Operating semiconductor qubits without individual barrier gates
A. S. Ivlev, D. R. Crielaard, M. Meyer, W. I. L. Lawrie, N. W. Hendrickx, A. Sammak, Y. Matsumoto, L. M. K. Vandersypen, G. Scappucci, C. Déprez, M. Veldhorst
Semiconductor spin qubits have emerged as a promising platform for quantum computing, following a significant improvement in their control fidelities over recent years. Increasing the qubit count remains challenging, beginning with the fabrication of small features and complex fanouts. A particular challenge has been formed by the need for individual barrier gates to control the exchange interaction between adjacent spin qubits. Here, we propose a method to vary two-qubit interactions without applying pulses on individual barrier gates while also remaining insensitive to detuning noise in first order. Experimentally we find that changing plunger gate voltages over 300 mV can tune the exchange energy $J$ from 100 kHz to 60 MHz. This allows us to perform two-qubit operations without changing the barrier gate voltage. Based on these findings we conceptualize a spin qubit architecture without individual barrier gates, simplifying the fabrication while maintaining the control necessary for universal quantum computation.
We develop a new theoretical approach for modelling a wide range of semiconductor-superconductor structures with arbitrary potential barriers and a spatially-dependent superconducting order parameter. We demonstrate asymmetry in the conductance spectrum as a result of a Schottky barrier shape. We further show that Andreev reflection process can be significantly enhanced through resonant tunneling with appropriate barrier configuration, which can incorporate the Schottky barrier as a contributing component of the device. Moreover, we show that resonant tunneling can be achieved in superlattice …
We study the tunneling of conduction electrons through a (110)-oriented single-barrier heterostructure grown from III-V semiconductor compounds. It is shown that, due to low spatial symmetry of such a barrier, the tunneling current through the barrier leads to an electron spin polarization. The inverse effect, generation of a direct tunneling current by spin polarized electrons, is also predicted. We develop the microscopic theory of the effects and show that the spin polarization emerges due to the combined action of the Dresselhaus spin-orbit coupling within the barrier and the Rashba spin-o…
Charge transport in amorphous oxide semiconductors is often described as the band transport affected by disorder in the form of random potential barriers (RB). Theoretical studies in the framework of this approach neglected so far the percolation nature of the phenomenon. In this article, a recipe for theoretical description of charge transport in the RB model is formulated using percolation arguments. Comparison with the results published so far evidences the superiority of the percolation approach.
Two-dimensional (2D) semiconductors have shown great promise in (opto)electronic applications. However, their developments are limited by a large Schottky barrier (SB) at the metal-semiconductor junction (MSJ), which is difficult to tune by using conventional metals due to the strong Fermi level pinning (FLP) effect. Here we show that, this problem can be overcome by using 2D metals, which are bounded with 2D semiconductors through van der Waals (vdW) interaction. This success relies on a weak FLP at the vdW MSJ, which is attributed to the suppression of metal-induced gap states. Consequently,…
The Hamiltonian of electrons interacting with interface phonons in three-barrier resonant tunneling structure is established using the first principles within the models of effective mass and polarization continuum. Using the Green's functions method, the temperature shifts and decay rates of operating electron states are calculated depending on geometric design of three-barrier nano-structure GaAs/Al$_{x}$Ga$_{1-x}$As which is an active region of quantum cascade detector. It is established that independently of the temperature, the energy of quantum transition during the process of electromag…
A systematic treatment of graphene-semiconductor junction is presented. Finite density of states at the Fermi level of graphene leads to exotic electronic properties at graphene-semiconductor interface. Quite generally, the Schottky-Mott limit and the sum rule of barrier heights are violated due to the internal potential of graphene. By merging the principal characteristics of semiconductor-semiconductor and metal-semiconductor junctions, the graphene-semiconductor contact may be considered as an archetype of unprecedented \emph{semimetal-semiconductor junction}. Generalized interface equation…
In a recent paper [Phys. Rev. B 72, 153314 (2005)], the $k^{3}$-Dresselhaus term in the contacts and the full form of the current operator are considered for spin-dependent tunneling through a symmetric barrier. The authors found that the full form of the current operator has a much larger influence on the spin polarization than it was initially thought. In this Comment we will show that their treatment of the other problem, the $k^{3}$-Dresselhaus term in the contacts, is incorrect. Their proposed solution in the contacts simply does not obey the Schrödinger equation. In this context we also …
The Schottky barrier height (SBH) is mapped with nanoscale resolution at pure Au/Si(001) and mixed Au/Ag/Si(001) interfaces utilizing ballistic electron emission microscopy (BEEM) by acquiring and fitting spectra every 11.7 nm over a 1 micron by 1 micron area. The energetic distribution of the SBH for the mixed interfaces contain several local maximums indicative of a mixture of metal species at the interface. To estimate the composition at the interface, the distributions are fit to multiple Gaussians that account for the species, "pinch-off" effects, and defects. This electrostatic compositi…
A vertical transistor based on a double gated, atomically thin heterojunction is theoretically examined. Both p-type and n-type transistor operations can be conveniently achieved by using one of the two gates as the switching gate. The transistor shows excellent saturation of output I-V characteristics due to drain-induced depletion and lack of tunneling barrier layers. The subthreshold slope could be below the thermionic limit due to band filtering as the switching mechanism. The atomically thin vertical PN heterojunction can be electrostatically modulated from a type II heterojunction to a b…
We study spin-dependent transport properties in magnetic tunneling junctions (MTJs) with semiconductor barriers, Fe/CuInSe$_2$/Fe(001) and Fe/CuGaSe$_2$/Fe(001). By analyzing their transmittances at zero bias voltage on the basis of the first-principles calculations, we find that spin-dependent coherent tunneling transport of $Δ_1$ wave functions yields a relatively high magnetoresistance (MR) ratio in both the MTJs. We carry out a detailed analysis of the band-resolved transmittances in both the MTJs and find an absence of the selective transmission of $Δ_1$ wave functions in some energy regi…
Binary stochastic neurons (BSNs) are excellent hardware accelerators for machine learning. A popular platform for implementing them are low- or zero-energy barrier nanomagnets possessing in-plane magnetic anisotropy (e.g. circular disks or quasi-elliptical disks with very small eccentricity). Unfortunately, small geometric variations in the lateral shapes of such nanomagnets can produce large changes in the BSN response times if the nanomagnets are made of common metallic ferromagnets (Co, Ni, Fe) with large saturation magnetization. Additionally, the response times are also very sensitive to …
Within the model of rectangular potentials and different effective masses of electrons in different elements of plane two-barrier resonance tunnel structure there is developed a theory of spectral parameters of quasi-stationary states and active conductivity for the case of mono-energetic electronic current interacting with electromagnetic field. It is shown that the two-barrier resonance tunnel structure can be utilized as a separate or active element of quantum cascade laser or detector. For the experimentally studied In$_{0.53}$Ga$_{0.47}$As/In$_{0.52}$Al$_{0.48}$As nano-system it is establ…
We have studied the finite bias transport properties of a 2H-1T' MoS$_2$ lateral metal-semiconductor (M-S) junction by non-equilibrium Green's functions calculations, aimed at contacting the 2D channel in a field effect transistor. Our results indicate that (a) despite the fundamentally different electrostatics of line and planar dipoles, the Schottky barrier heights respond similarly to changes in doping and applied bias in 2D and 3D M-S junctions, (b) 2H-1T' MoS$_2$ lateral junctions are free from Fermi level pinning, (c) armchair interfaces have superior contacting properties vs.\ zigzag in…
In 2D-semiconductor-based field-effect transistors and optoelectronic devices, metal-semiconductor junctions are one of the crucial factors determining device performance. The Fermi-level (FL) pinning effect, which commonly caused by interfacial gap states, severely limits the tunability of junction characteristics, including barrier height and contact resistance. A tunneling contact scheme has been suggested to address the FL pinning issue in metal-2D-semiconductor junctions, whereas the experimental realization is still elusive. Here, we show that an oxidized-monolayer-enabled tunneling barr…
Epitaxially-grown semiconductor heterostructures give the possibility to tailor the potential landscape for the carriers in a very controlled way. In planar lattice-matched heterostructures, the potential has indeed a very simple and easily predictable behavior: it is constant everywhere except at the interfaces where there is a step (discontinuity) which only depends on the composition of the semiconductors in contact. In this paper, we show that this universally accepted picture can be invalid in nanoscale heterostructures (e.g., quantum dots, rods, nanowires) which can be presently fabricat…
Electronic processes in a semiconductor system consisting of some Resonant Tunnelling Structures, built in the depletion region of a Schottky barrier, are investigated. It is shown that the Schottky barrier can block or unblock the resonant tunnelling current effectively. Tunnelling processes do reveal the coherent character. Sharp nonlinear current-voltage characteristics are observed on both of the forward and the reverse branches.
We demonstrate a method to create potential barriers with polarized light beams for polaritons in semiconductor microcavities. The form of the barriers is engineered via the real space shape of a focalised beam on the sample. Their height can be determined by the visibility of the scattering waves generated in a polariton fluid interacting with them. This technique opens up the way to the creation of dynamical potentials and defects of any shape in semiconductor microcavities.
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…
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…
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.
The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)/Alq3/Si hybrid structures was systematically studied by means of atomic force microscopy, SQUID magnetometry and nuclear magnetic resonance (NMR). All used techniques pointed out that the barrier thickness of 2 nm is required to obtain a magnetically good cobalt layer on top of Alq3. 59Co NMR measurements revealed that the AlOx barrier gives rise to the formation of an interface layer with "defective" cobalt favouring growth of "bulk" cobalt with good magnetic properties.
A key challenge for semiconductor quantum-dot charge qubits is the realization of long-range qubit coupling and performing high-fidelity gates based on it. Here, we describe a new type of charge qubit formed by an electron confined in a triple-quantum-dot system, enabling single and two-qubit gates working in the dipolar and quadrupolar detuning sweet spots. We further present the form for the long-range dipolar coupling between the charge qubit and the superconducting resonator. Based on the hybrid system composed of the qubits and the resonator, we present two types of entangling gates: the …
The Stationary Phase Method for a Wave Packet in a Semiconductor Layered System. The applicability of the method
使用Bohm在他的书{ \em“量子理论“}中所做的形式分析，Dover Publications Inc. New York（1979），为了近似地计算通过势垒的透射和反射波包的相位时间，我们计算了由不同介观层形成的半导体系统的相位时间。对透射波包和反射波包进行了分析，并根据波包的平稳相位，讨论了该方法在不同情况下的适用性。对于固定相位法的适用性，在仅取决于相位的导数的透射波的情况下，得到一个表达式，直到三阶。该条件指示系统的参数是否允许通过其前导项来定义波包。一个多屏障系统的情况下显示的结果的说明。这种形式包括使用传递矩阵来描述中心层，无论它是由一个层（单势垒情况）形成的，还是由两个势垒和一个内阱（DBRT系统）形成的，但是可以假设该层可以包括任何数量或任何种类的半导体层。
We present here a semiconductor injection laser operating in continuous wave with an emission covering more than one octave in frequency, and displaying homogeneous power distribution among the lasing modes. The gain medium is based on a heterogeneous quantum cascade structure operating in the THz range. Laser emission in continuous wave takes place from 1.64 THz to 3.35 THz with optical powers in the mW range and more than 80 modes above threshold. Free-running beatnote investigations on narrow waveguides with linewidths of 980 Hz limited by jitter indicate frequency comb operation on a spect…
We report scanning tunneling microscopy studies of individual adatoms deposited on an InSb(110) surface. The adatoms can be reproducibly dropped off from the STM tip by voltage pulses, and impact tunneling into the surface by up to ~100x. The spatial extent and magnitude of the tunneling effect are widely tunable by imaging conditions such as bias voltage, set current and photoillumination. We attribute the effect to occupation of a (+/0) charge transition level, and switching of the associated adatom-induced band bending. The effect in STM topographic images is well reproduced by transport mo…
We propose a novel class of temperature-tunable semiconductor metamaterials that exhibit negative refraction in the terahertz spectral range. These metamaterials are based on doped semiconductor superlattices with ultrathin barriers of about 1 nm thickness. Due to the tunnel transparency of the barriers, layers of the superlattice cannot be considered as isolated and, therefore, the classical homogenization approach is inapplicable. We develop a theory of quantum homogenization which is based on the Kubo formula for conductivity. The proposed approach takes into account the wave functions of t…
The adiabatic topological pumping is proposed by periodically modulating a semiconductor nanowire double-quantum-dot chain. We demonstrate that the quantized charge transport can be achieved by a nontrivial modulation of the quantum-dot well and barrier potentials. When the quantum-dot well potential is replaced by a time-dependent staggered magnetic field, the topological spin pumping can be realized by periodically modulating the barrier potentials and magnetic field. We also demonstrate that in the presence of Rashba spin-orbit interaction, the double-quantum-dot chain can be used to implem…
The transition metal dichalcogenide ($MX_{2}$, where $M$=Mo, W and $X$=S, Se, Te) monolayers are of high interest for semiconducting applications at the nanoscale level; this interest is due to both their direct band gaps and high charge mobilities. In this regard, an in-depth understating of the related Schottky barrier heights, associated with the incorporation of $MX_{2}$ sheets into novel low-dimensional metal-semiconductor junctions, is of crucial importance. Herein, we generate and provide analysis of the Schottky barrier heights behavior to account for the metal-induced gap states conce…
We show the operation of a Cu/Al_2O_3/Cu/n-Si hot-electron transistor for the straightforward determination of a metal/semiconductor energy barrier height even at temperatures below carrier-freeze out in the semiconductor. The hot-electron spectroscopy measurements return a fairly temperature independent value for the Cu/n-Si barrier of 0.66 $\pm$ 0.04 eV at temperatures below 180 K, in substantial accordance with mainstream methods based on complex fittings of either current-voltage (I-V) and capacitance-voltage (C-V) measurements. The Cu/n-Si hot-electron transistors exhibit an OFF current o…
Tunneling spectroscopy is widely used to examine the subgap spectra in semiconductor-superconductor nanostructures when searching for Majorana zero modes (MZMs). Typically, semiconductor sections controlled by local gates at the ends of hybrids serve as tunnel barriers. Besides detecting states only at the hybrid ends, such gate-defined tunnel probes can cause the formation of non-topological subgap states that mimic MZMs. Here, we develop an alternative type of tunnel probes to overcome these limitations. After the growth of an InSb-Al hybrid nanowire, a precisely controlled in-situ oxidation…
Optically active quantum dots, for instance self-assembled InGaAs quantum dots, are potentially excellent single photon sources. The fidelity of the single photons is much improved using resonant rather than non-resonant excitation. With resonant excitation, the challenge is to distinguish between resonance fluorescence and scattered laser light. We have met this challenge by creating a polarization-based dark-field microscope to measure the resonance fluorescence from a single quantum dot at low temperature. We achieve a suppression of the scattered laser exceeding a factor of 10^7 and backgr…
Graphene based transistors relying on a conventional structure cannot switch properly because of the absence of an energy gap in graphene. To overcome this limitation, a barristor device was proposed, whose operation is based on the modulation of the graphene-semiconductor (GS) Schottky barrier by means of a top gate, and demonstrating an ON-OFF current ratio up to $10^5$. Such a large number is likely due to the realization of an ultra clean interface with virtually no interface trapped charge. However, it is indeed technologically relevant to know the impact that the interface trapped charge…
Two-dimensional (2D) semiconductors are promising material candidates for next-generation nanoelectronics. However, there are fundamental challenges related to their metal-semiconductor (MS) contacts which limit the performance potential for practical device applications. In this work, we exploit 2D monolayer hexagonal boron nitride (h-BN) as an ultrathin decorating layer to form a metal-insulator-semiconductor (MIS) contact, and demonstrate a novel diode-like selective enhancement of the carrier transport through it. Compared to the conventional MS contact, the MIS contact dominated by both t…
Monolayers of transition metal dichalcogenides (TMDCs) exhibit excellent electronic and optical properties. However, the performance of these two-dimensional (2D) devices are often limited by the large resistance offered by the metal contact interface. Till date, the carrier injection mechanism from metal to 2D TMDC layers remains unclear, with widely varying reports of Schottky barrier height (SBH) and contact resistance (Rc), particularly in the monolayer limit. In this work, we use a combination of theory and experiments in Au and Ni contacted monolayer MoS2 device to conclude the following…
The Schottky barrier of a metal-semiconductor junction is one of the key quantities affecting the charge transport in a transistor. The Schottky barrier height depends on several factors, such as work function difference, local atomic configuration in the interface, and impurity doping. We show that also the presence of interface states at 2D metal-semiconductor junctions can give rise to a large renormalization of the effective Schottky barrier determined from the temperature dependence of the current. We investigate the charge transport in n- and p-doped monolayer MoTe$_2$ 1T'-1H junctions u…
Recently demonstrated metal-semiconductor heterojunctions with few-atom thickness show their promise as 2D Schottky contacts for future integrated circuits and nanoelectronics. The theory for 3D Schottky contacts, however, fails on these low-dimensional systems. Here, we propose a new model that yields carrier distribution and potential profile across the 2D metal-semiconductor heterojunction under the equilibrium condition, based on the input from first-principle calculations. Our calculation also suggests that, at the same forward bias, the current density of a stack of 2D graphene-phosphore…
