High-temperature Electronic Transport Properties Research Articles

High-Temperature Electronic Transport Properties of PEDOT:PSS Top-Contact Molecular Junctions with Oligophenylene Dithiols

In this study, we investigated the high-temperature electronic transport behavior of spin-coated PEDOT:PSS top-contact molecular ensemble junctions based on self-assembled monolayers (SAMs) of oligophenylene dithiols. We observed irreversible temperature-dependent charge transport at the high-temperature regime over 320 K. The effective contact resistance and normalized resistance decreased with increasing temperature (320 to 400 K), whereas the tunneling attenuation factor was nearly constant irrespective of temperature change. These findings demonstrate that the high-temperature transport properties are not dominated by the integrity of SAMs in molecular junctions, but rather the PEDOT:PSS/SAMs contact. Transition voltage spectroscopy measurements indicated that the contact barrier height of the PEDOT:PSS/SAMs is lowered at elevated temperatures, which gives rise to a decrease in the contact resistance and normalized resistance. The high-temperature charge transport through these junctions is also related to an increase in the grain area of PEDOT cores after thermal treatment. Moreover, it was found that there was no significant change in either the current density or normalized resistance of the annealed junctions after 60 days of storage in ambient conditions.

Electronic Structure and Transport Properties of La<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> Pyrochlore from First Principles

The first principle calculation as well as the Boltzmann transport calculation have been employed to study the high temperature electronic transport properties of pyrochlore La2Zr2O7. Combing constant scattering time approximation and experiment data, the electronic thermal conductivity and electron concentration are calculated as a function of temperature. The electronic thermal conductivity is 2.6×10-4 W/(m.s) at 1270K and 7.2×10-3 W/(m.s) at 1770K. The electron concentration increase rapidly with when the temperature is above 1600K.

New insights on the synthesis and electronic transport in bulk polycrystalline Pr-doped SrTiO3−δ

Recently, we have reported a significant enhancement in the electronic and thermoelectric properties of bulk polycrystalline SrTiO3 ceramics via praseodymium doping. This improvement was originated from the simultaneous enhancement in the thermoelectric power factor and reduction in thermal conductivity, which was contributed to the non-uniform distribution of Pr dopants. In order to further understand the underlying mechanism, we herein investigate the role of praseodymium doping source (Pr2O3 versus Pr6O11) on the synthesis and electronic transport in Pr-doped SrTiO3 ceramics. It was observed that the high-temperature electronic transport properties are independent of the choice of praseodymium doping source for samples prepared following our synthesis strategy. Theoretical calculations were also performed in order to estimate the maximum achievable power factor and the corresponding optimal carrier concentration. The result suggests the possibility of further improvement of the power factor. This study should shed some light on the superior electronic transport in bulk polycrystalline Pr-doped SrTiO3 ceramics and provide new insight on further improvement of the thermoelectric power factor.

High-temperature electronic transport properties of (YCa)BaCo4O7 compounds

Y1−xCaxBaCo4O7 (0.0≤x≤1.0) samples were prepared by the solid-state reaction method and their high-temperature electronic transport properties were investigated in nitrogen and oxygen respectively. Phase structure of Y1−xCaxBaCo4O7 transforms from hexagonal symmetry for x ≤0.6 samples to orthorhombic symmetry for x≥0.8 samples. In nitrogen, Y1−xCaxBaCo4O7 samples evolve three kinds of electronic transport behaviors with the increase of Ca content: thermal activation conduction, small polaron hopping conduction, and a possible mixed conduction. Ca doping increases the hole concentration and thus decreases Seebeck coefficients. In oxygen, the temperature dependence of electrical resistivity and Seebeck coefficients of Y1−xCaxBaCo4O7 samples displays similar change to their respective thermogravimetric curve, showing their electronic transport behavior under the control of their oxygen adsoption/desorption process.

Electron transport in CaMnO3 − δ at elevated temperatures: a mobility analysis

The drift mobility of electron charge carriers in oxygen non-stoichiometric manganite CaMnO3−δ was cal- culated by combining the total electrical conductivity and oxygen non-stoichiometry data at 700-950 °С and oxygen partial pressure varying between 10 −6 and 1 atm. The carrier concentration changes with pressure and temperature were obtained with the help of the earlier-developed defect model involving reactions of oxygen exchange and thermal exci- tation of manganese sites. The activation energy for mobil- ity is found to increase with oxygen non-stoichiometry. High-temperature electron transport properties of the man- ganite CaMnO3−δ can be explained in terms of activated jumps of n-type small polarons in adiabatic regime. The relatively small mobility of charge carriers is explained by strong localization of polarons on manganese sites.

High temperature electron transport properties of AlGaN/GaN heterostructures based on unintentionally doped and semi-insulating GaN buffer layer

High temperature electron transport properties of Al0.35Ga0.65N/GaN heterostructures grown on unintentionally doped (UID) and semi-insulating (SI) GaN buffer layer (BL) have been investigated. Electron sheet density and electron mobility dependence on temperature have been systematically measured by Hall effect measurement. It is found that the electron mobility of AlGaN/GaN heterostructure is mainly limited by the longitudinal optical (LO) phonon scattering at high temperature. The sheet carrier density of AlGaN/GaN heterostructure based on UID-BL increases gradually with the increase of temperature, while it decreases firstly, and then increases for the sample based on SI-BL. It is indicated by the analysis of the high temperature resistance properties for the corresponding UID-GaN and SI-GaN films, that the effect of the background carrier-limited electron mobility increases gradually for the UID-BL sample, while for the SI-BL sample the electron mobility is mainly limited by the additional dislocation scattering around room temperature but the background carrier limited effect increases after 600 K. This is meaningful for investigating the high temperature property of the devices based on AlGaN/GaN heterostructure. Moreover, the theoretical calculation indicates that, the two dimensional gas (2DEG) has expanded to the AlGaN barrier layer and the deeper position of the GaN buffer layer, and the electron occupation in the first subband decreases to 81% at 700 K.

High temperature electron transport properties of AlGaN/GaN heterostructures based on unintentionally doped and semi-insulating GaN buffer layer

High temperature electron transport properties of AlGaN/GaN heterostructures based on unintentionally doped and semi-insulating GaN buffer layer

High temperature electron transport properties of AlGaN/GaN heterostructures with different Al-contents

Electron transport properties in AlGaN/GaN heterostructures with different Al-contents have been investigated from room temperature up to 680 K. The temperature dependencies of electron mobility have been systematically measured for the samples. The electron mobility at 680 K were measured as 154 and 182 cm2/V·s for Al0.15Ga0.85N/GaN and Al0.40Ga0.60N/GaN heterostructures, respectively. It was found that the electron mobility of low Al-content Al0.15Ga0.85N/GaN heterostructure was less than that of high Al-content Al0.40Ga0.60N/GaN heterostructure at high temperature of 680 K, which is different from that at room temperature. Detailed analysis showed that electron occupations in the first subband were 75% and 82% at 700 K for Al0.15Ga0.85N/GaN and Al0.40Ga0.60N/GaN heterostructures, respectively, and the two dimensional gas (2DEG) ratios in the whole electron system were 30% and near 60%, respectively. That indicated the 2DEG was better confined in the well, and was still dominant in the whole electron system for higher Al-content AlGaN/GaN heterostructure at 700 K, while lower one was not. Thus it had a higher electron mobility. So a higher Al-content AlGaN/GaN heterostructure is more suitable for high-temperature applications.

Microstructure and high temperature transport properties of high quality epitaxial SrFeO 3 − δ films

We report the high temperature electronic transport properties of SrFeO 3 − δ epitaxial thin films obtained by pulsed laser deposition on NdGaO 3(110) substrates. The films show total conductivity higher than the bulk material and apparent activation energy of about 0.12 eV in O 2, lower than reported values for SrFeO 3 − δ films. The conductivity dependence with oxygen partial pressure shows a power dependence with an exponent close to + 1/4, in agreement with expected point defect equilibrium. For a given oxygen partial pressure, the temperature coefficient of resistance (TCR) shows a low positive value of about 1.5–2.5 10 − 3 K − 1 , which is still suitable for resistive oxygen sensing applications. The transport properties of the films are discussed in view of their particular microstructure.

High-temperature electronic transport properties of Fe-doped YBaCo 2O 5+δ

The resistivity and Seebeck coefficients of YBaCo 2− x Fe x O 5+ δ (0⩽ x⩽0.8) were measured from room temperature (RT) to 800 °C. It was found that all compositions behave as p-type semiconductors in the temperature range of RT–300 °C. When temperature is above about 300 °C, both electrical resistivity and Seebeck coefficients increase with the increase of temperature due to oxygen desorption from the lattices. Fe doping tends to increase resistivity and Seebeck coefficients in all the temperature ranges measured. The calculated activation energy of conductivity is larger than that of thermopower in the semiconductor region, which means that the conduction mechanism may be determined as a small polaron hopping model.