We have investigated phase separation, silicon nanocrystal (Si NC) formation and optical properties of Si oxide (SiO x , 0Ͻ x Ͻ 2) films by high-vacuum annealing and dry oxidation. The SiO x films were deposited by plasma-enhanced chemical vapor deposition at different nitrous-oxide/silane flow ratios. The physical and optical properties of the SiO x films were studied as a result of high-vacuum annealing and thermal oxidation. X-ray photoelectron spectroscopy (XPS) reveals that the as-deposited films have a random-bonding or continuous-random-network structure with different oxidation states. After annealing at temperatures above 1000°C, the intermediate Si continuum in XPS spectra (referring to the suboxide) split to Si peaks corresponding to SiO 2 and elemental Si. This change indicates the phase separation of the SiO x into more stable SiO 2 and Si clusters. Raman, high-resolution transmission electron microscopy and optical absorption confirmed the phase separation and the formation of Si NCs in the films. The size of Si NCs increases with increasing Si concentration in the films and increasing annealing temperature. Two photoluminescence (PL) bands were observed in the films after annealing. The ultraviolet (UV)-range PL with a peak fixed at 370-380 nm is independent of Si concentration and annealing temperature, which is a characteristic of defect states. Strong PL in red range shows redshifts from ϳ600 to 900 nm with increasing Si concentration and annealing temperature, which supports the quantum confinement model. After oxidation of the high-temperature annealed films, the UV PL was almost quenched while the red PL shows continuous blueshifts with increasing oxidation time. The different oxidation behaviors further relate the UV PL to the defect states and the red PL to the recombination of quantum-confined excitions.
Breakdowns in ultrathin gate oxide (Gox) ranging from 16 -33 A were physically analyzed with transmission electron microscope after constant voltage stress. In the Gox of 25 and 33A, a dielectric breakdown induced epitaxy (DBIE) at the gate oxide region is detected for compliance current of 100 nA and above, regardless of breakdown hardness (1).The compliance cunent for the transition of soft breakdown (SBD) to hard breakdown (HBD) is found to be in the range of 10-100 el, whereas for the thinner Gox of 16 A, the upper compliance current limit of SBD to HBD is greatly reduced to around 1 -10 pA and SBD DBIE is hardly detected. The results clearly indicate that DBIE is always present in the HBD oxides regardless of its thickness. Its presence in the SBD oxides is an indication of the early stage of catastrophic failure process that poses a Cox reliability concern.
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