Direct Determination of Minority Carrier Diffusion Lengths at Axial GaAs Nanowire p–n Junctions

Axial GaAs nanowire p–n diodes, possibly one of the core elements of future nanowire solar cells and light emitters, were grown via the Au-assisted vapor–liquid–solid mode, contacted by electron beam lithography, and investigated using electron beam induced current measurements. The minority carrier diffusion lengths and dynamics of both, electrons and holes, were determined directly at the vicinity of the p–n junction. The generated photocurrent shows an exponential decay on both sides of the junction and the extracted diffusion lengths are about 1 order of magnitude lower compared to bulk material due to surface recombination. Moreover, the observed strong diameter-dependence is well in line with the surface-to-volume ratio of semiconductor nanowires. Estimating the surface recombination velocities clearly indicates a nonabrupt p–n junction, which is in essential agreement with the model of delayed dopant incorporation in the Au-assisted vapor–liquid–solid mechanism. Surface passivation using ammonium sulfide effectively reduces the surface recombination and thus leads to higher minority carrier diffusion lengths

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

Far-Field Imaging for Direct Visualization of Light Interferences in GaAs Nanowires

The optical and electrical characterization of nanostructures is crucial for all applications in nanophotonics. Particularly important is the knowledge of the optical near-field distribution for the design of future photonic devices. A common method to determine optical near-fields is scanning near-field optical microscopy (SNOM) which is slow and might distort the near-field. Here, we present a technique that permits sensing indirectly the infrared near-field in GaAs nanowires via its second-harmonic generated (SHG) signal utilizing a nonscanning far-field microscope. Using an incident light of 820 nm and the very short mean free path (16 nm) of the SHG signal in GaAs, we demonstrate a fast surface sensitive imaging technique without using a SNOM. We observe periodic intensity patterns in untapered and tapered GaAs nanowires that are attributed to the fundamental mode of a guided wave modulating the Mie-scattered incident light. The periodicity of the interferences permits to accurately determine the nanowires’ radii by just using optical microscopy, i.e., without requiring electron microscopy