Papers by Keyword: Nitrogen Doping

Abstract: Low resistivity p-type SiC bulk crystals were grown by the sublimation method with using aluminum and nitrogen co-doping. In the sublimation growth of 4H-SiC, to obtain low-resistive p-type crystals are not easy because of the instability of 4H-SiC polytype with highly Al-doping. We have grown < 90 mΩcm p-type 4H-SiC bulk crystals with the co-doping condition. The results of SIMS and Raman spectroscopy show that high concentration of nitrogen co-doping could be effective to the stabilization of 4H polytype with p-type SiC growth.

Abstract: Limitations in the very fast growth of 4H-SiC crystals are surveyed for a high-temperature gas source method. The evolution of macro-step bunching and void formation in crystal growth is investigated by changing the partial pressures of the source gases and crystal rotation speeds. The variation in macro-step formation depending on radial positions, where step-flow or spiral growth governs, of a grown crystal is also revealed. Based on the relation between growth conditions and macro-step bunching, a trade-off between growth rate enhancement and crystal quality and a method to improve such trade-off are discussed. Nitrogen at a high concentration under very high growth rates in the high-temperature gas source method is also investigated.

Abstract: The formation of basal plane stacking faults in highly nitrogen-doped 4H-SiC crystals was theoretically investigated. A novel theoretical model based on the so-called quantum well action (QWA) mechanism was proposed; the model considers several factors, which were overlooked in a previously proposed model, and explains well the annealing-induced formation of double layer Shockley-type stacking faults in highly nitrogen-doped 4H-SiC crystals. We further revised the model to consider the carrier distribution in the depletion regions adjacent to the stacking fault and were successful in explaining the shrinkage of stacking faults during annealing at even higher temperatures.

Abstract: A visible light active nitrogen (N) doped TiO₂ was prepared using commercially available TiO₂ P25. In this study, a simple N doped TiO₂ was prepared by mixing of TiO₂ powder with urea as N precursor under microwave irradiation instead of normal using muffle furnace as heating media. Prepared N doped TiO₂ samples shows active under visible light irradiation due to lower band gaps energy of N doped TiO₂ observed by UV/Vis-DRS. U3-800 was found as optimum N doped where photodegradation rate of RR4 dye under is 1.6 times faster compared with unmodified TiO₂ as well as control TiO₂ under normal 55-W fluorescent lamp. An active photo response under visible light was observed from U3-800 with 80 minutes of time irradiation to complete RR4 color removal, while no photocatalytic degradation was observed from unmodified and control TiO₂. As a result, this study contributed to purification of effluent specifically toxic dyes with complete mineralization under visible light irradiation. For the future work, the modification of TiO2 with nitrogen and metal doping will indirectly increase the efficiency of photodegradation under visible light. In future, the immobilized technique can also be applied for N doped TiO2 to overcome the reusability issue.

Abstract: This paper presents the results of the studies of the structure and chemical composition of nitrogen-doped titanium dioxide thin films obtained by reactive magnetron sputtering deposition. The XRD data show the changes of the structure and phase composition of titanium dioxide thin films due to the nitrogen doping. The change of the films structure increases with the growth of the nitrogen content. The reduction of crystallites size takes place at the increase of the nitrogen concentration. Chemical bonds present in the films were examined by FTIR spectroscopy.

Abstract: We address the problem of nitrogen incorporation during bulk crystal growth of 4H-SiC and 6H-SiC by seeded sublimation method. The partial pressure of nitrogen and temperature dependence were considered in bulk SiC crystals. Free carrier concentration and incorporated nitrogen were determined using Raman spectroscopy and Secondary Ion Mass Spectrometry, respectively. The incorporated nitrogen at the (000-1) C-face of 4H-SiC and 6H-SiC is found to be independent of the polytype of the crystal. Higher desorption rate at Si-face compared to C-face is found, using a Langmuir equation, which is attributed to the difference in bond density between the two polar faces. The increased nitrogen desorption when growth temperature increases is believed to be the most contributing factor, based on the temperature dependent trends.

Abstract: Application of dichlorosilane (DCS) in 4H-SiC epitaxial growth on 4° off-cut substrates has been studied. The effect of C/Si ratio and N₂ gas flow rate on epilayer properties is investigated in detail. It is found that the C/Si ratio has a significant influence on the growth rate, epilayer surface roughness (step-bunching), conversion of basal plane dislocations (BPDs), and generation of morphological defects and in-grown stacking faults. A wide range of doping concentration from p- to n+ can be controlled in DCS growth. High quality 4° off-cut SiC epilayers are achieved for C/Si=1.3 – 1.8. Addition of N₂ has no obvious influence on growth rate and defect densities. The BPD conversion greater than 99.8% is achieved independent of N doping without any pretreatment.

Abstract: This paper presents a comparative optical and vibrational spectroscopy study of diversely n-type 4H-SiC epilayers. It is shown that in order to determine the nitrogen doping in a wide range (10¹⁶ up to few 10¹⁹cm^-3) the two techniques are complementary. Moreover only the LTPL provides the information about the compensation and nature of the dopant species.

Abstract: The TiO₂ complex samples were produced through sol-gel method, using Ti (OC₄H₉)₄, rice husk (RH) and methenamine as the reactants. Some property analyses were conducted, including X-ray diffraction (XRD), Brunauer-Emmet-Teller (BET), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and photocatalytic experiments in the condition of visible-light. The results showed that the size of crystal was limited with the addition of RH in the complex of N/RH/TiO₂ samples. The particles of TiO₂ were dispersed on the surface of rice husk ash, the skeleton of RH after burning. It can inhibit the phenomenon of agglomerate. The ability of both adsorption and photocatalytie activity of complex samples increased as the surface area increased. Doping N into RH/TiO₂ samples can decrease the forming time, inhibit the transformation of crystal from anatase to rutile to some extent, improve the bond of Ti-O-Si to form and make the absorption spectrum move to the red light part. The complex samples exhibited a certain photocatalytic activity under the visible light region. The reaction rate of visible light photodegradation process arrived 0.0143 min^-1, meeting with the first-order reaction kinetics.

Abstract: Nitrogen-doped graphene (N-rGO) was synthesized in the process of preparation of reduced graphene oxide from the expanded graphite through the improved Hummers’ method. The morphology, structure and composition of nitrogen-doped graphene oxide (GO) and N-rGO were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The nitrogen content of N-rGO was approximately 5 at.%. The electrochemical performances of N-rGO as anode materials for lithium-ion batteries were evaluated in coin-type cells versus metallic lithium. Results showed that the obtained N-rGO exhibited a higher reversible specific capacity of 519 mAh g^-1 at a current density of 100 mA⋅g^-1 and 207.5 mAh⋅g^-1 at a current density of 2000 mA⋅g^-1. The excellent cycling stability and high-rate capability of N-rGO as anodes of lithium-ion battery were attributed to the large number of surface defects caused by the nitrogen doping, which facilitates the fast transport of Li-ion and electron on the interface of electrolyte/electrode.