Wet thermal oxidations of polycrystalline Si₀.₅₄Ge₀.₄₆ films at 600°C for 30 and 50 min were carried out. A stable mixed oxide was obtained for films that were oxidized for 50 min. For film oxidized for 30 min, however, a mixed oxide with Ge nanocrystallites embedded in the oxide matrix was obtained. A trilayer gate stack structure that consisted of tunnel oxide/oxidized polycrystalline Si₀.₅₄Ge₀.₄₆/rf sputtered SiO₂ layers was fabricated. We found that with a 30 min oxidized middle layer, annealing the structure in N₂ ambient results in the formation of germanium nanocrystals and the annealed structure exhibits memory effect. For a trilayer structure with middle layer oxidized for 50 min, annealing in N₂ showed no nanocrystal formation and also no memory effect. Annealing the structures with 30 or 50 min oxidized middle layer in forming gas ambient resulted in nanocrystals embedded in the oxide matrix but no memory effect. This suggests that the charge storage mechanism for the trilayer structure is closely related to the interfacial traps of the nanocrystals. / Singapore-MIT Alliance (SMA)

Growth and evolution of germanium (Ge) nanocrystals embedded into a silicon oxide (SiO₂) system have been studied based on the Ge content of co-sputtered Ge-SiO₂ films using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy. It was found that when the proportion of Ge relative to Ge oxide is 20%, TEM showed that annealing the samples at 800°C for 60 min resulted in the formation of a denuded region between the silicon/silicon oxide (Si/SiO₂) interface and a band of Ge nanocrystals towards the surface of the film. By introducing a 20nm thick thermal oxide barrier on top of the silicon (Si) substrate on which the film is deposited, no denuded region in the bulk of this sample is observed. It is proposed that this barrier is effective in reducing both Ge diffusion into the Si substrate and Si diffusion from the substrate into the film. Si diffusing from the Si substrate reduces the Ge oxide into Ge which can subsequently diffuse into the Si substrate. However, the oxide barrier is able to confine the Ge within the oxide matrix so that the denuded region in the bulk of the film cannot form. However the reduction in diffusion should be more significant for Ge as its diffusion coefficient is lower than Si due to its larger size. It is suggested that the denuded region consists of amorphous Ge diffusing towards the Si/SiO₂ interface. When the Ge content is increased to slightly more than 70%, TEM showed that Ge nanocrysyals formed after annealing at 800°C for only 30 min for samples with and without the oxide barrier. There is no denuded region between the Ge nanocrystals band and the Si/SiO₂ interface for both samples but it was observed that coarsening effects were more prominent in the film deposited on top of the oxide barrier. The reduction effect of Si on Ge oxide should not play a significant role in these samples as the Ge content is high. / Singapore-MIT Alliance (SMA)

A novel method of synthesizing and controlling the size of germanium nanocrystals was developed. A tri-layer structure comprising of a thin (~5nm) SiO₂ layer grown using rapid thermal oxidation (RTO), followed by a layer of Ge+SiO₂ of varying thickness (6 - 20 nm) deposited using the radio frequency (r.f.) co-sputtering technique and a SiO₂ cap layer (50nm) deposited using r.f. sputtering, was investigated. It was verified using TEM that germanium nanocrystals of sizes ranging from 6 – 20 nm were successfully fabricated after thermal annealing of the tri-layer structure under suitable conditions. The nanocrystals were found to be well confined by the RTO SiO₂ and the cap SiO₂ under specific annealing conditions. The electrical properties of the tri-layer structure have been characterized using MOS capacitor test devices. A significant hysteresis can be observed from the C-V measurements and this suggests the charge storage capability of the nanocrystals. The proposed technique has the potential for fabricating memory devices with controllable nanocrystals sizes. / Singapore-MIT Alliance (SMA)