Resonant Raman scattering and photoluminescence in ${\mathrm{SiO}}_x/\mathrm{CdSe}$ multiple quantum wells

Raman scattering, photoluminescence, and x-ray diffraction measurements have been carried out in ${\mathrm{SiO}}_x/\mathrm{CdSe}$ multilayers with varying CdSe sublayer thickness $d_w$ (2.5, 3.5, 4.0, 5.0, and 10.0 nm). The x-ray data have revealed that after annealing at 673 K in air, CdSe wurtzite type nanocrystals are formed having an average size smaller than the CdSe sublayer thickness. A strong increase in the intensity of 1 LO phonon Raman band of CdSe at room temperature has been observed for samples with $d_w$ 3.5 and 4.0 nm when excited by the 647.1 nm ${\mathrm{Kr}}^{+}$ laser line. This increase is attributed to resonant conditions for Raman scattering brought about by an increase of the optical band-gap energy $E_g$ of CdSe layers with decreasing sublayer thickness. It is argued that this $E_g$ increase is not an internal strain-related, but a quantum-size effect manifested by one-dimensional carrier confinement. Assuming such a confinement and taking into consideration electrical and optical measurements, a band diagram is produced for the ${\mathrm{SiO}}_x/\mathrm{CdSe}$ system, from which the $E_g$ values for each multilayer material is calculated. It is found that the calculated $E_g$ values corresponding to $d_w=3.5\mathrm{}$ and 4.0 nm differ by 59 and 19 meV, respectively, from the quantum energy of the 647.1 nm (1.916 eV) laser line, thus supporting the above arguments and the idea that these structures constitute multiquantum wells. Two photoluminescence bands of CdSe layers have been observed in all multilayers. The peak of the main band shifts from 1.76 to 1.95 eV for multilayers with $d_w$ from 10 to 2.5 nm. This shift provides further evidence that the $E_g$ increase with decreasing $d_w$ is due to quantum-size effect.