Filmes finos de TiO2 foram crescidos sobre silício (100) através do processo de deposição química de organometálicos em fase vapor (MOCVD). Os filmes foram crescidos a 400, 500, 600 e 700ºC em um equipamento horizontal tradicional. Tetraisopropóxido de titânio foi utilizado como fonte tanto de titânio como de oxigênio. Nitrogênio foi utilizado como gás de arraste e como gás de purga. Foram realizadas análises de difração de raios-x para a caracterização da estrutura cristalina. Microscopia eletrônica de varredura com canhão de emissão de campo foi utilizada para a avaliação da morfologia e da espessura dos filmes. Os filmes de TiO2 crescidos a 400 e a 500ºC apresentaram fase anatase. O filme crescido a 600ºC apresentou as fases anatase e rutilo, enquanto que o filme crescido a 700ºC apresentou, além de anatase e rutilo, a fase broquita. Para se avaliar o comportamento eletroquímico dos filmes foi utilizada a técnica de voltametria cíclica. Os testes indicaram um forte caráter capacitivo dos filmes de TiO2. O pico de corrente anódica é diretamente proporcional à raiz quadrada da velocidade de varredura para os filmes crescidos a 500ºC, sugerindo que o mecanismo predominante de transporte de cátions seja por difusão linear. Observou-se que o filme crescido por 60 minutos permitiu maior facilidade de intercalação e desintercalação de íons Na+. Os filmes crescidos nas demais condições não apresentaram pico de corrente anódica, embora o acúmulo de cargas se fizesse presente. / Titanium dioxide (TiO2) thin films were grown on silicon substrate (100) by MOCVD process (chemical deposition of organometallic vapor phase). The films were grown at 400, 500, 600 and 700 ° C in a conventional horizontal equipment. Titanium tetraisopropoxide was used as source of both oxygen and titanium. Nitrogen was used as carrier and purge gas. X-ray diffraction technique was used for the characterization of the crystalline structure. Scanning electron microscopy with field emission gun was used to evaluate the morphology and thickness of the films. The films grown at 400 and 500°C presented anatase phase. The film grown at 600ºC presented rutile besides anatase phase, while the film grown at 700°C showed, in addition to anatase and rutile, brookite phase. In order to evaluate the electrochemical behavior of the films cyclic voltammetry technique was used. The tests revealed that the TiO2 films formed exclusively by the anatase phase exhibit strong capacitive character. The anodic current peak is directly proportional to the square root of the scanning rate for films grown at 500ºC, suggesting that linear diffusion is the predominant mechanism of cations transport. It was observed that in the film grown during 60 minutes the Na+ ions intercalate and deintercalate easily. The films grown in the other conditions did not present the anodic current peak, although charge was accumulated in the film.

Filmes finos de TiO2 e N:TiO2, e multicamadas TiO2/N:TiO2 foram crescidos sobre substratos de aço AISI 316 e Si(100), por meio da técnica de deposição química de organometálicos em fase vapor (MOCVD). Foram produzidos filmes com diferentes espessuras, nas temperaturas de 400 e 500°C. Os filmes foram caracterizados utilizando-se técnicas de difração de raios X (DRX), espectroscopia de fotoelétrons excitados por raios x (XPS) e microscopia eletrônica de varredura (MEV). A resistência à corrosão foi avaliada por meio de testes de polarização potenciodinâmica em eletrólito 3,5%p NaCl. Filmes não dopados, crescidos a 400°C, apresentaram TiO2 anatase, enquanto que os crescidos a 500°C apresentaram a fase rutilo, além de anatase. Nos filmes dopados com nitrogênio (7,29 e 8,29 at% a 400 e 500°C, respectivamente), em ambas as temperaturas, houve a formação de TiO2 anatase, bem como de fases contendo nitrogênio. Os filmes de TiO2 crescidos a 400°C ofereceram melhor proteção contra a corrosão que os crescidos a 500°C. Filmes crescidos a 500°C apresentaram estrutura colunar, que representa alto nível de porosidade, enquanto que os filmes crescidos a 400°C apresentaram estrutura mais densa. A dopagem não foi eficiente para proteger o substrato contra corrosão, provavelmente devido à formação das fases contendo nitrogênio. Os resultados para os testes com filmes compostos por multicamadas sugerem que aqueles com mais interfaces apresentam melhor resistência à corrosão. O processo de corrosão das amostras se inicia na superfície do filme, que está em contato com o meio agressivo, originando pites, que permitem ao meio corrosivo acessar o substrato metálico. O metal é atacado e dissolvido sob o filme, e resulta na delaminação do filme. / TiO2 and N-doped TiO2 (N:TiO2) thin films, and TiO2/N:TiO2 multilayered films were grown on AISI 316 steel substrates, and Si (100) by using metallorganic chemical vapor deposition (MOCVD) technique. The growth of the films was carried out at 400 and 500°C, and films with different thicknesses and structures were obtained. Titanium dioxide films were produced by using only titanium isopropoxide IV as both titanium and oxygen sources. In order to obtain N:TiO2 films, NH3 was also added to the system. The films were characterized by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) The corrosion resistance was evaluated by potentiodynamic polarization tests in a 3.5wt% NaCl electrolyte. TiO2 undoped films, grown at 400°C, presented anatase, while those grown at 500°C showed the rutile phase, besides anatase. For nitrogendoped films (7.29 and 8.29 at% at 400 and 500°C, respectively), at both temperatures, TiO2 anatase was formed, as well as nitrogen-containing phases. TiO2 films grown at 400°C provided better protection against corrosion than those grown at 500°C. Films grown at 500°C showed a columnar structure, which represents a high level of porosity, while the films grown at 400°C presented a denser structure. Doping was not efficient to protect the substrate against corrosion, probably due to the formation of nitrogen containing phases. The corrosion results for tests with multilayered films suggest that those with more interfaces present better resistance to corrosion. The corrosion of the samples starts at the surface of the films, which is in contact with the aggressive medium, causing pitting in this film, which allows the corrosive medium to reach the metallic substrate. The metal is attacked and dissolved under the film, and results in film delamination.

Recently, there has been increasing demands for high dielectric materials to replace SiO2 for high-density dynamic random access memories with ultralarge scale integration. TiO2 and BaTiO3 are very promising insulators for applications to DRAMs, as they exhibit higher dielectric constant.The growths of TiO2 and BaTiO3 thin films on (100) silicon are studied by MOCVD using Ti(i-OC3H7)4, Ba(DPM)2(tetraene)2 and N2O as precursors. The growth was performed in a cold wall horizontal system in the temperature range of 350~700¢J. The growth rates of TiO2 and BaTiO3 films are affected by the Ti flow rate, growth temperature and reactor pressure. The structures of TiO2 and BaTiO3 films are polycrystalline by X-ray diffraction examinations. The dielectric constant of as-grown TiO2 can reach 85 and BaTiO3 can reach 300 derived by C-V curves with the contact area 3.14¡Ñ10-4 cm2. In addition, the influences of postannealing treatment under an O2 and N2 ambient with different annealing temperature and time on the structural and electrical properties of as-grown TiO2 films will be also studied. However, TiO2 and BaTiO3 films have columnar structures acted the paths of leakage current. We use thermal annealing to reduce the leakage current. In the future, to enhance the dielectric constant and reduce the leakage current of the films is the goal in our study.

In recent years, there has been increasing demands for high dielectric materials to replace SiO2 for high-density dynamic random access memories with ultra large scale integration (ULSI). As the dimensions of the charge storage node decrease in high-density dynamic random access memories (DRAMs), TiO2 and BaTiO3 are very promising candidates for applications with exhibiting higher dielectric constant, high refractive index and high chemical stability. The growth of TiO2 and BaTiO3 thin films on various substrates i.e. (100) silicon¡B(100) GaAs¡B(100) InP and (100) MgO are studied by MOCVD using Ti(i-OC3H7)4, Ba(DPM)2, N2O and O2 as precursors. The growth was performed in a cold wall horizontal system in the temperature range of 280~750¢J. The growth rates and structure of TiO2 and BaTiO3 films are affected by the substrate temperature and reactor pressure, etc. The phase transition properties of TiO2 were studied via X-ray diffraction measurements. X-ray diffraction examination shows that phase transition of TiO2 films are at the same temperature of 450 oC on different substrates. Phase-pure rutile is obtained down to 450¢J on InP (100) and GaAs (100), while phase-pure anatase is obtained up to 450¢Jon MgO (100). The optical and electrical properties are associated with the film structures. TiO2 single phase films with rutile (110) orientation were successfully grown on InP (100) at 500¢J. In-plane epitaxial relationship of anatase TiO2 (100) // MgO (100) is present between 300¢J and 375¢J. In addition, the influences of substrate temperature and oxidizer on the structural and electrical properties of BaTiO3 films will be also studied. However, TiO2 and BaTiO3 films have columnar structures acted the paths of leakage current resulting low dielectric constant. We use thermal annealing to improve the quality of TiO2 with respect to leakage current density and dielectric constant. Dielectric constants of annealed TiO2 films were as high as 110.08. Leakage current density reduced to 5 ¡Ñ 10-5 A/cm2. In the future, to improve the crystal structure of the films is the goal in our study.

The materials based on GaN have successfully developed on short-wavelength laser diodes (LDs), light-emitting diodes (LEDs) and ultraviolet photodetector. In this study, GaN epitaxial layers have been successfully grown on sapphire substrates. We used several methods including the pre-baking treatment for sapphire substrates before growing epilayer, the growth temperature of buffer layer and the growth temperature of GaN epilayer to study it. From the results of the photoluminescence (PL) measured at 77K, the X-Ray diffraction measurement, SEM cross sectional views to realize the characteristic and we get a better qualities of GaN epilayers after using the foregoing methods. In this study, the pre-baking treatment for sapphire substrates can influence the quality and morphology of GaN epilayers. According to the results of the experiments, we study the mechanisms of yellows luminescence and donor-acceptor pair (DAP).

Zugl.: Duisburg, Essen, Univ., Diss., 2007

Techn. University, Diss., 2003--Berlin.

No description available.

New semiconductor materials may enable next-generation â spintronicâ devices which exploit both the spin and charge of an electron for data processing, storage, and transfer. The realization of such devices would benefit greatly from room temperature ferromagnetic dilute magnetic semiconductors. Theoretical predictions have suggested that room temperature ferromagnetism may be possible in the wide bandgap semiconductors GaMnN and ZnMnO, though the existing models require input from the growth of high-quality materials. This work focuses on an experimental effort to develop high-quality materials in both of these wide bandgap materials systems. ZnMnO and ZnCoO single crystals have been grown by a modified melt growth technique. X-ray diffraction was used to examine the structural quality and demonstrate the single crystal character of these devices. Substitutional transition metal incorporation has been verified by optical transmission and electron paramagnetic resonance measurements. No indications of ferromagnetic hysteresis are observed from the bulk single crystal samples, and temperature dependent magnetization studies demonstrate a dominant antiferromagnetic exchange interaction. Efforts to introduce ferromagnetic ordering were only successful through processing techniques which significantly degraded the material quality. GaMnN thin films were grown by metalorganic chemical vapor deposition. Good crystalline quality and a consistent growth mode with Mn incorporation were verified by several independent characterization techniques. Substitutional incorporation of Mn on the Ga lattice site was confirmed by electron paramagnetic resonance. Mn acted as a deep acceptor in GaN. Nevertheless, ferromagnetic hysteresis was observed in the GaMnN films. The apparent strength of the magnetization correlated with the relative ratio of trivalent to divalent Mn. Valence state control through codoping with additional donors such as silicon was observed. Additional studies on GaFeN also showed a magnetic hysteresis. A comparison with implanted samples showed that the common origin to the apparent strong ferromagnetic hysteresis related to contribution from Mn substitutional ions. The observed magnetic hysteresis is due to the formation of Mn-rich regions during the growth process. This work demonstrated that the original intrinsic models for room temperature ferromagnetism in the wide bandgap semiconductors do not hold and the room temperature ferromagnetism in these materials results from extrinsic contributions.

The purpose of this study is using titanium dioxide (TiO2) as gate oxide on thin film transistor (TFT) and discussed with their physical, chemical and electrical properties. Amorphous silicon (a-Si) and polycrystalline silicon (poly-Si) are used as substrates. The metal-organic chemical vapor deposition (MOCVD) and the liquid phase deposition (LPD) are used as the TiO2 growth methods. About the LPD growth method, ammonium hexafluoro-titanate ((NH4)2TiF6) and hexafluorotitanic acid (H2TiF6) are used as Ti sources. We are interested in two parts: (1) the growth mechanisms, physics properties, chemical properties and electrical properties of MOS structure; (2) the fabrication processes and electrical properties of devices. In the first part, we discuss the thin films characteristics on a-Si and poly-Si substrates. For the MOCVD growth method, the MOCVD-TiO2 film tends to form the poly structure. Poly structure has a higher dielectric constant, however, higher traps and dangling bonds also exist at the grain boundaries. Thus, poly structure of TiO2 film has a higher leakage current. For the LPD growth method, the film tends to form the amorphous structure. Amorphous structure has lower leakage current but also has lower dielectric constant. The film that grown from the (NH)2TiF6 source is called LPD-TiO2 film. The film that grown from the (NH)2TiF6 source is called LPD-TixSi(1-x)Oy film. Both films are incorporated with OH and F ions during the growth, the OH and F ions can be outgassed during the low temperature annealing process. In addition, appropriate F ions in the film can passivate the traps and dangling bonds. The low temperature treatments in N2 or O2 ambient and post-metallization annealing (PMA) are adopted to improve the film characteristics. On the other hand, the substrate is not a prefect structure (not a single structure). Thus the film may be influenced by substrate during the annealing treatment. In the second part, the electrical properties of TFT devices were discussed under the coplanar structure. There are several differences of the operation principle in TFT and MOSFET. A-Si and poly-Si are the un-doped substrates with many traps in the bulk. The channel should be occurred through the full depletion mode. The full depletion region is the substrate that under the gate electrode. Thus, the key point is kept the suitable thickness. Too thick, the channel can not appear. Too thin, the substrate may be over-etched. For ion implantation, due to the thinner active layer, the ion implantation energy should be lowed. In addition, the activation temperature and activation time should be adjusted suitable. We have fabricated the TFT devices with the MOCVD-TiO2 as gate oxide on poly-Si substrate. From the I-V characteristics, the Kink effect can be observed. However, the Ion/Ioff ratio is still low. We must further study how to increase the Ion/Ioff ratio.