Growth and characterization of HfO2 high-k gate dielectric films by laser molecular beam epitaxy (LMBE)

The HfO₂ gate dielectric films were fabricated by the laser molecular beam epitaxy (LMBE) technique. High-resolution transmission electron microscopy (HRTEM) observation showed that under optimized condition, there is no detectable SiO₂ interfacial layer in the as-deposited film and a SiO₂ interfacial layer of about 0.4 nm was formed at the Si interface due to the post deposition annealing. Capacitance–voltage (C–V) measurement of the film revealed that the equivalent oxide thickness was about 1.3 nm. Such a film showed very low leakage current density of 1.5 × 10⁻² A cm⁻² at 1 V gate bias from the current–voltage (I–V) analysis. The conduction mechanisms as a function of temperature T and electric field E were also systematically studied.     

Study of the effect of thermal annealing on high k hafnium oxide thin film structure and electrical properties of MOS and MIM devices

The effect of rapid thermal annealing on structural and electrical properties of high k HfO₂ thin films is investigated. The films were initially deposited at pre-optimized sputtering voltage of 0.8 kV and substrate bias of 80 V in order to get optimized results for oxide charges and leakage current as a MOS device. The film properties were investigated for optimum annealing temperature in oxygen and optimum rapid thermal annealing temperature in nitrogen respectively to get the best electrical results as a MOS device structure. The film thickness, composition and microstructure is studied by Laser Ellipsometry, XRD and AFM and the effect of thermal annealing is shown. The electrical I–V and C–V characteristics of the annealed dielectric film were investigated employing Al-HfO₂-Si MOS capacitor structure. The flat-band voltage (V _fb) and oxide-charge density (Q _ox) were extracted from the high-frequency C–V curve. Dielectric study were further carried out on HfO₂ thin films having metal–insulator–metal (MIM) configuration over a wide temperature (300–500 K) and frequency (100 Hz to 1 MHz) range.     

A three-dimensional nanostructured PANI/MnOx porous microsphere and its capacitive performance

Three-dimensional nanostructured polyaniline (PANI) and manganese oxide (MnO _x ) composite porous microspheres were prepared by oxidizing aniline with KMnO₄ under interfacial chemical synthesis with 4-amino-thiophenol (4-ATP) as the structure-directing agent on the Au substrate. Surface morphology and chemical composition of PANI/MnO _x microsphere were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, thermo gravimetric-differential thermal analysis, and Fourier transform infrared spectrum. The result displayed that concentration of KMnO₄ played a key role in forming the 3D nanostructured porous microspheres. To obtain the regular shapes and uniform sizes of the porous microspheres, the optimal concentration of oxidant was 0.15 mol L⁻¹. The electrochemistry performances of PANI/MnO _x microsphere were determined by cyclic voltammograms, electrochemical impedance spectroscopy, and galvanostatic charge–discharge. The specific capacitance of the 3D nanostructured PANI–MnO _x porous microspheres exhibited a maximum value of 828 F g⁻¹ at current density of 2 mA cm⁻² over a potential range of 0.0–0.9 V versus SCE. It has improved 365 and 88 % comparing with that of PANI (178 F g⁻¹) and MnO _x (440 F g⁻¹) obtained at the similar condition. The charge–discharge tests showed the PANI/MnO _x microsphere possessed a good cycling stability. It maintained about 84.2 % of the initial capacitance after 1000 cycles at a current density of 2.0 mA cm⁻².     

Photoresponse of n -ZnO/ p -Si heterojunction towards ultraviolet/visible lights: thickness dependent behavior

A series of n-ZnO/p-Si thin film heterojunctions have been fabricated by a low cost sol–gel technique for different ZnO film thicknesses and the dark as well as photo current–voltage (I–V) characteristics have been investigated in details. The heterojunction with ZnO thickness of 0.46 μm shows the best diode characteristics in terms of rectification ratio, I _F/I _R = 5.7 × 10³ at 5 V and reverse leakage current density, J _R = 7.6 × 10⁻⁵ A cm⁻² at −5 V. From the photo I–V curves and wavelength dependent photocurrent of the heterojunctions, it is found that the junction with 0.46 μm ZnO thickness shows the highest sensitivity towards both UV and visible lights.     

Electrochemical and mechanical behavior of laser processed Ti–6Al–4V surface in Ringer’s physiological solution

Laser surface modification of Ti–6Al–4V with an existing calcium phosphate coating has been conducted to enhance the surface properties. The electrochemical and mechanical behaviors of calcium phosphate deposited on a Ti–6Al–4V surface and remelted using a Nd:YAG laser at varying laser power densities (25–50 W/mm²) have been studied and the results are presented. The electrochemical properties of the modified surfaces in Ringer’s physiological solution were evaluated by employing both potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. The potentiodynamic polarizations showed an increase in the passive current density of Ti–6Al–4V after laser modification at power densities up to 35 W/mm², after which it exhibited a decrease. A reduction in the passive current density (by more than an order) was observed with an increase in the laser power density from 25 to 50 W/mm². EIS studies at the open circuit potential (OCP) and in the passive region at 1.19 V showed that the polarization resistance increased from 8.274 × 10³ to 4.38 × 10⁵ Ω cm² with increasing laser power densities. However, the magnitudes remain lower than that of the untreated Ti–6Al–4V at OCP. The average hardness and modulus of the laser treated Ti–6Al–4V, evaluated by the nanoindentation method, were determined to be 5.4–6.5 GPa (with scatter <±0.976 GPa) and 124–155 GPa (with scatter <±13 GPa) respectively. The corresponding hardness and modulus of untreated Ti–6Al–4V were ~4.1 (±0.62) and ~148 (±7) GPa respectively. Laser processing at power densities >35 W/mm² enhanced the surface properties (as passive current density is reduced) so that the materials may be suitable for the biomedical applications.     

Interfacial interaction of solid cobalt with liquid Pb-free Sn–Bi–In–Zn–Sb soldering alloys

Dissolution kinetics of cobalt in liquid 87.5%Sn–7.5%Bi–3%In–1%Zn–1%Sb and 80%Sn–15%Bi–3%In–1%Zn–1%Sb soldering alloys and phase formation at the cobalt–solder interface have been investigated in the temperature range of 250–450 °C. The temperature dependence of the cobalt solubility in soldering alloys was found to obey a relation of the Arrhenius type c _s = 4.06 × 10² exp (−46300/RT) mass% for the former alloy and c _s = 5.46 × 10² exp (−49200/RT) mass% for the latter, where R is in J mol⁻¹ K⁻¹ and T in K. For tin, the appropriate equation is c _s = 4.08 × 10² exp (−45200/RT) mass%. The dissolution rate constants are rather close for these soldering alloys and vary in the range (1–9) × 10⁻⁵ m s⁻¹ at disc rotational speeds of 6.45–82.4 rad s⁻¹. For both alloys, the CoSn₃ intermetallic layer is formed at the interface of cobalt and the saturated or undersaturated solder melt at 250 °C and dipping times up to 1800 s, whereas the CoSn₂ intermetallic layer occurs at higher temperatures of 300–450 °C. Formation of an additional intermetallic layer (around 1.5 μm thick) of the CoSn compound was only observed at 450 °C and a dipping time of 1800 s. A simple mathematical equation is proposed to evaluate the intermetallic-layer thickness in the case of undersaturated melts. The tensile strength of the cobalt-to-solder joints is 95–107 MPa, with the relative elongation being 2.0–2.6%.     

Atmospheric effects on the thermally induced sintering of nanoparticulate gold films

Gold (Au) films were formed by sintering of Au nanoparticles (NPs) under gas flows of air, oxygen (O₂), nitrogen (N₂), or N₂ bubbled through formic acid (FA/N₂). The microstructure changes of the Au nanoparticulate films were studied when different atmospheres were applied. The Au film sintered under FA/N₂ showed the progressive agglomeration and grain growth with porosity in the film, while the film sintered under N₂ had NPs without participating grain growth. A necking between NPs was observed in the film, however, unnecked NPs were still found. The Au film sintered under O₂ atmosphere showed the NPs agglomeration with various sizes up to 50 nm. X-ray characteristic peaks of the (111)-preferred orientation were observed in all samples. All samples showed N–H stretching at 3200–3300 cm⁻¹ regardless of sintering atmosphere. Hydrocarbon chains (C–H) at 2850–3000 cm⁻¹ were detected in the film sintered under N₂. For the Au film sintered under O₂, C–H stretching at 2850–3000 cm⁻¹, C–H deformation at 1350–1470 cm⁻¹, and C–O stretching at 1200–1300 cm⁻¹ were observed. C–O stretching at 1600–1700 cm⁻¹ was observed for the film sintered under FA/N₂ atmosphere. The electrical resistance of the film was related with microstructures and organic residual materials left in the film. Even though either porosity or carbon residues were observed in the film, the sintering of NPs in FA/N₂ or N₂ showed the sheet resistance comparable to that of electroplated one.     

Effect of the frequency and temperature on the complex impedance spectroscopy (C–V and G–V) of p-ZnGa2Se4/n-Si nanostructure heterojunction diode

X-ray diffraction pattern and AFM results confirm the nanostructure of p-ZnGa₂Se₄/n-Si. The unit cell lattice parameters, the crystallite size L, the dislocation density δ, and the main internal strain ε were calculated. The temperature and frequency-dependent electrical characteristics of the Al/p-ZnGa₂Se₄/n-Si/Al heterojunction diode (HJD) have been investigated to determine the interface states which are responsible for the non-ideal behavior of the characteristics of the diode. The capacitance–voltage (C–V), conductance–voltage (G–V), and series resistance–voltage (R _s–V) characteristics of the diode have been analyzed in the frequency range of 5 kHz–1 MHz and temperature range of 303–423 K. The interfaces states of the diode were determined using conductance–voltage technique. The interface state density profile for the diode was obtained as a function of temperature and frequency. The values of the built-in potential V _bi, the doping concentration N _d and the barrier height φ _b(C–V) of the diode were calculated at different temperatures and frequencies. Our experimental results revealed that both the series resistance and interface state density values must be taken into account in studying the impedance spectroscopy of HJD to stand up their performance for electronic applications characteristics.     

Superheating of YBa2Cu3O7−x thin-film microbridges

It is found that the hysteresis that develops with decreasing temperature in the current-voltage characteristics of thin-film YBa₂Cu₃O_7−x microbridges is due to superheating of the bridges by the transport current. The heat transport coefficient is determined to be α=5500–7500 V/(cm²·K) and the thermal resistance of the YBa₂Cu₃O_7−x /MgO film-substrate interface is R _if=(1.3–1.8)×10₋₄ (cm²·K)/W. Pis’ma Zh. Tekh. Fiz. 23, 56–62 (June 26, 1997)     

Structural, optical, and electric properties of BNT–BT0.08 thin films processed by sol–gel technique

Thin films with the composition [(Bi_0.5Na_0.5)TiO₃]_0.92–[BaTiO₃]_0.08 (hereafter BNT–BT_0.08) were deposited on Pt–Si by spin-coating from a stable sol precursor. The BNT–BT_0.08 film, crystallized on the Bi_0.5Na_0.5TiO₃ rhombohedral lattice, was obtained after annealing the film-gel at 700 °C. The films have a smooth surface (Rms = 2.76 nm) and grains with ferroelectric domains. The film showed a bandgap of 3.25 eV and a refractive index of 2.20 at a wavelength of 630 nm. The dielectric characteristics of BNT–BT_0.08 thin films were measured at room temperature and 10 kHz the dielectric constant (ε _r) was 243 and the loss tangent (tanδ) was 0.38. The remnant polarization (P _r) was 0.87 μC/cm² and the coercive field (E _c) was 220 kV/cm at 10 kHz and room temperature. The current density was approximately 2.7 × 10⁻⁵ A/cm² at low electric fields (100 kV/cm). BNT–BT_0.08 thin films shown piezoelectric properties (d _33eff = 100 pm/V) comparable to those of PZT thin films.     

Microstructures and properties of Bi3.25La0.75Ti2.94V0.06O12 ferroelectric thin film deposited by sol–gel method

Bi_3.25La_0.75Ti_2.94V_0.06O₁₂ (BLTV) thin film was fabricated on the Pt/TiO₂/SiO₂/p-Si(100) substrate using sol–gel method. The microstructures and electrical properties of the film after cosubstitution of La and V were investigated. The BLTV thin film shows less highly c-axis oriented than the BIT thin film mainly with fine rod-like grains. Raman spectroscopy shows that TiO₆ (or VO₆) symmetry decreases and Ti–O (or V–O) hybridization increases for V substitution. The P _r and E _c values of the BLTV thin film are 26.3 μC/cm² and 98 kV/cm at a voltage of 12 V, respectively. The thin film also exhibits a very strong fatigue endurance up to 10¹⁰ cycles and low leakage current density. The excellent properties of the BLTV thin film are attributed to the effective decrease or suppression of oxygen vacancies after La and V cosubstitution in the thin film.     

Novel direct MOCVD growth of In x Ga1−x As and InP metamorphic layers on GaAs substrates

A review of the technique of direct growing high-quality In_xGa_1−xAs or InP buffer layers on GaAs substrates by metal-organic chemical vapor deposition (MOCVD) is given. This low-temperature growth method benefits the improvement of metamorphic device performance. In this work, a simple and novel method of directly deposited thin In_xGa_1−xAs or InP buffer layers (< 1 μm) on GaAs substrates is presented, instead of strained-layer superlattice, two-step, graded or CS (compliant substrates) methods, while maintaining low dislocations, high crystal quality, and uniform and mirror like surfaces. For the direct growth technique of In_xGa_1−xAs on GaAs, we found an excellent quality In_0.54Ga_0.46As buffer of rms surface roughness of only 0.686 nm by AFM and FWHM of 925 arcsec by XRD can be obtained at a low growth temperature of 440^∘C with a constant Ga/In_gas partial pressure of 5. The superior results are mainly due to the use of low temperature growth technology. In addition, we also found this growth technology is available to directly grow InP buffer layers on GaAs. Experimental results conclude that the growth temperature of 480^∘C in harmony with the V/III ratios range of 130–210 is a suitable window to directly grow InP on GaAs substrates.     

Effect of sintering process on electrical properties and ageing behavior of ZnO–V2O5–MnO2–Nb2O5 varistor ceramics

The effect of sintering process on microstructure, electrical properties, and ageing behavior of ZnO–V₂O₅–MnO₂–Nb₂O₅ (ZVMN) varistor ceramics was investigated at 875–950 °C. The sintered density decreased from 5.52 to 5.44 g/cm³ and the average grain size increased from 4.4 to 9.6 μm with the increase of sintering temperature. The breakdown field (E_{1 mA}) decreased from 6991 to 943 V/cm with the increase of sintering temperature. The ZVMN varistor ceramics sintered at 900 °C led to surprisingly high nonlinear coefficient (α = 50). The donor concentration (N_d) increased from 3.33 × 10¹⁷ cm⁻³ to 7.64 × 10¹⁷ cm⁻³ with the increase of sintering temperature and the barrier height (Φ_b) exhibited the maximum value (1.07 eV) at 900 °C. Concerning stability, the varistors sintered at 925 °C exhibited the most stable accelerated ageing characteristics, with %ΔE_{1 mA} = 1.5% and %Δα = 13.3% for DC accelerated ageing stress of 0.85 E_{1 mA}/85 °C/24 h.     

Corrosion characteristics of anodized Ti–(10–40wt%)Hf alloys for metallic biomaterials use

The effect of anodizing on corrosion resistance of Ti–xHf alloys has been investigated. Ti–xHf alloys were prepared and anodized at 120, 170 and 220 V in 1 M H₃PO₄ solution, and crystallized at 300 and 500°C. Corrosion experiments were carried out using a potentiostat in 0.15 M NaCl solution at 36.5 ± 1°C. The Ti–xHf alloys exhibited the α′ and anatase phases. The pore size on the anodized surface increases as the applied voltage is increased, whereas the pore size decreases as the Hf content is increased. The anodized Ti–xHf alloys exhibited better corrosion resistance than non-anodized Ti–xHf alloys.     

Silicon metal-dielectric-semiconductor varicaps with an yttrium oxide dielectric

This paper reports an investigation of the electrophysical properties of metal-dielectric-semiconductor varicaps with an yttrium oxide dielectric, prepared by resistive vacuum evaporation of the rare-earth metal with subsequent thermal oxidation of the film in air at 500–550 °C. It is found that the electrical conductivity of the samples follows the Poole-Frenkel law. High-frequency capacitance-voltage characteristics are used to determine the specific capacitance of the dielectric, C ₀=0.027–0.03 μF/cm², the slope of the capacitance-voltage characteristic, dC/dV=35–40 pF/V, the fixed charge in the dielectric, Q _f=(1.7−2.7)×10⁻⁸ C/cm², and the density of surface states at the flat-band potential, N _ss=(1−2)×10¹¹ cm⁻²·eV⁻¹. The capacitance tuning range factor for the metal-dielectric-semiconductor varicaps is 2.5–3. These structures are shown to be applicable as metal-dielectric-semiconductor varicaps with a low control voltage and a high quality factor. Pis’ma Zh. Tekh. Fiz. 23, 50–55 (June 26, 1997)     

Coarsening of δ-Ni<Subscript>2</Subscript>Si precipitates in a Cu–Ni–Si alloy

The coarsening behavior of rod-shaped and spherical δ-Ni₂Si precipitates in a Cu–1.86 wt% Ni–0.45 wt% Si alloy during aging at 823–948 K has been investigated by measuring both precipitate size by transmission electron microscopy (TEM) and solute concentration in the Cu matrix by electrical resistivity. The rod-shaped δ precipitates have an elongated shape along 〈[`5] 5 8 \overline{5} 5 8 〉_m and a {110}_m habit-plane facet. The coarsening theory of a spherical precipitate in a ternary alloy developed by Kuehmann and Voorhees (KV) has been modified to a case of rod-shaped precipitates. The coarsening kinetics of average size of the rod-shaped and spherical δ precipitates with aging time t obey the t ^1/3 time law, as predicted by the modified KV theory. The kinetics of depletion of the supersaturation with t are coincident with the predicted t ^−1/3 time law. Application of the modified KV theory has enabled calculation of the energies of sphere, {110}_m and rod-end interfaces from the data on coarsening alone. The energy of the {110}_m interface having a high degree of coherency to the Cu matrix is estimated to be 0.4 J m⁻², the incoherent sphere-interface energy 0.6 J m⁻², and the rod-end interface energy 5.2 J m⁻².     

Fabrication and comparative study of top-gate and bottom-gate ZnO–TFTs with various insulator layers

Transparent ZnO thin film transistors (ZnO–TFTs) with different structures and dielectric layers were fabricated by rf magnetron sputtering. The PbTiO₃, AlO _x , SiN _x and SiO _x films were attempted to serve as the gate dielectric layers in the devices, respectively, and XRD was employed to investigate the crystal structure of ZnO films deposited on these dielectric layers. The optical properties of transparent TFTs were measured and revealed the average transmittance ranged from 60 to 80% in the visible part of the spectrum. Electrical measurement shows the properties of the ZnO–TFTs have great relations with the device structure. The bottom-gate TFTs have better behaviors than top-gate ones with the mobility, threshold voltage and the current on/off ratio of 18.4 cm² V⁻¹ s⁻¹, −0.7 V and 10⁴, respectively. The electrical difference of the devices may be due to different character of the interface between the channel and dielectric layers.     

Microstructural analysis of Pd/Pt/Au/Pd ohmic contacts to InGaP/GaAs

Microstructural evolution during the annealing of Pd/Pt/Au/Pd p-ohmic contacts (with and without a thin layer of Zn) to InGaP/GaAs HBTs has been studied using transmission electron microscopy (TEM). Metal layers were deposited by electron beam evaporation directly onto the InGaP emitter layer with the intention of consuming the InGaP during annealing to contact the heavily C-doped p-type GaAs (3×10¹⁹ cm⁻³) base layer below. Initial reaction between Pd and Pt and InGaP formed a five-component amorphous layer (Pd, Pt, In, Ga and P), which crystallized to (Pt_xPd_1−x)5(In_yGa_1−y)P (0≤x, y≤1) at the interface between Pt and the amorphous layer. Annealing at temperatures ≥415°C caused complete decomposition of the InGaP and partial decomposition of the GaAs base layer, producing a contact consisting of (Pt_xPd_1−x)5(In_yGa_1−y)P, PtAs₂ and PdGa. The attainment of low contact resistances did not depend on the presence of Zn. Minimum values of 0.10−0.12 Ω mm were achieved after annealing at 415–440°C for contacts both with and without Zn. This revised version was published online in July 2006 with corrections to the Cover Date.     

Production of Ti–13Nb–13Zr alloy for surgical implants by powder metallurgy

The Ti–13Nb–13Zr near-β alloy was developed aiming the replacement of the traditional Ti–6Al–4V alloy in surgical implants owing to its larger biocompatibility. Samples of this alloy were obtained using the blended elemental technique from hydrided powders. The isochronal sintering of the compacts for 2 h was carried out in the range 900–1,400 °C with a heating rate of 20 °C min⁻¹. In this work, the behavior of the elementary powders during sintering and the corresponding microstructural evolution were investigated. The alloy was characterized by means of scanning electron microscopy (SEM) in the backscattered mode, X-ray diffraction, and density measurements. The results indicate that the homogenization of the alloy is diffusion-controlled. With increasing temperature, homogenization of the alloy takes place and a fine plate-like α + β structure is found throughout the microstructure in temperatures above 1,300 °C. The process variables were defined aiming to minimize interstitial pick-up (C, O, and N) and avoiding intensive grain growth.     

Barrier characteristics of Pt/Ru Schottky contacts on <Emphasis Type="Italic">n</Emphasis>-type GaN based on <Emphasis Type="Italic">I–V–T</Emphasis> and <Emphasis Type="Italic">C–V–T</Emphasis> measurements

We have investigated the current–voltage (I–V) and capacitance–voltage (C–V) characteristics of Ru/Pt/n-GaN Schottky diodes in the temperature range 100–420 K. The calculated values of barrier height and ideality factor for the Ru/Pt/n-GaN Schottky diode are 0·73 eV and 1·4 at 420 K, 0·18 eV and 4·2 at 100 K, respectively. The zero-bias barrier height (Φ_b0) calculated from I–V characteristics is found to be increased and the ideality factor (n) decreased with increasing temperature. Such a behaviour of Φ_b0 and n is attributed to Schottky barrier (SB) inhomogeneities by assuming a Gaussian distribution (GD) of barrier heights (BHs) at the metal/semiconductor interface. The current–voltage–temperature (I–V–T) characteristics of the Ru/Pt/n-GaN Schottky diode have shown a double Gaussian distribution having mean barrier heights ( [`(F)]<sub>\textb0</sub> {\bar{{\Phi}}_{\text{b}0}} ) of 1·001 eV and 0·4701 eV and standard deviations (σ <sub>0</sub>) of 0·1491 V and 0·0708 V, respectively. The modified ln (J<sub>0</sub> /T<sup>2</sup> )-( q<sup>2</sup>s <sub>0</sub><sup>2</sup>/2k<sup>2</sup>T<sup>2</sup> ){ln} ({{J}_{0} /{T}^{2}} )-( {{q}^{2}{\sigma} _{0}^{2}/{2}{k}^{2}{T}^{2}} ) vs 10<sup>3</sup>/T plot gives [`(F)]_\textb0 \bar{{\Phi}}_{\text{b}0} and Richardson constant values as 0·99 eV and 0·47 eV, and 27·83 and 10·29 A/cm²K², respectively without using the temperature coefficient of the barrier height. The difference between the apparent barrier heights (BHs) evaluated from the I–V and C–V methods has been attributed to the existence of Schottky barrier height inhomogeneities.