In Situ Diagnostics of VUV Laser CVD of Semiconductor Interfaces by FTIR Spectroscopy and Spectroscopic Ellipsometry
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Abstract The growth of high quality amorphous hydrogenated semiconductor films was explored with different in situ spectroscopic methods. Nucleation of ArF laser-induced CVD of a-Ge:H on different substrates was investigated by real time ellipsometry, whereas the F 2 laser (157nm) deposition of a-Si:H was monitored by FTIR transmission spectroscopy. The ellipsometric studies reveal a significant influence of the substrate surface on the nucleation stage, which in fact determines the electronic and mechanical properties of the bulk material. Coalescence of initial clusters occurs at a thickness of 16 A for atomically smooth hydrogen-terminated c-Si substrates, whereas on native oxide covered c-Si substrates the bulk volume void fractions are not reached until 35 A film thickness. For the first time we present a series of IR transmission spectra with monolayer resolution of the initial growth of a-Si:H. Hereby the film thickness was measured simultaneously using a quartz crystal microbalance with corresponding sensitivity. The results give evidence for cluster formation with a coalescence radius of about 20 A. Difference spectra calculated for layers at different depths with definite thickness reveal that the hydrogen-rich interface layer stays at the substrate surface and does not move with the surface of the growing film. The decrease of the Urbach energy switching from native oxide to H-terminated substrates suggests a strong influence of the interface morphology on the bulk material quality.
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Introduction
transmission spectra with monolayer sensitivity of the initial growth of amorphous hydrogenated silicon (a-Si:H). This yields bonding information In recent years, the study of the interfaces of complementary to the structural information obamorphous semiconductor films has attracted intained by UV-VIS ellipsometric studies. creasing interest since they substantially affect the materials properties. Real time spectroscopic ellipsometry (RTSE) provides insight into 2 ArF laser CVD of a-Ge:H the structure of ultra-thin films in the coalescence regime. Beside the optical constants the 2.1 Experimental method provides the film thickness and volume void fractions. The change of these parameters Amorphous hydrogenated germanium (a-Ge:H) with time gives insight into the structural evolu- was deposited using an ArF excimer laser tion of the film. Up to now FTIR transmission (193 nm), in parallel configuration (see Fig. 1). spectroscopy has been considered to be not sen- Since the absorption cross section of germane sitive enough for studies with nanoscale resolu- (GeH 4) is very small at 193 nmn, digermane was tion. For the first time we present a series of IR used as source gas [1]. The deposition process is
i81 Mat. Res. Soc. Symp. Proc. Vol. 397 01996 Materials Research Society
monitored in situ by real time spectroscopic ellipsometry to obtain the evolution of the optical functions [2, 3] and to get information about the microstructure and nucleation processes [4]. The deposited films were then further charact
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