Understanding Thermal Evolution and Monolayer Doping of Sulfur-Passivated GaAs(100)

摘要

Monolayer doping (MLD) is an attractive method to precisely tailor dopant profiles for nanoelectronic semiconductor devices. The approach has been demonstrated for a number of different dopant/substrate combinations, but the mechanistic understanding of reactions of dopant-containing monolayers, intermediate structures, and the role of capping layers has suffered from lack of in situ characterization. Here, we investigate the thermal evolution of sulfur-passivated GaAs(100) without any additional capping layers in the context of MLD, using a combination of in situ X-ray photoemission spectroscopy (XPS), low-energy ion scattering (LEIS), and infrared (IR) absorption spectroscopy. In the case of (NH4)(2)S-passivated GaAs(100), the intermediate structure that precedes subsurface diffusion is characterized by a (2 X 1) reconstruction that has previously been ascribed to several different dimer moieties. These dimer structures are currently unresolved yet could influence subsequent doping processes. By use of LEIS, temperature-dependent measurements provide unambiguous information on the chemical origin of the intermediate (2 x 1) reconstruction, originating from the dimerization of S atoms. Annealing beyond 813 K results in a loss of the surface S signal and is accompanied by free-carrier absorption in IR spectra, consistent with doping. The magnitude of this absorption and the carrier densities extracted from these data indicate that peak doping levels exceeding 10(20) cm(-3) can be achieved and that the free carrier concentration within a shallow profile can be tuned by adjusting the annealing time.