Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H26: Semiconducting QC: Spin Qubit Growth and Materials |
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Sponsoring Units: GQI Chair: Matt Borselli, HRL Laboratories Room: 289 |
Tuesday, March 14, 2017 2:30PM - 2:42PM |
H26.00001: Measurements of valley splitting in novel Si/SiGe heterostructures Samuel F. Neyens$^{*}$, Ryan H. Foote$^{*}$, T. J. Knapp$^{*}$, Brandur Thorgrimsson$^{*}$, L. M. K. Vandersypen$^{\dagger}$, Payam Amin$^{\ddagger}$, Antonio Rodolph B. Mei$^{\ddagger}$, Nicole K. Thomas$^{\ddagger}$, James S. Clarke$^{\ddagger}$, D. E. Savage$^{*}$, M. G. Lagally$^{*}$, Mark Friesen$^{*}$, S. N. Coppersmith$^{*}$, M. A. Eriksson$^{*}$ Achieving an appropriate valley splitting is important for making quantum dot qubits in Si/SiGe heterostructures. We measure valley splittings in novel heterostructures grown with an extra layer of Ge, $\sim$5 monolayers in thickness, between the Si well and the SiGe barrier. For one of these extra-Ge heterostructures, the CVD growth was interrupted between the Si well and the Ge layer to achieve a more abrupt change in composition. The other extra-Ge heterostructure was made with a continuous growth process. Using Hall bar devices on both of these extra-Ge samples as well as one standard sample with no extra Ge, we measure activation energies for valley splittings in the first and second Landau levels. For the $\nu=3$ valley splitting, we find the abrupt, extra-Ge sample has consistently the highest valley splitting across three different carrier densities. For these densities, the valley splitting in the abrupt, extra-Ge sample is $\sim50\%$ higher than that of the standard sample. [Preview Abstract] |
Tuesday, March 14, 2017 2:42PM - 2:54PM |
H26.00002: Mitigation of Small Valley Splitting Effects using Additional Electrons Aaron Jones Valley splitting in SiGe quantum dots may be limited due to a variety of effects, including imperfect Si/SiGe interfaces. The primary impact of a small valley splitting is a limited region of bias space at the (2,0)/(1,1) double-dot charge boundary supporting Pauli blockade, impairing singlet triplet measurements as well as the fidelity of singlet initialization. We report on mitigating this problem by operating in the (4,0)/(3,1) charge regime, which fills the lowest valley ground states in the problematic dot. We report that the additional electrons enable the observation of Rabi oscillations in accumulation-mode double- and triple-dot qubits, despite limited valley splitting as evaluated via photon-assisted tunneling (PAT) and measurements of Pauli blockade. We also present theoretical expectations for the influence of orbital states and valley mixing angles when using this methodology to enable qubit control in small-valley-splitting devices. [Preview Abstract] |
Tuesday, March 14, 2017 2:54PM - 3:06PM |
H26.00003: Silicon qubit performance in the presence of inhomogeneous strain N. Tobias Jacobson, Daniel R. Ward, Andrew D. Baczewski, John K. Gamble, Ines Montano, Martin Rudolph, Erik Nielsen, Malcolm Carroll While gate electrode voltages largely define the potential landscape experienced by electrons in quantum dot (QD) devices, mechanical strain also plays a role. Inhomogeneous strain established over the course of device fabrication, followed by mismatched contraction under cooling to cryogenic temperatures, may significantly perturb this potential. A recent investigation by Thorbeck & Zimmerman [AIP Adv. 5, 087107 (2015)] suggests that unintentional QDs may form as a result of the latter thermal contraction mismatch mechanism. In this work, we investigate the effects of inhomogeneous strain on QD tunnel barriers and other properties, from the perspective of QD and donor-based qubit performance. Through semiconductor process simulation, we estimate the relative magnitude of strain established during fabrication as compared with thermal expansion coefficient mismatch. Combining these predictions with multi-valley effective mass theory modeling of qubit characteristics, we identify whether strain effects may compel stricter than expected constraints on device dimensions. Finally, we investigate the degree to which strain and charge disorder effects may be distinguished. [Preview Abstract] |
Tuesday, March 14, 2017 3:06PM - 3:18PM |
H26.00004: Studying Si/SiGe disordered alloys within effective mass theory John Gamble, In\`{e}s Monta\~{n}o, Malcolm S. Carroll, Richard P. Muller Si/SiGe is an attractive material system for electrostatically-defined quantum dot qubits due to its high-quality crystalline quantum well interface. Modeling the properties of single-electron quantum dots in this system is complicated by the presence of alloy disorder, which typically requires atomistic techniques in order to treat properly. Here, we use the NEMO-3D empirical tight binding code to calibrate a multi-valley effective mass theory (MVEMT) to properly handle alloy disorder. The resulting MVEMT simulations give good insight into the essential physics of alloy disorder, while being extremely computationally efficient and well-suited to determining statistical properties. [Preview Abstract] |
Tuesday, March 14, 2017 3:18PM - 3:30PM |
H26.00005: Gate Induced Strain in Silicon MOS-based Tunnel Junction Devices Ryan Stein, Neil M. Zimmerman, M.D. Stewart The coefficient of thermal expansion mismatch between typical MOS gate materials, such as Aluminum, and the underlying silicon substrate is capable of inducing strain that modifies the local silicon conduction band. For quantum dot devices measured at low temperatures, the induced strain is strong enough to lead to the formation of unintentional quantum dots and affect the tunnel coupling between dots. We investigate the role of gate-induced strain in quantum dot devices by measuring the I-V characteristics of tunnel barriers at cryogenic temperatures fabricated with a variety of gate materials. We will discuss our results in the context of exploiting these affects to simplify gate layouts or mitigating them in quantum dot devices. [Preview Abstract] |
Tuesday, March 14, 2017 3:30PM - 3:42PM |
H26.00006: Interface induced spin-orbit interaction in silicon quantum dots and prospects of scalability Rifat Ferdous, Kok Wai, Menno Veldhorst, Jason Hwang, Henry Yang, Gerhard Klimeck, Andrew Dzurak, Rajib Rahman A scalable quantum computing architecture requires reproducibility over key qubit properties, like resonance frequency, coherence time etc. Randomness in these properties would necessitate individual knowledge of each qubit in a quantum computer. Spin qubits hosted in Silicon (Si) quantum dots (QD) is promising as a potential building block for a large-scale quantum computer, because of their longer coherence times. The Stark shift of the electron g-factor in these QDs has been used to selectively address multiple qubits. From atomistic tight-binding studies we investigated the effect of interface non-ideality on the Stark shift of the g-factor in a Si QD. We find that based on the location of a monoatomic step at the interface with respect to the dot center both the sign and magnitude of the Stark shift change. Thus the presence of interface steps in these devices will cause variability in electron g-factor and its Stark shift based on the location of the qubit. This behavior will also cause varying sensitivity to charge noise from one qubit to another, which will randomize the dephasing times $T_2^*$. This predicted device-to-device variability is experimentally observed recently in three qubits fabricated at a Si/Si0$_2$ interface, which validates the issues discussed. [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 3:54PM |
H26.00007: Undoped strained germanium quantum wells towards spin qubits Giordano Scappucci, Amir Sammak, LaReine Yeoh, Diego Sabbagh, Sonia Conesa-Boj, Sebastian Kolling, Peter Zaumseil, Giovanni Capellini Germanium is emerging as a promising material to implement spin qubits because of the key properties of high carrier mobility, strong spin-orbit coupling, long spin coherence times and compatibility with silicon technology. We report the deposition of undoped strained Ge/SiGe quantum wells of high structural quality in a reduced pressure chemical vapor deposition tool. Structural analysis of the Ge/SiGe heterostructures confirm sharp interfaces, full relaxation of the virtual substrate, and coherent deposition of the strained quantum well. Furthermore, we will discuss architectures towards the development of CMOS compatible spin qubits in laterally defined Ge quantum dots. [Preview Abstract] |
Tuesday, March 14, 2017 3:54PM - 4:06PM |
H26.00008: Annealing shallow traps in electron beam irradiated high mobility metal-oxide-silicon transistors Jin-Sung Kim, Alexei Tyryshkin, Stephen Lyon In metal-oxide-silicon (MOS) quantum devices, electron beam lithography (EBL) is known to create defects at the Si/SiO$_2$ interface which can be catastrophic for single electron control. Shallow traps ($\sim$meV), which only manifest themselves at low temperature ($\sim$4 K), are especially detrimental to quantum devices but little is known about annealing them. In this work, we use electron spin resonance (ESR) to measure the density of shallow traps in two sets of high mobility ($\mu$) MOS transistors. One set ($\mu$=14,000 cm$^2$/Vs) was irradiated with an EBL dose (10 kV, 40$\mu$C/cm$^2$) and was subsequently annealed in forming gas while the other remained unexposed ($\mu$=23,000 cm$^2$/Vs). Our ESR data show that the forming gas anneal is sufficient to remove shallow traps generated by the EBL dose over the measured shallow trap energy range (0.3-4 meV). We additionally fit these devices' conductivity data to a percolation transition model and extract a zero temperature percolation threshold density, $n_0$ ($\approx$ 9$\times$10$^{10}$ cm$^{-2}$ for both devices). We find that the extracted $n_0$ agrees within 15$\%$ with our lowest temperature (360 mK) ESR measurements, demonstrating agreement between two independent methods of evaluating the interface. [Preview Abstract] |
Tuesday, March 14, 2017 4:06PM - 4:18PM |
H26.00009: Thermodynamic implications of $^{29}$Si spin impurities on scalability of silicon-based quantum computing Pavel Lougovski, Nicholas A. Peters It is anticipated that $^{31}$P donors in silicon have the potential for realizing scalable quantum processors in analogue to classical computer chips\footnote{B. E. Kane, {\em Nature} {\bf 393}, 133 (1998). }. In classical computing, removing excess heat is a challenge that sets practical limits on performance. Here we consider what fundamental thermodynamic limits exist for the P-donor quantum computer in isotopically enriched $^{28}$Si. Specifically, we consider the effect of $^{31}$P nuclear spin rotation on the nuclear spin dynamics of the remaining $^{29}$Si impurity atoms within a single-qubit gate volume. Our simulations show that a $\pi$ rotation of $^{31}$P nuclear spin induces $^{29}$Si nuclear spin flipping resulting in an average energy decrease of the $^{29}$Si nuclear spin bath. For a gate volume of 5 nm$^3$ and $^{29}$Si concentration of 800 PPM at 250$\mu K$, the average energy decrease per single qubit rotation is 4.74$\times10^{-12} eV$. This suggests that the scalability of $^{31}$P-donor quantum computer will not be limited by energy dissipation from single qubit control pulses into the $^{29}$Si nuclear spin bath. Moreover, randomized single qubit rotation promises to be useful for cooling the $^{29}$Si nuclear spin bath. [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:30PM |
H26.00010: Refinement of ultra-enriched silicon for quantum electronics Joshua Pomeroy, Kevin Dwyer, Aruna Ramanayaka, Ke Tang, Hyun-soo Kim Nano-electronic device fabrication in epitaxial layers of 28-Si enriched at NIST to 99.99998{\%} has been hampered by an unacceptable density of nitrogen, carbon and oxygen, while other contaminants are essentially absent. Highly enriched silicon is recognized as a critical material for solid state quantum information by offering a ``semi-conductor vacuum'' yielding very long quantum coherence times. Our method of enriching silicon uses ionization combined with magnetic field separation that can allow us to target specific enrichments and map out the fundamental dependence of quantum coherence on the enrichment level. In recent months, process and equipment improvements have successively reduced the density of these deleterious gasses, and this talk will provide an update of the current state of the purity and report on results from electrical test devices formed from this enriched silicon. [Preview Abstract] |
Tuesday, March 14, 2017 4:30PM - 4:42PM |
H26.00011: UHV Ion Source for Highly Enriched and Purified $^{28}$Si Ke Tang, K.J. Dwyer, Hyun-Soo Kim, A.N. Ramanayaka, J.M. Pomeroy In order to improve the chemical purity of highly enriched $^{28}$Si deposited for solid state quantum information, we have developed a NIST-made Ultra-High Vacuum (UHV) compatible ion source to replace our traditional High-Vacuum (HV) Penning ion source. Highly enriched $^{28}$Si is a critical material for quantum information since the reduced $^{29}$Si nuclear spin allows for much longer coherence (T$_{2})$ times of qubits. We have successfully deposited epitaxial $^{28}$Si films with enrichments up to 99.99998{\%} (0.127ppm $^{29}$Si) using mass filtered ion beam deposition using natural abundance silane gas source. However, the chemical contamination levels of nitrogen, carbon and oxygen are unsatisfactorily high in the films we have grown, most likely due to the poor vacuum in our HV ion source. In this talk, we will present the design, performance and optimization of a new UHV ion source for this purpose. [Preview Abstract] |
Tuesday, March 14, 2017 4:42PM - 4:54PM |
H26.00012: Low-temperature magnetotransport in Si/SiGe heterostructures on 300 mm Si wafers Giordano Scappucci, L. Yeoh, D. Sabbagh, A. Sammak, J. Boter, G. Droulers, N. Kalhor, D. Brousse, M. Veldhorst, L. M. K. Vandersypen, N. Thomas, J. Roberts, R. Pillarisetty, P. Amin, H. C. George, K J Singh, J S Clarke Undoped Si/SiGe heterostructures are a promising material stack for the development of spin qubits in silicon. To deploy a qubit into high volume manufacturing in a quantum computer requires stringent control over substrate uniformity and quality. Electron mobility and valley splitting are two key electrical metrics of substrate quality relevant for qubits. Here we present low-temperature magnetotransport measurements of strained Si quantum wells with mobilities in excess of 100000 cm$^{2}$/Vs fabricated on 300 mm wafers within the framework of advanced semiconductor manufacturing. These results are benchmarked against the results obtained in Si quantum wells deposited on 100 mm Si wafers in an academic research environment. To ensure rapid progress in quantum wells quality we have implemented fast feedback loops from materials growth, to heterostructure FET fabrication, and low temperature characterisation. On this topic we will present recent progress in developing a cryogenic platform for high-throughput magnetotransport measurements. [Preview Abstract] |
(Author Not Attending)
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H26.00013: Electronic structure of Si:P delta doped layer Chin-Yi Chen, Federico Mazzola, Justin W. Wells, Rajib Rahman Densely doped Si:P delta layers are used to form many of the electronic components of qubit devices patterned by Scanning Tunneling Microscope (STM) lithography. A variety of methods, ranging from ab-initio to empirical and from atomistic to continuum, has been used to compute the band structures of such Si:P layers. However, the vastly varying results from these methods have not been verified by experiments. Here, we compare atomistic tight-binding (TB) calculations of band structures of Si:P layers with angle resolved photoemission spectroscopy (ARPES) measurements. The experimental data portrays a second set of gamma bands, in addition to the two typically predicted valley split gamma bands, separated by around 215 meVs. Our calculations show that the existence of these additional gamma bands can be explained by an increase of the effective dielectric constant to about 20. In addition, we study the non-parabolicity of the bands and spin splittings due to spin-orbit coupling. [Preview Abstract] |
Tuesday, March 14, 2017 5:06PM - 5:18PM |
H26.00014: Significance of Accurate Electronic Structure Calculation Methods in Designing Silicon Donor Qubits Fahd Mohiyaddin, Jacek Jakowski, Jingsong Huang, Milton Nance Ericson, Charles Britton, Franklin Curtis, Eugene Dumitrescu, Bobby Sumpter, Travis Humble Recent demonstrations of long-lived spin qubits with high control fidelity have enhanced the potential of silicon donors in quantum computing [1]. Verifying the design of prototype silicon qubit devices using computational models provides insight into their electrostatic potential landscape, donor electron wave functions, and spin dynamics [2]. Here, we examine the sensitivity of device verification to the underlying electronic structure model used for the donor. Within the context of a computational workflow, we observe a significant discrepancy in the amplitude of the donor wave function computed using density-functional theory versus tight-binding methods for the case of doped silicon nanocrystals. While both methods can be used to match experimental values for the hyperfine coupling, differences in the calculated electronic amplitude at the donor site suggest that more complicated interactions, e.g., electron-exchange, may become unreliable. Hence, an accurate understanding of the donor wave function in the donor vicinity is critical to device design, as it serves as a handle to vital parameters in donor based quantum computer architectures. [1] J. T. Muhonen et al., Nature Nanotechnology 9, 986-991(2014). [2] T. S. Humble et al., Nanotechnology 27, 42(2016). [Preview Abstract] |
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