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We have investigated the stability of nano-amorphous region of Ge2Sb2Te5 (GST), fabricated by Electron Beam Lithography (EBL), dry etching, and ion implantation. Nano-structures, less than 100 nm in diameter and 20 nm thick, were either embedded in a crystalline environment or just isolated. We have observed nano-structure crystallization by in situ Transmission Electron Microscopy (TEM) in the 75°C-150°C temperature range. Re-crystallization of amorphous dots embedded in a crystalline region (either in the cubic or hexagonal phase) occurs by the movement of the interface at relatively low temperature (about 90°C). Instead, in the isolated structures the transition occurs at about 145°C by nucleation and growth. These results might be of relevance for the data retention of sub-50nm devices. Indeed, the more stable amorphous phase in self-standing regions indicates the better retention properties of isolated cells with respect to the traditional mushroom cell configuration.
Nanoscale structures have been recently proposed as charge storage nodes due to their potential applications for future nanoscale memory devices. Our approach is based on the idea of using Si nanodots as discrete floating gates. To experimentally investigate such potential, we have fabricated MOS structures with Si nanocrystals. The dots have been deposited onto an ultra-thin tunnel oxide by chemical vapour deposition, and then annealed at 1000 °C for 40 s, to crystallize all the dots. After deposition the dots have been covered by a CVD SiO2 layer, thus resulting in dots completely embedded in stoichiometric silicon oxide. The nanocrystal density and size have been studied by energy filtered TEM (EFTEM) analysis. An electrostatic force microscope has been used to locally inject the charge. By applying a relatively large tip voltage a few dots have been charged, and the shift in the tip phase has been monitored. The shift in the phase is attributed to the presence of the charge in the sample. A comparison between n and p type samples is also shown.
Silver nanoparticles (10–20 nm) embedded into silica thin films have been obtained through the use of a silver organometallic precursor compound dissolved in Spin-On-Glass and subsequently spinned onto suitable substrates. In this paper we present a study of the shape, size, and distribution of silver particles through the use of microscopes, x-ray diffraction, and optical extinction. It has been observed that the obtained films are stable for annealing up to 500 °C with a progressive degradation above this temperature. Furthermore it is possible to obtain high-density silver particles up to 15% in weight without affecting the cluster size and shape.
We report detailed experimental results on the electrochemical selective etching of doped Si. By using transmission electron microscopy analyses and spreading resistance measurements we investigated the dependence of the etching selectivity on the different parameters of the electrochemical cell, i.e., bias voltage and chemical solution. In B-doped samples immersed in buffered HF, the increase of bias voltage from 0.5 to 1 V produces a slight improvement of the etching selectivity and a B concentration as low as 1 × 1017 cm−3 can be successfully delineated at 1 V. A further improvement is achieved by using HF:HNO3:CH3COOH or HF:HCl chemical mixtures for which the delineation sensitivity approaches the value of 1 × 1016 cm−3. In buffered HF As-doped regions can be delineated to a concentration of 2 × 1017 cm−3, independently of the bias voltage, in the range 2–4 V. These results were used to measure the 2D doping diffusion profiles in silicon wafers patterned with polycrystalline Si strips and implanted with As or B, by using different tilt and twist angles. The high resolution of the electrochemical delineation allowed us to evaluate very accurately the effects of the implant angles on the lateral doping distribution.
To form crystalline Si dots embedded in SiO2, we have deposited thin films of silicon rich oxide (SRO) by plasma-enhanced chemical vapor deposition of SiH4 and O2. Then the materials wereannealed in N2 ambient at temperatures between 950 and 1100 °C. Under such processing, the supersaturation of Si in the amorphous SRO film produces the formation of crystalline Si dots embedded in SiO2. The narrow dot size distributions, analyzed by transmission electron microscopy, are characterized by average grain radii and standard deviations down to about 1 nm. The memory function of such structures has been investigated in metal-oxidesemiconductor (MOS) capacitors with a SRO film sandwiched between two thin SiO2 layers as insulator and with an n+ polycrystalline silicon gate. The operations of write and storage are clearly detected by measurements of hysteresis in capacitance-voltage characteristics and they have been studied as a function of bias.
Semiconducting iron silicide dots with dimensions ranging between 5 and 100 nm can be obtained by ion implantation on Si wafers and exhibit interesting photo- and electro- luminescent properties.
In our study we use structural and optical characterization as well as theoretical modelling in order to: i) discriminate among intrinsic effects of FeSi2 dots and effects due to lattice damage and Si matrix; ii) identify the range of physical parameters (size, phase, electronic structure) corresponding to the luminescent dots.
We discuss the sample preparation technique for the two-dimensional dopant concentration profiling in silicon based on chemical staining and transmission electron microscopy analysis. The capability of characterizing ULSI silicon devices is demonstrated by showing an example of failure analysis performed on a high speed bipolar transistor. The high spatial resolution of the technique allows to reveal many details which are not detectable by other techniques. The limitations of this method concern the impossibility to simultaneously delineate n and p-type regions on the same sample, and the need to have a p+n junction for delineating a boron profile with an acceptable sensitivity. We demonstrate that these limitations can be overcome by developing a novel electrochemical staining procedure based on anodic oxidation of silicon.
We present new results on the ion-assisted amorphous to polycrystal transition in silicon. The grain growth velocity vg and the nucleation rate r exhibit quite different behaviours: i) ion irradiation enhances r by a factor which is several orders of magnitude larger than that observed for vg; ii) irradiation with lighter ions produces a decrease of vg and a strong increase of r; iii) the increase of dose rate produces a decrease of both vg and r, but is particulary severe for r. These results have been explained by assuming that ion irradiation produces three fundamental effects: i) generation of long living defects which increase the free energy of the amorphous phase to the value of the fully unrelaxed amorphous silicon, causing the decrease of the thermodynamic barrier to nucleation; ii) generation of defects promoting the transition kinetics at concentrations well above the thermal equilibrium value; iii) prompt amorphization of a small volume at the surface of each crystalline grain.
The early stages of the thermally induced epitaxial realignment of undoped and As-doped polycrystalline Si films deposited onto crystalline Si substrates were monitored by transmission electron microscopy. Under the effect of the heat treatment, the native oxide film at the poly-Si/c-Si interface begins to agglomerate into spherical beads. The grain boundary terminations at the interface are the preferred sites for the triggering of the realignment transformation which starts by the formation of epitaxial protuberances at these sites. This feature, in conjunction with the microstructure of the films during the first instants of the heat treatment, explains the occurrence of two different realignment modes. In undoped films the epitaxial protuberances, due to the fine grain structure, are closely distributed and grow together forming a rough interface moving toward the film's surface. For As-doped films, the larger grain size leaves a reduced density of realignment sites. Due to As doping some of these sites grow fast and form epitaxial columns that further grow laterally at the expense of the surrounding polycrystalline grains.
The several processes required to achieve Er luminescence in Si are investigated. In particular, the role of Er - O interactions to obtain the incorporation of high Er concentrations, electrically and optically active, in crystalline Si is investigated. It is found that a large enhancement in the electrical activation of Er (up to three orders of magnitude) is obtained by co-implanting Er with O at 573 K or at 77 K. However, the introduction of high concentrations of active Er in Si is not sufficient to obtain a large enhancement in the luminescence since an efficient pumping of the optically active sites is also required. The optical efficiency of this sample has been studied by photoluminescence. It is seen that an enhancement by a factor of ∼ 5 with respect to literature data is obtained. Moreover studies on the luminescence intensity as a function of the pump power gave important information on the mechanisms underlying Er luminescence in Si and its competing phenomena. These data are presented and discussed.
In this paper our recent work on erbium implantation for optical doping of silicon is reviewed. It is shown that O co-implantation plays a key role both in providing Er with the appropriate chemical surrounding and in allowing the incorporation of high Er concentrations in thick Si layers without the formation of twins and/or precipitates. The luminescence intensity in Er and O co-implanted samples shows a much weaker temperature dependence (a decrease by a factor of 30 from 77K to 300K) than in samples without O (a decrease by 3 orders of magnitude in the same temperature range). This allowed us to observe room temperature photo- and electro-luminescence in Er and O co-doped samples. The temperature dependence of the luminescence in these samples has been determined to be due to non-radiative de-excitation processes. These data are reported and discussed.
The evolution of pre-existing damage structures in Si under high energy ion irradiation is discussed. Different initial morphologies are investigated: a sample partially pre-damaged with heavy ions and a sample partially pre-damaged with light ions are compared within them and with an undamaged single crystal. It is shown that ion irradiation can produce either damage accumulation, in the form of amorphous regions, or damage annealing depending on the pre-existing damage morphology, on the substrate temperature, and on the doping content in the irradiated layer. These data are discussed and interpreted on the basis of the existing models on ion induced amorphization and crystallization.
The ion beam induced growth of isolated silicon grains has been studied in chemical vapor deposited amorphous layers. The crystal radius increases linearly with the 1on dose and the growth rate depends in a complex way on the irradiation temperature in the 320 - 480 °C investigated temperature range. The grain density does not depend on the ion dose but it increases exponentially with increasing irradiation temperature. The grain density obtained after a pure thermal process on similar samples is In any case larger than the density appearing after ion irradiation. These facts may be explained by assuming that during ion irradiation only pre-existing seeds whose size is larger than a critical value can grow. This critical cluster size is larger than the critical cluster size for a pure thermal process.
Ion-assisted regrowth of chemical vapor deposited amorphous Si layers was investigated for different cleaning procedures. The process was directly monitored by transient reflectivity measurements. The c-a interface stops at the deposited layer/substrate interface for doses depending on the effectiveness of the cleaning procedure in removing the native oxide. Small concentrations of twins are found in the regrown layer. Their amount is also correlated to the cleaning procedure. In oxygen implanted bare Si samples the ion-induced growth rate is reduced to 0.3 of the normal value at a peak O concentration of 1 X 1021/cm3. The results on the ion-induced regrowth of deposited layers are explained in terms of oxygen profile broadening during irradiation and retardation of the growth for the presence of dissolved O.
Thin layers of Si were chemical vapor deposited onto as - received p-type <100> Si wafers and implanted with 80 KeV of As or Ge to a fluence of 1 × 1015 /cm2. Irradiation at 450°C with 600 KeV Kr++ ions causes the epitaxial growth of the entire deposited and amorphized Si layer. At lower irradiation temperatures the regrowth rate of the deposited layers is substantially reduced with respect that of the implanted amorphous layers. The presence of As enhances the regrowth rate of a factor 2.5. The results are explained qualitatively in terms of a dynamical bond breaking of SiO2, and of a dopant influence on the migration energy of the defects responsible for the growth.
Damage formation during hot implants of 600 keV As or Ge ions into Si was investigated by changing the target temperature (>150 °C) and the ion fluence. The defect distributions, as obtained by channeling analysis, are characterized by a gaussian shape whose maximum coincide with the peak of the energy density deposition and with a width of 200 nm. The amount of damage is a factor of two higher for Ge than for As ion implants, and a similar result was found for the damage created by Ge implants into bare Si or Si doped with a near constant As concentration of 2×10 20/cm3. The transition to amorphous formation is quite sharp for As (around 120 °C) and quite broad for Ge implants. The different amount and kind of extended defects is probably due to an interaction of the mobile point defects, vacancies and interstitials, with As. The interaction probably increases the defects annihilation rate.
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