Presentation on theme: "A Landolt Börnstein b RWG Wyckoff, Crystal Structure Wiley, New York 1963) c G Kalpana, B Palanivel, RM Thomas, M Rajagopalan, Physica B 222 (1996) 223."— Presentation transcript:
a Landolt Börnstein b RWG Wyckoff, Crystal Structure Wiley, New York 1963) c G Kalpana, B Palanivel, RM Thomas, M Rajagopalan, Physica B 222 (1996) 223 d L Konczewicz, P Biegenwald, T Cloitre, M Chibane, R Ricou, P Testud, O Briot, RL Aulombard, J. Crystal Growth 159 (1996) 117 e C Verié, J. Electronic Materials 27 (1998) 782 f C Bradford, CB O'Donnell, B Urbszek, A Balocchi, C Morhain, KA Prior, BC Cavenett, Appl. Phys. Letts. 76 (2000) 3929 g For the NaCl structure, d = a/2 h Y Morinaga, H Okuyama, K Akimoto Japan J. Appl. Phys. 32 (1993) 678 i U Lunz, C Schumacher, J Nürnberger, K Schüll, A Gerhard, U Schüssler, B Jobst, W Faschinger, G Landwehr, Semicond. Sci. and Technol. 12 (1997) 970 j I Suemune, H Uesugi, H Suzuki, H Nashiki, M Arita, Phys. Status Solidi B 202 (1997) 845 Lattice parameter a / Å (300 K) Interatomic distance d / Å Band gap / eV (300 K) GaAs a ZnSe a (NaCl) MgS b g 3.6 c -X: 2.7 c ZB MgS5.46 c X: 3.7 c ZB MgS5.66 d h ZB MgS5.621 e to 5.0 i ZB MgS5.622 f j Zincblende MgS is a wide bandgap semiconductor (~5eV). MgS can now be grown by MBE (in the zincblende structure) close to lattice-matched on GaAs. Main conclusions of this work: Resonant micro-Raman scattering has been carried out on ZB MgS with improved spectral range (improved dielectric filters); Good agreement with calculated phonon dispersion and DOS. D Wolverson, L. C. Smith, University of Bath, UK C. Bradford, B. C. Cavenett, K. A. Prior Heriot-Watt University, UK Micro-Raman studies of zincblende MgS Calculated phonon frequency Measured value TO344 cm -1 (±1 cm -1 ) LO430 cm cm -1 (±1 cm -1 ) Experimental observations Overtones of the LO phonon seen at 849, 1261 and 1674 cm -1 (n = 2…4 respectively); they are strong because MgS is highly polar and the Raman cross- section depends quadratically on the polaron coupling constant; Fitting the overtones shows that their widths are proportional to n whilst their peak positions lie below nth multiples of the LO phonon by approximately (15n) cm -1 ; This shift with n is evidence of the increasing participation in the multiple scattering process of phonons with finite wavevectors (though phonons close to the point still dominate). The shifts of the overtone peaks are significant because of the substantial curvature of the LO phonon dispersion around the point in ZB MgS (see calculated dispersion above). The other features that can be identified are those at 504 cm -1 and 759 cm -1 which are numerically close to the second and third multiples of a strong feature in the phonon density of states at around 250 cm -1 (see panels (c,d) of the comparison to experiment, above left). This feature arises from the LA phonon branch in the energy region of all of the X, L and W critical points and, since all two-phonon overtone scattering is Raman- active in zincblende crystals, 2LA and 3LA processes offer a possible explanation of these bands. However, scattering by a sum of point LO and TO phonons (predicted Raman shift 773 cm -1 ) gives another possible interpretation of the band at 759 cm -1. Finally, a weak band (marked *) can be see in in the experimental spectrum (panel (a), above left) with Raman shift 692 cm -1, assigned to the 2nd overtone of the -point TO phonon (predicted frequency 688 cm -1 ). Renishaw Raman system ( nm) with UV-enhanced CCD. Leica DM LM microscope with OFR UV-B objectives and translation stage for mapping. Coherent FRED frequency doubled CW Ar + ion laser (up to 100mW, 244nm, 5.08eV). Calculations: PWSCF code (www.pwscf.org); Local density approximation; BHS pseudopotentials for Mg and S of norm-conserving type; Exchange-correlation effects accounted for via the Perdew- Zunger parametrization; Non-linear core correction was included for Mg (the Mg 2p states were here not included in the valence states); (6,6,6) Monkhorst-Pack grid ; Convergence was checked with respect to this and to the kinetic energy cutoff (30 Ry); Lattice parameter found from the minimization of the total energy per unit cell: Bohr (5.633ºA) (figure, above left); experimental values range from 5.66ºA to 5.622ºA; Phonon dispersion and density of states calculated via PWSCF. Agreement good - better than expected from the LDA and linear response approach - fortuitous! Th-P-26 Conclusions Best Raman data yet obtained on ZB MgS, and good agreement with detailed theoretical calculations.