Hirotaka Ohno

Heteroepitaxial growth of LiNbO3 single crystal films by ion plating method

Lithium niobate (LiNbO3) thin films were deposited on several different substrates such as glass (Corning 7059), α-Al2O3 (R- and Z-plate), MgO(111) and ZnO(001) by an ion plating method. The crystallinity and compositional fluctuation of the deposited films have been examined by X-ray diffraction, RHEED and SIMS. Consequently, it has been confirmed that the heteroepitaxial growth of LiNbO3 occurs on the above crystalline substrates, despite large lattice mismatches (≈8.2%). The orientation relationships between the epitaxial layers and the substrates are as follows: 1. (1) (012)LiNbO3⌈(012)α-Al2O3(R-plate), [100]LiNbO3⌈[100]α-Al2O3; 2. (2) (001)LiNbO3⌈(001)α-Al2O3(Z-plate), [100]LiNbO3⌈[100]α-Al2O3; 3. (3) (001)LiNbO3⌈(111)MgO,[110]LiNbO3⌈[211]MgO; 4. (4) (001)LiNbO3⌈(001)ZnO, [110]LiNbO3⌈[210]ZnO, respectively.

AFM Observation of Self-Assembled Monolayer Films on GaAs (110)

We have confirmed that a self-assembled monolayer (SAM) film of octadecanethiol (ODT), CH3(CH2)17SH, can be formed on a cleaved GaAs (110) surface, by using an atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). Circular depressions were observed on the surface after film formation. The area of the circular depressions increased with immersion time, indicating that the solution oxidized the interface between ODT molecules and the GaAs surface, resulting in removal of ODT molecules. The oxidation was considerably faster in pure ethanol solution than that in ODT solution, demonstrating that the SAM film protects the GaAs surface from oxidation. High-resolution lateral force microscope (LFM) images revealed a periodic structure that had two types of lines: periodic lines 0.57 nm apart and lines rotated 55° with respect to them. A structural model of the SAM successfully explained both the features in high-resolution LFM images and the depression depth observed in AFM images.

PREPARATION OF SELF-ASSEMBLED MERCAPTOALKANOIC ACID MULTILAYERS ON GAAS (110) SURFACES

Mono- and double layer films have been formed by the self-assembly of 16-mercapto-hexadecanoic acid, (MHDA), HS(CH2)15COOH, via selective ionic interaction. To understand the formation process in the nanometer length scale, atomic force microscopy (AFM) studies have been conducted in combination with micro X-ray photoelectron spectroscopy (µ-XPS), Auger electron spectroscopy (AES) and micro-fourier transform infrared spectroscopy with the attenuated total reflection method (µ-FTIR ATR). After the formation of a self-assembled monolayer (SAM) was completed, the surface treated with copper ions was immersed in a solution of MHDA. By this method, an atomically smooth double layer was successfully formed on cleaved GaAs (110) surfaces. The formation speed of the second layer was faster than that of the SAM, indicating stronger interaction of thiol functional groups with Cu2+ ions than with GaAs surfaces.

AES and XPS studies of surface of AlxGa1-xAs (110) treated by ammonium sulfide

We applied the technique of ammonium sulfide treatment to AlxGa1-xAs (110) surfaces and investigated the atomic composition of a surface by Auger electron spectroscopy and X-ray photoelectron spectroscopy. It was found that surface oxidation was significantly reduced by the sulfur treatment and that sulfur atoms were mainly bonded to Al atoms in an Al-enriched sample of AlxGa1-xAs (x=0.78). The surface compositions of the Al-enriched specimen (x=0.43, 0.78) were also found to be As-riched as a result of both sulfur treatment and the preferential etching of metallic atoms by an acidic solution, which prevents the initial oxidation of the surface of AlxGa1-xAs.

Improvement of high-power characteristics of 780-nm AlGaAs laser diode by (NH4)2S facet treatment

Coherent cw operation has been obtained with a 10x4 array of Grating Surface Emitting (GSE) lasers consisting of 40 lasers and 50 emitting sections. The array is a GaAs quantum well device, grown on an A1GaAs substrate, operating at 861 nm to which the substrate is transparent. It is mounted p-down to metallized traces on a BeO slab to provide isolated electrical contacts and thermal contact to a simple chilled-water cooler. A single spectral line 0.5 A wide indicates coupling of the 40 laser sections. More detailed measurements on a section of an array containing ten laterally coupled lasers, 20 outputs, show that it is operating at a junction temperature of 30°C, has a line width of 0.1 5A, and a measured coherence of >75%. The expected wavelength stabilization with temperature due to the DBR grating is found, with a value of =0.6A/°C. An array of 4 longitudinally coupled lasers produced a line width of 130MHz and evidence of high coherence.

Nanometer-Scale Wires of Monolayer Height Alkanethiols on AlGaAs/GaAs Heterostructures by Selective Chemisorption

Self-assembled nanometer-scale wires (SANWs) composed of octadecylthiol [ODT, CH3(CH2)17SH] have been selectively formed on the GaAs surface of an AlGaAs/GaAs heterostructure. The sample grown by metal organic chemical vapor deposition (MOCVD) was cleaved to expose the AlGaAs/GaAs heterostructure in a solution containing the ODT molecules. Both surface properties and morphology of these wires have been investigated simultaneously by scanning probe microscopy. SANWs as narrow as 10 nm in width and ~1.0 nm in height were observed. The inability to form an ODT monolayer on the AlGaAs surface was attributed to the formation of an oxide layer.

Pulse Injection-Locked TEA CO2 Laser for Long Pulse Operation with Feedback Stabilization

Stable, long pulse operation of a TEA CO2 laser is achieved with a pulse injection locking system using a hybrid CO2 laser as an injection source. With feedback stabilization loops incorporated, both single-longitudinal- mode and two-longitudinal-mode oscillations are realized. The optimum condition for the synchronization between the injection pulse and the initiation of the discharge of the main TEA CO2 laser is investigated. The effect of the detuning angle on the long pulse operation is also studied.

DIRECT OBSERVATION OF PRECIPITATES AND SELF-ORGANIZED NANOSTRUCTURES IN MOLECULAR-BEAM EPITAXY GROWN HEAVILY DOPED GAAS:SI

We report a cross‐sectional scanning tunneling microscopy investigation of heavily Si doped [001]‐oriented GaAs grown by molecular‐beam epitaxy. At a very high doping level (6×1019 cm−3), Si‐doping induced precipitates are directly observed in XSTM images of the as‐grown epitaxial layers. Most of the precipitates are found to have a characteristic oval shape with the long axis (∼80 A) along the growth direction. In contrast to the low diffusivity of randomly distributed Si dopants in the moderate doping regime, these precipitates are found to be highly mobile and spontaneously form ‘‘nanowires’’ during crystal growth.

Aes and Xps Studies of Sulfur-Treated AL x GA 1−x as Surfaces

Passivation of Al x Ga 1−x As surfaces by ammonium sulfide treatment has been investigated. Enhancement of photoluminescence intensity and dramatic reduction of oxide peaks in spectra acquired by X-ray photoelectron spectroscopy on Al x Ga 1−x .As surfaces prepared by this treatment lead to the reduction of surface state density. It was found that sulfur atoms were mainly bonded to Al atoms in an Al-enriched specimen of Al x Ga 1−x As(x=0.78). The surface compositions of Al-enriched specimen(x=0.43, 0.78) were also found to be As-rich as a result of both sulfur treatment and the preferential etching of metallic atoms by an acidic solution, which prevents the initial oxidation of the surface of A1 x Ga 1−x As.

Theoretical Model and Numerical Simulation of Two-Longitudinal-Mode Oscillation in an Injection-Locked TEA CO2 Laser

Injection locking of a TEA CO2 laser is studied theoretically from the point of view of analyzing its two-longitudinal-mode oscillation (TLMO) observed experimentally. A phenomenological theoretical model and a new method of numerical calculation are developed to simulate TLMO locked with a monochromatic injection field. In the proposed theoretical model, injected radiation is allowed to couple with two neighboring longitudinal modes with assumed coupling coefficients. By making use of the proposed algorithm, rapid temporal variation of population densities and cavity fields can be calculated numerically, maintaining the correct phase relation. The simulation results show good agreement with experimental results and the power-modulated output of the TEA CO2 laser is obtained theoretically.