Low Temperature Hall Effect Research Articles

Engineering Pt/Co/AlO x heterostructures to enhance the Dzyaloshinskii–Moriya interaction

The study of interfacial Dzyaloshinskii–Moriya interaction (DMI) in perpendicularly magnetized structurally asymmetric heavy metal/ferromagnet multilayer systems is of high importance due to the formation of chiral magnetic textures in the presence of DMI. Here, we report the impact of cobalt oxidation at the Co/AlO x interface in Pt/Co/AlO x trilayer structures on the DMI by varying the post-growth annealing time, Al thickness and substrate. To quantify DMI we employed magneto-optical imaging of the asymmetric domain wall expansion, hysteresis loop shift, and spin-wave spectroscopy techniques. We further correlated the Co oxidation with low-temperature Hall effect measurements and x-ray photoelectron spectroscopy. Our results emphasize the importance of full characterization of the magnetic films that could be used for magnetic random access memory technologies when subjected to the semiconductor temperature processing conditions, as the magnetic interactions are critical for device performance and can be highly sensitive to oxidation and other effects.

De-carbonization of self-assembled molecular monolayers doping in silicon

Unintentional C-related contamination can be readily introduced into the substrate in self-assembled molecular monolayer doping. These C contaminants can bind with dopants, forming interstitial defects, which will in return electrically deactivate the dopants. This issue will exacerbate when the dopant concentration is low. In this paper, a low temperature oxidation method (550 °C for 30 min) is introduced to remove carbon before the phosphorus dopants are driven into silicon in a rapid thermal annealing process. The samples with and without the pre-oxidation process are characterized by the Van der Pauw, low-temperature Hall effect measurements, and secondary ion mass spectrometry analysis. The results indicate that the surface carbon concentration in silicon is nearly completely removed with the pre-oxidation process, as a result of which the electrical activity of phosphorus is indeed increased.

Epitaxial Zn3N2 thin films by molecular beam epitaxy: Structural, electrical, and optical properties

Single-crystalline Zn3N2 thin films have been grown on MgO (100) and YSZ (100) substrates by plasma-assisted molecular beam epitaxy. Depending on growth conditions, the film orientation can be tuned from (100) to (111). For each orientation, x-ray diffraction and reflection high-energy electron diffraction are used to determine the epitaxial relationships and to quantify the structural quality. Using high-temperature x-ray diffraction, the Zn3N2 linear thermal expansion coefficient is measured with an average of (1.5 ± 0.1) × 10−5 K−1 in the range of 300–700 K. The Zn3N2 films are found to be systematically n-type and degenerate, with carrier concentrations of 1019–1021 cm−3 and electron mobilities ranging from 4 to 388 cm2 V−1 s−1. Low-temperature Hall effect measurements show that ionized impurity scattering is the main mechanism limiting the mobility. The large carrier densities lead to measured optical bandgaps in the range of 1.05–1.37 eV due to Moss–Burstein band filling, with an extrapolated value of 0.99 eV for actual bandgap energy.

Dopant activation and photoresponses of boron-doped silicon by self-assembled molecular monolayers

Self-assembled molecular monolayer doping is an emerging doping technique. In this work, we investigated the activation rate and photoresponses of boron doped silicon by self-assembled molecular monolayer doping. By using low temperature Hall effect measurements and by secondary ion mass spectroscopy, we find that the activation rate of boron in these samples is in the range of 91%–54%, depending on the doping concentration. Interestingly, the photoresponsivity of the boron doped samples is also significantly higher than that of the phosphorus doped samples even though the same doping technique is used. The intriguing photoresponses are closely related to the trapping of photogenerated minority carriers by the defects in the p-type silicon.

Spin-wave analysis of the low-temperature thermal Hall effect in the candidate Kitaev spin liquid α−RuCl3

Proposed effective Hamiltonians from the literature for the material $\alpha$-RuCl${}_3$ are used to compute the magnon thermal Hall conductivity, $\kappa_{xy}$, using linear spin wave theory for the magnetically ordered state. No model previously proposed that was tested explains published experimental data. Models with Kitaev interaction $K>0$ are seen to predict $\kappa_{xy}\gtrsim 0$, which is inconsistent with the data. Fluctuations toward a Kitaev-type spin liquid would have the wrong sign to explain the data. However, a slight variant of a previously proposed model predicts a large $\kappa_{xy}$, demonstrating that the low-temperature thermal Hall effect could be generated exclusively by the Berry curvature of the magnon bands. The experimental data of $\kappa_{xy}$ can therefore serve as another method to constrain a proposed effective Hamiltonian.

Tunneling Spin Valves Based on Fe3GeTe2/hBN/Fe3GeTe2 van der Waals Heterostructures.

Thin van der Waals (vdW) layered magnetic materials hold the possibility of realizing vdW heterostructures with new functionalities. Here, we report on the realization and investigation of tunneling spin valves based on van der Waals heterostructures consisting of an atomically thin hBN layer acting as tunnel barrier and two exfoliated Fe3GeTe2 crystals acting as ferromagnetic electrodes. Low-temperature anomalous Hall effect measurements show that thin Fe3GeTe2 crystals are metallic ferromagnets with an easy axis perpendicular to the layers and a very sharp magnetization switching at magnetic field values that depends slightly on their geometry. In Fe3GeTe2/hBN/Fe3GeTe2 heterostructures, we observe textbook behavior of the tunneling resistance, which is minimum (maximum) when the magnetization in the two electrodes is parallel (antiparallel) to each other. The magnetoresistance is 160% at low temperature, from which we determine the spin polarization of Fe3GeTe2 to be 0.66, corresponding to 83% and 17% of the majority and minority carriers, respectively. The measurements also show that, with increasing temperature, the evolution of the spin polarization extracted from the tunneling magnetoresistance is proportional to the temperature dependence of the magnetization extracted from the analysis of the anomalous Hall conductivity. This suggests that the magnetic properties of the surface are representative of those of the bulk, as may be expected for vdW materials.

Suppression of the Hall number due to charge density wave order in high- Tc cuprates

Understanding the pseudogap phase in hole-doped high temperature cuprate superconductors remains a central challenge in condensed matter physics. From a host of recent experiments there is now compelling evidence of translational symmetry breaking charge density wave (CDW) order in a wide range of doping inside this phase. Two distinct types of incommensurate charge order -- bidirectional at zero or low magnetic fields and unidirectional at high magnetic fields close to the upper critical field $H_{c2}$ -- have been reported so far in approximately the same doping range between $p\simeq 0.08$ and $p\simeq 0.16$. In concurrent developments, recent high field Hall experiments have also revealed two indirect but striking signatures of Fermi surface reconstruction in the pseudogap phase, namely, a sign change of the Hall coefficient to negative values at low temperatures at intermediate range of hole doping and a rapid suppression of the positive Hall number without change in sign near optimal doping $p \sim 0.19$. We show that the assumption of a unidirectional incommensurate CDW (with or without a coexisting weak bidirectional order) at high magnetic fields near optimal doping and a coexistence of both types of orders of approximately equal magnitude at high magnetic fields at intermediate range of doping may help explain the striking behavior of low temperature Hall effect in the entire pseudogap phase.

Phosphorus ionization in silicon doped by self-assembled macromolecular monolayers

Individual dopant atoms can be potentially controlled at large scale by the self-assembly of macromolecular dopant carriers. However, low concentration phosphorus dopants often suffer from a low ionization rate due to defects and impurities introduced by the carrier molecules. In this work, we demonstrated a nitrogen-free macromolecule doping technique and investigated the phosphorus ionization process by low temperature Hall effect measurements. It was found that the phosphorus dopants diffused into the silicon bulk are in nearly full ionization. However, the electrons ionized from the phosphorus dopants are mostly trapped by deep level defects that are likely carbon interstitials.

A silicon nanowire heater and thermometer

In the thermal conductivity measurements of thermoelectric materials, heaters and thermometers made of the same semiconducting materials under test, forming a homogeneous system, will significantly simplify fabrication and integration. In this work, we demonstrate a high-performance heater and thermometer made of single silicon nanowires (SiNWs). The SiNWs are patterned out of a silicon-on-insulator wafer by CMOS-compatible fabrication processes. The electronic properties of the nanowires are characterized by four-probe and low temperature Hall effect measurements. The I-V curves of the nanowires are linear at small voltage bias. The temperature dependence of the nanowire resistance allows the nanowire to be used as a highly sensitive thermometer. At high voltage bias, the I-V curves of the nanowire become nonlinear due to the effect of Joule heating. The temperature of the nanowire heater can be accurately monitored by the nanowire itself as a thermometer.

Controlled doping by self-assembled dendrimer-like macromolecules

Doping via self-assembled macromolecules might offer a solution for developing single atom electronics by precisely placing individual dopants at arbitrary location to meet the requirement for circuit design. Here we synthesize dendrimer-like polyglycerol macromolecules with each carrying one phosphorus atom in the core. The macromolecules are immobilized by the coupling reagent onto silicon surfaces that are pre-modified with a monolayer of undecylenic acid. Nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) are employed to characterize the synthesized macromolecules and the modified silicon surfaces, respectively. After rapid thermal annealing, the phosphorus atoms carried by the macromolecules diffuse into the silicon substrate, forming dopants at a concentration of 1017 cm−3. Low-temperature Hall effect measurements reveal that the ionization process is rather complicated. Unlike the widely reported simple ionization of phosphorus dopants, nitrogen and carbon are also involved in the electronic activities in the monolayer doped silicon.

Metal-insulator transition in tin doped indium oxide (ITO) thin films: Quantum correction to the electrical conductivity

Tin doped indium oxide (ITO) thin films are being used extensively as transparent conductors in several applications. In the present communication, we report the electrical transport in DC magnetron sputtered ITO thin films (prepared at 300 K and subsequently annealed at 673 K in vacuum for 60 minutes) in low temperatures (25-300 K). The low temperature Hall effect and resistivity measurements reveal that the ITO thin films are moderately dis-ordered (kFl∼1; kF is the Fermi wave vector and l is the electron mean free path) and degenerate semiconductors. The transport of charge carriers (electrons) in these disordered ITO thin films takes place via the de-localized states. The disorder effects lead to the well-known ‘metal-insulator transition’ (MIT) which is observed at 110 K in these ITO thin films. The MIT in ITO thin films is explained by the quantum correction to the conductivity (QCC); this approach is based on the inclusion of quantum-mechanical interference effects in Boltzmann’s expression of the conductivity of the disordered systems. The insulating behaviour observed in ITO thin films below the MIT temperature is attributed to the combined effect of the weak localization and the electron-electron interactions.

Studies on the disorder in DC magnetron sputtered Cu2ZnSnS4 (CZTS) thin films grown in sulfide plasma

Cu2ZnSnS4 (CZTS) is a widely studied material in thin film photovoltaics. The desirable property of copper poor and zinc rich in CZTS thin films introduces several point defects and secondary and tertiary phases irrespective of growth techniques and growth conditions; these defects and the phases introduce disorder in the lattice. In the present study, CZTS thin films are prepared at 520K with DC magnetron sputtering technique using CZT alloy target in the presence of hydrogen sulfide plasma. The CZTS thin films have been prepared at 30W and 40W magnetron powers. X-ray diffraction and Raman spectroscopy show that the CZTS films are polycrystalline and have partially disordered kesterite (PD-KS) phase. The CZTS films grown are copper poor and zinc rich as indicated by Energy Dispersive X-ray (EDX) analysis. The optical band gap of the films is 1.56eV. The disorder of PD-KS CZTS films has been studied by the low temperature (300−20K) electrical resistivity and Hall effect measurements. The transport of charge carriers in disordered p-CZTS takes place via hopping mechanisms. CZTS films prepared with 30W magnetron power are more disordered structure due to the copper vacancy (VCu). The surface work function of CZTS thin films measured by Kelvin probe technique is 4.7eV.

Understanding the role of mobility of ITO films for silicon heterojunction solar cell applications

Transparent conducting oxides (TCO) are crucial in amorphous/crystalline silicon heterojunction solar cells performance as they have to be highly conductive to ensure the transport of photogenerated charges while remaining highly transparent to the sun spectrum. The key parameter to improve the transparency/conductivity compromise of TCO is mobility, since it allows increasing conductivity without losing transparency in the infrared region. A deep understanding of the factors influencing the mobility is then fundamental, and this is the objective of this paper. For this study, thin films of tin-doped indium oxide (ITO) were prepared by magnetron sputtering with different oxygen content. They were deeply characterized by standard and Low Temperature Hall Effect measurement, scanning and transmission electron microscopy and X-ray diffraction. The impact of oxygen on the microstructure was studied and the electrical properties were deeply investigated. In particular, the different mechanisms impacting the mobility were quantified as a function of oxygen content during ITO deposition.

Comparison of Characterization Techniques for Measurements of Doping Concentrations in Compensated n-type Silicon

Nowadays, compensated silicon (Si) is used in photovoltaic (PV) processes, whether it is through intentional co-doping of resistivity-adjusted Czochralski ingots for high efficiency n-type Si solar cells, as a result of alternative Si purification processes for the production of low-cost Si feedstock, or as a result of recycling end-of-life materials. Whatever the origin of the compensated Si, the doping concentrations need to be accurately and quickly characterized in order to control such processes. In this work, a rapid and highly sensitive characterization technique based on low temperature Hall Effect measurements is described in scientific details and compared to three well-established chemical methods: Glow Discharge Mass Spectrometry (GDMS), Inductively-Coupled Plasma Mass Spectrometry (ICP-MS), and Secondary Ion Mass Spectrometry (SIMS). The characterized samples were extracted from the n-type top part of a casted solar grade Si ingot. A very good agreement is observed between the dopants densities extracted from the electrical method and from the standard methods. With the advantage of a very low detection limit combined with a short measurement time, the advanced Hall Effect technique is promising for the rapid and accurate characterization of dopant concentrations in compensated Si.

Nanoscale Nitrogen Doping in Silicon by Self-Assembled Monolayers.

This Report presents a nitrogen-doping method by chemically forming self-assembled monolayers on silicon. Van der Pauw technique, secondary-ion mass spectroscopy and low temperature Hall effect measurements are employed to characterize the nitrogen dopants. The experimental data show that the diffusion coefficient of nitrogen dopants is 3.66 × 10−15 cm2 s−1, 2 orders magnitude lower than that of phosphorus dopants in silicon. It is found that less than 1% of nitrogen dopants exhibit electrical activity. The analysis of Hall effect data at low temperatures indicates that the donor energy level for nitrogen dopants is located at 189 meV below the conduction band, consistent with the literature value.

High-temperature superconductivity stabilized by electron-hole interband coupling in collapsed tetragonal phase ofKFe2As2under high pressure

We report a high-pressure study of simultaneous low-temperature electrical resistivity and Hall effect measurements on high quality single-crystalline KFe2As2 using designer diamond anvil cell techniques with applied pressures up to 33 GPa. In the low pressure regime, we show that the superconducting transition temperature T_c finds a maximum onset value of 7 K near 2 GPa, in contrast to previous reports that find a minimum T_c and reversal of pressure dependence at this pressure. Upon applying higher pressures, this T_c is diminished until a sudden drastic enhancement occurs coincident with a first-order structural phase transition into a collapsed tetragonal phase. The appearance of a distinct superconducting phase above 13 GPa is also accompanied by a sudden reversal of dominant charge carrier sign, from hole- to electron-like, which agrees with our band calculations predicting the emergence of an electron pocket and diminishment of hole pockets upon Fermi surface reconstruction. Our results suggest the high-temperature superconducting phase in KFe2As2 is substantially enhanced by the presence of nested electron and hole pockets, providing the key ingredient of high-T_c superconductivity in iron pnictide superconductors.

Post-growth purification of Co nanostructures prepared by focused electron beam induced deposition

In the majority of cases nanostructures prepared by focused electron beam induced deposition (FEBID) employing an organometallic precursor contain predominantly carbon-based ligand dissociation products. This is unfortunate with regard to using this high-resolution direct-write approach for the preparation of nanostructures for various fields, such as mesoscopic physics, micromagnetism, electronic correlations, spin-dependent transport and numerous applications. Here we present an in situ cleaning approach to obtain pure Co-FEBID nanostructures. The purification procedure lies in the exposure of heated samples to a H2 atmosphere in conjunction with the irradiation by low-energy electrons. The key finding is that the combination of annealing at 300 °C, H2 exposure and electron irradiation leads to compact, carbon- and oxygen free Co layers down to a thickness of about 20 nm starting from as-deposited Co-FEBID structures. In addition to this, in temperature-dependent electrical resistance measurements on post-processed samples we find a typical metallic behavior. In low-temperature magnetoresistance and Hall effect measurements we observe ferromagnetic behavior.

Pressure-Induced Unconventional Superconducting Phase in the Topological InsulatorBi2Se3

Simultaneous low-temperature electrical resistivity and Hall effect measurements were performed on single-crystalline Bi2Se3 under applied pressures up to 50 GPa. As a function of pressure, superconductivity is observed to onset above 11 GPa with a transition temperature Tc and upper critical field Hc2 that both increase with pressure up to 30 GPa, where they reach maximum values of 7 K and 4 T, respectively. Upon further pressure increase, Tc remains anomalously constant up to the highest achieved pressure. Conversely, the carrier concentration increases continuously with pressure, including a tenfold increase over the pressure range where Tc remains constant. Together with a quasilinear temperature dependence of Hc2 that exceeds the orbital and Pauli limits, the anomalously stagnant pressure dependence of Tc points to an unconventional pressure-induced pairing state in Bi2Se3 that is unique among the superconducting topological insulators.

Effect of Proton Irradiation on 2DEG in AlGaN/GaN Heterostructures

Low temperature Hall effect measurements were carried on AlGaN/GaN micro-Hall effect sensors before and after irradiation with 380 keV and fluence of 1014 protons/cm2 protons. The sheet electron density after irradiation did not show significant changes but there was a dramatic decrease in the electron mobility of the heterostructures. Prior to irradiation, the observation of well-defined Landau plateaus in the Hall resistance and Shubnikov-de Haas oscillations (SdH) at 4.5 T was indicative of the high quality the heterojunction confining the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface of micro-Hall effect sensors. In contrast, the Landau plateaus disappeared after irradiation and the threshold magnetic field required for the observation of the SdH increased, which was accompanied by a decrease of the electron mobility. Temperature dependent magnetoresistance measurements were used to deduce the effective mass and the quantum scattering time before irradiation. A negative magnetoresistance was observed at low magnetic fields which is related to weak localization and parabolic negative magnetoresistance attributed to electron-electron interaction in both samples.

Low Temperature Hall Effect Investigation of Conducting Polymer-Carbon Nanotubes Composite Network

Polypyrrole (PPy) and polypyrrole-carboxylic functionalized multi wall carbon nanotube composites (PPy/f-MWCNT) were synthesized by in situ chemical oxidative polymerization of pyrrole on the carbon nanotubes (CNTs). The structure of the resulting complex nanotubes was characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The effects of f-MWCNT concentration on the electrical properties of the resulting composites were studied at temperatures between 100 K and 300 K. The Hall mobility and Hall coefficient of PPy and PPy/f-MWCNT composite samples with different concentrations of f-MWCNT were measured using the van der Pauw technique. The mobility decreased slightly with increasing temperature, while the conductivity was dominated by the gradually increasing carrier density.