Anisotropic Phonon Deformation Potentials

The shear strain component for the anisotropically strained fl-plane GaN films is zero when the axes of the laboratory system are chosen along the crystal principal axes (see Section 9.3). It follows from Equation 9 then that the splittings of the Ei and E2 phonons depend only on the magnitude of the in-plane strain anisotropy, Consequently, the difference between the 

The c phonon deformation potentials for the Ei(TO), Ei(LO), and Ej phonons can then be estimated using the experimental results for the strain components and the phonon frequency splittings [17, 18). The results obtained for anisotropically strained nonpolar a-plane and polar c-plane GaN films are listed in Table 9.5. 

A correlation between phonon mode frequencies and strain components in films with different strains can, in principle, allow determination of the isotropic deformation potentials, a and b (see Equations 9 and 8). a-Plane GaN films offer a unique opportunity to obtain the Ai(TO) phonon deformation potentials by GIRSE. This is particularly important in view of the existing discrepancy in the literature between the values of the Ai(TO) phonon deformation potentials determined by Raman scattering [54] and theory [14]. Further, the aEi(io) and fcii(LO) have not been experimentally determined yet. 

It has been suggested that the relatively large uncertainties of the deformation potentials (Table 9.6) stem mostly from the regression procedure itself [55]. 

The i i(LO) phonon defomiation potentials a and b have also been detennined by performing a regression analysis of the following equation [17]  

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Table 9.5 Anisotropic phonon deformation potentials, C ,(to)i C ,(LO). and Ce, in cm, and the respective standard deviations.

The inconsistency in the strain components in the a-plane QD samples reported in [57, 58] indicates a significant impact of the assumptions made in their estimation from the phonon frequencies. We note that in aU estimations of the strain components, the anisotropic phonon deformation potentials chj(to) and ce were assumed to be zero [57]. The results of the anisotropically strained GaN films presented in Sections 9.4.2 and 9.4.3 show clear splittings of the El and 2 indicating nonzero anisotropic deformation potentials for these phonon modes (Tables 9.4 and 9.5). We point out, however, that the values of the C j(xo) and c in Ref. 17 may be also affected by the assumptions made in the strain component determination by XRD (for instance the deviation from hexagonal symmetry is assumed to be small). The studies of the vibrational properties of nitride materials with nonpolar surface orientations are scarce and clearly, further experimental and theoretical investigations are needed to clarify these issues. 

Darakchieva V, Paskova T, Schubert M, Arwin H, Paskov PP, Monemar B, Hommel D, Heuken M, Off J, Scholz F, Haskell BA, Fini PT, Speck JS, Nakamura S (2007) Anisotropic strain and phonon deformation potentials in GaN. Phys Rev B 75 195217... 

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