In recent years there has been significant effort in employing quantum dot (QD) structures in far-infrared and infrared photo-detectors. Besides the electronic properties of the QDs, the performance of QD photo-detectors is directly affected by the doping level as well as the scheme of doping [1,2]. Further, the introduction of dopants during growth may also affect the dot density and size distribution. To get a good understanding of the electrical properties of the doped QD structures, techniques that provide electrical information, for instance doping, with nano-scale spatial resolution and at the same time being capable of imaging the QDs and/or the QD-layers are required. Scanning spreading resistance microscopy (SSRM) is one such method that has the potential to meet the above demands. Recently, the ability of SSRM to detect and profile carriers in quantum wells as narrow as 5 -10 nm has been demonstrated [3].
In this work, cross-sectional SSRM is used to investigate Si-doped In(Ga)As/GaAs QD photo-detector structures consisting of five stacked QD-layers and with different doping schemes. The structure is either nominally undoped, or with only the GaAs barrier doped or with only the QD-layer doped. Apart from delineating the different layers, it is demonstrated that SSRM is capable of detecting and resolving QDs. The undoped sample provided the best resolution and the measured average separation and diameter of the QDs is in good agreement with independently determined dot size and density obtained under similar growth conditions. In contrast, the QD layers in the doped samples appear continuously broadened in the SSRM images. The differences in the image contrast between the different samples are explained taking into account dopant-induced changes on the QD density, size and homogeneity and the memory effect of Si doping during growth.
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