Studies On The Properties Of Voltage-current And Chromism Of Cubic Boron Nitride Single Crystals

Cubic boron nitride (cBN) is a kind of synthetic crystal at high pressure and high temperature (HPHT) with wide band-gap of about 6.4eV, and belongs to the indirect band-gap semiconductor. Inartificial cBN has not been found in nature so far. Although the cBN crystal is the simplest III-V compound, it has a high hardness, which is next to diamond and its thermal and chemical stability against ferrous materials is superior to that of diamond. Therefore, cBN has been utilized for the industrial application as cutting tools and abrasives.cBN has zinc blende structure with 4 3m symmetry and second-order nonlinear optical susceptibility, so the cBN crystal is a kind of nonlinear optical materials. cBN can be made into both p- and n-type semiconductors when suitable impurities are doped. Furthermore, cBN is transparent within the range of all visible light, most infrared and ultraviolet spectra. Therefore it will have wide applications in optoelectronics and microelectronics.The synthesis of the cBN crystal is extremely difficult, and the synthesized monocrystal is very small, whose maximal size is only about 3mm. So the studies and applications on the semiconducting and optical properties of cBN crystals are infrequent. We have researched the electronic and optical properties of cBN single crystals under high voltage, measured the nonlinear voltage-current (I-V) curve and observed the electrochromism and thermochromism of cBN crystals.According to the characteristics of the I-V curve, we divide it into three parts, which are the conditions before breakdown, under breakdown and after breakdown. Before breakdown, the I-V curve of the cBN single obeys the rule of Poole-Frankel effect. Then, when the voltage on the cBN crystal raise up to a threshold, lots of electrons on the impurity energy levels are ionized and come into conduction band of the crystal. Resulting from the increasing of the free electrons in the conduction band, the resistance of the crystal decreases, and so the voltage on the cBN crystal decreases. As the current increase, the voltage on the crystal decrease after breakdown. At the same time, we also studied on the electronic properties of cBN crystal depending on the temperature, measured the I-V and I-T curves under different temperatures, and then obtained the location of the shallow donor-energy level——about 0.4eV below conduction band.During the studying on the I-V properties of the crystal, we discovered the electrochromism of the cBN single crystal, which is different from the typical one. The colour of the cBN crystal changed into black from amber after breakdown. We observed the phenomenon of the cBN crystal carefully and described it in detail in this paper. We measured the dependence of the difference of the absorption coefficient of the cBN crystal which came from the electrochromism of the crystal on the wavelength and the current, and characterized the crystal by confocal microscopy Raman spectra. The origin of the electrochromism is considered to be resulted from the impurities in the cBN crystal. Moreover, the coloration of the cBN also occurred under a high temperature. As the temperature rises up, the color of the cBN crystal becomes darker and darker. And when it increases above 700K, the crystal changes black finally. This is the thermochromism of the cBN single crystal. Both the eletrochromism and the thermochromism of the cBN are repeatable.Due to the electrochromism, the absorption coefficient of the cBN crystal is increased in visible region and infrared region. The large wavelength range of the absorption enhancement induced by the electrochromism of the cBN crystals indicates a wider region of operation wavelength than other electro-optic (EO) modulators. After the breakdown and electrochromism occur, the change of the absorption coefficient of the cBN crystal increases linearly with the current rising. The property of the absorption coefficient relevant to the breakdown is just suitable to fabricate optical switches. Furthermore, the absorption of the cBN crystal induced by the electrochromism is almost independent of the polarization of the incident light. These characteristics ensure the potential applications of cBN crystal in EO modulators, optical switches, and other optoelectronic devices.