Fabrication And Electrical Performance Of Energy Conversion Unit For Nuclear Battery Based On Epitaxial Monocrystalline Diamond

With the rapid development of Micro-nano Electro-Mechanical Systems?MEMS?the demand for stable and reliable micro-long-term energy sources is increasingly urgent.Because of its long life,unit high energy density,strong resistance to external interference,stability and reliability,no need for maintenance and replacement and ease of miniaturization and integration,Radioactive Isotope Battery is an ideal reliable and long-lasting miniature energy source,which has great prospects for MEMS and the areas in the field of new energy.The most important core in isotope battery is the semiconductor material in conversion unit.The boron-doped monocrystalline diamond is widely recognized as the best candidate for the isotope battery conversion unit because of its excellent comprehensive performance.In this paper,the whole process system of material-to-device research on the epitaxial growth of boron-doped single crystal diamond,the production of energy conversion unit of isotope battery based on epitaxial single-crystal diamond and electrical performance has been studied,which is a exploration of preliminary process flow,preparation methods and basic performance evaluation for high-performance radioactive isotope battery based on diamond material,which lays the foundation for further improvement and optimization.Firstly using microwave plasma chemical vapor deposition?MPCVD?technology and H₂,CH₄ and B₂H₆ as reactive gas sources,the effect on growth quality and surface morphology of homogeneous epitaxial single crystal diamond of different temperatures?950?,1050?,1150??and different amounts of borane?22.5 ppm,45 ppm,100ppm?has been systematically studied.The results show that the growth temperature not only affected the crystalline quality of the epitaxial films but also significantly changed the surface morphology of the diamond films.The crystal quality of the single crystal diamond films prepared at 1050?was the best and the surface was smoother.As the amount of borane in the gas source increased,the quality of the epitaxial film would gradually deteriorate,the resistivity would decrease and the conductivity would become better.On the basis of the high quality epitaxial single crystal diamond,an interdigitated Schottky electrode and ohmic electrode was prepared on the surface of the diamond films by respectively using photolithography method and mask baffle method,thereby forming an isotope battery conversion unit.The photolithography fabrication result shows that this process route could ensure that the photolithographic pattern on the surface of the epitaxial single crystal diamond film was clear in the early stage,but the Au Schottky metal electrode could not be stripped when ultrasonic vibration was used to remove the photolithographic and the damage was serious.After the pre-plating process and additional annealing at 120?for 20 min,the finger Au electrode could not be peeled off and there was still a severe fracture phenomenon.The reason is that there is no compound reaction between C and Au,and there is no mechanical locking effect at the film-based interface gold?Au?,therefore the single crystal diamond has poor adhesion and cannot withstand the external load during the ultrasonic degumming process.Aiming at the problem of extremely poor bonding force of the metal film on the surface of the epitaxial single crystal diamond film and difficulty in making the omplete interdigitated finger,a masked baffle method was used to directly form interdigital metal electrode on the surface of different boron-doped single crystal iamond.By designing the mask that combined with different patterns of stainless steel flakes,which directly attached to the surface of sample,the electrodes with different ate widths was formed.The result shows that the electrodes produced were intact and no appearance defects.Further three kinds of interdigital electrodes with different gate widths?100?m 150?m and 200?m?were formed on the surfaces of single-crystal iamond substrates grown at three different boron-doping concentrations?3000 ppm6000 ppm and 13300 ppm?,and finally formed the Schottky conversion unit,which was tested and analyzed for I-V electrical performance.The results show that at the boron doped concentrations of 13300 ppm and 6000 ppm,the I-V electrical erformance of the unit was linear and almost no diode rectification occurred.Only hen the boron-doped concentration is reduced to 3000 ppm the conversion unit had a certain diode characteristic but the rectification ratio is very low,of which the highest is only 2.For the conversion unit that made of boron-doped diamond grown at boron-doping concentrations of 3000 ppm,the diode characteristic of the device was best when the gate electrode width is 150?m.