Growth Of Epitaxial Films Research Articles

Machine-learning-assisted analysis of transition metal dichalcogenide thin-film growth

In situ reflective high-energy electron diffraction (RHEED) is widely used to monitor the surface crystalline state during thin-film growth by molecular beam epitaxy (MBE) and pulsed laser deposition. With the recent development of machine learning (ML), ML-assisted analysis of RHEED videos aids in interpreting the complete RHEED data of oxide thin films. The quantitative analysis of RHEED data allows us to characterize and categorize the growth modes step by step, and extract hidden knowledge of the epitaxial film growth process. In this study, we employed the ML-assisted RHEED analysis method to investigate the growth of 2D thin films of transition metal dichalcogenides (ReSe2) on graphene substrates by MBE. Principal component analysis (PCA) and K-means clustering were used to separate statistically important patterns and visualize the trend of pattern evolution without any notable loss of information. Using the modified PCA, we could monitor the diffraction intensity of solely the ReSe2 layers by filtering out the substrate contribution. These findings demonstrate that ML analysis can be successfully employed to examine and understand the film-growth dynamics of 2D materials. Further, the ML-based method can pave the way for the development of advanced real-time monitoring and autonomous material synthesis techniques.Graphical

Epitaxial growth and scanning tunneling microscopy of LiV2O4 thin films on SrTiO3(111)

LiV2O4 is a mixed-valent spinel oxide and one of a few transition-metal compounds to host a heavy fermion phase at low temperatures. Although numerous experimental studies have attempted to elucidate how its 3d electrons undergo giant mass renormalization, spectroscopic probes that may provide crucial hints, such as scanning tunneling microscopy (STM), remain to be applied. A prerequisite is atomically flat and pristine surfaces, which, in the case of LiV2O4, are difficult to obtain by the cleavage of small, three-dimensional crystals. We report the epitaxial growth of LiV2O4 thin films with bulklike properties on SrTiO3(111) via pulsed laser deposition and stable STM imaging of the LiV2O4(111) surface. The as-grown films were transferred ex situ to a room-temperature STM, where subsequent annealing with optional sputtering in ultrahigh vacuum enabled compact islands with smooth surfaces and a hexagonal 1 × 1 atomic lattice to be resolved. Our STM measurements provide insights into the growth mechanisms of LiV2O4 on SrTiO3(111) as well as demonstrate the feasibility of performing surface-sensitive measurements of this heavy fermion compound.

Epitaxial growth of β-Ga2O3 thin films on SrTiO3 (1 1 1) and (1 0 0) substrates by chemical vapor deposition

Due to good lattice matching and bipolar doping behavior, wide bandgap SrTiO3 (STO) is a promising substrate for the heteroepitaxial growth of β-Ga2O3 to construct heterojunctions for (opto-)electronic device applications. However, the investigation of the growth of β-Ga2O3 film on the STO (111) substrate is still lacking. In this work, for the first time, the β-Ga2O3 (2¯ 01) thin films were epitaxially grown on the supercell matched STO (111) substrates by low pressure chemical vapor deposition. The effects of the source and growth temperatures on the surface roughness and crystalline quality of the β-Ga2O3 films were systemically investigated. By controlling the supply of Ga source, the heteroepitaxial β-Ga2O3 film grown under the optimized conditions exhibits a narrow X-ray diffraction rocking curve of 0.75° and a low root-mean-square roughness of 1.10 nm. Furthermore, the epitaxial growth of β-Ga2O3 films on the STO (100) substrate was also investigated. The heteroepitaxy of β-Ga2O3 films on STO lays the foundation for future device applications of β-Ga2O3-based heterojunctions.

Epitaxial growth of a β-Ga2O3 (−201)-oriented thin film on a threefold symmetrical SrTiO3 (111) substrate for heterogeneous integration

Epitaxial growth of a wide bandgap semiconductor β-Ga2O3 thin film with high crystal quality plays a decisive role in constructing optical and electronic devices. However, except for the native substrate, the scarcity of appropriate non-native substrates or the poor crystallization of the deposit in thin film growth severely limits the fabrication and applicability of the final heterostructures and devices. Here, by taking the consistent symmetry and closely matched atomic spacing between β-Ga2O3 (−201) and the cubic perovskite (111)-oriented plane of SrTiO3, we realize the epitaxial growth of single crystal β-Ga2O3 (−201) thin films on the SrTiO3 (111) substrate by the pulsed laser deposition method, as confirmed by wide-range reciprocal-space mapping and high-resolution scanning transmission electron microscopy. The fabricated β-Ga2O3 (−201) photodetector device on the SrTiO3 (111) substrate exhibits excellent ultraviolet optical detection performance with large on/off switching ratios and a fast response speed. Moreover, the β-Ga2O3/SrTiO3 (111) heterojunction shows type-II heterostructure characteristics for energy band alignment, which displays superior ability for electron–hole pairs separation with large conduction and small valance band offsets of 1.68 and 0.09 eV, respectively. The results offer us a new way to obtain high-quality β-Ga2O3 (−201) thin film heterostructures on cubic SrTiO3 (111) substrates and fabricate β-Ga2O3-based optical and electronic devices.

Epitaxial growth of high-quality yttria-stabilized zirconia films with uniform thickness on silicon by the combination of PLD and RF sputtering

High-quality (100)-oriented yttria-stabilized zirconia (YSZ) films with uniform thickness were epitaxially grown on silicon substrates by a two-step process of pulsed laser deposition (PLD) and radio-frequency magnetron sputtering (RFS). First, we fabricated a YSZ-seed layer on (001)-oriented silicon substrates using a PLD apparatus, and then transferred the YSZ-seed substrate to an RFS equipment to complete the growth of a thicker YSZ film. The dependence of YSZ crystal quality on RFS-growth temperature was investigated in the range of 200 to 850 °C. All the resulting YSZ films exhibited pure (100) orientation, and the full width at half maximum of the YSZ (200) X-ray rocking curve was as low as 0.64° for the YSZ film grown at 850 °C, and increased to 1.02° with the RFS-growth temperature declining to 200 °C. The results evidenced that the YSZ film can be epitaxially grown at 200 °C in the presence of a YSZ-seed layer. However, the splitting of the YSZ (200) X-ray diffraction peak appeared at 500 °C and 200 °C, which may be attributed to a transformation of the intrinsic stress in the YSZ films from compressive to tensile stress with decreasing temperature. In addition, the YSZ film had a uniform thickness distribution with an inhomogeneity of less than 5 % and a very flat and dense surface with root-mean-square surface roughness below 1 nm.

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Epitaxial growth of flexible 1T-VSe2 thin films on mica by pulsed laser deposition

Recently, vanadium diselenide (VSe2), a member of transition metal dichalcogenides, has attracted a great deal of interest in spintronic devices and memory devices due to its unique physical properties. However, it is still a challenge to prepare a continuous VSe2 thin film which is critical for its potential application. Here, we report a continuous single-crystalline 1T-VSe2 thin film grown on mica by pulsed laser deposition. Both x-ray diffraction and high-resolution transmission electron microscopy verify the van der Waals epitaxy of the VSe2/mica heterostructure. Free-standing and flexible VSe2 thin films can be obtained and combined with integrated circuit technology, which is of great significance for the application of two-dimensional materials in the field of multifunctional flexible electronic devices.

Ultrawide bandgap willemite-type Zn2GeO4 epitaxial thin films

Willemite-type Zn2GeO4 is a promising ultrawide bandgap semiconductor material. To date, experimental results on growth and physical properties of epitaxial thin films of willemite-type Zn2GeO4 are not available. Here, we report the heteroepitaxial growth of (00.1)-oriented Zn2GeO4 thin films on c-plane sapphire substrates using pulsed laser deposition. The in-plane orientation relationships are [11.0] Zn2GeO4//[11.0] Al2O3 and [11¯.0] Zn2GeO4//[11¯.0] Al2O3. A 450 nm thick epitaxial film with a surface roughness of 2.5 nm deposited under 0.1 mbar oxygen partial pressure exhibits a full width at half maximum (FWHM) of rocking curve of (00.6) reflex of 0.35°. The direct bandgap is evaluated to be 4.9 ± 0.1 eV. The valence band maximum is determined to be 3.7 ± 0.1 eV below the Fermi level. Together with the density-functional theory band structure calculation, it is suggested that the O 2p orbital and Zn 3d orbital dominantly contribute to the valence band of Zn2GeO4. The steady-state photoluminescence (PL) spectra of the films under 266 nm excitation at room temperature exhibit a broad defect-related emission band centered at 2.62 eV with a FWHM of 0.55 eV. The origin of this native defect-related PL is suggested to correlate with Zn interstitials. This work advances the fundamental study on willemite-type Zn2GeO4 epitaxial thin films for potential device application.

Enhancing the UV Response of All-Inorganic Perovskite Photodetectors by Introducing the Mist-CVD-Grown Gallium Oxide Layer

All-inorganic perovskites, with their low-cost, simple processes and superior heat stability, have become potential candidate materials for photodetectors (PDs). However, they have no representative responsivity in the deep-ultraviolet (UV) wavelength region. As a new-generation semiconductor, gallium oxide (Ga2O3), which has an ultrawide bandgap, is appropriate for solar-blind (200 nm–280 nm) deep-UV detection. In this work, ultrawide-bandgap Ga2O3 was introduced into an inorganic perovskite device with a structure of sapphire/β-Ga2O3/Indium Zinc Oxide (IZO)/CsPbBr3. The performance of this perovskite PD was obviously enhanced in the deep UV region. A low-cost, vacuum-free Mist-CVD was used to realize the epitaxial growth of β-Ga2O3 film on sapphire. By introducing the Ga2O3 layer, the light current of this heterojunction PD was obviously enhanced from 10−8 to 10−7, which leds its detectivity (D*) to reach 1.04 × 1012 Jones under a 254 nm light illumination with an intensity of 500 μW/cm2 at a 5 V bias.

Decay estimate and blow-up for a fourth order parabolic equation modeling epitaxial thin film growth

<abstract><p>In this paper, we study a fourth order parabolic equation modeling epitaxial thin film growth. By using the potential well method and some inequality techniques, we obtain the decay estimate of weak solutions. Meanwhile, the blow-up time is estimated from above and below. The blow-up rate is also derived.</p></abstract>

Nanoscale compositional segregation in epitaxial AlScN on Si (111).

We report the growth of epitaxial wurtzite AlScN thin films on Si (111) substrates with a wide range of Sc concentrations using ultra-high vacuum reactive sputtering. Sc alloying in AlN enhances piezoelectricity and induces ferroelectricity, and epitaxial thin films facilitate systematic structure-based investigations of this important and emerging class of materials. Two main effects are observed as a function of increasing Sc concentration. First, increasing crystalline disorder is observed together with a structural transition from wurtzite to rocksalt at ∼30 at% Sc. Second, nanoscale compositional segregation consistent with spinodal decomposition occurs at intermediate compositions, before the wurtzite to rocksalt phase boundary is reached. Lamellar features arising from composition fluctuations are correlated with polarization domains in AlScN, suggesting that composition segregation can influence ferroelectric properties. The present results provide a route to the creation of single crystal AlScN films on Si (111), as well as a means for self-assembled composition modulation.

Liquid-phase epitaxial growth of multiple MOF thin films with high lattice mismatch

We have prepared a series of multiple (bi-varietal and tri-varietal) pillar-layered MOF thin films [Cu2L2dabco]n (L = bdc, bpc and tpdc) with high lattice mismatch (up to 77.8%) using a liquid-phase epitaxial layer by layer method.

Corundum-structured α-Fe2O3 substrates for α-Ga2O3 epitaxial growth

The present study demonstrated the growth of α-Ga2O3 epitaxial thin films on α-Fe2O3 substrates via mist chemical vapor deposition. XRD analysis revealed that c-plane α-Ga2O3 thin films were epitaxially grown on a c-plane α-Fe2O3 substrate. The full width at half maximum for asymmetric diffraction (10–14) of α-Ga2O3 using α-Fe2O3 was lower than that obtained using α-Al2O3. Moreover, the XRD pole figure results revealed that α-Ga2O3 on α-Fe2O3 exhibited no in-plane rotations. We believe that α-Fe2O3 wafers with small lattice mismatches are promising substrates for α-Ga2O3 without rotational domains.

Epitaxial growth of ultrathin two-dimensional pentacene film with standing-up molecular geometry on nano-size-curved graphene surface

Molecule packing behavior at organic and graphene interface is essential for graphene-based organic electronics since charge carriers are transported within the very few organic layers nearest to graphene substrate. Although organic molecules especially the ones with planar aromatic rings usually adopt recumbent geometry on graphene substrate owning to the maximized π-π interaction, the alignment of organic molecules may be influenced by modifying the interfacial nature between organic molecules and graphene. Here, epitaxy growth of two-dimensional ultra-thin pentacene film with standing-up molecular geometry was observed for the first time on graphene with periodic nano-sized buckling structure. The curvature and strain of graphene was found collectively responsible for the standing-up geometry of pentacene and its oriented growth on graphene surface. This study provides a feasible way for controlling molecular epitaxy on 2D materials interfaces toward functional heterostructures.

Epitaxial growth of high quality Mn3Sn thin films by pulsed laser deposition

Noncollinear antiferromagnet Weyl semimetal Mn3Sn has recently attracted great research interest. Although large anomalous Hall effect (AHE), anomalous Nernst effect (ANE), and magneto-optical effect have been observed in Mn3Sn, most studies are based on single crystals. So far, it is still challenging to grow high quality epitaxial Mn3Sn thin films with transport and optical properties comparable to their single crystal counterparts. Here, we report the structure and magneto-optical and transport properties of epitaxial Mn3Sn thin films fabricated by pulsed laser deposition (PLD). Highly oriented Mn3+xSn1−x (0001) and (112¯0) epitaxial films are growth on single crystalline Al2O3 and MgO substrates. Large anomalous Hall effect up to ΔρH=3.02 μΩ cm and longitudinal magneto-optical Kerr effect with |θK| = 38.1 mdeg at 633 nm wavelength are measured at 300 K, which are comparable to Mn3Sn single crystals. Our work demonstrates that high quality Mn3Sn epitaxial thin films can be fabricated by PLD, paving the way for future device applications.

Epitaxial growth, magnetoresistance, and electronic band structure of GdSb magnetic semimetal films

Motivated by observations of extreme magnetoresistance (XMR) in bulk crystals of rare-earth monopnictide (RE-V) compounds and emerging applications in novel spintronic and plasmonic devices based on thin-film semimetals, we have investigated the electronic band structure and transport behavior of epitaxial GdSb thin films grown on III-V semiconductor surfaces. The ${\mathrm{Gd}}^{3+}$ ion in GdSb has a high spin $S=7/2$ and no orbital angular momentum, serving as a model system for studying the effects of antiferromagnetic order and strong exchange coupling on the resulting Fermi surface and magnetotransport properties of RE-Vs. We present a surface and structural characterization study mapping the optimal synthesis window of thin epitaxial GdSb films grown on III-V lattice-matched buffer layers via molecular-beam epitaxy. To determine the factors limiting XMR in RE-V thin films and provide a benchmark for band-structure predictions of topological phases of RE-Vs, the electronic band structure of GdSb thin films is studied, comparing carrier densities extracted from magnetotransport, angle-resolved photoemission spectroscopy (ARPES), and density-functional theory (DFT) calculations. ARPES shows a hole-carrier rich, topologically trivial, semimetallic band structure close to complete electron-hole compensation, with quantum confinement effects in the thin films observed through the presence of quantum-well states. DFT-predicted Fermi wave vectors are in excellent agreement with values obtained from quantum oscillations observed in magnetic field-dependent resistivity measurements. An electron-rich Hall coefficient is measured despite the higher hole-carrier density, attributed to the higher electron Hall mobility. The carrier mobilities are limited by surface and interface scattering, resulting in lower magnetoresistance than that measured for bulk crystals.

Van der Waals Epitaxy of HgCdTe Thin Films for Flexible Infrared Optoelectronics

AbstractVan der Waals epitaxial (vdW) growth of semiconductor thin films on 2D layered substrates has recently attracted considerable attention since it provides a potential pathway for realizing monolithically integrated devices and flexible devices. In this work, direct growth of epitaxial HgCdTe (111) thin films on 2D layered transparent mica substrates is achieved via molecular beam epitaxy. The full width at half maximum of the ω‐mode X‐ray diffraction peak is measured to be around 306 arc sec. Mid‐wave infrared photoconductors based on the as‐grown HgCdTe thin films have been demonstrated and the self‐heating effect has been evaluated. A peak responsivity at the wavelength of around 3500 nm is measured to be about 110 V W−1 at 80 K and 8 V W−1 at room temperature under a bias of 25 V cm−1. Twinning defects are observed, limiting the crystallinity and mobility‐lifetime product in HgCdTe/mica. Benefiting from the vdW epitaxial growth, an etch‐free layer transfer process for lifting off the HgCdTe from the mica substrate has been demonstrated, leading to large area free‐standing HgCdTe thin films.

Preparation of Bi-2212/YBCO heterostructure and their interfacial characters

YBa 2 Cu 3 O 7-X (YBCO) thin films were prepared on LaAlO 3 (LAO) substrates by a sol-gel method, and the epitaxial growth of Bi-2212 thin films on YBCO thin films was investigated. Both YBCO and Bi 2 Sr 2 Ca 1 Cu 2 O 8+σ (Bi-2212) bilayer films exhibit good biaxial texture and superconducting properties. Afterward, a cross-shaped Bi-2212/YBCO heterostructure was fabricated, and its interfacial atomic arrangement and I - V characteristics were analyzed. Atomic-resolution STEM images obtained from a spherical-aberration-corrected transmission electron microscope show that two superconducting films exhibit a layered structure and the atoms inside the films are artfully arranged. Moreover, the order of the seven atomic layers between Bi-2212 and YBCO layers with a thickness of about 1.32 nm is misarranged. Among them, the Y-O layer of YBCO and the Sr-O layer of Bi-2212 share a CuO 2 layer. The I-V curves of Bi-2212/YBCO bilayer films show that the seven misarranged atomic layers at Bi-2212/YBCO interface acts as a barrier layer, which means that a Josephson junction can be fabricated using this interface characters.

Thermodynamic Analysis of Group-III-Nitride Alloying with Yttrium by Hybrid Chemical Vapor Deposition.

Group-IIIb-transition-metal-alloyed wurtzite Group-IIIa-nitride (IIIb-IIIa-N) thin films have higher piezoelectric characteristics than binary IIIa-N for a broad range of applications in photonic, electronic, sensing, and energy harvesting systems. We perform theoretical thermodynamic analysis for the deposition and epitaxial growth of Y-alloyed GaN and AlN films by a newly introduced growth technique of hybrid chemical vapor deposition (HybCVD), which can overcome the limitations of the conventional techniques. We investigate the equilibrium vapor pressures in the source zones to determine the dominant precursors of cations for the input of the mixing zone. Then, we study the driving force for the vapor-solid phase reactions of cation precursors in the growth zone to calculate the relationship between the solid composition of YxGa1-xN and YxAl1-xN and the relative amount of input precursors (Y vs. GaCl and AlCl3) in different deposition conditions, such as temperature, V/III precursor input ratio, and H2/inert-gas mixture ratio in the carrier gas. The xY composition in YAlN changes nearly linearly with the input ratio of cation precursors regardless of the growth conditions. However, YGaN composition changes non-linearly and is also substantially affected by the conditions. The thermodynamic analysis provides insight into the chemistry involved in the epitaxial growth of IIIa-IIIb-N by the HybCVD, as well as the information for suitable growth conditions, which will guide the way for ongoing experimental efforts on the improvement of piezoelectricity of the lead-free piezoelectric materials.

The ultra-thin AlN epitaxy on monolayer WS2 by helicon sputtering at 400 °C

This work demonstrates the epitaxial growth of ultra-thin AlN films grown on monolayer WS2/Sapphire by helicon sputtering. In our work, WS2 acts as a template to facilitate the initial two-dimensional (2D) AlN growth via van der Waals epitaxy (VDWE), and this growth method provides a promising semiconductor manufacturing process with low thermal budget (<450 °C) than the required for the backend-of-the-line (BEOL). The ultrathin AlN epitaxy on WS2 is characterized by synchrotron-based grazing-incidence wide-angle x-ray scattering (GIWAXS). There are two stages in AlN epitaxy on the WS2; the first is the layered AlN growth aligned with the underlying WS2 lattice. The subsequent development of AlN transforms into 3D growth, which is similar to Stranski–Krastanov (SK) mode. The ultrathin AlN films achieve atomic level smoothness and significant piezoelectric properties as indicated by the technique of atomic- and piezo force microscopy (AFM/PFM). This research broadens the application of 2D materials in assisting AlN epitaxy and has a potential avenue for hetero-epitaxial integration in developing future electronic devices.