AVS2010 Session SS2+EM-MoM: Semiconductor Surfaces and Interfaces

Laser induced ablation of multilayer Xe on Si has been studied employing molecular dynamics (MD) simulations. 5nsec long laser pulse at λ=337nm was applied to a Xe slab at thicknesses of 16 32 and 40ML (7744, 15488, 19360 atoms, respectively) adsorbed on top of a 8 layers 5408 atoms Si slab. Evaporative and explosive ablation thresholds were identified at absorbed laser power of 12 and 16MW/cm² which corresponds to surface temperature rise of 500 and 658K, respectively. Selective ablation was studied, where only a fraction of the lateral dimension of the computation cell was actually ablated. Extremely strong lateral dissipation among the Xe layers, has led the ablation threshold to shift to higher laser power as the fraction of heated area shrinks. Heated fraction (HF) less than 10% results in practically no ablation at laser power below substrate damage threshold. The MD studies were assessed and verified by experimental laser ablation measurements. A 10nsec Nd:YAG laser pulse operating at λ=532nm was employed. It was found that for 80 and 160ML Xe layer thickness, full ablation was reached at laser power of 6.9 and 8.4MW/cm² which corresponds to surface temperature rise of 180 and 220K respectively. Line-edge profile resulting from fractional laser induce desorption- coverage grating formation followed by metallic lift-off experiments were compared to the MD simulations of selective ablation, revealing a remarkable similarity.

Surfaces and interfaces play an important role in development of modern microelectronics, optoelectronics, biosensing and other fields. This work will describe the approaches to tune the properties of interfaces, surfaces, and subsurface layers of participating materials by aminofunctionalization. The amino-groups of a general formula NH_x have been used in our group to control surface reactions on semiconductor surfaces, to promote deposition schemes on surfaces of thin solid diffusion barrier films, and to provide a reliable surface sites for biofunctionalization of self-assembled monolayers. In all of these cases, the reactivity of the amino-group can be designed to fit the required application. We will use selected temperature regimes, alkyl, aryl, and other substituents to alter the reactivity of amino-terminated surfaces and to reversibly tune the properties of surface and subsurface layers in thin solid films. Infrared spectroscopy was used to determine the chemical nature of the surface termination, X-ray photoelectron spectroscopy was applied to discover the stability of the surfaces and interfaces produced and to assist in assessing the chemical state of nitrogen-containing functional groups, microscopic techniques, including atomic force microscopy and transmission electron microscopy were employed to uncover the topographic properties and structure of the films based on titanium carbonitride that serve as a diffusion barrier and of the self-assembled amino-terminated layers utilized as platforms for biosensing devices. The preparation, structure, reactivity, and stability of these aminofunctionalized surfaces and interfaces will be discussed.

The surface structure and vibrational dynamics of CH₃–Si(111) and CD₃–Si(111) surfaces were measured using helium atom diffraction. The elastic diffraction patterns exhibited a lattice constant of 3.82 Å, in accordance with the spacing of the silicon underlayer. The high quality of the observed diffraction patterns indicates a high degree of long-range ordering for this novel interface. The vibrational dynamics were investigated by measurement of the Debye-Waller decay of the elastic diffraction peaks as the surface temperature was increased. The angular dependence of the specular (θ_i = θ_f) decay revealed perpendicular mean-square displacements and He-surface well depths of 1.0·10^-5 Ų K^-1 and 7.5 meV for the CH₃–Si(111) surface and 1.2·10^-5 Ų K^-1 and 6.0 meV for the CD₃­–Si(111) surface. Effective surface Debye temperatures of 983 K for CH₃ and 824 K for CD₃ were calculated. These unusually large Debye temperatures suggest that collisional energy accommodation at the surface occurs primarily through Si-C local mode. The parallel mean-square displacements were 4.3·10^-4 Ų K^-1 and 4.5·10^-4 Ų K^-1 for CH₃– and CD₃–Si(111) surfaces, respectively. The increase in thermal motion is consistent with interaction between the helium atoms and Si-CH₃ bending modes. These experiments yield new information on the dynamical properties of these robust and technologically interesting semiconductor interfaces.

Comprehensive descriptions of surface atomic structure have been developed over the years for a wide range of metals and covalent crystals, but this understanding has typically been obtained only after extensive trial and error. Unfortunately, experimental and theoretical characterization of surfaces is complicated significantly in systems that can exhibit metastable surface reconstructions or in alloy systems, where atomic size mismatch and lattice mismatch strains play an important role and can give rise to phase coexistence. Clearly, a systematic and rigorous approach to determining surface structure is needed in order to explore surface phenomena in alloy systems or away from equilibrium. We have developed an approach that uses prior knowledge about the surface atomic structure of a pure system, along with first principles energy calculations and statistical mechanical methods, to systematically and efficiently explore new ground-state and near-stable surface reconstructions, finite temperature behavior, and alloying effects. We describe the automated generation of III-V (001) surface reconstruction candidates in the group V-rich regime and discuss how our approach is used to study the complex surface structure of the In­­_xGa_1-xAs (001) alloy, which exhibits nanoscale coexistence domains and where an unexplained (nx3) reconstruction is observed over a wide range of conditions.

The formation of an oxide interlayer between a Si or Ge substrate and a metal oxide dielectric film has a direct influence on the physical and electrical properties of the field effect transistors made from these components. An oxide interlayer may form either during the deposition process or during a subsequent high temperature step. It is usually desirable to control or eliminate the formation of this oxide interlayer; one approach used is to create an oxide-free surface by chemically etching away the native oxide layer and adding a surface modifier such as hydrogen or halogens to inhibit further oxide formation.

In this work, we study the interlayer oxide formation on hydrogen-terminated silicon and halide-terminated germanium following TiO₂ atomic layer deposition (ALD). The surface analysis of TiO₂ films on silicon substrates is conducted immediately after the ALD process without exposure to ambient conditions by an integrated X-ray photoelectron spectroscopy (XPS)/ALD system. The results on hydrogen-terminated silicon show that no silicon oxide forms between the two materials during ALD at 100 ^oC. However, a silicon oxide interlayer is detected after annealing in ultrahigh vacuum. A concomitant depletion of oxygen in the TiO₂ films occurs, leading to generation of a Ti sub-oxide. The effect is shown to be correlated with both the annealing temperature and the thickness of the TiO₂ film. Control experiments carried out on TiO₂ film deposited by ALD on SiO₂-coated silicon show significantly less depletion of oxygen in the TiO₂ films. Our results indicate that TiO₂ is the source of O for Si oxidation, and that migration of oxygen to this interface is a driving force for oxygen depletion in the TiO₂ film. The TiO₂ films on Br- and Cl-terminated germanium substrates are deposited by ALD at temperatures in the range of 100-300 ^oC and analyzed using ex-situ synchrotron radiation photoemission spectroscopy (SR-PES). Formation of titanium germanate (TiGeO_x) was observed after annealing to 400 °C. Upon annealing to 700 °C, titanium sub-oxide formation is also observed for this system. However, this reduction was more pronounced in thinner TiO₂ films. The stability of these oxide structures upon annealing, and the prospect for eliminating the oxide interlayer in the TiO₂/Si and TiO₂/Ge systems will be discussed.