††thanks: These two authors contributed equally††thanks: These two authors
contributed equally

# Area-selective deposition and B $\delta$-doping of Si(100) with BCl3

K.J. Dwyer  Department of Physics, University of Maryland, College Park, MD
20742, USA.  S. Baek  Department of Physics, University of Maryland, College
Park, MD 20742, USA.  Azadeh Farzaneh  Department of Materials Science and
Engineering, University of Maryland, College Park, MD 20742, USA.  Michael
Dreyer  Department of Physics, University of Maryland, College Park, MD 20742,
USA.  J.R. Williams  Department of Physics, University of Maryland, College
Park, MD 20742, USA.  R.E. Butera<EMAIL_ADDRESS>Laboratory for Physical
Sciences, 8050 Greenmead Drive, College Park, MD 20740, USA.

###### Abstract

B-doped $\delta$-layers were fabricated in Si(100) using BCl3 as a dopant
precursor in ultrahigh vacuum. BCl3 adsorbed readily at room temperature, as
revealed by scanning tunneling microscopy (STM) imaging. Annealing at elevated
temperatures facilitated B incorporation into the Si substrate. Secondary ion
mass spectrometry (SIMS) depth profiling demonstrated a peak B concentration
$>$ 1.2(1) $\times$ 1021 cm-3 with a total areal dose of 1.85(1) $\times$ 1014
cm-2 resulting from a 30 L BCl3 dose at 150 ∘C. Hall bar measurements of a
similar sample were performed at 3.0 K revealing a sheet resistance of
$R_{\mathrm{s}}$ = 1.91 k$\Omega\square^{-1}$, a hole concentration of $n$ =
1.90 $\times$ 1014 cm-2 and a hole mobility of $\mu$ = 38.0 cm2V-1s-1 without
performing an incorporation anneal. Further, the conductivity of several
B-doped $\delta$-layers showed a log dependence on temperature suggestive of a
two-dimensional system. Selective-area deposition of BCl3 was also
demonstrated using both H- and Cl-based monatomic resists. In comparison to a
dosed area on bare Si, adsorption selectivity ratios for H and Cl resists were
determined by SIMS to be 310(10):1 and 1529(5):1, respectively, further
validating the use of BCl3 as a dopant precursor for atomic precision
fabrication of acceptor-doped devices in Si.

##  I Introduction

The realization of atomic-precision, advanced manufacturing (APAM) processes
using acceptor and donor doping is an essential step towards integrated
quantum circuitriesWard  _et al._ (2020). APAM process development, thus far,
has largely focused on donor dopant-based devices in Si, and in particular,
the precision placement of P atoms and their subsequent operation as
qubitsWeber  _et al._ (2014). More recently, acceptor-based Si devices have
been proposedSalfi  _et al._ (2014, 2016); Shim and Tahan (2014); Blase  _et
al._ (2009) and realizedŠkereň  _et al._ (2020) demonstrating the
opportunities of hole-type devices and the need for further development of
alternate chemistries for device fabrication. While APAM acceptor-doped
devices have been demonstrated in Si(100) using diborane (B2H6)Škereň  _et
al._ (2020), it results in the formation of electrically inactive dopant
complexes due to its inherent dimerized natureCampbell  _et al._ (2021),
prompting further investigations towards finding a more ideally suited B
dopant precursor.

The use of BCl3 as a dopant precursor has been well documented on Si
substratesHerner and Clark (2004); Consiglio  _et al._ (2016); Pilli  _et al._
(2018); Duvauchelle  _et al._ (2015). Moreover, it has been used to achieve
superconductivity in Si through gas immersion laser dopingCammilleri  _et al._
(2008). While these BCl3 doping demonstrations have all been performed at
temperatures or processing conditions that are inherently incompatible with
the current APAM process flow, they remain suggestive of the potential
viability of BCl3 as an APAM acceptor precursor that warrants further
investigation.

In this paper, we demonstrate the formation and characterization of B-doped
$\delta$-layers from BCl3-dosed Si(100). We find that BCl3 adsorbs readily
onto the Si(100) surface at room temperature. Further, the adsorption of BCl3
generates an active carrier concentration exceeding 1014 cm-2 without directly
annealing the substrate to facilitate activation. STM was used to verify both
BCl3 adsorption and B incorporation, while secondary ion mass spectrometry
(SIMS) depth profiling was used to quantify the B content of the
$\delta$-layers and verify resist selectivity. We find BCl3 to demonstrate
selectivity in adsorption on H- and Cl-terminated Si with respect to bare
Si(100), making it chemically compatible with current STM lithographic
processes. Hall bar measurements obtained at 3.0 K revealed similar values for
sheet resistance, carrier concentration, and carrier mobility to those
reported for the well developed process of P-doped $\delta$-layers formation
using PH3. This work serves as a stepping stone to enable further development
of acceptor-based devices, including complementary digital electronics at the
atomic-scale.

##  II Experimental Methods

###  II.1 B $\delta$-Layer Preparation

BCl3 dosing, STM imaging, and lithography were performed in a ScientaOmicron
VT-STM ultra-high vacuum (UHV) system with base pressure $P$
$<2.7~{}\times~{}10^{-9}$ Pa ($2.0~{}\times~{}10^{-11}$ Torr). The STM is run
using a ZyVector STM Lithography control system. The Si(100) wafers used in
these experiments were p-type (B-doped) obtained from ITME with resistivity
$\rho$ = 1 $\Omega\cdot$cm to 10 $\Omega\cdot$cm, and oriented within 0.5∘ of
(100). Si samples used for dosing tests and electrical measurements were cut
to 4 mm $\times$ 12 mm in size and cleaned by sonication in acetone, methanol,
and isopropanol. Samples were then immediately mounted on a ScientaOmicron XA
sample plate and loaded into the UHV system. Samples used for lithography
tests were prepared differently. Fiducial marks were etched in the surface to
allow a patterned area to be relocated in the STM (see Supplemental
Information). These samples were then diced to 5 mm $\times$ 9 mm and
chemically cleaned of metals and organics using a common cleaning recipe
consisting of piranha solution (H2SO4:H2O2), HF, and SC-2 (H2O:HCl:H2O2)
before being sonicated and loaded in a similar manner to other samples. Clean
Si(100)-(2$\times$1) surfaces were prepared in UHV following the procedure
described in Ref. Trenhaile  _et al._ , 2006 by flash annealing the sample to
1225 ∘C. Sample temperature was monitored with an optical pyrometer. Each
sample was initially scanned in the STM to check that it was free of metal and
carbon contaminants then re-flashed to produce a fresh Si(100) surface prior
to BCl3 dosing.

Samples were generally slightly above room temperature during sample transfer
to the dosing chamber and BCl3 dosing to minimize water contaminationYu  _et
al._ (2008) and, ultimately, O content within the B $\delta$-layer. BCl3 was
introduced into the UHV chamber via a precision leak valve with dosing
pressures ranging from 1.3 $\times~{}10^{-7}$ Pa ($1.0~{}\times~{}10^{-9}$
Torr) and 5.3 $\times~{}10^{-6}$ Pa ($4.0~{}\times~{}10^{-8}$ Torr). Dosing
values reported here are expressed in units of Langmuir (1 L = 10-6
Torr$\cdot$s) and generally have a relative uncertainty of $\sim$0.1. Dosing
parameters were varied for three samples used for electrical measurements,
denoted samples B2, B3, and B4. Sample B2 was dosed with 3.6 L BCl3 at room
temperature ($\sim$20 ∘C), sample B3 was dosed with 3.6 L BCl3 at 350 ∘C to
try to drive chemisorption and increase the B concentration beyond the room
temperature saturation, and sample B4 was dosed with 0.12 L BCl3 at room
temperature ($\sim$20 ∘C). B incorporation anneals were not performed on these
samples. Another sample used for SIMS analysis, denoted B1, was dosed with 30
L BCl3 at 150 ∘C to prevent any water adsorption then annealed at 350 ∘C to
incorporate the B. To form buried B $\delta$-layers and enable ex situ SIMS
and electrical measurements, BCl3-dosed samples were encapsulated with 20 nm
to 65 nm of Si at a typical rate of $\sim$0.016 nm/s using a MBE Komponenten
SUSI-63 sublimation source. Samples B1 and B4 was encapsulated with a
deposition rate of $\sim$0.004 nm/s at $\sim$240 ∘C. Other samples were not
heated deliberately during deposition, although thermal radiation from the
deposition source heated the sample to around 200 ∘C to 250 ∘C. SIMS depth
profiling (Eurofins EAG Materials Science) was used to quantify the B, Cl, and
H content of the B $\delta$-layers and Si films.

###  II.2 Electrical Measurements

Electrical transport properties of the B $\delta$-layers were characterized
with standard Hall bar measurements. Hall bars of width 25 $\mu$m and length
60 $\mu$m were patterned by electron beam lithography and the surrounding area
was etched to a depth of around 100 nm with a reactive ion etcher (RIE) (see
Supplemental Information). A 50 $\times$ 50 array of 1 $\mu$m sized squares
were also etched into the contact pad region to enhance the contact quality
between each pad and the B-doped $\delta$-layer. Ti/Au (5 nm/150 nm) was then
deposited using an electron-beam deposition system to define the contact pads.
Through this method, contact resistances of $<$ 1 k$\Omega$ at room
temperature were routinely achieved without any annealing process. Electrical
transport measurements of the B $\delta$-layers were performed at temperatures
ranging from 300 K to 1.9 K and magnetic fields from -14 T to +14 T to
characterize the sheet resistance ($R_{\mathrm{s}}$), hole carrier density
($n$), and mobility ($\mu$). The values for these parameters reported here
were measured at 3.0 K, where the measurement system was more stable.

##  III Results and Discussion

###  III.1 BCl3 Adsorption and Incorporation

Figure 1: Filled-state STM images (a: -2.30 V, 0.15 nA, b: -2.25 V, 0.50 nA,
c: -2.10 V, 0.15 nA) of a Si(100) surface dosed with 1.5 L of BCl3 at room
temperature before (a) and after annealing to 350 ∘C (b) or 500 ∘C (c) for 1
minute. (a) Various adsorbed BCl3-related species can be seen in the image as
bright, round features sitting on the Si dimer rows. Many are likely
chemisorbed BCl2 and Cl atoms. These larger adsorbates amount to a surface
coverage of $\sim$0.32. (b) A similarly dosed surface to (a) after annealing
to 350 ∘C showing fewer adsorbate features compared to (a). Dark dimer row
defects are also seen. (c) The same surface as in (a) is shown after annealing
to 500 ∘C. Short island chains (arrow) form from ejected Si during B
incorporation. Also visible are other defect features associated with
subsurface B atoms (circle).

Density functional theory calculations have determined that BCl3
dissociatively chemisorbs onto Si(100) forming Si-BCl2 and Si-Cl moieties on a
single Si dimerFerguson  _et al._ (2009). There are a number of papers
discussing the use of BCl3 as a dopant sourceDeutsch  _et al._ (1981); Kunii
_et al._ (2004), and there have also been a number of investigations of
adsorption on Si(111)Lapiano-Smith and McFeely (1992), but to the best of our
knowledge, no STM studies exist of BCl3 adsorption on the Si(100) surface.
Figure 1 shows filled-state STM images of a Si(100) surface exposed to BCl3
before (a) and after annealing to 350 ∘C (b) or 500 ∘C (c). The surface was
dosed with 1.5 L of BCl3 at room temperature ($\sim$20 ∘C) resulting in
numerous bright features due to adsorbates as seen in Fig. 1(a). BCl3 is
widely used in ALD applications and is known to stick readily to Si(100)Pilli
_et al._ (2018); Consiglio  _et al._ (2016); Ferguson  _et al._ (2002). We
also find BCl3 to adsorb readily onto the Si(100) surface at room temperature
forming a variety of surface bound structures. Many of these features are
likely chemisorbed BCl2 and as well as Cl atoms. The brighter BCl3 features in
(a) comprise a surface coverage of roughly 0.32.

After dosing, several samples were annealed at different temperatures to
observe the evolution of surface features with temperature, eventually leading
to incorporation of the B atoms into the substrate, which is typically done to
foster electrical activation. Figure 1(b) depicts the surface of a sample with
the same dose as in (a) after annealing at 350 ∘C for 1 minute. Some of the
smaller round BCl3 adsorbate features seen in (a) are gone while larger
features remain. This may indicate partial dissociation or incorporation into
the surface at this temperature. Also seen are darker dimer row defects, which
can indicate additional Cl on the surface.

Figure 1(c) depicts the surface shown in (a) after annealing at 500 ∘C for 1
minute. This anneal was chosen based on similar anneals used for B2H6 Škereň
_et al._ (2020) and AlCl3 incorporationRadue  _et al._ (2021). The annealed
surface appears quite differently with many of the bright round features in
(a) disappearing. Short Si chains running perpendicular to the underlying
dimer row direction can be seen on the surface (arrow) with their height
verified to be one atomic layer. This is a potential indicator that B atoms
were incorporated into the surface causing Si atoms to be ejected as adatoms,
forming the chains. It is also possible that some of the ejected Si observed
was the result of surface etching due to the presence of Cl on the surface
during the anneal step. Very little Cl is observed on the annealed surface in
Fig. 1(c), matching our expectation that Cl is removed at 500 ∘CRadue  _et
al._ (2021). Additionally, among the various defects that are visible, there
appears to be B-related surface defects (circle) which span three dimer rows
and are similar to features often observed on B-doped Si substrates caused by
B atoms several layers below the surfaceLiu  _et al._ (2008); Wang and Hamers
(1995). Annealing at intermediate temperatures of 400 ∘C and 450 ∘C produce
similar surfaces to the 350 ∘C surface in Fig. 1(b), though with additional Si
chains appearing at 450 ∘C, indicating that this temperature is closer to the
onset of B incorporation.

###  III.2 SIMS Characterization of B $\delta$-Layers

As discussed earlier, B $\delta$-doped samples were formed by exposing clean
Si(100) to BCl3 and then capping them with epitaxial Si. The $\delta$-layers
were characterized ex situ with SIMS to ascertain the estimated peak dopant
concentration, the areal dose, and the estimated thickness of the
$\delta$-doped layer. Figure 2 shows SIMS depth profiles of $\delta$-layer
samples B1 and B2. Sample B1 was capped with $\sim$19 nm Si and B2 was capped
with $\sim$62 nm Si after dosing. B profiles show in Fig. 2 represent the
total B, i.e. the sum of 10B and 11B. For B2 (dotted line), the peak B
concentration was measured to be 4.1(1) $\times$ 1020 cm-3 at the interface
with a total areal dose of 1.59(1) $\times$ 1014 cm-2. While the SIMS
measurement for B2 was performed using typical settings, $\delta$-layer sample
B1 was measured using a lower primary ion energy of 250 eV resulting in a
higher spacial resolution. The measurement on B1 was done as a reference to
more accurately gauge the peak B concentration and, importantly, the
$\delta$-layer thickness when eliminating some known SIMS measurement
artifactsGautier  _et al._ (1996). Areal dose accuracy, however, is consistent
between different measurement types, according to the analysis provider. We
believe the 3.6 L dose of B2 is at or near a saturation dose and thus the
higher 30 L dose of B1 should not lead to a significantly increased B content.
The high resolution measurement of B1 yielded a peak B concentration of 1.2(1)
$\times$ 1021 cm-3 with a total areal dose of 1.85(1) $\times$ 1014 cm-2,
similar to the areal dose of B2. Additionally, a SIMS measurement of B1 using
a primary ion energy of 1 keV, similar to that of B2, shows a significant
lowering of the peak to 6.3(1) $\times$ 1020 cm-3 (see Supplemental
Information). It can therefore be assumed that the peak B concentration for
sample B2 used in electrical measurements is closer to 1021 cm-3 than that
measurement indicated. This B concentration is similar to reported
concentrations for B and P $\delta$-layers deposited via a precursor of
roughly 1021 cm-3, and the areal dose is similar for PHagmann  _et al._
(2018); Škereň  _et al._ (2020). It is also consistent with a number of
reported B-doped $\delta$-layers fabricated using molecular beam epitaxy
(MBE)Weir  _et al._ (1994).

Figure 2: SIMS depth profiles of sample B2 dosed with 3.6 L BCl3 at room
temperature and sample B1 dosed with 30 L BCl3 at 150 ∘C. B2 was capped with
$\sim$62 nm Si while B1 had a $\sim$19 nm Si cap. The B profile for B2 (dotted
line) was measured under typical settings while a high resolution measurement
was done for B1 (solid line). B profiles show total B (10B + 11B). The peak B
concentration for B2 is 4.1(1) $\times$ 1020 cm-3 at the interface, and for
B1, the peak B concentration is 1.2(1) $\times$ 1021 cm-3. The B
$\delta$-layer thickness is measured to be $\sim$3.4 nm for B2 based on the
full width at half max of the peak and $\sim$1.2 nm for B1. A Cl depth profile
from B2 (dashed line) shows some Cl present at the B layer, with the majority
diffusing upwards towards the Si growth surface.

One measure of the thickness of the $\delta$-layers is determined from the
full width at half max (FWHM) of the B peak in Fig. 2. For sample B1, the
thickness of the B $\delta$-layer is estimated from the displayed depth
profile to be $<$ 1.2 nm. This value is close to the measurement limit of SIMS
even using high resolution settings and so may yet be an overestimate. The
lower resolution measurement of sample B2 resulted in a FWHM of $\sim$3.4 nm.
However, by comparing the SIMS data from multiple measurement energies, a
measure of the SIMS instrument artifacts that cause broadening of the
$\delta$-layer can be determinedGautier  _et al._ (1996). Then, an estimate of
the “true” thickness can be extrapolated for a situation with zero instrument
artifact effects (see Supplemental Information). Extrapolating the
measurements from sample B1 in this manner yielded a FWHM of $\sim$0.8 nm.
Further, by fitting the depth profiles at each energy to a $\delta$-layer
model that accounts for the instrument broadening, the standard deviation on
the distribution of B can also be extrapolated to reduce the broadening
effect. This type of analysis was done for sample B1 and resulted in a
standard deviation of the $\delta$-layer thickness of $<$ 0.5 nm, or within 4
monolayers (ML). Previous studies of B deposited as a $\delta$-layer by MBE
also found the B spreads over just a few MLWeir  _et al._ (1994). Taken
together, these estimates give us confidence to place an upper bounds on the
full thickness of our B $\delta$-layers of $<$ 1.0 nm. The thickness of the
electrically active region of a $\delta$-layer can also be measured by
studying the weak localization in the sample, as has been demonstrated with P
$\delta$-layersHagmann  _et al._ (2018, 2020), and will be explored in future
studies.

Figure 3: (a) and (b) Filled-state STM images (a: -2.22 V, 0.11 nA, b: -2.22
V, 0.13 nA) of a lithographically patterned H-Si(100) surface exposed to a 12
L BCl3 dose. (a) Shows an 80 nm $\times$ 80 nm square filled in with
BCl3-related adsorbates, with virtually none observed outside the pattern. (b)
A magnified image of the center of the dosed square in (a) showing BCl3
adsorbates and a surface step. (c) A SIMS depth profile of a H-Si(100) sample
dosed with 3.6 L of BCl3 and capped with $\sim$45 nm of Si. BCl3 adsorption is
blocked resulting in minimal B concentration at the interface of 6.0(2)
$\times$ 1017 cm-3. A small amount of Cl is detected in the film while the
large H content indicates an intact H resist. (d) and (e) Filled-state STM
images (d: -1.80 V, 0.15 nA, e: -1.80 V, 0.15 nA) of a lithographically
patterned Cl-Si(100) surface exposed to a 1.5 L BCl3 dose. (d) Shows a 25 nm
$\times$ 25 nm square filled in with BCl3-related adsorbates with virtually no
adsorbates observed outside the pattern. (e) A magnified view of the image in
(d) showing individual adsorbate features. (f) A SIMS depth profile of a Cl-
Si(100) sample dosed with 3.6 L of BCl3 and capped with $\sim$65 nm of Si.
BCl3 is blocked resulting in a peak B concentration of 1.8(2) $\times$ 1017
cm-3. The Cl content in the film results from the Cl resist diffusing upwards
during Si deposition.

A Cl depth profile (dashed line) from sample B2 was also measured and revealed
Cl present both at the buried interface layer and throughout the Si capping
layer in Fig. 2. At the buried interface, we find an areal dose of Cl of
1.4(1) $\times$ 1012 cm-2, or roughly 2 $\times$ 10-3 ML. While there is a
measured peak Cl concentration of 3.3(3) $\times$ 1018 cm-3 at the buried
interface, a substantially larger amount of Cl was found within the epitaxial
Si layer closer to the surface. Cl is expected to act as a surfactant during
Si growth and exchange places with deposited Si, altering the growth
kineticsPavlova  _et al._ (2019) in much the same way as H, leading to the
profile seen in Fig. 2. Surfactant-mediated growth is known to enhance layer-
by-layer growth in homoepitaxy and suppress three-dimensional (3D)
islandingZhang and Lagally (1994); Voigtländer and Zinner (1994). In addition,
theoretical investigations have suggested a pathway towards removing Cl from
the growth front due to lowered barriers to desorption for SiCl2Pavlova  _et
al._ (2019). Under the growth conditions utilized in these investigations, we
find evidence of Cl exchanging with Si adatoms and floating to the surface,
but are unable to quantify amount of Cl lost. Additional optimization of the
growth and incorporation parameters can significantly reduce the residual Cl
at the interface as well as in the Si film. These parameters will be explored
in future studies.

###  III.3 Selectivity of BCl3 with H and Cl Atomic Resists

For compatibility with existing APAM processes, BCl3 must demonstrate
selective adsorption with respect to an atomic resist. H-Si(100) has been the
standard substrate for atomically-precise device fabrication due in large part
to its ease of patterning and the use of PH3 and, more recently, B2H6 for
doping reactions. H-based dopant precursors experience no driving force to
react with the H-terminated substrate. In contrast, the use of BCl3 presents a
possible pathway for reaction with the H resist layer through abstractionFerng
_et al._ (2009); Li  _et al._ (2011) thereby compromising the selectivity
required for device fabrication with good pattern fidelity.

Recent experiments demonstrating compatibility of TiCl4 with a patterned
H-resistMitsui  _et al._ (1999); Dick  _et al._ (2016); Longo  _et al._ (2014)
at room temperature as well as density functional theory calculations
predicting PH3 compatibility with a Cl-resistPavlova  _et al._ (2018) suggest
BCl3 may be similarly compatible with H-Si(100). However, this has not yet
been explored experimentally prior to this work. Additionally, XPS
investigations of BCl3 adsorption on Si(100) found that Cl hindered further
adsorptionPilli  _et al._ (2018) and atomically precise lithographic
patterning of Cl-Si(100) was recently demonstratedDwyer  _et al._ (2019)
providing another potential resist for B-doped APAM applications. As such, the
selective adsorption of BCl3 with both H- and Cl-based resist layers and their
ability to mask B deposition on Si(100) was explored here using STM and SIMS.

Figure 3(a) and (d) show filled-state STM images of lithographically patterned
squares on H- and Cl-Si(100), respectively, after exposure to BCl3 at room
temperature ($\sim$20 ∘C). The H pattern in Fig. 3(a) consisted of a 80 nm
$\times$ 80 nm square (lithographic parameters: +7.0 V, 1.5 nA, 4 mC/cm) that
was exposed to a 12 L dose of BCl3. While this dose is beyond our typical
saturation dose, it provides a more robust test of the H pattern fidelity. The
Cl pattern in (d) consisted of a 25 nm $\times$ 25 nm square (lithographic
parameters: +10.0 V, 9.0 nA, 15 mC/cm) that was exposed to a more typical 1.5
L of BCl3. The initial H- and Cl-Si(100) surfaces were first scanned with STM
to verify full surface passivation. We observe that the areas outside of the
patterned squares in Fig. 3(a) and (d) appear as typical H- and Cl-terminated
surfaces, with the exception of some spurious dangling bonds from the
depassivation process. STM images obtained away from the patterned areas after
BCl3 exposure revealed that full H and Cl passivation was maintained with
negligible changes in the defect concentration (see Supplemental Information).
Higher magnification images of the centers of the dosed H and Cl patterns are
shown in Fig. 3(b) and (e), respectively. Within the patterned area, we find a
variety of features similar in appearance to those seen on the Si surface in
Fig. 1(a). The clear preservation of the lithographic patterns after dosing
verifies the ability of both H and Cl to mask the surface and validates their
use as atomic resists for area-selective deposition of BCl3. However, the STM
images do not provide a quantifiable measure of the selectivity of the
adsorption on the two resist surfaces.

A more precise measure of the adsorption selectivity of BCl3 on H- and Cl-
Si(100) compared to bare Si(100) was determined using SIMS. Figures 3(c) and
(f) present SIMS depth profiles of B, H, and Cl from BCl3-dosed H- and Cl-
Si(100) surfaces, respectively. Both fully-terminated surfaces were exposed to
3.6 L of BCl3 at room temperature ($\sim$20 ∘C) and encapsulated with Si to
bury the interface following the same procedure used to create the B-doped
$\delta$-layer samples, as discussed earlier. They were then removed from UHV
and analyzed using SIMS depth profiling ex situ. The H-Si(100) sample was not
heated before encapsulation with Si, while the Cl-Si(100) sample was heated to
$\sim$250 ∘C for 10 minutes in an effort to drive off potential physisorbed
speciesSilva-Quinones  _et al._ (2020). However, both sample processing is
still comparable as both were heated radiatively by the Si source during
deposition to roughly this temperature. The depth profiles show a background B
concentration in the substrate of $\sim$1017 cm-3 with buried interfaces
located $\sim$45 nm and $\sim$65 nm beneath the surface of the Si capping
layer on the H-Si(100) and Cl-Si(100) samples, respectively. At the buried
interface, a slight increase in the B concentration above 1017 cm-3 is
observed on both samples, but it is not possible to conclude from the B signal
alone whether or not this increase is due to BCl3 adsorption or surface
segregation of B from the substrate during sample preparationZhang  _et al._
(1996). We have observed similar peaks in the B concentration at the Si
overgrowth interface for some samples that were not exposed to BCl3 (see
Supplemental Information).

However, Cl is observed in Fig. 3(c) above 1017 cm-3 within the Si capping
layer indicating possible BCl3 reactions on H-Si(100). To test if the increase
in B was due to a reaction with H and not adsorption at single dangling bonds
present on the initial H-Si surface, a similarly prepared H-terminated Si
surface was exposed to the same dose of BCl3 at $\sim$250 ∘C. SIMS of this
sample revealed slightly higher concentrations of both B and Cl, indicating a
thermally activated reaction, as this dosing temperature is well below the
hydrogen desorption temperature (see Supplemental Information).

Cl is also observed at the interface of the Cl-Si(100) surface and in the Si
capping layer at over 1018 cm-3 in Fig. 3(f). Unlike for the case of the
H-Si(100) surface, it cannot be assumed that the presence this Cl is due to
reactions with BCl3 as the surface was already saturated with Cl. We cannot
determine quantitatively how much, if any, BCl3 reacts through the Cl resist
other than it appears negligible and is several orders of magnitude lower than
that seen on bare Si(100) as in Fig. 2. As Fig. 3(c) and (f) show, Cl readily
migrates from the interface towards the growth surface during Si overgrowth.
Even for the case of Cl-Si(100), with an initial Cl coverage approximately 1
ML, the buried interface after Si growth is left with and areal coverage of
just 5.1(2) $\times$ 1010 cm-2 of Cl, or $\sim$$10^{-4}$ ML. While the
residual Cl content at the buried interface is not insignificant, i.e. it is
above the background level in the substrate, the magneto-transport and device
measurements presented in the following section suggests Cl does not have a
detrimental impact on Hall bar measurements.

To more accurately determine the adsorption selectivity of H and Cl resists
with BCl3, we compare the total areal dose of B measured by SIMS for both
resists to that of bare Si(100) when all three are dosed with 3.6 L BCl3. The
SIMS measurement in Fig. 2 (lower resolution B2 measurement) gives the bare Si
B areal dose of 1.59(1) $\times$ 1014 cm-2, while the B areal doses on the
resist layers are measured from Fig. 3(c) and (f) to be 2.6(1) $\times$ 1011
cm-2 for H-Si(100) and 5.2(2) $\times$ 1010 cm-2 for Cl-Si(100). The
adsorption selectivity ratios for these resists is then calculated in a
similar manner as is done for area-selective depositionMackus  _et al._
(2019); Gladfelter (1993):

$\mathrm{Selectivity~{}Ratio}=\left[1-\left(\frac{\theta_{\mathrm{GA}}-\theta_{\mathrm{NGA}}}{\theta_{\mathrm{GA}}+\theta_{\mathrm{NGA}}}\right)\right]^{-1},$
(1)

where $\theta_{\mathrm{GA}}$ is taken to be the measured B dose in the growth
area (bare Si) and $\theta_{\mathrm{NGA}}$ is the B dose in the non-growth
area (H- and Cl-terminated Si). From Eq. (1), the adsorption selectivity is
found to be 310(10):1 and 1529(5):1 for H- and Cl-Si(100) at room temperature,
respectively.

Figure 4: Electric transport measurements of B-doped $\delta$-layers fabricated into 25 $\mu$m $\times$ 60 $\mu$m mesa etched Hall bars. (a) The temperature dependence of sheet resistance between 50 K and 1.9 K is shown for B2 (solid line), B3 (dashed line) and B4 (dotted line). Comparing dose temperature, sample B2 (room temperature dose) has lower $R_{\mathrm{sheet}}$ (higher conductivity) compared to sample B3 (350 ∘C dose). Comparing dose amount, sample B4 (0.12 L dose) has a higher $R_{\mathrm{sheet}}$ (lower conductivity) than either B2 or B3 (3.6 L doses). (b) and (c) Magnetic field dependence of the longitudinal, $R_{\mathrm{s}}$ (right axis), and transverse, $R_{\mathrm{H}}$ (left axis), resistance at 3.0 K for samples B2 and B4, respectively. Sample |  | BCl3 Dose |  |  $R_{\mathrm{s}}$ (k$\Omega\square^{-1}$) |  |  $n$ (1014 cm-2) |  |  $\mu$ (cm2V-1s-1)
---|---|---|---|---|---|---|---|---
B4 |  | 0.12 L, 20 ∘C |  | 7.04 |  | 0.395 |  | 21.4
B3 |  | 3.6 L, 350 ∘C |  | 3.69 |  | 1.13 |  | 15.0
B2 |  | 3.6 L, 20 ∘C |  | 1.91 |  | 1.90 |  | 38.0
Table 1: Electrical properties of B-doped $\delta$-layer samples measured at
3.0 K. The BCl3 dose parameters for samples B2, B3, and B4 are shown along
with the measured sheet resistance ($R_{\mathrm{s}}$), hole carrier density
($n$), and mobility ($\mu$).

A selectivity ratio of 1000:1 would be similar to values obtained for PH3 on
H-Si(100) and, as such, is the desired goal. However, a final dopant
concentration due to incorporation through the resist of $<$ 1018 cm-3 is
sufficient to ensure carrier freeze out of stray adsorbed BCl3 within the
resist layer at typical device measurement temperatures of ${\leq}$ 4 K. Both
H and Cl resist measurements presented here result in B concentrations below
that threshold, as seen in Fig. 3(c) and (f). Still, a small amount of BCl3
incorporation through the H resist could pose an issue for fabricating single-
atom devices, though that remains to be determined for the amount measured
here.

###  III.4 Electrical Characterization of B $\delta$-Layers

We studied the electrical transport characteristics of B-doped $\delta$-layer
samples B2, B3, and B4, which were fabricated into mesa-etched Hall bars 25
$\mu$m $\times$ 60 $\mu$m in size, as described earlier. An optical micrograph
of a Hall bar device is shown in the Supplemental Information. While full B
incorporation was demonstrated by annealing at 500 ∘C, as shown in Fig. 1(c),
$\delta$-layers discussed here were not annealed that hot, as described
earlier. Incorporation of dopants through annealing is typically considered
important for good electrical activation, but here we wanted to isolate the
effects of annealing on electrical properties and actually found that B dopant
activation was high despite no deliberate incorporation. The dose parameters
and measurement results of sheet resistance ($R_{\mathrm{s}}$), hole carrier
density ($n$), and mobility ($\mu$) for these samples are described in Table 1
with reported values measured at 3.0 K.

The temperature dependence of the sheet resistance between 50 K and 1.9 K is
shown in Fig. 4(a) for samples B2 (solid line), B3 (dashed line), and B4
(dotted line). The measured sheet resistance values of $R_{\mathrm{sheet}}<$
10 k$\Omega\square^{-1}$ below 30 K reflect the properties of the
$\delta$-layer as the resistance of the substrate and the Si capping layer
increases to over 10 M$\Omega\square^{-1}$ at these temperatures and thus do
not contribute to the measurement. Figure 4(b) and (c) show the magnetic field
dependence of the longitudinal magnetoresistance, $R_{\mathrm{s}}$ (left
axis), and transverse Hall resistance, $R_{\mathrm{H}}$ (right axis),
resistance of samples B2 and B4. The positive slope of $R_{\mathrm{H}}$
observed in (b) and (c) indicates that holes are the primary carriers in these
devices, verifying that an acceptor-doped region is being measured. Likewise,
the positive values of $R_{\mathrm{s}}$ measured for these samples also
indicates acceptor dopantsRamanayaka  _et al._ (2018), as expected for a B
$\delta$-layer. Since APAM processes requires tight control over the entire
doping and annealing process, achieving a minimum sheet resistance for sample
B2 of $R_{\mathrm{sheet}}$ = 1.91 k$\Omega\square^{-1}$ without an
incorporation annealing step validates the use of BCl3 as an acceptor doping
precursor. Comparing this result to $\delta$-layer devices dosed with PH3
shows they are electrically similar with the sheet resistances of P-doped
$\delta$-layers reported to be between $R_{\mathrm{sheet}}$ = 0.5
k$\Omega\square^{-1}$ and $R_{\mathrm{sheet}}$ = 5 k$\Omega\square^{-1}$ for
mildly dosed sampleGoh  _et al._ (2006); Weber  _et al._ (2012).

From the data in Fig. 4(b), the carrier density calculated for sample B2 is
$n$ = 1.90 $\times$ 1014 cm-2 with a hole mobility $\mu$ = 38.0 cm2V-1s-1.
This carrier density is slightly larger than the B areal dose measured by SIMS
of 1.59(1) $\times$ 1014 cm-2, implying full dopant activation and possibly
some additional electrical pathways such as vacancies or impurities in the
overgrowth layer. We compare sample B2 with B4, which was produced with a
smaller BCl3 dose. The sheet resistance of B4 was larger at
$R_{\mathrm{sheet}}$ = 7.04 k$\Omega\square^{-1}$ compared to B2. The carrier
density and mobility for sample B4 were both lower compared to B2 with $n$ =
3.95 $\times$ 1013 cm-2 and $\mu$ = 21.4 cm2V-1s-1. The increased carrier
density of B2 is only a factor of $\sim$5 times larger than for B4 despite the
BCl3 dose being a factor of 30 times larger. Suggesting that the surface of B2
may have been fully saturated by the 3.6 L dose.

Sample B3 was dosed with the same dose of BCl3 as B2 but at an elevated
temperature of 350 ∘C to see if BCl3 chemisorption could be facilitated in
order to increase B concentration in the $\delta$-layer. As shown in Fig.
1(b), BCl3 dissociation is enhanced at this temperature relative to room
temperature. However, the sheet resistance of B3 was observed to increase to
$R_{\mathrm{s}}$ = 3.69 k$\Omega\square^{-1}$ compared to the room
temperature-dosed ($\sim$20 ∘C) sample B2 as shown in Fig. 4(a), while the
hole carrier density and mobility for this sample was found to decrease at $n$
= 1.13 $\times$ 1014 cm-2 and $\mu$ = 15.0 cm2V-1s-1. As with B2, a SIMS depth
profile of B for B3 (not shown) measured a B areal dose similar to the carrier
density of 1.17(1) $\times$ 1014 cm-2. This indicates BCl3 adsorbs readily at
room temperature and more so than at 350 ∘C. It is possible that at 350 ∘C,
the sticking coefficient of BCl3 is decreased resulting in fewer molecules
chemisorbing. Alternatively, pairs of B atoms may be starting to dimerize as
they diffuse about and incorporate into the surface at this temperature,
becoming electrically inactive. It has been reported in literature that
P-doped $\delta$-layers fabricated by dosing Si(100) with PH3 at 550 ∘C showed
a similar increase in sheet resistance and decrease in carrier concentration
when compared to devices dosed at room temperatureGoh  _et al._ (2006).
Comparing the SIMS depth profile measurements of the B areal doses in samples
B2 and B3 indicates that the total amount of B was reduced by a factor of
0.26. This reduction does not fully explain the drop in carrier density in B3
by a factor of 0.41 compared to B2, indicating some B deactivation may also be
occurring.

Sample B2 with its more optimized dosing conditions has similar properties to
some P-doped $\delta$-layers with reported Hall carrier densities of $n<$ 2
$\times$ 1014 cm-2 and mobilities between $\mu$ = 30 cm2V-1s-1 and $\mu$ = 60
cm2V-1s-1Goh  _et al._ (2006); Weber  _et al._ (2012). Further, compared to
literature values for B-doped $\delta$-layer propertiesŠkereň  _et al._ (2020)
dosed with 1 L of B2H6 ($n$ $\approx 1~{}\times$ 1014 cm-2, $\mu\approx$ 20
cm2V-1s-1) and 170 L of B2H6 ($R_{\mathrm{sheet}}\approx$ 12
k$\Omega\square^{-1}$) then annealed to 250 ∘C, these BCl3-dosed samples show
lower values of sheet resistance with B2 also having a higher carrier density
and mobility without utilizing an incorporation anneal. This result supports
the notion that due to B2H6 being deposited as B dimers, many are electrically
inactive without high annealing temperatures, while deposition of single B via
BCl3 results in higher electrical activation.

Figure 5: Temperature dependence of the relative conductivity for samples B2
(squares), B3 (circles), and B4 (triangles). Temperature is plotted on a log
scale. The data for all three samples appears linear indicating a 2D nature to
the conduction. A 2D conductance model (Eq. (2)) was fit to the data to
extract the values of parameter $p$. The fits yield $p$ = 0.84 (B2), $p$ =
0.56 (B3), and $p$ = 0.33 (B4).

While these B-doped $\delta$-layers have favorable electrical properties,
their two-dimensional (2D) nature is also critical for use in APAM and quantum
information applications and requires investigation. The estimated
$\delta$-layer width from the SIMS depth profile of Fig. 2 is not necessarily
reliable evidence for a true 2D layer due to limitations of the SIMS
measurement. Therefore, we use the electrical measurements to ascertain the
dimensionality of the hole transport through the devices. We define a relative
conductivity, $\Delta\sigma$, as the change in conductivity from minimum
measurement temperature for each data set. This allows the data from multiple
samples to be plotted together for clarity. Figure 5 shows the temperature
dependence of the relative conductivity in the absence of a magnetic field
with temperature plotted on a log scale for samples B2 (squares), B3
(circles), and B4 (triangles).

We then fit the $\Delta\sigma(T)$ data as a function of $\ln(T)$ to a 2D
conductance modelLee and Ramakrishnan (1985).

$\sigma_{\mathrm{2D}}(T)=\sigma_{0}+\frac{pe^{2}}{2\hbar\pi^{2}}\ln{\frac{T}{T_{0}}}$
(2)

The dimensionless fitting parameter $p$ depends on the scattering mechanism of
the carriers in the device and the dephasing length. The fits to the data are
shown as solid lines in Fig. 5 with fit values for $p$ of $p$ = 0.84 for B2,
$p$ = 0.56 for B3, and $p$ = 0.33 for B4. Similar values of $p$ have been
reported for other B $\delta$-layers highly doped via MBEWeir  _et al._
(1994). Based on these fits, $\Delta$$\sigma$ is found to have a linear
relationship to $\ln(T)$, as seen in Fig. 5. This indicates that the transport
data is consistent with 2D conductionLee and Ramakrishnan (1985), however we
cannot rule out other mechanisms that contribute to a linear $\ln(T)$
dependence such as electron-electron interactions based on this data alone.
However, combined with the relatively thin B layer determined from the SIMS
data, this analysis leads us to conclude that B-doped $\delta$-layers formed
from BCl3 dosing described here likely produces a 2D hole system suitable for
fabrication of quantum devices in APAM applications. Obtaining a more accurate
measure of the dimensionality of conduction requires further investigations
such as weak localization measurement techniques, as have been done for
P-doped $\delta$-layersHagmann  _et al._ (2018, 2020).

##  IV Conclusion

In summary, we have demonstrated the viability of BCl3 as a molecular
precursor for acceptor-doping of B $\delta$-layers in Si(100) for the
fabrication of atomically-precise devices. B concentrations achieved in the
$\delta$-layers were shown to be similar to those achieved in the more well-
developed processes for P-doped $\delta$-layers. Importantly, BCl3 was shown
to be compatible with both H- and Cl-based resists for STM lithography-based
device fabrication. Cl content is not expected to be detrimental to device
operation as it moves out of the buried interface towards the top of the Si
epitaxial layer during growth, which can additionally be etched/polished back.
Moreover, the electrical properties demonstrated here on B-doped
$\delta$-layers rival those of P-doping from PH3 and B-doping from B2H6
without yet optimizing the incorporation anneal step. The compatibility of
BCl3 dosing with multiple resists ensures that it can be implemented in
process flows including PH3 for the formation of exotic atomic-scale devices
like pn junctions. The fact that BCl3 shows a slight preference for reacting
with H-Si over Cl-Si could be exploited for other process applications
involving halogen adsorption into patterned areas on H-Si(100)Ferng  _et al._
(2009) to serve as a negative resist. Overall, these results point towards the
suitability of BCl3 as a viable gaseous precursor for acceptor doping
processes associated with atomic precision advanced manufacturing of Si-based
devices.

##  V Acknowledgement

The authors would like to thank J. Foglebach, D. Ketchum, P. Hannah, J. Moody,
S. Brown and T. Olver for their assistance with BCl3 handling and
installation. We also acknowledge Matt Radue, George Wang, Ezra Bussmann, and
Scott Schmucker for thought provoking discussions. This work was supported in
part by the Laboratory Directed Research and Development program at Sandia
National Laboratories, a multimission laboratory managed and operated by
National Technology and Engineering Solutions of Sandia, LLC., a wholly owned
subsidiary of Honeywell International, Inc., for the U.S. Department of
Energy’s National Nuclear Security Administration under contract DE-
NA-0003525. This paper describes objective technical results and analysis. Any
subjective views or opinions that might be expressed in the paper do not
necessarily represent the views of the U.S. Department of Energy or the United
States Government.

## References

  * Ward  _et al._ (2020) D. R. Ward, S. W. Schmucker, E. M. Anderson, E. Bussmann, L. Tracy, T.-M. Lu, L. N. Maurer, A. Baczewski, D. M. Campbell, M. T. Marshall, and S. Misra,  (2020), arXiv:2002.11003 .
  * Weber  _et al._ (2014) B. Weber, Y. H. Tan, S. Mahapatra, T. F. Watson, H. Ryu, R. Rahman, L. C. Hollenberg, G. Klimeck, and M. Y. Simmons, Nature Nanotechnology 9, 430 (2014).
  * Salfi  _et al._ (2014) J. Salfi, M. Tong, S. Rogge, and D. Culcer, Nanotechnology 27, 244001 (2014).
  * Salfi  _et al._ (2016) J. Salfi, J. A. Mol, D. Culcer, and S. Rogge, Phys. Rev. Lett. 116, 246801 (2016).
  * Shim and Tahan (2014) Y.-P. Shim and C. Tahan, Nat. Commun. 5, 4225 (2014).
  * Blase  _et al._ (2009) X. Blase, E. Bustarret, C. Chapelier, T. Klein, and C. Marcenat, Nat. Mater. 8, 375 (2009).
  * Škereň  _et al._ (2020) T. Škereň, S. A. Köster, B. Douhard, C. Fleischmann, and A. Fuhrer, Nature Electronics , 2520 (2020).
  * Campbell  _et al._ (2021) Q. Campbell, J. A. Ivie, E. Bussmann, S. W. Schmucker, A. D. Baczewski, and S. Misra, Journal of Physical Chemistry C 125, 481 (2021).
  * Herner and Clark (2004) S. B. Herner and M. H. Clark, Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures 22, 1 (2004).
  * Consiglio  _et al._ (2016) S. Consiglio, R. D. Clark, D. O’Meara, C. S. Wajda, K. Tapily, and G. J. Leusink, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 34, 01A102 (2016).
  * Pilli  _et al._ (2018) A. Pilli, J. Jones, V. Lee, N. Chugh, J. Kelber, F. Pasquale, and A. LaVoie, Journal of Vacuum Science & Technology A 36, 061503 (2018).
  * Duvauchelle  _et al._ (2015) J. E. Duvauchelle, A. Francheteau, C. Marcenat, F. Chiodi, D. Débarre, K. Hasselbach, J. R. Kirtley, and F. Lefloch, Applied Physics Letters 107, 072601 (2015).
  * Cammilleri  _et al._ (2008) D. Cammilleri, F. Fossard, D. Débarre, C. T. Manh, C. Dubois, E. Bustarret, C. Marcenat, P. Achatz, D. Bouchier, and J. Boulmer, Thin Solid Films 517, 75 (2008).
  * Trenhaile  _et al._ (2006) B. R. Trenhaile, A. Agrawal, and J. H. Weaver, Appl. Phys. Lett. 89, 151917 (2006).
  * Yu  _et al._ (2008) S.-Y. Yu, H. Kim, and J.-Y. Koo, Phys. Rev. Lett. 100, 036107 (2008).
  * Ferguson  _et al._ (2009) G. A. Ferguson, U. Das, and K. Raghavachari, J. Phys. Chem. C 113, 10146 (2009).
  * Deutsch  _et al._ (1981) T. Deutsch, D. Ehrlich, D. Rathman, D. Silversmith, and R. Osgood Jr, Applied Physics Letters 39, 825 (1981).
  * Kunii  _et al._ (2004) Y. Kunii, Y. Inokuchi, A. Moriya, H. Kurokawa, and J. Murota, Applied surface science 224, 68 (2004).
  * Lapiano-Smith and McFeely (1992) D. Lapiano-Smith and F. McFeely, Journal of applied physics 72, 4907 (1992).
  * Ferguson  _et al._ (2002) J. D. Ferguson, A. W. Weimer, and S. M. George, Thin Solid Films 413, 16 (2002).
  * Radue  _et al._ (2021) M. S. Radue, S. Baek, A. Farzaneh, K. J. Dwyer, Q. Campbell, A. D. Baczewski, E. Bussmann, G. T. Wang, Y. Mo, S. Misra, and R. E. Butera,  (2021), arXiv:2101.09265 .
  * Liu  _et al._ (2008) Z. Liu, Z. Zhang, and X. Zhu, Physical Review B - Condensed Matter and Materials Physics 77, 035322 (2008).
  * Wang and Hamers (1995) Y. Wang and R. J. Hamers, Applied Physics Letters 66, 2057 (1995).
  * Gautier  _et al._ (1996) B. Gautier, R. Prost, G. Prudon, and J. C. Dupuy, Surface and Interface Analysis 24, 733 (1996).
  * Hagmann  _et al._ (2018) J. A. Hagmann, X. Wang, P. Namboodiri, J. Wyrick, R. Murray, M. D. Stewart, R. M. Silver, and C. A. Richter, Applied Physics Letters 112, 043102 (2018).
  * Weir  _et al._ (1994) B. E. Weir, L. C. Feldman, D. Monroe, H. J. Grossmann, R. L. Headrick, and T. R. Hart, Applied Physics Letters 65, 737 (1994).
  * Hagmann  _et al._ (2020) J. A. Hagmann, X. Wang, R. Kashid, P. Namboodiri, J. Wyrick, S. W. Schmucker, M. D. Stewart, R. M. Silver, and C. A. Richter, Physical Review B 101, 245419 (2020).
  * Pavlova  _et al._ (2019) T. V. Pavlova, E. S. Skorokhodov, G. M. Zhidomirov, and K. N. Eltsov, The Journal of Physical Chemistry C 123, 19806 (2019).
  * Zhang and Lagally (1994) Z. Zhang and M. G. Lagally, Physical review letters 72, 693 (1994).
  * Voigtländer and Zinner (1994) B. Voigtländer and A. Zinner, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 12, 1932 (1994).
  * Ferng  _et al._ (2009) S.-S. Ferng, S.-T. Wu, D.-S. Lin, and T. C. Chiang, J. Chem. Phys. 130, 164706 (2009).
  * Li  _et al._ (2011) H.-D. Li, C.-Y. Chang, L.-Y. Chien, S.-H. Chang, T.-C. Chiang, and D.-S. Lin, Phys. Rev. B 83, 075403 (2011).
  * Mitsui  _et al._ (1999) T. Mitsui, R. Curtis, and E. Ganz, J. Appl. Phys. 86, 1676 (1999).
  * Dick  _et al._ (2016) D. Dick, J. B. Ballard, R. C. Longo, J. N. Randall, K. Cho, and Y. J. Chabal, The Journal of Physical Chemistry C 120, 24213 (2016), https://doi.org/10.1021/acs.jpcc.6b08130 .
  * Longo  _et al._ (2014) R. C. Longo, S. McDonnell, D. Dick, R. M. Wallace, Y. J. Chabal, J. H. G. Owen, J. B. Ballard, J. N. Randall, and K. Cho, Journal of Vacuum Science & Technology B 32, 03D112 (2014), https://doi.org/10.1116/1.4864619 .
  * Pavlova  _et al._ (2018) T. V. Pavlova, G. M. Zhidomirov, and K. N. Eltsov, J. Phys. Chem. C 122, 1741 (2018).
  * Dwyer  _et al._ (2019) K. J. Dwyer, M. Dreyer, and R. E. Butera, J. Phys. Chem. A 123, 10793 (2019).
  * Silva-Quinones  _et al._ (2020) D. Silva-Quinones, C. He, R. E. Butera, G. T. Wang, and A. V. Teplyakov, Applied Surface Science , 146907 (2020).
  * Zhang  _et al._ (1996) Z. Zhang, M. A. Kulakov, B. Bullemer, I. Eisele, and A. V. Zotov, Applied Physics Letters 69, 494 (1996).
  * Mackus  _et al._ (2019) A. J. Mackus, M. J. Merkx, and W. M. Kessels, Chemistry of Materials 31, 2 (2019).
  * Gladfelter (1993) W. L. Gladfelter, Chemistry of Materials 5, 1372 (1993).
  * Ramanayaka  _et al._ (2018) A. N. Ramanayaka, H.-S. Kim, J. A. Hagmann, R. E. Murray, K. Tang, F. Meisenkothen, H. R. Zhang, L. A. Bendersky, A. V. Davydov, N. M. Zimmerman, C. A. Richter, and J. M. Pomeroy, AIP Advances 8, 075329 (2018), https://doi.org/10.1063/1.5045338 .
  * Goh  _et al._ (2006) K. Goh, L. Oberbeck, M. Simmons, A. Hamilton, and M. Butcher, Physical Review B 73, 035401 (2006).
  * Weber  _et al._ (2012) B. Weber, S. Mahapatra, H. Ryu, S. Lee, A. Fuhrer, T. Reusch, D. Thompson, W. Lee, G. Klimeck, L. C. Hollenberg, _et al._ , Science 335, 64 (2012).
  * Lee and Ramakrishnan (1985) P. A. Lee and T. Ramakrishnan, Reviews of Modern Physics 57, 287 (1985).