The Effect of Post-Deuteration on Metastability in a-Si:H
- PDF / 379,912 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 55 Downloads / 151 Views
THE EFFECT OF POST-DEUTERATION ON METASTABILITY INa-Si:H
N.H. NICKEL, W.B. JACKSON and C.C. TSAI Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 ABSTRACT
Hydrogenated amorphous silicon films were deuterated through a sequence of 1h exposures to a remote deuterium plasma at 350'C. The concentration profiles of hydrogen and deuterium were determined by SIMS at various times during the exposure sequence. The defect density in state A, after deuteration and after illumination with white light were determined using CPM measurements following each lh exposure sequence. We find that post-deuteration does not alter the defect density in state A, change the Urbach edge, nor significantly alter metastable defect formation. Intense light soaking increases the defect density by about 5X10 16 cm- 3 independent of the total H+D concentration. These results suggest that D always enters the sample in pairs pinning the hydrogen chemical potential which supports the idea of a negative U system for hydrogen and deuterium. Despite an increase of Si-H bonds by as much as 3x10 21cm- 3 , the annealed dangling bond density and the weak Si-Si bond density did not change.This suggests that the density of weak Si-Si bonds as well as the dangling bond density is determined by equilibration with strong Si-Si bonds through the interchange of H. The implications of these results for H bonding will be discussed. INTRODUCTION
Hydrogenated amorphous silicon (a-Si:H) owes its widespread use as the active layer in devices such as solar cells and thin-film transistors to the role of hydrogen in efficiently passivating unsaturated dangling bonds. Its major draw back, however, is the metastable defect formation caused either by strong illumination [11, carrier accumulation [2,31 doping [4,51 or rapid thermal quenching [6]. The physical mechanism underlaying metastability is believed to be hydrogen migration [7,8] since H diffuses relatively rapidly through the amorphous network at low temperatures [9] and doping [10] and illumination [111 even enhance hydrogen motion. In order to stabilize a-Si:H, higher deposition temperatures have been suggested recently [12], and the increased stability was attributed to a reduction in the hydrogen content of the a-Si:H film from 10 at.% to 4 at.%. On the other hand, an improved thermal stability for a-Si:H films deposited in a remote-hydrogen-plasma reactor at 4000C was reported. In these films the H content was maintained at 10 at.% using hydrogen dilution [131. The improvement was related to a change in the H migration kinetics and to a structural change in the random network of the silicon-hydrogen alloy. Johnson et al. [13] suggested that H is more stable incorporated in the network during a high temperature deposition and that the weak Si-Si bond density may be reduced. While the effects of hydrogen introduced during film growth on the equilibrium defect density, metastability and the Si-Si bond disorder in a-Si:H have been widely studied, little is known about the influence of ad
Data Loading...
