Multijunction thermal converters are routinely used at many primary standards laboratories for ac voltage measurements and calibrations. After nearly two decades of inactivity, the Primary Standards Laboratory at Sandia National Laboratories has reestablished the process of fabricating silicon based multijunction thermal converters. Initial results indicate the devices perform similarly to devices fabricated circa 2001 with ac-dc differences of less than 2 mu mathrm{V}/mathrm{V} over the frequency range of 20 Hz to 20 kHz. From 20 kHz to 1 MHz, the ac-dc difference was higher, but remained below 200 mu mathrm{V}/mathrm{V}. In addition to presenting these results, new design considerations, such as high-resistivity substrates for high-frequency applications, are discussed.
An electrodeposition process for void-free bottom-up filling of sub-millimeter scale through silicon vias (TSVs) with Cu is detailed. The 600 μm deep and nominally 125 μm diameter metallized vias were filled with Cu in less than 7 hours under potentiostatic control. The electrolyte is comprised of 1.25 mol/L CuSO4 - 0.25 mol/L CH3SO3H with polyether and halide additions that selectively suppress metal deposition on the free surface and side walls. A brief qualitative discussion of the procedures used to identify and optimize the bottom-up void-free feature filling is presented.
A methanesulfonic acid (MSA) electrolyte with a single suppressor additive was used for potentiostatic bottom-up filling of copper in mesoscale through silicon vias (TSVs). Conversly, galvanostatic deposition is desirable for production level full wafer plating tools as they are typically not equipped with reference electrodes which are required for potentiostatic plating. Potentiostatic deposition was used to determine the over-potential required for bottom-up TSV filling and the resultant current was measured to establish a range of current densities to investigate for galvanostatic deposition. Galvanostatic plating conditions were then optimized to achieve void-free bottom-up filling in mesoscale TSVs for a range of sample sizes.
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 specific regime into which current extreme-scale electronics fall. Here, we propose theoretically and validate experimentally a compact model for multielectrode tunnel barriers, suitable for design-rules-based engineering of tunnel junctions in quantum devices. We perform transport spectroscopy at approximately 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. Effects on threshold, such as metal-insulator transition and lateral confinement, are included because they influence parameters that determine barrier height and width (e.g., the Fermi energy and local electric fields). We compare these results to semiclassical solutions of Poisson's equation and find them to agree qualitatively. Finally, this characterization technique is applied to an efficient lateral tunnel barrier design that does not require an electrode directly above the barrier region in order to estimate barrier heights and widths.
The readout of semiconductor spin qubits based on spin blockade is fast but suffers from a small charge signal. Previous work suggested large benefits from additional charge mapping processes; however, uncertainties remain about the underlying mechanisms and achievable fidelity. In this work, we study the single-shot fidelity and limiting mechanisms for two variations of an enhanced latching readout. We achieve average single-shot readout fidelities greater than 99.3% and 99.86% for the conventional and enhanced readout, respectively, the latter being the highest to date for spin blockade. The signal amplitude is enhanced to a full one-electron signal while preserving the readout speed. Furthermore, layout constraints are relaxed because the charge sensor signal is no longer dependent on being aligned with the conventional (2,0)–(1,1) charge dipole. Silicon donor-quantum-dot qubits are used for this study, for which the dipole insensitivity substantially relaxes donor placement requirements. One of the readout variations also benefits from a parametric lifetime enhancement by replacing the spin-relaxation process with a charge-metastable one. This provides opportunities to further increase the fidelity. The relaxation mechanisms in the different regimes are investigated. This work demonstrates a readout that is fast, has a one-electron signal, and results in higher fidelity. As a result, it further predicts that going beyond 99.9% fidelity in a few microseconds of measurement time is within reach.
Individual donors in silicon chips are used as quantum bits with extremely low error rates. However, physical realizations have been limited to one donor because their atomic size causes fabrication challenges. Quantum dot qubits, in contrast, are highly adjustable using electrical gate voltages. This adjustability could be leveraged to deterministically couple donors to quantum dots in arrays of qubits. In this work, we demonstrate the coherent interaction of a 31P donor electron with the electron of a metal-oxide-semiconductor quantum dot. We form a logical qubit encoded in the spin singlet and triplet states of the two-electron system. We show that the donor nuclear spin drives coherent rotations between the electronic qubit states through the contact hyperfine interaction. This provides every key element for compact two-electron spin qubits requiring only a single dot and no additional magnetic field gradients, as well as a means to interact with the nuclear spin qubit.
Si-MOS based QD qubits are attractive due to their similarity to the current semiconductor industry. We introduce a highly tunable MOS foundry compatible qubit design that couples an electrostatic quantum dot (QD) with an implanted donor. We show for the first time coherent two-axis control of a two-electron spin logical qubit that evolves under the QD-donor exchange interaction and the hyperfine interaction with the donor nucleus. The two interactions are tuned electrically with surface gate voltages to provide control of both qubit axes. Qubit decoherence is influenced by charge noise, which is of similar strength as epitaxial systems like GaAs and Si/SiGe.
Enhancement-mode Si/SiGe electron quantum dots have been pursued extensively by many groups for their potential in quantum computing. Most of the reported dot designs utilize multiple metal-gate layers and use Si/SiGe heterostructures with Ge concentration close to 30%. Here, we report the fabrication and low-temperature characterization of quantum dots in the Si/Si0.8Ge0.2 heterostructures using only one metal-gate layer. We find that the threshold voltage of a channel narrower than 1 μm increases as the width decreases. The higher threshold can be attributed to the combination of quantum confinement and disorder. We also find that the lower Ge ratio used here leads to a narrower operational gate bias range. The higher threshold combined with the limited gate bias range constrains the device design of lithographic quantum dots. We incorporate such considerations in our device design and demonstrate a quantum dot that can be tuned from a single dot to a double dot. The device uses only a single metal-gate layer, greatly simplifying device design and fabrication.
Brener, Igal B.; Guo, Rui; Rusak, Evgenia; Staude, Isabelle; Dominguez, Jason J.; Decker, Manuel; Author, No; Rockstuhl, Carsten; Neshev, Dragomir N.; Kivshar, Yuri S.
We study functional hybrid metasurfaces consisting of metal-dielectric nanoantennas that direct light from an incident plane wave or from localized light sources into a preferential direction. The directionality is obtained by carefully balancing the multipolar contributions to the scattering response from the constituents of the metasurface. The hybrid nanoantennas are composed of a plasmonic gold nanorod acting as a feed element and a silicon nanodisk acting as a director element. In order to experimentally realize this design, we have developed a two-step electron-beam lithography process in combination with a precision alignment step. The optical response of the fabricated sample is measured and reveals distinct signatures of coupling between the plasmonic and the dielectric nanoantenna elements that ultimately leads to unidirectional radiation of light.