G. Ben Assayag

Manipulation of two-dimensional arrays of Si nanocrystals embedded in thin SiO2 layers by low energy ion implantation

In silicon nanocrystal based metal–oxide–semiconductor memory structures, tuning of the electron tunneling distance between the Si substrate and Si nanocrystals located in the gate oxide is a crucial requirement for the pinpointing of optimal device architectures. In this work it is demonstrated that this tuning of the “injection distance” can be achieved by varying the Si+ ion energy or the oxide thickness during the fabrication of Si nanocrystals by ultralow-energy silicon implantation. Using an accurate cross-section transmission electron microscopy (XTEM) method, it is demonstrated that two-dimensional arrays of Si nanocrystals cannot be positioned closer than 5 nm to the channel by increasing the implantation energy. It is shown that injection distances down to much smaller values (2 nm) can be achieved only by decreasing the nominal thickness of the gate oxide. Depth profiles of excess silicon measured by time-of-flight secondary ion mass spectroscopy and Si nanocrystal locations determined by XTEM ...

A transmission electron microscopy quantitative study of the growth kinetics of H platelets in Si

Proton implantation and thermal annealing of silicon result in the formation of a specific type of extended defects involving hydrogen, named “platelets” or “cavities.” These defects have been related to the exfoliation mechanism on which a newly developed process to transfer thin films of silicon onto various substrates is based. The density and the size of these platelets depend on the implantation and annealing conditions. In this letter, rigorous statistical methods based on transmission electron microscopy have been used to quantitatively study the thermal behavior of these defects. Upon annealing, it is shown that the cavities grow in size, reduce their density, while the overall volume they occupy remains constant. This phenomenon is due to a conservative ripening of the cavities. The transfer of hydrogen atoms from small to large cavities leads to a decrease of the elastic energy within the implanted layer while the strain locally increases around the projected range of the protons.

Nucleation, growth and dissolution of extended defects in implanted Si: impact on dopant diffusion

Abstract Transient Enhanced Diffusion (TED) of boron in silicon is driven by the large supersaturations of self-interstitial silicon atoms left after implantation which also often lead to the nucleation and subsequent growth, upon annealing, of extended defects. In this paper we review selected experimental results and concepts concerning boron diffusion and/or defect behavior which have recently emerged with the ion implantation community and briefly indicate how they are, or will be, currently used to improve “predictive simulations” softwares aimed at predicting TED. In a first part, we focuss our attention on TED and on the formation of defects in the case of “direct” implantation of boron in silicon. In a second part, we review our current knowledge of the defects and of the diffusion behavior of boron when annealing preamorphised Si. In a last part, we try to compare these two cases and to find out what are the reasons for some similarities and many differences in defect types and thermal evolution depending on whether boron is implanted in crystalline or amorphous silicon. While rising many more questions, we propose a “thermodynamical” vision of the nucleation and growth of clusters and extended defects and stress the interactions between these defects and the free Si self-interstitial atoms which surround them and are the source for TED in all cases. A pragmatic approach to the simulation of TED for various experimental conditions is proposed.

Kinetic study of group IV nanoparticles ion beam synthesized in SiO2

Abstract Most studies concerning group IV (Si, Ge) ion beam synthesized nanocrystals in SiO 2 have shown that a link exists between the observed physical properties and the characteristics of the “populations” of nanoparticles (size-distribution, density, volume fraction). The aim of this paper is to study the influence of the initial supersaturation and annealing conditions on these characteristics. For this, experimental methods have been developed, that allow accurate statistical studies. Different transmission electron microscopy (TEM) imaging conditions have been tested and the most adequate ones have been identified for each system. An original method for the measurement of the density of precipitates embedded in an amorphous matrix has been developed and tested for Ge precipitates in SiO 2 and has permitted to evidence a conservative Ostwald ripening during annealing. The kinetic behavior of Si nanoparticles has also been studied by coupling TEM measurements and “atomistic” simulations. During annealing, the growth of these nanoparticles is very slow but their size significantly increases when increasing the initial Si excess. Simulations are in perfect agreement with experiment when taking into account interaction effects between particles.

Transmission electron microscopy measurements of the injection distances in nanocrystal-based memories

The characteristics of nonvolatile memories making use of Si nanocrystals as charge storage elements buried in the gate oxide of regular metal–oxide–semiconductor transistors strongly depend on the distances between the nanocrystals and two electrodes, the channel and the gate. In this letter, we compare two transmission electron microscopy methods that can be used to extract such distances. We demonstrate by using image simulations that conventional electron microscopy under out-of-Bragg and strongly underfocused conditions is the fastest and most efficient technique to be used for routine measurements at a subnanometer resolution. Finally, we show that the injection oxide thickness of nanocrystal devices obtained by low-energy Si implantation into thin SiO2 layers and subsequent annealing can be precisely tuned from 8 to 5 nm by adjusting the implantation energy from 0.65 to 2 keV.

Detection and characterization of silicon nanocrystals embedded in thin oxide layers

Silicon nanocrystals embedded in a thin oxide layer can be used as charge storage elements in nonvolatile memory devices. The structural characteristics of the nanocrystals and their position in the oxide determine the electrical properties of the devices. In this work, silicon nanocrystals have been formed by ultralow-energy implantation (0.65–2.0 keV) of silicon in a 10 nm thin thermally grown SiO2 film on Si (100) followed by a thermal treatment. A time of flight secondary ion mass spectrometry (TOF-SIMS) methodology has been developed to detect the presence of silicon nanocrystals and to characterize them. The methodology allows one to obtain relevant information, such as the bandwidth and tunneling distance of Si nanocrystals. Chemical information about the presence of impurities introduced into the SiO2 layer during implantation and annealing have also been obtained. The advantages and disadvantages of this technique, based on TOF-SIMS in comparison with transmission electron microscopy, are discuss...

An electron microscopy study of the growth of Ge nanoparticles in SiO2

Ion implantation followed by high temperature annealing can be used to synthesize group IV semiconducting nanoparticles in SiO2. The density and the size distribution of these nanocrystals obviously depend on the implantation and annealing conditions. While their size can be measured by “classical” transmission electron microscopy techniques, their density cannot because no diffraction occurs in the amorphous matrix. In this letter, we use electron energy loss spectroscopy to overcome this problem. We have measured the evolution of the size distribution, the density, and the atomic fraction occupied by the Ge precipitates during annealing. We show that the nanocrystals grow in size and reduce their density, while the overall number of atoms they contain remains constant. This observation proves that the nanoparticles undergo a conservative ripening during annealing.

New characterization method of ion current‐density profile based on damage distribution of Ga+ focused‐ion beam implantation in GaAs

A new method is reported for characterizing focused ion probe current distributions based on the comparison between damage simulations and transmission electron microscopy observations. Several focused‐ion beam operation conditions were modeled, such as low‐to‐high source emission currents and variable beam acceptances. At low current and small acceptance, the ion spot exhibits a nearly Gaussian profile, otherwise larger tails are evidenced which can be modeled either by Pearson or ‘‘bi‐Gaussian’’ distributions. The sensitivity of the procedure to the tail extension is highlighted.

Oxidation of Si nanocrystals fabricated by ultralow-energy ion implantation in thin SiO2 layers

The effect of thermal treatments in nitrogen-diluted oxygen on the structural characteristics of two-dimensional arrays of Si nanocrystals (NCs) fabricated by ultralow-energy ion implantation (1 keV) in thin silicon dioxide layers is reported. The NC characteristics (size, density, and coverage) have been measured by spatially resolved electron-energy-loss spectroscopy by using the spectrum-imaging mode of a scanning transmission electron microscope. Their evolution has been studied as a function of thermal treatment duration at a temperature (900 °C) below the SiO2 viscoelastic point. An extended spherical Deal-Grove [J. Appl. Phys. 36, 3770 (1965)] model for self-limiting oxidation of embedded silicon NCs has been carried out. It proposes that the stress effects, due to oxide deformation, slow down the NC oxidation rate and lead to a self-limiting oxide growth. The model predictions show a good agreement with the experimental results. Soft oxidation appears to be a powerful way for manipulating the NC s...

Multi-dot floating-gates for nonvolatile semiconductor memories: Their ion beam synthesis and morphology

Scalability and performance of current flash memories can be improved substantially by replacing the floating polycrystalline-silicon gate by a layer of Si dots. Here, we present both experimental and theoretical studies on ion beam synthesis of multi-dot layers consisting of Si nanocrystals (NCs) embedded in the gate oxide. Former studies have suffered from the weak Z contrast between Si and SiO2 in transmission electron microscopy (TEM). This letter maps Si plasmon losses with a scanning TEM equipped with a parallel electron energy loss spectroscopy system. Kinetic Monte Carlo simulations of Si phase separation have been performed and compared with Si plasmon maps. Predicted and measured Si morphologies agree remarkably well, both change with increasing ion fluence from isolated NCs to spinodal pattern. However, the predicted fluences are lower than the experimental ones. We identify as the main reason of this discrepancy the partial oxidation of implanted Si by atmospheric humidity, which penetrates in...