Probing single electrons across 300 mm spin qubit wafers

Building a fault-tolerant quantum computer will require vast numbers of physical qubits. For qubit technologies based on solid state electronic devices, integrating millions of qubits in a single processor will require device fabrication to reach a scale comparable to that of the modern CMOS industry. Equally importantly, the scale of cryogenic device testing must keep pace to enable efficient device screening and to improve statistical metrics like qubit yield and process variation. Spin qubits have shown impressive control fidelities but have historically been challenged by yield and process variation. In this work, we present a testing process using a cryogenic 300 mm wafer prober to collect high-volume data on the performance of industry-manufactured spin qubit devices at 1.6 K. This testing method provides fast feedback to enable optimization of the CMOS-compatible fabrication process, leading to high yield and low process variation. Using this system, we automate measurements of the operating point of spin qubits and probe the transitions of single electrons across full wafers. We analyze the random variation in single-electron operating voltages and find that this fabrication process leads to low levels of disorder at the 300 mm scale. Together these results demonstrate the advances that can be achieved through the application of CMOS industry techniques to the fabrication and measurement of spin qubits.Comment: 15 pages, 4 figures, 7 extended data figure

Direct Synthesis of Graphene on Niobium and Niobium Nitride

Since its isolation by mechanical exfoliation in 2004, graphene has attracted enormous interest from the scientific community not the least because of its unique physical and electronic properties. Among these, graphene’s ballistic electron transport and proximity induced superconductivity make graphene-superconductor (GS) hybrid structures a scientifically promising area

Development of Graphene Synthesis and Characterization Techniques Toward CMOS Applications and Beyond

Graphene exhibits mechanical and electrical properties which, coupled with its two dimensional (2D) morphology, make it an attractive material component for inclusion in a wide range of industries. Since the discovery of graphene in 2004, industry adoption has been limited due to the demanding synthesis requirements for high quality and connected graphene as well as the difficulties associated with direct incorporation. Chemical vapor deposition (CVD) has emerged as the most cost efficient method for producing high quality graphene at scales suitable for mass production. However, the 1000°C temperatures and micrometer thick catalysts required for this process preclude direct inclusion in applications with topographically varied surfaces as graphene is produced in planar sheets that must be transferred. One attractive application for graphene is as a diffusion barrier in CMOS applications as the single atom thick material has shown significant ability to block copper diffusion at elevated temperatures. For realization of this application, both the required catalyst thicknesses and synthesis temperatures for graphene production must be reduced to enable direct graphene incorporation on these nanoscale and nonplanar surfaces without thermal damage to existing components. A second application in which graphene inclusion would be beneficial is the field of spintronics, in which the spin orientation of electrons are used as an additional degree of freedom for computation and information storage. This beyond-CMOS application represents an avenue for significant improvement over current technologies and graphene, with its weak spin orbit coupling and high electron mobility, displays potential as a long-distance spin transport component of future spintronic devices. Characterization of graphene’s spin transport properties has been primarily investigated in a nonlocal spin valve device (NLSV), resulting in experimental spin transport parameters orders of magnitude below those theoretical predicted. To advance graphene as a component for future spintronic applications, new device designs to explore spin transport phenomena not detectable in NLSV devices as well as scalable fabrication techniques will be needed. In this work, we develop graphene synthesis techniques to reduce required temperatures through hydrocarbon precursor control during plasma enhanced chemical vapor deposition (PECVD). Through manipulation of the size and ionization state of hydrocarbon precursors that interact with the growth catalyst, we demonstrate 95% few-to-monolayer graphene synthesis at 500°C on 50 nm catalysts, representing a 10-fold reduction in catalyst thickness requirements at temperatures approaching the limit for direct incorporation in CMOS applications. Additionally, we demonstrate manipulation of metal catalyst morphology and composition toward controlling graphene layer number, defect types, and uniformity. Characterization of trimetallic catalysts, compared to single metal or bimetallic catalysts traditionally examined in literature, reveal that low temperature graphene synthesis pathways can be manipulated through small additions of less reactive metals (Gold and Copper) to primarily high reactivity metal catalysts (Ni) through both energetic and surface modulation resulting in monolayer graphene synthesis. While low temperature graphene synthesis techniques are needed for graphene incorporation in current CMOS products, beyond-CMOS applications do not necessarily require temperature restrictions on synthesis as fabrication of these devices can implement planar graphene as the first device component. To characterize graphene as a spin transport channel, commercially available graphene grown at elevated temperatures is used to address spin transport properties through design of a novel device configuration, the hybrid drift diffusion spin valve (HDDSV), in which an additional transport channel is added to the standard NLSV. This device architecture has not been previously studied and is aimed at revealing magnetic contact effects on graphene spin transport as well as exploring drift and diffusion interactions with respect to achievable spin signals. Wafer scale fabrication of these devices is demonstrated and processing techniques are optimized to enable spin signal detection on arrays containing 120 individual devices. Characterization of the new HDDSV configuration reveals changes to detected spin signals in both the standard NLSV portion and the added channel, revealing spin signals as large as 865Ω in the additional transport channel compared to an average signal of 7.3Ω in the traditional configuration. The additional channels also exhibit detectable spin signal under a 3 point local measurement, representing a potential avenue toward long distance spin transport and enabling increased device complexity that will be necessary for the realization of graphene based spintronic devices. These findings represent the development of graphene synthesis and characterization techniques aimed at advancing fundamental understanding and enabling further practical application. The methods developed in this study serve as new avenues for continued improvement toward direct incorporation of a material that has the potential to revolutionize a number of fields

Infrared Radiography: Modeling X-ray Imaging Without Harmful Radiation

Planar x-ray imaging is a ubiquitous diagnostic tool and is routinely performed to diagnose conditions as varied as bone fractures and pneumonia. The underlying principle is that the varying attenuation coefficients of air, water, tissue, bone, or metal implants within the body result in non-uniform transmission of x-ray radiation. Through the detection of transmitted radiation, the spatial organization and composition of materials in the body can be ascertained. In this paper, we describe an original apparatus that teaches these concepts by utilizing near infrared radiation and an up-converting phosphorescent screen to safely probe the contents of an opaque enclosure

Plasma-Enhanced Chemical Vapor Deposition of Acetylene on Codeposited Bimetal Catalysts Increasing Graphene Sheet Continuity Under Low-Temperature Growth Conditions

Here, we report a novel method for low-temperature synthesis of monolayer graphene at 450 °C on a polycrystalline bimetal Ni-Au catalyst. In this study, low-temperature chemical vapor deposition synthesis of graphene was performed at 450 °C on codeposited Ni-Au which shows successful monolayer graphene formation without an extra annealing process. The experimental results suggest that electron beam codeposition of bimetal catalyst is the key procedure that enables the elimination of the pre-growth high-temperature annealing of the catalyst prior to graphene synthesis, an indispensable process, used in previous reports. The formation was further improved by plasma-assisted growth in which the inductively coupled plasma ionizes the carbon precursors that interact with codeposited Ni-Au catalyst of 50 nm in thickness at 450 °C. These combined growth conditions drastically increase the graphene’s sheet uniformity and area connectivity from 11.6% to 99%. These fabrication parameters enable the graphene formation that shifts from a bulk diffusion-based growth model towards a surface based reaction. The technique reported here opens the opportunity for the low-temperature growth of graphene for potential use in future CMOS applications

Simulation to Fabrication—Understanding the Effect of NiAuCu Alloy Catalysts for Controlled Growth of Graphene at Reduced Temperature

It is a significant challenge to grow large-scale, high quality, monolayer graphene at low temperature for the applications in industry, especially for the complementary metal oxide semiconductor fabrication process. To overcome this difficulty, we simulated the decomposition of acetylene (C2H2) on (100) surfaces of primarily nickel (Ni) catalysts with small mol fractions of gold (Au) and copper (Cu), using a 4 × 4 × 4 periodic supercell model. Based on the calculation of the reaction energies to decompose the C-H or C≡C bonds on different catalyst surfaces, a differential energy is proposed to clearly scale the decomposition difficulties such that larger differential energy leads to easier control of the monolayer growth. It is observed that on the NiAuCu alloy surface with a mol fraction 0.0313 of both Au and Cu, the differential energy of the C-H bonds and the C≡C bond are both positive, showing an obvious modulation effect on the decomposition of C2H2 and the catalytic activites. The simulation result is consistent with the growth of uniform monolayer graphene on silicon dioxide substrate at 500°C by plasma enhanced chemical vapor deposition with C2H2 precursor and Ni alloy catalysts with 1 wt% Au and 1 wt% Cu

Characterization and Manipulation of Carbon Precursor Species during Plasma Enhanced Chemical Vapor Deposition of Graphene

To develop a synthesis technique providing enhanced control of graphene film quality and uniformity, a systematic characterization and manipulation of hydrocarbon precursors generated during plasma enhanced chemical vapor deposition of graphene is presented. Remote ionization of acetylene is observed to generate a variety of neutral and ionized hydrocarbon precursors, while in situ manipulation of the size and reactivity of carbon species permitted to interact with the growth catalyst enables control of the resultant graphene morphology. Selective screening of high energy hydrocarbon ions coupled with a multistage bias growth regime results in the production of 90% few-to-monolayer graphene on 50 nm Ni/Cu alloy catalysts at 500 °C. Additionally, synthesis with low power secondary ionization processes is performed and reveals further control during the growth, enabling a 50% reduction in average defect densities throughout the film. Mass spectrometry and UV-Vis spectroscopy monitoring of the reaction environment in conjunction with Raman characterization of the synthesized graphene films facilitates correlation of the carbon species permitted to reach the catalyst surface to the ultimate quality, layer number, and uniformity of the graphene film. These findings reveal a robust technique to control graphene synthesis pathways during plasma enhanced chemical vapor deposition

Characterization and Manipulation of Carbon Precursor Species during Plasma Enhanced Chemical Vapor Deposition of Graphene

To develop a synthesis technique providing enhanced control of graphene film quality and uniformity, a systematic characterization and manipulation of hydrocarbon precursors generated during plasma enhanced chemical vapor deposition of graphene is presented. Remote ionization of acetylene is observed to generate a variety of neutral and ionized hydrocarbon precursors, while in situ manipulation of the size and reactivity of carbon species permitted to interact with the growth catalyst enables control of the resultant graphene morphology. Selective screening of high energy hydrocarbon ions coupled with a multistage bias growth regime results in the production of 90% few-to-monolayer graphene on 50 nm Ni/Cu alloy catalysts at 500 C. Additionally, synthesis with low power secondary ionization processes is performed and reveals further control during the growth, enabling a 50% reduction in average defect densities throughout the film. Mass spectrometry and UV-Vis spectroscopy monitoring of the reaction environment in conjunction with Raman characterization of the synthesized graphene films facilitates correlation of the carbon species permitted to reach the catalyst surface to the ultimate quality, layer number, and uniformity of the graphene film. These findings reveal a robust technique to control graphene synthesis pathways during plasma enhanced chemical vapor deposition

Impact of material and tunnel barrier quality on spin transport in a CVD graphene non-local spin valve device array

Wafer-scale graphene films produced via chemical vapor deposition (CVD) are now commercially available, however these films inherently contain randomly distributed defects such as adlayers and grain boundaries. This report discusses the impact of these defects on the signal integrity of an array of graphene-based non-local spin valves (NLSVs). It was found that critical spin parameters fluctuate drastically between adjacent identical devices. Investigation of the channel quality indicated that adlayers do not affect spin signal significantly even when located directly in the spin transport region of the device. In contrast, grain boundary defects within the spin transport region have significant impact on spin signal. Poor tunnel barrier integrity due to residue from the fabrication process also remains a dominant factor driving device variability