A new simple technique for the determination of the diffusion length in photoconductive insulators is presented. A steady-state photocarrier grating is created by two interfering laser beams, and the magnitude of the secondary photocurrent perpendicular to the grating fringes is measured. The measurement is then repeated when the two beams are incoherent. From a determination of the two photocurrents as a function of grating period the diffusion length of the photocarriers can be obtained. The method can yield accurate results to 5% of the laser wavelength.
The theory underlying the steady-state photocarrier grating technique is presented, including the effect of surface recombination. Experimental results for amorphous hydrogenated silicon and semi-insulating GaAs prove that diffusion lengths ranging from 200 Å to 10 μm can be measured with an accuracy of better than 5%.
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