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Large-Area Dry Transfer of Single-Crystalline Epitaxial Bismuth Thin Films Emily S. Walker, Seung Ryul Na, Daehwan Jung, Stephen D. March, Joon-Seok Kim, Tanuj Trivedi, Wei Li, Li Tao, Minjoo L. Lee, Kenneth M Liechti, Deji Akinwande, and Seth R. Bank Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02931 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 20, 2016

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Large-Area Dry Transfer of Single-Crystalline Epitaxial Bismuth Thin Films Emily S. Walker1, Seung Ryul Na2, Daehwan Jung3, Stephen D. March1, Joon-Seok Kim1, Tanuj Trivedi1, Wei Li1, Li Tao1, Minjoo L. Lee3,4, Kenneth M. Liechti2, Deji Akinwande*1, and Seth R. Bank*1 1. Microelectronics Research Center and Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78758, USA. 2. Research Center for the Mechanics of Solids, Structures and Materials and Department of Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin, Austin, TX,78712, USA. 3. Department of Electrical Engineering, Yale University, New Haven, CT 06520, USA. 4. Department of Electrical and Computer Engineering, The University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

KEYWORDS: Molecular Beam Epitaxy, Bismuth, Thin films, Transfer, 2-D electronics Corresponding Authors * Seth R. Bank ([email protected]) * Deji Akinwande ([email protected])

1 ACS Paragon Plus Environment

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Table of Contents Figure

Intensity (Arb. Units)

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Bi

1200

900

600

Exfoliated

Si

300

Original 0 0

100

200

300

400

Raman shift (cm-1)

500

600

ABSTRACT: We report the first direct dry transfer of a single-crystalline thin film grown by molecular beam epitaxy. A double cantilever beam fracture technique was used to transfer epitaxial bismuth thin films grown on silicon (111) to silicon strips coated with epoxy. The transferred bismuth films retained electrical, optical and structural properties comparable to the as-grown epitaxial films. Additionally, we isolated the bismuth thin films on freestanding flexible cured-epoxy post-transfer. The adhesion energy at the bismuth/silicon interface was measured to be ~1 J/m2, comparable to that of exfoliated and wet transferred graphene. This low adhesion energy and ease of transfer is unexpected for an epitaxially-grown film, and may enable the study of bismuth’s unique electronic and spintronic properties on arbitrary substrates. Moreover, this method suggests a route to integrate other group-V epitaxial films (i.e. phosphorus) with arbitrary substrates, as well as potentially to isolate bismuthene, the atomic thin-film limit of bismuth.

MAIN TEXT: Bismuth thin films have historically attracted attention due to their quantum size effects, long mean free path, low carrier density, and large spin-orbit coupling.1–5 The experimental room temperature carrier mobility of bismuth is extremely high, on the order of 20,000 cm2/V-s for 0.1 – 2 μm


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thin films3, with single-crystalline bulk bismuth exhibiting room temperature carrier mobility up to 5.7x106 cm2/V-s.6 Although bulk bismuth is a semi-metal with a band overlap of 30 - 40 meV,1,7 a transition to semiconducting behavior occurs through quantum confinement effects in bismuth films thinner than approximately 30 nm.7–10 In these films, a small, indirect band gap coexists with highly conductive surface states, which are observable in- and ex-situ.11–15 These surface states are resistant to oxidation and the introduction of non-magnetic impurities, raising the possibility that bismuth films at this scale may be topologically non-trivial,16 as theoretical studies predict freestanding and methyl-functionalized individual bismuth bilayers to be.17 The combination of high carrier mobility and a tunable band gap makes bismuth thin films attractive for electronic switching applications, while the presence of surface states and high spin-orbit coupling may enable integration with bismuth chalcogenide topological insulators and other materials for spintronic devices. Unlike many contemporary two-dimensional semiconductors, bismuth has been shown to be highly scalable and compatible with existing silicon-based technology. Indeed, full wafer single crystalline bismuth films as thin as 4 nm have been grown on Si(111) substrates through molecular beam epitaxy (MBE),18–22 while well-oriented polycrystalline films have been reported on Si(001) substrates through incorporation of an annealing step into the growth process.23 The Bi/Si(111) system is a unique case of semi-metal/semiconductor heteroepitaxy in both the absence of an intermixed and silicide zone at the immediate interface,23 and the dominance of planar growth rather than the rough, islanding morphology often observed for the Stranski-Krastanov growth mode.18 Its simple composition may also result in less susceptibility to environmental doping than has been observed in bismuth chalcogenide topological insulators, due to the reduced potential for stoichiometric vacancies and other defects in pure bismuth films.24,25


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We demonstrate that, unlike conventional Group-IV or III-V epitaxial films, Bi films grown on Si(111) substrates may be easily transferred in large, continuous areas from the host Si substrate to a secondary substrate. This transfer was accomplished through an epoxy-based dry transfer method originally developed to transfer and measure the adhesion energy of graphene grown by chemical vapor deposition (CVD) on copper surfaces.26–28 This technique is significantly simpler than the typical methods necessary for transferring epitaxial films, which include the incorporation of fabricated structures to facilitate cleaving,29 chemical removal of the host substrate after wafer bonding the epitaxial layer,30 and the use of an etch-release31–33 or low-adhesion energy intermediate layer34,35 to achieve epitaxial liftoff. Conversely, direct dry transfer of the epitaxial Bi is enabled by the surprisingly low adhesion energy at the Bi/Si(111) interface, which was extracted from load-displacement measurements conducted during transfer. The measured adhesion energy is lower than that of transferrable CVD-grown graphene on nickel or copper, which is an unexpected result for a single-crystalline thin film grown epitaxially by MBE. While chemical exfoliation and transfer of the topological insulator Bi2-xSbxTe3-ySey has recently been demonstrated,36 direct dry transfer of MBE-grown epitaxial compounds has not previously been achieved. The ability to easily transfer large areas of high quality epitaxial bismuth thin films to transparent, insulating, or flexible substrates using this method could enable optical, electrical, and strain effect studies of isolated singlecrystalline bismuth thin films for the first time, as well as allowing bismuth thin films to be utilized in novel heterostructures for electronic or spintronic devices. Additionally, the success of this method suggests that other group-V thin films that form puckered-layer monolayers, including antimony, arsenic and phosphorus37, may also have the potential for straightforward dry transfer. The latter two films are typically termed arsenene and phosphorene in the monolayer limit, respectively, and are experiencing rapidly growing contemporary interest.38–40 In this light, puckered-layer atomic thin films of bismuth can be called bismuthene.


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Growth of bismuth thin films on the 7x7 reconstruction of Si(111) by MBE has been shown to initiate through the formation of an initial disordered wetting layer, followed by an intermediate puckered-layer pseudocubic crystal structure with an {012} orientation.18–21 This thin layer of pseudocubic bismuth transitions to the [001] oriented hexagonal (alternatively defined as [111] rhombohedral) crystal structure of bulk Bi after approximately 4-6 ML of deposition.11,19–21 The hexagonal Bi(001) film has been shown to grow in a planar, layer-by-layer fashion, despite the extremely large lattice mismatch (~18%) between Bi and Si(111).18–21 Bismuth thin films with thicknesses between 6 – 50 nm were epitaxially grown on Si(111) substrates in a Varian Gen II MBE growth chamber. Prior to loading into the vacuum system, Si(111) substrates were briefly dipped in a solution of diluted hydrofluoric acid (HF) in order to remove the native oxide and passivate dangling surface bonds with hydrogen.41–43 The substrate was loaded into high vacuum within twenty minutes in order to limit re-oxidation, where it underwent a series of high temperature bakes in order to further reduce environmental contamination and to remove any remnants of the native oxide. Growth of Bi was initiated at room temperature at a rate of 0.2 Å/s. The growth rate was calibrated ex situ using transmission electron microscopy (TEM) and X-ray reflectivity (XRR) following growth. A representative TEM image of a nominally 15 nm thick Bi film grown using this method is shown in Figure 1a. The Bi atoms are arranged in a well-ordered, periodic structure, with a clearly defined interface between the Bi film and the Si(111) substrate. The high crystalline quality of the epitaxially grown bismuth thin films is further supported by the reflective high-energy electron diffraction (RHEED) images shown in Figures 1b and 1c, which were recorded in situ during growth. A transition in RHEED from an initial faint double line pattern previously observed by Nagao et al.18 to the 1x1 reconstruction characteristic of bulk bismuth was observed after approximately 4 nm of growth (Figure 1b), with some variation between 3 - 6 nm depending on the exact substrate temperature. The RHEED pattern remained


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clear and streaky throughout the entire growth (Figure 1c), suggesting planar epitaxial growth of high quality thin films.

a

b

Epoxy

13.5 nm

5 nm

Bi

c

12.3 nm

Si (111) 10 nm

d

e

f

-10 nm

Figure 1. Epitaxial growth of Bi thin films on Si(111) substrates. (a) Cross-sectional TEM image of a ~13 nm Bi film. The well-ordered crystal structure of the Bi indicates high-quality epitaxial growth on Si(111). (b) RHEED image of Bi film after 2 nm growth. (c) RHEED image of Bi film after 15 nm growth. (d) XRD scan of the Bi (003) peak showing alignment of the Bi film along the [001] hexagonal direction. (e) AFM images of 6 nm and (f) 50 nm thick Bi films. The films were relatively smooth, with RMS surface roughness of 0.397 and 1.93 nm, respectively.


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High-resolution X-ray diffraction (XRD) of the epitaxially grown Bi films (Figure 1d) indicates that the Bi films are oriented along the expected hexagonal (001) direction. The single-crystalline, epitaxial nature of the Bi films is supported by the presence of Pendellösung fringes suggestive of high-quality interfaces surrounding the (003) Bi peak. Representative atomic force microscopy (AFM) images for 6 nm and 50 nm thick films are shown in Figures 1e and 1f, respectively. As expected, the films were quite smooth, with root mean square (RMS) surface roughness on the order of 1 nm, with slight roughening observed in thicker films. This roughening process is likely to explain the gradual decrease in Pendellösung fringe definition with increasing film thickness. The Bi films were transferred using double cantilever beam (DCB) Mode I fracture.27,28 For this purpose, a rectangular strip of as-grown bismuth on the original epitaxial Si(111) substrate was bonded to a secondary bare Si(111) strip using a layer of low viscosity epoxy. The thickness of the epoxy layer varied between 3-10 μm. An identical Si(111) substrate was chosen for initial proof of concept transfer in order to avoid shear effects due to geometry effects. In order to obtain the maximum adhesion strength between epoxy and Bi, the laminated samples were cured at 100°C for 2 hours, followed by gluing aluminum loading fixtures onto the silicon strips. The final prepared DCB samples had a pre-notch acting as the initial crack, and a fully-bonded region over which the adhesion energy between the Bi and Si(111) was measured.


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Si (111)

a

G (J/m2)

1.50 1.50 1.25 1.25 1.00 1.00 0.75 0.75

2

G (J /m )

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0.50 0.50

0.25 0.25 0.00 0.00 10 10

15 15

20 20

25 25

30 30

35 35

a (m m )

b

a (mm)

Figure 2. (a) Schematic of the double cantilever beam method for dry transfer of the Bi films from their host substrate using epoxy. (b) The strain energy release rate (G) versus crack length (a) measured during transfer. The adhesion energy was calculated to be approximately 1 J/m2.

DCB fracture experiments conducted at a displacement rate of ~0.02 mm/s showed the successful transfer of as-grown Bi on Si (111) to the epoxy adhesive layer on the secondary substrate. A schematic of this procedure is shown in Figure 2a. The successful transfer of the epitaxial layer indicates that a crack initiated beneath the epoxy terminus and branched down into the Bi film, followed by interfacial cracking along the interface between the Bi film and the Si(111) substrate. During delamination, the reaction force


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(P) and applied displacement (Δ) at the loading fixture were measured for further fracture analysis. According to simple beam theory, the reaction force is related to the applied displacement through: 𝑃=

!"! ! !! !

∆

(Equation 1)

where E is the in-plane Young’s modulus of Si(111) (169 GPa), b and h are the width and thickness of the silicon strips, respectively, and a is the length of the crack from the loading point to the crack front. Thus, the crack length (a) was estimated simultaneously from the measurement of the reaction force and applied displacement. The energy release rate (G) is the energy available for the creation of new surface, and is given by:27 𝐺=

!"! ! ! ! !! ! ! !

.

(Equation 2)

The resistance of the Bi/Si(111) interface to crack propagation is obtained by plotting these two quantities (Figure 2b). It can be observed from this plot that the initial crack length was approximately 11 mm. The initially steep rise in the energy release rate reflects the fact that very little crack growth occurred below approximately 1 J/m2. This phase was followed by slow crack growth as the energy release rate climbed to a peak value of 1.25 J/m2. The subsequent drop in energy release rate corresponds to rapid crack growth culminating in the arrest of the crack. The cycle of fast growth and arrest repeated many times in what is known as stick-slip crack growth. Averaging the peak values of the energy release rate in each stick-slip event resulted in an average value of the adhesion energy (or fracture toughness of the interface) of 1 J/m2 for epitaxially grown Bi on Si(111). The uniformity of the energy release rate response at each stage of crack growth indicates that the Bi film was homogenously grown on the initial Si(111) substrate. The adhesion energy of the Bi/Si(111) interface is compared to experimental adhesion energy values measured using blister, nano-scratch and DCB fracture techniques for epitaxial and exfoliated interfaces in Table 1. The magnitude of the Bi/Si(111) adhesion energy is comparable to that of graphene transferred 9 ACS Paragon Plus Environment

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through exfoliation or wet transfer onto SiOx and is higher than the average adhesion energy between CVD-grown graphene and the metallic substrate. By contrast, single-crystalline epitaxially-grown semiconductors cannot typically be transferred from their host substrates easily due to the strong covalent bonding at the interface. In order to directly compare the adhesion energy of Bi on Si(111) to that of a typical III-V interface, a 15 nm film of aluminum arsenide (AlAs) with a 2 nm gallium arsenide (GaAs) cap was epitaxially grown on a GaAs substrate and subjected to the same DCB fracture experiment. Instead of transferring the epitaxial AlAs film, the epoxy was observed to delaminate from capping layer, corresponding to a lower adhesion energy bound of 7-8 J/m2 for the epitaxial AlAs/GaAs interface.

Table 1. Empirical Adhesion Energy Values

Interface

Adhesion Energy (J/m2)

Exfoliated Graphene / SiO2

0.15 – 0.4544–46

Epitaxial Bi / Si(111)

~1

Wet Transferred Graphene / SiOx

0.35 - 2.9847,48

CVD Graphene / seed Cu

0.72 – 6.026,27

Epitaxial AlAs / GaAs

>7-8

CVD Graphene / seed Ni

12.849


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This inability to transfer a conventional epitaxially-grown III-V film underscores the unique nature of the Bi/Si(111) interface. As formation of analogous nanoscale allotropes does not occur at the interfaces between MBE-grown III-V or group IV semiconductor films and their epitaxial substrates, it is likely that the initial formation of the puckered-layer phase of Bi19,20,50 plays a crucial role in enabling dry transfer in this material system. While conventional MBE-grown semiconductors form strong covalent bonds to their epitaxial substrates, by contrast, the puckered layer Bi allotrope that initially forms at the Si(111)/Bi interface has an inherent 2D layered structure similar to that of black phosphorus.19,20 The presence of buckled layers with even-layered stability in this structure has been previously shown by ab-initio calculation to indicate saturation of the out-of-plane dangling bonds.19 Consequently, it may be concluded that the interactions between the initial bismuthene layers and the Si(111) substrate are weak compared to the covalent bonding present at typical epitaxial heterointerfaces. As this unique Bi allotrope has been previously observed20,21 to undergo a structural transformation to bulk Bi(001) as growth proceeds, the exact relationship between the formation of the initial puckered-layer phase and the inherently low adhesion energy is currently unclear and merits further study. Likely potential mechanisms include modification of the bond structure between Bi(001) and Si(111) by the initial presence of the {012} allotrope19, and remnants of the allotrope at the interface in thicker films. In either case, successful transfer of epitaxial Bi films suggest that other group V films that form puckered-layer structures, including phosphorus and arsenic, may exhibit similarly low adhesion energy. Further studies are necessary in order to confirm the exact source of the unusually low adhesion energy of epitaxial Bi(001) to Si(111) substrates, and to determine whether similar effects are present in other group V epitaxial thin films37. Structural, electrical, and optical characterization of 15 and 30 nm Bi films before and after transfer confirmed that films of both thicknesses were successfully transferred in good condition, as they maintained their structural and electronic properties post-transfer.


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a

Si

Bi

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Transferred Bi Original Si

10 mm

b

c

d Figure 3. (a) Photograph of the transferred Bi film and the original epitaxial Si(111) substrate. (b) EDS spectra of the transferred Bi film bonded to the destination Si(111) wafer with epoxy. (c) EDS spectra of original Si(111) substrate after removal of the Bi film. No detectable Bi remains on the original substrate, indicating that the entirety of the film was transferred. (d) XRD pattern scans of the Bi (003) peak before 12 ACS Paragon Plus Environment

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and after transfer of 15 and 30 nm Bi films. The inset shows the clear shift in the (003) peak between the 30 nm Bi film strained to the original Si(111) substrate and the same film after transfer.

The Si(111) strips after Bi transfer and separation are shown in Figure 3a. The visible contrast difference between the bare Si substrate and the transferred epitaxial Bi roughly defines the region of transfer. Figure 3b and 3c present electron dispersive spectroscopy (EDS) measurements were used to confirm transfer of the Bi film to the epoxy-coated Si(111) strip. There is a clear distinction between the EDS spectrum of the transferred Bi film (3b) compared to the original Si(111) substrate post-transfer (3c). The EDS spectrum of the transferred bismuth film includes a main Bi peak, as well as carbon and oxygen peaks originating from the underlying epoxy. By contrast, no Bi peak is visible in the EDS spectrum of the epitaxial Si(111) substrate after transfer; with the exception of a native oxide signature, a similar EDS spectrum was exhibited by the bare secondary Si(111) strip prior to coating with epoxy. No attempt was made to remove the native oxide. This suggests that the entirety of the Bi film thickness was transferred, rather than delamination occurring internally partially through the Bi film. In order to confirm that this finding is representative of the majority of the Bi film and to determine the post-transfer crystal structure, XRD scans of the epitaxial Bi (003) and Si 111) peak pre- and post- transfer are compared in Figure 3d. The Bi (003) peak is preserved, which suggests that transfer of the complete film was successful. The full width half maximum of the Bi (003) peak (normalized with respect to the substrate peak) was found to broaden by approximately 5%. This implies that the structural integrity and crystalline orientation of the Bi film are robust to transfer. The slight shift in the position of the Bi (003) peak shown in the inset of Figure 3d is attributed to reduction of the already weak strain effects from the Si(111) substrate by the transfer process. Transferring the 30 nm Bi film was observed to reduce the out-ofplane hexagonal lattice constant calculated from the Bi (003) d-spacing from 11.92 Å in the original 13 ACS Paragon Plus Environment

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epitaxial film to 11.90 Å after transfer. The lattice constant post-transfer is closer to the bulk value, 11.86 Å,22 indicating slight relaxation of the Bi film induced by the transfer process. Reduction of the interfacial fringes after transfer may be due to increased disorder at the Bi/epoxy interface compared to the original high-quality interface between Bi and Si(111). The decreased intensity of the transferred films cannot be explained by the reduced area of the transferred sample, and could be a result of tiny gaps in the transferred film, or interference from the epoxy layer. The robustness of the material and electrical properties of the transferred Bi films were investigated via scanning electron microscopy (SEM), atomic force microscopy (AFM), and temperature dependent sheet resistance measurements and are summarized in Figure 4. These films were found to be uniform, smooth and featureless in large areas, except for regions in which bubbles in the epoxy locally prevented transfer of the Bi film. These areas were distinguishable as darker regions less than 1 μm2 in area in AFM images. The step height of these recessed areas measured by AFM corresponded well to the thickness of the original Bi film calibrated by XRR and TEM, providing further evidence that the complete Bi film was transferred. Further optimization of the DCB transfer method or utilization of another dry transfer technique may resolve or improve this issue. The sheet resistance of the transferred 30 nm Bi film was measured as a function of temperature using the van der Pauw method, and converted to resistivity using the film thickness determined by XRR. As expected for Bi films in this thickness regime,7–10 the electrical data showed semiconducting behavior in the temperature range studied. The original and transferred films exhibited similar resistivity at room temperature, with differences becoming more pronounced at lower temperatures, likely due to differences in the local dielectric environment. As is commonly observed for bismuth films thinner than 100 nm,10,11 the magnitude of the resistivity was higher than the resistivity of bulk bismuth, which is on the order of 1x10-4 Ω-cm51 for single-crystalline films at room temperature.


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a

Transferred Bi on Epoxy/Si 200 µm

10 nm

b 1.0 µm

c

-10 nm

Figure 4. Morphological and electrical characterization of the transferred Bi films. (a) SEM image of transferred 30 nm Bi film. The Bi film is shown to be transferred in areas on the order of 10 μm x 10 μm. (b) AFM image of transferred 15 nm Bi film. The step height of recessed areas is consistent with the film thickness. (c) Temperature dependent resistivity of original and transferred 30 nm films showing semiconducting behavior.


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The similar electrical properties post-transfer indicates that the Bi film is likely to maintain its unique transport properties, which is important for the integration of Bi thin films with diverse substrates through dry transfer. Additionally, the ability to successfully transfer whole Bi films in this manner affords access to the puckered bismuth layers at the initial heterointerface, potentially affording access to bismuthene layers for the first time. In addition to being easily transferrable from their original epitaxial substrate, single crystalline Bi films may also be isolated from the secondary Si(111) substrate post-transfer by manually peeling the epoxy layer from the Si(111) strip. In order to achieve separation of the epoxy layer from the secondary Si(111) substrate, the latter was first cleaved using a diamond scribe pen and a glass slide for support. While the Si(111) substrate cleaved easily due to its high degree of crystallinity, the epoxy was torn in order to grasp it, and then the epoxy and Bi were peeled from the secondary Si strip. As shown in the inset of Figure 5a, this method produced an almost freestanding continuous film of Bi on a flexible epoxy substrate. Raman spectra from samples before and after exfoliation of the Bi/epoxy layers from the Si(111) strip exhibited qualitatively identical Bi-related in-plane and out of plane-modes, as shown in Figure 5a, with the absence of the silicon peak being the only significant difference post-exfoliation. Raman spectra were measured using a Renishaw inVia Raman spectrometer capable of measuring down to 10 cm-1, allowing both characteristic Bi modes to be fully recorded. Additionally, no peaks corresponding to bismuth oxide were observed between 120-500cm-1,52 suggesting isolation of pure bismuth. A high angle annular dark field scanning transmission electron microscopy (HAADF STEM) image of the exfoliated Bi on epoxy layer is shown in Figure 5b.


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Bi

1200

a

b

Glue

Epoxy

c

900 600

Exfoliated

Si

300

Original

Bi

0 0

100

200

300

400

500

80 nm

80 nm

600

Raman shift (cm-1)

20 nm

20 nm 2 µm

2 µm 0 µm

-20 nm

d

e

Figure 5. (a) Raman spectrum of Bi exfoliated onto flexible epoxy through mechanical peeling. Inset: photograph of flexible Bi on epoxy post-exfoliation. (b) HAADF STEM image of exfoliated Bi film on epoxy. (c) Corresponding EDS image showing intact Bi film on epoxy. (d) AFM image of Bi after transfer to epoxy, showing a wrinkled, but continuous, Bi film. (e) FTIR reflectivity spectra of exfoliated Bi on epoxy (denoted “Exfoliated”) compared to that of the original Bi film after growth (“Original”), as well as the same Bi film immediately following initial transfer to the secondary Si(111) wafer (“Transferred”).

In order to prepare the Bi/epoxy STEM sample, a glue layer was used to bond two exfoliated Bi layers. The glued Bi layers were then sandwiched between two 500 μm indium phosphide (InP) substrates in order to support the exfoliated Bi films during mechanical grinding. As ion milling reduces the thickness of the Bi 17 ACS Paragon Plus Environment

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film, the glue bonding the two exfoliated Bi films weakens, resulting the inclusion of only one side of the exfoliated Bi film with glue in Figure 5b. The exfoliated Bi film on epoxy was continuous, indicating that the Bi film was isolated in good condition on the epoxy. Corresponding EDS analysis of the film and epoxy after exfoliation from the secondary Si substrate (Figure 5c) shows the clear signature of the Bi film on the epoxy, confirming that the Bi film is maintained as a continuous film post-exfoliation. Indeed, the AFM image of the Bi on epoxy after separation from the Si shown in Figure 5d confirms that the surface of the exfoliated Bi was continuous over a 4 μm2 area despite slight wrinkling of the Bi/epoxy flake. Fourier transform infrared spectroscopy (FTIR) reflectivity spectra of the mechanically exfoliated Bi on epoxy, the original Bi film after growth, and the same Bi film after the transfer are shown in Figure 5e. The transferred sample exhibited a Drude edge consistent with that of the original sample, with the addition of Fabry-Perot fringes due to the epoxy interlayer. The exfoliated sample displayed the same Drude edge and qualitatively similar Fabry-Perot fringes. Slight shifting in fringe periodicity is attributed to epoxy thickness change during the exfoliation process and the reduction in reflectivity at high wavenumbers is due to the loss of reflectivity from the back Bi/air interface after exfoliation. This result confirms the successful exfoliation of thin-film Bi with epoxy from the Si(111) substrate. Further investigations of the electrical and material properties of the exfoliated Bi film on epoxy are currently ongoing. This ability to isolate a single-crystalline, epitaxially grown film of Bi on a flexible epoxy substrate can enable future investigations of strain effects in single-crystalline Bi, as well as the development of flexible device applications for this material. Additionally, as the mechanical peeling method used to separate the epoxy from the Si(111) substrate results in complete exposure of one surface of the cured epoxy layer, we expect removal of the cured epoxy layer from the exfoliated Bi film to be straightforward using commerciallyavailable solvents, providing a potential pathway toward integration of single-crystalline Bi onto arbitrary substrates.


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In conclusion, the successful dry transfer of epitaxial Bi films has been demonstrated for the first time. Single-crystalline, semiconducting Bi films were transferred using a double cantilever beam method and shown to maintain their characteristic material and electrical properties post-transfer, as well as their crystalline orientation in the [001] direction. The unexpectedly low adhesion energy measured between these films and their epitaxial Si(111) substrates may be due to the unique growth mode of Bi, which initiates through the formation of an initial puckered-layer structure. Further work is necessary to expand this approach to other epitaxial group-V films with similar puckered-layer structures. The transferred Bi films were further isolated on the epoxy substrate layer post-transfer and continued to exhibit high crystallinity and semiconducting electrical properties, despite slight wrinkling of the flexible substrate. The ability to easily transfer epitaxial Bi can enable previously impossible measurements of single-crystalline Bi on transparent, flexible or topologically insulating substrates, and allows for the development of novel Bi-based electronic and spintronic devices. Additionally, as the dry transfer of a planar, single-crystalline thin film grown by MBE has not previously been demonstrated, these results open new possibilities for devices incorporating epitaxial films.


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Supporting Information. The following files are available free of charge. Supporting information including detailed descriptions of Bi film growth, sample preparation, and exfoliation, as well as additional RHEED, XRR, FTIR and XPS data (PDF) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Emily Walker acknowledges support from the Texas Instruments Graduate Fellowship. Deji Akinwande acknowledges support from the Army Research Office and the Presidential Early Career Award for Scientists and Engineers (W911NF-16-1-0277). Seth Bank acknowledges support from the Temple Foundation, a Multidisciplinary University Research Initiative from the Air Force Office of Scientific Research (AFOSR MURI Award No. FA9550-12-1-0488), and AFOSR YIP (FA9550-10-1-0182). Acknowledgements The authors would like to acknowledge Professor Jung-Fu Lin from the Department of Geological Science at the University of Texas at Austin for assistance with Raman spectroscopy. Abbreviations MBE, molecular beam epitaxy; Bi, bismuth; Si, silicon; CVD, chemical vapor deposition; HF, hydrofluoric acid; RHEED, reflective high-energy electron diffraction; TEM, transmission electron microscopy; XRR, X-ray reflectivity; XRD, X-ray diffraction; AFM, atomic force 20 ACS Paragon Plus Environment

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microscopy; DCB, double cantilever beam; AlAs, aluminum arsenide; GaAs, gallium arsenide; EDS, electron dispersive spectroscopy; XPS, X-ray photoelectron spectroscopy; FTIR, Fourier transform infrared spectroscopy


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