Single-Crystal Silicon Optical Fiber by Direct Laser ... - ACS Publications

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

Single-Crystal Silicon Optical Fiber by Direct Laser Crystallization Xiaoyu Ji, Shiming Lei, Shih-Ying Yu, Hiu Y. Cheng, Wenjun Liu, Nicolas Poilvert, Yihuang Xiong, Ismaila Dabo, Suzanne E. Mohney, John V. Badding, and Venkatraman Gopalan ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00584 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Photonics is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Single-Crystal Silicon Optical Fiber by Direct Laser Crystallization Xiaoyu Ji,ǂ, Shiming Lei,ǂ, Shih-Ying Yu,ǂ, Hiu Y. Cheng,,‖ Wenjun Liu,† Nicolas Poilvert,ǂ, Yihuang Xiong,ǂ, Ismaila Dabo,ǂ, Suzanne E. Mohney,ǂ, John V. Badding,*,ǂ,,‖,§ and Venkatraman Gopalan*,ǂ, ǂ

‖

§



Department of Materials Science and Engineering, Department of Chemistry, Department of Physics, and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States. †Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States KEYWORDS: optical fiber, silicon photonics, chemical vapor deposition, laser crystallization, crystal growth, optoelectronics ABSTRACT: Semiconductor core optical fibers with a silica cladding are of great interest in nonlinear photonics and optoelectronics applications. Laser crystallization has been recently demonstrated for crystallizing amorphous silicon fibers into crystalline form. Here we explore the underlying mechanism by which long single-crystal silicon fibers, which are novel platforms for silicon photonics, can be achieved by this process. Using finite element modeling, we construct a laser processing diagram that reveals a parameter space within which single crystals can be grown. Utilizing this diagram, we illustrate the creation of single-crystal silicon core fibers by laser crystallizing amorphous silicon deposited inside silica capillary fibers by high-pressure chemical vapor deposition. The single-crystal fibers, up to 5.1 mm long, have a very welldefined core/cladding interface and a chemically pure silicon core that leads to very low optical losses down to ~0.47-1 dB/cm at the standard telecommunication wavelength (1550 nm). It also exhibits a photosensitivity that is comparable to bulk silicon. Creating such laser processing diagrams can provide a general framework for developing single-crystal fibers in other materials of technological importance. The availability of high-purity semiconductors in fiber form could be transformative in bringing new classes of technologies to the table including the vision of all-fiber optoelectronics, where light is created, modulated and detected within an optical fiber. Silicon (Si) core optical fibers, in particular, have emerged in the past decade as a new platform for integrated optoelectronic devices.1–5 The Si transparency window is from 1.1 μm to 7 μm, which encompasses an atmospheric transmission window (3-5 µm) for free space communications, as well as the spectroscopic window for the chemical identification of molecules through their characteristic vibrations. Motivated by its large third order nonlinear coefficient χ(3),6 Si has been fabricated into both on-chip and fiber-based waveguides to explore nonlinear processes such as self-phase modulation,7,8 four-wave mixing9,10 and even supercontinuum generation.10,11 All-optical modulation12,13 also makes Si core fibers attractive for optical interconnect technology, which could step towards all-fiber optoelectronics. In comparison

with its amorphous counterpart, crystalline silicon (c-Si) is preferred for electronic devices because of its superior thermal and electronic properties.14 Various methods have been implemented to fabricate crystalline semiconductor core fibers, including direct thermal drawing,15,16 pressureassisted melt filling17 and post thermal annealing of high pressure chemical vapor deposited fibers.18 The thermal drawing process usually take place at temperatures higher than the melting points of semiconductor cores and the softening temperatures of cladding materials. Low optical

loss fibers made by such technique without any postprocessing usually have large core diameters, making them difficult to be used for nonlinear optical applications. The pressure-assisted melt filling technique has been proven to be effective in infiltrating germanium17 and arsenic sulfide19 into micron-sized pores inside silica fibers for supercontinuum generation. In the high pressure chemical vapor deposition (HPCVD) process, semiconductors can be infiltrated into micron-sized fiber pores at relatively low temperatures,20 and crystalline cores can be made either directly from deposition21 or by choosing a proper post-annealing method.3,18,22 While polycrystalline Si fibers with grain sizes ranging from 500 nm to a few microns have been successfully achieved, single crystalline fibers with even better performance are being pursued, since they should have lower optical losses due to the removal of grain boundaries, and this provides fundamental requirement for nonlinear optical applications or realizing efficient optical modulation in a fiber waveguide. Various annealing methods have been employed so far to create fibers with larger grain size. Large core diameter (100 μm) Si fiber containing single-crystal grains of ~ 9 mm has been made by treating molten core drawn Si fibers with rapid photothermal annealing.23 A laser annealing study by Healy et al.24 reported a polycrystalline Si core fiber with an average grain size of up to 200 μm with an optical propagation loss of 5.6 dB/cm at 1550 nm wavelength. Another recent study by Healy et al. used a CO2

ACS Paragon Plus Environment

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

laser (10.6 µm wavelength) to recrystallize the as-drawn 12 μm diameter crystalline Si core fibers by targeting the softening of the silica cladding and anneal the Si core via conductive heating; the optical loss of a 1.8 cm long single-crystal fiber at 1550 nm was reported to be 2 dB/cm.25 In general, crystallizing only the core and not softening the cladding has advantages in terms of avoiding oxygen contamination of the semiconductor core without using any interface modifiers such as calcium hydroxide,25 as well as retaining the rigidity of the cladding during the laser processing, since the cladding serves as a miniaturized crucible for the crystal growth. In this work, we focus on understanding and demonstrating laser-processing conditions for creating long single-crystal fibers using a 488 nm wavelength continuous wave argon ion laser that targets only the Si core that absorbs it, similar to the method used in Ref. 24 and Ref. 26. We construct a laser processing diagram based on a systematic experimental and finite element modeling study for the Si crystal growth in a fiber capillary in terms of two key processing parameters, namely, laser irradiation power and laser scanning speed. The diagram reveals a clear parameters window where single crystals can be achieved. We grow single crystals using this diagram as a guide, and demonstrate Si fibers with optical loss at 1550 nm of 0.47-1 dB/cm, that is comparable to that of the very best semiconductor-on-insulator (SOI) Si waveguides reported so far. Our single-crystal fibers are of small core diameter (1.7 µm), have chemically sharp interface with the silica cladding on the nanoscale, and possess optoelectronic properties close to bulk Si. We first describe the method using which our singlecrystal Si fibers are fabricated. HPCVD method pioneered by Badding et al.20 was used to deposit void-free amorphous Si (a-Si) inside the capillary hole of a silica fiber. Figure 1a schematically depicts the HPCVD process: a mixture of silane and hydrogen (SiH4/H2) gases at a total pressure between 30 and 35 MPa was configured to flow through a 1.7 μm inner diameter silica capillary heated in a furnace at 500 °C. When the capillary is heated, annular Si films are deposited onto the inner silica wall that merge to form a void-free fiber core.27 The carrier gas and byproduct H2 diffuse out through the silica wall.28 The asdeposited silicon was characterized by Raman spectroscopy and confirmed to be amorphous and not hydrogenated (Figure 1b). A 488 nm continuous wave argon ion laser, which has a penetration depth of 490 nm in Si,29 was later used for the crystallization of our a-Si fibers. A series of fibers were scanned through the focused laser beam by a computer controlled air-bearing linear translation stage (details given in Figure S1, Supporting Information) at different combinations of laser irradiation power and laser scanning speed, which are the two key parameters that are considered for the finite element modeling to build laser crystallization processing diagram. A laser crystallized Si fiber is more transparent under diascopic illumination in an optical microscope than an a-Si fiber because of its lower absorption coefficient, as seen in Figure 1c.29 The Lorentzian line width of the T2g Raman mode for the Si fiber (Figure 1d) is 2.72±0.2 cm-1 compared with 2.70±0.2 cm-1 for a Si bulk,

Page 2 of 10

Figure 1. Fabrication and morphology of silicon fibers. a) Schematic of the HPCVD process to synthesize void free a-Si fiber for laser crystallization. b) Micro-Raman spectrum of as-deposited a-Si fiber. The inset Raman spectrum is centered where usually Si-H stretching mode is, and it shows that the Si core is not hydrogenated. c) Transmitted light microscopy images of a-Si fiber and post laser annealed fiber showing that the laser crystallized silicon fiber is more transparent to visible light. d) Micro-Raman spectra of the laser crystallized Si fiber in comparison with a Si wafer reference. The tensile strain in the Si core is relaxed after etching out of the silica cladding, indicating it originates from the thermal expansion mismatch between the core and cladding.

suggesting that the crystallinity of the fiber is high. A redshift of Δω ~ 3 cm-1 of the Si T2g Raman mode relative to an unstrained Si wafer reference is observed in the infiber Si, and it shifts back to higher frequency for an unstrained state after etching away the silica cladding, which indicates that the tensile strain in the Si core is due to the thermal expansion mismatch between the core and the cladding.

Figure 2. Crystal orientation mappings of fibers crystallized at different laser processing conditions measured by X-ray μLaue diffraction.

We further examined the spatial uniformity of the crystalline orientation of these fibers using synchrotron X-ray micro-beam Laue diffraction (μ-Laue) (depicted in Supporting Information Figure S2). The X-ray beam was focused to a spot with a full width at half maximum of 600 nm, which can be considered as the lateral resolution of the scanned X-ray images (see Methods section). Figure 2 shows the local crystal orientation map of a series of laser crystallized Si fibers measured by the μ-Laue diffraction. Experimentally, ~ 6 mm long as-deposited a-Si fibers were used for the laser crystallization, and the laser heating was initially fixed at a starting location (about 2 mm away

2 Environment ACS Paragon Plus

Page 3 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

from one end of the fiber) to first establish an equilibrium temperature profile, and then the translation stage starts to move at pre-set speed to the other end of the fiber. To understand the mechanism by which a moving laser source creates a Si single crystal in a fiber geometry, we performed finite element modeling (FEM) on the temperature profile evolution (Figure 3) in the fiber heated by a moving laser source30,31 with parameters corresponding to each experimental condition. There is significant laser power loss during the laser crystallization process, such as the scattering loss from the silica cladding-air interface, the scattering from the amorphous core-cladding interface, and the reflection and scattering from liquid Si. Therefore, the net absorbed energy in the crystallization process is less than that of the incoming beam. A net absorption value of 0.081 W in our FEM calculations closely reproduces the results from a laser input power of 0.5 W. Therefore, a scaling factor of 16.2% is assumed in order to compare the FEM simulation results with experimental results. For convenience of discussion, we define “static state” as the initial equilibrium state when the translation stage is not moving. Once the stage starts to move, the initial temperature profile under static state is expected to change. Eventually it reaches a state where the temperature profile only depends on the laser irradiation power and laser scanning speed. We define this state as the “steady state”.

perature profile relative to higher speeds; however it leads to a larger temperature gradient than at higher speed. On the other hand, higher speed at the same power causes insufficient distributed energy per unit length and thus the temperature drops below the crystallization point of Si (1417 °C) shortly after the laser moves away from the heated position. This tradeoff therefore clearly indicates that an optimal speed range exists for a fixed power input. But at even higher laser irradiation power (Figure 3d), the surrounding silica cladding starts to deform and the Si inside starts to vaporize, resulting in a highly polycrystalline Si fiber. Therefore, for a given speed, an optimal laser power range exists as well. These simulation results are consistent with our experimental observations shown in Figure 2, in which a medium power and medium speed favors large crystal growth. By taking a closer look at the temperature gradient in the laser heated region and the laser beam center temperature near the core/cladding interface for each condition, the above conclusions can be further quantified. As shown in Figure 4, an optimum speed and an optimum power not only generate a relatively small temperature gradient, but they also guarantee the melting of the silicon core.

Figure 4. The temperature gradients and the beam center temperatures at core/cladding interface extracted from Figure 3. A scan speed of v=1 mm/s and a power of P=0.5W

result in low temperature gradient as well as a temperature that is just above the silicon crystallization point.

Figure 3. Temperature profile evolution in the Si core simulated using FEM at conditions (adjusted laser irradiation powers and experimental laser scanning speed) corresponding to each experimental conditions in Figure 2. The solid lines are temperature profiles in the center of the silicon core. The dotted lines are temperature profiles at the core/cladding interface, indicating whether the Si core can be fully melted or not.

In the initial static state, simulations show that the temperature profile is symmetric. It becomes asymmetric when the laser spot starts to move and the temperature gradient in the laser-heated regions is lowered. Similar to the Czochralski crystal growth method, the control of temperature gradient is extremely important.30 A smaller temperature gradient reduces the thermal stress, and is beneficial for a crack-free crystal growth.32 From Figure 3a-3c, we notice that at slower speeds, the fiber has a smaller transition length to establish a steady-state tem-

Towards developing a comprehensive laser-processing diagram for crystallizing long Si fibers, we performed more systematic FEM simulations for the temperature profiles at various combinations of laser scanning speeds and laser irradiation powers, and extracted the temperature gradient as well as the beam center temperature at the core/cladding interface (Figure S3). Figure 5a is such a laser processing diagram for long-length Si crystal growth in the speed-power parameter space. The grey area in the plot indicates the window that enables long lengths of single crystal growth. Outside that region, the fiber will end up being polycrystalline. Figure 5b illustrates how the temperature gradient boundaries are constructed. The boundaries based on the criteria of center temperature at core/cladding interface are constructed in a similar way (Figure 5c). We find that a positive temperature gradient less than 400 °C/mm is appropriate for single crystal growth, since it will not cause too much thermal stress and also guarantees that the crystal grows towards the direction of motion of the laser. An interface temperature at the beam center between 1400 °C and 1600 °C guarantees a full melting of the Si core and avoids the formation of SiO phase33,34 at the interface. The experi-

3 Environment ACS Paragon Plus

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 10

mental processing conditions under which we observed millimeters long single crystal growth falls right inside this growth window, illustrating a good consistency between theory and experiments. We expect that even longer single-crystal Si fiber can be grown by constructing such laser processing diagrams under steady state conditions.

Figure 6. Synchrotron X-ray Laue diffraction measurement on the laser crystallized Si fiber reveals its single crystallinity over length scale of 1600 μm. Stereographic projection shows the details of the (111) and (110) orientation distribution along the single crystal Si fiber.

Figure 5. Laser processing diagram for fabricating longlength single-crystal fibers using laser irradiation power and laser scanning speed as the two-dimensional parameter space. a) The grey area suggests the conditions under which long lengths of single crystal growth is favored. The stars mark the experimental conditions under which we observed large single crystal growth in this work. The polygons mark the experiment conditions under which we obtained polycrystalline fibers, and the square marks the conditions used in literature. b)-c) Illustration of how the boundaries based on the criteria of temperature gradient (two yellow lines) and beam center temperature at the core/cladding interface (two blue lines) are constructed. The blue dots represent the values extracted from temperature profiles simulated using FEM. The color represents the magnitude of the relevant quantity along the vertical axis. Red means maximum and blue means minimum.

The single-crystal fiber obtained under laser irradiation power of 0.5 W and laser scanning speed of 1 mm/s was characterized by synchrotron X-ray to spatially map its crystalline quality. A stereographic projection of the (111) diffraction peaks (zone axes that are closest to x direction as the sample was mounted) at 1601 locations along the fiber (processed at 0.5 W irradiation power and 1 mm/s scanning speed) length (Figure 6) shows that the crystallographic direction of cubic [111] is oriented closely perpendicular to the fiber wall (ranging from only 4° to 6.1° off the perpendicular direction) over a length of 1600 μm without any abrupt change. After reconstructing the cubic lattice orientation from the indexed Laue diffraction patterns, cubic [110] and [110] directions, which were out of the spatial detection range of the area detector, were found to align with the fiber axis. This was further confirmed by transmission electron microscopy (TEM) studies, which will be discussed later.

We proposed earlier that laser crystallization process using a visible wavelength laser could avoid oxygen contamination from the cladding. To test this hypothesis, we characterized the atomic scale structure and composition profile across the interface between the silicon core and the silica cladding using transmission electron microscopy (TEM). Figure 7a is a characteristic high-resolution TEM image of the Si/SiO2 interface, and the corresponding electron diffraction pattern is shown in the inset, confirming the X-ray results that the direction is closely aligned with the fiber axis. A field emission scanning electron microscope (FESEM) image of the surface of an etched laser crystallized fiber suggests that the interface is abrupt and smooth at the resolution limit of 2.5 nm (upper left inset in Figure 7a). The center of the fiber core region still possesses the same crystal orientation as that near the interface (Figure 7b), which illustrates the uniform high crystallinity of the fiber core. The Si and O elemental distribution across this interface was measured through an electron energy loss spectroscopy (EELS) line scan (Figure 7c). The scanning direction is indicated by the arrow in the high-angle annular dark-field (HAADF) inset image. From both Figure 7a and 7c we notice that the interface between the silicon core and silica cladding is remarkably well-defined, and no oxide precipitates were observed,35–37 which if present would increase optical scattering losses for waveguided light.38 The sharp change of both Si and O signals within a 5 nm region across the interface determines the upper limit of our transition region, given the imaging resolution limit. We varied the sample tilt to determine this 5 nm limit, since a slight tilt of the interface with respect to the electron beam would also increase the interface width; the actual transition region is therefore likely to be even narrower (Supporting Information, Figure S4). We attribute this phenomenon to the weaker interaction between the core and cladding because only the core material was heated by the laser, and a moving laser reduces the heating time for each location along the fiber as compared to the thermal drawing process. Silicon fibers by other techniques have reported much larger transition regions of ~ 40 nm – 2 μm due to the diffusion of oxygen from the cladding into the core.39,40

4 Environment ACS Paragon Plus

Page 5 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Figure 7. Transmission electron microscopy studies of the Si fiber illustrating a) the interfacial roughness is about 1 nm under TEM (top left inset: surface of an etched fiber viewed by FESEM) and b) crystal orientation along the fiber axis is close to and it remains the same across the core diameter (lower right insets in (a) and (b): electron diffraction patterns). c) An EELS line scan across the interface shows that any gradient in composition occurs over a distance of ≤ 5 nm. The inset EELS spectrum reveals no oxygen in the silicon core region indicated by the dashes line. d) The chemical element distribution uniformity along the fiber length is shown by an EDX area scan.

In contrast, note that in Figure 7c that the oxygen signal drops to the noise level within the Si core, indicating that our single-crystal Si core is essentially free of oxygen within our detection limits (Supporting Information, Figure S6b). Additionally, chemical analysis shows that there is no evidence for the formation of the SiO phase in our fiber, since no Si2+ peaks were detected in all EELS spectra. The interface sharpness is uniform along the fiber length, as shown in the energy dispersive X-ray (EDX) spectra map of Figure 7d. Single-crystal Si is expected to possess superior electronic and optical properties. The photosensitivity S = Gl/Pw, where G is electrical conductance, P is the power density of incident light, and l and w are the length and width of the wire respectively21 of the single-crystal fiber is characterized. The term G is proportional to the charge carrier mobility-lifetime product.41,42 This value for the single-crystal Si fiber (10-6 to 10-5 S cm2 W-1) is as good as a Si wafer reference (10-5 S cm2 W-1) at multiple wavelengths as shown in Figure 8a. It is one-to-two orders of magnitude higher than that of a polycrystalline Si fiber annealed in a furnace with grain size typically up to ~ 500 nm. This is due to the removal of any grain boundaries in a singlecrystal fiber, which can act as trapping, recombination and scattering sites for charge carriers.43 Thus our singlecrystal Si fiber has potential as active absorbers in solar cells in fiber geometries. In view of the excellent materials quality and single crystalline nature, the Si fiber is also expected to function

as a low-loss optical waveguide. Figure 8b shows the optical loss measured over the entire length of our 1.6 mm single-crystal fiber in comparison with other Si fibers and waveguides. The optical losses of the single-crystal Si optical fiber measured at wavelengths from 1125 nm to 1600 nm are lower than those of HPCVD hydrogenated amorphous Si fibers,3,10 HPCVD polycrystalline Si fibers,3,24 molten core-derived Si fibers15,44 and some SOI waveguides at certain wavelengths.45,46 The loss at telecommunication wavelength 1550 nm measured by the standard cutback method is 0.47 dB/cm, approaching the best SOI waveguide ever demonstrated for photonics applications.47 There is however, still some room for improvement when compared with the intrinsic absorption loss value of bulk Si, it is still somewhat higher at lower wavelengths. This is likely related to occasional (a few micron sized regions) planar defects we have observed near the Si/SiO2 interface in the fiber (Figure S5, Supporting Information). However the optical scattering (λ-4 dependence, characteristic of Rayleigh scattering, as shown in Figure S6, Supporting Information) is not significant at longer wavelengths of light.

Figure 8. Electronic and optical properties of single-crystal Si fiber. a) The high photosensitivity, S, of the single-crystal Si fiber indicates that the charge carrier mobility-lifetime product is comparable to that of the single crystal Si wafer, and much higher than that of a Si polycrystal fiber counterpart. b) Wavelength-dependent optical loss measurement of the 1.7 μm core diameter single-crystal Si fiber waveguide. Data for a reference SOI waveguide were taken from refs 45-47; for the HPCVD Si polycrystal fibers from refs 3,11; for the HPCVD a-Si:H fibers from refs 3,24, and for the molten core Si fibers from refs 15,44.

Using the laser processing diagram described above, we demonstrate that longer single-crystal Si fiber can be grown. Figure 9a shows the crystal orientation map along the length of a 1.7 μm core diameter laser crystallized Si fiber using laser irradiation power of 0.45 W and laser scanning speed of 1 mm/s (this set of parameter also falls in the crystal growth window as shown in Figure 5). Figure 9a clearly shows that a longer single crystal ~ 5.1 mm has been made (more X-ray diffraction results can be found in the supporting information Figure S8). The optical loss at 1550 nm wavelength is ≈ 1 dB/cm measured by a cutback method. The intensity profile of the guided mode observed at the fiber end is shown on the left panel in Figure 9b. Such fiber length may already be sufficient for certain Si photonics applications.10,48 For fabricating such fibers with much longer lengths for endoscopic imaging applications, further engineering improvements, such as

5 Environment ACS Paragon Plus

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

by spooling the fiber during laser crystallization, can result in much longer fibers approaching meters.

Figure 9. (a) Crystal orientation mapping of a longer single crystal Si fiber processed under the guidance of the laser processing diagram in Figure 5a. (b) Guided mode in the Si fiber at 1550 nm (left), the optical micrograph of the fiber end (middle) and a zoomed-in SEM image of the Si core (right).

In conclusion, we have demonstrated the fabrication of long-length single-crystal Si fibers using a precisely controlled visible laser annealing process, which can produce fibers with photoconductance as good as bulk single crystal, and optical losses at the telecommunication wavelength comparable to the best Si waveguide demonstrated thus far. We have also constructed a laser processing diagram that provides a comprehensive understanding of, and guidance for the processing conditions required for a general scanning laser fiber crystallization technique for Si. With these guidelines, and with further addition of an in-situ temperature monitoring during the laser annealing process,49 we expect longer single-crystal Si fibers to be grown in the future. These fibers could potentially be utilized as low-loss infrared waveguides for imaging spectroscopic endoscopes,50 for the demonstration of various third order nonlinear optical effects in Si, and even broadband supercontinuum generation at telecommunication wavelengths. Efficient supercontinuum generation requires the waveguide geometry being close to the single mode criterion,51 and our current fiber supports multimodes; however, previous works8,10 have shown that most of the coupled power is propagated in the fundamental mode by choosing appropriate light coupling method. It is also possible to fabricate single mode Si fiber by depositing Si in a much smaller core diameter fiber52 (core diameter less than 380 nm would allow for single-mode behavior at a wavelength of 1550 nm) using HPCVD followed by crystallization using laser annealing. We anticipate that the approach of HPCVD followed by a precisely controlled laser annealing process should be a low-cost, fast and flexible method for the production of a broad range of long single-crystal semiconductor optical fibers towards high performance fiber devices.

METHODS High-Pressure Chemical Vapor Deposition. Reservoirs used in the experiment were fabricated from highstrength stainless steel values, fittings, and tubing from

Page 6 of 10

High Pressure Equipment Company (HiP). The deposition temperature was set at 500 °C with a ramping rate ~ 6.25 °C/hour and the reaction was continued until the desired fully-infiltrated length was achieved. Laser Crystallization. The as-deposited a-Si fibers were first cleaned with acetone and then mounted on a custom-build air-bearing (NEWWAY) linear translation stage (motor provided by AEROTECH). The sample holder has x-y-z, yaw and pitch motions to help align the fiber along the translation direction. A linear encoder (Renishaw) is used to provide feedback at 10 nm resolution. The stage is then controlled by a motion controller from National Instrument (NI). The laser irradiation was focused by a 40×, 0.65 NA focusing objective to a spot size ~ 1 μm onto the silicon core. Materials Characterization. The synchrotron X-ray micro-beam Laue diffraction measurements were carried out at beamline 34-ID-E at the Advanced Photon Source at Argonne National Laboratory. Raman spectra were obtained in the backscattering geometry with 0.1 mW, 514.5 nm laser excitations using a Witec Alpha 300 S confocal Raman microscope. All Raman peaks were fitted with Voigt functions (to include instrument contribution), and Lorentzian peak linewidths as well as positions were extracted. The TEM sample was prepared in the FEI Helios NanoLab 660 Dual Beam FIB system. The TEM images, EELS line scan and EDX mappings were taken in a FEI Titan3 dual aberration corrected transmission electron microscope. A Fianium SC-450 supercontinuum laser combined with an acousto-optic tunable filter was used for photosensitivity and loss measurements as a function of wavelength using the single pass technique. Light was coupled into the fiber in free space using a 0.95 NA objective and collected onto a germanium power meter with an 80× objective. A cutback measurement was also made and a loss of 0.47±0.1 dB/cm at 1550 nm was obtained. Before the measurement, the fiber was mounted in a larger capillary and polished using standard techniques. The fiber output end was repeatedly removed by 500 μm each time via polishing during the cutback measurements. For photoresponse measurements, the Si wire was etched out of the two ends of the silica capillary using 48% buffered HF acid and contacts were made with aluminum in a Lab-18 thermal evaporator (Kurt. J. Lesker) followed by a thermal annealing in dry N2 at 430 °C for 30 minutes. The Si wafer reference used has a resistivity of ~ 1700 Ω·m, and was cleaved into a shape of 2 mm (width) × 5 mm (length) with aluminum contact pads separated by 2 mm along the length. The same Fianium laser was used as the illumination source, and a Keithley 6430 sub-femtoamp source meter was used to source voltage and measure current. Finite Element Modeling. The simulations are performed on the ANSYS®/Multiphysics platform. An axisymmetry is assumed, which effectively simplifies the practical three-dimensional thermal transport problem into a 2D axisymmetrical plane problem. For a realistic simulation of the experiment conditions of a moving heating source, a transient thermal analysis is performed. See supporting information for more modeling details.

SUPPORTING INFORMATION

6 Environment ACS Paragon Plus

Page 7 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

This material is available free of charge via the Internet at http://pubs.acs.org. (11)

AUTHOR INFORMATION Corresponding Author * Email: [email protected], [email protected]

(12)

Notes The authors declare no competing financial interest. (13)

ACKNOWLEDGMENT The authors acknowledge primary financial support from the Penn State Materials Research Science and Engineering Center for Nanoscale Science, grant #DMR 1420620, and partial support from grant #DMR 1107894. Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. X. Ji and V. Gopalan would like to thank beamline 34-ID-E at the Advanced Photon Source for providing the facilities for diffraction experiments. X. Ji would also like to thank A. Grede for helpful discussions on the photoconductivity measurement, G. Stone for discussions on the fiber laser crystallization setup and K. Wang for help in TEM characterization.

(14)

(15)

(16)

(17)

REFERENCES (1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

He, R.; Sazio, P. J. A.; Peacock, A. C.; Healy, N.; Sparks, J. R.; Krishnamurthi, M.; Gopalan, V.; Badding, J. V. Integration of Gigahertz-Bandwidth Semiconductor Devices inside Microstructured Optical Fibres. Nat. Photonics 2012, 6, 174–179. He, R.; Day, T. D.; Krishnamurthi, M.; Sparks, J. R.; Sazio, P. J. A.; Gopalan, V.; Badding, J. V. Silicon P-I-N Junction Fibers. Adv. Mater. 2013, 25, 1461–1467. Lagonigro, L.; Healy, N.; Sparks, J. R.; Baril, N. F.; Sazio, P. J. A.; Badding, J. V.; Peacock, A. C. Low Loss Silicon Fibers for Photonics Applications. Appl. Phys. Lett. 2010, 96, 41105. Martinsen, F. A.; Smeltzer, B. K.; Nord, M.; Hawkins, T.; Ballato, J.; Gibson, U. J. Silicon-Core Glass Fibres as Microwire Radial-Junction Solar Cells. Sci. Rep. 2014, 4, 6283. Huang, Y. P.; Wang, L. A. In-Line Silicon Schottky Photodetectors on Silicon Cored Fibers Working in 1550 Nm Wavelength Regimes. Appl. Phys. Lett. 2015, 106, 191106. Hon, N. K.; Soref, R.; Jalali, B. The Third-Order Nonlinear Optical Coefficients of Si, Ge, and Si1−xGex in the Midwave and Longwave Infrared. J. Appl. Phys. 2011, 110, 11301. Boyraz, O.; Indukuri, T.; Jalali, B. Self-PhaseModulation Induced Spectral Broadening in Silicon Waveguides. Opt. Express 2004, 12, 829. Peacock, A. C.; Mehta, P.; Horak, P.; Healy, N. Nonlinear Pulse Dynamics in Multimode Silicon Core Optical Fibers. Opt. Lett. 2012, 37, 3351. Fukuda, H.; Yamada, K.; Shoji, T.; Takahashi, M.; Tsuchizawa, T.; Watanabe, T.; Takahashi, J.; Itabashi, S. Four-Wave Mixing in Silicon Wire Waveguides. Opt. Express 2005, 13, 4629. Shen, L.; Healy, N.; Xu, L.; Cheng, H. Y.; Day, T. D.; Price, J. H. V.; Badding, J. V.; Peacock, A. C. FourWave Mixing and Octave-Spanning Supercontinuum Generation in a Small Core Hydrogenated Amorphous

(18)

(19)

(20)

(21)

(22)

(23)

(24)

(25)

(26)

Silicon Fiber Pumped in the Mid-Infrared. Opt. Lett. 2014, 39, 5721. Hsieh, I.-W.; Chen, X.; Liu, X.; Dadap, J. I.; Panoiu, N. C.; Chou, C.-Y.; Xia, F.; Green, W. M.; Vlasov, Y. A.; Osgood, R. M. Supercontinuum Generation in Silicon Photonic Wires. Opt. Express 2007, 15, 15242. Won, D.-J.; Ramirez, M. O.; Kang, H.; Gopalan, V.; Baril, N. F.; Calkins, J.; Badding, J. V.; Sazio, P. J. A. All-Optical Modulation of Laser Light in Amorphous Silicon-Filled Microstructured Optical Fibers. Appl. Phys. Lett. 2007, 91, 161112. Reed, G. T.; Mashanovich, G.; Gardes, F. Y.; Thomson, D. J. Silicon Optical Modulators. Nat. Photonics 2010, 4 , 518–526. Slaoui, A.; Siffert, P. Polycrystalline Silicon Films for Electronic Devices. In Silicon; Siffert, P., Krimmel, E. F., Eds.; Springer Berlin Heidelberg, 2004; pp 49–72. Ballato, J.; Hawkins, T.; Foy, P.; Stolen, R.; Kokuoz, B.; Ellison, M.; McMillen, C.; Reppert, J.; Rao, A. M.; Daw, M.; Sharma, S. R.; Shori, R.; Stafsudd, O.; Rice, R. R.; Powers, D. R. Silicon Optical Fiber. Opt. Express 2008, 16, 18675–18683. Danto, S.; Sorin, F.; Orf, N. D.; Wang, Z.; Speakman, S. A.; Joannopoulos, J. D.; Fink, Y. Fiber Field-Effect Device Via In Situ Channel Crystallization. Adv. Mater. 2010, 22, 4162–4166. Tyagi, H. K.; Schmidt, M. A.; Prill Sempere, L.; Russell, P. S. Optical Properties of Photonic Crystal Fiber with Integral Micron-Sized Ge Wire. Opt. Express 2008, 16, 17227. Chaudhuri, S.; Sparks, J. R.; Ji, X.; Krishnamurthi, M.; Shen, L.; Healy, N.; Peacock, A. C.; Gopalan, V.; Badding, J. V. Crystalline Silicon Optical Fibers with Low Optical Loss. ACS Photonics 2016, 3, 378–384. Xie, S.; Tani, F.; Travers, J. C.; Uebel, P.; Caillaud, C.; Troles, J.; Schmidt, M. A.; Russell, P. S. J. As2S3–silica Double-Nanospike Waveguide for Mid-Infrared Supercontinuum Generation. Opt. Lett. 2014, 39, 5216. Sazio, P. J. A.; Amezcua-Correa, A.; Finlayson, C. E.; Hayes, J. R.; Scheidemantel, T. J.; Baril, N. F.; Jackson, B. R.; Won, D.-J.; Zhang, F.; Margine, E. R.; Gopalan, V.; Crespi, V. H.; Badding, J. V. Microstructured Optical Fibers as High-Pressure Microfluidic Reactors. Science 2006, 311, 1583–1586. Sparks, J. R.; He, R.; Healy, N.; Krishnamurthi, M.; Peacock, A. C.; Sazio, P. J. A.; Gopalan, V.; Badding, J. V. Zinc Selenide Optical Fibers. Adv. Mater. 2011, 23, 1647–1651. Finlayson, C. E.; Amezcua-Correa, A.; Sazio, P. J. A.; Baril, N. F.; Badding, J. V. Electrical and Raman Characterization of Silicon and Germanium-Filled Microstructured Optical Fibers. Appl. Phys. Lett. 2007, 90, 132110. Gupta, N.; McMillen, C.; Singh, R.; Podila, R.; Rao, A. M.; Hawkins, T.; Foy, P.; Morris, S.; Rice, R.; Poole, K. F.; Zhu, L.; Ballato, J. Annealing of Silicon Optical Fibers. J. Appl. Phys. 2011, 110, 93107. Healy, N.; Mailis, S.; Bulgakova, N. M.; Sazio, P. J. A.; Day, T. D.; Sparks, J. R.; Cheng, H. Y.; Badding, J. V.; Peacock, A. C. Extreme Electronic Bandgap Modification in Laser-Crystallized Silicon Optical Fibres. Nat. Mater. 2014, 13, 1122–1127. Healy, N.; Fokine, M.; Franz, Y.; Hawkins, T.; Jones, M.; Ballato, J.; Peacock, A. C.; Gibson, U. J. CO2 LaserInduced Directional Recrystallization to Produce Single Crystal Silicon-Core Optical Fibers with Low Loss. Adv. Opt. Mater. 2016, 4, 1004–1008. Ji, X.; Page, R. L.; Chaudhuri, S.; Liu, W.; Yu, S.-Y.; Mohney, S. E.; Badding, J. V.; Gopalan, V. Single-

7 Environment ACS Paragon Plus

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(27)

(28)

(29) (30)

(31)

(32)

(33)

(34)

(35)

(36)

(37)

(38)

(39)

Crystal Germanium Core Optoelectronic Fibers. Adv. Opt. Mater. 2016, DOI:10.1002/adom.201600592. Baril, N. F.; Keshavarzi, B.; Sparks, J. R.; Krishnamurthi, M.; Temnykh, I.; Sazio, P. J. A.; Peacock, A. C.; Borhan, A.; Gopalan, V.; Badding, J. V. High-Pressure Chemical Deposition for Void-Free Filling of Extreme Aspect Ratio Templates. Adv. Mater. 2010, 22, 4605–4611. Sparks, J. R.; Sazio, P. J. A.; Gopalan, V.; Badding, J. V. Templated Chemically Deposited Semiconductor Optical Fiber Materials. Annu. Rev. Mater. Res. 2013, 43, 527– 557. Palik, E. D. Handbook of Optical Constants of Solids, Volume 1; Academic Press, 1985. Von Ammon, W.; Dornberger, E.; Oelkrug, H.; Weidner, H. The Dependence of Bulk Defects on the Axial Temperature Gradient of Silicon Crystals during Czochralski Growth. J. Cryst. Growth 1995, 151, 273–277. Dornberger, E.; Tomzig, E.; Seidl, A.; Schmitt, S.; Leister, H.-J.; Schmitt, C.; Müller, G. Thermal Simulation of the Czochralski Silicon Growth Process by Three Different Models and Comparison with Experimental Results. J. Cryst. Growth 1997, 180, 461–467. Furukawa, Y.; Kitamura, K.; Suzuki, E.; Niwa, K. Stoichiometric LiTaO3 Single Crystal Growth by Double Crucible Czochralski Method Using Automatic Powder Supply System. J. Cryst. Growth 1999, 197, 889–895. Huang, X.; Terashima, K.; Hoshikawa, K. SiO Vapor Pressure in an SiO2 Glass/Si Melt/SiO Gas Equilibrium System. Jpn. J. Appl. Phys. 1999, 38 (Part 2, No. 10B), L1153–L1155. Schnurre, S. M.; Gröbner, J.; Schmid-Fetzer, R. Thermodynamics and Phase Stability in the Si–O System. J. Non-Cryst. Solids 2004, 336, 1–25. Borghesi, A.; Pivac, B.; Sassella, A.; Stella, A. Oxygen Precipitation in Silicon. J. Appl. Phys. 1995, 77, 4169– 4244. Ponce, F. A.; Hahn, S. Microscopic Aspects of Oxygen Precipitation in Silicon. Mater. Sci. Eng. B 1989, 4, 11– 17. Yu, R.; Zhang, X. F.; Wang, Q.; Daggubati, M.; Paravi, H. TEM Study of Oxygen Precipitation in Si Wafers with Backside Layers. Microsc. Microanal. 2005, 11 (Supplement S02), 2070–2071. Ballato, J.; Hawkins, T.; Foy, P.; Yazgan-Kokuoz, B.; McMillen, C.; Burka, L.; Morris, S.; Stolen, R.; Rice, R. Advancements in Semiconductor Core Optical Fiber. Opt. Fiber Technol. 2010, 16, 399–408. Hou, C.; Jia, X.; Wei, L.; Tan, S.-C.; Zhao, X.; Joannopoulos, J. D.; Fink, Y. Crystalline Silicon Core Fibres from Aluminium Core Preforms. Nat. Commun. 2015, 6.

(40)

(41) (42)

(43)

(44)

(45)

(46)

(47)

(48)

(49)

(50)

(51)

(52)

Page 8 of 10 Ballato, J.; Hawkins, T.; Foy, P.; Yazgan-Kokuoz, B.; Stolen, R.; McMillen, C.; Hon, N. K.; Jalali, B.; Rice, R. Glass-Clad Single-Crystal Germanium Optical Fiber. Opt. Express 2009, 17, 8029–8035. Street, R. A. Hydrogenated Amorphous Silicon; Cambridge University Press: Cambridge, 1991. Beck, N.; Wyrsch, N.; Hof, C.; Shah, A. Mobility Lifetime product—A Tool for Correlating a‐Si:H Film Properties and Solar Cell Performances. J. Appl. Phys. 1996, 79 (12), 9361–9368. Harbeke, G. Polycrystalline Semiconductors: Physical Properties and Applications : Proceedings of the International School of Materials Science and Technology at the Ettore Majorana Centre, Erice, Italy, July 1-15, 1984; Springer series in solid-state sciences; Springer-Verlag: New York, 1985; Vol. 57. Morris, S.; Hawkins, T.; Foy, P.; Hudson, J.; Zhu, L.; Stolen, R.; Rice, R.; Ballato, J. On Loss in Silicon Core Optical Fibers. Opt. Mater. Express 2012, 2, 1511. Vlasov, Y. A.; McNab, S. J. Losses in Single-Mode Silicon-on-Insulator Strip Waveguides and Bends. Opt. Express 2004, 12, 1622. Lee, K. K.; Lim, D. R.; Kimerling, L. C.; Shin, J.; Cerrina, F. Fabrication of Ultralow-Loss Si/SiO_2 Waveguides by Roughness Reduction. Opt. Lett. 2001, 26, 1888. Cardenas, J.; Poitras, C. B.; Robinson, J. T.; Preston, K.; Chen, L.; Lipson, M. Low Loss Etchless Silicon Photonic Waveguides. Opt. Express 2009, 17, 4752–4757. Zhang, J.; Lin, Q.; Piredda, G.; Boyd, R. W.; Agrawal, G. P.; Fauchet, P. M. Optical Solitons in a Silicon Waveguide. Opt. Express 2007, 15, 7682. Kawamura, S.; Sakurai, J.; Nakano, M.; Takagi, M. Recrystallization of Si on Amorphous Substrates by Doughnut‐shaped Cw Ar Laser Beam. Appl. Phys. Lett. 1982, 40, 394–395. Ji, X.; Zhang, B.; Krishnamurthi, M.; Badding, J.; Gopalan, V. Mid-Infrared Spectroscopic Imaging Enabled by an Array of Ge-Filled Waveguides in a Microstructured Optical Fiber Probe. Opt. Express 2014, 22, 28459– 28466. Chemnitz, M.; Schmidt, M. A. Single Mode Criterion - a Benchmark Figure to Optimize the Performance of Nonlinear Fibers. Opt. Express 2016, 24, 16191. Zhao, W.; Bischof, J. L.; Hutasoit, J.; Liu, X.; Fitzgibbons, T. C.; Hayes, J. R.; Sazio, P. J. A.; Liu, C.; Jain, J. K.; Badding, J. V.; Chan, M. H. W. Single-Fluxon Controlled Resistance Switching in Centimeter-Long Superconducting Gallium–Indium Eutectic Nanowires. Nano Lett. 2015, 15, 153–158.

8 Environment ACS Paragon Plus

Page 9 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

For Table of Contents Use Only

Single-Crystal Silicon Optical Fiber by Direct Laser Crystallization Xiaoyu Ji, Shiming Lei, Shih-Ying Yu, Hiu Y. Cheng, Wenjun Liu, Nicolas Poilvert, Yihuang Xiong, Ismaila Dabo, Suzanne E. Mohney, John V. Badding, and Venkatraman Gopalan

Synthesis and fabrication of high quality, small core single-crystal silicon fibers that have low optical losses ~ 0.47-1 dB/cm at 1.55 μm. We explore appropriate laser processing conditions and demonstrate the growth of long silicon single-crystal fibers. These fibers have potential applications in fiber-based nonlinear optical devices, spectroscopic imaging, and solar cells.

9

ACS Paragon Plus Environment

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of content figure 32x12mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 10 of 10