Wavelength selective Photoluminescence (PL) imaging for probing kinetic energy dependence of carrier diffusion in a GaAs epilayer*

PL imaging is often used for investigating carrier diffusion in semiconductors. The measurement was typically carried out by measuring at one wavelength (e.g., at the band gap) or simply the whole emission band. At room temperature in a semiconductor like GaAs, the band-to-band PL emission, including the contributions of carriers of different kinetic energies, may occur in a spectral range over 200 meV, vastly exceeding the average thermal energy of about 26 meV. It is not a priori knowledge whether or not the carriers with different kinetic energies will diffuse together, i.e., with the same diffusion length. To investigate the potential dependence of the carrier diffusion on the carrier kinetic energy, we performed wavelength selective PL imaging on a high quality GaAs double hetero-structure in a spectral range from about 70 meV above to 50 meV below the bandgap, extracting the carrier diffusion lengths at different PL wavelengths by fitting the imaging data to a theoretical model. The results of this GaAs sample show that the locally generated carriers of different kinetic energies do mostly diffuse together, maintaining the same thermal distribution throughout the diffusion process.