High-quality Epitaxial Thin Films Research Articles

Electronic-grade epitaxial (111) KTaO3 heterostructures.

KTaO3 heterostructures have recently attracted attention as model systems to study the interplay of quantum paraelectricity, spin-orbit coupling, and superconductivity. However, the high and low vapor pressures of potassium and tantalum present processing challenges to creating heterostructure interfaces clean enough to reveal the intrinsic quantum properties. Here, we report superconducting heterostructures based on high-quality epitaxial (111) KTaO3 thin films using an adsorption-controlled hybrid PLD to overcome the vapor pressure mismatch. Electrical and structural characterizations reveal that the higher-quality heterostructure interface between amorphous LaAlO3 and KTaO3 thin films supports a two-dimensional electron gas with substantially higher electron mobility, superconducting transition temperature, and critical current density than that in bulk single-crystal KTaO3-based heterostructures. Our hybrid approach may enable epitaxial growth of other alkali metal-based oxides that lie beyond the capabilities of conventional methods.

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.

Epitaxial Strain Modulation of Photoelectrochemical Properties of Epitaxial Bismuth Vanadate Photoanodes

Solar fuels derived from photoelectrochemical water splitting have the potential to significantly contribute to achieving sustainability in our global energy infrastructure. Transition metal oxide (TMO) photoabsorbers are interesting candidates as photoelectrodes due to their excellent bandgap tunability, relative stability and cost-effectiveness. Further improvements in the photoelectrochemical performance of such photoelectrodes would entail strategies beyond the discovery of new compositions and structures [1]. Strain engineering in particular is a powerful tool to alter the material’s electronic structure and the resulting properties. For instance, optoelectronic properties (i.e., light absorption characteristics) [2] and surface catalytic properties [3] could be modulated by lattice-mismatch imposed epitaxial strain. Much less understood, however, is how epitaxial strain modulation affects the band bending and surface electronic structure; these properties dictate the photogenerated carrier separation at the photoelectrode/electrolyte interface, and thus the photoelectrochemical performance.To gain further insight into the impact of epitaxial strain on the photoelectrochemical performance of TMO photoelectrodes, monoclinic bismuth vanadate (BiVO4) was chosen as a material platform for study given its desirable (bandgap ~2.4 eV, favorable band edge positions and relatively long minority diffusion lengths) and undesirable (localized carrier transport, photocorrosion) characteristics [1]. High crystalline quality epitaxial BiVO4 thin films were used as a model system in order to minimize complications arising from grain boundaries and different crystalline orientations inherent in polycrystalline systems. BiVO4 films with an epitaxial indium tin oxide (ITO) conducting layer were deposited on (001)-oriented yttrium-stabilized zirconia (YSZ) single crystalline substrates. YSZ substrates with different dopant concentrations were used to impose different epitaxial strain states on the BiVO4 films. This strategy allowed us to conduct extensive photoelectrochemical measurements (voltammograms, incident photon-to-current efficiency, impedance spectroscopy) and x-ray photoelectron spectroscopic (XPS) measurements as a function of the strain state. Further investigation with varying BiVO4 film thickness was also performed to decouple any thickness-dependent contributions from the epitaxial strain effects. Through strain tensor decomposition analysis, Tauc bandgaps correlates well with deviatoric strains, which in turn seem to suggest the role of volume-preserving lattice distortions. Finally, we will show that the observed trends in photocurrents, incident photon-to-current efficiency spectra, flatband potentials obtained from Mott-Schottky analysis, and surface electronic structure obtained from XPS can be rationalized in terms of lattice-induced distortions on the BiO8 dodecahedra.

Pulsed-Mode Metalorganic Vapor-Phase Epitaxy of GaN on Graphene-Coated c-Sapphire for Freestanding GaN Thin Films.

We report the growth of high-quality GaN epitaxial thin films on graphene-coated c-sapphire substrates using pulsed-mode metalorganic vapor-phase epitaxy, together with the fabrication of freestanding GaN films by simple mechanical exfoliation for transferable light-emitting diodes (LEDs). High-quality GaN films grown on the graphene-coated sapphire substrates were easily lifted off by using thermal release tape and transferred onto foreign substrates. Furthermore, we revealed that the pulsed operation of ammonia flow during GaN growth was a critical factor for the fabrication of high-quality freestanding GaN films. These films, exhibiting excellent single crystallinity, were utilized to fabricate transferable GaN LEDs by heteroepitaxially growing InxGa1-xN/GaN multiple quantum wells and a p-GaN layer on the GaN films, showing their potential application in advanced optoelectronic devices.

Formation of quaternary [formula omitted] epilayers driven by thermally induced interdiffusion between spinel [formula omitted] epilayer and [formula omitted] substrate

Zinc aluminogallate, Zn(AlxGa1−x)2O4 (ZAGO), a single-phase spinel structure, offers considerable potential for high-performance electronic devices due to its expansive compositional miscibility range between aluminum (Al) and gallium (Ga). Direct growth of high-quality ZAGO epilayers however remains problematic due to the high volatility of zinc (Zn). This work highlights a novel synthesis process for high-quality epitaxial quaternary ZAGO thin films on sapphire substrates, achieved through thermal annealing of a ZnGa2O4 (ZGO) epilayer on sapphire in an ambient air setting. In-situ annealing x-ray diffraction measurements show that the incorporation of Al in the ZGO epilayer commenced at 850 °C. The Al content (x) in ZAGO epilayer gradually increased up to around 0.45 as the annealing temperature was raised to 1100 °C, which was confirmed by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy. X-ray rocking curve measurement revealed a small full width at half maximum value of 0.72 °, indicating the crystal quality preservation of the ZAGO epilayer with a high Al content. However, an epitaxial intermediate β–(AlxGa1−x)2O3 layer (β - AGO) was formed between the ZAGO and sapphire substrate. This is believed to be a consequence of the interdiffusion of Al and Ga between the ZGO thin film and sapphire substrate. Using density functional theory, the substitution cost of Ga in sapphire was determined to be about 0.5 eV lower than substitution cost of Al in ZGO. Motivated by this energetically favorable substitution, a formation mechanism of the ZAGO and AGO layers was proposed. Spectroscopic ellipsometry studies revealed an increase in total thickness of the film from 105.07 nm (ZGO) to 147.97 nm (ZAGO/AGO) after annealing to 1100 °C, which were corroborated using TEM. Furthermore, an observed increase in the direct (indirect) optical bandgap from 5.06 eV (4.7 eV) to 5.72 eV (5.45 eV) with an increasing Al content in the ZAGO layer further underpins the formation of a quaternary ZAGO alloy with a tunable composition.

Magnetotransport, spin reorientation, and anomalous ferrimagnetic-to-antiferromagnetic phase transition in epitaxial Mn2Sb alloy thin films

High-quality ferrimagnetic Mn2Sb epitaxial thin films have been successfully grown on SrTiO3 (001) single-crystal substrates via systematically optimizing the growth parameters using molecular beam epitaxy. Magnetotransport and magnetic measurements reveal that a spin reorientation transition occurs in the 260–150 K region where the direction of spins rotates from out-of-plane to in-plane upon cooling, resulting in the ferrimagnetic(II) phase, followed by a giant magnetoresistance associated anomalous ferrimagnetic(II)-to-canted antiferromagnetic (c-AFM) phase transition in the 150–115 K region, resulting in the c-AFM ground state, both of which are completely absent and have yet not been previously observed in Mn2Sb bulk and thin films. Temperature-dependent X-ray diffraction measurements reveal that the low-temperature c-AFM phase originates from the contraction of the out-of-plane lattice constant c, which would increase the exchange interaction between neighbouring magnetic sublattices and thus stabilize the c-AFM phase. DFT calculations reveal that substrate clamping is the cause of the unique c-axis contraction in Mn2Sb films. For the 24-nm films, there is almost no out-of-plane magnetization in the ground state, but exists a weak in-plane remanent magnetization (∼0.4 μB/f.u.) and anomalous Hall effects, implying spin canting within the ab plane. With decreasing film thickness from 64 to 8 nm, the out-of-plane saturation magnetization at 10 K increases by approximately 10 times, and for the 8-nm film, its saturation magnetization (4.8 μB/f.u.) is 2.8 times larger than that of Mn2Sb bulk (∼1.74 μB/f.u.), both of which are attributed to the interfacial strain effect. Our work demonstrates that Mn2Sb films grown on perovskite oxide substrates show anomalous spin-charge-lattice coupling phenomena, which may inspire more study of its basic properties and potential device applications.

Structural and electrochemical properties of high quality epitaxial titanium carbide thin films grown by pulsed laser deposition on MgO (111) and Al2O3 (001) substrates

The electrochemical behaviour of titanium carbide (TiCx, with 0.5 < x < 1) thin films is presented and discussed with respect to their structural properties. Epitaxial (111)-orientated TiC films are grown onto (111)-cut MgO and (001)-cut Al2O3 substrates by using TiC targets with various stoichiometry. When using a TiC target, the films grown on Al2O3 substrates exhibit cyclic voltammograms significantly different from those grown on MgO. As a matter of fact, their electrochemical properties are strikingly similar to films with TiC0.55 composition. It is demonstrated, by using high-resolution X-ray diffraction, that films grown on sapphire have a composition close to TiC0.55, the driving force for carbon deficiency being the relaxation of the compressive interfacial strain at the Al2O3/TiC interface. On the contrary, the interfacial strain at the MgO/TiC interface is tensile which cannot be compensated by variations in the Ti/C ratio.

Emergent weak antilocalization and wide-temperature-range electronic phase diagram in epitaxial RuO2 thin film

Binary ruthenium dioxide (RuO2) has gradually attracted much attention in condensed matter physics and material sciences due to its various intriguing physical properties, such as strain-induced superconductivity, anomalous Hall effect, collinear anti-ferromagnetism, etc. However, its complex emergent electronic states and the corresponding phase diagram over a wide temperature range remain unexplored, which is critically important to understanding the underlying physics and exploring its final physical properties and functionalities. Here, through optimizing the growth conditions by using versatile pulsed laser deposition, high-quality epitaxial RuO2 thin films with clear lattice structure are obtained, upon which the electronic transport is investigated, and emergent electronic states and the relevant physical properties are unveiled. Firstly, at a high-temperature range, it is the Bloch–Grüneisen state, instead of the common Fermi liquid metallic state, that dominates the electrical transport behavior. Moreover, the recently reported anomalous Hall effect is also revealed, which confirms the presence of the Berry phase in the energy band structure. More excitingly, we find that above the superconductivity transition temperature, a new positive magnetic resistance quantum coherent state with an unusual dip as well as an angel-dependent critical magnetic field emerges, which can be attributed to the weak antilocalization effect. Lastly, the complex phase diagram with multiple intriguing emergent electronic states over a wide temperature range is mapped. The results greatly promote the fundamental physics understanding of the binary oxide RuO2 and provide guidelines for its practical applications and functionalities.

Design of large horizontal gallium nitride hydride vapor-phase epitaxy equipment and optimization of process parameters

Gallium nitride (GaN) has been extensively investigated in the last two decades owing to its excellent characteristics and massive commercialization potential. GaN substrates have been increasingly investigated to enhance the quality and efficiency of GaN-based optoelectronic devices. Hydride vapor-phase epitaxy (HVPE) method is generally employed for preparing GaN substrates. Herein, a six-inch horizontal HVPE equipment was designed using computational fluid dynamics (CFD) for depositing GaN epitaxial thin films. First, the original model was structurally designed, and baffles and other structures were set to achieve uniform outlet velocity. Second, single-factor variable method was used to analyze the influence of process parameters (flow rate, chamber pressure, and rotating speed) on deposition rate. An orthogonal design was used to optimize the main process parameters for uniform film deposition on six-inch substrates. Numerical simulation results provide a theoretical basis for optimizing a large-size HVPE reaction cavity and process conditions, paving the way for high-quality large-size GaN epitaxial thin films.

Effect of a niobium-doped PZT interfacial layer thickness on the properties of epitaxial PMN-PT thin films

We are reporting on high quality epitaxial thin films of [Pb(Mg1/3Nb2/3)O3]0.67-(PbTiO3)0.33 [PMN-PT (67/33)]. These films were deposited on (001) oriented, vicinal SrTiO3 single crystal substrates, using 1 mol. % niobium-doped Pb(Zr0.52,Ti0.48)O3 (Nb-PZT) as an interfacial layer. The functional properties of the epitaxial PMN-PT (67/33) thin films were investigated as a function of the layer thickness of the Nb-PZT layer. The deposited hetero-structures are perovskite phase pure and fully (001)-oriented. The variation in Nb-PZT interfacial layer thickness results in an increasing trend change of the in-plane lattice parameter of that layer, which in turn causes a decrease in the c/a ratio of the PMN-PT film on top. The most noticeable effect related to this is a decrease in built-in-bias (imprint) voltage. Thus, the built-in bias can be tuned by changing the interfacial layer thickness. The ferroelectric capacitor properties are found to be most stable for the thinnest interfacial layers under a high number (108) of switching cycles.

Nd:YAG infrared laser as a viable alternative to excimer laser: YBCO case study

We report on the growth and characterization of epitaxial YBa_{2}Cu_{3}O_{7-delta } (YBCO) complex oxide thin films and related heterostructures exclusively by Pulsed Laser Deposition (PLD) and using first harmonic Nd:Y_{3}Al_{5}O_{12} (Nd:YAG) pulsed laser source (lambda = 1064 nm). High-quality epitaxial YBCO thin film heterostructures display superconducting properties with transition temperature sim 80 K. Compared with the excimer lasers, when using Nd:YAG lasers, the optimal growth conditions are achieved at a large target-to-substrate distance d. These results clearly demonstrate the potential use of the first harmonic Nd:YAG laser source as an alternative to the excimer lasers for the PLD thin film community. Its compactness as well as the absence of any safety issues related to poisonous gas represent a major breakthrough in the deposition of complex multi-element compounds in form of thin films.

Crossover from strong to weak exciton confinement in thickness-controlled epitaxial PbI2 thin films

Spatially confined excitons undergo two distinct quantization effects depending on the confinement length, the enhanced binding energy under strong confinement, and the center-of-mass quantization under weak one. However, the transition between them has not been experimentally identified in two-dimensional (2D) materials due to the lack of thin films satisfying large-scale uniformity and atomic-level flatness in a wide thickness range. Here, we reveal the crossover in high-quality epitaxial thin films of a 2D semiconductor PbI2 grown by molecular beam epitaxy. The absorption spectra exhibit oscillatory structures manifesting the exciton center-of-mass quantization, and the quantization energies show an additional blue shift associated with the strong confinement effect below five-layer thickness (35 Å). The precise control of exciton quantum states will lead to the further development of optoelectronic functionalities of 2D materials.

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.

Combining experimental and modelling approaches to understand the expansion of lattice parameter of epitaxial SrTi1-xTaxO3 (x = 0–0.1) films

Cubic SrTiO3 is widely used as a model perovskite oxide due to its exotic optical, electronic, magnetic, superconducting, and ferroelectric properties, which are dominantly influenced by its lattice parameter. This paper reports on the experimental observation of Ta5+ dopant-induced expansion of the perpendicular lattice parameter (a⊥) of (001) heteroepitaxial SrTixTa1-xO3 (x = 0–0.1) films grown at substrate temperature (Tg) of 750 °C under 5 × 10-5 mbar oxygen partial pressure (pO2). The experimental results demonstrate the synthesis of very high-quality epitaxial thin films in which almost all the Ta5+ dopants are substituted. However, not all the carriers are activated due to compensating defects. All these films have smooth surfaces and are stoichiometric in compositions. The value of a⊥ increases linearly with increasing Ta or free carrier concentrations. This expansion is modelled via density functional theory (DFT). The influences of three effects are considered in the present modelling: (i) ionic radius of Ta dopant (ii) deformation potential due to hydrostatic strain energy (iii) concentration of free carrier. Further calculations of a⊥ are also performed by considering the influence of substrate and the results overestimate the experimental values by ∼ 1 %. Further experiments on other perovskite systems are required to establish the efficacy of these modelling approaches.

Atomically Smooth Defect-Free III-As Heterostructures on InP(111) Substrate for Next-Generation Electronic Devices

Research into Zinc Blende III–V compound semiconductors has focused almost entirely on growth on (001)-oriented substrates for more than five decades. This is because high-quality epitaxial layers can be achieved relatively smoothly when III–Vs are grown on (001) substrates. However, emerging technologies generated renewed interest in the growth of high-quality III–Vs on (111)-oriented substrates. Prime examples of these applications are devices based on spin transport, and low-anisotropy tensile-strained quantum dots for quantum computing applications. Growth on (111) substrates could also help understand the growth of complex chalcogenides. Exploring these applications requires high-quality epitaxial thin films on (111) substrates. Here, we demonstrate for the first time the atomically smooth and defect-free growth of nanoscale InGaAs/InAlAs superlattice on InP(111)-oriented substrate using molecular beam epitaxy (MBE). This was achieved by optimizing the substrate misorientation angle and growth conditions. The growth optimization was guided by scanning transmission electron microscopy Moiré geometrical phase analysis and density functional theory calculations. It is shown that ensuring the step-flow growth mode for each ternary layer is the most effective way of achieving high-quality epitaxial structures on (111)-oriented substrates. This work sets the stage for developing InGaAs/InAlAs/InP(111) electronic and photonic structures with properties that can be tuned with strain-mediated piezoelectric field engineering in high-quality epitaxial thin films. The reported observations should help optimize epitaxy in other Zinc Blende material systems for the structure that could benefit from the tunable piezoelectric effects.

Phase Competition in High-Quality Epitaxial Antiferroelectric PbZrO3 Thin Films.

Antiferroelectric PbZrO3 has attracted renewed interest in recent years because of its unique properties and wide range of potential applications. However, the nature of antiferroelectricity and its evolution with the electric field and temperature remain controversial, mostly due to the difficulty of obtaining high-quality single-crystal samples. The lack of consensus regarding the phase transition in PbZrO3 is not only important on a fundamental side but also greatly hinders further applications. Herein, high-quality PbZrO3 epitaxial thin films are successfully fabricated by pulsed laser deposition. The structural and physical properties of the films are systematically studied via a combination of electric property measurements, X-ray diffraction, scanning transmission electron microscopy imaging, and second-harmonic generation studies. Our studies unveil the noncentrosymmetric nature of PbZrO3 films at room temperature. Moreover, the Curie temperature increased to 270°, ∼40° higher than that in the bulk, and no intermediate ferroelectric phase was observed. Besides, an incipient ferroelectric with relaxor-like behavior above the Curie temperature due to the existence of a local polar cluster in the high-temperature paraelectric phase is experimentally observed for the first time. Our studies provide a better understanding of PbZrO3 thin films and pave the way for practical applications of antiferroelectric material in modern electronic devices.

Ionic liquid modulation of exchange bias in epitaxial LaMnO3 thin films

The magnetic ground state of LaMnO3 (LMO) thin film is still a controversial issue, even though various mechanisms, such as cation/anion non-stoichiometry, epitaxial strain, interfacial charge reconstruction, and orbital ordering, have been proposed. Here, exchange bias (EB) was introduced into a high-quality epitaxial LMO thin film via relatively low oxygen growth pressure. The EB in LMO was modulated by +2 V gating via ionic liquid method with increased EB field (HEB), coercivity (HC), blocking temperature (TB), and reduced ferromagnetic (FM) magnetization. However, the −2 V gating has a much weaker tunability. By investigating the change of structure, surface morphology, and Mn oxidation state in LMO thin films, the modulation of magnetic properties is attributed to the creation/annihilation of oxygen vacancy in an LMO thin film. The suppressed FM phase in LMO can be ascribed to reduced Mn valence, structure disorders, and structure transition. However, the enriched antiferromagnetic phase results from the transition of the pseudocubic structure to the distorted orthorhombic structure. This work not only highlights the importance of functional defects in perovskite oxides but also sheds light on the potential of electric-field modulation of magnetism in spintronic devices.

CuInO2 epitaxial thin films on epi-GaN wafer: Fabrication and solar-blind photodetector

High-quality p-type CuInO2 epitaxial thin films with a bandgap of ∼3.99 eV were grown on epitaxial GaN (001) (epi-GaN) wafers by using pulsed laser deposition technology. At a constant growth temperature of 750 °C, the quality of the CuInO2 thin film increases with the decrease in oxygen pressure within the range of 0.09–1 Pa. For the single crystal film obtained at 0.09 Pa, the epitaxial relationship between the film and the substrate is determined to be CuInO2 (001) || GaN (001) with CuInO2 〈100〉 || GaN [100]. The self-powered ultraviolet photodetector prepared based on the highest quality film shows high photoresponsivity 0.31 mA/W and fast response speed (rise time: 0.34 s, decay time: 0.34 s) at zero voltage under 254-nm UV-light, meaning good solar-blind photoresponsivity.

Solid-phase epitaxy of a CuAlO2 template on c-Al2O3 for delafossite growth

Thin-film growth of ABO2 delafossites has recently attracted significant attention due to its attractive transport properties and potential applications. A fundamental requirement for achieving high-quality thin films is the availability of lattice matching substrates and chemical compatibility. However, there are still many obstacles to achieving high-quality thin films. Here, we report a process to further engineer a template ABO2 delafossite structure by solid-phase epitaxy of CuAlO2 on the surface of a commercial sapphire substrate, which offers a promising route to growing high-quality epitaxial thin films. The starting reagents involve a layer of polycrystalline Cu2O deposited on a c-Al2O3 substrate by pulsed laser deposition (PLD). Subsequent thermal treatment activates a solid-state interface reaction between the film and substrate, producing a CuAlO2 thin film. The reaction temperature and dwell time parameters were optimized in this study to prepare a phase diagram for CuAlO2 samples without phase impurities. This method provides an essential stepping-stone toward the approachability of a lattice matching template (i.e., substrate-buffer layer) for ABO2 heterostructures. An example of successful epitaxial growth of highly conducting PdCrO2 is also demonstrated by using a CuAlO2 buffer layer.

Optimization of the Growth Process of Double Perovskite Pr2-δNi1-xMn1+xO6-y Epitaxial Thin Films by RF Sputtering.

Epitaxial thin films of Pr2−δNi1−xMn1+xO6−y (PNMO) double perovskite were grown on (001)-oriented SrTiO3 substrates by RF magnetron sputtering. The influence of the growth parameters (oxygen pressure, substrate temperature, and annealing treatments) on the structural, magnetic and transport properties, and stoichiometry of the films was thoroughly investigated. It is found that high-quality epitaxial, insulating, and ferromagnetic PNMO thin films can only be obtained in a narrow deposition parameter window. It is shown that a careful selection of the growth conditions allows for obtaining a high degree of Ni/Mn cation ordering, which is reflected in the values of the Curie temperature, TC, and saturation magnetization, MS, which are very close to those of bulk material.