Liquid Injection MOCVD Research Articles

Heteroepitaxial growth of Ga2O3 on 4H-SiC by liquid-injection MOCVD for improved thermal management of Ga2O3 power devices

We report on the growth of monoclinic β- and orthorhombic κ-phase Ga2O3 thin films using liquid-injection metal-organic chemical vapor deposition on highly thermally conductive 4H-SiC substrates using gallium (III) acetylacetonate or tris(2,2,6,6-tetramethyl-3,5-heptanedionato) gallium (III). Both gallium precursors produced the β phase, while only the use of the latter led to growth of κ-Ga2O3. Regardless of the used precursor, best results for β-Ga2O3 were achieved at a growth temperature of 700 °C and O2 flows in the range of 600–800 SCCM. A relatively narrow growth window was found for κ-Ga2O3, and best results were achieved for growth temperatures of 600 °C and the O2 flow of 800 SCCM. While phase-pure β-Ga2O3 was prepared, κ-Ga2O3 showed various degrees of parasitic β phase inclusions. X-ray diffraction and transmission electron microscopy confirmed a highly textured structure of β- and κ-Ga2O3 layers resulting from the presence of multiple in-plane domain orientations. Thermal conductivities of 53 nm-thick β-Ga2O3 (2.13 + 0.29/−0.51 W/m K) and 45 nm-thick κ-Ga2O3 (1.23 + 0.22/−0.26 W/m K) were determined by transient thermoreflectance and implications for device applications were assessed. Presented results suggest great potential of heterointegration of Ga2O3 and SiC for improved thermal management and reliability of future Ga2O3-based high power devices.

Thermal stability of rhombohedral α- and monoclinic β-Ga2O3 grown on sapphire by liquid-injection MOCVD

We investigate the thermal stability of single-phase undoped epitaxial rhombohedral α- and bi-axially-symmetric, monoclinic β-Ga2O3 thin films grown on sapphire substrates by liquid-injection metalorganic chemical vapor deposition (LI-MOCVD) by means of in-situ high-temperature X-ray diffraction (HT-XRD) and room temperature (RT) cathodoluminescence (CL). Ga2O3 layers were vacuum annealed up to 1100 °C, while monitoring the respective 303‾0 and 6‾03 reflections of α- and β-phase Ga2O3. α-Ga2O3 layers were found to withstand the vacuum annealing at 700 °C/20 min, 750 °C/10 min, and 800 °C/8 min without notable degradation, however an indication of reversible biaxial in-plane compressive strain was observed for annealing temperatures close to 800 °C. Annealing at temperatures above 825 °C led to complete layer degradation and partial phase transformation to (010) β-Ga2O3. β-Ga2O3 layers showed no notable degradation or structural changes after the annealing at 1100 °C/30 min, however an indication of reversible biaxial in-plane tensile strain was observed for prolonged annealing at 1100 °C. RT CL spectra revealed the Ga2O3 emission consisted of several distinctive luminescence bands (red, green, blue, and two UV bands). Relatively weak but distinctive signal maxima with energy close to the expected bandgap energies of α- and β-Ga2O3, at 5.3 and 5.0 eV, respectively, were observed and attributed to their near band-edge (NBE) emissions.

Structure and Thermal Stability of ε/κ-Ga2O3 Films Deposited by Liquid-Injection MOCVD.

We report on crystal structure and thermal stability of epitaxial ε/κ-Ga2O3 thin films grown by liquid-injection metal-organic chemical vapor deposition (LI-MOCVD). Si-doped Ga2O3 films with a thickness of 120 nm and root mean square surface roughness of ~1 nm were grown using gallium-tetramethylheptanedionate (Ga(thd)3) and tetraethyl orthosilicate (TEOS) as Ga and Si precursor, respectively, on c-plane sapphire substrates at 600 °C. In particular, the possibility to discriminate between ε and κ-phase Ga2O3 using X-ray diffraction (XRD) φ-scan analysis or electron diffraction analysis using conventional TEM was investigated. It is shown that the hexagonal ε-phase can be unambiguously identified by XRD or TEM only in the case that the orthorhombic κ-phase is completely suppressed. Additionally, thermal stability of prepared ε/κ-Ga2O3 films was studied by in situ and ex situ XRD analysis and atomic force microscopy. The films were found to preserve their crystal structure at temperatures as high as 1100 °C for 5 min or annealing at 900 °C for 10 min in vacuum ambient (<1 mBar). Prolonged annealing at these temperatures led to partial transformation to β-phase Ga2O3 and possible amorphization of the films.

Low temperature Direct Liquid Injection MOCVD of amorphous CrCx coatings in large-scale reactors: An original route to nanostructured multilayer coatings

The effect of low temperature deposition on the quality and the microstructure of hard a -CrC x coatings grown by Direct Liquid Injection Metal-Organic Chemical Vapor Deposition (DLI-MOCVD) was investigated. At deposition temperatures higher than 525 °C, the coatings are bi-phased and polycrystalline , essentially composed of Cr 7 C 3 with Cr 3 C 2 as a minor phase. By decreasing the temperature in the 525–350 °C range, they are amorphous and exhibit a monolithic glassy-like microstructure without grain boundary. For temperatures lower than 350 °C, the uniform amorphous microstructure changes for a lamellar multilayer structure, while the composition of the gas phase was kept constant at the inlet of the reactor for all these CVD runs. The origin of this surprising lamellar structure has been investigated. After having discarded the idea of a self-organized multilayer growth, sometimes encountered in CVD processes, evidence was found for rapid changes in the deposition mechanism. They are induced by oscillations of the surface temperature around a critical value due to the regulation of the furnace used for such large-scale reactors. These almost periodic temperature changes lead to modulations in the surface composition of nutrient species, and subsequently, to the growth of multilayers. The individual composition of lamellae changes regularly, while the overall composition of the coatings remains identical to that of uniform monolithic coatings. For growth conditions leading to local temperature close to the critical value, a regular and controlled nanostructuring can be obtained with a period close to a hundred nanometers. Similar to monolithic a -CrC x coatings, these a -CrC x multilayer coatings exhibit a good barrier behavior against high temperature oxidation, according to the test performed. As future prospects, the main interest for these lamellar coatings could be their mechanical properties thanks to the numerous interfaces capable of deflecting and dissipating the propagation of cracks. • Chromium carbide coatings are deposited by DLI-MOCVD at very low temperatures. • A lamellar microstructure forms at temperatures as low as 325 °C. • Oxidation barrier behavior is fully conserved with the multilayer structure. • Changes in temperature and reaction mechanisms explain the lamellar growth.

Exploring the optical properties of Vernier phase yttrium oxyfluoride thin films grown by pulsed liquid injection MOCVD.

In this work, we report on the first successful deposition of Vernier phase yttrium oxyfluoride (V-YOF) thin films on Si (100) wafers using pulsed liquid injection metal organic chemical vapor deposition (PLI-MOCVD). The formation of V-YOF has been confirmed by X-ray diffraction measurements and electron probe microanalysis. The infrared phonon modes of V-YOF thin films and their corresponding optical constants as inferred from spectroscopic ellipsometry are reported here for the first time. The V-YOF thin films are featured by a broad absorption band centering around 370-373 cm-1; their refractive index is located between those of YF3 and Y2O3, and which shows an intimate relationship with the oxygen content in the film. The luminescence properties of Er3+ doped V-YOF thin films are finally examined and compared with that of YF3. Our results highlight that the as-deposited V-YOF thin films by PLI-MOCVD serve as promising candidates as efficient host material for spectral conversion for photovoltaic applications.

The quest towards epitaxial BaMgF4 thin films: exploring MOCVD as a chemical scalable approach for the deposition of complex metal fluoride films.

Conventional and Pulsed Liquid Injection MOCVD processes (C-MOCVD and PLI-MOCVD) have been explored as synthetic routes for the growth of BaMgF4 on Si (100) and single crystalline SrTiO3 (100) substrates. For the two applied approaches, the volatile, thermally stable β-diketonate complexes Ba(hfa)2tetraglyme and Mg(hfa)2(diglyme)2(H2O)2 have been used as single precursors (C-MOCVD) or as a solution multimetal source (PLI-MOCVD). Structural characterization through X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM) analyses confirmed the formation of epitaxial BaMgF4 films on SrTiO3 substrates. Energy dispersive X-ray (EDX) analyses have been used to confirm composition and purity of deposited films. The impact of process parameters on film properties has been addressed, highlighting the strong influence of precursor ratio, deposition temperature and oxygen partial pressure on composition, microstructure and morphology of the films. Both methods appear well suited for the growth of the BaMgF4 phase, but while PLI-MOCVD yields a more straightforward control of the precursor composition that reflects on film stoichiometry, C-MOCVD provides easier control of the degree of texturing as a function of temperature.

Heteroleptic titanium alkoxides as single-source precursors for MOCVD of micro-structured TiO2

[Ti(OR)4] (R=Pri or But) and their heteroleptic derivatives [Ti(OR)2(L)2] [L=bidentate donor-functionalized alkoxide: mmp (1-methoxy-2-methylpropan-2-olate) dmae (2-(dimethylamino)ethanoxide), dmop (2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)propan-2-olate) or dmom ((4,4-dimethyl-4,5-dihydrooxazol-2-yl)methoxide)] are investigated as single source precursors for liquid injection MOCVD of TiO2 on F-doped SnO2 substrates. The influence of the ligands on the phase and morphology of the deposited films is investigated, and [Ti(OPri)2(dmae)2] is found to be the most effective precursor for growth of nanostructured anatase. The identity of the monodentate alkoxide is found to have the greatest influence on the phase of the thin film: OPri-containing precursors deposit anatase under all conditions whereas OBut-containing precursors have a greater tendency to deposit rutile or mixed phases. The effect of film morphology on the photoinduced hydrophilic state is demonstrated. [Ti(OR)2(dmop)2] (R=Pri or But) have been characterised by single crystal X-ray diffraction.

Alumina particle reinforced TiO2 composite films grown by direct liquid injection MOCVD

The use of a liquid injection delivery system to form composite films containing nanoparticles was investigated. Al2O3–TiO2 films were grown on silicon substrates by direct liquid injection MOCVD (DLI-MOCVD) at 400 °C. The α-Al2O3 nanoparticles (α-Al2O3NPs) dispersed in TiO2 films resulted from co-deposition using colloidal α-Al2O3 solution and titanium tetraisopropoxide as titanium precursor. Scanning electron microscopy coupled with EDS as well as Raman spectroscopy confirmed the presence of α-Al2O3NPs aggregates embedded in the TiO2 matrix. The liquid injection system coupled with CVD technique can be promising to form composite films containing preformed nanoparticles.

Growth and structural characterization of YMnO3 thin films grown by pulsed liquid injection MOCVD on Si and SrTiO3 substrates

Optimum parameters for the growing of YMnO3 films by pulsed liquid injection metalorganic chemical vapor deposition have been studied. Si substrates were used for the optimization of the deposition process. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that polycrystalline single phase YMnO3 films can be obtained for an optimal ratio of Y and Mn on the injected solution and either amorphous, metastable orthorhombic, and/or hexagonal YMnO3 phases can be obtained depending on the deposition temperature and precursors ratio. In a second stage, YMnO3 films were grown on SrTiO3 substrates. Pure epitaxial orthorhombic YMnO3 phase was confirmed by XRD. The films microstructure, characterized by scanning electron microscopy and TEM, shows a columnar growth. Each columnar grain grows epitaxially with three possible orientations.

Growth of lanthanide-doped YF3 thin films by pulsed liquid injection MOCVD: Influence of deposition parameters on film microstructure

Yttrium fluoride (YF3) is a potential candidate as an ideal host material for rare-earth (RE) ions that allow the up- and down-conversion of light. We describe the deposition of Er/Yb co-doped YF3 thin films on Si(111) wafers using low pressure pulsed liquid injection MOCVD. Films were grown through the thermal decomposition of Y/RE(hfac)3 precursors dissolved in dimethoxyethane (0.05M) under 5torr of an O2/Ar atmosphere at different substrate temperatures between 450°C and 650°C. The film microstructure strongly depends on the operational conditions of the deposition and an optimization of the gas flow, oxygen content, pulse duration and deposition temperature was achieved. Under optimized conditions, films deposited at 590°C showed the best quality and no crack formation was observed on Si(111) for a film thickness <100nm. These films have been characterized by SEM, AFM, XRD and Raman techniques. Optical measurements, including the dispersion of the refractive index and light transmission, were performed on the Er/Yb:YF3 films grown under optimized conditions.

Interaction of nickel aminoalkoxide with samarium β-diketonate – Identification of new precursors for MOCVD synthesis of SmNiO3 perovskite films

The interaction of the nickel aminoalkoxide Ni(OCH(CH3)CH2NMe2)2 with commercial anhydrous samarium 2,2,6,6,-tetramethyl heptanedionate, Sm(thd)3, resulted in the isolation of mixed-ligand complexes, Sm2(OCH(CH3)CH2NMe2)2(thd)4 (1) and [Ni(OCH(CH3)CH2NMe2)(thd)(HOCH(CH3)CH2NMe2)·Ni(thd)2(HOCH(CH3)CH2NMe2)] (2), together with minor amounts of Ni(thd)2(HOCH(CH3)CH2NMe2)(H2O)0.5–x (3). The complexes have been structurally characterized by X-ray single crystal studies. A TGA investigation showed that both major products are volatile and evaporate essentially completely in the same temperature interval of 280–310°C, making mixed solutions of the nickel aminoalkoxide and Sm(thd)3, being potentially interesting as precursors for liquid injection MOCVD applications in the synthesis of SmNiO3 perovskite.

Liquid Injection MOCVD Grown Binary Oxides and Ternary Rare-Earth Oxide as Alternate Gate-Oxides for Logic Devices

ZrO2, HfO2 and DyScO3 thin films having thickness in the range 2-20 nm were grown on SiOx/Si(100) substrates in a multi-wafer planetary MOCVD reactor combined with a liquid delivery system using engineered precursors. Growth rate, surface morphology, crystal structure, crystal density of the as-deposited films were analysed as a function of deposition temperature. The influence of post deposition annealing on the densification and crystallization was studied. Electrical properties of MIS capacitor structures are also discussed. Results on the optimised gate stack of Pt/ZrO2/SiOx/Si, Pt/HfO2/SiOx/Si, Pt/DyScO3/SiOx/Si are finally compared; and DyScO3 seems to be promising high-k material candidate compared to Group-IVB oxides for the coming technology nodes.

Dimethylzinc adduct chemistry revisited: MOCVD of vertically aligned ZnO nanowires using the dimethylzinc 1,4-dioxane adduct

The dimethylzinc adduct [Me 2Zn(1,4-dioxane)] has been used for the growth of vertically aligned ZnO nanowires by liquid injection MOCVD. The ZnO nanowires were deposited at temperatures of 450 and 500 °C on Si(1 1 1) and F-doped SnO 2/glass substrates and XRD data showed that the ZnO nanowires were deposited in the wurtzitic phase. Room temperature PL data for the nanowires showed that they are of high crystalline quality with intense near band-edge emission at 3.28 eV and a very low intensity of defect-related green luminescence at 2.42 eV. Single crystal XRD data showed that the [Me 2Zn(1,4-dioxane)] adduct is polymeric with repeating [Me 2Zn] units bridged by monodentate 1,4-dioxane ligands.

Liquid Injection MOCVD Grown Binary Oxides and Ternary Rare-Earth Oxide as Alternate Gate-Oxides for Logic Devices

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Growth and Characterization of Ti‐Ta‐O Thin Films on Si Substrates by Liquid Injection MOCVD for High‐k Applications from Modified Titanium and Tantalum Precursors

AbstractTitanium oxide (TiO2) and titanium‐tantalum oxide (Ti‐Ta‐O) thin films are deposited by liquid injection (LI) metal‐organic (MO) CVD using metal amide‐malonate complexes, [Ti(NR2)2(dbml)2], and tantalum, [Ta(NMe2)4(dbml)] (R = Me, Et; dbml = di‐tert‐butylmalonato). TiO2 and Ti‐Ta‐O films are deposited on Si(100) in the temperature ranges 350–650 °C and 500–700 °C, respectively. The structure, morphology, and chemical composition of the films are evaluated by X‐ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), Rutherford backscattering spectroscopy (RBS), and X‐ray photoelectron spectroscopy (XPS). The electrical properties of the films, namely the dielectric properties, are assessed by carrying out capacitance‐voltage (C‐V) measurements on metal‐oxide‐semiconductor (MOS) capacitor structures.

Investigation of New 2,5‐Dimethylpyrrolyl Titanium Alkylamide and Alkoxide Complexes as Precursors for the Liquid Injection MOCVD of TiO2

AbstractThe new titanium 2,5‐dimethylpyrrolyl complexes [(Me2C4H2N)Ti(NMe2)3] and [(Me2C4H2N)Ti(µ2‐OiPr)(OiPr)2]2 and the previously reported titanium pyrrolyl complex [(C4H4N)Ti(µ2‐OiPr)(OiPr)2]2 are synthesized and chemically and structurally characterized. Single‐crystal X‐ray diffraction (XRD) data show that [(Me2C4H2N)Ti(NMe2)3] is monomeric with a pseudo‐tetrahedral geometry and contains an N‐σ bonded 2,5‐dimethylpyrrolyl ligand. The complexes [(Me2C4H2N)Ti (µ2‐OiPr)(OiPr)2]2 and [(C4H4N)Ti(µ2‐OiPr)(OiPr)2]2 are dimeric and contain two terminal and two bridging isopropoxy groups and terminal N‐σ‐bonded pyrrolyl ligands. The complexes [(Me2C4H2N)Ti(NMe2)3] and [(Me2C4H2N)Ti (µ2‐OiPr)(OiPr)2]2 are investigated as precursors for the deposition of TiO2 by liquid injection metal‐organic (MO)CVD. Titanium oxide films deposited at 450 °C from [(Me2C4H2N)Ti(NMe2)3] contain significantly more carbon impurities (C = 12.7 at.‐%) than those grown from [(Me2C4H2N)Ti(µ2‐OiPr)(OiPr)2]2 (C = 6.1 at.‐%). XRD analysis shows that the titanium oxide films deposited at 450 °C are amorphous.

Oxides heterostructures for nanoelectronics

We summarise in this paper the work of two groups focusing on the synthesis and characterisation of functional oxide for nanoelectronic applications. In the first section, we discuss the growth by liquid-injection MOCVD of oxides heterostructures. Interface engineering for the minimisation of silicate formation during the growth of polycrystalline SrTiO3 on Si is first presented. It is realised via the change of reactant flow or chemical nature at the Si surface. We then report on the epitaxy on oxide substrates of manganites films and superlattices and on their magnetic and electrical properties. La0.7Sr0.3MnO3 and La0.8MnO3-δ as well as multiferroic hexagonal ReMnO3 manganites are considered. We show that the film thickness and related strain may be used to tune the properties. Finally, we demonstrate the growth of MgO nanowires by CVD at a moderate temperature of 600°C, using gold as a catalyst. In the second section, we discuss the growth of epitaxial oxide heterostructures by MBE. First, the direct epitaxy of SrTiO3 on Si is considered. Issues and control of the SrTiO3/Si interface are discussed. An abrupt interface is achieved. We show that SrTiO3 on Si can be used as a buffer layer for the epitaxy of various perovskite oxides such as LaAlO3 or La0.7Sr0.3MnO3. La0.7Sr0.3MnO3 films are ferromagnetic and metallic at room temperature. The epitaxial growth of complex oxides on Si wafers opens up the route to the integration of a wide variety of functionalities in nanoelectronics. Finally, we discuss the monolithic integration of III-V compounds such as InP on Si using epitaxial SrTiO3 buffer layers for the future integration of optics on Si.

Preparation of High Permittivity GdScO3 Films by Liquid Injection MOCVD

We describe preparation and properties of GdScO3 thin films. The films were prepared by liquid injection metal-organic chemical vapor deposition, MOCVD at 600 ºC on (100) Si substrate. The as-deposited films were amorphous with a smooth surface and sharp GdScO3/Si interface. X-ray diffraction showed that the amorphous phase is well preserved upon rapid thermal annealing up to 1000 ºC. However, modification of the X-ray reflectivity pattern after annealing at 1000 ºC indicates increasing of the film thickness, presumably due to diffusion of silicon from the substrate into the whole volume of the film. Capacitance-voltage measurement resulted in dielectric constant of 22. It is shown, that exact stoichiometry of GdScO3 is not necessary to achieve dielectric constant above 20.

Flat-band Voltage and Structural Properties of Hafnium Dioxide Films Grown by Liquid-injection MOCVD

The flat-band voltage shift of bilayer SiO2/HfO2 MOS structures is investigated as a function of the structural properties of the HfO2 films. It was demonstrated that for amorphous HfO2 films, the flat-band voltage vs HfO2 thickness relationship is well described by a second-order polynomial resulting from a bilayer model including two interfacial charge densities and bulk high-k charges. When the crystalline structure of HfO2 is stabilized in a monoclinic phase, the flat-band voltage is shifted to positive voltages and becomes nearly independent on the HfO2 thickness. These results confirm that the high-k/SiO2 interface can significantly contribute to the flat-band voltage shift. However, this contribution appears greatly dependent on the microstructural properties of HfO2.

Remarkable Influence of molecular structure of N,N'-unsymmetrically substituted 1,3-amidinate and -guanidinate on the Volatility and the Thermal Stability of Precursors for HfO2 Films via Liquid Injection-MOCVD

The identification of appropriate ligand-metal combinations, which successfully present the features of : (i) having acceptable melting point, (ii) being more thermally stable than hafnium alkylamides, (iii) having sufficient volatility for vapor phase distribution and (iv) enabling the deposition by LI-MOCVD and ALD of hafnium containing thin films, should go through new generations of well-behaved molecular precursors. We report herein the chemical design and full characterizations of a portfolio of novel mono- and di- amidinate and guanidinate alkylamide hafnium complexes of N,N'-unsymmetrically substituted 1,3-amidinate and 1,3-guanidinate [R'-N=CX-NR"]xHf(NR2)4-x (R'≠ R" = Et, iPr, tBu ; R = Me or Et ; X = Me or NMe2 ; x = 1 or 2). We will exemplify the remarkable influence of the molecular structure on the melting point, the volatility and the thermal stability (see figure 1) and the nature (cubic, monoclinic) of HfO2 thin films by LI-MOCVD.