H. Amekura

Room-temperature photoluminescence from Tb ions implanted in SiO2 on Si

The room-temperature photoluminescence (PL) of Tb3+ ions has been studied. The Tb ions were implanted into 200 nm thick SiO2 on Si wafers. To achieve a uniform Tb distribution, the implantations were performed at 50, 100, and 190 keV to a total dose of 8.8×1014–1.3×1016 ions/cm2, resulting in Tb concentrations of 0.18–2.7 at. %. The PL spectrum consists of sharp lines due to the Tb3+ intra-4f transitions and a broadband due to SiO2 defects. The samples were annealed at temperatures ranging from 600 to 1050 °C. Up to 900 °C, the annealing procedure improves the PL yield; at temperatures higher than 1000 °C, the PL yield drops again at high dose. The PL spectra show noticeable influence of Tb–Tb crossrelaxation, which favors the green PL over the blue PL.

Fabrication of ZnO nanoparticles in SiO2 by ion implantation combined with thermal oxidation

Zinc-oxide (ZnO) nanoparticles (NPs) are fabricated in silica glasses (SiO2) by implantation of Zn+ ions of 60 keV up to 1.0×1017ions∕cm2 and following thermal oxidation. After the oxidation at 700 °C for 1 h, the absorption in the visible region due to Zn metallic NPs disappears and a new absorption edge due to ZnO appears at ∼3.25eV. Cross-sectional transmission electron microscopy confirms the formation of ZnO NPs of 5–10 nm in diameter within the near-surface region of ∼80nm thick and larger ZnO NPs on the surface. Under He–Cd laser excitation at λ=325nm, an exciton luminescence peak centered at 375 nm with FWHM of 113 meV was observed at room temperature.

Fabrication of nickel oxide nanoparticles in SiO2 by metal-ion implantation combined with thermal oxidation

A method is proposed to synthesize oxide nanoparticles in insulators, using metal-ion implantation and following thermal oxidation, which introduces less damage compared to the sequential implantation of metal ions and oxygen ions. Ni-oxide nanoparticles are formed in O2 gas flow at ∼800°C for 1h, through thermal oxidation of Ni metal nanoparticles, which were introduced in SiO2 by charging-free negative ion implantation of 60keV. After the oxidation, optical absorption in the visible region, which is due to Ni metal nanoparticles in the specimen, disappears, and a steep absorption edge of insulator NiO appears around ∼4eV. Simultaneously, the large magnetization of Ni metal nanoparticles changes to a weak magnetization of antiferromagnetic NiO nanoparticles. The nanoparticle formation is confirmed by transmission electron microscopy observation.

Cupric oxide nanoparticles in SiO2 fabricated by copper-ion implantation combined with thermal oxidation

Cupric oxide (CuO) nanoparticles (NPs) are fabricated in silica glasses (SiO2) by Cu-ion implantation and following thermal oxidation. First, Cu metal NPs were formed in SiO2 by the implantation of Cu negative ions of 60 keV to ∼6×1016ions∕cm2, and then the Cu NPs were oxidized to CuO NPs by annealing at 400–1000 °C in oxygen-gas flow. After the oxidation at 600 °C for 1 h, the surface plasmon resonance peak of metallic Cu NPs disappears. Grazing-incidence x-ray diffraction confirms the disappearance of Cu NPs and the formation of CuO NPs, but excludes the formation of Cu2O NPs which are thermodynamically less stable under atmospheric oxygen pressure. The CuO NPs show higher thermal stability up to ∼1000°C than Cu NPs.

Negative copper ion implantation into silica glasses at high dose rates and the optical measurements

Abstract A technique of high-current implantation, with negative heavy ions, has been developed to attain efficient high-density implantation, especially for metal modification of dielectrics. The Cu− ions were implanted into silica glasses to study the dose rate dependence at a fixed dose of 3.0 × 1016 ion/cm2, up to a current density 260 μA/cm2. To dissipate the heat load onto the substrate, a masking method was developed using a multi-hole Cu plate. Optical absorption and reflection in a wavelength range, λ = 200–900 nm, showed a surface plasmon peak of Cu colloids at λ = 560 nm and a broad band of E′ centers, etc. Although the spectral shapes were similar, irrespective of the dose rate, the absorbance and the reflectivity at the peak showed a dose rate dependence, i.e., a two-humped variation. For X-ray fluorescence, the dose rate dependence had a fair correlation with the Cu-L variation. Besides the dose rate dependent precipitation, the Cu amount retained in the substrates may be affected by the mass transport around the beam-solid interface.

Photoconductivity evolution due to carrier trapping by defects in 17 MeV‐proton irradiated silicon

Damage concentration dependence of dark and photoconductivity has been studied in crystalline Si, irradiated with 17 MeV proton. The photoconductivity spectra consist of a broad peak due to a band‐to‐band transition and a tail spectrum on the lower energy side. Two kinds of tail spectra are observed in impurity‐doped and/or damaged specimens, and the latter threshold is deeper than the former. While the main peak of the doped specimen does not greatly change up to a certain irradiation fluence φC, it steeply decreases beyond the φC, which depends on the shallow‐impurity concentration. The tail spectrum of the shallow impurity simultaneously vanishes at the φC and another tail spectrum grows above the φC. As compared to fluence dependence of photocurrent decay time, it is clarified that the drastic decrease in the main peak results from a drop in the decay time or the carrier density, not in the carrier mobility. The defect density for the critical fluence φC has good correlation with the dopant concentrat...

Radiation resistance of amorphous silicon in optoelectric properties under proton bombardment

Optoelectronic properties of semiconductors are generally affected by radiation damage. To possibly improve the radiation resistance, an amorphous phase of Si (a-Si:H) has been tested and compared to crystalline Si(c-Si). Photoconductivity spectra and radiation-induced conductivity (RIC) have been measured in situ, under 17 MeV proton irradiation, as a simulation of fusion environments. In c-Si, crucial deterioration of the optoelectronic properties is caused by radiation-induced defects, produced in the ordered lattice. On the contrary, a-Si:H shows remarkable radiation resistance against the proton bombardment. The stable photoconductivity in a-Si:H is accompanied by enhanced structural metastability under the irradiation. The stability does not result from stiffness, but from soft flexibility inherent in the amorphous structure. The good radiation resistance is also due to characteristics of high-energy protons, i.e., electronic excitation dominant over structural displacement damage formation. The mechanism is ascribed to spontaneous recombination of displaced atoms, promoted by the electronic excitation.

Atomic force microscopy and x-ray photoelectron spectroscopy studies of ZnO nanoparticles on SiO2 fabricated by ion implantation and thermal oxidation

The morphology and chemical composition of the surface of SiO2 that had been implanted with Zn ions of 60keV and annealed in two different atmospheres, i.e., oxygen gas and a vacuum, were compared. In the as-implanted state, the surface mainly consisted of SiO2 with low roughness due to radiation-induced smoothing. A large number of domelike structures of ZnO appeared on the surface of the SiO2 after annealing in oxygen gas at 600°C for 1h, and the size increased with the annealing temperature up to 800°C. After annealing at 900°C, the surface roughness steeply decreased and the composition changed to Zn2SiO4.

Melting of Zn nanoparticles embedded in SiO2 at high temperatures: Effects on surface plasmon resonances

Zn nanoparticles at room temperature show two absorption peaks in the near-infrared (NIR) and the ultraviolet (UV) regions, both of which satisfy the criterion of surface plasmon resonance (SPR). From x-ray diffraction at high temperatures, it was found that the Zn nanoparticles in SiO2 melt at 360–420 °C and solidify at 250–310 °C with a large temperature hysteresis. While the NIR peak disappears with melting, the UV peak shows sudden energy shift with melting but survives even after the melting. The first-principle band calculation ascribes the UV and NIR peaks to SPR-enhanced inter- and intraband transitions, respectively.

Fluence-dependent formation of Zn and ZnO nanoparticles by ion implantation and thermal oxidation: An attempt to control nanoparticle size

For possible control of the size of nanoparticles (NPs), the fluence-dependent formation of Zn and ZnO NPs by ion implantation with and without thermal oxidation was investigated by optical absorption spectroscopy, Rutherford backscattering spectrometry, and small-angle x-ray scattering (SAXS). The mean diameter and number density of Zn NPs in the as-implanted state in silica (SiO2) were determined by SAXS as 7 nm and 13×1017 cm−3, 12 nm and 3.8×1017 cm−3, and 12 nm and 3.2×1017 cm−3 for fluences of 0.50, 1.0, and 2.0×1017 ions/cm2, respectively. With increasing fluence, the mean diameter of the NPs increases and the number density decreases. However, an upper limit of the NP size and Zn concentration in SiO2 is observed above the fluence of 1.0×1017 ions/cm2 due to sputtering loss. Thermal annealing in oxygen gas at 700 °C for 1 h induces the transformation of Zn NPs to both ZnO NPs and the Zn2SiO4 phase. With decreasing fluence, the branching ratio to the ZnO component decreases. This is because the rea...