| Electro-optic detection techniques are powerful tools for measuring the internalelectric signal and the distribution of the electric field, which are non-intrusive andundamaging measurements, and have very strong abilities against electromagneticdisturbance and very high time resolutions. Since the integration level is improvedcontinuously, the electro-optic detection technique must possess very high spatialresolution which is mainly decided by the size of the probing light spot. Because ofthe limit of diffraction, the Full Width at Half Maximum (FWHM) of the focusinglight spot is about half the wavelength. For improving the spatial resolution, theshort-wavelength light is needed to be used, so the energy gap of the electro-opticmaterial must be wide enough. Cubic boron nitride (cBN) is one of synthesizedwide-bandgap semiconductors, whose energy gap is about6.4eV, and which is thelargest among III-V compounds. So if cBN is used as an electro-optic probe, thetheoretical spatial resolution of the electro-optic detection technique will be100nm(suppose that the wavelength of the probing beam is about200nm). However, cBNneeds to be synthesized at high temperature and high pressure, so it is very hard toobtain high-quality and large-size cBN crystals. Thus, people mainly apply cBN tomachining as abrasives and cutting tools because cBN has extreme hardness (secondonly to diamond), while the electro-optic property of cBN is ignored.Our group experimentally researched the second-order nonlinear optical effectsof cBN crystals previously, especially focused on the linear electro-optic effect ofcBN, and measured the linear electro-optic coefficient of plate-like octahedral cBNsamples. In previous experiments, cBN samples were not mechanically processed,the operating error was large, which affected the measurement precision.In this thesis, according to the knowledge of crystallography, minute cBNcrystals were processed into regular rectangular parallelepiped configurations bycleaving, abrasing and polishing. Then based on these regular cBN samples, a transverse electro-optic measurement system was set up, and the linear electro-opticcoefficient γ41of cBN was measured through comparison and elimination ofunknowns. Because the errors from multiple refractions and reflections of theprobing beam and the azimuth adjustment of samples were avoided, themeasurement precision of the linear electro-optic coefficient was improved greatly.In experiments, two kinds of transverse modulation structures were applied, one wasthat the electric field was perpendicularly added on (111) planes of cBN (the upperand lower larger planes of the plate-like cBN), and the probing light went normally_through (110) planes (smooth cleaving planes), on the contrary, the other was thatthe electric field was vertically applied to (110) planes, and the probing light wentthrough (111) planes. The measured linear electro-optic coefficients in two cases arealmost identical, respectively are3.68pm/V and3.95pm/V, which also indicates thatthe method of measuring the linear electro-optic coefficient is convenient andfeasible. According to the results, the linear electro-optic coefficient of cBN is largerthan those of other III-V compounds, and cBN is a kind of promising electro-opticmaterial.An interesting phenomenon was found in the course of measuring theelectro-optic effect of cBN. When the applied modulating voltage increases to acritical value, the electro-optic signal suddenly enlarges rapidly. At the same time,the current passing through cBN increases quickly too, and the specific I-Vcharacteristics of cBN indicates that the current-controlled differential negativeresistance effect takes place in cBN, and it can be observed through a microscopethat the color of cBN darkens obviously. Further experiments show that theelectro-optic signal is proportional to the current intensity after it rapidly increases,and it is independent of the polarization of the probing beam, in addition, it isconsiderably affected by the frequency of the modulating voltage. All evidencesreveal that this is a kind of electro-absorption modulation. Since the wavelength ofthe probing beam (1.3μm and0.65μm) is much larger than the absorption edge ofcBN, it could not be Franz-Keldysh effect. And it could not be the thermo-optic effect either for the frequency of the electro-optic signal is identical to that of themodulating voltage. The most possible cause is the free carrier plasma dispersioneffect (FCPDE). Based on the basic principle of FCPDE, the experimental formulasdescribing the dependence between the variations of the refractive index and theabsorption factor of cBN and the carrier density were put forward, that isΔα≈5.593×10-17ΔN, and the conductivity effective mass of carrier in cBN wasestimated, which is about0.18~0.28m0.At present, many researchers use cBN films as insulating gates of field effecttransistors or heat sinks of other high-power electronic devices, so it is very possibleto apply the linear electro-optic effect or FCPDE of cBN to the measurements on thedistributions of the electric field or free carriers. The research results in this thesiswill provide solid experimental basis for these applications. |
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