Bismuth trichloride as a molecular precursor for silicon doping

Dopant impurity species can be incorporated into the silicon (001) surface via the adsorption and dissociation of simple precursor molecules. Examples include phosphine (PH 3 ), arsine (AsH 3 ), and diborane (B 2 H 6 ) for the incorporation of phosphorus, arsenic, and boron, respectively. Through exploitation of precursor surface chemistry, the spatial locations of these incorporated dopants can be controlled at the atomic scale via the patterning of a hydrogen lithographic resist layer using scanning tunneling microscopy (STM). There is strong interest in the spatial control of bismuth atoms incorporated into silicon for quantum technological applications; however, there is currently no known precursor for the incorporation of bismuth that is compatible with this STM-based lithographic method. Here, we explore the precursor chemistry (adsorption, diffusion, and dissociation) of bismuth trichloride (BiCl 3 ) on Si(001). We show atomic-resolution STM images of BiCl 3 exposed Si(001) surfaces at low coverage and combine this with density functional theory calculations to produce a model of the surface processes and the observed features. Our results show that, at room temperature, BiCl 3 completely dissociates to

Electronic devices that operate at the atomic scale can be fabricated in silicon using a scanning tunneling microscope (STM) as a tool to deterministically position individual phosphorus dopant atoms with nanometer precision; 1 recent examples include the single atom transistor 2 and a two qubit gate. 3Dopant positioning is achieved via the use of a hydrogen termination layer on the silicon (001) surface as a lithographic resist, patterned by STM, to spatially control the adsorption and dissociation of the phosphorus precursor compound phosphine (PH 3 ).Recent work has demonstrated that arsenic donors and boron acceptors can be deterministically positioned into silicon with nanometer precision in an analogous manner, using arsine (AsH 3 ) 4 and diborane (B 2 H 6 ) 5 as the precursor species, respectively.Recently, the chloride compounds boron trichloride (BCl 3 ) 6,7 and aluminum trichloride (AlCl 3 ) 8 have been explored for acceptor positioning.Chlorine atoms, like hydrogen, are known to desorb from the silicon surface under thermal processing conditions and chlorine has been explored as an alternative resist layer. 9,10re, we present experiments reporting a precursor for spatially positioning bismuth into silicon using STM: bismuth trichloride (BiCl 3 ).][15] Using atomic-resolution STM imaging combined with density functional theory (DFT) geometry optimization and transition state calculations, we show that BiCl 3 adsorbs completely dissociatively on Si(001) to produce surface adsorbed chlorine and bismuth atoms, and thus, we find BiCl 3 to be a promising candidate for the controlled positioning of bismuth into silicon.That is, the dissociative adsorption of BiCl 3 into lithographically defined regions of clean Si(001) surface can be anticipated to produce single bismuth atoms in pre-defined locations in analogy to established processes for phosphine, arsine, and diborane. 1,4,5e prepared Si(001) samples (arsenic-doped, 0:04-0:06 X cm) by degassing in ultrahigh vacuum (<2 Â 10 À10 mbar) at 550 C overnight followed by flash annealing several times to 1150 C for 10 s.BiCl 3 is relatively nontoxic solid that we have obtained in powder form (Fisher Scientific) and evaporated in a low temperature effusion cell (MBE Komponenten) held at 90 C, with the silicon sample at room temperature.STM experiments were performed at room temperature in an Omicron GmbH LT-STM using chemically etched tungsten tips.
We have performed density functional theory (DFT) calculations using the hybrid exact-exchange functional B3LYP 16,17 and the Gaussian 16 software. 18A Si 33 H 28 cluster model was used to represent the Si(001) surface (see supplementary material Fig. 1).Twenty eight hydrogen atoms form a chemical termination for all silicon atoms other than the surface dimer atoms.These cluster-terminating hydrogen atoms were held in fixed positions during geometry optimization in order to emulate the strain that would be imposed by the surrounding surface and bulk atoms of an extended surface.Adsorption energies, E ads , are calculated relative to the clean Si(001) surface and a gas phase BiCl 3 molecule.A detailed description of our computational methodology can be found in the supplementary material accompanying this Letter.
Figures 1(a) and 1(b) show filled-and empty-state images of a surface that has been exposed to BiCl 3 for one second with the sample at room temperature.The silicon dimer rows that run along the ½110 direction are aligned running horizontally in the image.There are six key features in these images that can be attributed to the adsorption and dissociation of BiCl 3 .These features are highlighted by squares in Figs.1(a) and 1(b), and enlarged in Figs.1(c)-1(h), with corresponding structural schematics indicating our feature assignments.We did not identify any features other than these, and Figs.1(a) and 1(b) are representative of the surfaces that we imaged following the exposure of Si(001) to BiCl 3 .
The three features labeled Cl1, Cl2, and Cl3 have been described previously in experiments in which the Si(001) surface was exposed to molecular chlorine (Cl 2 ) at room temperature. 19These features are composed of one or two chlorine atoms attached to the surface as shown in the accompanying schematics.Cl1 is two chlorine atoms adsorbed to a single dimer in a monochloride structure; Cl2 is two chlorine atoms bound to two neighboring dimers and leaving two dangling bonds at the ends of each dimer; and Cl3 is a single chlorine bound to a dimer, where there is also some motion of the chlorine atom from one end of the dimer to the other as the image is scanned, producing a streaky appearance for the features. 19n addition to these chlorine features, we see bright elongated features on the surface, labeled g1 and shown in Fig. 1(f).Such features have been previously observed in STM experiments of Si(001) surfaces exposed to low coverages of elemental bismuth via evaporation and are attributed to bismuth ad-dimers. 20,21Four configurations of bismuth ad-dimers are known to be stable from STM experiments and DFT calculations, [22][23][24] where the ad-dimer bond is located either on top of the dimer row or in the trough between dimer rows, and the bond orientation is either parallel or perpendicular to the dimer row direction.In agreement with prior STM imaging 21 and x-ray standing wave measurements, 25 we find the most common configuration is on top of the dimer rows with the ad-dimer bond parallel to the dimer rows, as shown in the schematic in Fig. 1

(f).
The two other common features, labeled f1 and f2 in Figs.1(g) and 1(h), have not been previously reported in the literature.Both features take the space of two surface dimers and have a single protrusion feature adjacent to a depression.For reasons that will be explained below, we assign these two features to a single bismuth ad-atom adjacent to two chlorine atoms adsorbed to the surface and they differ in the relative positioning of the chlorine and bismuth atoms: feature f1 has two chlorine atoms bonded to the ends of two silicon dimers, analogous to the Cl2 feature, and with a bismuth ad-atom bridge bonding across the two silicon atoms at the other ends of these dimers [Fig.1(g)]; feature f2 has two chlorine atoms bound to a single dimer, analogous to the Cl1 feature, with a single bismuth ad-atom bridge bonding on the adjacent silicon dimer [Fig.1(h)].
We have counted the total number of features of each type occurring in a set of images and the results are shown in Table I.The first column in the table shows the raw count of the number of times the feature was observed, and the following two columns convert the raw counts to numbers of bismuth and chlorine atoms, respectively, based on the feature assignments described above.We find the ratio of chlorine to bismuth atoms is 3.29:1, which is to within 10% of the expected 3:1 ratio of the BiCl 3 stoichiometry, providing additional observational support to our feature assignments.
To understand how and why these features form, we have performed DFT calculations to determine the geometry optimized thermodynamic minimum energy configurations and transition-state barrier energies relevant to the adsorption, diffusion, and dissociation of BiCl 3 on Si(001).The dissociation is driven by an increased thermodynamic stability as the adsorbates break apart on the surface, while the energetic barriers determine how fast the adsorbates diffuse or dissociate, and which routes are preferred.The energetics and key structures necessary to explain our experimental observations are presented below, and a full summary of our computational results can be found in the supplementary material.
Our experimental data show that many of the chlorine and bismuth atoms on the surface are spatially separated from one another.This suggests the diffusion of molecular fragments BiCl x plays an important role in the dissociation pathway.With this in mind, it is instructive to first consider these BiCl x fragments in isolation and establish their capacity for surface diffusion.
Similar to PH 3 and AsH 3 , BiCl 3 has an electron lone pair that can readily form a dative bond with the electrophilic down-buckled atom of a silicon (001) surface dimer. 26However, unlike PH 3 and AsH 3 , we could not identify a configuration for molecularly adsorbed BiCl 3 .Thus, our calculations suggest that the adsorption of BiCl 3 is dissociative and barrierless.
The BiCl 2 fragment forms a single Bi-Si bond at the end of a surface dimer in what we refer to as the dimer-end position [b3; -1.58 eV, Fig. 2(a)].This structure can diffuse to an adjacent dimer via a transition state, where BiCl 2 forms a second bond to the silicon atom of the neighboring dimer in the same row [Fig.2(a)].The calculated activation energy for this transition is E A ¼ 0:55 eV, which, combined with our calculated attempt frequency for this transition, produces a short mean lifetime (i.e., the time the adsorbate resides on one dimer) of 2 Â 10 À3 s (at 300 K), suggesting this diffusion is rapid at room temperature.
The thermodynamic minimum energy structure for an isolated BiCl fragment is in an end-bridge configuration, where BiCl forms two Bi-Si bonds to the ends of two adjacent silicon dimers [d4, À2.21 eV, Fig. 2(b)].A slightly less stable configuration [e3; À2.20 eV, Fig. 2(b)], where BiCl is bridge-bound across a single surface dimer is separated by a transition state as shown in Fig. 2(b).Successive transitions between these configurations enable BiCl to diffuse along the dimer row with an overall activation barrier of 0.51 eV and mean lifetime of 8 Â 10 À5 s, implying rapid diffusion along the dimer row.
The dimer-bridge position [f7; À2.39 eV, Fig. 2(c)] is the preferred configuration for a bismuth ad-atom on the Si(001) surface.Bismuth ad-atom diffusion involves structure f9 [À2.01 eV, Fig. 2(c)], which is a closely related configuration, where the bismuth ad-atom forms a third bond to the adjacent dimer that we call the tripod structure.Diffusion of the bismuth ad-atom from one dimer to the next involves transitioning through two diagonally opposed tripod configurations, as illustrated in Fig. 2(c).The transition state separating these two structures is rate limiting with a barrier of 0.44 eV; correspondingly, the mean lifetime of a bismuth ad-atom with respect to diffusion is 4 Â 10 À4 s.The final step in the bismuth ad-atom diffusion is from structure f9 back to structure f7, but where the ad-atom is now bound to the next dimer along the dimer row.
The above results demonstrate that BiCl 2 , BiCl, and bismuth adatoms all exhibit low barriers for diffusion along the dimer rows, consistent with our experimental observations of bismuth and chlorine spatially separated on the surface.To complete the explanation of the features we observed experimentally on the BiCl 3 exposed Si(001) surface, we have also calculated the barriers for dissociation to describe how the BiCl x fragments break apart.A summary graph of the energetics is shown in Fig. 3(a).Also shown in this figure are the energies of the structures that we assign to the features observed in our experiments, namely, f1, f2, and g1, highlighted by solid circles.[The corresponding structural schematics were given in Figs.1(f

)-1(h).]
BiCl 2 [b3; À1.58 eV, Fig. 3(b)] can dissociate in two ways, by transferring the Cl atom either to the opposite end of the silicon dimer it is bound to (on-dimer dissociation) or to the silicon atom on the adjacent dimer (inter-dimer dissociation).On-dimer dissociation results in a more stable structure [c4, À1.91 eV, supplementary material Fig. 5(d)], but inter-dimer dissociation to structure c6 [À1.64 eV, Fig. 3(b)] is kinetically preferred due to lower activation energy (0.21 eV vs 0.35 eV).Furthermore, structure c6 readily stabilizes into structure d5 [À2.21 eV, Fig. 3(b)] that makes the dissociation reaction irreversible at room temperature.The lifetime of the BiCl 2 structure b3 with respect to dissociation is 3:2 Â 10 À9 s, which is too short-lived to be observable in our experiments, and is six orders of magnitude faster comparison to the diffusion of BiCl 2 discussed above.
BiCl can also dissociate to Bi þ Cl, which proceeds via a two-step process where the end-bridge BiCl [d4, À2.21 eV, Fig. 3(c)] first converts to a dimer-bridge configuration [e3, À2.20 eV, Fig. 3(c)], resulting in a BiCl bridge-bonded to the two atoms of a single dimer.

TABLE I.
Count of the number of each feature type within a set of STM images totaling a surface area of 5200 nm 2 .The raw feature counts have also been converted into numbers of bismuth and chlorine atoms based on the assignments shown in Fig. 1.The total ratio of chlorine to bismuth is 3:1 to within an uncertainty of 10%.Once in the dimer-bridge configuration, the chlorine atom can dissociate to the dimer-end position of the adjacent silicon dimer, leaving a bismuth ad-atom in the dimer-bridge position to produce structure f4 [À2.45 eV, Fig. 3(c)].This dissociated structure has gained 0.25 eV in stability relative to the BiCl configuration (d4).The rate limiting step for this dissociation of BiCl is the transition from the end-bridge to dimer-bridge configurations [d4-e3, shown in Fig. 2(b)], which is the same rate limiting step for BiCl diffusion.As such, the activation barriers for these two processes are the same [0.51eV; Fig. 3(a)].Thus, our calculations predict that BiCl 3 rapidly dissociates on Si(001), in agreement with our experimental observations of a range of features attributable to bismuth and chlorine atoms attached to the surface.Furthermore, isolated bismuth atoms and BiCl fragments have diffusion lifetimes much faster than typical STM image acquisition timescales, explaining why these species are not observed.Next, we explain how the features observed in our experiments arise.

Feature
The two successive dissociation steps BiCl 3 !BiCl 2 þ Cl, and BiCl 2 !BiCl þ Cl explain the observation of adsorbed chlorine atoms on the surface.Moreover, the barrier for BiCl 2 dissociation to BiCl is significantly smaller than the barrier for BiCl 2 diffusion, suggesting that BiCl 2 dissociates in-place without diffusion of the BiCl 2 fragment.Together, these observations explain the occurrence of the Cl1 and Cl2 features, both of which consist of a pair of adjacent surface bound chlorine atoms.
The complete dissociation of the BiCl 3 fragment and the low barrier for bismuth ad-atom diffusion provide the opportunity for two bismuth ad-atoms to encounter one another and form a bismuth ad-dimer.We have calculated the barrier to the formation of an ad-dimer starting from two adjacent bismuth ad-atoms in the dimer-bridge and tripod configurations [f10; Fig. 3(d)].The breaking of the third bond of the tripod structure and the formation of a bismuth-bismuth bond results in the bismuth ad-dimer, as shown in Fig. 3(d).This results in an overall energy barrier of 0.44 eV for the formation of a bismuth ad-dimer and an average lifetime of 4:9 Â 10 À6 s such that ad-dimers can be expected to readily form when two bismuth ad-atoms meet.The ad-dimer formation provides a significant increase in stability [g1, À3.12 eV, Fig. 3(d)] such that the reverse barrier is 1.43 eV and the process is irreversible at room temperature.As a result, the ad-dimers are stable and we observe these features in our STM images [Figs.1(a), 1(b), and 1(f)].
We have shown that isolated bismuth ad-atoms diffuse much too readily to be observed in an STM experiment.However, when a diffusing bismuth ad-atom encounters a pair of chlorine atoms in the Cl1 or Cl2 configurations, structures f1 or f2 are formed with an energy gain of 0.27 and 0.22 eV, respectively, [Figs.1(g), 1(h), and 3(a)].These increases in stability increase the barriers to escape these conformations which contribute to making these features long-lived and therefore observable in our STM experiments.However, it stands to reason that the bismuth atoms from features f1 and f2 can diffuse away via the f7-f7 diffusion barrier and then pair up with other chlorine atoms or other bismuth atoms, to form new f1 or f2 features, or bismuth addimers, respectively.Relative to f1 and f2 these barriers evaluate to 0.66 and 0.71 eV, which suggests that such processes would be apparent in our STM experiments.However, dynamical changes involving the f1 and f2 features have not been observed, implying that other higher barriers are involved in separating a bismuth atom from the f1 and f2 features.
Structure d1 [À2.63 eV; Fig. 3(a) and supplementary material Fig. S4(a)] is structurally similar to feature f1, with the modification that it has a BiCl, rather than a bismuth ad-atom, in the end-bridge location.The relatively strong thermodynamic stability of d1 and the relatively large barrier for its diffusion (1.04 eV assuming d4-d4 as rate limiting) suggest that it should be observable in STM images.However, our data are not consistent with the occurrence of both features f1 and d1.The absence of structure d1 in our experimental data can be attributed to the kinetic processes required for its formation.Calculation and comparison of the energetics and kinetics of all possible pathways is outside the scope of this manuscript.However, it is likely that it is the competition of diffusion and dissociation of BiCl that is the reason that d1 is not observed in experiment.
We have presented an investigation of the adsorption of bismuth trichloride on the silicon (001) surface.Using atomicresolution STM measurements and DFT calculations, we have demonstrated that the adsorption is completely dissociative, resulting surface bound bismuth and chlorine atoms.These results are promising for the use of BiCl 3 as a precursor for the deterministic positioning of single bismuth atoms into silicon because (1) BiCl 3 releases bismuth ad-atoms onto the Si(001) surface on adsorption and (2) the only other species introduced to the surface are chlorine atoms that do not pose a fundamental limitation on device fabrication; i.e., it is known that chlorine can be desorbed from Si(001) thermally, 27 or via direct stimulation using an STM tip. 9 Moreover, it has been recently shown that chlorine atoms segregate to the surface during Si(001) epitaxial growth, such that it may not be necessary to remove the chlorine at all. 28,29This desirable behavior for chlorine in regard to device fabrication can be contrasted to the behavior of carbon on Si(001), which readily forms a silicon carbide upon annealing and requires extremely high temperatures to remove.This suggests that carbon-based precursor molecules are unlikely to find application for device fabrication. 30otwithstanding these strong benefits of BiCl 3 and chloride compounds in general for dopant precursors, [6][7][8] there are several significant stages that remain to be demonstrated for atomically precise fabrication using BiCl 3 .Principal among these is the need to limit the formation of Bi-Bi ad-dimers.We suggest that this can be achieved through the use of an atomically precise resist to pattern areas of reactive silicon surface large enough to accommodate only one BiCl 3 adsorbate, e.g., a two-or three-dimer patch as is commonly used for atomically precise positioning of phosphorus dopants into silicon. 1,2his procedure would restrict bismuth atoms from diffusing and, therefore, inhibit Bi-Bi ad-dimer formation.The other key step that must be demonstrated is the substitutional incorporation of bismuth into the silicon surface.We anticipate this incorporation can be induced thermally by analogy to the behavior of the other group V donors, P and As on both Si(001) 1,4 and Ge(001), 31,32 and this is the focus of ongoing work in our laboratory.

FIG. 2 .
FIG. 2. Pathways for the dimer-row diffusion.(a) BiCl 2 diffusion takes place in a single step with BiCl 2 moving between the dimer-end position in two neighboring dimer via a transition state configuration where BiCl 2 forms bonds to both silicon atoms.(b) BiCl diffusion occurs in two stages with the BiCl moiety moving from the minimum energy end-bridge to a less stable dimer-end position.(c) Bismuth ad-atom diffusion occurs along the top of the dimer row with the bismuth atom moving between dimer-bridge and tripod configurations.