Research On Second-order Nonlinear Optic Properties And Electroluminescence Of Cubic Boron Nitride

Cubic boron nitride (cBN) is a kind of artificial synthetic crystal withband-gap of about 6.3 eV, which has zinc-blende structure and 4 3m. Itstheoretic value of the heat conductivity is up to 13W /cm?K, and its electricresistivity is up to 10 10??cm. Compared with diamond, cBN crystal hasbetter heat resistance, oxidation resistance, chemical stability andsemiconductivity, though its hardness is a little weaker than diamond hardness.cBN crystal can be made into both p-and n-type semiconductors whensuitable impurities are doped. Within the range of all visible light, mostinfrared and ultraviolet spectra, cBN crystal is transparent. Therefore it willhave wide applications in optoelectronics and microelectronics fields. Becausethe synthesis of cBN crystal is extremely difficult, and synthetic monocrystalis very small, whose maximal size is about 3 mm only. These stop the study oncharacteristics of cBN crystal. The second-order nonlinear opticalcharacteristics of cBN crystal have not been investigated by now. For the firsttime, we studied the second-order nonlinear optical properties of cBN crystal.In our study, we discovered the electroluminescence (380~400 nm) and thecurrent-controlled differential negative resistance phenomena of cBN crystal.The linear electro-optic effect (Pockels effect) is a special kind ofsecond-order nonlinear optic effects. It can happen in cBN crystal since cBNhas no symmetric center. Here, the two electric fields applied on a medium arean optical frequency electric field and a low frequency electrical field or zerofrequency electric field. The second-order nonlinear polarization is generatedby the interaction of the two electric fields on the medium. Because the cBNcrystal are very small and hard, it is difficult to cut a cBN cryastal intorectangular parallelepiped. Based on the practical form of the cBN crystal, weestablish the theoretical formula and experimental method mearusing thelinear electro-optic (EO) effect of the cBN crystal, and the normal transverseEO modulation is corrected. For the first time, the obvious EO effect of cBNcrystal is observed, and the linear relationship between the intensity of theoutput light and the modulation voltage was measured. Based on the measuredvalues, the half-wave voltage of measured cBN sample is observed, upon theabove results, the linear electro-optic tensor can be calculated atγ =1. 17×10?14m /V. In our experiment, there are two main innovations.Firstly, due to its small size and high half-wave voltage, the normal static anddynamic conditions were not fit to measure EO coefficient of cBN crystal. Theminimum (or null) transmission point measurement method was not fit forcBN crystal too, because the signal is too feeble to carry out. We use the setupthat it consists of two-crossed polarimeters (the input polarizer 'P' and theoutput analyzer 'A'), a cBN sample (S) and a quarter wave plate (Q) betweenthem, namely PSQA system. When applied the modulation voltageV~ = Vm sinωmt on the cBN sample along [111] direction, the intensity of theoutput light consists of two parts, a constant intensity and a variationalintensity with modulation voltage. Getting the ratio of the two parts, we cancalculate Vπ. Furthermore, its linear EO coefficient can be calculated. Secondly,the hardness of cBN crystal is just little less than that of diamond, it is difficultto cut a cBN sample into rectangular parallelepiped. The cBN crystals used inour study are irregular octahedron structure, whose outside surfaces are {111}planes. The top and bottom planes are bigger than the irregular side planes,and they cannot meet at right angles. Based on fact form of the cBN sample,we establish the theoretical formula and experimental method to fit the cBNsample. In this experiment, we don't need to get the absolute values but to getthe ratio two voltage-values, which are easy to be done in our experiment. Thelinear electro-optic effect of cBN crystal may be applied on electro-opticmeasuring and sample technique, in order to detect the high speed electricsignal in integrated circuits chip.Both second harmonic generation and optical rectification belong to thesecond-order nonlinear optics effects. The second harmonic generation is themost typical, important and elementary technique in nonlinear optics mixedfrequency. The optical rectification is a phenomenon that the DC componentof second-order nonlinear electrical polarization is produced with an opticalwave. In general, it is an inverse electro-optic effect or an inverse Pockels'effect. According to the nonlinear optics theory, that SHG and the opticalrectification should exist at the same time.We use Q-switching Nd:YAG laser, with the wavelength of 1064 nm, thepeak power more than 700 W, the repeated frequency of 2 kHz and the pulsewide less than 20 ns. The second-order harmonic generation of the cBN crystalwas observed, and the electric field or bias was generated in the sample. Thesignal of the optical rectification voltage was obtained by a lock-in amplifier.Its value was about 1μV. Due to the 4 3m symmetry of cBN crystal, therefractive index of cBN crystal is isotropic, and the phase matching cannot beachieved. However, the breakdown field of cBN crystal is very intensive, sowe can apply enough high voltage on it to make it be uniaxial. So that we mayachieve the condition of the phase matching. If the incoming laser is fs pulsetrain, we can get the THz wave or radiation. Because the cBN crystal istransparent within the whole visible and the most ultraviolet regions, we canobtain ultraviolet laser by SHG of cBN crystal using the visible laser. If theincoming laser is fs pulse train, we can get the THz wave or radiation by theoptical rectification. We are investigating the both works now.The absorption of many photons in semiconductors is being got greatattention, since it takes a great action on fundamental study of the nonlinearabsorption and the solid physics, especially on getting information of anenergy band structure. The effect of three-photon absorption is the high-ordernonlinear optic effect in the nonlinear optics. The three-photon absorption ofcBN crystal is investigated using Q-switching Nd:YAG laser, with thewavelength of 1064 nm. The infrared light (1064 nm) is focused on KTPcrystal. After passing KTP crystal, the frequency-doubled light is obtained.When the cBN crystal is exposed to the green laser with wavelength of 532 nm(the photon energy is 2.33 eV), we observed the photoconductance. This factmay be that it is happened by the three-photon absorption. Our study on this isin an elementary stage now. Further study on this will be supported by moretheories and experiments.Recently, the ultraviolet light of p-n junction of cBN is reported. However,the study of cBN's semiconductor characteristics is scarce, probably due to thelack of large-scale and high-quality single crystal. When we investigated theproperties of the nonintentionally doped cBN crystal, the electroluminescenseof cBN crystal is discovered. According to our knowledge, it is the first time todiscover the electroluminescence of cBN crystal that is produced by theintraband transition of electrons.The current-voltage curves of cBN crystal are measured using variouselectrodes. The shapes of the I-V curves for various electrodes are similar,and are nonlinear. The experimental phenomena for various cBN crystals arethe same. The nonlinear I-V characteristics of cBN crystal are explained usingspace charge limited current. When the field intensity inside cBN crystal getsto a adequate strength, the I-V curve becomes steep and does not fit the theoryof space charge limited current. At the same time, the light emission of cBNcrystal is observed. At the voltage at which the emission begins, the light is soweak that we cannot characterize it. The light intensity increases at the voltageincreases. When the electric field is strong enough, the electric breakdownoccurs inside whole cBN crystal. At this time, the light emission is thestrongest, and the electroluminescence is bright blue-violet. The light emissionis lasting for 5 hours. The electroluminescence spectrum of cBN crystal ismeasured. The light wavelength is from 380 to 400 nm. When measuring theI-V curve after breakdown of cBN crystal, we observed the current-controlleddifferential negative resistance phenomenon. The effect of cBN crystal differsfrom Gunn at all. According to the energy-band structure of cBN crystal, thesephenomena can be explained by the theories of avalanche breakdown and theelectronic transition between valleys of the conduction band of cBN crystal.The blue-violet light due to electroluminescence has great valuation in scienceand appliance. We are further investigating the mechanism ofelectroluminescence and try to make the light emission device.In short, we investigated the second-order nonlinear optics properties andthe electroluminensence of cBN crystal. The linear electro-optic coefficient ofcBN is determined. The second harmonic generation, the optical rectificationand the three-phonon absorption of cBN crystal are discovered. The electricproperties of cBN crystal are investigated. The nonlinear I-V curve, theelectric breakdown, the electroluminesence and the current-controlleddifferential negative resistance are analyzed and explained.