Study Of ZnO & GaN Wide Band-gap Semiconductors: Microstructure Modulation And Property Characterization

The development of material science is a symbol of human civilization.During the thousand years of human history,we experienced Anthropolithic Age,Bronze Age and Iron Age.Now we have entered the Information Age based on semiconductor materials and devices.With the rapid development of science and technology, people's demands on materials are increasing.So it is very important to explore and study new materials with superior properties.Wide band-gap semiconductor is one of them,which used to fabricate high power,high frequency and high temperature devices for future electronic devices.Since the discovery of nanomaterials in the 80s of 20^th century,people began to realize that the properties of materials are not only determined by the chemical compositions,but also have close relationship with the microstructures and aggregating states of materials.While the microstructures and aggregating states are affected by the synthetic procedure,techniques and engineering,etc.So it is important to investigate the influences of the macroscopic state,such as synthetic procedure, morphology,structure,etc,on the intrinsic structures and electron distributions to improve the physical properties of materials.It is also important to the development of materials science and practical applications.In this thesis,we chose zinc oxide and gallium nitride,which are two of the most important wide band-gap semiconductors,as the objective of our studies.We took chemical engineering and band-gap theory as the theoretical foundation on the synthesis and designing of ZnO and GaN materials.By systematically investigation of the relationship on synthetic procedures,microstructures and physical properties of ZnO and GaN to point against the key problems and hot spots on the scientific research and practical applications,we successfully modulated the microstructures and improved the physical properties of these materials.And our results have been further verified by theoretical calculations.We explored the basic rule on the relationship of procedure,structure and properties of ZnO and GaN,and provided an experimental sample and a theoretical foundation for large scale practical applications.In chapter one,we briefly introduced the categories,physical properties and development of semiconductor materials.And we also discussed the factors which can affect the properties of materials.Some basic information about ZnO and GaN are also presented.In chapter two,we synthesized ZnO nanorods and highly oriented ZnO nanorod arrays on FTO substrate by a simple hydrothermal method with PVA or PEG surfactant in the reaction.We performed X-ray diffractions,scanning electron microscopy,UV-vis spectra,photoluminescence,etc.to investigate the structures, morphologies and optical properties of our samples.We found that the addition of PVA or PEG as surfactant during the hydrothermal process can modulate the growth procedure and morphologies of ZnO materials.The growth along(100) and(101) directions can be prohibited and the agglomeration of ZnO nanorods can be reduced. Furthermore,the addition of surfactant can change the photoluminescence properties of ZnO materials,the intrinsic emission in UV region disappeared,and a new emission at visible violet region appeared.For the highly oriented ZnO nanorod arrays,the directions and the optical transmission of the ZnO nanorod arrays can be greatly improved by adding PVA as surfactant during the hydrothermal process.We investigated the growth process and the photoluminescence properties of ZnO nanorod arrays.The photoluminescence mechanism for the arrays was obtained by performing a series of annealing treatment under different atmospheres. Photocatalysis properties of the ZnO nanorod arrays were also studied.In chapter three,we designed and synthesized ZnO/In₂O₃ p-n hetero-nanostructures by co-precipitation method.We systematically investigated the influence of Zn/In in the starting materials,annealing temperatures,concentration of the precursor solutions on the compositions of our samples and the photocatalytic properties.According to our experimental analysis,the optimized condition for ZnO/In₂O₃ hetero-compositions are Zn/In=1:1 with the starting Zn²⁺ concentration of 20 mM,annealing under 800℃.We investigated the structures,morphologies and the interface of the heteronanostructure by various means.And the interface and the energy band structure of the ZnO/In₂O₃ composite were also investigated.The ZnO/In₂O₃ heteronanostructures are proven to be efficient in the separation of the photogenerated hole-electron pairs and have a high photocatalytic activities based on our systematic analysis.And the designing of the semiconductor heteronanostructures is regarded as a potential way in the development of future photocatalysts for particular applications.In chapter four,we synthesized GaN and ZnO@GaN solid solution nanomaterials by carbothermal nitridation method.By changing the initial Ga/C or Zn/C ratios in the starting materials,we synthesized a series of different samples.We investigated the structures,morphologies and growth process of GaN and ZnO@GaN materials.As we found,charcoal in the starting materials plays an important roles in controlling the morphologies of GaN and ZnO@GaN nanomaterials during the carbothermal nitridation procedure.We proposed a plausible growth mechanism of these nanomaterials based on our experimental data.According to our studies,we regarded carbothermal nitridation method a simple but effective way to modulate the morphologies of GaN and ZnO@GaN nanomaterials.Further investigations on the crystalline symmetry of ZnO@GaN solid solution nano-composites are under progress.In chapter five,we synthesized Mn/C codoped GaN nanomaterials by carbothermal nitridation method using charcoal as the carbon source.The bonding states of Mn and C atoms,characterized by XPS,confirm the doping of both Mn and C atoms into the GaN lattice.Nanostructures such as zigzag nanowires,nanoscrews, and hexagonal nanocones can be produced by controlling the reaction time and the C/Ga molar ratio in the starting mixture.Room-temperature magnetization measurements show that the saturation magnetization of Mn/C codoped GaN can be greater than the Mn doped GaN by a factor up to～40 and increases steadily with increasing Ga/C molar ratio in the starting mixture at a rate of～0.023 emu/g per C/Ga molar ratio.Further investigations are necessary to learn how to control the morphology and the ferromagnetism of Mn doped GaN more precisely.Our DFT calculations support the experimentally deduce suggestion that carbon doping in GaN favors the N sites over the Ga sites,Mn/C codoping strongly enhances the preference of FM coupling over the AFM coupling between the two doped Mn sites.Further investigations are necessary to learn how to control the morphology and the ferromagnetism of Mn doped GaN more precisely.In chapter six,we synthesized Fe,Ni doped GaN nanomaterials by the same carbothermal nitridation method.We characterized the structures,morphologies and ferromagnetic properties by various means.We found the saturation magnetization and the coercive force of Fe doped GaN and Ni doped GaN varied a lot with the change of Ga/C ratios in the starting materials.Further investigations are needed to learn the origins and relationship of the magnetic properties and the Ga/C ratios.In chapter seven,we summarized our work and discussed the problems remained to be solved.At last,we made a plan for the future work and looked forward to the futurity.Material science is one of the most important parts of natural science,and has profound influence on both the development of human civilization and the exploration of science.In this thesis,we investigated the relationship of synthetic procedures, microstructures,and physical properties of ZnO and GaN based nanomaterials,and modulated the microstructures to improve their properties.Our studies on the microstructure modulation and properties characterization of ZnO and GaN based nanomaterials are proven to be an important and efficient way to improve the properties these nanomaterials.And it is also important for the practical applications of ZnO and GaN based nanomaterials in the future.