Nearest-neighbor Hopping Conduction Research Articles

Negative Hall coefficient in band conduction region in heavily Al-doped 4H-SiC

At low temperatures, the Hall coefficients in heavily Al-doped 4H-SiC are reported to be negative in the band conduction region as well as in the hopping conduction regions (i.e., nearest-neighbor hopping conduction region and variable-range hopping conduction region). A physical model was proposed to explain the negative sign of RH(T) in the hopping conduction regions. However, the negative value of RH(T) in the conduction band region remains unexplained. This study proposed a physical model to explain the negative value of RH(T) in the conduction band region. In addition to the valence band, doping copious amounts of Al acceptors in 4H-SiC causes a strong overlap of the wave functions of the excited states of Al acceptors, which results in the formation of allowed bands, referred to as allowed minibands. Although the holes can flow freely through the valence band as well as the allowed minibands, the energy–momentum relationship in the valence band and the allowed minibands determines the sign of RH(T). As elucidated here, if the holes flow primarily in the lower parts of the allowed minibands, the RH(T) in the band conduction region becomes negative, whereas if the holes flow primarily in the upper parts of the allowed minibands and the valence band, the RH(T) becomes positive.

Electrical Properties of Heavily Al-Doped 4H-SiC

We investigate the temperature-dependent resistivity (ρ(T)) and Hall coefficient (RH(T)) of heavily Al-doped 4H-SiC and discuss the underlying conduction mechanisms. The sign of RH(T) changes from positive to negative in nearest-neighbor hopping (NNH) and variable-range hopping (VRH) conduction, whereas it is positive in band conduction because Al-doped 4H-SiC is a p-type semiconductor. We propose a general physical model to explain why RH(T) in hopping conduction becomes negative at low temperatures, which is applicable to both NNH and VRH conduction. Moreover, we elucidate why the activation energy for negative RH(T) becomes similar to that of ρ(T) in NNH conduction.

Electrically erasable writing properties of ZnS films by conductive atomic force microscopy

Resistive switching cycles were realized in Au/ZnS/substrate (indium–tin oxide (ITO), Cu, Si) structures, and electrically erasable writing operations were achieved in the Au/ZnS/Si structure using conductive atomic force microcopy. High-resolution transmission electron microscopy revealed that high resistance state was a mixture of amorphous and nanocrystalline state, while the frequency response of alternating current conductivity indicated that the low resistance state (LRS) was only nanocrystalline. Electric field and thermal effects contributed to the distribution of conductive defects in the ZnS film, and nearest-neighbor hopping conduction controlled the electrical resistance of the Au/ZnS/ITO structure. X-ray photoemission spectroscopy analysis of conductive defects of ZnS films in the LRS revealed that they were zinc-rich or sulfur-poor. This study confirms the intrinsic resistive switching characteristic of ZnS films, which can serve as nonoxide materials for nonvolatile memory application.

Enhanced electronic transport properties of Te roll-like nanostructures.

In this work, the electronic transport properties of Te roll-like nanostructures were investigated in a broad temperature range by fabricating single-nanostructure back-gated field-effect-transistors via photolithography. These one-dimensional nanostructures, with a unique roll-like morphology, were produced by a facile synthesis and extensively studied by scanning and transmission electron microscopy. The nanostructures are made of pure and crystalline Tellurium with trigonal structure (t-Te), and exhibit p-type conductivity with enhanced field-effect hole mobility between 273 cm2/Vs at 320 K and 881 cm2/Vs at 5 K. The thermal ionization of shallow acceptors, with small ionization energy between 2 and 4 meV, leads to free-hole conduction at high temperatures. The free-hole mobility follows a negative power-law temperature behavior, with an exponent between -1.28 and -1.42, indicating strong phonon scattering in this temperature range. At lower temperatures, the electronic conduction is dominated by nearest-neighbor hopping (NNH) conduction in the acceptor band, with a small activation energy E NNH ≈ 0.6 meV and an acceptor concentration of N A ≈ 1 × 1016 cm-3. These results demonstrate the enhanced electrical properties of these nanostructures, with a small disorder, and superior quality for nanodevice applications.

Modulation of bandgap and electrical conductivity in europium doped single ZnO nanorod device

We investigate the effect of europium doping on the optical properties and electrical conductivity of hydrothermally grown ZnO nanorods. The widening of optical bandgap with europium doping occurs because of the Burstein-Moss effect upto 0.8% and bandgap renormalization is observed at higher concentration. The temperature-dependent resistivity measurements of the individual nanorods reveal that the intrinsic electrical conductivity is a combination of thermally activated and nearest-neighbor hopping conduction processes at high and low-temperature regimes, respectively. We can successfully correlate that the modulation of carrier concentration in the defect D and D− bands can tune both bandgap and electron transport in Eu doped ZnO nanorods.

Multi-band simultaneous fits of transport data on p-type ScNiSb with including impurity conduction

The experimental data of the electrical conductivity [Formula: see text] and the Seebeck coefficient [Formula: see text] in the temperature range from 2 K to 950 K, together with those of the Hall coefficient [Formula: see text] in the temperature range from 2 K to 300 K, on the polycrystalline sample of [Formula: see text]-type ScNiSb reported by Ciesielski et al. [Phys. Rev. Appl. 14, 054046 (2020)] have been critically analyzed with including both nearest-neighbor and Mott variable-range hopping (VRH) conduction. Through the simultaneous fits to the experimental data of [Formula: see text], [Formula: see text] and [Formula: see text], various transport parameters such as the bandgap energy, the effective masses of holes and electrons, and the electron-to-hole mobility ratio, are deduced together with the ionization energy and the concentration of the acceptor level. It is shown that nearest-neighbor hopping (NNH) conduction exhibits a significant effect on the Seebeck coefficient at low temperatures while VRH conduction has an only small effect on it. Contrary to the speculation by Ciesielski et al., the electron mobility is proved to be lower than the hole mobility.

Magnetic and electrical transport studies of polycrystalline Sr1− x Bi x Fe12O19 (x = 0, 0.01, and 0.02)

Bismuth-substituted strontium hexaferrites, Sr1−x Bi x Fe12O19 for x = 0, 0.01 and 0.02, are studied via powder neutron diffraction (ND), magnetization (M) studies, Mössbauer spectroscopy, and electrical transport. ND results show an indication of increasing Fe2+ at 12k crystallographic sites (which is supported by Mössbauer results), with increasing Bi in the sample. They also suggest an increase in strain due to Bi substitution for the polyhedral associated with 2a and 2b spin-up and 4f1 spin-down sites. The M measurements over a wide temperature range (3–823 K), shows irreversibility in zero field cooled (ZFC) and field cooled data right below the Curie temperature, along with the Hopkinson peak in the ZFC data. The temperature dependence of saturated magnetization follows the Bloch relation but that of the coercive field shows unconventional behavior. The coercive field data is fitted using an equation devised by taking into consideration of all the three anisotropies. The critical exponents at the ferromagnetic–paramagnetic phase transition boundary, calculated using modified Arrott plots, are slightly overvalued as per mean-field theory. The temperature dependence of resistivity displays nearest-neighbor hopping conduction in all the three samples. The conductivity increases with increasing Bi in the sample, due to the increasing Fe2+ content, which facilitates the electron hopping between Fe sites. The magnetoresistance measured at various sub-room temperatures for all the compounds shows the interplay of anisotropy magnetoresistance (AMR) and giant magnetoresistance (GMR). Low temperature data are dominated by GMR and gradual participation of AMR increases as room temperature is approached.

Variable range hopping conduction mechanisms in reduced rutile TiO2

In this study, obtained samples via reducing Rutile TiO2 by Mg are analyzed to determine the titanium suboxide phases and the dominant structural phase in each sample. By increasing the heat treatment temperature or the amount of reducing agent (Mg), the amount of suboxide phases Ti n O2n−1 (1 ≤ n < 10) with the lower n values increases, and TiO is the main phase in the samples with a low electrical resistivity. The hopping conduction mechanism is also investigated in the temperature range of 11.5–300 K, and the characteristic parameters describing the conduction mechanism are determined and discussed. For samples with a low electrical resistivity, the nearest neighbor hopping (NNH) conduction mechanism governs the charge transport properties below 220 K, and a transition from the NNH to the Mott-variable range hopping (VRH) conduction regimes is observed at ∼65 K. In addition, the Efros-Shklovskii (ES)-VRH conduction process governs at low temperatures below 18 K, and the last crossover from the Mott-VRH to the ES-VRH models illustrates the strong electron-electron Coulomb interaction, which leads to the Coulomb gap (∼1 meV) at low temperatures. According to the obtained hopping parameters, the low resistivity samples are close to the metal-insulator transition. For samples with a high electrical resistivity, the hopping conduction relations are not fitted to the resistivity-temperature curves, and probably it is related to the resistive switching behavior of the material, which changes the nature and conductivity properties, and is depicted in the current-voltage (I-V) curve.

Role of H-bond along with oxygen and zinc vacancies in the enhancement of ferromagnetic behavior of ZnO films: An experimental and first principle-based study

Magnetic properties of H2-ambience annealed pure ZnO films have been reported. ZnO films prepared by using DC magnetron sputtering show room temperature ferromagnetism (RT-FM) after annealing in hydrogen atmosphere at two different temperatures (350 °C and 400 °C). Enhancement in O-H concentration has been observed by analyzing core-level spectra recorded using x-ray photoelectron spectroscopy (XPS). The valence band spectra suggest the enhancement in carrier concentration after annealing in H2-ambience. Enhancement in the vacancy type defects have been observed by positron annihilation spectroscopy and tailoring of defects have been observed by photoluminescence spectra. Electrical transport results also indicate rapid decrease in activation energies corresponding to thermally activated band and nearest neighbor hopping (NNH) conduction, i.e. increase in carrier concentration after annealing. Increase in saturation magnetization and Zn vacancy (evident from M-H and PL data) along with the O-H concentration, after annealing in H2-ambience, suggest that Zn vacancies along with O-H concentration play a crucial role behind the ferromagnetic nature of the films. Surface morphology as examined by atomic force microscopy (AFM) shows that there is no significant variation of grain size and surface roughness of the films after annealing. The experimental results are further verified theoretically by performing the density functional theory calculations. Based on both experimental results and theoretical model, it has been explained that the enhancement of magnetic moment of H2-ambience annealed ZnO is because of the increased carrier mediated exchange interaction among the localized magnetic moment at VZn sites.

Low resistance-drift characteristics in Cr2Ge2Te6-based phase change memory devices with a high-resistance crystalline phase

Resistance drift affects the reliability of data reading in phase change material (PCM)-based memory devices. In general, resistance drift occurs in high-resistance amorphous state in the PCM-based memory devices. The conduction mechanism such as Poole-Frenkel conduction (PF) in amorphous PCMs causes severe resistance drift in the device. Cr2Ge2Te6 (CrGT) exhibiting a phase transition between low-resistance amorphous and high-resistance crystalline phases demonstrates band conduction in the amorphous phase and mixed conduction with band conduction and nearest-neighbor hopping conduction in the crystalline phase. Since the conduction mechanisms of both phases in the CrGT are not governed by PF, the resistance-drift characteristics are expected to be different from that in conventional PCMs. In this study, the resistance-drift characteristics of the CrGT-based devices were investigated. The fabricated CrGT-based devices exhibited a cyclic endurance of ~7 × 105 times with a clear difference in resistance between the high- and low-resistance states. The drift coefficient, v, at 40 °C was evaluated for the CrGT- and Ge-Sb-Te (GST)-based devices; the value of v for the CrGT-based device was smaller than that for the GST-based device in both high- and low-resistance states. The current–voltage analysis revealed that mixed-conduction mechanism plays an important role to suppress the resistance drift in the high-resistance state of the CrGT-based devices. These findings provide new insights to realize a low resistance-drift PCM-based memory device.

Simple physical model for the sign of the Hall coefficient in variable-range hopping conduction in heavily Al-doped p-type 4H-SiC

We observe negative Hall coefficients (R H(T)) in the variable-range hopping (VRH) conduction region for heavily Al-doped p-type 4H-SiC. We have proposed a simple physical model in which the difference between the density of hopping sites for holes and the density of hopping sites for electrons under a magnetic field determines the sign of R H(T), and explained the negative R H(T) in nearest-neighbor hopping (NNH) conduction. Based on our proposed model for NNH conduction, here we propose a model in which R H(T) becomes positive when the density of localized states (g(E)) around the Fermi level (E F) increases with increasing energy (E), whereas R H(T) becomes negative when g(E) around E F decreases with increasing E. In heavily Al-doped p-type 4H-SiC where g(E) around E F is considered to decrease with increasing E, the proposed model can explain why R H(T) becomes negative in the VRH conduction region.

Hidden Vacancy Benefit in Monolayer 2D Semiconductors

Monolayer 2D semiconductors (e.g., MoS2 ) are of considerable interest for atomically thin transistors but generally limited by insufficient carrier mobility or driving current. Minimizing the lattice defects in 2D semiconductors represents a common strategy to improve their electronic properties, but has met with limited success to date. Herein, a hidden benefit of the atomic vacancies in monolayer 2D semiconductors to push their performance limit is reported. By purposely tailoring the sulfur vacancies (SVs) to an optimum density of 4.7% in monolayer MoS2 , an unusual mobility enhancement is obtained and a record-high carrier mobility (>115 cm2 V-1 s-1 ) is achieved, realizing monolayer MoS2 transistors with an exceptional current density (>0.60mA µm-1 ) and a record-high on/off ratio >1010 ,and enabling a logic inverter with an ultrahigh voltage gain >100.The systematic transport studies reveal that the counterintuitive vacancy-enhanced transport originates from a nearest-neighbor hopping conduction model, in which an optimum SV density is essential for maximizing the charge hopping probability. Lastly, the vacancy benefit into other monolayer 2D semiconductors is further generalized; thus, a general strategy for tailoring the charge transport properties of monolayer materials is defined.

Electrical Interface Characterization of Ultrathin Amorphous Silicon Layers on Crystalline Silicon

Heterojunctions of amorphous (a‐Si) and crystalline (c‐Si) silicon combine the favorable absorption characteristics of a‐Si for the solar spectrum, high energy conversion efficiencies, low processing temperatures, potential for low production costs, and a reduced amount of silicon used due to thin a‐Si films. To investigate a‐Si/c‐Si heterojunctions, amorphous p‐type silicon (a‐Si) with a thickness of 5 nm is deposited via radio frequency‐pulsed magnetron sputtering on p‐doped, (100)‐oriented, crystalline silicon (c‐Si) wafers. During deposition, the crystalline silicon wafers are kept at 430 °C and the hydrogen flow rate is varied from 0 to 50 sccm (standard cubic centimeters per minute). Temperature‐dependent current–voltage measurements are carried out to investigate the dominant transport mechanisms of electrical conduction. Moreover, the influence of hydrogen on physical properties such as barrier height, activation energy, and electrical conductivity is analyzed. A current–voltage dependence as predicted by the thermionic emission model for the low forward‐bias region below 0.25 V is observed. Temperature regimes for nearest neighbor hopping and band conduction are revealed by the Arrhenius plot and are found to depend on the hydrogen flow rate.

Mixed-conduction mechanism of Cr2Ge2Te6 film enabling positive temperature dependence of electrical conductivity and seebeck coefficient

A Cr2Ge2Te6 (CrGT) film is known to be a phase change material showing low-energy phase change between amorphous and crystalline phases. A crystalline Cr2Ge2Te6 (CrGT) film obtained from an amorphous phase via metastable crystalline phase has been reported to possess different local structure from the CrGT bulk material. The bulk CrGT is known to show a band conduction, while the conduction mechanism of the CrGT film has not been clear yet. In this study, the conduction mechanism of the CrGT film was discussed based on the temperature dependence of the conductivity (σ), Seebeck coefficient (S), and Hall properties. It was found that the CrGT film shows a variable range hopping conduction below 170 ​K, while it shows a mixed-conduction of band-like conduction and nearest-neighbor hopping conduction above 300 ​K. Such the mixed-conduction mechanism was found to cause the positive temperature dependence of the σ and S in the CrGT film. Furthermore, the thermal conductivity of the CrGT film was measured to be 0.16 ​W/mK, which is much lower than that of the bulk material. The mixed-conduction mechanism and the low thermal conductivity introduced by nanostructuring through sputtering method is suggested to promising strategy to enhance the thermoelectric properties of the CrGT material.

Anomalous Conduction between the Band and Nearest-Neighbor Hopping Conduction Regions in Heavily Al-Doped p-Type 4H-SiC

We measure the temperature-dependent resistivity (ρ(T)) for thick heavily Al- and Ncodoped p-type 4H-SiC samples grown by chemical vapor deposition (CVD), physical vapor transport (PVT), and solution growth (SG), and investigate their conduction mechanisms. For samples with an Al concentration (CAl) of 3.5×1019 to 1×1020 cm-3 grown by CVD, PVT, and SG, the conduction mechanisms at high and low temperatures are band and nearest-neighbor hopping (NNH) conduction, respectively. In the range CAl of 1×1019 to 3.5×1019 cm-3, on the other hand, an anomalous conduction, referred to as X conduction here, is observed between the band and NNH conduction regions for the samples grown by CVD and PVT, but not those grown by SG. One of the differences between the samples grown by CVD and PVT and those grown by SG is the off-orientation toward [11-20] of the (0001) 4H-SiC substrate. We discuss the reason for the appearance of X conduction, which appears to be consistent with dopant-concentration inhomogeneity model.

Sign of Hall coefficient in nearest-neighbor hopping conduction in heavily Al-doped p-type 4H-SiC

We have observed negative Hall coefficients [RH(T)] in a nearest-neighbor hopping (NNH) conduction region in epilayers of heavily Al-doped or Al–N co-doped p-type 4H-SiC grown on n-type 4H-SiC substrates by CVD or in wafers of heavily Al–N co-doped p-type 4H-SiC fabricated by solution growth. We propose a simple physical model to explain the sign of RH(T) in NNH conduction. According to this model, RH(T) becomes positive when the Fermi level (EF) is higher than the Al acceptor level (EAl), that is, the Fermi–Dirac distribution function f(EAl) is greater than 0.5, whereas RH(T) becomes negative when EF is lower than EAl, which occurs at low temperatures. Because the dominant conduction mechanisms in heavily Al-doped or Al–N co-doped p-type 4H-SiC with Al concentrations on the order of 1019 cm−3 are band and NNH conduction at high and low temperatures, respectively, the proposed model can explain why RH(T) becomes negative at low temperatures.

Deconvolution of local activation energy of hopping conductivity: Application to p-Ge

A new method for analyzing the local reduced activation energy [Formula: see text] of the impurity conductivity is applied to Ga-doped [Formula: see text]-Ge samples with a constant compensation ratio of [Formula: see text] reported by Zabrodskii and Andreev [Int. J. Mod. Phys. B 8, 883 (1994)]. Here, the local reduced activation energy is defined as [Formula: see text], where [Formula: see text] and [Formula: see text] denote conductivity and absolute temperature, respectively. The method enables us to deconvolute the [Formula: see text] curve into contributions from different conduction mechanisms such as free-hole conduction, nearest-neighbor hopping (NNH) conduction and variable-range hopping (VRH) conduction. As a result, it is shown that both the power exponents of the pre-exponential factors of NNH and VRH conductivity tend to decrease with increasing impurity concentration.

Cation impurity-defect complex induced ferromagnetism and hopping conduction in Sb-doped ZnO synthesized under high pressure

Sb-doped p-type ZnO was synthesized using the high-pressure method with a quasi-equilibrium thermodynamic process. Room temperature ferromagnetism was observed in Sb-doped ZnO. The ferromagnetic coupling was found to be associated with zinc vacancies (VZn) induced by Sb doping, which was supported by a strong ultraviolet emission peak relevant to VZn in low temperature photoluminescence. Hall effect measurements indicated that the Sb-doped ZnO with Sb content of 3.4 at% had a hole concentration of 1.4 × 1020 cm−3 and low resistivity of 7.4 × 10−2 Ω cm. Temperature-dependent resistance measurements suggest that the electrical conduction mechanism of the Sb-doped ZnO changed from thermal activation model and nearest-neighbor hopping (NNH) conduction at high temperature to Mott variable-range hopping (VRH) and Efros–Shklovskii VRH conduction at low temperature. First-principles calculations demonstrated that the Sb impurity substituting Zn (SbZn) combines two VZn defects to form acceptor-like impurity-defect complex of SbZn–2VZn in ZnO due to the coulomb attraction. The SbZn–2VZn complex in Sb-doped ZnO are strongly correlated with ferromagnetic stability and lead to the Mott and Efro–Shklovskii VRH conduction at low temperature due to the localization of carriers at the complex impurity band.

Transition of conduction mechanism from band to variable-range hopping conduction due to Al doping in heavily Al-doped 4H-SiC epilayers

We investigate the transition of the conduction mechanism from band and nearest-neighbor hopping (NNH) conduction to variable-range hopping (VRH) conduction in heavily Al-doped 4H-SiC epilayers with increasing Al concentration (CAl). In a sample with CAl of 1.8 × 1020 cm−3, the dominant conduction mechanisms at high and low temperatures were band and VRH conduction, respectively, whereas in samples with lower CAl values they were band and NNH conduction, respectively, and in samples with higher CAl values VRH conduction was dominant over the entire range of measurement temperatures examined (20–600 K).

Magnetotransport and conductivity mechanisms in Cu2ZnSnxGe1\u2212xS4 single crystals

Resistivity, ρ(T), and magnetoresistance (MR) are investigated in the Cu2ZnSnxGe1−xS4 single crystals, obtained by the chemical vapor transport method, between x = 0–0.70, in the temperature range of T ~ 50–300 K in pulsed magnetic field of B up to 20 T. The Mott variable-range hopping (VRH) conductivity is observed within broad temperature intervals, lying inside that of T ~ 80–180 K for different x. The nearest-neighbor hopping conductivity and the charge transfer, connected to activation of holes into the delocalized states of the acceptor band, are identified above and below the Mott VRH conduction domain, respectively. The microscopic electronic parameters, including width of the acceptor band, the localization radius and the density of the localized states at the Fermi level, as well as the acceptor concentration and the critical concentration of the metal-insulator transition, are obtained with the analysis of the ρ(T) and MR data. All the parameters above exhibit extremums near x = 0.13, which are attributable mainly to the transition from the stannite crystal structure at x = 0 to the kesterite-like structure near x = 0.13. The detailed analysis of the activation energy in the low-temperature interval permitted estimations of contributions from different crystal phases of the border compounds into the alloy structure at different compositions.