jbadding optoelectronics fiber

8/10/2019 JBadding Optoelectronics Fiber

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ures 1,2). 2 As a result there is now a unique opportunity to explore an alternate paradigm for optoelectronics; one wherethe currently dominant platforms of optical-fiber based information transfer and semiconductor-chip based information

processing can be more seamlessly integrated within a fiber. We can now envision all-fiber optoelectronics , whereelectronic devices can be built within the micro to nanoscale holes of a MOF, and light generation, modulation, anddetection can all be performed within a fiber (Figure 3).

Fig. 3. All-Fiber integrated optoelectronic concepts. Top: Optical modulation with semiconductor.Bottom: Mid infrared chalcogenide fiber laser.

Fig. 2. Microstructured optical fibers fabricated at the University of Southampton Optoelectronics Research Centre.Left: Solid core holey fiber. Right: Hollow core photonic bandgap fiber.

Proc. of SPIE Vol. 6475 64750N-2


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Chemical deposition onto the walls of the long, extremely narrow pores in a MOF does not seem feasible: the smallestdeviation from perfect conformal deposition anywhere along the length of the pore would prevent deposition, and masstransport of the reactants into and by-products out of such a confined space is too slow. However, fabrication of high-quality polycrystalline and single-crystal semiconductor structures within the microscale to nanoscale voids of MOFsvia a ultrahigh-pressure microfluidic chemical deposition technique is possible. High-pressure flow (2 to 100 MPa) al-lows for transport of reactants into the extreme aspect ratio pores, enabling uniform, dense and conformal annular depo-sition upon heating the fiber (Figure 4). 2 Figure 5 shows deposition of hexagonal tubes of silicon many centimeters longwithin the pores of a large air fraction microstructured optical fiber. The tubes are smooth and well developed.

1.1. Electronics in a Fiber

Fabrication of semiconductor structures inside MOFs allows for incorporation of electronic function into optical fibers.Fiber integrated electronics could be exploited for high speed optical modulators and detectors, for example. We havedemonstrated field effect transistors (FETs) from silicon and germanium wires inside MOFs. 2, 3 The FET serves as avaluable characterization tool that allows us to characterize semiconductor parameters such as carrier type,concentration, mobility and activation energy. If the carriers in a semiconductor material have high mobility, the optical

properties ar e likely to be good as well, as there will be fewer defects.

Fig. 4. Microfluidic chemical deposition process for fabricating semiconductor structures within micro to nanoscale pores of optical fibers .

Fig. 5. Empty (left) and silicon filled (right) pores within a large air fraction microstructured optical fiber.



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MOFs filled with silicon or germanium are etched with hydrofluoric acid to expose several hundred microns to a fewmm of semiconductor wire for the fabrication of ohmic contacts for the FETs. Aluminum is evaporated onto the wiresand sintered at 430 C for 30 minutes. Aluminum is known to form low resistance ohmic contacts to bulk crystalline n-type silicon and germanium. Next contacts were made to the aluminum with indium gallium eutectic. A capacitativelycoupled coaxial gate was fabricated from indium gallium eutectic placed around the silica cladding (Figure 6).

Making contacts to semiconductor micro and nanowires for accurate characterization of device performance remains an

ongoing challenge. For vapor-liquid-solid (VLS) grown wires that are etched out of a template, making good contactsoften requires an annealing step, which can lead to chemical diffusion along the wire. Such diffusion can alter thedoping type and level of the wire, as well as the chemical composition of the gate given its close proximity to thecontacts, making accurate interpretation of device characteristics difficult. Furthermore, the area of the contacts tolithographically processed loose VLS wires is often quite small, which makes it more difficult to avoid non-ohmiccontacts that give rise to non-gated transconduction. In contrast, contacts to the semiconductor wires contained withinthe MOFs can be made over a millimeter or more of length, increasing the chances that they will be truly ohmic. Thecontacts in the MOF devices are annealed, but the wire remains encased in silica that prevents any chemical diffusionfrom occurring along its length. The semiconductor gate region can be centimeters from either contact, and is againunaffected by annealing step. These two advantages of the MOF approach, the ease of making large ohmic contacts and

Fig. 6. Configuration of in-fiber semiconductor FET

Fig. 7. Current-Voltage characteristics for in-fiber germanium FET at different gate biases.



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freedom from chemical diffusion, make it easier to avoid difficulties that preclude obtaining a fundamentalunderstanding of device behavior.

For germanium wires deposited within the MOFs with a diameter of 5 microns and length of 11mm, the resistivity fromtwo terminal measurements is 5.6x10 -2 cm. Using a simple co-axial approximation the capacitance per unit length isgiven by 3:

C/L 2 0/ln(2h/r)

where is the dielectric permittivity of silica, h the radial thickness of the silica cladding, L the gate length and r theradius of the semiconductor core.

For our germanium samples, these values are = 3.8, h = 47 microns, L = 1.5 mm and r = 2.5 microns, resulting in acapacitance per unit length of 6 x 10 -11 Fm -1. As the gate bias voltage is varied from -100V to +100V (+/- 2.2x10 6 Vm -1), the slope of the I drain -V source curve increases (Figure 7), indicating that the carriers are n-type. The electron mobilitycan be calculated as follows:

dI drain /dV gate = (C/L2)V source-drain

where is the carrier mobility, which is 1.05 cm 2/Vs at room temperature. The carrier density can be estimated bycalculating the total charge in the germanium wire as Q = CV threshold , where C is the semiconductor wire capacitance andV threshold the voltage necessary to completely deplete the device. Thus the total charge density is:

N e = Q/e r 2

L.

Thus, taking, V threshold (pinch-off) as -100V and C = 9 x 10-14 F, N e 2 x 10

21 m -3 (or 2 x 10 15 cm -3).Figure 8 shows how I drain varies as a function of V gate . The transconductance of the sample begins to be pinched-off atvoltages in the range of -90 to -100V. Thus this sample is functioning as a fiber-based depletion-mode FET switch.While the experimentally available range of applied gate voltages limits characterization of the off regime of the FET,we can estimate that the on/off current ratio of the device is 10 4 or better. Room temperature electron mobilities for bulk

polycrystalline devices are typically ~200 cm 2/Vs. 4

Fig. 8. Drain current as a function of gate bias for in-fiber germanium FET.



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Fig. 9. Three terminal current-voltage characteristics for silicon in-fiber FET.

Fig. 10. Raman spectrum of silicon wire contained within fiber (top), after etching from fiber to bare silicon (mid-dle), and silicon reference (bottom). There is a low frequency tail on the Raman mode for the silicon wire in the fi-

ber. This tail remains af ter etching the wire from the silicon to release s train caused by bonding of the silica to thesilicon and their thermal expansion mismatch. This tail is indicative of the presence of nanocrystalline or defectivesilicon.



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We also made FETs from silicon. For a silicon sample annealed at a temperature of 700 C with a length of 18 mmand cladding diameter of 125 m, the measurements again indicate n-type carriers (Figure 9), with sample resistivity of0.21 cm and device mobility of 1.4x10 -2 cm 2/Vs. For comparison bulk crystalline silicon is typically 10 2-10 3 cm 2/Vs. 5 The present samples have considerable nanocrystalline grains, as evidenced by tailing in the Raman spectrum to lowfrequencies (Figure 10). 3 More careful thermal annealing should allow for considerable reduction in the amount of thisnanocrystalline silicon and improved mobility. Extrapolation of the effect of V gate on the transconductance allows us toestimate V pinch-off to be -320V, resulting in a free-carrier concentration of 6.6.x10

15cm -3. The mobility of the siliconsamples is insensitive to temperature (Figure 11). Thus extrinsic scattering mechanisms, such as grain boundaries,surface and charge traps are playing a dominant role in the transport, as expected from the presence of nanocrystallinesilicon in the samples determined from Raman spectroscopy. The conductivity ( ) vs. temperature (T) data for a

polycrystalline-Si sample can be fit to a function of the form (Figure 11):

(T) = A/ T exp(- /kT).

Here A is a semi-empirical constant and is an activation energy of the grain-barrier potential. The variation ofconductivity with temperature has been determined to vary in this way in the regime of grain-boundary limitedtransport, 6, 7 again in agreement with the conclusion that grain boundaries and defects currently dominate the transportin the silicon in-fiber fibers.

Reducing the magnitude of grain boundary scattering is key to further progress towards device development. Highertemperature annealing and/or alternative (e.g., laser based) annealing techniques are being explored towards this end.

1.2. Light Guiding in Fiber

We also characterized the light guiding properties of the silicon wires. Approximately 2 mm of the 125- m diametersilica cladding of a 5-cm-long sample was chemically etched away at one end. This was done to avoid the possibilitythat light would be guided in the cladding instead of the silicon wire itself. 633 nm laser light launched at the opposite

end of the fiber stops propagating at the point where the glass cladding ends (Figure 12). In contrast, 1550 nm lightfocused via a 0.65 NA 3.1mm focal length aspheric lens to 2 m at the core propagates all the way to the end of the fiber(Figure 12). 100 mW of 1550 nm light resulted in an output power of 0.03 mW, resulting in an upper bound of 7 dB/cmon the losses in the wire. Scattering from surface roughness is a critical component of the loss in planar siliconwaveguides because of the high index contrast between core and cladding. Our wires are fabricated within a templatethat has 0.1 nm RMS surface roughness, which should make their outer diameter also very smooth. The inner diameter

Fig. 11. Conductivity and mobility of the silicon wires as a function of temperature.



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becomes relatively unimportant because its surface area is so small in the fiber geometry. Thus losses due to surfaceroughness for semiconductor waveguides in the fiber geometry are easier to avoid.

1.3. Future Prospects for Integration

As the fundamental materials science described above to improve materials properties is pursued, an increased range ofdevices will become possible. Semiconductor (and metal) heterojunctions can be integrated within the fiber (Figure 13)

by sequentially altering the precursors employed during deposition, giving rise to the possibility for incorporating a

633nm light; Stops when cladding ends

1550 nm light; Propagates to the end of the fibre

Fig. 12. Top: Schematic of the light guiding experiment showing the 2 m diameter silicon core surrounded by theetched, tapered cladding at the left end. Middle: 633 nm light stops propagating at the core-cladding boundary.Bottom: 1550 nm light, in contrast, continues propagating to the end of the fiber. Some scatter is also evident atthe core-cladding boundary.

Fig. 13. Germanium-silicon radial heterojunction integrated into a microstructured optical fiber. Scale bar is 6 m.



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jbadding optoelectronics fiber

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