High-quality Epitaxial Films Research Articles

Tuning of thermally induced first-order semiconductor-to-metal transition in pulsed laser deposited VO2 epitaxial thin films

Vanadium oxide (VO2) thin films have drawn significant research and development interest in recent years because of their intriguing physical origin and wide range of functionalities useful for many potential applications, including infrared imaging, smart windows, and energy and information technologies. However, the growth of highly epitaxial films of VO2, with a sharp and distinct controllable transition, has remained a challenge. Here, we report the structural and electronic properties of high quality and reproducible epitaxial thin films of VO2, grown on c-axis oriented sapphire substrates using pulsed laser deposition at different deposition pressures and temperatures, followed by various annealing schedules. Our results demonstrate that the annealing of epitaxial VO2 films significantly enhances the Semiconductor to Metal Transition (SMT) to that of bulk VO2 transition. The effect of oxygen partial pressure during the growth of VO2 films creates a significant modulation of the SMT from around room temperature to as high as the theoretical value of 68 °C. We obtained a bulk order transition ≥104 while reducing the transition temperature close to 60 °C, which is comparatively less than the theoretical value of 68 °C, demonstrating a clear and drastic improvement in the SMT switching characteristics. The results reported here will open the door to fundamental studies of VO2, along with tuning of the transition temperatures for potential applications for multifunctional devices.

Optimisation of growth parameters to obtain epitaxial Y-doped BaZrO3 proton conducting thin films

We hereby report developments on the fabrication and characterization of epitaxial thin films of proton conducting Y-doped BaZrO3 (BZY) by pulsed laser deposition (PLD) on different single crystal substrates (MgO, GdScO3, SrTiO3, NdGaO3, LaAlO3 and sapphire) using Ni-free and 1% Ni-containing targets. Pure, high crystal quality epitaxial films of BZY are obtained on MgO and on perovskite-type substrates, despite the large lattice mismatch. The deposition conditions influence the morphology, cell parameters and chemical composition of the film, the oxygen partial pressure during film growth being the most determining. Film characterization was carried out using X-ray diffraction, transmission electron and atomic force microscopies, wavelength dispersive X-ray spectroscopy and angle-resolved X-ray photoelectron spectroscopy. All films show a slight tetragonal distortion that is not directly related to the substrate-induced strain. The proton conductivity of the films depends on deposition conditions and film thickness, and for the optimised conditions its total conductivity is slightly higher than the bulk conductivity of the target material (3mS/cm at 600°C, in wet 5% H2/Ar). The conductivities are, however, more than one order of magnitude lower than the highest reported in literature and possible reasoning is elucidated in terms of local and extended defects in the films.

Enhanced SAW characteristics of a-plane AlN epitaxial films using ZnO buffer layer

High-quality a-plane AlN epitaxial films were successfully deposited on r-plane sapphire using a thin ZnO buffer layer for surface acoustic wave (SAW) applications. Structural properties of AlN epitaxial films on different thicknesses of ZnO buffer and the related SAW characteristics were investigated systematically. The results demonstrate that AlN films were epitaxially grown on ZnO buffered-substrate with orientation relationship of $$(11\overline {2} 0){\text{ }}[10\overline {1} 0]$$ AlN// $$(11\overline {2} 0){\text{ }}[10\overline {1} 0]$$ ZnO// $$(01\overline {1} 2){\text{ }}[\overline {2} 110]$$ Al2O3. A thin ZnO buffer layer with moderate thickness significantly facilitated the crystal quality of a-plane AlN films, and in turn enhanced the SAW characteristics. High frequency SAW devices with a center frequency of ~ 1500 MHz corresponding to a high phase velocity of 6000 m/s were achieved on the obtained AlN films. The as-grown a-plane AlN epitaxial films with 40 nm optimized ZnO buffer exhibit high crystal quality with a FWHM value of the rocking curve of 0.76° and good performance of SAW devices with low insertion loss and large out-of-band rejection. This work offers an effective approach to achieve high-quality a-plane AlN epitaxial films on sapphire substrates for the AlN-based SAW devices applications.

Van der Waals epitaxy of SnS film on single crystal graphene buffer layer on amorphous SiO2/Si

Conventional hetero-epitaxial films are typically grown on lattice and symmetry matched single crystal substrates. We demonstrated the epitaxial growth of orthorhombic SnS film (∼500 nm thick) on single crystal, monolayer graphene that was transferred on the amorphous SiO2/Si substrate. Using X-ray pole figure analysis we examined the structure, quality and epitaxy relationship of the SnS film grown on the single crystal graphene and compared it with the SnS film grown on commercial polycrystalline graphene. We showed that the SnS films grown on both single crystal and polycrystalline graphene have two sets of orientation domains. However, the crystallinity and grain size of the SnS film improve when grown on the single crystal graphene. Reflection high-energy electron diffraction measurements show that the near surface texture has more phases as compared with that of the entire film. The surface texture of a film will influence the growth and quality of film grown on top of it as well as the interface formed. Our result offers an alternative approach to grow a hetero-epitaxial film on an amorphous substrate through a single crystal graphene buffer layer. This strategy of growing high quality epitaxial thin film has potential applications in optoelectronics.

Growth of high-quality AlN epitaxial film by optimizing the Si substrate surface

High-quality AlN epitaxial films have been grown on Si substrates by optimizing the hydrofluoric acid (HF) solution for cleaning of Si substrates. Effect of the Si substrate surface on the surface morphology and structural property of AlN epitaxial films is investigated in detail. It is revealed that as the concentration of HF solution increases from 0 to 2.0%, the surface morphology and the crystalline quality are initially improved and then get worse, and show an optimized value at 1.5%. The as-grown ∼200nm-thick AlN epitaxial films on Si substrates grown with HF solution of 1.5% reveal the root-mean-square (RMS) surface roughness of 0.49nm and the full-width at half-maximum for AlN(0002) X-ray rocking curve of 0.35°, indicating the smooth surface morphology and the high crystalline quality. The corresponding mechanism is proposed to interpret the effect of Si substrate surface on surface morphology and structural property of AlN epitaxial films, and provides an effective approach for the perspective fabrication of AlN-based devices.

Uniform sputter deposition of high-quality epitaxial complex oxide thin films

Uniform deposition of high-quality epitaxial complex oxide thin films over a large area is desirable not only for the large scale fabrication of oxide electronics but also for preparing multiple samples with the same growth conditions for various characterization techniques. However, it is particularly challenging to grow uniform thin films of multicomponent oxide systems containing volatile species such as Pb and Bi. By combining a misaligned parallel dual planar magnetron sputtering technique with substrate rotation, the authors have grown uniform epitaxial thin films piezoelectric 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 and multiferroic monodomain BiFeO3 with high-deposition rates over a 2 in. diameter area. These films have excellent uniformity of thickness, stoichiometric compositions, and electrical properties. This technique can be scaled to larger deposition areas by using larger sputtering targets and widely applied to various multicomponent complex oxide thin film heterostructures.

Solid-phase epitaxial film growth and optical properties of a ferroelectric oxide, Sr2Nb2O7

High-quality epitaxial films of a ferroelectric oxide Sr2Nb2O7 were successfully fabricated by solid phase epitaxy (SPE) on (110) LaAlO3 single crystal substrates. In the SPE method, amorphous Sr–Nb–O films are first deposited by pulsed laser deposition at room temperature and then annealed in vacuum at elevated temperatures, resulting in the crystallization of Sr2Nb2O7 with highly ordered atomic arrangement and an atomically flat surface. The refractive index of the resultant film was 2.1, indicating that the dielectric permittivity of the film was in between 20 and 80, which corresponds well with that of single crystal Sr2Nb2O7, demonstrating the effectiveness of the SPE method for the fabrication of high-quality epitaxial films of Sr2Nb2O7.

Evolution of magnetic properties in the vicinity of the Verwey transition in Fe3O4 thin films

We have systematically studied the evolution of magnetic properties, especially the coercivity and the remanence ratio in the vicinity of the Verwey transition temperature (TV ), of high-quality epitaxial Fe3O4 thin films grown on MgO (001), MgAl2O4 (MAO) (001), and SrTiO3 (STO) (001) substrates. We observed rapid change of magnetization, coercivity, and remanence ratio at TV , which are consistent with the behaviors of resistivity versus temperature [\r{ho}(T )] curves for the different thin films. In particular, we found quite different magnetic behaviors for the thin films onMgOfrom those onMAOand STO, inwhich the domain size and the strain state play very important roles. The coercivity is mainly determined by the domain size but the demagnetization process is mainly dependent on the strain state. Furthermore, we observed a reversal of remanence ratio at TV with thickness for the thin films grown on MgO: from a rapid enhancement for 40-nm- to a sharp drop for 200-nm-thick film, and the critical thickness is about 80 nm. Finally, we found an obvious hysteretic loop of coercivity (or remanence ratio) with temperature around TV , corresponding to the hysteretic loop of the \r{ho}(T ) curve, in Fe3O4 thin film grown on MgO.

Fabrication of atomically abrupt interfaces of single-phase TiH2 and Al2O3

We report the fabrication of atomically abrupt interfaces of titanium dihydride (δ-TiH2) films and α-Al2O3(001) substrates. With the assistance from reactive hydrogen in plasma, single-phase δ-TiH2 epitaxial thin films were grown on α-Al2O3(001) substrates using the reactive magnetron sputtering technique. Scanning transmission electron microscopy measurements revealed an atomically abrupt interface at the δ-TiH2(111) film and Al2O3(001) substrate. These results indicate that the reactive magnetron sputtering has great potential to deposit various epitaxial thin films of hydrides restricted by the hydrogenation limit. The fabrication of high-quality hydride epitaxial thin films with atomically controlled interfaces paves the way for future hydride electronics.

Optical and electrical performance of thermochromic V2O3 thin film fabricated by magnetron sputtering

V2O3 was recognized as one of the clearest examples of Mott-Hubbard physics but was rarely treated as transmittance modulating coatings based on the metal-insulator transition. Here, we deposited high quality epitaxial V2O3 thin films on sapphire (001) substrates as well as polycrystalline V2O3 films on glass or Si substrates, measured the optical properties during cooling and heating, and discussed the modulating ability using fully reversible optical hysteresis loops. Meanwhile, we also optimized the electrical performance of V2O3/SiNx/Si samples by W-Ti co-doping. The obtained co-doped V2O3 film shows a relatively high temperature coefficient resistance of −8.1%/K at 80 K without hysteresis loops, which exhibits great potential in high sensitivity thermal resistor devices. Our work provides a comprehensive scenario of the V2O3 thin film physics.

High quality epitaxial fluorine-doped SnO2 films by ultrasonic spray pyrolysis: Structural and physical property investigation

Despite its wide use in the display and photovoltaic industries, fluorine-doped tin oxide (F:SnO2, FTO) has been studied only in its polycrystalline form. In this work, we report on the first growth of epitaxial FTO thin film by ultrasonic spray pyrolysis – a simple chemical deposition method – and we reveal the structure-property interplay by investigating in details its growth, morphology and strain/defects. Epitaxial FTO films are successfully grown on (110) rutile TiO2 single crystals and form mosaic domains with an out-of-plane distribution smaller than 0.5°, showing high structural quality comparable to epitaxial films prepared by molecular beam epitaxy and pulsed-laser deposition. Owing to the large lattice mismatch with rutile TiO2, the FTO film develops significant structural defects to release the epitaxial strain and is consequently nearly fully relaxed with a slight residual strain of 0.1–0.2%. With the help of an innovative nano-beam precession electron diffraction technique, the strain distribution is mapped at the TiO2/FTO interface, from which we identify the interfacial and secondary strain relaxation taking place mainly in the first 22nm in the FTO film. The Hall-mobility of the epitaxial FTO films is close to the state-of-the-art and expected to improve further at lower doping concentrations.

Growth of high-quality AlGaN epitaxial films on Si substrates

High-quality AlGaN epitaxial films have been grown on Si(111) substrates by metal-organic chemical vapor deposition through introducing an AlN layer followed by a high Al composition AlGaN buffer layer, together with optimizing the growth temperature to promote the dislocations reduction. The as-grown ∼400nm-thick AlGaN epitaxial films with optimized growth temperature of 980°C reveal high crystalline quality with a full-width at half-maximum for AlGaN(0002) of 0.29° and smooth surface with a root-mean-square surface roughness of 0.76nm. This work provides an effective approach for the growth of high-quality AlGaN epitaxial films in the application of ultraviolet lasers and detectors.

Two-dimensional Frank–van-der-Merwe growth of functional oxide and nitride thin film superlattices by pulsed laser deposition

Abstract

Structure, site-specific magnetism, and magnetotransport properties of epitaxial D022 -structure Mn2FexGa thin films

Ferrimagnetic Mn$_2$Fe$_x$Ga $(0.26 \leq x \leq 1.12)$ thin films have been characterised by X-ray diffraction, SQUID magnetometry, X-ray absorption spectroscopy, X-ray magnetic circular dichroism and M\"{o}ssbauer spectroscopy with the aim of determining the structure and site-specific magnetism of this tetragonal, D0$_{22}$-structure Heusler compound. High-quality epitaxial films with low RMS surface roughness ($\sim 0.6$ nm) are grown by magnetron co-sputtering. The tetragonal distortion induces strong perpendicular magnetic anisotropy along the $c$-axis with a typical coercive field $\mu_0 H\sim 0.8$ T and an anisotropy field ranging from $6$ to $8$ T. Upon increasing the Fe content $x$, substantial uniaxial anisotropy, $K_\mathrm{u} \geq 1.0$ MJ/m$^3$ can be maintained over the full $x$ range, while the magnetisation of the compound is reduced from $400$ to $280$ kA/m. The total magnetisation is almost entirely given by the sum of the spin moments originating from the ferrimagnetic Mn and Fe sublattices, with the latter being coupled ferromagnetically to one of the former. The orbital magnetic moments are practically quenched, and have negligible contributions to the magnetisation. The films with $x=0.73$ exhibit a high anomalous Hall angle of $2.5$ % and a high Fermi-level spin polarisation, above $51$ %, as measured by point contact Andreev reflection. The Fe-substituted Mn$_2$Ga films are highly tunable with a unique combination of high anisotropy, low magnetisation, appreciable spin polarisation and low surface roughness, making them very strong candidates for thermally-stable spin-transfer-torque switching nanomagnets with lateral dimensions down to $10$ nm.

Epitaxial growth of BiFeO3 films on SrRuO3/SrTiO3

We have investigated the influence of growth parameters on the surface morphology of BiFeO3 (BFO) thin films prepared by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 buffer layers. High quality epitaxial BFO thin films are obtained by controlling the laser pulse energy (EL), substrate temperature (TS) and oxygen partial pressure (Po2). It is found that the surface morphology is sensitive to both TS and Po2, and the root-mean-square roughness decreases initially and increases subsequently with increasing TS or Po2. Epitaxial BFO thin films with atomically smooth surface are successfully prepared in step flow growth mode at EL=100mJ, TS=700°C and Po2=13Pa. Using X-ray diffraction, high-resolution cross-sectional transmission electron microscopy and piezoelectric force microscopy, the crystallinity, epitaxy and ferroelectricity of the BFO thin films prepared in optimal growth conditions have been studied. A growth diagram is constructed to delineate the growth window of high quality epitaxial BFO films with step and terrace topography.

Tuning the Curie temperature of epitaxial Nd0.6Sr0.4MnO3 thin films

NdxSr1−xMnO3 (0.2≤x≤0.5) systems are widely studied in magnetism, popular for high colossal magnetoresistance and are ferromagnetic oxides with TC ranging from 200K to 300K. Recently, many of such compounds are re-visited for exploring the correlation of spin, charge and lattice degrees of freedom. Although, manganite thin films are the ideal candidates for studying the electron-correlation effects, the puzzle of obtaining a high quality epitaxial thin films of NdxSr1−xMnO3 are still unsolved contrary to its sister compound LaxSr1−xMnO3. Hence, in this study, we demonstrate the growth of best quality of Nd0.6Sr0.4MnO3 (NSMO) epitaxial thin films. This is evident from the TC and a sharp insulator-to-metal transition (IMT) coinciding at as high as ∼255K against the bulk TC (∼270K). It is the highest reported TC in Nd0.6Sr0.4MnO3 thin films to date. Moreover, as-deposited films with in situ oxygen annealing are not enough to relax the lattice of NSMO films due to the significant Jahn-Teller distortion in the film. With ex situ annealing processes alongside the various deposition and in situ annealing conditions, we have extensively studied the growth of epitaxial NSMO thin films on LaAlO3 (001) and SrTiO3 (001) to investigate the evolution of lattice and its one-to-one correspondence with the magnetism and the electrical properties of thin films. Accordingly, the enhanced magnetization, reduced resistivity and the higher TC and IMT of the NSMO films obtained from our extensive growth analysis looks promising for the future applications across the TC and IMT.

Growth mechanisms of GaN epitaxial films grown on ex situ low-temperature AlN templates on Si substrates by the combination methods of PLD and MOCVD

Three kinds of ex situ low-temperature AlN (LT-AlN) templates grown on Si substrates by pulsed laser deposition (PLD) are employed to grow GaN epitaxial films by metal organic chemical vapor deposition (MOCVD). The single-crystalline ex situ LT-AlN template with smooth surface shows the greatest advantage to achieve high-quality GaN epitaxial films. It can effectively promote the lateral growth of high-temperature AlN layer to obtain coalesced and smooth surface, and therefore reduce the GaN nucleation energy barrier to facilitate GaN nucleation process. The as-grown truncated GaN nuclei with large sizes and low density are favorable for the two-dimensional growth of GaN. The ∼1.5 μm-thick GaN epitaxial film grown on this ex situ LT-AlN template shows a flat surface and the best crystalline quality with the minimum full-width at half maximums (FWHMs) of GaN(0002) and GaN(10–12) X-ray rocking curves (XRCs) as 504 and 566 arcsec, respectively, and the minimum FWHM of near band emission peak as 20.62 nm. According to the XRC results, compared with the GaN film grown without ex situ LT-AlN templates, the screw-type threading dislocations (TDs) density, edge- and mixed-type TDs density in as-grown GaN film are greatly reduced by 20% and 49%, respectively, proving the superiority of ex situ LT-AlN template. This work is instructional for the growth of high-quality nitride films by combination methods, and significant for achieving high-performance GaN-based devices.

Research Update: Van-der-Waals epitaxy of layered chalcogenide Sb2Te3 thin films grown by pulsed laser deposition

An attempt to deposit a high quality epitaxial thin film of a two-dimensionally bonded (layered) chalcogenide material with van-der-Waals (vdW) epitaxy is of strong interest for non-volatile memory application. In this paper, the epitaxial growth of an exemplary layered chalcogenide material, i.e., stoichiometric Sb2Te3 thin films, is reported. The films were produced on unreconstructed highly lattice-mismatched Si(111) substrates by pulsed laser deposition (PLD). The films were grown by vdW epitaxy in a two-dimensional mode. X-ray diffraction measurements and transmission electron microscopy revealed that the films possess a trigonal Sb2Te3 structure. The single atomic Sb/Te termination layer on the Si surface was formed initializing the thin film growth. This work demonstrates a straightforward method to deposit vdW-epitaxial layered chalcogenides and, at the same time, opens up the feasibility to fabricate chalcogenide vdW heterostructures by PLD.

Low Temperature Epitaxial Barium Titanate Thin Film Growth in High Vacuum CVD

Barium titanate is a promising perovskite material, exhibiting numerous materials properties advantageous for a range of applications, such as ferroelectricity, piezoelectricity, a high dielectric constant, and large electrooptic coefficients. It represents a potential candidate for the implementation of high bandwidth and low energy consuming modulators for optical communication in future integrated systems. Integration of the optical components with electronic circuits on silicon requires, however, suitable low temperature deposition processes for the growth of high quality epitaxial BaTiO3 films, which are still missing. For integration of novel materials into the complementary metal–oxide–semiconductor platform, a variety of prerequisites must be fulfilled; among them there is a strict limitation of the thermal budget of the growth process. Furthermore, high growth rates are desirable for enhanced throughput. Many chemical vapor deposition (CVD) processes allow growing thin films at high rates, but epitaxial BaTiO3 film formation typically requires high substrate temperatures. It is demonstrated that high vacuum CVD is capable of growing epitaxial barium titanate films on magnesium oxide, strontium titanate, and strontium titanate buffered silicon substrates at process temperatures of 400 °C and growth rates of up to 100 nm h−1 without the need of an additional annealing step; this is the lowest substrate temperature reported for a CVD processes.

Domain Evolution and Piezoelectric Response across Thermotropic Phase Boundary in (K,Na)NbO3-Based Epitaxial Thin Films.

Recent research progress in (K,Na)NbO3 (KNN)-based lead-free piezoelectric ceramics has attracted increasing attention for their applications to microsystems or microelectromechanical systems (MEMS) in the form of thin films. This work demonstrates that high-quality KNN-based epitaxial films can be synthesized by a conventional sol-gel method, whose phase structure and domain characteristics have been investigated with emphasis on the temperature effect. A monoclinic MC structure is observed at room temperature in KNN-based epitaxial films, which is close to but different from the orthorhombic phase in bulk counterparts. Piezoresponse force microscopy (PFM) at elevated temperatures reveals continuous changes of ferroelectric domains in KNN films during heating and cooling cycles between room temperature and 190 °C. A distinct change in domain morphology is observed upon heating to 110 °C, accompanied by a clear variation of dielectric permittivity suggesting a thermotropic phase transition, which is revealed to belong to a MC-MA phase transition on the basis of structural and PFM analysis on local ferroelectric and piezoelectric behaviors. Enhanced piezoelectric response at the thermotropic phase boundary is observed, which is attributed to active domains and/or nanodomains formed across the boundary. Domain engineering by utilizing the phase transition should be important and effective in KNN-based films not only for property enhancement but also for its textured ceramics.