Effect of oxygen on regulation of properties of moderately boron-doped diamond films

Regulation of oxygen on properties of moderately boron-doped diamond films is fully investigated. Results show that, with adding a small amount of oxygen (oxygen-to-carbon ratio < 5.0%), the crystal quality of diamond is improved, and a suppression effect of residual nitrogen is observed. With increasing ratio of O/C from 2.5% to 20.0%, the hole concentration is firstly increased then reduced. This change of hole concentration is also explained. Moreover, the results of Hall effect measurement with temperatures from 300 K to 825 K show that, with adding a small amount of oxygen, boron and oxygen complex structures (especially B₃O and B₄O) are formed and exhibit as shallow donor in diamond, which results in increase of donor concentration. With further increase of ratio of O/C, the inhibitory behaviors of oxygen on boron leads to decrease of acceptor concentration (the optical emission spectroscopy has shown that it is decreased with ratio of O/C more than 10.0%). This work demonstrates that oxygen-doping induced increasement of the crystalline and surface quality could be restored by the co-doping with oxygen. The technique could achieve boron-doped diamond films with both high quality and acceptable hole concentration, which is applicable to electronic level of usage.     

具有体液免疫和细胞免疫的反应扩散HIV病毒模型

本文基于HIV病毒在人体内的动态变化过程和人体的两类免疫机制,提出了一种具有体液免疫和细胞免疫的反应扩散HIV病毒模型.利用比较原理和极值原理证明了该模型非负解的存在性及有界性.通过定义不同患病阶段下两个关键阈值,并利用李亚普诺夫理论分析了无病平衡点和两类患病平衡点的全局动力学行为.最后,利用数值实例子验证了理论结果正确性.     

Significant suppression of residual nitrogen incorporation in diamond film with a novel susceptor geometry employed in MPCVD

This work proposed to change the structure of the sample susceptor of the microwave plasma chemical vapor deposition (MPCVD) reaction chamber, that is, to introduce a small hole in the center of the susceptor to study its suppression effect on the incorporation of residual nitrogen in the MPCVD diamond film. By using COMSOL multiphysics software simulation, the plasma characteristics and the concentration of chemical reactants in the cylindrical cavity of MPCVD system were studied, including electric field intensity, electron number density, electron temperature, the concentrations of atomic hydrogen, methyl, and nitrogenous substances, etc. After introducing a small hole in the center of the molybdenum support susceptor, we found that no significant changes were found in the center area of the plasma, but the electron state in the plasma changed greatly on the surface above the susceptor. The electron number density was reduced by about 40%, while the electron temperature was reduced by about 0.02 eV, and the concentration of atomic nitrogen was decreased by about an order of magnitude. Moreover, we found that if a specific lower microwave input power is used, and a susceptor structure without the small hole is introduced, the change results similar to those in the surface area of the susceptor will be obtained, but the spatial distribution of electromagnetic field and reactant concentration will be changed.     

Origin,characteristics, and suppression of residual nitrogen in MPCVD diamond growth reactor

Unintentional nitrogen incorporation has been observed in a set of microwave plasma chemical vapor deposition (MPCVD)-grown samples. No abnormality has been detected on the apparatus especially the base pressure and feeding gas purity. By a comprehensive investigation including the analysis of the plasma composition, we found that a minor leakage of the system could be significantly magnified by the thermal effect, resulting in a considerable residual nitrogen in the diamond material. Moreover, the doping mechanism of leaked air is different to pure nitrogen doping. The dosage of several ppm of pure nitrogen can lead to efficient nitrogen incorporation in diamond, while at least thousands ppm of leaked air is required for detecting obvious residual nitrogen. The difference of the dosage has been ascribed to the suppression effect of oxygen that consumes nitrogen. As the unintentional impurity is basically detrimental to the controllable fabrication of diamond for electronic application, we have provided an effective way to suppress the residual nitrogen in a slightly leaked system by modifying the susceptor geometry. This study indicates that even if a normal base pressure can be reached, the nitrogen residing in the chamber can be "activated" by the thermal effect and thus be incorporated in diamond material grown by a MPCVD reactor.