Spatially resolved analysis of dopant concentration in axial GaAs NW pn-contacts

The efficiency of novel opto-electronic devices e.g. solar cells crucially depends on controlling the doping levels in the device. Decreasing the size of the structure by employing nanowires is attractive for several reasons, for instance reduced material costs, but makes the characterization more challenging. There is still a lack of a precise and simultaneously rapid characterization technique for doping concentrations in nanostructures. In this work axial pn-junctions of GaAs-NWs were investigated electrically, by the utilization of a multi-tip scanning tunneling microscope (MT-STM) as a four point prober. Additionally, these NWs were probed optically at room-temperature by photoluminescence (PL) and cathodoluminescence (CL) microscopy spectroscopy. With both approaches we were able to achieve complementary information of the nanostructure and have developed a complete picture. By using a transport model (for MT-STM) and semi-empirical equations (for PL & CL) which take the Burstein-Moss shift and the band gap narrowing into account, the doping concentrations of the NWs were evaluated and compared. An axial variation in the electron concentration of the n-doped NW-top was resolved as well as a constant hole concentration in the p-doped NW-base. We demonstrate that the combination of both techniques will be necessary to improve the development of a suitable and precise model for the estimation of doping profiles by luminescence data.