CHARGE CARRIER RECOMBINATION IN AMORPHOUS SELENIUM FILMS
Date
1995-05
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Degree Level
Masters
Abstract
The charge transport characteristics of amorphous semiconductors determine how well they will perform in many applications. The product of the charge carrier drift mobility, ยต, and deep trapping lifetime, t, also known as the range of the carrier, represents the average distance per unit electric field that a charge carrier will travel in the conduction band before being trapped in deep localized states within the mobility gap. The irc product is therefore an important parameter in predicting the performance of an amorphous semiconductor based device. However, in applications where more than one polarity of electronic charge is involved, charge carrier recombination is also an important consideration. The use of amorphous selenium alloy based films in electroradiography is one such application. This work utilized three different experimental techniques to determine charge carrier drift mobility, deep trapping lifetime and recombination coefficient in stabilized amorphous selenium (a-Se:0.2%As+Cl in ppm) films,
suitable for use in electroradiographic systems. The conventional time-offlight (TOF) method was used to evaluate carrier drift mobility and the interrupted field time-of-flight (IFTOF) technique was used to determine the carrier deep trapping lifetime. The hole range of the amorphous selenium films used was wth=67.54x10-6 cm2V-1, and the electron range was
wre=2.68x10-6 cm2V-1. An ambipolar time-of-flight experiment was performed to measure bulk recombination by measuring the fractional change in hole photocurrent as an electron and hole charge packet passed through each other under the influence of an applied field.
TOF and IFTOF measurements were used to characterize charge transport in the samples before the recombination experiment was performed. The ambipolar TOF experiment indicated that the charge carrier recombination process within chlorinated a-Se:0.2%As follows the Langevin process, originally proposed for recombination of gaseous ions. The predicted Langevin recombination coefficient was Cer=35.2x10-9 cm3s-1, while the experimentally determined Langevin recombination coefficient was CrE=36.6x10-9 cm3s-1. This implies that the carrier mean free path in the films is much less than the recombination radius defined by Langevin. As
most photoinduced discharge theories ignore the effect of carrier recombination their development, the measured recombination coefficient can now be used to reformulate these models to include recombination.
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Degree
Master of Science (M.Sc.)
Department
Electrical and Computer Engineering
Program
Electrical Engineering