Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

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 FinFet^2,3, as well as for nanostructure based devices such as nanowire-based sensors and photovoltaics^4-7. 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 ....

Protocol

1. Surface Cleaning

1. Prepare acidic piranha solution by mixing 1:3 hydrogen peroxide (30%) and concentrated sulfuric acid.
   Caution: Piranha solutions are extremely strong and dangerous oxidizing agents and should be used with extreme caution. These solutions may explode in contact with organic solvents. Only qualified personnel with appropriate training and safety equipment may perform the procedure.
2. Place substrates (later used as donor and target substrates) in appropriate holder and insert into piranha solution for 15 min.
3. Rinse samples in DI water 3x.
4. Prepare base piranha solution by mixing 1:1:5 amm....

Results

Representative results for phosphorous-MLCD surface doping process are shown in Figure 1. Intrinsic silicon wafers were treated with phosphorous-MLCD, resulting in monotonic decrease in the sheet resistance values. Sheet resistance values decrease for longer anneal times and higher anneal temperatures as shown by the three traces in Figure 1. Sheet resistance values can be correlated to activated dopant concentration. Lower sheet resistance values indicate higher doping levels and vice v.......

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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