Effects of hydrogen on the growth of nanocrystalline silicon films by electron-beam excited plasma chemical vapor deposition

Abstract

It has been shown that nanocrystalline silicon films can be grown from silane gas without hydrogen dilution by electron-beam excited plasma chemical vapor deposition (EBEP–CVD). A high density of atomic hydrogen, which is derived from the dissociation of silane molecule, is confirmed in the plasma by optical emission spectroscopy. This fact is thought to be a reason for the growth of nanocrystalline silicon films without the introduction of hydrogen gas. Transmission electron spectroscopy reveals that crystallites are not distributed uniformly, but rather form the mosaic-like clusters in an amorphous silicon matrix in the film. Hydrogen gas is introduced into the EBEP–CVD silicon film growth so as to study the effects of the hydrogen gas. The growth rate increases proportionally to the hydrogen flow rate, and it is about 2.5 times greater than when no hydrogen gas is introduced. Also, a decrease in both the hydrogen content and the density of dangling bonds in the film is confirmed. These results imply that the generation of dangling bonds is suppressed by the introduction of hydrogen. The rate constants of dissociation reaction of silane molecules in the EBEP are estimated, and the change in density of radicals in the plasma by introducing hydrogen gas is calculated. A growth model is proposed that assumes the sticking coefficient of SiH3 precursor radical increases in proportion to atomic hydrogen flux. Using the calculation results and the model, the effects are explained to be due to an increase in the contribution ratio of SiH3 radicals to the growth, on account of the increase in the introduced hydrogen gas flow rate.

Effects of high hydrogen dilution on the optical and electrical properties in B-doped nc-Si:H thin films

“…Similar to mcSi [15,42], the growth species of nc-Si:H is generally believed to be the SiH y (y ¼ 023; mainly for y ¼ 3) precursors, which comes from the dissociation of the source gas SiH 4 by plasma. SiH 2 precursors can be inserted into any Si-H bonds and help to form dangling bonds [10], whereas SiH 3 can react only with sites associated with dangling bonds [42] to give the subsequent film growth. According to the 'surface diffusion model' [15], the atomic hydrogen affects the surface reaction kinetics.…”

“…where f a ; f c ; and f v are the volume fraction of the amorphous phase (mainly related to dangling bonds [10]), crystalline phase (mainly related to covalent bonds, and f c is equal to the crystalline volume fraction X c ), and voids (i.e., air), respectively, and…”

“…Especially, the effect of hydrogen dilution ratio (H 2 /SiH 4 ) has been demonstrated to play an important role in the structure and property of ncSi:H [7][8][9][10][11][12] and hydrogenated amorphous silicon (a-Si:H) [14]. To the best of our knowledge, as far as nc-Si:H deposited by PECVD is concerned, the effect of high hydrogen dilution (such as H 2 /SiH 4 from 62.5 to 142.9 in this article) on properties of nc-Si:H has been scarcely investigated, although some studies about the effect and role of low hydrogen dilution (H 2 /SiH 4 p40) are available in the literature [4,7].…”

“…It is obvious that different deposition conditions such as hydrogen dilution [4,7,8,[10][11][12], doping concentration [5,12], substrate temperature [13], and chamber pressure [9], may influence the optical and electrical properties of the nc-Si:H films. Especially, the effect of hydrogen dilution ratio (H 2 /SiH 4 ) has been demonstrated to play an important role in the structure and property of ncSi:H [7][8][9][10][11][12] and hydrogenated amorphous silicon (a-Si:H) [14].…”