Direct Recoil Spectroscopy of Compound Semiconductor Surfaces Covered with Organic Molecules

J.E. GAYONE ; E.A. SÁNCHEZ; L. RODRÍGUEZ; L.J. SALAZAR ALARCÓN; B. BLUM; R. SALVAREZZA; V.E. ESAULOV; O. GRIZZI

Cracovia

Conferencia; INTERNATIONAL CONFERENCE ON ATOMIC COLISIONS IN SOLIDS - ICACS24; 2010

Self assembled monolayers (SAMs) of thiol derived molecules on semiconductor surfaces are promising two-dimensional systems with interesting technological applications. In most of the applications a high and stable coverage by thiols is needed. In the literature on the adsorption nature of alkanethiols,  high quality SAMs have been proposed for thiols with a number of C atoms (Cn) equal or higher than C12., They are typically formed by transporting the molecules to the surface either through a solvent or directly via evaporation in vacuum.  In practical semiconductor device fabrication, SAM preparation from the vapor phase is highly desirable as such a process can be integrated with other dry processes such as molecular beam epitaxy and metal-organic chemical vapor deposition. In these cases, relatively short akanethiols are preferable because they exhibit a high vapor pressure, and the passivating layer may be removed at a relatively low temperature. In this work we present a Direct Recoil Spectroscopy study of the adsorption and thermal stability of alkanethiols and thiol terminated benzenedimethanethiol (BDMT) on GaAs and InP. The technique of Direct Recoil Spectroscopy with Time of Flight analysis (TOF-DRS) is particularly adequate to study these systems because of: 1) the high top layer sensitivity, 2) the low damage imparted to the film and substrate, and 3) the capability of detecting H.  The TOF-DRS measurements for hexanethiol (C6) deposited on GaAs(110) from vapour are consistent with the formation of a dense phase of  C6 molecules at relatively lower exposures. On the contrary, in solution preparation, dense phases are obtained on GaAs for long alkanethiols, and after lengthy immersions. The C6 phase has a first desorption peak at 325 K, where partial desorption of the intact molecule takes place. Fits to the desorption curves result in a 1 eV adsorption energy, in agreement with a chemisorption process, but significantly smaller than that measured for  C6 on Au(111). Increasing the temperature to 500 K results in the S-C bond scission and S deposition on the GaAs(110) surface. TOF-SIMS measurements confirm this result. Desorption studies by TOF-DRS on the GaAs(001) surface show a different behavior, i.e., without the first desorption peak.  Similar measurements for BDMT on InP(110) show formation of a standing up SAM with S atoms available at the SAM-vacuum interface. Investigation of the adsorption kinetics shows that a lying down phase is formed at low exposures, which precedes the SAM phase.  Optimal conditions for standing up SAM formation require exposures of the order of a mega Langmuir. A study of the SAM stability with temperature shows that the S terminated layer survives up to ~370 K, above this temperature a reordering of the layer takes place where S atoms are no longer available at the vacuum interface. Final desorption occurs around 500K. TOF-DRS results on the semiconductor will be compared to similar ones on the better known Au (111) substrate.