Phonons wurtzite-structure

The wurtzite-structure optical phonons at the / -point of the Brillouin zone belong to the following irreducible representation [32]... 

Both A - and Ei-modes are Raman and IR active. The two nonpolar E2-modes E and E are Raman active only. The Bi-modes are IR and Raman inactive (silent modes). Phonon dispersion curves of wurtzite-structure and rocksalt-structure ZnO throughout the Brillouin Zone were reported in [106-108]. For crystals with wurtzite crystal structure, pure longitudinal or... 

Table 3.2 summarizes the optical phonons of crystals with wurtzite structure and the scattering configurations, in which the optical phonons are predicted to produce backward signal in first-order Raman scattering. 

In the / -spectrum of the ZnO thin film, a similar plateau as in the 3 -spectrum of the ZnO bulk sample is present. However, the phonon modes of the sapphire substrate introduce additional features, for example atw 510, 630, and "-900 cm 1 [38,123]. The spectral feature at w 610 cm-1 is called the Berreman resonance, which is related to the excitation of surface polari-tons of transverse magnetic character at the boundary of two media [73]. In the spectral region of the Berreman resonance, IRSE provides high sensitivity to the A (LO)-mode parameters. For (OOOl)-oriented surfaces of crystals with wurtzite structure, linear-polarization-dependent spectroscopic... 

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... 

Fig. 3.13. Phonon-mode frequencies of wurtzite-structure PLD-grown Mg Zni- O thin films with Ai-symmetry (panel a, triangles) and Fi-symmetry (panel b, triangles), and of rocksalt-structure PLD-grown Mg Zni- O thin films (circles in both panels) vs. x [43,62,72,74], Open and solid symbols represent TO- and LO-modes, respectively. The dashed lines are linear approximations of the rocksalt-structure phonon modes from [74], the solid lines represent MREI calculations for the wurtzite-structure phonon modes redrawn from [132]. The shaded area, marks the composition range, where the phase transition occurs. Reprinted with permission from [74]...

AIN exists in two types the hexagonal (wurtzite structure) and the cubic (zincblende structure). The former is more stable, and has been investigated in more detail. The wurtzitic AIN has two formula units per unit cell (4 atoms per cell) and 9 optical branches to the phonon dispersion curves [1] ... 

Fig. 4.1-62 AIN (wurtzite structure). Phonon dispersion curves (left panel) and phonon density of states (right panel), from a rigid-ion model calculation [1.58,59]...

Figure 1.18 Schematic depiction of the phonon dispersion curves for zinc blende and Wz structures. Also shown are the F and A points of the zone in relation to the real-space hexagonal structure. Phonon branches along the [1 1 1] direction in the zinc blende structure are folded to approximate those of the wurtzite structure...

Vibrational properties of MgxZni 3(0 have been measured over the entire compositional range covering the wurtzite structure on the low end and cubic on the high end. The phonon mode frequencies of Mg Zni- O versus x, as obtained by utilizing... 

Fig. 3.2. Displacement patterns of the optical phonons of a lattice with wurtzite crystal structure. Reprinted with permission from [105]...

The phonon mode frequencies of wurtzite- and rocksalt-structure Mg Zni- O thin films vs. x, as obtained by combination of Raman scattering and IRSE, are plotted for 0 < x < 1 in Fig. 3.13. 

Infrared reflection and Raman spectroscopies have been employed to derive zone centre and some zone boundary phonon energies in wurtzite and in zincblende GaN (TABLE 1) [1-10]. Phonon and coupled modes have been employed to characterise stress conditions and carrier densities in thin films and device structures. 

Wurtzite ZnO structure with four atoms in the unit cell has a total of 12 phonon modes (one longitudinal acoustic (LA), two transverse acoustic (TA), three longitudinal optical (LO), and six transverse optical (TO) branches). The optical phonons at the r point of the Brillouin zone in their irreducible representation belong to Ai and Ei branches that are both Raman and infrared active, the two nonpolar 2 branches are only Raman active, and the Bi branches are inactive (silent modes). Furthermore, the Ai and Ei modes are each spht into LO and TO components with different frequencies. For the Ai and Ei mode lattice vibrations, the atoms move parallel and perpendicular to the c-axis, respectively. On the other hand, 2 modes are due to the vibration of only the Zn sublattice ( 2-low) or O sublattice ( 2-high). The expected Raman peaks for bulk ZnO are at 101 cm ( 2-low), 380 cm (Ai-TO), 407 cm ( i-TO), 437 cm ( 2-high), and 583 cm ( j-LO). 

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