Excess Carriers Induced in Indium Antimonide with a Carbon-Dioxide Laser

Abstract

Production of high excess-carrier densities, i.e., plasmas, in $n\ensuremath{-}\mathrm{InSb}$ irradiated with C${\mathrm{O}}_{2}$-laser light is accomplished by two-photon excitation of electron-hole pairs. The properties of such plasmas are found by photoconductivity, infrared-absorption, and recombination-radiation measurements, at liquid-${\mathrm{N}}_{2}$ and -He temperatures, with both 10.6- and 9.6-\ensuremath{\mu}m irradiation wavelengths. Maximum excess-carrier densities obtained are \ensuremath{\sim} ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ at 77\ifmmode^\circ\else\textdegree\fi{}K and \ensuremath{\sim} 5 \ifmmode\times\else\texttimes\fi{} ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ at 4\ifmmode^\circ\else\textdegree\fi{}K. In samples with thickness \ensuremath{\le} 0.3 mm, uniform carrier distributions are obtained throughout the bulk of the samples. This feature plus the absence of applied electric fields during the plasma-generation process make these plasmas unique. In samples with thicknesses \ensuremath{\ge} 0.3 mm, free-hole absorption of the C${\mathrm{O}}_{2}$-laser radiation causes the plasma density to be reduced in the far end of the sample. Absorption measurements yield a value for the free-hole-absorption cross section, $q \ensuremath{\approx}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$, which is in good agreement with the cross section measured for equilibrium holes by Kurnick and Powell. Shaping of the laser pulse during passage through the sample is observed and attributed to free-hole absorption which depends nonlinearly on the laser intensity.