Low Doped Samples Research Articles

Collapse of the low temperature insulating state in Cr-doped V2O3 thin films

We have grown epitaxial Cr-doped V2O3 thin films with Cr concentrations between 0% and 20% on (0001)-Al2O3 by oxygen-assisted molecular beam epitaxy. For the highly doped samples (>3%), a regular and monotonous increase of the resistance with decreasing temperature is measured. Strikingly, in the low doping samples (between 1% and 3%), a collapse of the insulating state is observed with a reduction of the low temperature resistivity by up to 5 orders of magnitude. A vacuum annealing at high temperature of the films recovers the low temperature insulating state for doping levels below 3% and increases the room temperature resistivity towards the values of Cr-doped V2O3 single crystals. It is well-know that oxygen excess stabilizes a metallic state in V2O3 single crystals. Hence, we propose that Cr doping promotes oxygen excess in our films during deposition, leading to the collapse of the low temperature insulating state at low Cr concentrations. These results suggest that slightly Cr-doped V2O3 films can be interesting candidates for field effect devices.

Optical temperature sensor based on the Nd3+ infrared thermalized emissions in a fluorotellurite glass

The temperature dependence of the infrared luminescence of a fluorotellurite glass doped with 0.01 and 2.5 mol% of Nd3+ ions was studied in order to use it as a high temperature sensing probe. For this purpose, the emission intensities of the (4S3/2, 4F7/2), (2H9/2, 4F5/2),4F3/2→4I9/2 transitions were measured in a wide range of temperatures from 300 upto 650 K. The changes in the emission profiles were calibrated by means of the fluorescence intensity ratio technique. The calibrations showed a strong dependence on the Nd3+ ions concentration, having the low-doped concentrated sample the best response to changes of temperature. The maximum value obtained for the thermal sensibility is 17×10−4 K−1 at 640 K, being one of the highest values found in the literature for Nd3+ optical temperature sensors. Finally, the experimental calibrations were compared with the theoretical temperature luminescence response calculated from the Judd–Ofelt theory.

Concentration effects on the FIR technique for temperature sensing

Different concentrations of Er3+ doped Y2O3 ceramics are synthesized. Under the 966nm excitation, it is found that the ratios of red to green emissions are obviously enhanced with temperature for high doped sample, which is attributed to the combined temperature-dependent processes. Besides that, variations of fluorescence intensity ratio of green upconversion emissions in Er3+:Y2O3 ceramics have been studied as a function of temperature. A maximum sensitivity of less than 0.003K−1 is obtained in the high doped sample, whereas it is above 0.005K−1 in the low doped sample. Concentration effects on the temperature sensing behaviors are discussed. In addition, similar phenomenon is found in Er3+/Yb3+ codoped NaYF4 nanocrystals.

Microstructure and Dielectric Properties of SnO2-doped CaCu3Ti4O12 Ceramics

Dielectric CaCu3Ti4O12-xSnO2 ceramics (x = 0, 0.003, 0.006, 0.010, 0.015 and 0.020) have been synthesized by solid-state reaction method. The microstructure was studied by X-ray diffraction, Raman spectrum and Scanning electron microscope. It is revealed that SnO2-doping results in the distortion of crystal structure and various grain growths for CaCu3Ti4O12-based ceramics. Dielectric measurement shows that lower-doped samples (x = 0.003 and 0.006) exhibit enhanced dielectric constants which are over 70,776 and 26,406 at the range of 40-104 Hz, respectively. In addition, the dielectric properties are explained in terms of internal barrier layer capacitance model by using the impedance spectroscopy analysis.

Determination of carrier lifetime and diffusion length in Al-doped 4H–SiC epilayers by time-resolved optical techniques

A series of p-type 4H–SiC epilayers with aluminium concentration ranging from 2 × 1016 to 8 × 1019 cm−3 were investigated by time-resolved optical techniques in order to determine the effect of aluminium doping on high-injection carrier lifetime at room temperature and the diffusion coefficient at different injections (from ≈3 × 1018 to ≈5 × 1019 cm−3) and temperatures (from 78 to 730 K). We find that the defect limited carrier lifetime τSRH decreases from 20 ns in the low-doped samples down to ≈0.6 ns in the heavily doped epilayers. Accordingly, the ambipolar diffusion coefficient decreases from Da = 3.5 cm2 s−1 down to ≈0.6 cm2 s−1, corresponding to the hole mobility of µh = 70 cm2 Vs−1 and 12 cm2 Vs−1, respectively. In the highly doped epilayers, the injection-induced decrease of the diffusion coefficient, due to the transition from the minority carrier diffusion to the ambipolar diffusion, provided the electron diffusion coefficient of De ≈ 3 cm2 s−1. The Al-doping resulted in the gradual decrease of the ambipolar diffusion length, from LD = 2.7 µm down to LD = 0.25 µm in the epilayers with the lowest and highest aluminium concentrations.

Low Sr doping effects on critical current density and pinning mechanism of YBa2Cu3O7-δ single crystals

We report on isotherm dc magnetization hysteresis loops, M(H) to 70K ≤ T ≤ 82.5K of YBa2-xSrxCu3O7-δ (x = 0, 0.02 and 0.1) single crystals with the purpose to study the influence of the low Sr doping effects on the YBa2Cu3O7-δ critical current density, JC(H) and normalized flux pinning force density, f(h). The M(H) measurements were performed in a commercial SQUID magnetometer with H ≤ 50kOe applied parallel to the c axis of the samples. The extended Bean critical state model was applied to JC(H) determination. The magnetic irreversibility fields, Hirr(T) were obtained from M(H) loops. The f(h) behavior was determined from FP/FP,max versus H/Hirr plots where FP is the pinning force density (FP = J × μ0H). The results show that the low Sr doped samples transport higher JC(H) than the pure one. The application of the Dew-Hughes scaling functions to the f(h) plots designates the core normal punctual type as majority pinning mechanism of samples. We suggest that the core normal punctual pinning mechanism, responsible to the enhancement of the JC(H) transported by YBa2-xSrxCu3O7-δ samples, is possibly connected to the segregation of Sr atoms precipitates at the crystalline structure of these samples.

Electrical and magnetic transport properties of Co-doped Bi2Se3 topological insulator crystals

Transition metal doped topological insulators (TI) Co x Bi 2-x Se 3 were prepared and the phase structure, electrical and magnetic transport properties, Hall mobility and Hall resistivity were studied. The lattice constant c decreased with the increasing Co concentration. All samples exhibited weak metallic resistivity. With increasing Co concentration, the value of ρxx increased, reaching a maximum at x = 0.02, and then decreased. The resistivity data could be fitted by different formulas below and above 30 K in all samples result from different mechanism corresponding to different temperature regions. The magnetoresistance (MR) of Bi 2 Se 3 TI showed a change dependence of concentration. The MR was enhanced by small quantity of Co doping, attaining a maximum value in Bi 1.98 Co 0.02 Se 3 crystal; but MR was suppressed in high-doped sample (x > 0.02). Those results were ascribed to the rivalry between the phonon scattering effect and the magnetic ordering in single crystals caused by Co ions. For low-doped samples doped, the contribution of phonon scattering was much higher than that of magnetic ordering, leading to larger MR value. However, in samples doped with higher Co concentrations, the magnetic ordering effect became dominant and caused the MR decreases. The MR increased with decreasing temperature and increasing magnetic field. The Hall mobility data indicated that incorporating Co in Bi 2 Se 3 single crystal can reduce native defect of Bi 2 Se 3.

Microstructural and dielectrical characterization of Ho doped BaTiO3 ceramics

The Ho doped BaTiO3 ceramics, with different Ho2O3 content, ranging from 0.01 to 1.0 wt % Ho, were investigated regarding their microstructural and dielectric characteristics. Doped BaTiO3 were prepared using conventional solid state reaction and sintered at 1380?C for four hours. SEM analysis of Ho/BaTiO3 doped ceramics showed that the low doped samples exhibit mainly fairly uniform and homogeneous microstructure with the grain size ranged from 20-40 ?m. In the samples with the higher dopant concentration the abnormal grain growth is inhibited and the grain size ranged between 2-10 ?m. Measurements of dielectric properties were carried out as a function of temperature up to 180 ?C at different frequencies. The samples doped with 0.01wt % of Ho, exhibit the high value of dielectric permittivity (?r = 2160) at room temperature. A nearly flat permittivity-response was obtained in specimens with higher additive content. Using a Curie-Weiss law and modified Curie-Weiss law the Curie constant (C), Curie temperature (Tc) and a critical exponent of nonlinearity (g) were calculated. The Curie temperature of doped samples were ranged from 128 to 130?C. The Curie constant for all series of samples decrease with increase of dopant concentration and the lowest values were observed on samples doped with 0.01 wt % of holmium.

Photoluminescence properties of Mg-doped InN nanowires

In this work, photoluminescence (PL) properties of nearly defect-free Mg-doped InN nanowires were investigated in detail. The low-doped sample exhibits two PL emission peaks up to 152 K, which can be ascribed to the band-to-band recombination and the Mg-acceptor energy level related recombination, respectively. For the high-doped sample, the Mg-acceptor energy level related transition dominates. Detailed power dependent PL studies further indicate that the Mg-acceptor energy level related PL emission is due to the donor-acceptor pair recombination process, which subsequently evolves into the free-to-acceptor recombination with increasing temperature.

Microstructure and dielectric properties of Dy/Mn doped BaTiO3 ceramics

Dy/Mn doped BaTiO3 with different Dy2O3 contents, ranging from 0.1 to 5.0at% Dy, were investigated regarding their microstructural and dielectric characteristics. The content of 0.05at% Mn was constant in all the investigated samples. The samples were prepared by the conventional solid state reaction and sintered at 1290°, and 1350°C in air atmosphere for 2h. The low doped samples (0.1 and 0.5at% Dy) exhibit mainly fairly uniform and homogeneous microstructure with average grain sizes ranged from 0.3μm to 3.0μm. At 1350°C, the appearance of secondary, abnormal, grains in the fine grain matrix and core–shell structure were observed in highly doped Dy/BaTiO3. Dielectric measurements were carried out as a function of temperature up to 180°C. The low doped samples sintered at 1350°C, display the high value of dielectric permittivity at room temperature, 5600 for 0.1Dy/BaTiO3. A nearly flat permittivity–temperature response was obtained in specimens with 2.0 and 5.0at% additive content. Using a Curie–Weiss and modified Curie–Weiss low, the Curie constant (C), Curie like constant (C′), Curie temperature (TC) and a critical exponent (γ) were calculated. The obtained values of γ pointed out the diffuse phase transformation in highly doped BaTiO3 samples.

Ellipsometric investigations of Fe3+-doped polyvinyl alcohol films

In this work, we study the inclusion mechanism of Fe3+ ions in a polyvinyl alcohol (PVA) matrix. Thin films of pure and FeCl3-doped PVA on silicon substrates, prepared by the spin-coating method, are investigated using spectroscopic ellipsometry (SE) and Fourier transform infrared (FT-IR) spectroscopy. SE measurements of PVA and Fe3+-doped thin films are carried out at an incidence angle of 75° over the wavelength range of 0.24–1.1 μm. An optical model is used to obtain the refractive index (n) and the extinction coefficient (k). The gap energy Eg is afterwards evaluated. The Fe3+ doping is found to affect strongly the optical parameters of the polymer films. In fact, an increase in the refractive index with doping is observed, resulting from the intermolecular hydrogen bonding between Fe3+ ions with the adjacent OH group of PVA. The increase of the thin films' absorption with doping is estimated by the k spectral profile analysis. The gap energy is then calculated and shows an important decrease with the Fe3+ filling, more pronounced for low doped samples. Such a behavior is confirmed by FT-IR analysis.

Microstructure and thermoelectric properties of tungsten trioxide ceramics doped with a low amount of terbium dioxide

The effect of the nonstoichiometric compound terbium dioxide (Tb4O7) on the thermoelectric properties of tungsten trioxide (WO3) ceramics was investigated. Among the sintered ceramics, the sample doped with 0.1 mol% Tb4O7 showed the maximum grain size and density. Doping with Tb4O7 also increased the electrical conductivity (σ) of the ceramics by about two orders of magnitude, and the sample doped with 0.1 mol% Tb4O7 showed the highest electrical conductivity. The absolute value of Seebeck coefficient (|S|) of the doped samples increased as well. Consequently, the power factor (σs 2) markedly increased. The sample doped with 2.0 mol% Tb4O7 demonstrated the maximum σs 2 of 2.88 μW m−1 K−2 at 973 K, which was larger than the highest recorded σs 2 for WO3 ceramics (2.71 μW m−1 K−2 at 1,023 K). In addition, the low-doped sample (0.1 mol%) exhibited excellent thermoelectric properties.

Enhanced contribution of surface state and modification of magnetoresistance in FexBi2−xSe3 topological insulator crystals

Magnetic atomic-doped topological insulator crystals with the formula FexBi2−xSe3 were fabricated using the self-flux method and their electrical and magnetic transport properties were studied. All samples showed weakly metallic resistivity and the resistivity increased monotonously as the Fe concentration increased. Conductivity was modified by Fe at high- and low-temperature regions via different mechanisms, which could be explained by the freezing of bulk carriers around 30 K. For highly doped samples, simultaneous crossovers in the curves of temperature dependent of resistivity (ρ-T) and temperature dependent of magnetoresistance (MR-T) at low temperature were observed, indicating that the contribution of the surface state was significantly improved. No obvious enhanced surface effect was observed in samples doped with low concentration of Fe, but the MR was enhanced significantly in low-doped samples and suppressed by high Fe content. Magnetic-field-induced metal-insulator transitions were observed in the samples with the largest MR values. The change in MR was related to three mechanisms: phonon scattering effect, magnetic ordering, and spin disorder scattering.

Crossover effect of inter-granular transport and quantum correction in Co-doping La2/3Ca1/3MnO3 manganites

The dependence of electrical transport on temperature and magnetic field is studied systemically in La2/3Ca1/3Mn1−xCoxO3 (x=0–0.15) systems between 2.5 and 300K in magnetic field ranging from 0 to 6T. The substitution of Co at Mn site results in weakening the ferromagnetism, a deterioration of the metallic conductivity. The PM–FM transition is broadened with increasing Co doping content x, and the magnetic spontaneous state has changed into a cluster glass state for the sample of x=0.15. The resistivity minima are clearly observed at low temperature (T<50K) for all the samples. An unusual modulation effect under magnetic field for low temperature resistivity minimum of the Co-doping samples is observed. The abnormal behavior of Co-doped samples is that the resistivity minimum temperature (Tmin) shifts to higher temperatures with the increase in magnetic field. It is distinctly different from undoped and low doped samples where Tmin shifts to lower temperature under higher magnetic fields. In other words, the resistivity upturn of Co-doped samples is enhanced by magnetic field instead of the general field suppression effect. The observed resistivity minimum at low temperatures is partly due to the coexistence of electron–electron interaction enhanced by an intrinsic magnetic inhomogeneity and spin polarization tunneling through grain boundaries. The electron–electron scattering due to the Coulomb interaction is the intrinsic mechanism for the low temperature resistivity upturn. Electrical resistivity behavior of the compounds as function of temperature at paramagnetic state could be explained on the basis of variable-range hopping model.

Fine structure analysis and sintering properties of Si-doped hydroxyapatite

Si-doped hydroxyapatite (Si-HA) has been intensively investigated as a promising bone biomaterial because of the biomineralization and bone formation benefit from silicon addition. In the current work, Si-HA samples were synthesized by the aqueous precipitation method. The sintering property of Si-HA was studied in terms of phase composition and transition, which was influenced by the Si doping percentage and sintering temperature. The results indicate that all the as-prepared and sintered low Si doping samples (Si% ≤ 1.6 wt%) possess HA phases; while the high Si doping samples (Si% ≥ 2 wt%) present amorphous phases as prepared, and largely or even completely transform to β-tricalcium phosphate (β-TCP) phase after sintering at 1250 °C. The Rietveld refinement on x-ray diffraction patterns was conducted to determine the particle sizes, microstrain factors and anisotropic distributions. According to the refined results, the calculated anisotropic particle sizes correspond to the observations made by transmission electron microscopy. The microstrains also present anisotropic distributions in all refinements. The accurate variation in the fine structure of Si-HA has been studied by the improved refinements after considering the anisotropic distribution.

Ho2O3 additive effects on BaTiO3 ceramics microstructure and dielectric properties

Doped BaTiO3-ceramics is very interesting for their application as PTCR resistors, multilayer ceramic capacitors, thermal sensors etc. Ho doped BaTiO3 ceramics, with different Ho2O3 content, ranging from 0.01 to 1.0 wt% Ho, were investigated regarding their microstructural and dielectric characteristics. The samples were prepared by the conventional solid state reaction and sintered at 1320? and 1380?C in an air atmosphere for 4 hours. The grain size and microstructure characteristics for various samples and their phase composition was carried out using a scanning electron microscope (SEM) equipped with EDS system. SEM analysis of Ho/BaTiO3 doped ceramics showed that in samples doped with a rare-earth ions low level, the grain size ranged from 20-30?m, while with the higher dopant concentration the abnormal grain growth is inhibited and the grain size ranged between 2- 10?m. Dielectric measurements were carried out as a function of temperature up to 180?C. The low doped samples sintered at 1380?C, display the high value of dielectric permittivity at room temperature, 2400 for 0.01Ho/BaTiO3. A nearly flat permittivity-response was obtained in specimens with higher additive content. Using a Curie-Weiss low and modified Curie-Weiss low the Curie constant (C), Curie temperature (Tc) and a critical exponent of nonlinearity (?) were calculated. The obtained value of ? pointed out that the specimens have almost sharp phase transition.

Temperature and concentration quenching of mid-IR photoluminescence in iron doped ZnSe and ZnS laser crystals

Concentration dependences of the mid-IR kinetic of luminescence at 5 E↔ 5 T 2 transition in Fe:ZnSe and Fe:ZnS laser samples were studied in 14–300 K temperature ranges. Radiation lifetime in Fe:ZnSe samples measured using low doped samples with iron concentration 0.1×10 18 cm −3 was estimated to be 57 μs. The magnetic susceptibility for higher doped ( C Fe=38 and 112×10 18 cm −3) Fe:ZnSe samples was found to consist of a paramagnetic Curie–Weiss behavior arising from the weakly interacting Fe 2+ ions and a diamagnetic ZnSe contribution plus a temperature-independent, field-dependent contribution possibly arising from very small amounts of aggregated Fe.

Enhancement of room-temperature magnetoresistance in La0.6Dy0.1Sr0.3MnO3/Ag x

The samples of La0.6Dy0.1Sr0.3MnO3/(Ag2O)x/2 (x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10, 0.20, 0.25, and 0.30) were prepared by using the solid-state reaction method. Their magnetic property, transport behavior, transport mechanism and magnetoresistance effect were studied through the measurements of magnetization-temperature (M-T) curves, ρ-T curves and the fitting of ρ-T curves. The results indicated that Ag could take part in the reaction when the doping amount is small. However, when the doping amount is comparatively large, Ag as metallic state mainly deposits on the grain boundary of La0.6Dy0.1Sr0.3MnO3, and then the system forms a two-phase composite. When the Ag doping amount is 30% mole ratio, the resistivity of the sample is one order of magnitude smaller than that of low doped samples, and its peak of magnetoresistance at 292 K and in the magnetic field of 0.2 T strengthens apparently and reaches 16.3%, which is over 7 times as large as 2.2% of La0.6Dy0.1Sr0.3MnO3. The two-phase composite system of magnetoresistance based on perovskite manganite consists of two parts: intrinsic magnetoresistance and extrinsic magnetoresistance. However, extrinsic magnetoresistance comes from spin-dependent scattering (SDS) and spin-polarized tunneling (SPT). Magnetoresistance near TC increases due to the contribution of intrinsic magnetoresistance and extrinsic magnetoresistance formed by SDS, and magnetoresistance at low temperature is extrinsic magnetoresistance formed by SPT.

Photomodulation raman spectroscopy as a pump-probe technique for surfaces and interfaces of semiconductors

Photomodulation Raman spectroscopy (PM-RS) has been employed to study the surface depletion electric field Es and to monitor the change in surface charge density in n-type GaAs, using the forbidden LO phonon scattering for low doping samples and coupled plasmon–LO phonon modes for high doping samples. In PM-RS, the photomodulating pumping beam (PB) is incident on the sample while the Raman measurements are in progress hence PM-RS can be viewed as a pump-probe technique. The photogenerated carriers partly neutralize the surface charges. Two different GaAs surfaces (011) with low and moderate doping density and (001) with high doping density were used. The total surface charge density has been obtained as a function of the PB intensity considering a constant depletion electric field for the lower doping sample of (011) surface and using the dependence of the unscreened LO phonon on the depletion width for the higher doping samples of (001) surface. The minority carrier's lifetime was also determined through dynamical measurements for the PM-RS of the low doping sample as ≈ 21 s, in a good agreement with other techniques

Enhancement of room-temperature magnetoresistance in La0.5Sm0.2Sr0.3MnO3/(Ag2O) x/2

La0.5Sm0.2Sr0.3MnO3/(Ag2O)x/2 (x = 0.00, 0.04, 0.08, 0.25, 0.30) samples were prepared by the solid-state reaction method, and their transport behaviors, transport mechanism, and magnetoresistance effect were studied through the measurement and fitting of ρ-T curves. The results show that the element Ag takes part in reaction when the doping amount is small. Ag is mainly distributed at the grain boundary of the host material and is in metallic state when the doping amount is relatively large; then the system becomes a two-phase composite. A small amount of Ag doping can apparently increase grain-boundary magnetoresistance induced by the spin-dependent scattering. The resistivity of the sample doped with 30 mol% Ag is one order of magnitude smaller than that of low-doped samples, and its magnetoresistance in the magnetic field of 0.5 T and at 300 K is strengthened apparently reaching 9.4%, which is connected not only with the improvement of the grain-boundary structure of the host material but also with the decrease of material resistivity.