N. C. Bishop

Enhancement-mode buried strained silicon channel quantum dot with tunable lateral geometry

We propose and demonstrate a relaxed-SiGe/strained-Si enhancement-mode gate stack for quantum dots. A mobility of 1.6u2009×u2009105 cm2/Vs at 5.8u2009×u20091011/cm2 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. The devices were fabricated within a 150 mm Si foundry setting that uses implanted ohmics and chemical-vapor-deposited dielectrics. A modified implant, polycrystalline silicon formation and annealing conditions were utilized to minimize the thermal budget that potentially leads to Ge/Si interdiffusion.

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 SNRu2009=u20091 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 5u2009μW 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.

Electron spin lifetime of a single antimony donor in silicon

We present measurements of the electron spin lifetime (T1) of a single Sb donor in Si. For a magnetic field (B) oriented along the [100] Si crystal direction and low temperature (T) such that kT≪gμB, we find T1−1=KB5, where K=1.7×10−3u2009Hzu2009T−5. The T1−1∝B5 dependence is expected for donor electron spin relaxation due to g-factor dependence on crystal strain. The magnitude of T1 is within a factor of two of theoretical estimates and is in close agreement with values obtained for bulk donor ensembles.

Single shot spin readout using a cryogenic high-electron-mobility transistor amplifier at sub-Kelvin temperatures

We use a cryogenic high-electron-mobility transistor circuit to amplify the current from a single electron transistor, allowing for demonstration of single shot readout of an electron spin on a single P donor in Si with 100u2009kHz bandwidth and a signal to noise ratio of ∼9. In order to reduce the impact of cable capacitance, the amplifier is located adjacent to the Si sample, at the mixing chamber stage of a dilution refrigerator. For a current gain of ∼2.7×103, the power dissipation of the amplifier is 13u2009μW, the bandwidth is ∼1.3u2009MHz, and for frequencies above 300u2009kHz the current noise referred to input is ≤70u2009fA/ Hz. With this amplification scheme, we are able to observe coherent oscillations of a P donor electron spin in isotopically enriched 28Si with 96% visibility.

Electrostatically defined silicon quantum dots with counted antimony donor implants

Deterministic control over the location and number of donors is crucial to donor spin quantum bits (qubits) in semiconductor based quantum computing. In this work, a focused ion beam is used to implant antimony donors in 100u2009nmu2009×u2009150u2009nm windows straddling quantum dots. Ion detectors are integrated next to the quantum dots to sense the implants. The numbers of donors implanted can be counted to a precision of a single ion. In low-temperature transport measurements, regular Coulomb blockade is observed from the quantum dots. Charge offsets indicative of donor ionization are also observed in devices with counted donor implants.

Transport spectroscopy of low disorder silicon tunnel barriers with and without Sb implants

We present transport measurements of silicon MOS split gate structures with and without Sb implants. We observe classical point contact (PC) behavior that is free of any pronounced unintentional resonances at liquid He temperatures. The implanted device has resonances superposed on the PC transport indicative of transport through the Sb donors. We fit the differential conductance to a rectangular tunnel barrier model with a linear barrier height dependence on source-drain voltage and non-linear dependence on gate bias. Effects such as Fowler-Nordheim (FN) tunneling and image charge barrier lowering (ICBL) are considered. Barrier heights and widths are estimated for the entire range of relevant biases. The barrier heights at the locations of some of the resonances for the implanted tunnel barrier are between 15-20 meV, which are consistent with transport through shallow partially hybridized Sb donors. The dependence of width and barrier height on gate voltage is found to be linear over a wide range of gate bias in the split gate geometry but deviates considerably when the barrier becomes large and is not described completely by standard 1D models such as FN or ICBL effects.

Enhancement-Mode Buried Strained Silicon Channel Double Quantum Dot with Integrated Electrometer

Silicon and silicon-germanium heterostructure devices are interesting test beds for exploring solid-state quantum computing. Electron spin lifetimes and coherence times in fully relaxed SiGe / strained Si (sSi) quantum well devices are much longer than competing systems (e.g. gallium- arsenide heterostructures), with further improvements expected. Additionally, highly crystalline and high mobility interfaces allow fabrication of few electron dots and sensitive electrometers more easily. We propose and demonstrate a relaxed-SiGe/sSi enhancement-mode gate stack for quantum dots. The wafers are grown to our specification using CVD process by Lawrence SQI. The devices were fabricated within a 150 mm Si foundry setting that uses implanted ohmics and chemical-vapor-deposited dielectrics. Polysilicon depletion gates are used to form few electron dots in the sSi quantum well. High density plasma silicon dioxide was used as a secondary dielectric, followed by a tungsten/titanium nitride enhancement gate to draw electrons into the system. A modified implant, polycrystalline silicon formation and annealing conditions were utilized to minimize the thermal budget that potentially leads to Ge/Si interdiffusion. A mobility of 1.6E5 cm^2/Vs at 5.8E11 /cm^2 is measured in Hall bars that witness the same device process flow as the quantum dot. Periodic Coulomb blockade conductance oscillations are measured in a single quantum dot nanostructure, evidence of discrete electron number. The Coulomb blockade diamonds increase to at least ±10 mV of dc voltage across the device after the last transition, a strong indication of single electron dot occupation. Charge transitions in a double quantum dot system are observed using the integrated electrometer, with tunable coupling between the dots. 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 Energys National Nuclear Security Administration under contract DE-AC04-94AL85000. References [1] A. M. Tyryshkin, S. A. Lyon, W. Jantsch, & F. Schaffler, Spin manipulation of free two- dimensional electrons in Si/SiGe quantum wells, Phys. Rev. Lett., vol 94, pp. 126802, 2005. [2] T. M. Lu, N. C. Bishop, T. Pluym, J. Means, P. G. Kotula, J. Cederberg, L. A. Tracy, J. Dominguez, M. P. Lilly, and M. S. Carroll, Enhancement-mode buried strained silicon channel quantum dot with tunable lateral geometry, Appl. Phys. Lett., vol. 99, no. 4, pp. 043101, 2011.

Process Development toward Enhancement-Mode Strained-Si/SiGe Double Quantum Dot

In this talk, we present our recent efforts in process development toward enhancement-mode strained Si/SiGe double quantum dots, utilizing either a 150mm Si foundry at wafer-level or a modern cleanroom setting at die-level. We focus on the following aspects: choice of gate insulator, device stability, and thermal budget.