The a-Si:H Growth Mechanism: Temperature Study of the SiH 3 Surface Reactivity and the Surface Silicon Hydride Compositi
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A9.3.1
The a-Si:H growth mechanism: Temperature study of the SiH3 surface reactivity and the surface silicon hydride composition during film growth W.M.M. Kessels, Y. Barrell, P.J. van den Oever, J.P.M. Hoefnagels, and M.C.M. van de Sanden Dept. of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
ABSTRACT We report on two experimental studies carried out to reveal insight into the interaction of SiH3 radicals with the a-Si:H surface as assumed essential in the a-Si:H growth mechanism. The surface reaction probability β of SiH3 on the a-Si:H has been investigated by spectroscopic means as a function of the substrate temperature (50 - 450 ºC) using the time-resolved cavity ringdown technique. The silicon hydrides –SiHx on the a-Si:H surface during deposition have been studied by the combination of in situ attenuated total reflection infrared spectroscopy and argon ion-induced desorption of surface hydrogen. For SiH3 dominated plasma conditions, it is found that the surface reactivity of SiH3 is independent of the substrate temperature with β = 0.30±0.03 whereas the silicon hydride composition on the a-Si:H surface changes drastically for increasing substrate temperature (from –SiH3 to =SiH2 to ≡SiH). The implications of these observations for the a-Si:H growth mechanism are addressed.
INTRODUCTION Although the efforts on constructing a growth model for a-Si:H have been fruitful and have contributed greatly to the understanding of the a-Si:H deposition process, there are several issues in the growth mechanism of a-Si:H that are still not completely unraveled [1]. The interaction of SiH3 radicals with the a-Si:H surface under different surface conditions (as e.g., determined by the substrate temperature) is one particular unresolved issue. Although recently several SiH3 surface reactions have been proposed on the basis of ab initio calculations and simulations such as density-functional theory (DFT) calculations and molecular dynamics (MD) simulations [2,3,4,5,6], there is still insufficient experimental data. Therefore we have carried out dedicated experiments using the expanding thermal plasma (ETP) technique [7] which is well-suited for these kind of studies: • the conditions in an Ar-H2-SiH4 plasma can be chosen such that a-Si:H film growth is approximately for 90% due to SiH3 radicals as revealed from cavity ringdown spectroscopy and threshold ionization mass spectrometry [8,9]; • ion bombardment does not play a role because the low electron temperature in the plasma leads to a very low self-bias during deposition (
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