Characterization of semiconductors from photoconductivity techniques: Photocarrier grating techniques
Résumé
In this chapter we detail photoconductivity techniques based on the illumination of a sample in a coplanar electrode configuration with a light intensity periodically modulated in space. Such a modulation is obtained from the interferences of two laser beams that illuminate the space between the electrodes. This illumination can be constant in time as in the steady-state photocarrier grating (SSPG) proposed by D. Ritter et al., modulated in time as in the modulated photocarrier grating (MPG) detailed by K. Hattori et al., moving at a constant speed perpendicularly to the electrodes in the moving grating technique (MGT) imagined by U. Haken et al., or oscillating perpendicularly to the electrodes as in the oscillating photocarrier grating (OPG) technique developed by F. Ventosinos et al.. All these experiments are practically and theoretically detailed. For each of them we present the experimental setup and the theoretical background from which transport parameters, such as the ambipolar diffusion length, can be extracted. We also show that, under some assumptions, information on the lifetimes and density of states (DOS) can be obtained, and, for some of them, a DOS spectroscopy can be achieved if the experiments are performed at various temperatures and/or generation rates. It is also shown that some of these techniques are complementary to other techniques, like the steady-state photoconductivity (SSPC) or the modulated photocurrent (MPC) experiments, the combination of these techniques bringing information that could not be obtained from a single one. All these techniques and their expected results are illustrated by means of numerical calculations as well as some experimental results.