ABSTRACT In the Zener tunneling regime of silicon (Si) nanoscale pn junctions with high dopant concentrations, small but evident quasi‐periodic current oscillations were found at low temperatures, superimposed on the conventional band‐to‐band tunneling (BTBT) current‐voltage characteristics. This oscillatory component is ascribed to the interference of the tunneling electron waves inside the depletion region, as supported by the experimental and analytical study of the temperature dependence. The amplitude of the current oscillations decreases with increasing temperature in a way consistent with enhanced decoherence (phase breaking) of electron waves by phonon scattering. The interference originates from the partial reflection of electron waves due to the irregularly shaped potential profile in the depletion region containing a step‐like potential shape induced by a dopant cluster. Statistical calculations of random dopant distributions for parameters consistent with the experiment suggest that donor clusters are the most likely origin of the potential irregularities.

As an extension of the continuous miniaturization trend for Si transistors, single-electron effects related to dopants in the nanoscale transistor channels have been recently considered for low-power and fundamental applications. Research on dopant-based single-electron tunneling (SET) functionality extends from single-donor quantum dots (QDs) to multiple-donor QDs. Different from using complex techniques for doping, we show that a simpler, uniform doping technique at high-concentration of nanoscale silicon-on-insulator (SOI) transistors can offer the statistical conditions for the formation of isolated multiple-donor clusters that allow SET functionality, even at room temperature.

Band-to-band tunneling (BTBT), with its key role as a transport mechanism in Esaki (tunnel) diodes, has been studied extensively for about 60 years. In such devices, it is expected that energy states of ionized dopants in the depletion-layer can affect the BTBT mechanism. In this paper, we introduce the observation and analysis of a novel transport mechanism in Si Esaki diodes: single-charge BTBT transport mediated by donor-cluster quantum-dots, statistically expected in the depletion-layer of nanoscale Esaki diodes. This demonstration can open new pathways for band-to-band tunneling devices.

The electrostatic potential of p+-n+ junctions, as in Esaki (tunnel) diodes, originates from the Coulomb potentials of ionized dopants in the depletion-layer, but it has been modeled so far based on uniform space-charge regions, ignoring the discrete and random dopant distribution. This model can explain well the band-to-band tunneling (BTBT) between the opposite bands of the quasineutral regions (conduction band in the n+-region and valence band in the p+-region). In this letter, we show that a BTBT transport model should contain the mechanism of tunneling via “inherent” localized bandgap-states, created by dopant-induced potential fluctuation, which becomes detectable as a parallel transport mechanism in nanoscale Esaki diodes. This is manifested by the observation of single-charge (SC) BTBT at 5.5 K in nanoscale Si Esaki diodes. Numerical analysis of nanoscale p+-n+ junctions with random dopant-atom distributions suggests that SC-BTBT is mediated by a potential dip created by a number of dopants “clustered” near each other, i.e., by a multiple-dopant cluster.

Coulomb-blockade transport via donors working as quantum dots (QDs) in Si nano-transistor channels opens new pathways for atomic-level applications, but position-control of such QDs remains challenging. Here, we demonstrate that multiple-donor cluster-QDs can be formed by CMOS-compatible selective-doping, as observed from Coulomb-blockade transport at low temperature (T = 5.5 K). While at high gate voltage electron tunneling takes place via extended QDs, likely due to line edge roughness of the nanoscale channel, at low gate voltage tunneling occurs via a cluster of intentionally-doped donors. For the interpretation, we introduce a model of an isolated donor-cluster as a QD with voltage-dependent tunnel resistances. (C) 2019 The Japan Society of Applied Physics

We have recently reported single-electron tunneling (SET) via a-few-donor QDs at high temperatures in high-concentration selectively-doped SOI-FETs. A central QD works by SET mechanism above 150 K at small source-drain bias due to enhanced tunnel barrier. For tuning the tunnel barrier, it becomes critical to understand the impact of the donor-QD location on the SET transport. Here, we report the possibility of probing donor-QDs from center to near the lead edge using high-bias stability diagrams. We also observe and model the changes due to purposely shifted positions of the selectively-doped area.

Single-electron tunneling (SET) transistors have been studied for the past several decades because they are promising for low-power consumption and fundamental-level control of charge. The quantum dots (QDs) that are the main part of an SET transistor have been demonstrated in a variety of materials, but recently dopant-atoms in silicon have also been shown to work as QDs. However, a single conventional dopant-atom has usually a shallow ground state energy level below the conduction band edge (∼45 meV). This means that the tunnel barrier is relatively low and thermally-activated current can flow over the barrier. Therefore, the operation of dopant-atom SET transistors remains limited to low temperatures. In this work, we statistically analyze the key factors for raising the SET operation temperature up to room temperature (>300 K).

Quantum dots formed by donor-atoms in Si nanodevices can provide a breakthrough for functionality at the atomic level with one-by-one control of electrons. However, single-electron effects in donor-atom devices have only been observed at low temperatures mainly due to the low tunnel barriers. If a few donor-atoms are closely coupled as a molecule to form a quantum dot, the ground-state energy level is significantly deepened, leading to higher tunnel barriers. Here, we demonstrate that such an a-few-donor quantum dot, formed by selective conventional doping of phosphorus (P) donors in a Si nano-channel, sustains Coulomb blockade behavior even at room temperature. In this work, such a quantum dot is formed by 3 P-donors located near the center of the selectively-doped area, which is consistent with a statistical analysis. This finding demonstrates practical conditions for atomic- and molecular-level electronics based on donor-atoms in silicon nanodevices.

As an extremely miniaturized Si transistor (MOSFET) close to the atomic scale, Dopant Atom Transistor is one of the promising candidates and the research field has been rapidly growing in the last decade. The dopant atom transistor consists of a dopant-induced quantum dot in the channel and its carrier transport is tunnelling of electrons or holes from the source to the drain through the single dopant atom. Until now, operation temperature has been mostly limited to 20 K or even below, since the ground state of the dopant is too shallow. In order to resolve this issue, we study potential deepening effect by application of a "cluster" or a "molecule" of dopant atoms, which is a number of dopant atoms closely gathering. As a result, it is shown that operation temperature approaches room temperature. In this work, a guiding principle for high temperature operation will be shown.

Inter-band tunneling in Si is a key mechanism for Esaki diodes and tunnel FETs. In nanoscale devices, the dopant states under high built-in electric field may significantly affect inter-band tunneling transport. Here, we introduce firsttime observations from measurements of nanoscale Si tunnel diodes of two main effects: (i) splitting of dopant minibands in high electric field, similarly to the Wannier-Stark ladder; (ii) single-charge tunneling transport via donor-acceptor pairs aligned by the electric field. These phenomena produce distinguishable effects to enhance inter-band tunneling current.

We report for ultra-thin Si tunnelling diodes that negative differential conductance (NDC) is dominated by the excess current at room temperature. This is attributed to the gap-states induced by the co-dopants in the pn junction. First-principles simulation shows that the presence of co-dopants in the pn junction region leads to an increase in the interband tunnelling current by two orders of magnitude. Furthermore co-dopants interaction plays a key role in the interband tunnelling. In the absence of dopants states in the pn junction region, raising the doping concentration at source region does not give an appreciable improvement in tunnelling current.

Inter-band tunneling current is an attractive transport mechanism for future generations of electronics. However, this mechanism is limited by the momentum conservation law which requires phonon assistance in tunneling due to the indirect-bandgap nature of Si. Here, we show that in low-dimensional pn Esaki diodes, inter-band tunneling current can be enhanced by the resonance of discrete dopants with deepened energy levels. Current enhancement is comparable with the background direct inter-band tunneling and can be modulated by the applied biases, suggesting a pathway for controlling atomic-level resonances for practical purposes.

Introduction Si pn junction diodes are fundamental devices for electronics, and their miniaturization into nanoscale is expected to reveal new physics related to the low-dimensionality and dopant individuality [1]. In previous reports, we identified the role of discrete dopants working as traps in downscaled pn diodes doped with moderate doping concentrations [2-4]. Here, we characterize high-concentration pn diodes (Esaki tunnel diodes), with band-to-band tunneling transport significantly affected by two factors: (i) discrete dopants; (ii) phonon assistance.

Introduction Dopant-atom transistors offer the ability to control carrier transport to the level of single atoms and single electrons. However, typical dopants in Si (such as P) have small barrier height and cannot sustain tunneling operation at practical temperatures. Here, we discuss an alternative of using strongly-coupled a few donors to form quantum dots (QDs) with larger barriers, allowing tunneling operation at room temperature.

Silicon tunnel devices are important for low power-consumption and fast-switching electronics. Inter-band tunneling current is, however, limited due to the indirect-bandgap nature of Si, but low-dimensionality of the devices may bring new effects into play. In this work, we analyze two-dimensional lateral Si Esaki diodes and first observe that inter-band tunneling is still largely mediated by phonon assistance. More importantly, however, we find that tunnel current is significantly enhanced by resonances involving dopant-atoms with deeper energy levels in the depletion region. These results experimentally illustrate the impact of atomistic effects in low-dimensional tunnel devices.