Diamond Growth by Atomic Vapor Deposition and CVD in low pressure ambient

为了揭示金刚石气相生长的基本过程，并了解生长温度（800 - 900 ℃）的物理意义，进行了两种实验。第一种实验是在10 Torr的氢气环境中，用去除碳团簇后得到的碳原子作为金刚石生长的源气体。在该系统中，金刚石颗粒被高分辨率TEM发现。如果使用含碳团簇的碳蒸气作为源气体，则即使在约850 ℃的升高的衬底温度下也能生长非晶碳膜。对于第二个实验，使用热灯丝CVD方法，在0.01-1 Torr的较低气体压力下使用甲烷和氢气的混合气体。当环境气压降低到0.01Torr左右时，金刚石的生长速率显著降低。在此压力以下，TEM观察无法发现金刚石生长。假设热丝周围气体温度为2000 C，计算氢分子、H、C、CH等活性物种的浓度。计算了气体温度为2000 ℃时H和C的平均自由程。实验结果表明，碳原子和氢原子到达衬底表面时，并不与其它组分发生碰撞，这两个实验结果表明，碳原子和氢原子是可以制备金刚石的。生长表面的加热不是金刚石结晶所必需的，而是氢从表面解离所必需的。剩下的重要任务是利用原子氢的氢提取反应来执行CVD工艺。这将实现从气相形成金刚石的低温过程。

Two kinds of experiments have been performed to reveal the fundamental process of diamond formation from the vapor phase and also to know the physical meaning of growing temperature such as 800-900C.For the first experiment, carbon atoms prepared by removing carbon clusters are used as the source gas for diamond deposition in a hydrogen ambient of 10 Torr. In this system, diamond particles are found by high resolution TEM. If carbon vapor containing carbon clusters is used as the source gas, amorphous carbon films grow even at the elevated substrate temperature of about 850C.For the second experiment, the hot-filament CVD method is used with the mixture gas of methane and hydrogen at the lower gas pressure of 0.01-1 Torr. The growth rate of diamond is remarkably reduced as the ambient gas pressure is lowered to about 0.01Torr. Below this pressure, diamond growth cannot be found by TEM observation.Assuming the gas temperature of 2000C around the hotfilament, the concentration of active species such as hydrog en molecules, H, C, CH etc. is calculated. The mean free path of H and C at the gas temperature of about 2000C is also calculated. From these results, it is found that atomic carbon and atomic hydrogen arrive to the substrate surface without collision to the other species.Considering these two experiments, it is shown that diamond can be grown preparing atomic carbon and hydrogen. The heating of growing surfaces is not required for the crystallization of diamond but for the dissociation of hydrogen from the surface. The remained important task is to perform the CVD process using the reaction of hydrogen extraction by atomic hydrogen. This will realize the low temperature process of diamond formation from the vapor phase.