Synthesis of metastable phases and compounds in a plasma assisted chemical vapor deposition technique
School of Materials Science and Engineering
Diamond is a wide band gap semiconductor of 5.5eV, and it has high carrier mobility, mechanical hardness, optical transparency, highest atom density, largest room temperature thermal conductivity and smallest thermal expansion. Pure diamond has low conductivity; but when intentionally doped with impurities (doping level from 1018 cm-3 to 1020 cm-3), diamond will be converted to n-type or p-type semiconductors. The surpassing properties of diamond and doped diamond crystals broadened its applications to protective and anti-corrosion coatings, electronic and optoelectronic device, electrochemistry and so on. Traditionally, diamond is synthesized in thermal quasi-equilibrium conditions provided by high temperature high pressure (HTHP, 3000oC, 150Kbar). Recently, many methods at low temperature low pressure have been developed to deposit diamond crystals; especially plasma enhanced chemical vapor deposition (PECVD) that plasma provides the non-equilibrium conditions for diamond growth. In this report, microwave discharge system is used to generate the plasma. A Microwave Plasma enhanced Chemical Vapor Deposition (MPCVD) system is designed in the laboratory to synthesize diamond crystals. It has a horizontal quartz tube reactor inside the microwave generator and a furnace wrapped out of the reaction chamber, to provide required temperature gradient as well as plasma intensity distribution. SEM, XRD, XPS and Raman spectroscopy are used to study the morphology, microstructure and quality of as-grown diamond crystals. Diamond crystals are successfully deposited on Silicon substrate in our self-designed equipment. The sizes of as-grown diamond individual crystals vary from 10μm to about 100μm. The growth rate of diamond crystal is about 2-3μm/h. Compared with the smooth (100) plane of as-grown CVD diamond, the (111) plane is rough and has some excrescences on the surface due to different growth mechanism. The influence of experiment conditions on diamond quality is studied. It is proved that experiment conditions, especially temperature, pressure and C/H ratio, are crucial to diamond growth. The best deposition conditions are found to be at 800oC, 30Torr. At other conditions, low quality diamond or even non-diamond carbon materials are formed, such as cauliflower-like carbon, carbon fiber and carbon nanowall. The influence of varies substrate pretreatment, such as scratching, covering with diamond like carbon film, and coating with platinum interlayer, have been tried to deposit large area diamond films. These methods have effectively enhanced the diamond nucleation. By combining some of the methods, large area continuous diamond films have been deposited on the silicon substrate, with a significant fraction of sp3 bonded carbon.
DRNTU::Engineering::Materials::Microelectronics and semiconductor materials::Thin films