Differential absorption spectroscopy of charge distributions in double-barrier tunnel structures

Differential absorption spectroscopy is used to determine the charge density in ${\mathrm{In}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/${\mathrm{In}}_{\mathrm{x}}$ ${\mathrm{Al}}_{1\mathrm{-}\mathrm{x}}$As double-barrier tunneling structures. We show the formation of accumulation and depletion regions on either side of the double barrier, and measure with an absolute calibration the accumulated charge in the 45-Å quantum-well region of the structures. Peak charge densities ≊5.4×${10}^{10}$ ${\mathrm{cm}}^{\mathrm{-}1}$ and ≊3.9×${10}^{10}$ ${\mathrm{cm}}^{\mathrm{-}2}$ are measured for barrier thicknesses of 70 and 56 Å, respectively. The corresponding calculated transit times are 70 and 20 ps.