A Comparative Study of Hydrogenated Amorphous Silicon Films Prepared by RF Sputtering in He/H 2 , Ar/H 2 and Xe/H 2 Mixt
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A COMPARATIVE STUDY OF HYDROGENATED AMORPHOUS SILICON FILMS PREPARED BY RF SPUTTERING IN He/H 2 , Ar/H 2 AND Xe/H 2 MIXTURES MARK L. ALBERS, H. R. SHANKS, AND J. SHINAR Ames Laboratory-USDOE and Department of Physics Iowa State University, Ames, Iowa 50011
ABSTRACT Preliminary results of a comparative study of some optical and ESR properties of aSi:H films prepared by rf sputtering on a cold substrate in 10 mtorr of either He, Ar, or Xe and 0.5 mtorr H2 are presented. In all cases the concentration of Si-H and Si-H2 bonds, the optical gap and the dangling bond spin density all generally increase as the rf power is 2 decreased from 3.3 to 0.27 W/cm . However, whereas the optical energy gap of He/H2 sputtered films ranges from 1.26 eV to 2.13 eV, the gap of Ar/H 2 and Xe/H2 films sputtered under these conditions only changes from 1.54 to 1.94 and 1.41 to 1.71 eV, respectively. The dangling bond spin densities 3 are lowest (_1017 cm- ) in the Ar/H 2 sputtered films at high rf power and 3 highest (-5x1018 cm- ) in Xe/H2 sputtered films at low power.
INTRODUCTION In almost all of the studies published until recently on rf sputtered hydrogenated amorphous silicon (aSi:H) films, the sputtering medium was a mixture of Ar and H2 [1]. Although Ross and Messier reported the results of a comparative study of films deposited by Ne/H 2 , Ar/H 2 , and Kr/H 2 [2], the partial pressures of the inert gases used in their work were generally high as compared to the generally accepted technologically optimal value of The rf sputtering power was also rather low about 10 mtorr [1], [21. (100 W). Not surprisingly, many of their films exhibited strong columnar morphology, and all contained a large concentration of Si-H2 and Si-H3 bonds (up to 63% of total H concentration) generally associated with the boundaries of these columns. These columns, and consequently their Si-H2 "signature," are generally considered detrimental for optoelectronic or photovoltaic applications. We were therefore motivated to compare the properties of films deposited by sputtering at the roughly "optimal" partial pressures of 10 mtorr inert gas and 0.5 mtorr H2 , in the lightest (He), "standard" (Ar), and heaviest (Xe) rare gases, at various rf power levels. This preliminary report describes some of the optical and electron spin resonance (ESR) results of this comparative study. The generally conceded technological figure of merit among these results is a low dangling bond spin density [3]. As we shall see below, films sputtered in 3 Ar/H2 at high rf power exhibit a spin density of approximately 1017 cm- . An earlier report of measurements on films sputtered in He/H has already been published [4], but the preparation conditions were slightly different. In particular, the substrates were placed on an aluminum foil, which resulted in a substrate temperature of about 150°C. The substrate temperature in this work was kept as close to room temperature as possible (see below).
EXPERIMENTAL PROCEDURE A detailed description of the experimental procedure has been publish
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