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We report the fabrication of a semiconductor germanium fiber with a borosilicate glass cladding. A non-ohmic voltage-current behavior has been observed in germanium fiber. The experiment of the photo-induced electrical response of germanium fibers has been carried out. The germanium fiber array with only three parallel-wired fibers shows a high-speed response to modulated 1.55μm laser irradiation with up to 100 kHz repetition rate. The concept of proof of high-speed photodetectors using fabric germanium array indicates that the glass-clad germanium fibers are promising for building flexible economic large-scale high-speed devices for detecting infrared radiation.
© 2017 Optical Society of America
1. Introduction
Micro-/nano-structured semiconductor materials are promising candidates for constructing next-generation of integrated optoelectronic devices. Amongst all types of structured semiconductor materials, one dimensional (1D) semiconductor micro-/nano-wires have generated growing attention recently [1–7], due to their large surface to volume ratio and versatile configurations for building compact flexible micro-/nano-scaled semiconductor optoelectronic devices. Glass-clad semiconductor-core optical fiber is one of the approaches to fabricate 1D semiconductor microwires with long yield [8–15]. High-pressure chemical vapor deposition [8], molten-core drawing method [9–13], and pressure-assisted melt-filling method [15] have been demonstrated to obtain continuous semiconductor fiber with the length up to hundreds of meters.
As the two major element materials dominant in semiconductor industry, silicon and germanium are the most attractive building blocks for constructing semiconductor fibers. This is because their excellent performances for applications in a wide range of research areas such as optoelectronics, near-infrared and mid-infrared optics, nonlinear optics, and so on [8–11, 14–16].
From the aspect of material properties, in comparison with silicon [17], semiconductor germanium is very promising in terms of its high refractive index (n = 4.0), excellent transmission window in the mid-infrared region (2-20 μm), and high nonlinear Kerr index (n2 = ~2x10−17 m2/W) [18, 19]. Flexible wire-shape germanium is thus promising to realize multiple functionalities for a wide range of mid-wave and long-wave IR photonics, as well as the semiconductor optoelectronics.
While many works concentrate on using semiconductor fibers for near-infrared and mid-infrared nonlinear optical applications [4, 15, 16], semiconductor optoelectronic fibers show promise for many novel applications such as high-speed photodetection and optical imaging, utilizing the unique advantages of fiber geometry, the long length and the large surface-to-volume ratio, over other conventional shapes of semiconductor optoelectronic devices [1–3, 7, 20]. A GHz-bandwidth semiconductor device has been proven using semiconductor silicon fiber with a built-in Pt/n-Si Schottky junction [20]. However, the semiconductor germanium high-speed photodetecting fiber has not been demonstrated yet. A flexible high-speed photodetecting germanium fiber device will be promising, since a varieties of germanium photodetectors have been reported for the usage in the broad range from near-infrared to far infrared regions, e.g., in 0.8 μm −1.8 μm [21, 22], at 10.6 μm [23], and even in the far-infrared (THz) region [24]. It will be a potentially economic approach to construct a flexible large-scale (1-100 cm2) fabric infrared photodetector device in a two-dimensional mesh format [1].
In this work, we report the fabrication of a glass-clad semiconductor germanium fiber. Borosilicate glass is used as the cladding. Continuous uniform germanium fiber with long yield has been drawn. The micro-Raman spectra have been measured to characterize the crystallinity nature of the fabricated germanium fiber. The current-voltage characteristics of the glass-clad germanium fiber are measured. The germanium fibers show high-speed low-noise photo-induced electrical response to modulated 1.55μm laser with up to 100 kHz repetition rate.
2. Fabrication of glass-clad germanium fiber
A 3.0 mm rod has been core-drilled from 99.999% pure, n-type, anitimony-doped single crystalline germanium with unknown dopant concentration (Sanjing Inc., China). A borosilicate glass (Schott Duran) tube with an inner diameter (ID) of 3.1 mm and outer diameter (OD) of 15.5 mm is used as the cladding. The cladding tube is pre-drawn to seal one end of the tube for allocating the germanium rod inside the tube. Fibers are then drawn at approximately 1000°C, which is above the melting temperature of germanium, 938 °C. Figure 1 illustrates the viscosity curves of the borosilicate glass cladding and the germanium core, based on the data provided in References [25, 26]. It is seen that at the drawing temperature, the germanium core is at molten state with a viscosity of ~0.1 poise (i.e., dPa⋅s), while the glass cladding is with 104.4 poise. It is thus seen that the glass tube here is not only working as the cladding but also as a deformable crucible of molten germanium during the fiber drawing. The liquideous core is encapsulated by the viscous glass cladding. This is different from the situation of drawing conventional glass fibers. This molten core drawing method originates from the well-known Taylor wire method [27], by which the metal wire with only a few micrometers in diameter can be fabricated by inserting metal wire inside a glass tube, which is normally also based on a borosilicate composition.
Fig. 1 Viscosity curves of borosilicate glass cladding [26] and germanium core [25] at high temperatures.
Approximately 50 meters of glass-clad germanium fiber with an OD of 600 μm and a core diameter of 125 μm has been directly drawn from the set of the above glass tube and germanium rod. Figure 2(a) shows the scanning electronic microscopic (SEM) photograph of the germanium fiber with a glass clad OD of 600 μm.
Fig. 2 SEM photographs of Ge fibers with (a) 600 μm OD and core diameter of 125 μm and (b) 260μm OD and core diameter of 21 μm.
By inserting the glass-clad germanium fiber with an OD of 600 μm into a second borosilicate glass tube with an inner diameter of 650 μm, glass-clad germanium fiber with small core size has been drawn with a yield of ~100 m. As seen from the SEM photo in Fig. 2(b), the fiber has an OD of 260 μm and a core diameter of 21 μm.
3. Characterization of crystalline germanium core in glass-clad fiber
The spontaneous Raman scattering (SRS) spectra of the single crystalline 3mm Ge rod, the glass-clad fibers with 125 μm germanium core and with 21 μm germanium core have been measured by a WItec alpha300 micro-Raman microscope at room temperature. A 532nm frequency-doubled Nd:YAG laser is used as the excitation source. With the assistance of a CCD camera on the Raman microscope, the laser beam is focused on the germanium area and the laser spot size is estimated to be 3-4 µm.
Figure 3 compares the measured SRS spectra of the germanium in these samples. One can see that all samples show only one strong Raman peak at 301.1 ± 0.7 cm−1 in Fig. 3(a)). This peak is the intrinsic first-order Raman line of diamond-structure germanium crystal [28]. The full width at half-maximum (FWHM) of this peak is 9.1 ± 0.1 cm−1, falling within the range given by Ref [28, 29]. for Ge single crystal at 300 K. In Fig. 3(b), weak bands are observed for all samples between 400 and 610 cm−1. These bands are associated with the second-order Raman scattering of germanium crystal [30, 31]. These high similarities between the starting Ge single crystal and the final Ge core in the fibers indicate a high degree of crystallinity and phase-purity of Ge core in the fibers made by the molten core method [11]. Mind that Raman spectroscopy is not capable of detecting grain boundaries and crystal orientation of a crystal sample.
Fig. 3 Spontaneous Raman scattering spectra of starting single crystalline 3mm Ge rod, glass-clad fibers with 125 μm Ge core and 21 μm Ge core in range of (a) 200-600 cm−1 and (b) 600-1000 cm−1 . Note that in the latter range, the intensity is multiplied by a factor of 10.
In addition, in Fig. 3(b), it is also seen that no Raman band exists at ~800 cm−1, which corresponds to the Ge-O vibrations [32], showing that the oxidation of Ge element or the oxygen diffusion from the glass cladding is negligible during the fiber drawing [11].
4. Characterization of electrical and optoelectronic properties of Ge fibers
The electrical conductivity of glass-clad Ge fibers has been measured with an Agilent 3446A digit multimeter. The end-faces of a 1cm-long fiber samples are placed into liquid zinc-gallium alloy, which has a melting temperature of 25 °C. The resistivity of germanium fibers is measured to be 75 ± 5 Ω·cm, compared to 47 Ω·cm in pure undoped crystalline Ge [15, 33]. The additional resistance might arise from the germanium quality using the molten-core drawing process and the metal contacts in the measurement. In addition, the resistivity of semiconductors also strongly depends on the presence of impurities in the material.
Two samples, single Ge fiber (1#) and parallel-wired three Ge fibers (2#), are prepared for optoelectronic photodetecting measurement. For each sample, the fiber length is 1 cm and the Ge core diameter is 125 µm, respectively. Note that the sample is fixed on a glass microscope slide.
Figure 4(a) shows the electrical connection of the prepared single glass-clad germanium fiber (1#). A CW 1.55 µm single-frequency fiber laser is used as the excitation source for Ge optoelectronic fiber. The single frequency laser is chosen here for having a stable and precise control on the laser irradiation. The output of the 1.55 µm laser is collimated to a spot with a diameter of 17 mm so that the whole length of the germanium sample can be exposed. Figure 4(b) illustrates the current-voltage (I-U) behavior of samples 1# with a laser irradiation density varying from 0 to 44 and 176 mW/cm2. It is seen that the sample shows a nonlinear I-U behavior rather than a linear Ohmic I-U relation. The reason for such nonlinearity is still unclear. One of the possibilities is due to the contributions from the germanium core and the used metal contacts. In addition, no difference has been observed on the I-U behavior with or without the irradiation of the visible light, indicating that the germanium fiber is not sensitive to the visible light.
Fig. 4 (a) Sketch of electrical contact to external circuit by connecting Ge fiber. (b) Current-voltage of single Ge fiber with CW 1.55μm laser irradiation.
Figure 5 shows the schematic of the experimental setup for measuring the optoelectronic response of samples 1# and 2# under high-speed modulated 1.55 μm laser irradiation. A home-made laser diode (LD) pumped 1.55 μm erbium doped fiber laser (EDFL) with a Distributed Bragg Reflector (DBR) cavity is employed to generate the single-frequency CW seed. The seed light is then modulated by an acousto-optic modulator (AOM), with a repetition rate tuning from 10 kHz to 1 MHz. The initial seed is modulated to a square wave with a duty cycle of 1:5. The modulated seed is then amplified by a 976nm LD pumped Yb:Er codoped silica fiber power amplifier. The output from the amplifier is collimated by a lens and launched onto the side of the Ge fiber sample. The power density of the laser irradiation is maintained at 176 mW/cm2. Both photo-induced voltage generated from the Ge sample and launched pulsed laser irradiation signal detected by the photodetector are monitored by an oscilloscope (Keysight Technologies DSO-X 3102T).
Fig. 5 Schematic of measuring optoelectronic response of samples 1# & 2# under modulated 1.55 µm laser.
Figure 6 plots the optoelectronic response of sample 1# with single Ge fiber, irradiated by modulated 1.55 μm laser with the repetition rates of 10 kHz, 100 kHz and 300 kHz. Figure 7 plots the optoelectronic response of sample 2# with parallel-wired three Ge fibers, irradiated by modulated 1.55 μm laser with the repetition rates of 10 kHz, 100 kHz and 1 MHz. It is seen that both samples can immediately give response to the modulated 1.55 μm irradiation up to 100 kHz repetition rate. In addition, it is seen that from Fig. 6(a) and 7(a), the rising part of the modulated 1.55 µm pulses with 10 kHz repetition rate is much steeper than the falling part, largely deviated from the square wave generated from the AOM. This is because at such a low repetition rate, there is no pulsed seed traveling into the fiber amplifier in 80% time of each period and the CW-pumped fiber amplifier accumulates high energy. Most of the energy is then quickly applied on the rising part of the next pulsed seed.
Fig. 6 Optoelectronic response of sample 1# with single Ge fiber under modulated 1.55 μm radiation with repetition rates of (a) 10 kHz, (b) 100 kHz and (c) 300 kHz.
Fig. 7 Optoelectronic response of sample 2# with parallel-wired three Ge fibers under modulated 1.55 μm radiation with repetition rates of (a) 10 kHz, (b) 100 kHz and (c) 1 MHz.
Sample 1# shows higher noise than sample 2#, mainly because the former resistance is three times higher than the latter one and also because the latter has three times more irradiated area than the former. The origin of the noise can be explained by the simple analytical model considering only the dark current fluctuations due to generation-recombination [2, 34, 35]. The amplitude of the noise is proportional to Idark1/2/d, where Idark is the dark current and d is the germanium core diameter respectively. Thus the parallel-wired germanium fiber shows lower noise on the photo-induced voltage response than the single Ge fiber. It is expected that the sensitivity of the germanium fiber sample can be largely enhanced by parallel-wiring a large amount of germanium fibers, for realizing the function as a high-speed low-noise photodetector or optical switch.
The noise on the photo-induced voltage traces of both samples enhances largely, when the repetition rate further increases to 300 kHz and 1 MHz for sample 1# and 2# respectively.
Figure 8 illustrates the definition of the noise intensity Inoise and the peak signal intensity Isignal, max in the generated voltage trace. The noise intensity Inoise is defined as the mean intensity contrast between the upper and lower envelopes. Once the signal to noise ratio (SNR), Isignal, max/Inoise is equal to 1.0, the measured signal is not convincible because the weight of the uncertain component is 50% in the measured intensities even at the signal peak. Second, in each period the response trace from the germanium fiber has a rising part due to the start of the excitation of the irradiation and then a fall part due to the end of the irradiation. The rising time τR and the fall time τF reflect how quick the germanium fiber can response with the rapidly modulated light. These parameters qualitatively represent the photodetecting performances including the responsivity, noise and sensitivity, of the germanium fiber samples.
Fig. 8 Definitions of noise intensity Inoise, peak signal intensity Isignal, max, rise time τR and decay time τD of generated voltage trace.
Table 1 summarizes the parameters of optoelectronic response of sample 1# and 2# under modulated 1.55 µm irradiation. It is seen that for sample 1#, the ratio of Isignal, max/Inoise is just above 1.00 when the repetition rate of the modulated laser is 300 kHz, indicating the sensitivity limit of sample 1# for high-speed modulated irradiation. Similarly, the detection limit of sample 2# is seen at 1 MHz, about 3 times faster than 1#, which is benefitted from the lower resistance and larger irradiation area of the parallel wired Ge fibers than the single Ge fiber. At the repetition of 100 kHz, the SNR of sample 1# and 2# is calculated to be 3.23 and 6.55 respectively, indicating that optoelectronic response with low noise is given by both samples. To the best of our knowledge, this is the first time that 100-kHz level optoelectronic response has been observed in semiconductor germanium photodetecting fibers. And the parallel wired Ge fiber gives nearly twice stronger response voltage and twice better SNR, in comparison with the single Ge fiber. It is therefore expected that by largely increasing the number of the parallel wired Ge fibers, the photodetecting performance, including the responsivity, noise and sensitivity, of germanium fibers can be further enhanced. In addition, the performance of the germanium fiber based optoelectronic devices can also be largely improved by carefully designing the electrical circuit connecting Ge fibers.
Table 1. Parameters of optoelectronic response of sample 1# and 2# under modulated 1.55 µm irradiation
In addition, it is seen from Table 1, Fig. 6 and Fig. 7 that when the repetition rate of the launched irradiation is above 100 kHz, there is a small time delay for the input pulse falling to the off-state. Such a delay is believed to be arising from the shutter speed of AOM when it modulates the pulsed seeds.
For each sample, the SNR decreases with the increase of the repetition rate because the pulse energy, i.e., the amount of the photons per pulse, launched onto the Ge fibers decreases. An array with a large amount of parallel wired Ge fibers is certainly a solution to largely lower the noise on the photo-induced electrical response for detecting the repetition rate beyond 100 kHz.
5. Summary
In conclusion, we report the fabrication of a glass-clad semiconductor germanium fiber. Using borosilicate glass as the cladding, continuous uniform germanium fiber with long yield has been fabricated. The measured micro-Raman spectrum of the fabricated glass-clad germanium core shows high-degree of crystallinity and phase-purity. The measured current-voltage performance of glass-clad germanium fiber shows non-Ohmic relation. As a concept of proof, the experiment of photo-induced electronic response of a glass-clad germanium photodetecting fiber array with only three parallel-wired fibers has been demonstrated. For the first time, the germanium photodetecting fibers have shown the low-noise high-speed responsivity when detecting pulsed laser irradiation at 1.55 μm with up to 100 kHz repetition rate. This opens way toward economic large-scale high-speed optoelectronic devices using flexible fabric germanium fiber array for infrared irradiation detection.
Funding
National Natural Science Foundation of China (NSFC, Nos. 61527822, 61235010 and 61307054); Beijing University of Technology, China; Beijing Overseas Talents Center.
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