Effect of deposition temperature on dielectric properties of PECVD Ta2O5 thin film

Tantalum oxide film formation by plasma-enhanced chemical vapour deposition (PECVD) using TaCl₅ as a source material was examined. The effects of deposition temperature on the formation, structure and electric properties of the Ta₂O₅ film were investigated for Al/Ta₂O₅/ p-Si (MTS) capacitors. The deposition rate and refractive index increased with increasing deposition temperature. It was found that the structure of Ta₂O₅ deposited by PECVD was amorphous as-deposited. However, crystalline -Ta₂O₅ of hexagonal structure was formed by a 700 °C, 1 h heat treatment in argon. Capacitance and relative dielectric constant of the PECVD Ta₂O₅ were found to be 2.54 fF m^–2 and 23.5, respectively. The PECVD films obtained in this study have higher dielectric constants and remarkably better general film characteristics than those obtained by other deposition methods.     

Charge Properties of a Condenser System Based on the Two‐Layer Dielectric SiO2–Ta2O5

A variant of a combined dielectric for the condensers of very largescale integrated circuits (VLSICs) has been proposed. Its electrophysical and mechanical properties have been analyzed. The influence of the thickness of each of the dielectrics on the effective builtin charge, the permittivity, and the threshold voltage has been considered. Special emphasis has been placed on the residual mechanical stresses in a threelayer dielectric system, namely, the influence of the thickness of a Ta₂O₅ film on the radius of curvature of the Si–SiO₂–Ta₂O₅ system has been analyzed and the dependence of the change in the effective builtin charge in the Ta₂O₅ film on the radius of curvature of the plate has been determined.     

Electrical characteristics of the MOD-derived SrBi2xTa2O9 and SrBi2.4(Ta,Nb)2O9 thin films

Ferroelectric and leakage current characteristics of the MOD-derived SrBi_2xTa₂O₉ (0.8 x 1.6) and SrBi_2.4(Ta_1-yNb_y)₂ O₉ (0 y 1) thin films were investigated. The SBT and SBTN films were fully crystallized to Bi-layered perovskite structure by annealing at 800°C for 1 hour in oxygen atmosphere. The ferroelectric characteristics of the SBT films were optimized at the Bi/Ta mole ratio x of 1.2. The leakage current density of the Bi-excess SBT films decreased remarkably by the post-metallization annealing at 800°C for 10 minutes in oxygen ambient. The ferroelectric characteristics of the SBTN films were optimized with the SBN content y of 0.25. The SrBi_2.4(Ta_0.75Nb_0.25)₂ O₉ film exhibited 2P_r and E_c of 19.04 C/cm² and 24.94 kV/cm at ±5 V, which were superior to 2P_r of 11.3 C/cm² and E_c of 39.6 kV/cm obtained for the SrBi_2.4Ta₂O₉ film after the post-metallization annealing. The MOD-derived SrBi_2.4(Ta_0.75Nb_0.25)₂O₉ film did not exhibit the polarization fatigue after 10¹¹ switching cycles at ±5 V.     

Modified mixed oxide perovskites 0.7Sr(Al1/2B1/2) O3·0.3LaAlO3 and 0.7Sr(Al1/2B1/2) O3·0.3NdGaO3 (B=Ta5+ or Nb5+) for high-T c superconductor substrate applications

Single-crystal fibres of modified strontium aluminium tantalum oxide (1-x)Sr(Al_1/2Ta_1/2) O₃·xLaAlO₃(SAT-LA) and (1-x)Sr(Al_1/2Ta_1/2)O₃·xNdGaO₃ (SAT·NG), and modified strontium aluminum niobium oxide (1-x)Sr(Al_1/2Nb_1/2)O₃·xNdGaO₃(SAT·NG) and (1-x)Sr (Al_1/2Nb_1/2)O₃·xLaAlO₃ (SAN·LA) were grown using a laser-heated pedestal growth technique. 0.7SAT·0.3LA grows congruently and retains a twin free simple cubic perovskite structure (as the SAT) when cooled down to room temperature. 0.7SAT·0.3LA crystals have a moderate dielectric constant ( = 21.7) and low dielectric loss (tan = 7.5 × 10^–5) at 10 kHz and 90 K. The reduction problem of Ta⁵⁺ is eliminated (which is common in the case of SAT growth). 0.7SAT·0.3NG and 0.7SAN·0.3NG have lower melting temperatures and crystal growth is easier. NdGaO₃ addition to the SAT and SAN enhances the potential of SAT and SAN as large-area substrates for high-T _c superconductor growth. However, the dielectric constants increased from -12 to -16(0.7SAT·0.3NG) and from 18 to 23 (0.7SAN·0.3NG) as a result of NdGaO₃ incorporation.     

Synthesis of carbon nanotube films by thermal CVD in the presence of supported catalyst particles. Part II: the nanotube film

Carbon nanotube films have been grown at 750° and 900 °C by thermal chemical vapor deposition (CVD) with acetylene (C₂H₂) and hydrogen on silicon (0 0 2) wafers supporting preformed (Fe,Si)₃O₄ particles. The reduction of the (Fe,Si)₃O₄ particles during CVD at 750 °C was accompanied by a disintegration leading to the formation of a high density of smaller (predominantly 5–15 nm) iron silicide (₁-Fe₂Si) particles that catalyzed the growth of a dense and aligned multi-wall carbon nanotube film. The tubes did not contain any inclusions apart from the catalytic particles present in the bottom part of the film, and it was concluded that the nanotubes grew via a base-growth mechanism. CVD at 900 °C resulted in a random growth of predominantly multi-wall carbon nanotubes. The film contained an increased number of amorphous carbon, or graphite, clusters containing particles that had been carbonized, the larger ones to cementite, -Fe₃C. Nanotubes were observed to grow from some of these clusters. Multi-wall carbon nanotube tips contained after CVD at 900 °C encapsulated -Fe₃C, or in a few cases - or -Fe, particles.     

Effect of Ta2O5 doping on the electrical properties of 0.99SnO2·0.01CoO ceramic

The effect of Ta₂O₅ doping in 0.99SnO₂·0.01CoO on the microstructure and electrical properties of this ceramic were analyzed in this study. The grain size was found to decrease from 6.87 m to 5.68 m when the Ta₂O₅ concentration increased from 0.050 to 0.075 mol%. DC electrical characterization showed a dramatic increase in the current loss and decrease in the non-linear coefficient with the increase of the Ta₂O₅ concentration. The conduction mechanism is by thermionic emission and the potential barriers are of Schottky type, separated by a thin film.     

Microstructure, electrochemical surface and electrocatalytic properties of IrO2+Ta2O5 oxide electrodes

X-ray diffraction (XRD) and scanning electron microscopy (SEM) have been used to characterize physical structure of IrO₂+Ta₂O₅ films over the whole composition range by thermodecomposition of chloride solutions heated at 450°C. Solid solubilization between Ta component and IrO₂ rutile in the mixed films was measured, and three typical surface morphologies of the oxide coatings were observed. The surface electrochemical properties of Ti/IrO₂-Ta₂O₅ electrodes were studied by cyclic voltammetry at varying potential scan rate, and a double-layer electrochemical structure containing the inner and outer layers has been distinguished. The voltammetric charge appears to decline with the decrease of grain size of oxide coatings as a result of the effect of surface tension. However, the coatings of 70% IrO₂+30% Ta₂O₅ with the finest grains still exhibit the highest apparent activity for oxygen evolution evaluated by the anodic current at a constant potential. This result is interpreted by the measurements of open-circuit potential (E _oc) and double-layer capacitance (C _dl) using electrochemical impedance spectroscopy (EIS). Thereby, the reliability of voltammetric charge obtained in double-layer potential region in determining the real electrocatalytic activity for O₂ evolution has been discussed.     

Dielectric properties of chemically vapour-deposited Si3N4

The dielectric properties of chemically vapour-deposited (CVD) amorphous and crystalline Si₃N₄ were measured in the temperature range from room temperature to 800° C. The a.c. conductivity ( _a.c.) of the amorphous CVD-Si₃N₄ was found to be less than that of the crystalline CVD-Si₃N₄ below 500° C, but became greater than that of the crystalline CVD-Si₃N₄ over 500° C due to the contribution of d.c. conductivity ( _d.c.). The measured loss factor () and dielectric constant () of the amorphous CVD-Si₃N₄ are smaller than those of the crystalline CVD-Si₃N₄ in all of the temperature and frequency ranges examined. The relationships of ^n-1, (- ) ^n-1 and/(- ) = cot (n/2) (were observed for the amorphous and crystalline specimens, where is angular frequency andn is a constant. The values ofn of amorphous and crystalline CVD-Si₃N₄ were 0.8 to 0.9 and 0.6 to 0.8, respectively. These results may indicate that the a.c. conduction observed for both of the above specimens is caused by hopping carriers. The values of loss tangent (tan) increased with increasing temperature. The relationship of log (tan) T was observed. The value of tan for the amorphous CVD-Si₃N₄ was smaller than that of the crystalline CVD-Si₃N₄.     

Study on the dielectric properties of oxide-doped Ba(Ti,Sn)O3 ceramics prepared from ultrafine powder

Ultrafine BaTiO₃ and BaSnO₃ powders were prepared by thermal decomposition of oxalates through synthesis by the method of co-precipitation; Dy₂O₃, Nb₂O₅ and ZnO, used as dopants, were added to the main constituents, known as the tailor-made Ba(Ti, Sn)O₃, according to the indicated compositions, then the batches were mixed until they became homogeneous. Careful treatment was necessary in later stages of the process of ceramic making. The dielectric properties of the ceramics were measured. It was found that high , low tan and low d/dt were characteristics of these ceramics. The results could be explained by the effects of peak shifting, broadening and grain-size inhibition. The relationship between composition, structure and properties is discussed.     

Preparation, characterization and dielectric properties of Sr5LnTi3Ta7O30 (Ln=La, Nd) ceramics

Sr₅LnTi₃Ta₇O₃₀ (Ln=La, Nd) ceramics were prepared by the conventional high temperature solid state reaction route. The sintered samples were characterized by X-ray diffraction and scanning electron microscopy methods. SLTT and SNTT belong to paraelectric phases of filled tetragonal TB structure at room temperature. SLTT and SNTT ceramics show high dielectric constant of 131 and 116 together with low dielectric loss 0.008 and 0.0036 at 1 MHz, respectively. In comparison with Ba-based ceramics with TB structure, the temperature coefficients of the dielectric constant () are significantly reduced.     

Low-temperature plasma-enhanced chemical vapor deposition of tungsten and tungsten nitride

Tungsten and tungsten nitride layers have been deposited by plasma-enhanced chemical vapor deposition (PECVD). Tungsten layers deposited at low deposition temperatures T150 °C using this method showed good uniformity over dielectric and silicon substrate areas. As the deposition temperature decreased, the silicon consumed during the deposition reaction decreased, at T150 °C no silicon consumption was measurable. PECVD tungsten nitride layers were deposited directly on oxidized silicon substrates with no requirement for a nucleation layer. As the NH₃ flow rate was increased, whilst maintaining all other parameters constant, deposited layers were found to change from metal tungsten to tungsten-rich amorphous layer to W₂N. The resistivity of the layers was found to be high compared to published literature for higher-temperature deposited layers. The high resistivity is attributed to the incorporation of fluorine into the layer at low deposition temperatures. A deposition process was established for smooth amorphous tungsten-rich W _x N layers at 150 °C.     

Origins of conductive losses at microwave frequencies in YBa2Cu3O7?δ/LaAlO3/YBa2Cu3O7?δ trilayers deposited by pulsed laser deposition

Oriented YBa₂Cu₃O_7–/LaAlO₃/YBa₂Cu₃O_7– trilayers were deposited by pulsed laser deposition (PLD) onto 100 MgO and LaAlO₃. Film thicknesses varied from 2000–5000 Å/ layer. A comparision of structure and transport data for the bottom and top superconducting layers indicated a slight decrease in film quality for the top superconducting layer. The critical temperature was lower for the top superconducting layer (90.5 vs. 90 K) and the microwave surface resistance was higher (increasing from 2 to 18 m at 36 GHz, 20 K). The resistivity of the dielectric was estimated to be 10⁶ cm, and the loss tangent of the dielectric film at microwave frequencies had an upper limit of 0.01. Cross-sectional TEM analysis of the trilayer structure showed a high density of threading dislocations in the dielectric layer that appeared to nucleate at steps in the underlying superconducting layer. The threading dislocations may serve as conduction paths in the LaAlO₃ layer.     

1 : 2 Long-range ordering and defect mechanism of WO3-doped perovskite Ba(Mg1/3Ta2/3)O3

The nature of the B-site cation ordering and the associated defect process necessary to stabilize the ordered domains were investigated using the WO₃-doped BaMg_1/3Ta_2/3O₃ BMT system as a typical example of BaB_1/3B_2/3O₃-type complex perovskites. It was shown that only the 1 : 2 long-range ordering of the B-site cation existed in both undoped and WO₃-doped BMT perovskites. The atomic defect mechanism associated with the stoichiometric 1 : 2 long-range ordering was systematically investigated. It is concluded that the substitution of W⁶⁺ for Ta⁵⁺ in the WO₃-doped BMT enhances the degree of the 1 : 2 long-range ordering and produces the positively charged W _Ta sites with a concomitant generation of tantalum vacancies V_Ta and mobile oxygen vacancies V _O for the ionic charge compensation.     

Barium–Strontium and Nickel–Cobalt Solid-State Interdiffusion between Hexagonal W-Ferrites BaM2Fe16O27and SrM2Fe16O27 (M = Ni2+, Co2+)

The Ni, Co, Ba, and Sr profiles in the diffusion zones produced between hexagonal W-ferrites (BaCo₂Fe₁₆O₂₇/BaNi₂Fe₁₆O₂₇, SrCo₂Fe₁₆O₂₇/SrNi₂Fe₁₆O₂₇, SrCo₂Fe₁₆O₂₇/BaCo₂Fe₁₆O₂₇, and SrCo₂Fe₁₆O₂₇/BaNi₂Fe₁₆O₂₇) by annealing at 1520 K were used to evaluate the interdiffusion coefficients of the cations involved by the Boltzmann–Matano method over the whole composition range.     

Direct current electrical conductivity of gamma ferric oxide

The study of the direct current electrical conductivity, , of freshly prepared -Fe₂O₃ and that of a sample stored for seven days in static air suggests that -Fe₂O₃ adsorbs oxygen and water from the atmosphere. From infra-red spectra it is deduced that the absorbed water in -Fe₂O₃ is present as the physically adsorbed water and as lattice water. The adsorbed oxygen and physically adsorbed water are removed by heating to 100 C, while the lattice water remains in -Fe₂O₃ even up to 280 C. The removal of lattice water is associated with a decomposition during which some of the hydrogen formed occupies the vacancy sites. This suggested formation of the hydrogen ferrite phase is based on the kink in the log against T ^–1 curve observed at 177 C. This kink is very well resolved for a sample equilibrated at 100 C in normal atmosphere, and the measurements of above 100 C of this sample are done in an N₂ atmosphere. The suggestion that the hydrogen ferrite phase is formed has been substantiated by comparison of the X-ray diffraction patterns of -Fe₂O₃ heated under the different atmospheres. From the log against T ^–1 plot for a sample heated under a nitrogen atmosphere the activation energy is small (< 0.05 eV) up to 215 C, and it is comparatively large (0.95 eV) above 215 C. These results suggest a hopping mechanism for the direct current electrical conductivity of -Fe₂O₃. This suggestion has been substantiated by data of the temperature variation of Seebeck voltage.     

Low-frequency dielectric spectroscopy and electrical properties of Ca[B<Subscript>3</Subscript>O<Subscript>4</Subscript>(OH)<Subscript>3</Subscript>]·H<Subscript>2</Subscript>O single crystals near the phase transition

The dielectric properties and polarization of ferroelectric Ca[B₃O₄(OH)₃]·H₂O (colemanite) crystals are studied near the Curie temperature. The real part of dielectric permittivity (), dielectric losses (tan), pyroelectric coefficient (), and thermally stimulated depolarization currents are measured from -50 to 50°C. To assess the detailed nature of the phase transition (Curie temperature t_C~-7°C), the temperature dependences of and tan and are measured at frequencies f = 0.8-20 kHz. The results are used to determine () (where =f ) and (T) and evaluate the activation energy. The (t) data indicate that colemanite undergoes a diffuse phase transition.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1489–1494.Original Russian Text Copyright © 2004 by Slabkaya, Lotonov, Gavrilova.     

Microstructural study of two LAS-type glass-ceramics and their parent glass

The two glass-ceramics studied here derive from the complex system (MgO,ZnO,Li₂O)-Al₂O₃-SiO₂ and are obtained by controlled devitrification of the same parent glass. Although they have the same chemical composition, one is a -quartz (or -eucryptite) type while the other one is a -spodumene glass-ceramic. A detailed microstructural analysis of these materials has been performed at different scales by several complementary characterization methods (SEM, TEM, DTA, XRD and FTIR). This extensive study has shown the great microstructure difference (grain distribution, grain size, nature of vitreous and crystalline phases) between these two glass-ceramics obtained from the same parent glass.     

Ba4Nd2Ti4Ta6O30 dielectric ceramics modified by Bi substitution for Nd

Modification of dielectric properties for Ba₄Nd₂Ti₄Ta₆O₃₀ ceramic was investigated through Bi partial substitution for Nd. The dielectric constant increased and the dielectric loss decreased with increasing concentration of Bi, and the dielectric constant reached 142, combined with a low dielectric loss of 10^–4 (at 1 MHz) for the composition Ba₄(Nd_0.975Bi_0.025)₂Ti₄Ta₆O₃₀. The temperature coefficient () can be slightly improved.     

Electrical characteristics of SrxBi2.4Ta2O9 thin film and Pt/Sr0.85Bi2.4Ta2O9/Al2O3/Si structure

Sr _x Bi_2.4Ta₂O₉ (0.7 x 1.3) thin films were processed by metalorganic decomposition and their ferroelectric characteristics were investigated. The Sr-deficient Sr _x Bi_2.4Ta₂O₉ films exhibited well-developed ferroelectric hysteresis curves compared to those of the Sr-excess films, and Sr_0.85Bi_2.4Ta₂O₉ film had the optimum electrical characteristics among Sr _x Bi_2.4Ta₂O₉ films. Electrical characteristics of the Pt/SBT/Al₂O₃/Si structure using Sr_0.85Bi_2.4Ta₂O₉(SBT) film were investigated for metalferroelectric-insulator-semiconductor field-effect-transistor (MFIS-FET) applications. Memory window of C-V hysteresis characteristics of the Pt/SBT/Al₂O₃/Si structure became large with decreasing the Al₂O₃ thickness, and the Pt/SBT(400 nm)/Al₂O₃ (10 nm)/Si structure gave memory window of 2.2 V at sweeping voltages of ±5 V. The Pt/SBT/Al₂O₃/Si structure can be proposed for MFIS-FET applications.     

Sinterability improvement of Ba(Mg1/3Ta2/3)O3 dielectric ceramics

The sinterability of Ba(Mg_1/3Ta_2/3)O₃ ceramics synthesized by solid-state-reaction methods was investigated. The poor sinterability of the present ceramics was found to be primarily due to the presence of satellite secondary phases of Ba₅Ta₄O₁₅ and Ba₄Ta₂O₉, and the nearly complete densification of Ba(Mg_1/3Ta_2/3)O₃ ceramics was accomplished successfully by controlling the phase constitution of the calcined powders. For this purpose, enhanced ball-milling processes were found to be extremely effective. Moreover, excellent microwave dielectric characteristics (_r = 24.5–24.7, Q = 26 000 at 9.8 GHz, and _f = 1.7 p.p.m. C^–1) were attained in the dense Ba(Mg_1/3Ta_2/3)O₃ ceramics without additives.