Padawer-Blatt, A.; Ducatel, J.; Korkusinski, M.; Bogan, A.; Gaudreau, L.; Zawadzki, P.; Austing, D.G.; Sachrajda, A.S.; Studenikin, S.; Tracy, Lisa A.; Reno, J.; Hargett, Terry H.
Difference in g factors in multidot structures can form the basis of dot-selective spin manipulation under global microwave irradiation. Employing electric dipole spin resonance facilitated by strong spin-orbit interaction (SOI), we observe differences in the extracted values of the single-hole effective g factors of the constituent quantum dots of a GaAs/AlGaAs double quantum dot device at the level of ∼5%-10%. We examine the continuous change in the hole g factor with electrical detuning over a wide range of interdot tunnel couplings and for different out-of-plane magnetic fields. The observed tendency of the quantum dot effective g factors to steadily increase on decreasing the interdot coupling or on increasing the magnetic field is attributed to the impact on the SOI of changing the dot confinement potential and heavy-hole light-hole mixing.
Padawer-Blatt, Aviv P.; Ducatel, Jordan D.; Bogan, Alex B.; Austing, Guy D.; Gaudreau, Louis G.; Zawadzki, Piotr Z.; Sachrajda, Andrew S.; Studenikin, Sergei S.; Tracy, Lisa A.; Reno, John R.; Hargett, Terry H.
The atomic precision advanced manufacturing (APAM) enabled vertical tunneling field effect transistor (TFET) presents a new opportunity in microelectronics thanks to the use of ultra-high doping and atomically abrupt doping profiles. We present modeling and assessment of the APAM TFET using TCAD Charon simulation. First, we show, through a combination of simulation and experiment, that we can achieve good control of the gated channel on top of a phosphorus layer made using APAM, an essential part of the APAM TFET. Then, we present simulation results of a preliminary APAM TFET that predict transistor-like current-voltage response despite low device performance caused by using large geometry dimensions. Future device simulations will be needed to optimize geometry and doping to guide device design for achieving superior device performance.
Hole spin qubits confined to lithographically - defined lateral quantum dots in Ge/SiGe heterostructures show great promise. On reason for this is the intrinsic spin - orbit coupling that allows all - electric control of the qubit. That same feature can be exploited as a coupling mechanism to coherently link spin qubits to a photon field in a superconducting resonator, which could, in principle, be used as a quantum bus to distribute quantum information. The work reported here advances the knowledge and technology required for such a demonstration. We discuss the device fabrication and characterization of different quantum dot designs and the demonstration of single hole occupation in multiple devices. Superconductor resonators fabricated using an outside vendor were found to have adequate performance and a path toward flip-chip integration with quantum devices is discussed. The results of an optical study exploring aspects of using implanted Ga as quantum memory in a Ge system are presented.
While it is likely practically a bad idea to shrink a transistor to the size of an atom, there is no arguing that it would be fantastic to have atomic-scale control over every aspect of a transistor – a kind of crystal ball to understand and evaluate new ideas. This project showed that it was possible to take a niche technique used to place dopants in silicon with atomic precision and apply it broadly to study opportunities and limitations in microelectronics. In addition, it laid the foundation to attaining atomic-scale control in semiconductor manufacturing more broadly.
Bogan, Alex; Studenikin, Sergei; Korkusinski, Marek; Gaudreau, Louis; Phoenix, Jason; Zawadzki, Piotr; Sachrajda, Andy; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.
We analyze experimentally and theoretically the transport spectra of a gated lateral GaAs double quantum dot containing two holes. The strong spin-orbit interaction present in the hole subband lifts the Pauli spin blockade and allows to map out the complete spectra of the two-hole system. By performing measurements in both source-drain voltage directions, at different detunings and magnetic fields, we carry out quantitative fitting to a Hubbard two-site model accounting for the tunnel coupling to the leads and the spin-flip relaxation process. We extract the singlet-triplet gap and the magnetic field corresponding to the singlet-triplet transition in the double-hole ground state. Additionally, at the singlet-triplet transition we find a resonant enhancement (in the blockaded direction) and suppression of current (in the conduction direction). The current enhancement stems from the multiple resonance of two-hole levels, opening several conduction channels at once. The current suppression arises from the quantum interference of spin-conserving and spin-flipping tunneling processes.
We report a detailed study of the tunnel barriers within a single-hole GaAs/AlGaAs double quantum dot device (DQD). For quantum information applications as well as fundamental studies, careful tuning and reliable measurements of the barriers are important requirements. In order to tune a DQD device adequately into the single-hole electric dipole spin resonance regime, one has to employ a variety of techniques to cover the extended range of tunnel couplings. In this work, we demonstrate four separate techniques, based upon charge sensing, quantum transport, time-resolved pulsing, and electron dipole spin resonance spectroscopy to determine the couplings as a function of relevant gate voltages and magnetic field. Measurements were performed under conditions of both symmetric and asymmetric tunnel couplings to the leads. Good agreement was observed between different techniques when measured under the same conditions. The results indicate that even in this relatively simple circuit, the requirement to tune multiple gates and the consequences of real potential profiles result in non-intuitive dependencies of the couplings as a function of the plunger gate voltage and the magnetic field.
There is rapidly expanding interest in exploiting the spin of valence-band holes rather than conduction-band electrons for spin qubit semiconductor circuits composed of coupled quantum dots. The hole platform offers stronger spin-orbit interaction (SOI), large difference between in-dot-plane and out-of-dot-plane g-factors, i.e. g-factor anisotropy, and a significantly reduced hyperfine coupling to nuclei in the host material. These attributes collectively can deliver fast all-electric coherent spin manipulation, efficient spin-flip inter-dot tunneling channels, a voltage tunable effective g-factor, a g-factor adjustable to nearly zero in an appropriately oriented external magnetic field, and long spin relaxation and coherence times. Here, we review our recent work on the physics of heavy holes confined in a planar GaAs/AlGaAs double quantum dot system with strong SOI. For a single-hole, we have performed resonant tunneling magneto-spectroscopy to extract spin-flip and spin-conserving tunneling strengths, implemented spin-flip Landau-Zener-Stückelberg-Majorana (LZSM) interferometry, determined the spin relaxation time T 1 as a function of magnetic field using a fast single-shot latched charge technique, electrically tuned the effective g-factor revealed by electric dipole spin resonance, and found signatures of the hyperfine interaction and dynamic nuclear polarization with holes. For two-holes, we have measured the energy spectrum in the presence of strong SOI (and so not limited by Pauli spin blockade), quantified the heavy-hole (HH) g-factor anisotropy on tilting the magnetic field, described a scheme to employ HHs whose g-factor is tunable to nearly zero for an in-plane magnetic field for a coherent photon-to-spin interface, and observed a well-defined LZSM interference pattern at small magnetic fields on pulsing through the singlet-triplet anti-crossing.
One big challenge of the emerging atomic precision advanced manufacturing (APAM) technology for microelectronics application is to realize APAM devices that operate at room temperature (RT). We demonstrate that semiclassical technology computer aided design (TCAD) device simulation tool can be employed to understand current leakage and improve APAM device design for RT operation. To establish the applicability of semiclassical simulation, we first show that a semiclassical impurity scattering model with the Fermi-Dirac statistics can explain the very low mobility in APAM devices quite well; we also show semiclassical TCAD reproduces measured sheet resistances when proper mobility values are used. We then apply semiclassical TCAD to simulate current leakage in realistic APAM wires. With insights from modeling, we were able to improve device design, fabricate Hall bars, and demonstrate RT operation for the very first time.
Atomic precision advanced manufacturing (APAM) offers creation of donor devices in an atomically thin layer doped beyond the solid solubility limit, enabling unique device physics. This presents an opportunity to use APAM as a pathfinding platform to investigate digital electronics at the atomic limit. Scaling to smaller transistors is increasingly difficult and expensive, necessitating the investigation of alternative fabrication paths that extend to the atomic scale. APAM donor devices can be created using a scanning tunneling microscope (STM). However, these devices are not currently compatible with industry standard fabrication processes. There exists a tradeoff between low thermal budget (LT) processes to limit dopant diffusion and high thermal budget (HT) processes to grow defect-free layers of epitaxial Si and gate oxide. To this end, we have developed an LT epitaxial Si cap and LT deposited Al2O3 gate oxide integrated with an atomically precise single-electron transistor (SET) that we use as an electrometer to characterize the quality of the gate stack. The surface-gated SET exhibits the expected Coulomb blockade behavior. However, the gate’s leverage over the SET is limited by defects in the layers above the SET, including interfaces between the Si and oxide, and structural and chemical defects in the Si cap. We propose a more sophisticated gate stack and process flow that is predicted to improve performance in future atomic precision devices.
Studenikin, Sergei S.; Korkusinski, Marek K.; Austing, Guy D.; Gaudreau, Louis G.; Zawadski, Piotr Z.; Sachrajda, Andy S.; Ducatel, Jordan D.; Padawer-Blatt, Aviv P.; Bogan, Alex B.; Takahashi, Motoi T.; Coish, Bill C.; Philippopoulos, Pericles P.; Hirayama, Yoshiro H.; Reno, J.L.; Tracy, Lisa A.; Hargett, Terry H.
Studenikin, Sergei S.; Austing, Guy D.; Sachrajda, Andrew S.; Ducatel, Jordan D.; Padawer-Blatt, Aviv P.; Bogan, Alex B.; Takahashi, Motoi T.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.; Coish, Bill C.; Philippopoulos, Pericles P.
Sachrajda, Andrew S.; Studenikin, Sergei S.; Austing, Guy D.; Gaudreau, Louis G.; Zawadski, Piotr Z.; Bogan, Alex B.; Takahashi, Motoi T.; Ducatel, Jordan D.; Padawer-Blatt, Aviv P.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.
Hole spins have recently emerged as attractive candidates for solid-state qubits for quantum computing. Their state can be manipulated electrically by taking advantage of the strong spin-orbit interaction (SOI). Crucially, these systems promise longer spin coherence lifetimes owing to their weak interactions with nuclear spins as compared to electron spin qubits. Here we measure the spin relaxation time T1 of a single hole in a GaAs gated lateral double quantum dot device. We propose a protocol converting the spin state into long-lived charge configurations by the SOI-assisted spin-flip tunneling between dots. By interrogating the system with a charge detector we extract the magnetic-field dependence of T1 ∝ B−5 for fields larger than B = 0.5 T, suggesting the phonon-assisted Dresselhaus SOI as the relaxation channel. This coupling limits the measured values of T1 from ~400 ns at B = 1.5 T up to ~60 μs at B = 0.5 T.
Quantum materials have long promised to revolutionize everything from energy transmission (high temperature superconductors) to both quantum and classical information systems (topological materials). However, their discovery and application has proceeded in an Edisonian fashion due to both an incomplete theoretical understanding and the difficulty of growing and purifying new materials. This project leverages Sandia's unique atomic precision advanced manufacturing (APAM) capability to design small-scale tunable arrays (designer materials) made of donors in silicon. Their low-energy electronic behavior can mimic quantum materials, and can be tuned by changing the fabrication parameters for the array, thereby enabling the discovery of materials systems which can't yet be synthesized. In this report, we detail three key advances we have made towards development of designer quantum materials. First are advances both in APAM technique and underlying mechanisms required to realize high-yielding donor arrays. Second is the first-ever observation of distinct phases in this material system, manifest in disordered 2D sheets of donors. Finally are advances in modeling the electronic structure of donor clusters and regular structures incorporating them, critical to understanding whether an array is expected to show interesting physics. Combined, these establish the baseline knowledge required to manifest the strongly-correlated phases of the Mott-Hubbard model in donor arrays, the first step to deploying APAM donor arrays as analogues of quantum materials.
Takahashi, Motoi T.; Studenikin, Sergei S.; Austing, Guy D.; Bogan, Alex B.; Gaudreau, Louis G.; Zawadski, Piotr Z.; Tracy, Lisa A.; Hargett, Terry H.; Reno, J.L.; Sachrajda, Andrew S.
Studenikin, Sergei S.; Austing, Guy D.; Gaudreau, Louis G.; Zawadski, Piotr Z.; Bogan, Alex B.; Ducatel, Jordan D.; Padawder-Blatt, Aviv P.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.
Tracy, Lisa A.; Studenikin, Sergei S.; Bogan, Alex B.; Takahashi, Motoi T.; Korkusinshki, Marek K.; Austing, Guy D.; Gaudrea, Louis G.; Zawadski, Piotr Z.; Sachrajda, Andrew S.; Fallahi, Saeed F.; Manfra, Michael J.; Reno, J.L.
We demonstrate the use of custom high electron mobility transistors (HEMTs) fabricated in GaAs/AlGaAs heterostructures to amplify current from quantum dot devices. The amplifier circuit is located adjacent to the quantum dot device, at sub-Kelvin temperatures, in order to reduce the impact of cable capacitance and environmental noise. Using this circuit, we show a current gain of 380 for 0.56 μW of power dissipation, with a bandwidth of 2.7 MHz and current noise referred to the input of 24 fA/Hz 1/2 for frequencies of 0.1-1 MHz. The power consumption required for similar gain is reduced by more than a factor of 20 compared to a previous demonstration using a commercial off-the-shelf HEMT. We also demonstrate integration of a HEMT amplifier circuit on-chip with a quantum dot device, which has the potential to reduce parasitics and should allow for more complex circuits with reduced footprints.
Studenikin, Sergei S.; Bogan, Alex B.; Takahashi, Motoi T.; Austing, Guy D.; Gaudreau, Louis G.; Zawadski, Piotr Z.; Sachrajda, Andy S.; Tracy, Lisa A.; Hargett, Terry H.; Reno, J.L.
Sachrajda, Andrew S.; Bogan, Alex B.; Studenikin, Sergei S.; Gaudreau, Louis G.; Takahashi, Motoi T.; Austing, Guy D.; Korkusinski, Marek K.; Ares, Geof A.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.
We demonstrate ultra-low power cryogenic high electron mobility transistor (HEMT) amplifiers for measurement of quantum devices. The low power consumption (few uWs) allows the amplifier to be located near the device, at the coldest cryostat stage (typically less than 100 mK). Such placement minimizes parasitic capacitance and reduces the impact of environmental noise (e.g. triboelectric noise in cabling), allowing for improvements in measurement gain, bandwidth and noise. We use custom high electron mobility transistors (HEMTs) in GaAs/A1GaAs heterostructures. These HEMTs are known to have excellent performance specifically at mK temperatures, with electron mobilities that can exceed 10 6 cm 2 /Vs, allowing for large gain with low power consumption. Low temperature measurements of custom HEMT amplifiers at T = 4 K show a current sensitivity of 50 pA at 1 MHz bandwidth for 5 mW power dissipation, which is an improvement upon performance of amplifiers using off-the-shelf HEMTs.
Sachrajda, Andrew S.; Bogan, Alex B.; Studenikin, Sergei S.; Gaudreau, Louis G.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.; Korkusinski, Marek K.; Aers, Geof A.
We perform Landau-Zener-Stückelberg-Majorana (LZSM) spectroscopy on a system with strong spin-orbit interaction (SOI), realized as a single hole confined in a gated double quantum dot. Analogous to electron systems, at a magnetic field B=0 and high modulation frequencies, we observe photon-assisted tunneling between dots, which smoothly evolves into the typical LZSM funnel-shaped interference pattern as the frequency is decreased. In contrast to electrons, the SOI enables an additional, efficient spin-flip interdot tunneling channel, introducing a distinct interference pattern at finite B. Magnetotransport spectra at low-frequency LZSM driving show the two channels to be equally coherent. High-frequency LZSM driving reveals complex photon-assisted tunneling pathways, both spin conserving and spin flip, which form closed loops at critical magnetic fields. In one such loop, an arbitrary hole spin state is inverted, opening the way toward its all-electrical manipulation.
Quantum Hall ferromagnetic transitions are typically achieved by increasing the Zeeman energy through in-situ sample rotation, while transitions in systems with pseudo-spin indices can be induced by gate control. We report here a gate-controlled quantum Hall ferromagnetic transition between two real spin states in a conventional two-dimensional system without any in-plane magnetic field. We show that the ratio of the Zeeman splitting to the cyclotron gap in a Ge two-dimensional hole system increases with decreasing density owing to inter-carrier interactions. Below a critical density of ~2.4 × 1010 cm-2, this ratio grows greater than 1, resulting in a ferromagnetic ground state at filling factor ν = 2. At the critical density, a resistance peak due to the formation of microscopic domains of opposite spin orientations is observed. Such gate-controlled spin-polarizations in the quantum Hall regime opens the door to realizing Majorana modes using two-dimensional systems in conventional, low-spin-orbit-coupling semiconductors.
Studenikin, Sergei S.; Bogan, Alex B.; Gaudreau, Louis G.; Korkusinski, Marek K.; Zawadski, Piotr Z.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.; Sachrajda, Andrew S.
Sachrajda, Andrew S.; Bogan, Alex B.; Studenikin, Alex S.; Korkusinski, Marek K.; Aers, Geoff A.; Gaudreau, Louis G.; Zawadski, Piotr Z.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.
Korkusinski, Marek K.; Bogan, Alex B.; Studenikin, Sergei S.; Aers, Geoff A.; Gaudreau, Louis G.; Zawadski, Piotr Z.; Sachrajda, Andrew S.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.
Sachrajda, Andrew S.; Bogan, Alex B.; Studenikin, Sergei S.; Korkusinski, Marek K.; Aers, Geoff A.; Gaudreau, Louis G.; Zawadski, Piotr Z.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.
Korkusinski, Marek K.; Bogan, Alex B.; Studeinkin, Sergei S.; Aers, Geoff A.; Gaudreau, Louis G.; Zawadski, Piotr Z.; Sachrajda, Andrew S.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.
Bogan, Alex B.; Studenikin, Sergei S.; Gaudreau, Louis G.; Korkusinski, Marek K.; Aers, Geof A.; Zawadski, Piotr Z.; Sachrajda, Andrew S.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.
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 100 kHz 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 × 10 3, the power dissipation of the amplifier is 13 μW, the bandwidth is ∼ 1.3 MHz, and for frequencies above 300 kHz the current noise referred to input is ≤ 70 fA/ 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.
Most spin qubit research to date has focused on manipulating single electron spins in quantum dots. However, hole spins are predicted to have some advantages over electron spins, such as reduced coupling to host semiconductor nuclear spins and the ability to control hole spins electrically using the large spin-orbit interaction. Building on recent advances in fabricating high-mobility 2D hole systems in GaAs/AlGaAs heterostructures at Sandia, we fabricate and characterize single hole transistors in GaAs. We demonstrate p-type double quantum dot devices with few-hole occupation, which could be used to study the physics of individual hole spins and control over coupling between hole spins, looking towards eventual applications in quantum computing. Intentionally left blank
We report the design, construction, and characterization of a biaxial sample rotation stage for use in a cryogenic system for orientation-dependent studies of anisotropic electronic transport phenomena at low temperatures and high magnetic fields. Our apparatus allows for continuous rotation of a sample about two axes, both independently and simultaneously.
We have compared simulations using solutions of Poisson's equation to detailed capacitance measurements on a double quantum dot structure. We tabulate the results and show which cases show good agreement and which do not. The capacitance values are also compared to those calculated by a solution of Laplace's equation. Electron density is plotted and discussed. In order to understand relevant potential barriers we compare simulations at 50 Kelvin to simulations at 15 Kelvin. We show that the charge density does not differ greatly, but that the conduction band potential does. However, a method of estimating the potential at 0 Kelvin based on the charge distribution at 50 Kelvin is shown to be close to the potential at 15 Kelvin. This method was used to estimate potential barriers at 0 Kelvin in two quantum dot structures.
There is significant interest in forming quantum bits (qubits) out of single electron devices for quantum information processing (QIP). Information can be encoded using properties like charge or spin. Spin is appealing because it is less strongly coupled to the solid-state environment so it is believed that the quantum state can better be preserved over longer times (i.e., that is longer decoherence times may be achieved). Long spin decoherence times would allow more complex qubit operations to be completed with higher accuracy. Recently spin qubits were demonstrated by several groups using electrostatically gated modulation doped GaAs double quantum dots (DQD) [1], which represented a significant breakthrough in the solid-state field. Although no Si spin qubit has been demonstrated to date, work on Si and SiGe based spin qubits is motivated by the observation that spin decoherence times can be significantly longer than in GaAs. Spin decoherence times in GaAs are in part limited by the random spectral diffusion of the non-zero nuclear spins of the Ga and As that couple to the electron spin through the hyperfine interaction. This effect can be greatly suppressed by using a semiconductor matrix with a near zero nuclear spin background. Near zero nuclear spin backgrounds can be engineered using Si by growing {sup 28}Si enriched epitaxy. In this talk, we will present fabrication details and electrical transport results of an accumulation mode double top gated Si metal insulator semiconductor (MIS) nanostructure, Fig 1 (a) & (b). We will describe how this single electron device structure represent a path towards forming a Si based spin qubit similar in design as that demonstrated in GaAs. Potential advantages of this novel qubit structure relative to previous approaches include the combination of: no doping (i.e., not modulation doped); variable two-dimensional electron gas (2DEG) density; CMOS compatible processes; and relatively small vertical length scales to achieve smaller dots. A primary concern in this structure is defects at the insulator-silicon interface. The Sandia National Laboratories 0.35 {micro}m fab line was used for critical processing steps including formation of the gate oxide to examine the utility of a standard CMOS quality oxide silicon interface for the purpose of fabricating Si qubits. Large area metal oxide silicon (MOS) structures showed a peak mobility of 15,000 cm{sup 2}/V-s at electron densities of {approx}1 x 10{sup 12} cm{sup -2} for an oxide thickness of 10 nm. Defect density measured using standard C-V techniques was found to be greater with decreasing oxide thickness suggesting a device design trade-off between oxide thickness and quantum dot size. The quantum dot structure is completed using electron beam lithography and poly-silicon etch to form the depletion gates, Fig 1 (a). The accumulation gate is added by introducing a second insulating Al{sub 2}O{sub 3} layer, deposited by atomic layer deposition, followed by an Al top gate deposition, Fig. 1 (b). Initial single electron transistor devices using SiO{sub 2} show significant disorder in structures with relatively large critical dimensions of the order of 200-300 nm, Fig 2. This is not uncommon for large silicon structures and has been cited in the literature [2]. Although smaller structures will likely minimize the effect of disorder and well controlled small Si SETs have been demonstrated [3], the design constraints presented by disorder combined with long term concerns about effects of defects on spin decoherence time (e.g., paramagnetic centers) motivates pursuit of a 2nd generation structure that uses a compound semiconductor approach, an epitaxial SiGe barrier as shown in Fig. 2 (c). SiGe may be used as an electron barrier when combined with tensilely strained Si. The introduction of strained-Si into the double top gated device structure, however, represents additional fabrication challenges. Thermal budget is potentially constrained due to concerns related to strain relaxation. Fabrication details related to the introduction of strained silicon on insulator and SiGe barrier formation into the Sandia National Laboratories 0.35 {micro}m fab line will also be presented.
We describe the development of a novel silicon quantum bit (qubit) device architecture that involves using materials that are compatible with a Sandia National Laboratories (SNL) 0.35 mum complementary metal oxide semiconductor (CMOS) process intended to operate at 100 mK. We describe how the qubit structure can be integrated with CMOS electronics, which is believed to have advantages for critical functions like fast single electron electrometry for readout compared to current approaches using radio frequency techniques. Critical materials properties are reviewed and preliminary characterization of the SNL CMOS devices at 4.2 K is presented.