Molecular Mechanism of Etching-Induced Faceting on Si(100

Article pubs.acs.org/JPCC

Molecular Mechanism of Etching-Induced Faceting on Si(100): Micromasking Is Not a Prerequisite for Pyramidal Texturing Erik S. Skibinski and Melissa A. Hines* Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853-1301, United States S Supporting Information *

ABSTRACT: Chemical control of crystalline surfaces during etching or growth finds application in a wide variety of areas, including the low-cost texturing of silicon solar cells to reduce reflectivity losses. Nevertheless, the kinetic processes that govern these morphological transformations are poorly understood. Here, we study the spontaneous nanoscale faceting of Si(100) surfaces during reaction with deoxygenated H2O using a combination of scanning tunneling microscopy, infrared spectroscopy, and kinetic Monte Carlo simulation. We show that this reaction is inherently unstable to kinetic faceting and that the flat-to-faceted transition is driven by the reactivity of a single chemical site present in a concentration of less than 0.4% of a monolayer. In contrast to previously postulated mechanisms, pyramidal faceting does not require “micromasks”: chemical heterogeneities, such as impurities, insoluble etch products, or H2 (g), that collect on the surface during etching and act as transient nanoscale etch masks. Instead, the etching reaction drives the formation of self-propagating, {111}-faced nanoscale hillocks. This study shows that the chemical control of surface morphology can be driven by minority species at concentrations far below the detection limit of surface spectroscopy. of an initially flat Si(100) wafer after an 18 h immersion in room temperature 0.10 M KCl (aq) which was adjusted to a pH of 7.5 with KOH (aq).8 After etching, the surface was entirely covered by a random array of ∼60 nm high faceted pyramids. Infrared absorption spectroscopy showed that the surface was terminated by a monolayer of H atoms. The characteristic energies of the Si−H stretching vibrations, shown in Figure 1c, are also sensitive probes of surface morphology. The infrared spectrum was dominated by two transitions: H bound to the Si(111) faces at 2083.4 cm−1 and H bound to the Si(110) edges at 2088.2 cm−1. The characteristic H/Si(100) modes were notably absent from the spectrum, indicating nearcomplete faceting. Rational improvement of pyramidal texturing has been stymied by a simple problem: the chemical and physical origins of the kinetic faceting transition are poorly understood. (In contrast, thermal faceting can be understood and predicted from surface tension anisotropies using the well-known Wulff construction.9) Early thinking was driven by the observation that etching-induced hillocks are often terminated by slow etching facets,10 which led to the plausible suggestion that fast etching faces may be inherently unstable to the formation of slow etching facets.11 Nevertheless, kinetic Monte Carlo (KMC) simulations have uncovered a number of cases where

1. INTRODUCTION Chemical control of crystalline surfaces finds application in a wide variety of fields spanning natural systems to applied materials to fundamental research; however, rational development of new processes is currently limited by our lack of understanding. The profound degree of control enabled by simple chemistry is perhaps best illustrated by nature.1 For example, brittlestar sea creatures grow exquisite single-crystal calcium carbonate lenses that are crystallographically aligned with photoreceptors and precisely tailored to minimize spherical aberration.2 The precision with which brittlestars and many other organisms control crystal structure far exceeds the best modern fabrication techniques both in quality and in cost, suggesting that improved understanding of these processes could lead to scientific and technological advances. More relevant to this work, chemical texturing of crystalline silicon surfaces is used in the photovoltaic industry to enable low-cost improvement in photoconversion efficiency.3 This application makes use of the chance discovery that some basic etching solutions transform initially flat Si(100) surfaces into highly textured surfaces decorated by random arrays of faceted nanoscale pyramids. The pyramidal texturing increases the number of light-surface interactions, as sketched in Figure 1a, thereby increasing conversion efficiency.4 Pyramidal texturing is predicted to increase the efficiency of silicon solar cells by up to 70%,5 and efficiency gains of 12% have been demonstrated.6 Nevertheless, the process suffers from irreproducibility and unpredictable time-dependent changes.7 This dramatic chemical transformation is illustrated by the atomic force micrograph in Figure 1b, which shows the surface © XXXX American Chemical Society

Special Issue: Steven J. Sibener Festschrift Received: June 25, 2014 Revised: September 6, 2014

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dx.doi.org/10.1021/jp5063385 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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This work uses analysis techniques developed during recent investigations of near-ideal H-terminated Si(100) surfaces produced by aqueous chemical etching. Because some regions of the surface studied here bear structural similarities to the near-ideal H/Si(100) surface, we summarize that structure briefly. An atomically flat H/Si(100) surface would be entirely dihydride-terminated; however, this structure would require H atoms on adjacent dihydrides to be only 0.15 nm apart, far below their 0.24 nm van der Waals diameter. To partially relieve this steric strain, NH4F-etched silicon surfaces adopt the alternating row morphology shown in Figure 2.28 This structure

Figure 1. Pyramidal texturing of an initially flat Si(100) surface. (a) Texturing reduces reflected light, (b) atomic force micrograph of etched Si(100) shows nanofaceted square pyramids, and (c) the Si−H stretch region of the infrared absorption spectrum of the textured surface is obtained with z-polarized radiation showing complete faceting.

fast etching surfaces show no such instability,12 suggesting that while this criterion may be necessary, it is definitely insufficient. A number of researchers have suggested that pyramidal faceting is driven by “micromasks”: chemical heterogeneities, such as impurities,13−15 insoluble etch products,16−19 or H2(g),20−24 that collect on the surface during etching and act as transient nanoscale etch masks.25,26 This suggestion is motivated by the well-known formation of Si{111} facets on masked Si(100) surfaces during anisotropic etching, a phenomenon that is often exploited in silicon micromachining.27 Nevertheless, detailed examination of this hypothesis raises a number of difficult-to-reconcile questions. While a more complicated model that invokes micromasking by semipermeable nanoscale particles (e.g., porous silicates) that selectively collect on the apexes and edges of the pyramids resolves some of these concerns,19 the existence of organized semipermeable etch products has not been experimentally verified. In this work, we used a different tactic to understand pyramidal faceting. Instead of studying fully faceted surfaces, we investigated a system that leads to a combination of flat Si(100) regions and nanoscale pyramids. Using morphological and spectroscopic probes in combination with atomistic simulations, we measured the site-specific reactivities of the various surface species and showed that the faceting transition was fully explained by the reactivity of a single chemical site that occurred primarily on pyramid apexes. Thus, micromasking is not required for faceting; some chemical reactions are inherently unstable. In addition, we showed that the pyramids were not static; they were self-propagating morphological features.

Figure 2. (a) Scanning tunneling microscopy (STM) image of Si(100) etched in NH4F(aq) showing alternating row morphology. Individual unstrained dihydrides are imaged as elliptical protrusions. (b) Molecular model of the alternating row morphology. Gray balls are H atoms, and blue balls are Si atoms. The trench dihydrides are sterically hindered and cant to partially relieve steric strain.

is formed by selective removal of every other row of Si atoms, which produces parallel rows of unstrained dihydrides. The trenches between these rows are also dihydride-terminated; however, the lattice structure leads to a 90° rotation of the trench dihydrides. The trench dihydrides are therefore highly strained, and both molecular simulations29−31 and spectroscopic measurements28 show that these dihydrides cant to partially relieve this strain. In the following, this surface structure will be referred to as the “alternating row morphology.” While structurally similar to the so-called “missing row” reconstruction observed on some clean transition metal surfaces,32 the alternating row morphology is a kinetic structure, whereas the missing row reconstruction is a thermodynamic (minimum energy) structure. B



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2. EXPERIMENT AND COMPUTATION 2.1. Experimental Methods. Prior to use, all Si wafers were thermally oxidized to a depth of 100 nm. Scanning tunneling microscopy (STM) samples were diced from n-type, 1−10 Ω cm Si(100), whereas Fourier transform infrared (FTIR) samples were cut into a 1.5 × 3.8 cm2 die from doubleside-polished, p-type, >1500 Ω cm, 500 μm thick float zone Si(100). The short sides of the FTIR samples were beveled at 45° for analysis in the multiple-internal-reflection geometry. All samples underwent a modified RCA clean33 as follows. Immediately before use, all labware was cleaned in an SC-1 bath of 1:1:5 by volume 30% H2O2(aq) (J.T. Baker, CMOS grade) to 28% NH3(aq) (BDH ACS grade) to ultrapure H2O (MilliQ) for 20 min at 80 °C to remove organic contamination. The labware was thoroughly rinsed with ultrapure water and then placed in an SC-2 bath of 1:1:5 by volume 30% H2O2(aq) to 37% HCl(aq) (BDH ACS grade) to ultrapure H2O for 20 min at 80 °C to remove metallic contamination. The sample was then cleaned sequentially in fresh SC-1 and SC-2 solutions for 15 min at 80 °C. The thermal oxide was removed with a 2 min immersion in 5:1 buffered oxide etch (BOE, J.T. Baker, CMOS grade). The sample was chemically oxidized in a fresh SC-1 bath for 15 min at 80 °C, followed by a 1 min immersion in BOE to produce a very clean, H-terminated sample. The surface morphology was determined by the final processing step, a 13 h immersion in deoxygenated ultrapure H2O under N2 purge (Airgas, ultrahigh purity). Prior to use, the H2O was deoxygenated without agitation for 9 h. After etching, an InGa ohmic contact was applied to the back of the STM samples, which were then load locked into an ultrahigh-vacuum STM. Infrared spectra were collected with a dry-air-purged Nicolet 670 FTIR spectrometer using a mercury−cadmium telluride detector and ZnSe grid polarizer (Molectron). Multiple-internal-reflection spectra were obtained with both s- and p-polarized radiation; then the spectra were computationally transformed to a Cartesian reference frame,34 with the surface normal defining the z-axis. Reference spectra were obtained by oxidizing the etched samples with O3 generated by a Hg pen lamp for 20 min. Interference fringes in the spectra were removed computationally.35 2.2. Kinetic Monte Carlo Model. The etching reaction was modeled using the atomistic, chemically realistic KMC model described in refs 12 and36. The surface was assumed to be entirely hydrogen-terminated; any “dangling bonds” created during etching were assumed to be immediately satisfied by hydrogen. Diffusion of surface atoms and redeposition of silicon from the solution were explicitly forbidden, as were undercutting reactions. These assumptions are well-justified for aqueous silicon etching as discussed in detail in ref 12. The model assumed that surface silicon atoms were sequentially etched in accordance with their site-specific etch rates, which were entirely determined by their local structure. As fully described in ref 12, the silicon atoms were first classified into bulk silicon, monohydrides, dihydrides, and trihydrides. The monohydrides and dihydrides were then subdivided according to their strain state. Finally, the dihydrides were further classified into three categories by their nearest-neighbor bonding. “Regular” dihydrides were bound to two bulk Si atoms, whereas α- and α2-dihydrides were bound to one or two silicon hydrides, respectively (ref 12 did not distinguish between α- and α2-dihydrides, which were

assumed to have identical etch rates in that study). A sample α2-dihydride is illustrated in Figure 3.

Figure 3. Molecular model of a sample α2-dihydride site. Here, the site decorates the apex of a {111}-faced square pyramid. The blue planes represent {111} and {110} facets.

The goal of the KMC simulations was to find the set of sitespecific etch rates that best fit the experimentally observed morphologies and spectra. Our approach was to first generate a large library of simulated flat and vicinal surfaces by independently varying each site-specific etch rate over many orders of magnitude and then to search this library for consistency with the experimental data. As described in the following, the final library contained 40,105 different sets of site-specific etch rates. For each set of site-specific etch rates, 50 monolayers of Si were etched from both a 96 × 96 nm2 flat surface and a 160 × 53 nm2 vicinal surface miscut by 3.5° toward the [011] direction; then the morphologies were stored. The etch rate space spanned by the library was chosen on the basis of previous research. Similar to NH4F(aq), deoxygenated H2O is a step-flow etchant of Si(111) surfaces.37−44 Since detailed investigations of the site-specific etching of Si(111) by H2O have not been performed, all simulations used the seven site-specific etch rates measured for NH4F/Si(111) etching.12 In other words, every set of site-specific etch rates in the library produced atomically flat Si(111). The initial library assumed that the etch rates of α- and α2-dihydrides were identical; the remaining six parameters were independently varied as described in ref 12. When none of the morphologies in this library were found to match the H2O experiments, the library was expanded to include cases where the etch rates of α- and α2-dihydrides were different (vide inf ra). More information on the library is contained in the Supporting Information.

3. RESULTS The morphology and infrared spectroscopy of H2O-etched Si(100) were first investigated by Faggin et al.,45 and our experimental results were consistent with many of their findings. The biggest difference was that our STM images were obtained at atomic resolution, displaying a far richer morphology than that observed in ref 45. In particular, regions of the H2O-etched surface displayed the alternating row morphology characteristic of NH4F-etched Si(100). In addition, the vibrational energies of strained canted dihydrides were unambiguously measured on the NH4F-etched Si(100) surface, which partially obviated the infrared mode assignments proposed by Faggin et al.45 In the following, we resolve these discrepancies, providing new insight into the mechanism of H2O/Si(100) etching. C



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3.1. Scanning Tunneling Microscopy. Consistent with Faggin et al.,45 the H2O-etched Si(100) surface displays three characteristic morphological features as seen in Figure 4. The most prominent features are ∼1.5 nm high square pyramids that are randomly distributed over the etched surface. Their faceted pyramidal shape is most apparent in rendered images such as Figure 4a. The pyramids are bounded by Si{111} nanofacets with Si{110} edges which lead to the well-resolved spectroscopic features discussed in section 3.3. In addition, variable-width raised stripes run parallel to the [011] and [01̅0] directions, often emanating from a pyramidal face. Finally, these features are set against a background of relatively flat terraces. The discrepancies with Faggin et al.45 became apparent at high resolution, where the “flat” terraces of ref 45 are shown to be weakly corrugated, corresponding to regions of the alternating row morphology characteristic of NH4F-etched Si(100) as shown by Figure 5. The long rows have a characteristic spacing of 0.76 nm, consistent with the removal of every other row of Si atoms. The rows themselves display elliptical features spaced by 0.38 nm, which is consistent with the imaging of close-packed silicon dihydrides on NH4F-etched Si(100).12 The long rows rotate by 90° on each successive terrace, in keeping with the crystallographic structure of Si(100). 3.2. KMC Simulations. KMC simulations were performed with the goal of reconciling the morphological and spectroscopic data while providing atomic-scale understanding. Two clues were used to search the simulated libraries. First, our spectroscopic data (vide inf ra) and those of Faggin et al.45 provided incontrovertible evidence of the formation of Si{111} and Si{110} microfacets. We therefore restricted our attention to those morphologies displaying ≥5% of a monolayer of Si{111} and ≥5% of a monolayer of Si{110}. (Because of uncertainties in the surface dielectric constant, the relevant oscillator strengths, and dynamic dipole coupling, these densities could not be directly measured from the infrared spectrum. We purposefully underestimated the site densities to avoid compromising the search.) Second, the high-resolution morphologies showed large regions of the alternating row morphology, which were comprised of an equal mixture of strained and unstrained dihydrides. We therefore restricted our attention to morphologies where the densities of strained and unstrained dihydrides were within 20% of one another. When these two search criteria were applied to the library described in ref 12 (which assumed that the α- and α2dihydride etch rates were identical), no candidate morphologies were found. In other words, the original model of site-specific Si(100) etching could not reproduce the experimental morphologies. From this, we hypothesized that the original model was incomplete. Prior research on kinetic hillock formation in two and three dimensions has pointed out the crucial role of the hillock apex site.13,26,46 If this site is too reactive, a nascent hillock etches away as soon as it is formed. In contrast, less reactive apexes stabilize hillock formation. Attempts to directly image the hillock apexes with atomic resolution were unsuccessful. Nevertheless, the {111}-faceted structure of the hillocks, seen both morphologically and spectroscopically, is most consistent with the unstrained-dihydride-terminated apex structure shown in Figure 3. This site is bound to two silicon monohydrides and is thus an α2-dihydride. To investigate the possible stabilizing role of the α2-dihydride site, the original simulation library was expanded to include

Figure 4. Experimental and simulated images of H2O-etched Si(100) displaying partially faceted morphology. (a) Rendered STM image showing nanofacets. (b) STM micrograph showing {111}-faceted hillocks, raised stripes, and relatively flat regions. Tip convolution effects increase the apparent size of the hillocks. (c) Best-fit KMC simulation of H2O-etched Si(100). Images b and c have the same scaling. The hillock densities are (b) 1.95 × 1012 cm−2 and (c) 1.72 × 1012 cm−2. D



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and simulations with an unacceptably high or low hillock density discarded. Figure 4 compares experiment to the best-fit simulation. A hillock density of 1.95 × 10 12 cm −2 was measured experimentally, whereas the best-fit simulation had a density of 1.72 × 1012 cm−2. The pyramids appeared to be somewhat larger in experiment than in simulation; however, this was almost certainly due to well-known tip convolution effects. The best-fit morphology displayed three characteristic structures. First, the majority of the surface was covered by the alternating row morphology sketched in Figure 2b. As expected, this was the same structure displayed by NH4Fetched Si(100). Second, the etched surface was decorated by randomly distributed nanoscale hillocks as sketched in Figure 7a. The hillocks had {111} faces, {110} edges, and α2-

Figure 5. High-resolution STM image of the H2O-etched Si(100) surface showing the alternating row morphology characteristic of flat regions.

cases where the etch rate of the α2-dihydride was lower than that of the α-dihydride (see the Supporting Information). When the expanded library was searched, 72 candidate morphologies were obtained, a representative sampling of which is shown in Figure 6. Some of these surfaces closely resembled the experimental morphology, whereas others did not. To find the best-fit morphology, the candidate morphologies were sorted by the average length of unstrained dihydride rows (a proxy for hillock density as seen in Figure 6)

Figure 7. Molecular models of characteristic features on H2O-etched Si(100). (a) A {111}-faceted square pyramid and (b) a stripe emanating from a pyramid. The (100) plane is in gray, whereas the {111} and {110} planes are in blue.

dihydride-terminated apexes. Finally, the stripes displayed an unexpected manifestation of nanofaceting as seen in Figure 7b. While the top of the stripes displayed the characteristic alternating row morphology, the sides of the stripes were bounded by {111} microfacets. The extent of Si{111} microfaceting was most easily seen in Figure 8, in which all of the {111} regions are highlighted in blue. 3.3. Infrared Spectroscopy. As first described by Faggin et al.,45 the infrared spectrum of H2O-etched Si(100) is characterized by seven resolved spectral bands plus an unknown number of unresolved bands, as seen in Figure 9. Interestingly, the congestion is primarily manifested in the inplane-polarized spectrum. Our assignment of the well-resolved bands is summarized in Table 1 and discussed in the following. Consistent with Faggin et al.,45 we assigned the two lowest energy modes, which were observed in both polarizations, unambiguously to H−Si{111} and H−Si{110} stretch vibrations. The mode at 2081.3 cm−1 was assigned to the

Figure 6. Representative sample of 72 simulated surfaces meeting initial search criteria, arranged in order of decreasing average stripe length which was (a) 33, (b) 17, (c) 16, and (d) 7 nm. The morphology in image a is the best fit to experiment. E



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inclination of the {111} and {110} planes, respectively, relative to the (100) face. The in-plane-polarized antisymmetric H− Si(110) stretch vibration at 2070.8 cm−1 was not resolved; its energy is denoted by a dashed line in Figure 9. On flat Si(100), the alternating row morphology gives rise to four Si−H stretch modes: the symmetric and antisymmetric stretch of unstrained dihydrides at 2103.7 cm−1 (z-polarized) and 2112.2 cm−1 (∥-polarized), respectively, and the isolated stretch vibrations of the lower and upper Si−H bonds on the strained canted dihydrides at 2084.9 cm−1 (∥-polarized) and 2142.3 cm−1 (z-polarized), respectively.28 On this basis, we assigned the modes at 2104 and 2137 cm−1 to the symmetric stretch of unstrained dihydrides and the upper Si−H stretch of strained canted dihydrides, respectively. The vibrational modes associated with the lower Si−H bond and the antisymmetric stretch of unstrained dihydrides, which are nominally ∥-polarized, were unresolved as indicated by the dashed lines in Figure 9. The KMC simulations also revealed a high density of strained monohydride sites, as seen in Figure 10. Some of these

Figure 8. Rendered image of best-fit simulation with {111} sites on the pyramids and stripes highlighted in blue.

Figure 9. Infrared absorption spectra of the H2O-etched surface. The spectra were obtained with s- and p-polarized radiation and then transformed into their Cartesian components. The dotted red lines indicate resolved spectral bands in Table 1, whereas the dashed gray lines indicate unresolved bands.

Table 1. Vibrational Assignments for Si−H Stretch Modes on H2O-Etched Si(100) Surfaces energy (cm−1)

a

principal polarization a

2070.8 2081.3 2084.9 2088.7 2092.4 2099

in plane both in planea both both in planea

2104 2110 2112 2115.8 2137

z both in planea z z

assignment H−Si{110}, antisymmetric H−Si{111} strained dihydride, lower Si−H H−Si{110}, symmetric pyramid mode strained opposing monohydrides, antisym. unstrained dihydride, symmetric monohydride strained by dihydride unstrained dihydride, antisymmetric strained opposing monohydrides, sym. strained dihydride, upper Si−H

Figure 10. Best-fit simulated image of H2O-etched Si(100) with strained monohydride sites highlighted in green.

sites were strained by adjacent dihydrides, whereas others were strained by adjacent monohydrides. On vicinal Si(111) surfaces, monohydrides strained by a single adjacent dihydride give rise to the C2 mode at 2101.3 cm−1.47,51 On Si(100), monohydrides were typically strained by an entire row of strained dihydrides, not a single dihydride, and were thus expected to have a higher vibrational energy. On this basis, we assigned the mode at 2110 cm−1 to a monohydride strained by adjacent dihydrides. In addition, the simulations showed a significant density of opposing monohydrides (monohydrides strained by monohydrides), which would give rise to two coupled modes. We therefore assigned the modes at 2099 cm−1 (in-plane-polarized) and 2115.8 cm−1 (z-polarized) to the antisymmetric and symmetric stretch vibrations of opposing monohydrides, respectively. One final mode at 2092.4 cm−1 was observed in both polarizations. Both the polarization and the singleton nature of

Unresolved.

monohydride stretch on {111} facets. This energy was redshifted by 2 and 1 cm−1 from that observed on flat H− Si(111)47 and 9°-miscut H−Si(111),48 which was consistent with the reduced dynamic dipole coupling on nanofacets.49 The mode at 2088.7 cm−1 was assigned to the symmetric H− Si{110} stretch vibration, in excellent agreement with the observed energy on flat H−Si(110).50 Both modes were observed in both polarizations, consistent with 54.7° and 45° F



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of a single surface site. In Figure 11, the locations of the α2dihydrides are indicated by green markers, which show that the α2-dihydrides appear preferentially on hillock apexes. The density of α2-dihydrides in Figure 11b was ∼200 times smaller than that in Figure 11a. In this system, other proposed mechanisms for pyramidal texturing can be ruled out. First, the characteristic fewnanometers spacing between nanoscale hillocks, seen in Figure 4, precludes micromasking by H2 bubbles, as bubbles of this diameter would be unstable due to their high internal pressures (hundreds of atmospheres).53 Second, these experiments have been reproduced over the past decade on different wafers, in different apparatuses, and in different laboratories. This high reproducibility rules out micromasking by contaminants. Finally, simulations of micromasking by transient etch products led to the production of spire-capped pyramids (Supporting Information) which were not observed experimentally. The mechanistic origin of slow α2-dihydride reactivity remains unclear; however, this may be a manifestation of long-range strain fields generated by the strained dihydride rows on the flat terraces. Most α-dihydride sites appeared on the highly strained terraces. Previous research on clean Si(100) surfaces showed that steric effects generate surprisingly longrange strain fields that significantly influence site energetics.54 In contrast, the α2-dihydrides selectively (although not exclusively) decorated sites on the nanoscale pyramids, whose Si{111} and {110} surfaces are essentially strain-free.55 4.2. Etch Hillocks are Self-Propagating Species. In this system, pyramidal faceting was not due to micromasking or pinning of the apex site. Instead, etch hillocks were selfpropagating, continuously etching species as seen in the accompanying QuickTime movies (Supporting Information). Etching of a hillock apex (α2-dihydride) led to the creation of a new apex site in the layer beneath the original. In addition, the faces of the nanoscale pyramids underwent the same step-flow etching reactions as flat Si(111) surfaces.56 This mechanism had an important consequence: pyramidal hillocks did not grow unbounded even when the α2-dihydride etch rate was set to zero, as the apex site was still removed by advancing steps on the Si{111} nanofacet. The corollary of this is that we have been unable to reproduce the micrometer-scale pyramids sometimes seen experimentally using a purely sitespecific model. This was likely a limitation of our current model, which did not include long-range (e.g., diffusional) effects, such as step bunching.57,58 4.3. Nanofaceting Not Restricted to Hillocks. The faceting mechanism was more complicated than simple nucleation and growth of pyramidal hillocks. Once formed, the pyramidal hillocks acted as pinning sites toward terrace etching. This pinning led to the generation of nanofaceted stripes which tended to form between adjacent hillocks. This mechanism is most easily understood by viewing the accompanying QuickTime movies (Supporting Information). Because of this, Si{111} nanofaceting was not restricted to the hillocks; it also occurred on the sides of the stripes as seen in Figure 8. 4.4. Implications for chemical control of morphology. This study shows that the reactivity of a single chemical site, the α2-dihydride, present in a concentration of less than 0.4% of a monolayer, dramatically influenced the etch morphology. This observation has profound implications for chemical understanding of morphological control, as spectroscopic analysis of single-crystal surfaces has a detection sensitivity of a few

the mode were suggestive of a monohydride stretch; however, the energy was significantly less than that of other strained monohydrides. On this basis we tentatively assigned this mode to a pyramid mode of unknown structure, perhaps arising from coupled monohydrides near the pyramid apexes. Clearly, characterizing unstrained monohydrides near the apexes as either H−Si{111} or H−Si{110} is overly simplistic. The accuracy of ab initio calculations of vibrational mode energies has improved in recent years; however, the errors inherent to the calculation of highly strained Si−H species on Si(100) remain too large for accurate mode assignments.31,52 For this reason, we did not test our assignments computationally but welcome the input of experts.

4. DISCUSSION These experiments showed that some chemical reactions are inherently unstable toward kinetic faceting even in the absence of chemical heterogeneities, such as impurities or etch products. While these experiments focused on etching reactions, similar phenomena are expected in growth reactions. 4.1. Kinetic Faceting Transition Controlled By Reactivity of a Single Site. In this system, the kinetic faceting transition was controlled by the reactivity of a single site: the α2-dihydride. While this site occurred preferentially on hillock apexes, the site was defined by its local structure, not its location. The effect of α2-dihydride reactivity is illustrated by Figure 11. The surface in Figure 11a was generated using the best-fit site-specific reaction rates, in which the reactivity of the α2dihydride was 1000 times less than the reactivity of the αdihydride. In contrast, when these two reaction rates were identical, no faceting was observed, as seen in Figure 11b. Thus, the morphological transition was driven solely by the reactivity

Figure 11. Effect of α2-dihydride reactivity on kinetic faceting. (a) When the α2-dihydride was 1000 times less reactive than the αdihydride, significant faceting was observed, whereas (b) no faceting was observed when the reactivity of the α- and α2-dihydrides were identical. Green markers denote the locations of the α2-dihydrides in both images, although none are visible in image b. G



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the NSF IGERT program (Grant DGE-0903653). This work made use of the Cornell Center for Materials Research Shared Facilities which were supported through the NSF MRSEC program (Grant DMR-1120296). We thank Marc Faggin and Kent Hallman for the AFM image of nanofaceted Si(100) and Prof. Kate Queeney for stimulating discussions.

percent of a monolayer. Therefore, the faceting transition is due to a spectroscopically “invisible” site. Compounding this problem, the most abundant surface sitesand thus the most easily detected siteshave the lowest etch rates and play the least important roles in determining the final morphology. This study illustrates the importance of multifaceted investigations of the chemical control of surface morphology which combine both experimental and computational techniques.

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(1) Addadi, L.; Weiner, S. Control and Design Principles in Biological Mineralization. Angew. Chem., Int. Ed. Engl. 1992, 31, 153− 169. (2) Aizenberg, J.; Tkachenko, A.; Weiner, S.; Addadi, L.; Hendler, G. Calcitic Microlenses as Part of the Photoreceptor System in Brittlestars. Nature 2001, 412, 819−822. (3) Chu, A. K.; Wang, J. S.; Tsai, Z. Y.; Lee, C. K. A Simple and CostEffective Approach for Fabricating Pyramids on Crystalline Silicon Wafers. Sol. Energy Mater. 2009, 93, 1276−1280. (4) Papet, P.; Nichiporuk, O.; Kaminski, A.; Rozier, Y.; Kraiem, J.; Lelievre, J. F.; Chaumartin, A.; Fave, A.; Lemiti, M. Pyramidal Texturing of Silicon Solar Cell With TMAH Chemical Anisotropic Etching. Sol. Energy Mater. 2006, 90, 2319−2328. (5) Saha, H.; Datta, S. K.; Mukhopadhyay, K.; Banerjee, S.; Mukherjee, M. Influence of Surface Texturization on the Light Trapping and Spectral Response of Silicon Solar Cells. IEEE Trans. Electron Devices 1992, 39, 1100−1107. (6) Summonte, Rizzoli, R.; Iencinella, D.; Centurioni, E.; Desalvo, A.; Zignani, F.; , Proceedings of PV in Europe: From PV Technology to Energy Solutions, Rome, Italy, 2002; pp 339−342. (7) Kashkoush, I.; Rieker, J.; Chen, C.; Nemeth, D.; Danel, A. How to Overcome the Effects of Silicon Build-Up During Solar Cell Wet Chemical Processing. Solid State Phenom. 2013, 195, 289−292. (8) Faggin, M.; Hallman, K.; Hines, M. A. Unpublished data. (9) Wortis, M. Equilibrium Crystal Shapes and Interfacial Phase Transitions. Chem. Phys. Solid Surf. 1988, 7, 367−405. (10) Batterman, B. W. Hillocks, Pits and Etch Rate in Germanium Crystals. Appl. Phys. Lett. 1957, 28, 1236−1241. (11) Jaccodine, R. J. Use of Modified Free Energy Theorems to Predict Equilibrium Growing and Etching Shapes. J. Appl. Phys. 1962, 33, 2643−2647. (12) Hines, M. A.; Faggin, M. F.; Gupta, A.; Aldinger, B. S.; Bao, K. Self-Propagating Surface Reactions Produce Near-Ideal Si(100) Surfaces. J. Phys. Chem. C 2012, 116, 18920−18929. (13) Gosálvez, M. A.; Nieminen, R. M. Surface Morphology During Anisotropic Wet Chemical Etching of Crystalline Silicon. New J. Phys. 2003, 5, 100.1−100.28. (14) Tanaka, H.; Abe, Y.; Toneyama, T.; Ishikawa, J.; Takenaka, O.; Inoue, K. Effects of Small Amount of Impurities on Etching of Silicon in Aqueous Potassium Hydroxide Solutions. Sens. Actuators, A 2000, 82, 270−273. (15) Bhatnagar, Y. K.; Nathan, A.; Lu, Y. New Observations on Pyramidal Hillocks in the Anisotropic Etching of ⟨100⟩ Silicon. Sens. Mater. 1996, 8, 423−429. (16) Bassous, E.; Baran, E. F. The Fabrication of High Precision Nozzles by the Anisotropic Etching of (100) Silicon. J. Electrochem. Soc. 1978, 125, 1321−1327. (17) Landsberger, L. M.; Naseh, S.; Kahrizi, M.; Paranjape, M. On Hillocks Generated During Anisotropic Etching of Si in TMAH. J. Microelectromech. Syst. 1996, 5, 106−116. (18) Tan, S.-S.; Reed, M. L.; Boudreau, R. Mechanisms of Etch Hillock Formation. J. Microelectromech. Syst. 1996, 5, 66−72. (19) Nijdam, A. J.; Van Veenendaal, E.; Cuppen, H. M.; Van Suchtelen, J.; Reed, M. L.; Gardeniers, J. G. E.; Van Enckevort, W. J. P.; Vlieg, E.; Elwenspoek, M. Formation and Stabilization of Pyramidal Etch Hillocks on Silicon {100} in Anisotropic Etchants: Experiments and Monte Carlo Simulation. J. Appl. Phys. 2001, 89, No. 4113. (20) Palik, E. D.; Glembocki, O. J.; Heard, I., Jr.; Burno, P. S.; Tenerz, L. Etching Roughness for (100) Silicon Surfaces in Aqueous KOH. J. Appl. Phys. 1991, 70, 3291−3300.

5. CONCLUSION Etching-induced faceting of H-terminated Si(100) surfaces by deoxygenated H2O was shown to be controlled by the reactivity of a single site using a combination of STM, infrared spectroscopy, and kinetic Monte Carlo simulation. In spite of its dominant morphological role, this site was present in vanishingly small concentrations (∼0.4% of a monolayer), well below that needed for spectroscopic detection. This special site was primarily found on the apexes of nanoscale, {111}-faced pyramidal etch hillocks, and the relatively low reactivity of the site was crucial to hillock stability. Importantly, the nanoscale pyramids were not static structures; they were evolving, selfpropagating features. In addition, nanoscale facet formation led to the creation of highly strained monohydride and dihydride sites at facet intersections. These highly strained sites had characteristic infrared transition energies and polarizations that were distinct from those on the flat and faceted regions. As a result, infrared spectroscopy can be used as a sensitive indicator of nanoscale morphology. In contrast to previously postulated mechanisms, faceting was driven entirely by site-specific chemical reactivity, not by micromasks or chemical heterogeneities, such as impurities, insoluble etch products, or gas bubbles. This research demonstrates that simple chemical reactions can produce surprisingly complex morphological features that, if understood and controlled, could have widespread scientific and technological impact.

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ASSOCIATED CONTENT

S Supporting Information *

Two movies in QuickTime format showing KMC simulations of H2O/Si(100) etching and the evolution of nanoscale pyramidal hillocks, text describing the simulations and a detailed comparison of the H2O- and NH4F-etched surfaces, figures showing the four dihydride strain states and their associated α-dihydride sites, simulated NH4F etching, and a rendered image of a small region of Figure S2 showing contaminant-capped asperities, and tables listing parameters used to describe step-flow etching of Si(111), parameter space spanned by the library used to model H2O etching of Si(100), and comparison of site densities for H2O- and NH4F-etched Si(100) surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1-607-255-3040. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF) under Award CHE-1303998. E.S.S. was supported by H



Article

(21) Campbell, S. A.; Cooper, K.; Dixon, L.; Earwaker, R.; Port, S. N.; Schiffrin, D. J. Inhibition of Pyramid Formation in the Etching of Si p ⟨100⟩ in Aqueous Potassium Hydroxide-Isopropanol. J. Micromech. Microeng. 1995, 5, 209−218. (22) Baum, T.; Schiffrin, D. J. AFM Study of Surface Finish Improvement By Ultrasound in the Anisotropic Etching of Si ⟨100⟩ in KOH for Micromachining Applications. J. Micromech. Microeng. 1997, 7, 338−342. (23) Baum, T.; Satherly, J.; Schiffrin, D. J. Contact Angle, Gas Bubble Detachment, and Surface Roughness in the Anisotropic Dissolution of Si(100) in Aqueous KOH. Langmuir 1998, 14, 2925−2928. (24) Vu, Q.-B.; Stricker, D. A.; Zavracky, P. M. Surface Characteristics of (100) Silicon Anisotropically Etched in Aqueous KOH. J. Electrochem. Soc. 1996, 143, 1372−1375. (25) Suárez, M. P.; Mirabella, D. A.; Aldao, C. M. Formation of Pyramidal Etch Hillocks in a Kossel Crystal. Surf. Sci. 2005, 599, 221− 229. (26) Mirabella, D. A.; Suárez, M. P.; Aldao, C. M. Hillock Sizes After Wet Etching in Silicon. Surf. Sci. 2009, 603, 3346−3349. (27) Elwenspoek, M.; Jansen, H. V. Silicon Micromachining; Cambridge University Press: Cambridge, U. K., 2004; p 420. (28) Clark, I. T.; Aldinger, B. S.; Gupta, A.; Hines, M. A. Aqueous Etching Produces Si(100) Surfaces of Near-Atomic Flatness: Stress Minimization Does Not Control Morphology. J. Phys. Chem. C 2010, 114, 423−428. (29) Ciraci, S.; Batra, I. P. Theory of Transition From the Dihydride to the Monohydride Phase on the Si(001) Surface. Surf. Sci. 1986, 178, 80−89. (30) Northrup, J. E. Structure of Si(100) H: Dependence on the H Chemical Potential. Phys. Rev. B 1991, 44, 1419−1422. (31) Freking, U.; Krüger, P.; Mazur, A.; Pollmann, J. Surface Phonons of Si(001)-(1 × 1) Dihydride. Phys. Rev. B 2004, 69, No. 035315. (32) Ho, K.-M.; Bohnen, K. P. Stability of the Missing-Row Reconstruction on FCC (110) Transition-Metal Surfaces. Phys. Rev. Lett. 1987, 59, 1833−1836. (33) Kern, W.; Puotinen, D. A. Cleaning Solutions Based on Hydrogen Peroxide for Use in Silicon Semiconductor Technology. RCA Rev. 1970, 31, 187−206. (34) Clark, I. T.; Aldinger, B. S.; Gupta, A.; Hines, M. A. Extracting Maximum Information From Polarized Surface Vibrational Spectra: Application to Etched, H-Terminated Si(110) Surfaces. J. Chem. Phys. 2008, 128, No. 144711. (35) Faggin, M. F.; Hines, M. A. Improved Algorithm for the Suppression of Interference Fringe in Absorption Spectroscopy. Rev. Sci. Instrum. 2004, 75, 4547−4553. (36) Gupta, A.; Aldinger, B. S.; Faggin, M. F.; Hines, M. A. Kinetic Monte Carlo Simulations of Anisotropic Si(100) Etching: Modeling the Chemical Origins of Characteristic Etch Morphologies. J. Chem. Phys. 2010, 133, No. 044710. (37) Watanabe, S.; Nakayama, N.; Ito, T. Homogeneous HydrogenTerminated Si(111) Surface Formed Using HF Solution and Water. Appl. Phys. Lett. 1991, 59, 1458−1460. (38) Pietsch, G. J.; Köhler, U.; Henzler, M. Direct Observation of Silicon Surface Etching by Water with Scanning Tunneling Microscopy. Chem. Phys. Lett. 1992, 197, 346−351. (39) Watanabe, S.; Sugita, Y. The Role of Dissolved Oxygen in Hot Water During Dissolving Oxides and Terminating Silicon Surfaces With Hydrogen. Surf. Sci. 1995, 327, 1−8. (40) Usuda, K.; Yamada, K. Atomic Force Microscopy Observations of Si Surfaces After Rinsing in Ultrapure Water With Low Dissolved Oxygen Concentration. J. Electrochem. Soc. 1997, 144, 3204−3207. (41) Watanabe, S. Chemical Structure and Surface Phonons Associated With H on Si. J. Chem. Phys. 1998, 108, 5965−5974. (42) Watanabe, S. Step Structure of Silicon Surface Hydrogenated in Solution as Revealed with Angle-Resolved Polarized Infrared Spectroscopy. Surf. Sci. 1998, 415, 385−391. (43) Fukidome, H.; Matsumura, M.; Komeda, T.; Namba, K.; Nishioka, Y. In-Situ Atomic Force Microscopy Observation of

Dissolution Process of Si(111) in Oxygen-Free Water At Room Temperature. Electrochem. Solid-State Lett. 1999, 2, 393−394. (44) Watanabe, S. Chemical Structure of Dihydride Phase on Saturated H-Chemisorbed Si Surfaces. J. Chem. Phys. 2000, 113, 2423−2429. (45) Faggin, M. F.; Green, S. K.; Clark, I. T.; Queeney, K. T.; Hines, M. A. Production of Highly Homogeneous Si(100) Surfaces By H2O Etching: Surface Morphology and the Role of Strain. J. Am. Chem. Soc. 2006, 128, 11455−11462. (46) Flidr, J.; Huang, Y.-C.; Hines, M. A. An Atomistic Mechanism for the Production of Two- and Three-Dimensional Etch Hillocks on Si(111) Surfaces. J. Chem. Phys. 1999, 111, 6970−6981. (47) Jakob, P.; Chabal, Y. J. Chemical Etching of Vicinal Si(111): Dependence of the Surface Structure and the Hydrogen Termination on the pH of the Etching Solutions. J. Chem. Phys. 1991, 95, 2897− 2909. (48) Jakob, P.; Chabal, Y. J.; Kuhnke, K.; Christman, S. B. Monohydride Structures on Chemically Prepared Silicon Surfaces. Surf. Sci. 1994, 302, 49−56. (49) Jakob, P.; Chabal, Y. J.; Raghavachari, K. Lineshape Analysis of the Si−H Stretching Mode of the Ideally H-Terminated Si(111) Surface: The Role of Dynamical Dipole Coupling. Chem. Phys. Lett. 1991, 187, 325−333. (50) Watanabe, S. Vibrational Study on Si(110) Surface Hydrogenated in Solutions. Surf. Sci. 1996, 351, 149−155. (51) Hines, M. A.; Chabal, Y. J.; Harris, T. D.; Harris, A. L. Measuring the Structure of Etched Silicon Surfaces With Raman Spectroscopy. J. Chem. Phys. 1994, 101, 8055−8072. (52) Gupta, A. Anisotropic etching of Si(100) in aqueous solutions. Ph.D. Thesis, Cornell University, Ithaca, New York, 2011, 184 pp. (53) Tyrrell, J. W. G.; Attard, P. Images of Nanobubbles on Hydrophobic Surfaces and Their Interactions. Phys. Rev. Lett. 2001, 87, No. 176104. (54) Webb, M. B. Strain Effects on Si(001). Surf. Sci. 1994, 299, 454−468. (55) Li, X.-P.; Vanderbilt, D. Calculation of Phonon-Phonon Interactions and Two-Phonon Bound States on the Si(111):H Surface. Phys. Rev. Lett. 1992, 69, 2543−2546. (56) Hines, M. A. In Search of Perfection: Understanding the Highly Defect-Selective Chemistry of Anisotropic Etching. Annu. Rev. Phys. Chem. 2003, 54, 29−56. (57) Garcia, S. P.; Bao, H.; Hines, M. A. Etchant Anisotropy Controls the Step Bunching Instability in KOH Etching of Silicon. Phys. Rev. Lett. 2004, 93, No. 166102. (58) Garcia, S. P.; Bao, H.; Hines, M. A. Effects of Diffusional Processes on Crystal Etching: Kinematic Theory Extended to Two Dimensions. J. Phys. Chem. B 2004, 108, 6062−6071.

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Molecular Mechanism of Etching-Induced Faceting on Si(100

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