Single-electron tunneling through an individual arsenic dopant in silicon

At a Glance

Metadata	Details
Publication Date	2016-11-24
Journal	Nanoscale
Authors	V. V. Shorokhov, D. Е. Presnov, Sergey V. Amitonov, Yu. A. Pashkin, V. A. Krupenin
Institutions	P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Lomonosov Moscow State University
Citations	56
Analysis	Full AI Review Included

Technical Documentation and Analysis: Single-Electron Tunneling in Individual Dopant Transistors

Executive Summary

This research successfully demonstrates a single-electron tunneling (SET) transistor where the active island is an individual Arsenic (As) dopant atom embedded within a silicon nanobridge structure. This device architecture represents a critical milestone toward single-atom electronics and high-performance quantum computing components.

Core Achievement: Fabrication and characterization of a single-atom SET using a CMOS-compatible top-down approach on Silicon-on-Insulator (SOI) wafers.
Active Element: A solitary As dopant atom acting as the quantum island, tunnel-coupled to highly doped source/drain electrodes.
Observed Phenomenon: Characteristic Coulomb diamonds were measured at 4.2 K, confirming single-electron tunneling through the discrete energy levels of the As dopant.
Quantum Significance: The demonstration of discrete single-particle energy levels (0.98-1.18 meV spacing) provides necessary insight for developing stable, high-performance quantum bits (qubits) and charge pumps.
Design Advantage: The fabrication method leaves the nanobridge surface open, mitigating detrimental effects from interfacing layers often found in conventional gated structures, enhancing its utility for sensing applications.
Material Opportunity: The findings emphasize the need for robust host materials with higher binding energies and larger bandgaps—a core capability provided by 6CCVD’s Boron-Doped Diamond (BDD) to achieve quantum transport at elevated temperatures.

Technical Specifications

Parameter	Value	Unit	Context
Host Material Base	Silicon-on-Insulator (SOI)	N/A	Commercially available starting wafer.
Active Si Layer Thickness	55	nm	Top Si layer, patterned for the nanobridge.
Oxide Layer Thickness (SiO₂)	145	nm	Isolation layer below active Si.
Nanobridge Dimensions (W/T)	~20	nm	Approximate width and thickness of the constriction.
Dopant Element	Arsenic (As⁺)	N/A	Donor atom acting as the single-electron island.
Ion Implantation Voltage	6	kV	Low acceleration voltage used for shallow doping.
Total Ion Dose	1.25 x 10¹⁵	cm^-2	Dose used to reach solubility limit in Si.
Annealing Temperature (RTA)	925	°C	Rapid Thermal Annealing for crystal restoration/dopant activation.
Annealing Duration	10	s	Rapid process to control dopant redistribution.
Resultant Doping Concentration	10¹⁹-10²⁰	cm^-3	Concentration in the top 30 nm of the active layer.
Sheet Resistance	~300	Ω sq^-1	Measured resistance of the active SOI layer.
Measurement Temperature	4.2	K	Liquid Helium temperature used for stability diagram mapping.
Maximum Bias Voltage (V)	± 120	mV	Range used for wide stability diagram mapping.
Maximum Gate Voltage (V_G)	70	V	Highest V_G before electrical breakdown.
Bias Division Factor (η)	≈ 0.36	N/A	Calculated ratio C_L/(C_L + C_R).
Lever Arm Factor (a_G)	≈ -0.0019	N/A	Efficiency of gate coupling.
Inter-Level Spacing (ΔE_αβ)	0.98	meV	Measured spacing between single-particle energy levels.
Final Ground State Energy Level	49.3	meV	Below the conduction band (bulk reference 54 meV).

Key Methodologies

The single-atom single-electron transistor was fabricated using a sophisticated top-down process, fully compatible with standard CMOS manufacturing techniques:

Material Preparation: Use of a commercially available SOI wafer (55 nm top Si layer, 145 nm SiO₂ layer, 725 µm substrate).
Ion Implantation: Heavy ion (As⁺) implantation performed at a low acceleration voltage (6 kV) to minimize the thickness of the highly doped region, using a total dose of 1.25 x 10¹⁵ cm^-2.
Dopant Activation: Rapid Thermal Annealing (RTA) at 925 °C for 10 s to restore crystal structure and achieve high dopant concentration (10¹⁹-10²⁰ cm^-3) in the top 30 nm.
Pattern Definition (EBL): Electron-beam lithography (EBL) was used with PMMA 950K resist to define the precise transistor structure, including the nanobridge and gates.
Metal Mask Creation: A 10 nm thick Aluminum (Al) film was deposited and lifted off to create a hard mask layer.
Nanobridge Fabrication (RIE): Anisotropic Reactive Ion Etching (RIE) using CF₄ plasma was performed on the active Si layer, using the Al layer as a mask. The Al mask was subsequently removed.
Fine Trimming: Consecutive short RIE steps (approximately 10 s each) were used at 77 K while monitoring resistance to gradually reduce the size of the nanobridge until transport was dominated by a single isolated dopant atom.

6CCVD Solutions & Capabilities: Enabling Next-Generation Single-Atom Electronics

The research highlights the limitations of silicon for realizing robust single-atom devices, specifically noting the objective of finding a combination that delivers a high Coulomb energy and thus a high operating temperature. MPCVD Diamond, particularly Boron-Doped Diamond (BDD), is the ideal material platform to extend and commercialize this quantum research.

Applicable Materials

The ideal 6CCVD material to replicate and significantly advance this research is highly conductive Boron-Doped Polycrystalline Diamond (BDD PCD) or Boron-Doped Single Crystal Diamond (BDD SCD).

Requirement/Benefit	6CCVD BDD Diamond Solution	Advantage over Silicon
High Operating Temperature	Diamond’s wide bandgap (~5.5 eV) and superior thermal conductivity.	Allows device operation far above cryogenic temperatures (4.2 K or 77 K).
Dopant Properties	Boron (B) dopants in diamond create shallow acceptors, or Nitrogen-Vacancy (NV) centers offer spin qubits.	Alternative dopant systems offer higher binding energies, potentially reducing inter-dopant distance requirements (as noted in the paper, section 4).
Conductivity	Heavy Boron Doping (approaching 10²¹ cm^-3) achieves metallic-like conductivity for source/drain contacts.	Provides a robust, highly conductive platform for the electrodes and quantum island, replicating the ‘metal-like’ contacts used in the Si experiment.
Material Quality	Optical Grade SCD or high-purity PCD.	Extremely low defect density, crucial for isolating a single dopant/qubit center without adjacent trap interference (a distortion noted in Figure 4 of the study).

Customization Potential

6CCVD provides the specialized engineering services required to move from prototype laboratory work (like the SOI device) to integrated diamond nanostructures:

Precision Substrates & Layer Control: We offer custom diamond substrates up to 125mm in size, with precise control over thickness (SCD or PCD from 0.1 µm to 500 µm). This replaces the standard SOI structure with an intrinsically superior diamond film suitable for device stacking or integration.
Custom Metalization Stacks: The Si device required ohmic contacts. 6CCVD offers full in-house metalization services, including Ti, W, Au, Pt, and Pd stacks, optimized for robust contact deposition on BDD/SCD materials, necessary for source, drain, and gate interfaces.
Nanofabrication Readiness: While the paper used RIE for etching, achieving nanobridge structures in diamond requires specific techniques. We provide materials with superior polishing (Ra < 1nm for SCD, Ra < 5nm for inch-size PCD), providing a mirror-smooth surface ideal for subsequent high-resolution electron-beam lithography (EBL) and patterning steps needed for the 20 nm scale features demonstrated here.
Engineering Support for Integration: Our laser cutting capabilities allow for complex mesa and feature definition, supporting the integration of diamond into packaging or custom cryogenic systems, critical for transport measurements.

Engineering Support

6CCVD’s in-house PhD team specializes in diamond material science and advanced heterostructures. We can assist researchers and engineers with material selection, doping density optimization, and surface preparation protocols necessary to transition single-atom tunneling transistor projects from silicon to diamond. Our expertise ensures reliable material specifications that meet the stringent requirements for quantum transport and high-temperature operation.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

We report the single-electron tunneling behaviour of a silicon nanobridge where the effective island is a single As dopant atom. The device is a gated silicon nanobridge with a thickness and width of ∼20 nm, fabricated from a commercially available silicon-on-insulator wafer, which was first doped with As atoms and then patterned using a unique CMOS-compatible technique. Transport measurements reveal characteristic Coulomb diamonds whose size decreases with gate voltage. Such a dependence indicates that the island of the single-electron transistor created is an individual arsenic dopant atom embedded in the silicon lattice between the source and drain electrodes, and furthermore, can be explained by the increase of the localisation region of the electron wavefunction when the higher energy levels of the dopant As atom become occupied. The charge stability diagram of the device shows features which can be attributed to adjacent dopants, localised in the nanobridge, acting as charge traps. From the measured device transport, we have evaluated the tunnel barrier properties and obtained characteristic device capacitances. The fabrication, control and understanding of such “single-atom” devices marks a further step towards the implementation of single-atom electronics.

Tech Support

Original Source

DOI: https://doi.org/10.1039/c6nr07258e