Chemical Vapor Deposition Syntheses of Wafer-Scale 2D Transition Metal Dichalcogenide Films toward Next-Generation Integrated Circuits Related Applications

2D semiconducting transition metal dichalcogenides (TMDCs), most with a formula of MX₂ (M=Mo, W; X=S, Se, etc.), have emerged as promising channel materials for next-generation integrated circuits, considering their dangling-bond-free surfaces, moderate bandgaps, relatively high carrier mobilities, etc. Wafer-scale preparation of 2D MX₂ films holds fundamental significance for realizing their applications. Chemical vapor deposition (CVD) is recognized as the most promising method for preparing electronic-grade 2D MX₂ films. This review hereby summarizes the recent progress in CVD syntheses of wafer-scale 2D MX₂ films and their applications in logic operations, data storage, and image capturing/processing related fields. The first part focuses on the wafer-scale syntheses of 2D MX₂ films through designing homogeneous metal precursor supply routes (e.g., precoating soluble precursor, feeding gaseous precursor, designing independent multisource supply or face-to-face metal precursor supply routes). The second part highlights the epitaxial growth of monolayer MX₂ single crystals on single-crystal Au substrates and well-designed sapphire substrates. The third part introduces various functional device/circuit related applications of CVD-derived 2D MX₂ wafers. Finally, challenges and prospects are discussed from the viewpoints of the controlled synthesis, reliable characterization, and damage-free transfer of 2D MX₂, as well as the fabrication and integration of high-performance devices.     

Wafer-Scale Growth of Aligned C60 Single Crystals via Solution-Phase Epitaxy for High-Performance Transistors

Fullerene (C₆₀) single crystals with exceptionally low defects and nearly perfect translational symmetry make them appealing in achieving high-performance n-type organic transistors. However, because of its natural 0D structure, control over continuous crystallization of C₆₀ over a large area is extremely challenging. Here, the authors report a solution-phase epitaxial approach for wafer-scale growth of continuously aligned C₆₀ single crystals. This method enables the rational control of the density of nucleation event at meniscus front by confining the size and shape of meniscus with a microchannel template. In this case, a single nucleus as seed crystal can be formed at the front of meniscus, and then epitaxial growth from the seed crystal occurs with continuous retreat of the meniscus. As a result, highly uniform C₆₀ single-crystal array with ultralow defect density is obtained on 2-inch substrate. Organic field-effect transistors made from the C₆₀ single-crystal array show a high average electron mobility of 2.17 cm² V⁻¹ s⁻¹, along with a maximum mobility of 5.09 cm² V⁻¹ s⁻¹, which is much superior to the C₆₀ polycrystalline film-based devices. This strategy opens new opportunities for the scalable fabrication of high-performance integrated devices based on organic crystals.     

3D/2D Perovskite Single Crystals Heterojunction for Suppressed Ions Migration in Hard X-Ray Detection

Halide perovskites exhibit diverse properties depending on their compositions. However, integrating desired properties into one material is still challenging. Here, a facile solution-processed epitaxial growth method to grow 2D perovskite single crystal on top of 3D perovskite single crystal, which can passivate the surface defects for improved device performance is reported. Short formamidine (FA⁺) ions are replaced by long organic cations, which can fully align and cover the single crystal surface to prevent the ions migration or short FA⁺ ions volatilization. The thickness of epitaxial layer can be finely adjusted by controlling the growth time. The defect density of single crystals heterojunction is only 3.18 × 10⁹ cm⁻³, and the carrier mobility is 80.43 cm² V⁻¹ s⁻¹, which is greater than that of the control 3D perovskite single crystal. This study for the first time realized large area 3D/2D perovskite single crystals heterojunction, which suppressed ions migration and exhibited advanced performance in hard X-rays detection applications. This strategy also provides a way to grow large area 2D perovskite single crystal from solution processes.     

Epitaxial Growth and Surface Roughness Control of Ferromagnetic Thin Films on Si by Sputter Deposition

Epitaxial growth of ferromagnetic (FM) thin-film electrodes on Si substrates is attracting increasing attention for magnetic tunnel junction (MTJ) studies as it enables both a high tunneling magnetoresistance (TMR) for practical applications and a well-defined junction structure for fundamental research. In this paper, we report epitaxial growth of both conventional FM and emerging Heusler FM electrodes by magnetron sputter deposition on Si substrates via appropriate buffer layers. Our primary focus is not only on the ability to grow epitaxial structures of the FM electrodes but also to achieve the desired smooth surfaces of these electrodes. The crystalline structures of the␣FM electrodes are known to play a critical role on the TMR of an MTJ. On the other hand, surface roughness of the FM electrode significantly degrades the performance of these junctions by interface scattering of spin-polarized electrons. We successfully grew various epitaxial face-centered cubic, hexagonal close-packed and body-centered cubic structures for Ni, Co, Fe, and their alloys in addition to the ordered intermetallic Heusler alloys, Co₂MnSi and Co₂MnAl, with appropriate buffer layers such as Ag between the FM layers and the Si substrates, followed by post-annealing. This annealing process greatly improves the quality of the epitaxy and the surface roughness of the FM layers deposited at room temperature. The crystallographic orientations of the ferromagnetic layers as well as the surface roughness were also found to vary with the orientation of the Si substrate.     

CdZnTe Substrate Surface Preparation Technology at ASELSAN,Inc. for Molecular Beam Epitaxy Growth of High Quality HgCdTe Epilayers

High quality CdZnTe substrates with 4% ZnTe mole fraction are used for epitaxial growth of HgCdTe infrared detector layers. Molecular Beam Epitaxy (MBE) growth of HgCdTe epilayers requires high quality surface layer with sub-nanometer surface roughness values as well. We report on the development of a CdZnTe substrate surface preparation process for MBE growth of high quality HgCdTe epilayers. Surface preparation processes were performed on both commercially available CdZnTe substrates and substrates obtained from in-house grown CdZnTe boules. We developed a multi-step substrate surface processing flow to achieve sub-nanometer surface roughness, low total thickness variation (TTV), and wax or slurry residue free CdZnTe substrate surfaces. Each process step was optimized with the aim of removing the subsurface damage caused by the previous process step; so that we reduce the amount of damaged layer needed to be etched prior to epitaxy. Our developed surface preparation process can be applicable to commercially available CdZnTe substrates with high surface roughness and high TTV. This process was also optimized for as-cut CdZnTe slices. We are capable of processing typically 25 mm?×?25 mm CdZnTe substrates with achievable surface roughness values (R_rms) down to below 0.5 nm.     

Epitaxial Growth of Large‐Scale Orthorhombic CsPbBr3 Perovskite Thin Films with Anisotropic Photoresponse Property

Inorganic cesium lead halide perovskite (CsPbX₃, X = Cl, Br, I) is a promising material for developing novel electronic and optoelectronic devices. Despite the substantial progress that has been made in the development of large perovskite single crystals, the fabrication of high‐quality 2D perovskite single‐crystal films, especially perovskite with a low symmetry, still remains a challenge. Herein, large‐scale orthorhombic CsPbBr₃ single‐crystal thin films on zinc‐blende ZnSe crystals are synthesized via vapor‐phase epitaxy. Structural characterizations reveal a “CsPbBr₃(110)//ZnSe(100), CsPbBr₃[?110]//ZnSe[001] and CsPbBr₃[001]//ZnSe[010]” heteroepitaxial relationship between the covering CsPbBr₃ layer and the ZnSe growth substrate. It is exciting that the epitaxial film presents an in‐plane anisotropic absorption property from 350 to 535 nm and polarization‐dependent photoluminescence. Photodetectors based on the epitaxial film exhibit a high photoresponsivity of 200 A W^?1, a large on/off current ratio exceeding 10⁴, a fast photoresponse time of about 20 ms, and good repeatability at room temperature. Importantly, a strong polarization‐dependent photoresponse is also found on the device fabricated using the epitaxial CsPbBr₃ film, making the orthorhombic perovskite promising building blocks for optoelectronic devices featured with anisotropy.     

CO2‐Laser‐Induced Growth of Epitaxial Graphene on 6H‐SiC(0001)

The thermal decomposition of SiC surface provides, perhaps, the most promising method for the epitaxial growth of graphene on a material useful in the electronics platform. Currently, efforts are focused on a reliable method for the growth of large‐area, low‐strain epitaxial graphene that is still lacking. Here, a novel method for the fast, single‐step epitaxial growth of large‐area homogeneous graphene film on the surface of SiC(0001) using an infrared CO₂ laser (10.6 μm) as the heating source is reported. Apart from enabling extreme heating and cooling rates, which can control the stacking order of epitaxial graphene, this method is cost‐effective in that it does not necessitate SiC pre‐treatment and/or high vacuum, it operates at low temperature and proceeds in the second time scale, thus providing a green solution to EG fabrication and a means to engineering graphene patterns on SiC by focused laser beams. Uniform, low–strain graphene film is demonstrated by scanning electron microscopy, X‐ray photoelectron spectroscopy, secondary ion‐mass spectroscopy, and Raman spectroscopy. Scalability to industrial level of the method described here appears to be realistic, in view of the high rate of CO₂‐laser‐induced graphene growth and the lack of strict sample–environment conditions.     

Specific features of formation of GaAs nanowire crystals during molecular beam epitaxy on different silicon surfaces

The results of experimental studies on the growth and the morphological and structural properties of GaAs nanowire crystals on different silicon surfaces are reported. It is shown that the nonplanar geometrical layout of growth allows the production of epitaxial nanowire crystals in a system with a large lattice mismatch. The growth on porous substrates, the role of the surface orientation, high-temperature annealing, and presence of an oxide layer at the surface, and some other effects typical of growth of III–V nanowire crystals on the Si surface are studied and analyzed. Intense emission from the array of GaAs nanowire crystals grown on the Si (111) surface is observed.     

Epitaxial SiC Growth Morphology and Extended Defects Investigated by Electron Backscatter Diffraction and Electron Channeling Contrast Imaging

Electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) were employed to investigate epitaxial SiC growth on 4H-SiC mesa structures. SiC polytypes were identified by indexing Kikuchi maps recorded from various points on the mesa surfaces. Orientation contrast was observed between different polytype surfaces using ECCI by forescattered electron detection. Extended defects in 3C-SiC were imaged directly by ECCI. Additionally, the ECCI technique was utilized to correlate dislocations with atomic step morphologies for various mesa surfaces. Evidence of vertical growth enhancement in the form of additional faceting was attributed to the presence of threading screw dislocations at mesa surfaces. Atomic steps were observed very near the edges of some mesa surfaces free of dislocations.     

Realizing the Intrinsic Anisotropic Growth of 1T′ ReS2 on Selected Au(101) Substrate toward Large-Scale Single Crystal Fabrication

Low-symmetry 2D materials with strong in-plane anisotropy are ideal platforms for building multifunctional optoelectronic devices. However, the random orientations and easy formation of multidomain structures lead to the single-crystal synthesis of these materials remains a big challenge. Herein, for the first time, the orientation-controlled synthesis of ReS₂, a typical low-symmetry 2D material, is explored via interface engineering based on the strong interaction between the material and Au substrates with different symmetries. It is revealed that the lattice orientation and growth behavior of ReS₂ are closely relevant to the lattice symmetry of Au facets. Single crystal ReS₂ domains with two and even one orientations are acquired on the four-fold symmetry Au(001) facet and the two-fold symmetry Au(101) facet, respectively. Combined with density functional theory calculations, it is demonstrated that the synergy of ultra-strong ReS₂-Au interfacial coupling and reduction of symmetry of Au facet is critical to realizing its intrinsic anisotropic growth. Furthermore, great enhancement of electrical and photoelectrical performances are acquired on the well-aligned single crystal ReS₂ device. The progress achieved in this work provides significant guidance for the controllable synthesis of wafer-scale single crystals of low-symmetry 2D materials for their practical device applications.     

Roughness improvement of the CoSi2/Si-interface for an application as buried silicide

The material CoSi₂ is preferred for the fabrication of buried silicide films between silicon device layer and buried oxide of SOI substrates for BICMOS integrations. Such an application needs excellent quality of the interface between the silicide and the silicon device layer. Using the conventional cobalt salicide process the roughness and waviness of the interface is too large for a device application. In this presentation three technologies to improve the CoSi₂/Si-interface quality were characterized. Using the first technology a very thin single crystalline CoSi₂ film was fabricated on a silicon substrate. This film acts as initial layer to produce thicker single crystalline silicide films. By the second technology an interlayer between cobalt and the silicon substrate was used to mediate an epitaxial CoSi₂ growth. Different types and materials were tested. Using the third technique a sacrificial layer of polycrystalline silicon between cobalt and the silicon substrate was consumed during the silicidation reaction. This method gives the best results with interface roughness values of less than 1 nm. The interface roughness was measured after CoSi₂ removal using AFM. A possible epitaxial growth of the silicide films was investigated with XRD analysis. Cross sectional SEM images were prepared to analyze the interface waviness and the CoSi₂ structure.     

Study of the optical and structural properties of multi quantum well structures grown on high-index surfaces

The MOVPE overgrowth of high [0 1¯ 1]-oriented ridges confined at sides by facets related to {n 1 1} crystallographic planes is reported. We studied the influence of the side tilt on the thickness of the AlGaAs and GaAs epitaxial layers grown under the condition of the kinetic growth mode. The multi quantum well (MQW) structures were prepared on the sides of ridges tilted at 54.7°, 45° and 30° to (1 0 0). The sidewall surface morphologies before and after epitaxial growth were evaluated and compared. We observed no tendency towards planarization towards a neighbouring high-index crystallographic plane, such as (2 1 1) and (3 1 1). We also showed that the quantum wells of the MQW structure make a smooth transition over the edge between the top surface and the facet as both AlGaAs and GaAs grew at similar rates on the surfaces.     

Compositional non-uniformities in selective area growth of GaInAs on InP grown by OMVPE

Selective area epitaxial growth of Ga_0.47In_0.53As on InP substrates patterned with silicon nitride was done by low pressure organometallic vapor phase epitaxy. Good surface morphology and clean side walls of the epitaxial layers were obtained in most of the areas of selective GalnAs growth. However, both GaAs incorporation and InAs incorporation increased near the edges of the selective growth areas due to the extra flux of Gacontaining and In-containing species migrating on the surface of silicon nitride. The increase in InAs incorporation was found at a higher rate when the adjacent silicon nitride area was large, hence, cross-hatching appeared near the edges. A characteristic length of adjacent silicon nitride seemed to be connected with the enhanced InAs incorporation, which was about 40μm at 600°. The non-uniformities in composition appeared in all wafers grown in the temperature range between 570 and 650°.     

Scanning electron microscopy and cathodoluminescence study of the epitaxial lateral overgrowth (ELO) process for gallium nitride

Traditional epitaxial growth of GaN by metalorganic vapor phase epitaxy (MOVPE) on mismatched substrates such as sapphire or SiC produces a columnar material consisting of many hexagonal grains ∼0.2–1.0 μm in diameter. The epitaxial-lateral-overgrowth (ELO) process for GaN creates a new material: single-crystal GaN. We have studied the ELO process for GaN grown by MOVPE in a vertical flow rotating substrate reactor. Characterization consisted of plan-view SEM and vertical-cross-section TEM studies, which revealed a large reduction in dislocation density in the overgrown regions of the GaN. Panchromatic and monochromatic cathodoluminescence images and spectra were used to study the spatial variation of the optical properties within the GaN ELO samples. The effects of growth temperature and stripe material on the overgrown layers were examined. Through the use of a higher substrate temperature during growth and the use of a SiN_x stripe material, the overgrown crystal shape has a smooth 2D top surface with vertical sidewalls. Applying a second ELO step, rotated by 60°, over a fully coalesced ELO layer yields a further reduction of defects in GaN overgrown surfaces.     

Enhanced Performance of Planar Perovskite Solar Cells Induced by Van Der Waals Epitaxial Growth of Mixed Perovskite Films on WS2 Flakes

Organic–inorganic metal halide perovskite solar cells (PSCs) have attracted much research interest owing to their high power conversion efficiency (PCE), solution processability, and the great potential for commercialization. However, the device performance is closely related to the quality of the perovskite film and the interface properties, which cannot be easily controlled by solution processes. Here, 2D WS₂ flakes with defect‐free surfaces are introduced as a template for van der Waals epitaxial growth of mixed perovskite films by solution process for the first time. The mixed perovskite films demonstrate a preferable growth along (001) direction on WS₂ surfaces. In addition, the WS₂/perovskite heterojunction forms a cascade energy alignment for efficient charge extraction and reduced interfacial recombination. The inverted PSCs with WS₂ interlayers show high PCEs up to 21.1%, which is among the highest efficiency of inverted planar PSCs. This work demonstrates that high‐mobility 2D materials can find important applications in PSCs as well as other perovskite‐based optoelectronic devices.     

Temperature Dependence of Epitaxial Graphene Formation on SiC(0001)

The formation of epitaxial graphene on SiC(0001) surfaces is studied using atomic force microscopy, Auger electron spectroscopy, electron diffraction, Raman spectroscopy, and electrical measurements. Starting from hydrogen-annealed surfaces, graphene formation by vacuum annealing is observed to begin at about 1150°C, with the overall step-terrace arrangement of the surface being preserved but with significant roughness (pit formation) on the terraces. At higher temperatures near 1250°C, the step morphology changes, with the terraces becoming more compact. At 1350°C and above, the surface morphology changes into relatively large flat terraces separated by step bunches. Features believed to arise from grain boundaries in the graphene are resolved on the terraces, as are fainter features attributed to atoms at the buried graphene/SiC interface.     

Determination of the fractal dimension for the epitaxial <Emphasis Type="Italic">n</Emphasis>-GaAs surface in the local limit

Atomic-force microscopy studies of epitaxial n-GaAs surfaces prepared to deposit barrier contacts showed that major relief for such surfaces is characterized by a roughness within 3–15 nm, although “surges” up to 30–70 nm are observed. Using three independent methods for determining the spatial dimension of the surface, based on the fractal analysis for the surface (triangulation method), its section contours in the horizontal plane, and the vertical section (surface profile), it was shown that the active surface for epitaxial n-GaAs obeys all main features of behavior for fractal Brownian surfaces and, in the local approximation, can be characterized by the fractal dimension D _f slightly differing for various measuring scales. The most accurate triangulation method showed that the fractal dimensions for the studied surface of epitaxial n-GaAs for measurement scales from 0.692 to 0.0186 μm are in the range D _f = 2.490?2.664. The real surface area S _real for n-GaAs epitaxial layers was estimated using a graphical method in the approximation δ → 0 δ is the measurement scale parameter). It was shown that the real surface area for epitaxial n-GaAs can significantly (ten times and more) exceed the area of the visible contact window.     

Surface morphology and crystallinity of single domain YBCO crystal

Large single domain YBa₂Cu₃O_7−x (YBCO) crystals were fabricated by the modified pulling method (SRL-CP method). The crystal growth from a sharpened YBCO seed crystal, which was cut into the shape of a reversed pyramid, was carried out for the purpose of obtaining a single domain crystal. The typical grown crystal was 13 mm² in size and 10 mm in length and the growth direction was the c-axis. In the ab plane of the as-grown crystal, a large spiral step was clearly observed. Morphology of this surface was investigated by atomic force microscopy (AFM). Both micro- and macro-steps were simultaneously observed with four-fold symmetry. This large concentric step in whole area indicate the single domain crystal has grown. The vertical height of the micro-step was less than 2 nm and almost equal to the c-axis unit cell height of the YBCO crystal. The spiral growth model explains the growth mechanism of YBCO crystals. The appearance of the macro-step was explained by the perturbation of the supersaturation. Furthermore, the crystallinity of YBCO single crystals was investigated by X-ray diffraction (XRD). The full width of half maximum (FWHM) of the X-ray rocking curve spectrum was about 0.11–0.12° and X-ray topographs also show the single domain crystal has grown. However, the dislocation hollow core was left in the central part along the growth direction.     

Topography induced preferential crystallization of thermally grown silicon dioxide films

We have investigated the crystallization of the oxide layer that grows on a deposited silicon film in a high temperature furnace. The growth of large SiO₂ crystal grains can be controlled by interfacial stress or surface topography. When the Si film is deposited on topographically patterned surfaces, the SiO₂ grains are nucleated along the edges and extremities of the relief structure. A competition in which faster growing grains terminate slower growing grains results in an average growth direction perpendicular to the edges. Single crystal grains of α-cristobalite up to hundreds of microns in length can be grown in this fashion.     

Visualizing Piezoelectricity on 2D Crystals Nanobubbles

2D crystals with noncentrosymmetric structures exhibit piezoelectric properties that show great potential for applications in energy conversion and electromechanical devices. Quantitative visualization of piezoelectric field spatial distribution is expected to offer a better understanding of macroscopic piezoelectricity, yet remains to be realized. Here, a technique of mapping piezoelectric potential on 2D materials bubbles based on the measurements of surface potential using kelvin probe force microscope is reported. By using odd number of layers hexagonal boron nitride and MoS₂ nanobubbles, strain-induced piezoelectric potential profiles are quantitatively visualized on the bubbles. The obtained piezoelectric coefficient is 3.4 ± 1.2 × 10⁻¹⁰ C m⁻¹ and 3.3 ± 0.2 × 10⁻¹⁰ C m⁻¹ for hBN and MoS₂, in agreement with the values reported. On the contrary, homogeneous distribution of surface potential is measured on even number of layers crystals bubbles where the crystal's inversion symmetry is restored. Using such technique, in situ visualization of photogenerated charge carrier separation under piezoelectric potential is also achieved, which offers a platform of investigating the coupling between piezoelectricity and photoelectric effect, and an approach of tuning piezoelectric field. The present work should aid the understanding of local piezoelectric potential and its various affecting factors including substrate doping and external stimuli, and give insights for designing piezoelectric nanodevices based on 2D nanobubbles.