M. G. Borselli

HRL Laboratories, LLC, Malibu, California, United States

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Publications (14)49.98 Total impact

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    ABSTRACT: We report on a quantum dot device design that combines the low disorder properties of undoped SiGe heterostructure materials with an overlapping gate stack in which each electrostatic gate has a dominant and unique function -- control of individual quantum dot occupancies and of lateral tunneling into and between dots. Control of the tunneling rate between a dot and an electron bath is demonstrated over more than nine orders of magnitude and independently confirmed by direct measurement within the bandwidth of our amplifiers. The inter-dot tunnel coupling at the (0,2)<-->(1,1) charge configuration anti-crossing is directly measured to quantify the control of a single inter-dot tunnel barrier gate. A simple exponential dependence is sufficient to describe each of these tunneling processes as a function of the controlling gate voltage.
    08/2014;
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    ABSTRACT: 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 μeV 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.
    02/2012;
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    ABSTRACT: Silicon is more than the dominant material in the conventional microelectronics industry: it also has potential as a host material for emerging quantum information technologies. Standard fabrication techniques already allow the isolation of single electron spins in silicon transistor-like devices. Although this is also possible in other materials, silicon-based systems have the advantage of interacting more weakly with nuclear spins. Reducing such interactions is important for the control of spin quantum bits because nuclear fluctuations limit quantum phase coherence, as seen in recent experiments in GaAs-based quantum dots. Advances in reducing nuclear decoherence effects by means of complex control still result in coherence times much shorter than those seen in experiments on large ensembles of impurity-bound electrons in bulk silicon crystals. Here we report coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure and show that this system has a nuclei-induced dephasing time of 360 nanoseconds, which is an increase by nearly two orders of magnitude over similar measurements in GaAs-based quantum dots. The degree of phase coherence observed, combined with fast, gated electrical initialization, read-out and control, should motivate future development of silicon-based quantum information processors.
    Nature 01/2012; 481(7381):344-7. · 38.60 Impact Factor
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    ABSTRACT: We demonstrate double quantum dots fabricated in undoped Si/SiGe heterostructures relying on a double top-gated design. Charge sensing shows that we can reliably deplete these devices to zero charge occupancy. Measurements and simulations confirm that the energetics are determined by the gate-induced electrostatic potentials. Pauli spin blockade has been observed via transport through the double dot in the two electron configuration, a critical step in performing coherent spin manipulations in Si.
    Applied Physics Letters 06/2011; · 3.79 Impact Factor
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    ABSTRACT: We have demonstrated few-electron quantum dots in Si/SiGe and InGaAs, with occupation number controllable from N = 0. These display a high degree of spatial symmetry and identifiable shell structure. Magnetospectroscopy measurements show that two Si-based devices possess a singlet N = 2 ground state at low magnetic field, and therefore, the twofold valley degeneracy is lifted. The valley splittings in these two devices were 270 and 120 μeV, suggesting the presence of atomically sharp interfaces in our heterostructures.
    Applied Physics Letters 03/2011; 98(12):123118-123118-3. · 3.79 Impact Factor
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    ABSTRACT: We have successfully formed a double quantum dot in the sSi/SiGe material system without need for intentional dopants. In our design, a two-dimensional electron gas is formed in a strained silicon well by forward biasing a global gate. Lateral definition of quantum dots is established with reverse-biased gates with ˜40 nm critical dimensions. Low-temperature capacitance and Hall measurements confirm electrons are confined in the Si-well with mobilities >10^4 cm^2/V-s. Further characterization identifies practical gate bias limits for this design and will be compared to simulation. Several double dot devices have been brought into the few-electron Coulomb blockade regime as measured by through-dot transport. Honeycomb diagrams and nonlinear through-dot transport measurements are used to quantify dot capacitances and addition energies of several meV. Sponsored by United States Department of Defense. Approved for Public Release, Distribution Unlimited.
    03/2011;
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    ABSTRACT: Quantum well heterostructures, widely used for the fabrication of quantum dots and related devices, typically make use of modulation doping. Removal of the dopants, by use of globally "field-gated" and/or back-gated heterostructure designs, eliminates the dominant sources of scattering, charge noise and instability in devices intended for low-temperature operation. In this talk we present recent progress in designing and fabricating undoped quantum well heterostructures in sSi/SiGe. A combination of simulation based modeling and experimental work has enabled us to successfully engineer materials for stable and quiet quantum dot operation. Specific topics to be presented include the important role of substrate and buffer layer background doping, concurrent MOS accumulation, leakage to front and back gates via barrier tunneling, and the expected range of electric fields that determine valley mixing in quantum dots. Sponsored by United States Department of Defense. Approved for Public Release, Distribution Unlimited.
    03/2011;
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    ABSTRACT: We report measurements of the spin-relaxation lifetime (T1) as a function of magnetic field in a strained-Si, accumulation-mode quantum dot. An integrated quantum-point contact (QPC) charge sensor was used to detect changes in dot occupancy as a function of bias applied to a single gate electrode. The addition spectra we obtained are consistent with theoretical predictions starting at N=0. The conductance of the charge sensor was measured by applying an AC voltage across the QPC and a 3 kphi resistor. Lifetime measurements were conducted using a three-pulse technique consisting of a load, read, and flush sequence. T1 was measured by observing the decay of the spin bump amplitude as a function of the load pulse length. We measured decay times ranging from approximately 75 msec at 2T to 12 msec at 3T, consistent with previous reports and theoretical predictions. Sponsored by United States Department of Defense. Approved for Public Release, Distribution Unlimited.
    03/2011;
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    ABSTRACT: We have measured charging spectra and charge dynamics of few-electron quantum dots made using Si/SiGe heterostructures. In the standard depletion-mode design, an excited state with Zeeman splitting consistent with a g-factor of 2.0±0.1 was identified on the lowest observed transition. The lifetime was 615 msec at 1.2T and had close to a B7 dependence on magnetic field, in good agreement with T1 spin relaxation estimates. We have also developed Si/SiGe accumulation-mode dots based on a double-well heterostructure in which electrons are localized in the top, nominally empty well by forward biasing a small gate. We have measured charging spectra from N=0 up to N=15, with addition energies as high as 4.5 meV. Magnetospectroscopy and charge dynamics are utilized to characterize valley splitting in these devices. Sponsored by United States Department of Defense Approved for Public Release, Distribution Unlimited.
    03/2010;
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    ABSTRACT: We present the results of electronic structure calculations of the ground and excited state spectra of accumulation-mode semiconductor quantum dots (QD) in both the Si/SiGe and InAlAs/InGaAs material systems. Devices are modeled using a real-space Poisson-Schr"odinger code coupled to a full configuration interaction (FCI) method in which both spin and valley degrees of freedom are explicitly included. Good agreement is found with measured ground state addition spectra allowing us to conclude that valleys play an essential role in Si QDs and that we have conclusively demonstrated single electron quantum dots in Si/SiGe. Calculations of the multi-electron excited-state spectra for both III-V and Si/SiGe accumulation-mode quantum dots will be presented along with predictions for transverse magneto-spectroscopy and comparisons with recent experimental data. Sponsored by United States Department of Defense Approved for Public Release, Distribution Unlimited.
    03/2010;
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    ABSTRACT: We report on the fabrication and characterization of a few-electron quantum dot controlled by a single gate electrode. Our device has a double-quantum-well design, in which the doping controls the occupancy of the lower well while the upper well remains empty under the free surface. A small air-bridged gate contacts the surface, and is positively biased to draw laterally confined electrons into the upper well. Electrons tunneling between this accumulation-mode dot and the lower well are detected using a quantum point contact (QPC), located slightly offset from the dot gate. The charge state of the dot is measured by monitoring the differential transconductance of the QPC near pinch-off. Addition spectra starting with N=0 were observed as a function of gate voltage. DC sensitivity to single electrons was determined to be as high as 8.6%, resulting in a signal-to-noise ratio of ~9:1 with an equivalent noise bandwidth of 12.1 kHz. Analysis of random telegraph signals associated with the zero to one electron transition allowed a measurement of the lifetimes for the filled and empty states of the one-electron dot: 0.38 ms and 0.22 ms, respectively, for a device with a 10 nm AlInAs tunnel barrier between the two wells. Comment: 3 pages, 3 figures
    Applied Physics Letters 10/2009; · 3.79 Impact Factor
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    ABSTRACT: We have observed the Zeeman-split excited state of a spin-1/2 multi-electron Si/SiGe depletion quantum dot and measured its spin relaxation time T1 in magnetic fields up to 2 T. Using a new step-and-reach technique, we have experimentally verified the g-value of 2.0 +/- 0.1 for the observed Zeeman doublet. We have also measured T1 of single- and multi-electron spins in InGaAs quantum dots. The lifetimes of the Si/SiGe system are appreciably longer than those for InGaAs dots for comparable magnetic field strengths, but both approach one second at sufficiently low fields (< 1 T for Si, and < 0.2 T for InGaAs). Comment: 5 pages, 5 figures
    08/2009;
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    ABSTRACT: We have developed a quantum-dot device based on a double-well heterostructure in which electrons are localized in the top, mostly empty well by forward biasing a small circular gate. Charge occupancy changes in the dot are monitored by measuring current confined to a narrow channel in the bottom well. In this design, dot occupancy is primarily controlled by a single gate and interacting dots can be straightforwardly fabricated. We have successfully fabricated and characterized single-dot devices of this design in AlGaAs/InGaAs, and are extending the design to SiGe/Si heterostructures. We have measured charging spectra of III-V versions of the device down to zero electron occupancy. Charging spectra show enhanced stability for n=2, 6, 12, and 20 electrons. We have measured the tunneling times as a function of bias to map out excited states of a two-electron dot.
    03/2009;
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    ABSTRACT: We have demonstrated few-electron quantum dots in Si/SiGe and InGaAs, with occupation number controllable from N = 0. These display a high degree of spatial symmetry and identifiable shell structure. Magnetospectroscopy measurements show that two Si-based devices possess a singlet N =2 ground state at low magnetic field and therefore the two-fold valley degeneracy is lifted. The valley splittings in these two devices were 120 and 270 {\mu}eV, suggesting the presence of atomically sharp interfaces in our heterostructures.