In a wide range of semiconductor applications, the characterization of the local conductivity is crucial for the final device performance. To fulfill this goal, four-point probe measurements are relevant, due to the low requirements on sample preparation. But with the ongoing reduction in size of the devices, standard probes can become very invasive and  the dominant source of scattering at the nanoscale. Therefore, the use of probes operated with a scanning tunneling microscope (STM) has considerable potential, as it does not need any extra electrodes on the specimen and enables  arbitrary arrangements of probe electrodes with the highest spatial resolution. Based on the atomic precision of motioncontrol of STM and, real-time and fast imaging capability of scanning electron microscopy (SEM), we will show  how this instrument can be used to characterize semiconductor materials and gives insight into the contribution of surface effects in their transport properties.

Carrier multiplication, the generation of an electron-hole pair from a hot electron, is of prime importance to master amplification phenomena in materials and multiply the output of opto-electronic devices and solar cells. As a result, the talk will first discuss the applicability of this technique for measuring the quantumyield of impact ionization. We will describe  how electron and holes can be respectively measured, enabling the direct and unambiguous determination of the quantum yield. Thanks to the scanning ability of the STM tips, spatial mapping of the carrier multiplication efficiency will be achieved, giving access to the combined effect of impact ionization, carrier diffusion and recombination in Si p-njunctions at the nanoscale.

As the excitation of materials with the incident electrons of a SEM can stimulate a wealth of reactions that are known to  modify the material properties, this talk will then discuss how electronic irradiations affect the transport properties of single semiconductor nanowires and nanocrystals. Upon irradiation, these nanostructures show a significant and reproducible increase of their conductivity, effect that will be explained by discriminating the surface and bulk contributions to the  electrical transport. Solutions to improve the tolerance of semiconductor nanostructures to irradiations will be finally suggested.