Infrared photoconductivity in heavily nitrogen doped a-Si:H
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1153-A02-02
Infrared photoconductivity in heavily nitrogen doped a-Si:H David J. Shelton1, James. C. Ginn1, Kevin R. Coffey2, and Glenn D. Boreman1 1 2
CREOL, University of Central Florida, Orlando, FL 32816, USA AMPAC, University of Central Florida, Orlando, FL 32816, USA
ABSTRACT High frequency steady-state photoconductivity in nitrogen doped hydrogenated amorphous silicon (a-Si:H-N) films has been demonstrated at infrared (IR) frequencies of 650 to 2000 cm-1. This allows IR photoconductivity to be excited using a simple thermal source. In order to produce high frequency photoconductivity effects, the plasma frequency must be increased to the desired device operation frequency or higher as described by the Drude model. IR ellipsometry was used to measure the steady-state permittivity of the a-Si:H-N films as a function of pump illumination intensity. The largest permittivity change was found to be ∆εr = 2 resulting from a photo-carrier concentration on the order of 1022 cm-3. IR photoconductivity is shown to be limited by the effective electron mobility. INTRODUCTION Thin film systems with IR conductivity or permittivity that may be actively tuned with the application of a DC electric field, have been of interest for some time to IR designers. As an alternative, photoconductive devices have been proposed for active IR systems. The carrier concentration can be actively changed by illuminating a-Si:H with source energy above the band gap and thus out of the IR band. This illumination results in the generation of electron-hole pairs, and a sufficient density of these carriers will result in a change in the material’s permittivity in the IR frequency range. Thus, by varying out-ofband pump power, an active IR system may be achieved. Photoconductive elements have been used for optically generated grid arrays and as switches for reconfigurable antennas at 40 GHz [1]. In these low frequency designs high resistivity Si wafers have been used as the photoconducting elements. Due to the nanoscale size of IR systems patterned a-Si:H thin films must be used for photoconducting elements, and a higher carrier concentration is required for a contrast in permittivity. The generated electron-hole pairs form a pseudo-metallic plasma with behavior described by the Drude model. Eq. 1 gives the permittivity of the photoconductive semiconductor as the difference between the dark permittivity εL(ω) and a photo-plasma term
ε r (ω ) = ε L (ω ) −
ω p2
i × 1 + 1 ω 2 − 2 ωτ
(1)
τ
where ωp is the plasma frequency, ω is the IR radiation frequency, and τ is the electronic relaxation time [2]. The plasma frequency depends upon the photo-carrier density in equation 2
q2
nilum (2) ε0 m* where q is the charge on the electron, ε0 is the permittivity of free space, and m* is the effective mass of the photo-carrier, and nilum is the photo-carrier density as a function of power from the thermal pump source. nilum should be greater than 1020 cm-3 for significant IR photoconductivity to occur.
ωp2 =
×
THEORY The photo-carrier den
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