Comparative analysis on resistance profiling along tapered semiconductor nanowires: multi-tip technique versus transmission line method.

The detection of doping dependent values like contact- and path resistances along nanowires (NWs) still proves to be rather challenging compared to planar structures. Unfortunately, the usually used and well established TLM (transmission line measurement) setup exhibits some drawbacks. Complex preliminary preparation steps and the necessity of ohmic contacts limit the investigation to certain semiconductor materials. The simultaneous determination of contact- and path resistances with an unknown distribution makes an analysis on complex structures like tapered nanowires very challenging. Our approach is the utilization of a multi-tip scanning tunneling microscope (MT-STM) as a four point prober, which allows the investigation of freestanding nanowires with an increased spatial resolution. Here, the used measurement setup allows a local separation of current injection and potential measurement and thus a highly precise determination of path resistances. Tapered p-doped GaAs-NWs were used to compare both techniques. Whereas the evaluation of the axial doping profile by MT-STM was rather simple, correction factors had to be introduced for the TLM measurement to calculate the specific resistances and transfer length. By comparing the results of both methods for the very same NW-sample, the precision and accuracy of MT-STM measurements was demonstrated. We found an agreement, which allows the conclusion that both methods exhibit advantages; however the MT-STM was determined as the more precise setup, which enables additional characterization capabilities, such as surface, temperature or light dependent measurements.

High-speed GaN/GaInN nanowire array light-emitting diode on silicon(111).

The high speed on-off performance of GaN-based light-emitting diodes (LEDs) grown in c-plane direction is limited by long carrier lifetimes caused by spontaneous and piezoelectric polarization. This work demonstrates that this limitation can be overcome by m-planar core-shell InGaN/GaN nanowire LEDs grown on Si(111). Time-resolved electroluminescence studies exhibit 90-10% rise- and fall-times of about 220 ps under GHz electrical excitation. The data underline the potential of these devices for optical data communication in polymer fibers and free space.

Effect of contaminations and surface preparation on the work function of single layer MoS2.

Thinning out MoS2 crystals to atomically thin layers results in the transition from an indirect to a direct bandgap material. This makes single layer MoS2 an exciting new material for electronic devices. In MoS2 devices it has been observed that the choice of materials, in particular for contact and gate, is crucial for their performance. This makes it very important to study the interaction between ultrathin MoS2 layers and materials employed in electronic devices in order to optimize their performance. In this work we used NC-AFM in combination with quantitative KPFM to study the influence of the substrate material and the processing on single layer MoS2 during device fabrication. We find a strong influence of contaminations caused by the processing on the surface potential of MoS2. It is shown that the charge transfer from the substrate is able to change the work function of MoS2 by about 40 meV. Our findings suggest two things. First, the necessity to properly clean devices after processing as contaminations have a great impact on the surface potential. Second, that by choosing appropriate materials the work function can be modified to reduce contact resistance.