a Landolt Börnstein b RWG Wyckoff, Crystal Structure Wiley, New York 1963)

1. Lattice parameter a / Å (300 K) Interatomic distance d / Å Band gap / eV (300 K) GaAs 5.6533a 2.448 ZnSe 5.6687a 2.450 (NaCl) MgS 5.2033b 2.602g 3.6c G-X: 2.7c ZB MgS 5.46c 2.36 G-X: 3.7c ZB MgS 5.66d 2.45 4.50h ZB MgS 5.621e 2.434 4.8 to 5.0i ZB MgS 5.622f 2.434 5.27-5.47j Renishaw Raman system (244-520nm) 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). Micro-Raman studies of zincblende MgS Th-P-26 D Wolverson, L. C. Smith, University of Bath, UK d.wolverson@bath.ac.uk; C. Bradford, B. C. Cavenett, K. A. Prior Heriot-Watt University, UK • 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. 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 G point still dominate). The shifts of the overtone peaks are significant because of the substantial curvature of the LO phonon dispersion around the G 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 G 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 G-point TO phonon (predicted frequency 688 cm-1). • 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: 10.65 Bohr (5.633ºA) (figure, above left); • experimental values range from 5.66ºA to 5.622ºA; • Phonon dispersion and density of states calculated via PWSCF. • 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 • Conclusions • Best Raman data yet obtained on ZB MgS, and • good agreement with detailed theoretical calculations. Agreement good - better than expected from the LDA and linear response approach - fortuitous!