Symmetry elements thin films

Figure 3.12 shows typical Raman spectra of several doped ZnO thin films. Additional modes (AM), occurring at to 275, 510, 582, 643, and 856 cm-1 (the first four of them are shown and marked by vertical solid lines in Fig. 3.12), were first assigned to N-incorporation [49-51], because the intensity of these modes was reported to increase with increasing N-content [50], However, the AMs appear also in Raman spectra of ZnO samples doped with other elements (Fig. 3.12a), [48,52,53]). Therefore, it was suggested that the AMs are related to defect-induced modes [48]. Theoretical considerations confirmed this assignment [131]. It was discussed that the AMs could be related to modes of ZnO, which are Raman-inactive within a perfect crystal. Upon doping-induced defect formation, the translational crystal symmetry can be broken, and Raman-inactive modes may become Raman-active. The Raman spectra of the ZnO thin films with transition metals in Fig. 3.12b show a different behavior than those in Fig. 3.12a [43,48], Raman spectra of Fe0.08Zn0.92O contain the above described AMs, but with different intensity ratios. For MnZnO, CoZnO, and NiZnO a broad band between iv 500 cm-1...

For the wurtzite-structure Mg Zni- O (x < 0.53) thin films, an one-mode behavior with a further weak mode between the TO- and LO-mode for the phonons with E - and Ai-symmetry was found. The Ai(TO)-, Ai(LO)-, and the upper branch of the Ai(LO)-modes of the wurtzite-structure thin films show an almost linear behavior, whereas the lower branch of the Eq(LO)-modes and the two Ai(TO)-branches exhibit a nonlinear behavior. In [132] the modified random element isodisplacement (MREI) model was suggested... 

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