Study On Vacuum Ultraviolet Emission From Cubic Boron Nitride Crystal Under Strong Electric Field

Cubic boron nitride crystal (cBN) is a semiconductor material with an indirect bandgap, whose bottom of the conduction band is at the X valley and the top of valence band at the Г point. The band-gap width is 6.3eV. In this paper, we mainly study the electroluminescence characteristics of cBN crystals. Our detailed researches are concerned with the facilitation of cBN crystal under strong electric field on gas discharge in the atmosphere and low pressure environment, and the phenomena of vacuum ultraviolet (VUV) emissions from cBN crystals in the environment with high vacuum degree.First of all, we investigated the facilitation of cBN crystal under strong electric field on gas discharge in the atmosphere and low pressure environment. Since the electric field between the needle-plate electrodes is extremely non-uniform, theoretically, the electric field near the tip of the needle is infinitely small, where the electric field reaches infinity. A high voltage applied on the needle-plate electrodes can lead to the ionization of the gas gap between the electrodes, and then the discharge phenomenon occurs. In our experiments, we found that in the pressure condition with magnitude of ten thousands, in which the gas discharge itself relative easily occurs under the strong electric field, the contribution of the electroluminescence of the cBN crystal to the gas discharge was not very obvious. With the increase of the vacuum degree, we found that the gas discharge was relatively difficult, and the cBN crystal under the strong electric field would contribute to the gas discharge. We consider that the electroluminescence of the cBN crystal under the strong electric field ionized the gas molecules and their recombination resulted in the phenomenon of gas discharge.Next, in order to study the electroluminescence of cBN crystals and avoid the effect of air discharge on the electroradioscence of cBN crystal, we investigated the curren -voltage (I-V) characteristics of the cBN crystal under high vacuum. Using the structure of the needle electrode-cBN crystal-plate electrode, we studied the I-V properties of cBN crystals with the needle electrode contacting to different zones with different colors on the boron (B) face and the nitrogen (N) face, respectively. The experimental results show that under the same bias, when the needle electrode contacted to the transparent zone of the cBN crystal, the currents were small, indicating that there exist less impurities and defects and thus the higher resistivity in the transparent zone; while the needle electrode contacted to the amber zone of the cBN crystal, the currents were large, indicating that there exist more impurities and defects and thus the lower resistivity in the amber zone. The I-Ⅴ characteristics of cBN crystal under high vacuum will be helpful for further research on the mechanism of electroradioscence from cBN crystals under strong electric field.Finally, in the case of the vacuum degree of about 2×10-3Pa, we studied the phenomena of electroluminescence of cBN crystals with needle-plate electrode structures. We used a homemade fluorescent cover coated with PDP phosphor sensitive to 130-200 nm wavelengths to detect the electroluminescence from the cBN crystal under strong electric field in vacuum, and we observed the green fluorescence stimulated by the VUV from electroluminescence of the cBN crystal with naked eyes. The VUV wave length of cBN crystal which is about 149 nm was measured by a vacuum spectrometer. As to the mechanisms of electroluminescent VUV from the cBN crystal under strong electric field, one possible explanation we proposed is the electron transition between the interbands, the electron transfer in the intraband, and the direct recombination between the electrons in the Г valley and the holes on the top of the valence band Г point. There are three physical mechanisms of generated electrons and holes in the recombination process. One is the intrinsic avalanche breakdown of the cBN crystal under strong electric field, which results in electrons on the top of the valence band Г point transiting to the bottom of the conduction band (X Valley), and the holes left on the top of the valence band Г point; the second one is the double injection of carriers via the electrodes, in which the electrons are injected into the X Valley via the cathode and the holes to the top of the valence band via anode; the third one is the contribution of the intermediate levels related to the impurities and defects in the band gap, on which the electrons are stimulated to the X valley and the holes to the top of the valence band. The electrons in the X valley arising from the above three possible physical processes subsequently transfer to the Гvalley under the strong electric field, then the electrons in the Г valley transit to the top of the valence band Г point and recombine with the holes, and radiate the VUV corresponding to the wavelength of 149nm. The exact physical mechanism of the electroluminescent VUV from the cBN crystal under strong electric field is still under investigation.