Nitrogen implantation in GaAs1−xPx (x=0.4; 0.65)

Nitrogen ions were implanted in GaAs_1−xP_x (x=0.4; 0.65) at room temperature at various doses from 5×10¹² cm⁻² to 5×10¹⁵ cm⁻² and annealed at temperatures from 600°C up to 950°C using a sputtered SiO₂ encapsulation to investigate the possibility of creating isoelectronic traps by ion implantation. Photoluminescence and channeling measurements were performed to characterize implanted layers. The effects of damage induced by optically inactive neon ion implantation on photoluminescence spectrum were also investigated. By channeling measurements it was found that damage induced by nitrogen implantation is removed by annealing at 800°C. A nitrogen induced emission intensity comparable to the intensity of band gap emission for unimplanted material was observed for implanted GaAs_0.6P_0.4 after annealing at 850°C, while an enhancement of the emission intensity by a factor of 180 as compared with an unimplanted material was observed for implanted GaAs_0.35P_0.65 after annealing at 950°C. An anomalous diffusion of nitrogen atoms was found for implanted GaAs_0.6P_0.4 after annealing at and above 900°C.     

Fundamental absorption edge of evaporated amorphous WO3 films

The fundamental absorption edge of evaporated WO₃ films is investigated. The optical gap of the virgin film is estimated to be 3.41 eV at room temperature and it decreases with increase of annealing temperature up to 200°C. Annealing at 300°C leads to change in the spectral shape, which is caused by crystallization. For the films annealed at 200°C, temperature coefficient of the optical gap is estimated to be −2×10⁻⁴ eV/K and the slope of Urbach's tail is found to be independent of measuring temperature up to 200°C. With electrolytic coloration, shift of the optical gap toward higher energy is observed. Magnitude of this shift is estimated to be 0.05 eV at the color center concentration of 7.5×10²¹ cm⁻³ when H⁺ electrolyte is used. If Li⁺ electrolyte is used, the magnitude of this shift is about three times larger than in the case of H⁺ electrolyte. This fact is interpreted by a small change in the host matrix structure owing to the injection of proton or Li⁺ during coloration.     

Structural, electrical, and surface characteristics of La0.5Sr0.5CoO3 thin films prepared by pulsed-laser deposition

A parametric study of the growth of La_0.5Sr_0.5CoO₃ (LSCO) thin films on (100) MgO substrates by pulsed-laser deposition (PLD) is reported. Films are grown under a wide range of substrate temperature (450–800 °C), oxygen pressure (0.1–0.9 mbar), and incident laser fluence (0.8–2.6 J/cm²). The optimum ranges of temperature, oxygen pressure, and laser fluence to produce c-axis oriented films with smooth surface morphology and high metallic conductivity are identified. Films deposited at low temperature (500 °C) and post-annealed in situ at higher temperatures (600–800 °C) are also investigated with respect to their structure, surface morphology, and electrical conductivity. Received: 20 November 1998 / Accepted: 6 July 1999 / Published online: 21 October 1999     

CW CO2-laser annealing of arsenic implanted silicon

CW CO₂-laser annealing of arsenic implanted silicon was investigated in comparison with thermal annealing. Ion channeling, ellipsometry, and Hall effect measurements were performed to characterize the annealed layers and a correlation among the different methods was made. The laser annealing was done with power densities of 100 to 640 W cm⁻² for 1 to 20 s. It was found that the lattice disorder produced during implantation can be completely annealed out by laser annealing with a power density of 500 W cm⁻² and the arsenic atoms are brought on lattice sites up to 96±2%. The maximum sheet carrier concentration of 6×10¹⁵ cm⁻² was obtained for 1×10¹⁶ cm⁻² implantation after laser annealing, which was up to 33% higher than that after thermal annealing at 600 to 900°C for 30 min.     

Pulsed laser deposition of Zr–N codoped <Emphasis Type="Italic">p</Emphasis>-type ZnO thin films

Present p-type ZnO films tend to exhibit high resistivity and low carrier concentration, and they revert to their natural n-type state within days after deposition. One approach to grow higher quality p-type ZnO is by codoping the ZnO during growth. This article describes recent results from the growth and characterization of Zr–N codoped p-type ZnO thin films by pulsed laser deposition (PLD) on (0001) sapphire substrates. For this work, both N-doped and Zr–N codoped p-type ZnO films were grown for comparison purposes at substrate temperatures ranging between 400 to 700 °C and N₂O background pressures between 10⁻⁵ to 10⁻² Torr. The carrier type and conduction were found to be very sensitive to substrate temperature and N₂O deposition pressure. P-type conduction was observed for films grown at pressures between 10⁻³ to 10⁻² Torr. The Zr–N codoped ZnO films grown at 550 °C in 1×10⁻³ Torr of N₂O show p-type conduction behavior with a very low resistivity of 0.89 Ω-cm, a carrier concentration of 5.0×10¹⁸ cm⁻³, and a Hall mobility of 1.4 cm² V⁻¹ s⁻¹. The structure, morphology and optical properties were also evaluated for both N-doped and Zr–N codoped ZnO films.     

Electrical transport properties of CuWO4

The temperature dependence of the electrical conductivity, thermoelectric power and dielectric constant of the antiferromagnetic CuWO₄ have been studied in the temperature range 300–1000 K. The conductivity results can be summarised by the equations σ_I=6.31 × 10⁻³ exp (−0.29 eV/kT) ohm⁻¹ cm⁻¹ in the temperature range 300–600 K and σ_II=3.16 × 10⁵ exp (−1.48 eV/kT) ohm⁻¹ cm⁻¹ between 600 K and 1000 K. The thermoelectric power can be expressed byθ=[− 1.25 (10³/T) + 3.9] mV/K. Initially dielectric constant increases slowly but for high temperatures its increase is fast.     

Sodium superionic conductor sputter-deposited coatings

Thin sodium superionic conductor (Nasicon) coatings are deposited on rotating substrates by co-sputtering in the reactive mode of a Zr-Si and a Na₃PO₄ target. The influence of the discharge current and of the target-to-substrate distance is investigated owing to the targeted Na₃Zr₂Si₂PO₁₂ stoichiometry. A thermo-structural analysis shows that the amorphous as-deposited coating of convenient composition crystallises around 700 °C in the monoclinic structure. Electrical measurements performed at room temperature after various annealing treatments indicate a ionic conductivity of about 2·10⁻³ S·cm⁻¹, consistent with that of bulk Nasicon. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Swift heavy ion irradiation induced modification of BiFeO3 thin films prepared by sol-gel method

We report the preparation of multiferroic BiFeO₃ thin films on ITO coated glass substrates through sol-gel spin coating method followed by thermal annealing and their modification by swift heavy ion (SHI) irradiation. X-ray diffraction and Raman spectroscopy studies revealed amorphous nature of the as deposited films. Rhombohedral crystalline phase of BiFeO₃ evolved on annealing the films at 550°C. Both XRD and Raman studies indicated that SHI irradiation by 200 MeV Au ions result in fragmentation of particles and progressive amorphization with increasing irradiation fluence. The average crystallite size estimated from the XRD line width decreased from 38 nm in pristine sample annealed at 550°C to 29 nm on irradiating these films by 200 MeV Au ions at 1 × 10¹¹ ions cm⁻². Complete amorphization of the rhombohedral BiFeO₃ phase occurs at a fluence of 1 × 10¹² ions.cm⁻². Irradiation by another ion (200 MeV Ag) had the similar effect. For both the ions, the electronic energy loss exceeds the threshold electronic energy loss for creation of amorphized latent tracks in BiFeO₃.     

Recrystallization of amorphous nanotracks and uniform layers generated by swift-ion-beam irradiation in lithium niobate

The thermal annealing of amorphous tracks of nanometer-size diameter generated in lithium niobate (LiNbO₃) by Bromine ions at 45 MeV, i.e., in the electronic stopping regime, has been investigated by RBS/C spectrometry in the temperature range from 250°C to 350°C. Relatively low fluences have been used (<10¹² cm⁻²) to produce isolated tracks. However, the possible effect of track overlapping has been investigated by varying the fluence between 3×10¹¹ cm⁻² and 10¹² cm⁻². The annealing process follows a two-step kinetics. In a first stage (I) the track radius decreases linearly with the annealing time. It obeys an Arrhenius-type dependence on annealing temperature with activation energy around 1.5 eV. The second stage (II) operates after the track radius has decreased down to around 2.5 nm and shows a much lower radial velocity. The data for stage I appear consistent with a solid-phase epitaxial process that yields a constant recrystallization rate at the amorphous-crystalline boundary. HRTEM has been used to monitor the existence and the size of the annealed isolated tracks in the second stage. On the other hand, the thermal annealing of homogeneous (buried) amorphous layers has been investigated within the same temperature range, on samples irradiated with Fluorine at 20 MeV and fluences of ∼10¹⁴ cm⁻². Optical techniques are very suitable for this case and have been used to monitor the recrystallization of the layers. The annealing process induces a displacement of the crystalline-amorphous boundary that is also linear with annealing time, and the recrystallization rates are consistent with those measured for tracks. The comparison of these data with those previously obtained for the heavily damaged (amorphous) layers produced by elastic nuclear collisions is summarily discussed.     

Fatigue-free Pb(Zr0.52Ti0.48)O3 thin films on indium tin oxide coated substrates by a sol-gel process

Fatigue-free Pb(Zr_0.52Ti_0.48)O₃ (PZT) ferroelectric thin films were successfully prepared on indium tin oxide (ITO) coated glass substrates using the sol-gel method combined with a rapid thermal annealing process (RTA). The films post-annealed at a temperature of 700 °C for 2 min by RTA process formed (110)-oriented Pb(Zr_0.52Ti_0.48)O₃ thin films with pure perovskite structure, and had a good morphology as well. The good ferroelectricity of the prepared PZT films was confirmed by P–E hysteresis loop measurements. Fatigue characteristics showed stable behavior. Degradation in polarization was not found while the repeating cycles were up to 10¹¹, and a low leakage current density of 10⁻⁸ A/cm² was also obtained from the highly fatigue-resisted PZT thin films on ITO/glass substrates. Received: 19 October 1998 / Accepted: 29 March 1999 / Published online: 26 May 1999     

H2/Pt/Ce0.9Gd0.1O1.95/Pt/O2 fuel cell operated in the intermediate temperature range 500 – 700°C

Ce_0.9Gd_0.1O_1.95 (GCO), is one of the potential candidate electrolytes for intermediate temperature Solid Oxide Fuel Cells (ITSOFC). GCO has high oxide ion conductivity in the intermediate temperature range (500 – 700 °C) compared to other Ce_1−yGd_yO_2-2/y compositions and the Gd³⁺ ion is the most appropriate dopant material compared to other rare earth materials such as Sm³⁺, Y³⁺, Zr³⁺, etc. Our results show that the fuel cell H₂/Pt/Ce_0.9Gd_0.1O_1.95/Pt/O₂ operated in the temperature range 500 – 700°C gives the maximum power densities 0.0049 W/cm² at 500 °C and 0.0126 W/cm² at 650 °C for cell voltages 0.6275 V and 0.6278 V, respectively, where the electrolyte was kept in 5% H₂ (+ Argon) for 12 hours before use in the fuel cell. Maximum power densities are 0.0038 W/cm² at 500 °C and 0.0270 W/cm² at 650 °C for cell voltages 0.5986 and 0.5913 V, respectively, where the electrolyte was kept in 2 % O₂ (+ Argon) for 12 hours before use in the fuel cell. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

XRD, conductivity and FTIR studies on LiI-Li2WO4-Li3PO4 prepared by low temperature sintering

Different amounts of Li₃PO₄ were mixed to a fixed ratio of LiI:Li₂WO₄, ground and pelletised before subjected to sintering at 70°C for 7 days. XRD shows that the product formed after sintering process is most likely Li₆P₄W₈O₃₂ due to peaks present at 10.6°, 22.4°, 24.0°, 24.4, 26.2°, 32.4° and 34.0°. Conductivity studies show that the sample with 25 wt.% Li₃PO₄ exhibits the highest room temperature conductivity of 3.42×10⁻³ Scm⁻¹. Conductivity is expected to occur through channel-like structures which could have formed due to corner or edge sharing of polyhedra. FTIR studies have shown the existence of WO₄ tetrahedra and WO₆ octahedral at 850 cm⁻¹ and 952 cm⁻¹, and phosphate tetrahedral at 564 cm⁻¹, 700 cm⁻¹, 890 cm⁻¹ and 1030 cm⁻¹.     

Structure and electrical properties of HfO<Subscript>2</Subscript> high-<Emphasis Type="Italic">k</Emphasis> films prepared by pulsed laser deposition on Si (100)

High-k gate dielectric hafnium dioxide films were grown on Si (100) substrate by pulsed laser deposition at room temperature. The as-deposited films were amorphous and that were monoclinic and orthorhombic after annealed at 500°C in air and N₂ atmosphere, respectively. After annealed, the accumulation capacitance values increase rapidly and the flat-band voltage shifts from −1.34 V to 0.449 V due to the generation of negative charges via post-annealing. The dielectric constant is in the range of 8–40 depending on the microstructure. The I–V curve indicates that the films possess of a promising low leakage current density of 4.2×10⁻⁸ A/cm² at the applied voltage of −1.5 V.     

Thermomechanical and conductivity studies of doped niobium titanates as possible current collector material in the SOFC anode

In the reducing atmosphere of the SOFC anode at operating temperatures of 800 °C and above Nb₂TiO₇ is reduced to Nb_1.33Ti_0.67O₄. This material displays very high electronic conductivity of >100 Scm⁻¹, suitable for use in such applications as a current collector. It has a low thermal expansion coefficient of 3 × 10⁻⁶ K⁻¹, however, which may cause problems due to mismatch with other SOFC components, e.g. YSZ. Doping with Fe₂O₃ successfully increased the thermal expansion to a maximum of 6 × 10⁻⁶ K⁻¹. A conductivity of 140 Scm⁻¹ at 900 °C in dry 5% H₂/Ar, with an activation energy of 0.18 eV, was achieved for the Nb_1.344Ti_0.642Fe_0.014O₄, making it suitable for the use as a current collector. Conductivity runs in wet 5%H₂/Ar showed lower conductivities of 15–18 Scm⁻¹ and lower activation energies of 0.08 − 0.09 eV. Single cell tests of Nb_1.33Ti_0.67O₄ showed power outputs of 5.5 − 7.2 mW·cm⁻² at 850 °C, lower than for Ni with 150 − 200 mW·cm⁻² at 850 °C, however, this material displayed much better stability at high temperatures than Ni. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Implantation doping of germanium with Sb,As, and P

The ions of Sb, As, and P have been implanted into germanium at energies ranging from 200 keV to 700 keV. Annealing was performed at 400°C, 550°C, and 650°C. The doping profile was determined by differentialCV-measurements. Strong outdiffusion (80%) and diffusion into the bulk material was observed after annealing. The remaining doping concentration and the diffusion constants were determined by a computer fit at 650°C. We foundD _Sb=1.8×10⁻¹³ cm²/s,D _As=9×10⁻¹⁴ cm²/s andD _P=4×10⁻¹⁴ cm²/s. Lower values of the diffusion constant were determined when the samples were covered with a SiO₂ layer.     

Oxygen thermodynamics and ion conductivity in the solid solution La1−xSrxCoO3−δ at large strontium content

The equilibrium oxygen content was measured in the model system and important oxygen permeable material La_1−xSr_xCoO_3−δ, where x=0.6, in the temperature range 650–900 °C and oxygen partial pressure range between 10⁻⁵ and 1 atm. The data were utilized to obtain changes in the partial entropy and enthalpy of oxygen in the solid as a function of the oxygen content. It is shown that the initially cubic perovskite undergoes to a phase transition to a tetragonal structure at δ >0.3. The oxygen permeation of L_0.4Sr_0.6CoO_3−δ at 700–900 °C is found to be controlled by bulk solid state processes. The activation energy equals about 0.8 eV at high oxygen pressure and small oxygen nonstoichiometry. Increasing oxygen deficiency results in a rapid increase in the activation energy. In combination with thermodynamic data, these changes can be explained as resulting from the intrinsic spatial inhomogeneouty in oxygen vacancy distribution which varies both with temperature and oxygen nonstoichiometry. It is shown that, when the oxygen deficiency increases at constant temperature, the oxygen vacancies form locally ordered microdomains (clusters), which eventually results in a transition of the cubic perovskite structure to the tetragonal structure. The oxygen ion conductivity depends strongly on the development of the ordering. Paper presented at the 6th Euroconference on Solid state Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Ac-conductivity and dielectric relaxations above glass transition temperature for parylene-C thin films

45% semi-crystalline parylene-C (–H₂C–C₆H₃Cl–CH₂–) _n thin films (5.8 μm) polymers have been investigated by broadband dielectric spectroscopy for temperatures above the glass transition (T _g =90°C). Good insulating properties of parylene-C were obtained until operating temperatures as high as 200°C. Thus, low-frequency conductivities from 10⁻¹⁵ to 10⁻¹² S/cm were obtained for temperatures varying from 90 to 185°C, respectively. This conductivity is at the origin of a significant increase in the dielectric constant at low frequency and at high temperature. As a consequence, Maxwell–Wagner–Sillars (MWS) polarization at the amorphous/crystalline interfaces is put in evidence with activation energy of 1.5 eV. Coupled TGA (Thermogravimetric analysis) and DTA (differential thermal analysis) revealed that the material is stable up to 400°C. This is particularly interesting to integrate this material for new applications as organic field effect transistors (OFETs). Electric conductivity measured at temperatures up to 200°C obeys to the well-known Jonscher law. The plateau observed in the low frequency part of this conductivity is temperature-dependent and follows Arrhenius behavior with activation energy of 0.97 eV (deep traps).     

Production and characterization of Nd,Cr:GSGG thin films on Si(001) grown by pulsed laser ablation

Nd,Cr:Gd₃Sc₂Ga₃O₁₂ (GSGG) thin films have been produced for the first time. They were grown on Si(001) substrates at 650 °C by pulsed laser ablation at 248 nm of a crystalline Nd,Cr:GSGG target rod. The laser plume was analyzed using time-of-flight quadrupole mass spectroscopy, and consisted of elemental and metal oxide fragments with kinetic energies typically in the range 10 to 40 eV, though extending up to 100 eV. Although films deposited in vacuum using laser fluences of 0.8±0.1 J cm⁻² reproduced the Nd,Cr:GSGG bulk stoichiometry, those deposited using fluences above ≈3 J cm⁻² resulted in noncongruent material transfer and were deficient in Ga and Cr. Attempts to grow films using synchronized oxygen or oxygen/argon pulses yielded mixed oxide phases. Under optimal growth conditions, the films were heteroepitaxial, with GSGG(001)[100]∥Si(001)[100], and exhibited Volmer–Weber-type growth. Room-temperature emission spectra of the films suggest efficient non-radiative energy transfer between Cr³⁺ and Nd³⁺ ions, similar to that of the bulk crystal. Received: 1 October 1999 / Accepted: 15 October 1999 / Published online: 23 February 2000     

Experimental and computational study of the passage of an electron beam through the curvilinear element of the Drakon stellarator system in a tenuous plasma

Results are presented from the first stage of studies on the passage of an electron beam with energy 100–500 eV in a magnetic field of 300–700 Oe through the curvilinear solenoid of the KRéL unit, the latter being a prototype of the closing segment of the Drakon stellarator system, in the plasma-beam discharge regime. The ion density at the end of the curvilinear part of the chamber, n _i ≈8×10⁸–10¹⁰ cm⁻³, the electron temperature T _e ≈4–15 eV, and the positions at which the beam hits the target for different distances from it to the electron source are determined experimentally. The motion of the electron beam is computationally modeled with allowance for the space charge created by the beam and the secondary plasma. From a comparison of the experimentally measured trajectories and trajectories calculated for different values of the space charge, we have obtained an estimate for the unneutralized ion density of the order of 5×10⁷ cm⁻³. Zh. Tekh. Fiz. 69, 22–26 (February 1999)     

Electrochemical study of P(VDF-HFP)/PMMA blended polymer electrolyte with high-temperature stability for polymer lithium secondary batteries

In the present study, a kind of solid polymer electrolyte (SPE) based on poly(vinylidene difluoride-co-hexafluoropropylene)/poly(methyl methacrylate) blends was prepared by a casting method to solve the safety problem of lithium secondary batteries. Owing to being plasticized with a room temperature ionic liquid, N-butyl-N′-methyl-imidiazolium hexafluorophosphate, the obtained SPE shows a thermal decomposition temperature over 300°C and an ionic conductivity close to 10⁻³ S cm⁻¹. The SPE-3 sample, in which the weight of two polymers is equivalent, possesses an ionic conductivity of 0.45 × 10⁻³ S cm⁻¹ at 25°C and presents an electrochemical window of 4.43 V. The ionic conductivity of the SPE-3 is as high as 1.73 × 10⁻³ S cm⁻¹ at 75°C approaching to that of liquid electrolyte. The electrochemical performances of the Li/LiFePO₄ cells confirmed its feasibility in lithium secondary batteries.