Mechanism of Phosphorus Transport through Silicon Oxide during

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Mechanism of Phosphorus Transport through Silicon Oxide during Phosphonic Acid Monolayer Doping Roberto C. Longo, Kyeongjae Cho, Peter Thissen, and Siegfried Hohmann J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02545 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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The Journal of Physical Chemistry

Mechanism of Phosphorus Transport Through Silicon Oxide During Phosphonic Acid Monolayer Doping

Roberto C. Longo,1 Kyeongjae Cho,1 Siegfried Hohmann2, and Peter Thissen2,* 1

Department of Materials Science and Engineering, University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080, USA

2

Karlsruher Institut für Technologie (KIT), Institut für Funktionelle Grenzflächen (IFG), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany *corresponding author: [email protected]

Abstract Monolayer doping (MLD) is a relatively new method to incorporate shallow dopants from an adsorbed organic monolayer. To prevent evaporation of the dopant-containing organic layer during thermal processing, an oxide capping layer has typically been used, without clearly understanding surface mass transport. In this work, we investigate the thermal evolution of a phosphoruscontaining organic layer grafted on oxide silicon surfaces, to determine whether phosphorus can diffuse through the oxide into silicon in the absence of a capping oxide layer. Self-assembled monolayers (SAM) of phosphonic acid are grown by tethering by aggregation and growth (T-BAG) on native oxide-terminated silicon wafers, and in situ characterization is performed by infrared spectroscopy and X-ray photoelectron spectroscopy, with complementary ex situ time-of-flight secondary ion mass spectrometry and impedance spectroscopy measurements, supported by ab initio density-functional theory (DFT) calculations. We find that annealing to 700 K initiates a selfdecomposition of the chemisorbed phosphonic acid molecules at the SAM/oxide interface. As the temperature is further increased, the P-C bond, which is the weakest link of the adsorbed molecule, breaks and releases the organic ligand, followed by a molecular rearrangement of the bonding configuration. Then, phosphorus transport through the silicon oxide is mediated by PO3-x species, further driven by the transformation of the native silicon oxide to a thermal silicon oxide phase. At 1023 K, diffusion of phosphorus into the sub-surface region of silicon is finally observed, without evidence for P desorption or C contamination. Our DFT results provide a mechanistic understanding of the pathway followed by the phosphorus atoms. Together, these findings provide a fundamental platform for MLD of silicon and other semiconductors in general.

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Introduction The scaling of the dimensions of modern electronic devices as low as the atomic or molecular regime raises new fundamental and technological challenges such as the formation and control of welldefined interfaces, contacts, composition and electrical properties at the atomic scale 1. Frequently, the electronic properties need to be modified after fabrication. For example, to increase the conductivity, dopant atoms are incorporated and activated in the semiconductor crystal structure 2. In fact, semiconductor doping is of utmost importance in many fields, such as nanoelectronics 3, photovoltaics 4, sensors 5, organic light emitting diodes and many other. Controlling the dopant profiles at the nanoscale represents an enormous challenge for both the semiconductor industry and materials science community. Moreover, the transition to non-planar architectures, like FinFETs (fin-shaped field effect transistors) or nanowire-based transistors, requires spatially more confined doping, e.g., reproducible shallow doping with narrow profiles, which are not attainable with conventional doping methods such as ion implantation 6. Among more recent methods such as spin-on doping, solid-source diffusion or even in situ CVD (chemical vapor deposition), monolayer doping (MLD) presents an attractive option with better control of the atomic composition at the nanoscale. Promising results have therefore generated strong interest in the scientific community 7,8. In MLD, the doping step is separated from the nanoscale building block synthesis, thus avoiding damage to the crystal lattice, which is critical for nanometer scale doping. The MLD process comprises several steps: i) preparation of an oxide-free semiconductor surface, ii) functionalization of the surface with a molecule containing the desired (p or n) dopant, and iii) thermal diffusion of the dopant into the near surface region (~nm). The self-limiting nature of selfassembled monolayers (SAMs) of MLD provides a uniform coverage of a well-defined quantity of dopant atoms. A subsequent high temperature annealing step drives the dopant atoms into the semiconductor via a diffusion mechanism. To avoid SAM desorption, an oxide capping layer (e.g., SiO2 or Al2O3) is typically deposited on top of the SAM, which unfortunately also traps carbon from the organic molecule, leading to the deactivation of the electrical activity of the phosphorus dopants 9 . Alternatively, the silica-like architecture or some molecular precursors allows the in situ generation of a SiO2 capping nanolayer before annealing 10. It was recently shown that a capping layer may not be necessary if the SAM is adsorbed directly on an oxide-free semiconductor surface, such as H-terminated Si surfaces. Specifically, organophosphonic or -arsenic acid SAMs were grafted on atomically flat Si(111) surfaces to achieve shallow P or As doping 11,12. In this case, the structure of the surface is very well defined, and the reaction pathway leading to P incorporation into the crystalline Si substrate could be determined using ab initio density-functional theory (DFT) calculations. Moreover, we have also shown how stepwise reactions of group-V-molecular precursors can be used to tailor the electronic properties of the target semiconductor substrate due to the instability of the corresponding binary and ternary oxides after annealing at high-temperatures 13. The requirement for an oxide-free surface, however, is prohibitive for many applications. For Si, it is typically accomplished using HF and/or NH4F solutions, that are highly toxic and corrosive chemicals and represent a challenge for high-volume manufacturing 14. More fundamentally, it is unclear that dopant atoms can penetrate through native oxides, since oxides are used to cap dopant-containing SAMs in conventional MLD processes. In this work, we investigate the feasibility of performing MLD directly on oxidized Si substrates using otherwise conventional SAM-grafting procedures followed by spike annealing. We used the wellACS Paragon Plus Environment

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characterized tethering by aggregation and growth (T-BAG) method for grafting of octadecylphosphonic acid (ODPA) on oxidized Si(111) surfaces 15,16 as a prototypical system for Pdoping of Si using a SAM grafted onto its native oxide. In order to develop an overall picture of the MLD process, here we combine in situ infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) with ex situ time-of-flight secondary ion mass spectrometry (ToF-SIMS) and impedance spectroscopy (IS), we experimentally monitor the chemical state of the adsorbed SAM, the electronic properties of the substrate, and the resulting diffusion profiles at every step of the annealing process. Furthermore, the atomistic diffusion pathway and driving force for P atoms through the SiO2 into the Si bulk is modeled in detail by first-principles DFT calculations. We unambiguously demonstrate that MLD of Si can be achieved without removing the native oxide or depositing a thick SiO2 ad-layer cap on the molecular monolayer at the beginning. Oxide-MLD also offers several advantages over conventional MLD, such as ease of sample processing, superior ambient stability, and minimal carbon contamination 17. The fundamental understanding of the MLD mechanism at various stages of the process provides a quantitative platform for achieving conformal coatings and subsequent shallow doping of semiconductors in the future.



Materials and Methods Sample Preparation 3 × 1 cm2 samples diced from lightly doped (0.1-1 Ω·cm), double-side polished, Float Zone (FZ) Si (111) wafers were first degreased by sequential sonications in dichloromethane, acetone, and methanol for 10 minute each, and subsequently immersed in piranha solution [H2SO4 (Fisher 98%):H2O2 (Fisher 30 %) 3:1] for 30 min at ≈80 °C to obtain highly hydrophilic, OH-terminated and clean surfaces. The T-BAG treatment was performed using a previously reported method 12; briefly, the sample was immersed in a mM solution of octadecylphosphonic acid (ODPA powder, Aldrich 97%) in tetrahydrofuran (THF, Aldrich 99.9%), below the critical micelle concentration (CMC) in a glass tube. The solution was heated in a water bath at 60 °C until complete evaporation was achieved (≈12 h). Following evaporation, the sample was loaded into an ultrahigh vacuum (UHV) chamber and resistively heated to 150 °C for 1 min to chemically absorb the ODPA to the SiO2 surface. The sample was subsequently taken out and rinsed in THF to remove any weakly adsorbed ODPA, before being loaded into the UHV system. Fourier Transform Infrared Spectroscopy FTIR measurements were performed using several different optical configurations. Ex situ measurements were performed in a transmission mode with the IR beam incident at 74° with respect to the surface normal (i.e., Brewster’s angle for Si) using a Bruker Vertex 70 spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector situated inside a N2-circulating glovebox. In situ measurements were performed in an UHV chamber (base pressure 4×10-10 torr) equipped with differentially pumped KBr windows for IR absorption measurements. The experiments were performed in the Brewster’s angle transmission geometry with a Bruker Vertex 70 spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector. The sample temperature was controlled via resistive heating of the Si substrate, and the temperature was monitored with a K-type (chromel-alumel alloy) thermocouple spot-welded onto a Ta clip that was attached in the middle of the long edge of the substrate. The spectra were generated from 512 co-added scans collected at 4 cm-1 resolution. The samples were annealed at a rate of 50-100K/min, with 60s dwell time at the target temperature. X-Ray Photoelectron Spectroscopy XPS analyses were performed with a XPS/AES/UPS-system of the company PREVAC , Poland equipped with a hemispherical analyzer R4000 of VG Scienta Ltd., UK under ultra-high vacuum conditions (10−10 mbar) using an Al Kα1,2 (1486,3 eV) x-ray radiation source. Spectra were recorded with a pass energy of 200 eV for survey and 100 eV for detailed high-resolution spectra of the P2p, Si2p, O1s, and C1s core level regions at 0° take-off angle with respect to the surface. The step width and the dwell time per step were defined as 0.05 eV and 100 ms, respectively. To compensate for charging a flood gun with electron energy of 2eV was used. Deconvolution of peaks was done using CASA XPS software whereas peaks were fitted using Shirley background, Gaussian/Lorentzian (GL) line shapes and a Marquardt-Levenberg optimization algorithm. Surface carbon was used to calibrate the binding energy of C 1s (C-C, C-H) to 285 eV. Time-of-Flight Secondary-Ion-Mass-Spectrometry


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The ToF-SIMS analysis was carried out on a gridless reflectron-based ToF-SIMS V (ION-TOF GmbH, Muenster, Germany), equipped with a bismuth-cluster ion source. All spectra and images were obtained using Bi+3 primary ions at 25 keV energy in the high current bunched mode, with a mass resolution of m/Δm ≥ 6000. The beam diameter was about 3 − 5 μm. The 2pA Bi primary beam was rastered on a 300×300 µm2 field of view, and 128×128 pixel were recorded. Depth profiling was performed in full interlaced mode with a 104 nA and 1 keV energy Cs+ beam, rastered across 600×600 µm2. Impedance Spectroscopy Impedance spectroscopy of the ODPA-functionalized and annealed samples was performed using an IMPEDANCE ANALYZER IM3570 (HIOKI) R, L, C measuring device operated at room temperature in the frequency range of 5 Hz–500 MHz, using titanium clamping contacts with the diameter of 1 mm. The measurements were performed in a glovebox. Computational Methods The calculations were performed using DFT within the generalized gradient approximation (GGA) as implemented in the Vienna ab initio simulation package (VASP) 18,19. The electron-ion interaction was described within the projector augmented wave (PAW) scheme 20 and the electronic wave functions were expanded into plane waves up to a kinetic energy of 400 eV. The surface used in the calculations is the Si(111), with a bulk-like reconstruction. The lateral dimensions of the supercell are 11.78x11.78 Å2, with a 120° angle between the x and y axis. The surface was modeled by periodically repeated slabs, with the supercell consisting of 8 atomic layers of Si plus one monolayer of hydrogenterminated SiO2 (with a surface density of 0.06 OH/Å2) and the corresponding adsorbed molecules. The vacuum region is equivalent to 16 atomic layers, in order to avoid interactions between neighboring images due to periodic boundary conditions. The 7 uppermost layers of Si as well as the SiO2 and the adsorbate degrees of freedom were allowed to relax until the forces on the atoms were below 10 meV/Å. The Brillouin zone integration was performed using a 4x4x1 mesh within the Monkhorst-Pack scheme 21, and the PBE functional was used to describe the electron exchange and correlation energy within the GGA 22. The Eigenmodes were calculated with the force constant approach, by diagonalizing the mass-weighted second derivative matrix (Hessian) for the adsorbed species and the top SiO2 layer. The restriction to the atoms of the top layer and the adsorbed species is well justified because the Eigenmodes of these atoms do not overlap with the Eigenmodes of the bulk material. The kinetic barriers were calculated with the Climbing Image Nudged Elastic Band (cNEB) method 23,24, using a string of geometric configurations to describe the reaction pathway of the system. A spring interaction between every configuration ensured the continuity of the reaction pathway.




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Results and Discussion Temperature-dependent IR, XPS, ToF-SIMS and IS measurements IR spectra of ODPA-grafted SiO2/Si are collected as a function of the annealing temperature under UHV, and are all differential, i.e., referenced to the spectrum taken before the last annealing step. The spectrum taken from the as-grafted surface shows CHx stretching modes (2800-2950 cm-1) typical of the aliphatic chain of ODPA. More detailed insight into the initial bonding configuration is derived from the lower frequency region (900-1300 cm-1), corresponding to the P-O stretching region. The observation of the band at 1250 cm-1, associated to P=O stretching, rules out a tridentate configuration, while the lack of any bands in the 900-1000 cm-1 region of the P-O-H deformation rules out a monodentate binding. The data therefore suggest that the as-grafted ODPA binds primarily in a bidentate configuration, as previously reported 15. The dominant broad band within the 1000-1200 cm-1 region corresponds to the P-O-Si stretching vibration, and therefore confirms the chemical grafting of ODPA.

Figure 1. Differential IR spectra of ODPA-grafted SiO2/Si as a function of temperature. Following annealing to 700K, differential spectrum referenced to the as-grafted sample shows the loss of the aliphatic CH2 and CH3 modes, in conjunction with the appearance of several features in the 900-1300 cm-1 region. Since no negative features are observed in the P-O stretching region, the data imply that the phosphonate anchor group remains intact and that the ODPA molecules break at the P-C connection. The positive bands at 1020 and 1240 cm-1 signify perturbation of the longitudinal optical (LO) phonon mode and transversal optical (TO) phonon mode of the underlying SiO2 25. These changes, which we have routinely observed when annealing SiO2/Si in the absence of any grafted overlayer, arise due to a densification of the oxide, and continue to evolve as the sample is subjected to higher annealing temperatures. The feature that emerges at 1240 cm-1 in particular is at the same frequency as the high-frequency component of the doublet from the P=O stretch, making changes in this region difficult to interpret. However, the appearance of the new mode at 1175 cm-1, a frequency region intermediate between P=O and P-O stretching, originates from the change in the molecular structure following the P-C bond breaking. The frequency of this mode is comparable to



that reported for dative P=O bonds observed on InP(100) such bonds.

26

, and it likely implies the formation of

Subsequent annealing to 800K results in additional loss of the hydrocarbon tail evidenced by further losses in the C-H stretching region, accompanied by a loss at 1100 cm-1, signifying either a loss of the P-O bonds of the initial phosphonate functional groups or a change in the intensity associated with the modes. Annealing to 900K resulted in further perturbation of the SiO2 layer, evidenced by further growth of the modes at 990 and 1240 cm-1. The C-H stretching region showed a growth of the CH2 and CH3 stretching modes due to outgassing of the sample holder when subjected to high temperature, not indicative of chemistry intrinsic to the MLD process. Further losses in the 10001200 cm-1 region were observed, indicative of additional loss of phosphonate bonds. Finally, upon annealing to 1000 K, further loss of the phosphonate P-O modes (1000-1200 cm-1) was observed, in conjunction with additional modification of the SiO2 LO and TO modes. In summary, the IR data indicate the temperature at which the ODPA molecule decomposes, with the release of the hydrocarbon chain preceding the formation of a dative P=O bond and followed ultimately by the elimination of the signal from the phosphonate functional groups. However, it does not show the state of the P atoms, i.e., whether the phosphonate groups desorb or decompose. In order to track the presence of P on the surface and also monitor the state of the substrate, we performed further measurements. Following annealing to 1000 K, the core-level spectra show an apparent enhancement of the O 1s and Si 2p peaks (both 4+ satellite and 0+ fundamental) due to reduced attenuation of the photoelectrons from these core levels following removal of the ODPA overlayer. The C 1s signal was almost completely removed; the residual C signal arises from recontamination of the surface due to outgassing of the sample holder during prolonged annealing. We have verified in other experiments that all C associated with the ODPA layer is removed, and that residual C contamination approaches the XPS detection limit for sufficiently short annealing times.

A


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B

Figure 2. A: Si 2p spectra of Si wafers that have undergone the ODPA-MLD process. B: To further verify that the process leads to doping, we performed additional experiments in which ODPAfunctionalized and annealed Si samples were compared directly to reference Si samples with the same nominal dopant concentration but not treated with the ODPA MLD process. The samples were cleaned by sputtering with 3 keV Ar+ to remove the native oxide at the surface and therefore eliminate any possible charging effects. The spectra of the Si 2p spectra are depicted in Figure 2B (red line MLD, black line reference). Importantly, the Si4+ satellite depicted as green line in Figure 2A confirms that the oxide overlayer remained intact during the annealing treatment. Of particular interest is the apparent shift of the Si0+ peak relative to the Si4+ satellite (associated with the SiO2 overlayer) following the annealing treatment. The Si0+ component of the Si 2p spectrum (depicted as red line in Figure 2A) shows a marked shift of 0.3 eV to lower binding energy, consistent with the incorporation of an n-type dopant. We note that this shift could alternatively be interpreted as a complex charging effect that differently impacts the native oxide vs. the Si substrate. To further verify that the process does indeed lead to a doping of the near surface region, we performed additional experiments in which ODPA-functionalized and annealed Si samples were compared directly to reference Si samples with the same nominal dopant concentration but not treated with the ODPA MLD process. The samples were cleaned by sputtering with 3 keV Ar+ to remove the native oxide at the surface and therefore eliminate any possible charging effects. Subsequent XPS measurements of the two samples were collected side-by-side, and the spectra of the Si 2p levels are shown in Figure 2B. Here the 2p1/2 and 2p3/2 components are not resolved due to inhomogeneous broadening induced by the Ar+ sputtering. However, a clear shift of ~0.3 eV toward higher binding energy is observed in the case of Si treated with the MLD process. In addition, the Si 2p level in the ML-doped sample shows a broader, heavily asymmetric profile with a pronounced tail on the high binding energy side, which could originate from one or both of the following effects: i) a concentration gradient of the dopant throughout the XPS escape depth, or ii) coupling of the photoelectron to a continuum of electron-hole pair excitations 3, giving a Fano-like profile that is routinely observed in the photoemission spectra of conducting materials. Thus, the XPS data provide unambiguous evidence of diffusion of P through the native oxide layer and into the Si bulk to provide significant n-type doping.



Figure 3. ToF-SIMS of the Si/SiO2/ODPA interface after heating the sample to 1200K. TOF-SIMS profiles of P in ODPA ML-Doped Si(111) obtained using the P concentration. Clear incorporation of phosphorus into the Si after heating the ODPA terminated sample can be observed. In this case, most of the P is detected with SIMS. Figure 3 shows the ToF-SIMS depth profile of phosphorus dopant atoms in silicon wafers. The phosphorus concentration, 2x1020 P atoms/cm3, can further be integrated over the whole depth profile, thus giving a surface coverage similar to the reference value of 0.9 nmol ODPA/cm² 28. Given that these results rule out P desorption, the data unambiguously show that phosphonic acid-based MLD of Si can be accomplished simply by grafting the prescursor molecule onto the native oxide surface, and that a capping step is not required.

Figure 4: Electrical Characterization of ODPA ML-Doped Si(111) as a function of temperature. Next, we performed ex situ IS measurements on the ODPA-functionalized SiO2/Si surfaces as a function of annealing temperature to probe the electrical activity of the phosphorus inside the substrate in contact with silicon at high temperature, initiating a self-decomposition at 800 K. In Figure 4, the x-axis represents the temperature reached during heating; the heating rate was ~50K/s and the final temperature was maintained for 1s. The y-axis represents the relative sheet resistance. The total sheet resistance of the first measurement was set to 0. Then, only temperature-caused changes at the interface can be seen. The red, blue and black points represent three different series of measurements. The increase of sheet resistance between 400 and 700 K can be correlated to the organic release of the ODPA. From 700 to 900 K a sharp decrease of the resistance can be noted, ACS Paragon Plus Environment

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which can be correlated to the doping process. Concerning the IR-data, the SiO2 is still intact after all the heating cycles. From the data shown in Figure 4, we can clearly conclude that the phosphorus: i) was transported through the silicon oxide barrier, ii) has then diffused inside the oxide free silicon, iii) finally modified the electrical activity of the sample.



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DFT calculations and Reaction Mechanism The most important steps of the mass transport of phosphorus from phosphonic acid adsorption onto the SiO2/Si surface to single atom diffusion into the Si bulk are depicted in Figure 5.

Figure 5. Mechanism of Phosphorus Transport through Silicon Oxide during phosphonic acid monolayer doping: a) desorbed ODPA molecule at room temperature, b) adsorption in a bidentate configuration at 400 K, c) at 800 K the PO3 radical transforms into a tridentate configuration after the loss of the CH4 group (see text for details), and d) finally, at 1025 K, P diffusion into the silicon and transformation of the SiO2 into a thermal oxide. Yellow spheres represent Si atoms; white, H; blue, C, red, O and brown spheres, P atoms, respectively. Two different reactions have been considered for the initial adsorption of phosphonic acid molecules onto the SiO2/Si surface, which is mainly driven by kinetics: Si—SiO2 (1) Si—SiO2 (2)

+

+

CH3OP(OH)2

CH3OP(OH)2

→

→

Si—SiO—OCH3OPOH

Si--(SiO+SiO)—OCH3OPOH

+

H2O

+

2H2O

The reaction (1) corresponds to a monodentate configuration, whereas (2) represents a bidentate adsorbed structure (see Figure 6). Both configurations are nearly equienergetic, with the monodentate structure being slightly more favorable with a small energy difference of only 0.2 eV. The reason for this small difference is the large distance between the two surface Si atoms forming ACS Paragon Plus Environment

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the bidentate bonding structure (4.62 Å), which increases the surface stress energy, thus counteracting the energy gain from the additional Si-O bond. The results from the following calculations shown below correspond to the bidendate structure, identified as the dominant initial structure from our experimental IR spectra. Another important point is that, contrary to the case of phosphonic acid adsorption on hydrogen-passivated Si, both the monodentate and bidentate configurations show a slightly negative reaction energy, meaning that ODPA adsorption on SiO2/Si becomes a thermodynamically exothermic process upon release of H2O molecules. Despite the fact that adsorption of phosphonate molecules is energetically favorable, SAMs of ODPA cannot be formed spontaneously on either clean or hydroxyl-terminated SiO2 under ambient conditions, since a moderately high-temperature (≈410 K) is needed to overcome the barrier for the reactions (1) or (2). Furthermore, the phosphonate/SiO2 interface is thermodynamically stable only under very dry conditions. In ambient conditions, as the chemical potential of water increases, H2O molecules start to diffuse to the interface, hydrolyzing the system. Then, the potential energy of the H2O/SiO2 interface drastically increases, due to its capability to form a dense network of hydroxyl groups on the surface, enhancing its stability and, hence, making it less reactive. Consequently, both a thermal input and dry conditions are necessary to break the hydroxyl network and allow the otherwise exothermic reaction between the ODPA molecules and the SiO2/Si surface.

Figure 6. a) Bidentate and b) monodentate configurations for ODPA adsorbed on the SiO2/Si(111) surface. Yellow spheres represent Si atoms; white, H; blue, C, red, O and brown spheres, P atoms, respectively. We performed DFT frequency calculations to correlate the experimental spectra with the atomic scale structure of the grafted layer as a function of temperature. Here, the adlayer is modeled for simplicity as a methylphosphonate group bonded in various configurations to a monolayer of silicon oxide on a silicon substrate. The primary P-O stretching frequencies of the bidentate starting structure are summarized in Table 1. In addition to the P=O stretch at 1069 cm-1, the calculated spectra also show modes due to asymmetric and symmetric P-O stretching at 926 and 916 cm-1, respectively. These values are in relatively good agreement with the experimental spectrum of the as-grafted monolayer, showing bands at 1090 and 1037 cm-1.



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Table 1. Calculated IR frequency modes (cm-1) for ODPA adsorption on the SiO2/Si surface.

Configuration

P-O

Si-O-P

CH3

P-C

P-O-O

Monodentate- 1149 adsorbate

991,948

1438, 1427

1319

920

Bidentateadsorbate

926,916

1414, 1408

1313

893

1069

MonodentateCH4 loss Bidentate-CH4 loss

1064, 927

981,961

Tridentate-CH4 1004 loss

1240

799

873,822

Following the grafting of the initial surface, the next step in the MLD process consists of the organicligand release. Our previous ab initio MD simulations found that in the similar case of phosphonic acids on oxide-free Si(111), this occurs at ≈800 K. To identify the corresponding temperature, range for ODPA on silicon oxide, we obtained the cNEB desorption path of the methane CH4 molecule for the ODPA/SiO2/Si system in a bidentate configuration (see Figure 7), as the most likely reaction product of the organic-ligand release. The result of the calculation (2.4 eV) and the endothermic character of the process (reaction energy of 1.22 eV) confirm the MD-calculated release temperature, also agreeing quite well with that obtained from our experimental IR spectra.


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Figure 7. Kinetic barrier for the CH3 desorption from the ODPA molecule adsorbed on the SiO2/Si(111) surface. The insets show the initial and final states. Yellow spheres represent Si atoms; white, H; blue, C, red, O and brown spheres, P atoms, respectively. At the temperature at which the organic-ligand release occurs, the native silicon oxide concurrently undergoes a structural rearrangement leading to a denser, more ordered state. This densification process, which occurs independently of any grafted ad-layer, opens a number of possible reactive sites by which the phosphonate radical that remains after the loss of the organic tail can react. We performed calculations of the vibrational modes of the phosphonate group in the as-grafted (bidentate) bonding configuration and following release of the organic tail to further understand the intermediate structures that precede P insertion into the silicon. Following desorption of the organic tail, the remaining phosphonate anchor group bound in a bidentate configuration is a radical. However, our experimental data show that the oxide overlayer undergoes considerable structural rearrangement in this temperature window, providing reactive sites that could allow the radical to transform to a more energetically favorable (by ≈2 eV) tridentate structure (see Figure 8). The results of our frequency calculations for both the radical bidentate and tridentate structures (Table 1) show that the calculated frequency of the P=O stretch in the case of the tridentate structure (1004 cm-1) agrees more closely with the experimental value (1177 cm-1) than the calculated value frequency of the bidentate radical structure (split modes at 981 cm-1 and 961 cm-1, average 971 cm-1). Furthermore, the next highest frequency of the tridentate structure associated with asymmetric P-O stretching is calculated as 873 cm-1, within the experimental range of values present until the final annealing step to 1000 K. The corresponding mode calculated for the bidentate phosphonate is 799 ACS Paragon Plus Environment


cm-1. Such a mode would produce a distinct feature in the experimental spectra, but no bands are observed in this region. Therefore, the combined experimental and calculated vibrational data point to the formation of a tridentate phosphonate as an intermediate structure before complete diffusion into the bulk Si. The remaining spectral changes observed in the 800-1000 K temperature window can be ascribed to diffusion of the PO3 groups within the SiO2 bulk, and subsequent reduction (discussed below) to result in diffusion of P into the Si bulk.

Figure 8. PO3-x- surface species after CH3 desorption from the ODPA molecule adsorbed on the SiO2/Si(111) surface, corresponding to a) bidentate and b) tridentate radicals. Yellow spheres represent Si atoms; white, H; blue, C, red, O and brown spheres, P atoms, respectively. Finally, we show that the driving force for P diffusion into bulk Si through the SiO2 monolayer is the reduction of the phosphonate radical, resulting in simultaneous formation of SiO2 and P-Si3 tetrahedra, which show a larger formation energy compared to the surface P-Ox species. Two different possibilities must be considered (see Figure 8): as was shown previously 13, the reaction to dissociate PO2 and obtain P4+ diffusion is slightly endothermic, (~1.20 eV), with a medium kinetic barrier (2.27 eV). Conversely, for the PO−3 oxide, the negative charge of the three O ions can induce the formation of SiO2 by breaking two of the surface Si dimers, with the P dopant occupying the corresponding Si lattice site prior to its diffusion through the silicon oxide. This reaction is slightly exothermic (0.20 eV), with a low kinetic barrier (1.66 eV). As depicted in Figure 8, most of the surface species are of the PO−3-x type, due to the fact mentioned earlier: the large potential energy of the H2O/SiO2 interface makes it very stable and, in order to become reactive with the ODPA molecules, a high-temperature regime becomes necessary to break such network. Then, the surface hydroxyl groups mostly react with each other, releasing H2O. Finally, after losing the organic tail, most of the P ACS Paragon Plus Environment

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dopants will be in a PO−3-x configuration. Therefore, the phosphonate radical reduction at the Si/SiO2 interface via SiO2 formation is the most likely pathway for P diffusion through the silicon oxide. A similar behavior has also been shown in our recent work on O impurities at the InP/HfO2 interface 29 . However, the energy required to break the P-Ox bonds is still relatively large. Thus a hightemperature regime is necessary in order to start the P diffusion through the SiO2 into the bulk Si. Once started, the transport of phosphorus across the SiO2 layer is relatively fast, with a kinetic barrier of only 1.5 eV and a slight endothermic reaction energy of 0.68 eV for P diffusion into the Si bulk, see Figure 9.

Figure 9. Kinetic barrier for P diffusion through the silicon oxide into the Si bulk. The insets show the initial and final states. Yellow spheres represent Si atoms; white, H; blue, C, red, O and brown spheres, P atoms, respectively.



Conclusions In conclusion, we have found that P-MLD of Si can be achieved without the need for wet chemical removal of its native oxide by grafting a P-containing SAM directly onto the oxide surface. In situ IR measurements as a function of the annealing temperature indicate that decomposition of the hydrocarbon tail followed by dative P=O bond formation and subsequent P-O bond breaking precede diffusion of P atoms into the SiO2 substrate. ToF-SIMS and IS measurements show that, after the diffusion of P is initiated at 900K, no remaining P is detectable at the surface after annealing to 1000K. Importantly, XPS measurements indicate that the oxide remains intact at this point. Also, XPS and IS further demonstrate that P atoms diffuse into the substrate and have a doping effect, directly evidenced through shifts of the Si 2p level. Our atomistic DFT calculations show that the reduction of PO−3 phosphonate radicals at the Si/SiO2 interface is the driving force leading to P diffusion through the silicon oxide into the Si bulk. The results of this study can undoubtedly open a new platform for the obtaining of shallow doped semiconductor surfaces without costly wet chemical pre-treatments and the risk of organic contamination.

Acknowledgments The results presented in this paper have been gained within the DFG-funded project TH 1566/4-1 and TH 1566/5-1. The authors also acknowledge the Texas Advanced Computing Center (TACC) for providing computational resources.

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TOC Graphic


Mechanism of Phosphorus Transport through Silicon Oxide during

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