Self-activated, self-limiting reactions on Si surfaces

The direct thermally activated reactions of oxygen and ammonia with Si surfaces in furnaces have been used for a very long time in the semiconductor industry for the growth of thick oxides and nitride layers respectively. The oxidation mechanism was described in the Deal-Grove model as a diffusion limited transport of oxygen to the oxide/silicon interface. For thin oxides the deal-Grove growth rate is initially constant, but for ultrathin oxides (a couple of nm thick) this is not true and the Deal-Grove model does not explain the mechanism.

In a series of recent reports we have found a new mechanism for the direct growth of ultrathin films (0-3 nm) of oxides and nitrides under ultrahigh vacuum conditions. Neutral oxygen and a microwave excited nitrogen plasma interact directly with Si surfaces kept at different temperatures during the reaction. The gas pressures are around 10^-6 Torr, and the temperatures vary from room temperature to 1000⁰C.The growth is in these cases self-limiting, with the optimal oxide thickness around 0.7-0.8 nm, at 500⁰C, and up to a few nm for nitride. The self-limiting oxide case was recently predicted by Alex Demkov in a structural optimization to minimise the total energy of an oxide system, which happened for an ordered structure, at a thickness of 0.7-0.8 nm. Thus this thin oxide structure has definite crystalline features.

We have closely monitored the reaction kinetics with normal x-ray induced photoelectron spectroscopies, and also the structure, composition and electrical properties of the system, with surface sensitive, high resolution core level photoelectron spectroscopy. The growth kinetics is well fitted by a Hill function, with parameters, which give information about the character of the process. This function describes a self-activated process. Thus the impact in the surface region of oxygen or nitrogen is to enhance the further uptake. The process also has the character of being ballistic, without any intermediate surface adsorption, and it is self-limiting. The ultrathin oxide formed has crystalline characteristics, in contrast to the normal case for thicker oxides, which are always amorphous (except near the interface, where some coordination with the underlying surface is inevitable). For the nitrides, the structure becomes ordered, with microcrystalline or even single crystal (epitaxial) characteristics for the Si (111) surface, when growing above 500⁰C.