Impurity Content and Defect Density in 42 a-Si:H Films
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IMPURITY CONTENT AND DEFECT DENSITY IN 42 a-Si:H FILMS 3 2 1 1 NAKATA , S. WAGNER , C.W. MAGEE , T.M. PETERSON , AND H.-R.PARK MASAMI 1 Department of Electrical Engineering, Princeton University, Princeton, NJ 08544 2 3Evans East, Plainsboro, NJ 08536 Electric Power Research Institute, Palo Alto, CA 94303 4 Mokpo National University, Muan, Korea.
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ABSTRACT In an earlier study of 37 a-Si:H films we identified positive correlations between the saturated light-induced defect density N.,t, the hydrogen content C. measured by IR absorption, and the optical gap. Impurities also have been implicated in the production of metastable defects. Yet no conclusive evidence has been produced that non-dopant impurities cause defects. This situation led us to analyze our samples for impurity content. Our present 42 samples were made in six laboratories by ten combinations of deposition technique. The concentrations of H, B, C, N, 0, F, Na, Cr, and Ge were determined by secondary ion mass spectroscopy. Na, Cr, or Ge were not detected. The content of C, N and 0 varies by a factor -1,000 and depends on deposition system and source gas. We report a comprehensive evaluation of our data with emphasis on their correlation with optoelectronic properties. We find the annealed state defect density N. and the saturated light-induced defect density N,~ surprisingly independent of impurity content. INTRODUCTION As early as 1977 Carlson added atmospheric contaminants to the SiH4 discharge to identify their effect on a-Si:H solar cell efficiency [1]. In 1980, Magee and Carlson [2] published a study of the effects of atmospheric impurities on the light-induced degradation of a-Si:H solar cells, and in 1984 Carlson wrote a first review of work on impurities in a-Si:H [3]. The effect of unwanted impurities on the optoelectronic properties of a-Si:H and its devices has been the subject of study and debate ever since. The early work had several motives. Based on the experience with crystalline semiconductors, impurities were assumed to degrade a-Si:H device performance. Therefore, one wanted to know the impurity content of a-Si:H, and how it is affected by feedstock gas contamination and by deposition procedures. Typical are studies of chlorine and of oxygen, brought in with silane as monochlorosilane [4] and as siloxane [5]. A second motive was the search for, and selection of, doping and alloying elements, which also identified impurities that degrade a-Si:H. Many more impurity studies were carried out over the past decade, either on isolated films or on solar cell devices. The work on solar cells focused on initial efficiency and on light-induced changes [4,6-8]. In films, the optical gap, the dark- and photoconductivity, their thermal activation energies [9-16], the photoresponse [16], the density of neutral and of charged dangling bonds [8-11,13,14], and the photoluminescence [17] were studied. The effect of impurities on SiH 2 bonding, and thence on light-induced metastability, also was investigated [15]. In addition, mechanisms of defect cre
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