Remote Hydrogen Plasma Research Articles

High responsivity and stability of MSM structured MoS2 photodetectors by remote hydrogen plasma treatment and alternating growth of Al2O3/HfO2 passivation layers

Molybdenum disulfide (MoS2) is a representative material of transition metal dichalcogenides with a wide range of potential practical applications. However, the low responsivity and relatively short lifespan of MoS2 photodetectors...

Doping-induced Ti3+ state and oxygen vacancies in TiO2: A single-chip combinatorial investigation

TiO2 is widely recognized as a high performance photocatalyst for photocatalytic applications. Oxygen vacancies (VO) and titanium defects are known to make a significant contribution to the enhanced photocatalytic efficiency of TiO2, accordingly control of these two centers is pivotal to optimizing the surface photochemical activity. In this work, the formation of VO and Ti3+ ions induced by n-type doping of TiO2 with Nb is demonstrated using a combinatorial investigation approach. Chemical Beam Vapour Deposition (CBVD) is used to fabricate Nb concentration-graded (NbxTi1-x)O2 anatase films with 0.009 ≤ x ≤ 0.033 over a 5 mm length on a single chip. Cathodoluminescence (CL) microanalysis shows the luminescence intensity decreases with increasing Nb concentration; however, the presence of a self-trapped exciton (STE) emission, characteristic of anatase TiO2, indicates that the Nb incorporation does not affect the structural properties of TiO2. Notably, the introduction of Nb donors induces the formation of the signature luminescence bands of VO and Ti3+ at 2.05 and 2.60 eV at 80 K, respectively, and their intensities rise by up to 28% with increasing Nb content. The formation of VO and Ti3+ is attributed to the effect of excess electrons locally produced by Nb donor doping. The thermal activation energies for the STE and defect-related emissions are found to be identical at 32 ± 4 meV, suggesting that the recombination kinetics is mediated by Nb donors. Significantly, we demonstrate that the near-surface VO density can be further increased by two different post-growth treatment approaches: (i) remote hydrogen plasma and (ii) low-energy electron beam irradiation (LEEBI), without effecting the valence state of Ti. Our results open opportunities to optimize the photocatalytic properties of TiO2 by defect engineering.

Spatially‐Modulated Silicon Interface Energetics Via Hydrogen Plasma‐Assisted Atomic Layer Deposition of Ultrathin Alumina

AbstractAtomic layer deposition (ALD) is a key technique for the continued scaling of semiconductor devices, which increasingly relies on scalable processes for interface manipulation of structured surfaces on the atomic level. While ALD allows the synthesis of conformal films with utmost control over the thickness, atomically‐defined closed coatings and surface modifications are challenging to achieve because of 3D growth during nucleation. Here, a route is presented toward the sub‐nanometer thin and continuous aluminum oxide (AlOx) coatings on silicon substrates for the spatial control of the surface charge density and interface energetics. Trimethylaluminum in combination with remote hydrogen plasma is used instead of a gas‐phase oxidant for the transformation of silicon dioxide (SiO2) into alumina. Depending on the number of ALD cycles, the SiO2 can be partially or fully transformed, which is exploited to deposit ultrathin AlOx layers in selected regions defined by lithographic patterning. The resulting patterned surfaces are characterized by lateral AlOx/SiO2 interfaces possessing 0.3 nm step heights and surface potential steps exceeding 0.4 V. In addition, the introduction of fixed negative charges of 9 × 1012 cm−2 enables modulation of the surface band bending, which is relevant to the field‐effect passivation of silicon and low‐impedance charge transfer across contact interfaces.

Remote Plasma-Induced Synthesis of Self-Assembled MoS2/Carbon Nanowall Nanocomposites and Their Application as High-Performance Active Materials for Supercapacitors.

The objective of this study is to investigate the synthesis and influence of MoS2 on carbon nanowalls (CNWs) as supercapacitor electrodes. The synthesis of MoS2 on CNW was achieved by the introduction of hydrogen remote plasma from ammonium tetrathiomolybdate (ATTM) without deterioration of the CNWs. The topographical surface structures and electrochemical characteristics of the MoS2–CNW composite electrodes were explored using two ATTM-dispersed organic solvents—acetonitrile and dimethylformamide (DMF). In this study, CNW and MoS2 were synthesized using an electron cyclotron resonance plasma. However, hydrogen radicals, which transform ATTM into MoS2, were provided in the form of a remote plasma source. The electrochemical performances of MoS2–CNW hybrid electrodes with various morphologies—depending on the solvent and ATTM concentration—were evaluated using a three-electrode system. The results revealed that the morphology of the synthesized MoS2 was influenced by the organic solvent used and affected both the electrochemical performance and topographical characteristics. Notably, considerable enhancement of the specific capacitance was observed for the MoS2 with open top edges synthesized from DMF. These encouraging results may motivate additional research on hybrid supercapacitor electrodes and the rapid synthesis of MoS2 and other transition metal dichalcogenides.

Passivation of thermally-induced defects with hydrogen in float-zone silicon

In this study, passivation of thermally-activated recombination centers with hydrogen in n-type float zone (FZ) Si containing nitrogen has been investigated. Prior to hydrogenation samples were heated to 550 °C using rapid thermal annealing and conventional furnaces. A large decrease in minority carrier lifetime occurred upon the heat-treatments confirming previous reports. A sequence of electron traps created in this process have been detected in the deep level transient spectra and characterized. Significant changes in the spectra have occurred after treatments in remote hydrogen plasma and subsequent annealing of the hydrogenated samples in the temperature range 100 °C–400 °C. A total elimination of electrical activity of the thermally induced defects has been observed in the hydrogenated samples subjected to annealing in the temperature range 150 °C–300 °C. The results obtained suggest a simple way for an effective cure of the degraded FZ-Si-based solar cells. Possible defect reactions occurring in the FZ-Si crystals and the role of nitrogen and carbon upon the performed treatments are discussed.

Edge-Trimmed Nanogaps in 2D Materials for Robust, Scalable, and Tunable Lateral Tunnel Junctions

Lateral tunnel junctions are fundamental building blocks for molecular electronics and novel sensors, but current fabrication approaches achieve device yields below 10%, which limits their appeal for circuit integration and large-scale application. We here demonstrate a new approach to reliably form nanometer-sized gaps between electrodes with high precision and unprecedented control. This advance in nanogap production is enabled by the unique properties of 2D materials-based contacts. The large difference in reactivity of 2D materials’ edges compared to their basal plane results in a sequential removal of atoms from the contact perimeter. The resulting trimming of exposed graphene edges in a remote hydrogen plasma proceeds at speeds of less than 1 nm per minute, permitting accurate control of the nanogap dimension through the etching process. Carrier transport measurements reveal the high quality of the nanogap, thus-produced tunnel junctions with a 97% yield rate, which represents a tenfold increase in productivity compared to previous reports. Moreover, 70% of tunnel junctions fall within a nanogap range of only 0.5 nm, representing an unprecedented uniformity in dimension. The presented edge-trimming approach enables the conformal narrowing of gaps and produces novel one-dimensional nano-trench geometries that can sustain larger tunneling currents than conventional 0D nano-junctions. Finally, the potential of our approach for future electronics was demonstrated by the realization of an atom-based memory.

Hard silicon carbonitride thin‐film coatings produced by remote hydrogen plasma chemical vapor deposition using aminosilane and silazane precursors. 1: Deposition mechanism, chemical structure, and surface morphology

AbstractAmorphous silicon carbonitride films were produced by remote hydrogen microwave plasma chemical vapor deposition (RP‐CVD) using aminosilanes: (dimethylamino)dimethylsilane, bis(dimethylamino)methylsilane, and tris(dimethylamino)silane, as well as disilazanes: 1,1,3,3‐tetramethyldisilazane and 1,3‐bis(dimethylsilyl)‐2,2,4,4‐tetramethylcyclodisilazane as single‐source precursors. The effect of substrate temperature (TS) on the rate and yield of the RP‐CVD process, chemical composition, chemical structure, and surface morphology of resulting films is reported. The temperature dependencies of thickness‐based growth rate and growth yield of the film imply that for a low substrate temperature range (TS = 30–200°C), deposition rate and yield are limited by desorption of film‐forming precursors from a growth surface, whereas for a high substrate temperature range (TS = 200–400°C), the rate and yield values, independent of the temperature, are nearly constant and the RP‐CVD is a nonthermally activated process. The increase of the substrate temperature from 30°C to 400°C causes the elimination of organic groups from the film and the formation of a silicon carbonitride network structure with a predominant content of Si–N nitridic and Si–C carbidic bonds. On the basis of structural study and literature data, hypothetical elementary reactions contributing to the film formation process are postulated. The films were found to be morphologically homogeneous, defect‐free materials exhibiting very small surface roughness, which varied in a narrow range of values.

Hard silicon carbonitride thin‐film coatings by remote hydrogen plasma chemical vapor deposition using aminosilane and silazane precursors. 2: Physical, optical, and mechanical properties of deposited films

AbstractPhysical, optical, and mechanical properties of silicon carbonitride (a‐SiCN) films produced by remote hydrogen plasma chemical vapor deposition (RP‐CVD) using aminosilane and disilazane precursors are examined in relation to their chemical structure. The films deposited at different temperatures (30–400°C) were characterized in terms of their density, refractive index, optical bandgap, photoluminescence, adhesion to a substrate, hardness, elastic modulus resistance to wear (predicted from the “plasticity index” values), and friction coefficient. Reasonable structural dependencies of film properties were determined using the relative integrated intensities of the infrared absorption bands from the Si−N and Si−C bonds, and the X‐ray photoelectron spectroscopy Si2p band from the Si−C bonds (controlled by substrate temperature) evaluated in the first part of this study. In view of their good mechanical properties, a‐SiCN films seem to be useful coatings for improving surface mechanics of engineering materials for advanced technology.

Hydrogenated Graphene Improves Neuronal Network Maturation and Excitatory Transmission.

Graphene is regarded as a viable bio-interface for neuroscience due to its biocompatibility and electrical conductivity, which would contribute to efficient neuronal network signaling. Here, monolayer graphene grown via chemical vapor deposition is treated with remote hydrogen plasma to demonstrate that hydrogenated graphene (HGr) fosters improved cell-to-cell communication with respect to pristine graphene in primary cortical neurons. When transferred to polyethylene terephthalate, HGr exhibits higher wettability than graphene (water contact angle of 83.7° vs 40.7°), while preserving electrical conductivity (≈3 kΩ □-1 ). A rich and mature network is observed to develop onto HGr. The intrinsic excitability and firing properties of neurons plated onto HGr appears unaltered, while the basic passive and active membrane properties are fully preserved. The formation of excitatory synaptic connections increases in HGr with respect to pristine graphene, leading to a doubled miniature excitatory postsynaptic current frequency. This study supports the use of hydrogenation for tailoring graphene into an improved neuronal interface, indicating that wettability, more than electrical conductivity, is the key parameter to be controlled. The use of HGr can bring about a deeper understanding of neuronal behavior on artificial bio-interfaces and provide new insight for graphene-based biomedical applications.

Characterization of Magnetic-Field Dependent Electron Transport of Fe3Si Nanodots by Using a Magnetic AFM Probe

We successfully fabricated Fe3Si-nanodots (NDs) on ultrathin SiO2 and evaluated their electron transport characteristics under magnetic field application by using atomic force microscopy (AFM) with a CoPtCr-coated cantilever. High density Fe3Si-NDs with an areal density as high as ~1011 cm-2 were formed on thermally grown SiO2 layer by exposing ultrathin Fe film on pre-grown Si-NDs to a remote hydrogen plasma without external heating. From local electron transport properties measured by the magnetic AFM tip, a clear change in resistance depending on the relative magnetization direction of Fe3Si-NDs with respect to the CoPtCr-coated AFM tip was confirmed at room temperature.

Characterization of hydrogen plasma defined graphene edges

We investigate the quality of hydrogen plasma defined graphene edges by Raman spectroscopy, atomic resolution AFM and low temperature electronic transport measurements. The exposure of graphite samples to a remote hydrogen plasma leads to the formation of hexagonal shaped etch pits, reflecting the anisotropy of the etch. Atomic resolution AFM reveals that the sides of these hexagons are oriented along the zigzag direction of the graphite crystal lattice and the absence of a D-peak within the noise background in the Raman spectra seems to suggest rather high quality zigzag edges. In a second step of the experiment, we investigate hexagon edges created in single layer graphene on hexagonal boron nitride and find a substantial D-peak intensity. Polarization dependent Raman measurements reveal that hydrogen plasma defined edges consist of a mixture of zigzag and armchair segments. Furthermore, electronic transport measurements were performed on hydrogen plasma defined graphene nanoribbons which indicate a high quality of the bulk but a relatively low edge quality, in agreement with the Raman data. These findings are supported by tight-binding transport simulations. Hence, further optimization of the hydrogen plasma etching technique is required to obtain pure crystalline graphene edges.

Optically addressing interaction of Mg/MgO plasmonic systems with hydrogen

Magnesium-based films and nanostructures are being studied in order to improve hydrogen reversibility, storage capacity, and kinetics, because of their potential in the hydrogen economy. Some challenges with magnesium (Mg) samples are their unavoidable oxidation by air exposure and lack of direct in situ real time measurements of hydrogen interaction with Mg and MgO surfaces and Mg plasmonic nanoparticles. Given these challenges, the present article investigates direct interaction of Mg with hydrogen, as well as implications of its inevitable oxidation by real-time spectroscopic ellipsometry for exploiting the optical properties of Mg, MgH2 and MgO. The direct hydrogenation measurements have been performed in a reactor that combines a remote hydrogen plasma source with an in situ spectroscopic ellipsometer, which allows optical monitoring of the hydrogen interaction and results in optical property modification. The hydrogen plasma dual use is to provide the hydrogen-atoms and to reduce barriers to heterogeneous hydrogen reactions.

Competitive interplay of deposition and etching processes in atomic layer growth of cobalt and nickel metal films

Abstract

Precise control of graphene etching by remote hydrogen plasma

Graphene with atomically smooth and configuration-specific edges plays the key role in the performance of graphene-based electronic devices. Remote hydrogen plasma etching of graphene has been proven to be an effective way to create smooth edges with a specific zigzag configuration. However, the etching process is still poorly understood. In this study, with the aid of a custom-made plasma-enhanced hydrogen etching (PEHE) system, a detailed graphene etching process by remote hydrogen plasma is presented. Specifically, we find that hydrogen plasma etching of graphene shows strong thickness and temperature dependence. The etching process of single-layer graphene is isotropic. This is opposite to the anisotropic etching effect observed for bilayer and thicker graphene with an obvious dependence on temperature. On the basis of these observations, a geometrical model was built to illustrate the configuration evolution of graphene edges during etching, which reveals the origin of the anisotropic etching effect. By further utilizing this model, armchair graphene edges were also prepared in a controlled manner for the first time. These investigations offer a better understanding of the etching process for graphene, which should facilitate the fabrication of graphene-based electronic devices with controlled edges and the exploration of more interesting properties of graphene.

Evaluating root cause: The distinct roles of hydrogen and firing in activating light- and elevated temperature-induced degradation

The root cause of light- and elevated temperature-induced degradation (LeTID) in multicrystalline silicon p-type passivated emitter and rear cell (PERC) devices is still unknown. Microwave-induced remote hydrogen plasma (MIRHP) is employed to vary the concentration of bulk hydrogen and to separate the effects of hydrogen and firing temperature in LeTID-affected wafers. We find that hydrogen is required for degradation to occur, and that samples fired prior to the introduction of hydrogen do not degrade. Importantly, samples with hydrogen that have not been fired do degrade, implying that the firing time-temperature profile does not cause LeTID. Together, these results suggest that the LeTID defect reaction consists of at least two reactants: hydrogen and one or more defects that can be separately modified by high-temperature firing. We assess the leading hypotheses for LeTID in the context of our new understanding of the necessary reactants.

Band alignment at the CdTe/InSb (001) heterointerface

CdTe/InSb heterojunctions have attracted considerable attention because of its almost perfect lattice match and the presence of nonoctal interface bonding. This heterojunction is a model heterovalent system to describe band offsets. In this research, molecular beam epitaxy was used to deposit a ∼5 nm epitaxial CdTe (001) layer on an InSb (001) surface. Monochromatic x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy were used to characterize the electronic states of clean InSb and CdTe surfaces and CdTe/InSb (001) heterostructures. A room temperature remote hydrogen-plasma process was used to clean the surfaces prior to characterization. The results indicate a valence band offset of 0.89 eV and a type-I (straddling gap) alignment for the CdTe/InSb (001) heterostructure interface. In addition, In-Te bonding was observed at the interface. Downward band bending of the InSb is attributed to excess electrons introduced by nonoctal In-Te interface bonding.

Silicon Carbide, Silicon Carbonitride, and Silicon Oxycarbide Thin Films Formed by Remote Hydrogen Microwave Plasma CVD

Background: Silicon-based thin films produced by remote microwave hydrogen plasma chemical vapor deposition (RP-CVD) from 1,1,4,4-tetramethyldisilaethylene, 1,1,3,3-tetramethyldisiloxane and 1,1,3,3- tetramethyldisilazane precursors are compared. The structure of these compounds differs in atomic composition of the central unit which links the two dimethylhydrosilane moieties. Objective: The effect of temperature on the kinetics of film growth rate induced by RP CVD process, chemical composition and structure as well as same characteristic parameters of resulting amorphous hydrogenated silicon carbide (a-SiC:H), silicon oxycarbide (a-SiOC:H), and silicon carbonitride (a-SiCN:H) films is discussed. Conclusion: It was found that RP CVD process is controlled by adsorption, in which there are two temperature ranges for the formation of the CVD layers. At low-temperature range soft polymer-like material is formed containing organic groups whereas at high-temperature range a transformation to high-density ceramic- like material occurs. Based upon the results of the studies, mechanisms of the initiation step, as well as growth and crosslinking steps in the film formation during RP CVD deposition are proposed. The films were also characterized in terms of their representative properties, such as density, refractive index and biocompatibility. Owing to the high density, defect-free surface and excellent optical parameters the films may be useful as coatings for optoelectronics applications and in medicine for steel implants. Keywords: Remote hydrogen plasma, CVD process, silicon-based precursor, silicon carbide film, silicon oxycarbide film, silicon carbonitride film, properties of thin films, biocompatibility.

Formation of Mn-germanide nanodots on ultrathin SiO2 induced by remote hydrogen plasma

High-density Mn-germanide nanodots [MnxGe1−x (x = 0.8) NDs] with an areal dot density as high as ∼1012 cm−2 were formed on thermally grown SiO2 by exposing ∼1.0-nm-thick Mn/∼1.0-nm-thick amorphous Ge (a-Ge) bilayer stacked structures to a remote H2 plasma (H2-RP) without external heating. The germanidation reaction of the Mn/a-Ge bilayer was observed through high resolution X-ray photoelectron spectroscopy measurements. Electrical isolation among the MnxGe1−x (x = 0.8) NDs was verified from the changes in surface potential after charge injection using an atomic force microscope/Kelvin probe technique. As seen from the VHF input power and exposure time dependence of the ultrathin bilayer stacked structures, control of thickness and H-radical flux are important parameters for the high-density formation of MnGe alloy NDs induced by H2-RP.

Anisotropic etching of graphite and graphene in a remote hydrogen plasma

We investigate the etching of a pure hydrogen plasma on graphite samples and graphene flakes on SiO2 and hexagonal boron-nitride substrates. The pressure and distance dependence of the graphite exposure experiments reveals the existence of two distinct plasma regimes: the direct and the remote plasma regime. Graphite surfaces exposed directly to the hydrogen plasma exhibit numerous etch pits of various size and depth, indicating continuous defect creation throughout the etching process. In contrast, anisotropic etching forming regular and symmetric hexagons starting only from preexisting defects and edges is seen in the remote plasma regime, where the sample is located downstream, outside of the glowing plasma. This regime is possible in a narrow window of parameters where essentially all ions have already recombined, yet a flux of H-radicals performing anisotropic etching is still present. At the required process pressures, the radicals can recombine only on surfaces, not in the gas itself. Thus, the tube material needs to exhibit a sufficiently low H radical recombination coefficient, such as found for quartz or pyrex. In the remote regime, we investigate the etching of single layer and bilayer graphene on SiO2 and hexagonal boron-nitride substrates. We find isotropic etching for single layer graphene on SiO2, whereas we observe highly anisotropic etching for graphene on a hexagonal boron-nitride substrate. For bilayer graphene, anisotropic etching is observed on both substrates. Finally, we demonstrate the use of artificial defects to create well defined graphene nanostructures with clean crystallographic edges.

Single electron transistors with hydrogen treatment of ALD SiO2 in nanoscale metal–insulator–metal tunnel junctions

Over the past five years, fabrication of metal–insulator–metal (MIM) single electron transistors (SET) featuring atomic layer deposition (ALD) of ultrathin tunnel barrier dielectrics (SiO2, Al2O3) has been reported. However, the performance of fabricated devices was significantly compromised by the presence of native metal oxide and problems associated with the nucleation of ALD dielectrics on metal substrates. To overcome the difficulty of dielectric ALD nucleation on metal substrates, we recently developed a fabrication technique in which the native metal oxide naturally forming in the presence of the ALD oxidant precursor is first used to promote the nucleation of ALD dielectrics, and then is chemically reduced by forming gas anneal (FGA) at temperatures near 400 °C. However, despite the elimination of native oxide, low temperature characterization of the devices fabricated using FGA reveals excess ‘switching’ noise of a very large magnitude resulting from charged defects within the junctions. It has been previously reported that remote hydrogen plasma (RHP) treatment of SiO2 thin films effectively eradicates fabrication defects. This work reports a comparative study of Ni-based MIM SET treated with FGA and/or RHP. We show that, using a combination of FGA and RHP treatments, it is possible to obtain MIM junctions free of switching noise and without a detectable contribution of native oxide.