RESONANT RAMAN-SCATTERING AND PHOTOLUMINESCENCE IN SIOX CDSE MULTIPLE-QUANTUM WELLS/

Raman scattering, photoluminescence, and x-ray diffraction measurement s have beer, carried out in SiOx/CdSe multilayers with varying CdSe su blayer thickness d(w) (2.5, 3.5, 4.0, 5.0, and 10.0 nm). The x-ray dat a have revealed that after annealing at 673 K in air, CdSe wurtzite ty pe nanocrystals are formed having an average size smaller than the CdS e sublayer thickness. A strong increase in the intensity of 1 LO phono n 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 Kr+ laser line. This increase is attributed to resonant conditions for Raman scatteri ng 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 ar. internal strain-related, but a quantum-size e ffect manifested by one-dimensional carrier confinement. Assuming such a confinement and taking into consideration electrical and optical me asurements, a band diagram is produced for the SiOx/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 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 argum ents 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. [S0163-1829(98)06836-2].