Growth and morphology of ultra-thin Al films on liquid substrates studied by atomic force microscopy

We study the growth process and morphology evolution of the ultra-thin Al films deposited on silicone oil surfaces by using atomic force microscopy. Initially, the deposited atoms nucleate and form compact clusters on the liquid surfaces. Then the clusters perform Brownian motion and adhere upon impact, which results in the ramified islands. Finally a continuous film forms as the nominal film thickness d increases. The mean size of the grains in the compact clusters and ramified islands is of the order of 10¹ nm. The ultra-thin Al films exhibit a self-affine surface morphology and therefore the dynamic scaling analysis is performed. It is found that the growth exponent β = 0.23 ± 0.05. In the range d = 0.1-1.0 nm, the roughness exponent α varies from α ≥ 1 to < 1. The physical interpretation for the crossover of the scaling behavior is presented.     

Surface morphology and growth mechanisms for sputtered amorphous silicon nitride thin films

Evolution of surface of sputter-deposited amorphous Si₃N₄ films growth on Si (100) substrates was investigated using atomic force microscopy (AFM). The scaling behaviors of the AFM topographical profiles were analyzed using the one-dimensional power spectral density. The results of root-mean-square surface height variation showed that there is a power law relationship between the surface roughness and deposition time. It is interesting to note that the growth exponent can be divided into one region and two regions, respectively, when Si₃N₄ films are deposited at different working pressures. A very low growth exponent of β = 0.07 ± 0.01 was found when Si₃N₄ films were deposited at a working pressure of 1.6 × 10^− 1 Pa. However, the growth exponent β can be divided into two regions, which is β₁ = 0.09 ± 0.01, β₂ = 0.24 ± 0.03 and β₁ = 0.09 ± 0.01, β₂ = 0.33 ± 0.04, when the films were deposited at a working pressure of 2.1 × 10^− 1 Pa and 2.7 × 10^− 1 Pa, respectively. The mechanisms of anomalous dynamic scaling exponents of Si₃N₄ films deposited at different working pressures were discussed.     

An investigation on palladium sulphide (PdS) thin films as a photovoltaic material

Polycrystalline PdS thin films with tetragonal structure have been grown by direct sulphuration of Pd layers. They are formed by crystallites of size ∼ 50 nm. As-grown PdS films show a Seebeck coefficient, S = − 250 ± 30 μV/K, which indicates an n-type conductivity. Electrical resistivity of the samples, measured by the four contact probe, is (6.0 ± 0.6) × 10^− 2 Ω·cm. Hall effect measurements, confirms n-type conductivity with a negative carrier density n = (8.0 ± 2.0) × 10¹⁸ cm^− 3 and electron mobility μ of (20 ± 2) cm²/V s. Band gap energy (E_g) and absorption coefficient (α) are determined from the optical transmission and reflectance of the films. A direct transition with energy gap E_g = (1.60 ± 0.01) eV is obtained. Optical absorption coefficient in the range of photon energies hν > 2.0 eV is higher than 10⁵ cm^− 1. All these properties make PdS thin films a good alternative material for solar applications.     

Microstructural evolution in nanostructured gold films

Thin Au films in the thickness range t = 1.5-126 nm were coated by DC sputtering on SiO_x/Si substrates at room temperature inside a vacuum chamber with a base pressure of about 1 × 10^− 2 mbar (1 Pa). The film structure, nanograin characteristics, and the surface roughness as a function of thickness were analyzed using X-ray diffraction, scanning tunneling microscopy and transmission electron microscopy. The results reflect the microstructural evolution with film thickness. They help us to understand the mode of grain growth, which is monomodal-normal columnar as well as spherical. By determination of the dynamic scaling exponent derived from power law dependence of the mean grain size and film thickness, the prevailed mechanism of grain growth is deduced to be the diffusion of mobile Au atoms in grain boundaries. The surface roughness increases with the film thickness following a power law: R_rms ~ t^b. The linear fitted value for b is 0.60.     

Pyroelectric Ta-modified LiNbO3 thin films and devices for thermal infrared detection

High-performance pyroelectric infrared (IR) detectors have been fabricated using tantalate-doped lithium niobate LiNb_1 − xTa_xO₃ (abbreviated as LNT, with x = 0-1.0) thin films deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by diol-based sol-gel processing, in which, tantalate (Ta) is adopted as dopant in lithium niobate. The randomly oriented LNT thin film exhibits a relatively small dielectric constant and a large pyroelectric coefficient. The pyroelectric characteristics of detectors with various tantalate contents, as a function of modulation frequency, were investigated. It was found that LiNb_0.8Ta_0.2O₃ had the largest voltage responsivity of 7020 (V/W) at 70 Hz, and a specific detectivity (D^?) of 7.76 × 10⁷ cm Hz^1/2/W at 200 Hz. These results indicate that the LNT thin film with x = 0.20 is most suitable for application as high-performance pyroelectric thin-film detectors.     

Bulge testing and fracture properties of plasma-enhanced chemical vapor deposited silicon nitride thin films

The mechanical properties and fracture behavior of silicon nitride (SiN_x) thin film fabricated by plasma-enhanced chemical vapor deposition is reported. Plane-strain moduli, prestresses, and fracture strengths of silicon nitride thin films deposited both on a bare Si substrate and on a thermally oxidized Si substrate were extracted using bulge testing combined with a refined load-deflection model of long rectangular membranes. The plane-strain moduli and prestresses of SiN_x thin films have little dependence on the substrates, that is, for the bare Si substrate, they are 133 ± 19 GPa and 178 ± 22 MPa, respectively, while for the thermally oxidized substrate, they are 140 ± 26 GPa and 194 ± 34 MPa, respectively. However, the fracture strength values of SiN_x films grown on the two substrates are quite different, i.e., 1.53 ± 0.33 GPa and 3.08 ± 0.79 GPa for the bare Si substrate and the oxidized Si substrate, respectively. The reference stresses were computed by integrating the local stress of the membrane at the fracture over the edge, surface, and volume of the specimens and fitted with the Weibull distribution function. For SiN_x thin film produced on the bare Si substrate, the volume integration gave a significantly better agreement between data and model, implying that the volume flaws are the dominant fracture origin. For SiN_x thin film grown on the oxidized Si substrate, the fit quality of surface and edge integration was significantly better than the volume integration, and the dominant surface and edge flaws could be caused by buffered HF attacking the SiN_x layer during SiO₂ removal.     

Epitaxial multi-component rare earth oxide for high-K application

We studied the growth and electrical properties of single crystalline mixed (Nd_1 − xGd_x)₂O₃ (NGO) thin films and compared the results with those of the binary Gd₂O₃ and Nd₂O₃ thin films, respectively. Epitaxial ternary NGO thin films were grown on Si(100) substrates using modified solid state molecular beam epitaxy. The films were characterized physically using various techniques. The capacitance equivalent oxide thickness of a 4.5 nm NGO thin film extracted from capacitance-voltage (C-V) characteristics was 0.9 nm, which is lower than all values reported earlier for other crystalline oxides. The leakage current density and the density of interface traps were 0.3 mA/cm² at |Vg − V_FB| =  1 V and 1.4 × 10¹²/cm², respectively. These excellent electrical properties of NGO thin films demonstrate that such ternary oxides could be one of the promising candidates for gate dielectrics in the upcoming generations of complementary metal oxide semiconductor (CMOS) devices.     

Growth of β-FeSi2 thin films on β-FeSi2 (110) substrates by molecular beam epitaxy

We have investigated the preparation of β-FeSi₂ substrate and growth condition of β-FeSi₂ thin film on β-FeSi₂ (110) substrate by molecular beam epitaxy. The surface of the substrate was prepared by a wet-etching using HF(50%):HNO₃(60%):H₂O = 1:1:5 solution at 25 °C. It is clear that the optimal etching period to obtain a flat surface was 3 min. The β-FeSi₂ thin film with streak RHEED pattern was obtained at Si/Fe flux ratio of 2.9. Average surface roughness (Ra) of the β-FeSi₂ film was about 0.5 nm in 5 × 5 μm² area.     

Comparison of growth mechanisms of silicon thin films prepared by HWCVD with PECVD

Hot-wire chemical vapor deposition (HWCVD) and plasma-enhanced chemical vapor deposition (PECVD) of Si thin films show different growth kinetic processes. According to the fractal analysis, the root-mean-square surface roughness δ and the film thickness d have the relation of δ ∼ d^β, where β is the dynamic scaling exponent related to the film growth mechanism. It was found that β is 0.44 for Si films prepared by HWCVD and 0.24 by PECVD. The former refers to a stochastic deposition while the latter corresponds to the finite diffusion of the radicals. Monte Carlo simulations indicate that the sticking process of growth radicals play an important role in determining the morphology of Si films.     

Structural and magnetic properties of Co-N thin films synthesized by direct current magnetron sputtering

Influence of nitrogen fractions [N_f = N₂/(N₂ + Ar)] and sputtering powers (P_s) on the structural and magnetic properties of Co-N thin films synthesized by direct current magnetron sputtering have been studied. With increasing N_f from 0 to 20%, a series of phases from β-Co, β-Co (N), Co₄N to Co₃N were obtained. However, when N_f was fixed at 10%, only Co₄N phase with different Co contents in the films was prepared, whose values of saturation magnetism (M_s) increased from 12.9 ± 8.2 Am²/kg to 103.9 ± 6.1 Am²/kg with the increase of P_s. Interstitial nitrogen caused the decrease of coercivity from 24.12 kAm^− 1 (for β-Co film) to 2.71 kAm^− 1. However, the addition of interstitial nitrogen was not observed to increase the M_s of β-Co.     

Nd-substituted SrBi2Ta2O9 ferroelectric thin films prepared by radio frequency magnetron sputtering

Nd-substituted SrBi₂Ta₂O₉ (SNBT) thin films are sputtered on Pt/Ta/SiO₂/Si substrates. X-ray diffraction and x-ray photoelectron spectroscopy studies indicate that Nd³⁺ is substituted into the bismuth layered perovskite structure, preferentially at the Sr²⁺ site. The annealed thin film is polycrystalline with plate/needle-like grain microstructure. Secondary ion mass spectrometry results show that elements in SNBT thin film homogeneously distribute along film depth and interfacial diffusion takes place during post annealing. The Nd substitution leads to enhanced remnant polarization (2P_r = 18 μC/cm²) and reduced coercivity (2E_c = 64 kV/cm) at 180 kV/cm measured at 25 °C. After 10¹⁰ switching cycles, around 9% remnant polarization is decreased.     

Effect of TiOx seed layer on the texture and electric properties in La and Ca modified PbTiO3 thin films

Lanthanum-and calcium-modified PbTiO₃ (PLCT) ferroelectric thin films were successfully prepared on Pt(111)/Ti/SiO₂/Si substrates by pulsed laser deposition. Influence of TiO_x seed layer on texture and electric properties of PLCT films was investigated. It is found the PLCT films without seed layer exhibited highly (100)-textured, while using about 9 nm TiO_x as seed layer lead to highly (301)-textured. The PLCT film with TiO_x seed layer possess higher remnant polarization (Pr = 26 µC/cm²), better pyroelectric coefficient and figure of merit at room temperature (p = 370 µC/m²k, F_d = 190 × 10^− 5 Pa^− 1/2) than that of film without seed layer. The mechanism of the enhanced electric properties was also discussed.     

Sputter-epitaxy and electric properties of multiferroic Bim + 1Fem-3Ti3O3m + 3 thin films

Growth conditions suitable for sputter-epitaxy of Bi_m + 1Fe_m-3Ti₃O_3m + 3 (BFTO) thin films with layered structure have been investigated. The amount of oxygen during deposition was found to be specifically essential for obtaining a good-quality thin film of BFTO with a large m. The (001) epitaxial thin films of BFTO with m of nearly 10 which is expected to retain magnetic order up to room temperature have been successfully grown on (001) SrTiO₃ substrates under the determined optimum condition. The film exhibited leakage current as low as order of 10⁻²-10⁻¹ A/m² limited by Schottky emission at the interfaces between the electrodes and the film. In addition, the film showed a ferroelectric polarization curve with Pr = 6 μC/cm² for applied field of 35 MV/m at room temperature though the curve was unsaturated. These indicate that the BFTO (m = 10) thin films are promising as multiferroics at room temperature.     

Surface mound formation during epitaxial growth of CrN(001)

Single crystal CrN(001) layers, 10 to 160 nm thick, were grown on MgO(001) by reactive magnetron sputtering at growth temperatures T_s = 600 and 800 °C. Insitu scanning tunneling microscopy shows that all layer surfaces exhibit mounds with atomically smooth terraces that are separated by monolayer-high step edges aligned along ( 110) directions, indicating N-rich surface islands. For T_s = 600 °C, the root mean square surface roughness σ initially increases sharply from 0.7 ± 0.2 for a thickness t = 10 nm to 2.4 ± 0.5 nm for t = 20 nm, but then remains constant at σ = 2.43 ± 0.13 nm for t = 40, 80 and 160 nm. The mounds exhibit square shapes with edges along ( 110) directions for t ≤ 40 nm, but develop dendritic shapes at t = 80 nm which revert back to squares at t = 160 nm. This is associated with a lateral mound growth that is followed by coarsening, yielding a decrease in the mound density from 5700 to 700 µm⁻² and an initial increase in the lateral coherence length ξ from 7.2 ± 0.6 to 16.3 ± 0.8 to 24 ± 3 nm for t = 10, 20, and 40 nm, respectively, followed by a drop in ξ to 22 ± 2 and 16 ± 2 nm for t = 80 and 160 nm, respectively. Growth at T_s = 800 °C results in opposite trends: σ and ξ decrease by a factor of 2, from 2.0 ± 0.4 and 20 ± 4 nm for t = 10 nm to 0.92 ± 0.07 and 10.3 ± 0.4 nm for t = 20 nm, respectively, while the mound density remains approximately constant at 900 μm⁻². This unexpected trend is associated with mounds that elongate and join along ( 100) directions, yielding long chains of interconnected square mounds for t = 40 nm. However, coalescence during continued growth to t = 160 nm reduces the mound density to 100 µm⁻² and increases σ and ξ to 2.5 ± 0.1 and 40 ± 2 nm, respectively.     

Nanoindentation study of single-crystal NaTh2(PO4)3

The single crystals of sodium dithorium orthophosphate NaTh₂(PO₄)₃ (NThP) were studied by means of micro/nanoindentation. The NThP hardness was found to be Н_N = 8.76 ± 0.18 GPa and the elastic modulus Е_N = 144 ± 1 GPa. Microhardness anisotropy of the NThP crystal unequal faces is insignificant. The non-uniformity of plastic strain observed for the NThP is caused by fracture initiation and growth in the imprint. The average fracture toughness index (K_Ic) for the NThP is estimated to be equal to 0.56 MPa m^0.5.     

Nickel diffusion in polycrystalline CuInSe2 thin films with a <112> fiber texture

Nickel diffusion in CuInSe₂ thin films was studied in the temperature range 430-520 °C. Thin films of copper indium diselenide (CuInSe₂) were prepared by selenization of CuInSe₂-Cu-In multilayered structure on glass substrate. A thin film of Nickel was deposited and annealed at different temperatures. Surface morphologies of the Ni diffused and undiffused CuInSe2 films were investigated using scanning electron microscope. The alteration of Nickel concentration in the CuInSe₂ thin film was measured by Energy Dispersive X-Ray Fluorescence (EDXRF) technique. These measurements were fitted to a complementary error function solution and the diffusion coefficients at four different temperatures were evaluated. The diffusion coefficients of Ni in CuInSe₂ films were estimated from concentration profiles at temperatures 430-520 °C as D = 1.86 × 10^− 7(cm²s^− 1)exp[− 0.68(eV)/kT].     

Piezoelectric properties of PZT films prepared by the sol-gel method and their application in MEMS

Pb(Zr_0.52Ti_0.48)O₃ (PZT) piezoelectric thin films with thickness of 0.7-2.2 μm were prepared on Pt/Ti-coated SiO₂/Si (0.5 μm/300 μm) substrates by the sol-gel method. The piezoelectric coefficient e₃₁ of the PZT thin film was − 12.5 ± 0.3 C/m², which is evaluated by measuring the tip displacement of PZT-coated cantilevers of the dimensions 50 mm long, 4 mm wide, and 0.3 mm thick. The prepared PZT films demonstrated excellent piezoelectric properties and have large potential applications in MEMS devices. Micro-machined ultrasonic sensors, in which PZT-coated membrane functioned as sensing element, were fabricated and their properties were also characterized.     

Experiments and theory on pentacene in the thin film phase: structural, electronic, transport properties, and gas response to oxygen, nitrogen, and ambient air

We investigated the morphological, structural, electronic, and transport properties of pentacene thin films grown by vacuum thermal evaporation on different inert substrates at room temperature. The results of our atomic force microscopy (AFM), X-ray diffraction and scanning tunnelling microscopy (STM) analysis show a structure in the so called “thin film phase” with 1-2 μm sized grains. Atomic terraces are clearly evidenced with AFM and give an inter-planar spacing of 1.54 nm corresponding to the (001) distance. The Scanning Tunneling Spectroscopy measurements show an HOMO-LUMO gap of 2.2 eV. After vacuum thermal evaporation on patterned substrates with different inter-electrodes distances, we have performed in situ measurements of the electrical response of such thin films. We found for these films a resistivity of ρ = 4.7 ± 0.2 · 10⁴ Ω m, that is an order of magnitude lower than the value reported to date in literature for single crystals of pentacene. This value is not affected by the presence of grain boundaries. The resistivity is further reduced by a factor 8.9 ± 0.7, 14 ± 1, 2.3 ± 0.3 upon exposure to oxygen, nitrogen and ambient air, respectively. In addition density functional theory calculations have been performed to investigate the electronic structure of pentacene in this specific phase, focusing on the effects on the relevant electronic properties of the relative orientation of the molecules within the crystalline unit cell, so far experimentally unknown. Our results show that the energy bandwidth and band-gap are crucially affected by the molecular stacking. Furthermore, by comparing our theoretical spectra with the scanning tunneling spectroscopy (STS) measurements, we propose a molecular arrangement that gives a good agreement with experiments as far as the relevant orbitals are concerned. For this polymorph, we find a HOMO and LUMO bandwidth of ≈ 0.7 eV and ≈ 0.8 eV, respectively, which are significantly larger than those obtained for the pentacene bulk-phase and are consistent with the larger conductivity experimentally observed in pentacene thin films.     

CuInSe2 monograin growth in the liquid phase of potassium iodide

The results of monograin CuInSe₂ synthesis from Cu-In alloy and Se in liquid KI are presented. The amounts of CuInSe₂ and KI were nearly equal to fulfil the criterion for the monograin growth (all free volume between the particles has to be filled with liquid). All the grown powder materials with narrow-disperse granularity were chalcopyrite CuInSe₂. The grown crystallites had tetrahedral shapes and homogeneous composition. Particle size distribution was used to describe the growth process. The activation energy of linear growth of crystals was E_d = 0.25 ± 0.05 eV, and the power of time dependence of the crystal growth was l/n = 0.26 ± 0.06. The solubility of CuInSe₂ in KI at 990 K was 0.17 ± 0.05 wt. %. The solubility of potassium and iodine in CuInSe₂ at 990 K was 0.094 wt. %, and 0.0086 wt. %, respectively. As a result, homogeneous p-type CuInSe₂ monograin materials were synthesised in KI solvent.     

Yttria-stabilized zirconia-based composites with adaptive thermal conductivity

Thermal conductivity trends in a “chameleon coating” thin film were characterized with a time-domain thermoreflectance (TDTR) technique. A yttria-stabilized zirconia (YSZ)-based nanocomposite material containing ∼21 vol.% silver (Ag) was employed for this study. The thermal conductivity (k) of the as-deposited composite film was measured with TDTR and found to have a value of 7.4 ± 1.4 W m⁻¹ K⁻¹. The film was then annealed at 500 °C for 1 h to stimulate Ag flow from within the composite to the surface via diffusion. The Ag that coalesced on the surface during annealing was removed to expose the underlying porous YSZ matrix, and the sample was reexamined with the TDTR technique. The thermal conductivity of the porous nanocomposite YSZ material was then measured to be 1.6 ± 0.2 W m⁻¹ K⁻¹, which is significantly lower than a fully dense control sample of pure nanocrystalline YSZ (2.0 ± 0.1 W m⁻¹ K⁻¹). The annealed film displayed a 20% reduction in thermal conductivity as compared to the control sample and a 4–5-fold reduction in thermal conductivity as compared to the as-deposited material. The experiments demonstrate temperature triggering of a composite material, resulting in self-modifying thermal conductivity and diffusion-controlled porosity. These aspects can be used to enhance or restrict thermal transport (i.e., a thermal switch). The applicability of the TDTR technique to measurements of thin, nanoporous film materials is also demonstrated.