Characterization of AlxGa1−xAs/GaAs heterojunction bipolar transistor structures using cross-sectional scanning force microscopy

We have characterized base-layer width and dopant distributions on cleaved cross-sections of ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{As/GaAs}$ heterojunction bipolar transistor (HBT) structures using a variation of electrostatic force microscopy. The contrast observed is sensitive to the local dopant concentration through variations in the depletion layer depth extending into the sample surface, and enables delineation of individual device regions within the epitaxial layer structure with nanoscale spatial resolution. In two epitaxially grown HBT structures, one with 50 nm base width and the other with 120 nm base width, we are able to delineate clearly the emitter, base, collector, and subcollector regions, and to distinguish regions within the collector differing in dopant concentration by a factor of two. We have also distinguished clearly between the base widths in these samples and have precisely measured the difference to be $\text{63±3\hspace{0.05em}}\mathrm{nm},$ in excellent agreement with the nominal difference of $\text{70±7\hspace{0.05em}}\mathrm{nm}.$