Nobutoshi Arai

Nanocrystal Formation of Metals in Thermally Grown Thin Silicon Dioxide Layer by Ion Implantation and Thermal Diffusion of Implanted Atoms in Heat Treatment

Ion implantation technique is useful method to dope atoms in a thin layer to make nanoparticles. However, the thermal annealing is required for recovering ion-induced damages and growing NPs. Therefore, the redistribution of implanted atoms in the layer is very important in the formation process of nanoparticles in size and position. We have investigated the redistribution of Ag atoms implanted in a thermally grown 25-nm-thick SiO2 layer on silicon substrate. Ag negative ions were implanted at 10 keV in the thin oxide layer, where the projected ranges are calculated 12 nm. Samples were annealed at a temperature of 800 degrees or less for 1 h in vacuum with Ar gas flow (50 ml/m). Depth profiles of implanted atoms were investigated by high-resolution Rutherford backscattering spectrometry (HRBS). The formed nanoparticles in the layer were studied by high-resolution cross-sectional transmission electron microscope (XTEM). The samples implanted with Ag negative ions at 10 keV with 5 ? 1015 ions/cm2 were measured by HRBS at scattering angle of 80 degree with He ions at 400 keV. Ag atoms distributed at the surface and at a depth corresponded to the calculated profile after annealing at a temperature below 500 ?C. It is expected that the surface accumulation of Ag atoms resulted from thermal diffusion of implanted atoms during implantation. At 500 ?C, the very small peak in concentration was observed at a depth of 22 nm. This means that a diffusion barrier for Ag atoms exits in this depth. The diffused atoms accumulated at this depth. At 700 ?C, the main peak of concentration was appeared at 20 nm in depth, in which FWHM was 7 nm. The XTEM observation showed that the Ag NPs aligned at the same depth of 20 nm along the interface of SiO2/Si, and that they were nanocrystals (NCs). This mono-layered Ag NCs well corresponded to the HRBS spectra. Thus, we have formed almost mono-layered nanocrystals for Ag implantation in thin silicon dioxide layer.

Negative-ion implantation into thin SiO2 layer for defined nanoparticle formation

Two methods to form nanoparticles at a certain depth in a thin oxide layer by negative-ion implantation into the oxide layer of silicon substrate have been investigated. One method is by implantation at a low energy and the other is by a thermal diffusion after implantation. Regarding the low-energy implantation, about 1keV of ion energy is required. In general, a surface charge-up of the oxide layer arises from a positive-ion implantation to affect ion penetration depth. In this research, we used negative ion implantation because of its advantage of almost “charge-up-free” feature, even for insulating materials. We obtained delta-layered gold nanoparticles (Au NPs) in a 25nm thick SiO2 layer on Si by the low-energy implantation method of gold negative ions at 1keV. The center depth and an average diameter of the delta-layered Au NPs were 5nm and 7nm, respectively. As by the thermal diffusion after implantation, silver negative ions were implanted into 25nm thick SiO2∕Si at 10keV with 5×1015ions∕cm2 at ro...

Germanium nanoparticles formed in silicon dioxide layer by multi-energy implantation and oxidation state of Ge atoms

Ge nanoparticles (NPs) embedded silicon oxide is expected to be promising light emission source, especially, UV – blue light region. We have tried to form Ge NPs in a 100- nm-thick SiO2 layer on Si substrate by multi-energy implantation of Ge negative ions with energies of 50, 20 and 10 keV and doses of 1.4 × 1016, 3.2 × 1015 and 2.2 × 1015 ions/cm2, respectively. Samples were annealed for 1 h at a temperature less than 900oC. By this implantation, Formations of Ge nanoparticles in a surface 50-nm depth region were expected. The depth distribution of implanted Ge atoms in the oxide was measured by XPS (Ge 2p, O 1s, Si 2p) with monochromatic Al K.. and Ar etching at 4 keV. The depth profiles were well agreed with the cross-sectional TEM image. But some extent of Ge atoms diffused to the SiO2/Si interface at 900 oC. The chemical sifted spectra of Ge 2p3/2 showed about 60 % of the oxidation of Ge atom around the end of the range (EOR) even in the as-implanted sample. This oxidation was considered to be due to the excess oxygen atoms near EOR by forward of sputtered oxygen atoms from SiO2 layer. Raman spectra supported this oxidation. In a preliminary investigation of cathode luminescence, the Ge-implanted sample with annealing at 600oC showed CL peak at 3.12 eV (397 nm in wavelength) in UV-blue region at room temperature. This means the Ge-implanted sample has a possibility for light emission in the UV-blue region.

Novel Nonvolatile Random-Access Memory with Si Nanocrystals for Ultralow-Power Scheme

A novel nonvolatile memory with Si nanocrystals has been developed. It has a floating gate consisting of a thin poly Si film and Si nanocrystals. It could be written or erased when a low voltage of +3 V or -3 V was supplied to the gate electrode. On the other hand, no hysteresis was observed at +1 V or -1 V. These characteristics enable the realization of a random-access memory that operates with extremely low power consumption. In this paper, a memory cell array suitable for novel memory devices is also described. The cell array features well bitline technology and the cell area is reduced to 4 F2 (F: feature size) by winding trench isolation. Applying the novel device to the cell array enables realization of high-density, random-access, nondestructive readout and ultralow-power memories.

Luminescence properties of Ge-implanted SiO 2 layer on Si substrate for blue-UV light source with low-voltage drive

The luminescence properties from shallowly Ge-implanted SiO2 layer were investigated for the purpose of fabrication of optical light source in blue-UV range at very low voltage operation. Germanium negative ions were implanted into 50-nm SiO2 layer on Si at very shallow depth from the surface to several tens nm at 10-50 keV and different incident angles. After post annealing, we measured photoluminescence and electroluminescence properties. We have obtained PL peaks at 290 nm and 390 nm in wavelength. In the EL measurement by applying AC voltage between the comb-shape transparent ITO electrode deposited on the SiO2 surface and the Al electrode on Si rear side, the considerably strong EL peak was obtained at around 390 nm in wavelength at AC 20 volts.

Cathode and Photo Luminescence of Silicon Dioxide Layer Implanted with Ge Negative Ions at Multi-Energy

Ge- ions were implanted into SiO2 layer three times by changing the energies of 50, 20 and 10 keV to form Germanium nanoparticles at a relatively wide-depth region. Then, the samples were annealed at 600–900°C for 1 h. Although Ge-nanoparticle formation was confirmed by cross-sectional TEM observation, XPS analysis showed about 30–60 % of the Ge atoms in SiO2 on average were oxidized. In cathode and Photo luminescence measurements, the emissions of around 400 nm in wavelength from the samples were observed. The position of cathode luminescence peak was independent of Ge fluence in the implantation, and temperature in the measurement. These results suggest that the luminescence mechanism is not due to quantum confinement effect of Ge nanoparticles, it is due to the oxygen defect center of oxidized germanium. The luminescence intensity changed dramatically with varying Ge fluence in the implantation.

Thermal Diffusion Barrier for Ag Atoms Implanted in Silicon Dioxide Layer on Silicon Substrate and Monolayer Formation of Nanoparticles

We have investigated thermal diffusion behavior of implanted Ag atoms in SiO2 by using a high‐resolution RBS method in the formation process of monolayered Ag nanoparticles. Ag atoms were implanted by negative ion implantation at 10 keV with 5×1015 ions/cm2 into the 25 nm‐thick SiO2/Si. Samples were annealed at 500 – 800°C for 1 h under Ar gas flow. At annealing temperature of 500°C, implanted Ag atoms distributed at the surface and at a depth corresponded to the calculated profile. It is expected that the surface accumulation of Ag atoms resulted from thermal diffusion of implanted atoms during implantation. At 500°C, the very small peak in concentration was observed at a depth of 22 nm. This means that a diffusion barrier for Ag atoms exits in this depth. The diffused atoms accumulated at this depth. At 700°C, the main peak of concentration was appeared at 20 nm in depth, where FWHM was 7 nm. These results well corresponded to the mono‐layered Ag nanocrystals observed by HR‐TEM.

Germanium Nanoparticle Formation into Thin SiO2 Films by Negative Ion Implantation and Their Electric Characteristics

Germanium nanoparticles in a thin SiO2 film on Si have been formed by negative ion implantation for the development of very low power consumption electron devices using nanoparticles. Their electrical properties of 25‐nm‐SiO2/Si films including Ge nanoparticles were investigated with CV method after subsequent annealing at various temperatures. Ge atoms were implanted at 10 keV with fluencies of 1×1015 and 5×1015 ions/cm2. Samples were annealed at 300, 500, 700 and 900°C for 1 h. Depth profiles of implanted Ge atoms in the SiO2 films were measured by using a high‐resolution RBS technique. The formed Ge nanoparticles were studied by cross‐sectional TEM observation. After annealing at less than 700°C, Ge nanoparticles were confirmed in the film. After 300°C‐annealing, a CV curve had so small hysteresis that could not be applied to memory devices. After 500°C‐annealing, both samples with 1×1015 ions/cm2 and with 5×1015 ions/cm2 had obvious hysteresis curves. Calculations of charge and nanoparticle intensity ...

Silver nanoparticles formation in thermal oxide on silicon by negative-ion implantation

Nanoparticles in insulators exhibit unique electrical properties due to single electron effect. This paper studied about formation of nanoparticles in thin silicon dioxide film by negative ion implantation and electrical properties, such as Coulomb blockade. By silver negative-ion implantation of 30 keV, 1 /spl times/ 10 ions/cm/sup 2/ into 50-nm-thick SiO/sub 2/ film on Si, Ag nanoparticles with diameter of 3 nm were created in the film. After annealing at 700/spl deg/C, the film showed considerably clear steps in I-V curve measured at room temperature. These steps are considered to be due to Coulomb blockade of the Ag nanoparticles. Thus, negative ion implantation was found to be applicable to form metal nanoparticles with sufficiently small size for obtaining Coulomb blockade phenomena at room temperature.