Phonon Mode Broadening Parameters

The phonon mode broadening parameter of the ZnO bulk sample2 in Fig. 3.8a is 7 = 13 1 cm-1. Similar values were reported for the (0001) ZnO thin film on (0001) sapphire in Fig. 3.8b 7 l = 10 lcm 1, and the (1120) ZnO thin film on a-plane sapphire in Fig. 3.9 7 = 15.1 0.2 cm-1, 71 = 10.7 0.5 cm Accordingly, the crystal quality of these ZnO thin films are comparable with that of the ZnO bulk sample, which was confirmed by X-ray diffraction [43]. 

Typical phonon mode broadening parameters for a set of ZnO thin films grown on silicon by PLD with varying oxygen partial pressure and/or substrate heater power are shown in Fig. 3.14. Heitsch et al. [30] observed that the 

2 For (0001)-oriented ZnO films, IRSE data are not sensitive to the A1 (TO) mode. 

Accordingly, the broadening parameter 7 cannot be determined. For setting up the MDF parameters, it is then often assumed 7 71, which has no influence 

In addition to defects and impurity incorporation, alloy-induced disorder further increases the phonon mode broadening parameters, as discussed 

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By evaluation of the phonon-mode frequencies, information about strain [29] or about the incorporation of doping or alloying atoms can be derived. Besides the phonon-mode frequency, the phonon-mode broadening parameter provides information about crystal quality [30], because scattering due to a lower crystal quality or due to alloying makes the phonon-mode broadening parameter larger. 

Fig. 3.14. Phonon mode broadening parameters, as determined by IRSE, vs. oxygen partial pressure for a set of PLD-grown ZnO thin films on (111) silicon. Triangles and squares represent data of thin films grown with substrate heater power of P = 400 W and P = 600 W, respectively. Reprinted with permission from [30]...

Fig. 3.15. Phonon mode broadening parameters of PLD-grown rocksalt-structure MgzZni-zO thin films on sapphire. Reprinted with permission from [74]...

While there are no experimental data available for cubic ZnO at atmospheric pressure, ab initio calculations for phonon properties of cubic ZnO, which relied on experimental data of rocksalt ZnO studied under high pressures ( 8 GPa) as input parameters, have been undertaken [55]. The predictions by such an exercise for a>ro and cOlo lead to 235 cm and 528 cm, respectively, for cubic ZnO. The values are smaller than those obtained by extrapolating the IRSE analysis. However, it should be pointed out that both extrapolations follow the same trend in predicting phonon mode frequencies and that they are smaller than those of hexagonal ZnO. The width of phonon modes depends on sample quality and processes that lead to broadening. A discussion of phonon mode broadening parameters can be found in Ref. [26, 27]. 

In this chapter some of the presently known optical properties of zinc oxide are reviewed. In particular, the anisotropic dielectric functions (DFs) of ZnO and related compounds from the far-infrared (FIR) to the vacuum-ultraviolet (VUV) spectral range are studied. Thereupon, many fundamental physical parameters can be derived, such as the optical phonon-mode frequencies and their broadening values, the free-charge-carrier parameters, the static and high-frequency dielectric constants, the dispersion of the indices of refraction within the band-gap region, the fundamental and above-band-gap band-to-band transition energies and their excitonic contributions. 

Table 9.4 Best-fit values and standard deviations of the , (TO), (LO), and phonon frequencies, to, and broadening parameters, y, for polarizations along the GaN [1120] (/ = x) and [1100] / = y) directions for an o-plane GaN film. Forthe ] modes, j denotes the phonon polarization, while for the 2 modes, j denotes the polarization of the incident light in the parallel and cross-polarized configurations, respectively.

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