Textured Thin Films Research Articles

High-quality textured SnSe thin films for self-powered, rapid-response photothermoelectric application

Abstract Tin selenide (SnSe) has attracted great attention in thermoelectrics (TE) thanks to its record high figure of merit (ZT) discovered in single crystal (SC). However, the practical application of SnSe SC is limited due to its extreme growth condition. With the development of flexible integrated electronics, using flexible polycrystalline TE films to substitute rigid SC becomes a main trend. Nevertheless, large resistances of TE films restrict their application as power devices. Here, self-powered SnSe-based photothermoelectric (PTE) detectors are demonstrated. The PTE detectors are sensitive to wide-spectrum and exhibit higher responsivity and respnse speed comparing with SC, due to the small heating volume, small conductive heat flux and large dissipation content of thin films. High-quality (400)-textured SnSe polycrystalline thin films are one-step synthesized by chemical vapor deposition. In-situ in-plane TE measurement shows that the obtained SnSe films possess a dimensionless ZT of 0.15 and a power factor of 0.322 mW m−1 K−2 at 550 K, showing SC level TE performance at low temperatures.

Scaling, rotation, and channeling behavior of helical and skyrmion spin textures in thin films of Te-doped Cu2OSeO3.

Topologically nontrivial spin textures such as vortices, skyrmions, and monopoles are promising candidates as information carriers for future quantum information science. Their controlled manipulation including creation and annihilation remains an important challenge toward practical applications and further exploration of their emergent phenomena. Here, we report controlled evolution of the helical and skyrmion phases in thin films of multiferroic Te-doped Cu2OSeO3 as a function of material thickness, dopant, temperature, and magnetic field using in situ Lorentz phase microscopy. We report two previously unknown phenomena in chiral spin textures in multiferroic Cu2OSeO3: anisotropic scaling and channeling with a fixed-Q state. The skyrmion channeling effectively suppresses the recently reported second skyrmion phase formation at low temperature. Our study provides a viable way toward controlled manipulation of skyrmion lattices, envisaging chirality-controlled skyrmion flow circuits and enabling precise measurement of emergent electromagnetic induction and topological Hall effects in skyrmion lattices.

Fabrication and growth of c-axis textured Nd2Fe14B thin films by high-rate sputtering

To apply Nd–Fe–B thin films for mass-produced heat-assisted magnetic recording media, we investigated the high-rate sputtering conditions required to obtain c-axis textured Nd2Fe14B thin films and analyzed the growth mechanism. Magnetization curves indicated that higher substrate temperatures and sputtering rates resulted in a higher degree of perpendicular magnetic anisotropy; a Nd–Fe–B layer deposited at a substrate temperature (Tsub_0) of 600 °C and a sputtering rate (Rsp) of 2.6 nm/s had the easy axis perpendicular to the film plane. The dependence of the magnetic properties on the sputtering rate was due to a decrease in the substrate temperature during sputtering; there was a threshold for obtaining a high squareness ratio. X-ray diffraction analysis and transmission electron microscopy (TEM) images showed that the c-axis textured Nd2Fe14B crystal phase was formed in the Nd–Fe–B layer deposited at Tsub_0 = 600 °C and Rsp = 2.6 nm/s, which resulted in the highly perpendicular magnetic anisotropy. In addition, the TEM images showed a layer of Nd2Fe14B with no obvious lattice fringes near the interface between the Nd–Fe–B layer and Mo underlayer, while the lattice fringes of the Nd2Fe14B phase were not parallel to this interface but gently curved along the Mo cap layer. We propose that the c-axis orientation was achieved by the rotation of the c plane, which has the lowest surface energy in the Nd2Fe14B phase, toward the vacuum-side surface during sputtering.

Polymorphic chiral squaraine crystallites in textured thin films.

An enantiomerically pure (R)-2-methylpyrrolidine-based anilino squaraine crystallizes in two chiral polymorphs adopting a monoclinic C2 and an orthorhombic P21 21 21 structure, respectively. By various thin-film preparation techniques, a control of the polymorph formation is targeted. The local texture of the resulting textured thin films is connected to the corresponding optical properties. Special attention is paid to an unusual Davydov splitting, the anisotropic chiroptical response arising from preferred out-of-plane orientation of the crystallites, and the impact of the polymorph specific excitonic coupling.

Highly textured Pt thin film grown at very low temperature using Ca2Nb3O10 nanosheets as seed layer

The decrease of the growth temperature of platinum (Pt) thin film on silicon substrate was studied using Ca2Nb3O10 nanosheets (CNOns) as seed layer. These nanosheets were obtained by the delamination of the layered perovskite KCa2Nb3O10 and they were deposited on silicon substrates by the Langmuir–Blodgett method. Pt thin films were sputtered on silicon coated by CNOns (CNOns/SiO2/Si), and on TiO2/SiO2/Si substrates for comparison, at temperatures ranging from room temperature up to 625 °C. X-ray diffraction, scanning electron microscopy, and atomic force microscopy were used to characterize the crystalline quality, thickness, surface morphology and roughness of the Pt thin films. Highly (111) textured Pt thin films were obtained on CNOns/SiO2/Si at substrate temperature as low as 200 °C. The full width at half maximum of the rocking curve of the (111) X-ray peak was about one degree, indicating a high crystalline orientation. The resistivity was measured at room temperature by the four point probes method to confirm the quality of Pt thin films elaborated at low temperatures. These results pave the way for easier integration of highly textured platinum thin film in low temperature microelectronic processes.

Influence of III–V substrates on the texture, structural, and optical properties of CdS thin films deposited by chemical bath deposition

We have studied the influence of texture distribution and crystalline structure of the CdS thin films on their optical properties. CdS samples were grown by chemical epitaxy on III–V substrates. Ga-V materials (c-GaAs and h-GaN) were selected due to the newly renovated interest in CdS/III–V heterostructures and their importance in the development of current optical devices, such as light emitter diodes and solar cells. Texture analysis was realized in pole diagrams obtained in a High-Resolution X-ray diffractometer. Measurements were carried out at 2θ = 26.6° and 2θ = 43.9° because these 2θ angles correspond to diffraction signal for cubic and hexagonal CdS phases, respectively. Pole diagrams show fourfold symmetry in the CdS layers deposited on GaAs (100) substrates and sixfold symmetry in CdS layers deposited on GaN (0002) substrates, which indicates that epitaxial CdS films with both crystal structures is possible by a low-cost technique (Chemical Bath Deposition). The structural study was completed with a surface analysis of the CdS samples by atomic force microscopy; the images obtained showed a greater surface roughness in the CdS layers grown on GaN substrates, this result agrees with the texture distribution observed in pole diagrams. Optical properties of radiative and non-radiative recombination were studied by Raman and photoluminescence (PL) spectroscopies. According to PL results, cubic CdS films have better efficiency in the recombination of radiative bands. In addition, we determined that the CdS–A1LO phonon for the cubic monocrystalline phase is located at 302.35 cm−1, which is 0.84 cm−1 smaller than hexagonal structure.

Influence of Substrate Surface Properties on Spin Dewetting, Texture, and Phase Transitions of 5CB Liquid-Crystal Thin Films.

We report how the texture and stability of a nematic liquid-crystal (LC) thin film of 5CB vary as a function of UV-ozone (UVO) exposure of the underlying poly(methyl methacrylate) (PMMA) substrate. UVO exposure of the PMMA substrate not only increases its surface energy, making it more wettable, but also results in the generation of oxygen-containing polar functional groups on the PMMA surface due to photolysis of ester. While the stability of the 5CB films is expectedly enhanced on UVO-exposed PMMA substrates against thermally induced dewetting, the texture of the film also changes as a function of the UV exposure time (tE). We show that the films continue to exhibit the nematic Schlieren texture for tE ≤ 20 min, although the disclination point density (|m|) gradually reduces with an increase in tE. However, the texture changes completely to a spherulite or fanlike texture in tE ≥ 20 min due to enhanced anchoring of the 5CB molecules on the substrates. In addition, enhanced wettability and stronger anchoring by the UVO-exposed PMMA substrates also suppress the tendency of spin dewetting of the 5CB films due to spontaneous rupture of the dispensed solution layer during spin coating, particularly when the solute concentration (Cn) is very low. The latter observation allows possible creation of thinner LC films, which are otherwise difficult to form by spin coating due to enhanced cohesive interactions between the anisotropic LC molecules. Finally, we show that in continuous films, the nematic-to-isotropic (N → I) and I → N phase transitions with gradual heating and cooling remain completely reversible, irrespective of the texture of the film and wettability of the substrate.

Toward van der Waals epitaxy of transferable ferroelectric barium titanate films via a graphene monolayer

Transferable highly (001)-oriented textured ferroelectric BaTiO3 thin films have been achieved on a graphene monolayer for wearable devices like sensors and actuators for future “Internet of Things” era.

Evaluating the accuracy of common γ-Al2O3 structure models by selected area electron diffraction from high-quality crystalline γ-Al2O3

Single crystal and textured polycrystalline γ-Al2O3 thin films were synthesized by oxidation of NiAl(110) in air at 850 °C for 1 and 2 h, respectively, and used to evaluate the accuracy of two spinel-based and two nonspinel models by comparison of selected-area electron diffraction (SAED). The lattice interplanar distances derived from the polycrystalline SAED pattern most closely matched the cubic spinel γ-Al2O3 model. The single-crystal SAED spot pattern showed symmetry consistent with both the cubic spinel and tetragonal nonspinel models, however, the Al cation distribution better matched the cubic spinel model based on the relative intensities of diffraction spots. Our work indicates that the traditional cubic spinel model is a more accurate model of γ-Al2O3 than the other models considered. The spinel-based monoclinic model is also more accurate than the monoclinic nonspinel model. The understanding of the relative accuracy of the different models is key for simulating γ-Al2O3 containing systems and is of general interest for the metal oxide and ceramic communities.

Effect of oxygen partial pressure on crystal quality and electrical properties of RF sputtered PZT thin films under the fixed Ar flow and sputtering pressure

Pb(Zr0·52Ti0.48)O3 films have been deposited on Pt/Ti/SiO2/Si (100) substrates by radio frequency (RF) magnetron sputtering method based on a sol-gel derived Pb(Zr0.30,Ti0.70)O3 (PZT) seed layer. Oxygen partial pressure-dependent crystal texture and electrical performance of PZT thin films, under the fixed Ar flow and sputtering pressure during deposition, were investigated. X-ray diffraction (XRD) analysis indicates that the major phase was transformed from perovskite phase containing little pyrochlore phase to pure perovskite phase with increasing oxygen partial pressure during sputtering. Atomic force microscopy (AFM) shows that the film processed by an O2/Ar composition of 10/90 demonstrated a compact and smooth surface. Dense perovskite structure without defects at the interface between seed layer and PZT film was observed utilizing Scanning electron microscope (SEM). Enhanced dielectricity (ε = 920.3, tan δ = 0.02 at 1 kHz) and ferroelectricity (2Pr = 22.8 μCcm−2, 2Ec = 97.9 kV/cm) were generated in the film processed with an O2/Ar composition of 10/90. Moreover, current-voltage (J-V) characteristic was also improved significantly with an O2/Ar composition of 10/90.

Texture and interface characterization of iridium thin films grown on MgO substrates with different orientations

Iridium thin films are grown by direct-current plasma magnetron sputtering, on MgO single-crystal substrates with various surface orientations, i.e. (100), (111), and (110). The surface morphology, the crystalline properties of the films, and the substrate–thin-film interface are investigated by atomic force microscopy, X-ray diffraction (XRD), focused ion beam scanning electron microscopy, and high-resolution transmission electron microscopy, respectively. The results reveal that hetero-epitaxial thin films with different crystallographic orientation and notable atomic scale smooth surface are obtained. From the XRD analysis, the following epitaxial relations are obtained: (1) (100)Ir||(100)MgO out-of-plane and [001]Ir||[001]MgO in-plane for Ir grown on MgO(100), (2) (110)Ir||(110)MgO out-of-plane and [1-10]Ir||[1-10]MgO in-plane for Ir grown on MgO(110), and (3) (111)Ir||(111)MgO out-of-plane and two variants for in-plane orientation [1-10]Ir||[1-10]MgO and [1-10]Ir||[10-1]MgO, respectively, for Ir grown on MgO(111). Because of the large misfit strain (9.7%), the thin films are found to grow in a strain-relaxed state with the formation of geometrical misfit dislocations with a ~ 2.8-nm spacing, whereas thermal strain is stored upon cooling down from the growth temperature (600 °C). The best structural characteristics are obtained for the (111)-oriented films with a mosaicity of 0.3° and vanishingly small lattice distortions. The (100)- and (110)-oriented films exhibit mosaicities of ~ 1.2° and lattice distortions of ~ 1% which can be explained by the larger surface energy of these planes as compared to (111).

Growth of (001) preferentially oriented BiFeO3 films on Si substrate by sol-gel method

Obtaining high-quality oriented/textured BFO thin films on traditional Si substrates by low-cost manageable sol-gel method is still challenging and currently relevant. The paper presents a comprehensive set of experimental investigations on the role of the buffer-layer (None, Pt/Ti/SiO2, and LNO) and the thickness of LNO & BFO films on the crystalline structure and microstructure involution of sol-gel based spin-coated BFO films on Si substrates. All BFO and LNO films exhibit good crystallinity and pure perovskite phase. No obvious preferential orientation was found in BFO films fabricated on bare Si(100) and Pt(111)/Ti/SiO2/Si substrates. While highly (001) preferred orientation was obtained for BFO film spin-coated on LNO(00l)/Si substrate. It demonstrated that highly oriented LNO buffer-layer with a certain thickness can promise the growth of high quality oriented /textured BFO thin films. In addition, the crystallization, orientation, phase, grain size, strain state and dislocation are all dependent on the film thickness. The XRD, SEM and W-H equation theoretical analysis results illuminate that the thickness of the BFO films may affect the stress relaxation state, and then the grain size, dislocation and orientation. We demonstrate that suitable buffer-layer materials, optimized thickness of buffer-layer and ferroelectric film have relevant role on the development of high-quality oriented BFO thin films on Si prepared by sol-gel and derived silicon based devices.

Morphology induced spectral reflectance lineshapes in VO2 thin films

In this work, we study the spectral reflectance of VO2 thin films and identify the specific contributions of the morphology and phase transition to optical spectra. The formation of highly [011] textured VO2 thin films on Si was achieved by an oxidation process starting with a metallic V thin film grown on an [001] Si substrate by an evaporation technique. Structural (XRD and Raman) and spectroscopic (XPS) characterization results indicate high purity VO2 formation with different sizes at various annealing temperatures without any change in the composition. Temperature dependent spectral reflectance distributions reveal that the insulator-to-metal transition (IMT) phase transition temperature of the VO2 nanostructures shows a slight size-dependence (∼3 °C), but this feature can be overshadowed by morphology that can lead to the misinterpretation of transition characteristics. The spectral line shape of the reflectance curves in the visible and near-infrared regions show substantially different characteristics for the samples annealed at different temperatures. Using numerical scattering calculations, we conclude that the changes in the optical response can be explained by morphological effects instead of changes in the intrinsic material properties such as a shift in the IMT temperature. Furthermore, the main mechanism leading to different spectral line shapes is the morphological differences leading to diffuse and specular reflectance.

Crystallinity and texture of molybdenum thin films obliquely deposited at room temperature

Controlling the texture of a nanostructured thin film is an important issue for numerous applications. Although the texture formation of thin films grown by oblique angle deposition has been investigated for a large number of materials, a general trend is missing so far. Thus, the texture formation must be evaluated on a case-by-case basis. In the present study, Molybdenum thin films consisting of separated tilted columns are fabricated by electron beam evaporation at a highly oblique deposition geometry at room temperature on natively and thermally oxidized Si(100) substrates. The film thickness is varied between 10 nm and 2.5 μm. The film morphology is investigated by scanning electron microscopy. Various transmission electron microscope (TEM) measurements on individual Molybdenum columns reveal a single crystalline structure of those columns. It is discussed why the tilt angle of the Molybdenum columns is not equal to the tilt angle of the crystals´ [110] direction. Reflection high-energy electron diffraction, X-ray diffraction in-plane pole figure measurements and cross-sectional TEM specimen are used to further investigate the crystallinity as well as the texture evolution of the Molybdenum thin films depending on the film thickness. The texture changes with increasing film thickness from arbitrarily oriented crystallites to a biaxially textured thin film with a minor fiber texture contribution. The selection of crystal orientations with increasing film thickness is discussed with view to the evolutionary selection process among the growing columns.

Spin fluctuations, geometrical size effects, and zero-field topological order in textured MnSi thin films

Spin fluctuations, geometrical size effects, and zero-field topological order in textured MnSi thin films

Thickness dependent correlation between structural and optical properties of textured CdSe thin film

Nano-crystalline CdSe thin films of different thicknesses under sub-micron range were deposited on glass substrate via thermal evaporation route. A gradual deterioration in film crystallinity confirmed by XRD line profile analysis has been accompanied by a reduction in Cd to Se molar ratio as the film thickness decreases. A coordinated microstructural and crystallographic orientation distribution analysis explicitly demonstrated that CdSe tends to grow in nano-sized columns with hexagonal c-axis parallel to its growth direction on glass substrate. A thickness dependence of structural evolution was discussed in terms of aspect ratio of the columnar structure and dispersion in orientation of hexagonal (002) basal plane. The variation in the spectra of optical constants [n(λ), k(λ)] obtained from Swanepoel envelop method was interpreted in terms of crystallographic defects arising from stoichiometric disorder which was also accounted for the observed thickness dependent shifts in band gap and valence band split energy. The bathochromic shifts in dielectric and energy loss functions, optical conductivity, skin depth and cut-off energy were discussed in detail along with the variations in their spectral shapes in connection with the dispersion in the real and imaginary parts of complex refractive index, which might shed a new light upon holistic comprehension of thickness dependent optical properties of other chalcogenide semiconducting thin films.

Growth, characterization, and analysis of the nanostructures of ZnO:B thin films grown on ITO glass substrates by a LPCVD: a study on the effects of boron doping

In this work, boron-doped ZnO (ZnO:B) thin films grown by low-pressure chemical vapor deposition (LPCVD) on ITO glass substrates with film thicknesses of 1, 2, and 3 µm are analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic-force microscopy (AFM), cathodoluminescence (CL), and light transmittance measurements. The effects of boron doping on the grain crystallization, grain sizes, film strains, surface texture, and light transmittance of ZnO:B thin films are studied from the analysis of these measurements. XRD results show that ZnO:B and undoped ZnO thin films both have the largest percentage of the (110) crystalline grains which form ridge-like structures at the surfaces of the thin films. These ridge-like structures of (110) grains provide sharper surface textures than the polygon structures of (100) grains. Hence, ZnO:B and undoped ZnO thin films grown on ITO glass substrates both have good-quality surface textures which can provide light-trapping functions for solar-cell applications. In addition, the surfaces of ZnO:B thin films are less sharp and rough than those of undoped ZnO thin films due to their smaller grain sizes. Boron-doping decreases the ZnO grain sizes and reduces the surface textures of ZnO thin films grown on ITO glass substrates. Since ZnO:B thin films have smaller average grain sizes than undoped ZnO thin films, ZnO:B thin films have larger quantum-size effects and hence larger band-gap energies and shorter CL emission wavelengths. Furthermore, thicker films have more ZnO(110) grains with ridge-like structures and have better surface textures which can provide better light trapping than thinner ones. However, thicker films are shown to have poorer optical transparency. The results of this work provide useful information for growth of textured well-faceted ZnO:B films on ITO glass substrates that can potentially be utilized for various applications, such as solar cells.

Optical properties and charge transport of textured Sc2O3 thin films obtained by atomic layer deposition

Scandium oxide films with thickness d = 20–100 nm were deposited by atomic layer deposition (ALD) using tris(methylcyclopentadienyl) scandium (III) Sc(MeCp)3 and water vapors as precursors at reactor temperature range of 200–400 °C. The ALD-window was detected in the range of 230–370 °C. Growth per cycle remained constant and was 0.080–0.084 nm/cycle in this range. O/Sc ratio was found to be 1.49 which is close to stoichiometric Sc2O3. The films were polycrystalline. The grains were more apparent for the films grown at higher temperatures. The grain size increased with thickness. Cubic Sc2O3 phase was determined by FTIR, Raman spectroscopy and X-rays diffraction.In general, different crystallographic directions are more preferential for different process conditions. Films were textured and their growth occurred mainly in the (400) direction. The refractive index dispersions of deposited films were well described by the Cauchy model. Optical properties depended weakly on process parameters (temperature or thickness). As it was detected for the films with close thickness the refractive index slightly increased with deposition temperature increase. The Nasyrov-Gritsenko (N-G) multiphonon trap ionization model well describes the charge transport. The trap parameters were determined to be: concentration N = 1.5 × 1019 cm−3, thermal energy Wt = 0.83 eV, and optical energy Wopt = 1.66 eV.

Tailoring the texture of titanium thin films deposited by high-power pulsed magnetron sputtering

Titanium (Ti) thin films with (002) or (100) texture are favored for many applications. In this paper, Ti thin films were prepared by high-power pulsed magnetron sputtering, and the texture of Ti thin films was successfully tailored by adjusting the pulse width, substrate bias and magnetic field strength. It is found that the peak power and average power of the Ti target are increased by increasing sputtering pulse width and decreasing magnetic field strength, which raise the Ti plasma flux and ion/atom ratio of Ti ions in front of the substrate simultaneously. Ti thin films with a highly (002) out-of-plane texture can be obtained with higher pulse width and lower magnetic field strength. The Ti thin films with highly (100) out-of-plane texture can be achieved with shorter pulse width, lower magnetic field strength and substrate bias.

Chemo-mechanical degradation in V2O5 thin film cathodes of Li-ion batteries during electrochemical cycling

We have devised an approach to fabricate dense textured V2O5 thin films, which allows us to scrutinize the root cause of capacity fade in V2O5 cathodes of Li-ion batteries.