Etching of GaAs(1 0 0) surfaces by HCl: density functional calculations to the mechanisms

Using the density functional method B3P86 and molecular clusters modelling typical local structures of the GaAs(1 0 0) surfaces, reaction energies and related barrier heights for the chemisorption/desorption of HCl, H2, Cl2, Ga2, As2, GaH, AsH, GaCl, and AsCl are calculated. The data allow one to set up etching mechanisms and to interpret experimental findings. The HCl etching is compared with the Cl2 etching. HCl, as Cl2, can be chemisorbed dissociatively at Ga surface dimers with no potential barrier and at As surface dimers with a barrier. However, from the formed structures the HCl is preferentially removed contrary to the Cl2 chemisorption where GaCl is desorbed. The HCl etching starts with the dissociative chemisorption at Ga–As bonds of steps where Cl binds to Ga atoms and H to As atoms without barriers. From the resulting structures, H2 is desorbed by recombination of hydrogen atoms from neighbouring As atoms. Finally, GaCl desorbs from the Ga-rich and As2 from the As-rich surface. The As2 removal is the rate-limiting step. Contrary to Cl2, HCl cannot form higher hydrogen- and chlorine-coordinated Ga and As atoms which are able to desorb easily. The opposite attacks with Cl at As and H at Ga proceed with barriers. The preferred reaction of these structures is the HCl desorption.