We have suggested that a small momentum spread of photoelectrons and high quantum efficiency can ... more We have suggested that a small momentum spread of photoelectrons and high quantum efficiency can be obtained concurrently by a photocathode using a semiconductor with a superlattice instead of a bulk. We have begun to search for the suitable semiconductor material of a superlattice photocathode for maintaining the surface with a negative electron affinity state for a long time. We measured quantum efficiency degradation of photocathodes using GaAs and AlGaAs semiconductors with various electron affinities. The AlGaAs semiconductor had a quantum efficiency lifetime of 10 times long compared with the GaAs semiconductor. We found that the AlGaAs semiconductor was the suitable material for the superlattice photocathode with the surface maintaining a negative electron affinity state for a long time.
The ErSi 2 nanowires were formed on Si(1 1 0) substrate by a self-assembly process. The wires wer... more The ErSi 2 nanowires were formed on Si(1 1 0) substrate by a self-assembly process. The wires were highly parallel and grew along the Sih1 1 0i direction. For size controlling of the wires, the deposited layer thickness of Er and the annealing temperature were changed. It was revealed that the annealing temperature is more important for reducing the wire width. The ErSi 2 nanowires were epitaxially grown on Si substrates and had smooth surface originally. It became rough after oxidizing in air. Furthermore, the crystal phase of ErSi 2 turned into amorphous due to the oxidation. The originally grown ErSi 2 wires, did not give rise to light emission. In contrast, an emission peak at wavelength around 1.5 lm was observed from the oxidized samples, indicating the Er-Si-O compound formation. Sharp photoluminescence peaks related to inner core transitions of Er 3+ ions were observed from the oxidized samples synthesized at high temperatures.
Textural drift in the laser recrystallized silicon-on-insulator (SOI) structure is successfully r... more Textural drift in the laser recrystallized silicon-on-insulator (SOI) structure is successfully reduced by using a liquid encapsulation technique. Liquid encapsulation with polyethylene gylcol during the laser recrystallization provides an effective heat release from the surface as compared with the conventional technique. Therefore, thermal gradient in the vertical direction of the silicon film can be controlled by this method. An improved thermal profile in the vertical direction is considered to be a main reason for the reduced textural drift.
Very long ErSi2 nanowires were formed on a Si(110) substrate by a self-assembly process. The wire... more Very long ErSi2 nanowires were formed on a Si(110) substrate by a self-assembly process. The wires were highly parallel and grew in the Si direction. Nanowires with lengths more than 50 mum were fabricated. It was observed by transmission electron microscopy that the wires were hexagonal crystal type, having their c-axis ( direction) perpendicular to the Si(112) plane. Then the
Layer-by-layer growth on nominally oriented (1ll)A GaAs substrate has been performed by atomic la... more Layer-by-layer growth on nominally oriented (1ll)A GaAs substrate has been performed by atomic layer epitaxy (ALE). Under enough ASH, feeding, the growth rate saturation of one-fourth monolayer per cycle was observed at the Tp range between 560°C and 600°C and three-eighths of monolayer per cycle at the Tp range below 550°C on (1ll)A substrate. The drastic change of the growth rate saturation at around 550°C indicates some kinds of surface reconstructions or site occupation on (1ll)A surface during ASH, supply.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2004
ABSTRACT The laser cooling transition of silicon atoms was observed in a discharge with a hollow ... more ABSTRACT The laser cooling transition of silicon atoms was observed in a discharge with a hollow cathode containing silicon using the third harmonic coherent light of a nanosecond Ti:sapphire laser. The optogalvanic spectrum obtained was investigated by scanning the wavelength of the coherent light from 250.3 nm to 253.3 nm. Four lines originating from the 3s(2) 3p(2) P-3 --> 3s(2) 3p4s P-3(0) transition of silicon atoms were observed in the spectrum, while the spectral resolution was limited to 0.1 nm by the linewidth of the coherent light. Optogalvanic signals increased in intensity as the discharge current decreased. This spectroscopic technique will help us to address the accurate frequency of the light source for the laser cooling of silicon atoms. (C) 2003 Published by Elsevier B.V.
We developed AlGaAs photocathodes with low electron affinity for long negative electron affinity ... more We developed AlGaAs photocathodes with low electron affinity for long negative electron affinity (NEA) lifetime. AlGaAs photocathodes achieved 10 times longer NEA lifetime than conventional NEA-GaAs photocathodes. We estimated the appropriate superlattice structure for small conduction mini band width, high density of state in the conduction band and high splitting energy between the heavy-and lighthole bands by theoretical energy band calculation. We conclude that the AlGaAs-GaAs superlattice semiconductor is a suitable NEA-GaAs photocathode that not only has acceptable NEA lifetime but also fulfills the requirements of small energy spread of photoelectrons, high quantum yield and highly spin-polarized electrons.
We found high crystallographic selectivity in atomic layer epitaxy (ALE) growth of GaAs, in compa... more We found high crystallographic selectivity in atomic layer epitaxy (ALE) growth of GaAs, in comparison with other epitaxial growth methods. In the temperature dependence of the GaAs growth rate, no GaAs growth on the GaAs (111)A and (110) planes was observed in the high temperature range under the condition of GaAs ALE growth on GaAs (100) plane. Also we discussed the mechanism of GaAs growth selectivity, which was believed to be caused by growth limitation due to As desorption. Due to the self-limiting effect and the high selectivity of ALE growth, trapezoidal-shaped GaAs/AlGaAs quantum wire structures, with 20 nm thickness and 50 nm width, were successfully realized.
Laser atomic layer epitaxy technique is reviewed, and growth characteristics and surface processe... more Laser atomic layer epitaxy technique is reviewed, and growth characteristics and surface processes in laser Atomic Layer Epitaxy (ALE) are described. The self-limiting mechanism is the key to attain the ideal layer-by-layer controllability in growth rate. The self-limited growth is obtained in a wide range of several growth parameters. The self-limiting mechanism is induced by the site-selective decomposition of alkylgallium precursors (alkylgallium precursors decompose only at the As surface, but not at the Ga surface), and the direct absorption of laser light in the chemisorbed alkylgallium layer provides the site selectivity.
Digest of Papers Microprocesses and Nanotechnology 2005, 2005
ABSTRACT The synthesis of rare-earth silicide nano-structures on Si substrate has been an interes... more ABSTRACT The synthesis of rare-earth silicide nano-structures on Si substrate has been an interesting topic because of their potential in forming low-dimensional devices. In this paper, self-assembled growth has been used to form wire-like structures based on the lattice mismatch between the wires and the semiconductor substrates.
Ion-beam irradiation causes a polytetrafluoroethylene (PTFE) surface to be covered with a large n... more Ion-beam irradiation causes a polytetrafluoroethylene (PTFE) surface to be covered with a large number of small protrusions perpendicular to the surface. The protrusions have a very large aspect ratio (the average diameter of the top is 100 nm and the height is 70 μm), and the shapes were produced only by irradiation. In this study, we investigated an unknown mechanism that produces PTFE surface changes, induced by N + ionbeam irradiation. The PTFE surfaces were irradiated with N + ion fluences and were observed by scanning electron microscopy (SEM). In-situ observations were also performed using a focused ion beam, equipped with a scanning ion microscope (FIB/SIM), to investigate the mechanism of morphological changes. The irradiated surfaces were analyzed by attenuated total reflectance/Fourier transform infrared (ATR/FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). At the fluence of 1 × 10 15 ions/cm 2 , micropores and a melted layer, with CfO and OH bonds, were formed on the surface. The melted layer acted as a mask against irradiation. Increasing the fluence, the melted layer gradually diminished and the micropores enlarged. When the melted layer vanished, many protrusions were produced on the surface and the functional groups could be observed at lower fluences. In-situ observations showed stretching and evaporation at the top of the protrusions. It was clarified that the protrusions were not formed by deposition of sputtered monomers from the substrate, but by stretching of the PTFE itself.
We have suggested that a small momentum spread of photoelectrons and high quantum efficiency can ... more We have suggested that a small momentum spread of photoelectrons and high quantum efficiency can be obtained concurrently by a photocathode using a semiconductor with a superlattice instead of a bulk. We have begun to search for the suitable semiconductor material of a superlattice photocathode for maintaining the surface with a negative electron affinity state for a long time. We measured quantum efficiency degradation of photocathodes using GaAs and AlGaAs semiconductors with various electron affinities. The AlGaAs semiconductor had a quantum efficiency lifetime of 10 times long compared with the GaAs semiconductor. We found that the AlGaAs semiconductor was the suitable material for the superlattice photocathode with the surface maintaining a negative electron affinity state for a long time.
The ErSi 2 nanowires were formed on Si(1 1 0) substrate by a self-assembly process. The wires wer... more The ErSi 2 nanowires were formed on Si(1 1 0) substrate by a self-assembly process. The wires were highly parallel and grew along the Sih1 1 0i direction. For size controlling of the wires, the deposited layer thickness of Er and the annealing temperature were changed. It was revealed that the annealing temperature is more important for reducing the wire width. The ErSi 2 nanowires were epitaxially grown on Si substrates and had smooth surface originally. It became rough after oxidizing in air. Furthermore, the crystal phase of ErSi 2 turned into amorphous due to the oxidation. The originally grown ErSi 2 wires, did not give rise to light emission. In contrast, an emission peak at wavelength around 1.5 lm was observed from the oxidized samples, indicating the Er-Si-O compound formation. Sharp photoluminescence peaks related to inner core transitions of Er 3+ ions were observed from the oxidized samples synthesized at high temperatures.
Textural drift in the laser recrystallized silicon-on-insulator (SOI) structure is successfully r... more Textural drift in the laser recrystallized silicon-on-insulator (SOI) structure is successfully reduced by using a liquid encapsulation technique. Liquid encapsulation with polyethylene gylcol during the laser recrystallization provides an effective heat release from the surface as compared with the conventional technique. Therefore, thermal gradient in the vertical direction of the silicon film can be controlled by this method. An improved thermal profile in the vertical direction is considered to be a main reason for the reduced textural drift.
Very long ErSi2 nanowires were formed on a Si(110) substrate by a self-assembly process. The wire... more Very long ErSi2 nanowires were formed on a Si(110) substrate by a self-assembly process. The wires were highly parallel and grew in the Si direction. Nanowires with lengths more than 50 mum were fabricated. It was observed by transmission electron microscopy that the wires were hexagonal crystal type, having their c-axis ( direction) perpendicular to the Si(112) plane. Then the
Layer-by-layer growth on nominally oriented (1ll)A GaAs substrate has been performed by atomic la... more Layer-by-layer growth on nominally oriented (1ll)A GaAs substrate has been performed by atomic layer epitaxy (ALE). Under enough ASH, feeding, the growth rate saturation of one-fourth monolayer per cycle was observed at the Tp range between 560°C and 600°C and three-eighths of monolayer per cycle at the Tp range below 550°C on (1ll)A substrate. The drastic change of the growth rate saturation at around 550°C indicates some kinds of surface reconstructions or site occupation on (1ll)A surface during ASH, supply.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2004
ABSTRACT The laser cooling transition of silicon atoms was observed in a discharge with a hollow ... more ABSTRACT The laser cooling transition of silicon atoms was observed in a discharge with a hollow cathode containing silicon using the third harmonic coherent light of a nanosecond Ti:sapphire laser. The optogalvanic spectrum obtained was investigated by scanning the wavelength of the coherent light from 250.3 nm to 253.3 nm. Four lines originating from the 3s(2) 3p(2) P-3 --> 3s(2) 3p4s P-3(0) transition of silicon atoms were observed in the spectrum, while the spectral resolution was limited to 0.1 nm by the linewidth of the coherent light. Optogalvanic signals increased in intensity as the discharge current decreased. This spectroscopic technique will help us to address the accurate frequency of the light source for the laser cooling of silicon atoms. (C) 2003 Published by Elsevier B.V.
We developed AlGaAs photocathodes with low electron affinity for long negative electron affinity ... more We developed AlGaAs photocathodes with low electron affinity for long negative electron affinity (NEA) lifetime. AlGaAs photocathodes achieved 10 times longer NEA lifetime than conventional NEA-GaAs photocathodes. We estimated the appropriate superlattice structure for small conduction mini band width, high density of state in the conduction band and high splitting energy between the heavy-and lighthole bands by theoretical energy band calculation. We conclude that the AlGaAs-GaAs superlattice semiconductor is a suitable NEA-GaAs photocathode that not only has acceptable NEA lifetime but also fulfills the requirements of small energy spread of photoelectrons, high quantum yield and highly spin-polarized electrons.
We found high crystallographic selectivity in atomic layer epitaxy (ALE) growth of GaAs, in compa... more We found high crystallographic selectivity in atomic layer epitaxy (ALE) growth of GaAs, in comparison with other epitaxial growth methods. In the temperature dependence of the GaAs growth rate, no GaAs growth on the GaAs (111)A and (110) planes was observed in the high temperature range under the condition of GaAs ALE growth on GaAs (100) plane. Also we discussed the mechanism of GaAs growth selectivity, which was believed to be caused by growth limitation due to As desorption. Due to the self-limiting effect and the high selectivity of ALE growth, trapezoidal-shaped GaAs/AlGaAs quantum wire structures, with 20 nm thickness and 50 nm width, were successfully realized.
Laser atomic layer epitaxy technique is reviewed, and growth characteristics and surface processe... more Laser atomic layer epitaxy technique is reviewed, and growth characteristics and surface processes in laser Atomic Layer Epitaxy (ALE) are described. The self-limiting mechanism is the key to attain the ideal layer-by-layer controllability in growth rate. The self-limited growth is obtained in a wide range of several growth parameters. The self-limiting mechanism is induced by the site-selective decomposition of alkylgallium precursors (alkylgallium precursors decompose only at the As surface, but not at the Ga surface), and the direct absorption of laser light in the chemisorbed alkylgallium layer provides the site selectivity.
Digest of Papers Microprocesses and Nanotechnology 2005, 2005
ABSTRACT The synthesis of rare-earth silicide nano-structures on Si substrate has been an interes... more ABSTRACT The synthesis of rare-earth silicide nano-structures on Si substrate has been an interesting topic because of their potential in forming low-dimensional devices. In this paper, self-assembled growth has been used to form wire-like structures based on the lattice mismatch between the wires and the semiconductor substrates.
Ion-beam irradiation causes a polytetrafluoroethylene (PTFE) surface to be covered with a large n... more Ion-beam irradiation causes a polytetrafluoroethylene (PTFE) surface to be covered with a large number of small protrusions perpendicular to the surface. The protrusions have a very large aspect ratio (the average diameter of the top is 100 nm and the height is 70 μm), and the shapes were produced only by irradiation. In this study, we investigated an unknown mechanism that produces PTFE surface changes, induced by N + ionbeam irradiation. The PTFE surfaces were irradiated with N + ion fluences and were observed by scanning electron microscopy (SEM). In-situ observations were also performed using a focused ion beam, equipped with a scanning ion microscope (FIB/SIM), to investigate the mechanism of morphological changes. The irradiated surfaces were analyzed by attenuated total reflectance/Fourier transform infrared (ATR/FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). At the fluence of 1 × 10 15 ions/cm 2 , micropores and a melted layer, with CfO and OH bonds, were formed on the surface. The melted layer acted as a mask against irradiation. Increasing the fluence, the melted layer gradually diminished and the micropores enlarged. When the melted layer vanished, many protrusions were produced on the surface and the functional groups could be observed at lower fluences. In-situ observations showed stretching and evaporation at the top of the protrusions. It was clarified that the protrusions were not formed by deposition of sputtered monomers from the substrate, but by stretching of the PTFE itself.
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Papers by Takashi Meguro