(High pressure) Raman study of the phonons / phonon-polaritons of ZnTe-based semiconductor mixed crystals

Replacement of electrons by photons as the carrier of information, leading to the development of photonics besides electronics, is highly promising in view to ultimately accelerate the signal processing in matter. On this way one resorts to elementary crystal excitations able to couple with light. The phonon-polaritons of semiconductor crystals with zinblende (cubic) and wurtzite (hexagonal) structures offer an interesting option in the THz spectral range. In fact, the anisotropic wurtzite structure generates a wide variety of phonon-polaritons, as recently shown with GaN [1]. In contrast, the phonon-polaritons of the pure zincblende compounds are difficult to handle because the phonon decoupling occurs as soon as penetrating deep into the phonon-polariton regime. Alloying seems an interesting option in this case since the additional alloy-related phonon-polariton branch preserves a pronounced phonon character throughout its dispersion [2].

Our team has gained an expertise in the detection of the phonon-polaritons of mixed crystals using near-forward Raman scattering, and in the modeling of the phonon-polariton Raman spectra within the dielectric approach. Up to now our attention focused onto the zincblende II-VI semiconductor mixed crystals based on ZnSe, that exhibits a large optical band gap (2.70 eV, needed to perform near-forward Raman measurements). In a recent extension to the wurtzite-type ZnMgSe mixed crystal, also based on ZnSe, it was possible to penetrate ultimately deep inside the phonon-polariton regime [2]. In each crystal structure a prerequisite to the discussion of the phonon-polaritons is a refined description of the underlying phonons in mixed crystals, generally a debated issue in the literature. This is achieved in terms of a generic 1-bond→2-phonon behavior of mixed crystals as formalized within our so-called phenomenological percolation scheme [2].

In this PhD project, the attention will be centered on the Raman study of the phonon-polaritons propagating in volume of II-VI semiconductor mixed crystals with zincblende structure based on ZnTe – that also exhibits a large optical band gap (2.26 eV), such as Cd_1-xZn_xTe. The phonon mode behavior of the latter system is controversial [3], and its phonon-polariton modes remain unexplored. As a sideline, the PhD project also includes a Raman study of phonon-polaritons of one nitride-based III-V compound with wurtzite structure characterized by a large vibrational anisotropy, namely AlN, supporting by a modeling of the corresponding Raman cross section.

The phonon-polariton Raman studies of the Cd_1‑xZn_xTe-zincblende and AlN-wurtzite crystals will be done at ambient pressure and at high pressure [4] (using a diamond anvil cell), with ab initio calculations of the underlying phonon modes in support.

1.            G. Irmer, C. Röder, C. Himcinschi and J. Kortus, Phys. Rev. B 88, 104303 (2013)

2.            H. Dicko, O. Pagès, M.B. Shoker, F. Firszt, K. Strzalkowski, A. Maillard, A. Polian, Y. Battie, L. Broch, A.                    EnNaciri, A.V. Postnikov, W. Paszkowicz and J.P. Itié, Scientific Reports 9, 7817 (2019).

3.            S.P. Kozyrev, Semiconductors, 49, 885 (2015).

4.            M.B. Shoker, O. Pagès, V.J.B. Torres, A. Polian, J.P. Itié, G.K. Pradhan, C. Narayana, M.N. Rao, R. Rao,                  C. Gardiennet, G. Kervern, K. Strzalkowski and F. Firszt, Scientific Reports 10, 19803 (2020)

Contact :             Resp. O. Pagès, LCP-A2MC, Université de Lorraine, Metz, France

                           Co-resp. A. Postnikov, LCP-A2MC, Université de Lorraine, Metz, France

PhD-Grant :        Circa. 1400 € net salary per month over 36 months

Web site of the Doctoral School (C2MP, Lorraine University) where candidates can apply :

http://doctorat.univ-lorraine.fr/fr/les-ecoles-doctorales/c2mp/offres-de-these

Funding category: Contrat doctoral

concours annuel pôle scientifique

PHD title: Doctorat de Physique

PHD Country: France