Physical Properties of Two Metastable States of Amorphous Silicon
- PDF / 433,654 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 106 Downloads / 214 Views
PHYSICAL PROPERTIES OF TWO METASTABLE STATES OF AMORPHOUS SILICON G.K. HUBLER*, C.N. WADDELL**, W.G. SPITZER**, J.E. FREDRICKSON***, AND T.A. KENNEDY* *Naval Research Laboratory, Washington, D.C.; **Physics and Materials Science Departments, University of Southern California, Los Angeles, California; ***Physics-Astronomy Department, California State University, Long Beach, California ABSTRACT Characterization of the two metastable states of amorphous Si produced by ion implantation is extended to include electron paramagnetic resonance, fundamental absorption edge, and density measurements in addition to infrared reflection. It is found that the properties of the two a-Si states are not dependent upon the mass of the 12 29 incident ion ( C, Si, Tp, 120Sn) or upon the anneal temperature for 400 0 ASI-0.30-0.o0 ASI-O.•8- 0.5O
,
density,
AP/pc
for
C, Si, and Sn implanted samples annealed at 4000C as a function of anneal time.
.96
0
0aS
T "300•C 6
10
20 itllh)
30
04-0SA
40
Fig.3. Comparison of the of in index changes refraction, fD, for anneal temperatures
4000,0 600 C.
5000,
TA
-
5500
3000,
and
increase of -28% in the average bond polarizability for the change from c-Si to a-Si-I, and a decrease of -8% for the change from a-Si-I to a-Si-Il. The observed decrease in the near band edge absorption after the a-Si-I-->a-Si-II transition occurs appears to be consistent with the proposal of a network reorganization in which there is a reduction in disorder [5]. The absorption in this region is often associated with the fluctuations in the atomic configurations [10] which cause shifts in electronic states and give rise to band tails. The reduction in disorder by network reorganization thus appears to be the cause for the reduction in absorption and the -8% decrease in average bond polarizability. The decrease in dangling bond density seems to be directly related to the annealing of defects within the amorphous structure. If the transition to a-Si-Il is a phase transition, one might expect a release of heat during the phase change. Using differential scanning calorimetry, Donovan et al. [11] has measured the heat of crystallization of a-Si and a-Ge prepared by ion implantation. For a-Ge, they observe a substantial heat of stabilization prior to crystallization, (1/2 of the heat released during crystallization) but no heat of stabilization is observed for a-Si prior to crystallization. Our preliminary refractive index data for as-implanted and thermally stabilized a-Ge [12] shows a similar relaxation to that occurring in Si. It is therefore puzzling that there is no heat of stabilization measured for Si and indicates that the mechanism producing the relaxation is different for the two materials. Given that two distinct states of a-Si can be produced, it is appropriate to speculate on the effects that the two states may have on
222
For example, in pulse laser annealing, the other material properties. coupling of the radiation prior to melting would change due to changes in the optical constants, especi
Data Loading...
