Scanning Tunnelling Microscopy Study of the Growth Mechanism for Hydrogenated Amorphous Silicon Produced by Plasma Enhan
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Mat. Res. Soc. Symp. Proc. Vol. 507 01998 Materials Research Society
EXPERIMENT The vacuum system used in this study has been described in some detail elsewhere [6] but essentially consists of two chambers - a capacitatively coupled PECVD deposition chamber and a chamber for the STM - which are connected by a UHV gate valve. Samples were deposited onto Sif I I I) substrates 1 mm wide and 8 mm long. These substrates were first flash heated to 1475 K in the STM chamber at a pressure of less than 3x10-7 Pa to induce a 7x7 reconstruction of the surface, producing an atomically flat substrate, and to remove any native oxide layer. The substrate was then transferred to the PECVD chamber, where a thin film of a-Si:H was deposited using the following optimised conditions: a pure silane gas flow rate of 50 sccm, a gas pressure of 36.7 Pa, a substrate temperature of 598 K and an rf power of 526 W m-2 . The a-Si:H thickness was controlled by varying the deposition time. Following the deposition, the sample was allowed to cool to ambient temperatures before being imaged with the STM using a tip bias of -2 V and a tunnelling current of -50 pA. Once imaging was complete, the sample was returned to the PECVD chamber, and further layers of a-Si:H deposited onto the first to construct a dynamic picture of material growth. RESULTS The STM was used to image the a-Si:H sample surface when it was 15, 30, 60, 120, 240 and 480 nm thick. Three typical constant current imaging (CC1) topography scans are shown in figure 1. An island structure is clearly observed on the sample surface at the earliest stage of growth (figure la) and these islands develop with growth, reducing in size as the a-Si:H layer gets thicker. The random attachment of species to a surface, known as stochastic roughening, is the simplest mechanism through which a surface can form. However, the presence of distinguishable features shows that a more sophisticated process underlies the growth of a-Si:H. Different growth mechanisms will affect the size of topographic features present [1]. Consider two spherical clusters on the surface of a material, where the radius of the second cluster, R2, is X. times bigger than the radius of the first, R 1. If a given growth mechanism takes a time At , to produce a given change in the shape of the first cluster and a time At 2 to produce a geometrically similar change in the shape of the second cluster, then
500x500x2.59 nm r=15 nm 100x100x3.03 nm '=240 nm 100x100x5.56 nm '-=480 nm Figure 1. Three constant current imaging topography scans of the (a) 15 nm thick, (b) 240 nm thick and (c) 480 nm thick a-Si:H samples. 572
(1)
At 2 " AAt)
In Fourier transform frequency space, this becomes [2] 1 F(Iql)
(2)
- -i
Iqi'
where q is the reciprocal space vector, F denotes the Fourier transform and i is a constant, which is dependent on the growth mechanism, known for the purposes of this work as the Fourier index. A graph of log{F(Iql)} against log{IqI) should produce a straight line whose gradient is equal to -i [7]. Herring [1] s
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