Steady-State Photocarrier Grating Technique Applied to a-Si:H Thin Film Transistors
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STEADY-STATE PHOTOCARRIER GRATING TECHNIQUE APPLIED TO a-Si:H THIN FILM TRANSISTORS F. WANG, M. REISSNER, T. FISCHER, S. GREBNER, and R. SCHWARZ Technical University of Munich, Physics Department E16, W-8046 Garching, Germany ABSTRACT The transport properties of a-Si:H based thin film transistors have been characterized by the ambipolar diffusion length (Lab) which was measured by the steady-state photocarrier grating technique (SSPG). The results show that with increasing positive gate voltage (4g) the photoconductivity increases, but Lamb decreases. The above results can be consistently explained in terms of the shift of the Fermi level towards the conduction band edge with increasing V1. On the other hand, from the analysis of the SSPG results it is found that the factor -y,, which is related to the interface recombination velocity, increases with V.. This indicates that when narrowing the conductive channel the recombination of the carriers at the interface increases.
INTRODUCTION The transport properties of semiconductors are strongly dependent on the Fermi level position Ef in the forbidden gap. In hydrogenated amorphous silicon (a-Si:H) the Fermi level dependence of the photoconductivity (arp,), which is dominated by majority carriers, was already investigated in films with different doping level [1]. Recently some comprehensive studies of the effect on both majority and minority carrier transport properties in doped a-Si:H films have been published [2, 3]. However, there are two disadvantages when using doped films for such a study: First, the Fermi level shift is small since the doping efficiency in a-Si:H is very low and, secondly, doping introduces new defect states and changes the DOS (the density of states) distribution in the gap [4]. Thin film transistors (TFT's) provide an alternative way for the study of Fermi level effects because E1 can be moved conveniently by changing the gate voltage (V,) without changing the defect density and DOS distribution. Some work has been done on the dependence of the majority carrier transport properties on Vg [5]. However, only very few authors [6] have discussed minority carrier transport in TFT structures probably due to the invalidity of conventional experimental methods. But from the viewpoint of both basic physics and applications it is necessary to understand the effect of the Fermi level on minority carriers transport properties. The steady-state photocarrier grating technique (SSPG) developed by Ritter et al. [8] provides a reliable method to measure the ambipolar diffusion length (La,,b) which is mainly determined by the mobility-lifetime product ( 6ar) of minority carriers. On the other hand, it is possible to study the interface properties with TFT's since more and more carriers stay near the interface as Vg increases. The aim of this work is to study the Fermi level effect on the minority carrier pr-product by means of SSPG method in a-Si:H TFT's. SAMPLES AND EXPERIMENTS Undoped a-Si:H films of 0.7 pm thickness were deposited on silicon dioxide (about 0
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