Chemistry and kinetics of the GaN formation by plasma nitridation of GaAs: Anin situreal-time ellipsometric study

The chemistry and kinetics of the nitridation of GaAs (100) surfaces by ${\mathrm{N}}_2,$ ${\mathrm{N}}_2{-\mathrm{H}}_2,$ and ${\mathrm{N}}_2{-\mathrm{N}\mathrm{H}}_3$ radio-frequency 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 °C and of the gas-phase chemistry on both the nitridation kinetics and the composition of the resulting GaN layer are highlighted. Pure ${\mathrm{N}}_2$ 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 Å) are obtained by ${\mathrm{N}}_2{-\mathrm{H}}_2$ and ${\mathrm{N}}_2{-\mathrm{N}\mathrm{H}}_3$ plasmas, since hydrogen reduces the As segregation by the formation and desorption of ${\mathrm{AsH}}_x$ 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 $T>~600°\mathrm{C},$ which is accompanied by the GaAs substrate decomposition and yields both a rough and Ga-rich GaN layer.