Sub-Surface Equilibration of Hydrogen with the a-Si:H Network Under Film Growth Conditions
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SUB-SURFACE EQUILIBRATION OF HYDROGEN WITH THE a-Si:H NETWORK UNDER FILM GROWTH CONDITIONS ILSIN AN, Y. M. LI, C. R. WRONSKI, and R. W. COLLINS Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802. ABSTRACT In this study we characterize hydrogen diffusion and reaction processes in the near-surface (top 200 A) of a-Si:H that lead to network equilibration under standard conditions of plasmaenhanced chemical vapor deposition (PECVD). Real time spectroscopic ellipsometry (SE) is used to provide continuous kinetic information on the near-surface conversion of Si-Si to Si-H bonds during exposure of in situ-prepared films at 250'C to filament-generated atomic H. We have found that for optimum PECVD a-Si:H, the formation of additional Si-H 1bonds2is limited by the capture of H at trapping sites, and the rapid diffusion process (D>10- 4 cm /s) by which H reaches the site is not detected optically. Deep trapping occurs at a rate of -10-3 s- under our 3 to generate -100 cm- mobile H in the near-surface of the 7film. filament conditions, estimated 3 Finally, more than 1021 cm- additional H atoms are trapped with emission rates 2.0 eV. Shallower traps are also detected at lower concentration. INTRODUCTION Models of a-Si:H growth by PECVD have usually emphasized (i) the Si-containing precursors arriving at the surface, (ii) the surface diffusion length of the precursors, and (iii) the processes by which attachment and cross-linking reactions are promoted at the top monolayers of the film [1,2]. It has been proposed that such features establish the important structural and electronic properties of a-Si:H. More recently, however, the importance of the interaction of atomic hydrogen with the near-surface of a-Si:H in the growth environment has been stressed [3]. Because H can diffuse into the network and react well below the surface, one must also consider the possibility that H equilibration in this region affects the a-Si:H properties as well. Thus, it is important to develop a better understanding of H diffusion and reaction in the near-surface of a-Si:H in the growth environment. An understanding of these processes has typically come from analyses of secondary ion mass spectrometry (SIMS) profiles. Here we describe the results of a new measurement approach based on spectroscopic ellipsometry (SE). The advantage of this technique is that it provides continuous real time information on the kinetics of the conversion of Si-Si to Si-H bonds throughout the optical penetration depth (OPD). The disadvantage is its lower sensitivity: it is a linear probe with a detection limit of no better than 1020 cm- 3 for changes in bonded H concentration. As a result, information is restricted to the processes that determine the ultimate hydrogen content and optical gap of the material. EXPERIMENTAL DETAILS The a-Si:H films were deposited onto Si wafers in an rf PECVD system with windows for optical access to the growing surface. Unless stated otherwise, optimum preparation conditions were used (minimum rf po
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