Mechanism of Arsenic Monolayer Doping of Oxide-Free Si (111)

Communication pubs.acs.org/cm

Mechanism of Arsenic Monolayer Doping of Oxide-Free Si(111) Roberto C. Longo,†,§ Eric C. Mattson,†,§ Abraham Vega,†,§ Wilfredo Cabrera,† Kyeongjae Cho,† Yves J. Chabal,† and Peter Thissen*,‡ †

Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States Karlsruher Institut für Technologie (KIT), Institut für Funktionelle Grenzflächen (IFG), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

‡

S Supporting Information *

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examine the surface evolution without concern of possible inhomogeneities. Clean, oxidized Si(111) samples are initially H-terminated by a 30 s dip in 10% HF(aq), followed by a 2.5 min dip in 40% NH4F(aq), and a final rinse in H2O for 30 s. This procedure produces an atomically smooth (111)-oriented Si surface from tens to hundreds of nanometers.20,21 The deposition of the MAsA on H-terminated Si(111) is done in one simple wet chemical step (immersion of the sample in 10−3 mol MAsA solution in methanol at 65 °C for 12 h in a glovebox), resulting in monodentate grafting of the surface with ∼65% coverage, without any detectable oxidation of the Si. The sample is then transferred to a UHV cluster, where it can be heated resistively from RT to 1173 K, for in situ characterization by infrared (IR) spectroscopy, X-ray photemission spectroscopy (XPS), and low energy ion scattering (LEIS). The heating rates are ∼100−200 K/min, with 10 s spent at each reported temperature. The details of the density functional theory (DFT) modeling and characterization procedures are given in the Supporting Information. Briefly, supercells with 8 atomic Si(111) layers with one adsorbed molecule are used, together with 4 × 4 × 1 meshes in the reciprocal space and electronic wave functions expanded into plane waves up to a cutoff energy of 360 eV. Previous work has shown that, in the case of phosphonates (e.g., using methylphosphonic acid or MPA), the weak link is the P−C bond,22 breaking at relatively low temperature (∼700 K). Therefore, the proposed scheme for arsenate is believed to provide a low-temperature pathway for As incorporation without carbon contamination, a central problem in all previous MLD doping reports.11 Furthermore, arsenate self-decomposition occurs at lower temperature than MPA due to the thermodynamic instability of binary arsenic oxides in contact with Si.23 A theoretical comparison of the interaction between MAsA and MPA molecules with H-terminated Si(111), using nudge elastic band (NEB) calculations (see Figure S2 of the Supporting Information), reveals that they both physisorb with negligible adsorption barriers (0 and 0.11 eV, respectively) and the reaction barrier for MAsA (1.36 eV) is lower than for MPA (2.03 eV).22 The reason for this difference mainly lies in the fact that the strength of the P−O bond (1.59 Å) is considerably larger than that of the As−O bond (1.78 Å), thus facilitating the release of the H2 molecule and the chemical grafting of the MAsA. The rate

ensity scaling and subsequent device dimension reduction continue to drive significant advances in the materials processing and architecture of advanced electronic devices.1 As gate lengths approach the sub-10 nm regime, junction doping has become an increasing concern due to its importance in controlling short channel effects.2 Source/drain junction depths must be extremely shallow and abrupt, typically around 1/3 of the gate length.3 Moreover, the transition to nonplanar architectures requires innovative methods to conformally dope the semiconductor material with fine control and reproducibility.4 Unfortunately, the conventional techniques for junction doping, such as ion implantation and anneal5−7 or plasma doping,8−10 cannot produce uniform and abrupt junctions shallower than 10 nm in depth, due to fundamental broadening of dopant distribution, random dopant fluctuations, and ioninduced damage. Recently, monolayer doping (MLD) has emerged as a facile approach and a promising technique for creating ultrashallow junctions.11−13 MLD comprises several steps: functionalization of the semiconductor surface with a p- or n-dopant-containing molecule and thermal diffusion of the dopant into the surface.11−13 The self-limiting nature of self-assembled monolayers (SAMs) of MLD provides a uniform and specified coverage of dopant. Then, subsequent high temperature annealing drives the dopant atoms into the semiconductor via a diffusion mechanism. Alternatively, monolayer contact doping (MLCD), in which the monolayer used for doping is formed on a separate substrate (not intended for doping), has also shown promising results.14,15 In all these cases, a capping layer (e.g., SiO2, Al2O3) is typically deposited to prevent SAM desorption.11 Although successful doping has been confirmed by electrical and depth-profiling measurements using MLD for a number of systems,16−18 the atomistic mechanism for dopant diffusion is not understood. Therefore, the process cannot be well controlled, particularly with regards to chemical contamination of the surface region. In this Communication, we investigate the MLD process by grafting methylarsenic acid (MAsA) directly on oxygenterminated Si(111) surfaces. Although we have also performed MLD doping on Si(001) surfaces (see Figure S1 of the Supporting Information), the focus is on atomically flat, oxidefree Si(111) surfaces because they provide a well-defined model system to derive atomistic mechanisms from spectroscopic and theoretical analyses.19 Importantly, the grafting process does not roughen the surface and first-principles calculations can then © XXXX American Chemical Society

Received: November 12, 2015 Revised: March 14, 2016

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DOI: 10.1021/acs.chemmater.5b04394 Chem. Mater. XXXX, XXX, XXX−XXX

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surface Si−H. The incorporation of O into Si is supported by the appearance of SiOx-related vibrational modes: TO (1022 cm−1) and LO (1078 cm−1) phonon modes. Concurrently, there is the loss of the As−O−H bands observed at 873 and 817 cm−1, suggesting that process (3) is kinetically favored. After the sample is annealed to 700 K for 10 s, the organic ligands are completely released and more oxygen atoms from the AsOx are incorporated into the Si backbond (O−Si−H at 2161 cm−1, O2−Si−H at 2200 cm−1, and O3−Si−H at 2270 cm−1). After 800 K, H is fully removed from the surface, with some incorporation of O to form substoichiometric SiOx, as evidenced by the appearance of LO (1178 cm−1) and TO (960 cm−1) phonon modes, with an estimated average thickness of ∼0.4 nm.24,25 While IR spectroscopy gives direct evidence for release of the organic tail and surface H, it cannot provide information on the behavior of As atoms (e.g., diffusion into the Si substrate). Therefore, complementary XPS measurements are performed at each step of the surface modification within the same UHV cluster system to track the oxidation state of the As (Figure 2).

of the grafting process (assuming similar prefactors for both MPA and MAsA molecules) is proportional to the exponential of the kinetic barrier, which implies that a 0.67 eV higher barrier entails a difference of several orders of magnitude between the rate of both chemical grafting processes (and the corresponding required temperatures). Of course these values are not sufficient to extract the exact rate of MAsA and MPA grafting to Hterminated Si(111) surfaces, because there are many more factors involved in the chemisorption, such as the dependence of the rate on coverage due to an increase in strain energy at higher coverage of MAsA molecules (see below), but at least it provides a first estimate of the rates of both MAsA and MPA grafting and final As- or P-doping. We also find that the thermodynamic ground state of MAsA adsorbed on H-terminated Si(111) surfaces is a 2/3 MLterminated ring-patterning, similarly to MPA,22 with the MAsA grafted via monodentate bonds to the surface.24,25 Higher coverage cannot be achieved due to steric hindering associated with the large footprint of the molecules (0.23 nm2 for monoand bidentate and 0.45 nm2 for tridentate configurations). Since MLD of MAsA grafted on oxide-free Si is performed by direct heating of the wafer, several surface sensitive measurement techniques are then necessary to unravel the As diffusion mechanism into the Si surface as a function of temperature. In situ IR absorption spectra (see Supporting Information for description of the experimental setup), recorded as a function of temperature, make it possible to examine the behavior of H on the surface (Figure 1). After the reaction of the MAsA solution

Figure 2. XPS detailed spectra of the C 1s, O 1s, As 2p, and Si 2p region of MAsA grafted on H-terminated Si(111) as a function of temperature. Figure 1. Differential transmission IR spectra of MAsA grafted on Hterminated Si(111) as a function of temperature. All spectra are referenced to spectra of lower temperature.

Within the initially chemisorbed MAsA molecule, the As atom has an oxidation state of +5, as evidenced by the observed energy of the As 2p core level (1327 eV), and O and C core levels are at 285 eV (associated with CHx) and 532 eV (associated with As− O). The changes occurring upon annealing confirm the IR measurements (e.g., loss of C at 600 K without shift) and support the associated under-coordination of As (shift-broadening of As core level), with rearrangement of the O atoms (shift from As−O at 532 eV to Si−O at 533 eV). Correspondingly, the Si 2p core level exhibits a small shift (0.5 eV) toward higher binding energies, consistent with Si−O formation. The most dramatic change is observed for the As core level, with a complete shift from 1327 to 1323 eV after annealing above 600 K, i.e., a transition from +5 to +1 oxidation state, together with a decrease of its intensity. These changes are consistent with O migration from MAsA into Si and strongly suggest the incorporation of As into the substrate. To explore this last hypothesis, we turn to LEIS.

with an H-terminated Si(111) surface, the remaining Si−H is characterized by a ν(Si−H) absorption band at 2079 cm−1 with ∼1/3 of the integrated intensity of the initial fully H-terminated Si(111) surface (see Figure S3 of the Supporting Information). This is consistent with the 2/3 ring-configuration calculated for the monodentate MAsA adsorption. After annealing to 600 K, the CHx stretch bands (2968 cm−1, 2927 cm−1, and 2852 cm−1) are removed, consistent with the release of the organic tail and with under-coordination of MAsA. Several mechanisms are possible for removing the organic radical: (1) polymerization with another radical, (2) reaction with H from other Si−H surface bond, and (3) MAsA tilting to allow reaction with H from the As−O−H bond. After 600 K annealing, there is also a marked shift of the ν(Si− H) band from 2079 to 2127 cm−1, which corresponds to ν(O− Si−H), i.e., when one O atom has migrated to the backbond of B

DOI: 10.1021/acs.chemmater.5b04394 Chem. Mater. XXXX, XXX, XXX−XXX

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on the LEIS subsurface signal is 3 × 1021/cm3 (see Figure S5 of the Supporting Information), as expected if all the As penetrates into the substrate (i.e., no loss through evaporation). Similarly, the additional low energy tail that develops upon annealing to 1173 K is indicative of As atoms localized yet further below the surface. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is typically used to quantify dopant concentration in thin films. However, since such measurements need to be done ex situ, the near surface region can oxidize and modify the chemical environment of the As, particularly as As is known to catalyze Si oxidation.28,29 Nonetheless, TOF-SIMS measurements (see Figure S7 of the Supporting Information) confirm that As has diffused into the sample, and its concentration is peaked at ∼0.7 nm below the surface after 1173 K anneal, which is consistent with LEIS measurements. To avoid the oxidation problem also present for ex situ electrical measurements, the electrical nature of these buried As atoms can be determined in part from in situ broadband IR measurements, which provide a complementary measure of the doping effect on the near surface region. As the MAsA SAM decomposes and eventually n-dopes the surface following progressive annealing, the Fermi level of the substrate is increased, resulting in a raised density of free carriers. Such carriers produce a Drude-like functional form in an IR absorption spectrum. Figure 3b shows the absorption spectrum of the sample following annealing to 973 K referenced to the spectrum collected following annealing to 573 K. Importantly, when no baseline correction is applied, the spectrum clearly shows a Drude-like absorption below 3000 cm−1 that closely mimics the absorption spectrum of bulk As-doped Si.30 In addition, we observe the development of a broad absorption band centered at 3800 cm−1, which has previously been assigned to an indirect interband transition associated specifically with n-type doping.30 Quantification of the absorbance change allows us to estimate the change in the conductivity of the substrate due to the MLD. We fit the differential absorption spectrum31 to a two-component dielectric function comprised of Lorentz oscillators describing the free-carrier and 3800 cm−1 features and assume that a 4 nm slab is responsible for the absorbance change, as indicated by the TOF-SIMS depth profiles. The extracted optical conductivity, when extrapolated to zero-frequency, corresponds to an increase in the d.c. conductivity of 256 Ω−1·cm−1. We note, however, that the surface oxide is still present at this temperature; as expected, this oxide will limit the doping effect of the As and the extent of the conductivity increase. Consequently, the measured increase of 256 Ω−1·cm−1 is lower than expected for a concentration of 1021 estimated from the LEIS measurements. Upon annealing to higher temperatures, however, the surface oxide is removed (see Figure S4 of the Supporting Information) and we therefore expect the ultimate conductivity change to be larger than the quoted value of 256 Ω−1·cm−1. Unfortunately, the IR spectra collected at these elevated temperatures are obscured by diffusion of N impurities present in the FZ Si. Figure 4 summarizes the complete MLD mechanism extracted from the data above and the first-principles calculations, starting from an MAsA molecule chemisorbed on H-terminated Si(111) surfaces and ending at 1050 K with subsurface As. We find that the breakup/desorption of the CH3 from the MAsA molecule happens spontaneously at around 800 K. With a medium kinetic barrier of 2.5 eV (see Figure S8 of the Supporting Information), CH4 formation is likely to happen before the organic tail finally desorbs from the surface. Note that the reaction energy is almost

LEIS is particularly useful to determine the As distribution within a shallow surface region (∼1 nm) of the Si(111) sample upon annealing. Figure 3a shows the As LEIS signal as a function

Figure 3. Subsurface diffusion and As-doping of Si. A: Temperaturedependent LEIS spectra in the As binary collision region. B: broadband IR response following annealing to 973 K overlaid with a fit of the differential absorbance to a two-component dielectric function.

of annealing temperature in the temperature region in which insertion into the bulk is believed to occur (>973 K). Up to 973 K annealing, the spectrum is characterized by a single peak at 2365 eV in the As binary collision region, corresponding to surface As, as is observed for the as-prepared surface (see Supporting Information). As the sample is sequentially annealed, this Asrelated peak shifts and weakens in a very specific manner that provides important information: at 1023 K, a low-energy shoulder develops at 2295 eV, then its intensity decreases between 1073 and 1123 K, with the initial peak at 2365 eV falling below the detection limit, and only the 2295 eV peak remaining; upon 1173 K annealing, the 2295 eV peak intensity further decreases and an additional lower-energy tail appears. This behavior is directly interpreted as subsurface penetration (i.e., it provides unambigious proof of the As incorporation into oxidefree Si), since ions that penetrate into the bulk lose additional energy to the substrate atoms above the As atoms through scattering and subsequent reionization at the surface.26 Note that potential screening by O or other molecules can be ruled out since O is observed to desorb as do any initial surface contaminants between 973 and 1025 K (see Figure S4 of the Supporting Information). To quantitatively understand the nature of the red-shifted As signal, we perform LEIS simulations using the T-Rutherford backscattering (TRBS) code,27 an implementation of the transport of ions in matter (TRIM) code designed for simulation of RBS data (see Supporting Information for details). Through a comprehensive analysis of model multilayer structures, we find that a 70 eV shift of the As signal corresponds to As atoms at a depth of 0.6 nm below the surface. At that point, our estimate of the As concentration based C

DOI: 10.1021/acs.chemmater.5b04394 Chem. Mater. XXXX, XXX, XXX−XXX

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migrates into Si. At a temperature of 1050 K, the subsurface diffusion of As into the Si is observed. The overall driving force was found to be the thermodynamic instability of As5+ in contact with Si at high temperature, initiating a self-decomposition at 800 K. Since there is no evidence for As desorption and no remaining C contamination, we conclude that a capping layer is not necessary for MLD based on organo-phosphonic or -arsenic acid grafted on oxide-free Si.

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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/acs.chemmater.5b04394. Complete information on the kinetics and energetics of the As MLD process, together with the description of the experimental setup and the computational methodology and additional TOF-SIMS and electrical characterization (PDF)

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Figure 4. Molecular process of MLD, with the precursor methylarsenic acid (MAsA) molecule adsorbed on H-terminated Si(111), starting at room temperature and ending at 1050 K. Yellow spheres represent Si atoms; white, H; blue, C, red, O; and pink, As atoms, respectively.

AUTHOR INFORMATION

Corresponding Author

*(P.T.) E-mail: [email protected]. Author Contributions §

(R.C.L., E.C.M., and A.V.) Equal contribution.

negligible (i.e., the energy of the initial and final states is practically the same). Consequently, the readsorption of the CH4 molecule is a very unlikely process although the kinetic barrier is relatively low. The remaining part of the MAsA molecule now changes its configuration from a monodentate to a bidentate structure, as subsurface diffusion of As into Si is initiated. We then consider the most probable pathways for the doping inclusion of As into the Si(111) surface, together with the effect of the different paths on the electronic structure. Specifically, we consider four different pathways (see Figure S9 of the Supporting Information), leading to four different possibilities for As doping into the Si(111) surface: adsorption on the surface (AS), subsurface diffusion (SSD), surface interchange with one Si atom (SI), or subsurface interchange (SSI). The most thermodynamically favorable structure is the adsorption of the As atom on the Si surface (AS), whereas the other three mechanisms are characterized by relatively similar adsorption energies (Figure S9). The results show that the activation barrier associated with As incorporation is 1.90 eV, once C and O have left the initial MAsA molecule, which is consistent with the annealing temperatures needed for As incorporation (see Figure S10 of the Supporting Information). In summary, the complete atomic reaction pathway for MLD of MAsA of Si(111) leading from adsorption over selfdecomposition and finally subsurface diffusion has been investigated for the first time both experimental and theoretically. The kinetic barrier for chemical grafting of MAsA to the Hterminated Si(111) surface is 1.36 eV, much lower than the value obtained for MPA chemisorption to the same Si surface, 2.03 eV. The reason for this difference lies in the fact that the strength of the P−O bond (1.59 Å) is considerably larger than that of the As−O bond (1.78 Å), thus making easier the release of the H2 molecule and the chemical grafting of the MAsA. At a temperature of 800 K, the release of the organic tail is complete. The remaining part of the MAsA molecule now changes its configuration from a monodentate to a bidentate structure, as O

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The IR, XPS, and LEIS work was fully supported by the National Science Foundation (Grant CHE-1300180). The authors would like to acknowledge very helpful discussions with Prof. W. G. Schmidt. The results presented in this paper have been gained within the DFG-funded project TH 1566/3-1. The authors also acknowledge the Texas Advanced Computing Center (TACC) for providing computational resources. The authors thank Dr. Elaine Zhou for assistance with the TOF-SIMS measurements.

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