Investigation of the Interfaces in Schottky Diodes Using Equivalent Circuit Models

The metal–semiconductor contact is one of the most critical factors that determine the performance of semiconductor devices such as Schottky barrier diodes (SBDs). SBDs between conductive carbon thin films and silicon have attracted attention due to their high performance and potential low cost of fabrication. Here, we introduce impedance spectroscopy (IS) as a powerful technique to characterize such SBDs. The electrical and structural characteristics of carbon–silicon SBDs between silicon and two different types of conductive carbon thin films have been investigated. Modeling the data with an extended equivalent circuit model reveals the effects of the metal electrode contacts of SBDs for the first time. From dc current–voltage measurements, diode parameters including the ideality factor, the Schottky barrier height, and the series resistance are extracted. Through use of analysis with IS, additional information on the Schottky contact is obtained, such as the built-in potential and more reliable barrier height values. Thus, IS can be utilized to analyze interfaces between metals and semiconductors in great detail by electrical means

Chemically Modulated Graphene Diodes

We report the manufacture of novel graphene diode sensors (GDS), which are composed of monolayer graphene on silicon substrates, allowing exposure to liquids and gases. Parameter changes in the diode can be correlated with charge transfer from various adsorbates. The GDS allows for investigation and tuning of extrinsic doping of graphene with great reliability. The demonstrated recovery and long-term stability qualifies the GDS as a new platform for gas, environmental, and biocompatible sensors

Effects of Excitonic Resonance on Second and Third Order Nonlinear Scattering from Few-Layer MoS₂

Nonlinear optical scattering from single- and few-layer MoS₂ contains important information about the orientation, inversion symmetry, and degree of interlayer coupling between the layers. We simultaneously map second harmonic generation (SHG) and four wave mixing (FWM) signals in chemical vapor deposition (CVD) grown 2H-phase MoS₂ from single to five layers. We tune the excitation wavelengths to compare cases where the nonlinear signals are on and off resonance with the <i>A</i>-exciton band. The SHG signal shows the expected 4-fold symmetry, however, the FWM signal depends on the incident laser polarization only, and is independent of the crystallographic orientation. We show using the symmetry of the χ⁽³⁾ tensor that this results from out of plane FWM dipoles. We explore the scaling of SHG and FWM signals with layer number on and off excitonic resonance When a nonlinear scattered signal overlaps with the <i>A</i> excitonic band, the scaling of the signals with layer number deviates from the expected values, due to the layer dependent red shift in the exciton absorption peak due to interlayer coupling. Finally we show that circularly polarized excitation significantly enhances nonlinear scattering which overlaps with the <i>A</i> excitonic band and indicates the presence of spin splitting of valence bands at the energy degenerate points (<i>K</i>, <i>K</i>′) of the Brillouin zone

Effects of Excitonic Resonance on Second and Third Order Nonlinear Scattering from Few-Layer MoS₂

Nonlinear optical scattering from single- and few-layer MoS₂ contains important information about the orientation, inversion symmetry, and degree of interlayer coupling between the layers. We simultaneously map second harmonic generation (SHG) and four wave mixing (FWM) signals in chemical vapor deposition (CVD) grown 2H-phase MoS₂ from single to five layers. We tune the excitation wavelengths to compare cases where the nonlinear signals are on and off resonance with the <i>A</i>-exciton band. The SHG signal shows the expected 4-fold symmetry, however, the FWM signal depends on the incident laser polarization only, and is independent of the crystallographic orientation. We show using the symmetry of the χ⁽³⁾ tensor that this results from out of plane FWM dipoles. We explore the scaling of SHG and FWM signals with layer number on and off excitonic resonance When a nonlinear scattered signal overlaps with the <i>A</i> excitonic band, the scaling of the signals with layer number deviates from the expected values, due to the layer dependent red shift in the exciton absorption peak due to interlayer coupling. Finally we show that circularly polarized excitation significantly enhances nonlinear scattering which overlaps with the <i>A</i> excitonic band and indicates the presence of spin splitting of valence bands at the energy degenerate points (<i>K</i>, <i>K</i>′) of the Brillouin zone

Effects of Excitonic Resonance on Second and Third Order Nonlinear Scattering from Few-Layer MoS₂

Nonlinear optical scattering from single- and few-layer MoS₂ contains important information about the orientation, inversion symmetry, and degree of interlayer coupling between the layers. We simultaneously map second harmonic generation (SHG) and four wave mixing (FWM) signals in chemical vapor deposition (CVD) grown 2H-phase MoS₂ from single to five layers. We tune the excitation wavelengths to compare cases where the nonlinear signals are on and off resonance with the <i>A</i>-exciton band. The SHG signal shows the expected 4-fold symmetry, however, the FWM signal depends on the incident laser polarization only, and is independent of the crystallographic orientation. We show using the symmetry of the χ⁽³⁾ tensor that this results from out of plane FWM dipoles. We explore the scaling of SHG and FWM signals with layer number on and off excitonic resonance When a nonlinear scattered signal overlaps with the <i>A</i> excitonic band, the scaling of the signals with layer number deviates from the expected values, due to the layer dependent red shift in the exciton absorption peak due to interlayer coupling. Finally we show that circularly polarized excitation significantly enhances nonlinear scattering which overlaps with the <i>A</i> excitonic band and indicates the presence of spin splitting of valence bands at the energy degenerate points (<i>K</i>, <i>K</i>′) of the Brillouin zone

Saturation of Two-Photon Absorption in Layered Transition Metal Dichalcogenides: Experiment and Theory

The saturation of two-photon absorption (TPA) in four types of layered transition metal dichalcogenides (TMDCs) (MoS₂, WS₂, MoSe₂, WSe₂) was systemically studied both experimentally and theoretically. It was demonstrated that the TPA coefficient is decreased when either the incident pulse intensity or the thickness of the TMDC nanofilms increases, while TPA saturation intensity has the opposite behavior, under the excitation of 1.2 eV photons with a pulse width of 350 fs. A three-level excitonic dynamics simulation indicates that the fast relaxation of the excitonic dark states, the exciton–exciton annihilation, and the depletion of electrons in the ground state contribute significantly to TPA saturation in TMDC nanofilms. Large third-order nonlinear optical responses make these layered 2D semiconductors strong candidate materials for optical modulation and other photonic applications

Effects of Excitonic Resonance on Second and Third Order Nonlinear Scattering from Few-Layer MoS₂

Nonlinear optical scattering from single- and few-layer MoS₂ contains important information about the orientation, inversion symmetry, and degree of interlayer coupling between the layers. We simultaneously map second harmonic generation (SHG) and four wave mixing (FWM) signals in chemical vapor deposition (CVD) grown 2H-phase MoS₂ from single to five layers. We tune the excitation wavelengths to compare cases where the nonlinear signals are on and off resonance with the <i>A</i>-exciton band. The SHG signal shows the expected 4-fold symmetry, however, the FWM signal depends on the incident laser polarization only, and is independent of the crystallographic orientation. We show using the symmetry of the χ⁽³⁾ tensor that this results from out of plane FWM dipoles. We explore the scaling of SHG and FWM signals with layer number on and off excitonic resonance When a nonlinear scattered signal overlaps with the <i>A</i> excitonic band, the scaling of the signals with layer number deviates from the expected values, due to the layer dependent red shift in the exciton absorption peak due to interlayer coupling. Finally we show that circularly polarized excitation significantly enhances nonlinear scattering which overlaps with the <i>A</i> excitonic band and indicates the presence of spin splitting of valence bands at the energy degenerate points (<i>K</i>, <i>K</i>′) of the Brillouin zone

A New 2H-2H′/1T Cophase in Polycrystalline MoS₂ and MoSe₂ Thin Films

We report on 2H-2H′/1T phase conversion of MoS₂ and MoSe₂ polycrystalline films grown by thermally assisted conversion. The structural conversion of the transition metal dichalcogenides was successfully carried out by organolithium treatment on chip. As a result we obtained a new 2H-2H′/1T cophase system of the TMDs thin films which was verified by Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The conversion was successfully carried out on selected areas yielding a lateral heterostructure between the pristine 2H phase and the 2H′/1T cophase regions. Scanning electron microscopy and atomic force microscopy revealed changes in the surface morphology and work function of the cophase system in comparison to the pristine films, with a surprisingly sharp lateral interface region

Highly Sensitive Electromechanical Piezoresistive Pressure Sensors Based on Large-Area Layered PtSe₂ Films

Two-dimensional (2D) layered materials are ideal for micro- and nanoelectromechanical systems (MEMS/NEMS) due to their ultimate thinness. Platinum diselenide (PtSe₂), an exciting and unexplored 2D transition metal dichalcogenide material, is particularly interesting because its low temperature growth process is scalable and compatible with silicon technology. Here, we report the potential of thin PtSe₂ films as electromechanical piezoresistive sensors. All experiments have been conducted with semimetallic PtSe₂ films grown by thermally assisted conversion of platinum at a complementary metal–oxide–semiconductor (CMOS)-compatible temperature of 400 °C. We report high negative gauge factors of up to −85 obtained experimentally from PtSe₂ strain gauges in a bending cantilever beam setup. Integrated NEMS piezoresistive pressure sensors with freestanding PMMA/PtSe₂ membranes confirm the negative gauge factor and exhibit very high sensitivity, outperforming previously reported values by orders of magnitude. We employ density functional theory calculations to understand the origin of the measured negative gauge factor. Our results suggest PtSe₂ as a very promising candidate for future NEMS applications, including integration into CMOS production lines

High-Performance Hybrid Electronic Devices from Layered PtSe₂ Films Grown at Low Temperature

Layered two-dimensional (2D) materials display great potential for a range of applications, particularly in electronics. We report the large-scale synthesis of thin films of platinum diselenide (PtSe₂), a thus far scarcely investigated transition metal dichalcogenide. Importantly, the synthesis by thermally assisted conversion is performed at 400 °C, representing a breakthrough for the direct integration of this material with silicon (Si) technology. Besides the thorough characterization of this 2D material, we demonstrate its promise for applications in high-performance gas sensing with extremely short response and recovery times observed due to the 2D nature of the films. Furthermore, we realized vertically stacked heterostructures of PtSe₂ on Si which act as both photodiodes and photovoltaic cells. Thus, this study establishes PtSe₂ as a potential candidate for next-generation sensors and (opto-)­electronic devices, using fabrication protocols compatible with established Si technologies