Laser-Induced Infrared Absorption in Silicon

Abstract

Absorption of intense light, of photon energy greater than the bandgap, produces changes in the optical properties of semiconductors. The induced absorption produced by 1.06‐_μ laser light in high‐resistivity single‐crystal silicon wafers was measured, by use of a monitor flash lamp, at monitor wavelengths λ_m = 1.2 to 2.7 _μ. The Q‐spoiled glass: Nd³⁺ laser produced pulses of typically 25‐nsec duration. The induced absorption increased with monitor wavelength, as expected for a free carrier mechanism. As a function of laser‐pulse‐energy density, the transmission immediately following the laser pulse was of the form T(λ) = exp (−σ_λE), with σ_λ = 1.7×10⁻¹⁷ cm² at λ_m = 2.6 _μ for Si at room temperature. The exponential dependence is in agreement with an analysis of the induced absorption process, and the value of σ_λ is in approximate accord with that calculated from measurements of infrared absorption in doped silicon.