Amplitude Of Shubnikov-de Haas Oscillations Research Articles

The comprehensive investigation of barrier layers on power loss mechanisms in AlGaN/GaN HEMT structures

The electron relaxation mechanism of the electrons in the pseudo triangle quantum well located in the Al0.3Ga0.7N/GaN heterostructure interface grown by the Molecular Beam Epitaxy (MBE) is studied by Shubnikov de-Haas (SdH) oscillations. Four different sample types were used in the experiments; the carrier density in the quantum well is the same for all samples, whereas the top layer structures are different. The effects of a spacer layer placed between the barrier layer and the quantum well layer, a doped barrier layer placed behind the undoped barrier layer, and a passivation layer placed as a top layer on the energy relaxation processes of the carriers in the quantum well were investigated. The SdH oscillation measurements were carried out under a magnetic field of up to 11 T at a temperature range of 1.8–20 K. The electron temperatures and energy relaxation mechanisms are obtained by comparing the change in the relative amplitude of SdH oscillations in different applied electric fields and lattice temperatures. It is deduced that the samples with a passivation layer and doped barrier layer have higher electron temperatures at lower applied electric fields. The hot electrons relax with acoustic phonon scattering for these samples, including deformation and piezoelectric interactions in the low-temperature region determined theoretically. However, the hot electrons relax with acoustic phonon scattering for samples with spacer layer and spacer/passivation layers in the high-temperature region determined theoretically. Furthermore, it was observed that the spacer and passivation layers lead to faster relaxation of hot electrons.

Growth of AlxGa1-xN/InyGa1-yN hetero structure on AlN/sapphire templates exhibiting Shubnikov-de Haas oscillation

We fabricated successfully heterostructures of AlGaN/GaN and AlGaN/InGaN/GaN on AlN template/c-sapphire exhibiting the Shubnikov-de Haas (SdH) oscillation in the magnetic field at lower temperatures. In AlxGa1-xN/GaN system, the carrier density of two-dimensional electron gas (2DEG) was controlled from 4.0 × 1012 to 9.0 × 1012 cm−2, which was consistent with the theoretical simulation, with increase of Al composition in AlGaN barrier layer. In AlGaN/InGaN/GaN system, the surface morphology and alloy disorder on the InGaN were properly defined. The effective mass of 2DEG at the Al0.22Ga0.78N/In0.04Ga0.96N interface was estimated to be 0.24 m0 from the temperature dependence of SdH oscillation amplitude. Although the effective mass was comparable to that of AlGaN/GaN heterostructure, the mobility of 2DEG at the InGaN was drastically degraded in spite of the slight amount of indium incorporation. This reduction was attributed to larger scattering probably due to the interface roughness of InGaN layer caused by the step meandering and/or the potential fluctuation caused by random incorporation of indium atoms.

Power loss mechanisms in n-type modulation-doped AlGaAs/GaAsBi quantum well heterostructures

We report on the power loss mechanisms of hot electrons in as-grown and annealed n-type modulation-doped Al0.15Ga0.85As/GaAs1-xBix (x = 0 and 0.04) quantum well structures considering acoustic phonon interactions via the deformation potential (non-polar) and piezoelectric (polar) scatterings. The two-dimensional (2D) electron gas is heated by applying various electric fields under a steady-state magnetic field, and the effect of the applied electric field on the Shubnikov de Haas (SdH) oscillations is analyzed to investigate the power loss mechanism. The temperature of hot electrons (Te) has been obtained by comparing the lattice temperature and applied electric field dependencies of the SdH oscillation amplitude. The hot electron temperature is almost the same for both Bi-free and Bi-containing samples except for the sample annealed at a higher temperature (700 °C) than the growth temperature of GaAsBi. The electron temperature dependence of power loss is analyzed using current theoretical analytic models derived for 2D semiconductors. We find that energy relaxation occurs in the intermediate temperature regime, including mixing of piezoelectric and deformation potential scattering. The power loss of hot electrons is found to be proportional to () with γ in the range from 2.4 to 4.2, which indicates that the hot electron relaxation is due to acoustic phonon scatterings via unscreened deformation potential and piezoelectric scattering. It is found that deformation potential scattering is dominant over piezoelectric scattering in the Bi-free sample, while the incorporation of Bi into the GaAs lattice makes these processes comparable. After thermal annealing at lower than growth temperature (350 °C), the scattering mechanism switches from deformation potential to piezoelectric scattering. After thermal annealing at higher than growth temperature (700 °C), the theoretical model does not fit to the experimental results due to degradation of the sample.

Light effective hole mass in undoped Ge/SiGe quantum wells

We report density-dependent effective hole mass measurements in undoped germanium quantum wells. We are able to span a large range of densities ($2.0-11\times10^{11}$ cm$^{-2}$) in top-gated field effect transistors by positioning the strained buried Ge channel at different depths of 12 and 44 nm from the surface. From the thermal damping of the amplitude of Shubnikov-de Haas oscillations, we measure a light mass of $0.061m_e$ at a density of $2.2\times10^{11}$ cm$^{-2}$. We confirm the theoretically predicted dependence of increasing mass with density and by extrapolation we find an effective mass of $\sim0.05m_e$ at zero density, the lightest effective mass for a planar platform that demonstrated spin qubits in quantum dots.

Puzzle of non-surface related 2D electron gas in n-InN epitaxial samples

We have studied multicarrier contributions to the conductivity in low concentration (with Hall concentration below 1×1018 cm−3) nondoped bulk n-InN samples using Shubnikov-de Haas (SdH) in the tilted magnetic field as well as variable field resistivity tensor measurements. In some samples, the Shubnikov-de Haas effect also revealed, besides 3D electron gas, high mobility 2D electron contributions (with transport and quantum mobilities reaching the values of 5060 cm2/V s and 1800 cm2/V s, respectively, for one of our samples), which could hardly be assigned to the surface electrons as the latter are commonly believed to have mobility too low to be detected at magnetic fields not exceeding 12 T in our SdH measurements. The values of the effective masses derived from the temperature dependences of the SdH oscillation amplitudes scale with the concentration of 2D channels and are typical for low concentration InN, thus confirming that these contributions are presumably located on the InN side of some interfaces. This is one of the first experimental evaluations of the effective mass of nonsurface-related 2D electron gas in InN grown on GaN.

The transport properties of Dirac fermions in chemical vapour-deposited single-layer graphene

The electronic transport properties of Dirac fermions in chemical vapour-deposited single-layer epitaxial graphene on anSiO2/Si substrate have been investigated using the Shubnikov–de Haas (SdH) oscillations technique. The magnetoresistance measurements were performed in the temperature range between 1.8 and 43 K and at magnetic fields up to 11 T. The 2D carrier density and the Fermi energy have been determined from the period of the SdH oscillations. In addition, the in-plane effective mass as well as the quantum lifetime of 2D carriers have been calculated from the temperature and magnetic field dependences of the SdH oscillation amplitude. The sheet carrier density (1.42 × 1013 cm−2 at 1.8 K), obtained from the low-field Hall Effect measurements, is larger than that of 2D carrier density (8.13 × 1012 cm−2). On the other hand, the magnetoresistance includes strong magnetic field dependent positive, non-oscillatory background magnetoresistance. The strong magnetic field dependence of the magnetoresistance and the differences between sheet carrier and 2D carrier density can be attributed to the 3D carriers between the graphene sheet and the SiO2/Si substrate.

Quantum oscillations and interference effects in strained n- and p-type modulation doped GaInNAs/GaAs quantum wells

We have performed magnetoresistance measurements on n- and p-type modulation doped GaInNAs/GaAs quantum well (QW) structures in both the weak (B < 0.08 T) and the high magnetic field (up to 18 T) at 75 mK and 6 K. We observe that the quantum oscillations in and quantum Hall effect (QHE) plateaus in are affected from the presence of the nitrogen in the III–V lattice. The enhancement of N-related scatterings and electron effective mass with increasing nitrogen causes lower electron mobility and higher two-dimensional (2D) electron density, leading to suppressed QHE plateaus in up to 7 T at 6 K. The Shubnikov de Haas (SdH) oscillations develop at lower magnetic fields for higher mobility samples at 6 K and the amplitude of SdH oscillations decreases with increasing nitrogen composition. The well-pronounced QHE plateaus are observed at 75 mK and at higher magnetic fields up to 18 T, for the p-type sample. For n-type samples, the observed anomalies in the characteristic of QHE is attributed the nitrogen-related disorders and overlapping of fluctuating Landau levels. The low magnetic field measurements at 75 mK reveal that the n-type samples exhibit weak antilocalization, whereas weak localization is observed for the p-type sample. The observation of weak antilocalization is an indication of strong electron spin–orbit interactions. The low field magnetoresistance traces are used to extract the spin coherence, phase coherence and elastic scattering times as well Rashba parameters and spin-splitting energy. The calculated Rashba parameters for nitrogen containing samples reveal that the nitrogen composition is a significant parameter to determine the degree of the spin–orbit interactions. Consequently, GaInNAs-based QW structures with various nitrogen compositions can be beneficial to adjust the spin–orbit coupling strength and may be used as a candidate for spintronics applications.

Transport Properties of Topological Insulator Bi0.83Sb0.17 Nanowires

ABSTRACTWe have investigated the transport properties of topological insulator based on single-crystal Bi0.83Sb0.17 nanowires. The single-crystal nanowire samples in the diameter range 200 nm – 1.1 μm were prepared by the high frequency liquid phase casting in a glass capillary using an improved Ulitovsky technique; they were cylindrical single-crystals with (1011) orientation along the wire axis. In this orientation, the wire axis makes an angle of 19.5o with the bisector axis C1 in the bisector-trigonal plane. Bi0.83Sb0.17 is a narrow gap semiconductor with energy gap at L point of Brillouin zone ΔE= 21 meV. In accordance with the measurements of the temperature dependence of the resistivity of the samples resistance increases with decreasing temperature, but at low temperatures decrease in the resistance is observed. This effect, decrease in the resistance, is a clear manifestation of the interesting properties of topological insulators - the presence on its surface of a highly conducting zone. The Arrhenius plot of resistance R in samples with diameter d=1.1 µm and d=200 nm indicates a thermal activation behavior with an activation gap ΔE= 21 and 35 meV, respectively, which proves the presence of the quantum size effect in these samples. We found that in the range of diameter 1100 nm - 200 nm when the diameter decreases the energy gap is growing as 1/d. We have investigated magnetoresistance of Bi0.83Sb0.17 nanowires at various magnetic field orientations. From the temperature dependences of Shubnikov de Haas oscillation amplitude for different orientation of magnetic field we have calculated the cyclotron mass mc and Dingle temperature TD for longitudinal and transverse (B||C3 and B||C2) directions of magnetic fields, which equal 1.96*10-2m0, 9.8 K, 8.5*10-3m0 , 9.4 K and 1.5*10-1m0 , 2.8 K respectively. The observed effects are discussed.

Magneto-intersubband oscillations of the kinetic coefficients of a two-dimensional system with a spin-orbit interaction

The temperature dependence of the Shubnikov–de Haas oscillations of longitudinal magnetoresistance of a two-dimensional electron system with the Rashba spin-orbit interaction is analyzed by using the expression for conductivity derived by S. G. Novokshonov and A. G. Groshev, Phys. Rev. B 74, 245333 (2006). It is shown that in the above system there exist magneto-intersubband oscillations of the kinetic coefficients characteristic for multisubband conductors. The period of these oscillations is determined by the spin precession frequency at the Fermi level ΩBF. In the case under consideration, the intersubband spin splitting depends on electron energy. This results in a weak temperature dependence of the magneto-intersubband oscillation amplitude, which is determined by Λ* = (2π2kBT/ℏωc)ℏ ΩBF / εF. This dependence is considerably weaker in comparison to the temperature smoothing of the Shubnikov–de Haas oscillation amplitude due to a small parameter (ℏ ΩBF /εF ≪ 1).

Effective mass of electron in monolayer graphene: Electron-phonon interaction

Shubnikov-de Haas (SdH) and Hall effect measurements performed in a temperature range between 1.8 and 275 K, at an electric field up to 35 kV m−1 and magnetic fields up to 11 T, have been used to investigate the electronic transport properties of monolayer graphene on SiC substrate. The number of layers was determined by the use of the Raman spectroscopy. The carrier density and in-plane effective mass of electrons have been obtained from the periods and temperature dependencies of the amplitude of the SdH oscillations, respectively. The effective mass is in good agreement with the current results in the literature. The two-dimensional (2D) electron energy relaxations in monolayer graphene were also investigated experimentally. The electron temperature (Te) of hot electrons was obtained from the lattice temperature (TL) and the applied electric field dependencies of the amplitude of SdH oscillations. The experimental results for the electron temperature dependence of power loss indicate that the energy relaxation of electrons is due to acoustic phonon emission via mixed unscreened piezoelectric interaction and deformation-potential scattering.

Electron and hole energy relaxation rates in GaInNAs/GaAs quantum wells via deformation potential and piezoelectric scattering

AbstractThe two‐dimensional (2D) electron and hole energy relaxation associated with acoustic phonon emission in n‐ and p‐type modulation doped Ga0.7In0.3NyAs1–y/GaAs quantum wells has been investigated experimentally using Shubnikov‐de Haas (SdH) effect measurements performed as a function of lattice temperature and applied electric field. Nitrogen concentration dependence of the effective mass and quantum lifetime of the 2D electrons in the n‐type samples have been determined from the temperature and magnetic field dependencies of the amplitude of SdH oscillations, respectively. The in‐plane effective mass of the 2D electrons increases when the nitrogen mole fraction is increased from y = 0.004 to 0.010 but remains the same when it is increased to y = 0.015. The values obtained for quantum lifetime suggest that interface roughness is the dominating scattering mechanism in n‐type Ga0.7In0.3NyAs1–y/GaAs quantum wells. The electron temperature (Te) of hot electrons and the hole temperature (Th) of hot holes have been obtained from the lattice temperature (TL) and applied electric field dependencies of amplitude of SdH oscillations. The experimental results for the electron/hole temperature dependence of power loss are compared with the current theoretical models for power loss in 2D semiconductors, which include both piezoelectric and deformation‐potential scattering. For n‐type samples, the power loss from the electrons is found to be proportional to (T–T) with γ in the range from 3.48 to 4.66, indicating that the energy relaxation of electrons is due to acoustic phonon emission via unscreened piezoelectric interaction. For the p‐type sample, the power loss is approximately proportional to (T–T). This behavior is in accord with the theoretical predictions for the variation of power loss with hole temperature for a 2D hole gas in the equipartition regime.

Energy Relaxation Rates in AlInN/AlN/GaN Heterostructures

The two-dimensional (2D) electron energy relaxation in Al0.83In0.17N/AlN/GaN heterostructures has been investigated experimentally. Shubnikov–de Haas (SdH) effect measurements were employed in the investigations. The electron temperature (T e) of hot electrons was obtained from the lattice temperature (T L) and the applied electric field dependencies of the amplitude of SdH oscillations. The experimental results for the electron temperature dependence of power loss are also compared with current theoretical models for power loss in 2D semiconductors. The power loss from the electrons was found to be proportional to (T 3e − T 3L ) for electron temperatures in the range 1.8 K < T e < 14 K, indicating that the energy relaxation of electrons is due to acoustic phonon emission via unscreened piezoelectric interaction. The effective mass and quantum lifetime of the 2D electrons have been determined from the temperature and magnetic field dependencies of the amplitude of SdH oscillations, respectively. The values obtained for quantum lifetime suggest that remote ionized impurity scattering is the dominant scattering mechanism in Al0.83In0.17N/AlN/GaN heterostructures.

Temperature dependent energy relaxation time in AlGaN/AlN/GaN heterostructures

The two-dimensional (2D) electron energy relaxation in Al0.25Ga0.75N/AlN/GaN heterostructures was investigated experimentally by using two experimental techniques; Shubnikov-de Haas (SdH) effect and classical Hall Effect. The electron temperature (Te) of hot electrons was obtained from the lattice temperature (TL) and the applied electric field dependencies of the amplitude of SdH oscillations and Hall mobility. The experimental results for the electron temperature dependence of power loss are also compared with the current theoretical models for power loss in 2D semiconductors. The power loss that was determined from the SdH measurements indicates that the energy relaxation of electrons is due to acoustic phonon emission via unscreened piezoelectric interaction. In addition, the power loss from the electrons obtained from Hall mobility for electron temperatures in the range Te>100K is associated with optical phonon emission. The temperature dependent energy relaxation time in Al0.25Ga0.75N/AlN/GaN heterostructures that was determined from the power loss data indicates that hot electrons relax spontaneously with MHz to THz emission with increasing temperatures.

Energy relaxation for hot Dirac fermions in graphene and breakdown of the quantum Hall effect

Energy loss rates for hot carriers in graphene are experimentally investigated by observing the amplitude of Shubnikov-de Haas oscillations as a function of electric field. The carrier energy loss in graphene follows the predictions of deformation potential coupling going as $\ensuremath{\sim}$T${}^{4}$ at carrier temperatures up to $\ensuremath{\sim}$100 K, and that deformation potential theory, when modified with a limiting phonon relaxation time, is valid up to several hundred Kelvin. Additionally we investigate the breakdown of the quantum Hall effect and show that energy loss rates in graphene are around ten times larger than GaAs at low temperatures. This leads to significantly higher breakdown currents per micrometer, and we report a measured breakdown current of 8 $\ensuremath{\mu}$A/$\ensuremath{\mu}$m.

Charge density wave transport in NbSe3 at low temperatures under high magnetic field

Charge density wave (CDW) depinning and sliding regimes have been studied in NbSe3 at low temperatures down to 1.5 K under magnetic field of 19 T oriented along the c-axis. We found that the threshold field for CDW depinning becomes temperature independent below T0 ≈ 15 K. Also CDW current to frequency ratio characterizing CDW sliding regime increases by factor 1.7 below this temperature. The results are discussed as a crossover from thermal fluctuation to tunneling CDW depinning at T < T0. Besides, we found that CDW sliding strongly suppresses the amplitude of Shubnikov-de Haas oscillations of magnetoresistance.

Phase study of oscillatory resistances in microwave-irradiated- and dark-GaAs/AlGaAs devices: Indications of an unfamiliar class of the integral quantum Hall effect

We report the experimental results from a dark study and a photoexcited study of the high-mobility GaAs/AlGaAs system at large filling factors, $\ensuremath{\nu}$. At large $\ensuremath{\nu}$, the dark study indicates several distinct phase relations (``type 1,'' ``type 2,'' and ``type 3'') between the oscillatory diagonal and Hall resistances, as the canonical integral quantum Hall effect (IQHE) is manifested in the type 1 case of approximately orthogonal diagonal and Hall resistance oscillations. Surprisingly, the investigation indicates quantum Hall plateaus also in the type 3 case characterized by approximately ``antiphase'' Hall and diagonal resistance oscillations, suggesting an unfamiliar and distinct class of IQHE. Transport studies under microwave photoexcitation exhibit radiation-induced magnetoresistance oscillations in both the diagonal, ${R}_{xx}$, and off-diagonal, ${R}_{xy}$, resistances. Further, when the radiation-induced magnetoresistance oscillations extend into the quantum Hall regime, there occurs a radiation-induced nonmonotonic variation in the amplitude of Shubnikov--de Haas (SdH) oscillations in ${R}_{xx}$ vs $B$, and a nonmonotonic variation in the width of the quantum Hall plateaus in ${R}_{xy}$. The latter effect leads into the vanishing of IQHE at the minima of the radiation-induced ${R}_{xx}$ oscillations with increased photoexcitation. We reason that the mechanism which is responsible for producing the nonmonotonic variation in the amplitude of SdH oscillations in ${R}_{xx}$ under photoexcitation is also responsible for eliminating, under photoexcitation, the type 3 associated IQHE in the high-mobility specimen.

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.

A new method of investigating the quantum channel surface

A new method of investigating the quantum channel surface was used to study the germaniumquantum well in a SiGe/Ge/SiGe p-type heterostructure, with a hole concentrationpH = 5.68 × 1011 cm−2 andmobility μ = 4.68 × 104 cm2 V−1 s−1 in magnetic fields up to 15 T and in the temperature range 50 mK–3 K using Shubnikov–deHaas (SdH) oscillations. The method is based on the deviation from theory describing theSdH-related conductivity oscillations. This deviation appears due to extra broadening ofthe Landau levels, which is attributed to the existence of an inhomogeneous distributionof the carrier concentration in the two-dimensional hole gas layer and, hence,a spread of energy. It is assumed that extra broadening is due to the naturalvariation of the well width equal to the interatomic distance. The effective hole mass(m* = 0.112m0) was found from the temperature dependence of the SdH oscillation amplitudes. Thequantum scattering time, fluctuations of the carrier concentration and quantum wellroughness were estimated from the magnetic field dependence of these oscillationamplitudes.

Electrical characterization of AlGaN/GaN on AlN substrates

AbstractThe results of an electrical transport study of an AlGaN/GaN heterostructure grown on a bulk AlN substrate are presented. Temperature dependent Hall effect and magnetoresistance measurements were made on an MOCVD grown heterostructure. The temperature dependences of the carrier concentration and mobility are characteristic of a two‐dimensional electron gas (2DEG). The carrier concentration was 1.19x1013 cm–2 at room temperature and 1.02x1013 cm–2 at 77 K while the mobility was 1100 cm2/Vs at room temperature and 5280 cm2/Vs at 77 K. The magnetoresistance measurements revealed the presence of Shubnikov – de Haas (SdH) oscillations. Illumination of the sample resulted in a persistent photocurrent, and an enhanced SdH oscillation amplitude with an increased quantum scattering time. The ratio of the classical to quantum scattering times, τc/τq, was 4.3, which is similar to values for AlGaN/GaN structures on other substrates. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The investigation of transport properties of mesoscopic graphite in high magnetic field

Abstract The electrical transport properties of mesoscopic graphite have been investigated in a gate voltage configuration. Few layer graphene structures made from Kish graphite exhibit Shubnikov-de Haas (SdH) oscillations in magnetic fields up to 33 T, with a strong gate voltage dependence. A two band model can be used to explain the linear dependence of the SdH frequency on the gate voltage. The temperature dependence of the SdH oscillation amplitude allows the determination of the effective masses of the carriers, which remain comparable between mesoscopic and bulk graphite samples. However, mesoscopic graphite thinner than 130 nm does not exhibit the field induced charge density wave transition seen in bulk samples above 25 T at low temperatures.