We present the essential findings of the screening theory of the integer quantum Hall effect (IQHE) considering a quantum point contact (QPC). Our approach is to solve the Poisson and the Schroedinger equations self-consistently, taking into account electron interactions, within a Hartree type approximation for a two dimensional electron gas (2DEG) subject to high perpendicular magnetic fields. The Coulomb interaction between the electrons separates 2DEG into two co-existing regions, namely quasi-metallic compressible and quasi-insulating incompressible regions, which exhibit peculiar screening and transport properties. In the presence of an external current, we show that this current is confined into the incompressible regions where the drift velocity is finite. In particular, we investigate the distribution of these incompressible strips and their relation with the quantum Hall plateaus considering a quasi 1D constriction, i.e. a QPC.
We report a new structure, called the Shielded Ohmic Contact (ShOC) rectifier which utilizes trenches filled with a high barrier metal to shield an Ohmic contact during the reverse bias. When the device is forward biased, the Ohmic contact conducts with a low forward drop. However, when reverse biased, the Ohmic contact is completely shielded by the high barrier Schottky contact resulting in a low reverse leakage current. Two dimensional numerical simulation is used to evaluate and explain the superior performance of the proposed ShOC rectifier.
Ohmic contacts are crucial elements of electron optics that have not received a clear theoretical description yet. We propose a model of an Ohmic contact as a piece of metal of the finite capacitance $C$ attached to a quantum Hall edge. It is shown that charged quantum Hall edge states may have weak coupling to neutral excitations in an Ohmic contact. Consequently, despite being a reservoir of neutral excitations, an Ohmic contact is not able to efficiently equilibrate edge states if its temperature is smaller than $\hbarΩ_c$, where $Ω_c$ is the inverse RC time of the contact. This energy scal…
We report on ultra-wide bandgap (UWBG) Al0.75Ga0.25N channel metal-insulator-semiconductor field-effect transistors (MISFET) with heterostructure engineered low-resistance ohmic contacts. The low intrinsic electron affinity of AlN (0.6 eV) leads to large Schottky barriers at metal-AlGaN interface, resulting in highly resistive ohmic contacts. In this work, we use reverse compositional graded n++ AlGaN contact layer to achieve upward electron affinity grading, leading to a low specific contact resistance of 1.9x10-6 ohm.cm2 to n-Al0.75Ga0.25N channels (bandgap ~ 5.3 eV) with non-alloyed contact…
We study dephasing in an electronic Mach-Zehnder (MZ) interferometer based on quantum Hall (QH) edge states by a micromiter-sized Ohmic contact embedded in one of its arms. We find that at the filling factor $ν=1$, as well as in the case where an Ohmic contact is connected to an MZ interfeoremter by a quantum point contact (QPC) that transmits only one electron channel, the phase coherence may not be fully suppressed. Namely, if the voltage bias $Δμ$ and the temperature $T$ are small compared to the charging energy of the Ohmic contact $E_C$, the free fermion picture is manifested, and the vis…
For carbon nanotube transistors, as for graphene, the electrical contacts are a key factor limiting device performance. We calculate the device characteristics as a function of nanotube diameter and metal workfunction. Although the on-state current varies continuously, the transfer characteristics reveal a relatively abrupt crossover from Schottky to ohmic contacts. We find that typical high-performance devices fall surprisingly close to the crossover. Surprisingly, tunneling plays an important role even in this regime, so that current fails to saturate with gate voltage as was expected due to…
III-V nanowires are comprehensively studied because of their suitability for optoelectronic quantum technology applications. However, their small dimensions and the spatial separation of carriers from the wire surface render electrical contacting difficult. Systematically studying ohmic contact formation by diffusion to $n$-doped GaAs nanowires, we provide a set of optimal annealing parameters for Pd/Ge/Au ohmic contacts. We reproducibly achieve low specific contact resistances of $\sim2\times10^{-7}\,Ω\text{cm}^2$ at room temperature becoming an order of magnitude higher at $T\simeq4.2\,$K. W…
Modulation-doped AlGaAs/GaAs heterostructures are utilized extensively in the study of quantum transport in nanostructures, but charge fluctuations associated with remote ionized dopants often produce deleterious effects. Electric field-induced carrier systems offer an attractive alternative if certain challenges can be overcome. We demonstrate a field-effect transistor in which the active channel is locally devoid of modulation-doping, but silicon dopant atoms are retained in the ohmic contact region to facilitate reliable low-resistance contacts. A high quality two-dimensional electron gas i…
The electrical contact to two-dimensional (2D)-semiconductor materials are decisive to the electronic performance of 2D-semiconductor field-effect devices (FEDs). The presence of a Schottky barrier often leads to a large contact resistance, which seriously limits the channel conductance and carrier mobility measured in a two-terminal geometry. In contrast, ohmic contact is desirable and can be achieved by the presence of a nonrectifying or tunneling barrier. Here, we demonstrate that an nonrectifying barrier can be realized by contacting indium (In), a low work function metal, with layered InS…
A novel technique to make Ohmic contact to a buried two-dimensional electron gas in a molecular-beam-epitaxy grown $GaAs/Al_{0.3}Ga_{0.7}As$ heterostructure with Mn $δ$-doping
High contact resistances have blocked the progress of devices based on MX2 (M = Mo,W; X = S,Se,Te) 2D semiconductors. Interface states formed at MX2/metal contacts pin the Fermi level, leading to sizable Schottky barriers for p-type contacts in particular. We show that (i) one can remove the interface states by covering the metal by a 2D layer, which is van der Waals-bonded to the MX2 layer, and (ii) one can choose the buffer layer such, that it yields a p-type contact with a zero Schottky barrier height. We identify possible buffer layers such as graphene, a monolayer of h-BN, or an oxide lay…
The study of electron transport in low-dimensional systems is of importance, not only from a fundamental point of view, but also for future electronic and spintronic devices. In this context heterostructures containing a two-dimensional electron gas (2DEG) are a key technology. In particular GaAs/AlGaAs heterostructures, with a 2DEG at typically 100 nm below the surface, are widely studied. In order to explore electron transport in such systems, low-resistance ohmic contacts are required that connect the 2DEG to macroscopic measurement leads at the surface. Here we report on designing and meas…
The discovery of graphene has ignited intensive investigation on two dimensional (2D) materials. Among them, transition metal dichalcogenide (TMDC), a typical representative, attracts much attention due to the excellent performance in field effect transistor (FET) related measurements and applications. Particularly, when TMDC eventually reaches few-layer dimension, a wide range of electronic and optical properties, in striking contrast to bulk samples, are detected. In this Letter, we synthesized single crystalline WS2 nanoflakes by physical vapor deposition (PVD) method and carried out a seri…
The formation of an ideal van der Waals (vdW) contacts at metal/transition-metal dichalcogenide (TMDC) interfaces is a critical step for the development of high-performance and energy-efficient electronic and optoelectronic applications based on the two-dimensional (2D) semiconductors. In overcoming the key chal-lenges of the conventional metal deposition process that leads to an uncontrol-lable Schottky barrier height and high contact resistance, notable advances were recently made by transferring atomically flat metal thin films or thermally evapo-rating indium/gold alloy. However, the reali…
Current voltage and Kelvin Probe Force Microscopy (KPFM) measurements were performed on single ZnO nanowires. Measurements are shown to be strongly correlated with the contact behavior, either ohmic or Schottky. The ZnO nanowires were obtained by metallo-organic chemical vapor deposition (MOCVD) and contacted using electronic-beam lithography. Depending on the contact geometry, good quality ohmic contacts (linear I V behavior) or non-linear (diode like) Schottky contacts were obtained. Current voltage and KPFM measurements on both types of contacted ZnO nanowires were performed in order to inv…
With the decrease of the dimensions of electronic devices, the role played by electrical contacts is ever increasing, eventually coming to dominate the overall device volume and total resistance. This is especially problematic for monolayers of semiconducting transition metal dichalcogenides (TMDs), which are promising candidates for atomically thin electronics. Ideal electrical contacts to them would require the use of similarly thin electrode materials while maintaining low contact resistances. Here we report a scalable method to fabricate ohmic graphene edge contacts to two representative m…
Achieving good quality Ohmic contacts to van der Waals materials is a challenge, since at the interface between metal and van der Waals material, different conditions can occur, ranging from the presence of a large energy barrier between the two materials to the metallization of the layered material below the contacts. In black phosphorus (bP), a further challenge is its high reactivity to oxygen and moisture, since the presence of uncontrolled oxidation can substantially change the behavior of the contacts. In this study, we investigate the influence of the metal used for the contacts to bP a…
We report a new strategy for fabricating 2D/2D low-resistance ohmic contacts for a variety of transition metal dichalcogenides (TMDs) using van der Waals assembly of substitutionally doped TMDs as drain/source contacts and TMDs with no intentional doping as channel materials. We demonstrate that few-layer WSe2 field-effect transistors (FETs) with 2D/2D contacts exhibit low contact resistances of ~ 0.3 k ohm.um, high on/off ratios up to > 109, and high drive currents exceeding 320 uA um-1. These favorable characteristics are combined with a two-terminal field-effect hole mobility ~ 2x102 cm2 V-…
In search of an improved strategy to form low resistance contacts to MoS2 and related semiconducting transition metal dichalcogenides, we use ab initio density functional electronic structure calculations in order to determine the equilibrium geometry and electronic structure of MoO3/MoS2 and MoO2/MoS2 bilayers. Our results indicate that, besides a rigid band shift associated with charge transfer, the presence of molybdenum oxide modifies the electronic structure of MoS2 very little. We find that the charge transfer in the bilayer provides a sufficient degree of hole doping to MoS2, resulting …
Microwave induced photoconductivity of a two-dimensional electron gas (2DEG) in selectively-doped GaAs/AlAs heterostructures has been studied using the Corbino geometry with capacitive contacts at a temperature T=1.6K and magnetic field B up to 0.5 T. Zero-conductance states have been observed in the samples under study subject to microwave radiation, similarly to the samples with ohmic contacts. It has been shown that ohmic contacts do not play a significant role for observation of zero-conductance states induced by microwave radiation
1 School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia 2 Australian Research Council Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), Monash University, Clayton, Victoria 3800, Australia 3 Department of Mechanical Engineering, Columbia University, New York, New York 10027, United States 4 Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States 5 School of Physics, the University of Melbourne, Melbourne, VIC 3010, Australia 6 Research Center for Functional Materials, National …
Low carrier mobility and high electrical contact resistance are two major obstacles prohibiting explorations of quantum transport in TMDCs. Here, we demonstrate an effective method to establish low-temperature Ohmic contacts in boron nitride encapsulated TMDC devices based on selective etching and conventional electron-beam evaporation of metal electrodes. This method works for most extensively studied TMDCs in recent years, including MoS2, MoSe2, WSe2, WS2, and 2H-MoTe2. Low electrical contact resistance is achieved at 2 K. All of the few-layer TMDC devices studied show excellent performance …
We report the fabrication of hexagonal-boron-nitride (hBN) encapsulated multi-terminal WSe$_2$ Hall bars with 2D/2D low-temperature Ohmic contacts as a platform for investigating the two-dimensional (2D) metal-insulator transition. We demonstrate that the WSe$_2$ devices exhibit Ohmic behavior down to 0.25 K and at low enough excitation voltages to avoid current-heating effects. Additionally, the high-quality hBN-encapsulated WSe$_2$ devices in ideal Hall-bar geometry enable us to accurately determine the carrier density. Measurements of the temperature ($T$) and density ($n_s$) dependence of …
The metal-semiconductor contact is a major factor limiting the shrinking of transistor dimension to further increase device performance. In-plane edge contacts have the potential to achieve lower contact resistance due to stronger orbital hybridization compared to conventional top contacts. However, a quantitative understanding of the electron transport properties in the edge contact is still lacking. In this work, we present full-band atomistic quantum transport simulations of the graphene/MoS$_2$ edge contact. By using a Wannier function basis to accurately describe the electronic bands, tog…
Two-dimensional material-based field effect transistors (2DM-FETs) are playing a revolutionary role in electronic devices. However, after years of development, no device model can match the Pao-Sah model for standard silicon-based transistors in terms of physical accuracy and computational efficiency to support large-scale integrated circuit design. One remaining critical obstacle is the contacts of 2DM-FETs. In order to self-consistently include the contact effect in the current model, it is necessary to perform self-consistent calculations, which is a fatal flaw for applications that priorit…
We present the theoretical model of the "quantum ammeter", a device that is able to measure the full counting statistics of an electron current at quantum time scales. It consists of an Ohmic contact, perfectly coupled to chiral quantum Hall channels, and of a quantum dot attached to one of the outgoing channels. At energies small compared to its charging energy, the Ohmic contact fractionalizes each incoming electron and redistributes it between outgoing channels. By monitoring the resonant tunneling current through the quantum dot, one gets an access to the moment generator of the current in…
We have developed a novel technique for detection of spin polarization with a quantum dot weakly coupled to the objective device. The disturbance to the object in this technique is very small since the detection is performed through sampling of single electrons in the object with very slow rate. We have applied the method to a quantum point contact (QPC) under a spin-orbit interaction. A high degree of spin polarization in the vicinity of the QPC was detected when the conductance stayed on a plateau at a half of the unit conductance quantum ($G_{\rm q}/2\equiv e^2/h$), and also on another plat…
In a recent experimental paper [1] a qualitative confirmation of the existence of upstream neutral modes at $ν= 2/3$ quantum Hall edge was reported. Using the chiral Luttinger liquid theory of quantum Hall edge we develop a quantitative model of the experiment [1]. A good quantitative agreement of our theory with the experimental data reinforces the conclusion of existence of the upstream neutral mode. Our model also enables us to extract important quantitative information about non-equilibrium processes in Ohmic and tunneling contacts from the experimental data. In particular, for $ν= 2/3$, w…
We study the thermal drag between two mesoscopic quantum Hall (QH) circuits. Each circuit consists of Ohmic contact perfectly coupled to quantum Hall edge states. The drag is caused by strong capacitive coupling between Ohmic contacts. The non-equilibirum conditions and the electron-electron interaction are taken into account by using the non-equilibrium bosonization technique. The thermal drag current in the passive circuit, the noise power of the corresponding heat current, and the Fano factor are calculated and analyzed.
InAs/GaSb double quantum wells (QWs) separated by a 100 Å AlSb middle barrier are grown by molecular beam epitaxy. We report a nanofabrication technique that utilizes the surface Fermi level pinning position in InAs $[E_f^s(\rm InAs)]$ for realizing independent electric contacts to each well. In particular, separate ohmic contacts to the upper InAs quantum well are achieved by selectively etching down to the InAs, while contacts to the lower GaSb quantum well are obtained by the depletion method. For the latter, the upper InAs quantum well is locally pinched off by top etched trenches capped w…
We propose a scheme to manipulate the electron-hole excitation in the voltage pulse electron source, which can be realized by a voltage-driven Ohmic contact connecting to a quantum hall edge channel. It has been known that the electron-hole excitation can be suppressed via Lorentzian pulses, leading to noiseless electron current. We show that, instead of the Lorentzian pulses, driven via the voltage pulse $V(t) = 2 \frac{\hbar}{e} \sqrt{\frac{\sqrt{3}}π k_{\rm B} T_h} \arctanh( \frac{t - t_0}{t_0} )$ with duration $t_0$, the electron-hole excitation can be tuned so that the corresponding energ…
We report on noise and thermal conductance measurements taken in order to determine an upper bound on the performance of graphene as a terahertz photon detector. The main mechanism for sensitive terahertz detection in graphene is bolometric heating of the electron system. To study the properties of a device using this mechanism to detect terahertz photons, we perform Johnson noise thermometry measurements on graphene samples. These measurements probe the electron-phonon behavior of graphene on silicon dioxide at low temperatures. Because the electron-phonon coupling is weak in graphene, superc…
The influence of the incompressible strips on the integer quantized Hall effect (IQHE) is investigated, considering a cleaved-edge overgrown (CEO) sample as an experimentally realizable sharp edge system. We propose a set of experiments to clarify the distinction between the large-sample limit when bulk disorder defines the IQHE plateau width and the small-sample limit smaller than the disorder correlation length, when self-consistent edge electrostatics define the IQHE plateau width. The large-sample or bulk QH regime is described by the usual localization picture, whereas the small-sample or…
We study solar cell properties of single silicon wires connected at their ends to two dissimilar metals of different work functions. Effects of wire dimensions, the work functions of the metals, and minority carrier lifetimes on short circuit current as well as open circuit voltage are studied. The most efficient photovoltaic behavior is found to occur when one metal makes a Schottky contact with the wire, and the other makes an Ohmic contact. As wire length increases, both short circuit current and open circuit voltage increase before saturation occurs. Depending on the work function differen…
Strong spin-orbit coupling, the resulting large $g$ factor, and small effective mass make InAs an attractive material platform for inducing topological superconductivity. The surface Fermi level pinning in the conduction band enables highly transparent ohmic contact without excessive doping. We investigate electrostatically defined quantum point contacts (QPCs) in a deep-well InAs two-dimensional electron gas. Despite the 3.3% lattice mismatch between the InAs quantum well and the InP substrate, we report clean QPCs with up to eight pronounced quantized conductance plateaus at zero magnetic fi…
Spin-orbit coupling is a source of strong spin dephasing in two- and three-dimensional semiconducting systems. We report that spin dephasing in a two-dimensional electron gas can be suppressed by introducing a quantum point contact. Surprisingly, this suppression was not limited to the vicinity of the contact but extended to the entire two-dimensional electron gas. This facilitates the electrical control of the spin degree of freedom in a two-dimensional electron gas through spin-orbit coupling.
We have succeeded in fully describing dynamic properties of spin current including the different spin absorption mechanism for longitudinal and transverse spins in lateral spin valves, which enables to elucidate intrinsic spin transport and relaxation mechanism in the nonmagnet. The deduced spin lifetimes are found independent of the contact type. From the transit-time distribution of spin current extracted from the Fourier transform in Hanle measurement data, the velocity of the spin current in Ag with Py/Ag Ohmic contact turns out much faster than that expected from the widely used model.
Observations of electron-hole asymmetry in transport through graphene devices at high magnetic field challenge prevalent models of the graphene quantum Hall effect. Here, we study this asymmetry both in conventional magnetotransport and in scanning gate microscopy maps measured in an encapsulated graphene constriction. We reveal that the presence of upstream modes and local doping in the vicinity of electrical contacts leads to a totally different picture of topological breakdown for electrons and holes, explaining the observed asymmetry.
We investigate theoretically the performance advantages of all-graphene nanoribbon field-effect transistors (GNRFETs) whose channel and source/drain (contact) regions are patterned monolithically from a two-dimensional single sheet of graphene. In our simulated devices, the source/drain and interconnect regions are composed of wide graphene nanoribbon (GNR) sections that are semimetallic, while the channel regions consist of narrow GNR sections that open semiconducting bandgaps. Our simulation employs a fully atomistic model of the device, contact and interfacial regions using tight-binding th…
Electrons in tight-binding lattice driven by DC electric field dissipate their energy through on-site fermionic thermostats. Due to the translational invariance in the transport direction, the problem can be block-diagonalized. We solve this time-dependent quadratic problem and demonstrate that the problem has an oscillatory steady-state. The steady-state occupation number shows that the Fermi surface disappears for any damping from the thermostats and any finite electric field. Despite the lack of momentum scattering, the conductivity takes the same form as the semi-classical Ohmic expression…
In ultra-thin two-dimensional (2-D) materials, the formation of ohmic contacts with top metallic layers is a challenging task that involves different processes than in bulk-like structures. Besides the Schottky barrier height, the transfer length of electrons between metals and 2-D monolayers is a highly relevant parameter. For MoS$_2$, both short ($\le$30 nm) and long ($\ge$0.5 $μ$m) values have been reported, corresponding to either an abrupt carrier injection at the contact edge or a more gradual transfer of electrons over a large contact area. Here we use \textit{ab initio} quantum transpo…
We theoretically study the orbital magnetic susceptibility produced by electron states near a crossing point of two band-contact lines in a crystal. It is shown that this susceptibility can have an unusual dependence on the Fermi level and can change noticeably with the temperature when the Fermi level is in the vicinity of the crossing point. These features of the magnetic susceptibility can be useful in detecting the crossing points in crystal. The obtained results explain the well-known temperature dependence of the magnetic susceptibility of V$_3$Si.
We report the fabrication of both n-type and p-type WSe2 field effect transistors with hexagonal boron nitride passivated channels and ionic-liquid (IL)-gated graphene contacts. Our transport measurements reveal intrinsic channel properties including a metal-insulator transition at a characteristic conductivity close to the quantum conductance e2/h, a high ON/OFF ratio of >107 at 170 K, and large electron and hole mobility of ~200 cm2V-1s-1 at 160 K. Decreasing the temperature to 77 K increases mobility of electrons to ~330 cm2V-1s-1 and that of holes to ~270 cm2V-1s-1. We attribute our abilit…
