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Article Dans Une Revue Physical Review B: Condensed Matter and Materials Physics (1998-2015) Année : 2002

High-pressure Raman spectroscopy study of wurtzite ZnO

Résumé

The high pressure behavior of optical phonons in wurtzite zinc oxide (w-ZnO) has been studied using room temperature Raman spectroscopy and ab-initio calculations based on a plane wave pseudopotential method within the density functional theory. The pressure dependence of the zone-center phonons (E2, A1 and E1) was measured for the wurtzite structure up to the hexagonal→cubic transition near 9 GPa. Above this pressure no active mode was observed. The only negative Grüneisen parameter is that of the E low 2 mode. E1(LO) and (TO) frequencies increase with increasing pressure. The corresponding perpendicular tensor component of the Born's transverse dynamic charge e * T is experimentally found to increase under compression like e * T (P) = 2.02 + 6.4 · 10 −3 .P whereas calculations give e * T (P) = 2.09−2.5·10 −3 .P (in units of the elementary charge e, P in GPa). In both cases, the pressure variation is small, indicating a weak dependence of the bond ionicity with pressure. The pressure dependence of the optical mode energies is also compared with the prediction of a model that treats the wurtzite-to-rocksalt transition as an homogeneous shear strain. There is no evidence of anomaly in the E2 and A1 modes behavior before the phase transition. PACS numbers: PACS: 78.30.-j, 64.70.Dv, 91.60.GF Zinc oxide belongs to the wide band-gap semiconductor family with large ionic characters of chemical bonds 2. The ionic size or relative electronegativity has been used to explain high pressure structure changes in A N B 8−N semiconductors. First principles calculations have shown that the zinc-blende (or wurtzite)→rocksalt→ β-Sn transition sequence is a common feature for most semiconductors. However, recent experimental identification of new intermediate phases (like cinnabar in CdTe for example) and the systematic absence of some phases (rocksalt (rs) for covalent compounds and β-Sn for all except the most covalent) invalidate the traditional transition sequence and, consequently, question the standard theoretical approach. In a recent Letter, Ozoli¸nšOzoli¸nš and Zunger 3 suggested that phase transitions are accompanied by phonon softening, whose instability has to be taken into account in the calculation to correctly predict the phase diagram. The latter outcome can also be discussed in terms of the transition mechanism, where the presence of negative Grüneisen parameters of phonon modes may play an important role. Recently, a new intermediate structure has been proposed in the wurtzite to rocksalt transformation path of GaN, along which the optical A 1 and E high 2 modes are expected to be affected by the new bond formations 4. Wurtzite ZnO, which transforms under pressure into the rs phase at 9 GPa, may provide a good example of this trend. Wurtzite ZnO belongs to the C 4 6v (P6 3 mc) space group. The primitive cell includes two formula units, with all atoms occupying 2b sites of symmetry C 3v. At the Γ point of the Brillouin zone, group theory predicts the existence of the following optics modes: Γ opt =A 1 +2B 1 +E 1 +2E 2. Frequency from the B low 1 and B high 1 silent modes has been calculated at 260 and 540 cm −1 respectively. A 1 , E 1 and E 2 modes are Raman active. In addition A 1 and E 1 are infrared active, and therefore split into longitudinal and transverse components (LO and TO). The mode assignment at ambient conditions is well established in the literature 5. To the best of our knowledge, no calculation and only two experimental attempts have been made to study the w-ZnO phonon frequency shift under pressure 6,7. In the first reference , the authors report the evolution of two phonon frequencies, E high 2 and E low 2 , over a relative small pressure range (0-1 GPa). In the second one, the pressure dependence of four Raman modes (E high 2
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Dates et versions

hal-01921305 , version 1 (13-11-2018)

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Frédéric Decremps, Julio Pellicer-Porres, A. Marco M Saitta, Jean-Claude Chervin, Alain Polian. High-pressure Raman spectroscopy study of wurtzite ZnO. Physical Review B: Condensed Matter and Materials Physics (1998-2015), 2002, 65 (9), ⟨10.1103/PhysRevB.65.092101⟩. ⟨hal-01921305⟩
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