Time-resolved cavity ringdown study of the Si and SiH3 surface reaction probability during plasma deposition of a-Si:H at different substrate temperatures

Abstract

Time-resolved cavity ringdown spectroscopy (τ-CRDS) has been applied to determine the surface reaction probability β of Si and SiH3 radicals during plasma deposition of hydrogenated amorphous silicon (a-Si:H). In an innovative approach, our remote Ar-H2-SiH4 plasma is modulated by applying pulsed rf power to the substrate and the resulting time-dependent radical densities are monitored to yield the radical loss rates. It is demonstrated that the loss rates obtained with this τ-CRDS technique equal the loss rates in the undisturbed plasma and the determination of the gas phase reaction rates of Si and SiH3 as well as their surface reaction probability β is discussed in detail. It is shown that Si is mainly lost in the gas phase to SiH4 [reaction rate kr=(3.0±0.6)×10−16m3s−1], while the probability for Si to react at an a-Si:H surface is 0.95<βSi<1 for a substrate temperature of 200°C. SiH3 is only lost in reactions with the surface and measurements of β of SiH3 for substrate temperatures in the range of 50–450°C show that βSiH3=(0.30±0.03), independent of the substrate temperature. The implications for a-Si:H film growth are discussed.

Hydrogenated amorphous silicon deposited under accurately controlled ion bombardment using pulse-shaped substrate biasing

“…For the growth flux this indicates substrate biasing has a clear effect on the gas phase, leading to additional SiH 3 production in the plasma. An increase in SiH 3 production has been observed in previous studies for rf biasing 5,26 and attributed to additional gas-phase reactions in the secondary plasma around the substrate holder. It was not observed in a recent study on PSB by Martin et al, 6 however, they assumed a constant mass density of 2.25 g / cm 3 and constant IR refractive index for all samples, while we include changes in both mass density and IR refractive index in our analysis via the Clausius-Mossotti relation.…”

Laser applications in thin-film photovoltaics

“…The importance of a silane radical with respect to the growth of Si films is assessed on the basis of its density in the plasma and its surface reaction probability. For conditions under which device-grade material is grown, it is found that SiH 3 is the most abundant radical in PECVD reactors [34]. Radicals with a higher Si hydrogenation are, however, less reactive.…”

“…By means of a simple chemical and surface reaction model, they deduced that Si and SiH radicals contribute to the film growth mechanism by about 0.2% and 5% only, respectively. From time-resolved CRD measurements in the afterglow of an ETP, Hoefnagels et al [34] showed that Si radicals have a higher gas-phase reactivity and, consequently, a shorter lifetime than SiH 3 radicals. Hoefnagels et al also derived a surface reaction probability of >0.95 and 0.3 for Si and SiH 3 , respectively.…”

“…Radicals with a higher Si hydrogenation are, however, less reactive. Overall, SiH 3 is believed to be the most significant film growth precursor [34,35].…”

Plasma monitoring and PECVD process control in thin film silicon-based solar cell manufacturing

“…Each property is usually detected by a special diagnostic technique, each with its advantages and limitations. Among the plasma diagnostics proven to be useful for investigations of thin film silicon growth are optical emission spectroscopy (OES) [14][15][16][17], laser-induced fluorescence (LIF) [14,18], Fourier-transform infra-red spectroscopy (FTIR) [16,19], tunable diode laser absorption spectroscopy (TDLAS) [20,21], quadrupole mass spectrometry (QMS) [15,22,23], or cavity ring down spectroscopy (CRDS) [24]. Measurements of the electron density and the dynamics of electrons in the plasma gas phase, however, remain challenging under PECVD conditions.…”