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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., Comment: 4 pages, 5 figures

Three coupled quantum dots in isotopically purified silicon enable all-electrical qubit control with long coherence time., Like modern microprocessors today, future processors of quantum information may be implemented using all-electrical control of silicon-based devices. A semiconductor spin qubit may be controlled without the use of magnetic fields by using three electrons in three tunnel-coupled quantum dots. Triple dots have previously been implemented in GaAs, but this material suffers from intrinsic nuclear magnetic noise. Reduction of this noise is possible by fabricating devices using isotopically purified silicon. We demonstrate universal coherent control of a triple-quantum-dot qubit implemented in an isotopically enhanced Si/SiGe heterostructure. Composite pulses are used to implement spin-echo type sequences, and differential charge sensing enables single-shot state readout. These experiments demonstrate sufficient control with sufficiently low noise to enable the long pulse sequences required for exchange-only two-qubit logic and randomized benchmarking.

We adapt the level set method to simulate the growth of thin films described by the motion of island boundaries. This island dynamics model involves a continuum in the lateral directions, but retains atomic scale discreteness in the growth direction. Several choices for the island boundary velocity are discussed, and computations of the island dynamics model using the level set method are presented.

The development of new semiconductor device materials is currently dominated by costly and time-consuming experimental investigations in which growth parameters are varied and correlated, ex situ, to device characteristics. Model-based process development has the potential to assist greatly in optimizing growth parameters and providing models that are suitable for use in in-situ process control. Researchers have used this approach sparingly, however, because robust, predictive models that describe the growth of semiconductor materials are neither well-developed nor validated. This situation is changing, however, due to the availability of high-resolution microscopy and improved modeling capabilities resulting from greatly increased computer power and more efficient numerical algorithms. The focus in this paper is on III-V semiconductor molecular beam epitaxy (MBE), which is the preferred technique for growing complex heterostructure device materials with many critical interfaces. The challenge is to understand the connection between macroscopic reactor conditions and microscopic film properties such as interface thickness and morphology.

We present experimental data for the morphological evolution of InAs buffer layers which are interpreted using continuum equations of motion and kinetic Monte Carlo simulations. Our analysis reveals the presence of an instability even as an initially rough surface smooths during growth. This instability is due to the step-edge barrier and causes a characteristic length to emerge while the surface roughness is decreasing, well before the formation of the mounds. The smoothing is well described by a linear continuum equation identical to that which describes the early stages of spinodal decomposition and has important practical implications for the growth of device buffer layers.

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., 3 pages, 3 figures

We present a general methodology for developing predictive atomistic models of III-V semiconductor surfaces. We apply this methodology to the development of a kinetic Monte Carlo model that describes both equilibrium and growth of InAs(001) surfaces. Thermodynamic equilibrium behavior is solely based on ab initio density-functional theory calculations. Inclusion of kinetic processes and a large set of atomistic configurations on this highly complex surface enables the correct prediction of several experimental trends, including two thermodynamically stable surface reconstructions in the As-rich regime.

Molecular Beam Epitaxy is a method for growing atomically thin films of material. During epitaxial growth, atoms are deposited on a surface, where they hop randomly until attaching at the edges of partially completed atomic monolayers. This process has practical application to the fabrication of high speed semiconductor electronic devices.

Combining theoretical and experimental methods, we investigate the influence of group V fluxes of As2 and Sb2 on the InAs(001) surface. We find that equilibrated surfaces under As2 flux change their surface stoichiometry continuously for InAs in the technologically relevant α2(2×4)-β2(2×4) reconstruction region and that the As-dimer density increases with increasing As2 flux. The change of the surface morphology under Sb2 exposure is also studied and discussed specifically in the context of interface formation. The existence of a common α2(2×4) reconstruction allows for the possibility of keeping the In sublattice unchanged when switching from As2 to Sb2 flux.

Coherent charge-photon and spin-photon coupling has recently been achieved in silicon double quantum dots (DQDs). Here, we demonstrate a versatile split-gate cavity-coupler that allows more than one DQD to be coupled to the same microwave cavity. Measurements of the cavity transmission as a function of level detuning yield a charge cavity coupling rate of g c / 2 π = 58 MHz, a charge decoherence rate of γ c / 2 π = 36 MHz, and a cavity decay rate of κ / 2 π = 1.2 MHz. The charge cavity coupling rate is in good agreement with device simulations. Our coupling technique can be extended to enable simultaneous coupling of multiple DQDs to the same cavity mode, opening the door to long-range coupling of semiconductor qubits using microwave frequency photons.