Electrical and Structural characterization of single core-shell semiconductor nanowires

• Applicant: Professor Dr. Ullrich Pietsch
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 413135326

The nanowire approach offers the possibility to form hetero-structures between highly lattice-mismatched systems, because the large surface to volume ratio allows relieving strain more effectively. Additionally, it offers the possibility to combine III V semiconductor materials with silicon which is a large step towards modern semiconductor technology. Understanding and control of electrical properties at the individual NW level is necessary for their integration into devices. We investigate GaAs/(In,Ga)As/GaAs and nip-doped (Ga,In)P/(Al,Ga)P radial core-shell NW heterostructures fabricated by MBE and MOCVD onto pre-patternede silicon (111) substrates for next generation optoelectronic devices. The performance of such structures is strongly related to crystallographic perfection which has significant impact on charge carrier mobility. In a core-shell system there might be several channels for charge carrier transport, such as the shell with lower band gap compared to the core, or along the oxidized surface of the outer shell. Additionally, it is suggested that structural defects like stacking-faults have strong impact on transport properties. We aim to understand the electrical properties of such radial core-shell NWs and their correlation to the particular defect structure. Conductivity measurements will be perfromed at single NWs in their as-grown geometry on the substrate, by means of FIB/SEM and low-temperature STM or AFM systems and by 4ppt STM using either tngsten nano-manipulator probe installed inside the FIB/SEM or the sharp metallic tip of the conductive AFM/STM. The I-V characteristics of selected core shell NWs will then be correlated to the detailed structural properties of the same nano objects: The phase balance betweeen wurtzite (WZ) and zincblende (ZB) units and structural defects within a NW will be determined by x-ray nano-diffraction and coherent diffraction imaging (CXDI) using synchrotron radiation at 3rd generation facilities PETRA III and the first 4th generation synchrotrin Max IV. Along with the ex-situ characterisation on single NWs,even in-situ X-ray diffrcation studies during simulataneous I-V measurement is planned. This in-situ experiments will be realized both at PETRA III and Max IV using a portable combined AFM/STM instrument, which has been designed specifically for the X-ray nano-focus beamline environment. Such in-situ studies will not only provide direct correlation of structural and electrical properties of individual NWs, but furthermore investigate the influence of local stress (applied by the AFM tip) and Joule´s heating(due to current flow) on NW structure and conductivity.

• DFG Programme: Research Grants
• International Connection: Sweden
• Co-Investigator: Professor Dr. Rainer Timm