ESR Study of Crystallization of Hydrogenated Amorphous Silicon Thin Films

Abstract

Solid-phase crystallization and the subsequent re-hydrogenation of the amorphous silicon thin films provides a low cost approach for thin-film crystalline Si:H-based photovoltaic devices. During the hydrogen effusion, significant lattice reconstruction occurs, as hydrogen is driven out of the film, accompanied by creation and migration of a large number of dangling bonds. We used electron-spin-resonance (ESR) to study evolution of the local order surrounding these dangling bonds during crystallization. When samples made by both plasma enhanced chemical vapor deposition (PECVD) and the and hot wire CVD (HWCVD) are heated to 560°C, hydrogen effuses within 30 min, giving rise to H-effused defect densities of about 5x1018 cm-3. Further heating at 560°C results in crystallizati°n in the HWCVD sample after about 200 min. On the other hand, PECVD samples crystallize only when heated up to 580°C, and then only after much longer times (Dt ~ 1300 min) [1,2]. ESR defects in both samples persist at the 5x10¹⁸ cm^-3 level as long as the sample remains amorphous during the grain nucleation period. As the crystallites appear, the defect densities gradually decrease and saturate at about 3x10¹⁷ cm^-3 as the crystallization is completed, both in HWCVD and PECVD samples.

In the H-effused states before crystallization, the ESR signals for both the HWCVD and PECVD samples show significant exchange-narrowing, suggesting that the defects are probably clustered. As the sample crystallizes, the defect clustering largely disappears, yet the line-widths in fully crystallized films are somewhat narrower than those in typical micro-crystalline silicon thin films as reported earlier [3]. This difference is probably due the specific structures of the grain boundaries in the present study. The effect of re-hydrogenation on both the H-effused amorphous and crystallized states will be discussed.