Nanopatterning of Group V Elements for Tailoring the Electronic

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Nanopatterning of Group-V-Elements for Tailoring the Electronic Properties of Semiconductors by Monolayer Doping Peter Thissen, Kyeongjae Cho, and Roberto C. Longo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13276 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 22, 2016

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ACS Applied Materials & Interfaces

Nanopatterning of Group-V-Elements for Tailoring the Electronic Properties of Semiconductors by Monolayer Doping Peter Thissen,∗,† Kyeongjae Cho,‡ and Roberto C. Longo∗,‡ †Karlsruher Institut f¨ ur Technologie (KIT), Institut f¨ ur Funktionelle Grenzfl¨ achen (IFG), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA E-mail: [email protected]; [email protected] Abstract The control of the electronic properties of semiconductors is primarily achieved through doping. While scaling down the device dimensions to the molecular regime presents an increasing number of difficulties, doping control at the nanoscale is still regarded as one of the major challenges of the electronic industry. Within this context, new techniques such as Monolayer Doping (MLD) represent a substantial improvement towards surface doping with atomic and specific doping dose control at the nanoscale. Our previous work has explained in detail the atomistic mechanism behind MLD by means of density-functional theory calculations (Chem. Mater. 2016, 28, 1975). Here, we address the key questions that will ultimately allow to optimize the scalability of the MLD process. First, we show that dopant coverage control cannot be achieved by simultaneous reactions of several group-V elements, but stepwise reactions make it possible. Second, using ab initio molecular dynamics, we investigate the thermal decomposition

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of the molecular precursors, together with the stability of the corresponding binary and ternary dopant oxides, prior to the dopant diffusion into the semiconductor surface. Finally, the effect of the coverage and type of dopant on the electronic properties of the semiconductor is also analyzed. Furthermore, the atomistic characterization of the MLD process raises unexpected questions regarding possible crystal damage effects by dopant exchange with the semiconductor ions, or the final distribution of the doping impurities within the crystal structure. By combining all our results, optimization recipes to create ultra-shallow doped junctions at the nanoscale are finally proposed.

keywords: semiconductors, density-functional theory, monolayer doping, electronic structure, self-assembled monolayers

1

Introduction

Scaling device dimensions down to the molecular regime presents fundamental and technological challenges for fabricating well-defined structures with controlled atomic composition. 1 The control of the electronic properties of semiconductors is primarily achieved through the inclusion of impurities or dopants, 2 which modify the electronic structure of the semiconductor by creating excess electrons (n-type) or holes (p-type). The controlled doping of semiconductors is in the foundations of modern microelectronics, 3 photovoltaics 4 and nanostructured devices, 2,5,6 among others. However, as gate lengths approach the sub-10 nm regime, junction doping is becoming the most important concern, owing to its importance in controlling short channel effects. 7 Indeed, sharp junctions and localized dopant profiles are the key to control the electronic structure and the properties at the nanoscale, which represents a major challenge in the semiconductor industry and in materials science research. 8–10 Moreover, the transition to non-planar architectures such as FinFETs (fin-shaped field effect transistors) and nanowirebased devices, requires new methods of doping with fine control and reproducibility. 6,11 Conventional doping methods such as ion implantation, 12–14 plasma doping 15–17 or solid2


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source diffusion for controlled nanometer scale surface doping 6 are challenging due to several important limitations. For instance, they can not produce uniform and sharp junctions due to nanoscale, ion-induced lattice damage, broadening of the dopant distribution or random dopant fluctuations. Even commonly used techniques within the context of semiconductors such as in situ CVD (chemical vapor deposition) doping suffer from non homogeneous dopant distributions, resulting from continuous exposure of the growing semiconductor surface to the dopant precursor along the CVD synthesis process. 6,18 Monolayer Doping (MLD), a substantial improvement toward surface doping with control at the nanoscale, has recently been showing great popularity among the scientific community. 2,6,11,19–21 The most noteworthy characteristic of the MLD approach is that the doping step is separated from the nanoscale building block synthesis, 2 thus avoiding damage to the crystal lattice, which is critical for nanometer scale doping. To achieve this, the semiconductor substrate is functionalized with a p- or n-dopant containing molecule, forming self-assembled monolayers (SAMs). The intrinsic nature of SAMs provides uniform coverages and specific concentration of dopant atoms. Then, the substrate, SAM and a capping layer deposited to avoid SAM desorption (typically SiO2 or Al2 O3 ) are thermally annealed at elevated temperatures, where decomposition of the SAM and subsequent dopant diffusion into the substrate can be achieved. This approach has been previously shown to create ultra-shallow junctions at interfaces of nanostructures and further confirmed by electrical measurements for a variety of systems. 19–21 Moreover, our recent work has shown that MLD based on organo-phosphonic or -arsenic acid does not need a capping step, 11,22 an important breakthrough in order to optimize the scalability of the MLD process. The atomistic mechanism behind MLD has also been explained in detail. 11,22 Important questions for the control of the MLD process, especially to avoid chemical contamination of the semiconductor surface by the organic ligands of the SAM, can be well-addressed by ab initio density-functional theory (DFT). However, three important atomistic aspects of MLD must still be carefully considered: First, the coverage controls the doping dose, i.e., the

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amount of available dopants is given by the number of molecular precursors forming the SAM. Second, such coverage is driven by the formation of nanopatterning molecular structures, i.e., during deposition, the molecular precursors feature specific adsorbed configurations driven by intermolecular van der Waals forces 23 and, third, the nanopatterning structures can represent either 1/3 or 2/3 of total coverage, which is the key to control the doping density. Total coverage of the target semiconductor substrate is obviously not possible because of steric repulsion between adsorbates. Here, we investigate the MLD process by grafting group-V-element-molecular precursors on an atomically flat, oxide-free Si(111) surface, which provides a well-defined model system to explore the characteristics of the molecular precursor deposition on the semiconductor surface and the resulting doping after thermal annealing. First, we show that the dopant coverage control can not be done by simultaneous reaction of group-V-elements, but stepwise reactions from the H-terminated to 1/3 to 2/3 nanopatterning makes it possible. Second, by means of ab initio Molecular Dynamics (AIMD), we study the molecular precursor decomposition process and the relative stability of their binary oxides in contact with Si. Further annealing at elevated temperatures releases the organic ligands and starts the dopant diffusion into the target substrate. Third, we demonstrate that, by controlling the coverage of the corresponding molecular precursor, the electronic properties of the target semiconductor substrate can be properly tailored. As will be shown and explained in detail, the atomistic characterization of the doping process itself raises unexpected questions regarding possible crystal lattice damage effects by atomic exchange with the dopant impurities, which has not been addressed before in previous MLD publications. Furthermore, the doping distribution (whether the dopants distribute uniformly in the crystal structure or they show a clustering behavior) within the semiconductor host material and its effect on the electronic properties needs also to be properly analyzed.

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Methods and Calculation Details

DFT calculations were carried out within the generalized gradient approximation (GGA) as implemented in the Vienna ab initio simulation package (VASP). 24,25 The electron-ion interaction was described within the projector-augmented wave (PAW) scheme 26 and the electronic wave functions were expanded into plane waves up to a kinetic energy of 500 eV. The PBE functional was used to describe the electron exchange and correlation energy within the GGA. 27 In addition, the DFT-D3 functional 28 was used in order to account for the dispersive, weak interaction forces leading to molecular physisorption on the Si(111) surface, and for the interaction between molecules within the SAMs. Obviously, van der Waals forces have no effect on the dopant diffusion into the semiconductor surface. The Si(111) surface was represented by periodically repeated slabs of 8 atomic layers. The supercell used here comprises the Si surface, the adsorbed molecules and a vacuum region equivalent to 16 atomic layers. The 7 uppermost layers of Si as well as the molecular precursors were relaxed until a tolerance of 10−5 eV and 10 meV/Åin the energy and forces, respectively. The Brillouin zone integration was performed using a 4×4×1 k -mesh within the Monkhorst-Pack scheme. 29 AIMD simulations were performed using the velocity Verlet algorithm coupled with the Nos´e thermostat to solve the equations of motion. Starting at 0 K, the temperature was increased in steps of 50 K and, at each temperature, a time step of 0.5 fs was used during an equilibration period of 10000 steps, for a total simulation time of 5 ps. The kinetic diffusion barriers were obtained with the nudged elastic band (NEB) method, using a string of geometric configurations to describe the reaction pathway of each reaction process studied. 30,31

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Results and Discussion

3.1

Molecular Adsorption and Coverage Control

P and As molecular precursors are methylphosphonic (CH3 OP(OH)2 ) and methylarsenic acid (CH3 OAs(OH)2 ), respectively (hereafter MPA and MAsA). The first step of the MLD is the molecular adsorption on the Si surface, which is primarily driven by kinetics. The grafting process on the H-terminated Si(111) surface takes place via the well-known reaction:

Si − H + CH3 OM(OH)2 −→ Si − OCH3 OMOH + H2 ,

(1)

with M= P or As. The difference in kinetics determines to a large extent the adsorption rate of the molecular precursors. However, no significant differences should be expected, since the adsorption reaction is basically the formation of a Si-O bond and the release of a H2 molecule. 11 Another important aspect is that the steric repulsion between adsorbed MAsA and MPA molecules, together with the dimer-like termination of the Si(111) surface, hinder and limit the molecular adsorption to a specific coverage (conforming the nature of the SAM), forming nanopatternings that are determined to a large extent by the molecular precursor. Such coverage control has been demonstrated for both MAsA and MPA molecules. 11,22 However, it cannot be achieved by simultaneous reaction of group V elements. Different approximations include the use of two different substrates (one at the bottom and other at the top of the target system) in order to get two different types of dopants into the system. 2 A much more simple way to achieve this goal consists of having a stepwise reaction from 1/3- to 2/3-nanopatterning coverage, i.e., to make use of the MAsA- and MPA-SAM properties to reach a 2/3 nanopattern with both As and P molecular precursors. Here, given the lower energy barrier for MAsA adsorption, 11,22 we propose the design of a hypothetical experiment consisting of the initial adsorption of MAsA corresponding to 1/3 of the available surface lattice sites (which can be easily obtained from the surface area), followed by the 6


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adsorption of a similar amount of the MPA molecular precursor. The total coverage is 2/3 of the surface and represents the upper limit for MAsA- and MPA-like precursors. We have obtained the most thermodynamically favorable pathway leading to the adsorption of MAsA and MPA representing a 2/3 coverage of the Si(111) surface. The final 2/3-nanopatterning configuration is shown in Figure 1. The most thermodynamically favorable structure is a hexagonal-shaped ring with two different domains for MAsA and MPA molecules. We have considered all the possible intermediate configurations leading to the final adsorption of a 2/3nanopatterning coverage, obtaining their respective DFT-adsorption energies. The structures obtained in our calculations are shown in the Figure S1 of the Supporting Information. As can be seen in the picture, there are intermediate configurations with lower adsorption energies than those leading to the final 2/3-nanopatterning ring-shaped configuration shown in Figure 1. The key aspect of the entire adsorption process is that the difference in energy between the intermediate configurations is much smaller than the difference between the ring-shaped final configuration with different MAsA and MPA domains and other possible final structures. Therefore, we have focused this study on the molecular adsorption process leading to the 2/3-nanopatterning hexagonal configuration shown in Figure 1. The adsorption kinetics depends basically on the relative structural arrangement of the molecules and the surface coverage. Moreover, nanopatterns are usually the result of an ordering process accompanying the adsorption reactions, due to the dependence of the kinetic barriers on the occupation of the neighboring lattice sites of the surface. 32 Figure 2 shows the kinetic evolution from the adsorption of a single MAsA molecule to the ring-shaped, 2/3-(MAsA-MPA)-nanopatterning coverage of the H-terminated Si(111) surface and Table 1 shows the corresponding adsorption energies and kinetic barriers of the six intermediate steps. A few interesting remarks can be extracted from our DFT-results. First, the overall adsorption process is exothermic, i.e., it releases H2 molecules and energy, but it is also clearly divided into two separate parts, marked by blue and red shaded areas in Figure 2: the first part (blue shade) is strongly exothermic and corresponds to the adsorption of all the

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Figure 1: The most thermodynamically favorable adsorbed structure of MAsA and MPA molecules on the H-terminated Si(111) surface, forming a 2/3-nanopatterning configuration. Yellow spheres represent Si atoms; white, H; blue, C, red, O; green, P and pink spheres, As atoms.

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MAsA precursors and the first MPA molecule (∼66% of the nanopatterning). The second part (red shade) is slightly endothermic and corresponds to the closing of the ring-shaped final structure with the last two MPA molecules (∼33% of the nanopatterning). Second, the adsorption kinetic barrier for the MAsA molecules (∼50% of the nanopatterning) is relatively similar, as expected. Only the adsorption of the last MAsA molecule shows a slightly lower kinetic barrier (∼0.2 eV), owing to the van der Waals interaction with the OH groups of neighboring MAsA molecules. During this first stage, there is also a shallow physisorption interaction between the MAsA precursors and the Si(111) surface. Third, the adsorption of the MPA molecules shows two different regions. The first MPA molecule undergoes a strong coulombic interaction with neighboring, adsorbed-MAsA’s, leading to a deep physisorbed state (-0.81 eV), which could also be regarded as the initial chemisorbed state of the second SAM growth. The final adsorption energy is 1.24 eV, the largest among the six molecules of the ring-shaped final configuration and the kinetic barrier is only 0.32 eV (but 1.11 eV from the physisorbed state). The ring-closing adsorption of the last two MPA molecules shows an endothermic character with increasing kinetic barrier and adsorption energy (more endothermic character). Finally, with and overall, exothermic, adsorption energy of 2.37 eV and an initial, moderate kinetic barrier of 1.36 eV, the final 2/3-(MAsAMPA)-nanopatterning coverage could be achievable at room temperature. 32 Table 1: MAsAx and MPAy physisorption and adsorption energies of the reaction leading to the ring-shaped, 2/3-nanopatterning configuration shown in Figure 1. The reaction product is always H2 and both energies are given with respect to the previous step. The corresponding kinetic energy barriers are also given. (x,y) (1,0) (2,0) (3,0) (3,1) (3,2) (3,3)

Physisorption energy (eV) Adsorption energy (eV) -0.11 -0.58 -0.11 -0.62 -0.23 -0.82 -0.81 -1.24 -0.23 +0.02 – +0.87 a Kinetic barrier with respect to the physisorbed state

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Kinetic barrier (eV) 1.36 1.37 1.03 0.32/1.11a 1.06 1.74


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Figure 2: Kinetic evolution from the adsorption of a single MAsA molecule to the ringshaped, 2/3-(MAsA-MPA)-nanopatterning coverage of the H-terminated Si(111) surface. Yellow spheres represent Si atoms; white, H; blue, C, red, O; green, P and pink spheres, As atoms. The peaks show the kinetic barriers of the reactions leading to the corresponding stable configurations (which correspond to the valleys between peaks, represented by the insets).

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3.2

Thermal Annealing and MLD

MLD is achieved by annealing the nanopatterning to a desired temperature, at which the organic tails of the adsorbed molecules forming the SAM have been already dettached from the surface, a binary oxide is subsequently formed and, finally, the dopant atoms diffuse into the Si surface. The entire MLD process have been modeled by means of AIMD, starting from MAsA and MPA adsorbed on Si(111) at room temperature and ending at 1100 K, with the Si doped by both As and P. Figure 3 shows the most important steps of the AIMD-annealing process. At ∼1000 K, we find the temperature for the breakup/desorption of the organic CH3 from the MAsA and MPA molecules. This process always happens spontaneously, just by increasing the annealing temperature. Moreover, CH4 formation (with one H from the OH group of the same molecule) is likely to happen before the organic tail finally desorbs from the surface. Owing to the proximity of the molecules of the nanopatterning (see Figure 1), a Hydrogen bond is formed between neighboring adsorbates (with average bond length of ∼1.5 Å), and such bond needs to be broken in order to lose CH4 as the initial reaction product of the thermal annealing. In addition, atomic H desorbs from the surface (the nanopatterning only occupies 2/3 of the available surface lattice sites). The kinetic barrier for H desorption is not very high, 2.42 eV (see Figure S2 of the Supporting Information), similar to other Si surface orientations. 33 The difference between the two processes (CH4 and H desorption) originates from the absence of kinetic barrier for the re-adsorption of H (CH4 desorption shows a similar kinetic barrier of ∼2.5 eV but negligible reaction energy 22 ). In the supercell where the calculations have been carried out, it comes to a competition between these two processes (desorption and re-adsorption), with no kinetic barrier for one of them. However, at a temperature of 1000 K, the partial pressure of H is low enough to avoid its re-adsorption on the Si surface and can then be disregarded in the following. Therefore, during this first high temperature step, we have reached a complete organic release. The remaining part of some of the MAsA and MPA molecules now change their configura11



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tions from monodentate to bidentate structures (in contrast to low-coverage- formed SAMs, 22 there are no available surface lattice sites in the high-coverage-nanopatterning model considered here for all the MAsA and MPA molecules to form bidentate structures), and the corresponding ternary oxides are formed on the Si(111) surface.

Figure 4: Kinetic energy barrier along the minimum energy path for PO2 (red curve) and PO− 3 (black curve) dissociation and subsequent P diffusion into the Si bulk. Yellow spheres represent Si atoms; red, O and green, P atoms. At the high-temperature regime of 1100-1200 K (see Figure 3), the kinetic release of O2 molecules is combined with the sub-surface diffusion of As and P and a probable (7×7) reconstruction of the Si surface, 34 with partial SiO2 and SiO terminations. After some O2 molecules have been partially released from the surface, subsequent annealing drives the As and P into the Si via a diffusion mechanism. The time scale of this step is obviously well 13



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beyond the reach of any practical AIMD method. Therefore, to study the diffusion of As and P into the Si bulk, we obtained the kinetic barriers for such atomic diffusion pathways and, then, heated the system again up to 1200 K, thus obtaining both the thermodynamic and kinetic driving forces for P and As sub-surface diffusion. Two different pathways were considered for each dopant: P atoms can form stable PO2 or unstable PO− 3 , whereas the stable As oxide (As2 O3 ) can not be formed within the present high-coverage-nanopatterning − structure due to the distance between adsorbed molecules. Then, only AsO− 2 AsO3 can

appear as surface As reaction products of the annealing. Therefore, the instability of P5+ , As4+ and As5+ may drive both dopants into the Si, whereas the mechanism for PO2 dissociation and subsequent P diffusion must be carefully examined. Combining NEB and AIMD calculations, we have obtained the reaction energies and the most probable pathways for the oxide dissociation and the following doping inclusion of P and As into the Si bulk, starting from the four different oxides and ending with the corresponding dopant as a sub-surface impurity. The high temperature regime is the necessary activation mechanism required for the decomposition of the oxides and the sub-surface diffusion of P and As dopants. The results are shown in Figure 4. The main conclusions are as follows. The reaction to dissociate PO2 and obtain P4+ diffusion into the Si is slightly endothermic, as expected (1.20 eV), with a medium kinetic barrier (2.27 eV). As such, the high temperature regime is the likely activation mechanism for interlayer doping. On the contrary, 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 (see Figure 4). This reaction is slightly exothermic (0.20 eV), with a low kinetic barrier (1.66 eV). Therefore, it can be regarded as the most likely pathway for P substitutional doping of Si. On the other hand, as the atomic radius of As is slightly larger than that of P (119 pm vs. 106 pm, with 111 pm for Si), it is much more difficult for As atoms to occupy Si lattice − sites, and the dissociation of both AsO− 2 and AsO3 oxides produces As diffusion into Si and

interlayer doping. Figure 5 shows that the reaction for AsO− 2 dissociation and subsequent As 14


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diffusion into Si is moderately endothermic (1.35 eV), with a medium kinetic barrier (2.62 eV), whereas the dissociation of AsO− 3 will cost as much as 2.63 eV, with an even larger activation barrier of 2.93 eV. Then, we can conclude by saying that the loss of O2 will definitely facilitate As doping, whereas P doping can be easily achieved at high temperature regimes without the need to desorb O2 molecules.

Figure 5: Kinetic energy barrier along the minimum energy path for AsO2 (red curve) and AsO− 3 (black curve) dissociation and subsequent As diffusion into the Si bulk. Yellow spheres represent Si atoms; red, O and pink, As atoms.

3.3

Tailoring the Electrical Activity of Semiconductors by MLD

A different way to monitor the MLD process is given by the electronic structure characterization of the diffusion pathways followed by the doping impurities into the semiconductor 15



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surface, which can be performed by a variety of experimental techniques, such as AngleResolved Photoemission Spectroscopy (ARPES). 35 A detailed analysis of the band structure of the final, doped-Si(111) will help us to investigate the effectiveness of shallow MLD doping and, at the same time, to answer important questions such as whether there is a limit for doping, to explore the electrical activity as a function of the number of doping elements and whether As/P doping is more effective or not than As or P standalone doping. To do that, we have considered several MLD models (shown in Figures 6 and S3 of the Supporting Information), which correspond to upper and lower limits of As, P and As/P doping concentrations. The electronic band structures and charge densities (projected on a plane containing the dopants, parallel to the surface) of As, P and As/P doped-Si(111) in the upper limit of doping concentration are shown in Figure 6. For comparison, the band structure of the H-passivated Si(111) surface is also shown. Some interesting features can be extracted from the plots. First, the dopants create new states in the band gap, thus narrowing it. The orbitals corresponding to these new states tend to be more localized than those of the host semiconductor, which are delocalized over the Si atoms. Localized orbitals are more sensitive to the atomic motion. Then, during annealing at high temperatures, it results in larger changes in the electronic density around the dopants. On the contrary, delocalized orbitals provide random contributions to the changes in the electronic density, which ultimately results in negligible variations of the charge density around the Si atoms of the semiconductor host during the annealing. Although not relevant for the success of the MLD process itself, a detailed analysis of the charge dynamics evolution with temperature (which is beyond the scope of this work 36 ), would be extremely useful to optimize the performance of photovoltaic devices. 4 Second, As-doping transforms the Si(111) surface gap into a direct-type band gap, also decreasing the gap substantially (to 0.28 eV, at the Γ point, although the actual value could be larger, due to the underestimation of band gaps at the GGA level of theory. However, although the resulting band gaps are probably strongly underestimated, this will not affect the main conclusions of this

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work, given the proximity of the impurity states to the band edges). As can be seen in the charge density projection, the depth reached by the As atoms in the final, doped-Si(111) is not the same (i.e., the plane containing the As atoms is not parallel to the surface) which, although still shallow, makes the doping process more effective, because the dopant-related electronic charge density is more distributed throughout the shallow area reached in the MLD process. Third, P-doping also narrows the band gap (to 0.42 eV, at the L point), of a direct-type as well, but the symmetry of the Valence Band Maximum (VBM) is inverted with respect to that of the As-doped Si(111) surface. As described in the previous section, P atoms tend to occupy lattice sites of Si (inducing slight atomic Si displacements), sharing a common plane (see the charge density projection in Figure 6 and the insets of Figure 4), which could make the doping less effective than that of As atoms. Finally, similarly to P, As/P combined doping slightly reduces the band gap (0.61 eV), but the combination of both dopants increases the dispersion of the VBM considerably, which can be detrimental for the electron mobility. Another important aspect is that, within this upper limit of doping concentration (which would correspond to a doping concentration of ∼18×1020 atoms/cm3 , of the same order of magnitude as obtained using a capping step 6 ), both As and P doping impurities tend to clusterize, i.e., the thermodynamic stability of the system increases significantly if the average distance between doping impurities is similar to that of the Si network (∼2.41 Å), forming a triangular structure. The dopant-cluster structures have been obtained with the system annealed at 1200 K. Subsequent MD quenching to 0 K and structural relaxation did not alter the atomic configuration substantially. On the other hand, such clusterization has little effect on the Si structure (except a small displacement of the neighboring Si atoms in the case of P doping), given the relatively low amount of doping atoms coming from the SAMs. For comparison, Figure S3 of the Supporting Information shows the electronic band structure corresponding to the low limit of doping concentration (∼3×1020 atoms/cm3 ). The picture shows that the effect of As and P doping is relatively similar, as expected, in

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Figure 6: Upper panel: Electronic charge density (in eV/Å2 ) of the As, P and As/P-doped Si(111) surface, corresponding to the upper limit of doping concentration. The charge density is projected on a plane containing the dopants, parallel to the surface (shown in the inset). The green color (pointed by arrows) indicates the electronic cloud associated with the dopants. Lower Panel: Electronic band structures (blue lines) corresponding to the upper limit of As, P and As/P doping concentration of the Si(111) surface. For comparison, the band structure of the hydrogen-terminated Si(111) surface is also shown (red lines).

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terms of band structure and carrier mobility, but the combined As/P doping also results in large dispersion of the VBM, analogously to the upper limit of doping concentration case previously discussed. To summarize, one can say that although As/P combined doping introduces additional charge carriers (electrons), thus increasing the charge carrier density, there seems to be an upper limit for the effectiveness of the MLD process and the corresponding mobility (electrical activity) of the extra carriers introduced in the Si(111) by the dopants. From this point of view, the tendency of P atoms to occupy Si lattice sites makes P a less suitable candidate than As to optimize the MLD process, although the overall electrical activity will still benefit from the combined As/P doping of the Si(111) surface. SAMs of dopantcontaining molecules can then be used to achieve ultra-shallow doping profiles of less than 10 nm in depth, provided that the anneal times at high temperatures are short enough to just overcome the kinetic barriers of the two processes aforementioned: dissociation of the binary oxides and subsequent diffusion of the dopants into the host semiconductor.

4

Conclusions

In conclusion, we have shown that stepwise reactions of group-V (P and As) molecular precursors make possible to achieve a 2/3 nanopatterning coverage of the Si(111) semiconductor surface. The overall reaction is exothermic, which makes the formation of the nanopatterning structure possible at room temperature. After annealed at high temperature regimes, the instability of the corresponding binary and ternary oxides is the driving force for dopant diffusion into the semiconductor surface. Our results show that, whereas the loss of O2 facilitates As doping, P doping can be easily achieved at high temperatures just by dissociation of the binary PO2 oxide. The changes in the electronic structure of the semiconductor surface induced by the doping impurities allow us to say that, although As/P combined doping introduces additional

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charge carriers (electrons), there seems to be an upper limit for the effectiveness of the MLD process and the corresponding mobility of the extra carriers introduced in the Si(111) by the corresponding dopant. From this point of view, the tendency of P atoms to occupy Si lattice sites might cause crystal damage, making P a less suitable candidate than As to optimize the MLD process, although the overall electrical activity will still benefit from the combined As/P doping of the Si(111) surface. Then, according to our calculations, the use of groupV-elements at the same time can reinforce the positive effects (As, mobility) and weaken the negative ones (P, structural stability). Within the proposed model, the maximum reachable As/P doping density is 18×1020 atoms/cm3 , approximately one order of magnitude lower than the Si bulk density. The exact doping density will obviously depend on many other factors, such as temperature and time of annealing. The experiments leading to stepwise reactions of different elements are very challenging. In our previous publications, 11,22,23 major parts of such experiments have already been shown. For instance, a 2/3 nanopatterning coverage of the semiconductor surface, was achieved and how shallow doping can be subsequently obtained was also demonstrated. The use of different dopants is what will ultimately allow a more complete control of the semiconductor properties (for instance, to form shallow p-n junctions) and, thus, this work opens a new door to such designs. Although theoretically possible, the design of MLD ultra-shallow p-n junctions raises additional difficulties that are currently under study.

Acknowledgement The results presented in this paper have been gained within the DFG-funded project TH 1566/3-2. The authors also acknowledge the Texas Advanced Computing Center (TACC) for providing computational resources.

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Supporting Information Available “Complete information on the adsorbed MPA and MAsA structures for the MLD process, together with the electronic band structures correspondig to the lower limit of P and/or As doping.” This material is available free of charge via the Internet at http://pubs.acs.org/.

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Nanopatterning of Group V Elements for Tailoring the Electronic

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