Comparative study of the luminescence of structures with Ge nanocrystals formed by dryand wet oxidation of SiGe films

Abstract

The luminescence emission of structures containing Ge nanocrystals embedded in adielectric matrix obtained by dry and wet oxidation of polycrystalline SiGe layers has beenstudied as a function of the oxidation time and initial SiGe layer thickness. A clearrelationship between the intensity of the luminescence, the structure of the sample, theformation of Ge nanocrystals and the oxidation process parameters that allows us toselect the appropriate process conditions to get the most efficient emission hasbeen established. The evolution of the composition and thickness of the growingoxides and the remaining SiGe layer during the oxidation processes has beencharacterized using Raman spectroscopy, x-ray diffraction, Fourier-transform infraredspectroscopy, Rutherford backscattering spectrometry and transmission electronmicroscopy. For dry oxidation, the luminescence appears suddenly, regardless ofthe initial SiGe layer thickness, when all the Si of the SiGe has been oxidizedand the remaining layer of the segregated Ge starts to be oxidized forming Genanocrystals. Luminescence is observed as long as Ge nanocrystals are present. For wetoxidation, the luminescence appears from the first stages of the oxidation, and isrelated to the formation of Ge-rich nanoclusters trapped in the mixed (Si and Ge)growing oxide. A sharp increase of the luminescence intensity for long oxidationtimes is also observed, due to the formation of Ge nanocrystals by the oxidationof the layer of segregated Ge. For both processes the luminescence is quenchedwhen the oxidation time is long enough to cause the full oxidation of the Genanocrystals. The intensity of the luminescence in the dry oxidized samples is about tentimes higher than in the wet oxidized ones for equal initial thickness of the SiGelayer.