Session H30: Semiconductor Qubits - Device Development

Screening of charged impurities with multi-electron singlet-triplet spin qubits in quantum dots

Charged impurities in semiconductor quantum dots comprise one of the main obstacles to achieving scalable fabrication and manipulation of singlet-triplet spin qubits. We theoretically show that using dots that contain several electrons each can help to overcome this problem through the screening of the rough and noisy impurity potential by the excess electrons [1]. We demonstrate how the desired screening properties turn on as the number of electrons is increased, and we characterize the properties of a double quantum dot singlet-triplet qubit for small odd numbers of electrons per dot. We show that the sensitivity of the multi-electron qubit to charge noise may be an order of magnitude smaller than that of the two-electron qubit. \\[4pt] [1] E. Barnes, J.P. Kestner, N.T.T. Nguyen, and S. Das Sarma, arXiv:1108.1399.   

Measurements of undoped accumulation-mode SiGe quantum dot devices

We report transport measurements of undoped single-well accumulation-mode SiGe quantum dot devices with an integrated dot charge sensor. The device is designed so that individual forward-biased circular gates have dominant control of dot charge occupancy, and separate intervening gates have dominant control of tunnel rates and exchange coupling. We have demonstrated controlled loading of the first electron in single and double quantum dots. We used magneto-spectroscopy to measure singlet-triplet splittings in our quantum dots: values are typically $\sim $0.1 meV. Tunnel rates of single electrons to the baths can be controlled from less than 1 Hz to greater than 10 MHz. We are able to control the (0,2) to (1,1) coupling in a double quantum dot from under-coupled (t$_{c} \quad <$ kT$\sim $ 5$\mu $eV) to over-coupled (t$_{c} \quad \sim $ 0.1 meV) with a bias control of one exchange gate. Sponsored by the United States Department of Defense. Approved for Public Release, Distribution Unlimited. The views expressed are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government.   

Fabrication of dual-gated devices on undoped Si/SiGe heterostructures

Undoped accumulation mode Si/SiGe heterostructures have recently emerged as a promising platform for the fabrication of few-electron silicon spin qubits. Spin blockade has been observed in an accumulation mode double dot [1] and a record mobility of 1.6 million cm$^2$/(Vs) has been achieved in undoped wafers grown at National Taiwan University [2]. We develop a fabrication process for dual-gated accumulation mode structures and form a stable two-dimensional electron gas by applying positive bias to a global top gate. The resulting 2DEG has charge densities of $2-5\times10^{11}$/cm$^{2}$ and mobilities up to 200,000 cm$^2$/(Vs). We present preliminary data from quantum point contacts fabricated in this geometry. \\ References:\\ \noindent [1] M. G. Borselli \textit{et al.}, Appl. Phys. Lett. {\bf99}, 063109 (2011).\\ \noindent [2] T. M. Lu \textit{et al.}, Appl. Phys. Lett. {\bf94}, 182102 (2009).\\   

Optimization of realistic silicon double quantum dots through simulation

We present results obtained using a newly developed semiclassical and Poisson-Schrodinger simulation tool which is able to simultaneously optimize many solution parameters. We discuss the benefit this capability has on realistic device design, and report general trends seen when targeting few-electron quantum dots in silicon and silicon-germanium structures. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Anatomy of the exchange gate action in undoped accumulation-mode SiGe quantum dot devices

We discuss simulations of an undoped accumulation-mode SiGe device containing an electrostatically formed double quantum dot in its active area. We validate our virtual model by extensive device characterization (in terms of gate actions, dot addition energies, etc.) and quantitative comparisons to concurrent experimental data. Next, we trace and map in detail the turn-on of the inter-dot exchange interaction by the exchange gate located between the dot gates. Of primary interest is the ability to control (i.e., both to completely shut off and to gradually modulate in the neV to $\mu$eV range) the exchange energy between the two separated electrons. We identify a potential obstacle to proper device operation, the formation of additional dot states under the progressively more forward-biased exchange gate. This effect is limited, however, to the case of large dot gate diameter and/or large dot-dot separation. Lastly we quantify and analyze the consequences of cross-capacitance between adjacent exchange and dot gates. Sponsored by the United States Department of Defense. Approved for public release, distribution unlimited.   

Effect of fixed charge gate oxide defects on the exchange energy of a multi-valley silicon double quantum dot

Exchange interaction at the MOS interface has been proposed as a qubit coupling approach for both MOS quantum dots and donor qubits. An intrinsic source of disorder in the MOS system is the charge defects in silicon dioxide, nanometers away from the interface and qubit electrons. The presence of a charge defect so near the qubit can significantly perturb the confinement potential and alter the intended coupling. Using a large-scale atomistic tight-binding method coupled to a full configuration interaction technique, we investigate the role of these defects on the two-electron coupling of a double quantum dot (DQD) as a function of detuning bias. We show how the multi-valley character of silicon is manifested in the two-electron spectrum, and hence in the exchange energies of excited triplet states corresponding to different valley configurations. Our results show that defects near the tunnel barrier can adversely affect the tunability of the DQD, while defects distributed asymmetrically relative to the two dots can act as an additional detuning source.   

Modeling split gate tunnel barriers in lateral double top gated Si-MOS nanostructures

Reliable interpretation of quantum dot and donor transport experiments depends critically on understanding the tunnel barriers separating the localized electron state from the 2DEG regions which serve as source and drain. We analyze transport measurements through split gate point contacts, defined in a double gate enhancement mode Si-MOS device structure. We use a square barrier WKB model which accounts for barrier height dependence on applied voltage. This constant interaction model is found to produce a self-consistent characterization of barrier height and width over a wide range of applied source-drain and gate bias. The model produces similar results for many different split gate structures. We discuss this models potential for mapping between experiment and barrier simulations. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. DOE's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Enhancement-mode buried strained-silicon channel double quantum dot

We demonstrate a relaxed-SiGe/strained-Si enhancement-mode gate stack for quantum dots. The devices were fabricated within a 150 mm Si foundry setting that uses implanted ohmics and chemical-vapor-deposited dielectrics. Thermal budget was minimized to prevent Ge/Si interdiffusion and strain relaxation. A mobility of 1.6x10$^{5}$ cm$^{2}$/Vs at 5.8x10$^{11}$/cm$^{2}$ is measured in Hall bars that witness the same device process flow as the quantum dot. Periodic Coulomb blockade measured in a double-top-gated lateral quantum dot nanostructure terminates with open diamonds up to +/- 10 mV of dc voltage across the device. Charge sensing indicates a lithographically defined double quantum dot with tunable coupling. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Hybrid Donor-Dot Devices made using Top-down Ion Implantation for Quantum Computing

We present progress towards fabricating hybrid donor -- quantum dots (QD) for quantum computing. These devices will exploit the long coherence time of the donor system and the surface state manipulation associated with a QD. Fabrication requires detection of single ions implanted with 10's of nanometer precision. We show in this talk, 100{\%} detection efficiency for single ions using a single ion Geiger mode avalanche (SIGMA) detector integrated into a Si MOS QD process flow. The NanoImplanter (nI) a focused ion beam system is used for precision top-down placement of the implanted ion. This machine has a 10 nm resolution combined with a mass velocity filter, allowing for the use of multi-species liquid metal ion sources (LMIS) to implant P and Sb ions, and a fast blanking and chopping system for single ion implants. The combination of the nI and integration of the SIGMA with the MOS QD process flow establishes a path to fabricate hybrid single donor-dot devices. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.~   

Computer assisted design of poly-silicon gated enhancement-mode, lateral double quantum dot devices for quantum computing

We discuss trade-offs of different double quantum dot and charge sensor lay-outs using computer assisted design (CAD). We use primarily a semi-classical model, augmented with a self-consistent configuration interaction method. Although CAD for quantum dots is difficult due to uncontrolled factors (e.g., disorder), different ideal designs can still be compared. Comparisons of simulation and measured dot characteristics, such as capacitance, show that CAD can agree well with experiment for relevant cases. CAD results comparing several different designs will be discussed including a comparison to measurement results from the same designs. Trade-offs between poly-silicon and metal gate lay-outs will also be discussed. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Correlation functions of the electric and magnetic fields in the vicinity of a metal surface

The Johnson noise-induced relaxation rate of a charge or spin qubit for a transition at a particular frequency in the vicinity of a metal boundary is proportional to the temporal Fourier component at that frequency of the electric or magnetic correlation function evaluated at the position of the qubit. These correlation functions are shown to be greatly enhanced compared to the blackbody result in the near vicinity of the metal due to the contribution of evanescent waves. As such, we expect a measurable enhancement of qubit decoherence due to the contribution of evanescent waves. We use a Green's dyadic approach to calculate the correlation functions of the fluctuating electric and magnetic fields in the vicinity of a conducting surface. In a local treatment of the dielectric properties of the metal this enhancement diverges as the inverse cube of the distance from the boundary, and for distances less than the order of the Fermi wavelength of the metal a nonlocal treatment is necessary to obtain an accurate result. We present a calculation of the correlation function for the full range of distances.   

Fabrication and measurement of quantum dots in double gated, dopantless Si/SiGe heterostructures

Significant progress has been made towards quantum dot spin qubits in Si/SiGe single and double quantum dots. In the past, these structures have been created by depleting a modulation-doped 2DEG that forms at the Si/SiGe interface. The modulation doping in such devices is believed to be a source of charge noise. Recently, undoped structures have been explored for the formation of both 2DEGs and quantum dots in Si/SiGe. Here we discuss measurements on double gated, dopantless quantum dots in Si/SiGe heterostructures. The devices are based on a new ``island mesa'' design incorporating micro-ohmic contacts. We present transport measurements on a double quantum dot showing a smooth transition from single dot to double dot behavior.   

Unintentional Quantum Dots in Silicon: Deducing the Location and Cause

When attempting to use local gates to electrostatically define quantum dots in silicon, additional unintentional quantum dots (U-QDs) that are not defined by the gates are often observed. U-QDs are typically blamed on random charged defects such as dopants or interface traps. We use measured gate capacitances and a capacitance simulator to determine the location of the U-QDs with a precision of a few nanometers. Since we have observed U-QDs in similar locations in multiple devices, we suggest that some U-QDs are not caused by random charged defects instead are a systematic but unanticipated consequence of the fabrication. We will discuss strain as a potential cause of the U-QDs. This allows us to suggest methods to reduce the frequency of U-QDs in future devices. Given the variety of groups suffering from U-QDs and the simplicity of this technique, we think that many groups might benefit from our methods.   

The divacancy in SiC: A new solid-state qubit

The nitrogen-vacancy center in diamond has attracted interest due to promising applications as a room-temperature solid-state qubit (the basic unit of a quantum computer). It is, however, desirable to identify defects that possess similar properties, but in alternative semiconductors that are either cheaper or more technologically mature. One notable defect system is the divacancy in 4H-SiC, which has recently been the subject of extensive experimental investigation. In this work, we employ advanced computational methods, particularly density functional theory using a hybrid functional, to investigate the stability and excitation energies of multiple forms of the divacancy in the various polytypes of silicon carbide. The hybrid functional gives band gaps and lattice parameters that are in excellent agreement with experiments. This allows for quantitative predictions of defect levels and zero-phonon line energies for excitation and emission processes, aiding in experimental identification of these defects.   

Large Stark effect for Li donor spins in Si

We study the effect of a static electric field on lithium donor spins in silicon. The anisotropy of the effective mass leads to the anisotropy of the quadratic Stark susceptibility, which we determined using the Dalgarno-Lewis exact summation method [1]. The theory is asymptotically exact in the field domain below Li-donor ionization threshold, relevant to the Stark-tuning Electron Spin Resonance (ESR) experiments [2]. With the calculated Stark susceptibilities at hand, we were able to predict and analyze several important physical effects. In particular, we demonstrate that the Stark effect anisotropy, combined with unique valley-orbit splitting of a Li donor in Si, spin-orbit interaction and specially tuned external stress, may lead to a very strong modulation of the donor spin g-factor by the electric field. Also we investigate the influence of random strains on the g-factor shifts and quantify the random strain limits and requirements to Si material purity necessary to observe the ESR-Stark shifts experimentally. Finally, we discuss possible applications of our results to quantum information processing with Li spin qubits in Si. \\[4pt] [1] A. Dalgarno and J. T. Lewis, Proc. Roy. Soc. 233, 70 (1955).\\[0pt] [2] F. R. Bradbury et al. Phys. Rev. Lett. 97, 176404 (2006).