ARTICLE pubs.acs.org/JPCC
Si2H6 Dissociative Chemisorption and Dissociation on Si(100)-(21) and Ge(100)-(21) Jean-Francois Veyan,† Heesung Choi,† Min Huang,† R.C. Longo,† Josh B. Ballard,‡ Stephen McDonnell,† Manori P. Nadesalingam,† Hong Dong,† Irinder S. Chopra,† James H. G. Owen,‡ Wiley P. Kirk,† John N. Randall,‡ Robert M. Wallace,† Kyeongjae Cho,† and Yves J. Chabal*,† †
Material Science and Engineering Department, University of Texas at Dallas, 800 W Campbell Road, Richardson, Texas 75080, United States ‡ Zyvex Laboratories, LLC, 1321 North Plano Road, Richardson, Texas 75081, United States
bS Supporting Information ABSTRACT: Atomically controlled epitaxy of semiconductor-onsemiconductor is important for the construction of nanometerscale devices such as quantum dots, qubits for quantum computing, nanoelectromechanical systems (NEMS) oscillators, and nanobiomedical devices. For instance, patterned atomic layer epitaxy (ALE) using a scanning tunneling microscopy tip to depassivate an area of a H-terminated Si(001) surface, creating a pattern for subsequent Si growth, should lead to a new generation of atomically precise structures. Vapor phase epitaxy is wellsuited to achieve controlled deposition, as the precursors are not reactive with the H-terminated background, unlike Si atoms from solid-source evaporation. Disilane (Si2H6) is arguably the best precursor for Si ALE on Si or Ge surfaces at moderate temperatures; yet, its adsorption configuration and subsequent decomposition pathways are not well understood. Combining experimental data from in situ infrared absorption spectroscopy (IRAS), scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS) after saturation by disilane of both Si(100)-(21) and Ge(100)-(21) surfaces, with first-principles calculations of candidate surface structures, we show that Si2H6 chemisorbs through a β-hydride elimination pathway as Si2H5 and H, instead of the previously proposed SiH3, and subsequently decomposes into an ad-monohydride dimer. The initial chemisorption process takes place on a single dimer and produces a monohydride ad-dimer oriented perpendicular to the substrate dimer rows. The ad-dimer can be located either in between two adjacent, initially clean dimers from the same dimer row, or in a bridging position over the trench between two adjacent dimer rows. These findings provide clear guidance for the formation of atomic size structures defined by local removal of hydrogen on H-terminated Si surfaces.
’ INTRODUCTION Nanoscale patterning of a hydrogen-terminated Si(100) surface using scanning tunneling microscopy (STM) tip-based H-depassivation1 is being developed to be a key process in atomically precise three-dimensional (3-D) nanostructure manufacturing.2 Starting from an atomically flat HSi(100)-(21) surface, H atoms are removed locally on selected small areas leaving unsaturated Si bonds on dimers. These clean areas are then accessible for further chemistry, as, for instance, Si or Ge atomic layer epitaxy (ALE) growth using chemical vapor deposition (CVD) techniques, thus enabling the growth of 3-D structures for manufacturing nanometer-scale features (∼30 nm3). Molecular disilane (Si2H6) has been shown to be a good candidate for silicon ALE growth on Si substrates because it readily adsorbs on clean Si(100).315 However, the chemisorption pathway of the Si2H6 molecule onto the Si(100)-(21) surface is still not well understood. At temperatures below 120 K, it has been demonstrated4,16 that Si2H6 molecules adsorb only r 2011 American Chemical Society
weakly (i.e., physisorb) on clean Si(100) surfaces. At room temperature, other experimental studies58,1416 have suggested a dissociative adsorption, involving a H3SiSiH3 bond cleavage process (Figure 1c) as the initial decomposition mechanism of the disilane molecule, leading to the chemisorption of two SiH3• on the two surface Sid atoms with dangling bonds (Sid = surface atom on the Si substrate) of a single dimer unit (intradimer chemisorption). From this starting configuration, various reaction pathways have been proposed, eventually leading to islands of clean Si dimers.14 In 2000, Niwano et al.3,5 proposed an interdimer chemisorption process of two SiH3• (i.e., on Sid from two adjacent dimers), resulting in the formation of a dihydride dimer H2SiSiH2 structure. In that model (Figure 1b), the Si2H4 molecule bonds to two Si atoms from consecutive dimers in a bridging configuration: SidSiH2H2SiSid. In 2005, Received: July 25, 2011 Revised: October 30, 2011 Published: November 01, 2011 24534
dx.doi.org/10.1021/jp207086u | J. Phys. Chem. C 2011, 115, 24534–24548
The Journal of Physical Chemistry C
Figure 1. Models of dissociation and initial chemisorption of the Si2H6 molecule on Si(100)-(21). (a) Configuration A: SiH bond cleavage1013 (β-HEp) forming SidSi2H5 + SidH in an intradimer configuration. (b) Configuration B: SidSi2H4Sid +2 SidH in an interdimer bridging configuration2,4 resulting from the coupling of two SiH3 chemisorbed on two consecutive dimers after SiSi bond cleavage. (c) Configuration X: SiSi bond cleavage38 forming two SidSiH3 in an intradimer configuration.
Smardon et al.10 suggested in a theoretical study that the β-hydride elimination pathway (β-HEp) (Figure 1a) initially proposed by Xia et al.11 in 1995 was a possible mechanism for Si2H6 chemisorption on the Si(100) surface at relatively low temperatures (120 K < T < 300 K). More recently, Kang’s group12,13 in a series of theoretical publications concluded that the β-HEp decomposition through a SiH3SiH2• + H• precursor state is more favorable than the H3SiSiH3 bond cleavage mechanism. Many of the discrepancies reported among the earlier work may be due to the choice of experimental conditions. Apart from the work of Xia et al.11 in 1995, all previous experimental studies have been performed at temperatures above 300 K or below 120 K. There have been no subsequent investigations performed at intermediate temperatures (between 120 and 300 K). Furthermore, the previous detailed STM and STM/density functional theory (DFT) studies14,15 performed at or above room temperature all involved low coverages or fluxes, with no limitation on the surface reaction sites available to each dissociating molecule. Finally, the number of characterization techniques used in all previous work has been limited as well. Consequently, only partial information could be derived, leading to apparent contradictions. In our work, we have explored both the adsorption and thermal decomposition mechanisms of the disilane over the critical temperature range (170800 K), from chemisorption to complete H desorption, under a high flux/coverage regime, using a combination of in situ infrared absorption spectroscopy (IRAS), density functional theory (DFT) calculations, and scanning tunneling microscopy (STM). To gain a better understanding about the mechanisms involved in the chemisorption and thermal decomposition processes, IR spectroscopy experiments using a Ge(100)-(21) substrate have been also performed, coupled with DFT calculations and X-ray photoelectron spectroscopy (XPS). Crystalline Ge and Si have the same diamond structure, a very similar lattice parameter (Ge: 565.75 pm, Si: 543.09 pm, lattice mismatch: 4%) and the same (21) buckled-dimer reconstruction for the (100) surface. Furthermore, Si2H6 has been shown to chemisorb on Ge(100)7,13,17,18 as well. For instance, Akazawa et al.17 have shown that below 625 K, the Si2H6 adsorption on Ge(100) is a self-limited process, while above 625 K, growth could be sustained by continued chemisorption at a constant rate. As in
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the case of Si, none of these previous studies have been performed below 300 K nor have they been directly compared to similar adsorption on Si(100) surfaces. Our results present unambiguous evidence that β-HEp constitutes the initial step in the Si2H6 chemisorption on both Si(100) and Ge(100) surfaces, ruling out other previously proposed models. The adsorbates resulting from this process, SidSi2H5 and SidH or GedSi2H5 and GedH (Ged = surface Ge atom) on the Ge substrate, are stabilized only at low temperatures (173 K). Upon thermal annealing, the data show that further decomposition of Si2H5 occurs, leading to the formation of hydrides as follows: Si2H5 f H2SiSiH2 f SiHSiH2 f HSiSiH. The next step involves the formation of an ad-dimer aligned along the main axes of either the underlying dimer row or the trench between two adjacent dimer rows. All these steps generate additional SidH on Si and GedH on Ge. Interestingly, the desorption of the resultant chemisorbed hydrogen depends on the initial substrate: on Si(100), the complete removal of all H atoms from the surface occurs at ∼800 K, while for Ge(100) it takes place at ∼625 K, with clear evidence that Si ad-atoms diffuse into the Ge lattice at ∼625 K leaving a well-reconstructed H-free Ge surface (see Supporting Information). This detailed mechanistic information reconciles previous discrepancies in reported mechanisms and provides a guide for Si ALE in spatially restricted areas.
’ EXPERIMENTAL SECTION Infrared Absorption Measurements. In situ IR absorption measurements are performed in an ultrahigh vacuum (UHV) chamber at a base pressure of 2 1010 Torr. The 3.8 1.5 cm2 Ge and Si samples are precleaned ex situ as described below and then rapidly (
