Synthesis And Characterization Of Boron Doped Diamond Single Crystal In Oxygen-enriched Environment At High Pressure And High Temperature

As an ultimate functional material,diamond has great application value in petroleum exploration,precision instrument processing,medical treatment,semiconductor and other related fields.Especially in the semiconductor field,diamond is called"the ultimate semiconductor material"because of its excellent properties such as ultra-wide band gap（～5.47 e V）,high thermal conductivity（120 W/cm·k）and high breakdown electric field（2×10⁷ V/cm）.The conductivity of boron-doped diamond can be from high insulation to close to that of metal.P-type semiconductor devices made of boron-doped diamond or diamond film have been actually applied to wastewater treatment and medical treatment.However,due to the lack of n-type semiconductor diamond,single p-type semiconductor diamond is difficult to prepare more complex electronic devices.The results show that the introduction of oxygen into diamond doping can improve the electrical properties of diamond.Boron and oxygen ion implantation into diamond films can realize n-type conductivity.Moreover,in the study of boron-doped diamond single crystal,it was found that the boron-oxygen composite impurity contained in the surface layer of the crystal made the surface layer show n-type conductivity.These studies show that oxygen plays a key role in the transformation of electrical properties of boron-doped diamond from p to n type.Therefore,it is extremely necessary to study the synthesis of boron-doped diamond in oxygen-rich environment,which is of great significance not only for exploring the preparation and application of n-type semiconductor diamond,but also for understanding the formation mechanism of natural boron and oxygen-containing diamond in the earth.Based on this,this topic puts forward the research of boron-doped diamond with different oxygen-containing additives by using the temperature gradient method on a domestic hexagonal top press.The specific experimental contents and conclusions are as follows:1.Diamond single crystal was synthesized by using amorphous boron as boron source in FeNi-C system.The results show that with the increase of boron doping amount,the crystal form will change from hexao-ctahedron to octahedron.When the doping amount of boron is less than 4 wt.%,the growth rate will generally increase,but the crystal quality will decrease.When the boron content is 5 wt.%and 6 wt.%,it is impossible to synthesize high-quality diamond crystals.Raman spectra show that with the increase of boron doping amount,the peak position Xc of Raman peak shifts to the direction of low energy,and at the same time,the half-peak width gradually widens,indicating that the internal stress of the crystal increases and the crystallinity decreases.In the Raman spectrum of boron-doped diamond,there are absorption peaks related to the change of phonon density of diamond,which indicates that high concentration boron-doped diamond was synthesized at this doping amount.This provides an important reference for the following experiments to select appropriate doping amount,synthesis conditions and performance comparison.2.Diamond single crystal was synthesized by adding 0-6 wt.%B₂O₃ into FeNi-C and FeNi-Ti-C systems.The results show that with the increase of B₂O₃ doping amount,the carbon oxide and oxygen in the synthesis cavity will increase the P-T conditions required for diamond synthesis.Infrared spectra show that it is difficult to synthesize boron-containing diamond by adding B₂O₃ in FeNi-C system,and B₂O₃doping in this system has little effect on the nitrogen content in diamond.In FeNi-Ti-C system,the introduction of titanium releases the boron source and oxygen source in B₂O₃,and boron impurities are doped into the diamond lattice.XPS test found that boron-oxygen compound impurity appeared on the crystal surface,and the peak related to oxygen also appeared in the crystal,which indicated that the oxygen impurity doped into the diamond.The mechanism of boron and oxygen impurities produced by the reaction of Ti and B₂O₃ at high temperature was further revealed through experiments.Although the growth of boron and oxygen doped diamond has been preliminarily realized,the introduction of Ti impurity in the cavity and the fixed proportion of boron and oxygen impurity in B₂O₃ restrict the research on the growth of boron and oxygen doped diamond with different proportions.3.Based on the above research,we used 3 wt.%Ni₂O₃ as oxygen-containing additive in FeNi-C system to build an oxygen-rich environment for diamond growth,and then added different proportions of amorphous boron to the oxygen-rich environment to regulate the growth of diamond single crystal.The results show that the synthesized diamonds are all high-quality crystals with complete crystal form and few surface defects.When the doping amounts of Ni₂O₃ and boron are 3 wt.%and 5 wt.%respectively,we have synthesized high-quality diamond crystals containing boron and oxygen composite impurities.The large-size（6.5 mm）diamond single crystal synthesized with this doping amount was tested by four-probe method,and the results showed that it showed p-type semiconductor characteristics,and the carrier concentration reached 2.02×10¹⁸ cm^-3.