Superlattice Thin Films Research Articles

Effects of MeV Si ions bombardment on the thermoelectric generator from SiO 2/SiO 2 + Cu and SiO 2/SiO 2 + Au nanolayered multilayer films

The defects and disorder in the thin films caused by MeV ions bombardment and the grain boundaries of these nanoscale clusters increase phonon scattering and increase the chance of an inelastic interaction and phonon annihilation. We prepared the thermoelectric generator devices from 100 alternating layers of SiO 2/SiO 2 + Cu multi-nano layered superlattice films at the total thickness of 382 nm and 50 alternating layers of SiO 2/SiO 2 + Au multi-nano layered superlattice films at the total thickness of 147 nm using the physical vapor deposition (PVD). Rutherford Backscattering Spectrometry (RBS) and RUMP simulation have been used to determine the stoichiometry of the elements of SiO 2, Cu and Au in the multilayer films and the thickness of the grown multi-layer films. The 5 MeV Si ions bombardments have been performed using the AAMU-Center for Irradiation of Materials (CIM) Pelletron ion beam accelerator to make quantum (nano) dots and/or quantum (quantum) clusters in the multilayered superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity. To characterize the thermoelectric generator devices before and after Si ion bombardments we have measured Seebeck coefficient, cross-plane electrical conductivity, and thermal conductivity in the cross-plane geometry for different fluences.

Tunneling Magnetoresistance with Sign Inversion in Junctions Based on Iron Oxide Nanocrystal Superlattices

Magnetic tunnel junctions sandwiching a superlattice thin film of iron oxide nanocrystals (NCs) have been investigated. The transport was found to be controlled by Coulomb blockade and single-electron tunneling, already at room temperature. A good correlation was identified to hold between the tunnel magnetoresistance (TMR), the expected magnetic properties of the NC arrays, the charging energies evaluated from current-voltage curves, and the temperature dependence of the junction resistance. Notably, for the first time, a switching from negative to positive TMR was observed across the Verwey transition, with a strong enhancement of TMR at low temperatures.

Fabrication of LiNbO3/LiTaO3 Superlattice Thin Films and Their Second Harmonic Generation Characteristics

LiNbO3(LN)/LiTaO3(LT) superlattice thin films are fabricated by the pulsed laser deposition method with several LT/(LN+LT) ratios including LN and LT mono-layered films. These films are found to be epitaxial and have clear superlattice structure by x-ray diffraction technique with both in-plane and out-of-plane configurations. However, SHG microscopic observations of these films reveal that the point group symmetry is monoclinic m, different from 3 m of LN and LT bulk crystals.

Syntheses of LiTaO3/LiNbO3 Super-Lattice Thin Films and Their Optical Properties

LiTaO3(LT)/LiNbO3(LN) super-lattice thin films with different LT/LN ratios are fabricated on (0001) sapphire (α-Al2O3) substrates using the pulsed laser deposition method. Well-defined X-ray super-lattice reflections are observed, and the SL period are calculated from these super-lattice positions, which coincide well with design ones estimated from the deposition velocities of LN and LT. X-ray θ–2θ and in-plane φ-scans confirm the epitaxial growth of the films. Photon energy dependence of complex refractive indices n* of these super-lattice thin films are measured by a spectroscopic ellipsometer. The blue shift of n* with increase of LT ratio is observed.

The Optical Properties of a-Si:H/a-SiGex:H Superlattice Structure to Apply Intrinsic Layer in Solar Cell

The hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon-germanium (a-SiGe:H) alloys have been prepared by the plasma enhanced chemical vapor deposition (PECVD) method. The films were grown on corning #1737 glass from the gas mixture of silane and germane. Superlattice thin films consisted of a-Si:H/a-SiGe:H multilayer structure. The UV-VIS Spectrometer analysis of a-SiGex:H alloys and superlattices thin films showed that the bandgap absorption spectra of a-SiGex:H alloys shift to lower photon energy with an increase in the GeH4 flow rate [0–300 sccm] and various a-Si:H [2−10 nm]/a-SiGex:H [2 nm] thickness. The optical bandgap had decreased by between 1.24∼1.71 eV in regards to a-SiGe:H and superlattice thin films.

Piezoelectric thin-film superlattices without using piezoelectric materials

In this paper we show that experimentally realizable apparently piezoelectric thin-film superlattices can be created from nonpiezoelectric materials provided an odd-order (e.g., trilayer) stacking sequence is used. The size-dependent mechanism of flexoelectricity, which couples gradients of strain to polarization, allows such a possibility. We present closed-form analytical expressions for the response of various thin-film and superlattice configurations. We also clarify some of the subtleties that arise in considering interface boundary conditions in the theory of flexoelectricity as well as the relationship of flexoelectricity to the frequently used polarization gradient terms used in modeling ferroelectrics. We find that for certain (optimum) material combinations and length scales, thin-film superlattices yielding apparent piezoelectricity close to 75% of ferroelectric barium titanate may be achievable.

In-plane thermal and thermoelectric properties of misfit-layered [(PbSe)0.99]x(WSe2)x superlattice thin films

The in-plane thermal conductivity is measured to be three times lower in misfit-layered [(PbSe)0.99]x(WSe2)x superlattice thin films than disordered-layered WSe2 because of interface scattering despite a higher cross-plane value in the former than the latter. While having little effect on the in-plane thermal conductivity, annealing the p-type [(PbSe)0.99]2(WSe2)2 films in Se increases the in-plane Seebeck coefficient and electrical conductivity because of decreased defect and hole concentrations. Increasing interface density of the annealed films by decreasing x from 4 to 2 has weak influence on the in-plane thermal conductivity but increases the Seebeck coefficient and decreases the room-temperature electrical conductivity.

Electronic states of Ag thin films with a laterally periodic insertion of stacking faults

Electronic states of Ag thin films containing stacking faults periodically in a direction parallel to the surface are studied theoretically. Such Ag stacking-fault superlattice thin films were recently fabricated on the $\text{Si}(111)4\ifmmode\times\else\texttimes\fi{}1\text{-In}$ surface, and the formation mechanism of the one-dimensional electronic states observed in photoemission experiments has not been clarified. These systems are suitable for studying stacking faults on nanometer scales in well-defined conditions. Main results obtained in the present study are threefold. One is that the double stacking-fault model proposed in this paper reproduces better the observed dispersions than the originally proposed model. In addition the fact that the stacking-fault thin films have the (111) surface is the origin of the strong deformation near the bottom of the quantized subbands. Second is that the tilt of the stacking-fault plane with respect to the normal of the surface causes mixing of states in different subbands, which is an important factor to form the flat band structures observed. Third is the discussion on the mechanism of the anomalous perfect transmission through the bulk stacking faults of noble metals. In contrast to usual potential scattering, stacking faults change only the phase of orbitals. The condition for perfect transmission is that the effects of scattering are canceled out by renormalization using single phase factor, which is excellently satisfied in the case of noble metals. In addition the symmetry in atomic positions between regular and stacking-fault structures with respect to an axis is the origin of the directional dependence of transmission property.

A Study of Phonon Transport in Si/Ge Superlattice Thin Films Using a Fast MC Solver

We employ a fast Monte Carlo solver, which takes advantage of the geometric symmetry existent in the system to reduce the computational effort, to investigate the phonon transport phenomena in the cross-plane and in-plane directions of Si/Ge superlattice thin films. The simulation results show that the heterogeneous interfaces perpendicular to the heat flow direction create a much stronger thermal resistance than the parallel ones. The cross-plane and in-plane thermal conductivities of a Si/Ge superlattice thin film increase monotonically with the smoothness of the interfaces and the superlattice period. Most of all, a minimum of the in-plane thermal conductivity is observed as the ratio of the layer thicknesses is varied for fixed period. This is due to the effective layer thermal conductivities being determined not only by the phonon intrinsic properties but also by the strong interface scattering. A detailed examination shows that the minimum appears when the effective layer thermal conductivities of the Si and Ge layers are about the same.

A Facile Route to ZnO Nanoparticle Superlattices: Synthesis, Functionalization, and Self-Assembly

Highly ordered, free-standing, complex, two- or three-dimensional (2D or 3D) nanoparticle superlattice arrays were created by either evaporation-induced self-assembly or precipitation-induced self-assembly from the size-monodispersed ZnO nanoparticles functionalized by carboxylic and alkylthiol ligands. The synthesis of the constituent ZnO nanoparticles is both novel and facile and is suggested as a generic means of producing oxide nanomaterials by reaction of metal cations and oxide anions in alcohol solutions at room temperature. The regularity of the nanoparticle products and their chemical functionalization provides the means to achieve superlattice thin films on indium tin oxide (ITO)-coated glass slides. Low-angle XRD, SEM, and TEM confirm the formation of these highly organized, 2D or 3D self-assembled superlattices that adopt an unusual primitive cubic structure. The properties of the nanoparticles and the superlattices are also described in the paper.

Effects of MeV Si Ions Modification on the Thermoelectric Properties of SiO2/SiO2+Cu Multilayer Thin Films

AbstractThe performance of the thermoelectric materials and devices is shown by a dimensionless figure of merit, ZT = S2σT/K, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature and K is the thermal conductivity. ZT can be increased by increasing S, increasing σ, or decreasing K. We have prepared 100 alternating multi-nano layer of SiO2/SiO2+Cu superlattice films using the ion beam assisted deposition (IBAD). The 5 MeV Si ions bombardments have been performed at the different fluences using the AAMU Pelletron ion beam accelerator to make quantum clusters in the multi-layer superlattice thin films to decrease the cross plane thermal conductivity increase the cross plane Seebeck coefficient and cross plane electrical conductivity. To characterize the thermoelectric thin films before and after Si ion bombardments we have measured the cross-plane Seebeck coefficient, the cross-plane electrical conductivity, and the cross-plane thermal conductivity for different fluences.

Interface-mediated ultrafast carrier conduction in oxide thin films and superlattices for energy

Some recent observations on carrier (electronic and ionic) transport in thin film and nanostructured metal oxides and superlattices, where interfaces play a dominant role in overall conductivity response, are reviewed. Conduction in nominally pure oxides is compared to heavily aliovalently doped oxides (that correspond to changes in the Debye length over few orders of magnitude). The role of dopant segregation and their spatial variation in affecting transport is pointed out. The ability to utilize interfaces in enabling ultrafast-ion conduction as well as the creation of novel interfacial materials and implications in energy technologies as well as some scientific questions that may warrant further attention are highlighted.

On-chip cooling by superlattice-based thin-film thermoelectrics

There is a significant need for site-specific and on-demand cooling in electronic, optoelectronic and bioanalytical devices, where cooling is currently achieved by the use of bulky and/or over-designed system-level solutions. Thermoelectric devices can address these limitations while also enabling energy-efficient solutions, and significant progress has been made in the development of nanostructured thermoelectric materials with enhanced figures-of-merit. However, fully functional practical thermoelectric coolers have not been made from these nanomaterials due to the enormous difficulties in integrating nanoscale materials into microscale devices and packaged macroscale systems. Here, we show the integration of thermoelectric coolers fabricated from nanostructured Bi2Te3-based thin-film superlattices into state-of-the-art electronic packages. We report cooling of as much as 15 degrees C at the targeted region on a silicon chip with a high ( approximately 1,300 W cm-2) heat flux. This is the first demonstration of viable chip-scale refrigeration technology and has the potential to enable a wide range of currently thermally limited applications.

Properties of MTiO3(M = Sr, Ba) and PbM'O3(M'= Ti, Zr) Superlattice Thin Films Fabricated by Laser Ablation

/Si substrate by the pulsed laser deposition process. The morphologies and physical properties of deposited films were characterized by using X-ray diffractometer, HR-SEM, and Impedance Analyzer. XRD data and SEM images of the films indicate that each layer was well deposited alternatively in the superlattice structure. The dielectric constant of BaTiO

Preparation and laser induced thermoelectric voltage effect of ［（SrTiO3）n/（SrTi0.8Nb0.2O3）m］20/LaAlO3 superlattices

The （SrTiO3）n/（SrTi0.8Nb0.2O3）m superlattices on vicinal-cut LaAlO3 （100） single crystal substrate with tilting angle 10° have been prepared using pulsed laser deposition. The superlattices were characterized by X-ray diffraction, in which regularly distributed satellite peak structure was observed. According to the position of satellite peaks, the superperiod of superlattices was calculated and the deposition rates of SrTiO3 and SrTi0.8Nb0．2Ｏ3 were deduced as 0.78 /pulse for SrTiO3 and 0.57 /pulse for SrTi0．8Nb0．2Ｏ3. In addition, laser induced thermoelectric voltage （LITV） effect was measured for the first time in these superlattice thin films. This phenomenon indieales that this kind of artificial atomic layer thermopile structure possesses Seebeck anisotropy. It was found that when the thickness of dielectric SrTiO3 layer n=46.8 nm, for conductive SrTi0．8Nb0．2Ｏ3 layer with m=19.0 nm and m=11.4 nm, the average peak voltage of LITV signals reaches the maximum U—P=0.7 V/mJ·mm, and the minimum average response time is τ—=124 ns, respectively.

Rapid Thermal Annealing of Sputtered Silicon-Rich Oxide/SiO[sub 2] Superlattice Structure

Silicon-rich oxide/silicon dioxide (SiO x /SiO 2 ), x = 1.2, superlattice thin films, deposited by the radio-frequency magnetron-sputtering technique, was rapid thermal annealed at temperatures ranging from 500 to 1000°C. The photoluminescence (PL) spectra of samples after rapid thermal annealing (RTA) at 1000°C consists of oxygen-deficiency-related PL bands around ∼400 and ∼515 nm, and silicon (Si) nanocrystal-related PL bands around ∼768 nm. Absorption bands at 364 nm (3.41 eV) and 302 nm (4.1 eV) are considered to be related to Si nanocrystals. On the basis of PL and PL excitation spectra, an energy-band diagram of a RTA-treated SiO x /SiO 2 superlattice sample was constructed.

STEM による La ドープ SrTiO<sub>3</sub> 人工超格子薄膜の原子構造解析

STEM による La ドープ SrTiO<sub>3</sub> 人工超格子薄膜の原子構造解析

Thermoelectric Generators of Sequentially Deposited Si/Si+Ge Nano-layered Superlattices

AbstractEffective thermoelectric materials have a low thermal conductivity and a high electrical conductivity. The performance of the thermoelectric materials and devices is shown by a dimensionless figure of merit, ZT = S2sσ/ KTC, σ is the electrical conductivity T/KTC, where S is the Seebeck coefficient, T is the absolute temperature and KTC is the thermal conductivity. In this study we have prepared the thermoelectric generator device of Si/Si+Ge multi-layer superlattice films using the ion beam assisted deposition (IBAD). To determine the stoichiometry of the elements of Si and Ge in the grown multilayer films and the thickness of the grown multi-layer films Rutherford Backscattering Spectrometry (RBS) and RUMP simulation software package were used. The 5 MeV Si ion bombardments were performed to make quantum clusters in the multi-layer superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity.Keywords: Ion bombardment, thermoelectric properties, multi-nanolayers, Figure of merit.

Fabrication and Characterization of Thermoelectric Generators From SiO2/SiO2+Au Nano-layered Superlattices

AbstractThe efficiency of the thermoelectric devices is limited by the properties of n- and p-type semiconductors. Effective thermoelectric materials have a low thermal conductivity and a high electrical conductivity. The performance of the thermoelectric materials and devices is shown by a dimensionless figure of merit, ZT = S2σT/K, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature and K is the thermal conductivity. In this study we prepared the thermoelectric generator device of SiO2/SiO2+Au multi-layer super-lattice films using the ion beam assisted deposition (IBAD). In order to determine the stoichiometry of the elements of SiO2 and Au in the grown multilayer films and the thickness of the grown multi-layer films Rutherford Backscattering Spectrometry (RBS) and RUMP simulation software package was used. The 5 MeV Si ion bombardments was performed to make quantum clusters in the multi-layer super-lattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity. To characterize the thermoelectric generator devices before and after Si ion bombardments we measured the cross-plane Seebeck coefficient, the cross-plane electrical conductivity, and the cross-plane thermal conductivity for different fluences.

Tribological and mechanical behaviors of CrN/a-CN x superlattice thin films

CrN/a-CN x superlattice thin films, which consisted of a superhard coating combined with a lamination of a hard transition metal nitride and lubrication material, were synthesized by Closed Field Unbalanced Magnetron Sputtering (CFUBMS). The bilayer period ( λ) was controlled in the range from 3.1 to 37.7 nm via the rotational speed of the substrate holder. The preferred orientation of the CrN/a-CN x superlattice thin films was CrN (200). Nano-indentation testing showed that the hardness of the coatings varied from 20 to 32 GPa as the bilayer period ( λ) was varied from 3.1 to 37.7 nm with the highest hardness being obtained with a bilayer period of about 6.3 nm. In this case, the lowest friction coefficient was also obtained, due to high plastic deformation resistance.