The eXciton Franz Keldysh effect: Spectrally measuring electric fields using the electron-hole interaction in wide band gap materials
Roberto C. Myers
Professor of Materials Science, Electrical Engineering, and Physics, The Ohio State University
Abstract: Wide band gap materials can sustain large electric fields up to a point. When the field is large enough, electrons and holes and tunnel between the typically well separated conduction and valence band states causing an avalanche of current, which destroys the device. This dielectric breakdown occurs at local positions in devices where the field lines are concentrated. Unfortunately, a voltmeter cannot be used to find these field hotspots since it measures only the potential difference across two electrodes (ΔV) and does not provide information on the slope of the potential (band bending), i.e. the E-field (∇V = F). Given the extraordinary sophistication and success of device physics and simulation it is remarkable that the basic electrostatics within such devices are based on modeling rather than direct measurements. We found that the photocurrent response of a wide band gap semiconductor to photons with energies below the band gap provides a spectral signature of the local electric field due to excitons (coupled electron and hole states). Typically ignored in optoelectronic devices, excitons provide a pathway to measure the electric field through the spectral shift of their absorption peak with field, called the exciton Franz-Keldysh effect, which was originally pointed out more than 50 years ago. We demonstrate such a field detection in an ultra-wide band gap material, β-Ga2O3, which exhibits 2D-like exciton states similar to 2D semiconductors, but with the added tunability associated with its high dielectric breakdown field limit. A variety of field-dependent effects including energy level shifting (exciton Stark effect) and field-dependent ionization are quantitatively fit to extract previously unknown material properties. This exciton-based field detection method provides a pathway to develop electric field mapping microscopy, and could be implemented in other materials with strong exciton binding, such as 2D semiconductors, where exciton dynamics also dominate the optoelectronic properties.
Host: Walter Lambrecht