Anisotropy in Hydrogenated Amorphous Silicon Films as Observed Using Polarized Ftir-Atr Spectroscopy
- PDF / 1,635,762 Bytes
- 6 Pages / 382.5 x 615.6 pts Page_size
- 36 Downloads / 287 Views
© 1999
Materials Research Society
IR sourc
r ::=::• j:i:,!:•-4 '
10
ATR subat a-SiHx film
a a-SiH. film
Polazer L'
_-
To IR detector
SEM cleavage plane Fig. 1. Schematic drawing of the attenuated total reflection configuration for infrared spectroscopy of a-SiHx films. The cleavage plane for SEM measurements on the Si ATR substrates is also shown. these cases there is no wavelength dependence of the ATR-absorbance band intensities [3], although dispersion in n2 may have some effect on the ATR-IR absorbance band shapes of acSiHx films on Si substrates [4]. Unlike transmission spectroscopy, ATR spectroscopy allows polarization of the incident light in the film growth direction, as well as in the film plane (Fig. 1). An early work pioneering the application of ATR spectroscopy to the analysis of a-SiHx films [5] described the measurement of the first overtone absorbance of the Si-H stretching vibration near 3950 cm-', as well as the absorbance from 3900-5400 cm-' corresponding to highpressure molecular hydrogen in microvoids in the films. To observe these weak absorbance bands, an optical path of 6.3 cm through the Si ATR substrates was used. However, at such long path lengths, impurity, lattice, and free-carrier absorption in the Si substrates limited their transparency to frequencies higher than about 1500 cm- [5,6]. In this work, we used short-pathlength (0.7-1.4 cm) Si and CdTe ATR substrates to extend the range of our spectroscopic measurements to frequencies as low as 400 cm1. EXPERIMENT Thin films of hydrogenated amorphous silicon were deposited on both of the faces of crystalline silicon ATR substrates and on one face of polycrystalline cadmium telluride ATR substrates, as shown in Fig. 1. These substrates were blown clean with dry nitrogen before film deposition, but were otherwise used as received from the manufacturer (Harrick Scientific Co.), as is our typical practice in preparing a-SiHx films on Si substrates for IR analysis of H content. The gaps in film coverage of the substrate (Fig. 1) result from the clamp used to secure the substrates to the heater can in the deposition system [1]. The clamp also shielded the 450 faces from the film deposition reaction. The ATR substrates were of the 450 single-pass parallelepiped (SPP) type [4], and were either 10 x 5 x 1 mm or 5 x 5 x 1 mm in size. The orientation of Si and other cubic semiconductors is commonly used for ATR substrates, since it facilitates producing them in the 450 SPP geometry. We used hot-wire chemical vapor deposition (HWCVD) [1] and plasma-enhanced chemical vapor deposition (PECVD, or glow-discharge) [2] methods to deposit the a-SiHx films. We prepared other HWCVD a-SiHx films on crystalline Si wafers. These samples were analyzed both by clamping them to an uncoated Si ATR substrate and by IR transmission. A Harrick Model 4XV 4x beam condenser equipped with a Perkin-Elmer wire-grid polarizer was operated with average incidence angle set to 45 + 15 degrees to the film plane to collect the FTIR-ATR spectra. The wide range of actual
Data Loading...
