Local Doping Profiles for Height-Selective Emitters Determined by Scanning Spreading Resistance Microscopy (SSRM)

In order to confirm the existence of a height-selective emitter, we compared local doping profiles of such emitters with profiles of a standard homogeneous emitter. The concept of a height-selective emitter is based on the classical selective emitter but in a smaller scale. The pyramid tips are highly doped. The sides and valleys are lowly doped. The comparison of the doping profiles was addressed by using a high-spatial-resolution analysis method: scanning spreading resistance microscopy. The measurement was performed on fully produced cells with a height-selective emitter and a standard homogeneous emitter, both with random pyramid textured surfaces. We prepared samples from these cells and investigated the cross sections. A representative pyramid of each type of emitter was selected. We found that for height-selective emitters the surface concentration can strongly vary depending on the measured position of the selected pyramid. The tip of the pyramid is heavily doped, while the bottom is lowly doped. For standard cells with a homogeneous emitter, the doping profiles do not differ dramatically as for the sample with a height-selective emitter. We calculated the local sheet resistance by using the measured local emitter profiles and a doping-dependent mobility model for phosphorus-doped silicon.