Transient Photocurrent Measurements on Current-Stressed a-Si:H Schottky Diodes
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ABSTRACT The characteristics of a-Si:H based switches, as used in liquid crystal displays, degrade as a result of current injection. To assess this degradation we investigated the effects of current injection on the electron mobility, lifetime and the built-in electric field profile in a-Si:H Schottky diodes. We present steady-state current-voltage and transient photocurrent (TP) measurements on Mo and Pd a-Si:H Schottky diodes. The mobility, lifetime and electric field profile were determined from TP measurements. The electric field profile was calculated by fitting simulations from a non-linear model of the measurement to the measured TP currents. The electron drift mobility is not affected by current stressing, the electron lifetime reduces slightly, the built-in voltage decreases significantly. The widths of the built-in fields of Schottky and ohmic back contact are reduced with approximately the same factor, corresponding with a spatially homogeneous increase of the defect density.
INTRODUCTION Degradation by current injection in amorphous silicon is a topic of both technological and fundamental importance, since it affects applications using a-Si:H switching devices. In liquid crystal displays for example, the switching characteristics of individual pixel switches degrade which results in a decrease of contrast and homogeneity. Current injection increases the densities of free carriers. The recombination of these carriers supplies the amorphous silicon with the energy necessary for creating defects [1]. Since the density of minority carriers limits the recombination, the rate of defect creation in Schottky diodes depends on the barrier height for minority carriers [2]. Also, these authors showed that the current-induced degradation does not affect the barrier itself. In this paper we investigate the changes in the steady state current-voltage (IV) characteristics and the electron transport parameters lifetime, mobility and built-in field of a-Si:H Schottky diodes due to current stressing. The transient photocurrent (TP) method is used to determine lifetime, mobility and internal field profile. The electric field profile was determined by fitting the TP measurements with transients calculated by a consistent model of the measurement. In this way, we can include the effects of diffusion, trapping kinetics and other non-linear properties.
SAMPLE PREPARATION AND EXPERIMENTAL SETUP Samples were prepared in a single-chamber PECVD system. The structure of the diodes is: corning 7059, 100nm Cr, 30nm n + a-Si:H, 1.71in intrinsic a-Si:H., 3Onm Pd or Mo. The Pd 491 Mat. Res. Soc. Symp. Proc. Vol. 377 @1995 Materials Research Society
and Mo dots form Schottky diodes with an area of 6.4-10-3 cm 2 , The highly doped n+ layer forms an ohmic contact with Cr. The density of defect states of the intrinsic layer has been measured with dual beam photoconductivity and was 4.1016 cm- . We annealed the diodes at 170' C for 30 minutes to improve the IV characteristics. IV-characteristics between 40' C and 100' C were measured af
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