Manipulating the Hydrogen-Bonding Configuration in ETP-CVD a-Si:H
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0989-A22-04
Manipulating the Hydrogen-Bonding Configuration in ETP-CVD a-Si:H M. A. Wank1, R. A. C. M. M. van Swaaij1, and M. C. M. van de Sanden2 1 DIMES-ECTM, Delft University of Technology, P. O. Box 5053, Delft, 2600 GB, Netherlands 2 Department of Applied Physics, Eindhoven University of Technology, P.O.Box 513, Eindhoven, 5600 MB, Netherlands
ABSTRACT The effect of ion bombardment on the relationship between the critical hydrogen concentration ccrit and the reactor pressure has been investigated for hydrogenated amorphous silicon (a-Si:H) deposited with the expanding thermal plasma-CVD (ETP-CVD) method. By increasing the reactor pressure, in particular above 0.24 mbar, the ionic cluster formation in the plasma can be increased, resulting in a decrease of ccrit. It is observed that this decrease of ccrit with increasing reactor pressure can not be compensated by ion bombardment at 14V biasing. Biasing with 20V however increases ccrit nearly up to the value obtained at low pressures. This observation indicates that the incorporation of ionic clusters formed at elevated reactor pressures can be reduced by substrate biasing, possibly due to break-up upon impact on the substrate surface or due to processes occurring in the secondary plasma close to the substrate. The onset of void formation in the film is found to depend on reactor pressure and substrate biasing, indicating that the maximum hydrogen solubility for ETP material might be affected by these deposition parameters. INTRODUCTION Hydrogen plays an important role in a-Si:H. It passivates dangling bonds in the a-Si:H network, reducing the defect density significantly compared to a-Si and permits the application of a-Si:H as material in, e.g., solar-cell devices. However, hydrogen is also involved in metastability mechanisms and a study of the hydrogen bonding configuration is therefore important to assess the material quality. Hydrogen bonding configuration is usually studied by Fourier transform infrared (FTIR) absorption spectroscopy. Three vibration modes of SiHx hydrides can be observed: a wagging mode at 640 cm-1, bending modes in the range of 840890 cm-1, and two stretching modes at 1980-2030 cm-1 (low stretching mode, LSM) and 20602160 cm-1 (high stretching mode, HSM). The LSM absorption is attributed to monohydrides in mono- and di-vacancies, whereas the HSM is mainly ascribed to Si-H bonds located at void surfaces [1]. From film density measurements, it was recently concluded that when the total hydrogen concentration, cH, is above a critical value, ccrit, of 14 at.% the material is regarded as voiddominated and below 14 at.% as vacancy-dominated [1]. In the vacancy dominated region the integrated absorption of the LSM mode, ILSM, is larger than that of the HSM mode, IHSM, and vice versa in the void dominated region. Therefore, the critical hydrogen concentration, ccrit, can be determined as that cH for which ILSM = IHSM. Recently, Petit [4] has investigated the dependence of ccrit on the reactor pressure during deposition. It was shown that incre
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