Control of Doping Level in Semiconductors via Self-Limited Grafting of

Dec 4, 2017 - An effective bottom-up technology for precisely controlling the amount of dopant atoms tethered on silicon substrates is presented. Poly...
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Control of Doping Level in Semiconductors via Self-Limited Grafting of Phosphorus EndTerminated Polymers Michele Perego,*,† Gabriele Seguini,† Elisa Arduca,†,‡ Andrea Nomellini,‡ Katia Sparnacci,*,§ Diego Antonioli,§ Valentina Gianotti,§ and Michele Laus§ †

Laboratorio MDM, IMM-CNR, Via C. Olivetti 2, I-20864 Agrate Brianza, Italy Università degli Studi di Milano, Via G. Celoria 16, I-20133 Milano, Italy § Università del Piemonte Orientale ‘‘A. Avogadro’’, Viale T. Michel 11, I-15121 Alessandria, Italy ‡

S Supporting Information *

ABSTRACT: An effective bottom-up technology for precisely controlling the amount of dopant atoms tethered on silicon substrates is presented. Polystyrene and poly(methyl methacrylate) polymers with narrow molecular weight distribution and end-terminated with a P-containing moiety were synthesized with different molar mass. The polymers were spin coated and subsequently end-grafted onto nondeglazed silicon substrates. P atoms were bonded to the surface during the grafting reaction, and their surface density was set by the polymer molar mass, according to the self-limiting nature of the “grafting to” reaction. Polymeric material was removed by O2 plasma hashing without affecting the tethered P-containing moieties on the surface. Repeated cycles of polymer grafting followed by plasma hashing led to a cumulative increase, at constant steps, in the dose of P atoms grafted to the silicon surface. P injection in the silicon substrate was promoted and precisely controlled by hightemperature thermal treatments. Sheet resistance measurements demonstrated effective doping of silicon substrate. KEYWORDS: doping, semiconductors, polymers, phosphorus, self-assembled monolayers conformal coverage of semiconductor surfaces.10 Consequently, it is fully compatible with nonplanar, restricted-dimension nanostructured materials, providing control and uniformity of doping profiles. The formation of sub-5 nm ultrashallow junctions in planar silicon was reported using both B, P, and As impurities.8,11−14 Furthermore, n- and p-type doping of semiconductors other than silicon (InAs,15 InGaAs,16−18 InP,19 Ge20) was achieved using different dopant impurities. Unfortunately, this methodology demonstrated limited capability to control the dopant areal dose, that is usually of the order of 1014 atoms/cm2, corresponding to a doping concentration of 1020 atoms/cm3 in the silicon substrate.8 Actually surface density of dopant-containing molecules in MLD is intrinsically limited by steric hindrance effects and availability of reactive sites on the pristine surface. For Pcontaining molecules, a maximum areal dose of ∼8 × 1014

D

oping represents the cornerstone of current semiconductor technology. Nowadays, precise control over dopant dose and distribution at the nanoscale is required to support further evolution in microelectronics,1 photovoltaics,2−4 solar fuel conversion, sensors, and quantum computation.5,6 Conventional doping technology by ion implantation process provides the capability to precisely control the dopant dose but induces severe crystal damages that make it incompatible with nanostructured materials. In addition, this technology cannot be applied to substrates presenting complex three-dimensional (3D) geometries. Conversely, diffusion approaches, using solid- and gas-phase sources, represent a gentle doping solution but lack of the desired uniformity and control over the areal dose of dopants for miniaturized device fabrication. Moreover, the presence of undesirable residues is a significant limitation of this technology.7 Recently, a technology based on self-assembled monolayers of dopant-containing molecules was proposed as a promising solution to overcome the limitations of conventional doping approaches.8,9 This nondestructive technique, usually referred to as monolayer doping (MLD), guarantees uniform and © XXXX American Chemical Society

Received: August 1, 2017 Accepted: December 4, 2017 Published: December 4, 2017 A

DOI: 10.1021/acsnano.7b05459 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of the doping process. Polymers end-terminated with a dopant-containing moiety are grafted on nondeglazed silicon substrates forming a self-assembled monolayer. After hashing of the polymeric chains, a SiO2 capping layer is deposited on the δ-layer of dopant atoms. Dopant drive-in and activation is induced by annealing the samples under various temperature and time conditions. Finally, the capping layer is removed by dipping in HF solution.

atoms/cm2 was reported.8,21 This value is close to the maximum number of P impurities that can be accommodated on silicon surfaces considering the density of available reactive sites.22,23 Slightly lower areal doses of dopant impurities were obtained using bulkier dopant-containing molecules with different chemical composition due to steric hindrance effects.8 However, optimization of the grafting process is required for each specific dopant-containing molecule to account for its thermal stability and specific chemical interaction with the target substrate. So far tuning of doping concentration in monolayer doping has been limited to the adjustment of process parameters.21 To overcome this limitation, alternative strategies were proposed. Modulation of dopant areal density of more than 1 order of magnitude was achieved by grafting dopant-containing and dopant-free molecules at various ratios.24 The dopant-free molecules reduce the density of dopant-containing molecules grafted to the substrate. Although this approach may be useful to control dopant concentration, the effective dopant dose on the semiconductor substrate is essentially determined by the processing conditions and not intrinsically regulated by the self-limiting nature of the grafting reaction. The final areal dose is the result of a competition between the grafting species and could be significantly influenced by the different grafting kinetics.25 Recently, spinon organic dopants for silicon have been proposed as an hybrid solution between monolayer doping technique and traditional inorganic spin-on dopants.26 This approach provides a simple and low-cost methodology to control the doping by properly adjusting the thickness of the polymeric layer and tuning the polymer chemistry. However, this approach is far from ideal, since accuracy in the dopant surface concentration is limited and consistent residues of the polymeric film are present on the silicon surface after the thermally promoted drive-in process.

In this work, an alternative approach is proposed (Figure 1) to precisely control the effective dose of impurity atoms in the dopant source by means of a self-limited “grafting to” reaction. In fact, the grafting process of functional polymers to a reactive surface nearly stops when the thickness of the brush layer approaches two times the polymer gyration radius in bulk due to entropic reasons.27 Accordingly, polymers with narrow molecular weight distribution and end-capped with a dopantcontaining moiety are specifically designed, prepared, and processed to form a polymeric brush layer onto nondeglazed silicon surfaces. By properly adjusting the molar mass (Mn) of the functional polymers, precise tuning of the dopant dose is expected. After removal of the polymer chains by hashing in O2 plasma and deposition of a SiO2 capping layer, dopant atoms can be driven-in by high-temperature thermal treatments to promote effective doping of the underlying silicon substrate. The possibility to further increase the dopant dose by means of repeated cycles of hashing-grafting is explored to investigate the capability to control the areal dose by iterated self-limited reactions. In particular polystyrene (PS) and poly(methyl methacrylate) (PMMA) polymers end-capped with a Pcontaining moieties were employed. A detailed study of the relation between the polymer characteristics, the monolayer features at the surface, and the dopant dose inside the silicon substrate upon annealing is presented.

RESULTS Materials. Hydroxy-terminated PS and PMMA polymers with different Mn were prepared by ARGET ATRP and subsequently reacted with diethyl chlorophosphate leading to the diethylphosphate (DPP) end-capped samples PSn-P and PMMAn-P where n indicates the Mn value. The yield of the coupling reaction with diethyl chlorophosphate is higher than 99%, as revealed by nuclear magnetic resonance (NMR) B

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all the polymers, and h increases rapidly over a short time period and then very slowly as the time increases, approaching a limiting thickness value (H). The value of the limiting grafting thickness H of the brush layer formed by tethered polymers is determined for each specific Mn (Figure 2b). H values are approximately twice the radius of gyration of the grafted chains in perfect agreement with previous results on the grafting of not entangled hydroxy-terminated random copolymers25 and homopolymers.28,29 Perfectly equivalent results were obtained by grafting the PSn-P samples onto deglazed silicon substrates. Starting from the measured H values and assuming that the density ρ of the polymers in the grafted film is equal to the one in bulk (ρ = 1.04 g/cm3), surface grafting density (Σ) of the PSn-P polymers is determined for all the grafted samples (Figure 2b). Σ values ranging from 2.8 × 1013 to 8.4 × 1013 chains/cm2 were obtained. PMMAn-P polymers exhibit similar evolution of h as a function of time (Figure S4), with H values of 5.7 ± 0.3 and 7.4 ± 0.6 nm for the PMMA8.6-P and PMMA14.1-P polymers, respectively. Correspondingly, Σ values of 4.7 ± 0.4 × 1013 and 3.7 ± 0.3 × 1013 chains/cm2 were calculated, assuming ρ = 1.18 g/cm3 for the PMMA films (Figure 2b). Phosphorus Quantification. After grafting, tethered polymeric chains were removed by O2 plasma hashing. The duration of the plasma treatment was optimized to ensure complete removal of the polymer brush layer (Figure S5). Finally, the samples were capped with a protective 10 nm-thick SiO2 film, resulting in the formation of P δ-layers embedded in a SiO2 matrix. Experimental details about SiO2 deposition are reported in the Methods section. Time of flight secondary ion mass spectrometry (ToF-SIMS) depth profiles were performed to quantify the amount of P atoms in the δ-layer (Figure 3a). Experimental data were fitted with a Gaussian distribution function. The fwhm of the fitting curve is ∼2.4 ± 0.5 nm for all the calibrated ToF-SIMS profiles. This fairly constant value is compatible with the depth resolution of the ToF-SIMS instrument and perfectly consistent with P atoms confined in

spectroscopy (Supporting Information, Figures S1 and S2). The chemical structures of the DPP end-capped PSn-P and PMMAn-P samples are shown in Figure S3. In particular, five PSn-P and two PMMAn-P samples were employed in this work. Their structural and molecular characterization is reported in the Supporting Information. The characteristics of the PSn-P and PMMAn-P samples are shown in Table 1. Mn values of the Table 1. Characteristics of Diethylphosphate End-Capped PS and PMMA Polymersa Polymer PS2.3-P PS3.0-P PS5.8-P PS14.2-P PS25.4-P PMMA8.6-P PMMA14.1-P

Mn 2.3 3.0 5.8 14.2 25.4 8.6 14.1

± ± ± ± ± ± ±

PDI 0.3 0.3 0.2 0.6 0.4 0.3 0.3

1.05 1.07 1.05 1.06 1.12 1.19 1.20

± ± ± ± ± ± ±

0.01 0.01 0.01 0.02 0.02 0.01 0.01

N 17 26 50 136 243 85 140

± ± ± ± ± ± ±

3 3 2 6 4 3 3

a

Mn is the number average molar mass (kg/mol), PDI is the polydispersity index, while N is the degree of polymerization.

PSn-P polymers range from 2.3 to 25.4 kg/mol, while Mn values of the PMMAn-P polymers are 8.6 and 14.1 kg/mol. All of the polymers exhibit narrow molecular weight distribution, with PDI values lower than 1.2. The degree of polymerization N for each polymer was also included in Table 1. Grafting Process. 30 nm-thick films of PSn-P or PMMAn-P polymers were spin coated onto 10 nm-thick SiO2 films on Si (100) wafers, upon cleaning of the SiO2 surface according to the procedure described in the Methods section. The polymercoated substrates were then annealed in ultrapure N 2 atmosphere at 250 °C for different times by means of a rapid thermal processing (RTP) machine to promote grafting. Finally, unattached polymer chains were removed by rinsing with toluene. The thicknesses (h) of the resulting polymer brush layers were measured by ellipsometry as a function of annealing time (Figure 2a). The h evolution is very similar for

Figure 2. (a) Thickness (h) of the grafted layer as a function of annealing time for PSn-P with different molar mass (Mn). Grafting was performed at 250 °C in a rapid thermal processing (RTP) machine. (b) Limiting thickness values (H) and corresponding grafting densities (Σ) for the different brush layers are reported for PSn-P (close symbols) and PMMAn-P (open symbols) as a function of Mn. C

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Figure 3. (a) Calibrated ToF-SIMS depth profiles of P δ-layer in SiO2 matrix for PSn-P polymers with different molar mass (Mn). Experimental data were fitted (dashed line) with a Gaussian distribution function. (b) P dose in the P δ-layer as a function of the grafting density (Σ) in the brush layer of PSn-P polymers. Data for the PMMA8.6-P and PMMA14.1-P polymers are reported as well. (c) Carbon contaminations in SiO2 as a function of Σ in the PSn-P and PMMAn-P brush layers. Data were normalized on the carbon contamination in reference samples prepared using conventional MLD technique.

Figure 4. (a) Calibrated ToF-SIMS depth profiles of P δ-layers obtained after repeated grafting-hashing cycles of PS2.3-P. Experimental data were fitted (dashed line) with a Gaussian distribution function. (b) P dose in the P δ-layers as a function of the number of grafting-hashing cycles for PS2.3-P and PMMA14.1-P. (c) Carbon contaminations in the SiO2 layer as a function of the number of grafting-hashing cycles for the same samples. Data were normalized on the carbon contamination in reference samples prepared using conventional MLD technique.

effective P areal doses in the δ-layers directly correlate with the grafting density of the PMMAn-P polymers in the corresponding brush layers (Figure 3b). Carbon contaminations in conventional MLD process are limited to the first atomic layer of the silicon substrate and consequently can be easily removed.30 However, they have to be minimized in order to avoid dopant deactivation due to C− P pair diffusion into the silicon substrate.31 Rutherford back scattering analysis detected very low carbon signals in the overall SiO2 layer for P δ-layers prepared using conventional MLD. The intensity of the carbon signal is of the order of

a δ-layer embedded in the SiO2 matrix. A progressive increase in P concentration was observed when decreasing Mn of the PSn-P polymers. Integrating the calibrated ToF-SIMS profiles, the effective P areal doses in the δ-layer were calculated (Figure 3b). A linear correlation between the P areal doses and the grafting density of the PSn-P polymers was observed. The amount of P atoms in the δ-layer can be controlled by proper selection of Mn for the PSn-P polymers, with P areal dose values ranging from 3 × 1013 to 8 × 1013 atoms/cm2. ToF-SIMS calibrated depth profiles of the δ-layers obtained using PMMAn-P polymers were acquired as well (Figure S6). The D

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areal dose of P atoms in the δ-layer (Figure 5). Doping profiles can be readily tuned by adjusting the initial areal dose of P

magnitude of adventitious carbon deposited on the surface of the samples during air exposure, thus preventing proper quantification of the carbon contamination introduced during δ-layer formation. Carbon contamination profiles in the SiO2 layer were monitored by ToF-SIMS for the P δ-layers prepared using DPP end-capped polymers. The level of carbon contamination is constant and independent of the thickness of the original polymer brush layers (Figure 3c). Data were normalized on carbon contamination detected in a reference sample prepared using conventional MLD technique (Figure S7). Interestingly, detected carbon contaminations are much lower than in the reference sample, with carbon signals that are only ∼20% of those observed in the case of conventional MLD (Figure 3c). These data further demonstrate complete removal of the polymers prior to SiO2 capping layer deposition. Duration of plasma treatment was optimized for each specific polymer to minimize organic residues on the surface. Overexposure of the samples to prolonged O2 plasma treatments does not modify the dopant dose in the samples, demonstrating a stable bond between grafted P atoms and SiO2 substrate. ToF-SIMS depth profiles were performed to quantify the amount of P atoms in the P δ-layers obtained by repeated grafting-hashing cycles of PS2.3-P polymer (Figure 4a). Integrating the calibrated ToF-SIMS profiles, the effective P areal doses in the P δ-layers were calculated. Stepwise increase in the amount of P atoms in the δ-layer from ∼8 × 1013 to ∼3 × 1014 atoms/cm2 was observed (Figure 4b). Fine tuning of the P areal dose is possible by using DPP end-capped polymers with larger Mn. ToF-SIMS calibrated depth profiles of the P δlayers obtained using the PMMA14.1-P polymer samples were acquired as well (Figure S8) and integrated to determine the effective P areal doses in the δ-layers. Interestingly, stepwise increase in the amount of P atoms in the δ-layer from ∼3 × 1013 to ∼8 × 1013 atoms/cm2 was obtained by repeated grafting-hashing cycles of PMMA14.1-P polymer (Figure 4b). Moreover, residual organic contaminations were monitored as a function of the number of grafting-hashing cycles, demonstrating that, irrespective of the number of iterated grafting-hashing cycles, the level of carbon contamination is roughly constant ∼20% of the carbon contamination detected in the reference sample prepared using conventional MLD (Figure 4c). Phosphorus Diffusion. To demonstrate effectual doping of silicon, P δ-layers with different initial P areal doses were prepared by repeated grafting-hashing cycles of PS2.3-P polymers on nondeglazed Si (100) substrates. Once completing the grafting-hashing cycles, 10 nm-thick SiO2 films were deposited as a protective capping layer by e-beam evaporation. P atoms were diffused into the silicon substrate by thermal treatments in a RTP machine at different temperatures for 5 s with heating rate of 50 °C/s. High annealing temperatures (TA) ranging from 1000 to 1250 °C were used to overcome the diffusion barrier for P atoms that is associated with the intervening native oxide layer.32 ToF-SIMS measurements were carried out to characterize the doping profile of the thermally diffused P atoms in silicon. No P drive-in was observed by ToFSIMS depth profiling in samples thermally treated at temperatures TA < 1100 °C due to the low P diffusivity in SiO232 which restricts P concentration in the silicon substrate below the sensitivity limit of ToF-SIMS instrument. Conversely, samples annealed at TA ≥ 1100 °C exhibit a high surface concentration that sharply decreases to 1018 atoms/cm3 at depths ranging from 30 to 120 nm depending on the initial

Figure 5. ToF-SIMS calibrated depth profile showing P distribution in the silicon substrate upon annealing at 1200 °C for 5 s. The dopant source was obtained by five cycles of grafting-hashing on a not deglazed Si (100) substrate using the PS-P2.3 polymer. Grafting was performed at 250 °C in N2 atmosphere. A 10 nm-thick SiO2 capping layer was deposited on the sample to prevent P outdiffusion during the drive-in process. SiO3− and 30Si− secondary ion signals are used to identify the SiO2 matrix and the underlying Si substrate. P calibrated profile was fitted (black dashed line) assuming a limited source model.

atoms in the δ-layer and the RTP processing conditions. P profiles fit the trend expected for limited source modeling. Diffusivity coefficients DP at different temperatures were extracted from the fitting of the P profiles obtained at TA ≥ 1100 °C for tA = 5 s. The temperature evolution of DP is consistent with the Arrhenius’ dependence DP(TA) = DP0 exp(−ED/kTA) with ED = 3.2 ± 0.3 eV in perfect agreement with literature data.7 The total dose of P atoms injected into the silicon substrate was obtained by integration of ToF-SIMS P concentration profiles. The dose of injected P atoms was monitored as a function of TA (Figure 6a) for P δ-layers obtained by 1, 3, and 5 repeated grafting-hashing cycles. For all of the samples, a progressive increase in the dose of injected P atoms is observed as TA increases. The maximal dose of P atoms, corresponding to 50% of the initial areal dose, was injected into the silicon substrate by annealing the substrate at TA = 1250 °C for 5 s. Assuming a symmetric P diffusion in the SiO2 matrix, this value corresponds to the maximum dose of P atoms that can be injected into the substrate considering a limited reservoir of dopants in the P δ-layer. Data demonstrate that the maximum dose injected into the Si substrate is directly related to the amount of P initially available in the δ-layer. Electrical Testing. In order to confirm the incorporation of electrically active impurities into the nearly intrinsic Si (100) substrate, four points probe measurements were carried out obtaining sheet resistance (Rs) at various processing conditions. The evolution of Rs values in P doped Si substrates was monitored as a function of TA (Figure 6b). The experimentally E

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Figure 6. (a) P dose injected in the silicon substrate as a function of the annealing temperature for different P δ-layers obtained with 1, 3, and 5 grafting-hashing cycles of PS2.3-P polymer. (b) Sheet resistance measurements as a function of the annealing temperature for P δ-layer with an initial area dose of 8 × 1013 atoms/cm2 obtained with a single cycle of grafting-hashing of PS2.3-P polymer.

determined Rs value of the pristine Si (100) substrate is 8 × 105 Ω/□. Upon thermal treatment, Rs values sharply decrease approaching a saturation value of 1.4 × 103 Ω/□ for annealing at TA = 1250 °C for 5 s, owing to the limited P reservoir in the δ-layer. Evolution of measured Rs values is perfectly consistent with data reported in the literature for mixed monolayer doping,24 spin on doping,26 or monolayer contact doping.33

sacrificial elements acting as spacers to tune the grafting density of the dopant-containing molecules. These molecules are used as carriers to effectively deliver dopant impurities onto silicon surfaces. Fine tuning of the spacer molecules allows modulating the density of dopant atoms on the silicon substrate without affecting the chemistry of grafting that can be independently optimized by proper selection of the carrier molecules to address specific substrate composition. The stacking of spacer and dopant carrier molecules guarantees accurate control of the areal dose of P atoms through the self-limiting nature of the grafting process. In particular, operating in the range of Mn values that allows avoiding entanglement of the polymeric chains in the deposited film, it is possible to increase the P areal dose from ∼3 × 1013 to ∼8 × 1013 atoms/cm2 by reducing the Mn of the PSn-P polymers used as spacer from 25.2 to 2.3 kg/ mol. In principle, additional increase in the P areal dose by further reduction of Mn is still possible. However, the possibility to repeat the grafting process after O2 plasma hashing, in combination with the control of the grafting density of the dopant-containing polymers provided by proper selection of the Mn value, guarantees a very fine-tuning of the areal dose of P atoms on nondeglazed silicon surfaces. The spacer molecules can be easily removed by O2 plasma hashing without affecting the distribution of the dopant-containing molecules covalently bonded to the substrate. Their removal exposes the reactive sites present on the underlying substrate, making them available for further grafting processes. A stepwise increase in the dopant areal density can be accomplished by repeated grafting-hashing cycles until saturation of the reaction sites on the substrate. The discrete amounts of P atoms bonded to the surface at each cycle can be precisely determined beforehand by selecting the appropriate combination of Mn values for the polymers used during each grafting cycle. In conventional MLD, tuning of the dopant areal dose is possible by changing the dopant-containing molecules. In the original paper by Ho et al., two different P-containing molecules are tested corresponding to a variation of the areal dose from

DISCUSSION The proposed doping approach based on the use of functional polymers to be grafted to the silicon substrate by means of dopant-containing moieties has several distinct advantages over standard MLD. The original procedure proposed by Ho et al. is based on a “grafting to” process from solution.8 This wet approach is intrinsically slow, since the processing temperature is limited by the presence of liquid solvent. This limitation is removed in the case of a “grafting to” process from melt, like the one we developed using PSn-P and PMMAn-P polymers. Grafting at relatively high temperature (T = 250 °C) leads to a polymer brush layer featuring a uniform surface coverage in the time scale of few minutes.34 Moreover the “grafting to” process from melt has been widely explored to form polymeric brush layers over solid surfaces, demonstrating the capability to effectively tune the density of the grafted molecules over a quite broad range of values that are set by constrains of entropic origin.27 Actually at saturation, the penetration of polymeric chains into the brush layer and their reaction with the underlying substrates are naturally inhibited by the entropic penalty associated with the stretching of the grafted polymeric chains.35 As a consequence, provided that there is a sufficient amount of reactive sites on the substrate, the density of the grafted molecules in the brush layer is inversely proportional to the Mn of the grafting polymers. In this respect, polymers end-terminated with dopantcontaining moieties provide more precise control of the dopant areal dose than conventional spin on dopant approaches and guarantee the tunability of the dopant areal dose over almost 2 orders of magnitude. In this approach, the polymeric chains are F

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ACS Nano 7.9 × 1014 at/cm2 to 1.3 × 1014 at/cm2, using diethyl 1propylphosphonate (DPP) and trioctylphosphine oxide (TOP), respectively.8 Apart from the limited variation of dopant areal dose compared to the one that can be achieved with the proposed methodology, the main drawback of this approach is that different molecules have to be designed and tested to modify the dose, impacting on the thermodynamic and kinetics of the grafting reactions. Optimization of the grafting process would be required for each specific dopantcontaining molecule to account for its thermal stability and reactivity, making dose variation extremely complex. The mild approach based on polymers end-terminated with dopant-containing moieties presents all the advantages of conventional solid-state diffusion methodologies together with the accurate and precise control of the dopant dose that is usually guaranteed by ion implantation and represents a significant step forward to deterministic doping of semiconductors. Actually, the combination of this approach with conventional lithographic technology is expected to provide accurate control of dopant lateral distribution over the surface. The possibility to integrate standard MLD process with lithographic technology to control lateral distribution of dopant impurities at microscale has already been demonstrated.10,36 Recent works proved that grafting of polymers on selected areas of the substrate is feasible with nanometer resolution upon proper functionalization using conventional lithographic approaches.10 Accordingly we expect that the proposed doping technology, based on functional polymers end-capped with a dopant-containing moiety, can be easily integrated in lithographic production lines, providing control over the lateral distribution of dopant atoms with sub-10 nm precision.

ethyl]amine (Me6TREN), for PS, or CuBr2/tris(2-pyridylmethyl)amine (TPMA), for PMMA, complexes in the presence of tin(II) 2ethylhexanoate (Sn(EH)2) as the reducing agent following the procedure previously reported.25 Polymers synthesized with this initiator are terminated on one end with a moiety containing a hydroxyl (OH) group and on the other end with a Br group. In all samples, the initiator/copper catalyst/ligand/reducing agent molar ratios were kept constant (1/0.02/0.22/0.20 [HEBIB]0/[CuBr2]0/ [Me6TREN]0/[Sn(EH)2]0 for PS and 1/0.04/0.24/0.20 [HEBIB]0/ [CuBr2]0/[TPMA]0/[Sn(EH)2]0 for PMMA), whereas by varying the reaction time and the monomer/initiator molar ratio, polymers with different molar masses, ranging from 2.3 to 25.4 kg/mol, were obtained.25 The synthetic details for the various samples are listed in Table 1S. The hydroxyl terminal group is subsequently reacted with a phosphorylating agent, namely diethyl chlorophosphate, to obtain a diethylphosphate (DPP) functional group covalently attached to the polymer chain end. In detail, 1.0 g of hydroxyl-terminated polymer was dissolved in 15 mL of anhydrous dichloromethane in a 50 mL threenecked flask, under magnetic stirring and nitrogen flux, then a 10-fold molar amount of distilled triethyl amine was added, and the system was cooled to 0 °C. The appropriate amount of diethyl chlorophosphate (10-fold molar ratio with respect to the polymer) was added dropwise, then the system was allowed to reach room temperature, and the reaction was performed overnight. The reaction mixture was diluted with 30.0 mL of dichloromethane, washed with 50.0 mL of an ice-cold aqueous solution (1.0 wt %) of NaOH, followed by ultrapure water until neutral pH, then the organic solution was dried over anhydrous sodium sulfate, and the polymer was recovered by precipitation into ice cold methanol. The DPP endcapped polymers were then washed three times with methanol, to remove any contaminants, and then dried under vacuum at room temperature. All polymers were characterized by gel permeation chromatography (GPC) analysis to obtain information on their molar mass and DPI. Nuclear magnetic resonance (NMR) spectroscopy was performed, collecting 13C NMR and 31P NMR spectra to assess the yield of phosphorylation reaction (Supporting Information). Wafer Cleaning. Single-side polished Si (100) wafers were cleaved into 1 × 1 cm2 pieces and cleaned by sonication in isopropyl alcohol for 300 s, followed by O2 plasma treatment for 300 s. The thickness of the native oxide layer was measured by a M-200U spectroscopy ellipsometer (J.A. Wallom Co., Inc.) using a xenon lamp at a 70° incident angle. The thickness of the native oxide layer was found to be 1.7 ± 0.2 nm. Polymer Grafting. The polymers (18.0 mg in solution with 2.00 mL of toluene) were spin coated on the substrates for 30 s at 3000 rpm resulting in ∼30 nm-thick films. Before polymer deposition, the substrates were cleaned by sonication in isopropyl alcohol and subsequently treated in O2 plasma. In order to promote the grafting reaction, the samples were then annealed using a rapid thermal processing (RTP) apparatus for different time periods at 250 °C in a N2 atmosphere. Thermal stability of the polymers end-capped with a P-containing moiety was tested, demonstrating no evidence of degradation at this specific grafting temperature (Figure S9). Samples were washed in an ultrasonic bath in toluene to remove the nongrafted chains. Finally, the grafted substrates were dried under N2 flow. The thicknesses of the initial polymers films and of the resulting grafted layers were measured by means of a spectroscopy ellipsometer. Silicon Dioxide Deposition and Thermal Treatments. SiO2 films were deposited in an electron-beam evaporation system operating in the high-vacuum regime (base pressure 5 × 107 mbar). The thickness of the evaporated SiO2 films was in situ monitored by a quartz microbalance and subsequently checked by spectroscopy ellipsometer analysis. The RTP treatments for the grafting of the polymers were performed in a Jipelec, JetFirst Series system. The process consists of a three-step treatment (a heating ramp, a plateau, and a cooling ramp) in N2 atmosphere. In all of the thermal treatments, the heating ramp was set at 18 °C s−1. In order to promote P diffusion through the SiO2 barrier into the Si substrate, RTP treatments were performed at different target temperatures ranging from 1000 to 1250 °C. The samples were heated at 50 °C/s and kept

CONCLUSIONS In conclusion, this work demonstrates that functional polymers end-terminated with a dopant-containing moiety can be used for the synthesis of a dopant source. Taking advantage of the self-limiting nature of the “grafting to” process from melt, precise control of the areal dose and spatial distribution of dopants can be achieved by selection of the polymer Mn. Polymer hashing in O2 plasma is extremely effective in removing the polymer chains, preserving the dopant covalently attached to the sample surface and limiting carbon contaminations. Stepwise increase of the areal dose is demonstrated by repeated cycles of grafting-hashing. Efficient injection of dopant atoms into the underlying substrate is possible by proper selection of the annealing conditions during the drive-in process. The method was demonstrated for P-containing molecules on planar deglazed and nondeglazed Si substrates, but the it can be readily implemented to other types of dopants and semiconductor substrates by using suitable combination of dopant-containing moieties and surface chemistry. An easy extension of the approach to nanostructured materials featuring a wide range of geometries may be envisioned. The high stability of covalently attached monolayers and the relative mild conditions required for their formation suggest the approach may find direct application to the doping of semiconductor materials for nanoelectronic and solar energy related devices. METHODS Materials. Polystyrene (PS) and poly(methyl methacrylate) (PMMA) polymers with narrow molecular weight distribution were synthesized by ARGET-ATRP initiated by 2-hydroxyethyl(2-bromoisobutyrate) (HEBIB) and catalyzed by CuBr2/tris[2-(dimethylamino)G

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ACS Nano at the target temperature for 5 s before cooling down to room temperature. Phosphorus Depth Profiling. Time of flight secondary ion mass spectrometry (ToF-SIMS) analysis was performed using a dual beam ION-TOF IV system. Sputtering was accomplished by Cs+ ions at 1 keV and 80 nA. The analysis was performed in negative polarity by using Ga+ ions operating at 25 keV and 1 pA. Accurate time-to-depth conversions for SiO2 and Si were performed by measuring the average sputter velocity in a 20 nm-thick SiO2 and in a 16 nm-thick silicon on insulator reference samples, respectively.37 Calibration of the 31P− signals in the SiO2 and Si matrices was performed, applying protocols previously developed and described in detail elsewhere.32,38 Electrical Characterization. Van der Pauw measurements were performed on square-shaped 1 × 1 cm2 samples on which tungsten probe tips were exactly located at the four corners. Keithley 6221 current source and Keithley 2182A nanovoltemeter units and a custom-written Labview script were employed to generate and collect current/voltage data.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05459. More details about polymer characterization by GPC and NMR. ToF-SIMS depth profiles of P δ-layers obtained by PMMAn-P polymers. Representative TGA-GC-MC data about thermal stability of the polymer films (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Michele Perego: 0000-0001-7431-1969 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors want to acknowledge Prof. Enrico Napolitani e Prof. Davide De Salvador (University of Padova) for RBS measurements and for fruitful discussions and Fabio Zanenga (CNR) for the help in the calibration of the etching conditions. REFERENCES (1) International Technology Roadmap for Semiconductors (ITRS) Executive Summary, 2013. http://www.itrs2.net/2013-itrs.html, accessed July 22, 2017. (2) Elbersen, R.; Vijselaar, W.; Tiggelaar, R. M.; Gardeniers, H.; Huskens, J. Fabrication and Doping Methods for Silicon Nano- and Micropillar Arrays for Solar-Cell Applications: A Review. Adv. Mater. 2015, 27, 6781−6796. (3) Caccamo, S.; Puglisi, R. A.; Di Franco, S.; D’Urso, L.; Indelicato, V.; Italia, M.; Pannitteri, S.; La Magna, A. Silicon Doped by Molecular Doping Technique: Role of the Surface Layers of Doped Si on the Electrical Characteristics. Mater. Sci. Semicond. Process. 2016, 42, 200− 203. (4) Puglisi, R. A.; Garozzo, C.; Bongiorno, C.; Di Franco, S.; Italia, M.; Mannino, G.; Scalese, S.; La Magna, A. Molecular Doping Applied to Si Nanowires Array Based Solar Cells. Sol. Energy Mater. Sol. Cells 2015, 132, 118−122. (5) Prati, E.; Kumagai, K.; Hori, M.; Shinada, T. Band Transport across a Chain of Dopant Sites in Silicon over Micron Distances and High Temperatures. Sci. Rep. 2016, 6, 19704. H

DOI: 10.1021/acsnano.7b05459 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.7b05459 ACS Nano XXXX, XXX, XXX−XXX