Four-probe Electrical Transport Measurements Research Articles

The Application of Photoelectric Effect in Superconducting Materials

In this paper, the history of superconducting materials and the superconducting phenomena are first briefly described, followed by a corresponding introduction to the characterization techniques that may be used in the study of superconducting materials. For example, scanning tunneling microscope is used to demonstrate the hole Fermi and electron Fermi surfaces in Fe-based superconducting materials. Fourier-transform infrared (FTIR) spectroscopy is used to study the symmetry of the superconducting band gap in Fe-based superconducting materials. Four-probe electrical transport measurements are used to measure the resistivity of superconducting materials. Angle-resolved photoemission spectroscopy (ARPES) is used to study the multi-orbital-to-electron structure of Fe-based superconducting materials, the symmetry of the superconducting band gap and its size, the various ordered states to electron. The use of ARPES to study the multi-orbital pair electron structure of Fe-based superconducting materials, the symmetry and size of the superconducting energy band gap, the structure of the various ordered states and the possible electronic coupling modes, etc. have made a great contribution to the exploration and study of superconducting materials.

Evidence for Superconductivity above 260K in Lanthanum Superhydride at Megabar Pressures.

Recent predictions and experimental observations of high T_{c} superconductivity in hydrogen-rich materials at very high pressures are driving the search for superconductivity in the vicinity of room temperature. We have developed a novel preparation technique that is optimally suited for megabar pressure syntheses of superhydrides using modulated laser heating while maintaining the integrity of sample-probe contacts for electrical transport measurements to 200GPa. We detail the synthesis and characterization of lanthanum superhydride samples, including four-probe electrical transport measurements that display significant drops in resistivity on cooling up to 260K and 180-200GPa, and resistivity transitions at both lower and higher temperatures in other experiments. Additional current-voltage measurements, critical current estimates, and low-temperature x-ray diffraction are also obtained. We suggest that the transitions represent signatures of superconductivity to near room temperature in phases of lanthanum superhydride, in good agreement with density functional structure search and BCS theory calculations.

Stable and scalable 1T MoS2 with low temperature-coefficient of resistance

Monolithic realization of metallic 1T and semiconducting 2H phases makes MoS2 a potential candidate for future microelectronic circuits. A method for engineering a stable 1T phase from the 2H phase in a scalable manner and an in-depth electrical characterization of the 1T phase is wanting at large. Here we demonstrate a controllable and scalable 2H to 1T phase engineering technique for MoS2 using microwave plasma. Our method allows lithographically defining 1T regions on a 2H sample. The 1T samples show excellent temporal and thermal stability making it suitable for standard device fabrication techniques. We conduct both two-probe and four-probe electrical transport measurements on devices with back-gated field effect transistor geometry in a temperature range of 4 K to 300 K. The 1T samples exhibit Ohmic current-voltage characteristics in all temperature ranges without any dependence to the gate voltage, a signature of a metallic state. The sheet resistance of our 1T MoS2 sample is considerably lower and the carrier concentration is a few orders of magnitude higher than that of the 2H samples. In addition, our samples show negligible temperature dependence of resistance from 4 K to 300 K ruling out any hoping mediated or activated electrical transport.

Characterization of material parameters of La0.33Pr0.34Ca0.33MnO3 thin film by terahertz time-domain spectroscopy

We present a systemic study of the terahertz (THz) optical conductivity of a strongly correlated La0.33Pr0.34Ca0.33MnO3 (LPCMO) thin film on a LaAlO3 substrate. The measurements are carried out by THz time-domain spectroscopy in the temperature regime from 15 to 105 K. The frequency-dependent optical conductivity in the metallic phase region of the samples exhibits a non-Drude-like response. We find that below 105 K, both the real and imaginary parts of the complex conductivity can be reproduced by the Drude–Smith model. The important sample and material parameters of the LPCMO thin film (such as the persistence of velocity, the ratio of carrier density to effective mass, and electronic scattering time) can be determined by fitting experimental data. The results obtained agree with those obtained from four-probe electrical transport measurements.

Quantum-interference transport through surface layers of indium-doped ZnO nanowires

We have fabricated indium-doped ZnO (IZO) nanowires (NWs) and carried out four-probe electrical-transport measurements on two individual NWs with geometric diameters of ≈70 and ≈90 nm in a wide temperature T interval of 1–70 K. The NWs reveal overall charge conduction behavior characteristic of disordered metals. In addition to the T dependence of resistance R, we have measured the magnetoresistance (MR) in magnetic fields applied either perpendicular or parallel to the NW axis. Our R(T) and MR data in different T intervals are consistent with the theoretical predictions of the one- (1D), two- (2D) or three-dimensional (3D) weak-localization (WL) and the electron–electron interaction (EEI) effects. In particular, a few dimensionality crossovers in the two effects are observed. These crossover phenomena are consistent with the model of a ‘core–shell-like structure’ in individual IZO NWs, where an outer shell of thickness t (≃15–17 nm) is responsible for the quantum-interference transport. In the WL effect, as the electron dephasing length Lφ gradually decreases with increasing T from the lowest measurement temperatures, a 1D-to-2D dimensionality crossover takes place around a characteristic temperature where Lφ approximately equals d, an effective NW diameter which is slightly smaller than the geometric diameter. As T further increases, a 2D-to-3D dimensionality crossover occurs around another characteristic temperature where Lφ approximately equals t (<d). In the EEI effect, a 2D-to-3D dimensionality crossover takes place when the thermal diffusion length LT progressively decreases with increasing T and approaches t. However, a crossover to the 1D EEI effect is not seen because LT < d even at T = 1 K in our IZO NWs. Furthermore, we explain the various inelastic electron scattering processes which govern Lφ. This work demonstrates the complex and rich nature of the charge conduction properties of group-III metal-doped ZnO NWs. This work also strongly indicates that the surface-related conduction processes are essential to doped semiconductor nanostructures.

Four-probe electrical-transport measurements on single indium tin oxide nanowires between1.5 and 300 K

Single-crystalline indium tin oxide (ITO) nanowires (NWs) were grown by the standard thermalevaporation method. The as-grown NWs were typically 100–300 nm in diameter and a fewµm long. Four-probe submicron Ti/Au electrodes on individual NWs were fabricated by theelectron-beam lithography technique. The resistivities of several single NWs have beenmeasured from 300 down to 1.5 K. The results indicate that the as-grown ITONWs are metallic, but disordered. The overall temperature behavior of resistivitycan be described by the Bloch–Grüneisen law plus a low-temperature correctiondue to the scattering of electrons off dynamic point defects. This observationsuggests the existence of numerous dynamic point defects in as-grown ITO NWs.

Ex situ doping of silicon nanowires with boron

An ex situ proximity technique is demonstrated for the electrical doping of silicon nanowires with spin on dopant (SOD) used as the boron source. The technique is based on solid-state diffusion and is comprised of two stages: predeposition and drive in. During predeposition, a predetermined amount of boron is introduced into the near surface region of the nanowires by holding the SOD source in close proximity to the nanowires. The boron concentration in the nanowires is controlled by the appropriate selection of predeposition temperature and time, with 800 and 950 °C and 5–10 min used in the present studies. The boron is then diffused further into the nanowires during the drive-in stage. The doped nanowires were characterized using scanning electron microscopy, secondary ion mass spectrometry, transmission electron microscopy, and four-probe electrical transport measurements. The high temperatures employed in this doping process do not result in any observable damage to these 120–180 nm diameter nanowires and good control over the dopant concentration in the range from 1018 to 1020 cm−3 is obtained. This ex situ doping technique provides a useful alternative to the methods currently available for electrical doping of nanowires, which are predominantly in situ techniques.

Quantum size effects in solitary wires of bismuth

We have performed four-probe electrical transport measurements on solitary highly crystalline wires of semimetallic bismuth with aspect ratios up to 60 at room and at cryogenic temperatures. By proper choice of the substrate material and the film deposition parameters, lithographic wires with lateral dimensions of down to one single grain, $\ensuremath{\sim}250\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, were fabricated. The electrical resistance of each wire was measured against its thickness through successive reactive ion etching of the self-same wire. Quantum size effects revealed themselves as regular oscillations in the electrical resistance. Some evidence for the semimetal-to-semiconductor phase transition has been detected. The measured data are discussed within the framework of the existing theoretical models.

Four-probe electrical transport measurements on individual metallic nanowires

This work presents nanoscale four-probe measurements on metallic nanowires usingindependently controlled scanning tunnelling microscope tips. This technique has allowedus to follow the change in resistance with probe separation. Gold, zinc and nickel nanowireswere grown by electrodeposition within porous polycarbonate membranes. Their structureand composition were studied by transmission electron microscopy. Four-probe electricaltransport measurements were taken using four independently controlled scanning tunnellingmicroscope tips positioned using a high resolution scanning electron microscope. MultipleI–V measurements were taken at varying tip separations, on each nanowire, and the change inresistance with separation was observed to be in good agreement with predictions based onthe nanowire geometry. The resistivity values of the nanowires were found to be close tobulk values.

Low-temperature growth and orientational control in RuO 2 thin films by metal-organic chemical vapor deposition

For growth temperatures in the range of 275°C to 425°C, highly conductive RuO 2 thin films with either (110)- or (101)-textured orientations have been grown by metal-organic chemical vapor deposition (MOCVD) on both SiO 2/Si(001) and Pt/Ti/SiO 2/Si(001) substrates. Both the growth temperature and growth rate were used to control the type and degree of orientational texture of the RuO 2 films. In the upper part of this growth temperature range (∼ 350°C) and at a low growth rate (< 3.0 nm/min.), the RuO 2 films favored a (110)-textured orientation. In contrast, at the lower part of this growth temperature range (∼ 300°C) and at a high growth rate (> 3.0 nm/min.), the RuO 2 films favored a (101)-textured orientation. In contrast, higher growth temperatures (> 425°C) always produced randomly-oriented polycrystalline films. For either of these low-temperature growth processes, the films produced were crack-free, well-adhered to the substrates, and had smooth, specular surfaces. Atomic force microscopy showed that the films had a dense microstructure with an average grain size of 50–80 nm and a rms. surface roughness of ∼ 3–10 nm. Four-probe electrical transport measurements showed that the films were highly conductive with resistivities of 34–40 μΩ-cm (at 25°C).