We study the kinetics of silane pyrolysis via confined high-pressure chemical vapor deposition (HPCVD) at pressures of 20−33 MPa in a microcapillary of 5.9 μm inner diameter. We find the growth rate to be first order with respect to silane concentration, with an activation energy of 53.7 ± 2.9 kcal/mol and a pre-exponential factor of 1.5 × 10 10 m/s. The obtained activation energy is in the range of activation energies reported for hydrogen desorption from c-Si surfaces, suggesting that hydrogen desorption from the surface is the ratelimiting step in film growth. To further investigate this finding, reactive molecular dynamics simulations of thermal decomposition of silane on clean and hydrogen-passivated c-Si were performed. Homogeneous reactions were not observed in any of the simulations, in support of the hypothesis that heterogeneous silane decomposition on the silicon surface is the dominant mechanism for film deposition. In silane pyrolysis simulations on clean c-Si surfaces, almost all available silicon surface sites (i.e., dangling bonds) were occupied by silicon-hydrides (mostly tri-and dihydrides) upon exposure to gas-phase silane, whereas no reaction was observed during silane decomposition simulations on the hydrogen-passivated c-Si. Therefore, the results of the reactive molecular dynamics simulations indicate that the availability of dangling bonds resulting from hydrogen desorption from the surface is the rate-limiting step in film growth at high pressure.

The optical fiber geometry is known for rugged, high power laser sources that are preferred for many applications, but is typically limited to the visible and near-infrared regions of the electromagnetic spectrum due to the transmission limits of silica (< 2 µm). This wavelength range could be extended into the mid-infrared using transition metal doped, crystalline II-VI optical gain media, but these materials cannot be fabricated into optical fibers using conventional glass drawing methods. An in-situ high pressure chemical vapor deposition method for the fabrication of silica-cladded ZnSe fiber cores uniformly doped with Cr 2 + is reported. Optical pumping experiments reveal that these doped fibers exhibit threshold behavior and thus function as mid-infrared optical fiber lasers. Finite element calculations show that undesirable thermal effects common in bulk II-VI crystals are mitigated in the fiber geometry.

Today fiber lasers in the visible to near-infrared region of the spectrum are well known, however mid-infrared fiber lasers have only recently approached the same commercial availability and power output. There has been a push to fabricate optical fiber lasers out of crystalline materials which have superior mid-IR performance and the ability to directly generate mid-IR light. However, these materials cannot currently be fabricated into an optical fiber via traditional means. We have used high pressure chemical vapor deposition (HPCVD) to deposit Fe2+:ZnSe into a silica optical fiber template. These deposited structures have been found to exhibit laser threshold behavior and emit CW mid-IR laser light with a central wavelength of 4.12 µm. This is the first reported solid state fiber laser with direct laser emission generated beyond 4 µm and represents a new frontier of possibility in mid-IR laser development.

Polycrystalline zinc selenide optical fibers and fiber lasers are expected to provide powerful capabilities for infrared waveguiding and laser technology. High pressure chemical vapor deposition, which is the only technique currently capable of producing zinc selenide optical fibers, leaves a geometric imperfection in the form of a central pore which is detrimental to mode quality. Chemical vapor transport with large temperature and pressure gradients not only fills this central pore but also encourages polycrystalline grain growth. Increased grain size and a reduction in defects such as twinning are demonstrated with transmission electron microscopy, Raman spectroscopy, and X-ray diffraction, supporting that high-quality material is produced from this method. Finally, the mode structure of the waveguide is improved allowing most of the guided optical intensity to be centrally positioned in the fiber core. Loss as low as 0.22 dB/cm at 1908nm is demonstrated as a result of the material improvement.

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