Semiconductor Substrate Research Articles

Structural measurement of electron-phonon coupling and electronic thermal transport across a metal-semiconductor interface

Scattering of energetic charge carriers and their coupling to lattice vibrations (phonons) in dielectric materials and semiconductors are crucial processes that determine the functional limits of optoelectronics, photovoltaics, and photocatalysts. The strength of these energy exchanges is often described by the electron-phonon coupling coefficient, which is difficult to measure due to the microscopic time- and length-scales involved. In the present study, we propose an alternate means to quantify the coupling parameter along with thermal boundary resistance and electron conductivity by performing a high angular-resolution time-resolved X-ray diffraction measurement of propagating lattice deformation following laser excitation of a nanoscale, polycrystalline metal film on a semiconductor substrate. Our data present direct experimental evidence for identifying the ballistic and diffusive transport components occurring at the interface, where only the latter participates in thermal diffusion. This approach provides a robust measurement that can be applied to investigate microscopic energy transport in various solid-state materials.

Fast and Regulated Zinc Deposition in a Semiconductor Substrate toward High‐Performance Aqueous Rechargeable Batteries

AbstractRechargeable aqueous zinc‐ion batteries are considered as ideal candidates for large‐scale energy storage due to their high safety, eco‐friendliness, and low cost. However, Zn anode invites dendrite growth and parasitic reactions at anode‐electrolyte interface, impeding the practical realization of the battery. In this study, the electrochemical performance of the Zn‐metal anode is proposed to improve by using a 3D ZnTe semiconductor substrate. The substrate features high zincophilicity, high electronic conductivity and electron affinity, and a low Zn nucleation energy barrier to promote dendrite‐proof Zn deposition along the (002) crystal plane, while it also maintains high chemical stability against parasitic metal corrosion and hydrogen evolution reactions at surface, and a stable skeleton structure against the volume variation of anode. A Zn‐metal anode based on the telluride substrate shows a long cycle life of over 3300 h with a small voltage hysteresis of 48 and 320 mV at 1 and 30 mA cm−2, respectively. A zinc telluride@Zn//MnO2 full cell can operate for over 500 cycles under practical conditions in terms of lean electrolyte (18 µL mAh−1) and limited Zn metal ( negative/positive capacity ratio of 3:1, and a high mass loading of the cathode.

Facet junction of CeO2 with high SERS activity for sensitive detection of ATP

Crystal facet engineering is proven to improve charge transfer of semiconductors. However, the facet-dependent properties and potential mechanisms of CeO2 for surface-enhanced Raman scattering (SERS) performance have not been explored. SERS effect is a surface sensitive technology, so it is very important to study the relationship between semiconductor surface topography and SERS activity. Here, we synthesized CeO2 (J2) with co-exposed {100} and {111} facet junction to effectively promote charge transfer (CT) between CeO2 (J2) substrate and methylene blue (MB), so that the SERS activity is significantly enhanced compared to CeO2 with only one exposed facets, which had a higher SERS activity with an enhancement factor (EF) of 3.77 × 106. The phenomenon is due to that the facet junction of CeO2 (J2) can promote electron-hole (e--h+) separation under light irradiation for more efficient CT. In addition, the oxygen vacancies enriched in CeO2 (J2) can provide an extra charge transfer path, which also facilitates the enhancement of SERS activity. Furthermore, the CeO2 (J2) with excellent SERS activity was used to detect ATP with a detection limit of 6.90 nM. This work provides a new SERS platform and opens up a perspective for preparation of SERS semiconductor substrates.

Regulating the work function of Cu2O films via crystal facet engineering with enhanced charge transfer and SERS activity

Semiconductors have been extensively studied for SERS applications owing to the advantage of inexpensive, nontoxic and easy fabrication. However, the facile design of uniformity substrates with enhanced charge transfer and SERS activity are highly desirable. Herein, we reported the fabrication of Cu2O films with (100) and (111) facets exposed by adjusting the pH value of electrolyte. When using 4–NBT as probe molecule, the Cu2O(100) exhibits an improvement of enhancement factor about 2.7 times than Cu2O(111), as well as relatively low limit of detection up to 1 × 10−7 M. All calculated relative standard deviation values are lower than 5 %, which demonstrates the uniformity and reproducibility of as–prepared Cu2O films. UPS analysis reveals that (100) facet possesses lower electron work function, and can promote the efficient interfacial charge transfer. Furthermore, First–principles calculation confirms that (100) facet possess the better adsorption ability for 4–NBT, which can transfer more electrons from Cu2O to the molecule due to its lower work function, and thus resulting in the remarkable improvement of SERS activity. This research will offer more choices for the design of uniformity semiconductor substrates with enhanced SERS sensitivity based on crystal facet engineering.

Capacitance–voltage characterization of metal–insulator–semiconductor capacitors formed on wide-bandgap semiconductors with deep dopants such as diamond

As diamond possesses only deep dopants, certain conventional physics and characterization methods are not applicable to diamond devices, owing to the explicit or implicit assumption of shallow dopants. To resolve this limitation, the capacitance–voltage (C–V) characteristics of metal–insulator–semiconductor (MIS) capacitors formed on a semiconductor substrate with deep and compensating dopants were successfully formulated. Based on these equations, methods for accurately estimating the MIS capacitor properties were developed and validated through their application in the analysis of an actual MIS capacitor formed on a boron-doped hydrogen-terminated diamond substrate. The high-frequency C–V characteristic of the capacitor exhibited a prominent dip specific to deep dopants. However, the dip depth was considerably shallower than theoretically expected. This C–V characteristic was accurately reproduced theoretically, assuming the presence of a surficial diamond layer that contains acceptors with an activation energy of 0.23 eV, which is less than the value 0.37 eV for boron, and has a thickness of the extrinsic Debye length (40 nm in this study) or larger. The insulator charge of the MIS capacitor was estimated as −4.6 × 1012 cm−2 in units of electronic charge, which is sufficiently large to induce two-dimensional hole gas. The interface-state density was 1.4 × 1012 cm−2 eV−1 for interface-state energies of 0.3–0.5 eV above the valence band maximum. Hence, the proposed methodology and the possible presence of the reduced activation energy layer will guide the development of diamond-based devices.

A first-principles study on remote van der Waals epitaxy through a graphene monolayer on semiconductor substrates

To investigate the mechanism of remote epitaxy, where the overlayer can follow the same crystalline structure as the underlying semiconductor substrate through a thin two-dimensional interlayer, we systematically study the potential fluctuations of graphene covered Si, GaAs, and GaN substrates from first-principles. We find that the uneven semiconductor surface, the distorted graphene, and the non-uniform interface charge transfer make significant contributions to the potential fluctuation. The semiconductor substrate with different surface reconstructions and orientations will generate different potential fluctuations through the graphene interlayer. We also calculate and compare the adsorption of adatoms on graphene covered substrates. The adsorption energies of adatoms not only depend on their distances to the underlying semiconductor surface, but are also sensitive to the direction of the charge transfer at the graphene/substrate interface. Changing the semiconductor reconstruction or orientation could even reverse the order of the adsorption energies of cation and anion adatoms by reversing the interface charge transfer direction, leading to a change in the growth orientation of the overlayer. Our study improves the understanding of the mechanism of remote epitaxy, and reveals that it is possible to control the initial nucleation and orientation of overlayers by changing the semiconductor reconstructions and/or orientations in remote epitaxy.

Bi2Se3 Growth on (001) GaAs Substrates for Terahertz Integrated Systems.

Terahertz (THz) technologies have been of interest for many years due to the variety of applications including gas sensing, nonionizing imaging of biological systems, security and defense, and so forth. To date, scientists have used different classes of materials to perform different THz functions. However, to assemble an on-chip THz integrated system, we must understand how to integrate these different materials. Here, we explore the growth of Bi2Se3, a topological insulator material that could serve as a plasmonic waveguide in THz integrated devices, on technologically important GaAs(001) substrates. We explore surface treatments and find that an atomically smooth GaAs surface is critical to achieving high-quality Bi2Se3 films despite the relatively weak film/substrate interaction. Calculations indicate that the Bi2Se3/GaAs interface is likely selenium-terminated and shows no evidence of chemical bonding between the Bi2Se3 and the substrate. These results are a guide for integrating van der Waals materials with conventional semiconductor substrates and serve as the first steps toward achieving an on-chip THz integrated system.

Beam size estimation method for mid-infrared lasers using silicon-based photonic semiconductor image sensor

A beam size estimation method for mid-infrared lasers is proposed. The light-receiving surface of a silicon-based photonic semiconductor image sensor is irradiated with a mid-infrared laser beam to be measured. The semiconductor substrate is heated and excites thermal carriers at the laser incident spot. The distribution of excited carriers is captured and visualized as a monochromatic image by the image sensor. Demonstration using a carbon dioxide laser showed a correlation between the diameter of distributed thermal carriers and that of the incident laser beam with Gaussian distribution. The results indicated that the proposed method is effective for the estimation of beam diameter of mid-infrared lasers.

Pit rim decomposition into multiple quantum dots on surfaces of epitaxial thin films grown on pit-patterned substrates

We report results of dynamical simulations according to an experimentally validated surface morphological evolution model on the formation of regular arrays of quantum dot molecules (QDMs) consisting of 1D arrays of smaller interacting quantum dots (QDs). These QD arrays form along the sides of each pit rim on the surface of a coherently strained thin film epitaxially deposited on a semiconductor substrate, the surface of which consists of a periodic pattern of inverted prismatic pits with rectangular pit openings. We find that this complex QDM pattern results from the decomposition of the pit rim from a “quantum fortress” with four elongated QDs into four 1D arrays of multiple smaller QDs arranged along each side of the pit rim. Systematic parametric analysis indicates that varying the pit opening dimensions and the pit wall inclination directly impacts the number of QDs in the resulting QDM pattern, while varying the pit depth only affects the dimensions of the QDs in the nanostructure pattern. Therefore, the number, arrangement, and sizes of QDs in the resulting pattern of QDMs on the film surface can be engineered precisely by proper tuning of the pit design parameters. Our simulation results are supported by predictions of morphological stability analysis, which explains the pit rim decomposition into multiple QDs as the outcome of a tip-splitting instability and provides a fundamental characterization of the post-instability nanostructure pattern. Our theoretical findings can play a vital role in designing optimal semiconductor surface patterns toward enabling future nanofabrication technologies.

Coupling light emission of single-photon sources into single-mode fibers: mode matching, coupling efficiencies, and thermo-optical effects.

We discuss the coupling efficiency of single-photon sources into single-mode fibers using 3D printed micro-optical lens designs. Using the wave propagation method, we optimize lens systems for two different quantum light sources and assess the results in terms of maximum coupling efficiencies, misalignment effects, and thermo-optical influences. Thereby, we compare singlet lens designs with one lens printed onto the fiber with doublet lens designs with an additional lens printed onto the semiconductor substrate. The single-photon sources are quantum dots based on microlenses and circular Bragg grating cavities at 930 nm and 1550 nm, respectively.

Band Structure Engineering within Two-Dimensional Borocarbonitride Nanosheets for Surface-Enhanced Raman Scattering.

Herein, with two-dimensional (2D) borocarbonitride (BCN) as a metal- and plasmon-free surface-enhanced Raman scattering (SERS) platform, we demonstrate a band structure engineering strategy to facilitate the charge transfer process for an enhanced SERS response. Especially, when the conduction band of the BCN substrate is tuned to align with the LUMO of the target molecule, remarkable SERS performance is achieved, ascribed to the borrowing effect from the vibronic coupling of resonances through the Herzberg-Teller coupling term. Meanwhile, fluorescence quenching is achieved due to the efficient charge transfer between the BCN substrate and target molecule. Consequently, BCN can accurately detect 20 kinds of trace chemical and bioactive analytes. Moreover, BCN exhibits excellent thermal and chemical stability, which can not only withstand high-temperature (300 °C) heating in the air but also resist long-term corrosion in harsh acid (pH = 0, HCl) and base (pH = 14, NaOH). This work provides new insight into band structure engineering in promoting the SERS performance of plasmon- and metal-free semiconductor substrates.

The Theory of Surface-Enhanced Raman Spectroscopy on Organic Semiconductors: Graphene.

Drawing on a theoretical expression previously derived for general semiconductor substrates, we examine the surface-enhancement of the Raman signal (SERS) when the substrate is chosen to be monolayer graphene. The underlying theory involves vibronic coupling, originally proposed by Herzberg and Teller. Vibronic coupling of the allowed molecular transitions with the charge-transfer transitions between the molecule and the substrate has been shown to be responsible for the SERS enhancement in semiconductor substrates. We then examine such an expression for the Raman enhancement in monolayer graphene, which is dependent on the square of the derivative of the density of states of the graphene. On integration, we find that the discontinuity of the density-of-states function leads to a singularity in the SERS intensity. Knowledge of the location of this resonance allows us to maximize the Raman intensity by careful alignment of the doping level of the graphene substrate with the charge-transfer transition.

High-Speed Interconnects: History, Evolution, and the Road Ahead

An integrated circuit (IC), or chip, is a set of electronic circuits and components placed on a tiny planar silicon (Si) semiconductor substrate. These electronics circuits and components are electrically connected to one another with the help of planar and vertical conductor elements known as <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">interconnects</i> . This article presents a detailed overview of the evolution of interconnects, highlighting the technological milestones that have been achieved during the past four decades. Interconnects form the backbone of signal communication among devices on a chip and between chips on a printed circuit board (PCB). The layout and design of interconnects on an IC are vital to the chip’s performance, functionality, reliability, efficiency, and overall fabrication yield.

Tuning lower dimensional superconductivity with hybridization at a superconducting-semiconducting interface

The influence of interface electronic structure is vital to control lower dimensional superconductivity and its applications to gated superconducting electronics, and superconducting layered heterostructures. Lower dimensional superconductors are typically synthesized on insulating substrates to reduce interfacial driven effects that destroy superconductivity and delocalize the confined wavefunction. Here, we demonstrate that the hybrid electronic structure formed at the interface between a lead film and a semiconducting and highly anisotropic black phosphorus substrate significantly renormalizes the superconductivity in the lead film. Using ultra-low temperature scanning tunneling microscopy and spectroscopy, we characterize the renormalization of lead’s quantum well states, its superconducting gap, and its vortex structure which show strong anisotropic characteristics. Density functional theory calculations confirm that the renormalization of superconductivity is driven by hybridization at the interface which modifies the confinement potential and imprints the anisotropic characteristics of the semiconductor substrate on selected regions of the Fermi surface of lead. Using an analytical model, we link the modulated superconductivity to an anisotropy that selectively tunes the superconducting order parameter in reciprocal space. These results illustrate that interfacial hybridization can be used to tune superconductivity in quantum technologies based on lower dimensional superconducting electronics.

Nonstoichiometric tungsten oxide nanosheets with abundant oxygen vacancies for defects‐driven SERS sensing

AbstractSemiconductor materials have gradually developed into attractive candidate substrates for surface‐enhanced Raman spectroscopy (SERS) due to their unique characteristics, exhibiting great application potential in a wide range of scenarios. The introduction of oxygen vacancies plays a key role in improving the performance of semiconductor substrates by effectively modulating the surface state and electronic structure of semiconductor materials. Here, a reduction method is employed to prepare nonstoichiometric WO3−x nanosheets with abundant oxygen vacancies, which as a substrate material possesses excellent SERS sensing performance and outstanding signal reproducibility. The presence of surface deficiency sites generates an effective photoinduced charge transfer resonance at the interface between the substrate and the analyte molecules by offering more charge transfer pathways as well as strong vibration coupling, thereby achieving significantly improved SERS sensitivity on the WO3−x substrate. Further investigations suggest that the defect structure is beneficial to facilitate the recognition of multiple probe molecules, which indicates semiconductor materials have great potential in SERS detection.

Near-infrared sensitive two-wave mixing adaptive interferometer based on a liquid crystal light valve with a semiconductor substrate.

Two-wave mixing adaptive interferometer based on a liquid crystal light valve with a semiconductor GaAs substrate is realized and studied at 1064 nm wavelength. The local response of the dynamic hologram recorded in the liquid crystal layer of the light valve allows for detection of small phase modulations of the object beam. The characteristics of the interferometer are estimated experimentally. The temporal adaptability lies in the subsecond range. The large optical nonlinearity of the cell is favorable for measurements of small displacements with high sensitivity.

Molybdenum Nitride Porous Prisms with a Strong Plasmon Resonance Effect in the Visible Region for Surface-Enhanced Raman Spectroscopy

In surface-enhanced Raman spectroscopy (SERS) detection, the structure of the Raman-scattering substrate is critical to the sensitivity and stability of the detector. Herein, molybdenum nitride (MoN) porous structures with a well-defined hexagonal prism shape were synthesized via a precursor nitriding route. As a typical metallic transition-metal nitride (TMN), these molybdenum nitride porous hexagonal prisms exhibit a rare strong SPR effect in the visible region, with a resonance peak centered at 534 nm. Benefiting from the strong SPR effect and their huge surface area and porosity, these MoN porous hexagonal prisms exhibit surface-enhanced Raman scattering effects comparable to those of noble metals, with a Raman enhancement factor of 5.5 × 106. More importantly, these MoN SERS substrates exhibit ultrahigh chemical stabilities that noble metal and semiconductor substrates do not possess, which can prevent corrosion by strong acids, alkalis, and high-temperature oxidation.

Zinc Titanium Nitride Semiconductor toward Durable Photoelectrochemical Applications.

Photoelectrochemical fuel generation is a promising route to sustainable liquid fuels produced from water and captured carbon dioxide with sunlight as the energy input. Development of these technologies requires photoelectrode materials that are both photocatalytically active and operationally stable in harsh oxidative and/or reductive electrochemical environments. Such photocatalysts can be discovered based on co-design principles, wherein design for stability is based on the propensity for the photocatalyst to self-passivate under operating conditions and design for photoactivity is based on the ability to integrate the photocatalyst with established semiconductor substrates. Here, we report on the synthesis and characterization of zinc titanium nitride (ZnTiN2) that follows these design rules by having a wurtzite-derived crystal structure and showing self-passivating surface oxides created by electrochemical polarization. The sputtered ZnTiN2 thin films have optical absorption onsets below 2 eV and n-type electrical conduction of 3 S/cm. The band gap of this material is reduced from the 3.36 eV theoretical value by cation-site disorder, and the impact of cation antisites on the band structure of ZnTiN2 is explored using density functional theory. Under electrochemical polarization, the ZnTiN2 surfaces have TiO2- or ZnO-like character, consistent with Materials Project Pourbaix calculations predicting the formation of stable solid phases under near-neutral pH. These results show that ZnTiN2 is a promising candidate for photoelectrochemical liquid fuel generation and demonstrate a new materials design approach to other photoelectrodes with self-passivating native operational surface chemistry.

Mixed valence Ce-doped TiO2 with multiple energy levels and efficient charge transfer for boosted SERS performance

Considering the variable valence characteristics of rare earth elements, they can be in a variety of valence forms coexistence. Doping of rare earth element with different valence states may produce different energy levels to tune the semiconductor energy band structure. We utilize rare earth element Ce doping TiO2 for the development of high-performance semiconductor surface-enhanced Raman scattering (SERS) substrates based on an energy-level tuning strategy. Ce doping not only forms multiple energy levels including Ce3+ and Ce4+ metal doping energy levels in the bandgap of TiO2, but also enriches the surface state level of TiO2 itself, which together promote the separation of photogenerated carriers and improve charge transfer efficiency between substrates and absorbed molecules. This endows TiO2 semiconductor substrate with a higher SERS enhancement factor, which can reach 2.2 × 106. The detectable concentration of methylene blue can be as low as 10−10 mol/L. Moreover, the semiconductor substrate exhibits excellent uniformity and stability. This study not only provides a new strategy to develop excellent semiconductor SERS substrate with multiple energy levels, but also lays the foundation for promising practical application of semiconductor substrate.

Tracking interfacial changes of graphene/Ge(1 1 0) during in-vacuum annealing

Graphene quality indicators obtained by Raman spectroscopy have been correlated to the structural changes of the graphene/germanium interface as a function of in-vacuum thermal annealing. Specifically, it was found that graphene becomes markedly defective at 650 °C. By combining scanning tunneling microscopy, X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure spectroscopy, we concluded that these defects are due to the release of H2 gas trapped at the graphene/germanium interface. The H2 gas was produced following the transition from the as-grown hydrogen-termination of the Ge(110) surface to the emergence of surface reconstructions in the substrate. Interestingly, a complete self-healing process was observed in graphene upon annealing at 800 °C. The identified subtle interplay between the microscopic changes occurring at the graphene/germanium interface and graphene’s defect density is integral to advancing the understanding of graphene growth directly on semiconductor substrates, controlled 2D-3D heterogeneous materials interfacing and integrated fabrication technology.