Thermal Quenching of Photoconductivity and the Sign of Photocarriers in Doped a-Si:H
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THERMAL QUENCHING OF PHOTOCONDUCTIVITY AND THE SIGN OF PHOTOCARRIERS IN DOPED a-Si:H B.-G. Yoon,* H. FRITZSCHE,** M. Q. TRAN** AND D.-Z. CHI** * Department of Physics, University of Ulsan, Ulsan, 680-749, Republic of Korea **James Franck Institute, The University of Chicago, 5640 Ellis Ave., Chicago, IL 60637,USA
ABSTRACT Thermal quenching (TQ) of photoconductivity ap, occurs when the demarkation level of minority carriers passes through recombination centers having small capture cross section for majority carriers compared to other centers present but normal cross section for minority carriers. The photoconductivity becomes superlinear with light intensity at the temperature of maximum TQ. We discovered TQ not only in n-type but also p-type a-Si:H. This cannot happen with the same centers unless the sign of the majority carriers changes. We present evidence that in p-type and undoped films majority carriers are electrons at T below TQ and holes above TQ. The nature of these special centers will be discussed. INTRODUCTION Measurements of the photoconductivity .', contain important information about the optical excitation of mobile carriers, their recombination by radiative or nonradiative processes, and their transport in extended states or by hopping between localized band tail states. The large number of relevant parameters and physical processes makes it often difficult to identify the principal conduction and recombination processes and how they depend on temperature, light intensity, and doping. The present work explores in greater detail than done before [1-5] the doping dependence of thermal quenching (TQ) of the photoconductivity in hydrogenated amorphous silicon, a-Si:H, and the sign of the dominant photocarriers. A photoconductor whose photoconductivity decreases with increasing T in a limited T-range (thermal quenching) also exhibits infrared quenching of .yp below the temperature of TQ and a supralinear dependence of up on the electron-hole generation rate G. These phenomena can occur when two recombination channels exist with significantly different cross sections for the majority photocarriers [6]. Since thermal quenching is observed in both n-type and p-type a-Si:H samples, this cannot happen with the same set of recombination centers when the majority photocarriers change sign. One believes that they change sign because a increases with decreasing conductivity activation energy for both p-type and n-type samples [7]. The present study addresses these conflicting interpretations. Our attempts to determine the sign of the photocarriers by photo-thermopower measurements failed because of the presence of temperature dependent photovoltages. We therefore deduced the sign from the magnitude of the drift mobility of the photocarriers. EXPERIMENTAL DETAILS About 1M m thick samples were deposited at 500K and a pressure of 65mTorr in a 13.6 MHz glow discharge reactor from a flow of pure SiHl4 mixed with the desired dopant gases PH 3 or B2 H6 . The samples are labeled with their gas phase doping ratio. The de
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