Light-activated telegraph noise in AlGaAs tunnel barriers: Optical probing of a single defect

We observe light-activated switching between discrete resistance states in GaAs/AlGaAs single-barrier tunnel structures. This switching is caused by a change in charge state of a single hole trap which modulates the current through a small region of low barrier height generated by a crystalline dislocation propagating through the barrier. Each switching event corresponds to the capture or subsequent release of a single photogenerated hole. We are therefore able to probe this single charge trap by optically inducing changes in the occupancy of its two charge states.