Application of microwave scanning probes to photovoltaic materials

We demonstrate that near-field scanning microwave microscopy (NSMM) can be used to detect photoresponse in photovoltaic materials with potential for submicrometer lateral resolution. In this approach, a radio-frequency scanning tunneling microscopy (RF-STM) tip is coupled to a resonant microwave cavity with a resonance near 4.5 GHz. The presence of additional mobile carriers in the vicinity of the probe tip changes the load on the cavity. These changes are quantified through the shifts in both the resonance frequency and quality factor Q. The changes in conductivity under illumination with blue light are easily detected for Cu(In,Ga)Se₂ and GaAs solar cell test material.