The Investigation On The Growth Mechanism And Photodetectors Of AlGaN One-Dimensional Nanostructure

AlGaN semiconductors which exhibit the large direct band gap which is tunable from 3.4e V to 6.02 e V,remarkable optical properties,high thermal stability and conductivity,good dielectric properties and good mechanical strength can be widely used in the fabrication of high-performance electronic and optoelectronic devices.Nanomaterials have some superior properties to the corresponding bulk material due to the destruction of the periodic lattice.To date,the AlGaN nanomaterials and devices are still under laboratory research.Much more have to be done to improve the quality of AlGaN nanomaterials prepared via different techniques and to understand the growth mechanism.In this dissertation,we mainly studied the synthesize of AlGaN one-dimensional nanostructures,the preparation and function of the catalyst particles,the growth mechanism and the ultraviolet photodetectors based on AlGaN nanowires.The conclusions are as follows:Firstly,we synthesized high-quality Al N and AlGaN 1-D nanostructures via chemical vapor deposition using different catalysts.We obtained the evolution of the nanostructures by tuning the parameters during the growth and explained the evolution based on mass transport theory.Through the research on the growth time,the presence of the nanocones at the beginning of the growth indicated that some precursor impinged on the substrate and could not diffuse to the catalyst or L-S interface.Eventually,they might directly react and deposit on the side wall.When the growth continued,the length of the nanowires became much greater than the average diffusion distance.Therefore,the tip of nanowires was no longer cone-shape.But,as the growth time increased,the axial growth rate would reduce due to the loss of the catalyst and the lateral growth rate will be greater than the axial growth rate.So,when the growth time increases,the diameter of nanowires was much greater and the cone-shape tip could be observed.Through the research on the growth temperature and the flow of ammonia,we found that when the temperature or the flow of ammonia was too low,the reaction between precursors were hard to occur and no nanostructures were synthesized.In the ammonia-rich condition,the products turn out to be stacked clusters.Secondly,the annealing of the Ni,Au and Pd catalysts and the role of the catalyst played during the growth process were studied systematically.Through the research on the catalyst annealing,we found that the annealing process of Au and Pd on Si could be explained invoking Ostwald ripening and the Ni and catalyst on sapphire maintained solid-phase during annealing.The further experiments revealed that he catalyst island is the key to achieve the growth of the nanowires and,with the competition between the elements in catalyst,products in different structures and element contents would be synthesized in different growth parameters.Thirdly,we explained the growth mechanism in detail with the researches on preparation of nanowires and the catalyst.When the growth driving force is limited by the rate of chemical reaction,the products prefer to deposit at VLS region or LS interface which lead the axial growth.Meanwhile,the products can also deposit at VS interface due to the gap of chemical potential between vapor and solid phase which lead the lateral growth.The morphology of nanostructures is determined by the different rates on the different interfaces.Fourthly,metal-semiconductor-metal(MSM)structure photodetectors based on the prepared AlGaN nanowires and ultra-long Ag nanowires were fabricated.At present,the reverse dark current maintained below 10?A and the largest light-to-dark-currents ratio was 40.