Electrical transport in heavy rare-earth vanadates

This paper reports measurements of electrical conductivity () and Seebeck coefficient (S) between 300 and 1250 K and differential thermal analysis (DTA) and thermogravimetric analysis (TGA) between 300 and 1200 K, together with X-ray diffraction studies of heavy rare-earth vanadates (RVO₄ with R=Tb, Dy, Ho, Er and Yb). All these vanadates have been found to have a tetragonal unit cell. The DTA study shows a flat dip in the temperature interval 1075 to 1300 K, indicating a possible structural phase transition of these compounds. Practically no weight loss has been observed in TGA from 300 to 1200 K in any of the vanadates. All RVO₄ are semiconducting materials with the room-temperature value lying in the range 10^–12 to 10^{–3 –1} m^–1, becoming of the order of 10^{–2 –1} m^–1 around 1000 K. The electrical conductivity of all vanadates exhibits an exponential increase in the temperature intervals 420 K toT ₁ andT ₁ toT ₂, with different values of the activation energy. A log againstT ^–1 plot shows a peak aroundT ₃ and drops to a minimum value aroundT ₄, before increasing again with temperature.T ₄ >T ₃ >T ₂ >T ₁ are different for different vanadates and these are termed break temperatures.T ₄ lies well within the temperature range of the DTA peak and can be termed the phase transition temperature. In the lower temperature interval the electrical conduction is essentially extrinsic. The localized charge carriers on defect centres conduct by a hopping mechanism. The defect centres are V⁴⁺ ions in all vanadates with R⁴⁺ centres in some of them. It is concluded that in the temperature intervalT ₁ <T <T ₂ the conduction mechanism is of the intrinsic band type, with oxygen 2p and vanadium 3d as the valence and conduction bands, respectively. Related parameters like the energy band gap and the mobilities of the charge carriers have also been evaluated. The low values of mobility suggest that large polarons with intermediate coupling are the charge carriers rather than bare electrons in the intrinsic region. All these vanadates tend to become metallic, but before this is achieved the phase change makes the conductivity smaller.     

Electrical transport in light rare-earth orthochromites

Electrical conductivity, , Seebeck coefficient, S, and dielectric constant, , measurements on the pressed pellets of six light rare-earth orthochromites, RCrO₃, where R = La, Pr, Nd, Sm, Eu and Gd, have been carried out in the temperature range 300 to 1000 K. These are essentially electronic conductors, exhibiting p-type extrinsic semiconducting nature in the studied temperature range. The extrinsic charge carriers (holes) originate from Cr⁴⁺ centres which are present due to native defects in these solids. Their room-temperature electrical conductivities lie in the range 10^–7 to 10^{–5 –1} cm^–1, which become of the order of 10^{–2 –1} cm^–1 near 1000 K. The conductivity is a maximum in LaCrO₃ and drops across the RCrO₃ series, with SmCrO₃ being an exception. The mechanism involved in the electrical transport is the hopping of holes from Cr⁴⁺ centres to neighbouring Cr³⁺ ions. The activation energy of transport is nearly 0.3 eV. Typical hopping mobility of the holes is of the order of 10² cm^–2 V^–1 sec^–1 at 325 K and of the order of 10 cm² V^–1 sec^–1 at 1000 K. The mobility activation energy of the holes in a typcial RCrO₃ decreases with temperature due to the smoothing of the potential barriers between Cr⁴⁺ and Cr³⁺ sites. Several discontinuities are observed in the T against T ^–1 and S against T ^–1 plots of RCrO₃. The anomalies which these discontinuities reflect here have also been indicated.     

Electrical transport in light rare-earth molybdates

Rare-earth molybdates of the type R₂(MoO₄)₃ with R=La, Ce, Pr, Nd, Sm and Eu were prepared and characterized, and the electrical conductivity, and Seebeck coefficient, S in the temperature range 450–1200 K were measured. These molybdates are concluded to be insulating solids with a band gap which increases slowly going down the series from 2.30 eV for La molybdate to 3.20 eV for Eu molybdate. The plots of log and S versus T ^–1 show, in general, three linear regions with two break temperatures T ₁ and T ₂ occurring due to a change in the conduction mechanism. At higher temperatures the intrinsic conduction in these solids occurs via a band mechanism. The O^2– 2p and Mo⁶⁺ 4d orbitals form the valence and conduction bands, respectively. These bands are the main support of conduction in La, Sm and Eu molybdates; however, for Ce, Pr and Nd molybdates 4f ⁿ levels fall within the band gap and become very effective in electrical conduction. The main charge-carrying entities seem to be electrons in Ce, Pr and Nd molybdates and holes in La, Sm and Eu molybdates. On the basis of mobility calculations of charge carriers it is concluded that the charge carriers in these bands become polarons which are, in fact, the charge carrying entities. At lower temperatures electrical conduction is mainly extrinsic. Cerium molybdate shows a semiconductor-semimetal transition around 940 K.     

Electrical transport in rare-earth orthochromites

The electrical conductivity (σ) and Seebeck coefficient (

) measurements on pressed pellets of La, Nd and Sm orthochromites are reported for the temperature range 300 to 1000K. These orthochromites exhibit extrinsic, p-type conduction in the whole temperature range. In both measurements, breaks define a transition temperature (T_B), which has been predicted to be due to some dielectric anomaly. Seebeck coefficient measurements show that thermal generation of charge carriers begins above T_B.     

Electrical conduction in light rare-earth tungstates

This paper reports the results of electrical conductivity (σ) and thermoelectric power (H) of light rare-earth tungstates in the temperature range 600–1200K. Holes are the dominant charge carriers over the whole studied temperature range for Nd, Sm and Gd tungstates. However, in the case of La, Ce and Pr tungstates, the conduction is dominated by electrons at lower temperatures, but above 950K in La, 800K in Ce and 950K in Pr the dominant charge carriers become holes. A sharp break and change in the log σ vs 1/T slope occurs in La, Ce and Pr tungstates around the same temperature at which the dominant charge carrier changes from electrons to holes. In the case of Nd, the conductivity anomaly occurs around 1020K without any change in the nature of the charge carrier. The data have been analysed using band theory.     

Electrical transport in heavy rare-earth iron garnets

The measurements of electrical conductivity () in the temperature range 450 to 1250 K and thermoelectric power (S) in the temperature range 600 to 1200 K of sintered pressed pellets of rare-earth iron garnets (REIG) with a general chemical formula RE₃Fe₅O₁₂ (where RE=Y, Gd, Dy, Ho, Er and Yb) are reported. Values corresponding to the crystalline state have been evaluated employing pore fraction correction. It is observed that plots of log T againstT ^–1 are linear with breaks in the slopes of temperatureT ₁ (lying between 560 and 578 K) andT ₂ (~ 1000 K). However, plots ofS againstT ^–1 are linear over the entire temperature range. The results have been discussed using the usual electrical transport theories and it has been concluded that electrical conduction in these solids up to a temperature of 1250 K is extrinsic in which holes localized on Fe³⁺ sites (Fe⁴⁺ centres created by native defects) conduct via a thermally activated hopping mechanism. Mobility activation energy, mobility and the number of such centres in each garnet are also evaluated.     

Electrical transport studies on heavy rare-earth tungstates

This paper reports the measurement of thermoelectric-power (S) at different temperatures (800–1100 K) and electrical conductivity (σ) as a function of electric field strength, time, ac signal frequency and temperature (650–1200 K) for pressed pellets of heavy rare-earth tungstates (HRET) with a general formula RE₂(WO₄)₃ [where RE = Tb, Dy, Ho, Er, Tm and Yb]. These tungstates are typical insulating compounds with room-temperature σ value less than 10^?10 ohm^?1 cm^?1 and become semiconductors at elevated temperature with σ values of the order of 10^?5 ohm^?1 cm^?1 at 1200 K. The S vs T^?1 and log σ vs T^?1 plots are linear, but a change in the slope of straight lines occurs at a temperature (T_B) which lies between 900–1025 K for different tungstates. These break temperatures are the same for both S and σ plots. It has been found that HRET are mixed ionic-electronic conductors. Above T_B the electronic conduction dominates over the ionic conduction, but below T_B both become comparable. The electronic conduction above T_B is intrinsic with large polaron holes as the principal charge carriers; they conduct via a band mechanism. The energy band gap lies in the range 3.2 to 4.0 eV, and the charge carrier mobility in the range 3.8×10^?2 to 2.5 cm²/V-Sec for the different tungstates. Below T_B both electronic and ionic conduction are extrinsic. The electrons conduct via a thermally activated hopping mechanism with an activation energy lying in the range 0.87 to 1.30 eV, and the holes via a diffusion process with an activation energy lying in the range 0.88 to 1.23 eV for the different tungstates.     

Electrical transport mechanisms of Neodymium-doped rare-earth semiconductors

Journal of Materials Science: Materials in Electronics - This study reports the electrical properties of Nd-doped cerium oxide (CeO2) films synthesized by microwave assisted hydrothermal using a...     

Electrical conduction in non-metallic rare-earth solids

Schematic energy band diagrams for the genesis of charge carriers in non-metallic rare-earth solids have been presented. It has been shown that positions of 4f bands have significant effect on the genesis and nature of charge carriers, their conduction mechanism and magnitude of electrical conductivity () and Seebeck coefficient (S) of the solid. Relevant relations have been given for both and S in different situations. Experimental data on rare-earth sesquioxides (R₂O₃), rare-earth tungstates [R₂(WO₄)₃] and rare-earth molybdates [R₂(MoO₄)₃] in the intrinsic range have been explained as examples for the validity of energy band diagrams.     

Electrical transport properties of fullerene peapods interacting with light

In this paper, we investigate the transport properties of field-effect transistors (FETs) based on various kinds of fullerene (C(60), C(70), and C(84)) peapods. The encapsulation of various fullerenes inside single-walled carbon nanotubes (SWNTs) is characterized by TEM and Raman spectroscopy. Our results indicate that the transport characteristics of p-type SWNTs are extremely sensitive to the encapsulation of various kinds of fullerene. In the absence of light illumination, the threshold voltage of p-type SWNTs shifts towards positive values after C(60) and C(70) encapsulation, and ambipolar transport characteristics are observed for C(84) peapods. The photoinduced electron transfer phenomenon is observed for fullerene peapods under light illumination. The optical response for C(60) and C(70) peapod FET devices is reflected in a shift of threshold voltage towards negative values, and a recoverable characteristic is observed when light is off. After a long period of light illumination, in contrast to p-type C(60) and C(70) peapods, an n-type transport characteristic is observed on C(84) peapods.     

Electrical transport in magnesium aluminate

The conductivity of MgAl₂O₄ has been measured at 1273, 1473 and 1673 K as a function of the partial pressure of oxygen ranging from 10⁵ to 10⁻¹⁴ Pa. The MgAl₂O₄ pellet, sandwiched between two platinum electrodes, was equilibrated with a flowing stream of either Ar + O₂, CO + CO₂ or Ar + H₂ + H₂O mixture of known composition. The gas mixture established a known oxygen partial pressure. All measurements were made at a frequency of 1 kHz. These measurements indicate pressure independent ionic conductivity in the range 1 to 10⁻¹⁴ Pa at 1273 K, 10⁻¹ to 10⁻¹² Pa at 1473 K and 10⁻¹ to 10⁻⁴ Pa at 1673 K. The activation energy for ionic conduction is 1·48 eV, close to that for self-diffusion of Mg²⁺ ion in MgAl₂O₄ calculated from the theoretical relation of Glyde. Using the model, the energy for cation vacancy formation and activation energy for migration are estimated.     

Electrical transport in cermet-type materials

An analytical method for determining the energy dependence of the electrical conductivity in poorly conducting materials is presented and applied to samples of Nb-Al₂O₃ cermet films. It is shown that the low temperature dependence of the resistivity is due to a set of discrete conductivity states which are narrow in energy and which lie close to the Fermi energy.     

Electrical transport in doped one-dimensional nanostructures

Mobility and noise are two important issues for electronic devices, and they have many new features in one-dimensional (1D) doped nanostructures. For the convenience of readers the background of solid state physics is reviewed first, and then the transport process in 3D crystal material is introduced. Velocity saturation is an important phenomenon in modern electronic devices, and it is analyzed in an intuitive approach. It is predicted FinFET will be the next generation MOSFET, and its structure and characteristics are introduced. With the reduction of device dimensions the mesoscopic phenomena begin to show up. A simple way to treat transport problem in this domain is the Landauer-Büttiker formula, and the basic equation is derived. Finally the 1D quantum wire structure grown from a bottom-up approach is reviewed. Owing to the good material quality the scattering is very weak, and the wave properties of the coherent transport are discussed. Engineering applications of nanostructures in electronic information processing that manipulates time varying signals often involve device characterizations in the time domain. Since carrier transport in nanostructures is inherently a random process and it causes random fluctuations in quantities like current and voltage, so background knowledge in the microscopic origins of noise and other related practical issues is important to identify enough noise margins for reliable system design. This subject is the focus of the second part of the review article.     

Electrical transport in ethyl cellulose-chloranil system

The charge-transport behaviour in pure and chloranil (Chl) doped ethyl cellulose (EC) system has been studied by measuring the dependence of current on field, temperature, electrode material and dopant concentration. The role of doping molecular concentration in the polymer matrix and modification in the conduction characteristics are studied. Lowering of the activation energy due to doping was observed. The current was found to increase with an increase in the chloranil concentration. An explanation for this has been attempted on the basis of formation of molecular aggregates between chloranil molecules and ethoxy groups of ethyl cellulose. It is suggested that chloranil occupies interstitial positions between the polymer chains and assists in carrier transportation by reducing the hopping barriers. The current-voltage characteristics of different samples are analyzed using space charge limited current theory and quantitative information about the transport parameters is derived. The values of effective drift mobility and trapped charge carrier concentration which result in the build up of space charge have been calculated.     

Pyroelectric and dielectric properties of some light rare-earth orthochromites

The measurement of pyroelectic coefficient (p) and dielectric constant (K) of rare-earth orthochromites RCrO₃, where R=praseodymium, neodymium, samarium, europium and gadolinium, have been reported in the temperature range 300 to 450 K. Pyroelectric data show that praseodymium and neodymium orthochromites do not have ferroelectric phase in this temperature range, whereas samarium, europium and gadolinium orthochromites are ferroelectric. The dielectric data support the above conclusion. Spontaneous polarization (P _s) for samarium, europium and gadolinium have also been evaluated. The maximum values forP _s are 4.6 × 10^–3 C m^–2 for SmCrO₃, 0.42 C m^–2 for EuCrO₃ and 0.72 C m^–2 for GdCrO₃.     

Electrical transport in copper chromite catalyst

The electrical conductivity, σ, and thermoelectric power, S, of copper chromite (CuCr₂O₄) are reported in the temperature range 295 to 815 K. A break (T _B) in the slope of the plot of log σ against T ^?1 was observed around 556 K. Apart from this break, the curves are linear, and their slopes correspond to activation energies of 0.60 eV (T _B<556 K) and 1.22 eV (T _B>556 K). A break (T′ _B) in the slope of the -S against T ^?1 plot was also observed round 556 K. Apart from the break at this temperature, the S against T ^?1 curves are linear. At T′ _B>556, S can be expressed by the relation $$S = - \left[ {\left( {\frac{{0.65 eV}}{{{\text{2}}eT}}} \right) + (0.42) mV K^{ - {\text{1}}} } \right]$$ The mechanism involved in the electrical transport is the hopping of holes from Cr⁴⁺ centres to neighbouring Cr³⁺ ions. The typical hopping mobility of the holes is of the order of 10⁶ m² V^?1 sec^?1. The mobility activation energy of the holes in CuCr₂O₄ decreases with temperature due to the smoothing of the potential barriers between Cr⁴⁺ and Cr³⁺ sites.     

Electrical transport properties of III-nitrides

Excellent n-type GaN layers have been grown by all of the major epitaxial techniques: MBE, MOCVD, and HVPE. In this work, we analyze the band conduction in such samples by temperature-dependent Hall-effect measurement and theory, and determine quantitative information on donor and acceptor concentrations, as well as donor activation energies. In HVPE layers it is necessary to take account of a degenerate n-type layer at the GaN/sapphire interface in order to correctly analyze the bulk material. We also investigate hopping conduction, which occurs at low temperatures in conductive material, and at both low and high temperatures in semi-insulating material. Finally, we show by analysis of electron-irradiation data that both the N vacancy and the N interstitial are electrically active, demonstrating donor and acceptor character, respectively.