Fine-grained 3C-SiC thick films prepared via hybrid laser chemical vapor deposition

An ultraviolet laser (λ = 266 nm) operated in pulsed mode and a diode laser (λ = 1060 nm) operated in continuous mode were simultaneously applied to create a hybrid laser chemical vapor deposition (CVD) approach. Fine-grained 3C-SiC thick films were prepared via hybrid laser CVD by using SiCl₄, CH₄ and H₂ as precursors. The effects of the ultraviolet laser on the preferred orientations, microstructures, microhardness values and deposition rates of 3C-SiC thick films were investigated. The 3C-SiC thick films that were prepared at 4 kPa via diode laser CVD exhibited <110>-orientations and 5-100 µm grain sizes, whereas those prepared via hybrid laser CVD were randomly oriented with 0.5-5 µm grain sizes. Compared to diode laser CVD, the additional irradiation of the ultraviolet laser in the hybrid laser CVD improved the Vickers microhardness values of the 3C-SiC thick films from 30 to 35 GPa, and the maximum deposition rate was also increased from 935 to 1230 µm/h.     

Microstructure and texture of polycrystalline 3C–SiC thick films characterized via EBSD

Cubic silicon carbide (3C–SiC) is an excellent protective film on graphite and has been fabricated via laser chemical vapor deposition (LCVD) with an extremely high deposition rate by our research group. To understand comprehensively its growth behavior, the polycrystalline 3C–SiC thick films with the preferred orientation of <111> and <110> were characterized by diverse measurements, especially electron back-scatter diffraction (EBSD). Along the growth direction of the <110>-oriented 3C–SiC, the microstructure changed from equiaxed grains to elongated grains with <111> orientation, and eventually the <110>-oriented columnar grains. The stacking faults in the <110>-oriented 3C–SiC could be marked as <11–20>-oriented 6H–SiC. On the other hand, in the <111>-oriented 3C–SiC films, the microstructure changed from mainly equiaxed grains to large columnar grains. The high-density stacking faults in <111>-oriented 3C–SiC films may lead to the identification of the nominal 2H, 4H and 6H polytypes by Raman spectra and EBSD. The (0001) planes of 2H-, 4H–SiC are perpendicular to the growth direction, while that of 6H–SiC are parallel.     

Formation of <111> fiber texture in β-SiC films deposited on Si(100) substrates

The evolution of the morphology and the texture of 3C-SiC films grown by chemical vapor deposition (CVD), using 1,3-disilabutane as precursor, on Si(100) substrates is investigated by transmission electron microscopy. Films were found to exhibit a columnar grain structure with a strong <111> fiber texture and a high density of stacking faults and twins. The columnar grains do not originate at the substrate surface but on a buffer layer about 3 to 5 nm thick, consisting of interconnected 3D-islands that initiate as epitaxial nuclei. The change from <100> epitaxial islands to <111> columnar grains can be understood in terms of anisotropic growth rates and multiple twinning. The observed <111> fiber texture, faulted substructure, faceted surface morphology and carbon enrichment of the growth surface are in agreement with the proposed growth model.     

High‐speed heteroepitaxial growth of 3C‐SiC (111) thick films on Si (110) by laser chemical vapor deposition

3C‐SiC (111) thick films were grown on Si (110) substrate via laser chemical vapor deposition (laser CVD) using hexamethyldisilane (HMDS) as precursor and argon (Ar) as dilution gas. The 3C‐SiC (111) polycrystalline films were prepared at deposition temperature (T_dep) of 1423‐1523 K, whereas the 3C‐SiC (111) epitaxial films were obtained at 1573‐1648 K with the thickness of 5.40 to 9.32 μm. The in‐plane relationship was 3C‐SiC [‐1‐12]//Si [001] and 3C‐SiC [‐110]//Si [‐110]. The deposition rates (R_dep) were 16.2‐28.0 μm/h, which are 2 to 100 times higher than that of 3C‐SiC (111) epi‐grown on Si (111) by conventional CVD. The growth mechanism of 3C‐SiC (111) epitaxial films has also been proposed.     

High‐Speed Preparation of <111>‐ and <110>‐Oriented β‐SiC Films by Laser Chemical Vapor Deposition

Highly oriented <111> and <110> β‐SiC films were prepared on Si(100) single crystal substrates by laser chemical vapor deposition using a diode laser (wavelength = 808 nm) and HMDS (Si(CH₃)₃–Si(CH₃)₃) as a precursor. The effects of laser power (P_L), total pressure (P_tot), and deposition temperature (T_dep) on the orientation, microstructure, and deposition rate (R_dep) were investigated. The orientation of the β‐SiC films changed from <111> to random to <110> with increasing P_L and P_tot. The <111>‐, randomly, and <110>‐oriented β‐SiC films exhibited dense, cauliflower‐like, and cone‐like microstructures, respectively. Stacking faults were observed in the <111>‐ and <110>‐oriented films, and aligned parallel to the (111) plane in the <111>‐oriented film, whereas they were perpendicular to the (110) plane in the <110>‐oriented film. The highest R_dep of the <111>‐oriented β‐SiC film was 200 μm/h at P_tot = 200 Pa and T_dep = 1420 K, whereas that of the <110>‐oriented film was 3600 μm/h at P_tot = 600 Pa and T_dep = 1605 K.     

Formation of qualified epitaxial graphene on Si substrates using two-step heteroexpitaxy of C-terminated 3C-SiC(-1-1-1) on Si(110)

Formation of epitaxial graphene (EG) on 3C–SiC films heteroepitaxially grown on Si substrates, otherwise known as graphene-on-silicon (GOS) technology, has a high potential in future nanocarbon-based electronics. The EG's quality in GOS however remains mediocre due mostly to the high density of crystal defects in the 3C–SiC/Si films caused by the large (~ 20%) lattice-mismatch between Si and 3C–SiC crystals. Resultant Si out-diffusion along the planar defects during the high-temperature (~ 1525 K) graphitization annealing can also account for the degradation. Here we propose a two-step growth technique that consists of seeding of rotated 3C-SiC(-1-1-1) crystallites on the Si(110) substrate, conducted in the high-temperature-low-pressure regime, followed by a rapid growth of SiC films in the low-temperature-high-pressure regime. We succeeded in forming an almost lattice-relaxed 3C-SiC(-1-1-1) film on Si(110), having a sufficient thickness (~ 200 nm) that we believe is able to suppress the Si out-diffusion during graphitization. A graphitization annealing applied to this epi-film yields an EG, whose domain size is increased by 60% as compared to that of conventional GOS films.     

Surface Chemistry Involved in Epitaxy of Graphene on 3C-SiC(111)/Si(111)

Surface chemistry involved in the epitaxy of graphene by sublimating Si atoms from the surface of epitaxial 3C-SiC(111) thin films on Si(111) has been studied. The change in the surface composition during graphene epitaxy is monitored by in situ temperature-programmed desorption spectroscopy using deuterium as a probe (D₂-TPD) and complementarily by ex situ Raman and C1s core-level spectroscopies. The surface of the 3C-SiC(111)/Si(111) is Si-terminated before the graphitization, and it becomes C-terminated via the formation of C-rich (6√3 × 6√3)R30° reconstruction as the graphitization proceeds, in a similar manner as the epitaxy of graphene on Si-terminated 6H-SiC(0001) proceeds.     

Origin of Different Growth Modes for Epitaxial Manganite Films

The microstructures of the Bi_0.4Ca_0.6MnO₃ (BCMO) and La_0.67Ca_0.33MnO₃ (LCMO) epitaxial films are investigated by transmission electron microscopy in detail. BCMO epitaxial films (~ 10 and ~ 40 nm) exhibit an island growth mode whereas the LCMO films (~ 6 and ~ 30 nm) follow a layer by layer growth mode. Combined with the critical thickness models for the expected onset of the misfit dislocations in epitaxial films, an atomic collapse model is introduced to explain their mechanism of formation in manganite films. At the beginning of deposition, the strain caused by the lattice mismatch between the epitaxial film and substrate can be accommodated by elastic deformation. With the increase of film thickness, the strain becomes larger and larger. When the film thickness reaches the critical thickness, the strain can only be relaxed by the formation of misfit dislocations. Meanwhile, the atomic configuration of the epitaxial film will reorganize and some atoms begin to collapse, thus an island morphology will be formed. Once the collapse morphology is formed, maintenance of this wave‐like morphology depends on atomic diffusion length of the deposited atoms. If the diffusion length of the deposited atoms is long, the island morphology will not be maintained. If the diffusion length of the deposited atoms is short, the island morphology will keep until the epitaxial film is thick enough. The results could shed light on the growth modes for other perovskite epitaxial films.     

Micro-Raman Mapping of 3C-SiC Thin Films Grown by Solid–Gas Phase Epitaxy on Si (111)

A series of 3C-SiC films have been grown by a novel method of solid–gas phase epitaxy and studied by Raman scattering and scanning electron microscopy (SEM). It is shown that during the epitaxial growth in an atmosphere of CO, 3C-SiC films of high crystalline quality, with a thickness of 20 nm up to few hundreds nanometers can be formed on a (111) Si wafer, with a simultaneous growth of voids in the silicon substrate under the SiC film. The presence of these voids has been confirmed by SEM and micro-Raman line-mapping experiments. A significant enhancement of the Raman signal was observed in SiC films grown above the voids, and the mechanisms responsible for this enhancement are discussed.     

Crystalline texture analyses of sputtered BiFeO3 thick films on Si with a large polarization

In our work last year (H. Zhu et al., Rhombohedral BiFeO₃ thick films integrated on Si with a giant electric polarization and prominent piezoelectricity, Acta Materialia 200 (2020) 305–314), it was demonstrated that the rhombohedral-like, (110)-textured BiFeO₃ thick films (～2 μm) sputter-deposited at 450 °C and 500 °C exhibited ultrahigh polarizations of P_r ～ 115 μC/cm² and 135 μC/cm², respectively. However, it is not sufficient to explain these ultra-high polarizations by a preferential growth mechanism and the effect of a moderate compressive strain. To further clarify the polarization enhancement of the films, the texture characteristics of these BFO thick films were quantitatively analyzed by fitting the rocking curves and pole figures to the March-Dollase model. The results showed that, in addition to the (110)-textured growth of a BFO thick film under a moderate compressive strain, the minority non-(110)-textured grains also contributed to the enhancement of the total polarization. Our study demonstrates that, the ultra-high polarizations of our BFO thick films can be well explained by adding the contribution from non-textured grains to the preferential growth of the film under a compressive strain.     

Effect of Pressure on Microstructure of <111>‐Oriented β‐SiC Films: Research via Electron Backscatter Diffraction

Scanning electron microscopy (SEM) and high‐resolution electron backscatter diffraction (EBSD) has been employed to study the microstructure development of <111 > ‐oriented β‐SiC films prepared by laser chemical vapor deposition (LCVD) with various total pressure (P_tot). The Surface morphology of films evolved from pyramids with sixfold symmetry to needlelike structure by increasing the P_tot. The EBSD results indicated that the higher P_tot (800 Pa) led to the lower neighbor‐pair misorientation and large in‐plane domains in β‐SiC films.     

Laser approaches for deposition of carbon nitride films — chemical vapour deposition and ablation

The deposition of carbon nitride films by laser-induced chemical vapour deposition (LCVD) from the system NH₃/CCl₄ and by laser ablation of a graphite target in nitrogen atmosphere has been studied. Two types of lasers were used: a copper bromide vapour laser for LCVD and an eximer KrF laser for the ablation process. The first laser source has a set of parameters (wavelengths, repetition rate, pulse duration) which makes it unique in material processing. During the ablation deposition an additional DC discharge (ignited by each shot) or RF discharge (permanent) was utilised to increase the reactivity of the nitrogen.The deposition rate for LCVD is almost one order of magnitude higher than for the laser ablation (0.050–0.350 and 0.025–0.040 nm/pulse, respectively) due to the direct laser impact on the substrate surface.The composition of the layers was studied by Auger electron spectroscopy (AES) and wavelength dispersive X-ray analysis (WDX). For the laser ablation the N/C ratio increases when an additional DC or RF discharge was used compared to the non-assisted process. Maximum values are reached at nitrogen pressures twice lower for RF discharge than for DC discharge assisted deposition. In the case of LCVD the composition depends mainly on the laser power.Chemical bonding was investigated by Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra of the films deposited by LCVD show well defined bands which are superimposed to a broad peak for samples prepared by laser ablation. At higher RF plasma power a weak band around 2200 cm⁻¹ appears in the spectra which can be attributed to the stretching mode of triple bonded C–N. Such a peak is not observed in cases of DC discharge and LCVD.     

Structural defects in (100) 3C-SiC heteroepitaxy: Influence of the buffer layer morphology on generation and propagation of stacking faults and microtwins

Growth of 3C-SiC on (100) Si has been performed via chemical vapor deposition under two pressure regimes (low and atmospheric pressure) in the early stage of growth. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) have been conducted to study the initial stage of growth while X-ray diffractometry (XRD) and TEM have been used to analyze thicker films and to detect and quantify defects, resulting in a comprehensive and detailed investigation of 3C-SiC structural defects. We have found out that the secondary nucleation of 3C-SiC island growth leads to a higher defect generation rate and, at the same time, to a more effective defect elimination rate. Hillocks found on the surface of thin samples grown under reduced pressure conditions are more pronounced as they seem to be a consequence of twins created in the early stage of growth. Finally, a different initial nucleation density (in the two pressure regimes considered) does not strongly influence stacking fault and microtwin density when growth of thick 3C-SiC films is performed. A very strong influence is indeed observed when 3C-SiC thickness is limited to hundreds of nanometers.     

Constraint Annealing of HfO2 Films on Silicon Substrate: Suppression of Si Outward Emission

Flexural tensile and compressive constraints were applied mechanically to the 7.5 nm thick HfO₂ films on Si substrates to investigate the influences of stress on the Si outward emission behavior in Si/HfO₂ during annealing. The constraint stress inhibited further growth of the interfacial layer (IL) between HfO₂ and Si, suppressing the IL‐growth‐induced Si outward emission. This fact was associated with atomic rearrangement that was induced during constrained annealing, resulting in the formation of a robust HfO₂ layer with low oxygen vacancy. Such an HfO₂ layer effectively suppressed the inward diffusion of oxygen, the IL growth and the Si out‐diffusion.     

Growth and characterization of epitaxial (La2/3Sr1/3)MnO3 films by pulsed laser deposition

High quality epitaxial (La_2/3Sr_1/3)MnO₃ (0 0 1) thin films were grown by pulsed laser deposition on SrTiO₃ (0 0 1) substrate at optimized growth parameters. The films quality was confirmed by both structural and physical properties characterization. Channeling Rutherford Backscattering Spectrometry characterization showed the minimal channeling coefficient as low as 4%. The LSMO thin films growth on SrTiO₃ substrate follows the island growth model. The Curie temperature of LSMO films is around 360 K, which is the one of the highest reported in literature. The resistivity of LSMO films showed the metal-insulate transition temperature coincides with the Curie temperature. This high quality LSMO is suitable for room temperature magnetic devices application.     

Crystallization characteristics of Li2O–Nb2O5 compound films on sapphire substrates and formation of crack-free and thick LiNbO3 epitaxial films

The formation of crystal phases in Li₂O–Nb₂O₅ compound films deposited on sapphire C-plane and A-plane substrates was studied by X-ray diffraction. Sufficiently oxidized samples with Li-deficient or stoichiometric compositions were prepared by co-sputtering from LiNbO₃ (LN) and Li₂O targets. Crystallization during deposition at elevated temperatures and solid-phase crystallization (SPC) of deposited amorphous films were investigated. For films on sapphire C-planes, nucleation into Li₃NbO₄(222) domains occurred at the onset temperature of crystallization. In the case of stoichiometric films, the LN(006) signal indicating epitaxial growth was the primary one for crystallization during deposition above 460 °C and SPC above 750 °C. Misoriented LN(104) domains tended to coexist with LN(006) domains. In the case of Li-deficient films, LiNb₃O₈(_602) domains coexisted with LN(006) at temperatures above 750 °C as a result of Li₂O loss. For films on sapphire A-planes, epitaxial LN(110) domains were predominant. Li₃NbO₄(222) domains were totally absent and the signal intensity of LiNb₃O₈(212) was less than 10% of that of LN(110) even for the Li-deficient films, which reflected fast crystallization of LN(110) domains. The SPC rate of stoichiometric film was considerably lower than that of Li-deficient film. As-crystallized LN film on the sapphire C-plane was strained with a narrow domain width. Thick film cracked as a result of stress caused by lattice mismatch with the substrate. In contrast, LN film processed by SPC was not strained and had large domains with flat film surface. Based on these results, crack-free 1-μm-thick LN epitaxial films on a sapphire C-plane were achieved. First, an amorphous LN buffer was subject to SPC to obtain a relaxed buffer layer. Subsequently, a stress-free thick LN overlayer was grown by co-sputtering from Li₂O and LN targets at 530 °C. The relaxed buffer layer effectively mitigated the strain caused by the lattice mismatch with the substrate.     

Diamond films with preferred <110> texture by hot filament CVD at low pressure

The effects of gas pressure on the textured growth of diamond films were investigated in a hot filament chemical vapor deposition (HFCVD) system. Diamond thin film with the growth rate of 1.3 μm/h and with high <110> texture was obtained at 5 Torr when lowering the gas pressure from 40 Torr to 1 Torr. The formation of high density nanocrystalline diamond nuclei elongated along the <110> direction in the nucleation stage and its consequent growth at lower pressure were considered to be responsible for the formation of <110> textured diamond thin film.     

Epitaxial gallium nitride thin films grown on silicon substrates utilizing gallium nitride seed-layer formed by liquid source precursor

Gallium nitride (GaN) epitaxial thin films were deposited on Si substrates by a modified hydride vapor phase epitaxy (MHVPE) technique utilizing the GaN seed-layer formed from liquid source precursor. Tris N,N-dimethyldithiocarbamato gallium(III) (Ga(mDTC)₃) powder was dissolved in chloroform (CHCl₃) to prepare the liquid source precursor for seed-layer formation. The developed method was found to be suitable for the epitaxial growth of GaN on Si in spite of the large mismatch in lattice constants and thermal expansion coefficients, resulting in device-quality epitaxial films with fairly smooth surface morphology. The epitaxial GaN films obtained in this study had a hexagonal structure with (0002) preferred orientation with the FWHM value of 428.6 arcsec of the (0002) GaN XRD peak. Photoluminescence spectra of GaN films exhibited a strong and sharp peak at 3.41 eV with the FWHM value of 107meV.     

Preparation by chemical solution deposition and transparent conducting properties of LaRh1-xNixO3 thin films

Transparent conducting thin films have been widely used in lots of fields. The absence of high-performance hole-type transparent conducting thin films, however, seriously limits the wider applications. LaRhO₃ as a type of perovskite material shows hole-type conduction with semiconductor-like properties and no investigations have been carried out about transparent conducting properties on LaRhO₃ thin films. Here, LaRh_1-xNi_xO₃ (x = 0, 0.05, 0.1) thin films were firstly deposited by chemical solution deposition, showing epitaxial growth on single crystal SrTiO₃ (001) substrates with the epitaxial relationship of LaRhO₃(001)[110]||SrTiO₃(001)[110]. With the doping of Ni element, the surface morphology became denser. Hall measurements confirmed that the hole concentration was enhanced with Ni doping, resulting in the decreased resistivity. Low resistivity of 17.3 mΩ cm at 300K was obtained for the LaRh_0.9Ni_0.1O₃ thin films. The electrical transport mechanisms were investigated, showing thermal activation at high temperatures and variable range hopping model for the doped thin films at low temperatures. The transmittance within the visible range for all thin films was higher than 50%. The results will provide a feasible route to deposit hole-type transparent conducting LaRhO₃-based thin films.     

Growth mechanism of 3C-SiC(lll) on Si without carbonization process

We have used rapid thermal chemical vapor deposition (RTCVD) technique to grow epitaxial SiC thin films on Si wafers without carbonization process by pyrolyzing tetramethylsilane (TMS). The growth rate of SiC films increases with TMS flow rate and temperature, but it decreases with temperature at higher TMS flow rates. The XRD spectra of the films indicate that the growth direction is along the (111) direction of β-SiC. IR and RBS measurements have been employed to analyze the chemical composition of the films. At 1100°C TMS molecules dissociate almost completely into Si atoms, CH₄ and C₂H₂ gases. The growth mechanism of SiC films on Si substrates without carbonization process has been proposed based on the analyses by TEM and QMS.