AVS2001 Session SC+SS-MoM: Oxidation of Semiconductors

Monday, October 29, 2001 10:00 AM in Room 122

Monday Morning

Start	Invited?	Item
10:00 AM		SC+SS-MoM-2 Coexistence of Active and Passive Oxidation Areas on the Si(100) Surface under Oxygen Cluster Beam Impact D.V. Daineka (A.F. Ioffe Physicotechnical Institute, Russia); F. Pradère, M. Chatelet (CNRS, Ecole Polytechnique, France); E. Fort (Universités Paris VI et Paris VII, France) The Si(100) oxidation by cluster beam impact has been studied in ultrahigh vacuum for surface temperatures from 850 to 1100°C. Neutral oxygen clusters with an average size of 2000 molecules and a translational energy of 0.26 eV/molecule were delivered by a supersonic beam with the maximal flux density of 10¹⁵ cluster/cm²s. The analysis of surface profiles after the beam exposure at T<1000°C shows that a circular groove is formed on the surface. The groove, resulting from surface etching via the reaction 2Si+O₂-> 2SiO(g), is characterized with a steep inner wall and a gradual decrease of its depth towards the outer edge. In the central part of the impact spot, surrounded by the groove, no surface etching occurs due to the formation of a protective oxide layer. The revealed effect is attributed to the gaussian flux density distribution in the cluster beam cross section. The abrupt drop of the etching rate at the inner wall of the groove corresponds to the transition from active to passive oxidation. Only active oxidation with formation of a single etch pit was observed at T>1000°C. The reaction is steady-state and close to first-order. It has been found that there is no pronounced temperature dependence of the etching rate, which is in contrast with the previous results obtained with molecular oxygen.^1,2 Etching rates as high as 6 µm/min were measured for T>1000°C. This enhanced reaction efficiency is attributed to the role of the oxygen clusters. The obtained results show that the knowledge of the flux density distribution in the beam is extremely important when supersonic sources are used to study surface reactions. ¹ Y. Ono, M. Tabe, H. Kageshima, Phys. Rev. B48(1993) 14291 ² S. Hildebrandt, A. Kraus, R. Kulla, H. Neddermeyer, Appl. Surf. Sci. 141(1999) 294 .
10:20 AM		SC+SS-MoM-3 An Ab Initio Study of the Initial Oxidation of the Si(100)-(2x1) Y. Widjaja, C.B. Musgrave (Stanford University) As the dimensions of metal-oxide-semiconductor (MOS) devices keep shrinking, O₂ molecule is increasingly used as the oxidizing species over H₂O as it oxidizes silicon more slowly and hence results in better control of film thickness. Here, we use density functional theory to investigate the detailed chemical mechanism of O₂ reaction with the Si(100)-(2x1) surface using cluster approximations, in which larger clusters are used to examine reactions across dimers as well as to investigate nonlocal effects. Our proposed mechanism confirms the trapping-mediated mechanism previously observed by molecular beam experiments. We find that O₂(g) initially adsorbs on the "up" silicon atom of the surface dimer with an adsorption energy of 31 kcal/mol. The adsorption is site specific and reaction on the "down" silicon atom is unstable. The adsorbed O₂(a) then reacts and forms a peroxide bridge structure, which subsequently dissociates and inserts into the dimer bond and the backbond. Reactions involving neighboring dimers also exhibit an adsorbed state in which the O₂(a) molecule is adsorbed in between the two silicon dimers. In addition to investigating the initial adsorption of oxygen molecules, we also study the atomistic mechanisms leading to the SiO(g) desorption observed at high temperature. The desorption barrier calculated is 65 kcal/mol, which explains the high thermal energy required before SiO(g) desorption occurs.
11:00 AM		SC+SS-MoM-5 Oxidation of Si(100): Mechanisms of Oxygen Insertion, Migration and Agglomeration K. Raghavachari (Agere Systems) Understanding the formation of thin oxides on silicon surfaces is of prime importance as developments in microelectronics demand oxide thicknesses of the order of a few atomic layers. We have carried out first-principles quantum chemical calculations with cluster models to investigate the structural and mechanistic aspects of the initial oxidation of a Si(100) surface. The microscopic steps related to the initial oxygen incorporation as well as oxygen migration and agglomeration on annealing are considered in detail. The calculated activation energy barriers suggest an interesting competition between the steps involved in oxygen insertion and oxygen migration and agglomeration. The presence of surface hydrogen causes significant perturbations on the calculated energy barriers and has important implications to the reaction mechanisms. Our results are used to provide novel interpretations of experimental infrared spectroscopic data.
11:20 AM		SC+SS-MoM-6 Fundamental Aspects of Silicon Oxidation: O₂ and H₂O Reaction with Si(100) and H-passivated Si Surfaces Y.J. Chabal (Agere Systems); A. Esteve (LAAS, France); X. Zhang, E. Garfunkel (Rutgers University); K. Raghavachari (Agere Systems) Examining the initial oxidation steps of both clean and H-passivated silicon surfaces is important to unravel the mechanism for oxygen insertion and oxide formation in realistic environments. We have combined high resolution infrared absorption spectroscopy (IRAS) with quantum chemical (QC) cluster calculations and kinetic Monte Carlo (KMC) simulations to determine the energetics and kinetics of O₂ and H₂O thermal oxidation of Si(100) and H-passivated Si(100) and Si(111) surfaces. Specific local structures are determined by comparing experimental IRAS data of both Si-O and Si-H vibrational modes with vibrational frequencies determined from first principles QC calculations of energetically stable model structures. KMC simulations are then used to analyze the cumulative effect of a series of elementary reaction steps on extended growth of an oxide layer. For the clean Si(100)-(2x1) surface, oxygen is readily incorporated into the surface from either O₂ or H₂O with a thermodynamic propensity to agglomerate but with different kinetics. KMC simulations show that oxide growth is governed by two fundamental phenomena: (i) charge transfer arising from oxygen insertion into the Si-Si bonds and (ii) hydrogen passivation and/or dangling bond formation at the surface. The charge transfer strongly affects the energetics (thermodynamics) of further oxygen agglomeration (the ability for an oxygen atom to leave an oxygenated dimer unit); the presence/absence of dangling bonds then compounds this effect by further modifying the oxygen migration kinetics. For H-passivated surfaces, both O₂ and H₂O are found to incorporate into the Si-SiH backbonds without loss of surface hydrogen. We find an oxygen insertion energy of 1.6 - 1.7 eV, while the oxidation kinetics of different surface structures appear to be dominated by O2 access to Si-Si bonds (locally blocked by unreactive Si-H species).
11:40 AM		SC+SS-MoM-7 Displacement of Surface As Atoms by Insertion of Oxygen Atoms into As-Ga Backbonds M.J. Hale, S.I. Yi, J.Z. Sexton, A.C. Kummel (University of California, San Diego) Stable and metastable oxide structures resulting from the reaction of GaAs(001)-(2x4) with O and O₂ are investigated using scanning tunneling microscopy (STM). The relative stability of these oxide structures is examined using density functional calculations. STM images show that when GaAs(001)-(2x4) is exposed to O atoms, the O atom will either remove an As atom from its original dimer position and take its place or insert into an As-Ga backbond and create a metastable state. As the O atom coverage increases, O atoms increasingly occupy the position of two As atoms across two neighboring dimers, while the number of metastable states remain constant. These experiments show that As is preferentially removed as a pair (As₂) with one removed As atom originating from each of two neighboring As-As dimers instead of two As atoms from the same As-As dimer. This displacement of As₂ is consistent with the propensity of the unit cell to relax into a charge-balanced morphology. Furthermore, the charge-imbalance from oxygen chemisorption is the driving force for As₂ displacement. DFT calculations demonstrate both the charge imbalances in the metastable states and the relative stability of the final chemisorption state. The displaced arsenic atoms form As_Ga antisites which pin the Fermi level and prevent thermal oxidation from forming an electrically passive interface on GaAs in contrast to vapor deposited oxides.