Research On Nigrescence In High Electric Field And Piezoelectric Effect Of Cubic Boron Nitride

Cubic boron nitride (cBN) is a kind of artificial synthetic crystal with band-gap of about 6.3 eV, which has zinc-blende structure and 4 3m.cBN is a kind of indirect transition semiconductor. Its theoretic value of the heat conductivity is up to 13 W/cm·K, and its electric resistivity is up to 1010 10 10Ω?cm. Compared with diamond, cBN crystal has better heat resistance, oxidation resistance, chemical stability and semiconductivity, though its hardness is a little weaker than diamond hardness. cBN crystal can be made into both p- and n-type semiconductors when suitable impurities are doped and different technics are applied . Within the range of all visible light, most infrared and ultraviolet spectra, cBN crystal is transparent. Therefore it will have wide applications in optoelectronics and microelectronics fields. Because the synthesis of cBN crystal is extremely difficult, and synthetic monocrystal is very small, whose maximal size is about 3 mm only. These limit the study on characteristics of cBN crystal. At the same time, because of its large bandgap and small size, Ohm contact is very hard to get. And it also limits researches on cBN.As far as now, the study on basic characteristics on cBN crystal is lacking. And we have never read the report of cBN's nigrescence in high electric field. When the electric field is strong enough, the electric breakdown occurs inside cBN crystal. At this time, the phenomenon of nigrescence is discovered. And through the process,we observe the piezoelectricity and electrostriction of cBN crystal.When the electric field is strong enough, about 105V/cm, the electric breakdown occurs inside cBN crystal. Of course the phenomenon of nigrescence is also discovered at the same time. And then, we repeat the same experiment in vacuum room. The air pressure is 10-4~10-3Pa. Finally we have the same result.Aim at result above, first of all the transmission spectrum of cBN crystal is measured before and after the breakdown of the cBN Crystal. Lamp-house is Xe-lamp. As a result, the light absorption coefficient don't change with the wavelength before and after the breakdown, when the wavelength is smaller than 500nm. But if the wavelength is larger than 500 nm, the absorption is significantly enhanced after the electric breakdown.Subsequently, we use the wavelength of 532 nm, 1064nm, 650nm in order to get the absorption with the current changes. We use Q-switching Nd:YAG laser, with the wavelength of 1064 nm, the peak power more than 700 W, the repeated frequency of 2 kHz and the pulse wide less than 20 ns. 532nm laser can be received by second-order harmonic generation. The wavelength of 650 nm is produced by semiconductor laser. And through the experiment, we can get the conclusion that the absorption is significantly enhanced with the current increase. The absorption of cBN crystal before and after the breakdown changes with currents.According to the absorption of different wavelengths, different currents, a conclusion can be got that absorption have significantly changes with wavelength when the wavelength is larger than 500 nm. That is, the absorption is relevant with carrier concentration. According to the theory of light absorption, it, the absorption of cBN crystal in strong electric field, is mainly free-carrier absorption. Through the analysis of cBN Raman spectroscopy, we can get that the source of free-carrier is the new energy level, which is introduced by impurities. In the high electric field, a large number of electrons are inspired to conduction band and this increase the absorption of light.In the experiment, we also observe that the cBN crystal have clear inverse piezoelectric and electrostriction effects. For kinds of solid, which can be polarized and deformed, in addition to the direct effects of electric field strength E and electric displacement D (or polarization P), the stress X and strain x , while there is coupling effect between machine and electricity. They are piezoelectric effect and electrostriction effect. Then, the theoretical analysis on the reverse piezoelectric effect and the effect of electrostriction have been carried out , and we get the theoretical expression.