A COMPARATIVE-STUDY OF GE NANOCRYSTALS IN SIXGEYOZ ALLOYS AND SIOX GEOY MULTILAYERS/

Amorphous SixGeyOz alloy films of different chemical composition and S iOx/GeOy multilayers are prepared by de magnetron sputtering. The form ation of Ge nanocrystals in the alloy matrix is induced by annealing a t temperatures ranging from 700 to 900 degrees C. In contrast, the cry stallization in the Ge sublayers in the multilayer structures occurs a t 500 degrees C. Using high resolution transmission electron microscop y, we can directly image the Ge nanoclusters in the as-prepared films as well as the Ge nanocrystals after crystallization in the annealed a lloy and multilayer samples. Raman spectra of the as-prepared and anne aled films demonstrate the strong similarity between the alloy and the multilayer films. In both cases, the crystallization of the Ge phase is observed by the narrowing and disappearing of the amorphous mode ar ound 270 cm(-1) and the appearance and increase of the mode below 300 cm(-1), typical for nanocrystallized Ge. The Ge cluster formation is s timulated by a high oxygen content of the alloy films. If the Si atoms are not saturated with oxygen, Si-Ge alloy regions are formed, result ing in an increase of the crystallization temperature of the films. A weak broad Raman mode at 390 cm(-1) is found as a signature of these r egions. Films containing small Ge nanocrystals show visible room tempe rature photoluminescence around 3.03 and 2.34 eV. Quantum confinement or the presence of luminescence centers (Si-2(0), Ge-2(0)) at the surf ace of the Ge nanocrystals are discussed as possible origins of the br ight blue luminescence, However, a peak shift to higher energies with decreasing crystal size is not detectable, contrary to the predictions of the quantum confinement model. (C) 1997 American Institute of Phys ics.