Preparation And Properties Of A-GaN Films And Alignment Of CNTs By LB Technique

This dissertation includes two parts: the first part is the research on preparation andoptical properties of amorphous GaN (a-GaN) films; and the second part is the alignment ofsingle-walled carbon nanotubes (SWNTs) by Langmuir-Blogdett (LB) technique and themechanism discussion.Recently, as a third-generation semiconductor material, gallium nitride (GaN) has beenconsidered to be the most promising optoelectronic material for many applications. Thisstudy is on preparation and optical properties of amorphous GaN (a-GaN) thin films.In this dissertation, the GaN films are prepared under different Ar pressures anddifferent substrate temperatures by DC sputtering method.The films were characterized by structural, optical and electronic analysis methods.The results show that the structure of the films is amorphous, and Ga-N bond is dominating.The 360 nm peak in photoluminance spectra is from band-to-band transition. No peaks inthe visible region indicate low density of gap states.UV-Vis spectra are recorded to obtain the absorption spectra, band gap, and refractiveindex. Fitting the Urbach tails by equations based of quantum time-dependent perturbationtheory, the results are quite consistent with the experimental data, which indicate that thea-GaN films have wide band gap, low refractive index and wide Urbach tail. The refractiveindexes are also examined by Spectroscopic Ellipsometry (SE), indicate that the films areamorphous.The influence of Ar flow ratio in the sputtering gas on the properties of a-GaN isstudied. The Ar flow ratio has a big influence on the deposition speed. The transmissionratio and optical gap are larger at low Ar flow ratio, indicating the presence of non-bondedmetallic Ga. The absorption coefficient is low at low energy region, which means lowdensity of gap states. The value of the fitting characteristic parameters indicates that theband tail is wider than the one of crystalline GaN.The influence of substrate temperatures on the properties of a-GaN is studied. Theresult shows: the higher temperature results in nanoparticles embedded in the amorphousfilm; at higher temperature the crystalline structure is improved, the band gap reduces obviously and less defect states exist.The influence of annealing at different temperatures and different gases on theproperties of a-GaN is studied. As the temperature of annealing in Oxygen increases, theband gap decreases, and the absorption edge turns steep. The annealing in NH₃ or N₂ hasless influence on the optical properties.As one-dimensional nano-material, carbon nanotubes (CNTs) have wide applicationpotential because of their extraordinary properties. However, the CNTs are absorbed byimpurities and randomly distributed after being prepared by ordinary methods. How topurify and arrange them in order by large scale is to be solved. In this dissertation, we purify,functionalize, and align the single-walled carbon nanotubes (SWNTs) by Langmuir-Blodgett (LB) technique using compression-transfer method. The conclusions are asfollows.A simple and effective purification for SWNTs synthesized by arc-discharge has beendemonstrated. The sodium dodecyl sulfate (SDS) could separate the impurities absorbed onthe SWNTs. The lower concentration nitric acid reacts with SWNTs slowly at lowertemperatures, which could benefit the purification because of less erosion. The hightemperature oxidation of SWNTs in air is studied. Pure SWNTs with smooth walls, lesserosion, perfect structure and good symmetry are obtained.By oxidizing with H₂SO₄ and HNO₃, the SWNTs are cut and introduced to carboxylicgroups. Mix acid treatment shows that as the react time increases, the damages will enhanceand the SWNTs are cut into shorter tubes.The amphiphilic octadecylamine (ODA) is attached by two different methods: one isby ways of reacting with thionyl chloride (SOCl₂); the other is heating directly. TheSWNTs are dissolved in organic solvent. The mechanics are discussed.Using LB technique, the aligned SWNT films are fabricated by compression-transfermethod. A mechanics model is constructed to demonstrate the motion of single SWNT onsubphase for the first time. The influence factors, such as temperatures, surface pressure,compression speed, contact angle and the aspect ratio are discussed. Using layer-by-layermethod, multilayers of SWNTs, which are controllable and have good deposition rate areobtained. UV-Vis spectra indicate that the absorption increases as the layer numberincreases, and the 700 nm characteristics are from the band-to-band transition. For theSWNTs aligned perpendicular to the electrodes, the I-V curves are nonlinear, because of the Schottky barrier introduced by the contact of SWNTs and electrodes. As the surfacepressure increases, the current increases. While the SWNTs aligned parallel to theelectrodes, the current decreases obviously.