Relaxation, after annealing

Recent studies of doped a-Si H have found that the background density of localized states, that is, the electrically active dopants and dangling bond defects, are metastable (Ast and Brodsky, 1979 Street et al., 1986, 1987a Muller et al., 1986). After annealing above 150°C in the dark, the dark conductivity at room temperature of n- and p-type doped a-Si H decreases by nearly a factor of two over a time scale of several weeks for n-type and several hours for p-type a-Si H. As shown in Fig. 9 (Street et al., 1987a), the relaxation rate of the occupied band tail density nBT is a sensitive function of temperature, so that the time to reach... 

Figure 11.11 shows the stress-strain curves of PET, PTT and PBT fibers [4], Both PTT and PBT have a knee or a plateau region at about 5 and 7 % strains respectively, whereas PET stress increases smoothly with strain and does not have the plateau region. Table 11.6 compares the moduli of the three polyesters before and after annealing. The modulus of PET is nearly four times higher than those of PTT and PBT. After annealing, the PET modulus decreased by nearly half due to relaxation and loss of orientation. However, the PTT modulus increased by... 

Co3+ complexes of cobalamin have been cryoreduced by y-irradiation of aqueous-organic solutions at 77K, and the resulting Co2+ complexes have been characterized by EPR spectroscopy.73,74 Various nonequilibrium forms kinetically stabilized at low temperature gradually relaxed to the normal reduced complexes after annealing at 15 OK and higher. 

Finally, after annealing at 1300 K another (1x1) structure with improved long-range order as compared with the low temperature (1x1) phase shows up. Different from the low temperature phase the surface is here Ni terminated and no measurable relaxation could be found - NiAl(100)-(lxl)f i. 

Fig. 10.44. Structures of (a) Sieo and (b) Sii23 after annealing. All bonds below 2.8A are drawn out. The structures are fully relaxed, with the root-mean-square force to be 0.015 eV A [85].

In a-Si H NMR techniques have been useful primarily for the determination of local structural arrangements. Studies of H NMR at elevated temperatures, or at lower temperatures after annealing at elevated temperatures, have probed the details of hydrogen evolution in a-Si H. In undoped films both hydrogen (in the form of H and D) and silicon ( Si) have been investigated. In doped films studies have been reported for boron ( B), phosphorus ( P), and fluorine ( F). A second useful result of the H NMR measurements in a-Si H has been the inferred existence of trapped H2 molecules from spin-lattice relaxation measurements. The presence of these molecules has been very difficult to observe by other experimental techniques. 

It is clear from the foregoing that the observed phenomena cannot be explained by a static distribution of surface states but that this distribution changes with time following chemical surface reactions and strain relaxations. The time scale of these reactions and relaxations can span many hours and even days as shown by the time dependence of surface spins (Street and Knights, 1981) and of the sheet conductance after annealing (Ast and Brodsky, 1979)and after adsorption (Tanielian eta/., 1978,1980 Tanielian, 1982). 

---