Role of the hydrogen plasma treatment in layer-by-layer deposition of microcrystalline silicon

Abstract

We have investigated the role of hydrogen in hydrogenated microcrystalline silicon (μc-Si:H) formation using hydrogen plasma treatments, in particular examining the possibility of subsurface reaction due to permeating hydrogen atoms, which leads to the crystallization of hydrogenated amorphous silicon (a-Si:H). It is demonstrated that the hydrogen plasma treatment of a-Si:H film on the anode using a cathode covered by a-Si:H film, which is inevitably coated during the deposition period, gives rise to the deposition of μc-Si:H over the a-Si:H layer, i.e., chemical transport takes place. It is also found that the pure hydrogen plasma treatment using a clean cathode induces only etching of the a-Si:H layer. These results imply that the present hydrogen plasma condition does not cause crystallization of a-Si:H but only etching, and that careful experimentation is required to determine the real subsurface reaction due to atomic hydrogen.

Chemical sputtering by H2+ and H3+ ions during silicon deposition

“…The process in which silicon is etched from the powered electrode and redeposited on the grounded electrode is known as chemical transport. 17 In this process, the etch rate is lower than the gross deposition rate at the grounded electrode and at the powered electrode the etch rate is higher than the deposition rate. The net deposition rate on the grounded electrode can have several reasons, among others: a difference in the H y þ ion flux between the powered and grounded electrode, a difference in temperature, and an ion energy dependent etch yield.…”

Probing the phase composition of silicon films in situ by etch product detection

“…The H 2 plasma treatment time was set at Ͻ60 s to diminish H-induced material modification 11,14,18 and the influence of redeposition. 19,20 The time averaged baseline corrected SiH * emission at 414.3 nm proved to scale with the SiH 4 density 13,21 and is used here as a measure for the abundance of etch products during H 2 plasma treatment. After H 2 treatment the next cycle commences with the deposition of a fresh silicon film on top of the previous film using a different SiH 4 flow.…”

Connection between hydrogen plasma treatment and etching of amorphous phase in the layer-by-layer technique with very high frequency plasma excitation

“…6,7 Recent results showed that chemical transport within the deposition system also has to be taken into account and that under conditions where this mechanism is excluded the hydrogen treatment of a-Si:H causes only etching but no structural change. 8 This could mean that the growth of c-Si:H in the LbL process is a surface phenomenon and not caused by a volume phase transformation. To further investigate this we applied plasma excitation frequencies ex from the very high frequency ͑VHF͒ range to the LbL technique.…”

“…4,5 In the next deposition step the crystalline structure is preserved for a few monolayers, which means the crystallite columns grow. Once the crystallites are exposed at the surface, other processes like enhanced mobility of film precursors due to hydrogen coverage 6,7 and chemical transport within the deposition chamber 8 further promote crystallite formation.…”