Dong-Gu Lee

Semiconducting/Semi-Insulating Reversibility in Bulk GaAs

Bulk, liquid‐encapsulated Czochralski GaAs may be reversibly changed from semiconducting (ρ∼1 Ω cm) to semi‐insulating (ρ∼107 Ω cm) by slow or fast cooling, respectively, following a 5 h, 950 °C soak in an evacuated quartz ampoule. This effect has been studied by temperature‐dependent Hall‐effect, photoluminescence, infrared absorption, mass spectroscopy, and deep level transient spectroscopy measurements. Except for boron, the samples are very pure, with carbon and silicon concentrations less than 3×1014 cm−3. Donor and acceptor concentrations, on the other hand, are in the mid 1015 cm−3 range, which means that the compensation is primarily determined by native defects, not impurities. A tentative model includes a donor at EC−0.13 eV, attributed to VAs−AsGa, and an acceptor at EV+0.07 eV, attributed to VGa−GaAs.

Synthesis of (111) oriented diamond thin films by electrophoretic deposition process

A method for (111) oriented diamond film synthesis has been developed using controlled seeding of micron-sized diamond particles by electrophoresis. Different sizes of diamond powders (0.25 and 5μm) were electrophoretically seeded on silicon substrates using diamond suspensions in organic solvents (acetone, methanol, and ethanol). The seeded samples were then consolidated by the hot filament chemical vapor deposition process. Diamond suspension in acetone was found to be the most suitable for obtaining uniform diamond seeding in electrophoresis. A preferred (111) orientation was obtained for a monolayer of 5 μm seeds. However, when smaller seeds (<1 μm) were used, randomly oriented films were obtained. The surface morphology, crystal orientation, and quality of diamond films were investigated using scanning electron microscopy, x-ray diffractometry, and Raman spectroscopy.

Excimer laser-assisted planarization of thick diamond films

The planarization of polycrystalline diamond films is critical for a large number of industrial applications. We have investigated a laser-assisted method for planarization of thick diamond films. This method is based on the application of excimer laser combined with simultaneous rotation of the sample. Thick diamond films (average surface roughness: ∼20 μm and thickness ∼500 μm) were fabricated by plasma jet chemical vapor deposition process. The planarization of diamond films was found to be critically dependent on the angle of incidence of laser beam. Smoother surfaces were obtained at higher incidence angles (θ = 80°). However, by combination of sample rotation with laser irradiation at higher incidence angles (θ = 80°), maximum surface planarization was achieved. Under optimum conditions, the surface roughness of the samples were reduced from 20 to 0.1 μm. The mechanisms for surface planarization of thick diamond films are discussed.

Novel method for adherent diamond coatings on cemented carbide substrates

Abstract Adhesion of diamond films on cemented carbide substrates is a critical issue due to its large thermal expansion mismatch. This large difference in thermal expansion results in the generation of very high stresses in the coating that may lead to delamination, cracking, or other deleterious effects. A method to increase the adherence of diamond coatings on cemented carbide substrates is reported, based on a substrate-modification process that creates a three-dimensional thermally and compositionally graded interface. Indentation tests on diamond coated tungsten carbide substrate showed that the adhesion of diamond films significantly improved with increasing the surface roughness of cemented carbides.

Novel in situ production of smooth diamond films

We have developed a unique method to produce smooth diamond films using a modified microwave plasma process system. This method consists of sequential in situ deposition and planarization in an electron cyclotron resonance plasma system. Diamond films were deposited to a thickness of 3.0 μm in this system at a pressure of 1.000 Torr from gas mixtures of methanol and hydrogen. Deposition was followed by planarization using a two-grid ion beam extraction process with a pure oxygen plasma at 10 mTorr. The average roughness of the diamond films so produced was as low as 30 nm, which was a factor of two lower than that of the as-deposited diamond films.

Grown‐in defects in multi‐epilayer GaAs grown by metalorganic chemical vapor deposition under different growth conditions

Studies of the grown‐in defects in multi‐epilayer GaAs (with/without a buffer layer) grown by metalorganic chemical vapor deposition under different [AsH3]/[TMGa] ratios, growth temperatures, and growth rates have been made in this letter by the deep level transient spectroscopy method. For samples without a buffer layer, two electron traps with activation energies of Ec−0.83 eV (EL2a) and Ec−0.74 eV are observed, whereas for samples with 6‐μm‐thick buffer layers, only EL2a level is found. The concentration of the deep level traps is found closely related to the [AsH3]/[TMGa] ratio, the growth temperature, and the growth rate (mainly at lower growth rate). The results show that, for samples without a buffer layer, the background dopant density profile is closely related to the deep level trap density profile in the epilayers, whereas for samples with a buffer layer, the profile of background dopant density is less influenced by the presence of the deep level trap.

Diamond Coatings on Surface-Modified Carbide Tools Using KrF Pulsed Laser

The surface of a cemented carbide was modified using a KrF pulsed laser process to achieve a microrough structure, leading to stronger adherence of diamond films to cemented carbide substrates. The surface morphology and roughness were investigated with laser conditions. After the surface modification with the laser and the etching of the modified surface, heat treatment was performed prior to deposition of diamond film in order to observe changes in surface morphology and adhesion. Diamond films were grown by chemical vapor deposition process. The results indicated that the heat treatment of the modified cemented carbide improved the adhesion of diamond films by the recrystallization of tungsten carbides into the fine and long grains. As laser energy for the modification of a cemented carbide increased, the surface roughness increased and tungsten carbide (WC) was transformed to WC 1−x (X =0–0.3) and then to W 2 C.

Particle-Assisted Oriented Deposition of Diamond Thin Films

We have developed a method for oriented diamond film synthesis using micron-sized diamond particles. Different size of diamond powders were electrophoretically seeded on silicon substrates using diamond suspensions in organic solvents (acetone, methanol, and ethanol). Diamond suspension in acetone was found to be the best for obtaining uniform diamond seeding by electrophoresis. The thickness of diamond seeded films was changed by varying the applied voltage to observe the effect on the orientation of diamond particles. Then diamond films were deposited by the hot filament chemical vapor deposition (HFCVD) process. A preferred orientation with direction normal to the substrate was obtained for monolayer coatings. The surface morphology, crystal orientation, and quality of diamond films were investigated using scanning electron microscopy, x-ray diffractometry, and Raman spectroscopy.