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May 19, 2015 - ABSTRACT: Designing new approaches to incorporate dopant impurities in semi- conductor materials is essential in keeping pace with ...
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Functionalization of Silica Nanoparticles and Native Silicon Oxide with Tailored Boron-Molecular Precursors for Efficient and Predictive p‑Doping of Silicon Laurent Mathey,†,‡,§,¶ Thibault Alphazan,†,‡,§,¶ Maxence Valla,∥ Laurent Veyre,§ Hervé Fontaine,†,‡ Virginie Enyedi,†,‡ Karim Yckache,†,‡ Marianne Danielou,†,‡ Sébastien Kerdiles,†,‡ Jean Guerrero,†,‡ Jean-Paul Barnes,†,‡ Marc Veillerot,†,‡ Nicolas Chevalier,†,‡ Denis Mariolle,†,‡ François Bertin,†,‡ Corentin Durand,⊥,# Maxime Berthe,⊥ Jolien Dendooven,▽ François Martin,†,‡ Chloé Thieuleux,*,§ Bruno Grandidier,*,⊥ and Christophe Copéret*,∥ †

Université Grenoble Alpes, F-38000 Grenoble, France CEA, LETI, MINATEC Campus, F-38054 Grenoble, France § C2P2, CPE Lyon, 43 Bd du 11 Nov. 1918, Villeurbanne 69616 Cedex, France ∥ Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg. 1-5, CH-8093 Zürich, Switzerland ⊥ Institut d’Electronique et de Microélectronique et de Nanotechnologies (IEMN), CNRS, UMR 8520, Département ISEN, 41 bd Vauban, Lille 59046 Cedex, France ▽ Department of Solid State Sciences, Ghent University, Krijgslaan 281−S1, B-9000 Gent, Belgium ‡

S Supporting Information *

ABSTRACT: Designing new approaches to incorporate dopant impurities in semiconductor materials is essential in keeping pace with electronics miniaturization without device performance degradation. On the basis of a mild solution-phase synthetic approach to functionalize silica nanoparticles, we were able to graft tailor-made boronmolecular precursors and control the thermal release of boron in the silica framework. The molecular-level description of the surface structure lays the foundation for a structure−property relationship approach, which is readily and successfully implemented to dope non-deglazed silicon wafers. As the method does not require an additional oxide capping step and shows minimal risk of carbon contamination, as demonstrated by compositional and electrical characterizations of the wafers, it is perfectly adapted to advanced microelectronics manufacturing processes.

nanowire structures.15,16 However, transferring this process to the microelectronic industry is difficult because it requires a rather nonmanufacturing-friendly substrate (hydrofluoric acid deglazed silicon) and the use of a low-temperature, conformal SiO2 evaporation capping to avoid dopant evaporation upon annealing. To circumvent this capping issue, the monolayer contact doping (MLCD) concept was recently proposed.17,18 While providing good results in the formation of (ultra)shallow junctions, the use of two substrates (donor and acceptor) makes this ex situ technique not as straightforward as approaches related to MLD, relying on only one substrate. In this work, we show how to avoid these issues by the use of a simple bottom-up methodology based on the grafting of tailored boron-containing molecular precursors (anchoring/ self-protected) on the thin layer of native silica present on top of silicon wafers. This method takes advantage of (i) the presence of a tunable coverage/density of silanols on silica as

INTRODUCTION Dopants play a critical role in semiconductor devices and are therefore a major focus of research.1−3 For instance, several doping strategies have emerged and led to significant improvements to drive impurities (dopant) inside pure substrates and to reduce the variability of targeted electronic devices, together with increased performances for nanoobjects.4−11 However, improving dopant incorporation, avoiding randomness of concentration, and investigating diffusion phenomena still represent an outstanding challenge today, in particular with the development of smaller nanosized devices.8 Indeed, increasing the performance in microelectronics has been directly related to the continuing shrinking of transistors12 and thus to their optimized properties. The control of dopant concentration and distribution in semiconductors is essential,8 and several methods such as single-ion implantation,6 chemisorption of dopant from gaseous hydride molecules,9,10 spin-on doping, 11 or single-atom doping13 have been investigated. The monolayer doping (MLD) concept14 appeared to be well-adapted to the medium doping range targeted for transistors or junctions in Fins as well as in © 2015 American Chemical Society

Received: April 9, 2015 Revised: May 18, 2015 Published: May 19, 2015 13750

DOI: 10.1021/acs.jpcc.5b03408 J. Phys. Chem. C 2015, 119, 13750−13757


The Journal of Physical Chemistry C

Figure 1. Strategy for a mild two-step doping process without postcapping, for 3D and 2D substrates.

with CaF2 windows under an inert atmosphere. FT-IR spectra recorded in transmission mode were obtained using a Bruker Alpha FT-IR spectrometer under an inert atmosphere. Typically, 32 scans were acquired (4 cm−1 resolution). Liquid-state NMR spectra were recorded using a Bruker Spectrospin 300 MHz spectrometer. Solid-state NMR spectra were measured on either a 400 or 700 MHz ultrashielded Bruker NMR spectrometer. A 3.2 mm probe head and a 1.3 mm probe head were used for measuring 11B/13C spectra on the 400 MHz and 1H spectra on the 700 MHz, respectively. A background suppression sequence was used to measure the 11B spectrum with a 100 kHz radio frequency field. A crosspolarization magic angle spinning was used to record the 13C spectrum. The radio frequency field for 1H was always set to 100 kHz. Dehydroxylation of Silicon Wafers. Wafers were cut in 20 cm2 (2.5 × 8 cm2) pieces from n-doped (1015 P·cm−2) 20 cm diameter silicon wafers. They were then calcined for 2 h under air at 500 °C (6 °C·min−1), partially dehydroxylated at 500 °C under high vacuum (10−5 mbar) for 12 h, and stored in the glovebox. Grafting of C3v on Wafers, When Using Dichloromethane. After solubilizing C3v into dichloromethane (45 mL), the solution was contacted with the wafer pieces and left to react for 2 h. Then, after removing the solution, 15 mL of dichloromethane was added to wash the sample. Afterwards, the solution was removed, and all volatiles were removed in vacuo before putting the wafer inside the glovebox. There, the sample was washed twice with toluene (5 mL) and twice with pentane (5 mL) before being dried in vacuo. RTP Annealing of the Wafers. The wafer was placed in the furnace, and the thermocouples were removed in order to prevent them from melting. The atmosphere inside the furnace was first pumped out to work in a low-pressure N2 atmosphere (200 sccm). Infrared lamps warmed the atmosphere until stabilization at 300 °C. The furnace was then heated to 985 °C at a rate of 10 °C·s−1, and the temperature was held for 4 s before reaching 1000 °C, with a stabilization time of 1 s. Afterward, the furnace was cooled to 20 °C. ICP-MS Measurement of the Silica Thin Layer Covering Wafers. The vapor phase decomposition (VPD) method as described in the literature was used in order to recover the totality of the species present in the silica layer.29−31 The condensation and trace metals present on the etched wafer were collected by a 0.1 mL drop of a solution of HNO3 50%.32 The droplet was then dropped into a small Teflon vial, and the volume was adjusted to reach a 1 mL solution. The solution

anchoring surface groups, (ii) the self-limiting property of sterically hindered molecular precursors over a surface, (iii) their self-capping ability to avoid dopant evaporation upon annealing, without the need for any additional oxide capping steps (Figure 1), and (iv) the minimization of carbon contaminants that could diffuse upon thermal annealing. This molecular approach has been successfully applied for oxide surface functionalization19 or for the design and development of numerous well-defined supported catalysts20−25 but has been more scarcely used for electronic materials.26−28 One example from our group was the controlled grafting of Cu-molecular precursors to regulate the density of Cu nanoparticles (NPs) for the growth of silicon nanowires.28 One advantage of this approach is the use of silica NPs (high surface area threedimensional (3D) substrates) as models to help the characterization of the surface and ease the characterization and development of the final object through a structure−property relationship approach.

EXPERIMENTAL SECTION All experiments were carried out under a dry and oxygen-free Ar atmosphere using either standard Schlenk or glovebox techniques for the organometallic synthesis. For syntheses and treatments of surface species, reactions were carried out using high vacuum lines (10−5 mbar). Elemental analyses were performed at “Mikroanalytisches Labor Pascher” in Germany. Toluene, dichloromethane, and pentane were dried using an MBraun solvent purification system, contacted with 4 Å molecular sieves and degassed under vacuum. THF (Aldrich) was distilled over Na/K and benzophenone. BH3·THF in THF (1 M) was purchased from Sigma-Aldrich and used as received. Phenanthrenedione, also purchased from Sigma-Aldrich, was evacuated in vacuo and stored in the glovebox. The wafers were 20 cm in diameter, n-doped (1015 P·cm−3), and single-face polished. RTP and ICP-MS analyses on wafers were carried out in a clean room in LETI, CEA Grenoble. RTP annealing experiments were performed using a JetFirst 200 by Jipelec using a BT or HT type optical pyrometer and 2 K type (Chromel/Alumel), Ø 0.127 mm thermocouples. An Agilent 7500cs was used for the ICP-MS measurements with the following parameters: analysis hot plasma mode (1600 W) under Argon (no He and no H2 were introduced into the octopole); integration time was 2 s on mass 11 for boron. Magnetic SIMS analyses were performed using a SC ULTRA instrument from CAMECA. FT-IR spectra recorded in diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) mode were obtained using a Nicolet 6700 FT-IR spectrometer. The samples were loaded into a custom DRIFT cell equipped 13751

DOI: 10.1021/acs.jpcc.5b03408 J. Phys. Chem. C 2015, 119, 13750−13757


The Journal of Physical Chemistry C

Scheme 1. Reaction of Phenanthro[9,10-d][1,3,2]dioxaborole (C2) with Silica Nanoparticles Partially Dehydroxylated at 500 °C (SiO2‑(500)) and Proposed Surface Species

Figure 2. IR spectra of partially dehydroxylated SiO2‑(500) NPs at 500 °C and C2/SiO2 ((A), transmission mode) or C3v/SiO2 ((C), diffuse reflectance infrared fourier transform mode). 11B MAS solid-state NMR spectra and their decomposition for C2/SiO2 ((B), 400 MHz, ω/2π = 10 kHz, 5k scans, recycling delay of 1 s) and C3v/SiO2 ((D), 400 MHz, ω/2π = 15 kHz, 41k scans, recycling delay of 1 s). Background suppression was obtained through a 90−180−180 pulse sequence.

Electrical Measurements. The electrical measurements were performed on HF etched samples, in an ultrahigh vacuum (UHV) system (base pressure lower than 5 × 10−10 mbar), equipped with a multiple probe scanning tunneling microscope combined with a scanning electron microscope (SEM) (Nanoprobe, Omicron Nanotechnology).33 W tips were prepared by an electrochemical etching in NaOH and thoroughly cleaned in UHV. AFM Measurements of Grafted Wafers. The topographical measurements were performed in tapping mode on a

was then analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Magnetic SIMS Measurement of HF Etched Wafers. Immediately after removing the silica thin layer by an HF vapor treatment (VPD), wafer pieces were cut into 1 × 1 cm2 samples and stored in a glovebox before analysis. SIMS analyses were performed at different impact energies of an O2+ beam, from 250 eV up to high energies of 3000 eV for boron depth profiling. For contaminants (C, Cl) depth profiling, a 500 eV Cs+ incident beam was used. 13752

DOI: 10.1021/acs.jpcc.5b03408 J. Phys. Chem. C 2015, 119, 13750−13757


The Journal of Physical Chemistry C

Scheme 2. Reaction of a Tris(2-hydroxyphenyl)methane-borate Tetrahydrofuran Adduct (C3v) with Silica Nanoparticles Partially Dehydroxylated at 500 °C (SiO2‑(500)) and Proposed Surface Species

observed by 13C CP-MAS NMR, probably because the corresponding species is present in small amounts and does not bear a proton needed for efficient CP enhancement. Finally, the 11B MAS NMR spectrum (Figure 2B) displays two peaks at 16 and 11 ppm in a ca. 1:1 ratio, both characteristic of trigonal B(OR1)(OR2)(OR3) species.43,44 They probably correspond to mono- and bis-grafted surface species, A and B (Scheme 1), which are often formed upon grafting on silica partially dehydroxylated at 500 °C due to the presence of isolated and vicinal silanols.23,40−42,45,46 Overall, the precursor C2 allows boron chemisorption onto the silica surface up to about 1.39 × 1014 B·cm−2 as a mixture of organic capped B-surface species anchored to the surface via B−O bonds. For C3v,47 grafting provides a material named C3v/SiO2, containing 0.29 wt % of boron, which corresponds to 0.81 × 1014 B atom·cm−2 and a consumption of 41% of surface OH groups. IR spectroscopy (Figure 2C) reveals that most isolated surface SiOH (3743 cm−1) disappear, while ν(C−H) bands associated with the organic functionalities appear. Note however the appearance of a broad OH band at 3573 cm−1, probably associated with OH interacting with adjacent organic functionalities. Since C3v can chemisorb on silica through coordination on oxygen functionalities (surface OH/O, species C, Scheme S1, Supporting Information) or react by grafting through the reaction of the surface silanols with B−OLigand bond(s) (species D and E, Scheme 2), we also monitored the reaction of C3v with 18O-doped silica (see Supporting Information), which showed that grafting occurred through cleavage of B−OLigand bond(s) (Scheme 2). The 1H MAS and 13 C CP-MAS NMR spectra (Figures S3 and S4, Supporting Information) show that the organic ligand is still present (see Supporting Information for a more precise assignment), along with trace amounts of adsorbed tetrahydrofuran. The 11B solidstate NMR spectrum (Figure 2D) displays two peaks at 4 ppm (37%) and 11 ppm (63%), which are attributed to tetra- and tricoordinated boron-surface species according to their respective 11B NMR chemical shifts.43,44 We propose that the tricoordinated B-species is associated with the bis-grafted species because of the opening of two B−O bonds, while the tetra-coordinated species corresponds to the monografted species because the rigid scaffold favors such geometry and the coordination of the OH group to B (Scheme 2, species D and E). Overall, the grafting process generates chemisorbed Bsurface species, having a large capping ligand. Considering that the amount of boron quantified on SiO2 NPs (1.39 × 1014 to 0.81 × 1014 B atom·cm−2) lies within the expected medium doping range for electronic devices, we transferred this grafting methodology to silicon wafers covered

Bruker Multimode AFM using a silicon probe (Budget Sensors TAP300Al) having a spring constant of about 40 N·m−1 and a resonant frequency of about 340 kHz.

RESULTS AND DISCUSSION Here, we use the strategy depicted in Figure 1 to chemisorb boron dopants at the surface of silica NPs from B-containing molecular precursors with two types of protective capping organic scaffold and reactive functionalities, referred to as precursors C2 and C3v because of the symmetry of the molecules.34 We show that silica NPs act as a relevant test bed to gain insight into the surface functionalization, the removal of the protective organic groups, and the quantification of boron impurities incorporated in the NPs. We further demonstrate that this chemical approach is transferrable to silicon wafers enabling their efficient doping with limited side effects. The molecular precursor C2 was grafted on SiO2 NPs (NP diameter ≈ 20 nm, 200 m2·g−1) partially dehydroxylated at 500 °C under vacuum, which contains mostly isolated silanols and a OH density of about 2.0 × 1014 OH·cm−2 (Scheme 1).35−37 Grafting carried out in a toluene solution yielded a solid, quoted C2/SiO2, containing 0.50 wt % of boron corresponding to 1.39 × 1014 B atom·cm−2, thus showing that about 70% of surface silanols reacted. This is confirmed by infrared (IR) spectroscopy (Figure 2A), which shows the occurrence of ν(C−H) bands consistent with the presence of organic ligands and the concomitant disappearance of the isolated silanols35−37 at 3744 cm−1. Instead, a broad band at 3611 cm−1 is visible, which is associated with residual OH interacting with aromatic ligands.38 Remarkably, the absence of the B−H band shows that grafting occurs by the reaction of the surface silanols with the B−H group (Scheme 1, species A). In addition, note the presence of two bands of low intensity at 1944 and 1695 cm−1, which are attributed to a combination band39 and to the vibration mode of an α,β-conjugated carbonyl group (CO), respectively; the latter species is likely in equilibrium with its most stable enol form (Scheme 1, species B). They show that some borole rings are opened during the grafting, probably through the reaction of gem- and vicinal silanols.40−42 This is confirmed by 1H and 13C cross-polarization magic angle spinning (CP-MAS) nuclear magnetic resonance (NMR) spectroscopy (Figures S1 and S2, Supporting Information), which shows the presence of the aromatic carbons with three intense peaks at 121, 126, and 139 ppm as well as the peak of weak intensity at 81 ppm consistent with the presence of a carbon belonging to a benzylic ether (ArCHxO). Note however that the carbon belonging to this CO group was not directly 13753

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The Journal of Physical Chemistry C

Figure 3. Magnetic secondary ion mass spectrometry (SIMS) depth profiles of boron for C3v/wafer: (A; C) grafted in toluene or (B; D) grafted in dichloromethane (DCM). (A; B) As obtained for B-doped wafer, over the first 12 nm, for two different areas, at low impact energy (250 eV). (C; D) As obtained for the control wafer and C3v/wafer (after subtraction of the “control” depth profile) at high impact energy (3000 eV).

On the wafer, the incorporation of boron in the silicon wafer was performed by rapid thermal processing (RTP) of C2/wafer and C3v/wafer. Spikes (1 s) at 1000 and 1050 °C were carried out under a nitrogen atmosphere to drive in boron atoms. Note that oxygen addition to the gas mixture could have allowed higher annealing temperatures, without SiO volatilization, but would have also led to a detrimental regrowth of the silica layer; hence, it was not investigated. The sample surface was subsequently analyzed by atomic force microscopy (AFM) to observe the integrity of the silica surface. As a pitting effect was observed at 1050 °C (holes with a mean depth of about 11 nm, Figure S7, Supporting Information), further RTP experiments were performed at a reduced temperature of 1000 °C. After annealing both wafers at 1000 °C, the boron content of the oxide layer was investigated by the VPD-ICP/MS method, while the amount of boron that diffused inside the silicon matrix was quantified by secondary ion mass spectrometry (SIMS) analysis immediately after hydrofluoric acid (HF) etching of the oxide surface layer of the wafer. As shown in Figure 3, boron readily diffuses into the Si matrix. For C2/ wafer, 0.20 × 1014 B atom·cm−2 were still present in the SiO2 layer, while 0.04 × 1014 B atom·cm−2 diffused inside the Si matrix over the first 12 nm; it corresponds overall (i.e., B quantified in SiO2 and Si) to only 9% of the initial boron content. For C3v/wafer prepared in dichloromethane, 0.67 × 1014 and 0.32 × 1014 B atom·cm−2 are measured in SiO2 and in Si (at a depth of 12 nm), respectively, thus leading to an overall recovery of 39% (Table 1). Such a higher yield indicates an enhanced self-capping ability provided by the C3v ligand to keep boron on the substrate for RTP at 1000 °C. In addition, SIMS analyses performed at a greater depth (115 nm) revealed that a

with a native oxide layer. Following a thermal treatment similar to that used for SiO2 NPs (500 °C, 10−5 mbar), n-type lowdoped Si wafers were contacted with a 10 mM solution of C2 or C3v in toluene, giving C2/wafer or C3v/wafer, respectively. The boron content on the oxide layer of each wafer was quantified by VPD followed by ICP-MS revealing the presence of 2.78 × 1014 B atom·cm−2 for C2/wafer and an unexpectedly high Bcoverage (14.68 × 1014 B atom·cm−2) for C3v/Wafer. With C3v, grafting in dichloromethane yields a lower concentration of boron at the surface of the SiO2 layer (2.55 × 1014 B atom· cm−2). Noteworthy, B-density is similar for wafers (2D) and SiO2 NPs (3D). The robustness of Si−O−B bonds was then addressed by thermal treatment of the boron-doped SiO2 NPs. Monitoring of the thermal treatment of C2/SiO2 and C3v/SiO2 at 500 °C under dry air by IR spectroscopy (Figure S6, Supporting Information) shows in both cases a total disappearance of the bands associated with the organic ligand and the reappearance of the ν(SiO−H) mode alongside a new sharp band at 3702 cm−1, assigned to isolated boranol species BO−H bonded to silica by borosiloxane bridges.35,48 No boron losses were observed according to elemental analysis (see section SI 1 of the Supporting Information), while a previous work using chlorinated organoboron molecular precursors25 showed a 20% loss of boron under the same experimental conditions. It probably originates from the more robust linkage between C2-/C3v-based surface species and silica, leading to boranol species inserted in the SiO2 framework after calcination. In addition, most carbon was removed upon calcination (>99%), while about 15% are still observed on the silica surface in the aforementioned work. 13754

DOI: 10.1021/acs.jpcc.5b03408 J. Phys. Chem. C 2015, 119, 13750−13757


The Journal of Physical Chemistry C Table 1. Boron Quantified in the Thin SiO2 Layer or in the Si Matrix of Wafers, Before and After RTP, as a Function of the B-Containing Molecule Used for Doping boron content (× 1014 B atom·cm−2) after RTP (1000 °C)

B-containing molecule

on SiO2a

in SiO2a

in Si matrix

overall recovery (%)

C2 in toluene C3v in dichloromethane

2.78 2.55

0.20 0.67



0.04b 0.32b 1.40c 0.75c

9b 39b 81c 11c

C3v in toluene a

As quantified by VPD-ICPMS, reference was substrated from all values. bAs quantified by SIMS over the first 12 nm, reference was substrated. cAs quantified by SIMS over 115 nm, reference was substracted.

Figure 4. Tunneling spectra of C2/wafer, C3v/wafer (black), and control n-type wafer (red). Tunneling conditions: VS = 1.3 V, ITunnel = 1000 pA (black curves); VS = −3.0 V, ITunnel = 500 pA (red curve). Inset: Transport measurements performed with two STM tips in electrical contact with the surface of the wafers.

substantial amount of boron diffuses into the wafer. Indeed, 1.40 × 1014 B atom·cm−2 was found in Si, leading to an overall recovery of 81% (Table 1). The highest amount of boron recovered in the present study (inert atmosphere) is thus in sharp contrast with the aforementioned work25 reporting only 65% of overall recovery (air). Moreover, we emphasize that the boron dose that effectively diffuses in silicon after RTP (55%, i.e., B atoms in Si vs initial B content) is higher than previously obtained in published works (33%) using monolayer doping14,49 or spin-on polymer doping strategies.50 We also note that the incorporation of boron is higher when grafting occurs in dichloromethane rather than in toluene. Such a result demonstrates the need to choose the “right” solvent to graft the appropriate B-containing molecular precursor. Knowing that aryl-based molecules are prone to π-bonding, toluene probably favors the formation of multilayer or patches of surface species (physisorption) contrary to dichloromethane (a more polar solvent). While such multilayers give a high concentration of boron at the wafer surface, such boron species can easily desorb upon the RTP step and prevent boron from efficiently diffusing into the bulk material. Additional SIMS analyses performed on different locations of the C3v/wafer prepared with dichloromethane also reveal a greater homogeneity of the depth profiles in comparison with the C3v/wafer prepared with toluene (Figure 3B vs 3A). Eventually, the SIMS analysis of other impurities such as C, O, and Cl shows a very low amount of contaminants incorporated into the borondoped Si matrix with respect to the untreated n-type doped wafer (Figure S9, Supporting Information), avoiding detrimental compensation effects or premature aging by those contaminants once the device is operating.51 In order to ensure that boron impurities are electrically active in the Si matrix, the electronic properties of C2/wafer and C3v/ wafer were investigated with scanning tunneling microscopy (STM). Indeed, when no surface states exist in the forbidden gap of a semiconductor surface, n- and p-type layers can be clearly distinguished in the local current−voltage characteristics.52,53 Tunneling spectra were thus acquired in ultrahigh vacuum after a brief etching and passivation of the surface layer in dilute HF solution. Comparison of the spectra between the untreated n-type doped wafer, C2/wafer, and C3v/wafer shows an asymmetry of the I (V) characteristics with respect to the Fermi level (EF) (Figure 4), indicating different electrical behaviors. For the low-doped untreated wafer (1015 P atom· cm−3), the strongest component of the current is seen at positive bias. As this component corresponds to the flow of

electrons from the STM tip into the empty conduction band states and starts already at EF, this characteristic is consistent with an n-type doped wafer.54 An opposite behavior is clearly seen for C2/wafer and C3v/wafer. The strongest component corresponds this time to the flow of electrons from the occupied valence band states into the STM tip, indicating the formation of a p-type layer below the surface of C2/wafer and C3v/wafer. Furthermore, on the low-doped untreated wafer, a small component is first observed at negative bias, due to the tip-induced weak accumulation layer in the conduction band, before the contribution of the valence band occupied states increases the current for voltages lower than −1.5 V.54 The absence of such a weak component in the tunneling spectrum of C2/wafer reveals a strong hole accumulation layer below the surface that hides the onset between the contribution of the valence band states and the conduction band states at positive bias. It suggests a very high doping level, which is clearly highlighted by the transport measurements performed with two STM tips in contact with the surface of C2/wafer (inset of Figure 4). Indeed, a straight line through the origin is observed that sharply contrasts with the zero conductance measured over 3.8 V for the low-doped untreated wafer. This characteristic is typical of an ohmic conductor, in agreement with the formation of a degenerated p-type doped subsurface layer. Remarkably, the current branch at positive bias in the tunneling spectrum of C3v/wafer is even stronger, yielding a quasi-metallic behavior to this sample, in agreement with the high boron concentration measured by SIMS analysis.

CONCLUSION The use of boron molecular precursors that combine tailored structure (bulky ligand and capping protection) with anchoring ability on the surface of silica layers of non-deglazed Si wafers is beneficial to incorporate controlled doses of boron without carbon contamination. This functionalization approach allows for a molecular level description of the surface species using silica nanoparticles as models; it is thus amiable to structure− property relationship, and its transferability to dope nondeglazed Si wafers makes this process perfectly adapted to advanced microelectronics manufacturing processes (convenient doping dose, no degradation of electrical performances) without any additional oxide-capping step. 13755

DOI: 10.1021/acs.jpcc.5b03408 J. Phys. Chem. C 2015, 119, 13750−13757


The Journal of Physical Chemistry C

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S Supporting Information *

Experimental details; AFM images; IR and solid-state NMR spectra; schemes of surface species. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03408.


Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Present Address #

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6487, USA. Author Contributions ¶

These authors contributed equally.


The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the transversal CEA project Zero POVA and EQUIPEX program Excelsior for funding, S. Deleonibus (DRT/LETI), M. Sanquer (DSM/INAC), and L. Vandroux (DRT/LETI) for their continuous support. ETH Zürich, CPE Lyon, and CNRS are also acknowledged for their financial support and access provided to the facilities (Nanochemistry platform). J.D. is a postdoctoral fellow of the FWO-Flanders. C. Durand acknowledges the financial support of the DGA.


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