Resonant and non-resonant Raman spectra of CdTe/ZnTe self-assembled quantum dots

Nebojsa Romcevic 1Maja Romcevic 1Radmila Kostic 1Dusanka Stojanovic 1Grzegorz Karczewski 2Robert Rafal Galazka 2

1. Institute of Physics (IF), Pregrevica 118, Belgrade 11080, Serbia
2. Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland

Abstract

We present Raman scattering results of CdTe/ZnTe self assembled quantum dots (SAQD). Investigated samples were grown by molecular beam epitaxy on GaAs substrate. A CdTe buffer layer 4.5mm thick was deposited on the substrate. After a 1mm thick ZnTe layer, 3 monolayers (ml) of CdTe were deposited to form a random distribution of quantum dots. The dot layer was covered by 50 nm ZnTe capping layer.

Raman spectra were obtained in the backscattering geometry using lines of Ar-ion laser. Multiphonon (MP) resonant process can be described by the following equation: Ein-Kħw=E; Ein and E are the incident photon energy and electron transition energy, respectively. ħw is the phonon energy and K is the MP order. MP emission is not registered at T=300K. One-phonon spectrum shows weak intensity line at 201.6 cm-1. MP emission processes are registered at temperature below T=200K (for 2.41eV laser energy, K=4). However, one-phonon spectrum consists of two lines. Besides mentioned line at 201.6 cm-1 new line at 204.2 cm-1 appears and takes part in resonance process. Resonance with line at 201.6 cm-1 is not observed. At T=20K resonance order depend on excitation energy and its values are 2, 4, 5 and 7 for laser lines 2.41eV, 2.47eV, 2.49eV and 2.54eV, respectively. One phonon spectra is now more complicated. Besides mentioned, new lines are registered at 200.4, 210.3, 217 and 386 cm-1.

We attribute the peak 204.2 cm-1 to confined longitudinal-optical (LO) phonons from bulk ZnTe (209 cm-1). Raman features at 210.3 and 200.4 cm-1 are ascribed to the ZnTe LO modes confined by degenerate superlattices. We identified the peaks at 217 and 386 cm-1 as ZnTe (TA+LO) modes and (LO+TO(Г)) modes, respectively. We connected mode at 201.6 cm-1 with specific structure of ZnTe in the isolated islands of CdTe neighbourhood.

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Presentation: Poster at E-MRS Fall Meeting 2008, Symposium A, by Maja Romcevic
See On-line Journal of E-MRS Fall Meeting 2008

Submitted: 2008-05-09 12:32
Revised:   2008-06-26 17:21
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