arXiv subjects

Evan M. Anderson

Publications and source records attributed to Evan M. Anderson.

10 recordsLinked to original sources

arXiv2026

No band gap, no problem: Defects in InAs using a band-avoiding occupation-constrained density functional theory

Density functional theory (DFT) underestimates the experimental band gap---the infamous band gap problem. As the band gap defines the energy scale of defect levels, this complicates computation of charge transition energies for atomic defects. In the extreme case of narrow-gap semiconductors, the DFT band gap collapses to zero, seemingly precluding quantitative predictions of defect levels. We present a band-avoiding occupation-constrained DFT (ba-occ-DFT) approach that prevents spurious occupation of band-edge states and enables reliable total energy calculations of atomic defects. Application to indium arsenide (InAs) shows that ba-occ-DFT circumvents the band gap problem, separates band-edge errors from defect level calculations, and enables rigorous defect level predictions in a narrow-gap semiconductor despite a zero DFT band gap.

cond-mat.mtrl-sci↗

arXiv2025

Comparison of time-resolved photoluminescence and deep-level transient spectroscopy defect evaluations in an InAs nBn detector subjected to in-situ and ex-situ 63 MeV proton irradiation

Deep-level transient spectroscopy and temperature-dependent time-resolved photoluminescence experiments are performed on identical InAs nBn photodetector structures as a function of in-situ and ex-situ 63 MeV proton irradiation to assess their generation and recombination dynamics. Pre-irradiation, the n-type InAs absorbing region exhibits a steadily increasing minority carrier lifetime with increasing temperature, providing evidence that excited minority carriers may be recombining via shallow defect levels. From deep-level transient spectroscopy, two features are found between 10 K and 275 K: a low temperature broad shoulder, which suggests emission from multiple shallow electron defect levels with energies < 29 meV, and a high temperature minimum occurring at approximately 230 K with an activation energy of 539 meV, which suggests a defect in the barrier layer in the device. Two similar nBn detectors are then subjected to 63 MeV proton irradiation in step doses and measured between steps. One experiment is performed in-situ with an nBn held at approximately 10 K during dosing, and the other experiment is performed ex-situ with a similar nBn held at room temperature for dosing. The ex-situ dosing results in an evaluation of the defect introduction rate that is three to four times lower than in-situ due to partial annealing of the proton-induced displacement damage at room temperature. The results for these two experiments are then compared with the dose-dependent recombination rate analysis, resulting in an estimated recombination defect cross-section of 1.6x10^(-13) cm^2 for the shallow shoulder defect.

physics.app-ph↗

arXiv2025

Optical characterization of deep level defects in n-type Al$_x$In$_y$Ga$_{1-x-y}$P for development of solid-state photomultiplier analogs

Characterizing intrinsic defects is an important step in evaluating materials for new optoelectronic device applications. For photomultipliers, suppressing dark currents is critical, but there exists a tradeoff between maximizing the band gap while remaining sensitive to the wavelength of interest, and minimizing the incorporation of new defects by growing not-yet-optimized alloys. We present a series of capacitance-based measurements, including deep level optical spectroscopy, steady-state photocapacitance and illuminated capacitance-voltage, on photodiodes with lightly \textit{n}-type Al$_x$In$_y$Ga$_{1-x-y}$P absorber regions. Several deep levels are identified, including one near midgap. While the inclusion of aluminum increases each trap density by approximately 10x, the hole capture cross section also appears to decrease, suggesting that Shockley-Read-Hall dark currents may be suppressed. These materials may be good candidates for development into silicon photomultiplier analogs with wider bandgap for scintillator applications.

physics.app-ph↗

arXiv2025

First principles band structure of interacting phosphorus and boron/aluminum $\delta$-doped layers in silicon

Silicon can be heavily doped with phosphorus in a single atomic layer (a $\delta$ layer), significantly altering the electronic structure of the conduction bands within the material. Recent progress has also made it possible to further dope silicon with acceptor-based $\delta$ layers using either boron or aluminum, making it feasible to create devices with interacting $\delta$ layers with opposite polarity. Using Density Functional Theory, we calculate the electronic structure of a phosphorus-based $\delta$ layer interacting with a boron or aluminum $\delta$ layer, varying the distances between the $\delta$ layers. At separations 1 nm and smaller, the dopant potentials overlap and largely cancel each other out, leading to an electronic structure closely mimicking intrinsic silicon. At separations greater than 1 nm, the two $\delta$ layers behave independently of one another, with an equivalent electronic structure to a p-n diode with an intrinsic layer taking the place of the depletion region. One mechanism for charge transfer between $\delta$ layers at larger distances could be tunneling, where we see a tunneling probability exceeding what would be seen for a standard silicon 1.1 eV triangular barrier, indicating that the interaction between delta layers may enhance tunneling compared to a traditional junction.

cond-mat.mtrl-sci↗

arXiv2025

Roadmap on Atomic-scale Semiconductor Devices

Spin states in semiconductors provide exceptionally stable and noise-resistant environments for qubits, positioning them as optimal candidates for reliable quantum computing technologies. The proposal to use nuclear and electronic spins of donor atoms in silicon, introduced by Kane in 1998, sparked a new research field focused on the precise positioning of individual impurity atoms for quantum devices, utilising scanning tunnelling microscopy and ion implantation. This roadmap article reviews the advancements in the 25 years since Kane's proposal, the current challenges, and the future directions in atomic-scale semiconductor device fabrication and measurement. It covers the quest to create a silicon-based quantum computer and expands to include diverse material systems and fabrication techniques, highlighting the potential for a broad range of semiconductor quantum technological applications. Key developments include phosphorus in silicon devices such as single-atom transistors, arrayed few-donor devices, one- and two-qubit gates, three-dimensional architectures, and the development of a toolbox for future quantum integrated circuits. The roadmap also explores new impurity species like arsenic and antimony for enhanced scalability and higher-dimensional spin systems, new chemistry for dopant precursors and lithographic resists, and the potential for germanium-based devices. Emerging methods, such as photon-based lithography and electron beam manipulation, are discussed for their disruptive potential. This roadmap charts the path toward scalable quantum computing and advanced semiconductor quantum technologies, emphasising the critical intersections of experiment, technological development, and theory.

quant-ph↗

arXiv2025

A reduced-temperature process for preparing atomically clean Si(100) and SiGe(100) surfaces with vapor HF

Silicon processing techniques such as atomic precision advanced manufacturing (APAM) and epitaxial growth require surface preparations that activate oxide desorption (typically >1000 $^{\circ}$C) and promote surface reconstruction toward atomically-clean, flat, and ordered Si(100)-2$\times$1. We compare aqueous and vapor phase cleaning of Si and Si/SiGe surfaces to prepare APAM-ready and epitaxy-ready surfaces at lower temperatures. Angle resolved X-ray photoelectron spectroscopy (ARXPS) and Fourier transform infrared (FTIR) spectroscopy indicate that vapor hydrogen fluoride (VHF) cleans dramatically reduce carbon surface contamination and allow the chemically prepared surface to reconstruct at lower temperatures, 600 $^{\circ}$C for Si and 580 $^{\circ}$C for a Si/Si$_{0.7}$Ge$_{0.3}$ heterostructures, into an ordered atomic terrace structure indicated by scanning tunneling microscopy (STM). After thermal treatment and vacuum hydrogen termination, we demonstrate STM hydrogen desorption lithography (HDL) on VHF-treated Si samples, creating reactive zones that enable area-selective chemistry using a thermal budget similar to CMOS process flows. We anticipate these results will establish new pathways to integrate APAM with Si foundry processing.

cond-mat.mtrl-sci↗

arXiv2023

Suppression of mid-infrared plasma resonance due to quantum confinement in delta-doped silicon

The classical Drude model provides an accurate description of the plasma resonance of three-dimensional materials, but only partially explains two-dimensional systems where quantum mechanical effects dominate such as P:$δ$-layers - atomically thin sheets of phosphorus dopants in silicon that induce novel electronic properties beyond traditional doping. Previously it was shown that P:$δ$-layers produce a distinct Drude tail feature in ellipsometry measurements. However, the ellipsometric spectra could not be properly fit by modeling the $δ$-layer as discrete layer of classical Drude metal. In particular, even for large broadening corresponding to extremely short relaxation times, a plasma resonance feature was anticipated but not evident in the experimental data. In this work, we develop a physically accurate description of this system, which reveals a general approach to designing thin films with intentionally suppressed plasma resonances. Our model takes into account the strong charge density confinement and resulting quantum mechanical description of a P:$δ$-layer. We show that the absence of a plasma resonance feature results from a combination of two factors: i), the sharply varying charge density profile due to strong confinement in the direction of growth; and ii), the effective mass and relaxation time anisotropy due to valley degeneracy. The plasma resonance reappears when the atoms composing the $δ$-layer are allowed to diffuse out from the plane of the layer, destroying its well-confined two-dimensional character that is critical to its novel electronic properties.

cond-mat.mes-hall↗

arXiv2022

Accelerated Lifetime Testing and Analysis of Delta-doped Silicon Test Structures

As transistor features shrink beyond the 2 nm node, studying and designing for atomic scale effects become essential. Being able to combine conventional CMOS with new atomic scale fabrication routes capable of creating 2D patterns of highly doped phosphorus layers with atomic precision has implications for the future of digital electronics. This work establishes the accelerated lifetime tests of such doped layers, showing that these materials survive high current (>3.0 MA/cm2) and 300$^{\circ}$C for greater than 70 days and are still electrically conductive. The doped layers compare well to failures in traditional metal layers like aluminum and copper where mean time to failure at these temperatures and current densities would occur within hours. It also establishes that these materials are more stable than metal features, paving the way toward their integration with operational CMOS.

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↗