Neil F. Baril

Microstructured optical fibers as high-pressure microfluidic reactors

Deposition of semiconductors and metals from chemical precursors onto planar substrates is a well-developed science and technology for microelectronics. Optical fibers are an established platform for both communications technology and fundamental research in photonics. Here, we describe a hybrid technology that integrates key aspects of both engineering disciplines, demonstrating the fabrication of tubes, solid nanowires, coaxial heterojunctions, and longitudinally patterned structures composed of metals, single-crystal semiconductors, and polycrystalline elemental or compound semiconductors within microstructured silica optical fibers. Because the optical fibers are constructed and the functional materials are chemically deposited in distinct and independent steps, the full design flexibilities of both platforms can now be exploited simultaneously for fiber-integrated optoelectronic materials and devices.

Low loss silicon fibers for photonics applications

Silicon fibers are fabricated using a high pressure chemical deposition technique to deposit the semiconductor material inside a silica capillary. The silicon is deposited in an amorphous state into pure silica capillaries and can be crystallized to polysilicon after the deposition via a high temperature anneal. Optical transmission measurements of various amorphous and polycrystalline core materials were performed in order to determine their linear losses. Incorporating silicon functionality inside the fiber geometry opens up new possibilities for the next generation of integrated silicon photonics devices.

Nonlinear transmission properties of hydrogenated amorphous silicon core optical fibers.

The nonlinear properties of a low loss hydrogenated amorphous silicon core fiber have been characterized for transmission of high power pulses at 1540 nm. Numerical modelling of the pulse propagation in the amorphous core material was used to establish the two-photon absorption, free-carrier absorption and the nonlinear refractive index, which were found to be larger than the values typical for crystalline silicon. Calculation of a nonlinear figure of merit demonstrates the potential for these hydrogenated amorphous silicon core fibers to be used in nonlinear silicon photonics applications.

All-optical modulation of laser light in amorphous silicon-filled microstructured optical fibers

Amorphous silicon is deposited within optical fibers by a high pressure microfluidic deposition process and characterized via Raman spectroscopy. All-optical modulation of 1.55 µm light guided through the silicon core is demonstrated using the free carrier absorption generated by a 532 nm pump pulse. Modulation depths of up to 8.26 dB and modulation frequencies of up to 1.4 MHz are demonstrated.

Electrical and Raman characterization of silicon and germanium-filled microstructured optical fibers

Extreme aspect ratio tubes and wires of polycrystalline silicon and germanium have been deposited within silica microstructured optical fibers using high-pressure precursors, demonstrating the potential of a platform technology for the development of in-fiber optoelectronics. Microstructural studies of the deposited material using Raman spectroscopy show effects due to strain between core and cladding and the presence of amorphous and polycrystalline phases for silicon. Germanium, in contrast, is more crystalline and less strained. This in-fiber device geometry is utilized for two- and three-terminal electrical characterization of the key parameters of resistivity and carrier type, mobility and concentration

Mid-infrared transmission properties of amorphous germanium optical fibers

Germanium optical fibers have been fabricated using a high pressure chemical deposition technique to deposit the semiconductor material inside a silica capillary. The amorphous germanium core material has a small percentage of hydrogen that saturates the dangling bonds to reduce absorption loss. Optical transmission measurements were performed to determine the linear losses over a broad mid-infrared wavelength range with the lowest loss recorded at 10.6 µm. The extended transmission range measured in the germanium fibers demonstrates their potential for use in mid-infrared applications.

Confined high-pressure chemical deposition of hydrogenated amorphous silicon

Hydrogenated amorphous silicon (a-Si:H) is one of the most technologically important semiconductors. The challenge in producing it from SiH(4) precursor is to overcome a significant kinetic barrier to decomposition at a low enough temperature to allow for hydrogen incorporation into a deposited film. The use of high precursor concentrations is one possible means to increase reaction rates at low enough temperatures, but in conventional reactors such an approach produces large numbers of homogeneously nucleated particles in the gas phase, rather than the desired heterogeneous deposition on a surface. We report that deposition in confined micro-/nanoreactors overcomes this difficulty, allowing for the use of silane concentrations many orders of magnitude higher than conventionally employed while still realizing well-developed films. a-Si:H micro-/nanowires can be deposited in this way in extreme aspect ratio, small-diameter optical fiber capillary templates. The semiconductor materials deposited have ~0.5 atom% hydrogen with passivated dangling bonds and good electronic properties. They should be suitable for a wide range of photonic and electronic applications such as nonlinear optical fibers and solar cells.

High‐Pressure Chemical Deposition for Void‐Free Filling of Extreme Aspect Ratio Templates

Extreme aspect ratio semiconductor structures are critical to modern optoelectronic technology because of their ability to waveguide light and transport electrons. Waveguides formed from almost any material by conventional micro/nanofabrication techniques typically have significant surface roughness that scatters light and is a constraining factor in most optoelectronic devices. For example, fabricated planar silica waveguides have optical losses 3 to 5 orders of magnitude higher than silica fibers, in part due to surface roughness. For these reasons silica nanofibers have been proposed as alternatives to fabricated silica or semiconductor channels for waveguiding of light in miniaturized optical devices, as they meet the strict requirements for surface roughness and diameter uniformity required for low loss. An additional advantage of these silica fibers is that they have a circular cross section that can simultaneously guide both transverse electric (TE) and transverse magnetic (TM) polarizations without cutoff. In contrast the rectilinear cross sections of microfabricated planar waveguides can effectively guide only one polarization without cutoff. However, semiconductors in general exhibit a far broader range of useful optoelectronic function than silica glass because of their ability to form hetero and homojunctions, serve as optical gain media over a broad range of wavelengths, and their superior non-linear optical properties.

Bulk InAsxSb1-x nBn photodetectors with greater than 5μm cutoff on GaSb

Mid-wavelength infrared nBn photodetectors based on bulk InAsxSb1-x absorbers with a greater than 5 μm cutoff grown on GaSb substrates are demonstrated. The extended cutoff was achieved by increasing the lattice constant of the substrate from 6.09 to 6.13 A using a 1.5 μm thick AlSb buffer layer to enable the growth of bulk InAs0.81Sb0.19 absorber material. Transitioning the lattice to 6.13 A also enables the use of a simple binary AlSb layer as a unipolar barrier to block majority carrier electrons and reduce dark current noise. Individual test devices with 4 μm thick absorbers displayed 150 K dark current density, cutoff wavelength, and quantum efficiency of 3 × 10−5 A/cm2, 5.31 μm, and 44% at 3.4 μm, respectively. The instantaneous dark current activation energy at a given bias and temperature is determined via Arrhenius analysis from the Dark current vs. temperature and bias data, and a discussion of valence band alignment between the InAsxSb1-x absorber and AlSb barrier layers is presented.

Optimization of thickness and doping of heterojunction unipolar barrier layer for dark current suppression in long wavelength strain layer superlattice infrared detectors

Suppression of generation-recombination dark current and bias stability in long wavelength infrared (LWIR) strained layer superlattice (SLS) detectors, consisting of a lightly doped p-type absorber layer and a wide bandgap hole barrier, are investigated with respect to the wide bandgap barrier layer thickness and doping profile. Dark current IV, photoresponse, and theoretical modeling are used to correlate device performance with the widegap barrier design parameters. Decreased dark current density and increased operating bias were observed as the barrier thickness was increased. This study also identifies key device parameters responsible for optimal performance of heterojunction based SLS LWIR detector.