Kinetics of solid phase crystallization in amorphous silicon

Abstract

In this review we have examined the crystallization behavior of a-Si over the temperaturerange from 500 °C to ∼ 1380°C. We have shown that SPE is a thermally activated process which is characterized by a single activation energy (2.7 eV) over the temperature range from ∼ 470 °C to ∼ 1350 °C. The activation energies for intrinsic SPE in ion-implanted and e-beam deposited layers on Si(100) substrates were found to be the same, implying that the interfacial bond breaking and rearrangement processes responsible for SPE in those layers is the same in spite of possible differences in microstructure. We showed that solid phase, transformations can occur in a-Si at temperatures far in excess of the a-Si melting point, T_al, and that the competition between solid phase crystallization and melting is a function of heating conditions (thermal rise-time, heating duration) and properties of the sample (amorphous film thickness). We evaluated kinetics of random nucleation and growth over a temperature range from 650 to 1380°C and showed that the random crystallization process is a well-behaved function of temperature over that temperature range with an activation energy of 4 eV. We found that random nucleation and growth becomes the dominant solid phase crystallization process at high temperatures ( > 1330 °C) in agreement with predictions based on differences between the activation energies for random crystallization and SPE.

The effects of doping and non-doping impurities on the kinetics of SPE and randomcrystallization were investigated as a function of temperature and impurity concentration. We showed that a variety of phenomena, including precipitation and impurity segregation can alter the intrinsic crystallization kinetics, and we identified time-temperature-concentration windows in which specific processes dominate. We concentrated on the investigation of crystallization behavior in layers containing impurities which illustrate the wide range of temperature- and concentration-dependent phenomena that can occur during heating of an amorphous thin film. Simple rate-enhancement and -retardation processes produced by doping (B and P) and non-doping impurities (F) were contrasted with the complex rate changes that can occur when processes such as impurity clustering and phase separation compete either individually or collectively with SPE in layers containing As, In and Au. We showed that random crystallization rates can be enhanced enormously by the presence of certain impurities and that impurity-enhanced nucleation can give rise to strong competition between random crystallization and SPE at temperatures much lower than observed in intrinsic layers. In the case of certain impurities (e.g. fluorine) the effects due to enhanced random crystallization in the high-temperature regime can be predicted from nucleation rate measurements performed at low temperatures. In other cases (e.g. arsenic), a different enhancement mechanism must be invoked to rationalize the experimental observations.

The competition between melting and solid phase crystallization was investigated at T ≥ T_al. We showed that melting at temperatures near T_al (1200°C) can be observed under microsecond duration heating conditions in thick (2600 A) films, whereas solid phase crystallization at temperatures in excess of 1300 °C is observed in thin (1000 Å) films. The differences in observed melting behavior have not yet been fully reconciled. A model which rationalizes the melting behavior in terms of amorphous phase relaxation during heating has been proposed [138], but more quantitative information concerning the thermodynamics of a-Si and 1-Si in the very high temperature regime is required before this model can be fully tested. Taken together with the results of nanosecond duration heating experiments, the results obtained using cw Ar and fl-pumped dye laser heating suggest that the kinetics of melt nucleation may play a pivotal role in determining the conditions under which melting of a-Si at T_al will occur.

We have shown that the interplay among solid phase crystallization and competitive processes in a-Si depends strongly on temperature. The combination of laser heating and in situ diagnostics provides a powerful capability for investigating crystallization behavior at temperatures that are inaccessible with conventional techniques. The ability to access the high-temperature regime has allowed us to obtain new information concerning the kinetics and thermodynamics of phase transformations and to characterize the competition that can occur among the various solid phase processes during heating of amorphous silicon.