Alkane thiol and dithiol films grown on GaAs and InP under UHV conditions

L.J. SALAZAR ALARCÓN, L.J. CRISTINA, E.A. SÁNCHEZ, O. GRIZZI AND J. E. GAYONE

La Plata, Buenos Aires

Workshop; 1st International Workshop on Semiconductor Devices Modeling and Electronic Materials and IEEE EDS 2010 MINI-COLLOQUIUM; 2010

1. Introduction Self assembled monolayers (SAMs) of thiol derived molecules on semiconductor surfaces are promising two-dimensional systems [1] with interesting technological applications in surface pasivation [2], chemical and bio sensing [3] and hybrid molecule-semiconductor devices [4]. In some of the applications a high and stable coverage by thiols is needed. Thiol deposition on GaAs surfaces for example is possible from both vapor [5] and liquid [6] phases. However 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 this work, we have deposited alkanethiol molecules with different chain lengths on GaAs(110) and thiol terminated benzenedimethanethiol (BDMT) on InP(110) from vapor phase under UHV conditions. The Adsorption kinetics was followed in situ step by step with an ion based technique, which allowed determination of coverage and of film composition. Results from thermal annealing of the films are also reported. 2. Manuscript layout Clean GaAs(110) samples were exposed to the vapors of alkanethiols (Aldrich, 98 % purity) at partial pressures in the range of 1x10-9 – 1x10-7 Torr. Prior to exposure, the thiols were purified by freeze-pump-thaw cycles. The technique of Direct Recoil Spectroscopy with Time of Flight analysis (TOF-DRS) [7] was used 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. Integration of the direct recoil (DR) peaks after background subtraction [7] provides information on the adsorption kinetics, and when this technique is combined with sample annealing  it also provides information on the desorption process. TOF-DRS shows that the adsorption of Ethanethiol (figure 1), Propanethiol, Hexanethiol and dipropyl disulfide on GaAs(110) proceeds directly towards a dense standing up phase without passing through a stable phase of lying down molecules, as is the case of Au substrates [8]. We found that the desorption processes differ depending on the initial coverage (figure 2) and the length of the hydrocarbon chain (figure 3). For higher starting coverage, the drop in the coverage yields a first desorption peak around 300 K where partial desorption of the intact molecule takes place, and a second desorption at 500 K results in the S-C bond scission and the S deposition on the surface. The first desorption temperature increases with increasing hydrocarbon chain (figure 3). On the contrary, the behavior of dipropyl disulfide on GaAs(110) and BDMT on InP(110) [9]  is markedly different (figure 4); the first desorption peak is absent in these cases. The disulfide is adsorbed as C3H7S at initial exposures via scission of the S-S bond and desorbs as C3H7S and C3H7 at ~ 550 K [10]. These results suggest that the first desorption peak corresponds to recombination of the thiolates with surface hydrogen to desorbs as a molecular thiol, and the second one to the dissociation of the C–S bond leaving S on the GaAs(110) surface [7].   3. Conclusion This work describes a clean method to obtain high quality thiol and dithiol films on compound semiconductor surfaces. It also shows the capability of an ion based technique to study the properties during both growing and annealing of the films. 5. References [1] McGuiness, C. L.; Shaporenko, A.; Mars, C. K.; Uppili, S.; Zharnikov, M.; Allara, D.L. J. Am. Chem. Soc. 2006; 128, 5231. [2] Moumanis, K.; Ding, X.; Dubowski, J. J.; Frost, E. H. J. Appl. Phys. 2006, 100, 34702. [3] Ding, X.; Moumanis, K.; Dubowski, J. J.; Frost, E. H.; Escher, E. Appl. Phys. A 2006, 83, 357. [4] Lodha, S.; Janes, D. B. J. Appl. Phys. 2006, 100, 024503. [5] Donev, S.; Brack, N.; Paris, N. J.; Pigram, P. J.; Singh, N. K.;Usher, B. F. Langmuir 2005, 21, 1866. [6] McGuiness, C. L.; Shaporenko, A.; Mars, C. K.; Uppili, S.;Zharnikov, M.; Allara, D. L. J. Am. Chem. Soc. 2006, 128, 5231. [7] L.M. Rodríguez, J.E. Gayone, E.A. Sánchez, O. Grizzi, Bárbara Blum, R.C. Salvarezza, L. Xi, and W. Ming Lau, J. Am. Chem. Soc. 2007, 129, 7807. [8] L.M. Rodríguez, J.E. Gayone, M.L. Martiarena, E.A. Sánchez, O. Grizzi, B. Blum, R.C. Salvarezza, L. Xi, W.M. Lau, Nucl. Instrum. Methods Phys. Res. B,  2007,183, 258. [9] L.J. Salazar Alarcón, L. Chen, V. E. Esaulov, J. E. Gayone, E. A. Sánchez, and O. Grizzi, submitted to  J. Chem. Phys. [10] T.P. Huang, T.H. Lin, T.F. Teng, Y.H. Lai, W.H. Hung, Surf. Sci. 2009, 603, 1244.