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.     

Properties of screen-printed Ho-Ba-Cu-O films on YSZ substrates

High-T_c screen-printed Ho-Ba-Cu-O films were prepared on YSZ substrates by a melt processing method. The films were fired at T_s = 1000-1050 °C for 5 min and cooled to 450 °C by two steps in flowing O₂. The maximum critical current density J_c (77 K, 0 T) of 2.0 × 10³ A cm^− 2 was only attained under much limited firing conditions; T_s = 1020 °C and cooled to 800 °C at a cooling rate of 400 °C h^− 1.     

Study of SrTiO3 thin films grown by sputtering technique for tunnel barriers in quasiparticle injection contacts

High-quality, c-axis oriented YBa₂Cu₃O_7 − x/SrTiO₃/Au (YBCO/STO/Au) planar structures were fabricated in situ by direct current/radiofrequency inverted-cylinder magnetron sputtering on (001) STO oriented substrates. The sandwich-type structures were patterned to transistor dimensions by standard ultraviolet-photolithography and Ar etching. The current transport mechanism in the very thin STO barriers (2-30 nm) was examined by measuring the tunneling G as function of temperature (T), and bias voltage (V). It was found that resonant tunneling and hopping via a small number of localized states (LS) are responsible for electronic conduction in the insulating material. Elastic tunneling was observed for the case of a nominal 2 nm thick STO-barrier with an energy gap Δ ≈ 20 meV in the (001) direction of YBCO. On the other hand, inelastic hopping transport via n-LS dominated for STO barrier thickness d > 2 nm. G of the lowest-order hopping channel (hopping via two LS) exhibits the characteristic T and V dependences: G₂^hop(T) ∝ T^4/3, G₂^hop(V) ∝ V^4/3, respectively. Increasing the thickness of the STO barriers, hopping channels of higher order contribute more and more to the current transport as proven by measuring the T and V dependences. A crossover to variable range hopping behavior has been observed for junctions with thicker barriers (d ≥ 20 nm) in the high-V or high-T regime. By fitting the experimental data to theoretical models, physical parameters of the LS could be determined. For instance, the value of the localization length or radius of the localized state was determined to be ~ 4.6 × 10^− 8 cm which corresponds to the lattice constant of the STO unit cell. A value of ~ 6 × 10¹⁹ (eV)^− 1 cm^− 3 was calculated for the density of LS and the average barrier height was estimated as ~ 0.4 eV.     

Annealing effect on dielectric and leakage current characteristics of Mn-doped Ba0.6Sr0.4TiO3 thin films as gate insulators for low voltage ZnO thin film transistor

We report on the dielectric properties and leakage current characteristics of 3 mol% Mn-doped Ba_0.6Sr_0.4TiO₃ (BST) thin films post-annealed up to 600 °C following room temperature deposition. The suitability of 3 mol% Mn-doped BST films as gate insulators for low voltage ZnO thin film transistors (TFTs) is investigated. The dielectric constant of 3 mol% Mn-doped BST films increased from 24 at in-situ deposition up to 260 at an annealing temperature of 600 °C due to increased crystallinity and the formation of perovskite phase. The measured leakage current density of 3 mol% Mn-doped BST films remained on the order of 5 × 10^− 9 to 10^− 8 A/cm² without further reduction as the annealing temperature increased, thereby demonstrating significant improvement in the leakage current characteristics of in-situ grown Mn-doped BST films as compared to that (5 × 10^− 4 A/cm² at 5 V) of pure BST films. All room temperature processed ZnO-TFTs using a 3 mol% Mn-doped BST gate insulator exhibited a field effect mobility of 1.0 cm²/Vs and low voltage device performance of less than 7 V.     

Properties of transparent conducting oxides formed from CdO alloyed with In2O3

The structure, optical and electrical properties of transparent conducting oxide films depend greatly on the methods of preparation, heat treatment, type and level of dopant. Thin films of (CdO)_1−x(In₂O₃)_x have been grown by electron beam evaporation technique for different concentrations of In₂O₃ (x = 0, 0.05, 0.1, 0.15 and 0.2). Increase of doping led to increased carrier concentration as derived from optical data and hence to increased electrical conductivity, which degraded the transparency of the films. An improvement of the electrical and optical properties of Cadmium indium oxide (CdIn₂O₄) has been achieved by post-deposition annealing. A resistivity value of 7 × 10^− 5 Ω cm and transmittance of 92% in the near infrared region and 82% in the visible region have been obtained after annealing at 300 °C for 90 min in air.     

Stability and effect of annealing on the optical properties of plasma-deposited Ta2O5 and Nb2O5 films

Tantalum and niobium oxide optical thin films were prepared at room temperature by plasma-enhanced chemical vapor deposition using tantalum and niobium pentaethoxide (M(OC₂H₅)₅) precursors. We studied the evolution of their optical and microstructural properties as a result of annealing over a broad temperature range from room temperature up to 900 °C. The as-deposited films were amorphous; their refractive index, n, and extinction coefficient, k, at 550 nm were n = 2.13 and k < 10^− 4 for Ta₂O₅, and n = 2.24 and k < 10^− 4 for Nb₂O₅. The films contained a small amount of residual carbon (∼ 2-6 at.%) bonded mostly to oxygen. During annealing, the onset of crystallization was observed at approximately T_C1 = 650 °C for Ta₂O₅ and at T_C1 = 450 °C for Nb₂O₅. Upon annealing close to T₁ (300 °C for Nb₂O₅ and 400 °C for Ta₂O₅), n at 550 nm decreased by less than 1%. This was correlated with the decrease of carbon content, as suggested by Fourier transform infrared spectroscopy, elastic recoil detection and static secondary ion mass spectroscopy (SIMS) results. During annealing, we observed phase transition from the δ- (hexagonal) phase to the L- (orthorhombic) phase between 800 °C and 900 °C for Ta₂O₅, and between 600 °C and 700 °C for Nb₂O₅. The structural changes were also marked by silicon diffusion from the substrate into the oxide layer at annealing temperatures above 500 °C for Ta₂O₅ and above 400 °C for Nb₂O₅. As a consequence of oxygen, silicon and metal interdiffusion, the interface between the Si substrate and the metal oxide (Ta₂O₅ or Nb₂O₅) is characterized by its broadening, well documented by spectroscopic ellipsometry and SIMS data.     

Dielectric properties and substitution mechanism of samarium-doped Ba0.68Sr0.32TiO3 ceramics

Ba_0.68Sr_0.32TiO₃ ceramics of perovskite structure are prepared by solid state reaction method with addition of x mol% Sm₂O₃, and their dielectric properties are investigated. It is found that, integrating with the lattice parameters and tolerance factor t, there is an alternation of substitution preference of Sm³⁺ for the host cations in perovskite lattice. Owing to the replacement of Sm³⁺ ions for Ba²⁺ ions in the A site, T_c rises with the increase of Sm₂O₃ doping when the doping content is below 0.1 mol%; meanwhile, when the content is more than 0.1 mol%, Sm³⁺ ions tend to occupy the B-site, causing a drop of T_c. Owing to the modifications of Sm³⁺ doping, dielectric constant, dissipation factor and temperature stability of dissipation factor are influenced remarkably, making it a superior candidate for environment-friendly applications. Moreover, the creation of oxygen vacancies controls the dielectric constant when the addition is above 0.1 mol%, so the dielectric constant decreases with increasing of samarium.     

Synthesis of V-doped TiO2 films by chemical bath deposition and the effect of post-annealing on their properties

Amorphous composite films, composed of a Ti_1 − xV_xO₂ solid-solution phase and a V₂O₅ phase, were produced by chemical bath deposition and subsequently air-annealed at various temperatures up to 550 °C. The microstructure and chemical composition of the as-prepared and annealed films were investigated by a combinatorial experimental approach using Scanning electron microscopy, X-ray powder diffraction and X-ray photoelectron spectroscopy. Ultraviolet-Visible Spectrometry was applied to determine the optical band gap of the as-prepared and annealed films. It followed that the incorporation of vanadium in the as-deposited films reduces the optical band gap of TiO₂ from about 3.8 eV to 3.2 eV. Annealing of the films up to 350 °C leads to slight increase of band gap, as attributed to a reduction of the defect density in the initially amorphous oxide films due to the gradual development of long-range order and a concurrent reduction of the V⁴⁺-dopant concentration in the Ti_1 − xV_xO₂ solid-solution phase. The films crystallized upon annealing in air at 550 °C, which resulted in drastic changes of the phase constitution, optical absorbance and surface morphology. Due to the lower solubility of V⁴⁺ in crystalline TiO₂, V⁴⁺ segregates out of the crystallizing Ti_1 − xV_xO₂ solid-solution phase, forming crystalline V₂O₅ at the film surface.     

Ohmic contacts and n-type doping on TixCr2 − xO3 films and the temperature dependence of their transport properties

A procedure to dope n-type Cr_2 − xTi_xO₃ thin films is proposed. Besides doping the material, at the same time the method forms ohmic contacts on Ti_xCr_2 − xO₃ films. It consists on the deposition of 10 nm Ti and 50 nm Au, followed by thermal annealing at 1000 °C for 20 min in N₂ atmosphere. Ohmic contacts were formed on three samples with different composition: x = 0.17, 0.41 and 1.07 in a van der Pauw geometry for Hall effect measurements. These measurements are done between 35 K and 373 K. All samples showed n-type nature, with a charge carrier density (n) on the order of 10²⁰ cm^− 3, decreasing as x increased. As a function of temperature, n shows a minimum around 150 K, while the mobilities have an almost constant value of 11, 28 and 7 cm²V^− 1 s^− 1 for x = 0.17, 0.41 and 1.07, respectively.     

High frequency characteristics of tin oxide thin films on Si

High frequency characteristics of tin oxide (SnO₂) thin films were studied. SnO₂ thin films have been successfully grown on n-type Si (111) substrates by using a spray deposition technique. The capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics of the metal-oxide-semiconductor (Au/SnO₂/n-Si) Schottky diodes were investigated in the high frequency range from 300 kHz to 5 MHz. It has been shown that the interface state density, D_it, ranges from 2.44 × 10¹³ cm⁻² eV⁻¹ at 300 kHz to 0.57 × 10¹³ cm⁻² eV⁻¹ at 5 MHz and exponentially decreases with increasing frequency. The C-V and G/ω-V characteristics confirm that the interface state density and series resistance of the diode are important parameters that strongly influence the electrical parameters exhibited by the metal-oxide-semiconductor structure.     

Structural and electrical properties of La2−xBaxMo2O9−x/2 (x = 0-0.18)

La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) have been prepared by solid state reaction method. The lattice parameter of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18-0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La₂Mo₂O₉ is defined in range of 0.18 < x < 0.2. As the replacement degree of La³⁺ by Ba²⁺ increases, the bulk conductivity of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) decreases initially and then increases, a minimum value at La_1.9Ba_0.1Mo₂O_8.95 exists. Hebb-Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173 K. The average thermal expansion coefficient of La_1.84Ba_0.16Mo₂O_8.92 determined by high-temperature XRD was deduced as great as 17.5 × 10⁻⁶ K⁻¹ between 298 and 1173 K.     

Transparent amorphous In-Ga-Zn-O thin film as function of various gas flows for TFT applications

The electrical and optical properties of amorphous indium gallium zinc oxide (a-IGZO) films, which can be used as a channel layer, deposited by radio frequency (rf) magnetron sputtering system at room temperature (RT), were investigated as function of various gas flows. The optical transmittance of films deposited under Ar, O₂ / Ar + O₂ and O₂ / Ar-4% H₂ + O₂ atmospheres in the visible wavelength was consistently above 90% at a wavelength of 550 nm at all gas flows, although the film deposited under Ar-4% H₂ atmosphere exhibited a transmittance of below 50%. The carrier concentration and mobility of the a-IGZO films fabricated under Ar and Ar-4% H₂ were observed slight decrease as a function of the flow, respectively. The thin film transistors (TFTs) with an a-IGZO channel deposited under Ar and Ar-4% H₂ atmosphere exhibited the following good characteristics: V_th of 0.34 V, µ_FE of 3.6 cm² V^− 1 s^− 1, on/off ratio of 10⁶, and S value of 0.04 V decade^− 1.     

Cadmium oxide, indium oxide and cadmium indate thin films obtained by the sol-gel technique

Thin films of the mixed CdO-In₂O₃ system were deposited on glass substrates by the sol-gel technique. The precursor solution was obtained starting from the mixture of two precursor solutions of CdO and In₂O₃ prepared separately at room temperature. The In atomic concentration percentages (X) in the precursor solution with respect to Cd (1 − X), were: 0, 16, 33, 50, 67, 84 and 100. The films were sintered at two different sintering temperatures (Ts) 450 and 550 °C, and after that, annealed in a 96:4 N₂/H₂ gas mixture at 350 °C. X-ray diffraction patterns showed three types of films, excluding those constituted only of CdO and In₂O₃ crystals: i) For X ≤ 50 at.%, the films were constituted of CdO + CdIn₂O₄ crystals, ii) For X = 67 at.%, the films were only formed of CdIn₂O₄ crystals and iii) For X = 84 at.% the films were constituted of In₂O₃ + CdIn₂O₄ crystals. In all films in the 0 < X < 100 range, the formation CdIn₂O₄ crystals of this material was prioritized with respect to the formation of CdO and In₂O₃ materials. All films showed high optical transmission and an increase of the direct band gap value from 2.4 (for CdO) to 3.6 eV (for In₂O₃), as the X value increases. The resistivity values obtained were in the interval of 8 × 10^− 4 Ω cm to 10⁶ Ω cm. The CdIn₂O₄ films had a resistivity value of 8 × 10^− 3 Ω cm and a band gap value of 3.3 eV.     

Deterioration and recovery in the resistivity of Al-doped ZnO films prepared by the plasma sputtering

A hot-cathode plasma sputtering technique was used for fabricating the highly transparent and conducting aluminum-doped zinc oxide (AZO) films on glass substrates from a disk-shaped AZO (Al₂O₃: 2 wt.%) target. Under particular conditions where the target voltage was V_T = −200 V and the plasma excitation pressure was P_S = 1.5 × 10⁻³ Torr, the lowest resistivity of 4.2 × 10⁻⁴ Ω cm was obtained at 400 nm, and this was associated with a carrier density of 8.7 × 10²⁰ cm⁻³ and a Hall mobility of 17 cm²/V s. From the annealing experiment of the AZO films in the oxygen and nitrogen gases of the atmospheric pressure it was revealed that both the oxygen vacancies and the grain boundaries in the polycrystalline AZO film played an important role in the electrical properties of the film.     

Transport properties and thermal expansion of La2Mo2O9-based solid electrolytes

The oxygen ion transference numbers of La_1.7Bi_0.3Mo₂O₉, La₂Mo_1.7W_0.3O₉ and La₂Mo_1.95V_0.05O₉ ceramics, determined by modified faradaic efficiency and e.m.f. methods at 973-1173 K, vary in the range 0.995-0.977 in air, decreasing when temperature increases. The activation energies for the ionic and electronic transport are 61-71 kJ/mol and 123-141 kJ/mol, respectively. Reducing oxygen chemical potential leads to increasing n-type electronic contribution to the total conductivity, which remains, however, essentially p(O₂)-independent down to oxygen pressures of 10⁻⁴-10⁻³ atm and exhibits reversible drop on further reduction, probably due to phase decomposition. Doping La₂Mo₂O₉ with calcium results in segregation of a CaMoO₄-based phase, accompanied with increasing electronic transport. The average thermal expansion coefficients of La₂Mo₂O₉-based materials, calculated from dilatometric data in air, are (14.4-14.8) × 10⁻⁶ K⁻¹ at 300-700 K and (16.4-22.5) × 10⁻⁶ K⁻¹ at 700-1070 K.     

Nitrogen doped p-type SnO thin films deposited via sputtering

p-Type SnO thin films were fabricated via reactive RF magnetron sputtering on borosilicate substrates with an Sn target and Ar/O₂/N₂ gas mixture. The undoped SnO thin film consisted of a polycrystalline SnO phase with a preferred (1 0 1) orientation; however, with nitrogen doping, the preferred orientation was suppressed and the grain size decreased. The electrical conductivity of the undoped SnO thin films demonstrated a relatively low p-type conductivity of 0.05 Ω⁻¹ cm⁻¹ and it was lowered slightly with nitrogen doping to 0.039 Ω⁻¹ cm⁻¹. The results of the X-ray photoelectron spectroscopy suggested that the nitrogen doping created donor defects in the SnO thin films causing lower electrical conductivity. Lastly, both the undoped and doped SnO thin films had poor optical transmittance in the visible range.     

The effect of deposition parameters on the properties of SrCu2O2 films fabricated by pulsed laser deposition

SrCu₂O₂ (SCO) thin films have been fabricated by pulsed laser deposition at oxygen partial pressures between 5 × 10^− 5-5 × 10^− 2 mbar and substrate temperatures from 300 °C to 500 °C. All films were single-phase SrCu₂O₂, p-type materials. Films deposited at a substrate temperature of 300 °C and oxygen pressure 5 × 10^− 4 mbar exhibited the highest transparency (∼ 80%), having conductivity 10^− 3 S/cm and carrier concentration around 10¹³ cm^− 3. Films deposited at oxygen partial pressure higher than 10^− 3 mbar exhibited higher conductivity and carrier concentration but lower transmittance. Depositions at substrate temperatures higher than 300 °C gave films of high crystallinity and transmittance even for films as thick as 800 nm. The energy gap of SrCu₂O₂ thin films was found to be around 3.3 eV.     

Fabrication and photocatalytic property of Pt-intercalated layered perovskite niobates H1−xLaNb2−xMoxO7 (x = 0–0.15)

H_1−xLaNb_2−xMo_xO₇ was prepared by solid-state reaction followed by an ion-exchange reaction. Pt was incorporated in the interlayer of H_1−xLaNb_2−xMo_xO₇ by the stepwise intercalation reaction. The H_1−xLaNb_2−xMo_xO₇ showed hydrogen production activity and the activities were greatly enhanced by Pt co-incorporating. The x value in H_1−xLaNb_2−xMo_xO₇ had an important effect on the photocatalytic activity of the catalyst. When the x = 0.05, the H_1−xLaNb_2−xMo_xO₇/Pt showed a photocatalytic activity of 80 cm³ h⁻¹ g⁻¹ hydrogen evolution rate in 10 vol.% methanol solution under irradiation from a 100 W mercury lamp at 333 K.     

Temperature shift of the optical gap in some PbO-ZnO-P2O5 glasses

The optical gap (E_g) between 4.54 eV-4.88 eV at room temperature was determined for PbO-ZnO-P₂O₅ glasses with the formal content of P₂O₅ in the region of 30 to 50 mol% and with the formal content of PbO in the region of 50 to 45 mol%, respectively. The temperature (T) dependence of the optical gap (E_g(T)) in the region 80 < T[K] < 600 was determined, and an electron-phonon interaction is suggested to be a major contribution to the temperature shift of the optical gap. In the temperature region of 300-600 K, the E_g(T) dependence can be approximated by a simple linear relation with the temperature coefficient (γ) of the optical gap in the region 6.04 ≤ γ × 10⁴ [eV/K] ≤ 7.39.     

Structural and electrical properties of HfO2/Dy2O3 gate stacks on Ge substrates

In the present work we report on the structural and electrical properties of metal-oxide-semiconductor (MOS) devices with HfO₂/Dy₂O₃ gate stack dielectrics, deposited by molecular beam deposition on p-type germanium (Ge) substrates. Structural characterization by means of high-resolution Transmission Electron Microscopy (TEM) and X-ray diffraction measurements demonstrate the nanocrystalline nature of the films. Moreover, the interpretation of the X-ray reflectivity measurements reveals the spontaneous growth of an ultrathin germanium oxide interfacial layer which was also confirmed by TEM. Subsequent electrical characterization measurements on Pt/HfO₂/Dy₂O₃/p-Ge MOS diodes show that a combination of a thin Dy₂O₃ buffer layer with a thicker HfO₂ on top can give very good results, such as equivalent oxide thickness values as low as 1.9 nm, low density of interfacial defects (2-5 × 10¹² eV^− 1 cm^− 2) and leakage currents with typical current density values around 15 nA/cm² at V_g = V_FB − 1V.