R. Edrei

Evolution and properties of nanodiamond films deposited by direct current glow discharge

Nanocrystalline carbon films possessing a prevailing diamond character are deposited by a direct current glow discharge chemical vapor deposition method using a 9:91 vol % methane to hydrogen gas mixture. In the present work the evolution and properties of nanodiamond films deposited by this method onto silicon substrates as a function of time were studied by various complementary techniques. Our analysis showed that prior to formation and growth of continuous films of a predominantly nanodiamond character, a graphitic phase is formed. After the nanodiamond phase is stabilized, near edge x-ray adsorption fine structure measurements proved the predominant diamond character of the film to be about 80%. By electron energy loss spectroscopy analysis the sp2-like character of the nanodiamond grain boundaries has been determined. The nanodiamond films were found to be thermally stable up to temperatures of ∼950 °C as established by vacuum heating. By scanning electron microscopy and atomic force microscopy the ...

SiO2∕Si3N4∕Al2O3 stacks for scaled-down memory devices: Effects of interfaces and thermal annealing

Effects of interfaces and thermal annealing on the electrical performance of the SiO2∕Si3N4∕Al2O3 (ONA) stacks in nonvolatile memory devices were investigated. The results demonstrated the principal role of Si3N4∕Al2O3 and Al2O3/metal-gate interfaces in controlling charge retention properties of memory cells. Memory devices that employ both electron and hole trappings were fabricated using a controlled oxidation of nitride surface prior to the Al2O3 growth, a high-temperature annealing of the ONA stack in the N2+O2 atmosphere, and a metal gate electrode having a high work function (Pt). These devices exhibited electrical performance superior to that of their existing SiO2∕Si3N4∕SiO2 analogs.

Study of single silicon quantum dots' band gap and single-electron charging energies by room temperature scanning tunneling microscopy.

Scanning tunneling spectroscopy in the shell-filling regime was carried out at room temperature to investigate the size dependence of the band gap and single-electron charging energy of single Si quantum dots (QDs). The results are compared with model calculation. A 12-fold multiple staircase structure was observed for a QD of about 4.3 nm diameter, reflecting the degeneracy of the first energy level, as expected from theoretical calculations. The systematic broadening of the tunneling spectroscopy peaks with decreasing dot diameter is attributed to the reduced barrier height for smaller dot sizes and to the splitting of the first energy level.

Oxide-free InSb (100) surfaces by molecular hydrogen cleaning

We report that annealing of an oxidized InSb (100) single-crystal sample at 250°C under molecular hydrogen flow [molecular hydrogen cleaning (MHC)] results in complete desorption of the surface oxides. Following this process, the surface morphology is found to be very smooth at the nanometric scale without any droplet structure and a nearly 1:1 In:Sb stoichiometry. MHC was applied to remove the native oxide of an epi-ready InSb(100) substrate used for molecular beam epitaxy growth of InSb films. These results suggest that MHC of InSb can be used as a very effective cleaning process for epitaxial film growth.

Atomic force microscope study of amorphous silicon and polysilicon low-pressure chemical-vapor-deposited implanted layers

The roughness of the poly/interpoly oxide interface plays a most important role in the performance of devices; it is expected that for a smoother interface, the double-polysilicon structure will present better electrical properties, such as higher breakdown voltage, and will be more reliable. To obtain the best electrical properties of the oxide layer, it is, therefore, essential to control the polysilicon morphological properties. The overall performances will be affected by the postdeposition process: implantation (dose, energy, and ion), oxidation (temperature, time, ambient), and preoxidation cleaning procedures. In this study, polysilicon and amorphous silicon films were produced under different controlled process conditions and were analyzed using atomic force microscopy (AFM). Significant differences in morphology between polysilicon and amorphous silicon films were obtained. Polysilicon roughness is an order of magnitude higher than amorphous silicon. Roughness of amorphous silicon films increased...

Desorption of InSb(001) native oxide and surface smoothing induced by low temperature annealing under molecular hydrogen flow

The preparation of InSb (001) oxygen-free surfaces by thermal annealing at relatively low temperatures under molecular hydrogen flow is reported. This process is compared with thermal oxide desorption (TOD) at 400°C under ultrahigh vacuum conditions. Molecular hydrogen cleaning (MHC) at substrate temperature of 250°C and at hydrogen pressure of 5×10−6Torr resulted in complete desorption of the native oxide layer. Furthermore, no carbon contamination was observed on the surface following this treatment. The surface morphology of the samples following this process was found to be very smooth without any droplet structure. The In:Sb surface stoichiometry was nearly 1:1 along the MHC process. In addition, annealing the sample at 400°C in vacuum after oxide removal by MHC maintains the smoothness and the stoichiometry of the surface. In contrast, TOD at 400°C of an oxidized InSb surface in vacuum does not result in complete oxide removal from the surface. Furthermore, small droplets associated with In are prod...

Chemical bonding and interdiffusion in scaled down SiO2∕Si3N4∕SiO2 stacks with top oxide formed by thermal ed copyoxidation

The influence of thermal oxidation on the composition of silicon nitride films in SiO2∕Si3N4∕SiO2 stacks for advanced nonvolatile memories is reported. X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analyses lead to the conclusion that wet thermal (pyrogenic) oxidation of silicon nitride enhances the incorporation of oxygen into the silicon nitride layer and creates a silicon oxynitride layer. In the oxynitride layer formed by wet oxidation, O is mostly bonded to N, whereas in the native oxynitride at the silicon nitride surface, O is preferentially bonded to Si. Dry oxidation (1200°C) results in an even higher amount of oxygen incorporation into the silicon nitride layer as compared with the pyrogenic process. After both pyrogenic and dry oxidation, hydrogen concentration decreases in the bulk of the silicon nitride layer. Following wet oxidation, hydrogen was found to accumulate at the surface layers of the grown oxynitride film. Oxygen penetration into the nitride l...

Nitrogen diffusion and accumulation at the Si∕SiO2 interface in SiO2∕Si3N4∕SiO2 structures for nonvolatile semiconductor memories

Time-of-flight secondary ion mass spectrometry (SIMS) depth profiling was used to study nitrogen distribution in SiO2∕Si3N4∕SiO2 (ONO) structures employed in advanced semiconductor memories. We have investigated different factors that affect nitrogen accumulation at the Si∕SiO2 interface of the ONO structure. To isolate the impact of ion beam enhanced nitrogen diffusion towards the Si∕SiO2 interface in SIMS measurements, the top silicon oxide and silicon nitride layers were chemically etched before the SIMS procedure. Thermal diffusion effects were investigated by comparing specimens with different thermal budgets and different thickness of layers in the ONO stack. Our results unambiguously suggest that nitrogen can be accumulated at the Si∕SiO2 interface during ONO fabrication.

Evolution of Surface Topography of as-Grown Si Films near Amorphous-to-Polycrystalline Transition

The evolution of the surface topography and microstructure of silicon films deposited near the amorphous-to-polycrystalline transition was investigated, and was found to be accompanied by the appearance of crystalline aggregates whose density and size distribution strongly depend on deposition parameters. The films were deposited by low-pressure chemical vapor deposition, and the parameters varied were temperature, pressure, film thickness, and substrate material. Both temperature and pressure have a dramatic effect on the density and size of surface aggregates. A transition from amorphous to polycrystalline silicon at the critical temperature, T C 580°C, is evident from the abrupt change in surface roughness. This effect was far more pronounced for thin (55 nm) silicon films than for their thicker (150 nm) counterparts and was generally larger for films grown directly on c-Si rather than on SiOx substrates. Evidence for nanocrystalline silicon particles embedded into the amorphous matrix below T C was found. In this study, it is suggested that the amorphous-polycrystalline silicon transition and consequently the surface topography evolution of the films is fluctuation assisted. The transition manifests itself in an enhanced population of Si nucleation centers, which are mostly formed at the upper surface of the deposited film.

Low thermal budget SiO2∕Si3N4∕SiO2 stacks for advanced SONOS memories

SiO2∕Si3N4∕SiO2 (ONO) stacks are used as trapping media in industrial semiconductor memories. This memory device uses deep traps in the silicon oxynitride interlayer between the silicon nitride layer and the top oxide. In this research, low-thermal budget (low temperature) ONO layers were designed suitable for integration into advanced scaled down semiconductor technologies. The alternative growing procedure included oxidation of silicon nitride surface in oxygen plasma. Low temperature oxidation of the silicon nitride layer in oxygen plasma allows incorporation of oxygen into the silicon nitride layer, thus forming a silicon oxynitride layer. The chemical composition of the silicon oxynitride interlayer was similar to the oxynitride of thermally grown ONO. The plasma procedure did not cause any surface roughening. The memory devices with the low-temperature-oxygen-plasma ONO stack showed similar performance to memories with standard high temperature ONO. ONO structures were characterized chemically and s...