Enhanced Atomic Precision Fabrication by Adsorption of Phosphine into Engineered Dangling Bonds on H-Si Using STM and DFT.

The doping of Si using the scanning probe hydrogen depassivation lithography technique has been shown to enable placing and positioning small numbers of P atoms with nanometer accuracy. Several groups have now used this capability to build devices that exhibit desired quantum behavior determined by their atomistic details. What remains elusive, however, is the ability to control the precise number of atoms placed at a chosen site with 100% yield, thereby limiting the complexity and degree of perfection achievable. As an important step toward precise control of dopant number, we explore the adsorption of the P precursor molecule, phosphine, into atomically perfect dangling bond patches of intentionally varied size consisting of three adjacent Si dimers along a dimer row, two adjacent dimers, and one single dimer. Using low temperature scanning tunneling microscopy, we identify the adsorption products by generating and comparing to a catalog of simulated images, explore atomic manipulation after adsorption in select cases, and follow up with incorporation of P into the substrate. For one-dimer patches, we demonstrate that manipulation of the adsorbed species leads to single P incorporation in 12 out of 12 attempts. Based on the observations made in this study, we propose this one-dimer patch method as a robust approach that can be used to fabricate devices where it is ensured that each site of interest has exactly one P atom.

Experimental realization of an extended Fermi-Hubbard model using a 2D lattice of dopant-based quantum dots.

The Hubbard model is an essential tool for understanding many-body physics in condensed matter systems. Artificial lattices of dopants in silicon are a promising method for the analog quantum simulation of extended Fermi-Hubbard Hamiltonians in the strong interaction regime. However, complex atom-based device fabrication requirements have meant emulating a tunable two-dimensional Fermi-Hubbard Hamiltonian in silicon has not been achieved. Here, we fabricate 3 × 3 arrays of single/few-dopant quantum dots with finite disorder and demonstrate tuning of the electron ensemble using gates and probe the many-body states using quantum transport measurements. By controlling the lattice constants, we tune the hopping amplitude and long-range interactions and observe the finite-size analogue of a transition from metallic to Mott insulating behavior. We simulate thermally activated hopping and Hubbard band formation using increased temperatures. As atomically precise fabrication continues to improve, these results enable a new class of engineered artificial lattices to simulate interactive fermionic models.

Open-loop amplitude-modulation Kelvin probe force microscopy operated in single-pass PeakForce tapping mode.

The open-loop (OL) variant of Kelvin probe force microscopy (KPFM) provides access to the voltage response of the electrostatic interaction between a conductive atomic force microscopy (AFM) probe and the investigated sample. The measured response can be analyzed a posteriori, modeled, and interpreted to include various contributions from the probe geometry and imaged features of the sample. In contrast to this, the currently implemented closed-loop (CL) variants of KPFM, either amplitude-modulation (AM) or frequency-modulation (FM), solely report on their final product in terms of the tip-sample contact potential difference. In ambient atmosphere, both CL AM-KPFM and CL FM-KPFM work at their best during the lift part of a two-pass scanning mode to avoid the direct contact with the surface of the sample. In this work, a new OL AM-KPFM mode was implemented in the single-pass scan of the PeakForce Tapping (PFT) mode. The topographical and electrical components were combined in a single pass by applying the electrical modulation only in between the PFT tip-sample contacts, when the AFM probe separates from the sample. In this way, any contact and tunneling discharges are avoided and, yet, the location of the measured electrical tip-sample interaction is directly affixed to the topography rendered by the mechanical PFT modulation at each tap. Furthermore, because the detailed response of the cantilever to the bias stimulation was recorded, it was possible to analyze and separate an average contribution of the cantilever to the determined local contact potential difference between the AFM probe and the imaged sample. The removal of this unwanted contribution greatly improved the accuracy of the AM-KPFM measurements to the level of the FM-KPFM counterpart.

Low-Resistance, High-Yield Electrical Contacts to Atom Scale Si:P Devices Using Palladium Silicide.

Scanning tunneling microscopy (STM) enables the fabrication of two-dimensional δ-doped structures in Si with atomistic precision, with applications from tunnel field-effect transistors to qubits. The combination of a very small contact area and the restrictive thermal budget necessary to maintain the integrity of the δ layer make developing a robust electrical contact method a significant challenge to realizing the potential of atomically precise devices. We demonstrate a method for electrical contact using Pd2Si formed at the temperature of silicon overgrowth (250 °C), minimizing the diffusive impact on the δ layer. We use the transfer length method to show our Pd2Si contacts have very high yield (99.7% +0.2% -1.5%) and low resistivity (272±41Ωµm) in contacting mesa-etched Si:P δ layers. We also present three terminal measurements of low contact resistance (<1 kΩ) to devices written by STM hydrogen depassivation lithography with similarly high yield (100% +0% -3.2%).

Atom-by-Atom Construction of a Cyclic Artificial Molecule in Silicon.

Hydrogen atoms on a silicon surface, H-Si (100), behave as a resist that can be patterned with perfect atomic precision using a scanning tunneling microscope. When a hydrogen atom is removed in this manner, the underlying silicon presents a chemically active site, commonly referred to as a dangling bond. It has been predicted that individual dangling bonds function as artificial atoms, which, if grouped together, can form designer molecules on the H-Si (100) surface. Here, we present an artificial ring structure molecule spanning three dimer rows, constructed from dangling bonds, and verified by spectroscopic measurement of its molecular orbitals. We found that removing 8 hydrogen atoms resulted in a molecular analog to 1,4-disilylene-hexasilabenzene (Si8H8). Scanning tunneling spectroscopic measurements reveal molecular π and π* orbitals that agree with those expected for the same molecule in a vacuum; this is validated by density functional theory calculations of the dangling bond system on a silicon slab that show direct links both to the experimental results and to calculations for the isolated molecule. We believe the unique electronic structure of artificial molecules constructed in this manner can be engineered to enable future molecule-based electronics, surface catalytic functionality, and templating for subsequent site-selective deposition.

Quantifying atom-scale dopant movement and electrical activation in Si:P monolayers.

Advanced hydrogen lithography techniques and low-temperature epitaxial overgrowth enable the patterning of highly phosphorus-doped silicon (Si:P) monolayers (ML) with atomic precision. This approach to device fabrication has made Si:P monolayer systems a testbed for multiqubit quantum computing architectures and atomically precise 2-D superlattice designs whose behaviors are directly tied to the deterministic placement of single dopants. However, dopant segregation, diffusion, surface roughening, and defect formation during the encapsulation overgrowth introduce large uncertainties to the exact dopant placement and activation ratio. In this study, we develop a unique method by combining dopant segregation/diffusion models with sputter profiling simulation to monitor and control, at the atomic scale, dopant movement using room-temperature grown locking layers (LLs). We explore the impact of LL growth rate, thickness, rapid thermal annealing, surface accumulation, and growth front roughness on dopant confinement, local crystalline quality, and electrical activation within Si:P 2-D systems. We demonstrate that dopant movement can be more efficiently suppressed by increasing the LL growth rate than by increasing the LL thickness. We find that the dopant segregation length can be suppressed below a single Si lattice constant by increasing the LL growth rates at room temperature while maintaining epitaxy. Although dopant diffusivity within the LL is found to remain high (on the order of 10-17 cm2 s-1) even below the hydrogen desorption temperature, we demonstrate that exceptionally sharp dopant confinement with high electrical quality within Si:P monolayers can be achieved by combining a high LL growth rate with low-temperature LL rapid thermal annealing. The method developed in this study provides a key tool for 2-D fabrication techniques that require precise dopant placement to suppress, quantify, and predict a single dopant's movement at the atomic scale.

Silicon epitaxy on H-terminated Si (100) surfaces at 250 °C.

Low temperature Si epitaxy has become increasingly important due to its critical role in the encapsulation and performance of buried nanoscale dopant devices. We demonstrate epitaxial growth up to nominally 25 nm, at 250°C, with analysis at successive growth steps using STM and cross section TEM to reveal the nature and quality of the epitaxial growth. STM images indicate that growth morphology of both Si on Si and Si on H-terminated Si (H: Si) is epitaxial in nature at temperatures as low as 250 °C. For Si on Si growth at 250 °C, we show that the Si epitaxial growth front maintains a constant morphology after reaching a specific thickness threshold. Although the in-plane mobility of silicon is affected on the H: Si surface due to the presence of H atoms during initial sub-monolayer growth, STM images reveal long range order and demonstrate that growth proceeds by epitaxial island growth albeit with noticeable surface roughening.

Atomically Traceable Nanostructure Fabrication.

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process being used in order to maintain exquisite control over both feature size and feature density. Here, we demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top-down atomic control over nanofabrication. Hydrogen depassivation lithography is the first step of the nanoscale fabrication process followed by selective atomic layer deposition of up to 2.8 nm of titania to make a nanoscale etch mask. Contrast with the background is shown, indicating different mechanisms for growth on the desired patterns and on the H passivated background. The patterns are then transferred into the bulk using reactive ion etching to form 20 nm tall nanostructures with linewidths down to ~6 nm. To illustrate the limitations of this process, arrays of holes and lines are fabricated. The various nanofabrication process steps are performed at disparate locations, so process integration is discussed. Related issues are discussed including using fiducial marks for finding nanostructures on a macroscopic sample and protecting the chemically reactive patterned Si(100)-H surface against degradation due to atmospheric exposure.