Defect levels and interface space charge area responsible for negative photovoltage component in InAs/GaAs quantum dot photodetector structure

InAs/GaAs quantum dot photodetector photoresponse spectra consist of quantum dot, wetting layer and GaAs components, however, they frequently contain features attributed to defect levels. In this study we focus on the origin of an unwanted negative photovoltage component in order to find a design allowing to eliminate these undesirable effects. Photoelectric properties of vertical InAs/GaAs quantum dot structures grown on semi-insulating (si) and n+-GaAs wafers are analyzed. It is found that the photoresponse drop above 1.37 eV in the photovoltage spectrum, observed with si-GaAs substrate, is originated from shallow defect levels in the unintentional space-charge area created at the interface between the substrate and n+-GaAs buffer. The intrinsic field in that area is opposite to the top pn junction, according to modelling calculations, and it reduces the photoelectric response. Photoelectric characterization of the heterostructure grown on n+-GaAs indicates that the utilization of n+-GaAs substrate helps to avoid this band bending and to eliminate its negative effects, increasing the total photoresponse.