Epitaxial Thin Films Research Articles

Structural degeneracy and formation of crystallographic domains in epitaxial LaFeO3 films revealed by machine-learning assisted 4D-STEM

Structural domains and domain walls, inherent in single crystalline perovskite oxides, can significantly influence the properties of the material and therefore must be considered as a vital part of the design of the epitaxial oxide thin films. We employ 4D-STEM combined with machine learning (ML) to comprehensively characterize domain structures at both high spatial resolution and over a significant spatial extent. Using orthorhombic LaFeO3 as a model system, we explore the application of unsupervised and supervised ML in domain mapping, which demonstrates robustness against experiment uncertainties. The results reveal the consequential formation of multiple domains due to the structural degeneracy when LaFeO3 film is grown on cubic SrTiO3. In situ annealing of the film shows the mechanism of domain coarsening that potentially links to phase transition of LaFeO3 at high temperatures. Moreover, synthesis of LaFeO3 on DyScO3 illustrates that a less symmetric orthorhombic substrate inhibits the formation of domain walls, thereby contributing to the mitigation of structural degeneracy. High fidelity of our approach also highlights the potential for the domain mapping of other complicated materials and thin films.

Infrared laser deposition of high-quality CsBH4 epitaxial thin films stable under atmospheric conditions

We have demonstrated the growth of CsBH4 (001) epitaxial thin films utilizing an IR laser deposition technique. The thin film exhibited coherent growth with a tensile strain from the substrate, resulting in its crystallinity superior to other kinds of metal borohydride thin films reported in earlier studies. Moreover, the notable chemical stability of CsBH4 enabled surface morphological and optical characterizations even under atmospheric conditions. The optical transition energy values estimated from a Tauc plot were found smaller than the theoretical ones previously reported, and DFT calculations suggested a partial deficiency of Cs+ or [BH4]− created in the thin films.

First-order phase transition versus spin-state quantum-critical scenarios in strain-tuned epitaxial cobaltite thin films

First-order phase transition versus spin-state quantum-critical scenarios in strain-tuned epitaxial cobaltite thin films

A Hybrid Pulsed Laser Deposition Approach to Grow Thin Films of Chalcogenides.

Vapor-pressure mismatched materials such as transition metal chalcogenides have emerged as electronic, photonic, and quantum materials with scientific and technological importance. However, epitaxial growth of vapor-pressure mismatched materials are challenging due to differences in the reactivity, sticking coefficient, and surface adatom mobility of the mismatched species constituting the material, especially sulfur containing compounds. Here, a novel approach is reported to grow chalcogenides-hybrid pulsed laser deposition-wherein an organosulfur precursor is used as a sulfur source in conjunction with pulsed laser deposition to regulate the stoichiometry of the deposited films. Epitaxial or textured thin films of sulfides with variety of structure and chemistry such as alkaline metal chalcogenides, main group chalcogenides, transition metal chalcogenides, and chalcogenide perovskites are demonstrated, and structural characterization reveal improvement in thin film crystallinity, and surface and interface roughness compared to the state-of-the-art. The growth method can be broadened to other vapor-pressure mismatched chalcogenides such as selenides and tellurides. This work opens up opportunities for broader epitaxial growth of chalcogenides, especially sulfide-based thin film technological applications.

Unusual tilted growth and epitaxial relationship of NaCl B1-structured NiO and CrN on r-plane Al2O3

Epitaxial NiO and CrN thin films were deposited on a single-crystal Al2O3(11¯02) (r-plane sapphire) using magnetron sputtering. The two materials were intentionally deposited into two different deposition chamber designs and under different conditions (temperature, pressure, gases, and energy of sputtered particles). Despite the differences in the deposition condition and material system, both materials had the same feature with uncommon tilted epitaxial growth. Through an in-depth x-ray diffraction analysis of the NaCl (B1)-structured materials on r-plane sapphire, the full twin domain epitaxial relations were determined and can be described as (110)NaCl(B1)∥(44¯03)Al2O3 and [11¯2]NaCl(B1)∥[1¯1¯20]Al2O3. This relationship differs from the previously observed orientation of (100)NaCl(B1)∥(11¯02)Al2O3 and [100]NaCl(B1)∥[101¯0]Al2O3. These results are of general relevance for the growth of the extended NaCl (B1)-structured cubic material family onto a r-plane sapphire substrate where similar epitaxial growth can be expected.

X-ray reciprocal space mapping analysis of ferromagnetic GdN films grown by pulsed laser epitaxy

Epitaxial thin films of ferromagnetic rare-earth nitride, GdN, were synthesized using pulsed laser deposition (PLD) on (001) MgO, pseudo-cubic (001) YAlO3, and (001) TiN buffered (001) MgO substrates. X-ray high-resolution reciprocal space mappings confirmed the epitaxial relationship between GdN and the substrates. The use of a TiN buffer layer changed the growth direction of the GdN films from (001) to (111). The ferromagnetic behavior of the films was characterized, and it was found that the magnetic easy axis could be tuned according to the crystal growth direction. These results suggest that PLD is a viable method for synthesizing epitaxial GdN films with tunable magnetic properties. The ability to control the crystal growth direction and magnetic easy axis of GdN films could be useful for developing spintronic devices.

Atomic Layer Deposition of Epitaxial Ferroelectric Hf0.5Zr0.5O2 Thin Films

AbstractThe groundbreaking discovery of unconventional ferroelectricity in HfO2 opens exciting prospects for next‐generation memory devices. However, the practical implementation, particularly its epitaxial stabilization and a clearer understanding of its intrinsic ferroelectricity has been a significant challenge. The study arouses the potential importance of atomic layer deposition (ALD) for mass production in modern industries, demonstrating its proficiency in achieving epitaxial growth of ferroelectric Hf0.5Zr0.5O2 (HZO) thin films on Yttria‐stabilized zirconia (YSZ) substrates. Moreover, with distinct ferroelectric switching currents, the work reveals the ferroelectric characteristics of epitaxial HZO thin films deposited through ALD on YSZ‐buffered Si substrates, which aligns well with CMOS technology. Overall, the results pave the way for a scalable synthesis system for ferroelectric HfO2‐based materials, hinting at a bright future for low‐temperature epitaxial nanoelectronics.

Surface triggered stabilization of metastable charge-ordered phase in SrTiO3

Charge ordering (CO), characterized by a periodic modulation of electron density and lattice distortion, has been a fundamental topic in condensed matter physics, serving as a potential platform for inducing novel functional properties. The charge-ordered phase is known to occur in a doped system with high d-electron occupancy, rather than low occupancy. Here, we report the realization of the charge-ordered phase in electron-doped (100) SrTiO3 epitaxial thin films that have the lowest d-electron occupancy i.e., d1-d0. Theoretical calculation predicts the presence of a metastable CO state in the bulk state of electron-doped SrTiO3. Atomic scale analysis reveals that (100) surface distortion favors electron-lattice coupling for the charge-ordered state, and triggering the stabilization of the CO phase from a correlated metal state. This stabilization extends up to six unit cells from the top surface to the interior. Our approach offers an insight into the means of stabilizing a new phase of matter, extending CO phase to the lowest electron occupancy and encompassing a wide range of 3d transition metal oxides.

Integration of epitaxial LiNbO3 thin films with silicon technology

Development of bulk acoustic wave filters with ultra-wide pass bands and operating at high frequencies for 5th and 6th generation telecommunication applications and micro-scale actuators, energy harvesters and sensors requires lead-free piezoelectric thin films with high electromechanical coupling and compatible with Si technology. In this paper, the epitaxial growth of 36°Y-X and 30°X-Y LiNbO3 films by direct liquid injection chemical vapour deposition on Si substrates by using epitaxial SrTiO3 layers, grown by molecular beam epitaxy, has been demonstrated. The stability of the interfaces and chemical interactions between SrTiO3, LiNbO3 and Si were studied experimentally and by thermodynamical calculations. The experimental conditions for pure 36°Y-X orientation growth have been optimized. The piezoelectricity of epitaxial 36°Y-X LiNbO3/SrTiO3/Si films was confirmed by means of piezoelectric force microscopy measurements and the ferroelectric domain inversion was attained at 85 kV.cm−1 as expected for the nearly stoichiometric LiNbO3. According to the theoretical calculations, 36°Y-X LiNbO3 films on Si could offer an electromechanical coupling of 24.4% for thickness extension excitation of bulk acoustic waves and a comparable figure of merit of actuators and vibrational energy harvesters to that of standard PbZr1−x Ti x O3 films.

Near room-temperature ferromagnetism in BixCr1−xTe2 epitaxial thin films grown on GaAs(111)B by molecular beam epitaxy

Two-dimensional (2D) ferromagnetic materials (FMs) are potentially the material foundation for future spintronics devices. However, at present, the Curie temperature (TC) of most 2D FM is relatively low and cannot meet the need for practical applications. Nowadays, CrTe2 thin films grown by molecular beam epitaxy (MBE) are reported to be room-temperature ferromagnetic only on graphene substrate instead of 3D substrates. In this work, we report high-quality Bi-doped CrTe2 (BixCr1−xTe2) thin films grown on conventional substrates of GaAs(111)B by MBE. Magnetotransport measurements reveal strong ferromagnetism of all the films, with out-of-plane magnetic anisotropy. More importantly, as more Bi atoms are doped into the film, the Curie temperature increases and reaches 305 K at x = 0.1. This improvement is a step forward for its application in spintronics and other fields.

Impact of optimized growth conditions for magnetic phase transition and magnetic domain evolution in epitaxial FeRh thin films

FeRh undergoes a first-order magnetic phase transition, transitioning from a low-temperature antiferromagnetic state to a high-temperature ferromagnetic state around 370 K. This magnetic phase transition is profoundly affected by external parameters, such as composition and strain, which can be precisely controlled by varying growth conditions. Here, we present an investigation of FeRh thin films grown under various conditions, including annealing time and sputtering gun power. FeRh film grown at the optimal conditions yields a sharp and steep transition behavior, with maximal magnetization change between the antiferromagnetic and ferromagnetic phases. Magnetic force microscopy reveals that the optimal film displays directional domain growth, aligned with the crystallographic direction, while the non-optimal film shows random domain nucleation. Furthermore, we observe that the optimal film exhibits no significant correlation between surface morphology and magnetic domains, in contrast to the non-optimal film, where the surface morphology and magnetic domains are closely correlated. Our results highlight the critical interplay between growth conditions and film quality, emphasizing the importance of film optimization in the study of FeRh's magnetic phase transition. This comprehensive investigation provides valuable insights into the magnetic properties of FeRh, paving the way for future technological applications.

Ultrafast Bragg coherent diffraction imaging of epitaxial thin films using deep complex-valued neural networks

Domain wall structures form spontaneously due to epitaxial misfit during thin film growth. Imaging the dynamics of domains and domain walls at ultrafast timescales can provide fundamental clues to features that impact electrical transport in electronic devices. Recently, deep learning based methods showed promising phase retrieval (PR) performance, allowing intensity-only measurements to be transformed into snapshot real space images. While the Fourier imaging model involves complex-valued quantities, most existing deep learning based methods solve the PR problem with real-valued based models, where the connection between amplitude and phase is ignored. To this end, we involve complex numbers operation in the neural network to preserve the amplitude and phase connection. Therefore, we employ the complex-valued neural network for solving the PR problem and evaluate it on Bragg coherent diffraction data streams collected from an epitaxial La2-xSrxCuO4 (LSCO) thin film using an X-ray Free Electron Laser (XFEL). Our proposed complex-valued neural network based approach outperforms the traditional real-valued neural network methods in both supervised and unsupervised learning manner. Phase domains are also observed from the LSCO thin film at an ultrafast timescale using the complex-valued neural network.

Measuring dielectric and electro-optic responses of thin films using plasmonic devices.

This paper introduces a simple method for the measurement of the relative permittivity and the Pockels coefficient of electro-optic (EO) materials in a waveguide up to sub-THz frequencies. By miniaturizing the device and making use of plasmonics, the complexities of traditional methods are mitigated. This work elaborates the fabrication tolerance and simplicity of the method, and highlights its applicability to various materials, substrates and configurations. The method is showcased using drop-casted perovskite barium titanate (BaTiO3, BTO) nano-particle thin-films and it has previously been used to measure epitaxial thin film BTO. In this work we show the effective relative permittivity of drop casted BTO to be εeff ∼ 30 at 200 MHz, dropping to ∼ 18 at 67 GHz and similarly, the effective Pockels coefficient was found to be reff ∼ 16 at 350 MHz and ∼ 8 at 70 GHz. These values are a factor > 50 below the values found for thin film BTO. Yet, the fact that the method can be applied to such different samples and Pockels strengths gives testimony to its versatility and sensitivity.

High harmonic generation in epitaxially grown zinc oxide films

High harmonic generation (HHG) in solids has been identified as a promising mechanism for light source generation and for spectroscopy of materials. HHG from bulk solids, however, often suffers from nonlinear propagation effects, resulting in a loss of spectral coherence and the skewing of spectroscopic measurements. Here, we study HHG in epitaxial ZnO thin films grown on Al2O3 substrates using atomic layer deposition. We find that the HHG emission consists of narrow spectral peaks, in contrast to those seen in bulk, and that the dependence of the harmonic yield on the film thickness differs for above-gap and below-gap harmonics, which can be understood from analytical models based on the linear and nonlinear response of the medium. The measured harmonic spectra depend qualitatively on the preparation of the films, with as-grown films generating even harmonic orders, which are absent in annealed films. The results are interpreted using transmission electron microscopy measurements, which indicate different morphologies for the as-grown and annealed films.

Heteroepitaxial growth of a-, m-, and r-plane α-Ga2O3 thin films on rh-ITO electrodes for vertical device applications

α-Ga2O3 has garnered significant attention as a promising next-generation material for applications in power-switching and deep ultraviolet optoelectronics owing to its ultrawide bandgap. However, fabricating α-Ga2O3-based devices with a vertical structure is challenging because α-Ga2O3 requires heteroepitaxial growth on a heterogeneous substrate, typically insulating sapphire. In this study, we investigated the heteroepitaxial growth of a-, m-, and r-plane α-Ga2O3 on rhombohedral indium tin oxide (rh-ITO) electrodes via mist chemical vapor deposition. X-ray diffraction (XRD) 2θ-ω and φ scan analyses revealed that a-, m-, and r-plane α-Ga2O3 epitaxial thin films were successfully grown on rh-ITO bottom electrodes when α-Fe2O3 buffer layers were inserted. No phase separation or in-plane rotation was observed in the XRD results of the α-Ga2O3 thin films. The α-Fe2O3 buffer layers facilitated the preferential growth of α-Ga2O3 on various planes of rh-ITO. The surface morphology of the a-, m-, and r-plane α-Ga2O3, examined via field emission scanning electron microscopy, differed depending on the crystal plane. The results obtained in this study have potential applications in the development of vertical-structured devices with α-Ga2O3 thin films.

Evaluation of Sputtering Processes in Strontium Iridate Thin Films

The growth of epitaxial thin films from the Ruddlesden–Popper series of strontium iridates by magnetron sputtering is analyzed. It was found that, even using a non-stoichiometric target, the films formed under various conditions were consistently of the perovskite-like n = ∞ SrIrO3 phase, with no evidence of other RP series phases. A detailed inspection of the temperature–oxygen phase diagram underscored that kinetics mechanisms prevail over thermodynamics considerations. The analysis of the angular distribution of sputtered iridium and strontium species indicated clearly different spatial distribution patterns. Additionally, significant backsputtering was detected at elevated temperatures. Thus, it is assumed that the interplay between these two kinetic phenomena is at the origin of the preferential nucleation of the SrIrO3 phase. In addition, strategies for controlling cation stoichiometry off-axis have also been explored. Finally, the long-term stability of the films has been demonstrated.

High-performance p-type V2O3 films by spray pyrolysis for transparent conducting oxide applications.

High-quality epitaxial p-type V2O3 thin films have been synthesized by spray pyrolysis. The films exhibited excellent electrical performance, with measurable mobility and high carrier concentration. The conductivity of the films varied between 115 and 1079 Scm-1 while the optical transparency of the films ranged from 32 to 65% in the visible region. The observed limitations in thinner films' mobility were attributed to the nanosized granular structure and the presence of two preferred growth orientations. The 60nm thick V2O3 film demonstrated a highly competitive transparency-conductivity figure of merit compared to the state-of-the-art.

Structural and Photoelectronic Properties of κ-Ga2O3 Thin Films Grown on Polycrystalline Diamond Substrates.

Orthorhombic κ-Ga2O3 thin films were grown for the first time on polycrystalline diamond free-standing substrates by metal-organic vapor phase epitaxy at a temperature of 650 °C. Structural, morphological, electrical, and photoelectronic properties of the obtained heterostructures were evaluated by optical microscopy, X-ray diffraction, current-voltage measurements, and spectral photoconductivity, respectively. Results show that a very slow cooling, performed at low pressure (100 mbar) under a controlled He flow soon after the growth process, is mandatory to improve the quality of the κ-Ga2O3 epitaxial thin film, ensuring a good adhesion to the diamond substrate, an optimal morphology, and a lower density of electrically active defects. This paves the way for the future development of novel hybrid architectures for UV and ionizing radiation detection, exploiting the unique features of gallium oxide and diamond as wide-bandgap semiconductors.

Effects of phase transition driven by cation vacancies on magnetism and electricity in LaxMnO3 thin films

The attractive physical properties of perovskite manganese make them fundamental building blocks for oxide spintronic devices. However, defects and lattice distortions can lead to extrinsic properties, especially cation vacancies, severely compromising device performance. In particularly, many intriguing properties, such as the emergence of magnetism and two-dimensional high-mobility electron gases, are closely related to cation vacancies. In this study, we deposited a series of LaxMnO3 epitaxial thin films with varying contents of La3+ vacancies on SrTiO3 (100) substrates, with the presence of La3+ vacancies verified by using X-ray diffraction, X-ray photoemission spectroscopy, X-ray absorption spectroscopy, and energy dispersive spectroscopy spectrum. The reduction in La3+ vacancies led to stronger ferromagnetism with an easy axis oriented along the (110) direction in LaMnO3 thin films, along with the appearance of a monoclinic phase, indicating that the ferromagnetism possibly originated from the three-dimensional d3z2−r2/dx2−y2-alternated orbital order rather than the Mn3+/Mn4+ double exchange effect. This result implies that the consistent regulation of cation vacancies by pulsed laser deposition can modulate both the structure and physical properties of manganate films, thereby enabling control over device properties.

Investigation of In2SexOy/In2Se3/Si dual-junction photodiode for self-powered broadband photodetection

We report a dual-junction strategy for fabricating a high-performance In2SexOy/In2Se3/Si heterojunction photodiode by oxidizing the epitaxial In2Se3 thin films. The device exhibits a suppressed dark current (4.2 × 10−11 A) and enhanced photocurrent at zero bias, benefiting from the double built-in electric fields. Consequently, it demonstrates excellent and uniform self-powered broadband (255–1050 nm) photodetection performance with the typical responsivity of several hundred mA/W, and detectivity of over 5 × 1011 Jones. Moreover, a fast response speed with a response time of 0.20 ms is achieved. Our investigation offers a potential route to construct full-spectrum photodetectors.