Shuhei Tanaka

Nonlinear optical property of CdTe microcrystallites doped glasses fabricated by laser evaporation method

Samples of CdTe microcrystallites doped glasses were fabricated by a high energy pulsed laser evaporation method. In order to fabricate a CdTe doped glass, synthesis of CdTe microcrystallites and formation of SiO2 films were carried out alternately on a fused silica glass substrate. The absorption edge of the CdTe doped glasses shifted to a higher energy region than that of the bulk CdTe due to the quantum size effect as the particle size of CdTe microcrystallites decreased. The third‐order nonlinear susceptibility of χ(3) was estimated to be 4×10−7 esu at 580 nm using the method of degenerate four wave mixing.

Formation of nanoscale phosphorus colloids in implanted SiO2 glass

Phosphorus ions were implanted to SiO 2 glasses at an energy of 180 keV to doses from 3 × 10 15 to 5 × 10 17 ions/cm 2 at room temperature. TEM observation revealed that amorphous red phosphorus (P red ) colloid particles of 2–4 nm in diameter are precipitated in the substrates implanted to doses 17 ions/cm 2 . Dissociation of the resulting P red into P 2 molecules was observed by optical absorption measured at temperatures > 550°C. After once heating to ∼ 600°C, the particle size of P red increased over two orders of magnitude and the optical band gap decreased from 2.5 eV to 2.2 eV, which is close to that of bulk P red .

Fabrication of microcrystallites of II–IV compound semiconductors by laser ablation method

Abstract Microcrystallites of CdTe and CdS were obtained by pulsed laser ablation in argon gas. Average particle size depended on laser power, and on gas pressure during ablation. Particle diameter of CdTe could be controlled from 4 to 10 nm and their particles dispersed in methanol showed a quantum size effect on measurements of absorption property. Photoluminescence spectra of microcrystallites of CdS had a strong band edge emission related to impurities or defects. We confirmed that the laser ablation method to fabricate microcrystallites of II-VI compound semiconductors was useful.

Paramagnetic resonance of E′-type centers in Si-implanted amorphous SiO2. Si29 hyperfine structure and characteristics of Zeeman resonances☆

Abstract Electron paramagnetic resonance spectra were measured on SiO2 glasses implanted with Si ions to a fluence of 6 × 1016 cm−2 at an acceleration voltage of 160 kV. Three sets of doublets with different separation were observed in Si29-implanted substrates and were ascribed to primary hyperfine structures due to a Si29 nucleus ( nuclear spin = 1 2 ). The doublets with separation of 44.0 and 9.0 mT were attributed to ·Si29 ≡ O3 (E′-center, where the dot and three parallel lines denote the unpaired electron and three separate bonds, respectively) and ·Si29 ≡ Si3 (similar to D center in amorphous Si or Pb center in Si/SiO2 interface) radicals, respectively, and the doublet with a separation of ∼ 23 mT is tentatively assigned to a ·Si29 ≡ SinO3 − n (n = 1 or 2). The area intensity ratio of these three doublets was approximately 2.5 (44.0 mT): 2.5 (23.0 mT): 1.0 (9.0 mT). Zeeman resonances of E′ centers have a broader spread in g2 and are less saturable to microwave power than those of normal E′ resonances. It is suggested that these are common characteristics of E′ resonances in compacted amorphous SiO2.

Ultrafast nonlinear optical effect in CdTe-doped glasses fabricated by the laser evaporation method

Abstract The laser evaporation method is an attractive technology to synthesize composite materials. We tried to apply the laser evaporation method on fabrication of CdTe-doped glasses for the first time. CdTe microcrystallites embedded in SiO2 films were confirmed by transmission electron microscope images. The third-order nonlinear susceptibility χ(3) and the decay time τ of CdTe-doped glasses were measured by three-beam forward type degenerate four-wave mixing. The maximum value of χ(3) was estimated 4.2 x 10-7 esu when the absorption coefficient was 6000 cm-1. From the limitation of pulse duration of 5 ps, τ was not exactly measured but imagined to be shorter than 10 ps, which was much faster than the other semiconductor-doped glasses ever reported. The figure of merit defined as χ(3)/ατ is probably larger than 7.

Growth of CdS by atmospheric pressure metalorganic vapor‐phase epitaxy at low temperature

Single‐crystalline epilayers (epitaxial layers) of undoped, phosphorus‐, and iodine‐doped CdS have been grown at 250–400u2009°C on (100)‐oriented GaAs substrates by atmospheric pressure metalorganic vapor‐phase epitaxy using dimethylcadmium and hydrogen sulfide as source materials. The premature reaction typically encountered with this source combination, as well as in the case of ZnSe epitaxial growth using dimethylzinc and hydrogen selenide, can be eliminated completely even at atmospheric pressure by controlling the respective flow velocities of the carrier gases, including the source gases, independently. The crystallographic structure of CdS epilayers is largely related to the VI/II source gas mole ratio, and changes from hexagonal into cubic (100) structure with decreasing VI/II ratio. From the x‐ray‐diffraction and photoluminescence measurements, an epilayer grown at 350u2009°C with a VI/II ratio of 2 has excellent crystalline quality with complete single‐cubic (100)‐type epitaxial structure. Also in corre...

Epitaxial growth of ZnS grown at low temperatures by atmospheric pressure metalorganic vapor phase epitaxy

Single crystalline epitaxial layers of undoped ZnS have been grown at 250–300 °C on (100)‐oriented GaAs substrates by atmospheric pressure metalorganic vapor phase epitaxy using dimethylzinc (DMZ) and hydrogen sulfide as source materials. The premature reaction typically encountered with this source combination can be eliminated completely as well as in the case of ZnSe epitaxial growth using DMZ and hydrogen selenide, even at atmospheric pressure, by controlling the source gas velocity and the mole ratio. ZnS heteroepitaxial layers (heteroepilayers) with excellent mirrorlike surface morphologies, which were grown at 250 °C for the source gas mole (VI/II) ratio below 10, were obtained. The VI/II ratio dependence of the ZnS heteroepilayers is very different from that of ZnSe heteroepilayers, whose surface morphologies are mirrorlike at a VI/II ratio above 10. Also the surface morphologies of the ZnS heteroepilayers gradually begin to degrade as the growth temperature increases above 250 °C. Moreover, below 220 °C, the crystalline quality abruptly changes to polycrystal, as with bad hazy morphology, which is similar to that seen in ZnSe heteroepilayers grown below 210 °C. It is shown that it is necessary to grow ZnS layers epitaxially at least higher than 220u2009°C.Single crystalline epitaxial layers of undoped ZnS have been grown at 250–300 °C on (100)‐oriented GaAs substrates by atmospheric pressure metalorganic vapor phase epitaxy using dimethylzinc (DMZ) and hydrogen sulfide as source materials. The premature reaction typically encountered with this source combination can be eliminated completely as well as in the case of ZnSe epitaxial growth using DMZ and hydrogen selenide, even at atmospheric pressure, by controlling the source gas velocity and the mole ratio. ZnS heteroepitaxial layers (heteroepilayers) with excellent mirrorlike surface morphologies, which were grown at 250 °C for the source gas mole (VI/II) ratio below 10, were obtained. The VI/II ratio dependence of the ZnS heteroepilayers is very different from that of ZnSe heteroepilayers, whose surface morphologies are mirrorlike at a VI/II ratio above 10. Also the surface morphologies of the ZnS heteroepilayers gradually begin to degrade as the growth temperature increases above 250 °C. Moreover, below...

Photoluminescence study of Li‐ and Na‐implanted ZnSe epitaxial layers grown by atmospheric pressure metalorganic vapor‐phase epitaxy

We have investigated p‐type doping of Li and Na impurities by ion implantation into ZnSe heteroepitaxial layers grown at very low temperatures (250u2009°C) by atmospheric pressure metalorganic vapor‐phase epitaxy. The activation rates of shallow acceptor levels, the density of self‐activated centers in the implanted‐ZnSe epitaxial layers (epilayers), and the degradation of crystalline quality due to radiation damage are dependent on the ion implantation and annealing conditions. As the dose density changes, so do the optimum annealing conditions for the degrees of activation and the recovery of radiation damage. Also the depth profile of the implanted ions and damage is greatly influenced by the acceleration energy. It is difficult to decide on the optimum annealing conditions at the different acceleration energies and to assess the crystalline quality of the implanted epilayers with a nonuniform depth profile of the implanted ions, because the region analyzed is restricted near to the surface and does not al...

The Dependence of Field Effect Mobilities on Substrate Temperature for Amorphous Silicon Deposition for Amorphous Silicon Thin Film Transistors

We evaluated the field effect mobility ( µFE) for a-Si TFTs at different temperatures for a-Si deposition (Tsub). The µFE showed a maximum in the temperature range of 200~250°C. We estimated the tail localized state distribution of the a-Si films for each Tsub value from theoretical curves modified by the measurement temperature dependence of µFE. The result of the fittings showed that the a-Si tail localized state was suppressed in the Tsub range 200~250°C.

Ion implantation for large-area optoelectronics on glass substrates

Abstract The authors have investigated the possibility of large-area optoelectronics on glass substrates fabricated by ion implantation, particularly AM-LCD (active-matrix liquid crystal display) and highly intelligent functional devices. Basic knowledge and technologies have been obtained for the formation of SiON- and PSG-layer underneath the glass surfaces by ion implantation. These layers can prevent sodium diffusion into the optoelectronic devices fabricated on glass substrates. Planarization of glass surfaces by ion implantation and the crystallization of amorphous Si by ion implantation without external heating are also discussed.