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Abstract
Tellurium (Te) semiconductor core optical fibers with silicate glass cladding were drawn by the molten core method. The as-drawn precursor fiber has a large core diameter of about 123 µm, which was found to be polycrystalline. What is more, a Bridgman-type fiber postprocessing technique was constructed and used for the first time to anneal the polycrystalline Te semiconductor core optical fibers. The Te core in precursor fiber was melted and recrystallized to single crystal Te with c-axis orientation parallel to fiber axis, which was confirmed by X-ray diffraction, single crystal X-ray diffraction, micro-Raman spectra, and transmission electron microscope measurement results. Enhanced conductivities were observed in single crystal Te semiconductor core optical fibers under illuminated and stress states, respectively. This work demonstrates that the Bridgman-type fiber postprocessing technique could be an effective way to fabricate single crystal semiconductor core optical fibers with large core diameters (∼ 100 µm) and long lengths (a few centimeters).
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, glass-clad semiconductor core optical fibers have attracted much attention due to their extensive promising applications in nonlinear photonics, lasers, sensing, biomedicine, power delivery, energy conversion, and so on [1–6]. There are three established fabrication methods for the production of glass-clad amorphous or crystalline semiconductor core optical fibers (precursor fibers), the molten core drawing (MCD), the high pressure chemical vapor deposition (HPCVD), and the pressure assisted melt filling (PAMF) [2,3]. Compared with amorphous semiconductor in optical fibers, crystalline semiconductor has superior thermal and electronic properties, making the glass-clad crystalline semiconductor core optical fibers more attractive for all-fiber-optoelectronics [7–9]. However, the as-drawn glass-clad crystalline semiconductor core optical fibers usually contain polycrystalline core, which will hinder light transmission and carrier migration [7,10–14]. A low optical loss of less than 1 dB/cm is often considered necessary for many photonic and optoelectronic practical applications at near to mid-infrared wavelengths in areas such as nonlinear photonics, laser, and in-fiber photodetectors [10].
In a push to further improve the crystal quality of semiconductor core in precursor fibers produced by the MCD, HPCVD, and PAMF methods, a number of postprocessing technologies have been developed to fabricate low loss glass-clad crystalline semiconductor core optical fibers at infrared band [7,10,15–18]. The most two dominating postprocessing techniques are oven-annealing and laser annealing. An increase in the polycrystalline grain size and a decrease in the defects in the polycrystalline semiconductor core can be realized by oven-annealing amorphous semiconductor core optical fibers [10,19,20]. Nevertheless, glass-clad single crystal semiconductor core optical fibers have not been obtained by the oven-annealing postprocessing method. The laser annealing includes direct laser crystallizing amorphous semiconductor core optical fibers into crystalline state and laser-induced directional recrystallization of polycrystalline semiconductor core to produce single crystal semiconductor core in fibers [7,10,17,21]. There are two primary premises for the laser annealing: core melting via direct optical absorption and core melting via thermal conduction from a laser heated the glass cladding [18]. Selective heating of the semiconductor core using a tightly focused visible laser beam (to which the glass cladding is transparent) has been shown to both crystallize and alter the local stress and related properties of an elemental silicon fiber core after fabrication [22]. Infrared laser treatment, in which the glass cladding absorbs the energy, has also been used to fabricate low loss single crystal semiconductor core optical fibers, whilst simultaneously relieving the stresses at the core/cladding interface [21,23,24]. More recently, the preparation of long single crystals with uniform composition, as well as fabrication of compositional microstructures within the fiber core have been realized by tailoring the recrystallization conditions in SiGe alloy semiconductor core optical fibers [25]. However, a unidirectional laser was usually used as the heating source in the process of preparing single crystal semiconductor core optical fibers by the laser annealing postprocesing technique. The single-direction laser annealing postprocessing technique cannot provide a symmetrically distributed temperature field due to the cylindrical geometry of fiber, resulting in an uneven distribution of residual stress in fiber core after the laser annealing process, which will limit the fabrication of high quality single crystal in fiber core. It is worth noting that high quality crystalline Ge core fibers can be obtained by annealing Ge core fibers using a 360° axially symmetric distribution CO2 laser with different power [26]. It proved that the residual stress distribution at the Ge core cross section is relative to the way of laser irradiation [26].
Te is a narrow bandgap (direct bandgap energy of about 0.35 eV) semiconductor material, which has many useful properties, including relatively high transparency at mid-infrared wavelength region, nonlinear optical responses, thermoelectric and piezoelectric properties, making it an attractive core material for fiber-integrated optoelectronic devices [27–29]. Compared to semiconductor silicon and germanium, Te has extended infrared transparency range and higher Raman gain so that it has great potential for application in the important mid-infrared wavelength regime [27]. Therefore, we aim to fabricate high quality of glass-clad single crystal Te semiconductor core optical fibers in this work. Compared with the HPCVD and PAMF techniques, the molten core method is more versatile and practical, which can be used to fabricate long kilometer lengths of glass-clad amorphous or crystalline semiconductor core optical fibers (precursor fibers) with large core diameters [30–33]. Hence, polycrystalline Te semiconductor core optical fibers with silicate glass cladding were successfully drawn by using the molten core method, which have a large core diameter of about 123 µm. More importantly, a Bridgman-type fiber postprocessing technique with symmetric heating, which can provide symmetrically and evenly distributed temperature field, was used for the first time to fabricate Te single crystal in fiber core. The morphology, phase, structure, and electrical properties of the single crystal Te semiconductor core optical fibers were systematically investigated.
2. Experiment
A commercial cylindrical silicate glass (BK7 Schott) rod with a diameter of 30 mm was drilled a 3 mm diameter hole along the centerline. Then the hole with one end closed was mechanically polished, followed by a chemical etching process to obtain a high surface quality. The diameter of the inner hole after polishing and etching is 3.4 mm. The other end of the glass tube was sealed after the high-purity Te powder (99.999%, Aladdin Industrial Corporation, Shanghai, China) was filled in. This process was done in an Ar-filled glove box. Then the precursor fibers were drawn by using a commercial optical fiber drawing tower at approximately 900 °C. At this temperature the cladding glass can be deformed, but is still viscous enough to contain the molten Te semiconductor material and maintain its cylindrical geometry when being drawn. In addition, the raise of heating temperature was kept slow and gradual during the fabrication of the precursor fibers in order to reduce the tendency of glass cracking. The as-drawn fibers (precursor fibers) have outer diameter of 600∼900 µm, which yielded core diameter of 100∼150 µm. The size of the precursor fibers was controlled by the drawing temperature, the feeding speed of the preform, and the drawing tension.
The schematic diagram of the Bridgman-type fiber postprocessing technique is shown in Fig. 1. Only one ring resistance wire was used as heating source to provide a large temperature gradient field, which is beneficial to maintain the stability of the crystal growth interface [34]. One end of the precursor fiber was fixed to a holding tool coupled with a vertical displacement stage and the other end passed through the center of the resistance wire for about 3 cm depth to ensure the bottom core to stay away from the high-temperature zone and keep rigidity in the postprocessing. Then the precursor fiber was lowered at a constant speed of 5 mm/h, which was referenced from the growth speed of bulk Te single crystal, and gradually crossed the high-temperature zone to recrystallize the Te core [35]. The high-temperature zone is 10∼60 °C higher than the melting point of Te (450 °C) semiconductor core while much lower than the softening point of the silicate cladding glass (719 °C) [36]. The temperature gradient is about 100 °C/cm in vertical direction in the high-temperature zone. In this postprocessing method, the glass cladding kept rigid enough and served as a container to shape the molten liquid semiconductor material into fiber geometry during the recrystallization process.
The cross section of the precursor fiber was observed by an electron microscopy, which was performed using an electro-probe X-ray micro-analyzer (EPMA-1600, Shimadzu, Japan) operated at 20 kV under vacuum atmosphere. The crystalline phase and crystal orientation in fiber core were identified by X-ray powder diffractometer (X’Pert PRO, Cu Kα) and micro-Raman spectra (Renishaw RM2000). Single crystal X-ray diffraction was measured using Rigaku R-AXIS SPIDER IP diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). A gallium ion beam was applied to etch the core/clad interface on a polished fiber cross section to obtain an ultra-thin chip sample by using a focused ion beam instrument (FEI Helios 450S, dual beam FIB). Transmission electron microscopy (TEM) measurements were carried on the prepared ultra-thin chip sample by using a transmission electron microscope (FEI Titan Themis 200) with a high angle annular dark field (HAADF) detector. The current-voltage characteristics of the fibers were recorded using Keithley series 2400 source meter.
3. Results and discussions
Figure 2(a) shows the electron micrograph image of the cross section of the polished precursor fiber, which has an inner diameter of ∼ 123 µm and an outer diameter of ∼ 800 µm. It can be clearly observed that there are no obvious discontinuities at the core/clad interface and no obvious cracks or signs of bubbles in the core, indicating the silicate glass is an appropriate cladding for Te semiconductor core, which can be successfully drawn into optical fibers by using the molten core method. It is worth mentioning that a substantial drawback of the use of phosphate glass as the cladding for Te semiconductor core optical fiber in the previous work is its relative low softening temperature, which is not suitable for the Bridgman-type fiber postprocessing method. In addition, compared with the silicate glass, the phosphate glass has lower stability, which will be degraded when exposed to air moisture over time [27,37]. The wavelength dispersive spectrometer (WDS) mapping distributions of element silicon (Si), oxygen (O), and Te are illustrated in Figs. 2(b)–2(d), respectively. The distribution boundary of each element forms a circle. The distributions of O and Si are mainly in the cladding region, while the core is mainly comprised of Te. These results indicate that the core/clad structure of the precursor fiber is preserved completely.
Fig. 2. (a) Electron micrograph image of the as-drawn Te core fiber. (b)-(d) EPMA images of the marked area in (a).
Figure 3 displays the XRD spectra of the as-drawn and the annealed Te core. The Te core, which is also called Te microwire, was obtained by etching the fibers in HF for stripping the silicate glass cladding. Since a single Te microwire is not sufficient to collect the diffraction intensity, a dozen of Te microwire, which was cut from a 10-cm-long Te microwire, was tightly paved into a layer on the surface of a glass panel for the XRD measurement. It can be seen that all peaks in the pattern of the as-drawn Te core can be indexed to polycrystalline Te with some irregular preferred orientation. It is worth noting that unusually strong (hk0) diffraction peaks including (100), (110), (200), and (300) are observed in the XRD pattern of the annealed Te core, while the other orientation peaks disappear, suggesting that the annealed Te microwire has a preferential orientation of [001] [28,29,38,39]. The inset of the Fig. 3 shows the hexagonal lattice occupied by the trigonal phase of Te, which has helical chains of Te atoms packed parallel to each other along the c-axis [40,41]. The XRD results show that the polycrystalline Te semiconductor in fiber core have been converted to single crystal Te with c-axis orientation parallel to fiber axis by using the Bridgman-type fiber postprocessing method. It can also be confirmed by the single crystal X-ray diffraction that the lattice constants of the annealed Te core are calculated to be a = b=0.4431 nm and c=0.5927 nm, which are consistent with the standard values of a = b=0.4458 nm and c=0.5927 nm (JCPDS Card No. 36-1452) [42].
Fig. 3. XRD spectra of the as-drawn and annealed Te core. The inset of the Fig. 3 shows the crystal structure of trigonal Te.
Figure 4 presents the micro-Raman spectra (532 nm excitation) for the Te powder and Te core, which were excited by a linear polarized laser in different incident directions (k), including perpendicular to the cylindrical surface (k⊥core axis) and end face (k//core axis). The laser polarization is aligned along Te core axis for the cylindrical surface test. There are three active Raman phonon modes shown in the Raman spectrum of polycrystalline Te powder. The Raman peaks of Te located at 92 cm−1, 119 cm−1, 139 cm−1, which can be assigned to E1, A1, and E2 modes, respectively [43,44]. According to the previous report, single crystal Te has optical anisotropy that the E1 mode will disappear when the laser polarization direction is along c-axis and maximizes to equal to E2 mode when the laser incident direction is along c-axis (k//c-axis) [45]. It can be found that the intensity of the E1 mode at 92 cm−1 is lower than that of the E2 mode at 140 cm−1 for the as-drawn Te core under the excitation of different polarized lights, indicating the crystal orientation of the as-drawn Te core along the core axis is not parallel to c-axis. However, for the annealed Te core, the E1 mode disappears when the laser polarization direction is along core axis and equals to the E2 mode when the laser incident direction is along core axis (k//core axis), suggesting the crystal orientation of the annealed Te core along the core axis is parallel to c-axis. The micro-Raman measurements show that the annealed Te core has c-axis orientation parallel to core axis, which is similar to the large single crystal Te prepared by the Bridgman method without seed [35].
To further determine the structure of the crystal in the annealed fiber core, TEM measurement was carried on an ultra-thin chip sample, which was obtained from the cross section of the annealed fiber by using a FIB instrument. Figure 5(a) shows the TEM image of the ultra-thin chip sample. The white area in Fig. 5(a) indicates that the core/clad interface has been broken, which was caused by the large stress generated during the FIB processing. Figures 5(b)–5(d) illustrate the energy-dispersive X-ray spectroscopy (EDX) mappings on Fig. 5(a). The yellow, red, and cyan colors denote to element Si, O, and Te, respectively. It can be seen that the core is composed of Te, while Si and O dominate the cladding area. Figures 5(e)–5(h) show the high-resolution transmission electron microscopy (HR-TEM) images of the annealed Te core at locations 1-4 marked in the Fig. 5(a), respectively. There is no obvious change in these four figures and all demonstrate the same crystal lattice fringe with the spacing d value of ∼ 0.223 nm, corresponding to the (110) crystal facet of the trigonal phase of Te. The selected area electron diffraction (SAED) patterns of the annealed Te core at locations 1-4 marked in the Fig. 5(a) are shown in the inset of the Figs. 5(e)–5(h), respectively, demonstrating the single-crystalline nature of the annealed Te core. It is worth mentioning that the ultra-thin chip sample and the chosen areas in the Fig. 5(a) are randomly selected for the TEM measurements. The XRD spectra, micro-Raman spectra, and TEM measurements together verify that Te core in the precursor fibers was polycrystalline and single crystal Te with c-axis orientation parallel to fiber axis was produced in fiber core by using a Bridgman-type fiber postprocessing technique. The mechanism of forming single crystal is the melting growth without seed crystal. The size of the fiber is much smaller than that of the bulk single crystal, so that it is easy to obtain single crystal by the geometry eliminating mechanism in self-nucleated growth, which is a classical method for bulk single crystal growth.
Fig. 5. (a) TEM images of the annealed Te semiconductor core optical fibers. (b)-(d) EDX mappings on (a), yellow, red, and cyan denote Si, O, and Te, respectively. (e)-(h) The HR-TEM images of the annealed Te core at the four labeled locations in (a). The insets in (e)-(h) show the SAED patterns of the annealed Te core at the four labeled locations in (a).
Figure 6 compares the voltage-current curves of five-cm-long Te semiconductor core optical fibers between dark and illuminated states (under illumination from a 200 mW/cm2 532 nm laser diode). The inset of the Fig. 6 shows the schematic of the current-voltage characteristic test under illuminated state. First, platinum (Pt) was sprayed on the end faces of the Te semiconductor core optical fiber to reduce the contact resistance. Then, the Te semiconductor core optical fibers can be easily connected with external circuit by coating silver (Ag) paste to both ends of a fiber segment, which were then used for the voltage-current characteristics measurement under dark and illuminated state, respectively. For the voltage-current measurement, the 532 nm laser was irradiated to the fiber directly without being focused. It can be observed that there is no obvious change in conductivity between dark and illuminated states in the as-drawn Te semiconductor core optical fiber, which can be interpreted as the polycrystalline Te in fiber core hinders the migration of carrier and increases the reflection of electrons at its grain boundaries [46]. Compared with the as-drawn Te semiconductor core optical fiber, however, the conductivity of the annealed Te semiconductor core optical fiber is 23% higher in dark state and higher by 32% in illuminated state. In previous report, polycrystalline Te has approximately the same resistance as a single crystal Te and there is not a strongly preferred direction for electrical conductivity in the single crystal Te [47,48]. The increased conductivity in the annealed Te semiconductor core optical fiber can be attributed to the significant reduction of grain boundaries and defects in the single crystal Te core. It is well-known that Te has a narrow bandgap of ∼ 0.35 eV, which can detect radiation at a wavelength less than 3.5 µm by photoconduction [49]. An enhanced electrical conductivity was obtained in the annealed Te semiconductor core optical fiber through a steady irradiation of 532 nm laser. Considering the absorption edge of the silicate glass cladding, the annealed Te semiconductor core optical fibers have potential application for in-fiber photodetector at entire near-infrared waveband, which is much wider than that in other single crystal semiconductor core fibers [7,17,21]. The propagation loss of the annealed Te semiconductor core optical fibers was measured by using the cut-back method. Several 10-cm-long single crystal Te core fibers have been measured. The end faces of the fiber were polished. The minimum length of the truncated fiber is about 2 cm. However, the transmission loss of the single crystal Te semiconductor core optical fiber at 10.6 µm is too large to be measured. The large loss may be caused by the large Fresnel reflection, intrinsic absorption, defect absorption, and interfacial scatting between the semiconductor core and glass cladding.
Fig. 6. Current-voltage characteristics of Te semiconductor core optical fibers in the dark and under illumination. The inset of the Fig. 6 shows the schematic of current-voltage characteristic test under illuminated state.
Figure 7 displays the current-voltage characteristics of the five-cm-long annealed Te semiconductor core optical fibers without and under stress states. The annealed Te semiconductor core optical fibers used in Fig. 6 were not flexible enough for the I-V test under stress state. Therefore, the Te microwire with enhanced flexibility, which was obtained by etching the fibers in HF for stripping the silicate glass cladding, was used for the I-V test under stress state. Both ends of the Te microwire were coated with Ag paste to connect with external circuits. The stress was put at the middle of the Te microwire to make sure it has a shape change from the straight line to arc-shaped line with a curvature radius of about 3 cm, as is shown in the inset of the Fig. 7. However, the pressure value was not quantitatively measured. It is noted that there is no change in the conductivity of the as-drawn Te semiconductor core optical fiber without and under stress states. The interface between the Te microwire and Ag paste forms Schottky contacts, resulting in a large interfacial resistance. However, the sample of Te semiconductor core optical fiber with Te-Pt-Ag structure in Fig. 6 forms Ohmic contacts, which has a very small interfacial resistance. Therefore, the blue curves in both figures (Fig. 6 and Fig. 7) are different due to the different contact resistance. Although the contact resistance of the Te-Ag contact is larger than that in Te-Pt-Ag contact, the measured I-V curve under stress in Fig. 7 is nearly symmetrical and linear, confirming the good contact between the Te semiconductor and Ag metal. It is noteworthy that the current was limited to 10 mA to protect the I-V measuring instrument. However, the conductivity of the annealed Te semiconductor core optical fiber under stress is much larger than that without stress. The large difference in conductivity of the annealed Te core fiber under stress and without stress states suggests it could be used for all-fiber stress sensors, which is contrast to the conventional fiber stress sensing that require the use of optical probing signals [50–52]. Compared with the as-drawn optical fibers with polycrystalline Te core, the annealed Te semiconductor core optical fibers has enhanced electrical conductivity under the illuminated or stress state, verifying that single crystal Te has been produced in fiber core by using the Bridgman-type fiber postprocessing technique.
Fig. 7. Current-voltage characteristics of the annealed Te core fibers without and under stress states. The inset of the Fig. 7 shows the test schematic.
4. Conclusion
In conclusion, polycrystalline Te semiconductor core optical fibers with silicate glass cladding were fabricated by using the molten core method. Moreover, long-length single crystal Te with a large diameter of about 123 µm was successfully produced in fiber core by a Bridgman-type fiber postprocessing, which was confirmed by the XRD, single crystal X-ray diffraction, micro-Raman spectra, and TEM measurements. Compared with the polycrystalline Te semiconductor core optical fiber, the conductivity was obviously increased in the single crystal Te semiconductor core optical fiber under illuminated or stress state, suggesting it could be used for in-fiber photodetector and stress sensor. Perhaps more importantly, this is a proof-of-concept work to verify that the firstly constructed Bridgman-type fiber postprocessing technique can be used to produce long-length (a few centimeters) single crystal semiconductor in fiber core with a large diameter (∼ 100 µm). We anticipate that the molten core method followed by the Bridgman-type fiber postprocessing technique can be a versatile, low-cost, and practical technical route for the fabrication of long-length single crystal semiconductor core optical fibers with large core diameters, which could be used for all-fiber optoelectronics.
Funding
China Postdoctoral Science Foundation (2018M640777); Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01X137); Science and Technology Planning Project of Guangdong Province (2017B030314005); Fundamental Research Funds for the Central Universities (D2192750); Guangdong Key Research and Development Program (2018B090904001, 2018B090904003); Key Special Projects of the National Key R&D Program “Development of Major Scientific Instruments and Equipments” (2017YFF0104504); National Natural Science Foundation of China (U1601205).
Disclosures
The authors declare no conflicts of interest.
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