Table of contents

A novel type of flat electron emission diode with a high emission efficiency has been fabricated using single crystalline diamond thin films homoepitaxially grown on thick high-pressure-synthesized diamond. For the formation of the buried electrode, 180-keV N⁺ ions were implanted into the homoepitaxial layer grown by microwave plasma chemical-vapor-deposition (CVD) method to a dose 1×10¹⁶ ions/cm² at room temperature. Since this process created a significant damage in the specimen surface layer working as the electron emission surface, a high quality diamond layer was subsequently overgrown to recover the damaged surface. Applying voltages of sub-kV between the hydrogenated surface and the buried electrode results in an efficient electron emission (>10%).

The surface morphologies of SrTiO₃ substrates have been investigated by atomic force microscopy. Periodic step arrays could be developed on both normal and vicinal SrTiO₃ (100) substrates by a new method employed both buffered NH₄F-HF (BHF) solution and O₂ annealing. An explanation is given for the surface morphologies observed which involves the surface stoichiometrical change upon BHF solution and surface diffusion upon oxygen annealing.

It is demonstrated that the refractive-index-controllable nature of luminescent porous silicon (PS) is directly applicable to the development of a three-dimensionally buried optical waveguide. The PS waveguide is fabricated on a p-type silicon wafer by monolithic processes such as photolithography, ion implantation, anodization, and thermal oxidation. An induced high contrast of refractive indices leads to efficient confinement and propagation of visible light. When the active core layer is partially excited by a He-Cd (325 nm) laser, blue emission is observed from a cleaved facet. The PS waveguide is potentially useful as a component of silicon-based photonic integration.

An InGaN multiquantum-well (MQW)-structure laser diode (LD) was grown on an epitaxially laterally overgrown GaN on sapphire. The lowest threshold current densities between 1.2 and 2.8 kA/cm² were obtained when the number of InGaN well layers was two. The InGaN MQW LD was grown on a free-standing GaN substrate that was obtained by removing the sapphire substrate. The LDs with cleaved mirror facets showed an output power as high as 30 mW under room-temperature continuous-wave (CW) operation. The stable fundamental transverse mode was observed by reducing the ridge width to a value as small as 2 µm. The lifetime of the LDs at a constant output power of 5 mW was about 160 h under CW operation at an ambient temperature of 50°C, due to a high threshold current density of 6 kA/cm².

We have grown 1.5 µm-thick GaN layers on (0001) sapphire substrates by metalorganic chemical vapor deposition with various flow rates of trimethylgallium. The lattice constants a and c in the layer were estimated by X-ray diffraction. The threshold power density for stimulated emission was measured by photopumping the layers. It was found that the grown layers were elastically deformed and that the residual strain can be sustained by decreasing the flow rate of trimethylgallium. It was also found that the threshold power density of stimulated emission decreases as the lattice constant a decreases. This tendency indicates that the strain relaxation mechanism is associated with enhancement of nonradiative recombination.

The structure of polycrystalline silicon films was controlled by selecting deposition conditions, especially gas mixing ratio of H₂/SiF₄, for plasma enhanced chemical vapor deposition on glass at a low temperature of 360°C. Under the H₂/SiF₄ ratio of 10/90 sccms, (111), (220), (311) and (400) diffraction peaks were observed by X-ray diffraction for a 0.5 µm thick film and (400) oriented crystallites grew with film thickness. The sharpness of the Raman peak around 520 cm^-1 and the pseudo-dielectric function measured by an ellipsometer indicated that the crystallites have excellent regularity of the Si–Si bond compared with (220) oriented films and also have small surface roughness, which are preferable features for device applications. The selective growth of (400) oriented grains is thought to partly originate from selective etching due to fluorine related species generated under low hydrogen conditions.

A heat treatment with high-pressure H₂O vapor at temperatures of 190 °C to 270 °C was applied to improve the characteristics of n-channel polycrystalline silicon thin-film transistors. The heating at 190 °C with 3.6×10⁵ Pa H₂O vapor increased the carrier mobility from 50 cm²V^-1· s^-1 (as fabricated) to 412 cm²V^-1· s^-1 and reduced the threshold voltage from 2.2 V to 1.1 V. The drain current at negative gate voltages caused by the heating with high-pressure H₂O vapor was reduced by additional heating at 300 °C as well as at 350 °C with H₂O vapor at one atmospheric pressure. This annealing process resulted in an on/off drain current ratio of 10⁷.

The metalorganic molecular beam epitaxial growth of CuInSe₂ layers on a GaAs(100) substrate was performed at 550°C using cyclopentadienylcoppertriethylphosphine (CpCuTEP), triethylindium and selenium as source precursors. CuInSe₂ layers with a flat surface were successfully grown. The epilayers were examined by X-ray diffraction, photoluminescence (PL) and photoreflectance measurements. The CuInSe₂ layer grown under optimum conditions exhibited an exciton emission in the PL spectrum.

At the same input power (1000 W), inductive coupled plasma (ICP) and ultrahigh-frequency (UHF) plasma sources produced electron densities of 1 ×10¹¹ cm^-3 at 3.5 mTorr, yet the UHF plasma was much less dissociated (30%) than the ICP plasma (70%). This can be attributed to differences in the electron energy distribution functions in the UHF and ICP plasmas, especially at low pressure. Under these conditions, Al etching profiles were investigated to understand the influences of the degree of dissociation on the etching reactions. UHF plasmas could completely accomplish anisotropic etching with just Cl₂ as the feed gas, whereas the ICP produced isotropic etching profiles under the same conditions. This implies that the degree of dissociation strongly influences etching of the Al sidewall, as well as the anisotropic etching rate in a high density Cl₂ plasma.

In the AlGaAs/GaAs two-section multiple-quantum-well (MQW) laser system, Q-switched optical pulses whose width is shorter than the cavity round-trip time were obtained by driving the gain section with 200 ps electrical pulses. The pulse-width characteristics were measured while varying the cavity length, and the optimum cavity length for obtaining the shortest optical pulse was found. When the cavity length was 170–200 µm, the shortest optical pulse with a width of 6–8 ps was obtained. We also found a profile change in the second-harmonic-generation (SHG) auto-correlation trace when the cavity length was changed.

We measured the temperature dependence of the zero-dispersion wavelength of dispersion-shifted fibers using a four-wave mixing process in the range of 22°C–59°C. From the experimental results, we found that the zero-dispersion wavelength increases linearly as the temperature around dispersion-shifted fiber(DSF) increases, and its slope is 0.032 nm/°C.

An optical pulse train with a repetition rate in the gigahertz range has been successfully generated by an FM fiber ring laser that uses an optical circulator and a fiber grating for frequency filtering. The FM laser generates optical output with maximum chirping rate of more than 1.3×10²¹ Hz/s under intracavity phase modulation at 3.3 GHz. By shifting the frequency filtering window of fiber grating, the pulse repetition rate can be doubled, and optical pulses with a repetition rate of 6.6 GHz rate are obtained.

We have two-dimensionally visualized a photoinduced planar waveguide produced in a photorefractive LiNbO₃ crystal. Temporal and spatial evolution of the waveguide was investigated by the interferometric imaging technique. The guiding of a probe He–Ne laser in the waveguide was also experimentally observed.

One period of a three-dimensional photonic crystal operating in the 5∼10-µm-wavelength region is developed, where four layers having a striped pattern are stacked using a wafer fusion and the alignment method based on a laser beam diffraction pattern observation technique. By measuring the transmission spectrum, considerable attenuation (of about 16 dB) is successfully achieved and is in good agreement with the theoretical calculation.

The first MeV hydrogen ion implanted waveguide in KTiOPO₄ is reported. The planar optical waveguide in KTiOPO₄ was fabricated by 1.0 MeV hydrogen ions to a dose of 2×10¹⁶ ions/cm² at room temperature. The dark modes were observed by prism coupling. Both refractive index profiles before and after annealing show a typical barrier waveguide in KTiOPO₄ formed by 1.0 MeV hydrogen ion implantation. After annealing at 200°C for 30 min, a 0.4% decrease in the refractive index was observed. Loss is estimated to be less than 2.2 dB/cm.

The growth of β-C₃N₄ crystallites is studied at various substrate temperatures by an inductively-coupled plasma sputtering method using 500 W of radio frequency power to enhance the gas dissociation. The crystallites deposited are demonstrated to be β-C₃N₄ phase rather than other phases from the transmission electron diffraction and the X-ray photoelectron spectroscopy results. Upon increasing the substrate temperature from 400°C to 800°C, β-C₃N₄ crystallite size increases from 0.02 µm to 0.2 µm, but the [N]/[C] atomic ratio in the film decreases slightly from 1.0 to 0.85, suggesting that the film contains larger β-C₃N₄ crystallites in a less nitrogenated amorphous carbon matrix at a higher temperature. The film deposited at 800°C exhibits a highly spotty transmission electron diffraction pattern and contains a high percentage (90%) of sp³ C-N bonding as estimated from X-ray photoelectron spectroscopy. The results suggest that a high substrate temperature enhances the formation of β-C₃N₄ crystallites at a high degree of gas dissociation.

The growth of thick ZnSe films with high crystalline quality on a Si substrate is impaired by the 4.4% lattice mismatch at the interface. Based on the concept of van der Waals epitaxy, an InSe buffer layer was inserted between ZnSe and Si. The three-monolayer-thick buffer layer was grown by molecular beam epitaxy on a hydrogen-terminated Si(111) substrate. Despite a lattice mismatch of 4.2%, the InSe film grew with high crystalline quality and without lattice distortions. ZnSe was subsequently grown on the InSe layer. The lattice mismatch of 0.2% at this interface appeared to be favorable for the growth of high-quality ZnSe films. In situ reflection high-energy electron diffraction and ex situ atomic force microscopy studies in the thin-film growth regime are presented.

We have proposed a sequential sputtering/selenization technique and apparatus for the growth of CuInSe₂ (CIS)-based thin films. The apparatus consists of a cylindrical rotating drum for holding substrates and three horizontally interconnected subchambers for Cu, In, and Se fluxes. The serious problem associated with hybrid sputtering of metal target contamination by Se flux has been greatly reduced by the current geometric design. In this method, a very thin Cu/In stacked layer is first sputter-deposited, and then selenized with thermally evaporated Se vapor at each rotation of the drum. Polycrystalline CIS films for solar cells were grown by sequentially repeating these steps, which prevented the formation of the micron-sized voids usually observed in CIS-based thin films grown by selenization.

We have observed that it is possible to direct bond copper to copper at low temperature. For sputtered Cu surface on silicon wafer without any further surface planarization, we determined that 100°C temperature and uniform pressure (10⁷ N/m²) condition for three hours can produce a very strong Cu–Cu bond. A straight-pull test is completed and the results revealed that an adhesion promoter layer is required at the copper-silicon interface.

Polycrystalline CuInSe₂ thin films were prepared on Mo-coated soda-lime glass substrates by the ionized cluster beam (ICB) technique, in which Cu, In and Se vapors were ionized and accelerated. The dependence of the film properties on acceleration voltage were studied. The substrate temperature was maintained below 350°C. The films were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), an electron-probe microanalyzer (EPMA) and the Rutherford backscattering spectrometry (RBS). It was found that polycrystalline films with improved grain size and uniformity were obtained when the acceleration voltage exceeded 4 kV, and the acceleration voltage played an important role in the formation of the ternary compound during the crystal growth.

In the force detection of the evanescent field using a semiconductor tip, the force gradient is affected not only by the surface potential change due to the evanescent field, but also by the contact potential difference (CPD) between the tip and the sample which is not uniform on the surface. In this paper, we propose a novel method to measure the evanescent field without the CPD effect using the Kelvin probe technique. Simultaneous images of the topography, the CPD and the force gradient due to the evanescent field were obtained on a 15-nm thickness sputtered Au surface. These images showed no correlation in several areas. The lateral resolution of the force gradient due to the evanescent field was better than 15 nm (λ/33).

A low-temperature, uniform, and high-density microwave plasma (2.45 GHz) is produced utilizing a spokewise antenna with no use of magnetic field. The plasma maintains a uniform state in Ar low pressure of 10 mTorr with high electron density, >10¹¹ cm^-3, and temperature less than of 2.5 eV within ±6% over a 16 cm diameter. Highly crystallized, photoconductive, hydrogenated microcrystalline silicon, µc-Si:H film is produced from dichlorosilane (SiH₂Cl₂), H₂ and Ar mixture at a high deposition rate of more than 5 Å/s. This plasma source has high potential not only for etching but also for the large-area film deposition processes.

Cubic boron nitride (c-BN) thin films are synthesized by reactive sputtering. Pure boron is used as the sputtering target, which is dc-biassed in an Ar/N₂ electron cyclotron resonance plasma. Substrates are rf-biased with a frequency of 13.56 MHz. BN films with a dominant cubic phase are obtained in the case of a high ion-to-boron flux ratio of 12 at the substrate self-bias higher than -175 V; the transferred momentum per atom is about 1260 (eV amu)^1/2, which is larger than the value predicted using the momentum transfer model for c-BN synthesis by a factor of 4. An intermediate layer between the c-BN layer and the substrate improves the adhesion of the c-BN layer and prevent a exfoliation. This intermediate layer is deposited under an Ar/N₂ gas mixing ratio of 9 without rf bias.

Nickel coated carbon nanotubes have been converted to diamonds with high conversion ratio at 8 GPa and 1600°C. Nickel coated on the surface of each and every carbon nanotube might act as catalyst and play the key role for high conversion ratio of carbon nanotubes to diamond. Pure carbon nanotubes can only be converted to high oriented graphite under same high pressure and high temperature condition.

A new method of fabricating Si microstructures on thin insulating films is described. It combines selective-area growth of Si with electron-beam direct patterning of a SiO₂/SiN_x bilayer mask. The chemical composition of the top SiO₂ layer of the mask can be modified locally by electron-beam irradiation. The different chemical properties between irradiated and nonirradiated surfaces make it possible to deposit Si only on the irradiated surface. The bottom SiN_x layer is stable against electron-beam irradiation, and thus insulates electrically the deposited Si microstructure from the Si substrate. Selective-area growth of Si was performed by ultrahigh-vacuum chemical vapor deposition (UHVCVD).

Spatially selective electrochemical deposition of a metal (Ni) into the ordered nanohole-array structure of anodic porous alumina was performed using a microelectrode. A patterned deposit consisting of fine Ni cylinders of 70 nm diameter was obtained in the hole-array structure with spatial resolution on the order of tens of micrometers, and its size was in good agreement with that of the microelectrode used as a tip.