Abstract

Exciton saturation adversely limits the switching speed of GaAs/Al_xGa_1-xAs quantum well electroabsorptive optical modulators. We have studied the dependence of the exciton saturation intensity I_sx on the strength of the applied field and the height and width of the AlGaAs barriers. Five PIN photodiodes were grown by molecular beam epitaxy with the quantum wells comprising the intrinsic region. The well width was kept nominally constant, but x ranged from 0.2 to 0.4 and the barrier thickness L_b varied between 31 and 95 Å. We found that I_sx increases with applied field, and also depends strongly on x and L_b.For L_b = 65 Å at zero applied field, I_sx increased from 1.3 to 43 kW cm⁻² on reducing x from 0.4 to 0.2. Similarly, for x = 0.3 at zero bias, I_sx increased from 3.1 to 18 kW cm⁻² on reducing L_b from 95 to 31 Å. These results can be explained in terms of a reduction of the carrier lifetime caused by barrier and field dependent tunneling rates. The field dependence of the carrier escape time for the sample with x = 0.31 and L_b = 31 Å was measured by the time-resolved electroabsorption technique.^1,2 The measured escape time is ~100 ps at fields below 30 kV cm⁻¹ and decreases to ~10 ps at fields above 60 kV cm⁻¹.

© 1989 Optical Society of America