Shot Noise in Coherently Coupled GaAs-AlGaAs Double-Well Resonant-Tunneling Diodes

We have found experimentally that, regarding shot-noise, there is no difference between coherent and sequential resonant tunneling, in contrast with calculations that predict that the suppression of noise in a coherent-tunneling process should be larger. Our measurements were done at 4.2 K in GaAs-AlGaAs Double-Well Resonant-Tunneling Diodes (DWRTD) grown by molecular beam epitaxy on n$^+$GaAs substrates. Each AlGaAs barrier at the two ends of each structure was 100 \AA\,, while the central barriers were 60 \AA\, (uncoupled DWRTD), 20 \AA\, and 15 \AA\, (coupled DWRTDs) thick, depending on the sample. In all cases, the two quantum wells between the barriers were 53 \AA\, and 80 \AA\, wide, each well having two confined quantum states. The current-voltage characteristics of the diodes exhibited current peaks associated with voltage-induced alignment of states in adjacent wells. In the coupled DWRTDs each peak split into two, as a result of symmetric and anti- symmetric wavefunctions being extended to both wells. Noise measurements revealed a shot noise power spectrum up to about $50\%$ smaller than that of Poissonian noise $2eI$, regardless of whether the wells were coupled or uncoupled. Our results support those calculations in multiple-barrier structures that predict that the shot-noise reduction should be independent of whether the electronic transport is sequential or coherent.