Precursors For Chemical Vapor Deposition Research Articles

The synthesis, characterization, and thermal properties of a titanium (III) amidinate compound and its potential as a single chemical vapor deposition precursor for N/C-doped TiO2 film

A titanium(III) amidinate compound had been successfully obtained using a salt elimination reaction between TiCl3(3THF) and [Li(i Prn BuAMD)] in a 65% yield, and the characterization of the compound was conducted using various characterization methods. The reported compound’s thermal performances were studied through thermal gravimetric analysis (TGA), and the result indicated that the reported titanium(III) amidinate compound exhibited a suitable volatility, high thermal stability, and sufficient vapor pressure. Furthermore, film material deposit was performed in a CVD reactor under low pressure, and a N/C-doped TiO2 film was successfully deposited.

Influence of the Carrier Gas Flow in the CVD Synthesis of 2-Dimensional MoS2 Based on the Spin-Coating of Liquid Molybdenum Precursors.

Atomically thin molybdenum disulfide (MoS2) is a two-dimensional semiconductor with versatile applications. The recent adoption of liquid molybdenum precursors in chemical vapor deposition has contributed significantly to the reproducible wafer-scale synthesis of MoS2 monolayer and few-layer films. In this work, we study the effects of the carrier gas flow rate on the properties of two-dimensional molybdenum disulfide grown by liquid-precursor-intermediate chemical vapor deposition on SiO2/Si substrates. We characterized the samples using Optical Microscopy, Scanning Electron Microscopy, Raman spectroscopy, and Photoluminescence spectroscopy. We analyzed samples grown with different nitrogen carrier flows, ranging from 150 to 300 sccm, and discussed the effect of carrier gas flows on their properties. We found a correlation between MoS2 flake lateral size, shape, and number of layers, and we present a qualitative growth model based on changes in sulfur provision caused by different carrier flows. We show how the use of liquid precursors can allow for the synthesis of homogeneous, single-layer flakes up to 100 µm in lateral size by optimizing the gas flow rate. These results are essential for gaining a deeper understanding of the growth process of MoS2.

Molecular Transformations for Direct Synthesis of Thorium Dioxide Films

New Th(IV) complexes with high volatility were synthesized and applied as molecular single‐source precursors for chemical vapor deposition (CVD) of ThO2 thin films. The change in the steric profile of alkoxide ligands and concomitant reduction in the molecular weight of [Th(L)2(OR)2] (1‐OR, R = iso‐propyl (1‐OiPr) or tert‐butyl (1‐OtBu), L = N‐(4,4,4‐trifluorobut‐1‐ene‐3‐one)‐methoxyethylamide)) had a profound effect on the vapor pressure (at 10‐5 mbar) evident in the drop in sublimation temperature to 100 °C for the iso‐propoxide derivative against 130 °C observed for the tert‐(1‐OtBu). Hirshfeld surface analysis of both complexes showed different degrees of intermolecular H⋅⋅⋅F interactions responsible for the observed differences in the volatilities of the two complexes. Both mixed‐ligand compounds (1‐OiPr, 1‐OtBu) were applied in the CVD process to deposit thorium oxide thin films without any carrier or reactive gas that verified their suitability as single‐source precursors to ThO2 coatings. Thin films of ThO2 were grown at 500 °C and 600 °C on Si/SiO2 substrates, which showed presence of C, N, and F presumably originating from the incorporation of ligand fragments in the growing CVD deposits. Subsequent calcination of CVD‐grown ThO2 films at 800 °C in air led to phase pure ThO2 coatings with uniform morphology.

Surface engineering of CuO-Cu2O heterojunction thin films for improved photoelectrochemical water splitting

The efficient production of green hydrogen poses a significant challenge that requires the development of active, low-cost, and stable catalysts. Copper oxide has emerged as a highly effective photocatalyst for water splitting, offering the advantages of being nontoxic, abundant, and inexpensive. Nonetheless, the photoelectrochemical water splitting (PEC) process can be enhanced through surface modification. In this study, we utilized single-source precursors for aerosol-assisted chemical vapor deposition (AACVD) in order to deposit thin films of copper oxide (CuO-Cu2O) heterojunctions onto a substrate composed of fluorine-doped tin oxide (FTO). The chosen precursor for this purpose was copper (II) nitrate hemi(pentahydrate). To further optimize the CuO-Cu2O films, a novel acid treatment involving the addition of varying quantities of acetic acid (AA) to the precursor solution was employed. Remarkably, this acid treatment resulted in a significant improvement in the photocurrent density of the CuO-Cu2O heterojunction. Notably, a 3 mL acid-treated CuO-Cu2O thin film exhibited an impressive photocurrent measure of −6.8 mA/cm2 at 0.0 V vs RHE in 0.5 M Na2SO4 electrolyte, compared to the performance of the untreated CuO-Cu2O thin film (−2.8 mA/cm2 at 0.0 V vs RHE). This catalyst demonstrated stability and a threefold increase in hydrogen production performance. The increase in photocurrent densities is closely linked to the influence of AA on two key factors: particle growth and structural defects, such as oxygen vacancies. This relationship has been confirmed through the examination of SEM, AFM, and PL spectra results. The findings of this study not only offer a novel approach to catalyst design but also propose a practical method for photoelectrochemical water splitting. This method holds great promise for achieving sustainable and renewable energy sources, as well as efficient green hydrogen production.

Understanding vapor phase growth of hexagonal boron nitride.

Hexagonal boron nitride (hBN), with its atomically flat structure, excellent chemical stability, and large band gap energy (∼6 eV), serves as an exemplary 2D insulator in electronics. Additionally, it offers exceptional attributes for the growth and encapsulation of semiconductor transition metal dichalcogenides (TMDCs). Current methodologies for producing hBN thin films primarily involve exfoliating multi-layer or bulk crystals and thin film growth via chemical vapor deposition (CVD), which entails the thermal decomposition and surface reaction of molecular precursors like ammonia boranes (NH3BH3) and borazine (B3N3H6). These molecular precursors contain pre-existing B-N bonds, thus promoting the nucleation of BN. However, the quality and phase purity of resulting BN films are greatly influenced by the film preparation and deposition process conditions that remain a substantial concern. This study aims to comprehensively investigate the impact of varied CVD systems, parameters, and precursor chemistry on the synthesis of high-quality, large scale hBN on both catalytic and non-catalytic substrates. The comparative analysis provided new insights into most effective approaches concerning both quality and scalability of vapor phase grown hBN films.

Synthesis, Characterization, and Thermal Properties of Liquid Iron Amidinates and Their Potential as Chemical Vapor Deposition Precursors for Producing Iron-Containing Films

Synthesis, Characterization, and Thermal Properties of Liquid Iron Amidinates and Their Potential as Chemical Vapor Deposition Precursors for Producing Iron-Containing Films

Targeting Manganese Amidinate and ß-Ketoiminate Complexes as Precursors for Mn-Based Thin Film Deposition.

With a focus on Mn based organometallic compounds with suitable physico-chemical properties to serve as precursors for chemical vapor deposition (CVD) and atomic layer deposition (ALD) of Mn-containing materials, systematic synthetic approaches with ligand variation, detailed characterization, and theoretical input from density functional theory (DFT) studies are presented. A series of new homoleptic all-nitrogen and mixed oxygen/nitrogen-coordinated Mn(II) complexes bearing the acetamidinate, formamidinate, guanidinate and ß-ketoiminate ligands have been successfully synthesized for the first time. The specific choice of these ligand classes with changes in structure and coordination sphere and side chain variations result in significant structural differences whereby mononuclear and dinuclear complexes are formed. This was supported by density functional theory (DFT) studies. The compounds were thoroughly characterized by single crystal X-ray diffraction, magnetic measurements, mass spectrometry and elemental analysis. To evaluate their suitability as precursors for deposition of Mn-based materials, the thermal properties were investigated in detail. Mn(II) complexes possessing the most promising thermal properties, namely Bis(N,N'-ditertbutylformamidinato)manganese(II) (IV) and Bis(4-(isopropylamino)pent-3-en-2-onato)manganese(II) (ßIII) were used in reactivity studies with DFT to explore their interaction with oxidizing co-reactants such as oxygen and water which will guide future CVD and ALD process development.

Compounds with Chelating 3-Dimethylamino-1-propyl Ligands as Chemical Vapor Deposition Precursors. Synthesis and Characterization of M[(CH2)3NMe2]2 Complexes of Nickel(II), Palladium(II), and Platinum(II)

Compounds with Chelating 3-Dimethylamino-1-propyl Ligands as Chemical Vapor Deposition Precursors. Synthesis and Characterization of M[(CH₂)₃NMe₂]₂ Complexes of Nickel(II), Palladium(II), and Platinum(II)

In situ mass spectrometric investigation to probe GeSn growth dynamics and mechanisms in the chemical vapor deposition processes

GeSn has attracted increasing attention due to its tunable bandgap from indirect to direct resulting in unique electronic and optoelectronic capabilities. Chemical vapor deposition (CVD) is well acknowledged as an advanced growth method for GeSn, demonstrating its capability with grown materials for infrared lasers and detector development. As an in situ diagnostics of the CVD process, a residual gas analyzer (RGA) could enable the detection of all gaseous species during growth, thus probing the growth dynamics and mechanism. Therefore, it is highly desirable but is lacking in the (Si)GeSn research community. This work utilized an RGA equipped with a specialized differential vacuum pumping system to analyze the mass spectra of the GeSn CVD precursors of SnCl4, GeH4, and their combination. The spectra of SnCl4 collected from gas samples at a standard base pressure of 0.3 Torr display consistent fragments with central mass-to-charge ratios (m/z) of 78, 120, 155, 225, and 260, corresponding to Sn+, SnCl2+, SnCl+, SnCl2+, SnCl3+, and SnCl4+, respectively. These profiles closely resemble those observed in the National Institute of Standards and Technology and Wiley and Matsumoto et al. The simultaneous introduction of SnCl4 and GeH4 at a combined pressure of several tens of Torr yields spectra, indicating a chemical reaction that produces GeCl4 at room temperature in the vacuum chamber and tubing. Utilizing Gaussian16 and ORCA codes, the ab initio and density functional thermochemistry computations were employed to predict potential essential reactions and validate the experimental findings.

Volatile N,N′-dialkyl-β-dialdiminate complexes of magnesium and zinc as possible chemical vapor deposition precursors

A series of volatile magnesium(II) and zinc(II) bis(β-dialdiminato) complexes (of stoichiometry M[RN(CH)3NR]2 where R = methyl, ethyl, isopropyl, or tert-butyl) have been prepared and investigated as potential precursors for the chemical vapor deposition of thin films. Several of the compounds have been characterized crystallographically; the metal centers adopt pseudo-tetrahedral geometries with the two ligands forming six-membered rings with the metal center. All eight compounds sublime cleanly both in vacuum and under 1 atm of N2, and the most volatile compounds (those having N-methyl substituents) have vapor pressures of 1 Torr at 88 (Mg) and 90 °C (Zn). In general, metal complexes of N,N′-dialkyl-β-dialdiminate ligands are more volatile than their corresponding N,N′-dialkyl-β-diketiminate and N,N-dialkylamidinate analogues.

Lightweight, freestanding hybrids of graphene and hexagonal boron nitride foams

Lightweight boron carbon nitride foams which are a hybrid of graphene and hexagonal boron nitride (hBN), are produced using the nickel template method and remain freestanding after the etching of nickel without the stabilization of a polymer during etching. Chemical vapor deposition (CVD) precursors with varying carbon content were utilized, but only a 10% range of carbon atomic percentage variation is observed in the resulting boron carbon nitride foams due to the high carbon solubility of nickel. However, the cooling rate during the CVD process has a much more significant effect on the carbon content. Moreover, a 6.5-fold increase in resistivity is observed for our foams compared to a graphene foam with an overall trend of decreasing resistance with increasing carbon content. Furthermore, when the foam is infiltrated with epoxy to form a composite at 0.3% volumetric percentage concentration, there is a 27% increase in the thermal conductivity over hBN foam composites. These foams are suitable to use as a substitute for graphene foams when a lower electrical conductivity is desired and could potentially be used as a thermal interface material if a higher pore density nickel template is utilized.

Guiding Principles for the Design of a Chemical Vapor Deposition Process for Highly Crystalline Transition Metal Dichalcogenides

2D transition metal dichalcogenides (TMDs) for advanced logic transistor technologies are deposited by various modifications of the chemical vapor deposition (CVD) method using a wide variety of precursors. Being a major electrical performance limiter, the TMD crystal grain size strongly differs between the various CVD precursor chemistries from nano‐ to millimeter‐sized crystals. However, it remains unclear how the CVD precursor chemistry affects the nucleation density and resulting TMD crystal grain size. This work postulates guiding principles to design a CVD process for highly crystalline TMD deposition using a quantitative analytical model benchmarked against the literature. The TMD nucleation density reduces favorably under low supersaturation conditions where the metal precursor sorption on the starting surface is reversible and the corresponding metal precursor desorption rate exceeds the overall deposition rate. Such reversible precursor adsorption guarantees efficient long‐range gas‐phase lateral diffusion of precursor species in addition to short‐range surface diffusion, which vitally increases crystal grain size. The proposed model explains the large spread in experimentally observed TMD nucleation densities and crystal grain sizes for state‐of‐the‐art CVD chemistries. Ultimately, it empowers the reader to interpret and modulate precursor adsorption and diffusion reactions through designing CVD precursor chemistries.

The investigation of initial decomposition paths of methyltrichlorosilane on (0001) and (000[formula omitted]) surfaces of 4H-SiC: A DFT study

Methyltrichlorosilane (CH3SiCl3, MTS), a favored precursor for chemical vapor deposition (CVD) process of silicon carbide (SiC) epitaxial layer fabrication in a wide variety of temperatures, has been investigated prevalently. In this work, the investigation focused on the initial decomposition paths of MTS with active surface sites on 4H-SiC surfaces. The surfaces employed in this study include both (0001) face and (0001¯) face, which represent Si face and C face, respectively. The decomposition pathways of MTS decomposed with the active surface sites of 4H-SiC surfaces were researched theoretically with employment of transition state theory (TST). The barriers of Gibbs free energies and the estimated reaction rate constants were acquired to discuss the decomposition reactivity of MTS with the active sites on 4H-SiC surfaces. This work shows that MTS has tendency to decompose on both (0001) face and (0001¯) face of 4H-SiC, the H and Cl atoms can be abstracted by active surface sites on (0001) face and (0001¯) face, and impingement of MTS to active surface sites on (0001) and (0001¯) surfaces can yield C-containing and Si-containing surface species.

Area-selective atomic layer deposition on 2D monolayer lateral superlattices

The advanced patterning process is the basis of integration technology to realize the development of next-generation high-speed, low-power consumption devices. Recently, area-selective atomic layer deposition (AS-ALD), which allows the direct deposition of target materials on the desired area using a deposition barrier, has emerged as an alternative patterning process. However, the AS-ALD process remains challenging to use for the improvement of patterning resolution and selectivity. In this study, we report a superlattice-based AS-ALD (SAS-ALD) process using a two-dimensional (2D) MoS2-MoSe2 lateral superlattice as a pre-defining template. We achieved a minimum half pitch size of a sub-10 nm scale for the resulting AS-ALD on the 2D superlattice template by controlling the duration time of chemical vapor deposition (CVD) precursors. SAS-ALD introduces a mechanism that enables selectivity through the adsorption and diffusion processes of ALD precursors, distinctly different from conventional AS-ALD method. This technique facilitates selective deposition even on small pattern sizes and is compatible with the use of highly reactive precursors like trimethyl aluminum. Moreover, it allows for the selective deposition of a variety of materials, including Al2O3, HfO2, Ru, Te, and Sb2Se3.

Early events in the mechanism of single-source chemical vapor deposition of zirconium and hafnium diboride: a computational investigation.

Chemical vapor deposition (CVD) of group 4 metal-diboride ceramics from a single source is a versatile technique that finds many applications from hypersonic flight to microelectronics. Though the kinetics of CVD have been studied extensively-allowing significant process improvements-a mechanistic understanding of the process has yet to be attained. Computations suggest two plausible reaction pathways-one higher-energy and the second lower-that correlate well with experimental results reported in the literature, explaining phenomena such as high-temperature deposition resulting in films overstoichiometric in boron. These insights offer a new perspective that may be instrumental in the rational design of new precursors for single-source CVD.

Synthesis, Characterization, and Single-Crystal X-ray Structures of Refractory Metal Compounds as Precursors for the Single-Source Chemical Vapor Deposition of Metal Nitrides.

The chemical vapor deposition of refractory metal nitrides requires volatile precursors and has previously been achieved by using metal complexes containing a variety of imide ligands. Recently, the 1,4-di-tert-butyl-1,3-diazabutadiene (DAD) adduct of bis(tert-butylimide)dichloridemolybdenum(VI) was shown to be an excellent precursor for the single-source CVD of Mo2N thin films. Leveraging the success of this work, we prepared chromium and tungsten compounds with the same framework. Additionally, the framework has been modified slightly to allow the isolation of mono(tert-butylimide)trichloride complexes of vanadium, niobium, tantalum, and molybdenum(V) to extend the search for new vapor-phase precursors. These compounds were all fully characterized using the standard methods of multinuclear magnetic resonance spectroscopy, combustion analysis, and single-crystal X-ray diffraction. Their thermal properties were determined by using thermogravimetric analysis and differential scanning colorimetry to assess their utility as vapor-phase precursors. Finally, preliminary deposition studies were carried out to investigate their potential as single-source CVD precursors.

Complexes of Silver 1,1,1,5,5,6,6,6-Octafluorohexane-2,4-dionate with π-Donor Ligands: Synthesis, Structure, and Thermal Properties

Two new Ag(I) complexes with 1,1,1,5,5,6,6,6-octafluorohexane-2,4-dionate ion (Ofhac) and π‑donor neutral ligands, vinyltriethylsilane (VTES) or cycloocta-1,5-diene (COD), were synthesized with the goal to expand the library of silver precursors for chemical vapor deposition. The products were characterized by elemental analysis and IR and NMR spectroscopy. The complex [Ag(VTES)(Ofhac)] (I) was liquid under standard conditions; the temperature of its crystallization was below –20°C. Treatment of I with benzene gave rise to crystals of [Ag4(C6H6)2(Ofhac)4]∞ (II), which was confirmed by NMR and X-ray diffraction (CCDC no. 2232810). The structure of [Ag(COD)(Ofhac)]2 (III) was established by X-ray diffraction (CCDC no. 2232809). The binuclear molecules are formed due to the μ2-κ1(O):κ1(O') function of the Ofhac ligands (Ag–O, 2.458(2)–2.461(2) Å), while COD is κ2-η2:η2-coordinated (Ag–C, 2.420(17)–2.684(11) Å). The thermal properties of I and III in comparison with analogues containing 1,1,1,5,5,5-hexafluoropentane-2,4-dionate ion (Hfac) were studied by thermogravimetry.

Toward an understanding of the mechanism of mixed-salt-mediated CVD growth of MoSe2

The use of liquid precursors in chemical vapor deposition (CVD) techniques is advantageous for growing large-area, uniform two-dimensional (2D) transition-metal dichalcogenides (TMDs) compared to conventional methods using solid precursors. While various liquid precursors have been explored, recent studies highlight the use of mixed-salt precursors for growing uniform and wafer-scale TMDs. In this study, we propose a growth mechanism and present our findings on the epitaxial growth of MoSe2 domains as a function of annealing/growth time and H2 flow rate using Na2MoO4 and Na2SeO3 mixed-salt precursors. We confirm that the increase in the annealing time enhances the distribution of spin-coated precursors, leading to a rise in flake number density. On the other hand, prolonged growth time results in better-aligned MoSe2 flakes along the c-sapphire substrate step-edges. A significant finding is the dynamic diffusion of dendritic structures within large domains over the growth period, owing to the constant dissolution and recrystallization in the presence of residual liquid alloys. An increase in the H2 flow during CVD growth yields small, triangular domains aligned with the step edges, a result of the efficient reduction of precursor alloys and subsequent selenization. Our results provide an insight on achieving uniform and aligned morphology in CVD growth of 2D TMDs using liquid-phase precursors, a crucial step toward large-area fabrication.

In Pursuit of Next Generation N-Heterocyclic Carbene-Stabilized Copper and Silver Precursors for Metalorganic Chemical Vapor Deposition and Atomic Layer Deposition Processes

Volatile, reactive, and thermally stable organometallic copper and silver complexes are of significant interest as precursors for the metalorganic chemical vapor deposition (MOCVD) and atomic layer deposition (ALD) of ultra-thin metallic films. Well-established CuI and AgI precursors are commonly stabilized by halogens, phosphorous, silicon, and oxygen, potentially leading to the incorporation of these elements as impurities in the thin films. These precursors are typically stabilized by a neutral and anionic ligand. Recent advancements were established by the stabilization of these complexes using N-heterocyclic carbenes (NHCs) as neutral ligands. To further enhance the reactivity, in this study the anionic ligand is sequentially changed from β-diketonates to β-ketoiminates and β-diketiminates, yielding two new CuI and two new AgI NHC-stabilized complexes in the general form of [M(NHC) (R)] (M = Cu, Ag; R = β-ketoiminate, β-diketiminate). The synthesized complexes were comparatively analyzed in solid, dissolved, and gaseous states. Furthermore, the thermal properties were investigated to assess their potential application in MOCVD or ALD. Among the newly synthesized complexes, the β-diketiminate-based [Cu(tBuNHC) (NacNacMe)] was identified to be the most suitable candidate as a precursor for Cu thin film deposition. The resulting halogen-, oxygen-, and silicon-free CuI and AgI precursors for MOCVD and ALD applications are established for the first time and set a new baseline for coinage metal precursors.

Role of density gradients in the growth dynamics of 2-dimensional MoS2 using liquid phase molybdenum precursor in chemical vapor deposition

The key objective of this work was the comparison of MoS2 2-dimensional monolayer flakes grown by Chemical Vapour Deposition with different density gradients in the liquid precursors solution. Structures grown using solutions with iodixanol or glycerol were characterized by Optical Microscopy, Atomic Force Microscopy, Raman spectroscopy and Photoluminescence. Furthermore, electrical properties of structures were studied by characterizing backgated Field-Effect Transistors fabricated on individual flakes.The fundamental outcome of this study is that there are relevant differences between the structures grown with the two different liquid solutions: MoS2 structures grown with glycerol present a large variability in the lateral size of the sharp-corner triangular structure with a good crystalline quality while the employment of iodixanol (the most used substance) causes a poorer crystalline quality, demonstrated by the presence of nanometric pores, with a more homogeneous sizing of the triangular structures.The poor crystalline quality of the iodixanol samples results in reduced light emission efficiency and lower mobility.