Offset in the dark current characteristics of photovoltaic double barrier quantum well infrared photodetectors

The double barrier quantum well (DBQW) structure based on AlGaAs/AlAs/GaAs represents a promising design for infrared detectors operating in the 3-5 μm range. Nevertheless, this structure is affected by some unexpected and intriguing features which must be taken into account for its practical operation, i.e. the remarkable photovoltaic (PV) effect observed in detectors doped in the QW and the appearance of anomalies in the dark current (Id) characteristics at T < 60 K. In this work, we focus on the study of the anomalies in Id, in particular the presence of a non-zero current (offset) at zero bias. We report that the offsets are time independent, with negligible hysteresis effects. Additionally we find a closely link between the offsets and the unintentional PV response. Although so far we do not have any clear explanation for the offset, we have found that its appearance may relate to a change in the preferential escape direction of electrons in the dark, which is related to a change from tunnelling-assisted thermionic emission to tunnelling emission. This interpretation is based on the experimental result that the temperature at which the offset first appears agrees with the transition temperature in the mechanisms controlling Id and with the temperature where a reversal in the electron flux direction is found. These results are reproducibly found in all the DBQW samples we have studied.