The Schottky barrier transistor in emerging electronic devices.

This paper explores how the Schottky barrier (SB) transistor is used in a variety of applications and material systems. A discussion of SB formation, current transport processes, and an overview of modeling are first considered. Three discussions follow, which detail the role of SB transistors in high performance, ubiquitous and cryogenic electronics. For high performance computing, the SB typically needs to be minimized to achieve optimal performance and we explore the methods adopted in carbon nanotube technology and two-dimensional electronics. On the contrary for ubiquitous electronics, the SB can be used advantageously in source-gated transistors and reconfigurable field-effect transistors (FETs) for sensors, neuromorphic hardware and security applications. Similarly, judicious use of an SB can be an asset for applications involving Josephson junction FETs.

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.

Monolithically Integrated Perovskite Semiconductor Lasers on Silicon Photonic Chips by Scalable Top-Down Fabrication.

Metal-halide perovskites are promising lasing materials for the realization of monolithically integrated laser sources, the key components of silicon photonic integrated circuits (PICs). Perovskites can be deposited from solution and require only low-temperature processing, leading to significant cost reduction and enabling new PIC architectures compared to state-of-the-art lasers realized through the costly and inefficient hybrid integration of III-V semiconductors. Until now, however, due to the chemical sensitivity of perovskites, no microfabrication process based on optical lithography (and, therefore, on existing semiconductor manufacturing infrastructure) has been established. Here, the first methylammonium lead iodide perovskite microdisc lasers monolithically integrated into silicon nitride PICs by such a top-down process are presented. The lasers show a record low lasing threshold of 4.7 µJcm-2 at room temperature for monolithically integrated lasers, which are complementary metal-oxide-semiconductor compatible and can be integrated in the back-end-of-line processes.

Noninvasive Scanning Raman Spectroscopy and Tomography for Graphene Membrane Characterization.

Graphene has extraordinary mechanical and electronic properties, making it a promising material for membrane-based nanoelectromechanical systems (NEMS). Here, chemical-vapor-deposited graphene is transferred onto target substrates to suspend it over cavities and trenches for pressure-sensor applications. The development of such devices requires suitable metrology methods, i.e., large-scale characterization techniques, to confirm and analyze successful graphene transfer with intact suspended graphene membranes. We propose fast and noninvasive Raman spectroscopy mapping to distinguish between free-standing and substrate-supported graphene, utilizing the different strain and doping levels. The technique is expanded to combine two-dimensional area scans with cross-sectional Raman spectroscopy, resulting in three-dimensional Raman tomography of membrane-based graphene NEMS. The potential of Raman tomography for in-line monitoring is further demonstrated with a methodology for automated data analysis to spatially resolve the material composition in micrometer-scale integrated devices, including free-standing and substrate-supported graphene. Raman tomography may be applied to devices composed of other two-dimensional materials as well as silicon micro- and nanoelectromechanical systems.

Growth orientation dependent hardness for epitaxial wurtzite InN films.

We report the hardness of wurtzite InN thin film grown with different crystalline orientations of heteroepitaxial s-plane (1101) and a-plane (1120) on r-plane (1102) Al2O3, and homoepitaxial film on c-plane (0001) Al2O3. Hardness values along with elastic properties are studied using nanoindentation technique. Maximum hardness is reported for c-plane (approximately 8 GPa), which is followed by s-plane (approximately 5.5 GPa) and then a-plane (approximately 4 GPa) oriented InN Films. Peierls force in the slip system of different crystalline orientations for wurtzite sample is calculated for explaining the systematic variations in hardness values of these films grown with different crystalline orientations.

Electron Transport across Vertical Silicon/MoS2/Graphene Heterostructures: Towards Efficient Emitter Diodes for Graphene Base Hot Electron Transistors.

Heterostructures comprising silicon, molybdenum disulfide (MoS2), and graphene are investigated with respect to the vertical current conduction mechanism. The measured current-voltage (I-V) characteristics exhibit temperature-dependent asymmetric current, indicating thermally activated charge carrier transport. The data are compared and fitted to a current transport model that confirms thermionic emission as the responsible transport mechanism across devices. Theoretical calculations in combination with the experimental data suggest that the heterojunction barrier from Si to MoS2 is linearly temperature-dependent for T = 200-300 K with a positive temperature coefficient. The temperature dependence may be attributed to a change in band gap difference between Si and MoS2, strain at the Si/MoS2 interface, or different electron effective masses in Si and MoS2, leading to a possible entropy change stemming from variation in density of states as electrons move from Si to MoS2. The low barrier formed between Si and MoS2 and the resultant thermionic emission demonstrated here make the present devices potential candidates as the emitter diode of graphene base hot electron transistors for future high-speed electronics.

Role of Substrate Surface Morphology on the Performance of Graphene Inks for Flexible Electronics.

Two-dimensional (2D) materials, such as graphene, are seen as potential candidates for fabricating electronic devices and circuits on flexible substrates. Inks or dispersions of 2D materials can be deposited on flexible substrates by large-scale coating techniques, such as inkjet printing and spray coating. One of the main issues in coating processes is nonuniform deposition of inks, which may lead to large variations of properties across the substrates. Here, we investigate the role of surface morphology on the performance of graphene ink deposited on different paper substrates with specific top coatings. Substrates with good wetting properties result in reproducible thin films and electrical properties with low sheet resistance. The correct choice of surface morphology enables high-performance films without postdeposition annealing or treatment. Scanning terahertz time-domain spectroscopy (THz-TDS) is introduced to evaluate both the uniformity and the local conductivity of graphene inks on paper. A paper-based strain gauge is demonstrated and a variable resistor acts as an on-off switch for operating an LED. Customized surfaces can thus help in unleashing the full potential of ink-based 2D materials.

Dielectric Properties and Ion Transport in Layered MoS2 Grown by Vapor-Phase Sulfurization for Potential Applications in Nanoelectronics.

Electronic and dielectric properties of vapor-phase grown MoS2 have been investigated in metal/MoS2/silicon capacitor structures by capacitance-voltage and conductance-voltage techniques. Analytical methods confirm the MoS2 layered structure, the presence of interfacial silicon oxide (SiO x ) and the composition of the films. Electrical characteristics in combination with theoretical considerations quantify the concentration of electron states at the interface between Si and a 2.5-3 nm thick silicon oxide interlayer between Si and MoS2. Measurements under electric field stress indicate the existence of mobile ions in MoS2 that interact with interface states. On the basis of time-of-flight secondary ion mass spectrometry, we propose OH- ions as probable candidates responsible for the observations. The dielectric constant of the vapor-phase grown MoS2 extracted from CV measurements at 100 kHz is 2.6 to 2.9. The present study advances the understanding of defects and interface states in MoS2. It also indicates opportunities for ion-based plasticity in 2D material devices for neuromorphic computing applications.

Direct observation of grain boundaries in graphene through vapor hydrofluoric acid (VHF) exposure.

The shape and density of grain boundary defects in graphene strongly influence its electrical, mechanical, and chemical properties. However, it is difficult and elaborate to gain information about the large-area distribution of grain boundary defects in graphene. An approach is presented that allows fast visualization of the large-area distribution of grain boundary-based line defects in chemical vapor deposition graphene after transferring graphene from the original copper substrate to a silicon dioxide surface. The approach is based on exposing graphene to vapor hydrofluoric acid (VHF), causing partial etching of the silicon dioxide underneath the graphene as VHF diffuses through graphene defects. The defects can then be identified using optical microscopy, scanning electron microscopy, or Raman spectroscopy. The methodology enables simple evaluation of the grain sizes in polycrystalline graphene and can therefore be a valuable procedure for optimizing graphene synthesis processes.

High Photocurrent in Gated Graphene-Silicon Hybrid Photodiodes.

Graphene/silicon (G/Si) heterojunction based devices have been demonstrated as high responsivity photodetectors that are potentially compatible with semiconductor technology. Such G/Si Schottky junction diodes are typically in parallel with gated G/silicon dioxide (SiO2)/Si areas, where the graphene is contacted. Here, we utilize scanning photocurrent measurements to investigate the spatial distribution and explain the physical origin of photocurrent generation in these devices. We observe distinctly higher photocurrents underneath the isolating region of graphene on SiO2 adjacent to the Schottky junction of G/Si. A certain threshold voltage (VT) is required before this can be observed, and its origins are similar to that of the threshold voltage in metal oxide semiconductor field effect transistors. A physical model serves to explain the large photocurrents underneath SiO2 by the formation of an inversion layer in Si. Our findings contribute to a basic understanding of graphene/semiconductor hybrid devices which, in turn, can help in designing efficient optoelectronic devices and systems based on such 2D/3D heterojunctions.

Resistive graphene humidity sensors with rapid and direct electrical readout.

We demonstrate humidity sensing using a change of the electrical resistance of single-layer chemical vapor deposited (CVD) graphene that is placed on top of a SiO2 layer on a Si wafer. To investigate the selectivity of the sensor towards the most common constituents in air, its signal response was characterized individually for water vapor (H2O), nitrogen (N2), oxygen (O2), and argon (Ar). In order to assess the humidity sensing effect for a range from 1% relative humidity (RH) to 96% RH, the devices were characterized both in a vacuum chamber and in a humidity chamber at atmospheric pressure. The measured response and recovery times of the graphene humidity sensors are on the order of several hundred milliseconds. Density functional theory simulations are employed to further investigate the sensitivity of the graphene devices towards water vapor. The interaction between the electrostatic dipole moment of the water and the impurity bands in the SiO2 substrate leads to electrostatic doping of the graphene layer. The proposed graphene sensor provides rapid response direct electrical readout and is compatible with back end of the line (BEOL) integration on top of CMOS-based integrated circuits.

Residual metallic contamination of transferred chemical vapor deposited graphene.

Integration of graphene with Si microelectronics is very appealing by offering a potentially broad range of new functionalities. New materials to be integrated with the Si platform must conform to stringent purity standards. Here, we investigate graphene layers grown on copper foils by chemical vapor deposition and transferred to silicon wafers by wet etching and electrochemical delamination methods with respect to residual submonolayer metallic contaminations. Regardless of the transfer method and associated cleaning scheme, time-of-flight secondary ion mass spectrometry and total reflection X-ray fluorescence measurements indicate that the graphene sheets are contaminated with residual metals (copper, iron) with a concentration exceeding 10(13) atoms/cm(2). These metal impurities appear to be partially mobile upon thermal treatment, as shown by depth profiling and reduction of the minority charge carrier diffusion length in the silicon substrate. As residual metallic impurities can significantly alter electronic and electrochemical properties of graphene and can severely impede the process of integration with silicon microelectronics, these results reveal that further progress in synthesis, handling, and cleaning of graphene is required to advance electronic and optoelectronic applications.

Band gap engineering of chemical vapor deposited graphene by in situ BN doping.

Band gap opening and engineering is one of the high priority goals in the development of graphene electronics. Here, we report on the opening and scaling of band gap in BN doped graphene (BNG) films grown by low-pressure chemical vapor deposition method. High resolution transmission electron microscopy is employed to resolve the graphene and h-BN domain formation in great detail. X-ray photoelectron, micro-Raman, and UV-vis spectroscopy studies revealed a distinct structural and phase evolution in BNG films at low BN concentration. Synchrotron radiation based XAS-XES measurements concluded a gap opening in BNG films, which is also confirmed by field effect transistor measurements. For the first time, a significant band gap as high as 600 meV is observed for low BN concentrations and is attributed to the opening of the π-π* band gap of graphene due to isoelectronic BN doping. As-grown films exhibit structural evolution from homogeneously dispersed small BN clusters to large sized BN domains with embedded diminutive graphene domains. The evolution is described in terms of competitive growth among h-BN and graphene domains with increasing BN concentration. The present results pave way for the development of band gap engineered BN doped graphene-based devices.