Inelastic light scattering and photoluminescence in semiconductors and their heterostructures

Eunkyung Suh, Purdue University

Abstract

Spectroscopic techniques provide deep insights into the physics of films, epilayers, and quantum well structures. The mismatch of the elastic, electronic, and magnetic properties of the constituents forming a given heterostructure; lattice mismatch and the resultant "strained" superlattice; the vibrational, electronic, and magnetic excitations--collective or localized in character and sustained within a constituent, propagating throughout the heterostructure or along interfaces--these are some of the fundamental issues which have been addressed with great precision and detail using Raman scattering in superlattices of the Mn-based diluted magnetic semiconductors Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te/Cd$\sb{\rm 1-y}$Mn$\sb{\rm y}$Te and ZnSe/Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se. In GaAs/Al$\sb{\rm x}$Ga$\sb{\rm 1-x}$Te single quantum wells, confined optical phonons are observed in the first-order Raman spectrum when the scattered radiation comes into the resonance with the electronic transitions of the quantum well. The confined LO phonons have wavevectors extending to a significant fraction of the Brillouin zone and their frequencies agree well with those deduced from the bulk dispersion curve. The observation of the s-d exchange interaction enhanced spin-flip Raman shift from electrons in Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te:In epilayers and modulation doped Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te:In/CdTe superlattices is studied to demonstrate successful controlled doping during the growth by photoassisted molecular beam epitaxy. The resonance Raman enhancement, confinement of electrons to the CdTe well in the superlattices, the penetration of the electronic wavefunctions into the Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te barriers, the spatial location of the intentionally introduced donors, and the strength of the s-d interactions are investigated. Optical phonons of Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se (0 $\leq$ $\times$ $\leq$ 0.33) in the zinc-blende phase and Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se and Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$S in the wurtzite phase are investigated by Raman scattering. The zone-center optical phonons of Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se exhibit "two-mode" behavior whereas those of Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se and Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$S exhibit behavior intermediate between the "two-mode" and "one-mode" situations. Under hydrostatic pressure, in contrast to pure ZnSe, the Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se mixed crystals undergo a high pressure phase transition between 28 and 52 kbar depending on the composition. The energy of the Mn$\sp{2+}$ internal transition shows a negative pressure coefficient.

Degree

Ph.D.

Advisors

Ramdas, Purdue University.

Subject Area

Condensation

COinS