Takeshi Takamori

Photoluminescence from AlGaAs-GaAs single quantum wells grown on variously oriented GaAs substrates by MBE

Abstract Photoluminescence (PL) measurements at low temperatures have been carried out for AlGaAs-GaAs single quantum well (SQW) structures grown on (100), (110), (111)B, (311)A and (311)B GaAs substrates by MBE. A mirror smooth surface was obtained under each optimal growth condition except for (111)B and (110) samples. The (110) surface is cloudy due to the columnar growth. PL results suggest that the height of column is in the order of 10 nm. The film on the (111)B surface consists of a lot of trigonal pyramids, which give a rather broad line width of PL peaks from a SQW compared with the (100) sample. The high optical quality for both (311)A and (311)B SQWs, which is comparable to that for (100) SQWs, is achieved in optimal growth conditions. PL study determines the heavy-hole effective mass of about 0.5 m 0 for the [311] direction.

Structure–composition variation in rf‐sputtered films of Ge caused by process parameter changes

It has been shown that noncrystalline Ge‐sputtered films prepared under a range of conditions display a wide variation in structure, composition, and film properties depending on the sputtering process parameters. Data are presented in which optical absorption edge, stress, density, argon and oxygen contents, and other film properties can be varied continuously as a function of preparation parameters. The range of changes which can be brought about in these properties is enormous: changes of 20% in density, 500% in Ar content, and positive to negative in macroscopic stress are achieved within ’’normal’’ operating conditions. The data demonstrate unequivocally that the term ’’sputtered film’’ can refer to only a specific sample, not a reproducible material.

Thermal properties of Na2O·3SiO2 glasses with high water content

Abstract Na 2 O·3SiO 2 glasses with various water contents (up to ∼8wt%) were prepared under high pressure, hydrothermal conditions and their thermal properties, were analyzed by DTA and dilatometry. The glass transition temperature (Tg) determined by the two methods agreed reasonably well. Exsolution of dissolved water occurs at temperature above Tg and is accompanied by foaming. Both the glass transition temperature and the foaming temperature decrease with increasing water content, while the thermal expansion coefficient increases with increasing water content. The change of the glass transition temperature with water content was compared with equations applicable to polymer plasticizers, and was found to be best described by an equation which takes the different contribution of hydroxyl ions and molecular water into account. It was concluded that the hydroxyl ions lower Tg drastically while the molecular water lowers it only slightly by merely diluting the glasses.

Electrical and Optical Properties of Si Doped GaAs Grown by Molecular Beam Epitaxy on (311) Substrates

The electrical and optical properties of Si-doped GaAs grown on GaAs (100), (311)A and (311)B substrates by molecular beam epitaxy (MBE) have been studied. A mirror smooth surface is observed for each orientation at low Si atom concentration. Electrical and optical characteristics of the (311)A sample are similar to Si-doped p-type GaAs grown by liquid phase epitaxy, while (311)B samples show almost the same characteristics as (100) samples. Even for a Si atomic concentration up to 1×1020 cm-3, which is too high to keep good surface morphologies of (100) and (311)B samples, excellent morphologies of (311)A samples were obtained. This orientation dependence may be due to different site occupations of Si atoms and surface atom configurations around Si atoms during MBE growth.

Structure and orientation of films prepared by reactive sputtering of Ni, Fe, and NiFe in Ar/N2 mixtures

Using controlled N2/Ar sputtering gas mixtures, rf‐sputtered films were prepared from Fe, Ni, and Ni81Fe19 targets, and their structure and orientation were studied by x‐ray diffraction. When no N2 was introduced during sputtering, all the films were oriented in the highest density planes; i.e., in (110), (111), and (111) for α‐Fe, Ni, and γ‐Ni81Fe19, respectively. With increasing N2 introduction, however, the orientation shifted to lower density planes, and eventually nitrides were formed. Multilayer films of metal and nitride were prepared with the Ni81Fe19 target by repetitive supply of N2 for short periods during sputtering. Strong orientation effects were observed depending on the number of layers grown. In addition, the x‐ray diffraction of these films presented evidence of epitaxial growth of the nitrides (Ni,Fe)4N on the alloy layers.

Thermally stimulated studies of bismuth silicon oxide crystal

Thermally stimulated current (TSC) of nominally undoped bismuth silicon oxide crystals was measured after UV excitation at liquid nitrogen temperature. The results were compared with the data in the literature, and the origins of some discrepancy discussed. The TSC measurement was very sensitive to the crystal lot; that is probably to the structural defects.

Viscous flow and crystallization of a metallic glass

Abstract The isothermal viscosity of a metallic glass, Fe 40 Ni 40 P 14 B 6 , increases over a large span of time. When the glass is crystallized, the viscosity increases still more rapidly than in the glassy state resulting in s-shaped viscosity-time curves. The viscosity at constant heating rate decreases rapidly with temperature, but it suddenly increases upon crystallization.

Room-temperature photo-pumped operation of 1.58-μm vertical-cavity lasers fabricated on Si substrates using wafer bonding

Long-wavelength vertical cavity lasers have been successfully fabricated on Si substrates using direct wafer bonding. InGaAs-InGaAsP multiquantum-well active layers with 40.5-pair InGaAsP-InP stacked mirrors have been directly bonded on 3.5-pair Al/sub 2/O/sub 3//a-Si mirrors deposited on Si substrates. The sample has been optically pumped at room temperature and lasing operation at 1.58-/spl mu/m has been achieved.

Thermally stimulated current studies of hole transport in a layered organic photoconductor

Thermally stimulated current (TSC) measurements have been peformed on a layered organic photoconductor based on p‐diethylaminobenzaldehyde diphenylhydrazone (DEH) dispersed in bisphenol A polycarbonate. Well‐defined transient current peaks were observed, indicative of a hole transport process having an activation energy of ∼0.5 eV at an applied electric field of 10 V/μm. This TSC feature is attributed to carriers which are thermally activated from electron donor states associated with the DEH molecules responsible for transport in this medium. This transport peak is clearly distinguished from the higher temperature secondary structure which is attributed to trap states. Measurements of the steady‐state photoconductivity over a range of temperatures also yield approximately the same activation energy for the transport process.

Surface-mountable silicon microlens for low-cost laser modules

A surface-mountable silicon microlens that can be passively aligned in a silicon V-groove is proposed. The microlens has a diffractive optical element on the end facet of a semicylindrical structure whose diameter is identical to that of a single-mode fiber (SMF). The experimental result of coupling efficiency between a distributed feedback laser diode and SMF shows low loss (-3.0 dB) as determined using the microlens. Monte Carlo simulations indicate that the microlens is suitable for low-cost and high-performance optical modules for metro networks.