arXiv subjects

S. M. Carr

Publications and source records attributed to S. M. Carr.

8 recordsLinked to original sources

arXiv2019

Quantum dots with split enhancement gate tunnel barrier control

We introduce a silicon metal-oxide-semiconductor quantum dot architecture based on a single polysilicon gate stack. The elementary structure consists of two enhancement gates separated spatially by a gap, one gate forming a reservoir and the other a quantum dot. We demonstrate, in three devices based on two different versions of this elementary structure, that a wide range of tunnel rates is attainable while maintaining single-electron occupation. A characteristic change in slope of the charge transitions as a function of the reservoir gate voltage, attributed to screening from charges in the reservoir, is observed in all devices, and is expected to play a role in the sizable tuning orthogonality of the split enhancement gate structure. The all-silicon process is expected to minimize strain gradients from electrode thermal mismatch, while the single gate layer should avoid issues related to overlayers (e.g., additional dielectric charge noise) and help improve yield. Finally, reservoir gate control of the tunnel barrier has implications for initialization, manipulation and readout schemes in multi-quantum dot architectures.

cond-mat.mes-hall↗

arXiv2019

Single-Shot Readout Performance of Two Heterojunction-Bipolar-Transistor Amplification Circuits at Millikelvin Temperatures

High-fidelity single-shot readout of spin qubits requires distinguishing states much faster than the T1 time of the spin state. One approach to improving readout fidelity and bandwidth (BW) is cryogenic amplification, where the signal from the qubit is amplified before noise sources are introduced and room-temperature amplifiers can operate at lower gain and higher BW. We compare the performance of two cryogenic amplification circuits: a current-biased heterojunction bipolar transistor circuit (CB-HBT), and an AC-coupled HBT circuit (AC-HBT). Both circuits are mounted on the mixing-chamber stage of a dilution refrigerator and are connected to silicon metal oxide semiconductor (Si-MOS) quantum dot devices on a printed circuit board (PCB). The power dissipated by the CB-HBT ranges from 0.1 to 1 μW whereas the power of the AC-HBT ranges from 1 to 20 μW. Referred to the input, the noise spectral density is low for both circuits, in the 15 to 30 fA/$\sqrt{\textrm{Hz}}$ range. The charge sensitivity for the CB-HBT and AC-HBT is 330 μe/$\sqrt{\textrm{Hz}}$ and 400 μe/$\sqrt{\textrm{Hz}}$, respectively. For the single-shot readout performed, less than 10 μs is required for both circuits to achieve bit error rates below $10^{-3}$, which is a putative threshold for quantum error correction.

cond-mat.mes-hall↗

arXiv2017

Spectroscopy of multi-electrode tunnel barriers

Despite their ubiquity in nanoscale electronic devices, the physics of tunnel barriers has not been developed to the extent necessary for the engineering of devices in the few-electron regime. This problem is of urgent interest, as this is the precise regime into which current, extreme-scale electronics fall. Here, we propose theoretically and validate experimentally a compact model for multi-electrode tunnel barriers, suitable for design-rules-based engineering of tunnel junctions in quantum devices. We perform transport spectroscopy at $T=4$ K, extracting effective barrier heights and widths for a wide range of biases, using an efficient Landauer-Büttiker tunneling model to perform the analysis. We find that the barrier height shows several regimes of voltage dependence, either linear or approximately exponential. The exponential dependence approximately correlates with the formation of an electron channel below an electrode. Effects on transport threshold, such as metal-insulator-transition and lateral confinement are non-negligible and included. We compare these results to semi-classical solutions of Poisson's equation and find them to agree qualitatively. Finally, we characterize the sensitivity of a tunnel barrier that is raised or lowered without an electrode being directly above the barrier region.

cond-mat.mes-hall↗

arXiv2015

Cryogenic preamplification of a single-electron-transistor using a silicon-germanium heterojunction-bipolar-transistor

We examine a silicon-germanium heterojunction bipolar transistor (HBT) for cryogenic pre-amplification of a single electron transistor (SET). The SET current modulates the base current of the HBT directly. The HBT-SET circuit is immersed in liquid helium, and its frequency response from low frequency to several MHz is measured. The current gain and the noise spectrum with the HBT result in a signal-to-noise-ratio (SNR) that is a factor of 10-100 larger than without the HBT at lower frequencies. The transition frequency defined by SNR = 1 has been extended by as much as a factor of 10 compared to without the HBT amplification. The power dissipated by the HBT cryogenic pre-amplifier is approximately 5 nW to 5 μW for the investigated range of operation. The circuit is also operated in a single electron charge read-out configuration in the time-domain as a proof-of-principle demonstration of the amplification approach for single spin read-out.

cond-mat.mes-hall↗

arXiv2014

Imaging and registration of buried atomic-precision donor devices using scanning capacitance microscopy

We show that a scanning capacitance microscope (SCM) can image buried delta-doped donor nanostructures fabricated in Si via a recently developed atomic-precision scanning tunneling microscopy (STM) lithography technique. A critical challenge in completing atomic-precision nanoelectronic devices is to accurately align mesoscopic metal contacts to the STM defined nanostructures. Utilizing the SCMs ability to image buried dopant nanostructures, we have developed a technique by which we are able to position metal electrodes on the surface to form contacts to underlying STM fabricated donor nanostructures with a measured accuracy of 300 nm. Low temperature (T=4K) transport measurements confirm successful placement of the contacts to the donor nanostructures.

cond-mat.mtrl-sci↗

arXiv2014

Probing limits of STM field emission patterned Si:P $δ$-doped devices

Recently, a single atom transistor was deterministically fabricated using phosphorus in Si by H-desorption lithography with a scanning tunneling microscope (STM). This milestone in precision, achieved by operating the STM in the conventional tunneling mode, typically utilizes very slow ($\sim\!10^2~\mathrm{nm^2/s}$) patterning speeds. By contrast, using the STM in a high voltage ($>10~\mathrm{V}$) field emission mode, patterning speeds can be increased by orders of magnitude to $\gtrsim\!10^4~\mathrm{nm^2/s}$. We show that the rapid patterning negligibly affects the functionality of relatively large micron-sized features, which act as contacting pads on these devices. For nanoscale structures, we show that the resulting transport is consistent with the donor incorporation chemistry enhancing the device definition to a scale of $10~\mathrm{nm}$ even though the pattering spot size is $40~\mathrm{nm}$.

cond-mat.mes-hall↗

arXiv2005

Quantum teleportation between nanomechanical modes

We study a quantum teleportation scheme between two nanomechanical modes without local interaction. The nanomechanical modes are linearly coupled to and connected by the continuous variable modes of a superconducting circuit consisting of a transmission line and Josephson junctions. We calculate the fidelity of transferring Gaussian states at finite temperature and non-unit detector efficiency. For coherent state, a fidelity above the classical limit of 1/2 can be achieved for a large range of parameters.

quant-ph↗

arXiv2001

Quantum Effects in the Mechanical Properties of Suspended Nanomechanical Systems

We explore the quantum aspects of an elastic bar supported at both ends and subject to compression. If strain rather than stress is held fixed, the system remains stable beyond the buckling instability, supporting two potential minima. The classical equilibrium transverse displacement is analogous to a Ginsburg-Landau order parameter, with strain playing the role of temperature. We calculate the quantum fluctuations about the classical value as a function of strain. Excitation energies and quantum fluctuation amplitudes are compared for silicon beams and carbon nanotubes.

cond-mat.mes-hall↗