De Haas Measurements Research Articles

Electron transport in N-polar GaN-based heterostructures

Electron transport in N-polar GaN-based high-electron-mobility transistor (HEMT) structures with a combination of In0.18Al0.82N-AlN as the barrier was studied via temperature-dependent van der Pauw Hall and Shubnikov de Haas measurements. In contrast to Ga-polar HEMT structures, no persistent photoconductivity could be detected. In a sample with 10 nm thick InAlN, only one oscillation frequency was observed, demonstrating that a single sublevel is present. From the oscillations, a two-dimensional electron gas carrier density of 8.54 × 1012 cm−2 and a mobility of 4970 cm2/V s were extracted at 1.7 K. This sample was further investigated using ionic liquid gating. The charge density was varied from 7.5 × 1012 cm−2 to 9.6 × 1012 cm−2. The electron mobility significantly declined with decreasing charge density. This is in contrast to Ga-polar HEMT structures, where the electron mobility typically increases slightly as the charge density decreases.

Shubnikov–de Haas measurements on a high mobility monolayer graphene flake sandwiched between boron nitride sheets

A flake of monolayer graphene was sandwiched between boron nitride sheets. Temperature dependent Shubnikov–de Haas measurements were performed to access how this technique influences the electronic properties of the graphene sample. The maximum mobility found in this configuration was approximately 105 cm2 Vs −1. From the phase of the oscillations a Berry phase β of 1/2 was obtained indicating the presence of Dirac fermions. We obtained Fermi velocities around m s−1 which imply hopping energies close to 2.5 eV. Furthermore, the carrier lifetime is typically higher than that found in graphene supported by SiO2, reaching values higher than 700 fs.

Fermi surface evolution of Na-doped PbTe studied through density functional theory calculations and Shubnikov–de Haas measurements

We present a combined experimental and theoretical study of the evolution of the low-temperature Fermi surface of lead telluride, PbTe, when holes are introduced through sodium substitution on the lead site. Our Shubnikov-de-Haas measurements for samples with carrier concentrations up to $9.4\times10^{19}$cm$^{-3}$ (0.62 Na atomic $\%$) show the qualitative features of the Fermi surface evolution (topology and effective mass) predicted by our density functional (DFT) calculations within the generalized gradient approximation (GGA): we obtain perfect ellipsoidal L-pockets at low and intermediate carrier concentrations, evolution away from ideal ellipsoidicity for the highest doping studied, and cyclotron effective masses increasing monotonically with doping level, implying deviations from perfect parabolicity throughout the whole band. Our measurements show, however, that standard DFT calculations underestimate the energy difference between the L-point and $\Sigma$-line valence band maxima, since our data are consistent with occupation of a single Fermi surface pocket over the entire doping range studied, whereas the calculations predict an occupation of the $\Sigma$-pockets at higher doping. Our results for low and intermediate compositions are consistent with a non-parabolic Kane-model dispersion, in which the L-pockets are ellipsoids of fixed anisotropy throughout the band, but the effective masses depend strongly on Fermi energy.

Berry phase in cuprate superconductors

Geometrical Berry phase is recognized as having profound implications for the properties of electronic systems. Over the last decade, Berry phase has been essential to our understanding of new materials, including graphene and topological insulators. The Berry phase can be accessed via its contribution to the phase mismatch in quantum oscillation experiments, where electrons accumulate a phase as they traverse closed cyclotron orbits in momentum space. The high-temperature cuprate superconductors are a class of materials where the Berry phase is thus far unknown despite the large body of existing quantum oscillations data. In this report we present a systematic Berry phase analysis of Shubnikov - de Haas measurements on the hole-doped cuprates YBa$_2$Cu$_3$O$_{y}$, YBa$_2$Cu$_4$O$_8$, HgBa$_2$CuO$_{4 + \delta}$, and the electron-doped cuprate Nd$_{2-x}$Ce$_x$CuO$_4$. For the hole-doped materials, a trivial Berry phase of 0 mod $2\pi$ is systematically observed whereas the electron-doped Nd$_{2-x}$Ce$_x$CuO$_4$ exhibits a significant non-zero Berry phase. These observations set constraints on the nature of the high-field normal state of the cuprates and points towards contrasting behaviour between hole-doped and electron-doped materials. We discuss this difference in light of recent developments related to charge density-wave and broken time-reversal symmetry states.

Gate voltage control of the Rashba effect in a p-type GaSb quantum well and application in a complementary device

The gate voltage dependence of the Rashba effect in a p-type quantum well was investigated by using Shubnikov–de Haas measurements. The GaSb-based p-type quantum well has a large Rashba spin–orbit interaction parameter of 1.71×10−11eVm for a zero gate voltage and exhibits gate controllability. We also propose a complementary logic device using n- and p-type spin transistors that simultaneously utilize charge and spin currents to improve the signal margin.

Study of Shubnikov–de Haas oscillations and measurement of hole effective mass in compressively strained In XGa 1−XSb quantum wells

In X Ga 1− X Sb has the highest hole mobility amongst all III–V semiconductors which can be enhanced further with the use of strain. The use of confinement and strain in In X Ga 1− X Sb quantum wells lifts the degeneracy between the light and heavy hole bands which leads to reduction in the hole effective mass in the lowest occupied band and an increase in the mobility. We present magnetotransport measurements on compressively strained In X Ga 1− X Sb and GaSb quantum wells. Hall-bar and Van de Pauw structures were fabricated and Shubnikov–de Haas oscillations in the temperature range of T = 2–10 K for magnetic fields of B = 0–9 T were measured. The reduction of effective hole mass with strain was quantified. These results are in excellent agreement with modeling results from band structure calculations of the effective hole mass in the presence of strain and confinement.

Formula omitted] C-NMR study of [formula omitted] Cu(NCS) [formula omitted] under pressure

We measured the 13C-NMR spectrum and T 1 of κ -(BEDT-TTF) 2Cu(NCS) 2 under pressure. Both of Knight shift and 1 / T 1 T showed the temperature independent behavior at low temperature under all pressures, therefore it is suggested that this material behaves as Fermi liquid at this region. Pressure dependence of Knight shift at low temperature cannot account the strong pressure dependence of the m c * , which observed by Shubnikov–de Haas (S–dH) measurement. On the other hand, pressure dependence of Korringa factor, calculated from Knight shift and 1 / T 1 T , is similar to that of the m c * . These results suggest that anti-ferromagnetic (AF) fluctuations contribute to the superconductivity in this material.

Electron transport in gated InGaAs and InAsP quantum well wires in selectively grown InP ridge structures

The purpose of this work is to fabricate ribbon-like InGaAs and InAsP wires embedded in InP ridge structures and investigate their transport properties. The InP ridge structures that contain the wires are selectively grown by chemical beam epitaxy (CBE) on pre-patterned InP substrates. To optimize the growth and micro-fabrication processes for electronic transport, we explore the Ohmic contact resistance, the electron density, and the mobility as a function of the wire width using standard transport and Shubnikov–de Haas measurements. At low temperatures the ridge structures reveal reproducible mesoscopic conductance fluctuations. We also fabricate ridge structures with submicron gate electrodes that exhibit non-leaky gating and good pinch-off characteristics acceptable for device operation. Using such wrap gate electrodes, we demonstrate that the wires can be split to form quantum dots evidenced by Coulomb blockade oscillations in transport measurements.

Energy relaxation probed by weak antilocalization measurements in GaN heterostructures

Energy relaxation and electron-phonon (e-p) interaction are investigated in wurtzite Al0.15Ga0.85N/AlN/GaN and Al0.83In0.17N/AlN/GaN heterostructures with polarization induced two-dimensional electron gases in the Bloch–Grüneisen regime. Weak antilocalization (WAL) and Shubnikov–de Haas measurements were performed on gated Hall bar structures at temperatures down to 0.3 K. We used WAL as a thermometer to measure the electron temperature Te as a function of the dc bias current. We found that the power dissipated per electron, Pe, was proportional to Te4 due to piezoelectric acoustic phonon emission by hot electrons. We calculated Pe as a function of Te without any adjustable parameters for both the static and the dynamic screening cases of piezoelectric e-p coupling. In the temperature range of this experiment, the static screening case was expected to be applicable; however, our data was in better agreement with the dynamic screening case.

Large zero-field spin splitting in AlGaN/AlN/GaN/AlN heterostructures

This work describes Shubnikov–de Haas (SdH) measurements in Al0.22Ga0.78N/AlN/GaN/AlN heterostructures. Our experiments coupled with the analysis of the Hall data at various temperatures confirm the formation of a two-dimensional electron gas (2DEG) at the AlN/GaN interface. A beating pattern in the SdH oscillations is also observed and attributed to a zero-field spin splitting of the 2DEG first energy subband. The values of the effective spin-orbit coupling parameter and zero-field spin-split energy are estimated and compared with those reported in the literature. We show that zero-field spin-split energy tends to increase with increasing sheet electron density and that our value (12.75 meV) is the largest one reported in the literature for GaN-based heterostructures.

Influence of the illumination on the subband structure and occupation in Al x Ga1−x N/GaN heterostructures

The subband structure and occupation in the triangular quantum well at Al x Ga1−x N/GaN heterointerfaces have been investigated by means of temperature dependent Shubnikov–de Haas (SdH) measurements at low temperatures and high magnetic fields under illumination. After the illumination of the heterostructures, the total two-dimensional electron gas concentration increases, and the SdH oscillation amplitudes are enhanced when there is no additional subband occupation. It is also found that the energy separation between the subbands decreases after the illumination. We suggest that the illumination decreases the electric field and thus weakens the quantum confinement of the triangular quantum well at Al x Ga1−x N/GaN heterointerfaces. The GaN layer is thought to be the primary contributor of the excited electrons by the illumination.

Scattering times in the two-dimensional electron gas of AlxGa1−xN/AlN/GaN heterostructures

Low-field Hall and Shubnikov–de Haas (SdH) measurements were performed on two-dimensional electron gas of Al0.24Ga0.76N/GaN and Al0.24Ga0.76N/AlN/GaN heterostructures at a low temperature. A dramatic improvement in electron mobility is observed in Al0.24Ga0.76N/AlN/GaN heterostructures with 1 and 2 nm thick AlN interlayers. A further increase in the AlN thickness degrades the transport behaviour. Transport and quantum scattering times were derived from the corresponding Hall mobility and the oscillatory part of the SdH signal. The ratio of the transport scattering time and the quantum scattering time increases from 3.87 in the heterostructure without the AlN interlayer to 14.34 with the 1 nm AlN interlayer and to 12.45 with the 2 nm AlN interlayer. A modified scattering calculation due to charged dislocation with a finite length suggests that dislocation related long-range scattering event tends to dominate both transport and quantum scattering times when a thin AlN interlayer is introduced.

Zero-field spin splitting in AlxGa1−xN/GaN heterostructures with various Al compositions

The zero-field spin splitting in AlxGa1−xN/GaN heterostructures with various Al compositions has been investigated at low temperatures and high magnetic fields. The zero-field spin-splitting energy and the spin-orbit coupling parameter are obtained by means of beating pattern Shubnikov–de Haas measurements. It is found that the spin-orbit coupling parameter can be tuned by the polarization-induced electric field. The AlxGa1−xN/GaN heterostructure is one of the promising materials for the spin-polarized field effect transistor.

Negative photoconductivity in In 0.52Al 0.48As/In 0.7Ga 0.3As heterostructures

We investigated the transport of a two-dimensional electron gas (2DEG) in an In 0.52Al 0.48As/In 0.7Ga 0.3As heterostructure when exposed to light from infrared laser diodes ( λ=0.78, 1.3 μm) by means of both Shubnikov–de Haas and Hall-effect measurements. We observed negative photoconductivity due to a reduction in the number of electrons in the 2DEG when they were illuminated by photons at λ=1.3 μm. We speculate that the negative photoconductivity originates from the diffusion and trapping of photo-induced hot electrons at deep impurity levels in the InAlAs barrier layer near the InAlAs/InGaAs interface.

Room-temperature spin-oriented photocurrent under near-infrared irradiation and comparison of optical means with Shubnikov de-Haas measurements in AlxGa1−xN∕GaN heterostructures

The admixture of linear and circular photogalvanic effects and (CPGEs) in AlxGa1−xN∕GaN heterostructures has been investigated quantitatively by near-infrared irradiation at room temperature. The spin-based photocurrent that the authors have observed solidly indicates the sizable spin-orbital interaction of the two-dimensional electron gas in the heterostructures. Further analysis shows consistency between studies by optical and magnetic (Shubnikov de-Haas) measurements on the spin-orbital coupling effects among different AlxGa1−xN∕GaN heterostructures, indicating that the CPGE measurement is a good way to investigate the spin splitting and the spin polarization in semiconductors.

Magnetotransport characterization of AlGaN/GaN interfaces

AbstractMagnetotransport studies of high quality AlGaN/GaN heterojunctions are reported. The transport and quantum lifetimes were respectively deduced from low temperature mobility and Shubnikov de Haas measurements as a function of carrier density in metal organic vapor phase epitaxy grown AlGaN/GaN/sapphire heterostructures. It is found that contrary to theoretical predictions both quantum and transport lifetimes have the same bell shape behavior versus carrier density. We determined experimental ratio of quantum to transport lifetime varying from 9 to 16 for carrier densities in 1–9 × 1012 cm–2 range. We show that the quantum lifetime is very sensitive to interface scattering at high carrier density. Therefore, our work shows how Shubnikov de Haas experiments can be used to characterize the quality of AlGaN/GaN interfaces used in modern light emitting devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of Field Induced Spin Density Waves in (TMTSF)2ReO4 by Shubnikov de Haas Measurements

SdH — measurements were performed on the organic metal (TMTSF)2ReO4 under a pressure of 8.5 kbar. Fast oscillations (FO) with a frequency of 330T could be reproduced already above 4.5T for temperatures below 4.5K. It could be shown, that these oscillations are not influenced by field induced spin-density-waves (FISDW’s) with a frequency of 28T. The effective carrier mass could be calculated to be mFO=1.4 me. The observation of the FISDW’s is strongly temperature dependent. The temperature and magnetic field dependent phase diagram for the FISDW’s is discussed.

Magnetotransport study on the defect levels of delta-doped In0.22Ga0.78As∕GaAs quantum wells

We have studied the electronic properties of delta-doped In0.22Ga0.78As∕GaAs quantum wells (QWs) by van der Pauw Hall measurements and Shubnikov–de Haas measurements. From the temperature-dependent van der Pauw Hall measurements, we observed two kinds of donors, which have binding energies of 104±7 and 9.6±0.1meV. After inserting In0.1Ga0.9As layers between the In0.22Ga0.78As and GaAs layers, a single donor with binding energy of 50±2meV was observed. The carrier concentration determined by SdH measurements did not change after the QWs were illuminated at low temperature, which indicates that these deep donors could not produce a persistent photoconductivity in delta-doped In0.22Ga0.78As∕GaAs QWs.

Magnetic-field- and temperature-dependent Fermi surface of CeBiPt

The half-Heusler compounds CeBiPt and LaBiPt are semimetals with very low charge-carrier concentrations as evidenced by Shubnikov–de Haas (SdH) and Hall-effect measurements. Neutron-scattering results reveal a simple antiferromagnetic structure in CeBiPt below TN = 1.15 K. The band structure of CeBiPt sensitively depends on temperature, magnetic field and stoichiometry. Above a certain, sample-dependent, threshold field (B>25 T), the SdH signal disappears and the Hall coefficient reduces significantly. These effects are absent in the non-4f compound LaBiPt. Electronic-band-structure calculations can well explain the observed behaviour by a 4f-polarization-induced Fermi-surface modification.

Transport properties of the two‐dimensional electron gas in GaN/AlGaN heterostructures grown by ammonia molecular‐beam epitaxy

AbstractTransport properties of the two‐dimensional electron gas in AlGaN/GaN heterostructures grown by ammonia molecular‐beam epitaxy are experimentally investigated. Conventional Hall and Shubnikov–de Haas measurements as well as investigations of quantum transport phenomena are reported. It is found that negative magnetoresistance (NMR) caused by weak localization demonstrates an unusual behavior at low temperature (1.8 K). The observed NMR cannot be described by the ordinary theory of quantum corrections to conductivity based on a single phase breaking time τϕ . The anomalous NMR behavior can be explained by the presence of two occupied quantum subbands, characterized by their own phase breaking times τ ϕ 1 and τ ϕ 2. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)