碳纳米管场致发射中的空间电荷效应

Space Charge Effect in Carbon Nano-tube Field Emission

采用微波等离子体化学气相沉积(MWPCVD)方法成功制备以碳纳米管束为单元的场致发射阵列,获得很好的场致发射电流发射特性,在电流密度较大时,发现I-V特性偏离由Fowler-Nordheim公式计算出的结果。采用Electron Beam Simulation(EBS)软件进行模拟分析发现:在电流密度较低时,I-V特性能很好与F-N公式吻合。但碳纳米管尖端电流密度大于106A/cm2时,碳纳米管尖端处的有效电场强度受空间电荷的影响比较明显,进而对碳纳米管的场致发射特性显现出不可忽略的影响,此时碳纳米管的发射电流密度开始受到空间电荷的限制。

High aspect ratio, small diameter, inertness, and high heat conductivity make CNTs promising field emitters. With the advancement in the fabrication technology, the emission current density of field emission arrays is increasing steadily. At high current density, the measured F-N plots often show deviations from linearity. Apart from the intrinsic effects of the material of the field emission cathode, another reason explaining this phenomenon could be the effect of space charge. Moreover, the performance of CNT-emitters has been improved greatly recently： this means that the macroscopic current density can reach several amperes per square centimeter and the corresponding microscopic current density at the tip of CNTs is so high that space charge affects the field emission of CNT-emitters noticeably. This leads to a lower current density from the CNT-arrays as compared to the situation where space charge effects do not play a role. In this paper, we try to simulate the influence of the space charge effect in carbon nanotube field emitter. A fabrication of carbon nanotube bundle field emission arrays （FEAs） grown by microwave plasma chemical vapor deposition （MWPCVD） was reported in this paper. This prepared FEAs present good field emission performance. To simulate the field emission characteristics of these FEAs, a model was built based on the microstructure of our carbon nanotubo bundle field emission arrays firstly. Then the influence of the space charge effect on the character of carbon nanotubo field emitter was simulated using the Electron Beam Simulation （EBS） software, including the distribution of electric field on the surface of emitter, emission current density, I-V behavior and the expending angle of emitting electron beam. The simulation results show that the I-V characteristics of CNT emitters act in well accord with F-N equation when the current density at the tip of CNTs is low. However, when the current density at the tip of CNTs is more than 106 A/cm^2, the effective electric-field intensity on carbon nanotube tip is impacted evidently by the space charges, which influence the characteristic of carbon nanotube field emission evidently. By comparing the electric-field intensity of tip in free space with the field of tip in space that is influenced by space charges, we can find that the tip field, with the continuous increase of applied voltage, is linear with the applied voltage in free space. However, they are nonlinear in space influenced by space charges. When space charges appear, the field of tip is lower than that in free space. The reduction of tip field will lead to the reduction of emission current density on the tip. As a result the current density of carbon nanotube is limited by the space charges. Through the simulation, we also find that the space charge can lead the expending angle of emitting electron beam to be larger.