Hard X-rays can be used to investigate electronic and optoelectronic devices in more or less realistic operational conditions, and modern X-ray optics reach the relevant length scales for nanoelectronic devices. We have developed methods to combine electrical measurements with nanofocused X-rays, where the X-rays are used both as pump and probe of semiconductor nanowires.
In the first experiment, local pumping with X-rays was used to investigate carrier collection properties of single nanowire solar cells. Single InP and InGaP nanowires with p-i-n junctions were contacted electrically, and locally excited with a 50-nm diameter X-ray beam. The absorbed X-rays excite electrons and holes in the semiconductor, which generate a current when they are separated by the internal electric field. This X-ray beam induced current (XBIC) was investigated as function of position, flux and applied electric bias. The subsequent analysis revealed the depletion region and the minority carrier diffusion lengths, as function of excitation and external bias. Furthermore, the secondary X-ray fluorescence was used to quantitatively map the p-doping.
In the second project, nanofocused hard X-rays were used to quantitatively probe both strain and bending in a single nanowire device under electric bias. Scanning X-ray diffraction was performed with 100 nm real-space resolution along the nanowire axis, also within the metal contacts. The device was then exposed to increasing bias voltages until breakdown, while simultaneously measuring the electrical current and performing scanning X-ray Bragg diffraction. The 3D shape of the nanowire was reconstructed from the XRD data. We observed that the nanowire changed shape, correlated with a reduction in electrical conductance.