Investigation of the Surface Morphology of a-Si:H by Atomic Force Microscopy and In-Situ Ellipsometry
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based semiconductors, providing detailed insight into their surface structures. Therefore these direct methods bear the potential for significantly improving the interpretation of ellipsometry data, because they yield much more accurate information about the surface roughness, and about thickness and volume density data of the respective layers that are urgently needed for fixing some of the parameters of the optical models for ellipsometry evaluation. EXPERIMENTAL For studying substrate induced effects crystalline silicon and fused silica have been used as substrate materials during our investigation. The roughness parameter Ra is as usual defined to be the arithmetic mean of the height differences to the overall average of the deviation profile that has been determined from the AFM. Over the area under investigation (0.25 pm 2 ) c-Si exhibited a roughness of 1 - 1.5
A,
and fused silica one of about 2.5 A. The native oxide
layer thickness on c-Si has been checked by spectroscopic ellipsometry, and turned out to be approx. 20A. Amorphous silicon thin films have been deposited from a dc glow discharge (substrate temperature 520 K, silane flow 6 sccm and pressure 20 Pa, plasma power 30 mW/cm 3), resulting in a growth rate of 3 A/s and in device quality material. In-situ ellipsometry has been performed using a phase-modulated [11] instrument (JobinYvon, UVISEL). Kinetic measurements were carried out at a photon energy of 4eV in order to maximize surface sensitivity. Spectroscopic data were recorded from 1.5 to 5.5eV. Ellipsometric data were evaluated by applying multilayer analyses [12] and the Bruggeman effective medium approximation [13]. For atomic force microscopy we used the contact mode of a TopoMetrix instrument employing a Si 3N4 tip with a spring constant of 0.3 N/m. Each image has a line scan resolution of 200 at a scan rate of 1.5 pm/s. For infrared absorption measurements we used a Bruker IFS 48 spectrometer in the range of 400 to 2400 wavenumbers. The spectra have been evaluated in the coherent approximation to fully account for interference effects [14]. RESULTS AND DISCUSSION Fig. I shows an in-situ single wavelength ellipsometric trajectory at a photon energy of 4eV taken during the deposition
Start
142 ...
Experiment
138 S.. pimuint Simulation -..
135
\2s-
to" 2O "
132,-
'5s 128
3000s
27.0
27.5
28.0
of a-Si:H on crystalline silicon; the labels indicate the deposition time t (t=Os corresponds to the removal of the proximity shutter). The initial stage of the evolution is well known [1]. The trajectory starts at the optical parameters of the substrate and finally settles at a film thickness of about 0.9-1 pm, indicating that no further changes in surface morphology occur.
28.5
S(deg.)
In order to monitor the progress in Fig. 1: In-situ ellipsometry trajectory at a photon d i mevral re a dditionaly energy of 4 eV, taken during a-Si:H deposition on individual several prepared additionally c-Si and numerical simulation.
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samples and finished film growth after the resp. deposition
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