arXiv subjects

Shashank Misra

Publications and source records attributed to Shashank Misra.

29 records · Page 2Linked to original sources

arXiv2021

The impact of stochastic incorporation on atomic-precision Si:P arrays

Scanning tunneling microscope lithography can be used to create nanoelectronic devices in which dopant atoms are precisely positioned in a Si lattice within $\sim$1 nm of a target position. This exquisite precision is promising for realizing various quantum technologies. However, a potentially impactful form of disorder is due to incorporation kinetics, in which the number of P atoms that incorporate into a single lithographic window is manifestly uncertain. We present experimental results indicating that the likelihood of incorporating into an ideally written three-dimer single-donor window is $63 \pm 10\%$ for room-temperature dosing, and corroborate these results with a model for the incorporation kinetics. Nevertheless, further analysis of this model suggests conditions that might raise the incorporation rate to near-deterministic levels. We simulate bias spectroscopy on a chain of comparable dimensions to the array in our yield study, indicating that such an experiment may help confirm the inferred incorporation rate.

cond-mat.mes-hall↗

arXiv2021

AlCl$_{3}$-dosed Si(100)-2$\times$1: Adsorbates, chlorinated Al chains, and incorporated Al

The adsorption of AlCl$_{3}$ on Si(100) and the effect of annealing the AlCl$_{3}$-dosed substrate was studied to reveal key surface processes for the development of atomic-precision acceptor-doping techniques. This investigation was performed via scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. At room temperature, AlCl$_{3}$ readily adsorbed to the Si substrate dimers and dissociated to form a variety of species. Annealing of the AlCl$_{3}$-dosed substrate at temperatures below 450 $^{\circ}$C produced unique chlorinated aluminum chains (CACs) elongated along the Si(100) dimer row direction. An atomic model for the chains is proposed with supporting DFT calculations. Al was incorporated into the Si substrate upon annealing at 450 $^{\circ}$C and above, and Cl desorption was observed for temperatures beyond 450 $^{\circ}$C. Al-incorporated samples were encapsulated in Si and characterized by secondary ion mass spectrometry (SIMS) depth profiling to quantify the Al atom concentration, which was found to be in excess of 10$^{20}$ cm$^{-3}$ across a $\sim$2.7 nm thick $δ$-doped region. The Al concentration achieved here and the processing parameters utilized promote AlCl$_{3}$ as a viable gaseous precursor for novel acceptor-doped Si materials and devices for quantum computing.

cond-mat.mtrl-sci↗

arXiv2020

A model for atomic precision p-type doping with diborane on Si(100)-2$\times$1

Diborane (B$_2$H$_6$) is a promising molecular precursor for atomic precision p-type doping of silicon that has recently been experimentally demonstrated [T. {Š}kere{ň}, \textit{et al.,} Nature Electronics (2020)]. We use density functional theory (DFT) calculations to determine the reaction pathway for diborane dissociating into a species that will incorporate as electrically active substitutional boron after adsorbing onto the Si(100)-2$\times$1 surface. Our calculations indicate that diborane must overcome an energy barrier to adsorb, explaining the experimentally observed low sticking coefficient ($< 10^{-4}$ at room temperature) and suggesting that heating can be used to increase the adsorption rate. Upon sticking, diborane has an $\sim 50\%$ chance of splitting into two BH$_3$ fragments versus merely losing hydrogen to form a dimer such as B$_2$H$_4$. As boron dimers are likely electrically inactive, whether this latter reaction occurs is shown to be predictive of the incorporation rate. The dissociation process proceeds with significant energy barriers, necessitating the use of high temperatures for incorporation. Using the barriers calculated from DFT, we parameterize a Kinetic Monte Carlo model that predicts the incorporation statistics of boron as a function of the initial depassivation geometry, dose, and anneal temperature. Our results suggest that the dimer nature of diborane inherently limits its doping density as an acceptor precursor, and furthermore that heating the boron dimers to split before exposure to silicon can lead to poor selectivity on hydrogen and halogen resists. This suggests that while diborane works as an atomic precision acceptor precursor, other non-dimerized acceptor precursors may lead to higher incorporation rates at lower temperatures.

cond-mat.mtrl-sci↗

arXiv2020

Low Thermal Budget High-k/Metal Surface Gate for Buried Donor-Based Devices

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 leverage of the gate 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.

cond-mat.mtrl-sci↗

arXiv2020

Atomic Precision Advanced Manufacturing for Digital Electronics

An exponential increase in the performance of silicon microelectronics and the demand to manufacture in great volumes has created an ecosystem that requires increasingly complex tools to fabricate and characterize the next generation of chips. However, the cost to develop and produce the next generation of these tools has also risen exponentially, to the point where the risk associated with progressing to smaller feature sizes has created pain points throughout the ecosystem. The present challenge includes shrinking the smallest features from nanometers to atoms (10 nm corresponds to 30 silicon atoms). Relaxing the requirement for achieving scalable manufacturing creates the opportunity to evaluate ideas not one or two generations into the future, but at the absolute physical limit of atoms themselves. This article describes recent advances in atomic precision advanced manufacturing (APAM) that open the possibility of exploring opportunities in digital electronics. Doing so will require advancing the complexity of APAM devices and integrating APAM with CMOS.

physics.app-ph↗

arXiv2019

Quantum Simulators: Architectures and Opportunities

Quantum simulators are a promising technology on the spectrum of quantum devices from specialized quantum experiments to universal quantum computers. These quantum devices utilize entanglement and many-particle behaviors to explore and solve hard scientific, engineering, and computational problems. Rapid development over the last two decades has produced more than 300 quantum simulators in operation worldwide using a wide variety of experimental platforms. Recent advances in several physical architectures promise a golden age of quantum simulators ranging from highly optimized special purpose simulators to flexible programmable devices. These developments have enabled a convergence of ideas drawn from fundamental physics, computer science, and device engineering. They have strong potential to address problems of societal importance, ranging from understanding vital chemical processes, to enabling the design of new materials with enhanced performance, to solving complex computational problems. It is the position of the community, as represented by participants of the NSF workshop on "Programmable Quantum Simulators," that investment in a national quantum simulator program is a high priority in order to accelerate the progress in this field and to result in the first practical applications of quantum machines. Such a program should address two areas of emphasis: (1) support for creating quantum simulator prototypes usable by the broader scientific community, complementary to the present universal quantum computer effort in industry; and (2) support for fundamental research carried out by a blend of multi-investigator, multi-disciplinary collaborations with resources for quantum simulator software, hardware, and education.

quant-ph↗

arXiv2017

All-optical lithography process for contacting atomically-precise devices

We describe an all-optical lithography process that can be used to make electrical contact to atomic-precision donor devices made in silicon using scanning tunneling microscopy (STM). This is accomplished by implementing a cleaning procedure in the STM that allows the integration of metal alignment marks and ion-implanted contacts at the wafer level. Low-temperature transport measurements of a patterned device establish the viability of the process.

physics.app-ph↗

arXiv2013

Design and performance of an ultra-high vacuum scanning tunneling microscope operating at dilution refrigerator temperatures and high magnetic fields

We describe the construction and performance of a scanning tunneling microscope (STM) capable of taking maps of the tunneling density of states with sub-atomic spatial resolution at dilution refrigerator temperatures and high (14 T) magnetic fields. The fully ultra-high vacuum system features visual access to a two-sample microscope stage at the end of a bottom-loading dilution refrigerator, which facilitates the transfer of in situ prepared tips and samples. The two-sample stage enables location of the best area of the sample under study and extends the experiment lifetime. The successful thermal anchoring of the microscope, described in detail, is confirmed through a base temperature reading of 20 mK, along with a measured electron temperature of 250 mK. Atomically-resolved images, along with complementary vibration measurements, are presented to confirm the effectiveness of the vibration isolation scheme in this instrument. Finally, we demonstrate that the microscope is capable of the same level of performance as typical machines with more modest refrigeration by measuring spectroscopic maps at base temperature both at zero field and in an applied magnetic field.

physics.ins-det↗

arXiv2013

Visualizing Nodal Heavy Fermion Superconductivity in CeCoIn5

Understanding the origin of superconductivity in strongly correlated electron systems continues to be at the forefront of unsolved problems in all of physics. Among the heavy f-electron systems, CeCoIn5 is one of the most fascinating, as it shares many of the characteristics of correlated d-electron high-Tc cuprate and pnictide superconductors, including the competition between antiferromagnetism and superconductivity. While there has been evidence for unconventional pairing in this compound, high-resolution spectroscopic measurements of the superconducting state have been lacking. Previously, we have used high-resolution scanning tunneling microscopy techniques to visualize the emergence of heavy-fermion excitations in CeCoIn5 and demonstrate the composite nature of these excitations well above Tc. Here we extend these techniques to much lower temperatures to investigate how superconductivity develops within a strongly correlated band of composite excitations. We find the spectrum of heavy excitations to be strongly modified just prior to the onset of superconductivity by a suppression of the spectral weight near the Fermi energy, reminiscent of the pseudogap state in the cuprates. By measuring the response of superconductivity to various perturbations, through both quasiparticle interference and local pair-breaking experiments, we demonstrate the nodal d-wave character of superconducting pairing in CeCoIn5.

cond-mat.supr-con↗

arXiv2012

Evidence for a universal minimum superfluid response in field-tuned disordered superconducting films measured using low frequency ac conductivity

Our measurements of the low frequency ac conductivity in strongly disordered two-dimensional films near the magnetic field-tuned superconductor-to-insulator transition show a sudden drop in the phase stiffness of superconducting order with either increased temperature or magnetic field. Surprisingly, for two different material systems, the abrupt drop in the superfluid density in a magnetic field has the same universal value as that expected for a Berezinskii-Kosterlitz-Thouless transition in zero magnetic field. The characteristic temperature at which phase stiffness is suddenly lost can be tuned to zero at a critical magnetic field, following a power-law behavior with a critical exponent consistent with that obtained in previous dc transport studies on the dissipative side of the transition.

cond-mat.supr-con↗

arXiv2004

Local Ordering in the Pseudogap State of the High-T$_c$ Superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

We report atomic scale characterization of the pseudogap state in a high-T$_c$ superconductor, Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. The electronic states at low energies within the pseudogap exhibit spatial modulations having an energy-independent incommensurate periodicity. These patterns, which are oriented along the copper-oxygen bond directions, appear to be a consequence of an electronic ordering phenomenon--the observation of which correlates with the pseudogap in the density of electronic states. Our results provide a stringent test for various ordering scenarios in the cuprates, which have been central in the debate on the nature of the pseudogap and the complex electronic phase diagram of these compounds.

cond-mat.supr-con↗