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

Monday, October 18, 2010 8:20 AM in Room Santa Ana

Monday Morning

Time Period MoM Sessions | Abstract Timeline | Topic SS Sessions | Time Periods | Topics | AVS2010 Schedule

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8:20 AM SS2+EM-MoM-1 Spontaneous Microfaceting and Pyramid Growth during Si(100) Etching
Marc F. Faggin, Ankush Gupta, Melissa A. Hines (Cornell University)
The spontaneous, etching-induced transformation of an initially flat Si(100) surface to a completely nanofaceted morphology consisting of overlapping pyramidal hillocks has been observed using a combination of morphological and spectroscopic probes and modeled using a fully-atomistic kinetic Monte Carlo (KMC) simulator of Si(100) etching. A novel silicon etchant has been developed that catalyzes the complete chemical transformation of a Si(100) surfaces into H-terminated Si{111} and Si{110} nanofacets. This finding was confirmed by infrared absorption spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM). The formation of pyramidal hillocks is highly reproducible and occurs on a time scale of several hours, enabling detailed studies of initial hillock formation and subsequent growth. The formation of microfaceted pyramidal hillocks during etching of Si(100) has previously been attributed to local masking on the surface by deposited impurities, etch products or gas bubbles. These mechanisms assume that an adsorbed impurity or gas bubble decorates the apex of every pyramid. Our atomistic simulations uncovered a second mechanism, one that is intrinsic to the etchant and that generates dynamically self-propagating pyramidal structures. Attempts to distinguish between these two mechanisms through rational modifications of the etchant chemistry will be described. For example, the kinetics of pyramid growth were followed spectroscopically, enabling quantitative assessment of the effects of chemical additives. These observations are more than an intellectual curiosity, as the silicon solar cell industry is actively searching for inexpensive, environmentally-friendly means of pyramidally texturing Si(100) surfaces to reduce reflection losses. Conversely, in microfabricated devices, suppression of pyramid formation is critical to high-yield manufacturing processes. An understanding of the hillock formation process may lead to the rational design of better etchants.
8:40 AM SS2+EM-MoM-2 Selective Ablation of Xe on Silicon Surfaces: MD Simulation and Experimental Laser Patterning
Ori Stein, Micha Asscher (The Hebrew University in Jerusalem, Israel)

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.

Key words: Molecular Dynamics Simulations, Laser Ablation, adsorbed Xe on Si, Coverage Grating.
9:00 AM SS2+EM-MoM-3 Tuning Properties of Thin Films by Aminofunctionalization
Andrew Teplyakov (University of Delaware)

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 NHx 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.

9:20 AM SS2+EM-MoM-4 Helium Atom Diffraction Measurements of the Surface Structure and Vibrational Dynamics of CH3–Si(111) and CD3–Si(111) Surfaces
James S. Becker, Ryan D. Brown (University of Chicago); Erik Johansson, Nathan S. Lewis (California Institute of Technology); Steven J. Sibener (University of Chicago)

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 (θi = θf) decay revealed perpendicular mean-square displacements and He-surface well depths of 1.0·10-5 Å2 K-1 and 7.5 meV for the CH3–Si(111) surface and 1.2·10-5 Å2 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 Å2 K-1 and 4.5·10-4 Å2 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.

9:40 AM SS2+EM-MoM-5 Comprehensive Descriptions of Surface Atomic Structure
John C. Thomas, Joanna Mirecki Millunchick (The University of Michigan, Ann Arbor); Normand A. Modine (Sandia National Laboratories); Anton Van der Ven (The University of Michigan, Ann Arbor)

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 In­­xGa1-xAs (001) alloy, which exhibits nanoscale coexistence domains and where an unexplained (nx3) reconstruction is observed over a wide range of conditions.

10:20 AM BREAK
10:40 AM SS2+EM-MoM-8 The Structure of Metal-Rich (001) Surfaces of InAs and InSb
Jacek Kolodziej (Jagiellonian University, Poland)
Based on electron diffraction experimental observations the reconstructions of the indium-rich indium antimonide and indium arsenide (001) surfaces are assigned as c(8x2) or closely related c(8x2)/(4x2). At room temperature scanning tunneling and atomic force microscopy studies also evidence highly symmetric c(8x2) or (4x2) reconstructions. However, microscopic studies done at cryogenic temperatures indicate lowering of the surface structures symmetry from c2mm/p2mm to p2, structural domains, disorder and fluctuations. In the present paper we will show that the surfaces are well described by a so called zeta-family models with certain atomic rows occupied slightly above 50%. Atomic vacancies are confined to these rows and rapidly fluctuate at room temperature. Averaging effects cause that experiments done at elevated temperatures using slow methods evidence symmetric structures. In contrast, at low temperatures, the vacancies stabilize to form regular double-period structures along the rows, but this spontanoeusly leads to the complete surface structure having only p2 symmetry group, structural domains and partial disorder on the surface. We have also identified a variety of local structures appearing at the domain walls. This complex surface structure is inherent to the thermodynamic equilibrium of the system as indicated by failed attempts to increase surface indium content by adsorption of In atoms from a gas phase. Density functional theory calculations confirm that the new surface structure is a minimum energy configuration at fixed stoichiometry. Simulated scanning tunneling microscopy images confirm proposed model of the structure.
11:00 AM SS2+EM-MoM-9 Monolayer Passivation of Ge(100) Surface via Nitridation and Oxidation
Joon Sung Lee, Sarah Bishop, Tobin Kaufman-Osborn, Andrew C. Kummel (University of California at San Diego)
The monolayer passivation of Ge(100) surface via formation of Ge-N and Ge-O surface species was studied using scanning tunneling microscopy (STM) and density functional theory (DFT) to develop a process of minimizing interface defect density between Ge and a high-k dielectric layer in a highly scaled device. Direct nitridation was performed on a Ge(100) surface using an electron cyclotron resonance (ECR) plasma source with pure N2 gas. It was hypothesized that plasma nitridation at elevated temperature (500oC) would form an ordered nitride structure that would combine the low defect density of GeO2 with the higher thermal stability of GeON via formation of a Ge-N ordered structure. Experimental and theoretical modeling showed that bandgap states are produced from the ordered nitride structure resulting in Fermi level pinning of the surface; however, it is predicted that H-passivation on the nitride structure would unpin the Fermi level by reducing the dangling bonds and the bond strain. The best method to passivate a Ge(100) surface is to form a layer of GeO2 which is free of suboxides. However, this process is difficult to scale using thermal oxidation by O2, so alternative oxidants, H2O and GeO2, were studied. At room temperature, the H2O-dosed Ge surface showed Ge-OH sites with very few Ge adatoms, while the e-beam deposition of GeO2 formed semi-ordered Ge-O structures and Ge ad-species. It is likely the H2O dosing produces an ideal passivation layer since it displaces few surface Ge atoms. Nevertheless, annealing above 300oC converts the surface oxides into suboxide rows on both H2O and GeO2 dosed Ge surfaces due to the reactivity of GeO2 with Ge. Scanning tunneling spectroscopy (STS) shows that the Fermi level of the n-type Ge surfaces covered by suboxides is near the valence band edge, consistent with formation of Ge suboxide rows likely causing Fermi level pinning. The atomic layer deposition (ALD) of GeON and SiO2 are being studied to form a monolayer or bilayer of passivation with a minimum defect density and the improved thermal stability.
11:20 AM SS2+EM-MoM-10 Formation of Titanium Sub-Oxide from TiO2 ALD Films on Si and Ge Substrates after Vacuum Anneal
Rungthiwa Methaapanon, Pendar Ardalan, Stacey Bent (Stanford University)

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 (TiGeOx) 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.

11:40 AM SS2+EM-MoM-11 Interfacial Effects of Near-surface Dopant Diffusion and Electrical Activation in Silicon
Prashun Gorai, Yevgeniy Kondratenko, Edmund G. Seebauer (University of Illinois at Urbana-Champaign)
Defect behavior in silicon can be controlled by manipulating the chemical state of nearby surfaces and solid-solid interfaces, with important implications for transistor fabrication by ion implantation and annealing. Silicon interstitials formed during the ion implantation step are responsible for unwanted transient enhanced diffusion (TED) of dopants, and affect the degree of dopant activation as well. Earlier work in our laboratory has shown that certain chemical treatments of surfaces and interfaces changes its ability to act as sinks for interstitials. The fundamental kinetic quantity describing “sink” behavior can be described by an annihilation probability (S). Yet surfaces and interfaces also support electrically charged defects, which create local strong electric fields that influence the local motion of interstitials that are charged. The degree of charge buildup can be quantified by an electric potential (Vi). The combined effects of S and Vi not only influence the annihilation of interstitials, but lead under some conditions to the pile up of electrically active dopant near the surface or interface. However, up to now, the precise nature of the interplay, including the most relevant time scales during annealing, has never been quantified. Through continuum modeling on the nanometer length scale, the present work provides such quantification. Differential equations describing the diffusion and reaction of silicon and boron interstitials are solved to yield the time evolution of boron profiles that are compared in important cases to experiment.
Time Period MoM Sessions | Abstract Timeline | Topic SS Sessions | Time Periods | Topics | AVS2010 Schedule