High-quality semiconductor fibres via mechanical design – Nature

Materials

Fused silica tubes (purity 99.99%) of 2.1 mm inner diameter (ID) and 12 mm outer diameter (OD) were purchased from Runtu Glass Products. Three sizes (2.1 mm ID and 3.6 mm OD, 9.1 mm ID and 11.3 mm OD, and 13.2 mm ID and 16 mm OD) of ASG tubes (SCHOTT 8253) and rods of 3.75 mm diameter were purchased from SCHOTT AG. Undoped Si (purity >99.999%, resistivity >1.0 Ω cm) and Ge (purity >99.999%) rods of 2 ± 0.127 mm diameter were purchased from Lattice Materials. P- and n-type Si rods (purity >99.999%, resistivity <0.02 Ω cm) of the same size were purchased from the same supplier. Carbon-filled polycarbonate (10²−10⁶ Ω sq⁻¹) film (thickness 125 μm) was purchased from Boedeker Plastics. Copper (50 μm diameter) and tungsten (30 μm diameter) wires (purity 99.999%) were purchased from Xionglin Metals. Polycarbonate slabs (24 × 8 × 300 mm) were purchased from Tiannuo Polymers. Si and Ge rods were soaked in a 10% hydrofluoric acid solution to remove native oxide before use. Pre-drying of polycarbonate and carbon-filled polycarbonate (CPC) were done in a vacuum oven at 80 °C for 48 h before being used in fibre drawing. Other materials were used without any further treatment.

Fabrication and characterization of semiconductor fibres

Si rod was inserted into a fused silica tube and sealed in vacuum (1 × 10⁻² mbar) using an oxyhydrogen flame. Si/silica fibres were fabricated by drawing the preforms at 1,950 °C via the molten-core method, with the feed and draw rates set to 0.002 cm s⁻¹ and 3.2 cm s⁻¹, respectively. For Ge/ASG fibres, the fabrication started with preform assembling. The ASG tubes of 13.2 mm ID and 16 mm OD were drawn into thinner tubes with 8.9 mm ID and 11.1 mm OD at 1,190 °C. The ASG tubes of 9.1 mm ID and 11.3 mm OD were drawn into three sizes (6.7 mm ID and 8.8 mm OD, 4.9 mm ID and 6.6 mm OD, and 3.7 mm ID and 4.8 mm OD) through the same process. The 3.75-mm-diameter ASG rod was drawn down to 2 mm diameter. The total five sizes of tubes were jacketed to form a preform of 2.1 mm ID and 11.1 mm O, into which a Ge rod was inserted. ASG rods with 2 mm diameter were used in the vacuum (1 × 10⁻² mbar) sealing of the assembly to finalize the preform. The preform was then drawn at 1,150 °C to obtain Ge/ASG fibres with the feed rate and draw rate set to be 0.002 cm s⁻¹ and 3.2 cm s⁻¹, respectively. No deoxidizer was used in the process. Standalone Si and Ge fibre were exposed by hydrofluoric acid etching. Before the etching process, glass-clad Si and Ge fibres were cut into segments with a length of 80 cm, limited by the dimension of the acid tank.

Raman spectra were collected using a Witec UHT S300 system (excitation wavelength 532 nm). Fibre lateral- and cross-sections were prepared with embedding samples in epoxy resin (EpoxiCure 2 and EpoThin 2) and then polished with 600, 1200, 2500 and 4000 grit silicon carbide grinding papers. Raman spectroscopy cannot precisely reveal the pristine stress value due to the necessary polish for sample preparation, but it can serve as a reference for qualitative analysis, as the precisely quantitative comparison between modelling (or simulation) and experimental results to determine the individual contribution of solidification expansion and thermal mismatch is currently challenging owing to the extreme working conditions. Through the mechanical optimization for the molten-core method, high-quality glass-clad Si and Ge fibres were obtained. Material characterizations indicate the polycrystalline nature of the fibres with limited oxygen content and free of cracks. Oxygen in the core results from thermally activated dissolution and diffusion from the glass cladding. Extremely low oxygen semiconductor fibres can be achieved by introducing deoxidizer, and laser recrystallization can be applied when the single crystal is desired. Scanning electron microscopy and energy-dispersive X-ray spectroscopy measurements were conducted with the accelerating voltage of 20 kV and working distance of 10 mm using a JEOL JSM-7800F. X-ray diffraction data were extracted by the azimuthal integration of two-dimensional wide-angle X-ray scattering collected by a Xenocs Nanoinxider (sample-detector distance 79.84 mm, wavelength 1.54189 Å, beam size 200 μm, exposure time 60 s and Psi rotation at every 18°). The transmission electron microscopy lamellae were prepared by focused-ion-beam milling. High-resolution transmission electron microscopy and selected area electron diffraction images were collected with a double-tilt holder using a JEOL 2100F 200-kV field-emission transmission electron microscope.

Fabrication of optoelectronic fibres

Convergence fibre drawing is a modified thermal drawing technique that expands the material selections that are not limited by the process compatibility owing to the drawing temperature. Using this method, the polymers transform into viscous flows and converge to the semiconductor fibres and metal wires that retain solidity when being drawn together down to the fibre dimension.

In our design of an optoelectronic fibre device, standalone Si or Ge fibre was placed in the centre of a transparent polycarbonate cladding and sandwiched by two copper or tungsten wires, while conducting CPC was used to close the gap between semiconductors and metals. Back-to-back Schottky contacts were constructed at the transverse plane between CPC and semiconductors, and further connected with two metal buses to enable decent conductivity both across the microscale transverse plane and along the metre-scale fibre axis. The semiconductor core was slightly thicker than the opaque electrodes to enable pseudo-omnidirectional response to incident light with a beam size larger than the opaque electrodes. It is worth noting that both post-processing techniques, such as laser recrystallization, and semiconductor manufacturing methods, such as doping and lithography, are applicable to the semiconductor fibres and may lead to enhanced performance.

Preparation of the preform used in convergence fibre drawing of the optoelectronic fibres started with the milling of two polycarbonate slabs to create three hemispherical channels with a radius of 2 mm and a distance of 1 mm to each other in the centre, along the preform length. Then, the polycarbonate in the 1 mm space between the three channels was machined off, and two 1 mm square CPC slabs were placed. The preform was then consolidated in a vacuum oven (3 h at 170 °C). The preform was drawn at 300 °C, and the feed rates and draw rates were 0.002 cm s⁻¹ and 3.2 cm s⁻¹, respectively. Copper wires (for single-core fibre) and tungsten wires (for dual-core fibre) were fed into the two side channels, and Si or Ge fibre (two fibres in the case of dual-core fibre) was fed into the centre channel during the draw.

Simulations

The finite element analysis for the solidification and cooling stages was implemented with the software suite Abaqus/Standard. In all the simulations, axisymmetric structures were adopted to reduce the computation cost. The parameters used in the simulations are listed in Supplementary Tables 1 and 4.

In the first stage, viscoelastic analysis was carried out for the stress distributions after solidification, and the glass claddings were considered as viscoelastic materials with the Maxwell model³³. The normalized shear relaxation modulus of the claddings was chosen as a value (0.99) close to 1. The volume expansion of semiconductors at solidification was applied in the form of an isotropic eigenstrain induced by a virtual temperature change. As drawing a long fibre at constant velocity is a steady-state process, the liquid–solid interface remained at the same position after Δt. Interfacial sliding was allowed over the portion of the solid core–cladding interface formed during Δt. The choice of the value of Δt does not affect stress evolution in the core if it is small compared with the stress relaxation time (Extended Data Fig. 7). Our conclusions also hold for the other fibres with different ratios of core radius to fibre radius used in the experiment (Extended Data Fig. 7).

In the cooling stage, the viscous effect of the cladding was neglected, considering its much larger viscosities at low temperatures, and accordingly the cladding was modelled as a linear elastic material. Thermal residual stresses started to accumulate in the solidified core when the temperature reached the annealing point of the cladding (\({T}_{{\rm{a}}}^{{\rm{clad}}}\)), above which the residual stresses would have relaxed rapidly. Therefore, the thermal mismatch was calculated in the temperature range from \(T=\min \left({T}_{{\rm{m}}}^{{\rm{core}}},{T}_{{\rm{a}}}^{{\rm{clad}}}\right)\) to the ambient temperature (26 °C). The total thermal strain in this temperature range was obtained by integrating the temperature-dependent data of the linear thermal expansion coefficient for Si, Ge and silicon dioxide^34,35. Both terminating cross-sections of the fibre were constrained such that the core and the cladding deformed synchronously in the axial direction. The effect of the drawing force is neglected, as it produces much smaller fibre tensions (10–20 MPa) than those induced by the solidification expansion and thermal mismatch.

Capillary instability calculation is described in Supplementary Note 2, and parameters used in the calculations are listed in Supplementary Table 5. Electric field distribution was calculated in the COMSOL electrostatics module with extremely fine mesh.

Performance characterization

A 532-nm laser diode (Thorlabs DJ-532) and a 1,550,nm InGaAsP laser diode (NEC NX5504EK) were used as the illumination sources. A laser diode current controller (Thorlabs LDC205C), temperature controller (Thorlabs TED200C) and function generator (Agilent 33250 A) were used to operate and modulate the illumination source. The laser beam was focused by a spherical lens and further shaped into an ellipse via a planoconcave lens to cover the whole aperture of the optoelectronic fibres, which was placed in the beam centre. Laser power was monitored by a power meter (Thorlabs PM100). In the measurement of responsivity and I–V curves, a source meter (Keithley 6517B) was used for power supply and current monitoring. Electrical connections to the fibres were established to the metal wire electrodes exposed by a fibre stripper. A transimpedance amplifier circuit (amplifier OPA380, load resistance 5 kΩ) was used for NEP and rise-time measurements. Waveforms were collected through an oscilloscope (Tektronix DPO5104B), and MATLAB (MathWorks) rise-time and pwelch functions were employed. A bias voltage of 2 V was applied to the fibres in all tests.

Linear translational stages (Thorlabs NRT150) were used in tensile and compression tests with a 20-N force gauge (Yisida DS2-20N) mounted. The impact strength was collected from unnotched Charpy impact tests. Torsional strength testing was conducted on a customized testing machine. The cyclic bending test was performed on a Prtronic FT2000 flexible electronic tester. A commercial washing machine was used in the washability test. Following the ISO 6330 standard, ten washing cycles were applied to the functional fabric.

Wireless optoelectronic fibre system

A 32 × 48 mm customized printed circuit board (PCB) was used as the interface board for data acquisition, processing and wireless transmission modules. Two GS8554 (four channels) operational amplifiers were used in the transimpedance amplifier circuit, allowing the installation of eight optoelectronic fibres on the PCB. An analogue-to-digital-converter (ADS7828) was used for onboard data processing, and the wireless transmission to cell phone by Bluetooth. A coin cell was used as a power supply. A customized mobile application for visualization was developed.

The functional beanie was achieved by interknitting with eight Ge optoelectronic fibres, which connected to the PCB placed in the inner top space inside the beanie. The demonstration of outdoor use was recorded at a pedestrian crossing at noon on a sunny day with the most intense sunlight of the day using a 30-mW 1,550-nm laser pointer as a signal source at a distance of 1.5 m to the beanie in Supplementary Video 2. Similarly, Si optoelectronic fibres were interknitted into a sweater and used to demonstrate a wearable receiver for an indoor Li-Fi communication system. A photo of the building (the Learning Hub at Nanyang Technological University) was encoded by a customized algorithm into the light from an LED, which was received and converted into electrical signals by the sweater and decoded by a customized algorithm to restore the photo. Mini 532-nm LED strips and Si optoelectronic fibre were integrated together into the watchband. A small portion of the green light related to the volume changes of blood vessels caused by heartbeats was scattered back from the wrist to the optoelectronic fibre and generated an electrical signal reflecting the heart rate. A conventional sensor BIOFY SFH 7070 was used in the same configuration for comparison. In the demonstration of underwater use, Si optoelectronic fibres were conformally (over right-angled steps) glued to the outer surface of the mini-submarine. The PCB interface board was installed beneath the mini-submarine and protected by a waterproof plastic box.