many M/S nanocomposite photocatalysts with different structures have been developed for the application in environmental remediation, selective organic transformation, hydrogen evolution, and disinfection. Herein, we will give a review on the M/S nanocomposite photocatalysts, regarding their fundamentals, structures (as well as their typical synthetic approaches), applications and properties. Finally, we will also present our perspective on the future development of M/S nanocomposite photocatalysts.

Piezo-phototronic enhanced photoresponsivity based on single CdTe nanowire photodetector

The piezo-phototronic effect utilizes the piezo-polarization charge to modify the energy band diagram at the local interface/junction and manipulate the optoelectronic processes of charge carriers, which have provided a promising approach to improve the performance of photoelectric devices. In this paper, we report on the fabrication of single ultra-fine CdTe nanowire (NW) piezo-phototronic photodetectors (PDs). The structure and morphology of the as-synthesized CdTe NWs is characterized in detail. The result shows that the CdTe NWs have a single crystalline zinc blende structure, with its diameter narrowing to about 20 nm. The flexible Ag-CdTe NW-Ag lateral PDs are prepared on the polyethylene terephthalate substrate, showing a broadband photoresponse from ultraviolet to near infrared (NIR) (325-808 nm). By introducing the piezo-phototronic effect, strain-induced piezoelectric polarization charges effectively enhance the performances of the NIR PDs (808 nm) by 430% in photocurrent and 427% in photoresponsivity. The physical mechanism is carefully investigated by analyzing the energy band diagrams at the local metal–semiconductor interface under mechanical deformations. The ultra-fine structure with a larger piezoelectric coefficient is attributed to the enhancement of photoresponsivity. This investigation demonstrates an efficient prototype of the broad-wavelength piezo-phototronic PD based on the ultra-fine CdTe NWs, which provides an effective route to enhance the performance of optoelectronic devices.

Large-Area Nanofabrication of Partially Embedded Nanostructures for Enhanced Plasmonic Hot-Carrier Extraction

When plasmonic nanoparticles are coupled with semiconductors, highly energetic hot carriers can be extracted from the metal–semiconductor interface for various applications in light energy conversion. However, the current quantum yields for hot-electron extraction are generally low. An approach for increasing the extraction efficiency consists of maximizing the contact area between the surface of the metal nanostructure and the electron-accepting material. In this work, we developed an innovative, simple, and scalable fabrication technique that partially embeds colloidal plasmonic nanostructures within a semiconductor TiO2 layer without utilizing any complex top-down nanofabrication method. The successful embedding is confirmed by scanning electron microscopy and atomic force microscopy imaging. Using visible-pump, near-IR probe transient absorption spectroscopy, we also provide evidence that the increase in the surface contact area between the nanostructures and the electron-accepting material leads to a...

Nanoplasmonics and its Applied Devices

Nanoplasmonics makes a connection to conventional optics to the nanoworld. Interesting performance like subwavelength focusing to invisibility cloaking, nanoplasmonics have profound applications in science and engineering world from biophotonics to nanocircuitry. Metal and dielectric have free d-shell electrons. When metal and dielectric of different refractive index come in contact, these free electrons get accumulated in a region at the metal-semiconductor interface forming nanoplasmons. Practical implementation of nano device fabrication is the most challenging task due to the dissipative losses in metal. The optimum operating condition can be achieved by the efficient use of optical gain. We review here the ongoing progress in the field of nanoplasmonic research. Journal of Nanotechnology and Smart Materials J Nanotech Smart Mater 2014 | Vol 1:402

Melting process and the size depression of the eutectic temperature in Ag/Ge and Ge/Ag/Ge layered films

Melting temperature is one of the principal parameters of phase equilibria thermodynamics and its value affects the majority of materials properties. Despite previous studies, the fundamental mechanisms of interphase interactions at melting in two-component nanomaterials are under consideration. We address these issues in a comprehensive experimental study of phase transformations in nanosized Ag-Ge eutectic system by means of in situ TEM techniques. The Ge/Ag/Ge and Ag/Ge layered films with thin Ag layer (1–50 nm) on top or in-between amorphous Ge films have been used as a model of a binary system of variable size. Morphology, crystalline structure and composition of Ag-Ge films have been examined in situ and ex situ by advanced (S)TEM techniques, the eutectic temperature (TE) and the Ge crystallization temperature have been measured as a function of Ag film thickness. A few hundred degrees lowering of TE with the film thickness reduction has been registered. Complex changes of the Ge/Ag/Ge films morphology below the eutectic temperature driven by Ag-mediated crystallization of amorphous Ge have been observed and related to the formation of metastable (liquid and/or Ag-Ge hcp) phases at the metal-semiconductor interface at 250 °C already.

Analysis of the carrier conduction mechanism in 100 MeV O7+ ion irradiated Ti/n-Si Schottky barrier structures

The electrical transport properties of Ti/n-Si Schottky barrier structure irradiated by 100 MeV O7+ ion has been studied between 120 K and 320 K and compared with unirradiated Ti/n-Si structure to gain the understanding of carrier transport mechanism in irradiated devices. The barrier parameters have been extracted using the current-voltage (I–V) characteristics in forwarding bias. It is found that Schottky barrier height (zero bias) increases while the value of ideality factor becomes smaller with increasing temperature. Recent models consisting of inhomogeneous barrier potential at the metal-semiconductor (MS) interface have been used to explain the behavior of barrier parameters. The role played by the high energy ions at MS interface during irradiation is considered to understand the observed behavior.

Combined thermionic emission and tunneling mechanisms for the analysis of the leakage current for Ga2O3 Schottky barrier diodes

A new analysis method of reverse leakage current for β-Ga2O3 Schottky barrier diodes is performed by using two models: bias dependence and no bias dependence of barrier height. The method incorporates both the current induced by the tunneling of carriers through the Schottky barrier and the current induced by the thermionic emission of carriers across the metal–semiconductor interface. The experimental reverse transition voltage between thermionic emission and tunneling process can be determined from the intersection of the two components that were separated from the total current. Below the reverse transition voltage, the thermionic emission current dominates, and above it, the tunneling current dominates, while near the reverse transition voltage, neither tunneling nor thermionic emission accurately describes the conduction process because the both currents have the same order of magnitude; therefore, the both mechanisms must be combined together. The experimental reverse transition voltage (bias-dependent model) increases for low and high temperatures and decreases at intermediate temperatures for β-Ga2O3 Schottky barrier diodes. The bias dependence of the barrier height model shows that the barrier height is strongly dependent (increases) on the reverse bias, in particular at low temperatures and low reverse bias. This model can explain the discrepancy between the experimental characteristics and those calculated by no bias dependence of barrier height model.

High performance lateral Schottky diodes based on quasi-degenerated Ga2O3**Project supported by the National Key R&D Program of China (Grant No. 2017YFB0403003), the National Natural Science Foundation of China (Grant Nos. 61774081, 61322403, and 91850112), the State Key R&D Project of Jiangsu, China (Grant No. BE2018115), Shenzhen Fundamental Research Project, China (Grant Nos. 201773239 and 201888588), State Key Laboratory ofWide-Bandgap Semiconductor

Ni/β-Ga2O3 lateral Schottky barrier diodes (SBDs) were fabricated on a Sn-doped quasi-degenerate n+-Ga2O3 bulk substrate. The resultant diodes with an area of 7.85×10−5 cm2 exhibited excellent rectifying characteristics with an ideality factor of 1.21, a forward current density (J) of 127.4 A/cm2 at 1.4 V, a specific on-state resistance (Ron,sp) of 1.54 mΩ.cm2, and an ultra-high on/off ratio of 2.1×1011 at ±1 V. Due to a small depletion region in the highly-doped substrate, a breakdown feature was observed at −23 V, which corresponded to a breakdown field of 2.1 MV/cm and a power figure-of-merit of 3.4×105 W/cm2. Forward current–voltage characteristics were described well by the thermionic emission theory while thermionic field emission and trap-assisted tunneling were the dominant transport mechanisms at low and high reverse biases, respectively, which was a result of the contribution of deep–level traps at the metal–semiconductor interface. The presence of interfacial traps also caused the difference in Schottky barrier heights of 1.31 eV and 1.64 eV respectively determined by current–voltage and capacitance–voltage characteristics. With reduced trapping effect and incorporation of drift layers, the β-Ga2O3 SBDs could further provide promising materials for delivering both high current output and high breakdown voltage.

Synthesis of BiVO4 nanoflakes decorated with AuPd nanoparticles as selective oxidation photocatalysts

The ultrathin BiVO4 nanoflakes decorated with Pd and AuPd nanoparticles (NPs) were respectively synthesized and optimized for the enhanced photocatalysis towards selective oxidation of aromatic alcohols. The monometallic Pd(x)-BiVO4 samples presented hump-like variation in the photocatalytic activity with increasing Pd amount (x) from 0 to 2.0 wt%. Subsequently, coupling Au with Pd on BiVO4 nanoflakes resulted in a further improvement in the photocatalysis, with retaining the high selectivity (>99%) for aldehyde production. By tuning metal loading, the typical Au(0.5)Pd(0.5)-BiVO4 photocatalyst exhibited the highest benzaldehyde yield of 887.7 μmol·g−1·h−1, which was 6.0 times that of bare BiVO4 nanoflakes and 1.35 times that of Pd(1.0)-BiVO4 photocatalyst. A series of characterizations and DFT calculations confirmed the enhanced light harvesting and charge separation of the Au(0.5)Pd(0.5)-BiVO4 material, owing to the strong electronic couplings in AuPd NPs and its remarkable influence on the band structure of BiVO4. The photocatalytic mechanism studies indicated that the selective oxidation of aromatic alcohols was achieved by the cooperation of photogenerated holes and O2− radical, and this process was promoted by the interfacial synergism between AuPd NPs and BiVO4 nanoflakes. This work demonstrates a systematic study on optimizing photocatalysts to improve their performance in light-driven organic transformations as well as highlights the synergistic effect of metal-metal coupling and metal-semiconductor interface on photocatalysis.

Impact on photon-assisted charge carrier transport by engineering electrodes of GaN based UV photodetectors

A scheme illustrating electrode-engineering in metal-semiconductor-metal geometry is employed for fabricating GaN-based high-performance ultraviolet photodetectors. The effect of different metal-semiconductor (Au, Ti/Al and Al) junctions on photon-assisted charge-carrier transport and the current-conduction is explored. Depending upon work function of the metals, barrier height varies which contributed towards different characteristics of the device revealing linear and non-linear behaviour of the electrical measurements from the non-rectifying and rectifying contact at the metal-semiconductor interface respectively. A maximum photoresponsivity & efficiency of 280.4 mA/W and 107.04% respectively, under 5 V applied bias in the visible-blind region was measured from the device with rectifying Au electrodes. While, the photoresponsivity values of 219 mA/W and 200 mA/W under the same bias has been achieved from devices measured with near-Ohmic contacts using Ti/Al and Al electrodes, respectively. The enhancement in detector's performance with Au electrode has been attributed to the built-in electric field as well as the presence of internal gain which can promote efficient transport of charge carriers through the metal-semiconductor interface. Thus, an approach has been adopted to enhance the photoresponsivity of the fabricated GaN-based optoelectronic devices which could be utilized for wide range of industrial, military, environmental and biological applications.

Plasmonic Ag nanoparticles decorated Bi2S3 nanorods and nanoflowers: Their comparative assessment for photoelectrochemical water splitting

In the present work, nanostructures comprising of plasmonic silver (Ag) nanoparticles (NPs) decorated on the hydrothermally grown bismuth sulfide nanorods (Bi2S3–NR) and hierarchical nanoflowers (Bi2S3NF) have been successfully synthesized by chemical route. We have fabricated the nanostructured photoanodes and demonstrated their enhanced photoelectrochemical (PEC) performance for water splitting applications. Bismuth sulfide nanoflowers have shown appreciable increased PEC activity due to its hierarchical structure as compared to the nanorods facilitated by the faster charge transport due to the availability of more interfacial sites. Among all, Ag/Bi2S3NF photoanode has exhibited highest photocurrent density (12.34 mA/cm2 at 2 V vs. Ag/AgCl) which is ∼2 fold higher as compared to Bi2S3 nanoflowers. A ∼3 fold enhancement in the incident photon-to-current conversion efficiency is achieved by Ag/Bi2S3NF photoanode as compared to Bi2S3NF photoanode. The remarkable enhancement in the photocurrent density of Bi2S3 after grafting metal plasmons is attributed to its enhanced photoresponse, faster transfer of plasmon mediated hot electrons from excited state of Ag NPs to Bi2S3 surface and higher separation efficiency of photogenerated charge carriers facilitated by the metal-semiconductor interface. A plausible mechanism is also proposed for the improved PEC water splitting over Ag/Bi2S3NF photoanode.

Electrical Doping Effect of Vacancies on Monolayer MoS2

Doping of transition-metal dichalcogenides (TMDCs) is an effective way to tune the Fermi level to facilitate the band engineering required for different types of devices. For TMDCs, a controversy abounds with regard to the doping role played by vacancy-type defects. Here, we report a detailed study based on first-principles calculations proposing that the native sulfur vacancies (VS) can significantly alter the electrical doping level in MoS2 and tune the material to exhibit conventional n- or p-type semiconductor characteristics. In particular, we reveal that the lower concentration of the single VS (2.8 and 6.3%) yields p-type characteristics, whereas the higher concentration of the single VS or a cluster of VS (12.5, 18.8, and 25.0%) yields n-type characteristics. The trend is consistent with previous X-ray photoelectron spectroscopy and scanning tunneling microscopy results. Employing this method of tuning the electron doping level, we modeled a commonly used metal–semiconductor interface to demonstra...

Silicon-Based Embedded Trenches of Active Antennas for High-Responsivity Omnidirectional Photodetection at Telecommunication Wavelengths.

Although the use of plasmonic nanostructures for photodetection below the band gap energy of the semiconductor has been intensively investigated recently, efficiencies of such hot electron-based devices have, unfortunately, remained low because of the inevitable energy loss of the hot electrons as they move and transfer in active antennas based on metallic nanostructures. In this work, we demonstrate the concept of high-refractive-index material-embedded trench-like (ETL) active antennas that could be used to achieve almost 100% absorbance within the ultrashallow region (approximately 10 nm) beneath the metal-semiconductor interface, which is a much smaller distance compared with the hot electrons' mean free path in the noble metal layer. Taking advantage of these ETL-based active antennas, we obtained photoresponsivities under zero bias at wavelengths of 1310 and 1550 nm of 5854and 693 nA mW-1, respectively-values higher than most those previously reported for active antenna-based silicon (Si) photodetectors that operate at optical telecommunication wavelengths. Furthermore, the ETL antenna strategy allowed us to preserve an omnidirectional and broadband photoresponse, with a superior degree of detection linearity of R2 = 0.98889 under the light of low power density (down to 11.1 μW cm-2). The photoresponses of the ETL antenna-based device varied by less than 10% upon changing the incident angle from normal incidence to 60°. Because these ETL-based devices provide high responsivity and omnidirectional detection over a broad bandwidth, they show promising potentials for use in hot electron-based optoelectronics for many applications (e.g., Si photonics, energy harvesting, photocatalysis, and sensing devices).

Impact of chemical interface damping on surface plasmon dephasing.

The excellent light harvesting ability of plasmonic nanoparticles makes them promising materials for a variety of technologies that rely on the conversion of photons to energetic charge carriers. In such applications, including photovoltaics and photocatalysis, the excitation of surface plasmons must induce charge transfer across the metal-adsorbate or metal-semiconductor interface. However, there is currently a lack of molecular level understanding of how the presence of a chemical interface impacts surface plasmon dephasing pathways. Here, we report an approach to quantitatively measure the influence of molecular adsorption on the spectral shape and intensity of the extinction, scattering, and absorption cross-sections for nanostructured plasmonic surfaces. This is demonstrated for the case of thiophenol adsorption on lithographically patterned gold nanodisk arrays. The results show that the formation of a chemical interface between thiophenol and Au causes surface plasmons to decay more prominently through photon absorption rather than photon scattering, as compared to the bare metal. We propose that this effect is a result of the introduction of adsorbate-induced allowable electronic transitions at the interface, which facilitate surface plasmon dephasing via photon absorption. The results suggest that designed chemical interfaces with well-defined electronic structures may enable engineering of hot electron distributions, which could be important for understanding and controlling plasmon-mediated photocatalysis and, more generally, hot carrier transfer processes.

Leakage Current by Poole–Frenkel Emission in Pt Schottky Contacts on () β-Ga2O3 Grown by Edge-Defined Film-Fed Growth

We report an investigation of the leakage current in Pt/β-Ga2O3 Schottky contacts on an () substrate using a temperature-dependent current–voltage (I-V-T) measurement in a temperature range of 300 K to 425 K. It was revealed that the main process of the reverse leakage current flow is the emission of electrons through trap states located near the metal-semiconductor interface, governed by the Poole-Frenkel emission (PFE). Based on an etch pits analysis, the density of the defects at the surface of the () β-Ga2O3 substrates was estimated to be ∼2 × 105 cm−2. The trap states associated with these high-density defects, which could be the main culprit for the reverse leakage current in the Pt/β-Ga2O3 Schottky contacts, were identified at ∼ 0.7 eV below the conduction band.

Determination of Surface States Energy Density Distributions and Relaxation Times for a Metal-Polymer-Semiconductor Structure

The capacity/conductivity-voltage (C/G-V) data of the PVP interlayered metal-semiconductor structure were examined at ±4V biases and 1-500 kHz frequency interval at room temperature. The surface states ( $N_{ss}$ ) in the insulation layer and the metal-semiconductor interface are seen as the main causes of high capacitance and conductivity values at low frequencies. AC signals, which result in an increase in capacitance and conductivity values, are easily monitored by $N_{ss}$ at low frequencies. The graph extracted against the conductivity and frequency logarithm ( $G_{p}$ /ω-log(f)) indicates a peak appearance as a result of the $N_{ss}$ effect. In the energy range of ( $E_{c}$ −0.423) − ( $E_{c}$ −0.604), surface states ( $N_{ss}$ ) and relaxation times (τ) values alter from 8.71 × 1011 to 5.84 × 1011 eV−1 cm−2 and 6.54 × 10−6 s to 1.10 × 10−4 s, respectively.

Comparative study of n-GaN transition group refractory metal Ohmic electrode

Ohmic contact is directly related to the performance of GaN device and is one of the important factors affecting device performance. In recent years, many research groups have studied the electrode materials and annealing conditions of n-type GaN Ohmic contacts. In this paper, the ohmic contact properties and structural characteristics of the Hf/Al electrode of a transition group metal refractory metal Hf system under different annealing conditions are studied, and compared with those of the Ti-based ohmic contact Ti/Al electrode. The specific contact resistivity of each electrode is measured by a dot-type transmission line model, and the structural characteristics of the electrode are analyzed by using an Auger electron spectrometer which can be analyzed in depth. The results show that the Hf/Al electrode under the same annealing condition exhibits superior ohmic contact performance compared with the conventional Ti/Al electrode. At the same time, the lowest specific contact resistivity of the Hf/Al electrode annealed in an N<sub>2</sub> atmosphere at a low temperature of 650 ℃ for 60 s is 4.28×10<sup>–5</sup> Ω·cm<sup>2</sup>. The in-depth analysis of Auger electron spectrum shows that the Hf/Al electrode has a solid phase reaction with the n-type GaN material. In addition, the cross section of each electrode is observed by auger electron spectroscopy. In the Hf/Al electrode sample, the metal-semiconductor interface does not show voids after annealing. This situation occurs at the sample interface where the Ti/Al electrode is annealed at 650 ℃ for 60 s in N<sub>2</sub> atmosphere and annealed at 850 ℃ for 30 s in N<sub>2</sub> atmosphere. This is one of the reasons why the Hf/Al electrode sample has a lower specific contact resistivity. At the same time, the surface of Hf/Al electrode and Ti/Al electrode annealed at 850 ℃ are characterized by using scanning electron microscope. It is found that the surfaces of both electrodes subject to high temperature annealing show a similar granular rough surface, and this rough surface has a certain influence on the electrical properties of the GaN device. The rough surface formed by the electrode under such high temperature annealing conditions is an urgent problem to be solved in the future research. In summary, the study in this paper indicates the use of Hf/Al to form an ohmic contact with n-type GaN under a low temperature annealing condition.

Ultrathin MoS2 nanosheets for high-performance photoelectrochemical applications via plasmonic coupling with Au nanocrystals.

In this work, we prepared ultrathin MoS2 nanosheets with exposed active edge sites and high electric conductivity that can sufficiently absorb light in the visible region to enable solar energy conversion. The gold nanocrystal-decorated MoS2 nanosheets facilitate sufficiently enhanced photoelectrochemical water splitting in the UV-visible region. Different Au nanostructures, such as Au nanoparticles and nanorods, were modified on the surface of MoS2 nanosheets to promote photoelectrochemical water decomposition. By spin-coating a synthetic gold-modified MoS2 hybrid photoanode on a FTO substrate, the efficiency of photoelectrochemical water oxidation was significantly enhanced, by 2 times (nanorods) and 3.5 times (nanoparticles) in the visible-infrared region; furthermore, the average optical resistance was reduced by a factor of two compared to the MoS2 photoanode without Au, and the photocurrent increases exponentially when the system bias was greater than 0.7 volts. The Au-MoS2 metal-semiconductor interface plays an important role in studying the surface plasmon interactions, charge transfer mechanism, and electric field amplification. This rational design for such a unique hybrid nanostructure explains the plasmon-enhanced photoelectrochemical water splitting. This current contribution provides a new path for using the plasmonic metal/semiconductor heterostructure to effectively harvest UV-visible light for solar fuel generation.