Excess Silicon Content Research Articles

Effect of natural aging and preaging on subsequent precipitation process of an AI-Mg-Si alloy with high excess silicon

The mechanism of the influence of natural aging (that occurs between solution heat treatment and artificial aging) and preaging immediately after quenching on the subsequent precipitation process in an AI-0.95Mg-1.55Si-0.3Mn-0.1Zr alloy was studied using differential scanning calorimetric analyses and microhardness measurements. The results show that natural aging has a distinct influence on the subsequent precipitation process. It is proposed that the formation of stable silicon clusters during natural aging is the major reason for such an effect. Preaging at 120 or 150°C for 5 min is effective in suppressing the clustering process, and thus inhibiting the deleterious effect of natural aging. The decrease in hardness due to the natural aging in the alloy studied is not as marked as that reported for balanced AI-Mg2Si alloys. It is proposed that a high excess silicon content reduces the detrimental effect of natural aging.

Optical properties of non-stoichiometric SiO 2 as a function of excess silicon content and thermal treatments

The infrared (IR) absorption spectra and the behavior of the refraction index of a two-phase non-stoichiometric SiO 2 film with excess Si have been studied as a function of the excess of Si and post-deposition thermal treatment. The oxides were deposited by low-pressure chemical vapor deposition using SiH 4 and N 2O as reactant gases at a substrate temperature in the range of 650 to 750 °C. Some of the films were given a final annealing treatment at temperatures ranging from 700 to 1100 °C in N 2 for 30 min. Both annealed and as-deposited oxides have IR absorption peaks associated with the bending, rocking and stretching modes of the Si-O-Si bonds in SiO 2, although the exact location of these peaks is different for different contents of excess silicon and it also depend on the post-deposition thermal treatment given to the oxides. Unannealed samples present a shift of the stretching peak towards low wavenumbers as the excess of Si is increased. The samples annealed at 1000 °C on the other hand do not present this shift. Unannealed samples with large content of Si also present an absorption peak at 890 cm −1 that could be associated with partially oxidized Si. It is suggested that at least part of the excess Si in the as-deposited samples is present in the form of an SiO x phase while in the annealed samples a clear separation occurs between a Si and a SiO 2 phase. The behavior of the refraction index is similar for both types of sample, increasing as the excess silicon is increased.

Characterization of plasma-enhanced chemically-vapor-deposited silicon-rich silicon dioxide/thermal silicon dioxide dual dielectric systems

Plasma enhanced chemically-vapor-deposited silicon-rich oxides (200 Å and 500 Å in thickness) of various excess silicon content were deposited onto thermal silicon dioxide (SiO2) layers (103, 207, and 530 Å in thickness) grown on a p-type silicon (Si) substrate. The dielectric constant, electron injection efficiency, current-voltage (I-V) reproducibility, and breakdown property of these composite structures were examined. The dielectric constants of Si-rich oxide were observed to increase with Si content from 3.8 for films deposited at a gas phase ratio (R0) of the concentration of nitrous oxide (N2O) to silane (SiH4) of 150 to ∼10 for films deposited with R0=0. The Si-rich oxides with R0≤5 were found to work as electron injectors. The average oxide field needed to induce a current of 4.8×10−7 A/cm2 through the SiO2 (530 Å in thickness) decreased about 40% in magnitude by adding a Si-rich oxide layer with the optimized R0(=1) compared to that of a control sample which had no Si-rich oxide layer. For thin SiO2 (103 Å and 207 Å in thickness) samples, the decrease of the average field was only 2% and 10% in magnitude with the optimized R0 (=2) layer, respectively, due to the relatively large voltage drop (≊−1 V) across the Si-rich oxide compared to that across the thermal oxide layer. The voltage drop across the oxide is discussed in terms of a dual dielectric model. The yeild, which was defined as the percentage of capacitors that required a field larger than 2 MV/cm to obtain a current of 9.6×10−4 A/cm2, on as-fabricated samples was larger than 90% for all samples with Si-rich oxide. The samples were not destroyed by the passage of a relatively high current density (1.21×10−2 A/cm2) through the oxide and subsequent measurements resulted in approximately the same field to produce the specified current as for the first measurement. The yield was found to have a maximum at R0=1–10 depending on the thickness of Si-rich oxides and SiO2. Current-voltage reproducibility was also found to be improved by the deposition of Si-rich oxide.

Charge transport and trapping phenomena in off-stoichiometric silicon dioxide films

The electrical characteristics of off-stoichiometric silicon dioxide films have been investigated. The off-stoichiometric oxide films studied had an excess atomic silicon (Si) content in the range of 1%–6%. Raman spectroscopy and photoconductivity measurements indicate that the excess Si is present as amorphous Si islands or small crystallites embedded in silicon dioxide (SiO2) forming a two-phase material. These films differ in structure from previously reported films where dual dielectric layers of stoichiometric SiO2 and Si-rich SiO2 with ≥13% excess atomic Si were used. These dual dielectric films were observed to produce electron injection from contacting electrodes via the Si-rich SiO2 layer into the SiO2 at lower average electric fields. This injection mechanism was believed to be due to localized electric field enhancement near the SiO2–Si-rich SiO2 interface caused by the curvature of the tiny Si islands in the SiO2 matrix. The current versus voltage characteristics of the off-stoichiometric oxide films which will be discussed here were found to be highly nonohmic, showing an increase in conductivity with increasing excess silcion content in the material. At low fields (≲1 MV/cm), these films have a very small conductance with leakage current densities smaller than 10−11 A/cm2 at room temperature. Furthermore, the effect of permanent charge trapping was observed to decrease in these films with increasing Si content. It is proposed that the electron transport across the film is controlled primarily by tunneling between the silicon islands with a reversible space charge region ≲150 Å in extent near the injecting contact, limiting the current measured in the external circuit. It is also proposed that the decrease in permanent electron trapping is associated with this conduction mechanism and/or the possibility of trapped electrons in the SiO2 phase tunneling to the Si islands. With the incorporation of these off-stoichiometric oxides into electrically-alterable read-only-memory devices, extended write/erase cycling (by at least four orders of magnitude) beyond that normally observed for equivalent devices using stoichiometric SiO2 layers is demonstrated due to the reduction in permanent electron trapping in the oxide layer and its effect on the electric fields.