Photoluminescence study of vertical transport in ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$/Si heterostructures

We have observed band-edge photoluminescence from excitons in the center ${\mathrm{Si}}_{0.75}$${\mathrm{Ge}}_{0.25}$ well of a triple-quantum-well structure, due to tunneling through the Si barriers from the two outer ${\mathrm{Si}}_{0.87}$${\mathrm{Ge}}_{0.13}$ wells. Most of the photogenerated carriers originated in the Si substrate and cap, and migrated to the nearest ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ wells (i.e., the outer wells), where in the absence of tunneling they became trapped. For Si barrier widths of 9 nm or less, center-well luminescence was observed, indicating the occurrence of tunneling. Time-decay measurements of the outer-well luminescence gave an estimated tunneling time of ≃325±100 ns for 9-nm barriers, at which point luminescence was seen from both the center and outer wells. We also observed thermal hopping of carriers from the outer wells over the Si barriers and into the center well at high temperatures.