High-resistivity Samples Research Articles

Crystal Structure, Luminescence and Electrical Conductivity of Pure and Mg2+-Doped β-Ga2O3-In2O3 Solid Solutions Synthesized in Oxygen or Argon Atmospheres.

Undoped and Mg2+-doped β-Ga2O3-20% In2O3 solid solution microcrystalline samples were synthesized using the high-temperature solid-state chemical reaction method to investigate the influence of native defects on structural, luminescent, and electrical properties. The synthesis process involved varying the oxygen partial pressure by synthesizing samples in either an oxygen or argon atmosphere. X-ray diffraction (XRD) analysis confirmed the monoclinic structure of the samples with the lattice parameters and unit cell volume fitting well to the general trends of the (Ga1-xInx)2O3 solid solution series. Broad emission spectra ranging from 1.5 to 3.5 eV were registered for all samples. Luminescence spectra showed violet, blue, and green emission elementary bands. The luminescence intensity was found to vary depending on the synthesis atmosphere. An argon synthesis atmosphere leads to increasing violet luminescence and decreasing green luminescence. Intense bands at about 4.5 and 5.0 eV and a low-intensity band at 3.3 eV are presented in the excitation spectra. The electrical conductivity of the samples was also determined depending on the synthesis atmosphere. The high-resistance samples obtained in an oxygen atmosphere exhibited activation energy of around 0.98 eV. Samples synthesized in an argon atmosphere demonstrated several orders of magnitude higher conductivity with an activation energy of 0.15 eV. The results suggest that the synthesis atmosphere is crucial in determining the luminescent and electrical properties of undoped β-Ga2O3-In2O3 solid solution samples, offering the potential for various optoelectronic applications.

Features of a Low-Temperature Charge Density Wave in the Monoclinic Phase of NbS3 Manifested in the NMR and in Transport Properties

The relaxation of the transverse nuclear magnetization in the monoclinic phase of NbS3 has been studied by the 93Nb nuclear magnetic resonance method near the temperature TP2 = 150 K, at which a low-temperature charge density wave is formed. It has been shown that the critical slowing down of one of the vibrational modes of the lattice, which is quite slow even above TP2, occurs slightly below TP2. The transition at TP2 occurs not only in low-resistance samples, as thought previously, but also in high-resistance ones, and involves Nb atoms in the bulk of a sample. The transport properties of high-resistance samples, namely, the smearing of the depinning threshold for the charge density wave below TP2, confirm that the phase transition in them occurs at TP2. It has been concluded that the distortion of the lattice at TP2 is not due to the Peierls mechanism and can be attributed to the Keldysh–Kopaev transition. Another possible mechanism is the fluctuation distortion of the lattice above TP2 that prevents the sliding of the charge density wave.

A Brief Study of Methods to Determine the Electrical Resistivity of Materials

<p class="Abstract">Present study deals with the study of methods to calculate the resistivity of materials. Different methods are discussed which determine the resistivity of specific materials on the basis of their shapes and sizes. Their working, structure, advantages and loopholes are discussed in a brief manner. Materials are categorized according to their magnitude of resistivity. The two probe method is helpful in measuring the resistivity of high resistive samples while four probe methods can be used for materials with low resistance. Pulse probe method measures the resistivity of materials having small physical dimensions. The Montgomery method is useful in finding the individual resistivities of anisotropic materials.</p>

New thermally stimulated emission spectrometer for the detection of ultra-shallow low-density traps

Electron/hole traps alter the optical and electrical properties of materials by creating additional recombination pathways, trapping or providing charge carriers and modifying exciton dynamics. Understanding the defect/trap dynamics is crucial to control the optoelectronic properties of materials, and measuring donor/acceptor ionization energy is critical in semiconductor research. Here, we developed a highly sensitive thermally stimulated emission (TSE) spectrometer for the low temperature regime of 9–325 K to detect and characterize shallow traps in bandgap materials with enhanced sensitivity. It provides a powerful characterization tool for a wide range of semiconductors and electronic and photonic materials. This technique is ideal where electrical methods cannot be used for donor/acceptor characterization as in powder, irregular shape and thickness, and high resistive samples. The performance of the spectrometer was tested on Ce doped Y3Al5O12 single crystals, and the measurements identified several shallow levels that cannot be detected with conventional methods. Then, its capabilities were further demonstrated by detecting a shallow level in Ga2O3, which is emerging as an exceptional semiconductor for high-power devices and optoelectronics. A sophisticated data analysis technique based on the three-point analysis (TPA) approach was applied to deconvolute the highly overlapped TSE signals. The developed ultra-low temperature spectrometer together with the TPA deconvolution method provides a unique tool for studying exciton dynamics in photonic materials and measuring donor/acceptor ionization energies and densities in luminescent semiconductors. It will advance material characterization and development for a wide range of applications including lasers, electronic and illumination devices, and detectors for medical diagnostic and nuclear applications.

Passivation of the beryllium acceptor in GaN and a possible route for p-type doping

Using theory and experiment, we address a possible p-type doping route of GaN using the Be acceptor. Hybrid functional calculations suggest that straightforward incorporation of Be in GaN will likely lead to compensated high resistivity samples. In this case, beryllium interstitials and nitrogen vacancies are the most likely compensating donors. However, calculations also show that simultaneous incorporation of beryllium and hydrogen in the nitrogen-rich growth regime is likely to lead to the formation of neutral BeGa-Hi complexes. These complexes are energetically favorable and do not exhibit any transition levels in the bandgap. Subsequent annealing in hydrogen-free or nitrogen ambient will dissociate these complexes and activate shallow BeGa acceptors in GaN. Experimental measurements, where an ultraviolet photoluminescence band related to the shallow BeGa acceptor emerges after annealing the Be-doped GaN, confirm these findings.

Using resistivity data to study the waterflooding process: A case study in tight sandstone reservoirs of the Ordos Basin, China

Water injection is one of the most common methods used to enhance the oil recovery of reservoirs with low permeability and strong heterogeneity. However, with the injection of water in the production process, the formation water salinity, oil saturation, and the distribution of the residual oil in the reservoir are altered, causing difficulties in the evaluation of the flooded layers. Consequently, it is crucial to understand the changes in reservoir resistivity when evaluating waterflooded layers with different wettabilities. Resistivity experiments for high-resistivity (HR) and normal-resistivity (NR) tight sandstone samples are carried out to simulate different types of waterflooding processes. Specifically, fresh water and the original formation water with higher salinity are separately used to displace oil in rock samples, including the HR and NR samples. Our results indicate that the salinity of injected water and the reservoir wettability can affect the reservoir resistivity after waterflooding. When fresh water is injected, the resistivity of NR and HR samples initially decreases, and then increases due to reduced oil saturation and increased salinity of the injected fresh water. In contrast, the resistivity of NR and HR samples decreases monotonically when the original formation water is injected. Our study also indicates that the formation water injection facilitates resistivity logging evaluation of high-resistance waterflooding layers better than fresh-water injection. To our knowledge, this is the first report of such a rich set of resistivity experiments in tight sandstone waterflooded reservoirs with complex wettability. Finally, combining waterflooding resistivity experiments with relative permeability, we have developed a fast, new, convenient method for evaluating the extent of waterflooding of a tight sandstone reservoir, which has been successfully applied to field-data examples for water-cut evaluation.

E-TCT characterization of a thinned, backside biased, irradiated HV-CMOS pixel test structure

This paper presents the results of edge-Transient Current Technique (e-TCT) measurements of a test structure of a High Voltage CMOS (HV-CMOS) pixel demonstrator, the H35DEMO. Several high resistivity (1000 Ω⋅cm) samples of the device were thinned to 100μm, processed for backside biasing, and irradiated with neutrons to fluences up to 2⋅1016neq⋅cm−2. The evolution of effective doping concentration with respect to fluence is studied. Samples irradiated to a fluence of 5⋅1014neq⋅cm−2 are fully depleted beyond −50V substrate bias voltage while samples irradiated to the highest fluence reach 30 μm depletion at −200V.

Thermoelectric measurements of high-resistance Janus monolayer transition-metal dichalcogenide

The broken reflection symmetry exhibited by the recently reported two-dimensional Janus SMoSe monolayers can lead to unique electronic, thermal, and thermoelectric properties. Thermoelectric measurements of these and other semiconducting materials are often complicated by a loading error associated with a high sample resistance. Here, we report a thermoelectric measurement of a monolayer SMoSe sample with a resistance exceeding 40 MΩ. The loading error associated with the high sample resistance and other contaminations is removed with the use of high-impedance electronics, discrete modulated heating, and Fourier transform analysis of the measured thermoelectric voltage. Under a 20 V gate voltage, the measured Seebeck coefficient of the Janus monolayer changes from −200 µV/K at 350 K to −220 µV/K at 425 K. These results are consistent with an n-type semiconductor behavior of the Janus monolayer sample and demonstrate the effectiveness of this method for thermoelectric measurements of high-resistance semiconductor samples.

Optical conductivity of granular aluminum films near the Mott metal-to-insulator transition

We report measurements of the energy gap of granular aluminum films by THz spectroscopy. We find that as the grains progressively decouple, the coupling ratio $2\Delta(0)/k_{B}T_{c}$ increases above the BCS weak coupling ratio $3.53$, and reaches values consistent with an approach to BCS-BEC crossover for the high resistivity samples, expected from the short coherence length. The Mattis-Bardeen theory describes remarkably well the behavior of $\sigma_{1,s}/\sigma_{1,n}$ for all samples up to very high normal state resistivities.

Effects of the resistivity of AZO film on the IV and CV characteristics of AZO/p-Si heterojunction

Al-doped ZnO (AZO) films with a range of resistivity were deposited on p-Si substrates in oxygen background by varying the laser fluence and substrate temperature using pulsed laser deposition. AZO films with roughness of 4.1–10.3 nm were deposited where the resistivity of the films range from 10−3 Ω cm to 10−1 Ω cm. Ohmic contacts were coated onto the AZO/p-Si heterojunctions to form Au/AZO/p-Si/Al structures and the samples were characterized using I-V and CV measurements. The samples with high resistivity AZO film (~ 10−1 Ω cm) deposited at room temperature exhibited MOS-like characteristics and AZO/p-Si heterojunction with low resistivity AZO films (~ 10−3 Ω cm) deposited at 100 °C to 300 °C showed p-n junction characteristics. Based on the junction formed, the barrier potential, depletion width, depletion capacitance and oxide thickness were deduced. The CV characteristics of the high resistivity samples deposited at room temperature were also shown to be affected by the presence of interface states within the junction.

Conductance Spectroscopy of Exfoliated Thin Flakes of Nb xBi2Se3.

We study unconventional superconductivity in exfoliated single crystals of a promising three-dimensional (3D) topological superconductor candidate, Nb-doped Bi2Se3 through differential conductance spectroscopy and magneto-transport. The strong anisotropy of the critical field along the out-of-plane direction suggests that the thin exfoliated flakes are in the quasi-2D limit. Normal metal-superconductor (NS) contacts with either high or low transparencies made by depositing gold leads onto Nb-doped Bi2Se3 flakes both show significant enhancement in zero bias conductance and coherence dips at the superconducting energy gap. Such behavior is inconsistent with conventional Blonder-Tinkham-Klapwijk theory. Instead, we discuss how our results are consistent with p-wave pairing symmetry, supporting the possibility of topological superconductivity in Nb-doped Bi2Se3. Finally, we observe signatures of multiple superconducting energy gaps, which could originate from multiple Fermi surfaces reported earlier in bulk crystals.

Optical characterization of high and low resistive silicon samples suitable for reconfigurable antenna design

AbstractHighly resistive (HR) silicon (Si) can behave as a switch when illuminated by optical source of suitable wavelength. Different reconfigurable passive structures, such as filters, waveguides, and antennas, can be constructed using such silicon switches. This letter presents experimental characterization of high and low resistive (HR & LR) silicon for switching application. In the experiment, HR Si is modeled on a switched transmission line by halogen lamp and a laser source. The experiment of utilizing halogen lamp for Si switch characterization is cost‐effective and can assist engineers in designing reconfigurable antennas. In future, this experiment could be utilized in designing novel reconfigurable antennas.

Potential Images of Ferroelectric Domain Structures Formed by Electron Beam in Lithium Niobate Crystals

Ferroelectric domain structures formed by an electron beam in lithium niobate crystals are studied using low-voltage SEM microscopy. The structures are formed in crystals with different conductivity, including samples with high-resistance congruent composition (CLN) and samples with conductivity increased by reductive annealing (RLN). The potential nature of the contrast of the domain structures observed in the secondary electron mode depending on the conductivity of the samples and the direction of spontaneous polarization of the domains is analyzed. It is assumed that the domain contrast in CLN crystals is associated with long-lived charges localized near domain walls and in the irradiated areas. The recorded domain structures in the CLN crystals are visualized on polar and nonpolar cuts. In the RLN crystals with improved conductivity compared to CLN, the potential contrast of the periodic domain structures is found only on the polar cuts, where vector Ps of the domains is perpendicular to the irradiated surface. This contrast is likely because the field of the spontaneous electric polarization charges influences the secondary electrons.

Потенциальные изображения сегнетоэлектрических доменных структур в кристаллах ниобата лития после формирования электронным лучом

AbstractFerroelectric domain structures formed by an electron beam in lithium niobate crystals are studied using low-voltage SEM microscopy. The structures are formed in crystals with different conductivity, including samples with high-resistance congruent composition (CLN) and samples with conductivity increased by reductive annealing (RLN). The potential nature of the contrast of the domain structures observed in the secondary electron mode depending on the conductivity of the samples and the direction of spontaneous polarization of the domains is analyzed. It is assumed that the domain contrast in CLN crystals is associated with long-lived charges localized near domain walls and in the irradiated areas. The recorded domain structures in the CLN crystals are visualized on polar and nonpolar cuts. In the RLN crystals with improved conductivity compared to CLN, the potential contrast of the periodic domain structures is found only on the polar cuts, where vector P _ s of the domains is perpendicular to the irradiated surface. This contrast is likely because the field of the spontaneous electric polarization charges influences the secondary electrons.

Temperature Dependence of the Seebeck Coefficient in Zinc Oxide Thin Films

Thermoelectric devices are reliable tools for converting waste heat into electricity as they last long, produce no noise or vibration, have no moving elements, and their light weight makes them suitable for the outer space usage. Materials with high thermoelectric figure of merit (zT) have the most important role in the fabrication of efficient thermoelectric devices. Metal oxide semiconductors, specially zinc oxide has recently received attention as a material suitable for sensor, optoelectronic and thermoelectric device applications because of their wide direct bandgap, chemical stability, high-energy radiation endurance, transparency and acceptable zT. Understanding the thermoelectric properties of the undoped ZnO thin films can help design better ZnO-based devices. Here, we report the results of our experimental work on the thermoelectric properties of the undoped polycrystalline ZnO thin films. These films are deposited on alumina substrates by thermal evaporation of zinc in vacuum followed by a controlled oxidation process in air carried out at the 350-500 °C temperature range. The experimental setup including gradient heaters, thermometry system and Seebeck voltage measurement equipment for high resistance samples is described. Seebeck voltage and electrical resistivity of the samples are measured at different conditions. The observed temperature dependence of the Seebeck coefficient is discussed.

Characterization of the CVD Graphene Monolayer as an Active Element of a One-Port Microwave Device

The one-port configuration of a microwave device is the simplest architecture for graphene ultrafast (photo-)resistors, (photo-)diodes, sensors, and photodetectors. Here, this configuration is realized by a segment of coplanar waveguide (CPW) loaded with a CVD graphene monolayer shorted to the ground. The magnitude and phase of the reflection coefficient ( ${S}_{11}$ parameter) measured in the $0.1~\div ~26$ GHz frequency range are presented for undisturbed structures as well as for structures illuminated by white light and electrostatically polarized with the backgate. A simple and robust de-embedding procedure based on the signal flow graph and allowing for a simultaneous extraction of the parameters characterizing the CPW segment and the graphene monolayer is proposed. The graphene impedance at microwave frequencies is in good agreement with the value of dc sheet resistance obtained from van der Pauw measurements. The back-gate voltage modulation is an efficient way to modulate both the dc sheet resistance and the impedance. In particular, the Dirac point can be achieved both at microwave frequencies and at dc. An equivalent circuit model consists of only the resistance and capacitance connected in parallel, because of good quality ohmic contacts and a negligible inductance as a result of low-current flow in high-resistance graphene samples.

Nuclear magnetic resonance features of low-permeability reservoirs with complex wettability

The nuclear magnetic resonance T2 spectra of low-permeability reservoirs with complex wettability were studied using the samples from the Chang 8 Member, Upper Triassic Yanchang Formation, Ordos Basin, China. Abnormally high resistivity and normal resistivity core samples were selected. NMR T2 spectra under different wettability and water saturation conditions, contact angles and Amott wettability indexes were designed and tested. The test results show that under fully brine-saturated condition, the NMR T2 spectra of normal resistivity core samples reflect surface relaxation of water, while the samples with abnormally high resistivity exhibit wide unimodal T2 spectrum, consisting of both surface and volume relaxation of water, which indicates that these cores are not fully water-wet after washing oil. In the process of oil displacing water, the NMR T2 spectra of normal resistivity core samples present bimodal feature, and those of abnormal high resistivity core samples (both non-ageing and ageing) mainly show the same unimodal feature as those measured under fully brine-saturated condition. Based on these results, it can be inferred that the wettability change of abnormally high resistivity core samples to oil-wet has basically completed during oil displacing water process, and the ageing process has little effect on the wettability of abnormally high resistivity core samples. In the process of water displacing oil to residual oil, the NMR T2 spectra of abnormally high resistivity core samples generally show trimodal feature, among which, the shortest relaxation time spectrum peaks coincide with that under irreducible water saturation condition, the moderate ones reflect surface and volume relaxation of residual oil, and the longest ones reflect surface and volume relaxation of water in large pores.

Direct observation of hopping induced spin polarization current in oxygen deficient Co-doped ZnO by Andreev reflection technique

Oxygen vacancy induced ferromagnetic coupling in diluted magnetic oxide (DMO) semiconductors have been reported in several studies, but technologically more crucial spin-polarized current (SPC) is still under-developed in DMOs. Few studies have claimed that VRH mechanism can originate the SPC, but, how VRH mechanism associated with percolation path, is not clearly understood. We used Point-contact Andreev reflection (PCAR) technique to probe the SPC in Co-doped ZnO (CZO) films. Since the high resistance samples cause broadening in conductance(G)-voltage(V) curves, which may result in an unreliable evaluation of spin polarization, we include two extra parameters, (i) effective temperature and (ii) spreading resistance, for the simulation to avoid the uncertainty in extracting spin polarization. The effective G-V curves and higher spin polarization can be obtained above a certain oxygen vacancy concentration. The number of completed and fragmentary percolation paths is proportional to the concentration of oxygen vacancies. For low oxygen vacancy samples, the Pb-tip has a higher probability of covering fragmentary percolation paths than the complete ones, due to its small contact size. The completed paths may remain independent of one another and get polarized in different directions, resulting in lower spin-polarization value. High oxygen vacancy samples provide a high density of completed path, most of them link to one another by crossing over, and gives rise to high spin-polarization value.

Deep levels in low resistive Zn3P2

The low resistivity p‐type polycrystalline Zn3P2 grown by means of closed tube vapor transport was investigated. Deep carrier traps have been studied by means of deep level transient spectroscopy (DLTS) and by photocapacitance transient measurements. From DLTS measurements parameters of four main hole traps were determined. The first with activation energy 0.17 eV above the top of the valence band, probably connected with zinc vacancy, the defect with activation energy 0.34 eV which origin is unknown, the defect with activation energy 0.46 eV, probably oxygen related and the defect with activation energy 0.77 eV with unknown origin. For the defects with energies 0.17 and 0.46 eV nonzero values of capture energies were found: 0.03 and 0.16 eV, respectively. From photocapacitance measurements three optical activation energies were determined as steps on the curve: 0.83, 0.99, and 1.25 eV. Defects with optical activation energies of 0.83 eV is probably the same defect as defect with activation energy 0.46 eV and energy barrier for capture 0.16 eV obtained from DLTS measurements, and may be responsible for persistent photoconductivity (PPC) efect in high resistive samples of Zn3P2.

Electric conductivity of polymer films filled with magnetic nanoparticles

The conductivity of polymer composites with magnetic nanoparticles (MNP) containing magnetite and other MNP (Ni, Cu–Ni) in the layers and planar cells with Al electrodes is studied. For soluble polymers (polyvinylpyrrolidone and polyvinyl alcohol) containing 1–10 wt % of magnetite MNP, a substantial effect of MNP on surface conductivity is detected over a wide range (from 10–10 to 10–3 Ω–1). It is shown that the addition of magnetite MNP not only results in a considerable change in cell conductivity, but also leads to its partially irreversible variation (by an order of magnitude or more) via minor modifications of the experimental conditions (temperature, electric field). For high-resistance samples with low probabilities of conducting chain formation, temperature current peaks are observed upon moderate heating (up to 350 K). These peaks are similar to the maxima observed upon polymer electret thermodischarges when the charges are captured by the deep centers associated with separate MNP or MNP aggregates. The type and position of the maxima are determined by the characteristics of the polymer matrix. For polyvinylpyrrolidone composites, the maxima are observed some time after heating (the echo effect). With composites based on solventborne polymers (polyalkanesterimides, soluble polyimide) and Ni, Cu–Ni MNP, no change in film conductivity measured electrophotographically is observed, due to the formation of a dielectric coating formed by polymer macromolecules adsorbed on the MNP surface. An explanation based on the possible formation of magnetic aggregates of magnetite MNP and conducting chains is proposed. Magnetic aggregation IPM is proposed as one way of controlling cell conductivity.