Mark A. Prelas

Wide band gap electronic materials

Preface. Diamond:- Growth and Doping. Characterization and Properties. Applications. Amorphous and Diamond-Like Carbon Films:- Growth and Doping. Characterization and Properties. Applications. Other Wide Band Gap Semiconductors:- Growth and Doping. Characterization and Properties. Applications. Oral Presentations. Poster Presentations. Author Index. Key Word Index.

Diamond nanoparticles as a support for Pt and PtRu catalysts for direct methanol fuel cells.

Diamond in nanoparticle form is a promising material that can be used as a robust and chemically stable catalyst support in fuel cells. It has been studied and characterized physically and electrochemically, in its thin film and powder forms, as reported in the literature. In the present work, the electrochemical properties of undoped and boron-doped diamond nanoparticle electrodes, fabricated using the ink-paste method, were investigated. Methanol oxidation experiments were carried out in both half-cell and full fuel cell modes. Platinum and ruthenium nanoparticles were chemically deposited on undoped and boron doped diamond nanoparticles through the use of NaBH(4) as reducing agent and sodium dodecyl benzene sulfonate (SDBS) as a surfactant. Before and after the reduction process, samples were characterized by electron microscopy and spectroscopic techniques. The ink-paste method was also used to prepare the membrane electrode assembly with Pt and Pt-Ru modified undoped and boron-doped diamond nanoparticle catalytic systems, to perform the electrochemical experiments in a direct methanol fuel cell system. The results obtained demonstrate that diamond supported catalyst nanomaterials are promising for methanol fuel cells.

DIFFUSION OF BORON, LITHIUM, OXYGEN, HYDROGEN, AND NITROGEN IN TYPE IIA NATURAL DIAMOND

Diffusion of boron, lithium, nitrogen, oxygen, and hydrogen into type IIa natural diamond was studied. The diffusion was performed in two steps. First, diffusion of Li and oxygen was performed in nitrogen atmosphere at 860 °C for one hour. The sample was then placed in a hot filament chemical vapor deposition (CVD) growth reactor and diffusion was performed for two hours in hydrogen atmosphere from a boron solid source placed on the surface of the sample. The condition of diffusion were those used routinely during CVD growth. After diffusion, the concentration of Li was of the order of 2×1016 cm−3 at the depth of 0.5 micrometer, and oxygen, nitrogen, and boron were found to be in the range (1–4)×1020 cm−3 at the same depth. The diffusion of hydrogen under conditions specific to CVD growth has also been studied for the first time and was found to be quite strong.

Prospective n-type impurities and methods of diamond doping

Abstract A major goal of diamond thin film technology research has been the reproducible production of p-n junctions, which are the basic units of many electronic devices. While p-type conductivity is relatively easily attained by boron doping, n-type conductivity has proved much harder to achieve. The experimental and theoretical results on prospective donor impurities are reviewed. In analogy with classical semiconductors, we will discuss the possibility of obtaining n-type diamond by using substitutional impurity atoms (nitrogen and phosphorus) and interstitial atoms (Li and Na). New methods of forced diffusion and ion assisted doping during growth are discussed. Methods of forced introduction of impurities into the diamond lattice have an important advantage over traditional ion implantation methods. Ion implantation introduces structural defects (vacancies, vacancy + interstitial, and their combinations) that are difficult to cure. Both methods, forced diffusion and ion assisted doping during growth, introduce no additional structural defects, except that inherent to the impurity itself.

FORCED DIFFUSION OF IMPURITIES IN NATURAL DIAMOND AND POLYCRYSTALLINE DIAMOND FILMS

A method is proposed for the determination of the state of an impurity (donor, acceptor, or deep level) in semiconductor lattice. To demonstrate the method boron was diffused into type Ia natural diamond under a dc electric field. The concentration and diffusion profiles of boron were affected by the applied field. Boron diffuses as a negative ion since it is an acceptor shallow enough to be partially ionized at the temperature of diffusion. The drift velocity of boron ions at the temperature of diffusion was also estimated. The diffusion of lithium and oxygen from a Li2CO3 source in chemical vapor deposited diamond films was performed under bias at 1000 °C in an argon atmosphere. After diffusion, the concentrations of Li, O, and H in the diamond films were found to be around (3–4)×1019 cm−3. No dependence of these concentrations on the applied bias was observed. It was found that the diffusion of Li goes primarily through grain boundaries, which may explain why it does not depend on the applied voltage. ...

Properties of diffused diamond films with n-type conductivity

Abstract High quality, freestanding “white” CVD diamond films, 230 μm thick, polished on both sides and with resistivity 10 14 Ω cm were used for diffusion of impurities to obtain n-type conductivity. Diffusion of lithium, oxygen and chlorine was performed under a bias. Auger analysis was used to determine the impurity concentrations. After diffusion, the concentrations of Li, O and Cl in the diamond films were found to be about (3–4) × 10 19 cm −3 . Raman scattering, cathodoluminescence, microwave photoconductivity, electron spin resonance, optical absorption, Hall effect and electrical conductivity measurements were employed for the film characterization. The measurements showed that the initial high structural quality of the film was not deteriorated after diffusion. The hall effect measurements showed n-type conductivity. The sheet resistance of the diffused layer was 10 5 Ω/□. The carrier mobility was estimated to be about 50 cm 2 V −1 s −1 .

New tilted-poles Wien filter with enhanced performance

The Wien filter is an E×B deflecting analyzer with the electrostatic field perpendicular to the magnetostatic field. The twofold functions of the Wien filter are as an energy analyzer as well as a mass analyzer. It has very high resolution for paraxial charged‐particle beams with V=E/B, the Wien velocity. Two Wien filters, a tilted‐poles Wien filter, and a classical parallel‐rectangular‐poles Wien filter were built and tested for electrons up to 3.5 keV and protons beams of 200 eV. (The tilted‐poles Wien filter is a new diagnostic developed by the authors.) The performance of the two is compared, and the tilted‐poles Wien filter has superior resolution to the classical Wien filter. Both Wien filters appear to have features useful for high‐temperature plasma diagnostics, including simultaneous measurement of energy and mass spectra, and high resolution.

Silver on diamond Schottky diodes formed on boron doped hot‐filament chemical vapor deposited polycrystalline diamond films

Schottky diodes were fabricated using sputter deposited silver contacts to boron doped polycrystalline diamond thin films grown by a hot‐filament chemical vapor deposition process with trimethyl borate as an in situ dopant source. High forward current density and a high forward‐to‐reverse current ratio were exhibited by these diodes. Current density‐voltage and capacitance‐voltage‐frequency characteristics of these diodes are very similar to those of Schottky diodes fabricated using a single‐crystal diamond substrate.

Diamond ultraviolet photovoltaic cell obtained by lithium and boron doping

Polycrystalline high quality freestanding 300-μm-thick diamond films were doped by diffusion of B and Li under electric bias in order to fabricate vertical p-n junctions. Circular contacts were obtained by high dose ion implantation of B and Li. The I–V characteristics were rectifying. When illuminated by deuterium lamp, an open circuit voltage was 2.6 eV. The shape of the I–V characteristic under illumination points to the existence of shunt and series resistances. The obtained structure is most probably a p-n junction with bad contacts.

Effect of mechanical stress on current-voltage characteristics of thin film polycrystalline diamond Schottky diodes

Schottky diodes utilized for mechanical stress effect studies were fabricated using aluminum contacts to polycrystalline diamond thin films grown by a hot‐filament‐assisted chemical vapor deposition process. Compressive stress was found to have a large effect on the forward biased current‐voltage characteristics of the diode, whereas the effect on the reverse biased characteristics was relatively small. This stress effect on the forward biased diamond Schottky diode was attributed to piezojunction and piezoresistance effects that dominated the diode current‐voltage characteristics in the small and large bias regions, respectively. At a large constant forward bias current, a good linear relationship between output voltage and applied force was observed for force of less than 10 N, as predicted by the piezoresistance effect. The measured force sensitivity of the diode was as high as 0.75 V/N at 1 mA forward bias. Compared to either silicon or germanium junction diodes and tunnel diodes, polycrystalline diam...