Comparison between the Oscillating Photocarrier Grating and the Moving Grating Techniques

VENTOSINOS, F.; LONGEAUD, C.; SCHMIDT, J. A.

Nara

Conferencia; 24th International Conference on Amorphous and Microcrystalline Semiconductors; 2011

ICANS 24 Science and Technology

Photocarrier grating techniques are based on making interferences between two coherent light beams to form a periodic pattern of light intensity. Areas of high and low light level result in high and low carrier concentrations, respectively, creating an internal electric field. In the moving grating technique (MGT), proposed by Haken et al.,1 one introduces a small shift between the beam frequencies by means of acousto-optic modulators, causing the grating to move at a constant velocity along the sample surface. The measured experimental quantity is a dc short circuit current. The oscillating photocarrier grating (OPG) technique, proposed by the authors,2 is based on controlling the phase of one of the beams by using of an electro-optic modulator (EOM). When the signal applied to the EOM is a triangular wave function, the resulting intensity grating moves with a constant velocity in one direction for the first half of each period (therefore for half a grating period) and then moves in the opposite direction for the second half of each period. In this way we can think of OPG as an ac version of MGT, allowing us to use synchronous detection by means of a lock-in amplifier. The purpose of this work is to verify the equivalence of both techniques, and to show how complementary they can be. The reduction in the noise level achieved by OPG allows the experiment to be performed at low levels of illumination or in samples that are not very photoconductive. The drawback of OPG is the necessity of a current amplifier, which always imposes some bandwidth limitations. On the other hand, MGT only requires the measurement of dc quantities, allowing the experiment to be performed at high grating velocities. We performed MGT and OPG measurements on the same hydrogenated amorphous silicon sample, for different levels of illumination. As can be seen in Fig. 1, both techniques agree when measured for the same photon flux, being MGT better suited for the highest photon fluxes and OPG for the lowest ones. We also show that, depending on the generation rate, the maximum of the curves corresponds to one characteristic time of the sample, the dielectric relaxation time Tdiel or the small signal lifetime T'. The combination of both techniques permits to measure accurately both times over a wide range of conditions. Furthermore, we demonstrate that the knowledge of T' can be used to achieve a density of states spectroscopy, since  T' contains information of the density of trapped carriers. References 1) U. Haken, M. Hundhausen and L. Ley, Appl. Phys. Lett. 63, 3063 (1993). 2) F. Ventosinos, C. Longeaud, N. Budini and J. A. Schmidt, J. Appl. Phys., submitted.