On the effects of a pressure induced amorphous silicon layer on consecutive spreading resistance microscopy scans of doped silicon
Résumé
In this study, the evolution of the Scanning Spreading Resistance Microscopy (SSRM) signal, as a function of the doping level and of the bias voltage applied for consecutive scans, is carefully analyzed for an intimate high force contact between the tip and a silicon staircase test-structure. Within our experimental set-up, the SSRM technique, performed in ambient air, is used to collect the overall current flowing through the tip-sample Schottky nanocontact, taking into account the local spreading resistance contribution and the current flowing along the surface around the conductive tip. Whereas no significant bias voltage sign dependence is measured in n-type epitaxially doped silicon, a large increase of the overall resistance is demonstrated in lightly doped p-type silicon for a negatively biased sample. In this regime, the Schottky nanocontact blocks the spreading current, and a surface current of minority carriers can be observed. Moreover, micro-Raman analysis shows that the topside silicon layer, which remains after SSRM scanning is amorphous and therefore that SSRM scanning promotes silicon surface amorphization around the tip. Hence, the surface leakage current is suppressed, which impacts the total measured current, especially on lightly doped p-type silicon where it may play a significant role. By using Peak Force Tapping Quantitative Nano-Mechanical (PF-QNM) mode, the micro-structural, nano-mechanical properties are determined for n- and p-type epitaxially doped silicon, after successive SSRM scans. Finally, two-dimensional axisymmetric device simulations have been performed and confirm the impact of the minority carrier induced current on pristine samples. Thus, differential measurement between the first and the second SSRM scan allow the surface current to be probed.