Uniaxial stress studies

Uniaxial Stress Studies of Lattice Defects Decorated with H 170... 

The implantation of hydrogen into silicon or crystal growth in a hydrogen atmosphere introduces vibrational bands that have been ascribed to lattice defects decorated with hydrogen. While IR experiments were begun —10 years before similar studies of passivated shallow impurities, the structures of the complexes that result from H+ implantation are not well understood. This subject has been reviewed previously by Pearton et al. (1987, 1989). Here, the central experimental results will be summarized. A recent uniaxial stress study (Bech Nielsen etal., 1989) of several of the vibrational features will be discussed in Section IV.3. 

The assignment of the 809 and 1560 cm-1 bands of the donor-H complexes were based upon the expected frequencies of the wagging and stretching modes and the 2 1 ratio of intensities (Bergman et al., 1988a). A uniaxial stress study of the vibrational bands (Bergman et al., 1988b) will be reviewed here. Only the As—H complex was studied under stress because the donor-H complexes have nearly identical vibrational spectra for the different donors and are expected to behave similarly under stress. 

IR absorption and Raman spectroscopy are also well suited to the application of uniaxial stress techniques. Stress-induced splittings of the vibrational bands give information about the symmetry of the defect. In favorable cases, it has been possible to determine the kinetics of the H motion between equivalent sites around an impurity from a study of the alignment of the H-related complexes that can be induced by stress. 

The uniaxial stress dependence of the IR absorption and Raman bands due to the B—H complex have been studied by Bergman et al. (1988b) and by Stutzmann and Herrero (1988a,b) and Herrero and Stutzmann (1988, 1989). The observed stress splittings and their interpretation for the two studies do not agree. Here, the two experiments and their interpretation will be summarized. While the differences between the experiments will be discussed, they will not be resolved. 

The most documented case is the Be-H complex in GaAs, which is characterized by a LVM at 2037 cm-1 (Nandhra et al., 1988). Stavola et al. (1989) have studied the effect of uniaxial stress on this LVM. The use of uniaxial stress allows the orientational degeneracies to lift and therefore gives the symmetry of the center evidenced by its LVM. 

The effect of uniaxial stress on several other LVMs has been studied (Clerjaud et al. 1988b). Their behavior under uniaxial stress is very complex and the nature of the lattice defects that are involved has not been determined yet. 

Uniaxial stress lowers the threshold current of a Ga-As junction laser and shifts the emission peak to shorter wavelengths This effect can be used to study the influence of crystal asymmetry of the spatial and spectral distributions of emitted photons. 

The elastic energy of inhomogeneous, anisotropic, ellipsoidal inclusions can be studied using Eshelby s equivalent-inclusion method. Chang and Allen studied coherent ellipsoidal inclusions in cubic crystals and determined energyminimizing shapes under a variety of conditions, including the presence of applied uniaxial stresses [11]. 

Clay system studied rigorously as a function of uniaxial stress p over a wide range of well-defined salt concentrations between 0.001 and 0.1 M... 

The main feature of Figure 8.8 is that the curves (b), (c) and (d), measured at p = 0, 0.04 and 0.07 atm, respectively, are very similar to each other. Comparison with Figure 8.3 also reveals that they are very similar to the corresponding curve obtained with c = 0.1-M H-salt, and the same structural analysis applies in each case. Before considering the details, it is clear that the dressed macroion structure is basically unaffected by uniaxial stress over the pressure regime studied. 

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