Structural properties of hot wire a-Si:H films deposited at rates in excess of 100 Å/s

Abstract

The structure of a-Si:H, deposited at rates in excess of 100 Å/s by the hot wire chemical vapor deposition technique, has been examined by x-ray diffraction (XRD), Raman spectroscopy, H evolution, and small-angle x-ray scattering (SAXS). The films examined in this study were chosen to have roughly the same bonded H content CH as probed by infrared spectroscopy. As the film deposition rate Rd is increased from 5 to >140 Å/s, we find that the short range order (from Raman), the medium range order (from XRD), and the peak position of the H evolution peak are invariant with respect to deposition rate, and exhibit structure consistent with a state-of-the-art, compact a-Si:H material deposited at low deposition rates. The only exception to this behavior is the SAXS signal, which increases by a factor of ∼100 over that for our best, low H content films deposited at ∼5 Å/s. We discuss the invariance of the short and medium range order in terms of growth models available in the literature, and relate changes in the film electronic structure (Urbach edge, as-grown defect density) to the increase in the SAXS signals. We also note the invariance of the saturated defect density versus Rd, measured after light soaking, and discuss possible reasons why the increase in the microvoid density apparently does not play a role in the Staebler–Wronski effect for this type of material.

Identification of Nucleation Center Sites in Thermally Annealed Hydrogenated Amorphous Silicon

“…[22] To obtain information on a larger scale (extended-range disorder), such as that associated with structural inhomogeneities such as microvoids, two techniques have been used, small angle X-ray scattering (SAXS), [24] and infrared (IR) measurements of the amount of Si-H stretch mode absorption in the 2070 cm À1 ''shifted'' mode, from which the microstructure parameter R Ã can be calculated. [25] While SAXS has examined the microvoid densities of high deposition rate HWCVD films, [26] it has been used primarily to examine the structure of device quality PECVD films and low C H HWCVD films. While it is still unclear whether such microvoids exist, or can be detected in device quality PECVD a-Si:H, the microvoid density (0.01%) in a low C H ($2 at.%) HWCVD a-Si:H film has been examined, and was found to be at the detection limit of the spectrometer used for these measurements.…”

Study of amorphous nanocrystalline thin silicon films by grazing-incidence small-angle X-ray scattering

“…In the case of thin film silicon, SAXS has been successfully applied to confirm the crucial role of nano-structure in light-induced degradation, in diffusion of hydrogen, in phase segregation, etc. (Prado et al, 1997;Williamson, 1995;Shinar et al, 1999;Mahan et al, 2001;Gurman et al, 2000). However, since grazing-incidence small-angle X-ray scattering (GISAXS) detects only the presence of objects which have an electron density that differs from the average electron density of the material ('particles'), for a complete analysis this technique has to be combined with other methods.…”

From Nanovoids to Blisters in Hydrogenated Amorphous Silicon

“…According to Refs. [7] and [8], voids and related H bonding configurations are also involved in thè Staebler-Wronsky effect [9], Le. degradation of thè hydrogenated a-Si properties upon illumination.…”