Passivation of defect states in Si-based and GaAs structures

Formation of defect states on semiconductor surfaces, at its interfaces with thin films and in semiconductor volumes is usually predetermined by such parameters as semiconductor growth process, surface treatment procedures, passivation, thin film growth kinetics, etc. This paper presents relation between processes leading to formation of defect states and their passivation in Si and GaAs related semiconductors and structures. Special focus is on oxidation kinetics of yttrium stabilized zirconium/SiO₂/Si and Sm/GaAs structures. Plasma anodic oxidation of yttrium stabilized zirconium based structures reduced size of polycrystalline silicon blocks localised at thin film/Si interface. Samarium deposited before oxidation on GaAs surface led to elimination of EL2 and/or ELO defects in MOS structures. Consequently, results of successful passivation of deep traps of interface region by CN⁻ atomic group using HCN solutions on oxynitride/Si and double oxide layer/Si structures are presented and discussed. By our knowledge, we are presenting for the first time the utilization of X-ray reflectivity method for determination of both density of SiO₂ based multilayer structure and corresponding roughnesses (interfaces and surfaces), respectively.     

The reaction of oxygen and water with iron films studied by X-Ray photoelectron spectroscopy

Adsorption of oxygen on iron at ambient temperature and low pressure is shown by XPS to give a chemisorbed species and the oxide, Fe₂O₃. At low temperatures a further adsorbed species is detected, similar to the nickel-oxygen system. Correction of the intensity of the oxygen signal for depth results in an oxidation curve in agreement with reported work using other techniques, i.e. oxidation is fast until about four layers of oxide are formed, at an exposure of ca. 10² L, and then proceeds slowly to about ten layers. Adsorption of water vapour produces an overlayer less than one layer in depth at an exposure of 10⁵ L. Comparison of the overlayer depths calculated from the decrease in unoxidised iron signal intensity and from the increase in oxygen intensity gives good agreement for the thick oxide film produced by oxygen adsorption, but not for the thin overlayer formed by exposure to water vapour. This suggests a difference in packing of the ions in the thin overlayer compared to the arrangement in the bulk oxide.     

Initial oxidation of the Ce–Ru(0001) interface

Initial oxidation of the Ce–Ru(0001) overlayer system has been studied by photoelectron spectroscopy and low energy electron diffraction. The Ce overlayer thicknesses ranged from 2 to 7 Å. The interface was studied at annealing temperatures up to 1000°C and it was oxidized at low oxygen exposures up to 68 l at 500°C. Interactions between Ce and Ru at the interface were indicated by intensity variations of the Ce3d4f² and Ce3d4f⁰ features with annealing temperature. No intermixing between Ce and Ru exceeding one monolayer was observed. Upon oxidation, trivalent Ce₂O₃ oxide was initially observed at the surface. With increased oxygen exposures, a conversion from trivalent oxide to ‘tetravalent’ CeO₂ was seen. The Ce overlayer had no catalytic effect on oxidation of the Ru substrate in contrast to previous studies on other refractory metal, such as Nb, Ta or W.     

Bistable mean-field kinetics of CO oxidation on Pt with oxide formation

The bistability of the kinetics of CO oxidation on Pt at sub-atmospheric pressures can be complicated by surface-oxide formation. We present a simple mean-field model making it possible to describe a transition from the low reactive reaction regime occurring via the conventional mechanism of CO oxidation at CO excess, to the high reactive regime including CO interaction with a fully developed surface-oxide overlayer at O₂ excess. In the latter case, the oxide is assumed to form islands, the CO₂ formation may run primarily on oxide, and in agreement with recent experiments the reaction rate may be several orders of magnitude lower than the CO adsorption rate on a bare metal surface.     

A NEW METHOD TO ENHANCE THE MAGNETISM OF Fe OVERLAYER ON GaAs(100): SULFUR PASSIVATION USING CH3CSNH2

Ferromagnetic resonance (FMR) has been used to investigat the magnetism of Fe overlayer on S-passivated GaAs(lO0) pretreated by CH₃CSNH₂. Comparing with the magnetism of Fe overlayer on clean GaAs(100), we find that sulfur passivation can prevent Aa diffusion into Fe overlayer and weaken the interaction of As and Fe. It results in enhancing the magnetism of Fe overlayer on GaAs(100). We also investigate the effects of the pre-annealing of S- pasaivated GaAs(100) substrate on the magnetism of Fe overlayers. The results show that the maximum effective magnetization can be obtained at annealing temperature of 400℃. According to the experimental results of synchrotron radiation photoemission, it can be explained by the change of chemical composition and surface structure of the passivation layer on GaAs(100) surface after the annealing.     

Oxidation of Ni(Pt)Si by molecular vs. atomic oxygen

X-ray photoelectron spectroscopy (XPS) has been used to characterize the oxidation of a clean Ni(Pt)Si surface under two distinct conditions: exposure to a mixed flux of atomic and molecular oxygen (O + O₂; PO+O₂ = 5 × 10⁻⁶ Torr) and pure molecular oxygen (O₂; PO₂ = 10⁻⁵ Torr) at ambient temperatures. Formation of the clean, stoichiometric (nickel monosilicide) phase under vacuum conditions results in the formation of a surface layer enriched in PtSi. Oxidation of this surface in the presence of atomic oxygen initially results in formation of a silicon oxide overlayer. At higher exposures, kinetically limited oxidation of Pt results in Pt silicate formation. No passivation of oxygen uptake of the sample is observed for total O + O₂ exposure <8 × 10⁴ L, at which point the average oxide/silicate overlayer thickness is 23 (3) Å (uncertainty in the last digit in parentheses). In contrast, exposure of the clean Ni(Pt)Si surface to molecular oxygen only (maximum exposure: 5 × 10⁵ L) results in slow growth of a silicon oxide overlayer, without silicate formation, and eventual passivation at a total average oxide thickness of 8(1) Å, compared to a oxide average thickness of 17(2) Å (no silicate formation) for the as-received sample (i.e., exposed to ambient.) The aggressive silicon oxidation by atomic oxygen, results in Ni-rich silicide formation in the substrate and the kinetically limited oxidation of the Pt.     

Kinetics of laser-induced surface melting and oxide removal in single-crystalline Ge

The process of oxide removal in crystalline Ge using a pulsed ultraviolet laser has been studied by means of real-time reflectivity measurements with nanosecond resolution. The interaction of laser radiation with a clean, oxide-free surface has been characterized and the inhomogeneous and homogeneous energy density melting thresholds of c-Ge for 193 nm radiation have been determined. The values are 180 and 370 mJ/cm², respectively. We have demonstrated that it is possible to remove an oxide overlayer by irradiation in vacuum and to produce a surface that shows the same response to laser radiation as a smooth, oxide-free, chemically cleaned surface. Under certain specific irradiation conditions it is even possible, after removing the oxide overlayer, to produce an enhanced crystalline quality in the near-surface region compared to that obtained upon chemical cleaning as evidenced by Rutherford backscattering/channeling measurements.     

Initial stages of oxidation of GaAs(111)2 × 2-Ga surfaces

The initial interaction of oxygen at room temperature with GaAs(111)2 × 2-Ga surfaces has been studied by quantitative Auger analysis and low-energy electron diffraction, under different electron irradiation and gas ionization conditions. Oxygen fills first the non-vacancy overlayer sites with a preferential bond to the Ga atoms. This adsorption phase is characterized by the absence of chemical shifts in the Ga Auger peaks that involve core levels. The oxidation stage begins with the occupation of the underlayer sites below the first Ga-As bilayer. For coverages lower than 2 monolayers oxygen adsorption and incorporation takes place without any loss of Ga or As atoms of the surface layers. Electron irradiation and gas ionization of the oxygen-covered surface increase the kinetics up to two orders of magnitude, but no changes in the adsorption sites and/or occupation sequence have been detected.     

The evanescent-wave cavity ring-down spectroscopy technique applied to the investigation of thermally grown oxides on Si(100)

We report about the contribution of thermally grown SiO_x overlayer on the SiO_x/Si interface (with oxidation states Siⁿ⁺, where n = 1, 2, 3, 4) to the optical losses of a resonant spectroscopic cavity. The experiments on Si oxide thin films were performed in evanescent wave for Si samples in contact with a total internal reflection surface of a BK7 prism. The evanescent field can be exploited to investigate properties and processes such as the absorption of thin film or solid/air interface reactions. The results show that the oxide overlayer thickness grows with the thermal exposure time and is limited after more than 7 h of treatment. Transmission electron microscopy has been used for the native oxide thickness measurement and angle-resolved X-ray photoelectron spectroscopy used to determine the thermal oxide thickness. A change of absorption coefficient Δα in the range 100–200 cm^?1 is obtained by evanescent-wave cavity ring-down spectroscopy (EW-CRDS) for thermal silicon oxide overlayer, in agreement with the general trend from literature. The evaluation from the EW-CRDS experiments presents the used setup as a competitive method for measuring the absorption properties of thin overlayer.     

Oxidation of Zn in UHV environment at low temperature

Thermal oxidation of polycrystalline Zn foils at 5 × 10⁻⁷ Torr oxygen pressure and at room temperature, 50 °C, 70 °C, 90 °C and 110 °C was studied. In situ photoemission spectroscopy using synchrotron light with photon energy of 57 eV was used to monitor the formation of ZnO and to determine the thickness of the oxide overlayer. At the initial oxidation, the oxidation rate follows a two-stage logarithmic equation and later trends to saturate at a certain thickness depending on the oxidation temperature. The saturated thickness was found to increase with the oxidation temperature. The two-stage oxidation process may be governed by two kinds of space charge presumably formed in the thin oxide overlayer.     

Combined RHEED-AES study of the thermal treatment of (001) GaAs surface prior to MBE growth

Auger analysis and reflection high energy electron diffraction (RHEED) have been used to study the UHV thermal cleaning procedure of different chemically treated (001) GaAs surfaces when heated in ultra high vacuum. It is shown that the ultimate surface composition of the substrate critically depends on the nature and the thickness of the oxide layer formed during chemical treatment. The oxygen removal mechanism has been studied and a comparative analysis of AES and RHEED observations has been drawn. A low residual carbon coverage cleaning procedure is fully investigated and it results that a carbon coverage as low as ∼6×10⁻² monolayer induces surface faceting by heating the GaAs substrate at temperatures higher than 570°C. A (001) GaAs surface heated in an arsenic flux up to 570°C is As-stabilized and (411) facets appear at a temperature ranged between 575 and 585°C.     

Dissociative chemisorption and localized oxidation states at titanium surfaces

Dissociative chemisorption of carbon monoxide and nitric oxide is shown to result in the formation of discrete localised oxidation states of titanium; Ti²⁺ with CO and Ti²⁺ and Ti³⁺ with NO. The results are compared with dioxygen (Ti²⁺, Ti³⁺ and Ti⁴⁺). A correlation is shown to exist between the extent of dissociative chemisorption i.e. overlayer growth and the emergence in a step-wise manner of Ti²⁺, Ti³⁺ and Ti⁴⁺. Attention is drawn to the significance of the Madelung energy in determining the oxidation states generated within the overlayer by this molecule-specific process.     

Oxidation of 4H-SiC covered with a SmSix surface alloy

Oxidation of Sm/4H-SiC is studied by X-ray photoemission spectroscopy (XPS) and low energy electron diffraction (LEED). In particular, we report kinetic information from the oxidation of a SmSi_x (1 × 1) surface alloy formed on (0 0 0 1) 4H-SiC. During the initial oxidation of the SmSi_x alloy, a (2 × 2)-LEED pattern is observed. Furthermore, the Sm 2+ valency observed from the clean SmSi_x surface alloy, which is related to surface samarium atoms, disappear at 15 L oxygen exposure. The oxygen atom is consequently deduced to be located at bridge or hollow sites involving one Sm atom. The initial oxidation result in an oxygen deficit SmSiO_x interface oxide, probably as a consequence of the high oxidation temperatures in this work (900-1050 °C). We report that in a prolonged oxidation (longer than 10 kL) a SiO₂ layer forms on top of the samarium silicon oxide interface layer.     

Detection of excess crystalline As in GaAs: Nation oxide overlayers by Raman scattering

Laser Raman spectroscopy was employed as a non-destructive probe for the detection and monitoring of crystalline arsenic in the native oxide films formed during heating of GaAs in air at various temperatures. Spectroscopy of oxide films formed after successive heating and etching treatments could confirm the location of arsenic to be near the top of the GaAs: native oxide overlayer.     

Valence variations in the monolayer regime of Sm on the Nb(110) surface

Sm overlayers in the monolayer regime, deposited on a Nb(110) single-crystal surface at room temperature, have been studied by means of high-resolution photoelectron spectroscopy of the Nb 3d_5/2 core level and the Sm valence band. In the submonolayer regime, the Sm valence varies from mainly divalent for very low coverages to essentially trivalent close to a complete monolayer. Above 1 ML, a new divalent component appears in the Sm 4f spectra, corresonding to divalent Sm in the second layer. The mixed valence in this overlayer system is concluded to be heterogeneous (all Sm atoms have integer but site-dependent valence). Sm forms ordered overlayers on Nb(110) and the Sm growth is consistent with a layer-by-layer growth mode (Frank–van der Merwe growth).     

A kinetic study of the initial oxidation of the Ni(110) surface by RHEED and X-ray emission

Nickel (110) surfaces, prepared by a combination of high temperature oxidation, argon ion bombardment and hydrogen reduction, were oxidized in pure O₂. The structural aspects of this interaction were studied by reflection high energy electron diffraction (RHEED) and the corresponding kinetics determined by electron excited X-ray emission spectroscopy. On exposure to oxygen the surface was observed to go through three ordered two-dimensional structures. These were a (2 × 1), a (3 × 1) and finally a (9 × 4) structure containing 0.015 microg/cm² of oxygen. From this surface NiO was produced in a (001) epitaxy which was compressed 4.5% in the plane of the surface. With oxidation beyond 0.060 microg/cm² the oxide strain was relieved and a complex oxide epitaxy developed which has been tentatively identified as a (117) oxide parallel to the nickel (110) surface. The kinetics have been explained on the basis of three distinct processes. (i) An initial chemisorption stage (0 to 0.015 microg/cm²) associated with the two-dimensional structures, (ii) Oxide nucleation and spreading to cover the surface, associated with the (001)-NiO epitaxy (0.015 to 0.06 microg/cm²), (iii) Logarithmic film thickening above 0.060 0.06 microg/cm² associated with the development of (117) epitaxy.     

Oxygen adsorption on an alkali metal-covered Ni(100) surface

The oxygen chemisorption on an alkali (Na, K, Cs) covered Ni(100) surface and its initial oxidation were studied by Auger and electron energy loss spectroscopy (ELS). It was found that in the presence of an alkali metal, the sticking coefficient S remains unity up to a given oxygen coverage of θ_O^cwhose value depends on the alkali overlayer concentration and the ionicity of the Ni-alkali metal bond. At a given oxygen coverage, the line shapes of Auger and loss spectra are almost the same for alkali-covered and clean Ni(100), which suggests that alkali metals cause no change in the character of the Ni-O bond. The effect of alkali metals is associated with increasing electron charge in the surface region, which facilitates oxygen chemisorption. The enhanced surface oxygen concentration in the presence of an alkali metal results in the formation of an oxide phase at lower oxygen exposures than is the case of clean Ni surfaces.     

Longitude magneto optical Kerr effect of Fe/GaAs (0 0 1) with Al overlayers

The longitude magneto optical Kerr effect (LMOKE) is investigated on ultrathin Fe film grown on GaAs (0 0 1) substrate with Al overlayer. The formula of the longitude magneto optical Kerr effect is derived to find out the influence of the Al overlayer on the magneto optical properties of Fe/GaAs (0 0 1) sample. Results obtained from this formula fit very well with the experimental data. The change of the Kerr rotation as a function of the incident angle is also given, which can be used to find a proper angle in sample measurement. A numerical simulation was carried out to find out the relation between Kerr rotation and optical properties of the overlayer. Results from this simulation can be used to select the best overlayer material to protect the Fe/GaAs (0 0 1) sample.     

Oxidation kinetics of epitaxial Ge-covered Si(100) surfaces

We compared oxidation kinetics on Ge-covered Si(100) surfaces grown at 350 and 600°C for 0.9 and 2.0 ML Ge overlayer thicknesses. The O_KLL intensities showed clear oxidation enhancement on the surfaces grown at 600°C. The oxygen interaction for the surface covered with 2 ML Ge formed at 350°C was weaker than for the Ge(100) surface, indicating that the compressive strain due to the lattice mismatch may suppress the oxygen interaction with surface Ge dimers.     

Grain boundary ridge formation during initial high temperature oxidation of Mn/Al TRIP steel

Confocal scanning laser microscopy (CSLM) was used in real-time observation of alloy element oxidation of a Mn/Al TRIP steel in an Ar–O₂ atmosphere. CSLM images reveal a marked role of grain boundaries in the overall initial oxidation kinetics of the alloy, and consequently in the morphology of the initial surface oxide. The oxidation on the alloy surface is dominated by the formation of Mn-rich oxide ridges along grain boundary traces on the surface. Oxide ridge formation kinetics was quantified by measurements on images extracted from real-time recordings of surface oxide evolution. Oxide ridge growth was found to take place at a constant rate. Scanning electron microscopy (SEM) images of the oxidized surfaces showed homogenous oxide ridges along straight grain boundary traces and heterogeneous oxide ridges along non-straight grain boundary traces. A transport mechanism of Mn to the surface is proposed, which relies on grain boundary segregation of Mn and on a relationship between grain boundary diffusivity and grain boundary character. It is suggested that when regarding alloys with significant grain boundary segregation of a solute, separate Wagner balances for internal vs. external oxidation is required for the grain lattices and the grain boundaries, respectively.