Optical and Magneto-Optical Studies of (lead, Europium)telluride Thin Films and Superlattices.
Optical properties of (Pb,Eu)Te thin films and superlattices grown by MBE have been studied through absorption and photolumines- cence techniques in external magnetic fields of up to 5T. Increasing Eu concentration (0 (LESSTHEQ) x (LESSTHEQ) 1) strongly affects the bandstructure of this compound. In the low x region, the optical gap increases approximately linearly with dE(,g)/dx (TURN) 5.8 eV thus indicating that PbTe-like bandedge states define the optical gap.(, )For 0.05 < x < 0.5, the slope dE(,g)/dx is substantially reduced and possibly reflects involvement of the (SIGMA) valence band extrema in determining the gap energy. For the high concentration range, where a large Stokes shift is measured between the absorption and emission spectra, states defining the optical gap are increasingly dominated by the Eu('++) 4f and 5d levels. Magnetic field induced energy shifts in the optical gap for x < .2 have been fit by assuming a Heisenberg-like exchange mechanism between the localized Eu('++) ion moments and spins of the extended states in this semimagnetic semiconductor. By this model, Eu-Eu antiferromagnetic interactions and the Eu('++) ion-extended states exchange interactions are much smaller than the analogous effects observed in II Mn VI compounds. For EuTe, the absence of large energy shifts in emission spectra when cooling below T < T(,N) suggests that the bound magnetic polaron contribution to the configurational relaxation is reduced in these higher quality MBE grown films when compared to previous results on bulk materials. Photoluminescence spectra from (Pb,Eu)(Se,Te) MQW structures show good agreement with a calculation for the n = 1 (--->) n = 1 quantum well transition which includes mass discontinuity at the heterointerfaces and band nonparabolicity. Values for the band offsets derived from this calculation are (DELTA)E(,c)/(DELTA)E (TURNEQ) 0.93 or 0.1. A model for luminescence lineshapes, which accounts for monolayer well width fluctuations, reveals electronic temperatures which are considerably above lattice temperatures even under low photo- excitation conditions. The techniques of luminescence and photo- modulated reflectance have also been employed to determine the optical gap of a novel strained layer binary superlattice EuTe/PbTe.
- Pub Date:
- Physics: Condensed Matter