Epitaxial Growth Of Thin Films Research Articles

Co2Fe(Ti0.5Al0.5) epitaxial thin films: Structural and magnetic properties of a Heusler alloy with Z-site transition metal substitution

We investigate the structural, static, and dynamic magnetic properties of epitaxial Heusler Co2Fe(Ti0.5Al0.5) (CFTA) alloy thin films with thickness varying from 6 nm to 80 nm grown by sputter beam epitaxy on cubic MgO(001), MgAl2O4(001), and hexagonal Al2O3(112̄0) substrates. X-ray diffraction measurements indicate epitaxial growth of CFTA thin films with B2 chemical ordering, with cubic [001] and [220] CFTA axes normal to the cubic and hexagonal substrates, respectively. Microstructure analysis of films grown on MgO substrates reveals a uniformly oriented epitaxial crystal with small variations consistent with strain distortions, providing an explanation for the relatively large X-ray rocking curve values found. Meanwhile, films on Al2O3(112̄0) substrates reveal columnar growth with frequent in-plane grain rotations. A pronounced four-fold magnetocrystalline anisotropy is observed in epitaxial thin films grown on cubic substrates. A pronounced uniaxial anisotropy for films grown on Al2O3(112̄0) substrates is observed. A saturation magnetization of ∼5.0μB/f.u. (where f.u. represents formula unit) is obtained at room temperature, slightly smaller compared to the expected value based on the Slater-Pauling rule. Ferromagnetic resonance spectroscopy finds an effective damping parameter and inhomogeneous linewidth broadening comparable to those found in parent compound Co2FeAl, which suggests that Ti substitution can be achieved without negatively affecting the magnetic properties of the system.

Epitaxial growth of NbN thin films for electrodes using atomic layer deposition

The epitaxial growth of NbN thin film was accomplished via atomic layer deposition (ALD) for the first time using NbCl5 and NH3 as the Nb precursor and nitrogen source at a deposition temperature of 450 ℃. The cubic NbN thin film was grown epitaxially on a cubic MgO crystal through the coherent lattice matching between NbN and MgO with a small lattice mismatch (∼2.8%). A high concentration of Cl impurity of 4–5% remained in NbN thin films grown on a SiO2 substrate using ALD. However, the Cl impurity concentration decreased to ∼ 2% in the epitaxially grown NbN thin films, which facilitated the epitaxial growth of NbN thin films on the MgO substrate. The origin was attributed to a residual strain at the NbN/MgO interface, which induced a bond length change in Nb-N-Cl. The bond length change may promote Cl desorption during NbN ALD because an in-plane compressive strain in the NbN film and an in-plane tensile strain in the MgO surface were observed. Finally, the epitaxially grown NbN thin film exhibited a 50% lower resistivity than that grown with a polycrystalline phase based on the enhanced carrier mobility owing to the improved crystallinity of epitaxial NbN thin films.

Thermal-strain driving sharp metal-to-insulate transition and island-grain growth of solution-derived NdNiO3 epitaxial thin films

An ultra-sharp metal-to-insulate transition (MIT) of 1.24 K−1 in the epitaxial perovskite NdNiO3 thin films was derived by the chemical solution deposition on the LaAlO3 substrates. The thermal strains from shrink, grain growth, and thermal expansion coefficient misfit play a key role in the film microstructure and electrical properties. The originally theoretical in-plane compressive epitaxial strain changes into a tensile one caused by the thermal driving force. It relaxes with improved grain growth via decreased oxygen vacancies with increasing annealing temperature, while the concurrently enhanced tensile strain from the thermal expansion coefficient misfit between the films and the substrate leads to the destabilization of Ni3+ and the higher MIT temperature. Nevertheless, too much higher tensile strain gives rise to island-grain growth in the films, leading to the weak and even disappeared MIT.

Tutorial: Metalorganic chemical vapor deposition of β-Ga2O3 thin films, alloys, and heterostructures

β-phase gallium oxide (Ga2O3) is an emerging ultrawide bandgap (UWBG) semiconductor with a bandgap energy of ∼ 4.8 eV and a predicted high critical electric field strength of ∼8 MV/cm, enabling promising applications in next generation high power electronics and deep ultraviolet optoelectronics. The advantages of Ga2O3 also stem from its availability of single crystal bulk native substrates synthesized from melt, and its well-controllable n-type doping from both bulk growth and thin film epitaxy. Among several thin film growth methods, metalorganic chemical vapor deposition (MOCVD) has been demonstrated as an enabling technology for developing high-quality epitaxy of Ga2O3 thin films, (AlxGa1−x)2O3 alloys, and heterostructures along various crystal orientations and with different phases. This tutorial summarizes the recent progresses in the epitaxial growth of β-Ga2O3 thin films via different growth methods, with a focus on the growth of Ga2O3 and its compositional alloys by MOCVD. The challenges for the epitaxial development of β-Ga2O3 are discussed, along with the opportunities of future works to enhance the state-of-the-art device performance based on this emerging UWBG semiconductor material system.

Epitaxial growth of gadolinium and samarium thin films and their subsequent facile nitridation at ambient temperatures

Lanthanide nitride species have recently been shown to be easily formed at ambient temperatures and low pressures via the reaction of clean lanthanide surfaces with molecular nitrogen. However, understanding and predicting the nature and efficiency of this process is still in its infancy. In this work we report on the nitridation of the surface of two lanthanide metals, gadolinium (Gd) and samarium (Sm). To that end, epitaxial gadolinium and samarium thin layers are grown on AlN(0001) by molecular beam epitaxy and exposed to molecular nitrogen in high vacuum conditions and ambient temperature. In situ reflection high-energy electron diffraction is used to monitor the growth of Gd in real time, as well as the subsequent exposure to nitrogen, showing a clear transition from a pure Gd surface to a gadolinium nitride (GdN) surface layer. The formation of a nitride layer is further reinforced by magnetic measurements showing clear contributions from the Gd metal and GdN surface layers. Formation of SmN is investigated using synchrotron X-ray photoelectron spectroscopy to probe the surface of Sm pre- and post-nitrogen exposure, showing a change in the surface valence from divalent to trivalent samarium, and confirming the nitridation of the pure Sm surface layer.

Epitaxial growth of (Ba0.7Sr0.3)TiO3 thin films on GdScO3 substrates by magnetron sputtering

Epitaxial growth of Ba0.7Sr0.3TiO3 (BST70) thin films on GdScO3 (GSO) substrates had been realized using the radio frequency magnetron sputtering system with the epitaxial alignment [001]BST70||[110]GSO and [010]BST70||[001]GSO. Reciprocal space mapping and transmission electron microscope results confirm the epitaxial growth without an impurity phase at the interface. The Fourier-filtered image shows that the BST70 thin film grew well with few dislocations. The out-of-plane parameter of the as-deposited film was elongated due to strain, which was induced by the differences in the thermal expansion coefficients between the film and the substrate, and oxygen vacancies. The highly strained as-deposited BST70 films could be relaxed by the post-annealing procedure at 800 °C in an O2-rich atmosphere for better epitaxial quality.

Structural Analysis of Mo Thin Films on Sapphire Substrates for Epitaxial Growth of AlN.

Aluminum nitride (AlN) thin film/molybdenum (Mo) electrode structures are typically required in microelectromechanical system applications. However, the growth of highly crystalline and c-axis-oriented AlN thin films on Mo electrodes remains challenging. In this study, we demonstrate the epitaxial growth of AlN thin films on Mo electrode/sapphire (0001) substrates and examine the structural characteristics of Mo thin films to determine the reason contributing to the epitaxial growth of AlN thin films on Mo thin films formed on sapphire. Two differently oriented crystals are obtained from Mo thin films grown on sapphire substrates: (110)- and (111)-oriented crystals. The dominant (111)-oriented crystals are single-domain, and the recessive (110)-oriented crystals comprise three in-plane domains rotated by 120° with respect to each other. The highly ordered Mo thin films formed on sapphire substrates serve as templates for the epitaxial growth by transferring the crystallographic information of the sapphire substrates to the AlN thin films. Consequently, the out-of-plane and in-plane orientation relationships among the AlN thin films, Mo thin films, and sapphire substrates are successfully defined.

Epitaxial growth of V2O3 thin films on Si(1 1 1) by molecular beam epitaxy

Vanadium sesquioxide (V2O3) is a strongly correlated electron material exhibiting two distinct metal–insulator transitions that can be tuned via strain, doping, or pressure, making it an interesting material for new-generation sensors or smart devices. For this purpose, it is required to achieve well-ordered epitaxial thin film growth with high-quality electrical and optical properties on technologically relevant substrates. We report the successful growth of epitaxial thin films of V2O3 via molecular beam epitaxy, in the paramagnetic insulating (PI) phase on the (111) plane of silicon, by tailoring the growth conditions. Extensive electrical, structural, and morphological characterization both in situ and ex situ has been performed on all samples. The structural analysis reveals that temperature plays a more impactful role in affecting the thin film microstructures than the oxygen partial pressure. When the epitaxy of V2O3 occurs on the unoxidized (111) plane of silicon, four equivalent epitaxial domains begin to form, leading to twin boundaries in the bulk of the film. The considerable lattice mismatch between silicon and V2O3 induces the growth of the corundum PI phase. Lastly, small deviations from stoichiometry due to different oxygen inflow during growth alter significantly the resistivity change upon cooling.

Demonstration of Bixbyite-Structured δ-Ga2O3 Thin Films Using β-Fe2O3 Buffer Layers by Mist Chemical Vapor Deposition

Gallium oxide (Ga2O3) possesses five polymorphs: α, β, γ, κ (ε), and δ. Although the first four polymorphs have been well-studied, there are few reports on δ-Ga2O3. Here, we demonstrate the epitaxial growth of metastable δ-Ga2O3 thin films by mist chemical vapor deposition using β-Fe2O3 buffer layers. X-ray diffraction (XRD) 2θ–ω scan pattern revealed that (004) κ-Ga2O3 grew on (111) yttria-stabilized zirconia (YSZ) without a buffer layer or with a bcc-In2O3 buffer layer, whereas (222) δ-Ga2O3 grew on (222) β-Fe2O3. The β-Fe2O3 buffer layer led to the epitaxial growth of the δ-Ga2O3 thin film. The lattice mismatch between the equivalent crystal structures of β-Fe2O3 and δ-Ga2O3 triggered this growth. XRD analysis shows that δ-Ga2O3 grew epitaxially on the β-Fe2O3 buffer layer/YSZ substrate in both the out-of-plane and in-plane orientations, and the lattice constant inferred from the diffraction peaks was estimated to be 9.255 Å. Reciprocal space mapping results indicated that the δ-Ga2O3 grown on β-Fe2O3 was fully relaxed. Selected area electron diffraction images confirmed that the δ-Ga2O3 exhibited a cubic bixbyite structure. The optical band gap of δ-Ga2O3 was 4.3 or 4.9 eV, as calculated from reflection electron energy loss spectroscopy. We successfully grew a δ-Ga2O3 epitaxial thin film for the first time.

Epitaxial Growth of π-SnS on PbS Substrates

Epitaxial growth of the newly discovered metastable cubic tin sulfide (π-SnS) material was studied theoretically and experimentally. Lead sulfide (PbS) substrates possess a small lattice mismatch with π-SnS and are therefore suitable substrates for epitaxial growth. Very recently, epitaxial growth of π-SnS(111) on PbS(111) and π-SnS(110) on PbS(110) was reported. The interfacial energies of π-SnS(111) on PbS(111), as well as π-SnS(100) on PbS(100), were calculated and found to be smaller than their corresponding pristine surface energies, which give rise to the stable epitaxial growth of π-SnS on PbS. Moreover, we experimentally confirmed the epitaxial growth of π-SnS(100) onto PbS(100) as predicted by the theoretical results. These results establish the pathway of epitaxial growth of π-SnS compact thin films in desired orientations and thus bring their applications closer.

Growth and electron transport characteristics of epitaxial thin strontium iridate films

The results of a study of epitaxial thin films of strontium iridate with the compositions Sr2IrO4 and SrIrO3 obtained by laser ablation and direct current cathode sputtering, respectively, are presented. Data on the growth technology, crystal structure, electrophysical parameters are given, and the activation energy for low-defect dielectric Sr2IrO4 films is calculated.

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 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.

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.

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>

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.