Article pubs.acs.org/JPCC
Identification of Surface Defects and Subsurface Dopants in a DeltaDoped System Using Simultaneous nc-AFM/STM and DFT E. J. Spadafora,† J. Berger,†,‡ P. Mutombo,† M. Telychko,† M. Švec,† Z. Majzik,† A. B. McLean,§ and P. Jelínek*,†,∥ †
Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, CZ-16200 Prague, Czech Republic Department of Physical Electronics, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Břehová 7, CZ-11519 Prague, Czech Republic § Department of Physics, Engineering Physics and Astronomy, Queen’s University, Kingston, Ontario K7L 3N6, Canada ∥ Graduate School of Engineering, Osaka University 2-1, Yamada-Oka, Suita, Osaka 565-0871, Japan ‡
ABSTRACT: We have studied the near surface defects of the delta-doped B:Si(111)(√3 × √3)R30° system using a combination of scanning tunneling microscopy, noncontact atomic force microscopy, Kelvin probe force spectroscopy, total energy DFT, and STM theoretical simulations. We positively identify and characterize two near surface defects: the adatom vacancy and a B substitutional defect located in the second Si bilayer. We also confirm the assignment of the dangling-bond defect. Additionally, the influence that the subsurface B dopants have on the surface electronic structure, the modulation of the surface potential and the chemical activity of the surface is investigated.
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INTRODUCTION Delta function-like doping profiles, defined by the fact that the extent of the dopant distribution in one spatial direction is smaller than the free carrier de Broglie wavelength, are a very appealing concept in semiconductor device design.1 The ultimate limit of delta doping is achieved when dopants are confined to a single atomic plane. Interestingly, it is possible to assemble a plane of B dopants in Si by segregating a third of a monolayer of bulk dopants into the B:Si(111)-(√3 × √3)R30° surface reconstruction (hereafter B√3).2−6 This has served as a very useful prototype for this class of systems. In the past few years, the B√3 surface reconstruction has also received attention because of its chemical passivity; it resists hydrogen etching7 and the cyclo-addition-like reactions that commonly occur when organic molecules are added to Si surfaces8−12 are noticeably absent: molecules do not form covalent bonds with the surface.13,14 Instead, molecules are bound to the surface by the van der Waals interaction and it has been possible to self-assemble small organic molecules into arrays on B√3 by taking advantage of intermolecular noncovalent interactions.15 The surface is, therefore, a suitable candidate for the nondissociative adsorption of organic molecules and the subsequent growth of supramolecular assemblies.16−19 However, despite the fact that local probes have now been used to study near-surface dopants in a wide range of semiconductors surfaces,20−23 the surface defects and dopantinduced near surface defects of the B√3 system still remain poorly understood. For example, even though there are a number of different surface defect geometries that can be constructed, to our knowledge, only the most commonly © 2014 American Chemical Society
occurring surface defect, the dangling bond (DB) defect, where a Si atom is located in the substitutional S5 site (Si−S5, see Results and Discussion), has been positively identified. Although studies of the surface using local probes have identified other surface defects, they have not yet been assigned to specific atomic geometries. Furthermore, this surface is now being used as a platform for assembling supra-molecular layers, but little is known about how the substitutional subsurface B dopants modify the surface energetics and affect the supramolecular assembly of small organic molecules. These lone dopants can also play a significant role on commercial device performances, as well as on the fundamental local properties of a semiconductor.21 The aim of the present study is, therefore, to provide a more complete understanding of the surface and subsurface defects states in B√3 using concurrent STM/nc-AFM, Kelvin probe force spectroscopy, and theoretical methods.
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EXPERIMENTAL METHODS The experiments were performed using a modified Omicron variable temperature AFM/STM, in UHV at a base pressure ≤10−10 mbar and at room temperature. STM and nc-AFM measurements were made using a qPlus sensor, fabricated by mounting an electrochemically etched tungsten tip to a quartz cantilever.24 This allows for the simultaneous acquisition of the average tunneling current flowing between the sample and an oscillating tip, and the average force acting upon the tip. The Received: April 7, 2014 Revised: June 27, 2014 Published: June 27, 2014 15744
dx.doi.org/10.1021/jp503410j | J. Phys. Chem. C 2014, 118, 15744−15753
The Journal of Physical Chemistry C
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The optimized geometry obtained during structural relaxation was used in simulations of STM images for the proposed structures. The model of the STM tip was a foursided tungsten pyramid derived from the W(100) surface structure. Our results were checked using several other tip geometries and, importantly, simulations that were performed using a Si terminated tip provided similar results. The STM simulations were carried out using the STM code, which uses Green’s function formalism to evaluate the tunneling current.33 The Green’s functions were calculated from a local basis set Hamiltonian obtained from the Fireball code using the optimized atomic structures of a given model. Constant height STM images, corresponding to a tip height of 4 Å over the surface and at a given bias voltage, were computed. Pseudoatomic functions with an enlarged cutoff radius R = 15 au were employed to derive the interatomic hopping (tunneling) probabilities between the surface atoms and an atom on the tip. The adatom layer and two layers below it were considered for tunneling. The first surface Brillouin zone was sampled by Γ-point for the purpose of STM simulations.
latter is inferred from the measured frequency shift. Crosstalk between the deflection channel and the tunneling current was eliminated by carefully modifying the internal wiring of the microscope and the qPlus tuning fork.25 The qPlus sensors used in this study typically had a resonant frequency ( f 0) of ∼55 ± 5 kHz, a Q factor ≥1700, an oscillation amplitude of ≤300 pm, and a stiffness of ≈3000 N·m−1. The latter has been determined from a thermal noise analysis of the sensor.26 The highly p-doped Si(111) wafers were manufactured, to our specifications, by Prime Wafers. The room temperature resistivity was less than 0.001 Ω·cm and the B concentration was greater than 1020 cm−3. To prepare the B√3 surface reconstruction, the samples were degassed in UHV at ∼550 °C for 12 h. Then, the sample was flash annealed to ∼1200 °C for 10 s and quickly cooled to about 800 °C. This cycle was repeated at least 5 times. Finally, the sample was cooled from 800 °C to room temperature at a rate of ∼10 °C s−1. In order to obtain the short-range interaction force FSR, a total interaction force was calculated from the average frequency shift spectroscopy Δf(z) using the procedure developed by Sader et al.27 For each set of curves, the shortrange interaction force FSR(z) was obtained by subtracting a fit to the total force in the long-range interaction region. The longrange region is defined from the free oscillation to the z position at which the curves start splitting and thus defining the onset of the short-range interaction.28 Following the method derived by Lantz et al.29 for a spherical tip and plane geometry, the long-range vdW forces can be best approximated by FvdW = AHR/6(z − z0)3, in which AH is the Hamaker constant, R is the mesoscopic tip radius, z is the sample displacement, and z0 is a constant that accounts for the height of the absorbed atom or cluster on the tip apex. An excellent fit to the data was found using the fitting parameters: AHR = 2.80 ± 0.02 × 10−36 J·m and z0 = −1.43 ± 0.01 nm.
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RESULTS AND DISCUSSION Filled and empty state topographical STM images taken from the same area of the B√3 surface are presented in Figure 1a
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THEORETICAL DETAILS Total energy calculations of the B√3 system were performed with the Fireball tight-binding ab initio code.30,31 Fireball uses an optimized spatially confined pseudoatomic orbital basis set. In our case, an s basis set32 was used for the H atoms and an spd basis set for the Si and B atoms. The cutoff radii of the pseudoatomic basis functions were as follows: R(H,s) = 4.0 au, R(Si,s) = 4.8 au, R(Si,p) = 5.48 au, R(Si,d) = 5.2 au, R(B,s) = 4.5 au, R(B,p) = 4.9 au, and R(B,d) = 5.2 au; local density approximation is used for the exchange correlation functional. The calculations were restricted to the Γ point of the first surface Brillouin zone. The bottom Si atomic layer as well as the hydrogen layer were kept fixed during the geometry optimization while all other atoms were allowed to relax freely into their equilibrium positions. The criterion for terminating the relaxation was that maximal forces on free atoms had to be below 0.05 eV/Å and the change of total energy between subsequent iterations had to be smaller than 10−4 eV per unit cell. Our atomic models of the Si(111) surface consisted of a slab consisting of 480 atoms, 13 atomic layers thick (including the adatom layer) with an additional passivating hydrogen layer on the underside. The lateral size of the supercell was 6 × 6 with respect to an unreconstructed Si(111) surface. The Si adatom layer was passivated by substituting the Si atoms directly under the adatoms with B atoms. Some of the models contained one unpassivated Si adatom, a Si adatom vacancy, and one more B substitutional defects in deeper layers.
Figure 1. STM topographical images of the (a) filled and (b) empty states of B√3, in which a Si DB, a subsurface B defect, and an Si adatom vacancy are highlighted by the blue, green, and red arrows, respectively. Image details: 21 nm × 21 nm, It = 0.5 nA, Vempty = +1.4 V, Vfilled = −0.5 V. The height scale is 0−3.5 Å.
and b, respectively. Boron atoms have moved into the S5 site that is located in the first Si bilayer, sweeping surface danglingbond (DB) states out of the energy gap (see Figure 2), passivating the surface, and stabilizing the B√3 surface reconstruction. However, despite the high structural quality of the surface, there are a number of defects. Namely, we identified three characteristic types of defects. They are particularly noticeable because their electronic contrasts in both filled and empty state images are different and are characteristic of each defect type. The brightest feature in the empty state image, labeled by the blue arrow in Figure 1b, is the dangling-bond defect. This has been previously identified and assigned to a Si atom in the S5 substitutional site (Si−S5) located below the Si adatom.34−38 We find that, in general, the DB defect is brighter in empty state images than in filled state images. Interestingly, in the filled states images, the DB feature is often surrounded by a dark region. The dark region has previously been attributed to Coulomb interactions between the charged DB and the free holes.35 15745
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triazine molecules were adsorbed onto B√3.13 This highlights the importance of understanding how native defects appear in STM images, which is the subject of this paper. We have shown previously on Si(111)-(7 × 7), a related system, that Si adatom vacancies can be difficult to discriminate from adsorbed molecules using STM measurements alone. However, this is straightforward if nc-AFM and STM measurements are performed concurrently.41 The precise control of the Si flashing and subsequent cooling rate can be used to control the number of unpassivated DB states as well as the underlying subsurface defect sites.42 Using the preparation procedure described above, the two most prominent defects that occur are the DBs and subsurface features (blue and green arrows, respectively, in Figure 1). Based on a statistical approximation of seven images encompassing a total area of ∼210 nm2 of the surface, the occurrence of the Si DB is approximately less than 0.5%, while subsurface B dopants occur more frequently at less than 4% of the B√3 sites. Lastly, the Si adatom vacancy rarely occurs, less than 0.05%. In order to identify the defects that we have observed on this surface, STM simulations are compared to the experimental STM images. Specifically, a comparison between experimental and calculated STM images of the three defect features is shown in Figure 2. The STM simulations were performed on optimized structures from the total energy DFT calculations with the W tip model. We note that similar results were obtained with a Si tip and a Si on W tip, suggesting that the STM simulations are not strongly dependent on the choice of the tip. The top and side views of the structural models that have been used to generate the simulations are presented in the right-hand panels of Figure 2. The top view model is overlaid on the STM simulation zoomed from the dashed black box. In the Si DB structure a Si atom occupies the S 5 substitutional site. This has been simulated in Figure 2a, and the simulation agrees well with experimental images: the DB defect is brighter in the empty state images than in filled state images. However, the simulated images did not produce a dark area surrounding the DB in the empty states, suggesting that this behavior may be due to surrounding subsurface B dopants. STM images of the second type of defect, shown in Figure 2b, are well reproduced if we consider an atomistic model comprised of two subsurface B atoms: one in the S5 site and one in the second bilayer. The intensity of the Si adatoms is brighter in filled state and darker in empty state images. Additionally, in the simulations, the presence of the subsurface B dopant also affects the neighboring Si adatoms anisotropically, in agreement with the STM observations. We calculated STM images for several structures with an additional substitutional B dopant placed in different subsurface layers to understand the influence of an extra B atom on the atomic STM contrast. Figure 3 shows the effect on the surface that a substitutional B dopant in the first three sublayers has (labeled B, C, and D in the model), as revealed through STM simulations. The spatial anisotropic intensity is not seen when the B atom is placed directly below the S5 site, as seen in the B position in Figure 3. We note that similar spatially anisotropic intensity distributions at the surface have been observed in other systems with charged subsurface B dopants.43−45 DFT calculations of a substitutional B dopant in the B, C, or D sites revealed that it is not energetically favorable for the B atom to sit in the B site. Here, the two opposing substitutional B atoms sitting the S5 and the B sites would enhance a repulsive
Figure 2. STM topography (16 Å × 16 Å, Is = 0.5 nA, Vempty = +1.4 V, Vfilled = −0.5 V), theoretical STM simulation and the model (side and top views) of the Si DB (a), the subsurface boron dopants (b), and the Si adatom vacancy (c) in the empty and filled states. In the model, the Si and B atoms are blue and green, respectively. The top view of the model is projected onto the area outlined by the dashed box in the corresponding STM simulation.
Another defect (green arrow in Figure 1), is assigned to subsurface B acceptors in the second bilayer (see Figures 2 and 3). This feature is bright in the filled state images but dark in the empty state images. A similar feature is found using STM to study subsurface B dopants in the Si(100)-(2 × 1) system,39,40 in which the local enhancement or darkening of the region near the B dopant site is caused by a local tip-induced band bending over the dopant sites. The occurrence of these subsurface features is more prevalent in the filled states than in the empty. Lastly, a Si adatom vacancy (red arrow) image as a hexagonal feature in the empty state and as a triangular feature in the filled state images. This feature is relatively rare on B√3 but similar features have been observed after 2; 4; 6-tri(2′-thienyl)-1; 3; 515746
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Figure 3. Model for the constant height STM simulation images (16 Å × 16 Å) using a W(100) tip with a substitutional B dopant in the first three sublayers (labeled B, C, and D) and line profiles through each figure. On the right-hand side is a comparable experimental constant height STM image (16 Å × 16 Å, Vempty = +1.4 V, Vfilled = −0.5 V) and normalized line profile.
constant at given bias voltage (+1.4 V and −0.5 V for empty and filled states, respectively). In the empty state images, the vacancy presents itself as a large protrusion over the B atom located in the S5 position. This has been corroborated with STM simulation, in which the protrusion is less diffuse, possibly due to the fact that the STM simulations were performed at 0 K or that the tip apex is larger in the experiment. To further explore how the subsurface B dopants alter the surface electronic structure, constant height nc-AFM/STM measurements were performed. Scanning at a constant height, the frequency shift (Δf) is predominantly used to identify the adatoms, while the tunneling current is used to detect changes in the surface states. To minimize the phantom force,46 the bias voltage has been reduced to 5 mV. Figure 4a,b shows a simultaneously acquired constant height AFM image of the frequency shift and tunneling current, respectively. These
electrostatic interaction between them. However, the total energy of a substitutional B atom in the C or D sites is comparable. The total energy difference between a substitutional B atom in the B and D sites is approximately 300 meV. Moreover, while in the fourth subsurface D layer, theoretical STM simulations shows a weaker contrast at the surface, as visible in the line profiles of Figure 3. The contrast becomes even more negligible in sequentially deeper layers, such that any surface contrast due to the subsurface B dopant becomes indistinguishable from the surrounding Si adatoms. To compare, an experimental constant height STM image has been recorded as seen on the right-hand side of Figure 3. This type of surface defect is the same as the ones in the constant current images in Figures 1 (green arrows) and 2b. The line profile through the central Si adatom reveals the anisotropic contrast behavior. Based on the match between experimental and theoretical STM images and normalized line profiles, in regard to the anisotropic characteristic and contrast difference, we conclude that the substitutional B defect is located in the second bilayer beneath the Si adatom. Here, it is most likely in the third (C) subsurface atomic layer, due to the contrast match with the experiment. However, due to the faint intensity contrast observed in the simulations of Figure 3, it might also be positioned in the subsequent bilayer, specifically the fourth (D) layer. The third defect observed is the Si adatom vacancy. Comparison of the experimental and theoretical STM images of the Si adatom vacancies, see Figure 2c, provides an excellent level of agreement. To the best of our knowledge, this kind of defect has not been previously identified. This is possibly because it is rarely found on B√3 and because the vacancy is not rendered as a dark feature centered on the position of the Si adatom in STM images, what one might intuitively expect. In constant current STM with the large tunneling current set point (i.e., close tip−sample distance), the tip tunnels directly into DB states present on three surrounding Si atoms in the first bilayer at the vacancy site. The presence of the 3 Si DBs produces higher current at the vacancy site than on the passivated Si layer. Consequently, the tip is retracted from the surface above the vacancy to keep the tunneling current
Figure 4. (a) Simultaneously acquired frequency shift Δf and (b) tunneling current It images using constant height AFM of the B√3 surface with V = +5 mV, Δf = −3 Hz, Aosc = 300 pm, and f 0 = 54.3 kHz. The red arrow indicates a Si adatom vacancy. (c, d) Line profiles taken over the region in images (a) and (b) which highlights the bright and dark regions in the tunneling current. 15747
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images were taken at a frequency shift set point Δf of −3 Hz and a vibrational amplitude A0 of 300 pm. Here, the B√3 surface displays a uniform frequency shift over the passivated Si sites, while revealing modulated electronic properties in the tunneling current, as seen in the line profile of Figure 4c,d. The bright feature, visible in the center of Figure 4a,b, is an isolated Si DB. Using nc-AFM, Kinoshita et al.47 found that the measured vertical displacement of a Si DB site relative to the normal Si B√3 sites was larger than the calculated height difference. A straightforward explanation for this difference is that the tip-site force interaction is larger over the Si−S5 sites, consistent with our spectroscopy findings (see Figures 5 and 6).
Figure 6. (a) Simultaneously acquired short-range force FSR(z) and (b) tunneling current It(z) spectra taken over the DB and over the Si adatoms in the bright and dark regions using a charged W tip. The inset in (b) shows the DB, bright and dark regions where these spectra were taken.
is several nA, (ii) over bright atoms, where the tunneling current is in the range 0.01−0.2 nA, and (iii) over the dark atoms, where the tunneling current is well below 0.01 nA. These bright and dark regions, highlighted by the line profile of Figure 4d, are caused by the inhomogeneous distribution of subsurface B dopants, beyond the second bilayer. Comparing the tunneling current values calculated for the ideal B√3 surface and the B√3 surface with an additional substitutional B dopant (see profiles plotted in Figure 3), we can identify the dark area as a region without substitutional B dopants in the near surface region. Vice versa, the bright region indicates areas rich of the substitutional B dopants in deeper atomic layers (see Figure 3). We acquired simultaneous site-specific force29,48 and tunneling49 current spectroscopy of different surface defects and areas. The F−z spectroscopy allows us to study directly the chemical reactivity of different surface sites. The site-specific F/ It−z spectroscopy is presented in Figure 5. Here, the frequency shift (Δf) induced by tip−sample interaction forces is recorded simultaneously with the tunneling current (It), as well as the dissipation energy, as a function of the tip−sample separation (z). The spectroscopy data presented is comprised of averaged measurements taken over four different surface sites: (i) DB site; (ii) a subsurface B dopant site which has been identified from the STM map of the same surface; and a given Si surface atom located in bright (iii) and dark (iv) area of the tunneling current (for detailed identification of specific sites see insets of Figure 5). In order to reduce the effect of electrostatic forces contributing to the spectroscopy measurements, the bias voltage was tuned to the contact potential difference (CPD) minimum, VCPD ≅ +0.7 V. The zero position of the horizontal axis corresponds to the tip−sample distance at which the frequency shift (Δf) set point is −6 Hz. A maximum short-range force FSR ≈ 0.8 nN was obtained over a single isolated Si DB, which is a similar magnitude to that obtained on the Si(111)-(7 × 7) surface.25 Above the subsurface B dopant site, a maximum short-range force ∼0.2 nN was obtained. This is approximately one-quarter of that obtained over the Si DB. The local maximum occurs at a tip− sample distance of
