Monolayer Kontakt Dotierung von Silizium-Oberflächen und Nanodrähte Mit Phosphororganische Verbindungen
Summary
A detailed procedure for surface doping of Silicon interfaces is provided. The ultra-shallow surface doping is demonstrated by using phosphorus containing monolayers and rapid annealing process. The method can be used for doping of macroscopic area surfaces as well as nanostructures.
Abstract
Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra shallow and sharp doping profiles at the nanometer scale. In MLCD process the dopant source is a monolayer containing dopant atoms.
In this article a detailed procedure for surface doping of silicon substrate as well as silicon nanowires is demonstrated. Phosphorus dopant source was formed using tetraethyl methylenediphosphonate monolayer on a silicon substrate. This monolayer containing substrate was brought to contact with a pristine intrinsic silicon target substrate and annealed while in contact. Sheet resistance of the target substrate was measured using 4 point probe. Intrinsic silicon nanowires were synthesized by chemical vapor deposition (CVD) process using a vapor-liquid-solid (VLS) mechanism; gold nanoparticles were used as catalyst for nanowire growth. The nanowires were suspended in ethanol by mild sonication. This suspension was used to dropcast the nanowires on silicon substrate with a silicon nitride dielectric top layer. These nanowires were doped with phosphorus in similar manner as used for the intrinsic silicon wafer. Standard photolithography process was used to fabricate metal electrodes for the formation of nanowire based field effect transistor (NW-FET). The electrical properties of a representative nanowire device were measured by a semiconductor device analyzer and a probe station.
Introduction
Controlled surface doping of semiconductor structures with macroscopic areas as well as at the nanoscale is important for advanced semiconductor device architectures such as FinFet2,3, as well as for nanostructure based devices such as nanowire-based sensors and photovoltaics4-7. We recently introduced monolayer contact doping (MLCD) for repeatable, uniform surface doping of silicon interfaces with macroscopic and nanometric dimensions with control over dopant dose and diffusion profile1. An important feature of MLCD is the restriction of monolayer formation to a substrate that is termed "donor substrate". MLCD simplifies some of the process steps required for Monolayer Contact Doping (MLCD) and provides complementary surface doping capabilities8. Once the donor substrate is loaded with the dopant containing monolayer by using self-limiting surface chemistry, the donor substrate is brought to contact with the substrate intended for doping, termed "target substrate", and both substrates are annealed while in contact. During the anneal process, dopant atoms diffuse to both donor and target substrates, and are activated at the elevated temperature. Since MLCD does not require high energy implantation of dopant atoms, no structural damage is caused to the semiconductor lattice during the process and no further anneal step is required. Good control over dopant diffusion is possible by controlling the rapid thermal process parameters. Ultra shallow and uniform dopant diffusion lengths down to a few nanometers are easily achieved. Separation of the monolayer from the process sequence simplifies the process, allow greater control over process parameters and open new possibilities for doping schemes that were not possible by using other methods. Achieving dopant level as high as the solubility limit of phosphorus in silicon is possible by multiple MLCD doping processes applied successively. In summary, traditional doping methods suffer from intrinsic limitations to fabricate ultra-shallow doping profiles. This is because of inherent statistical variations of source concentrations, overall dose and energy distribution, which are inherent to the low implantation energies required for ultra-shallow implantation. MLCD provides a simple means for surface doping, this is the result of the unique features of MLCD relying on the precise control of dopant dose and location at the atomic scale by utilizing robust surface chemistry for generating the dopant source with self-limiting monolayer chemistry formed exclusively at the semiconductor surface.
Protocol
Representative Results
Discussion
MLCD is a simple and reproducible method. However, attention to surface cleaning and monolayer formation must be taken. Piranha cleaning of the surfaces prior to the MLCD process is important not only for the purpose of avoiding possible impurities, but also for initialization of the surface for reproducible monolayer formation providing reproducible results between processes. The piranha treatment results in hydroxylation of surface groups which is required for binding of precursor molecules to the surface for the forma…
Disclosures
The authors have nothing to disclose.
Acknowledgements
This work was partially funded by the Farkas center for light-induced processes.
Materials
| High purity silicon wafers | Topsil | – | |
| 50 nm Si3N4/50 nm SiO2/Si wafers | Silicon Valley Microelectronics | – | |
| Sulfuric Acid 98% | BioLab | 19550523 | |
| Hydrogen Peroxide 30% | J.T. Baker | 2190-03 | |
| Ammonium Hydroxide 25% | J.T. Baker | 6051 | |
| Ethanol | J.T. Baker | 8025 | |
| Mesitylene | Sigma | M7200 | |
| Dichloromethane | Macron | 4881-06 | |
| Tetraethyl methylenediphosphonate | Aldrich | 359181 | |
| Mineral Oil | Sigma | M3516 | |
| Hydrofluoric Acid 49% | J.T. Baker | 9564-06 | |
| Isopropanol | J.T. Baker | 9079-05 | |
| N-Methyl-2-pyrrolidone | J.T. Baker | 9397-05 | |
| AZ nLOF2020 | AZ Electronic Materials | nLOF 2020 | |
| AZ 726 MIF | AZ Electronic Materials | 726 MIF | |
| Poly-L-Lysine solution | Sigma | P8920 | |
| Gold colloid solution | Ted Pella | 82160-80 | |
| RTA system | AnnealSys | MicroAS | |
| 4 point probe sheet resistance measurement system | Jandel | RM3-AR | |
| Mask aligner | Suss | MA06 | |
| e-Beam evaporator | VST | TFDS-141E | |
| Semiconductor analyzer | Agilent | B1500A | |
| CVD system | – | – | Home-built |
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