Correlative nm-Scale Nonuniformity of Active Charge Carriers and Electrical Potential along both the Plane-View and Depth Directions in Group-V-Doped CdTe Thin Films

We report nanometer-scale imaging on inhomogeneous distributions of active carrier and electrical potential in an As-doped CdTe film along both plane-view and film-depth directions. Despite Se grading, the scanning capacitance microscopy imaging does not show a clear variation of carrier concentration along the depth of the film. Instead, we observe carrier concentration variations of about 1 order of magnitude (high 10
¹⁵
to low 10
¹⁷
/cm
³
), with inhomogeneous spatial regions ranging from a few hundred nm to a few μm. This nonuniformity is distributed randomly in both the film lateral and vertical directions, independent of grain structure and grain boundaries (GBs). We further mapped the surface potential using Kelvin probe force microscopy. Higher potential was found on GBs, illustrating positive GB charging but not GB-specific carrier concentration. The results indicate that this suite of techniques can identify nonuniform carrier concentration and potential fluctuations that can contribute to open-circuit voltage deficits in group-V-doped CdTe devices.