**NEW** Characterization of doping in nanowires by off-axis electron holography

Nanowires (NWs) are structures with length typically on the order of microns and diameter on the order of tens to hundreds of nanometers. Group III-V semiconductor NWs are investigated for applications such as solar cells, photodetectors and sensors. GaAs NWs grown on a Si substrate from a Ga droplet catalyst are shown in Figure 1.

Figure 1. (a) SEM image of GaAs NWs vertically grown on a Si substrate and (b) TEM image of GaAs NWs which have been transferred on a TEM grid (Cu/Lacey carbon).

For solar cell NWs, the control of doping and electrostatic potential distribution is important and challenging. The doping can be assessed by off-axis electron holography in a transmission electron microscope. This is one of the most powerful methods for dopant assessment, since it enables to measure a spatially-resolved phase contrast from a specimen due to its electrostatic potential. A schematic of the off-axis electron holography technique is shown in Figure 2 and it can be performed at the FEI Analytical Titan 80-300ST TEM, located at DTU Cen.

An example of the electrostatic potential distribution measured by electron holography across a GaP NW is reported in Figure 3. The measured built-in potential from holography can be correlated with the forward turn-on voltage in p-n junction diodes, or with the open-circuit voltage in photovoltaic devices. Cross-sectional specimens for off-axis electron holography are prepared perpendicular to the growth direction of the NWs using focused ion beam (FIB) milling, which is a techniques available at the FEI Helios dual beam, located at DTU Cen.

Figure 2. Configuration of off-axis electron holography.

Figure 3. The electrostatic potential distribution across the core-shell p-n junction of a semiconducting GaP NW along the axial (a) and radial (b) direction measured by electron holography.

Learning objectives

  • Learn to use transmission electron microscopy (SEM and TEM), electron holography and FIB.
  • Understand the importance of nanowires and doping in the photovoltaics industry.
  • Quantitative analysis of electron microscopy images and holograms.

Open to: Bachelor, Master, PhD, Special Project students and 3-weeks courses


Elisabetta Maria Fiordaliso
DTU Nanolab