Temperature-dependent Electronic Transport Properties Research Articles

Correlation between charge transport and lattice dynamics in La- and Y-doped Ca2MnO4 perovskites

Transition metal oxides exhibit fascinating physical phenomena and hold potential for next-generation electronic devices; therefore, fundamental understanding of their conduction mechanisms is of prime technological importance. We investigate charge transport kinetics and elastic behavior of bulk polycrystalline Ca2-xRxMnO4 oxides, where R = Y or La and 0.01 ≤ x ≤ 0.20. Analysis of temperature dependent electronic transport properties considering the nearest neighbor small polaron hopping model in combination with first-principles calculations of elastic properties indicates tight correlation between charge transport and lattice elasticity. We find that the polaron hopping energies of Y-doped compounds are lower than their La-doped counterparts, and attain minimum values of 60 and 73 meV, respectively. This is associated to softening of interatomic bonds, which is more pronounced for Y-doping compared to La-doping. This is corroborated by a fundamental model for elastically isotropic materials, indicating that the elastic energy induced by small polarons corresponds with the measured polaron hopping energies. Accordingly, the lattice shear and Young moduli, sound velocities and Debye temperatures calculated for Y-doped cells are smaller than for La-doped ones. The non-monotonous dependence of hopping energies on dopant concentration is explained in terms of polaron-polaron separation and mutual electrostatic interactions across the Mn+3-O-Mn+4 conduction path. Our study shows how charge carrier dynamics are strongly coupled with elastic properties, implying that the latter can be used to predict charge carrier mobility in oxide semiconductors.

Weighted Mobility Ratio Engineering for High-Performance Bi-Te-Based Thermoelectric Materials via Suppression of Minority Carrier Transport.

Thermoelectrics, which can generate electricity from a temperature difference, or vice versa, is a key technology for solid-state cooling and energy harvesting; however, its applications are constrained owing to low efficiency. Since the conversion efficiency of thermoelectric devices is directly obtained via a figure of merit of materials, zT, which is related to the electronic and thermal transport characteristics, the aim here is to elucidate physical parameters that should be considered to understand transport phenomena in semiconducting materials. It is found that the weighted mobility ratio of the majority and minority carrier bands is an important parameter that determines zT. For nanograined Bi-Sb-Te alloy, the unremarked role of this parameter on temperature-dependent electronic transport properties is demonstrated. This analysis shows that the control of the weighted mobility ratio is a promising way to enhance zT of narrow bandgap thermoelectric materials.

Temperature dependent electronic transport properties of heterojunctions formed between perovskite SrTiO3 nanocubes and silicon

In this study, synthesized strontium titanate (SrTiO3) nanocubes were coated on n-Si semiconductor by spin coating to obtain a heterojunction device. Transmission electron microscopy image, scanning electron microscope image and x-ray diffraction patterns of thin film of SrTiO3 nanocubes coated on Si surface were taken for structural and morphological characterization of the material. The basic device parameters such as ideality factor (n) and barrier height (Φb) values of the reference Ni/Si/Al metal–semiconductor diode and of the Ni/SrTiO3/n-Si/Al heterojunction devices were calculated with the thermionic emission (TE) theory. The n and Φb values of the reference Ni/n-Si/Al device were calculated as 1.93 and 0.60 eV, respectively at room temperature. On the other hand, lower n and higher Φb values of Ni/SrTiO3/n-Si/Al heterojunction device were calculated as 1.34 and 0.63 eV, respectively. With these results, the current–voltage (I–V) measurements of the heterojunction device in the 80–400 K range were taken and the n, Φb, series resistance (Rs) values were calculated depending on the temperature by using different methods such as TE, Cheung and Norde functions. It was observed that while the temperature values decreased, n and Rs values of the device increase and Φb value decreases. The results obtained showed that the charge transport system is compatible with TE. The device parameters calculated from the Cheung and Norde methods also showed similar changes depending on the temperature. However, since the calculation method is different according to the TE method, different values were obtained in the device parameters. In addition, the curve Φb and (1/n)−1 against (1/2kT) was observed in accordance with the double Gauss distribution of the barrier heights. It was seen that the reverse bias I–V characteristics of the Ni/SrTiO3/n-Si/Al can be used in thermal sensors applications.

Fully Relativistic Temperature-Dependent Electronic Transport Properties of Magnetic Alloys From the First Principles

Ab initio calculations based on the fully relativistic Dirac approach and incorporating chemical disorder and temperature-induced atomic displacements (phonons) are presented. The tight-binding linear muffin-tin orbital method is used, and the multicomponent coherent potential approximation deals with the chemical disorder and phonons on the same level. Electrical resistivities calculated without and with spin–orbit interaction are compared, and the anomalous Hall effect for magnetic alloys is investigated. The developed technique is tested on pure nickel and on random binary Cu–Ni alloys. The calculated results are found to be in good agreement with experimental and other theoretical data. The combined effect of phonons and spin disorder, simulated within the disordered local moment state, has also been studied. The results confirm the validity of Matthiessen’s rule in a wide range of temperatures for the electrical resistivity of pure nickel.

Effect of Surface Passivation and Substrate on Proton Irradiated AlGaN/GaN HEMT Transport Properties

In this work, we fabricate Van der Pauw structures on AlGaN/GaN high electron mobility transistors (HEMTs) with both SiN and AlN passivation layers and both SiC and GaN substrates to study temperature dependent electron transport properties of 2 dimensional electron gases (2DEG) in as-fabricated and proton irradiated devices. We confirm that using a GaN substrate or AlN passivation layer enhances the mobility particularly at low temperatures, though this enhancement is greatly reduced upon irradiating the samples with high proton fluences. Our results suggest that the sheet density may be affected by Al diffusion in the sample creating electron donating impurities, which freeze out around 200 K and are more prominent in AlN passivated samples and minimized by using a GaN over a SiC substrate. Additionally, we show that a large dose of radiation forms traps in the AlGaN layer, which drop the sheet density at high proton fluences.

Effects of nanoscale surface roughness on the resistivity of ultrathin epitaxial copper films

The knowledge on the influence of surface roughness and the electron–phonon (el–ph) interaction on electrical transport properties of nanoscale metal films is important from both fundamental and technological points of view. Here we report a study of the temperature dependent electron transport properties of nanoscale copper films by measuring temperature dependent electrical resistivity with thickness ranging from 4 to 500 nm. We show that the residual resistivity, which is temperature independent, can be described quantitatively using both measured vertical surface root-mean-square roughness and lateral correlation length in the nanoscale, with no adjustable parameter, by a recent quasi-classical model developed by Chatterjee and Meyerovich (2010 Phys. Rev. B 81 245409–10). We also demonstrate that the temperature dependent component of the resistivity can be described using the Bloch–Grüneisen equation with a thickness dependent el–ph coupling constant and a thickness dependent Debye temperature. We show that the increase of the el–ph coupling constant with the decrease of film thickness gives rise to an enhancement of the temperature dependent component of the resistivity.

Field and temperature dependent electron transport properties of random network single walled and multi walled carbon nanotubes

Field and temperature dependent electron transport properties of random network single walled carbon nanotubes (SWCNTs) and multi walled carbon nanotubes (MWCNTs) were investigated and compared. The electrical characterizations of drop casted CNT samples were done by two probe measurements by varying temperatures from 80 K to 300 K in the field region 1–400 V cm−1. The charge transport mechanisms at low (<3.5 V) and high (>3.5 V) fields were analyzed from measured I–V characteristic curves at various temperatures (<300 K) with respect to applied field. At low field, the ohmic behavior was observed and at high field the charge transport appears to be Poole–Frenkel type in both types of CNTs network. Electron–electron and electron–phonon scatterings in the localized defect states dominate in SWCNTs, whereas in MWCNTs the delocalization of charge carriers as well as the scattering centers is responsible due to the presence of inner shells. Because of the different nature of chirality in random network, the SWCNTs displayed lower conduction when compared to MWCNTs. The variation in Poole–Frenkel co-efficient (β) (SWCNTs-0.193 × 10−22; MWCNTs-0.089 07 × 10−22 J V1/2 cm−1/2), activation energy (SWCNTs-90 meV; MWCNTs-60 meV for field of ) and trap energy levels (SWCNTs-109 meV; MWCNTs-37 meV) are discussed for both SWCNTs and MWCNTs.

Temperature-dependent electron transport in ZnO micro/nanowires

Temperature dependent electron transport properties of individual ZnO mirco/nanowires from 293 K to 473 K were investigated by a four-probe method, and the size dependence of activation energy was studied. The conductivity increased with the increasing temperature, which was attributed to the thermal activations of donors: shallow donors and deep donors. The activation energy related to shallow donors was effectively independent of the radius, suggesting that it was associated with the surface conduction channel of ZnO wire, while a nearly inverse dependence of the activation energy of deep donors on the wire diameter may result from the dielectric confinement effect.

Synthesis of reduced graphene oxide–Fe 3O 4 multifunctional freestanding membranes and their temperature dependent electronic transport properties

A simple and scalable method for the synthesis of reduced graphene oxide (RGO) based conductive and magnetic multifunctional films (membranes) is reported. A RGO–iron oxide (Fe 3O 4) freestanding film is fabricated using a versatile chemical route followed by vacuum infiltration. Temperature dependent electronic transport properties of the magnetic GO and RGO films were measured using a four probe technique from room temperature to 15 K. A conduction mechanism based on variable range hopping is suggested for explaining of the electronic conductivity variations. Possible applications of this multifunctional membrane are also discussed.

Deep donor state in InN: Temperature-dependent electron transport in the electron accumulation layers and its influence on Hall-effect measurements

Temperature-dependent electron transport properties in electron accumulation layers of InN are quantified by using the multilayer model. Room temperature electron densities in the electron accumulation layers are 5.83 × 1013 and 3.26 × 1014 cm−2 with Hall mobilities of 429 and 149 cm2/Vs in In- and N-polarity InN, respectively. A deep donor state with an activation energy of ∼80 meV, which is previously believed to exist in the InN bulk layer, is found to actually locate in the electron accumulation layers. The InN bulk layer shows single-shallow-donor behavior and its temperature-dependent electron mobility is in good agreement with the ensemble Monte Carlo simulation results.