Showing papers by "Solid State Physics Laboratory published in 2018"

Interactions and Magnetotransport through Spin-Valley Coupled Landau Levels in Monolayer MoS 2

Abstract: The strong spin-orbit coupling and the broken inversion symmetry in monolayer transition metal dichalcogenides results in spin-valley coupled band structures. Such a band structure leads to novel applications in the fields of electronics and optoelectronics. Density functional theory calculations as well as optical experiments have focused on spin-valley coupling in the valence band. Here we present magnetotransport experiments on high-quality $n$-type monolayer molybdenum disulphide (${\mathrm{MoS}}_{2}$) samples, displaying highly resolved Shubnikov--de Haas oscillations at magnetic fields as low as 2 T. We find the effective mass $0.7{m}_{e}$, about twice as large as theoretically predicted and almost independent of magnetic field and carrier density. We further detect the occupation of the second spin-orbit split band at an energy of about 15 meV, i.e., about a factor of 5 larger than predicted. In addition, we demonstrate an intricate Landau level spectrum arising from a complex interplay between a density-dependent Zeeman splitting and spin- and valley-split Landau levels. These observations, enabled by the high electronic quality of our samples, testify to the importance of interaction effects in the conduction band of monolayer ${\mathrm{MoS}}_{2}$.

Spin and Valley States in Gate-Defined Bilayer Graphene Quantum Dots

Abstract: Electrostatically defined nanostructures in a double layer of graphene localize single electrons for the first time, offering an important step towards graphene-based quantum computation that promises long coherence times.

Electrostatically Induced Quantum Point Contacts in Bilayer Graphene.

Abstract: We report the fabrication of electrostatically defined nanostructures in encapsulated bilayer graphene, with leakage resistances below depletion gates as high as R ∼ 10 GΩ. This exceeds previously reported values of R = 10–100 kΩ.1−3 We attribute this improvement to the use of a graphite back gate. We realize two split gate devices which define an electronic channel on the scale of the Fermi-wavelength. A channel gate covering the gap between the split gates varies the charge carrier density in the channel. We observe device-dependent conductance quantization of ΔG = 2e2/h and ΔG = 4e2/h. In quantizing magnetic fields normal to the sample plane, we recover the four-fold Landau level degeneracy of bilayer graphene. Unexpected mode crossings appear at the crossover between zero magnetic field and the quantum Hall regime.

Scanning gate microscopy in a viscous electron fluid

Abstract: We measure transport through a Ga[Al]As heterostructure at temperatures between $32\phantom{\rule{333333pt}{0ex}}\mathrm{mK}$ and $30\phantom{\rule{333333pt}{0ex}}\mathrm{K}$ Increasing the temperature enhances the electron-electron scattering rate and viscous effects in the two-dimensional electron gas arise To probe this regime we measure so-called vicinity voltages and use a voltage-biased scanning tip to induce a movable local perturbation We find that the scanning gate images differentiate reliably between the different regimes of electron transport Our data are in good agreement with recent theories for interacting electron liquids in the ballistic and viscous regimes stimulated by measurements in graphene However, the range of temperatures and densities where viscous effects are observable in Ga[Al]As are very distinct from the graphene material system

Automated tuning of inter-dot tunnel coupling in double quantum dots

Abstract: Semiconductor quantum dot arrays defined electrostatically in a 2D electron gas provide a scalable platform for quantum information processing and quantum simulations. For the operation of quantum dot arrays, appropriate voltages need to be applied to the gate electrodes that define the quantum dot potential landscape. Tuning the gate voltages has proven to be a time-consuming task, because of initial electrostatic disorder and capacitive cross-talk effects. Here, we report on the automated tuning of the inter-dot tunnel coupling in gate-defined semiconductor double quantum dots. The automation of the tuning of the inter-dot tunnel coupling is the next step forward in scalable and efficient control of larger quantum dot arrays. This work greatly reduces the effort of tuning semiconductor quantum dots for quantum information processing and quantum simulation.

Gate-tunable quantum dot in a high quality single layer MoS2 van der Waals heterostructure

Abstract: We have fabricated an encapsulated monolayer MoS2 device with metallic ohmic contacts through a pre-patterned hexagonal boron nitride (hBN) layer. In the bulk, we observe an electron mobility as high as 3000 cm2/Vs at a density of 7 × 1012 cm−2 at a temperature of 1.7 K. Shubnikov-de Haas oscillations start at magnetic fields as low as 3.3 T. By realizing a single quantum dot gate structure on top of hBN, we are able to confine electrons in MoS2 and observe the Coulomb blockade effect. By tuning the middle gate voltage, we reach a double dot regime where we observe the standard honeycomb pattern in the charge stability diagram.

Coupled Quantum Dots in Bilayer Graphene.

Abstract: Electrostatic confinement of charge carriers in bilayer graphene provides a unique platform for carbon-based spin, charge, or exchange qubits. By exploiting the possibility to induce a band gap with electrostatic gating, we form a versatile and widely tunable multiquantum dot system. We demonstrate the formation of single, double and triple quantum dots that are free of any sign of disorder. In bilayer graphene, we have the possibility to form tunnel barriers using different mechanisms. We can exploit the ambipolar nature of bilayer graphene where pn-junctions form natural tunnel barriers. Alternatively, we can use gates to form tunnel barriers, where we can vary the tunnel coupling by more than 2 orders of magnitude tuning between a deeply Coulomb blockaded system and a Fabry-Perot-like cavity. Demonstrating such tunability is an important step toward graphene-based quantum computation.

Polaron Polaritons in the Integer and Fractional Quantum Hall Regimes.

Abstract: Elementary quasiparticles in a two-dimensional electron system can be described as exciton polarons since electron-exciton interactions ensures dressing of excitons by Fermi-sea electron-hole pair excitations. A relevant open question is the modification of this description when the electrons occupy flat bands and electron-electron interactions become prominent. Here, we perform cavity spectroscopy of a two-dimensional electron system in the strong coupling regime, where polariton resonances carry signatures of strongly correlated quantum Hall phases. By measuring the evolution of the polariton splitting under an external magnetic field, we demonstrate the modification of polaron dressing that we associate with filling factor dependent electron-exciton interactions.

Enhanced Interactions between Dipolar Polaritons

Abstract: Nonperturbative coupling between cavity photons and excitons leads to the formation of hybrid light-matter excitations, termed polaritons. In structures where photon absorption leads to the creation of excitons with aligned permanent dipoles, the elementary excitations, termed dipolar polaritons, are expected to exhibit enhanced interactions. Here, we report a substantial increase in interaction strength between dipolar polaritons as the size of the dipole is increased by tuning the applied gate voltage. To this end, we use coupled quantum well structures embedded inside a microcavity where coherent electron tunneling between the wells creates the excitonic dipole. Modifications of the interaction strength are characterized by measuring the changes in the reflected light intensity when polaritons are driven with a resonant laser. The factor of 6.5 increase in the interaction-strength-to-linewidth ratio that we obtain indicates that dipolar polaritons could constitute an important step towards a demonstration of the polariton blockade effect, and thereby to form the building blocks of many-body states of light.

A 2 × 2 quantum dot array with controllable inter-dot tunnel couplings

Abstract: The interaction between electrons in arrays of electrostatically defined quantum dots is naturally described by a Fermi-Hubbard Hamiltonian. Moreover, the high degree of tunability of these systems makes them a powerful platform to simulate different regimes of the Hubbard model. However, most quantum dot array implementations have been limited to one-dimensional linear arrays. In this letter, we present a square lattice unit cell of 2 × 2 quantum dots defined electrostatically in an AlGaAs/GaAs heterostructure using a double-layer gate technique. We probe the properties of the array using nearby quantum dots operated as charge sensors. We show that we can deterministically and dynamically control the charge occupation in each quantum dot in the single- to few-electron regime. Additionally, we achieve simultaneous individual control of the nearest-neighbor tunnel couplings over a range of 0–40 μeV. Finally, we demonstrate fast (∼1 μs) single-shot readout of the spin state of electrons in the dots through ...

Gate-Defined Quantum Confinement in InSe-Based van der Waals Heterostructures.

Abstract: Indium selenide, a post-transition metal chalcogenide, is a novel two-dimensional (2D) semiconductor with interesting electronic properties. Its tunable band gap and high electron mobility have already attracted considerable research interest. Here we demonstrate strong quantum confinement and manipulation of single electrons in devices made from few-layer crystals of InSe using electrostatic gating. We report on gate-controlled quantum dots in the Coulomb blockade regime as well as one-dimensional quantization in point contacts, revealing multiple plateaus. The work represents an important milestone in the development of quality devices based on 2D materials and makes InSe a prime candidate for relevant electronic and optoelectronic applications.

Topologically Nontrivial Valley States in Bilayer Graphene Quantum Point Contacts

Abstract: We present measurements of quantized conductance in electrostatically induced quantum point contacts in bilayer graphene. The application of a perpendicular magnetic field leads to an intricate pattern of lifted and restored degeneracies with increasing field: at zero magnetic field the degeneracy of quantized one-dimensional subbands is four, because of a twofold spin and a twofold valley degeneracy. By switching on the magnetic field, the valley degeneracy is lifted. Because of the Berry curvature, states from different valleys split linearly in magnetic field. In the quantum Hall regime fourfold degenerate conductance plateaus reemerge. During the adiabatic transition to the quantum Hall regime, levels from one valley shift by two in quantum number with respect to the other valley, forming an interweaving pattern that can be reproduced by numerical calculations.

Signatures of transient Wannier-Stark localization in bulk gallium arsenide.

Abstract: Many properties of solids result from the fact that in a periodic crystal structure, electronic wave functions are delocalized over many lattice sites. Electrons should become increasingly localized when a strong electric field is applied. So far, this Wannier–Stark regime has been reached only in artificial superlattices. Here we show that extremely transient bias over the few-femtosecond period of phase-stable mid-infrared pulses may localize electrons even in a bulk semiconductor like GaAs. The complicated band structure of a three-dimensional crystal leads to a strong blurring of field-dependent steps in the Wannier–Stark ladder. Only the central step emerges strongly in interband electro-absorption because its energetic position is dictated by the electronic structure at an atomic level and therefore insensitive to the external bias. In this way, we demonstrate an extreme state of matter with potential applications due to e.g., its giant optical non-linearity or extremely high chemical reactivity. In strong enough electric fields the non-linear response of electrons in crystals is expected to lead to spatial localization but so far this has only been seen in artificial structures. Schmidt et al. present evidence of this Wannier-Stark localization effect in bulk GaAs driven by intense mid-infrared pulses.

Role of AlGaN/GaN interface traps on negative threshold voltage shift in AlGaN/GaN HEMT

Abstract: This article reports negative shift in the threshold-voltage in AlGaN/GaN high electron mobility transistor (HEMT) with application of reverse gate bias stress. The device is biased in strong pinch-off and low drain to source voltage condition for a fixed time duration (reverse gate bias stress), followed by measurement of transfer characteristics. Negative threshold voltage shift after application of reverse gate bias stress indicates the presence of more carriers in channel as compared to the unstressed condition. We propose the presence of AlGaN/GaN interface states to be the reason of negative threshold voltage shift, and developed a process to electrically characterize AlGaN/GaN interface states. We verified the results with Technology Computer Aided Design (TCAD) ATLAS simulation and got a good match with experimental measurements.

Growth and Comparison of Residual Stress of AlN Films on Silicon (100), (110) and (111) Substrates

Abstract: This paper reports on the comparison of residual stresses in AlN thin films sputter-deposited in identical conditions on Si (100) (110) and (111) substrates. The deposited films are of polycrystalline wurtzite structure with preferred orientation along the (002) direction. AlN film on the Si (111) substrate showed a vertical columnar structure, whereas films on Si (100) and (110) showed tilted columnar structures. Residual stress in the AlN films is estimated by x-ray diffraction (XRD), infra-red absorption method and wafer curvature technique. Films residual stress are found compressive and values are in the range of − 650 (± 50) MPa, − 730 (± 50) MPa and − 300 (± 50) MPa for the AlN films grown on Si (100), (110) and (111) substrates, respectively, with different techniques. The difference in residual stresses can be attributed to the microstructure of the films and mismatch between in plane atomic arrangements of the film and substrates.

Scanning gate experiments: From strongly to weakly invasive probes

Abstract: An open resonator fabricated in a two-dimensional electron gas is used to explore the transition from strongly invasive scanning gate microscopy to the perturbative regime of weak tip-induced potentials. With the help of numerical simulations that faithfully reproduce the main experimental findings, we quantify the extent of the perturbative regime in which the tip-induced conductance change is unambiguously determined by properties of the unperturbed system. The correspondence between the experimental and numerical results is established by analyzing the characteristic length scale and the amplitude modulation of the conductance change. In the perturbative regime, the former is shown to assume a disorder-dependent maximum value, while the latter linearly increases with the strength of a weak tip potential.

Design and Fabrication of Multi-finger Field Plate for Enhancement of AlGaN/GaN HEMT Breakdown Voltage

Abstract: The design and fabrication of gate/source connected multi-finger field plate structures using TCAD ATLAS simulation software is presented. The designed field plate structures are fabricated on indigenous AlGaN/GaN HEMT devices. AlGaN/GaN HEMT devices with field plate structures exhibit about three times improvement in breakdown voltage of device and are in close agreement with the simulation results. Integration of field plates in device have resulted in higher VDS (drain to source voltage) operation and improvement in output power of AlGaN/GaN HEMT devices. Incorporation of field plates also decrease the reverse leakage current of HEMT devices.

Effect of vacuum packaging on bandwidth of push–pull type capacitive accelerometer structure

Abstract: This paper presents the effect of vacuum packaging on the band-width of push–pull type capacitive accelerometer structure. The accelerometer structure (for ± 30 g application) consists of silicon proof mass (1000 μm × 1000 μm × 30 μm) suspended by four L-shaped beams over 1 μm deep cavity. The fixed electrodes (Au) are on Pyrex glass substrates which are anodically bonded to the Si substrate. The squeeze film damping factor (ς) arising due to the trapped air between the electrodes is found to be ~ 2600 at atmospheric pressure. Introduction of holes in the proof-mass reduces ς to 750. In such a highly damped environment, the operational bandwidth (3 dB) bandwidth is found to be only around 20 Hz which is not suitable for many of the navigation applications. With vacuum packaging (vacuum level 100–760 Torr), there is not much noticeable improvement in the bandwidth due to high level of ς (> 150). In the 1–100 Torr range, the bandwidth is found to be improving linearly (from 20 Hz to 2 kHz) with the improving vacuum level. Further improvement of vacuum level (< 1 Torr) leads to the under-damped (ς < 1) condition and the proof mass start oscillating which also undesirable. Thus, vacuum packaging in the 1–10 Torr range (having ς: 2–15) is found to be suitable for achieving optimum operation bandwidth for the current accelerometer sensor.

Gamma non-ionizing energy loss: Comparison with the damage factor in silicon devices

Abstract: The concept of non-ionizing energy loss (NIEL) has been demonstrated to be a successful approach to describe the displacement damage effects in silicon materials and devices. However, some discrepancies exist in the literature between experimental damage factors and theoretical NIELs. 60Co gamma rays having a low NIEL are an interesting particle source that can be used to validate the NIEL scaling approach. This paper presents different 60Co gamma ray NIEL values for silicon targets. They are compared with the radiation-induced increase in the thermal generation rate of carriers per unit fluence. The differences between the different models, including one using molecular dynamics, are discussed.

Direct observation of a Γ-X energy spectrum transition in narrow AlAs quantum wells

Abstract: Spectra of magnetoplasma excitations have been investigated in a two-dimensional electron systems in AlAs quantum wells (QWs) of different widths. The magnetoplasma spectrum have been found to change profoundly when the quantum well width became thinner than $5.5$~nm, indicating a drastic change in the conduction electron energy spectrum. The transformation can be interpreted in terms of transition from the in-plane strongly anisotropic $X_x - X_y$ valley occupation to the out-of-plane isotropic $X_z$ valley in the QW plane. Strong enhancement of the cyclotron effective mass over the band value in narrow AlAs QWs is reported.

Cavity-Mediated Coherent Coupling between Distant Quantum Dots.

Abstract: Scalable architectures for quantum information technologies require one to selectively couple long-distance qubits while suppressing environmental noise and cross talk. In semiconductor materials, the coherent coupling of a single spin on a quantum dot to a cavity hosting fermionic modes offers a new solution to this technological challenge. Here, we demonstrate coherent coupling between two spatially separated quantum dots using an electronic cavity design that takes advantage of whispering-gallery modes in a two-dimensional electron gas. The cavity-mediated, long-distance coupling effectively minimizes undesirable direct cross talk between the dots and defines a scalable architecture for all-electronic semiconductor-based quantum information processing.

Tunneling spectroscopy of graphene nanodevices coupled to large-gap superconductors

Abstract: We performed tunneling spectroscopy measurements of graphene coupled to niobium/niobium-nitride superconducting electrodes. Due to the proximity effect, the graphene density of states depends on the phase difference between the superconductors and exhibits a hard induced gap at zero phase, consistent with a continuum of Andreev bound states. At energies larger than the superconducting gap, we observed phase-dependent energy levels displaying the Coulomb blockade effect, which are interpreted as arising from spurious quantum dots, presumably embedded in the heterostructures and coupled to the proximitized graphene.

Spin-orbit coupling effects in the quantum Hall regime probed by electron spin resonance

Abstract: The spin resonance of two-dimensional (2D) electrons confined in a high-quality 4.5-nm AlAs quantum well was studied in the regime of the integer quantum Hall effect. The electron $g$-factor extracted from the magnetic field position of the spin resonance at a fixed microwave frequency demonstrated a strong nonlinear dependence on the magnetic field with discontinuities around even filling factors. The value of the $g$-factor tended to increase with a decrease of the magnetic field around each odd filling. Furthermore, the $g$-factor at the exactly odd filling factor $\ensuremath{ u}$ turned out to be dependent on $\ensuremath{ u}$, suggesting the entanglement between the spin degree of freedom and the orbital motion of the electrons in the regime of the integer quantum Hall effect. This suggestion is further supported, as all the experimental data are well described, when a Dresselhaus-type spin-orbit interaction is introduced into the Hamiltonian of a single electron in the quantizing magnetic field. Surprisingly, such excellent agreement was observed even in the case of tilted magnetic fields. The fitting procedure allowed us to determine the strength of the Dresselhaus spin-orbit term in a 2D electron system and to extract the fundamentally important Dresselhaus constant for bulk AlAs. Unexpectedly, not only was single spin resonance observed around even fillings, it tended to split into two well-resolved lines. Yet this finding remains a mystery and highlights the need for further experimental and theoretical efforts.

Flexible single walled nanotube based chemical sensor for 2,4-dinitrotoluene sensing

Abstract: In this work, we have attempted to fabricate flexible single walled carbon nanotube based sensor for detection of 2,4-dinitrotoluene (DNT) an explosive chemical. For analyte sensing study, the flexible sensor is fabricated by vacuum filtration method. These fabricated gas sensors are characterised by SEM and Raman spectroscopy. The sensor response is investigated toward the explosive chemicals which have NO2 group in their molecular structure. The fabricated sensor is able to detect the traces of DNT at room temperature. The sensor gives 0.28–0.32% repeatable response to 0.22 ppm of DNT. The response of sensor increases with increase in the vapour concentration of the DNT vapours.