Naoki Ohshima

Generation and suppression process of crystalline defects in GaP layers grown on misoriented Si(100) substrates

We have investigated the generation process of crystalline defects in GaP layers grown on Si substrates (GaP/Si) by molecular beam epitaxy (MBE) and migration enhanced epitaxy (MEE). Transmission electron microscopy observations revealed that few threading defects such as stacking faults and threading dislocations are detected in GaP/Si by MEE. In addition, a regular network of misfit dislocations was generated during the lattice relaxation process. On the other hand, stacking faults were generated in high density at the hetero-interface and threading dislocations as well as interfacial misfit dislocations were observed in GaP/Si by MBE. The generation of stacking faults would be related to the coalescence or expansion of isolated GaP islands and the presence of stacking faults would affect the generation of threading dislocations.

Reduction of threading dislocation density in InP‐on‐Si heteroepitaxy with strained short‐period superlattices

We applied multistrained short‐period superlattices (SSPSs) and GaP buffer layers to the InP‐on‐Si heteroepitaxy, in order to suppress the generation of threading dislocations. As a result, it was found that the density of threading dislocations in an InP/SSPSs/GaAs/SSPSs/GaP/Si structure including (InAs)m(GaAs)n SSPSs and (GaAs)i(GaP)j SSPSs was remarkably reduced, compared with that in the InP/GaP/Si structure. Misfit dislocations lying along the 〈011〉 directions were observed at heterointerfaces in the InP/SSPSs/GaAs/SSPSs/GaP/Si structure. Therefore, the lattice mismatch strain was stepwise accommodated by the generation of misfit dislocations at the heterointerfaces. From these results, it was clarified that multi‐SSPSs are effective for reducing the density of threading dislocations in heteroepitaxy with a large lattice mismatch.

Evolution process of cross-hatch patterns and reduction of surface roughness in (InAs)m(GaAs)n strained short-period superlattices and InGaAs alloy layers grown on GaAs

We investigated the formation process of cross-hatch patterns (CHPs) and the lattice relaxation process in the growth of an (InAs)1(GaAs)4 strained short-period superlattice (SSPS) and an In0.2Ga0.8As alloy layer on GaAs(100) substrates. By using x-ray diffraction and cross-sectional transmission electron microscopy, it was found that the lattice relaxation in the (InAs)1(GaAs)4SSPS proceeded as fast as that in the In0.2Ga0.8As alloy layer. The surfaces of the grown layers showed CHPs, and the surface roughness increased by means of the evolution of the CHPs. The surface roughness of the SSPSs was larger than that of the alloy layers, whereas no apparent difference was observed in the lattice relaxation process between the SSPSs and the alloy layers. Additionally, the height of surface ridges parallel to the [011] direction was higher than that parallel to the [011] direction. We observed the distribution of highly strained InAs and GaAs islands on the surfaces of strained InGaAs layers by using an atomi...

In-situ maskless selective area epitaxy of GaAs using a low-energy Ga focused ion beam

Abstract In-situ maskless selective area epitaxy of GaAs was performed for the first time with a low-energy Ga focused ion beam (FIB) and an As 4 molecular beam. A newly developed low-energy FIB system was employed in the experiments, which can be utilized in the retarding mode with a substrate bias of zero. The low energy Ga FIB and the As 4 molecular beam were supplied simultaneously on the GaAs(100) substrate. The incident energy ( E i ) of Ga FIB and the substrate temperature ( T s ) Were varied from 30 to 500 eV and from 270 to 500°C, respectively. The maskless selective deposition of GaAs with a diameter of ∽ 180 μm can be achieved below E i of 200 eV. The results of μm-RHEED measurement showed that the GaAs film was a single crystal above T s of 400°C. All the selectively grown GaAs had flat surfaces.

Suppression of threading dislocation generation in GaAs-on-Si with strained short-period superlattices

Abstract We investigated the suppression of threading dislocation generation in GaAs-on-Si(100) with strained short-period superlattices by reflection high-energy electron diffraction and transmission electron microscopy. A two-dimensional growth mode was kept during the lattice relaxation process. As a result, the density of threading dislocations in the GaAs epilayer was markedly reduced. A lattice mismatch of 4% was accommodated at hetero-interfaces by generating misfit dislocations in the directions of 〈011〉.

A study on in-situ maskless selective epitaxy of GaAs by a low-energy Ga focused ion beam with an As4 molecular beam

In-situ maskless selective epitaxy of GaAs was performed with a low-energy Ga focused ion beam (FIB) and an As 4 molecular beam. The low-energy Ga FIB and the As 4 were supplied simultaneously on GaAs(100) substrates. The incident energy (E i ) of the Ga FIB was varied from 30 to 300 eV. In this energy range, a maskless selective epitaxy of GaAs was successfully achieved on a rectangular region by deflecting the Ga FIB under the As 4 molecular beam irradiation. The growth rate was found to depend both on E i and As 4 pressure. The growth rate was decreased with the increase of E i and with the decrease of As 4 pressure. The As 4 pressure dependence on the growth rate suggests that the incident Ga ion could give its kinetic energy to arsenic molecules

Analog velocity sensing circuits based on bio-inspired correlation neural networks

We propose simple analog MOS circuits producing one-dimensional optical flows aiming at the realization of compact motion-sensing circuits. In the proposed circuit, the optical flow is computed by a number of local motion sensors, as in biological motion-sensing systems. Mimicking the structure of biological motion detectors made the circuit structure quite simple, compared with conventional velocity sensing circuits. Extensive simulation results using SPICE and experimental results indicated that the proposed circuits could compute local velocities of a moving light spot and showed direction selectivity for the moving spot, which implies that a high-resolution motion-sensing chip can be realized using standard analog very large-scale integration technology.

Reduction of Point Defects and Formation of Abrupt Hetero-Interfaces in Low-Temperature Molecular Beam Epitaxy of GaAs and GaP under Atomic Hydrogen Irradiation.

The removal effect of excess As and P atoms adsorbed on GaAs (100) and GaP (100) surfaces by atomic hydrogen (H) irradiation was investigated by reflection high-energy electron diffraction and X-ray photoelectron spectroscopy. It was found that the excess As and P atoms were effectively removed by atomic H irradiation at a low temperature of 350°C. Then, we attempted to obtain a high-quality GaAs epitaxial layer and an ordered (GaAs)1(GaP)3 strained short-period superlattice (SSPS) with abrupt GaAs/GaP hetero-interfaces in the low-temperature growth under atomic H irradiation. The quality of the GaAs epitaxial layer and the abruptness of the GaAs/GaP hetero-interfaces in the (GaAs)1(GaP)3 SSPS were evaluated by photoluminescence, deep-level transient spectroscopy and transmission electron microscopy. As a result, it was clarified that the density of point defects in the GaAs epitaxial layer was reduced and an abrupt GaAs/GaP hetero-interface of the (GaAs)1(GaP)3 SSPS was formed in the low-temperature growth under atomic H irradiation.

Defect-Controlled Selective Epitaxial Growth of GaP on Si by Migration-Enhanced Epitaxy under Atomic Hydrogen Irradiation

We investigated the properties of deposition of GaP on dry-SiO2 and SiNx masks using molecular beam epitaxy (MBE) or migration-enhanced epitaxy (MEE) under atomic hydrogen irradiation and attempted the selective epitaxial growth of GaP-on-Si. The critical substrate temperature, below which poly-GaP was deposited on a mask layer, was lower for dry-SiO2 than that for SiNx, and was lowered by MEE rather than MBE. As a result, the selective epitaxial growth of GaP was achieved by MEE using the dry-SiO2 mask. It was found that the formation of large anti-phase domains expanding into the surface was suppressed by forming a P-prelayer at low temperature. It was also confirmed that the density of misfit dislocations at the GaP–Si hetero-interface was remarkably reduced with a decrease in the growth area.

An optical adaptive device and its application to a competitive learning circuit

A mechanism suppressing the divergence of synaptic weights should be added to the network performing unsupervised learning algorithms with the Hebbian rule. In order to realize competitive learning in hardware, a synaptic connection circuit keeping a sum of synaptic weights constant was fabricated with the complementary field effect transistor (CMOS) floating gate process. The synaptic weights of each neuron were varied by applying the optical correction signals. The sum of synaptic weights was kept constant and the weights were memorized in a nonvolatile manner. A primitive competitive learning circuit was constructed with the synaptic connection circuits and a winner-take-all (WTA) circuit. The self-organization of the competitive circuit was confirmed experimentally and by means of simulations. The limitation of operating range and the feasibility of large-scale integration were discussed.