CURRENT NOISE SPECTROSCOPY OF DEEP ENERGY-LEVELS IN PHOTOCONDUCTORS

According to a barrier photoconductance theory recently developed, two current noise sources can be envisaged, related respectively to the f luctuation of the barrier height (photoinduced noise component) and to the trapping-detrapping processes in shallow states within the photoc onducting material (g-r and 1/f noise component). It is shown that the first noise component, which can be separated by the second one, give s information on the energy gap and on the photoionization cross secti on of the deep energy levels of the photoconducting material. In parti cular for thin films it will be shown that the photoionization cross s ection of the deep energy levels varies inversely to the total number of photons impinging on the photoconductor and proportionally to the n oise power spectral density as the photon energy changes. Such relatio nship suggests that the wavelength dependence of the noise power spect rum provides the correction to be taken into account if the constant p hotocurrent method is used and the variations of the minority carrier lifetime with the photon energy cannot be disregarded. Results concern ing the energy gap and its temperature dependence, in the interval ran ging from the room temperature to 200 K, are reported and compared wit h the results found in the literature for CdS based samples. The photo ionization cross section of the deep centers as a function of the phot on energy, at room temperature, is reported for the same sample. Such quantities have been obtained by measuring the spectral density of the photoinduced noise vs wavelength at constant photoconductance value. As for the other kinds of noise spectroscopy, the main advantage of th e present method is to work out in the operative condition of the semi conductor device. (C) 1996 American Institute of Physics.