Simultaneous Relaxation of Network and Defects in Silicon-Implanted a-Si:H
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3 3 1 1 J. NAKATA', 2, S. SHERMAN , S. WAGNER , P.A. STOLK , J.M. POATE 'Department of Electrical Engineering, Princeton University, Princeton, NJ 08544 2 On leave from Department of Electrical Engineering, Kinki University,
Higashiosaka, Osaka 577, Japan AT&T Bell Laboratories, Murray Hill, NJ 07974
3
ABSTRACT We report extensive optical and electronic transport data on silicon-implanted a-Si:H, annealed in steps in the dark or with additional illumination. All measured properties relax gradually with increasing annealing temperature. The dark conductivity of the as-implanted film is dominated by hopping conduction via midgap defects. This channel is pinched off during the initial stages of annealing. The midgap defect density and the Urbach energy follow an annealing path that agrees qualitatively with the trajectory postulated by the equilibrium theory of the dangling-bond density. Therefore, the silicon network and the defect density equilibrate continuously during network relaxation.
INTRODUCTION The perhaps most prominent question about hydrogenated amorphous silicon is the quantification of its structure. In the mid-'80s it became clear that a-Si:H prepared in different labs around the world had very similar, "device-quality" properties, which suggested the existence of a reproducible structure. Among the following discoveries was the freeze-in of the dangling-bond density upon cooling below -200TC [1], and the formulation of the thermodynamic equilibrium between the strained valence band tail states and the dangling bonds [2,3,4]. Later work included hydrogen equilibration with strained and dangling bonds [5]. Given the proper tools and data, it is likely that the equilibrium concept for the structure of a-Si:H will be extended to a complete, interlocking formulation of the silicon network including dangling bonds, distributed as well as clustered hydrogen, and voids. Such a formulation will need precise values for the equilibrium constants. * We
dedicate this paper to the memory of the late Professor Yoshio Inuishi of Kinki University. 173 Mat. Res. Soc. Symp. Proc. Vol. 377 0 1995 Materials Research Society
An equally important question is the role of hydrogen in the relaxation process of both, the silicon network and the silicon dangling bonds. The role of defects in network relaxation has been clarified in studies of pure, ion-beam amorphized silicon [6,7]. The general consensus is that network relaxation in non-hydrogenateda-Si is controlled by the migration and recombination of structural defects (presumably silicon dangling bonds). In the temperature range from RT to 5000 C, the defect density progressively decreases by a factor of -10, from 5x10 20 cm"3 to 5x1019 cm- 3 , the Raman TO-peak HWHM decreases from 44 to 36 cm- 1 , the Urbach energy decreases from 0.2 to 0.12 eV [8], and the Cody optical gap increases from 1.0 to 1.5 eV [9]. Thus, the relaxation of the metastable defect population in a-Si brings about significant changes in the optical and electronic properties. How does the relaxat
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