Variable Range Hopping Research Articles

Engineering lattice oxygen defects and polaronic transport in vanadium pentoxide via isovalent phosphorus doping

Oxygen vacancies are equilibrium defects in the vanadium pentoxide system that give rise to polaronic hopping transport via V4+ charge compensating defect. In this paper, we report the tunability of polaron formation, the hopping process, and their magnetic signature by substitution of isovalent (5+) phosphorus ions in the V5+ site. The powder x-ray diffraction data show a monotonous shift in lattice parameters with progressive P-doping, confirming the presence of a substitutional dopant. The polaron hopping energy reduced from 0.307 to 0.290 eV depicting a lower defect concentration in P-doping in V2O5. At low temperatures, it is found to obey the Efros–Shklovskii variable range hopping mechanism. The estimated hopping range increased to 1.6 ± 0.1 nm in doped V2O5 in contrast to ∼1.3 nm in the undoped one. The electron spin resonance measurements show a diminishing broad ferromagnetic signal and rising paramagnetic signal (g = 1.97) with progressive P-doping depicting predominant isolated electronic spins in the doped sample. The same is corroborated in room temperature M–H with a distinct hysteresis that diminishes with P-doping and a rise of a paramagnetic slope. Moreover, the reduced oxygen defects and lower V4+ relative occupancy together with fermi level fall toward intrinsic position are substantiated by photoelectron emission studies.

Double perovskite material of highly active, monoclinic, ordered Ni2ZnMoO6 for an effective quasi-solid state supercapacitor

In the present study, monoclinic Ni2ZnMoO6 double perovskites (monoclinic NZMO DP) electrodes were synthesized using hydrothermal method for supercapacitors application. The structural, elemental, vibrational, and optical characterization were analyzed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy-energy Dispersive X-ray analysis (HR-SEM & EDX), Transmission Electron Microscopy (TEM), Brunauer-Emmett-Teller (BET), Fourier Transform Infrared spectroscopy(FTIR) and UV-Visible Diffusion reflectance Spectroscopy (UV-DRS). A monoclinic structure with a space group of P 2/m and lattice parameters of a = 11.4377, b = 9.2961, c = 6.8301, and β = 97.710 was fitted using Rietveld refinement, and the cell volume was 719.6458 Å3. The surface area of monoclinic NZMO DP was found to be 50.45 m2 g−1 with a pore volume of 0.15 cm3 g−1. Electrochemical characterization was done using Cyclic voltammetry (CV), Electrochemical Impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD) measurements. An equivalent-circuit model R(CR)(CR) is iterated to interpret the EIS responses. The Rct found to be less than 100 ohms in the present study can be attributed to Mott variable range hopping (VRH) conduction. The GCD analysis of ASC monoclinic NZMO DP//AC in 6 M KOH at 0.5 Ag−1 in the potential window of 0–1.6 V yields a maximum specific capacitance of 160.45 Fg−1. Excellent electrochemical stability of the monoclinic NZMO DP was ascertained by the specific capacitance retention of the electrode material at 91.37 % after 6000 cycles. The ASC-NZMO monoclinic DP shows an excellent power density of 646.25 W/kg with an energy density of 20.53 Wh/kg. The excellent power storage efficiency and application possibilities of supercapacitors make them a promising technology in various fields.

Exploration of temperature driven conduction process and ferroelectric properties of Mn doped GdCrO3

This work delivers the research findings of the temperature dependent DC resistivity, AC impedance and ferroelectric polarization of GdCr1−xMnxO3 (x = 0 and 0.3). Mixed valence states of Cr (Cr3+ and Cr4+) are explored using the x-ray photoelectron spectroscopy analysis. The exponential decay of DC resistivity on escalating the temperature advocates the semiconducting-like nature for the probed samples. The DC resistivity data of these samples fit well into small-polaron hopping and variable range hopping models. The impedance attributes of these samples were scrutinized over a broad spectrum of temperatures at selected frequencies. The values of the real and imaginary parts of impedance unveil substantial reduction on raising the temperature, thereby signifying the increase in conductivity of the samples. Pristine sample displays an electrical relaxation peak at 65 °C, which translates towards the lower temperature at higher frequencies. Further, the semicircular behavior of Nyquist plots at higher temperatures indicates the reduction of the charge transfer resistance. The equivalent circuits of Nyquist plots are generated using Z-view software. From these plots, it is perceived that grain boundary resistance upsurges while the grain resistance and capacitance drops upon doping. The ferroelectric measurements reveal that the coercive field (Ec) values decrease whereas the values of maximum polarization (Pm), remnant polarization (Pr) and energy storage increase in 30% doped GdCrO3. These observations establish that electrical and ferroelectric properties of GdCrO3 system can be tuned with appropriate Mn doping.

Structural, dielectric and magnetic study of double perovskite La2CoMnO6

Polycrystalline double perovskite La2CoMnO6 sample was prepared via solid state reaction route and its structural, dielectric and magnetic properties were investigated. XRD confirmed the phase formation without any impurity. Rietveld refinement analysis suggested the presence of monoclinic (P21/n, ordered) as well as rhombohedral and orthorhombic (R3c &Pbnm, disordered) structures. Microstructural analysis using FESEM confirmed that average grain size lies in micrometer range. Dielectric constant (log ε′) and loss tangent (tanδ) both exhibited large values at lower frequencies as well as in higher temperature range. Nyquist impedance plot proposed NTCR (negative temperature coefficient of resistance) nature of double perovskite sample. DC conductivity showed increasing trend with increase in temperature and also, the conductivity is of the order of 10−2 S/m, which represent semiconducting behavior of prepared sample. Also, the analysis suggested the co-existence of thermal activation (TA), small polaron hopping (SPH) and variable range hopping (VRH) conduction mechanisms. Temperature dependent magnetic measurements (M-T) suggested that sample exhibit single magnetic transition at 226 K. Field dependent magnetic measurements (M-H) confirmed the ferromagnetic nature of the prepared sample. M-H loop is not well saturated even at 60 kOe which suggested the presence of anti-site disorder. Multi-domain magnetic structure of the prepared sample suggests its suitability for memory device applications.

Spin glass behavior and dielectric properties of Sm2CuMnO6 perovskites

Monoclinic Sm2CuMnO6 is a fascinating material that exhibits intriguing magnetic and dielectric properties. In this study, we investigated the magnetic transition, conduction mechanism, and dielectric behavior of the material. Mott's variable range hopping of polarons is proposed as a satisfactory explanation for the conduction process, shedding light on the underlying charge transport mechanism. Furthermore, the material displays a spin glass transition at a freezing temperature of Tf = 18 K, suggesting the presence of competing magnetic interactions. Dielectric measurements reveal intriguing temperature and frequency dependence of the dielectric constant (ε) in Monoclinic Sm2CuMnO6. Below 100 K, the dielectric constant remains unchanged with variations in both temperature and frequency, indicating a stable dielectric response. However, above 100 K, ε exhibits an increasing trend with rising temperature and also displays frequency dependence. This behavior suggests the contribution of both deformational and relaxation polarizations, emphasizing the complex nature of the dielectric response.

The effect of Si3N4/Al2O3 stacked structure of AlGaN/GaN HEMTs

Excessive gate leakage current and current collapse effects are the critical reliability issues for GaN-based high electron mobility transistors (HEMTs). In this paper, Si3N4 passivation layers was grown separately on p-GaN gate HRCL-HEMTs with or without Al2O3 film, and the gate leakage and current collapse effects of the devices and their intrinsic mechanisms were investigated. It is found that the device with Si3N4/Al2O3 stacked structure has better performance in terms of gate reverse leakage current, breakdown voltage, etc. The Al2O3 film blocking layer can reduce the H plasma damage to the channel, decrease the gate leakage, and the passivation can effectively suppress the increase of gate leakage current. Through variable temperature characteristic test, it is verified that the dominant mechanism of the device gate reverse leakage current is the two-dimensional variable range hopping (2D-VRH) model. The interface state between Si3N4 passivation layer and Al2O3 film increases the trap activation energy of the device, which is the reason for the reduction of gate reverse leakage current, and also causes the current collapse effect of the device.

Low-temperature electron transport of rutile-type GexSn1−xO2

Rutile-type wide and ultrawide band-gap oxide semiconductors are emerging materials for high-power electronics and deep ultraviolet optoelectronics applications. A rutile-type GeO2-SnO2 alloy (r-GexSn1–xO2) recently found is one of such materials. Herein, we report low-temperature electron transport properties of r-GexSn1−xO2 thin films with x = 0.28 and 0.41. Based on resistivity and magnetoresistance measurements, along with the theory of quantum interference, it is suggested that Efros–Shklovskii variable-range hopping, i.e., hopping over the states within the Coulomb gap, is dominant at lower temperatures (T ≤ 10 and 15 K) in both r-Ge0.41Sn0.59O2 and r-Ge0.28Sn0.72O2. The negative and positive magnetoresistances observed at low temperatures are attributable to the quantum interference and field-induced spin alignment, respectively. The magnetoresistance measurements at higher temperatures suggest that both Mott variable–range hopping and thermally activated band conduction occur at T < 100 K and that almost pure thermally activated band conduction takes place at T ≥ 150 K.

Microelectronics-compatible reduced graphene oxide based nanomechanical resonators capable of all-electrical actuation and read-out

Photolithographic patterning and deterministic transfer of reduced graphene oxide (rGO) are key innovations that can give way to cost-effective batch processing of nanoelectromechanical system (NEMS)-based resonators with high throughput. Here, by implementing the same, we report a facile fabrication process for NEMS-based resonators consisting of rGO as the active material for the structural layer. The structural properties of rGO thin film are confirmed using Raman spectroscopy and X-ray diffraction analysis. Temperature-dependent current–voltage characteristics of rGO film ensure the typical 2D Mott variable range hopping (VRH) transport with ambipolar charge conduction, further supporting the effectiveness of the fabrication process. All-electrical actuation and readout of the fabricated resonators are performed using both frequency modulation (FM) downconversion and piezoresistive mixing methods adeptly to achieve a resonance frequency of around 28–32 MHz. The fabrication process is compatible with the current microelectronic technology and can foster the prospect of on-chip implementation of rGO NEMS toward possible applications such as molecular sensors, mass detectors, etc.

Effects of Pb nanosheets substrates on the optical and electrical properties CrSe thin films

Herein the effects of lead nanosheets substrates on the optical and electrical properties of chromium selenide thin films were studied. Chromium selenide thin films were grown by vacuum thermal evaporation technique onto glass and 100 nm thick lead nanosheets substrates. The effects of Pb nanosheets on the structural, morphological, compositional, optical and electrical properties was explored. While films grown onto glass substrates showed compositional stoichiometry forming CrSe2 phase those coated onto Pb substrates preferred the growth CrSe phase. Both films were of amorphous structure and exhibited direct and indirect allowed optical transitions. Pb nanosheets enhanced the optical absorption of chromium selenide in the visible and ultraviolet ranges of light. Formation of films onto Pb nanosheets resulted in band gaps and band tails narrowing and decreased the room temperature electrical conductivity () as well. Deep analyses of the variations in the range of 25–340 K showed that the electronic transport is dominated by the thermal excitation and variable range hopping of charge carriers. It was observed that Pb nanosheets increased the average hopping energy and hopping range, decreased the density of localized states near the Fermi level and increased the degree of state disorder. The features of glass/CrSe2 and Pb/CrSe films are promising for using then in optoelectronic technology.

Low temperature magnetic and structural properties of Sr-doped La2CoMnO6 (La2-xSrxCoMnO6: 0 ≤ x ≤ 0.08) double perovskite nanoparticles

A study on crystal structure and physical properties has been performed on nanoparticles of lead-free double perovskites, La2CoMnO6 (LCMO) and Sr-doped La2-xSrxCoMnO6 (LSMO:0 ≤ x ≤ 0.08) prepared by the sol–gel synthesis method. In preparation for the LSMO nanoparticle, the La site of the LCMO double perovskite was replaced with an increasing concentration of Sr atom. Structural investigation done by employing the X-ray Diffraction technique indicates that the crystallite size decreases as the doping concentration increases. The Rietveld refinement on the XRD pattern shows the formation of the rhombohedral phase of LCMO (S.G. no. 161). Strain in the prepared samples estimated from Williamson Hall (W-H) analysis shows that the strain decreases as crystallite size decreases. The presence of compressive strain is also evident from the blue shift obtained in the Raman spectrum of the nanoparticles. The UV–Vis spectra reveal the maximum absorption in the near UV region of the electromagnetic spectrum. The Bandgap analysis from Tauc’s plot shows an increase in band gap with Sr doping in the LCMO host lattice (from 1.81 eV to 1.92 eV). This is attributed to the decrease in crystallite size and octahedral tilting with Sr doping. Resistivity versus temperature plots reveal the semiconducting nature of the compound. A linear nature graph between lnρ and T-14 indicates that the electric transport mechanism is governed by the variable range Hopping (VRH) model. The M−H curves show that the maximum magnetization decreases as a result of Sr doping due to the anti-site disorder introduced. dM/dT vs T curve shows two magnetic transitions at ∼ 215 K and at ∼ 173 K which are due to Co2+–O2--Mn4+ and Co3+–O2--Mn4+ ferromagnetic superexchange interactions, respectively.

Anomalous scaling regime for one-dimensional Mott variable-range hopping

Anomalous scaling regime for one-dimensional Mott variable-range hopping

THE STRUCTURAL AND ELECTRICAL PROPERTIES OF NON-STOICHIOMETRIC CADMIUM SELENIDE THIN FILMS

Non-stoichiometric CdxSe100-x (x=10, 20, 30 at. %) alloys were prepared by melt-quenching technique, while thin films of these alloys were prepared by vacuum thermal evaporation technique. The Differential thermal analysis (DTA) results showed that the composition affects the characteristic temperatures, for instance, the glass transition, and peak crystallization temperatures. X-ray diffraction patterns (XRD) of the as-deposited films confirmed its amorphous nature except for films with x=30 at. %. These results were confirmed by scanning electron microscopy (SEM) investigations and correlated to the rigidity percolation threshold of the lattice. Furthermore, XRD patterns indicated that amorphous structures transformed into crystalline structures for annealed thin films at 423 K for 30 minutes. The microstructure parameters (crystal size, microstrain, and dislocation density) of both Se and CdSe phases were calculated for annealed films. The temperature dependence of dark d.c. conductivity of thin films was studied in the temperature range (300-500 K). The electrical measurements results showed two types of conduction mechanisms. Conduction is due to extended state in the temperature range (370-500 K) and variable range hopping in the temperature range (300-370 K). Moreover, in the high temperature range, the results indicate the validity of Meyer-Neldel rule (MNR) in the studied samples.

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.

Change of orbital ordering in Fe3O4 probed through low frequency-low magnetic field magnetoimpedance effect mediated by magnetic inhomogeneity

Fe3O4 is a fascinating material due to its charge localization physics, variable range hopping (VRH) and its potential implications in the field of spintronics. In this investigation, we explore the formation and interaction of charge-ordered clusters and change of orbital order in cold-pressed Fe3O4, employing a comprehensive magnetoimpedance analysis in the low-frequency, low magnetic field domain spanning the Verwey transition. The physical and magnetic inhomogeneities due to particle size distribution and capacitive interparticle contact, are found to induce a substantial dispersion in the impedance response. Exploiting the variation in relaxation time as a function of temperature and magnetic field, we meticulously monitor the reorientation of orbitals and the consolidation of charge-ordered clusters, a consequence of localization phenomena. The shielding and deshielding of charge-ordered clusters are interpreted through AC magnetoresistance, which exhibits a sign inversion from negative to positive, synchronous with the dissolution of the charge order. The AC magnetoresistance showed the interaction of clusters of ‘charge -ion order’ in the higher and lower frequency regimes. This study reveals the chage of orbital order at around 39 K and 150 K along with the one at 120 K due to change in relaxation time associated with the impedance responce mediated by magbetic inhomogenity. Notably, across the studied frequency range, the charge conduction mechanism remains consistent, as elucidated by a variable range hopping model. Our study demonstrates that Fe3O4 nanoparticles exhibit a complex interplay of charge ordering and magnetic ordering, with implications for understanding the electrical properties of such materials. This research provides insight into the charge conduction processes and its coupling to change of orbital ordering in Fe3O4, a material of considerable interest for its promising electronic applications.

Carrier transport across PtSe2/n-type GaN heterojunction

We fabricated PtSe2/n-type GaN heterojunction diode by transferring a large-scale grown two-dimensional (2D) PtSe2 layered film onto a 3D GaN substrate. The PtSe2 film transferred onto GaN was ∼5.84-nm thick and had 11 layers with interlayer spacing of 0.52 nm. An exploration of the temperature-dependence of the current–voltage (I–V) characteristics of the PtSe2/n-type GaN heterojunction in the range 125–400 K revealed a strong dependency of the ideality factor and barrier height on measurement temperature. This indicates a presence of low- and high-barrier-height patches resembling barrier inhomogeneities along the interface. A barrier inhomogeneity analysis with a consideration for the Gaussian barrier heights distribution demonstrated the existence of a double Gaussian barrier distribution having a mean barrier height of 0.97 and 1.41 eV in low- and high-temperature regimes, respectively. An examination of the temperature-dependency of reverse leakage current indicated the dominance of variable-range hopping conduction in lower temperature region (<225 K) and Poole–Frenkel emission mechanism in higher temperature region (>250 K). The interface state density Nss of the PtSe2/n-type GaN heterojunction determined from the capacitance method was lesser than that attained from the forward I–V characteristics that can be accredited to the inhomogeneous spreading of the Nss.

Charge Carrier Transport in Iron Pyrite Thin Films: Disorder-Induced Variable-Range Hopping

Charge Carrier Transport in Iron Pyrite Thin Films: Disorder-Induced Variable-Range Hopping

Different reverse leakage current transport mechanisms of planar Schottky barrier diodes(SBDs) on sapphire and GaN substrate

The effects of different substrates on the off-state leakage current in gallium nitride (GaN) planar diodes are experimentally demonstrated and studied by analyzing temperature-dependent current–voltage characteristics. The two devices exhibit different leakage mechanisms despite being subjected to the same process conditions. The device with a sapphire substrate shows a more intricate leakage mechanism, implying the presence of additional leakage channels compared to the device with free-standing substrates. The electrical leakage characteristics in the planar GaN-on-sapphire device (referred to as utemp-SBD) and GaN substrate device (referred to as usub-SBD) Schottky barrier diodes (SBD) can be described by two distinct processes as the reverse bias gradually increases: utemp-SBD go through the thermionic emission (TE), variable range hopping (VRH), Frenkel–Poole (FP) emission and space-charge-limited conduction (SCLC) model; The leakage conduction mechanism of usub-SBD include TE, VRH, and FP emission mechanisms. It is noteworthy that in device 1, there is a rapid increase in leakage current within the voltage range of −36 to −40 V, primarily driven by the SCLC mechanism. This effect arises due to the presence of intrinsic traps in GaN grown on a heterogeneous substrate. Additionally, in the voltage range of −7 to −50 V (348 K to 448 K), the leakage mechanism shifts from FP emission to the VRH model, possibly due to variations in the Schottky barrier height.This study provides an in-depth analysis of the leakage mechanisms observed on different substrates and offers valuable insights for the design of planar gate transistors to minimize or avoid the occurrence of leakage channels.

Carrier transport mechanism of Mg/Pt/Au Ohmic contact on p-GaN/AlGaN/GaN platform with ultra-low resistivity

In this work, the carrier transport mechanism of Mg/Pt/Au Ohmic contact on p-GaN/AlGaN/GaN with a very low specific contact resistivity of 1.98 × 10−5 Ω cm2 is investigated. Secondary ion mass spectroscopy measurement results show that the Mg concentration near the p-GaN surface increases form 8 × 1019 to 7 × 1020/cm3 for Mg/Pt/Au contact after annealing. It indicates that Mg atoms from the Mg/Pt/Au metal stack diffuse into the p-GaN during annealing, forming a heavily Mg doped p++-GaN layer with a depth of about 3 nm. The sheet resistance Rsh depends on temperature for Mg/Pt/Au contacts on p-GaN/GaN, indicating that the influence of 2DHG on carrier transport mechanisms at the metal/p-GaN interface was eliminated. For Mg/Pt/Au contacts at ≥360 K, specific contact resistivity reasonably follows T−1, which indicates that the band conduction of Schottky theory dominates the carrier transport. For Mg/Pt/Au contacts at 200–360 K, the electrical resistivity reasonably follows T−1/4, indicating variable-range hopping (VRH) conduction through Mg-related deep-level defects (DLD). Based on the VRH conduction model, the effective barrier height (qφ) of the Mg-related DLD band is extracted as 0.265 eV, which well matches the excellent Ohmic contact.

Electric field and temperature dependent conductivity in PEDOT:PSS/PVA

The charge transport in conducting polymer blends of poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) [PEDOT:PSS] and polyvinyl alcohol (PVA) at 75, 50, 25, and 12 vol. % of PEDOT:PSS is investigated in the temperature range of 300–4.2 K. The scaling fit to percolative transport [σ ∝ |f − fc|t] shows that fc is around 10% and t = 2. The temperature dependent conductivity varies from the critical regime of metal–insulator transition to variable range hopping, along with the alterations in the networks of PEDOT:PSS in PVA. The variation in the exponent (n) in electric field dependent conductivity is more significant as the volume fraction of PEDOT:PSS decreases. The value of n decreases as the thermal contribution to transport dominates, showing the competing roles of both field and temperature.

Tuned Transport Path of Perovskite MAPbI3-based Memristor Structure.

In this study, the features of resistive random access memory (RRAM) employing a straightforward Cr/MAPbI3/FTO three-layer structure have been examined and clarified. The device displays various resistance switching (RS) behavior at various sweep voltages between 0.5 and 5 V. The RS effect has a conversion in the direction of the SET and RESET processes during sweeping for a number of cycles at a specific voltage. The directional change of the RS processes corresponds to the dominant transition between the generation/recombination of iodide ion and vacancy in the MAPbI3 perovskite layer and the electrochemical metallization of the Cr electrode under the influence of an electric field, which results in the conductive filament (CF) formation/rupture. At each stage, these processes are controlled by specific charge conduction mechanisms, including Ohmic conduction, space-charge-limited conduction (SCLC), and variable-range hopping (VRH). By identifying the biased voltage and the quantity of voltage sweep cycles, one can take a new approach to control or modulate the pathways for effective charge transport. This new approach is made possible by an understanding of the RS characteristics and the corresponding mechanisms causing the variation of RS behavior in the structure.