H2 Etching Research Articles

Influence of Carbon Source on the Buffer Layer for 4H-SiC Homoepitaxial Growth.

In this study, we systematically explore the impact of C/Si ratio, pre-carbonization time, H2 etching time, and growth pressure on the buffer layer and subsequent epitaxial layer of 6-inch 4H-SiC wafers. Our findings indicate that the buffer layer's C/Si ratio and growth pressure significantly influence the overall quality of the epitaxial wafer. Specifically, an optimal C/Si ratio of 0.5 and a growth pressure of 70 Torr yield higher-quality epitaxial layers. Additionally, the pre-carbonization time and H2 etching time primarily affect the uniformity and surface quality of the epitaxial wafer, with a pre-carbonization time of 3 s and an H2 etching time of 3 min found to enhance the surface quality of the epitaxial layer.

Improvement of electrical properties by insertion of AlGaN interlayer for N-polar AlGaN/AlN structures on sapphire substrates

In this study, we propose nitrogen-polar (N-polar) Al0.1Ga0.9N/Al0.9Ga0.1N/aluminum nitride (AlN) structures. N-polar Al0.1Ga0.9N/Al0.9Ga0.1N/AlN was grown on a sapphire substrate with a misorientation of 2° with respect to the m-axis using metal–organic vapor deposition. The effects of varying the Al0.9Ga0.1N interlayer thickness from 30 nm to 1 μm using pulsed H2 etching on the planarity and current–voltage characteristics of the samples were investigated. The current first improved upon increasing the interlayer thickness from 30 to 300 nm, owing to the reduction in interfacial impurities between (aluminum) gallium nitride (Al)GaN) and Al0.9Ga0.1N, but subsequently decreased upon further increasing the thickness because of the relaxation growth of the interlayer. Furthermore, pulsed H2 etching of the Al0.9Ga0.1N interlayer suppressed step bunching and improved planarity. Subsequently, the proposed method was employed to fabricate N-polar Al0.1Ga0.9N/Al0.9Ga0.1N/AlN heterostructure FETs, which demonstrated five times higher source-drain current (I DS) than that of conventional structures without an interlayer.

Hydrogen in-situ etching of GaN surface to reduce non-radiative recombination centers in 510-nm GaInN/GaN quantum-wells

This study aims to improve the internal quantum efficiency (IQE) of GaInN/GaN 510-nm green quantum-wells (QWs) by reducing the surface defects (SDs) at the n-type GaN surface using in-situ etching. For this purpose, we examined hydrogen (H2) and a mixture of H2 and ammonia (NH3) as the etching gases for surface etching of n-type GaN at various reactor temperatures and pressures. The results show that the in-situ etching performed effectively for the SD reduction, while the surface morphology of n-type GaN slightly deteriorated. Next, we examined the effect of in-situ etching on the IQE of QWs via the growth of 510-nm GaInN/GaN QWs on the surface-etched n-type GaN. The results show that the IQE at 300 K improved by a factor of 2.1 through the H2 + NH3 etching and 3.0 through the H2 etching, which is attributable to the fact that the SDs were more sufficiently removed using H2 as an etching gas, despite the slightly undesired shape of QW because they were grown on the roughed n-type GaN surface. Meanwhile, some SDs remained when H2 + NH3 was used as an etching gas.

Directional etching for high aspect ratio nano-trenches on hexagonal boron nitride by catalytic metal particles

Stimulated by the attractive performance of multi-dimensional heterostructures involving hexagonal boron nitride (hBN), intense attentions have been paid to creation of sharp boundary/interface, which could bring hBN nano-structures additional appealing physical properties. However, the lack of controllable synthesis limits further experimental investigation on hBN nano-structures. Here, the directional etching of transitional metal nano-particles (NPs) on the surface of hBN to produce nano-trenches with sharp edges was systematic investigated. It is found that, only Pt and Ir NPs can produce armchair-oriented nano-trenches at low H2 partial pressure, while other transitional metals lead to zigzag oriented nano-trenches. The density and width of the nano-trenches always increase with etching temperature and the pre-treated solution concentration while the trench orientation depends on both H2 partial pressure and etching temperature. The aspect ratio of nano-trenches may reach several thousand under optimized conditions. The method exhibited here shines a light on edge-selective patterning of 2D crystals.

Growth and Etching of the Grain Boundaries in Polygonal Graphene Islands.

The grain boundary is an intrinsic defect within mono- and multi-layer polygonal graphene islands during the chemical vapor deposition growth process. It greatly influences their mechanical and electronic properties. However, the precise characterization and formation mechanism of grain boundaries still remain unclear. In this work, H2 etching experiments show that a polygonal etched hole originates from the natural location of a grain boundary beyond the nucleation site in polygonal monolayer graphene. Furthermore, colorful Raman mapping provides a visualized method to explore the distribution of grain boundaries in polygonal bilayer graphene. Therefore, a deep understanding of the growth kinetics of mono- and bi-layer polygonal graphene was obtained through etching engineering combined with Raman spectroscopy.

Substrate Modification during Chemical Vapor Deposition of hBN on Sapphire.

A comparison of hexagonal boron nitride (hBN) layers grown by chemical vapor deposition on C-plane (0001) versus A-plane (112̅0) sapphire (α-Al2O3) substrate is reported. The high deposition temperature (>1200 °C) and hydrogen ambient used for hBN deposition on sapphire substantially alters the C-plane sapphire surface chemistry and leaves the top layer(s) oxygen deficient. The resulting surface morphology due to H2 etching of C-plane sapphire is inhomogeneous with increased surface roughness which causes non-uniform residual stress in the deposited hBN film. In contrast to C-plane, the A-plane of sapphire does not alter substantially under a similar high temperature H2 environment, thus providing a more stable alternative substrate for high quality hBN growth. The E2g Raman mode full width at half-maximum (FWHM) for hBN deposited on C-plane sapphire is 24.5 ± 2.1 cm-1 while for hBN on A-plane sapphire is 24.5 ± 0.7 cm-1. The lesser FWHM standard deviation on A-plane sapphire indicates uniform stress distribution across the film due to reduced undulations on the surface. The photoluminescence spectra of the hBN films at 300 and 3 K, obtained on C-plane and A-plane sapphire exhibit similar characteristics with peaks at 4.1 and 5.3 eV reported to be signature peaks associated with defects for hBN films deposited under lower V/III ratios. The dielectric breakdown field of hBN deposited on A-plane sapphire was measured to be 5 MV cm-1, agreeing well with reports on mechanically exfoliated hBN flakes. Thus, under the typical growth conditions required for high crystalline quality hBN growth, A-plane sapphire provides a more chemically stable substrate.

Deep ultraviolet emission from multiple quantum wells on flat N-polar AlN templates fabricated using periodical pulsed H2 etching

In a previous study, we successfully grew flat N-polar AlN layers on a c-plane sapphire substrate with a misorientation angle of 2.0° by metal-organic vapor phase epitaxy. However, its surface had undulations due to step bunching, and therefore further improvement of the surface flatness is required. In this study, we employed pulsed H2 etching during the growth of N-polar AlN layers to improve the surface flatness. Atomic force microscopy results indicated that the surface flatness was significantly improved, exhibiting a root mean square value of 0.4 nm. Further, the deep ultraviolet emission from AlGaN-based multiple quantum wells (MQWs) on the N-polar AlN layers was characterized, and the effect of the surface flatness on the optical characteristics was investigated. The surface flatness was found to play a crucial role in improving the optical characteristics of MQWs on N-polar AlN layers.

Constructing FeN4/graphitic nitrogen atomic interface for high-efficiency electrochemical CO2 reduction over a broad potential window

Summary Atomically dispersed Fe–N–C catalyst has shown great performance in the CO2-to-CO conversion, yet the high CO selectivity is achieved only within a rather narrow potential range, which cannot well meet the requirements for the following CO dimerization or hydrogenation. Here, we developed a hydrogen-pyrolysis etching strategy to precisely manipulate the uncoordinated N dopants in the Fe–N–C catalyst. This strategy could preferentially eliminate pyridinic and pyrrolic N atoms while leaving graphitic N. The resulting catalyst gave a CO faradaic efficiency (FECO) above 90% over a broad window from −0.3 to −0.8 V (versus RHE) (in particular, FECO > 97% at −0.6 V). In situ ATR-SEIRAS and first-principle calculations further revealed that this strategy can not only suppress the parasitic H2 generation but also promote CO2 activation and protonation with the assistance of co-adsorbed H2O on the “atomic interface” of “FeN4/graphitic N.”

Mobility improvement of 4H-SiC (0001) MOSFETs by a three-step process of H2 etching, SiO2 deposition, and interface nitridation

4H-SiC(0001) metal-oxide-semiconductor field-effect transistors (MOSFETs) and MOS capacitors were fabricated by the following procedures: H2 etching, SiO2 deposition, and nitridation, and their electrical characteristics were evaluated. Substantially low interface state densities (4–6 × 1010 cm−2 eV−1) and high channel mobilities (80–85 cm2 V−1 s−1) were achieved by N2 annealing or NO annealing after H2 etching and SiO2 deposition. The threshold voltage of the MOSFETs fabricated with N2 annealing was shifted negatively when the oxide was formed by deposition. On the other hand, normally-off operation and high channel mobility were compatible for the MOSFETs fabricated with NO annealing.

In-Situ Preparation of GaN Sacrificial Layers on Sapphire Substrate in MOVPE Reactor for Self-Separation of the Overgrown GaN Crystal

GaN layers on sapphire substrates were prepared by using metal organic vapor phase epitaxy (MOVPE) combined with an in-situ H2 etching process for the purpose of later self-separation of thick GaN crystals produced by hydride vapor phase epitaxy (HVPE) on such substrates. The etching process results in deep pits and long voids that formed on the surface and along the lower interface between GaN and sapphire, respectively. The pits, which were investigated by SEM analysis, can be modified in their aspect ratio and density by controlling the etching parameters. Using a proper set of in-situ etching parameters, a seed layer with internal voids can be prepared, which is suitable for HVPE overgrowth and the self-separation process. The quality of the in-situ-etched seed GaN layer and overgrown GaN crystal were characterized by X-ray diffraction (XRD) and defect selective etching (DSE). With the aid of atomic force microscopy (AFM) in tapping mode, the interface morphology of the separated GaN crystal was analyzed. The crystal quality of the separated HVPE-GaN crystal is comparable to the crystal grown on untreated GaN MOVPE-seed, which did not separate from the sapphire substrate. The introduced technique to promote the crystal separation during the HVPE process has no obvious drawback on the quality of the grown GaN crystals. Using this technique, the self-separation occurs more gently due to a weakened interface between GaN/sapphire. The conventional separation from an untreated seed by pure thermomechanical action results in higher mechanical forces on the crystal and consequently much higher risk of crystal breakage.

Formation of high-quality SiC(0001)/SiO2 structures by excluding oxidation process with H2 etching before SiO2 deposition and high-temperature N2 annealing

We formed SiC/SiO2 structures by various procedures that excluded an oxidation process. We found that a SiC/SiO2 interface with a low interface state density near the conduction band edge of SiC (D it ∼ 4 × 1010 cm−2 eV−1 at E c −0.2 eV) is obtained for a fabrication process consisting of H2 etching of the SiC surface, SiO2 deposition, and high-temperature N2 annealing. D it is rather high without H2 etching, indicating that etching before SiO2 deposition plays a significant role in reducing D it. The key to obtaining low D it may be the removal of oxidation-induced defects near the SiC surface.

Design and formation of SiC (0001)/SiO2 interfaces via Si deposition followed by low-temperature oxidation and high-temperature nitridation

We report an effective approach to reduce defects at a SiC/SiO2 interface. Since oxidation of SiC may inevitably lead to defect creation, the idea is to form the interface without oxidizing SiC. Our method consists of four steps: (i) H2 etching of SiC, (ii) Si deposition, (iii) low-temperature (∼750 °C) oxidation of Si to form SiO2, and (iv) high-temperature (∼1600 °C) N2 annealing to introduce nitrogen atoms. The interface state density estimated by a high (1 MHz)–low method is in the order of 1010 cm−2 eV−1, two orders of magnitude lower than that of an interface formed by SiC oxidation.

High Quality Graphene Thin Films Synthesized by Glow Discharge Method in A Chemical Vapor Deposition System Using Solid Carbon Source.

Arc discharge is traditionally used to synthesize randomly arranged graphene flakes. In this paper, we substantially modify it into a glow discharge method so that the discharge current is much more reduced. The H2 and/or Ar plasma etching of the graphitic electrode (used to ignite the plasma) is hence much gentler, rendering it possible to grow graphene in thin film format. During the growth at a few mbar, there is no external carbon gas precursor introduced. The carbon atoms and/or carbon containing particles as a result of the plasma etching are emitted in the chamber, some of which undergo gas phase scattering and deposit onto the metallic catalyst substrates (Cu-Ni alloy thin films or Cu foils) as graphene sheets. It is found that high quality monolayer graphene can be synthesized on Cu foil at 900 °C. On Cu-Ni, under the same growth condition, somewhat more bilayer regions are observed. It is observed that the material quality is almost indifferent to the gas ratios, which makes the optimization of the deposition process relatively easy. Detailed study on the deposition procedure and the material characterization have been carried out. This work reveals the possibility of producing thin film graphene by a gas discharge based process, not only from fundamental point of view, but it also provides an alternative technique other than standard chemical vapor deposition to synthesize graphene that is compatible with the semiconductor planar process. As the process uses solid graphite as a source material that is rich in the crust, it is a facile and relatively cheap method to obtain high quality graphene thin films in this respect.

Detecting the Repair of Sulfur Vacancies in CVD-Grown MoS2 Domains via Hydrogen Etching

Inherent sulfur vacancies in molybdenum disulfide (MoS2) films grown via chemical vapor deposition have restricted the practical application of MoS2 in optoelectronic devices. The repair of sulfur vacancies is important for improving the quality of MoS2 and enhancing the performance of devices based on MoS2 films. Here we report a route that uses hydrogen (H2) etching to detect the repair effect of sulfur vacancies. The domains with and without sulfur annealing were etched via H2 under the same conditions, and the changes of surface microstructures and properties were compared. It was found that the tolerance of MoS2 to H2 etching was enhanced after sulfur annealing. Results of the photoluminescence spectrum further proved that H2 etching could effectively detect the repair effect of sulfur vacancies. As a practical application, the optimum condition of sulfur annealing was obtained through H2 etching. Our work promotes the practical application of MoS2 in the field of new-generation electronics and optoelectronics.

Gas sensor based on defective graphene/pristine graphene hybrid towards high sensitivity detection of NO2

We propose an approach to improve the performance of graphene-based gas sensors by the integration of defective graphene with pristine graphene. The defect density of defective graphene is controlled by the fluence of Si+ implantation, and an H2 etching process is utilized to tune defect size. As defects are able to adsorb target gas efficiently, the response of graphene-based sensors was improved remarkably with the controllable defect density. The response sensitivity of a defective-graphene-based sensor to concentrations of NO2 at 100 ppm can be as high as 248%, 13 times higher than that of a sensor built using pristine graphene. In addition, defective-graphene-based sensors exhibit high response and recovery rates at room temperature, which is comparable to those of pristine graphene-based sensors and faster than conventional defect-decorated graphene sensors. Most importantly, defective-graphene-based gas sensors exhibit excellent reproducibility, stability, and selectivity. Our study suggests a simple and effective strategy for the mass production of high-performance graphene-based gas sensors for NO2 gas detection.

Increasing the electrochemical activity of basal plane sites in porous 3D edge rich MoS2 thin films for the hydrogen evolution reaction

Molybdenum disulfide (MoS2) has been extensively utilized as an electrocatalyst for the hydrogen evolution reaction (HER) with edges as the primary active catalytic sites. Previous work on edge-rich MoS2 nanoplatelet 3D porous films (3D-ER-MoS2) shows they are promising catalysts, yet have basal planes that are inactive. Here we demonstrate how hydrogen annealing and oxygen plasma etching of 3D-ER-MoS2 generates defects and increases active site density within the basal plane, leading to dramatic improvements in HER catalytic activity. We also explore the critical processing parameters for electrocatalytic enhancement. Significantly enriched edge density was revealed for both routes. O2 plasma treatment was more effective in increasing the number of edges by creating micro-cracks and local surface damage on the basal planes as well as structuring saw-toothed edges; H2 etching mainly introduced irregular shaped basal surface nanopores and strips. By controlling processing parameters, optimum surface area/active sites density enhancement can be achieved, together with the robust 3D porous architecture and superaerophobic surface. The defect-rich MoS2 catalysts exhibit excellent HER activity: 700 °C – H2 – MoS2 shows Tafel slope of 94 mV/dec and low onset overpotential of 193 mV; 15 min – O2 – MoS2 performs amongst the best with excellent exchange current density of 57 μA/cmgeo−2. Our defect engineered 3D-ER MoS2 exhibits jgeo (η = 0.5 mV) of 6-fold and 38-fold compared to monolayer MoS2 subject to similar process, for O2 plasma and H2 etching approaches respectively. Our study demonstrates an effective way to realize high performance Pt-free electrocatalysts for hydrogen generation by dual enhancement via edge enrichment and basal surface activation.

Ultra-flat AlN grown with a pulsed H2 etching condition

A 1.5 μm AlN epilayer with a root-mean-square surface roughness of 0.25 nm was grown by metal-organic chemical vapor deposition at the single substrate temperature below 1200 °C. The ultra-flat surface is achieved with 30 min annealing performed in the initial growth, during which abrupt AlN hillocks are removed by pulsed H2 etching controlled via the on/off duration of NH3 supply. The pulsed etching technique effectively tailors the island-like morphology of the AlN buffer, facilitating the lateral crystal nucleation in subsequent growth. This study provides a simple, feasible and cost-effective manufacturing method for high-quality AlN.

(Invited) SOI Wafer Technology for Advanced Mos Applications

Emerging microelectronic applications such as Internet-Of-Things (IOT) or mixed signal processing utilize fully depleted MOSFETs built on SOI wafers. Advanced FD devices such as planar MOSFETs with back gate biasing require wafers with thin Si top layer as well as ultra thin BOX layer. To sustain good electrostatic control of the channel and to keep back-gate voltages reasonably low, top Si layer thickness has to be in the 5-10 nm range and BOX thickness needs to be less than 20 nm. The SmartCutTM process is the only technology that allows transferring extremely thin layers of silicon on a wide range of handling substrates with industrial quality. As threshold voltage of FD SOI transistor depends on the thickness of Si layer, the thickness has to be controlled within extremely tight tolerance, typically + /- 5A in full range of spatial frequencies spanning from 10 nm till 300 mm [1]. This creates challenge not only for manufacturing technology, but requires development of new thickness metrology. Introduction of differential reflection microscopy (DRM) [2] allows in-line SOI thickness control in wide range of spatial frequencies with unprecedented throughput and precision. The principles and main applications of DRM will be discussed in the presentation. The key process steps with their influence on specific spatial frequencies are identified. One of the main contributors to mid range frequency (in mm range) thickness variation is the interaction of propagating fracture front with acoustic waves emitted by itself. The experimental verification of this mechanism as well as the results of numerical simulations will be presented. Another critical process in SOI technology is reduction of high frequency thickness variations during finishing steps. Possibilities of using pure surface diffusion as well as chemically enhanced high temperature smoothing will be demonstrated. It will be shown that during high temperature annealing in neutral atmosphere Si surface evolution can be described by Mullings-Herring equation with two extra stochastic terms, responsible for conservative and non-conservative noise, while addition of HCl to the annealing atmosphere changes surface behavior to surface relaxation described by the Edwards-Wilkinson model. General linear 4th order surface development model is proposed covering both mechanisms. Material parameters for the model are determined experimentally in the case of HCl(<1%)/H2 etching of silicon. For some advanced applications base wafer of SOI structure can play functional role in the final device, not limited to a mechanical support. One of such examples is RF SOI substrates specifically designed for RF analog applications, such as front end RF modules in smartphones. Introduction of thermally stable layer with high density of carrier traps between BOX layer and high resistivity Si substrate significantly improves high frequency electrical parameters of the devices built on such substrates [3]. Insertion loss, second harmonic generation of coplanar waveguides as well as quality factors of inductors are such main parameters. Empirical model is presented which allows prediction of the results of RF measurements from simple SRP data.

Defect-rich carbon fiber electrocatalysts with porous graphene skin for flexible solid-state zinc–air batteries

Rechargeable flexible Zn–air batteries have attracted great attentions as promising next-generation energy storage devices for portable and wearable electronics. Bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) electrocatalysts on the air electrode are critical for improving the energy storage performance of Zn–air batteries. Free-standing electrocatalysts with superb OER/ORR reactivity render promising flexible power sources for the wearable and stretchable devices. In this contribution, a metal-free electrocatalyst based on surface modification of flexible carbon cloth is proposed. A coaxial cable-like structure with carbon fiber skeleton coated by nanostructured porous and defect-rich graphene skin is in situ fabricated through a facile H2 etching approach. With abundant heteroatoms and defects as active sites, the nanocarbon shells coated carbon cloth exhibits excellent OER/ORR bifunctional activity. The OER and ORR current densities on graphene skin modified carbon fiber are 20 and 3 times higher than those of pristine carbon cloth, respectively. This emerging carbon cloth derived electrocatalyst with porous graphene skin also serves as the air electrode in a rechargeable flexible solid-state Zn air battery with polymer gel electrolyte, and demonstrates stable charge/discharge cycling even under bending. This strategy of constructing nanostructures directly on carbon fibers benefits the rational design of flexible and functionalized materials for electrocatalytic energy applications.

Directed growth of graphene nanomesh in purified argon via chemical vapor deposition

Graphene nanomeshes (GNMs), new graphene nanostructures with tunable bandgaps, are potential building blocks for future electronic or photonic devices, and energy storage and conversion materials. In previous works, GNMs have been successfully prepared on Cu foils by the H2 etching effect. In this paper, we investigated the effect of Ar on the preparation of GNMs, and how the mean density and shape of them vary with growth time. In addition, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (TEM) revealed the typical hexagonal structure of GNM. Atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) indicated that large copper oxide nanoparticles produced by oxidization in purified Ar can play an essential catalytic role in preparing GNMs. Then, we exhibited the key reaction details for each growth process and proposed a growth mechanism of GNMs in purified Ar.