A CVD reactor is a chamber in which gasses at high temperature and controlled pressure undergo chemical reactions that result in growing a thin film on a solid surface (called the substrate) inside the reactor. To grow a gallium arsenide film, you start with gasses containing gallium and arsenic. The gas usually used to supply arsenic, for instance, is arsine (AsH3), a highly toxic compound that requires extreme care in handling. The hazards and expense of handling arsine are one of the obstacles to production-scale CVD, and Mountziaris' research, among other things, aims at finding a safe substitute.
The major difficulties of CVD research have to do with understanding the complex brew of chemical reactions that occur as the source gasses diffuse toward the substrate. The gasses decompose into fragment molecules, which in turn react with each other and with the unreacted source gas. Some of the fragments stick to the surface, where they recombine to produce a film. At the same time, these surface reactions also release other gas fragments. "We need to know what kinds of reactions are occurring," says Mountziaris, "at what rates, and how they affect the growth and purity of the film. Understanding the chemistry is the most serious problem right now, and that's where my work comes into play."
To probe CVD chemistry while it is happening is very difficult since many different reactions are going on at the same time. To isolate the gas phase from the surface reactions, typical experiments involve injecting one gas at a time and studying how it decomposes when no film is grown. Even with no substrate, however, surface reactions occur on the hot wall of the reactor. Furthermore, says Mountziaris, the residence time of the gas in the reactor can be long enough for secondary reactions to occur. "What we see is the result of all these events. It's very hard to find out what kind of elementary, one-step processes are occurring."
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