H2 Etching Research Articles

Reduction of Dislocation Density of SiC Crystals Grown on Seeds after H<sub>2</sub> Etching

Three-inch 6H-SiC bulk crystals were grown by the PVT method on the seeds processed by different treatments. The influences of seed surface morphology and subsurface damage on the dislocation density were investigated. The seed surface morphology was characterized by atomic force microscopy (AFM). The extent of the subsurface damage was estimated by electron back-scattered diffraction (EBSD) and Band Contrast (BC) value. The distribution and density of the dislocations were observed by optical microscopy (OM). The results showed that the pit density performed by H2 1400°C etching was nearly one order of magnitude lower than that by mechanical polishing (MP) process. So H2 etching processed at 1400°C for 2h could completely remove the subsurface damage, compared with the MP process with the deep surface damage.

Insight into CO2 Etching Behavior for Efficiently Nanosizing Graphene

Chemical etching of graphene over catalytic metal surface holds great potential in nanosizing the graphene microstructure and thus modulating its properties. Herein, it has been demonstrated that gaseous CO2 can efficiently etch the monolayer graphene film grown by chemical vapor deposition catalyzed with the surface of Cu foil. During the etching process, the CO2 etching rate is monotonously dependent on the CO2 flowrate and the etching temperature, and is faster than the H2 etching which has been usually employed for the graphene patterning. Moreover, the resultant graphene flakes by the CO2 etching remain the originally high crystallinity and are free of being oxidized. Also, a 120° angle between the neighboring edges and hexagonal morphology of graphene flakes can be realized for the potential shape regulation of nanostructured graphene, which is similar as the anisotropic etching of H2. These results illustrate that the CO2 etching over the metal surface can provide a promising strategy for the precisely fabrication of nanostructured graphene materials.

A new model for in situ nitrogen incorporation into 4H-SiC during epitaxy

Nitrogen doping of 4H-SiC during vapor phase epitaxy is still lacking of a general model explaining the apparently contradictory trends obtained by different teams. In this paper, the evolutions of nitrogen incorporation (on both polar Si and C faces) as a function of the main growth parameters (C/Si ratio, temperature, pressure and growth rate) are reviewed and explained using a model based on surface exchanges between the gas phase and the uppermost 4H-SiC atomic layers. In this model, N incorporation is driven mainly by the transient formation of C vacancies, due to H2 etching, at the surface or near the surface. It is shown that all the growth parameters are influencing the probability of C vacancies formation in a similar manner as they do for N incorporation. The surface exchange model proposes a new framework for explaining the experimental results even beyond the commonly accepted reactor type dependency.

Nucleation and growth of single layer graphene on electrodeposited Cu by cold wall chemical vapor deposition

The nucleation density and average size of graphene crystallites grown using cold wall chemical vapor deposition (CVD) on 4 μm thick Cu films electrodeposited on W substrates can be tuned by varying growth parameters. Growth at a fixed substrate temperature of 1000 °C and total pressure of 700 Torr using Ar, H2 and CH4 mixtures enabled the contribution of total flow rate, CH4:H2 ratio and dilution of the CH4/H2 mixture by Ar to be identified. The largest variation in nucleation density was obtained by varying the CH4:H2 ratio. The observed morphological changes are analogous to those that would be expected if the deposition rate were varied at fixed substrate temperature for physical deposition using thermal evaporation. The graphene crystallite boundary morphology progresses from irregular/jagged through convex hexagonal to regular hexagonal as the effective C deposition rate decreases. This observation suggests that edge diffusion of C atoms along the crystallite boundaries, in addition to H2 etching, may contribute to shape evolution of the graphene crystallites. These results demonstrate that graphene grown using cold wall CVD follows a nucleation and growth mechanism similar to hot wall CVD. As a consequence, the vast knowledge base relevant to hot wall CVD may be exploited for graphene synthesis by the industrially preferable cold wall method.

On-axis Si-face 4H-SiC epitaxial growth with enhanced polytype stability by controlling micro-steps during the H2 etching process

In situ H2 etching and homoepitaxial growth of 4H-SiC were performed on on-axis Si-face substrates using low-pressure chemical vapor deposition. We investigated the effect of various etching temperatures and durations on the surface of the on-axis substrates and used optimized etching characteristics to enhance the polytype stability of the epitaxial layer. It was found that the micro-steps at the etched surface of the on-axis substrates play a vital role in enhancing the stability of the 4H-SiC polytype. Homoepitaxial 4H-SiC with up to 99% stability was successfully grown with spread-out micro-steps without step-bunching during the etching process.

Polymer-confined growth of perforated MoSe2 single-crystals on N-doped graphene toward enhanced hydrogen evolution.

The edge and corner atoms of 2D transition metal dichalcogenides (TMDCs) are the main electrocatalytically active sites for electrochemical reaction. Here, we demonstrate an approach to generate abundant edge/corner atoms in molybdenum diselenide (MoSe2) nanocrystals supported by nitrogen-doped graphene (NG) which consequently leads to significantly enhanced hydrogen evolution reaction (HER) activity. These structures were fabricated by firstly absorbing the Mo-containing precursor within polymer-functionalized graphene oxide, then selenized to obtain MoSe2 nanocrystals on the surface, and finally H2 etching was performed to form perforated structures. The use of a functional polymer as an absorption matrix efficiently mitigates aggregation which allows us to obtain MoSe2 single-crystals of ∼150 nm in lateral dimension, while maintaining high MoSe2 loading. We observed a remarkably enhanced electrocatalytic activity resulting from a significantly increased abundance of edge/corner atoms in hydrogen evolution measurements. Specifically, with this perforated MoSe2/NG-modified cathode, current densities of -1 and -10 mA cm-2 were realized with the overpotentials of only 30 and 106 mV, along with a small Tafel slope of 57 mV dec-1 and large exchange current density of 127.4 μA cm-2 in 0.5 M H2SO4. Such an efficient strategy also opens doors for the unparalleled design and fabrication of TMDC-based nanocomposites for electrochemical applications.

Chloride-based fast homoepitaxial growth of 4H-SiC films in a vertical hot-wall CVD**Project supported by the National High Technology R&D Program of China (No. 2014AA041402), the National Natural Science Foundation of China (Nos. 61474113, 61274007, 61574140), the Beijing Natural Science Foundation of China (Nos. 4132076, 4132074), the Program of State Grid Smart Grid Research Institute (No. SGRI-WD-71-14-004), and the Youth Innovation

Chloride-based fast homoepitaxial growth of 4H-SiC epilayers was performed on 4° off-axis 4H-SiC substrates in a home-made vertical hot-wall chemical vapor deposition (CVD) system using H2−SiH4−C2H4−HCl. The effect of the SiH4/H2 ratio and reactor pressure on the growth rate of 4H-SiC epilayers has been studied successively. The growth rate increase in proportion to the SiH4/H2 ratio and the influence mechanism of chlorine has been investigated. With the reactor pressure increasing from 40 to 100 Torr, the growth rate increased to 52 μm/hand then decreased to 47 μm/h, which is due to the joint effect of H2 and HCl etching as well as the formation of Si clusters at higher reactor pressure. The surface root mean square (RMS) roughness keeps around 1 nm with the growth rate increasing to 49 μm/h. The scanning electron microscope (SEM), Raman spectroscopy and X-ray diffraction (XRD) demonstrate that 96.7 μm thick 4H-SiC layers of good uniformity in thickness and doping with high crystal quality can be achieved. These results prove that chloride-based fast epitaxy is an advanced growth technique for 4H-SiC homoepitaxy.

Effect of hydrogen flow on growth of 3C-SiC heteroepitaxial layers on Si(111) substrates

3C-SiC thin films were grown on Si(111) substrates at 1250°C by horizontal low pressure chemical vapor deposition (LPCVD). We performed an exhaustive study on the effect of H2 flow rate on the crystalline quality, surface morphologies, growth rate, n-type doping of 3C-SiC thin films and the voids at the interface. The films show epitaxial nature with high crystal quality and surface morphology increase obviously with increasing H2 flow rate. The growth rate and n-type doping are also dependent on H2 flow rate. The properties of the voids at the interface are discussed based on the cross-sectional scanning electron microscope characterization. Transformation of voids with increasing H2 flow rate are attributed to higher 3C-SiC film growth rate and H2 etching rate. The mechanism of void formation is discussed based on our model, too. The results demonstrate that H2 flow rate plays a very important role in the heteroepitaxial growth of 3C-SiC films.

Use of hydrogen etching to remove existing dislocations in GaN epitaxial layers

In this paper, based on the anisotropic nature of hydrogen (H2) etching on GaN, we describe a new approach to the removal of threading dislocations in GaN layers. The top surfaces of c-plane (Ga-face) and a-plane GaNs are considered stable in H2; therefore, H2 etches only crystal imperfections such as dislocation and basal plane stacking fault (BSF) sites. We used H2 to etch undoped c-plane GaN, n-type c-plane GaN, a-plane GaN, and an InGaN/GaN multiple quantum well structure. Several examinations were performed, indicating deep cavities on the c-plane GaN samples after H2 etching; furthermore, gorge-like grooves were observed on the a-plane GaN samples. The deep cavities on the c-plane GaN were considered the etched dislocation sites, and the gorge-like grooves on the a-plane GaN were considered the etched BSF sites. Photoluminescence measurements were performed and the results indicated that the H2-etched samples demonstrate superior optoelectronic properties, probably because of the elimination of dislocations.

Investigation of giant step bunching in 4H-SiC homoepitaxial growth: Proposal of cluster effect model

We have investigated the H2 etching and growth conditions causing giant step bunching (GSB) in 4H-SiC homoepitaxial growth on 8° off-axis substrates by a chemical vapor deposition method and found that GSB does not occur during H2 etching under a wide range of experimental conditions, whereas GSB occurs during epitaxial growth at extremely low or high C/Si ratios, i.e., an excessive supply of SiH4 or C3H8. To explain these results, we have proposed a model taking into account the effect of Si or C cluster formation called “the cluster effect” model. We have shown that the cluster effect model can explain well our experimental result of GSB occurrence or nonoccurrence during etching and homoepitaxial growth of 4H-SiC.

Glide of threading edge dislocations after basal plane dislocation conversion during 4H–SiC epitaxial growth

Transmission electron microscopy (TEM) and KOH etching were used to analyze the motion of dislocations after the conversion of basal plane dislocations (BPDs) to threading edge dislocations (TEDs) during 4H–SiC epitaxy. The locations of TED etch pits on the epilayer surface were shifted compared to the original locations of BPD etch pits on the substrate surface. The shift of the TED etch pits was mostly along the BPD line directions towards the up-step direction. For converted screw type BPDs, the conversion points were located below the substrate/epilayer interface. The shift distances in the step-flow direction were proportional to the depths of the BPD–TED conversion points below the substrate/epilayer interface. For converted mixed type BPDs, the conversion points were exactly at the interface. Through TEM analysis, it was concluded that the dislocation shift is caused by a combined effect of H2 etching prior to growth and glide of the threading segments during high temperature epitaxy. The TED glide is only possible for converted pure screw type BPDs and could present a viable means for eliminating BPDs from the epilayer during growth by moving the conversion point below the substrate/epilayer interface.

Homoepitaxial growth and investigation of stacking faults of 4H-SiC C-face epitaxial layers with a 1° off-angle

We grew epitaxial layers on 4H-SiC C-face substrates with a 1° off-angle, and discussed important factors related to stacking fault (SF) density reduction by investigating the causes of SFs. Three types of SFs were generated, namely 3C inclusions, 8H-SFs and -SFs. The 3C inclusions were caused by 3C-SiC particles, which were present on the substrates before epitaxial growth, or which had fallen onto the substrates during epitaxial growth from the inside walls of a chemical vapor deposition reactor. The 3C-inclusion density decreased when the in-situ H2 etching depth exceeded 0.4 µm because the 3C-SiC particles, which were present on substrates before epitaxial growth, were removed. For 8H-SFs and -SFs, high dislocation density areas on the substrates rather than the dislocations themselves cause these SFs. To reduce the density of these SFs, it is important to suppress generation of the high dislocation density areas on the substrates.

Comparative Study of 4H-SiC Epitaxial Layers Grown on 4° Off-Axis Si- and C-Face Substrates Using Bistrimethylsilylmethane Precursor

In situ H2 etching and homoepitaxial growth of 4H-SiC have been carried out on 4° off-axis Si-face and C-face substrates by low-pressure chemical vapor deposition. We systematically analyzed H2 etching characteristics and epitaxial growth of 4H-SiC substrates on two different polarities using an organosilicon source material, bistrimethylsilylmethane (C7H20Si2). The effects of the growth conditions, such as growth temperature and source flow rate on the surface morphology, crystallinity, polytype conversion, defect generation, and structural imperfection of the epilayers on different polar surfaces, were investigated. High-quality epitaxial layers were successfully grown at low temperature range of 1320–1440°C on the Si-face and 1500°C on the C-face. A low source flow rate of 5–10 sccm was also preferred to grow defect-free epilayers.

Analysis on Generation of Localized Step-Bunchings on 4H-SiC(0001)Si Face by Synchrotron X-Ray Topography

Surface roughening regions running like scratches are often observed locally after epitaxy film grown on a very flat 4H-SiC wafer surfaces. We investigated generation mechanism of such roughening surface by using X-ray topography and confocal optical microscopy. We found that lattice defects were often introduced during CMP at local regions, and those local regions cannot be recognized by optical microscopy, since very flat surface can be observed. By H2 etching which is preprocess of epitaxy film growth, those lattice defects are almost etched off, but local rough surface consists of pits and step bunching regions appear like scratches, and those local pits and surface roughening regions grew up to step bunching during epitaxy film growth.

The effects of thin capping layers between quantum wells and barriers on the quantum efficiency enhancement in InGaN-based light emitting diodes

We discovered that adding H2 to the carrier gas in GaN barrier growth improved the light emitting diode (LED) peak quantum efficiency and shifted the efficiency maxima toward lower currents (∼20 mA). This implies that the Shockley–Read–Hall nonradiative process can be suppressed via the introduction of combination carrier gas (H2/N2) during barrier growth. Further, 1–2 nm thick Al-In-Ga-N alloys were adopted as capping layers to circumvent H2 etching effect during growth interruption. It was then revealed that quantum efficiency was effectively enhanced for LEDs employed with these thin large bandgap capping layers, particularly at low injection levels. Numerical simulation suggested that the improved quantum efficiency can be ascribed to the increased electron capture rate in the active region as well as enhanced electron and hole wavefunction overlap, which correlated well with experimental results.

Effect of Surface Morphology on the On-State Resistance of SiC Photoconductive Semiconductor Switches

The photoconductive semiconductor switches (PCSS) were fabricated on V-doped semi-insulating 6H-SiC. We studied the effect of surface morphology on the on-state resistance of SiC PCSS. The SiC wafers with quite similar physical properties were processed by mechanical polishing, chemical mechanical polishing and H2 etching for producing different surface morphologies. All the SiC PCSS were excited by a 355 nm laser with a frequency of 10 Hz and a pulse intensity of 132 μJ/mm2. We found that the surface morphology had an obvious effect on the on-state resistance. The PCSS fabricated on mechanical polished SiC wafer with an average surface roughness (rms) of 1.0 nm showed the largest on-state resistance of 45.6 ohms, while a low value of 13.3 ohms was observed for the wafer processed by H2 etching at high temperature of 1550 °C.

High-Resolution TEM Observation of AlN/GaN Grown on Si Substrates

A high-quality crack-free AlN cap layer on GaN layer has been achieved using an AlN buffer layer directly grown on a silicon substrate at high temperature by radio frequency (RF) plasma-assisted molecular beam epitaxy. A two dimensional (2D) growth process guide to AlN cap layer of high grade crystal quality. The nucleation and the growth dynamics have been studied by in situ reflection high energy electron diffraction (RHEED) and ex situ by high resolution transmission electron microscopy (HR-TEM). The microstructure was investigated by energy-dispersive X-ray spectroscopy (EDX). It was disclosed that AlN is single crystalline with low defect. High densities of V-shaped pits were not detected at the interface between AlN and GaN layers. Contradictory the earlier reported V-shaped defects in nitride-based alloys; these V-shaped pits were condensed on top of the AlN layer because of H2 etching of the surface when a high temperature growth discontinuity between AlN and GaN layers.

Polarity and Its Influence on Growth Mechanism during MOVPE Growth of GaN Sub-micrometer Rods

The influence of polarity during the MOVPE growth of GaN based sub-micrometer (sub-μm) rods has mostly been neglected up to now. In this paper we demonstrate that the surface polarity plays a crucial role for the morphology of the GaN sub-μm rods. Based on the differences between N-polar and Ga-polar surfaces, a model is suggested to explain the influence of various parameters on the morphology of GaN sub-μm rods for the first time. For Ga-polar GaN, the {10−11} r-planes, similar to N-polar (000−1) c-planes, are terminated by nitrogen atoms. These N-terminated surfaces can be passivated by hydrogen, which leads to a stable surface with low growth rates and therefore tends to keep a pyramidal shape with stable r-planes. For N-polar GaN, {10−1−1} r-planes can be modified by H2 etching, leading to the formation of more stable {1−100} m-planes, hence supporting the formation of sub-μm rods with vertical sidewalls.

InAlN/GaN heterostructure field-effect transistors on Fe-doped semi-insulating GaN substrates

InAlN/GaN heterostructure field-effect transistors (HFETs) have been grown and fabricated on Fe-doped semi-insulating c-plane GaN substrates. The problematic parasitic leakage caused by interface charge between the epitaxial layers and the GaN substrate as well as any adverse effect of the substrate surface damage caused by the mechanical chemical polish employed on the substrates has been circumvented by using a combination of inductively coupled plasma dry etching and in situ H2 etching. As a result, the current leakage for 100 μm separation mesa-to-mesa was reduced down to 3×10−9 A/mm at 10 V voltage bias for a 320 μm mesa pad width normal to the current flow direction and the corresponding GaN buffer resistivity was about 3.5×108 Ω cm. Owing to the good thermal conductivity of GaN substrates, the HFETs exhibit much less current degradation, compared to those on a sapphire substrate, at high drain biases. Likewise, the dc and pulsed I-V characteristics were reasonably similar, suggestive of negligible drain current lag. A dc saturation drain current density of 1.0 A/mm was achieved at zero gate bias. For HFETs with 1.1 μm gate length and 90 μm gate width, the maximum extrinsic dc transconductance was 275 mS/mm.

4H-SiC Homoepitaxial Growth on Vicinal-Off Angled Si-Face Substrate

We have carried out detailed investigations of 4H-SiC homoepitaxial growth on vicinal off-angled Si-face substrates. We found that the surface morphology of the substrate just after in-situ H2 etching was also affected by the value of the vicinal-off angle. Growth conditions consisting of a low C/Si ratio and a low growth temperature were effective in suppressing macro step bunching at the grown epilayer surface. We also demonstrated epitaxial growth without step bunching on a 2-inch 4H-SiC Si-face substrate with a vicinal off angle of 0.79o. Ni Schottky barrier diodes fabricated on an as-grown epilayer had a blocking voltage above 1000V and a leakage current of less than 5x10-7A/cm2. We also investigated the propagation of basal plane dislocation from the vicinal off angled substrate into the epitaxial layer.