OPEN Nanoscale Nitrogen Doping in Silicon by Self...

1Scientific RepoRts | 5:12641 | DOi: 10.1038/srep12641

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Nanoscale Nitrogen Doping in Silicon by Self-Assembled MonolayersBin Guan1, Hamidreza Siampour1, Zhao Fan1, Shun Wang2, Xiang Yang Kong3, Abdelmadjid Mesli4, Jian Zhang5 & Yaping Dan1

This Report presents a nitrogen-doping method by chemically forming self-assembled monolayers on silicon. Van der Pauw technique, secondary-ion mass spectroscopy and low temperature Hall effect measurements are employed to characterize the nitrogen dopants. The experimental data show that the diffusion coefficient of nitrogen dopants is 3.66 × 10−15 cm2 s−1, 2 orders magnitude lower than that of phosphorus dopants in silicon. It is found that less than 1% of nitrogen dopants exhibit electrical activity. The analysis of Hall effect data at low temperatures indicates that the donor energy level for nitrogen dopants is located at 189 meV below the conduction band, consistent with the literature value.

The successful development of the complementary metal-oxide-semiconductor (CMOS) technology in the past decades has generated a huge impact on the human society by offering devices with higher performances at lower cost1. The main driving force of this development is the constant downscaling of CMOS transistors, which, however, has become increasingly difficult. One of the main issues is the short channel effect that the transistor gate loses control as the size becomes smaller, resulting in malfunctions. Currently, the CMOS industry solves this issue by using a 3-dimensional “FIN” structure that allows the gate grabbing around the channel to enhance its control2. This solution unfortunately increases the complexity of the fabrication processes. The cost per device increases instead of continuously decreasing as Moore’s Law predicts3.

A second solution to alleviate the short channel effect is to form ultra-shallow junctions in the source and drain regions of the transistor. This is one of the main motivations of “gold rush” for developing tran-sistors based on the 2-dimensional atomic monolayers such as graphene and MoS2, which will facilitate the formation of atomically thin junctions4–6. Silicon, as the most commonly used substrate in CMOS industry, is unlikely to be replaced by any new materials in the foreseeable future7. With silicon as the substrate, it has been difficult to form ultra-shallow junctions on the silicon surface by the traditional ion implantation doping process. This is because the physical bombardment of accelerated ions turns the single-crystalline silicon surface (about 10 nm thick) into low quality amorphous silicon, resulting in significant degradation in the device performances8. In recent years, doping by self-assembled monolay-ers (SAMs) has proven to be a mild and controllable doping process9,10. The process consists of chemical assembly of dopant-containing molecules on silicon surfaces and subsequent thermal treatment that drives dopants to diffuse into the substrate. To form ultra-shallow junctions, spike thermal treatment is often required for this process11. A sub-10 nm junction can be formed for fast diffusing dopants like phosphorus by limiting the annealing time to as short as a few seconds. However, a very short pulse of high temperature might lead to problems such as low reproducibility. It is more reliable to form

1University of Michigan - Shanghai Jiao Tong University Joint Institute. 2Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Department of Physics and Astronomy. 3School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200040, China. 4Institut Matériaux Microélectronique Nanosciences de Provence, UMR 6242 CNRS, Université Aix-Marseille, 13397 Marseille Cedex 20, France. 5Faculty of Medicine, Shanghai Jiao Tong University, Shanghai, 200040, China. Correspondence and requests for materials should be addressed to J.Z. (email: [email protected]) or Y.D. (email: [email protected])

Received: 12 February 2015

accepted: 06 July 2015

Published: 31 July 2015

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ultra-shallow junctions using dopants with low thermal diffusion coefficient, because such dopants will allow extended periods of annealing time. Nitrogen, similar to phosphorus, is also a group V element acting as a donor-type dopant in silicon. Previous research on nitrogen diffusion in silicon has showed that nitrogen dopants introduced by implantation and rapid thermal annealing have low diffusivity12,13. Nitrogen doping by SAMs can potentially form ultra-shallow or even atomically thin junctions in silicon. It thus may provide a better solution to the short channel effect and allow for the continuous downscaling of the CMOS technology.

In this Report we investigate the properties of nitrogen doping by SAMs that carry doping elements such as phosphorus and nitrogen. The chemical composition of the SAMs and the doped silicon substrate were examined using X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectrometry (SIMS). Van der Pauw and Hall effect measurements were used to characterize the electronic properties of the dopants. We demonstrate the successful doping of nitrogen and phosphorus in intrinsic silicon wafer, extract the diffusion coefficients for both dopants and estimate the donor energy levels in the monolayer doped silicon samples.

Results and DiscussionsThe preparation of SAMs on silicon followed the so-called hydrosilylation strategy, as shown in Fig. 1. The hydrosilylation process on silicon, first reported by Linford and Chidsey14,15, refers to the addition of silicon hydride onto an unsaturated carbon-to-carbon bond. It has been considered the most robust method for producing covalent Si− C bound organic monolayers on silicon16–18. Hydride-terminated

Figure 1. Schematic illustration of surface modification (SAMs formation) and monolayer doping on silicon.



surfaces were reacted with a nitrogen-containing alkene species 1 to form a silicon-carbon bound mon-olayer under UV light radiation. The strategies to modify the silicon surfaces with other molecules are shown in the supplementary Fig. S1. Figure 2 shows XPS spectra acquired on the silicon surface with alk-ene species 1 modification. The survey spectrum indicates the presence of Si, C and O, which is in good agreement with the presence of an organic monolayer on the silicon substrate. The oxygen 1s emission at 532 eV is ascribed to oxygen from the monolayer, as well as adventitiously adsorbed oxygen. Oxygen does not usually give reliable signals in XPS due to the contamination by the ambient atmosphere. However, both carbon and nitrogen can provide useful information on the nature of surface-bound species. The narrow scan of the C 1s region (Fig. 2b) shows a broad band at 285 eV (1.4 eV FWHM), attributed to aliphatic carbon-carbon (C− C) bonding. The side shoulder of the C 1s band at 286.5 eV (1.6 eV FWHM) is assigned to carbon-oxygen (C− O) and carbon-nitrogen (C− N) bonds from the molecule 1. The peak at 289 eV (1.6 eV FWHM) accounts for the carbonyl (C= O) in the terminal group. These observed bind-ing energies are consistent with those reported for carbon species on Si(100) surfaces19. Moreover, the representative peak area ratio of C− C to C− O/C− N shows an experimental value of 2.88:1, which is in agreement with those predicted by the stoichiometry of the atoms on the putative surface product (2.5:1). The narrow scan spectrum of N 1s in Fig. 2c displays one component at 400.5 eV (1.6 eV FWHM), corre-sponding to the nitrogen atom at tert-Butoxycarbonyl (t-Boc) group of alkene 1. Therefore, it is suggested that an alkene species 1 monolayer is formed on the silicon surface. We also employed XPS to analyse other SAMs formed on silicon according to the scheme in supplementary Fig. S1. Some oxides have been detected on the modified silicon surface (supplementary Fig. S2), although it was reported that an oxide-free surface after hydrosilylation on Si(100) could be achieved20–22. The data and discussions can be found in the supplementary Fig. S2 and Fig. S3.

The monolayer modified samples were then coated with a 20-nm thick layer of silicon dioxide by atomic layer deposition (ALD) and subsequently annealed at 1050 °C for 2 min (Fig. 1). To evaluate the nitrogen doping process, Van der Pauw technique was employed to measure the sheet resistance of the silicon samples at room temperature. Table 1 lists the experimental results of the silicon samples with and without nitrogen doping, as well as the sample with both nitrogen and phosphorus doping

Figure 2. (a) XPS survey spectra of the alkene species 1 monolayer on silicon. (b) Narrow scan of the C 1s region. (c) Narrow scan of the N 1s region.



(for the I-V curves, refer to the supplementary Fig. S4). The sheet resistance of the original undoped silicon substrate was measured to be 143 ± 21 kΩ . A piece of silicon wafer labeled as “blank sample” was selected to undergo all the capping and annealing processes except the SAM formation. The sample shows a sheet resistance as high as 121 ± 13 kΩ , indicating negligible contamination from the capping SiO2 and the ambient environment of the annealing chamber. A control sample with the absence of doping atoms was prepared to test the possible influence of C, O and H to the doping process. No significant change was observed from the control sample with only C monolayer modification (see the scheme in Supplementary Fig. S1). This means that C and O atoms, when sitting at their natural sites, substitutional for carbon and interstitial for oxygen, are electrically inactive in silicon. The samples that were modified with the monolayers containing N or both N and P (Supplementary Fig. S1) experience a significant drop in the sheet resistance. Clearly, the drop in the sheet resistance is solely contributed by N or both N and P that are carried by the SAMs. Electrostatic force microscopy (EFM) was also applied on the monolayer doped silicon samples to obtain the static charge profile across the samples. The EFM images in Supplementary Fig. S5 showed no clear aggregation of dopants across the samples, suggesting a relatively uniform doping.

To access to the doping profiles, the doped samples were analysed by SIMS, as shown in Fig. 3. In sample #1 with only nitrogen as dopant, the dopant concentration drops rapidly from ~4 × 1019 cm−3 near the surface to ~1017 cm−3 within a distance of 100 nm. In sample #2 with both nitrogen and phos-phorus doping, nitrogen shows the same trend of diffusion residing in the top 100 nm of the substrate. Phosphorus dopants, however, diffuse deeper than nitrogen to 200 nm below the interface, leading to a lower dopant concentration near the surface than nitrogen as required by the mass conservation. These dopants are carried by the SAMs, the nature of which limits the total number of initial dopants on the surface. After high temperature annealing, the dopants will follow the limited-source diffusion process, which is governed by the equation below:

π( , ) =

−

( )

N x tN

DtxDt

exp2 1

02

where N0 is the initial surface concentration, x the diffusion distance, t the annealing time and D the diffusion constant which is temperature dependent.

By fitting equation (1) into the curves in Fig. 3, we find that the diffusion coefficient for nitrogen is 3.66 × 10−15 cm2 s−1 in sample #1 and 1.39 × 10−14 cm2 s−1 in sample #2. The diffusion coefficient for phosphorus in sample #2 is 1.69 × 10−13 cm2 s−1 which is comparable to the widely observed value for phosphorus in silicon23,24, but two orders of magnitude larger than that of nitrogen in intrinsic silicon (sample #1). The diffusion of nitrogen follows a complex behavior in which defects distribution and some other factors during annealing process play key roles25,26. It is likely that the diffusion of nitrogen dopants can be enhanced by interaction with the faster diffusing phosphorus dopants. This is in line with the aforementioned values showing that the diffusion coefficient of nitrogen in the phosphorus-contained silicon (sample #2) is relatively larger than that in sample #1 without phosphorus.

High temperature annealing aims at accomplishing two steps: (i) driving the dopant atoms from the surface towards the bulk; (ii) activating them electrically by allowing doping atoms to find substitutional

Si(100) Samples Sheet Resistance Rs

(kΩ)

Blank sample: intrinsic silicon sample without any surface modification 121 ± 13

Control: sample modified with alkyl

monolayer

116 ± 12

Sample #1: modified with monolayer containing nitrogen

35.6 ± 0.2

Sample #2: modified with monolayer containing both nitrogen and phosphorus

36.6 ± 0.3

Table 1. Sheet resistances measured by Van der Pauw technique.



sites in the lattice. The first step is verified by SIMS, while the second is measured by Van der Pauw tech-nique and Hall effect measurement. The comparison of the two approaches allows for informing about the activated versus total fraction of introduced dopants.

From the SIMS data in Fig. 3, we can calculate the total amount of dopant atoms that diffuse from the surface into the bulk. In sample #1 the average nitrogen dopant concentration (nd) per unit area is 2.36 × 1013 cm−2, and the average nitrogen and phosphorus dopant concentration per unit area in sample #2 are 9.35 × 1012 cm−2 and 4.3× 1012 cm−2, respectively. The amount of nitrogen diffusing into substrate in sample #2 outnumbers that of phosphorus by a factor of around 2, which is consistent with the fact that ~65% of phosphate molecules are coupled onto nitrogen modified surface (see supplementary Fig. S3). If the nitrogen dopants were 100% electrically active in sample #1, the sheet resistance of the sample would be around 188 Ω . This is lower by a factor of about 200 than the measured sheet resistance listed in Table 1, indicating that the nitrogen dopants were not fully electrically activated. The activation rate can be estimated by the ratio 188/35517 = 0.53%. Similar conclusions can be reached for sample #2. However, it is not straightforward to find the exact activation rate of the dopants from the resistivity measurements, because the carrier mobility is dependent on the concentration of the activated dopants that are highly non-uniform in our case. We therefore turn to the Hall effect measurements that can directly provide the carrier concentration without the information of carrier mobility.

For Hall effect measurements, the Hall voltage (VH) is governed by the following equation:

= ( )VR IB

d 2HH

where RH is the Hall coefficient which is equal to −ne1 for n-type semiconductors with the electron con-

centration n and unit charge e, I the current flow, B the magnetic field, and d the thickness of the sample.

Considering that the electron concentration is non-uniform from the surface to the bulk, Hall resist-ance can be written in the following expression in which nc is the electron concentration per unit area.

∫ ∫= = − = −

( )= −

( )= −

( )

VI

Rd

Bned

Bn x edx

Be n x dx

B Ben

1 1 1

3

H Hd d

c0 0

Figure 4 displays the Hall effect measurement results on both samples in Hall bar geometry at 300 K. The Hall resistance is linearly correlated with the magnetic field. It is worth pointing out that the meas-ured value of Hall resistance is influenced by the longitudinal misalignment and the thermoelectric volt-age between the two Hall contacts. Hence in Fig. 4 Hall resistance shows a background number when the magnetic field is zero, which however does not influence the extraction of the free carrier concentration. From the slope of the linear correlation, the free electron concentration per unit area (nc) is extracted to be 1.36 × 1011 cm−2 in sample #1 and 2.1× 1011 cm−2 in sample #2. Compared to the SIMS data, we find that the activation rate for N in sample #1 is only 0.58% (a value very close to the crude estimation 0.53% given above), and the average activation rate for N and P in sample #2 is approximately 1.54%. Less than 1% activation rate of nitrogen in our case is consistent with the research in 1970s27, as the fraction of substitutional nitrogen is small. It is known that phosphorus implanted silicon has a high degree of electrical activity compared to nitrogen. However, it was reported that the presence of nitrogen could retard phosphorus activation by forming inactive complexes of phosphorus and nitrogen atoms at a ratio

Figure 3. Nitrogen and phosphorus doping profiles in sample #1 (a) and sample #2 (b). Inset graphs show the nitrogen and phosphorus concentration in log scale.



of 1:128. That might be the cause of low activation rate for sample #2. It also has been shown previously that P dopants carried by SAMs (not containing N) have an ionization rate over 90%9. We conclude that the low activation rate is not intrinsic to this method, but highly dependent on other impurities in the substrate. For example, it was shown that the activation rate of N dopants by ion implantation could be significantly increased in a boron doped silicon substrate29. Those boron atoms residing at the lattice sites of silicon can be displaced by silicon self-interstitials and later nitrogen atoms can fill the vacancies to render nitrogen donors. As a result, the activation rate of nitrogen could be significantly improved. The work using boron-doped silicon substrates to increase the electrical activation of nitrogen dopants by the SAM method is under investigation.

To further investigate the properties of dopants, we performed low temperature Hall effect measure-ments, from which the activation energy of the dopants can be found. We first extracted the average free electron concentrations (nc) at different temperatures from the curves of Hall resistance dependence on magnetic field for both N doped sample and N&P doped sample (see the supplementary Table S1), which are similar to those in Fig. 4 except that they show some nonlinearity at low temperatures. The details are illustrated in the supplementary Fig. S6 and S7. Then we plot the surface concentration of nc as a function of kT in Fig. 5, where k is the Boltzmann constant and T is the absolute temperature. The dopant distribution is highly non-uniform within the thin-doped layer near the surface. But the average concentration of electrons (nc), holes (pc) and ionized dopants (nD

+) will always remain charge neutral. On the assumption that the holes are negligibly small in concentration compared to the electrons, which is true in our case, we can derive the electron concentration nc that is correlated with the temperature as the following expression (see the supplementary materials for details):

( )( )

=− + +

( )

Δ

Δn

N N N N exp

exp

8

4 4c

c c c DE

kT

EkT

2

Figure 4. Hall resistance dependence on the magnetic field at 300 K for sample #1 (a) and sample #2 (b).

Figure 5. Temperature dependence of the average free electron concentration for sample #1 and #2 .



where Nc is the effective density of states function which is defined as ( )≈ = ( )π //

⁎

N w kT2cm kTh

2 3 23 2n

2 with w being the constant related to the band structure of the semiconductor, ND the concentration of donors, and ΔE the activation energy which is equal to (Ec − Ed) with Ec and Ed being the conductance band edge and the donor energy level, respectively.

By fitting equation (4) into the curve in Fig. 5 for N doped sample, we can obtain the activation energy of the nitrogen dopants as 189 meV and some other parameters as listed in Table 2. In the N and P doped sample, there are two types of donors to generate free electrons. As phosphorus dopants in silicon have much lower ionization energy than nitrogen, which is ~44 meV30, P atoms that replace Si lattice remain complete ionization within the temperature range of our measurements. For the N and P sample, equation (4) can be rewritten as

=− + +

+( )

Δ

Δn

N N N N exp

expN

8

4 5c

c c c DE

kTE

kT

P

2

where all the parameters have the same meaning as in equation (4) except that NP is the average concen-tration of completely ionized P donors.

The fitting results of equation (5) are also displayed in Fig. 5 and Table 2. The concentration of N donors (ND) and the activation energy for N atoms (ΔE) derived from the two equations show similar results, ~3.4× 1011 cm−2 and ~189 meV, respectively. The activation energy we obtained here shows a deep energy level existing for N dopants in Si, which is in good agreement with the literature31.

According to the fitting values in Table 2, the concentration of N donors in N-doped sample is 2-order magnitude lower than the total nitrogen concentration detected by SIMS, indicating that only about 1% of the nitrogen atoms diffusing into silicon bulk have substituted the Si lattice. This is again consistent with the report that more than 95% implanted nitrogen in silicon is trapped on some non-substituional sites or defects in the form of molecular N2, silicon nitride precipitates or randomly distributed atomic nitrogen32. Similarly, only 1.4% of phosphorus atoms have substituted the Si lattice. The rest of P atoms may form the inactive nitrogen-phosphorus complexes with N atoms as mentioned previously28.

In conclusion, we have successfully demonstrated that the nitrogen-containing SAMs on intrinsic silicon surface leads to nitrogen doping in the substrate with the assistance of high temperature anneal-ing. The activation rate and diffusion coefficient of nitrogen dopants were found to be consistent with literature values. As both parameters are related to defects such as silicon interstitials in the substrate, nitrogen doping with a higher activation rate might be achieved on p-type Si substrates. Based on the Van der Pauw and Hall effect measurements, a deep donor energy level at 189 meV below the conduction band was found for N doped silicon, which is associated with substitutional nitrogen. In future, DLTS (deep level transient spectroscopy) measurements will be employed to obtain further information about dopants and defects in nitrogen-doped silicon.

MethodsWafer cleaning. Float Zone single-side polished silicon wafers, (100) surface orientation ((100) ± 0.05°), 500 ± 25 μ m thick, 9000–15000 Ω cm in resistivity, were cleaved into 1 cm by 1 cm pieces and cleaned with acetone and ethanol of CMOS grade in a sonication bath for 5 min, respectively. After rinsed with DI water, the Si samples were immersed in “piranha solution” (98% H2SO4:30% H2O2, 3:1 (v/v)) for 30 minutes at 90 °C, followed by rinsing with DI water again. The wafers were then etched in 2.5% hydrofluoric acid (HF) solution for 90 seconds to remove the oxide layer and rinsed with DI water, followed by drying under N2 stream.

UV-mediated hydrosilylation and surface functionalization. The freshly etched Si(100) sam-ples were immediately placed in the reaction tube, covered with 30–50 μ L of 50 mg/mL tert-Butyl N-Allylcarbamate (TCI America, > 98%) methanol solution and were exposed to the irradiation of a 254 nm UV lamp (0.36 mW/cm2) in an inert nitrogen atmosphere for 3 h, affording sample #1. Control

Fitting results Parameter ValueStandard

error

N doped sample (sample #1), fitted with equation (4)

ND (× 109 cm−2) 334 30

ΔE (eV) 0.189 0.003

N and P dope sample (sample #2), fitted with equation (5)

ND (× 109 cm−2) 348 47

ΔE (eV) 0.188 0.007

NP (× 109 cm−2) 61.8 4.6

Table 2. Fitting results of equations (4) and (5) to the plots of nc as the function of kT.



experiment was conducted on freshly prepared Si samples with neat undecylenic acid (J&K Scientific Ltd., > 97%) under 254 nm UV radiation overnight. The samples were then rinsed with copious dichlo-romethane and ethanol, and dried under nitrogen gas. To obtain samples with phosphorus functionaliza-tion (sample #2, see Supplementary Fig. S1), sample #1 was subjected to 25% trifluoroacetic acid (TFA) in dichloromethane for 3 h, followed by a 10 min rinse in 10% ammonium hydroxide (NH4OH) to form the primary amine terminated surface. The surface was immersed in an ethanol solution of bifunctinal crosslinker dicyclohexylcarbodiimide (DCC, 40 mΜ , Aldrich) and mono-Dodecyl phosphate (5 mΜ , Aldrich) for 6 h, affording sample #2.

Silicon dioxide deposition and thermal annealing. SiO2 films were grown by atomic layer depo-sition on Si samples with tris(dimethylamino)silane (TDMAS) and ozone as precursors. Thermal anneal-ing at 1050 °C for 120 s, with a ramp temperature of 100 °C/min, starting from 800 °C, was performed in an argon environment in a tube furnace (Thermo scientific Lindberg/Blue, USA). After annealing, the Si samples were immersed again in 2.5% HF solution to remove SiO2 film on the surfaces.

Surface Characterization. X-ray photoelectron spectroscopy (XPS) was carried on a Kratos AXIS UltraDLD spectrometer with a monochromated Al Kα source (1486.6 eV), a hybrid magnification mode analyser and a multichannel detector at a takeoff angle of 90° from the plane of the sample surface. Analysis chamber pressure is less than 5 × 10−9 Torr. All energies are reported as binding energies in eV and referenced to the C 1s signal (corrected to 285.0 eV) for aliphatic carbon on the analysed sample surface. Survey scans were carried out selecting 250 ms dwell time and analyser pass energy of 160 eV. High-resolution scans were run with 0.1 eV step size, dwell time of 100 ms and the analyser pass energy set to 40 eV. After background subtraction using the Shirley routine, XPS spectra were fitted with a convolution of Lorentzian and Gaussian profiles by using software Casa XPS. Secondary-ion Mass Spectroscopy (SIMS) was conducted to obtain dopant profile at the top 300 nm of substrate at Evans Analytical Group, NJ, USA. EFM experiments were performed on a Nanonavi E-Sweep system (Seiko, Japan) operated in tapping mode.

Electrical Characterization. The metal contacts on silicon for electrical measurements were realized by evaporating 200-nm aluminum or chromium/gold films in a thermal evaporation system (Angstrom Engineering, Canada). Van der Pauw measurements were performed on square-shaped samples on which the metal contacts are exactly located at the four corners. The custom-made probe station is equipped with four solid tungsten probe tips (the tip size < 1 μ m). Keithley 2400 sourcemeter units and a custom-written Labview script were employed to generate and collect current/voltage data. Hall effect measurements were performed on samples in Hall bar geometry in a Physical Property Measurement System (PPMS, Quantum Design, USA).

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AcknowledgementsThis research was financially supported by the national “1000 Young Scholars” program of the Chinese central government, the National Science Foundation of China (61376001), the “Pujiang Talent Program” of the Shanghai municipal government, and the 54th general fund of China Postdoctoral Science Foundation. We thank Dr. Limin Sun (Instrumental Analysis Center, Shanghai Jiao Tong University) for her help with XPS, Mr. Ang Wang and Mr. Xiaofei Zhou (University of Michigan - Shanghai Jiao Tong University Joint Institute) for wire bonding.

Author ContributionsY.D. conceived the idea and supervised the research. J.Z. advised on the monolayer surface modification. B.G. designed and performed the experiments and collected the data. B.G., Y.D. and H.S. analysed the data. F.Z. conducted the EFM experiment. X.Y.K. performed the ALD experiment. S.W. contributed to interpreting the results of Hall effect measurement. A.M. contributed to discussion of the results. B.G. and Y.D. wrote the manuscript. All authors reviewed the manuscript.

Additional InformationSupplementary information accompanies this paper at http://www.nature.com/srepCompeting financial interests: The authors declare no competing financial interests.How to cite this article: Guan, B. et al. Nanoscale Nitrogen Doping in Silicon by Self-Assembled Monolayers. Sci. Rep. 5, 12641; doi: 10.1038/srep12641 (2015).

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Supporting Information for

Nanoscale Nitrogen Doping in Silicon by Self-‐Assembled Monolayers

Bin Guan 1, Hamidreza Siampour 1, Zhao Fan 1, Shun Wang2, Xiangyang Kong3,

Abdelmadjid Mesli 4, Jian Zhang 5*, Yaping Dan 1*

1 University of Michigan -‐ Shanghai Jiao Tong University Joint Institute,

2Key Laboratory of Artificial Structures and Quantum Control (Ministry of

Education), Department of Physics and Astronomy,

3School of Materials Science and Engineering,

Shanghai Jiao Tong University, Shanghai, 200040, China.

4 Institut Matériaux Microélectronique Nanosciences de Provence, UMR 6242

CNRS, Université Aix-‐Marseille, 13397 Marseille Cedex 20, France

5 Faculty of Medicine,

Shanghai Jiao Tong University, Shanghai, 200040, China.

Figure S1. Surface modification routes for preparing monolayers without

nitrogen (control sample) and with both nitrogen and phosphorus

functionalization (sample #1 and #2). The freshly etched hydride-‐terminated Si

(100) samples were immersed in neat deoxygenated undecylenic acid 3 and

were reacted under the irradiation of a 254 nm UV lamp in an inert nitrogen

atmosphere for over 16 h. The samples were then rinsed with copious

dichloromethane and ethanol, and dried under argon gas, yielding control

sample. Sample #2 was prepared with further modification on alkene species 1

modified sample #1. Following by amine deprotection in 25% trifluoroacetic

acid, the sample was derivatized with phosphate 2 in the presence of cross-‐

linker DCC, providing monolayers containing both nitrogen and phosphorus

atoms.

H H H H

Si (100) Si (100)

1, 3 h

254 nm UV

HN

O

O

25% TFA, 2.5 h

NH4OH, 10 min

HOP

O

OHO

9

NH

P

OO

HO

Si (100)

2, DCC, 6 h

DCC: dicyclohexylcarbodiimide

N C N

3, overnight254 nm UV

OH

O7

H H H HH H

NH

OO

NH

OO

Si (100)

HH H

NH2 NH2

H H

Si (100)

H H

OHO

7

OHO

7

1 2 3

sample 1 sample 2

control sample

Figure S2. XPS spectra of silicon sample modified with undecylenic acid (control

sample). a) Survey scan; b) high-‐resolution scan of the Si 2p region, showing a

clear Si 2p1/2−Si 2p3/2 spin−orbit-‐split doublet (99.9 and 99.3 eV, respectively)

from the bulk silicon and some silicon oxides in the range of 102-‐104 eV. The

fractional monolayer coverage of oxidized silicon, calculated directly from the

oxidized-‐to-‐bulk Si 2p peak area ratio according to a conventional model,1 is

approximately 0.5 monolayer equivalents. The silicon oxide is likely caused by

the unreacted silicon hydride bonds underneath the monolayer being oxidized

again when exposed to air. As Si(100) surface is terminated with a mixture of

SiH, SiH2 and SiH3 species, the orientation of Si(100) may lead to the formation of

self-‐assembled monolayers (SAMs) not as densely packed as that on Si(111).

Several studies have shown that densely packed SAMs on silicon can prevent the

oxidation of silicon more efficiently.2-‐4 Therefore, it is easier for the unreacted

silicon hydride on Si(100) to be oxidized. Nevertheless, forming an oxide-‐free

surface on Si(100) after hydrosilylation is still possible as reported by other

researchers.5-‐7; c) C 1s narrow scan, revealing an asymmetric peak at 285.0 eV

attributed to aliphatic C−C carbons, a second peak at 286.5 eV assigned to carbon

adjacent to the carbonyl moiety (−CC(O)OH) and a peak at 290.0 eV for carbonyl

–C(O)OH. Experimentally the peak area ratio of aliphatic C−C and two other

peaks is 8 : 1 : 1, which is close to the value (9 : 1 : 1) predicted by the

stoichiometry of the atoms of undecylenic acid.

Figure S3. XPS spectra of chemically modified Si surfaces. a) The nitrogen 1s

spectrum of the N-‐Boc-‐allylamine monolayer on silicon, showing a single peak at

400.5 eV. After treatment in 25% TFA (b) two peaks are recorded. The NH3+ and

the primary amine NH2 have been assigned to the peaks at 402.0 eV and 400.2 eV,

respectively. This indicates amine-‐terminated surface has been formed. The

surface further reacted with phosphate shows a peak at 400.6 eV corresponding

to newly formed N−P bonding and a peak at 402.0 eV suggesting the unreacted

amine group on the surface (c). According to the ratio of two peak areas, the

phosphate coupling efficiency can be estimated as 65%. Hence, the ratio of

nitrogen atom and phosphorus atom on sample #2 is about 1.54. d) The

phosphorus 2s spectra of the N-‐Boc-‐allylamine modified sample displays a broad

peak at 185.0 eV, corresponding to silicon plasmon loss8. After the treatment of

25% TFA and phosphate, a new peak appears on the shoulder at 190.9 eV (e),

assigned to the phosphorus from N−P bonding on the surface. Note that the peak

area of N−P component in N 1s spectrum is equal to the peak area in P 2s

spectrum. The above results, consistent with literature,9 demonstrate the

successful modification on silicon surfaces.

Van der Pauw measurement

The shape of the samples for resistivity measurement is square with point ohmic

electrodes contacted at the four corners. A voltage is applied to flow current IAB

along one side of the square sample and the voltage along the opposite side, VCD,

is measured. The current IAB and voltage VCD are then plotted to obtain the

resistance for one side RAB,CD. After repeating the process to the four sides of the

square sample, the sheet resistance of the sample can be deduced from the

following equations.

exp −πRx

Rs

"

#$

%

&'+ exp −

πRy

Rs

"

#$

%

&'=1 (1)

Rx =12RAB,CD + RCD,AB( ),Ry =

12RBC,AD + RAD,BC( ) (2)

Figure S4. I-‐V curves from Van der Pauw measurements on blank, control

samples, sample #1 and sample #2.

Figure S5. Electrostatic force microscope images on control (left) and monolayer

nitrogen-‐doped samples (right). The top two images were recorded when no

voltage was applied on the probe tip. The bottom images were taken at positive

bias of 1 V. Under positive bias, the sample with different doping concentration

patterns induced different static force towards the probe tip and thus rendered a

phase shift profile. The image for nitrogen-‐doped sample with + 1 V bias shows

no abrupt change in the profile, suggesting the even distribution of dopants.

Low-‐temperature Hall measurement

Figure S6. Dependence of the Hall resistance of the nitrogen-‐doped sample #1

(a) and both N and P doped sample #2 (b) on magnetic flux density at different

temperature from 300 K to 200 K.

At low temperatures, the hall measurement results from sample #1 show non-‐

linear correlation between measured resistance and magnetic field in Fig. S6. It

was due to the magnetoresistance effect, as the resistance of a semiconductor

generally increases when the sample is placed in a magnetic field. The measured

resistance between point 1 and point 2 (R12) in the Hall bar is not the actual Hall

resistance (RH), but the combination of magnetoresistance (RM) and RH. To

extract the actual RH and free carrier concentration at low temperature, the

resistance between point 2 and point 3 (R23) was also recorded. For no Hall effect

should be detected between point 2 and point 3, R23 is consisted of its original

resistance (RS) and RM. As an example, the experimental results of R23 and R12 in

the function of magnetic field for sample #1 at 240 K are showed in Fig. S7.

Figure S7. Hall resistance Vs Magnetoresistance. a) Dependence of resistance

between point 1 and point 2 (R12) on the change of magnetic field. b)

Dependence of resistance between point 2 and point 3 (R23) on magnetic field.

The two curves in Fig. S7 were then fitted with the following equations,

respectively.

R12 = RM12 + RH = r12 a+ cx2 + ex4 + gx6 +!( )+ rH (bx + dx3 + fx5 +!) (3)

R23 = RM 23 + RS = r23 a+ cx2 + ex4 + gx6 +!( )+ rS (bx + dx3 + fx5 +!) (4)

where x is the magnetic flux density and parameters such as a, b, c, d, e, f, g … are

the same ones in equation (3) as in equation (4). The fitting result of

rH (bx + dx3 + fx5 +!) is the actual Hall resistance. Hence, the free carrier

concentration nc can be obtained by the equation (5).

rHb = −1nce,nc = −

1rHbe

(5)

Table S1. Free carrier concentration (nc) for sample #1 and #2 at different

temperature.

Sample #1 (nitrogen doping) Sample #2 (both nitrogen and

phosphorus)

Temperature (K) nc (× 1011 cm-‐2) Temperature (K) nc (× 1011 cm-‐2)

300 1.36 300 2.10

280 1.12 290 1.85

260 0.838 280 1.72

240 0.562 270 1.61

220 0.369 260 1.48

200 0.202 250 1.34

190 0.135 240 1.21

180 0.0790 230 1.09

170 0.0401 220 0.987

160 0.0155 210 0.898

200 0.843

The donor energy level in nitrogen doped silicon

In the monolayer doped silicon samples, the doping is highly non-‐uniform and

majority of the dopants are located within a thin layer below the surface.

However, the electron concentration still follows the universal equation below to

maintain charge neutral:

nc = nD+ + pc

where nc is the average electron concentration, nD+ the average concentration of

ionized dopants and pc the average hole concentration.

For our case, the thin doped layer is always n-‐type, and hence nc >> pc. Then, we

have

nc = nD+

→ nc = Nc expEF −Ec

kT#

$%

&

'(=

ND

1+ 2exp EF −Ed

kT#

$%

&

'(= nD

+ (a)

exp EF −Ec

kT"

#$

%

&'=

ncNc

where Nc is the effective density of states function, Nc ≈ 22πmn

*kTh2

"

#$

%

&'

3/2

= w kT( )3/2

with w being the constant related to the band structure of the semiconductor, ND

the concentration of donors, EF the Fermi level, Ec the conductance band edge

and Ed the donor energy level.

Further, the right part of equation (a) can be written as:

ND

1+ 2exp EF −Ed

kT"

#$

%

&'=

ND

1+ 2exp Ec −Ed

kT"

#$

%

&'exp

EF −Ec

kT"

#$

%

&'

In the end, equation (a) can be rewritten as:

nc = Nc expEF −Ec

kT"

#$

%

&'=

ND

1+ 2exp Ec −Ed

kT"

#$

%

&'ncNc

(b)

After further simplifying equation (b) and solving the 2-‐order equation, we will

find

)exp(4

)exp(82

kTE

kTENNNN

nDccc

c Δ

Δ++−

=

(c)

where the activation energy ΔE = Ec -‐ Ed.

Sheet resistance results from a different type of wafer

Si (111) wafer as doping substrate was also employed into the nitrogen doping

experiment. The sheet resistance results of the Si (111) sample before and after

monolayer doping is shown in Table S2. The result in line with that of Si (100)

also suggests a successfully nitrogen doping in Si (111) by self-‐assembled

monolayer.

Table S2. Sheet resistances measured by Van der Pauw technique

Si (111) Samples with an original resistivity of 15 kΩ.cm,

thickness of 300 μm

Sheet Resistance Rs

(kΩ)

Blank sample: intrinsic silicon sample without any surface

modification 457 ± 23

Sample #1: modified with monolayer containing nitrogen

165 ± 27 HN

O

O

Additional SIMS profiles for monolayer doped samples

Figure S8. The duplicate nitrogen profiles of the N doped sample by SIMS,

showing good reproducibility of the experimental results.

Figure S9. The duplicate phosphorus profiles of the N and P doped sample by

SIMS.

Table S3. Comparison of SiO2 formation methods§

Methods for SiO2 formation ALD Spin-‐on-‐glass

(SOG)

Thermal

evaporation

Silicon sample (measured sheet

resistance of 143 ± 21 kΩ) Sheet Resistance Rs (kΩ)

Control: unmodified wafer coated

with SiO2 and annealed 121±13 116±6 109*

N-‐doped: modified with N

monolayer, coated with SiO2 and

annealed

35.6±0.2 41.2±6.8 N.A.

§ Only the data by ALD-‐SiO2 capping are displayed in the manuscript, because

the experimental data using SOG and thermal evaporation to form SiO2 coating

are incomplete due to non-‐scientific reasons.

* Only one repeat was done for thermally evaporated SiO2.

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