Heterogeneity in microcrystalline-transition state: Origin of Si-nucleation and microcrystallization at higher rf power from Ar-diluted SiH4 plasma

Abstract

Using very high Ar-dilution to the SiH4 plasma, good quality amorphous Si:H films could be obtained at very low rf power. The a-Si:H film, prepared at a very low deposition rate of ∼10 Å/min, exhibited a σPh∼1×10−4 S cm−1, σPh/σD∼105, a notably wide optical gap of 2.10 eV and a very good stability against thermal annealing effects with reasonable light induced degradation. At higher rf power undoped μc-Si:H films were prepared with a high σD∼1×10−4 S cm−1, at a deposition rate of 30 Å/min from <1 sccm of SiH4. Micrograins were identified with several well-defined crystallographic orientations. However, porosity in the grain boundary zone contributed a significant amount of adsorbed effects on the electrical properties. At very high powers, the growth of a columnar network structure was demonstrated. Long-range structural relaxation permitted by the non-rigid and heterogeneous network structure associated with the physical vapor deposition-like growth at the microcrystalline-transition state, has been identified as the origin of nucleation to the Si network and microcrystallization at higher power. It is proposed that Ar* in the Ar-diluted plasma provides the energy required for nucleation and grain growth during microcrystallization, and plays an analogous role as atomic H does during chemical annealing in H2-diluted plasma.

Transport and stability studies on high band gap a-Si:H films prepared by argon dilution

“…As R is increased (SiH 4 flow rate is decreased) further, the deposition rate decreases drastically (1.3 A/s, for R = 630), resulting in a very low thickness of the film. Even for this argon dilution, the deposition rate is found to be much higher compared to that reported in the literature (0.5-0.83Å/s) [5]. For R ≤ 400 the band gap is estimated to be ∼1.85-1.90 eV, which increases to 2.16 eV for R = 630.…”

“…In the case of argon dilution other radicals like Si, SiH, SiH 2 are present in the plasma. Due to their low mobility and high reactivity these species react with Si-H bond on the surface resulting in columnar growth with high deposition rate [4,5,11]. In our case, when both hydrogen and argon are used as diluents, the presence of atomic hydrogen and Ar * causes effective dissociation of SiH 4 .…”

“…In order to increase the stability of these films, changes in deposition parameters including the dilution of the precursor gas silane (SiH 4 ) with hydrogen or argon have been tried. Whereas, considerably low deposition rate of films prepared using high hydrogen dilution of SiH 4 [3] puts a question towards cost-effectiveness of these films, columnar growth of argon dilution films [4,5] reduces the usefulness. Very high hydrogen and argon dilution further results in the polycrystalline/microcrystalline films [6][7][8].…”

Nanocrystalline silicon prepared at high growth rate using helium dilution

“…The average film deposition rate (R D ) as measured by Dektak 6M surface profiler, has been given in table 1. It has been observed that the film deposition rate follows a sublinear dependence with P, hence produces the SiH 4 depleted plasma which could initiate heterogeneity and promote nanocrystallization in the network (Knights 1980;Matsuda 1983;Das et al 2001).…”

“…He, Ar, Xe) dilution to SiH 4 is one way of producing high density plasma at low electrical excitation to increase the deposition rate. However, high Ar dilution leads to columnar growth and mostly produce defective network (Knights 1980;Das et al 2001), while Xe dilution maintains an amorphous nature of the network even at a very high RF power applied to the plasma (Matsuda et al 1991). He dilution undoubtedly increases the growth rate, however, it has not been reported to produce nanocrystalline silicon network (Tchakarov et al 2004).…”