Electron holography of optoelectronic materials under light excitation – ECHOMELO

The electric and magnetic fields in nanostructured semiconductor materials are affected by light absorption. The absorbed photons create charge carriers which induce changes in the local internal fields. For example, in III-N heterostructures used for light emitting diodes (LEDs), the internal electric field resulting from the quantum stark effect is screened by light absorption. In Cu(In,Ga)NSe solar cells (CIGS) light induces an electric field across the hetero-junction. These effects have been extensively studied on a macroscopic or mesoscopic scale. However, the behavior of a nanostructured material under light excitation is ruled by variation of the structural properties, such as defect density, strain, chemical inhomogeneity, low dimensionality (confinement) or interfaces. Therefore, studying the effect of light at the atomic scale is fundamental to understand, characterize and optimize their optical response. Imaging light absorption at the atomic scale will deeply increase our knowledge of optically active nanostructures.
Due to their picometer wavelength, fast electron imaging is not limited by diffraction and can be used to observe atomic structures. Different methods based on electron excitation are used to study optically active nanostructures. For example, cathodoluminescence (CL) spectroscopy monitors the optical response of these materials at the nanometer scale. It was used to study the role of defects14 and polarity15 in III-N nanowire luminescence, as well as the effect of grain boundaries for carrier diffusion in CIGS materials. However, CL spectroscopy is as of yet unable to give a measurement of fundamental optical properties, such as quantum efficiency, non-linear carrier dynamics and absorption spectrum in a spatially resolved fashion. All these properties have been extensively studied with photoluminescence (PL) spectroscopy above the diffraction limit. Electron holography is commonly used to image the electric and magnetic fields of nanoobjects and several studies have been performed on III-N nanowires and solar cells, but until now it was not used to study the effect of optical excitation at the atomic scale.
In ECHOMELO, we propose to determine the link between the atomic structure and light absorption efficiency at the nanoscale which is one of the key parameters for many semiconductors nanostructures. We aim to study the light absorption at the nanoscale, combining under light excitation electron holography imaging and luminescence spectroscopy (CL and PL). We will thus develop a light injection system on the sample into a transmission electron microscope designed for electron holography. Two types of materials will be investigated, III-N nanowires and CIGS solar cells. Each representing a class of materials that will greatly benefit from the imaging of light absorption at the nanoscale. Indeed, III-N nanowires are known for their strong internal electric field 2 MV/cm (i.e. 100 mV per atomic layer) as well as the sensitivity of this field to carriers. In the case of light excitation of non-contacted solar cells, the local short-circuit voltage can be derived from the accumulation of excited carriers.