Chemistry and kinetics of the GaN formation by plasma nitridation of GaAs: An in situ real-time ellipsometric study

The chemistry and kinetics of the nitridation of GaAs (100) surfaces by N-2, N-2-H-2, and N-2-NH3 radiofrequency plasmas, in a remote configuration, are investigated in situ and in real time using spectroscopic ellipsometry. The effects of the surface temperature in the range 70-700 degrees C and of the gas-phase chemistry on both the nitridation kinetics and the composition of the resulting GaN layer are highlighted. Pure N-2 plasmas yield stoichiometric and smooth GaN layers with As segregation at the GaN/GaAs interface. The As segregation inhibits GaAs nitridation, because the N atoms scavenge the free As, and thereby limits the GaN thickness to a few angstroms. Thicker GaN layers (>100 Angstrom) are obtained by N-2-H-2 and N-2-NH3 plasmas, since hydrogen reduces the As segregation by the formation and desorption of AsHx species. For the three plasma mixtures, the self-limiting nature of the GaAs nitridation process is revealed and explained using simple kinetic and chemical models based on the fact that the GaAs nitridation can be considered to be a topochemical reaction. Also demonstrated is the ineffectiveness of the nitridation at T greater than or equal to 600 degrees C, which is accompanied by the GaAs substrate decomposition and yields both a rough and Ga-rich GaN layer