Temperature Dependence Of Electron Transport Research Articles

Analysis of the high-temperature transport property in GaN-based single and double heterostructures

GaN-based heterostructures have been proved to be excellent candidates for high-voltage, large-power, high-frequency and high-temperature application fields, thanks to their superior material properties. To further improve the carriers’ confinement, using InGaN channel layer or introducing AlGaN back barrier layer is promoted. These structures are also proved to be favorable for the robust electron mobility at high temperature. However, the reason solely rests on the longitudinal optical (LO) phonon scattering and the behind mechanism is still absent. This work compares the temperature-dependent electronic transport properties in three groups of GaN-based heterostructures, including GaN-based single heterostructure (SH) with/without AlN interlayer, GaN-based SH and double heterostructure (DH), and GaN-based DH with AlGaN or GaN channel layer. The advantageous high-temperature mobility is attributed to the enhanced LO phonon energy in heterostructures. By self-consistent calculations of the Schrödinger–Poisson equations, a close relation between the narrow electron distribution in the quantum well with the high LO phonon energy, and consequently superior mobility at high temperature is implied. The results provide a simple strategy for the optimized design of the GaN-based heterostructures outperforming at high temperature.

Surface scattering-dependent electronic transport in ultrathin scandium nitride films

With the constant miniaturization of device technologies, it has become essential to understand and engineer the electronic properties of semiconductors in nanoscale dimensions. Scandium nitride (ScN), an emerging rock salt indirect bandgap semiconductor, has attracted significant interest for its interesting thermoelectric, plasmonic, neuromorphic computing, and Schottky barrier device applications. However, an in-depth understanding of the electronic transport, carrier scattering mechanism, and optical properties in ultrathin ScN films is still missing. Here, we show surface-scattering dominant electronic transport in epitaxial ScN films at nanoscale thicknesses. At the ultrathin dimensions, surface scattering increases significantly due to the large surface-to-volume ratio and growth-induced texturing. As a result, mobility decreases, and resistivity increases drastically with decreasing film thickness. Temperature-dependent electronic transport shows that the mobility of the ultrathin films decreases with increasing temperature due to the ionized-impurity and dislocation scattering. Electronic transport properties are further rationalized with x-ray diffraction and pole-figure analysis that shows that while the ultrathin films maintain their predominant 002 texture, their quality degrades with decreasing thickness. However, no significant changes are observed in the electronic structure of the films, as evidenced by x-ray photoelectron spectroscopy, photoemission measurements, and first-principles density functional theory calculations. Our results elucidate the impact of surface scattering on the ultrathin ScN films and would lead to miniaturized devices with improved efficiencies.

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.

Toward AlGaN channel HEMTs on AlN: Polarization-induced 2DEGs in AlN/AlGaN/AlN heterostructures

Due to its high breakdown electric field, the ultra-wide bandgap semiconductor AlGaN has garnered much attention recently as a promising channel material for next-generation high electron mobility transistors (HEMTs). A comprehensive experimental study of the effects of Al composition x on the transport and structural properties is lacking. We report the charge control and transport properties of polarization-induced 2D electron gases (2DEGs) in strained AlGaN quantum well channels in molecular-beam-epitaxy-grown AlN/AlxGa1−xN/AlN double heterostructures by systematically varying the Al content from x = 0 (GaN) to x = 0.74, spanning energy bandgaps of the conducting HEMT channels from 3.49 to 4.9 eV measured by photoluminescence. This results in a tunable 2DEG density from 0 to 3.7 × 1013 cm2. The room temperature mobilities of x ≥ 0.25 AlGaN channel HEMTs were limited by alloy disorder scattering to below 50 cm2/(V.s) for these 2DEG densities, leaving ample room for further heterostructure design improvements to boost mobilities. A characteristic alloy fluctuation energy of ≥1.8 eV for electron scattering in AlGaN alloy is estimated based on the temperature dependent electron transport experiments.

Doping dependent electronic and magnetic ordering in mixed-valent La1−x Sr x MnO3 thin films

We have investigated the collective electronic and magnetic orderings of a series of La1−x Sr x MnO3 thin films grown epitaxially strained to (001) oriented strontium titanate substrates as a function of doping, x, for 0 ≤ x ≤ 0.4. We find that the ground states of these crystalline thin films are, in general, consistent with that observed in bulk crystals and thin film samples synthesized under a multitude of techniques. Our systematic study, however, reveal subtle features in the temperature dependent electronic transport and magnetization measurements, which presumably arise due to Jahn-Teller type distortions in the lattice for particular doping levels. For the parent compound LaMnO3 (x = 0), we report evidence of a strain-induced ferromagnetic ordering in contrast to the antiferromagnetic ground state found in bulk crystals.

Structure-property relationships and mobility optimization in sputtered La-doped BaSnO3 films: Toward 100cm2V−1s−1 mobility

The wide band gap semiconducting perovskite $\mathrm{BaSn}{\mathrm{O}}_{3}$ is of high current interest due to outstanding room temperature mobility at high electron density, fueled by potential applications in oxide, transparent, and power electronics. Due in part to a lack of lattice-matched substrates, $\mathrm{BaSn}{\mathrm{O}}_{3}$ thin films suffer from high defect densities, however, limiting electron mobility. Additionally, the vast majority of $\mathrm{BaSn}{\mathrm{O}}_{3}$ thin film research has focused on pulsed laser deposition or molecular beam epitaxy. Here, we present an exhaustive optimization of the mobility of ${\mathrm{Ba}}_{0.98}{\mathrm{La}}_{0.02}\mathrm{Sn}{\mathrm{O}}_{3}$ films grown by a scalable, high-throughput method: high-pressure-oxygen sputter deposition. Considering target synthesis conditions, substrate selection, buffer layer structure, deposition temperature, deposition rate, thickness, and postdeposition annealing conditions, and by combining high-resolution x-ray diffraction, reciprocal space mapping, rocking curve analysis, scanning transmission electron microscopy, atomic force microscopy, and temperature-dependent electronic transport measurements, detailed understanding of synthesis-structure-property relationships is attained. Optimized room temperature mobility of $96\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$ is achieved in vacuum-annealed $\mathrm{GdSc}{\mathrm{O}}_{3}$(110)/$\mathrm{BaSn}{\mathrm{O}}_{3}$(120 nm)/${\mathrm{Ba}}_{0.98}{\mathrm{La}}_{0.02}\mathrm{Sn}{\mathrm{O}}_{3}$(200 nm) heterostructures, as well as $92\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$ on unbuffered substrates and $87\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$ without postdeposition annealing. These results, including important trends in defect densities and a surprising dependence of mobility on lattice mismatch, substantially expand the understanding of the interplay between deposition conditions, microstructure, and transport in doped $\mathrm{BaSn}{\mathrm{O}}_{3}$ films, establishing competitive mobilities in films fabricated via a scalable, high-throughput, industry-standard technique.

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.

Electronic degeneracy conduction in highly Si-doped Al0.6Ga0.4N layers based on the carrier compensation effect

Electronic degeneracy to express metallic conduction in Al-rich AlGaN for the electron injection layer enhances the efficiencies of deep ultraviolet light emitters. This study systematically demonstrates the Si doping range and conditions to realize degenerate n-type Al0.6Ga0.4N layers based on the electron compensation effect. The temperature-independent electron concentrations resulting from the degenerate band appear in high Si doping conditions to overcome the electron compensation due to carbon on nitrogen sites (CN). However, excessive Si doping of over 4.0 × 1019 cm−3 leads to the collapse of the electronic degeneracy and a switch to the temperature-dependent electron transport via the impurity bands, where the luminescence bands originating from III vacancy-Si complexes (VIII-nSi) are dominant. The key parameter is the effective donor concentration, Nd − Na, based on the reduction in electron concentrations via acceptor-like deep levels such as CN and VIII-nSi. The Hall-effect analyses for n-type Al0.6Ga0.4N layers with various Si concentrations yielded an Nd − Na value of (9.5 ± 2.9) × 1018 cm−3 to vanish the ionization energy of Si donors, which is approximately six times higher than that in GaN. The results suggest not only the optimal doping range to obtain an Al-rich AlGaN layer with metallic conduction but also the necessity of the growth condition to minimize electron compensation.

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.

First-principles calculations of finite temperature electronic structures and transport properties of Heusler alloy Co2MnSi

In this study, we calculate the temperature-dependent electronic structures and transport properties of the Heusler alloy Co2MnSi on the basis of the Korringa–Kohn–Rostoker Green's function method combined with the coherent potential approximation (CPA). Temperature effects often have a significant influence on the spin-polarization properties of Heusler alloys. To incorporate the contributions of temperature effects, we first consider lattice vibrations and spin fluctuations. Using CPA, we can replace them with random displacements due to local phonons and local magnetic moment disorders, respectively. In the Co2MnSi Heusler alloy, we found that the band structures are smeared by the electron–phonon scattering process and the half-metallic property is eliminated by magnon excitations from the spin-up to spin-down states. Furthermore, we can estimate the electrical resistivity as a function of temperature in the scheme of linear response theory. Including the local phonon disorder, local moment disorder, and Mn–Co antisite disorder in CPA, we can reproduce the temperature-dependent resistivity observed by experiments.

Charge Kondo effects in a quadruple quantum dot in spinless and spinful regimes

We theoretically study charge Kondo effects in a quadruple quantum dot. This system has been realized in a carbon nanotube [Nature (London) 535, 395 (2016)] by A. Hamo et al., while it can be also formed in a two-dimensional electron gas (2DEG). The system is in a particular situation where a quadruple dot has twofold degenerate ground states of $({n}_{\text{A}}=1,{n}_{\text{B}}=1,{n}_{\text{C}}=1,{n}_{\text{D}}=0)$ and (0,0,0,1) charge configurations, where ${n}_{\ensuremath{\lambda}}$ is the electron occupation number of the individual dot $\ensuremath{\lambda}=\text{A,B,C,D}$ of the quadruple dot. The two charge states behave as the pseudospin-1/2 states of a Kondo impurity. In the spinless regime, where the real spin of electrons is frozen, for example, by an external magnetic field, the quadruple dot can exhibit a single-channel charge Kondo effect in which coherent charge fluctuations massively occur between the two charge states with the help of electron tunneling between the quadruple dot and its electron reservoirs. The origin of the charge Kondo effect is similar to that of a negative-$U$ Anderson impurity. In the spinful regime, on the other hand, the real spin and charge degrees of freedom couple each other due to interdot electron tunneling between the dots A and B so that the spin singlet is formed in the charge state (1,1,1,0). In this regime, the low-energy Hamiltonian of the quadruple dot system can be mapped onto a two-channel Kondo Hamiltonian having channel anisotropy. In realistic situations of carbon nanotubes or GaAs 2DEGs, the channel anisotropy is so large that the quadruple dot shows a single-channel charge Kondo effect also in the spinful regime at experimentally available temperatures. We compute the temperature dependence of electron transport through the quadruple dot, which is useful for identifying the charge Kondo effects.

Theoretical and experimental evaluation of thermoelectric performance of alkaline earth filled skutterudite compounds

Skutterudite compounds are one of the most promising thermoelectric materials that can be used for intermediate power generation applications because of their excellent thermoelectric and mechanical properties. In spite of extensive research efforts during the past two decades, there is still a lack of full understanding of the electronic and thermal transport in these compounds. In the present study, we carry out a combined experimental and theoretical investigation of the electronic and thermal transport properties of alkaline earth-filled (Ca,Sr,Ba)0.2Co4Sb12 skutterudites in the temperature range from 300 ​K to 800 ​K. The experimental work includes structural properties probed by PXRD and EMPA for phase purity and composition analysis. Theoretical values of the temperature dependent electronic transport parameters were determined using the nearly-free electron approximation, and various phonon thermal conductivity contributions were studied using the Debye isotropic continuum model and the theory of Price for the bipolar contribution. The theoretical and experimental transport parameters were found in good agreement with each other. They confirm that both Ba and Sr filled skutterudites are highly degenerate semiconductors, while the heat transport in the Ca filled compound is dominated by the bipolar contribution at temperatures above 300 ​K. The Sr filled skutterudite shows the highest Seebeck coefficient, likely due to its lowest donor ionisation energy. From our detailed theoretical investigations, it is seen that the key mechanism to control the phonon thermal conduction is originated from the interaction between mass defects and phonons. The maximum ZT value obtained 0.14 for Sr0.2Co4Sb12 compound at 800 ​K likely indicate that Sr is more effective filler among the alkaline earth metals.

Nanoparticles in Bi0.5Sb1.5Te3: A prerequisite defect structure to scatter the mid-wavelength phonons between Rayleigh and geometry scatterings

Nanoparticles in thermoelectric alloys has been considered as one of the most important ingredients to enhance their thermoelectric figure of merit zT mainly by reducing the lattice thermal conductivity due to intensified phonon scattering. However, the scattering mechanism of phonon with respect to wavelengths, which provides the comprehensive design rules for nanocomposites with enhanced zT, has not been fully understood. Here, we report a critical role of nanoparticles for the lattice thermal conductivity reduction from the theoretical and experimental analysis of the temperature-dependent thermal and electronic transport properties of p-type Ag/Cu nanoparticles-embedded Bi0.5Sb1.5Te3 with respect to their electronic, bipolar, and lattice thermal conductivities. It was found that the introduction of the Ag/Cu nanoparticles reduced the lattice thermal conductivity through the additional phonon scattering based on the changeover between the Rayleigh and geometrical scatterings, indicating the indispensability of nanoparticles to scatter phonons that cannot be scattered effectively by either point defects or grain boundaries.

The temperature dependence of electron transport in a composite film of graphene oxide with single-wall carbon nanotubes: an analysis and comparison with a nanotube film

The work describes the results of low-temperature studies (5–291 K) of electron transport in composite films of graphene oxide with single-wall nanotubes (GO-SWNTs) obtained by vacuum filtration of their aqueous suspension. The emergence of conductivity in such films is shown to be related to nanotubes, since the GO film, unlike the nanotube film, has no conductivity. For a comparative analysis, the electrical conductivity of the SWNT film was also considered. The GO-SWNT and SWNT films exhibit a semiconductor behavior with a negative temperature coefficient of electrical conductivity. The temperature dependences of film resistance have been analyzed using the 3D Mott model that describes the motion of electrons (due to thermally activated tunneling through barriers) with variable-range hopping (the VRH model) in an interval of 5–240 K. The analysis of the dependences yielded estimates for the parameters of electron transport in the composite GO-SWNT film and SWNT nanotube film: the average hopping range and energy of the electron; their temperature dependences have been plotted. A comparison of these parameters for different films showed that nanotube contact with the GO surface hinders electron transport in the composite film. To describe the temperature dependence of film resistance at Т > 240 K, the Arrhenius model is used from which the potential barrier value has been obtained.

First Principle Study of Temperature-Dependent Magnetoresistance and Spin Filtration Effect in WS2 Nanoribbon.

An applicable use of density functional theory (DFT) along with nonequilibrium Green's function (NEGF) is done for exploring the temperature-dependent spin electron transport nature in a ferromagnetic tungsten disulfide (WS2) nanoribbon. To demonstrate the effect of temperature on spin filtration and spin Seebeck effect, we evaluated vital parameters such as spin-polarized current and spin filtration efficiency. Spin filtration efficiency of around ∼95% is obtained in the high-temperature difference range. The high temperature (TL) of the left electrode in comparison to the high temperature (TR) of the right electrode results in higher and lower spin filtration efficiency in parallel magnetization (PM) and antiparallel magnetization (APM), respectively. Transmission spectrum plots at equilibrium are also calculated in PM and APM to justify the temperature-dependent spin transport behavior in the WS2 nanoribbon. Giant thermal magnetoresistance around 1.934 × 103% is achieved. The temperature-dependent negative differential resistance behavior of the current plot has been observed. Huge value of thermal magnetoresistance (MR) and excellent spin filtration obtained for WS2 nanoribbon suggests the potential application of this material in spin caloritronic devices.

Dramatic Enhancement of Optoelectronic Properties of Electrophoretically Deposited C60-Graphene Hybrids.

Fullerene (C60) and multilayer graphene hybrid devices were fabricated using electrophoretic deposition, where the C60 clusters are electrically charged upon the application of an external bias in a polar solvent, acetonitrile, mixed with toluene, which facilitates their deposition on the graphene membranes. Raman spectroscopy unveiled the unique vibrational fingerprints associated with the A2g mode of the C60 molecules at ∼1453 cm-1, while blue shifts of ∼6 and ∼17 cm-1 were also attributed to the G- and 2D-bands of the hybrids relative to bare graphene, suggestive of p-doped graphene. The intensity ratio of the G- and the 2D-bands I2D/IG (hybrid) dropped to ∼0.18 from ∼0.3 (bare graphene), and this reduction in I2D/IG is also a signature of hole-doped graphene, consistent with the relatively strong electron accepting nature of C60. The electronic conductance of the two-terminal hybrid devices increased relative to bare graphene at room temperature which was attributed to the increased carrier density, and temperature-dependent electronic transport measurements were also conducted from ambient down to ∼5.8 K. Additionally, a low energy shift in the Fermi level, EF ≈ 140 meV, was calculated for the hybrids. When the hybrid devices were irradiated with a broadband white light source and a tunable laser source (with a wavelength λ ranging from ∼400-1100 nm), a strong photoresponse was evident, in contrast to the bare graphene devices which appeared unresponsive. The responsivity of the hybrids was measured to be ∼109 A/W at λ ≈ 400 nm and ∼298 K, while the detectivity and external quantum efficiency were also exceptional, ∼1015 jones and ∼109%, respectively, at ∼1 V and a light power density of ∼3 mW/cm2. The values are ∼10 times higher compared to other hybrid devices derived from graphene reported previously, such as quantum dot-graphene and few-layer MoS2-graphene heterostructures. The strong photoresponse of the C60-graphene hybrids reported here is attributed to the doping enhancement arising in graphene upon the adsorption of C60. This work demonstrates the exceptional potential of such hybrid nanocarbon-based structures for optoelectronics.

A novel method for predicting optimal gas sensing temperature of morphologically distinct nanostructured Schottky interfaces

Nanostructured metal-oxide Schottky interfaces enable substantially enhanced gas response for gas sensors. Although it is widely known that operating these gas sensors requires heating to an elevated temperature for optimal gas response, the fundamental principles that governs this temperature-dependent gas response has yet been well understood. In this work, we propose a novel technique based on thermionic field emission (TFE) theory for predicting the optimal gas sensing temperature of morphologically different nanostructured metal-semiconductor (Schottky) interfaces. Through the fabrication and characterization of Pt/MoO3 Schottky contacted sensor devices, we found the previously unreported correlation between TFE transport and optimal gas sensing temperature. By establishing the proposed technique on the connection between temperature-dependent gas response and electron transport, prediction of optimal sensing temperature can be performed via simple I–V measurements, often to a high accuracy of several degrees. Experimental results indicate that the technique is applicable across a wide variety of morphologically distinct devices.

Engineering SrSnO3 Phases and Electron Mobility at Room Temperature Using Epitaxial Strain.

High-speed electronics require epitaxial films with exceptionally high carrier mobility at room temperature (RT). Alkaline-earth stannates with high RT mobility show outstanding prospects for oxide electronics operating at ambient temperatures. However, despite significant progress over the last few years, mobility in stannate films has been limited by dislocations because of the inability to grow fully coherent films. Here, we demonstrate the growth of coherent, strain-engineered phases of epitaxial SrSnO3 (SSO) films using a radical-based molecular beam epitaxy approach. Compressive strain stabilized the high-symmetry tetragonal phase of SSO at RT, which, in bulk, exists only at temperatures between 1062 and 1295 K. We achieved a mobility enhancement of over 300% in doped films compared with the low-temperature orthorhombic polymorph. Using comprehensive temperature-dependent synchrotron-based X-ray measurements, electronic transport, and first principles calculations, crystal and electronic structures of SSO films were investigated as a function of strain. We argue that strain-engineered films of stannate will enable high mobility oxide electronics operating at RT with the added advantage of being optically transparent.

Temperature dependent percolation mechanism for conductivity in TiO3 revealed by a microstructure study

We have performed optical microscopy, micro-photoelectron spectroscopy, and micro-Raman scattering measurements on TiO3 single crystals in order to clarify the interplay between the microstructure and the temperature dependent electronic transport mechanisms in this material. Optical microscopy observations reveal dark and bright domain patterns on the surface with length scales of the order of several to a hundred micrometers showing a pronounced temperature dependent evolution. Spatially resolved photoelectron spectroscopy measurements show the different electronic character of these domains. Using micro-Raman spectroscopy, we observe a distinct temperature dependence of the crystal structure of these domains. On the basis of these findings the different domains are assigned to insulating and metallic volume fractions, respectively. By decreasing the temperature, the volume fraction of the conducting domains increases, hence allowing the electrons to percolate through the sample at temperatures lower than ∼150 K.

Temperature-dependent electronic charge transport characteristics at MoS2/p-type Ge heterojunctions

Significant effort has been devoted to constructing two-dimensional transition metal dichalcogenides-based hybrid heterojunctions with enhanced performance or unique functionalities for versatile device applications in emerging electronics and optoelectronics. In this study, we report the temperature-dependent electronic charge transport characteristics in MoS2/p-type Ge heterojunction diodes. From the current-voltage (I-V) characteristics of the heterojunction device, it is observed that different transport phenomena can occur depending upon the temperature and bias voltage. The charge transport is dominated by thermionic emission in the high-temperature regime above 300 K, whereas in low-temperature regime below 300 K, the charge transport mechanism transitions from the thermionic emission to the tunneling mechanism associated with trap sites. In particular, the I-V characteristics in the low-temperature regime show a transition from the direct tunneling at a low bias to the Fowler-Nordheim tunneling mechanism at a high bias. This could be well described by the electrical analyses on temperature-dependent I-V behavior and corresponding energy band diagrams.