Tailoring Materials for Mottronics: Excess Oxygen Doping of a Prototypical Mott Insulator

The Mott transistor is a paradigm for a new class of electronic devices—often referred to by the term Mottronics—which are based on charge correlations between the electrons. Since correlation‐induced insulating phases of most oxide compounds are usually very robust, new methods have to be developed to push such materials right to the boundary to the metallic phase in order to enable the metal–insulator transition to be switched by electric gating. Here, it is demonstrated that thin films of the prototypical Mott insulator LaTiO₃ grown by pulsed laser deposition under oxygen atmosphere are readily tuned by excess oxygen doping across the line of the band‐filling controlled Mott transition in the electronic phase diagram. The detected insulator to metal transition is characterized by a strong change in resistivity of several orders of magnitude. The use of suitable substrates and capping layers to inhibit oxygen diffusion facilitates full control of the oxygen content and renders the films stable against exposure to ambient conditions. These achievements represent a significant advancement in control and tuning of the electronic properties of LaTiO_3+x thin films making it a promising channel material in future Mottronic devices.     

On the compositional dependence of the optical gap and structural properties of amorphous In x Se1−x thin films

The optical gap of the amorphous In _x Se_1–x thin film has been observed to decrease with increasing content of indium in the film (x). An attempt is made to interpret the compositional dependence of the optical gap in amorphous In _x Se_1–x thin film in an alloy-like approach. It has been found that introduction of higher concentrations of indium imparts greater stability to the structure of the indium selenium alloy. From the study of the radial distribution function (RDF) by X-rays it is observed that there is a correlation between the coordination number and the optical gap of indium selenide thin films.     

Gasochromic switching of reactively sputtered molybdenumoxide films: A correlation between film properties and deposition pressure

Molybdenumoxide (MoO_x) thin films can change their optical properties upon exposure to hydrogen. Since the film properties strongly depend on process parameters we have studied how the films are affected by the total pressure during deposition. Stoichiometric and sub-stoichiometric MoO_x films were prepared by reactive direct current magnetron sputtering in an atmosphere of argon and oxygen. Substoichiometric films were coated with platinum as a catalyst and were colored in diluted hydrogen atmosphere and bleached in air. Optical spectroscopy, X-ray reectometry, spectroscopic ellipsometry and simulations of the measured spectra were used to characterize the films ex situ. In situ switching characteristics as revealed by optical spectroscopy and changes in stress were measured as well. We find that the total pressure during sputter deposition has a strong influence on the optical constants, the film density, and the sputter rate. The mechanical stresses and switching Preprint submitted to Elsevier Science 10 March 2006 cycles during the film coloration and bleaching also strongly depend on the total pressure. The influence of the sputter pressure on film properties is explained by the kinetics during the sputter process.     

Optical and mechanical characterization of zirconia-carbon nanocomposite films

The focus of the present work is the study of carbon co-deposition effect on the optical and mechanical properties of zirconia films. Optical and dielectric constant, band gap and transition lifetime of such composite systems were determined, as well as their elasticity properties. The thin ZrO_2−x-C films were sputter-deposited on silicon and polycarbonate, from a pure ZrO₂ and graphite targets in a radio-frequency argon plasma.Besides the zirconia phase and crystalline parameter changes induced by carbon addition, the electronic properties to the films were significantly modified: a drastical optical gap lowering was observed along an increased electronic dielectric constant and refractive index. The invariance of the film elasticity modulus and the similarity of the optical transition lifetime values with those of pure amorphous carbon films indicate an immiscibility of the ceramic and carbon components of the film structure.     

The changes produced in the electrical resistance of vacuum-condensed films of neodymium and praseodymium when they react with hydrogen and with air

When thin films of vacuum-condensed praseodymium and neodymium are exposed to hydrogen or to air the electrical resistance changes owing to the chemical reactions which occur. The progress of the reactions was studied by means of the resistance changes which occur and by means of electron diffraction.At room temperature, neodymium reacted less readily with hydrogen than did praseodymium and the reaction rate for both metals increased with increasing temperatures. The phases formed in the various experiments ranged from the double h.c.p. structure of the metals to the high resistance cubic structure of the hydrides LnH_x with x approaching 3. Cubic and hexagonal Nd₂O₃ formed when the specimens were strongly heated by the beam of the electron microscope.Exposure to air caused the films to oxidize. Thin films approximately 250 Å thick became insulators but the oxide layer formed on thicker films protected the interior of a given film from rapid oxidation so that the film did not become an insulator.     

A.c. photoconductivity and optical properties of bulk polycrystalline and amorphous In x Se1−x thin films

The a.c. photoconductivity of bulk polycrystalline In _x Se_1–x and the optical properties of amorphous In _x Se_1–x thin films were investigated. The effect of heat treatment on the absorption coefficient of InSe thin films was also studied. The spectral response of the photoconductivity of polycrystalline In _x Se_1–x related to the near-edge absorption spectrum, shows a well-defined maximum corresponding to the width of the forbidden gap. Polycrystalline In _x Se_1–x realizes the behaviour of photoconductors which are sensitive in a spectral range between 400 and 1700 nm. Optical absorption of amorphous In _x Se_1–x thin films was recorded as a function of photon energy and the data were used to deduce the value of optical energy gap of the films. It was found that the optical energy gap decreased with increasing indium content in both bulk polycrystalline and amorphous thin films. The effect of heat treatment on the optical gap of the film has been interpreted in terms of the density of states model of Mott and Davis.     

Effect of Zn doping on structure and ferroelectric properties of PST thin films prepared by sol–gel method

(Pb _y Sr_1−y )Zn _x Ti_1−x O_3−x thin films were prepared on ITO/glass substrate by sol–gel process using dip-coating method. The phase structure, morphology and ferroelectric property of the thin film were studied. All the thin films show the typical perovskite phase structure. Both the crystallinity and c/a ratio of the perovskite phase increases initially and then decreases gradually with doping Zn in the thin film. Ferroelectric properties of the Zn-doped PST thin films, including ferroelectric hysteresis-loop, remnant polarization and coercive force, decrease gradually with increasing Zn. And the effect of Zn on ferroelectric properties is more obvious in PST thin film with high content of Pb than that with low Pb although the high lead thin film exhibits high intrinsic ferroelectric properties.     

Oxygen Diode Formed in Nickelate Heterostructures by Chemical Potential Mismatch

Deliberate control of oxygen vacancy formation and migration in perovskite oxide thin films is important for developing novel electronic and iontronic devices. Here, it is found that the concentration of oxygen vacancies (V_O) formed in LaNiO₃ (LNO) during pulsed laser deposition is strongly affected by the chemical potential mismatch between the LNO film and its proximal layers. Increasing the V_O concentration in LNO significantly modifies the degree of orbital polarization and drives the metal–insulator transition. Changes in the nickel oxidization state and carrier concentration in the films are confirmed by soft X‐ray absorption spectroscopy and optical spectroscopy. The ability to unidirectional‐control the oxygen flow across the heterointerface, e.g., a so‐called “oxygen diode”, by exploiting chemical potential mismatch at interfaces provides a new avenue to tune the physical and electrochemical properties of complex oxides.     

Effect of deposition ambient on structural and optical properties of Mg<Subscript>x</Subscript>Zn<Subscript>1−x</Subscript>O alloy thin films grown by RF sputtering

Mg_xZn_1−xO thin films were deposited on Corning eagle 2,000 glass substrates by a RF magnetron sputtering using a ceramic target. The effect of Ar/O₂ ratios on structural and optical properties was investigated. The XRD results showed that the film demonstrated the best structural properties when the Ar/O₂ ratio equal 7:3. Sputtering ambient seemed to have minor effect on the optical properties of Mg_xZn_1−xO thin films.     

Topotactic Metal–Insulator Transition in Epitaxial SrFeOx Thin Films

Topotactic phase transformation enables structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for elucidating the redox reaction and oxygen diffusion within transition metal oxides. In addition, it enables tuning of the emergent physical properties of complex oxides, through strong interaction between the lattice and electronic degrees of freedom. In this communication, the electronic structure evolution of SrFeO_x epitaxial thin films is identified in real‐time, during the progress of reversible topotactic phase transformation. Using real‐time optical spectroscopy, the phase transition between the two structurally distinct phases (i.e., brownmillerite and perovskite) is quantitatively monitored, and a pressure–temperature phase diagram of the topotactic transformation is constructed for the first time. The transformation at relatively low temperatures is attributed to a markedly small difference in Gibbs free energy compared to the known similar class of materials to date. This study highlights the phase stability and reversibility of SrFeO_x thin films, which is highly relevant for energy and environmental applications exploiting the redox reactions.     

Deposition of magnesium hydride thin films using radio frequency reactive sputtering

Reactive sputter deposition of MgH_x thin films was performed using mixed hydrogen-argon plasma. This technique allows in-situ deposition of metal hydride films in contrast to the commonly applied ex-situ hydrogenation of metallic films. Partly transparent films were obtained and the formation of crystalline MgH₂ could be observed for thicknesses above about 200 nm. The formation of some metallic Mg in the films could not be avoided. Increased hydrogen loading by increased pressure, H₂:Ar ratio or reduced power produced films of porous structure that easily oxidise. More densely packed films remain stable for several months of air exposure. Post-deposition treatments in H-plasma showed evidence of hydrogenation of deposited films without the use of a catalysing capping film. Film properties are studied by X-ray diffraction, scanning electron microscopy and by optical and resistivity measurements.     

Effect of Al doping and deposition runs on structural and optical properties of In<Subscript>2</Subscript>S<Subscript>3</Subscript> thin films grown by CBD

β-In_2−x Al _x S₃ thin films have been grown on glass substrate by chemical bath deposition for different value of Al concentration y = (([Al])/([In]))_sol (0 ≤ y ≤ 5 at.%). Samples have been characterized using X-ray diffraction, atomic force microscopy and by spectrophotometric measurements. The influence of the increase of y ratio in the structural and optical properties are described and discussed in terms of crystallinity improvement. In order to increase film thickness of β-In_2−x Al _x S₃, we have been realized multi-deposition system. The structural, the surface morphology as well as the optical properties seem to be improved as the film thickness is of about 1200 nm.     

Absorbing TiOx thin film enabling laser welding of polyurethane membranes and polyamide fibers

Abstract

We report on the optical properties of thin titanium suboxide (TiO_x) films for applications in laser transmission welding of polymers. Non-absorbing fibers were coated with TiO_x coatings by reactive magnetron sputtering. Plasma process parameters influencing the chemical composition and morphology of the deposited thin films were investigated in order to optimize their absorption properties. Optical absorption spectroscopy showed that the oxygen content of the TiO_x coatings is the main parameter influencing the optical absorbance. Overtreatment (high power plasma input) of the fiber surface leads to high surface roughness and loss of mechanical stability of the fiber. The study shows that thin substoichiometric TiO_x films enable the welding of very thin polyurethane membranes and polyamide fibers with improved adhesion properties.     

Studies on electrosynthesized zinc mercury selenide alloys

Zn_1-x Hg _x Se ternary alloy thin films with x ranging from 0.14 to 0.5 have been deposited on conducting glass substrates by electrodeposition from aqueous bath containing ZnSO₄, HgCl₂ and SeO₂ at bath temperatures from 30 °C to 80 °C. The influence of deposition parameters on the crystallinity, composition, band gap and lattice constants of the film is studied. The films deposited at the potentials between −0.4 V and −1.0 V vs SCE at 70 ° C were characterized by X-ray diffraction (XRD), Energy dispersive X-ray (EDX), scanning electron microscope and optical absorption technique. Photoelectrochemical (PEC) solar cells based on Hg _x Zn_1-x Se thin film electrodes were constructed and the effects of annealing and photo etching on solar cell parameters were studied.     

Two‐Dimensional Mott Insulators in SrVO3 Ultrathin Films

Strongly correlated oxides that undergo a metal‐insulator transition (MIT) are a subject of great current interest for their potential application to future electronics as switches and sensors. Recent advances in thin film technology have opened up new avenues to tailor MIT for novel devices beyond conventional CMOS scaling. Here, dimensional‐crossover‐driven MITs are demonstrated in high‐quality epitaxial SrVO₃ (SVO) thin films grown by a pulsed electron‐beam deposition technique. Thick SVO films (∼25 nm) exhibit metallic behavior with the electrical resistivity following the T² law corresponding to a Fermi liquid system. A temperature driven MIT is induced in SVO ultrathin films with thicknesses below 6.5 nm. The transition temperature T_MIT is at 50 K for the 6.5 nm film, 120 K for the 5.7 nm film and 205 K for the 3 nm film. The emergence of the observed MIT can be attributed to the dimensional crossover from a three‐dimensional metal to a two‐dimensional Mott insulator, as the resulting reduction in the effective bandwidth W opens a band gap at the Fermi level. The magneto‐transport study of the SVO ultrathin films also confirm the observed MIT is due to the electron‐electron interactions other than disorder‐induced localization.     

Properties of Heavily Mn-doped GaMnAs with Curie Temperature of 172.5 K

The authors have investigated the magnetic properties of heavily Mn-doped ferromagnetic semiconductor Ga_1−x Mn _x As thin film with the Mn concentration x of 15.2% grown by molecular-beam epitaxy at relatively high growth temperature of 250 °C. Magnetic circular dichroism and the anomalous Hall effect measurements indicate that this thin film holds the intrinsic ferromagnetic semiconductor features. By low-temperature annealing, the resistivity was significantly decreased and the Curie temperature was largely enhanced from 95 K to 172.5 K.     

Selective Growth of Metal‐Free Metallic and Semiconducting Single‐Wall Carbon Nanotubes

A major obstacle for the use of single‐wall carbon nanotubes (SWCNTs) in electronic devices is their mixture of different types of electrical conductivity that strongly depends on their helical structure. The existence of metal impurities as a residue of a metallic growth catalyst may also lower the performance of SWCNT‐based devices. Here, it is shown that by using silicon oxide (SiO_x) nanoparticles as a catalyst, metal‐free semiconducting and metallic SWCNTs can be selectively synthesized by the chemical vapor deposition of ethanol. It is found that control over the nanoparticle size and the content of oxygen in the SiO_x catalyst plays a key role in the selective growth of SWCNTs. Furthermore, by using the as‐grown semiconducting and metallic SWCNTs as the channel material and source/drain electrodes, respectively, all‐SWCNT thin‐film transistors are fabricated to demonstrate the remarkable potential of these SWCNTs for electronic devices.     

Compositional and metal-insulator transition characteristics of sputtered vanadium oxide thin films on yttria-stabilized zirconia

Vanadium dioxide (VO₂) thin films have been shown to undergo a rapid electronic phase transition near 70 °C from a semiconductor to a metal, making it an interesting candidate for exploring potential application in high speed electronic devices such as optical switches, tunable capacitors, and field effect transistors. A critical aspect of lithographic fabrication in devices utilizing electric field effects in VO₂ is the ability to grow VO₂ over thin dielectric films. In this article, we study the properties of VO₂ grown on thin films of Yttria-Stabilized Zirconia (YSZ). Near room temperature, YSZ is a good insulator with a high dielectric constant ($\epsilon _{\rm r} > 25$\epsilon _{\rm r} > 25). We demonstrate the sputter growth of polycrystalline VO₂ on YSZ thin films, showing a three order resistivity transition near 70 °C with transition and hysteresis widths of approximately 7 °C each. We examine the relationship between chemical composition and transition characteristics of mixed phase vanadium oxide films. We investigate changes in composition induced by low temperature post-deposition annealing in oxidizing and reducing atmospheres, and report their effects on electronic properties.     

Polaron Absorption and the Insulator to Metal Transition in Nd2?xCexCuO4?y Cuprates

The optical conductivity of Nd_2–x Ce_xCuO_4–y single crystals with x ranging from 0 to 0.122 has been measured at differenttemperatures. The polaron band observed at 1100 cm–1 in Nd₂CuO_4–y shifts to lower frequencies when the Ce concentration xapproaches 0.125, namely, the critical value for the insulator to metal transition. Thisunexpected behavior indicates a progressive delocalization of polarons in theCu–O plane for increasing x and may explain the anomalous Drudebehavior reported for the carriers in the metallic phase of superconducting cuprates.     

Cd(S<Subscript>(1 − <Emphasis Type="Italic">x</Emphasis>)</Subscript> + CO<Subscript>3(<Emphasis Type="Italic">x</Emphasis>)</Subscript>) thin films by chemical synthesis

A microcrystalline mixture of cadmium carbonate (CdCO₃) and cadmium sulfide (CdS) were grown in the thin film format onto glass substrates by means of chemical bath. The temperature of the bath (T_d) was selected in the interval 23–80^∘C. At low temperatures, CdCO₃ is the compound predominant in the layers. At high temperatures CdS is the compound deposited on the substrate. At intermediate T_d-values a mixture of both materials are present, i.e., the gradual transition from an insulator (CdCO₃) to a semiconductor (CdS) growth occurs when T_d increases. Physical properties of films were studied by means of X-ray diffraction and optical absorption. The forbidden energy band gap of direct electronic transitions (E_g) was calculated by applying the α² ∝ (hν − E_g) relation to the optical absorption spectra.