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Growth Of ZnO Whiskers And TiO2 Single Crystal With Floating Zone Method And Their Optic Spectra

Posted on:2007-03-06Degree:DoctorType:Dissertation
Country:ChinaCandidate:X Y GuoFull Text:PDF
GTID:1101360185954784Subject:Condensed matter physics
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ZnO whiskers and TiO2 single crystal were grown by thefloating zone (FZ) method under high oxygen pressure. The crystalstructure of the samples was characterized by using X-ray diffractionmeasurements, which revealed that the samples have highlycrystallinity with c-axes orientation. The optical properties of thesamples were studied by the photoluminescence and Raman spectrum.They show that the samples have a good quality.1. The ZnO is one of wide-band-gap semiconductors, which has aband-gap of 3.37eV at room temperature (RT). It has a large excitonbinding energy of 60 meV, which should, in principle, favour theefficient excitonic emission processes at RT. Highly efficientexcitonic UV lasing action in ZnO nanowires and nanocrystalliteunder optical pumping at RT was observed. There is a strongcommercial desire to produce potentials efficient light emittingdiodes and short-wavelength semiconductor lasers. It is importantapplications in high density data storage systems, solid-state emittersoptical communications, UV optoelectronics, integrated sensors andso on. The transparency in visible light of ZnO provides opportunitiesto develop transparent electronics. The strong exciton binding energysuggests that the ZnO will resist the formation of photochemicaldefects under intensive light radiation. Whiskers as single crystalfiber has little defects, its strength and modulus near to the theoreticalvalue of the ideal crystal materials, it is the most strong form of thesolid that discovered up to now. Small-diameter ZnO whiskers haveattracted scientists great attention for industrial applications as newreinforced composite materials. Because the luminescence of ZnO isvery sensitive to its surface state, ZnO whiskers with a highsurface-to-bulk ratio are expected to have novel practical applicationsin electrooptical devices and chemical sensors. Recently, ZnO crystalwhiskers with a high aspect ratio have been successfully used as aprobing tip to develop new precise high-resolution imagingtechniques for atomic force microscopy and scanning tunnelingmicroscopy. At present, the research of ZnO single crystal hasbecome attractive attention by many groups in the internationalresearch. The research contents include preparation methods of largesingle crystals, nanofilms, nanorods, nanowires, nanotubes etc. andthe emitting mechanism. The spectrum of ZnO whiskers exists twoemitting band at room temperature (RT), one is the green light from480nm to 520nm, and the other at UV band around 370-410nm. Theintensity of peak and position of photoluminescence associated withmany factors, such as crystal shape, defects, grain size, excitationintensity, temperature etc.. ZnO whiskers were grown using thetemperature field effect of the FZ furnace. It is a self-assembleprocess, need not any substrate and ZnO seed crystal. The grain sizeis about 10um. The lattice parameters calculated according to theXRD pattern of the ZnO whiskers are a=0.3251nm and b=0.5220nm,it shows that the ZnO whiskers with hexagonal structure are growthalong c-axis orientation. This result is the same as that the growth onthe Si substrate using MBE, but the diameter of single crystals in thisstudy is larger than that one. For reducing oxygen defects andpreventing sublimation of ZnO at high temperature the high oxygenpressure as protective gas was used.In order to determine the optional pressure, we grew whiskers atvarious pressures from 0.1MPa to 0.7MPa. Through analyses of XRD,PL and Raman scattering spectra it is revealed that the oxygenpressure of 0.3MPa is the best choice.This work completed measurements of PL of ZnO whiskersusing cw He-Cd (46mw, 325nm) laser as excitation light, from 93Kto 303K. The free exciton emission bands at 93K were observed.They are Eex, I, P, 2LO, 3LO and 4LO. We found that these bandschange to narrow and peaks shift to blue with the temperaturedecreased, we believe that the main reason is the following twofactors dominates the PL spectra: one is due to decreasingtemperature reduced the number of population (hot electrons) ofconduction band;the other is that with temperature descent to reducethe crystal lattice expansion, it is shortening the average distance ofelection transitions, lead to the decreasing of the probability offormation bounding electrons in holes. The PL spectra of ZnOwhiskers under different excitation intensities (EI) at RT using a SHG(290nm) of pulsed YAG pumped R590 dye laser were measured. TheEI is changed from 1kW/cm2 to 1.5MW/cm2. At low EI (<1kW/cm2),there is only the green fluorescence (515nm) observed, it mainlycomes from oxygen vacancies and the zinc interstitials, and identifiedas luminescence from bound exciton transitions. The greenfluorescence of ZnO whiskers will be seen as long as the excitationphoton energy over 2.4eV, it has not power threshold. But when theEI increases up to 2kW/cm2, an additional emission band (3.130eV,denoted by P) appears in the UV range, it should be attributed theexcitons recombination of the near band, with the further increasingof EI, the intensity of the P band grows linearly. The driving force ofthe exciton recombination mainly comes from exciton-excitoncollision, therefore, the exciton densities must be getting to a criticalvalue (threshold). This phenomenon provides the experimentalsupport for understanding the reason of ZnO UV emission originatesfrom exciton recombination. The UV intensities of ZnO whiskersincrease with EI increasing. When the EI increases up to 20kW/cm2.The wavelength at 380nm (3.263eV) appears. When the EI increasesup to 500kW/cm2, the P-band inelastic exciton-exciton scatteringprocesses will dominate luminescence. When the EI over the 1MW,because of formation of the electron-hole plasma (EHP) in greatquantities, the EHP will dominate the UV emission, and the peakposition shifted to blue (3.152eV) when the EI increasing over 1.5meV, the PL intensity increase superlinearly with EI increasing, andthe line width (FWHM) changed to narrow down in the meantime. Itmeans that the phenomenon of stimulated emission take place.According to the sensitivity of Raman spectroscopy to latticestrain and doping, we studied the Raman spectrum of ZnO whiskersat RT and various temperature (93K-706K) using Jobin Yvon HR800Raman spectrometer. The experimental results of the ZnO whiskersare the same as the reported data. Among the various phonon modesof wurtzite crystal the E2high mode is Raman active and has been usedin the characterization of carrier. Therefore, detailed study of theRaman line shape of these phonon modes, including the FWHM andpeak position relation with temperature is very desirable.2. The rapid development of optical communications has raisedhigher requirements for the physical properties of the largebirefringence uniaxial crystals. Optically pure calcite and iceland sparsingle crystals have early been used as the materials for opticalpolarizer. Although many researchers have done many works on thegrowth of calcite single crystals, most studies are still far away frompractical applications.Rutile (TiO2) has higher birefringence and refractive indicesthan calcite, and its chemical stability is also better than that ofcalcite. However, it is very difficult to obtain the optical quality rutilesingle crystals for practical applications, because of its higher meltingpoint (1850℃), the defects and low angle grain boundaries caused byoxygen vacancies in the process of crystal growth. This has hinderedthe applications of the rutile single crystal in optics. The floating zone(FZ) method developed in recent years is one of the better ways toobtain high quality oxide single crystal. In this paper, we grew TiO2single crystals by using the FZ method in the higher oxygen pressure(0.5MPa). In the process of crystal growth, the existence of highoxygen pressure significantly reduces oxygen vacancies in the crystal,so the preparation of the optical quality rutile single crystals isdesirable. The as-grown rutile single crystal was black andcompletely opaque. After the post-growth annealing in air at 1000℃for 3 days, the crystal was restored to its intrinsic color of pale yellowand transparent.Lattice parameters calculated according to the XRD pattern ofthe TiO2 single crystal are a=0.459nm, c=0.296nm. This result showsthat TiO2 single crystal with tetragonal structure was grown alongc-axis orientation.This work studied the grown orientation of the TiO2 singlecrystal by Raman sprectroscopy. The Raman vibration modes ofrutile are A1g+B1g+B2g+Eg. The Raman peaks at 143, 450, 609 and826 cm-1 were all observed by us, they are belong to B1g, Eg, A1g andB2g, respectively. The Raman peak at 238 cm-1 is known to be thecompound vibration peak due to the multiple-phonon scatteringprocesses, which is also considered as a characteristic Raman peak ofrutile (TiO2).Because of the confinement of the experimental equipments, werecorded only Raman spectra of the rutile single crystal TiO2 at thetemperatures range from 113K to 673K. The Raman spectrum of therutile (TiO2) single crystal at various temperatures showed differentcharacteristics, including Raman frequency shifts, Raman peakintensities and FWHM. When the sample temperature increasedfrom 113K to 673K, the Eg mode shifted from 455 cm-1 to 431 cm-1.In view of the composition of the Raman activity, the Eg modebelongs to inharmonious vibration of oxygen atoms along the c-axis,which is the nonpolar mode. The result of increasing temperatureonly increased the vibration frequency of the oxygen atom, leading toRaman frequency shifting to higher frequency. The A1g mode alsoshifted to higher frequency with the temperature increasing, and theA1g mode is the result of stretching vibration of the bond Ti-O, whichbelongs to polar vibration mode. At the same time, the polarity ofelectric dipole moment reduces with increasing temperature. Whenthe temperature reached to 673K, the polarity of electric dipolemoment reduced to the minimum level, so the intensity of A1g modedecreased gradually and disappeared nearly. We also found that theintensities of all Raman peaks decrease with temperature increasing,which leads to the FWHM widening. It may be owing to thefollowing two factors: one is that the increasing temperature resultedin widening of the distribution of the bond length and its related bondangle of Ti-O;the other is that the increasing temperature decreasedthe population of molecular ground state level, leading to thedecrease of the stimulated particle densities, and the higher thetemperature, the more intense the effect. Moreover, the Raman peakat 238 cm-1 should attributed to the second-order Raman scatteringbecause this peak has some characteristics of typical second-ordermultiple-phonon processes. Its intensity change with temperaturevariation is not too obvious because the observed second-orderintensity changes only with wave vector, and the peak width is oftenextremely broad due to the multiple dispersions.The Raman peak at 690 cm-1 was observed, for the first time, inthis work. To our knowledge, there were no reports on this peak inthe related literatures previously. This peak may be generated by thesecond-order process.
Keywords/Search Tags:Floating zone, Rutile, X-ray diffraction, Raman scattering, whiskers, Photoluminescence
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