High Substrate Temperature a-Si:H Grown by DC Reactive Magnetron Sputtering
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HIGH SUBSTRATE TEMPERATURE a-Si:H GROWN BY DC REACTIVE MAGNETRON SPUTTERING Y. H. Liang, N. Maley and J. R. Abelson, Coordinated Science Laboratory and the Materials Science and Engineering Department, University of Illinois, Urbana, IL 61801
ABSTRACT We report the electronic properties, stability and microstructure of a-Si:H films grown at very high substrate temperature (Ts= 320-425'C) by DC reactive magnetron sputtering (RMS). The partial pressures of Ar and H 2 are fixed at 1.5 and 0.8 mT, respectively, during the deposition. The initial defect state density, determined by the constant photocurrent method (CPM), varies from 2-5x10 15 cm- 3 with H content changing from 15-10 at.% as Ts increases from 320375'C. For 100 hrs white light exposure at 1W/cm 2 , a heavily degraded state was obtained with mid-gap state density in the range 2-3xl0 1 6 cm- 3 over this Ts range. These are among the lowest values reported for intrinsic a-Si:H.
INTRODUCTION The metastable defect behavior in a-Si:H films appears to depend on the substrate temperature during deposition. Previous studies
have established
an optimal
growth
temperature,
Ts - 230-
250'C, for the as-deposited defect density. As Ts departs either above or below this temperature range, the defect density increases [1]; this holds for both silane plasma deposition and RMS in our group [2]. However, few studies have focussed on the stability of device quality a-Si:H films at higher Ts. Conceptually, the motivation for high temperature growth experiments is that the growth species should have larger surface mobility and find energetically preferable binding sites. Consequently, a less disordered Si network may be formed, and this should reduce metastable defect creation. Recently experiments have examined high Ts films in different deposition systems. Johnson et al. showed improved stability of aSi:H films deposited at 400'C in a remote H2 plasma deposition reactor [3]. Mahan et al. deposited a-Si:H by hot-wire assisted chemical vapor deposition (HWCVD) at very high Ts - 400'C, which can produce a-Si:H films with H content as low as 1 at.% [4]. In both cases the electronic properties are comparable with device quality aSi:H grown at 230°C.
Mat. Res. Soc. Symp. Proc. Vol. 297. ©1993 Materials Research Society
146
We present our studies on the effect of substrate temperature on the electronic properties and microstructure of a-Si:H films by DC reactive magnetron sputtering. For substrate temperatures in the range 320-375*C, the a-Si:H films show very good initial electronic properties. After a long intense light soaking, apparently saturated states were reached with mid-gap density of defect states (DOS) 2-3x10 16 cm- 3 .
EXPERIMENTAL Hydrogenated amorphous silicon films were prepared by DC magnetron sputtering of a crystalline Si target in an Ar + H 2 gas mixture in a UHV deposition system as described elsewhere [2]. The Ar pressure was kept constant at 1.5mT, and the H 2 pressure at 0.8 mT. The substrate temperature was varied from 230-425*C. The aSi:H film
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