The Moving photocarrier Grating Technique (MGT) allows the simultaneous determination of the photocarrier drift mobilities and the small-signal recombination lifetime of photoconductive semiconductors. The technique measures the direct current (DC) induced by a monochromatic illumination consisting of a moving interference pattern superimposed on a uniform background of much higher intensity. A drawback of the technique is the low level of the signal to be measured, which can be masked by the noise at low temperatures or low light intensities. In this work, we propose implementing an alternating current (AC) version of the MGT by chopping the weak beam in the standard configuration. We call this new technique the Chopped Moving photocarrier Grating (CMG). In CMG, the AC signal can be measured with a lock-in amplifier for electrical noise removal. In this way, the signal-to-noise ratio can be increased compared to the standard DC technique. Assuming a multiple-trapping model for charge transport, we find the theoretical expression for the current density induced by CMG at fundamental frequency. By using a numerical simulation with parameters typical for hydrogenated amorphous silicon, we verify the expected equivalence between both techniques for low enough chopping frequencies. Then, we test experimentally this equivalence for an undoped hydrogenated amorphous silicon sample. For low signal levels, we demonstrate the superior performance of CMG.
In this work, we present two new pairs of formulas to obtain a spectroscopy of the density of states (DOS) in each band tail of hydrogenated amorphous silicon (a-Si:H) from photoconductivity-based measurements. The formulas are based on the knowledge of the small-signal recombination lifetime s 0 , the characteristic decay time of the concentration of trapped carriers generated in excess by the illumination, and that can be measured by methods like the Oscillating Photocarrier Grating (OPG) or Moving Grating Technique (MGT). First, we deduce the formulas and test their accuracy by numerical simulations using typical a-Si:H parameters. Next, we characterize an a-Si:H sample using well-known methods, like Fourier transform photocurrent spectroscopy to evaluate the valence band tail and modulated photoconductivity to measure the conduction band tail. We also performed measurements of steady-state photoconductivity, steady-state photocurrent grating and MGT, for a range of generation rates. From these measurements-and taking typical values for the capture coefficients, the extended states mobilities and the DOS at the band edges-we apply the new formulas to get the band tails. We find that the results obtained from the application of our formulas are in good agreement with those found with the traditional methods for both band tails. Moreover, we show that MGT/OPG measurement to get s 0 can be avoided if one of the band tails is measured by one of the traditional methods, since the known band tail can be used to evaluate s 0 with one pair of equations, and then the other pair can be applied to get the other band tail. Published by AIP Publishing.
We propose to use steady state measurements and a teaching-learning-based optimization (TLBO) algorithm to get the complete set of material transport parameters of disordered semiconductors, taking undoped hydrogenated amorphous silicon as an example. First, the steady-state conductivity under illumination and the ambipolar diffusion length (L amb) are measured for several temperatures and generation rates. The steady-state photocarrier grating (SSPG) technique is used for the evaluation of L amb. Then, the TLBO algorithm is used for the obtainment of the material parameters that best satisfy the charge neutrality and the continuity equations. The use of this algorithm allowed us to get an excellent estimation of the valence band tail slope, as compared to the one obtained from measurements of the absorption coefficient by Fourier transform photocurrent spectroscopy and transmittance/reflectance. The dangling bonds and the conduction band tail parameters were also found to be in very good agreement with those measured from high frequency modulated photocurrent experiments (MPC). Numerical simulations show that the capture coefficients of the band tails and defects states can also be estimated, although with less precision than the DOS parameters.
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