| Summary: | 碩士 === 龍華科技大學 === 工程技術研究所 === 96 === To explore nano optotronic materials with improved properties,in this study,a hot wall LPCVD system,carried out at 550~650℃, was employed to grow nanostructured zinc oxide.Three methods were used. First,the impact of plasma treatment to the PL 380 nm peak intensity was investigated.Second,a set of LPCVD experiments with different deposition times were performed,with morphology variation observed and being indicated the growth mechanism of nanomaterials.The third part,uses in-situ doping of magnesium,to prepare Mg doped ZnO,with its structural and luminescence properties changes as a function of doping amount being investigated.FESEM,PL, XRD,and Raman were used to analyze the properties of deposits.Experimental data on the first part show encouraging results.After treated with RF plasma at a power of 100 watts,for 5 minutes,sets of CVD zinc oxide samples,which were deposited at 600oC,200 torr,and an oxygen concentration of 25%,show abrupt intensity increments of 72-1210% for PL UV peaks near 380 nm.However,FESEM observes almost no morphological change after treatment.Interestingly,the (002) preferential orientation becomes weaker after treatment for most samples.One plausible reason is that plasma treatment change the surface layer structure and eventually improves the electron-hole pair recombination ratio.
For the second part of study,data show deposition time plays an important role in evolving nanostructures.CVD runs were performed at 600oC,60 torr, and an oxygen concentration of 87%.SEM images show,on blank Si(100) and sputtered ZnO buffers, CVD zinc oxide crystals appears as flat hexagonal plates initially,with deposition times from 10 to 30 min. Subsequently,the deposits become superstructures of stacked plates.At a deposition time of a hour,ZnO becomes columnar structure.While on copper metal,unique one dimensional nanostructures were observed.One is nanocolumn,formed by a series of polyhedrons seemingly ZnO,with PL active mode detected.This appears mostly in short deposition time samples.The second is high density,straight nanolines,with diameters less than 100 nm,and length more than 12µm. XRD reveals various ZnO crystallographic orientations as basal plane (002), (100),(101),(110) and (102).Their PL peaks were weak.A survey of axial growth rate against deposition time shows self limiting phenomena.The axial growth rate of such nanocolmunar or lines drops sharply for deposition times more than 10 min.The function of PL peak intensity against time also show similar trend.One feasible reason is that accumulation of organic vapor in the CVD chamber deters decomposition of precursor and makes growth slow down.
Data of the third part of research,in-situ doping of Mg,has been proved to be a direct way to alter the band gap of ZnO,and also an efficient method to produce nanowires.For a doped of 5wt% Mg(C5H7O2)2 with 1 gram of Zn(C5H7O2)2,under a condition of 600oC,130 torr, with water vapor introduced,and an oxygen concentration of 87%,zinc oxide nanolines with diameters of 100 nm,and length of 5 µm formed.Mg doping also causes red shifts for PL near band edge emission lines. EDS indicates that Mg was introduced successfully into ZnO lattice. Raman show that, except modes of ZnO, two additional acoustic modes at 378cm-1 and 328cm-1 were detected.SEM also detected a unique growth mechanism for nanostructures on copper. This is a combination of normal line growth,followed by lateral growth subsequently.at the beginning, heterogeneous nucleation of zinc/cooper alloy occurs on the cooper substrate,which is followed by a growth of straight nanolines via VLS mechanism.Then,continuous growth of nanolines with finite diameter. This might be due to either anisotropic behavior for surface migration adatoms or site blocking due to ambient Hacac vapor,until the diameter and length achieve 50 nm and 10 �慆, respectively.Further,secondary nucleation and growth of zinc oxide happen on the side wall of nanolines.The shape of nuclei is a duplicate of ZnO wurtzite unit lattice. Repeated nucleation and two dimensional growth cause stacking of hexagonal plate grains.
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