INVESTIGADORES
SCHMIDT javier Alejandro
congresos y reuniones científicas
Título:
Theoretical analysis and experimental results on the modulated photocarrier grating technique
Autor/es:
SCHMIDT, J. A.; LONGEAUD, C.; VENTOSINOS, F.; GUEUNIER-FARRET, M. E.
Lugar:
Utrecht, Holanda
Reunión:
Conferencia; 23rd International Conference on Amorphous and Nanocrystalline Semiconductors; 2009
Institución organizadora:
Universidad de Uthecht
Resumen:
   The modulated photocarrier grating (MPG) technique [1], proposed in 1993 as an improvement of the steady-state photocarrier grating (SSPG) technique, involves the generation of photocarriers by a temporally oscillating interference pattern. The measurement is performed in the frequency domain, recording the modulus of the resulting photocurrent and its phase shift referred to the illumination. In this sense, the method is closely related to the modulated photocurrent (MPC) technique.    The theoretical analysis presented in the original publication [1] defined phenomenological parameters like the small signal trap-controlled mobilities and recombination lifetimes, working with the total carrier concentrations (mobile plus trapped). In that analysis, the density of states (DOS) does not appear in the equations that describe the MPG experiment.    In this work, we present a complete theoretical analysis of the MPG method, based on the general transport equations for mobile carriers and on the Taylor and Simmons’ statistics for the recombination terms. The simultaneous solution of the transport differential equations and of Poisson’s equation allows us to obtain the general solution of the problem under small modulation conditions. Therefore, we can numerically calculate the response of a “sample” defined through the DOS and the extended states mobilities. Our calculations show that the response of the sample will be not only a photocurrent modulated with the frequency f of the illumination, but also a response in dc and with frequency 2f. Since we can calculate the relative intensities of these photocurrents, we have a means to put to test our calculations.    For that purpose we have implemented experimentally the MPG technique, using a He-Ne laser and an electro-optic modulator, and we have applied it to a hydrogenated amorphous silicon sample. We measure the modulus and phase shift of the resulting photocurrent as a function of the modulation frequency, and we compare them with the results of our numerical calculations. We verify the existence of a response at frequency 2f, and we measure the relative intensity of this signal. We also calculate the diffusion lengths for majority and minority carriers in this sample. We explore the validity of the conditions of ambipolar transport, performing numerical calculations and experiments. We show that the MPG method provides complete information about the transport properties of a photoconducting material.      [1] K. Hattori, Y. Koji, S. Fukuda, W. Ma, H. Okamoto, and Y. Hamakawa, J. Appl. Phys. 73, 3846 (1993).