The Dangling Bond in Undoped Amorphous Hydrogenated Silicon: Trap or Recombination Center?
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THE DANGLING BOND IN UNDOPED AMORPHOUS HYDROGENATED SILICON: TRAP OR RECOMBINATION CENTER? M. A. Parker, K. A. Conrad, and E. A. Schiff Department of Physics, Syracuse University Syracuse, N. Y. 13244-1130 U. S. A. ABSTRACT
The role of the neutral dangling bond defect upon photocarrier processes in undoped amorphous hydrogenated silicon (a-Si:H) is discussed. The evidence that the dangling bond is a simple recombination center is reviewed, and it is shown that this model does not account for photocurrent response time measurements. Experimental data pertinent to the role of electrical contacts upon response time measurements are presented, and it is concluded that contact effects do not account for response-time measurements. The possibility that the dangling bond is primarily an electron trap is discussed. INTRODUCTION There have been several published accounts in recent years which suggest strongly that the neutral charge state of the dangling bond defect is a simple electron recombination center in undoped amorphous hydrogenated silicon (a-Si:H). A recombination center is distinguished from a true trap in the following sense (cf. [1]): a defect is an electron trap if a captured electron photocarrier is reemitted to the transport channel at a later time. Conversely, the defect is a recombination center if the captured electron is immobilized at the defect sufficiently long that a hole recombines with the electron prior to a reemission event. The distinction between the two terms is illustrated in the upper portion of Fig. 1. In this paper we first review the evidence from several experiments that the neutral dangling bond defect in undoped a-Si:H plays the role of a simple electron recombination center. We then illustrate that this model for electron photocarrier processes leads to what may be termed the response-time paradox for a-Si:H; observed photoconductivity response times vastly exceed the values expected from the recombination center model. This paradox has led Street to suggest that the response times may be due to contact artifacts. We discuss this suggestion, and present arguments and experimental results which indicate that contacts are not the origin of the long response times in undoped a-Si:H. We conclude with a brief discussion of an alternate model for the role of the dangling bond in undoped a-Si:H: the neutral dangling bond is primarily an electron trap. REVIEW:
EVIDENCE THAT THE DANGLING BOND IS A RECOMBINATION CENTER
The evidence that a neutral dangling bond (T 3 0 ) is a simple recombination center comes primarily from the following experiments: (i) Deep-level Transient CapacitanceSpectroscopy (DLTS): In a-Si:H which is moderately doped with phosphorus DLTS indicates that the binding energy of an electron captured by a neutral dangling bond (T 3 0 ) and hence forming *Supported by NSF Grant DMR 83 - 06083.
Mat. Res. Soc. Symp. Proc. Vol. 70. '1986 Materials Research Society
126
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Fig. 1: The upper
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