Low-temperature Oxidation Process Research Articles

In Situ Upgrading of Heavy Oil

Abstract Low temperature oxidation (LTO) of hydrocarbon liquids generally results in a more viscous end product; this has clearly been shown in the literature of the past 30 years. However, under the right conditions, LTO can be used to achieve viscosity reduction in heavy oils. The In Situ Combustion Group at the University of Calgary conceived of a two-stage LTO process whereby oil is contacted with air, first at low, then at elevated, temperatures. The first, low temperature, step incorporates oxygen into some of the hydrocarbons, yielding labile bonds that should break at lower-than-usual temperatures. Once these free radicals are formed, the second step promotes bond cleavage at higher temperatures, resulting in shorter chain hydrocarbons. In a field situation, this process would be analogous to first injecting air into a formation at low temperature, then starting a steam soak or steam flood. Experimental runs carried out on Athabasca bitumen examined the effects of oxygen partial pressure, temperature, reaction time, and the presence of rock and brine. On completion of each experiment, the gas composition was determined using gas chromatography, water acidity (pH) was measured, and the hydrocarbon products were analysed for coke and asphaltenes contents, viscosity, and density. Some instances of viscosity reduction have been observed; these are linked to lower oxygen partial pressures, higher second stage temperatures and longer run times. This paper discusses the experimental work, and estimates the optimum conditions for successful viscosity reduction of a given heavy oil. Introduction Heavy oil and oil sands are important hydrocarbon resources that total over 10 trillion barrels, nearly three times the conventional oil in place in the world. The oil sands of Alberta alone contain over two trillion barrels of oil. In Canada, approximately 20﹪ of oil production is from heavy oil and oil sand resources(1). The application of thermal energy to increase heavy oil recovery has become more popular as conventional reserves decline. Steam injection accounts for the majority of the thermal recovery projects currently in operation; however in situ combustion offers many theoretical advantages if the operational characteristics of the process are incorporated in the design and operation of the field project. A major difficulty encountered in operating in situ combustion processes is low temperature oxidation (LTO), which involves oxygen addition reactions that occur at temperatures lower than 300 °CDATA [C. Typically, the byproducts of these reactions are oxidized hydrocarbons that have an increased polarity. This makes them more viscous, and thus detrimental to the in situ combustion process. Because of the major impact that LTO can have on the performance of an in situ project, a significant number of investigations have been carried out on the nature and effect of LTO reactions(2-27). In some circumstances, however, it may be beneficial to subject oil to LTO. The experimental results of Cram and Redford showed that air/steam combinations can provide better recovery rates and better thermal efficiencies than steam alone, at comparable volumes of steam injected, when the process is carried out in the low temperature oxidation region((28). It is believed that energy generation by the exothermic oxidation reactions is a significant factor in the LTO process.

Improved Residual Light Oil Recovery by Air Injection (LTO Process)

Abstract A new air injection technique, low temperature oxidation (LTO) process, is described. Improved oil recovery from deep, light oil reservoirs is achieved by removing the oxygen in the injected air by LTO reactions with the residual oil in the reservoir. The product of the LTO reactions is a "flue gas," which displaces the oil. Preliminary results of LTO reaction kinetics and oil recovery have been obtained using four North Sea light oils. The paper also contains some discussion of the safety issues related to air injection offshore. Introduction Gas injection into light oil reservoirs is a proven improved oil recovery IOR technique. The IOR potential for gas injection in the United Kingdom Continental Shelf (UKCS) has been estimated at 1.4 bSTB(1). However, the application of gas injection is limited by gas availability and cost, particularly for many mature fields, with the prospect of abandonment unless economic methods can be developed to extend the field life. Therefore, there is now growing interest in air injection because of its availability. Air injection has been widely used in the past for production of viscous heavy oils, where the heat generated by in situ combustion is a necessary part of the recovery process. Air injection can also be used for the recovery of light oils, but in this case, heat generation is not necessary for the displacement. Some form of oxidation is only required in order to remove the oxygen from the air and prevent it from reaching the production wells. Yannimaras et al.(2) have discussed the benefits of air injection for IOR from deep, light oil reservoirs, wherein the principle objective was to generate flue gas (85﹪ N2, 15﹪ CO2) by in situ combustion. There are a number of ongoing successful air injection field projects, notably in the West Hackberry Field, Louisiana [Amoco(3)]; in Medicine Pole Hills Unit, North Dakota; Buffalo, South Dakota [Koch(4)]; most recently, in the Horse Creek Field, North Dakota [Total(5)]; and Total's proposed LTO pilot test in the H Field in Indonesia(6). In the latter case, core flooding studies were undertaken to investigate the effect of various parameters on oxygen uptake by the oil. Previous field projects and simulation studies have considered that high temperature oxidation (HTO, or in situ combustion) is needed to remove the oxygen and enhance oil recovery. Christopher(7) [see also Yannimaras et al.(8)] used an accelerating rate calorimeter (ARC) to screen light reservoir oils for continuous exothermicity. For light oils they found that about 20﹪ were good candidates for propagating full in situ combustion. This suggests that perhaps a majority of light oils will sustain only low temperature oxidation (LTO). Thus, when the primary objective is only to generate nitrogen and carbon dioxide in situ, then a less intensive oxidation process, without combustion, is sufficient. The focus is therefore on a spontaneous LTO process, which can be applied in all light oil reservoirs with sufficiently high reactivity to react with (and consume) oxygen in the injected air.

Two Step Process for the Growth of a Thin Layer of Silicon Dioxide for Tunneling Effect Applications

ABSTRACTIn today's main crystalline silicon (c-Si) applications in MOS (metal-oxide-silicon), MIS (metalinsulator-semiconductor) or SIS (Semiconductor-Insulator-Semiconductor), the growing of the oxide layer plays the main role, dictating the device performances, in particular if it has to be grown by a low temperature process. Of fundamental importance is the SiO2 interface with the c-Si. A very low defect density interface is desirable so that the number of trapping states can be reduced and the devices performance optimised.A two step low temperature oxidation process is proposed. The process consists of growing first a layer of oxide by a wet process and then treating the grown oxide with an oxygen plasma. The oxygen ions from the plasma bombard the oxide causing compaction of the oxide and a decrease in the interface roughness and defect density.Infrared spectroscopy and spectroscopic ellipsometry measurements were performed on the samples to determine the oxide thickness, optical and structural properties. SIS structures were built and capacitance measurements were performed under dark and illuminated conditions from which were inferred the interface defect density and correlated with the oxide growth process.

Oxidation Kinetics of North Sea Light Crude Oils at Reservoir Temperature

Air injection into deep, light oil reservoirs is a potential technique for large scale improved oil recovery. The feasibility of an air injection process relies on complete consumption of oxygen from the injected air to achieve a nitrogen flood. In this study, the oxidation kinetics of light crude oils was investigated at typical reservoir temperatures (90–140°C), and high pressures. The oxygen consumption rate was measured from the reduction in the oxygen partial pressure using a small batch reactor (SBR). Measured pressure data for different crude oils was used to establish a simple reaction rate model of acceptable accuracy for reservoir simulation. At the relatively low reservoir temperature of the experiments, the main gaseous product from the reaction was carbon dioxide. Additional experiments were performed in a high pressure oxidation tube (0.1 m diameter and 1.25 m long). The oxidation rate data obtained was used to validate the reaction rate model. The low temperature oxidation (LTO) scheme and its implications for the Air Injection LTO Process are discussed.

X-ray photoelectron spectroscopy study of low-temperature molybdenum oxidation process

The low-temperature oxidation during deposition by evaporation of molybdenum thin films has been investigated. Analysis by x-ray photoelectron spectroscopy and x-ray diffraction reveals that small differences in the substrate temperature during deposition may give rise to important changes in the final composition and structure of the molybdenum oxide. Changes in binding energy and line shape of the Mo 3d5/2−Mo 3d3/2 doublet attributed to oxygen incorporation have been studied. Two principal steps can be distinguished, with a transition temperature of ∼310 °C. Up to substrate temperatures of ∼310 °C, the superficial Mo remains almost unaffected, with some oxygen dissolved. At ∼310 °C, mixing of Mo0 metal and molybdenum oxide (Moδ+0<δ<4) clusters or islands is observed. Finally, above this temperature a surface layer of molybdenum oxide, Mo6+, is formed. In addition, an abrupt change in d100 interplanar parameter of Mo is observed.

Oxidation study of Co/Cu multilayers by resonant X-ray reflectivity

The outer oxide layer of magnetron-sputtered Co/Cu multilayers has been studied by means of resonant and classical X-ray reflectometry. The variation of the oxide layer thickness has been monitored versus the upper-Co layer thickness. The surface oxidation seems to stop at the Co/Cu interface and hence no Cu oxide is produced. The contact potential at the Co/Cu interface may be at the basis of this effect. In addition, the dependence of the oxide layer roughness with its thickness has been studied, varying the roughness inversely to the oxide layer thickness, something which results comprehensible on the basis of the low temperature oxidation process.

Precipitation of As in thermally oxidized ion-implanted Si crystals

The As precipitation occurring during thermal oxidation of As implanted Si crystals has been studied by extended x-ray absorption fine structure measurements. (100) Si wafers, implanted with 3×1015/cm2 and 3×1016/cm2 As+ ions at an energy of 70 keV, were oxidized either in H2O ambient (wet) at 920 °C or O2 (dry) at 1100 °C. Precipitation of monoclinic SiAs occurs at the SiO2/Si interface for low temperature oxidation processes. In the case of 3×1016 As/cm2, about 90% of the As forms SiAs precipitates, while for the lower dose a mixing of precipitates and As in substitutional-like sites is observed. On the other hand, when the high temperature oxidation is performed, most of the As (up to 90% for the 3×1015 As/cm2 sample) is found in a substitutional-like configuration.

Oxidation treatment of a sulphide-bearing scrubber dust from the Mansfeld Region, Germany: Organic and inorganic phase changes and multi-element partition coefficients between liquid and solid phases

A low temperature oxidation process has been developed to separate Pb from Zn in a complexly contaminated scrubber dust. The material consists of approx. 50% galena (PbS) and sphalerite/wurtzite (ZnS), and also an amorphous component, a variety of different silicate and carbonate phases as well as naturally-occurring radionuclides and oil and grease. The two-stage process consists of a grinding stage followed by placing this material into a hydrogen peroxide solution. The resultant oxidation reaction is violently exothermic — the temperature rises spontaneously to approx. 96°C. The amount of solid phases remaining after completion of the reaction was reduced by approx. 40% and consisted largely of insoluble lead sulphate which retained the bulk of the radionuclides. Almost all of the zinc was placed into solution by this process as was the Cd, Re and Cu. The total PAH content of the original Theisenschlamm (468 ppm) was reduced to 11.25 ppm in the residual sediment and the PCDD/PCDF concentrations were reduced by approx. 40%. The radionuclides are almost totally concentrated in the solid phase. The method shows considerable promise as a separation technique for very fine-grained sulphide-bearing residues.

Flare Pit Waste Remediation By Low Temperature Oxidation

Abstract Remediation of contaminated soil at oilfield sites, particularly flare pit residues, has become a focus of increasingly stringent environmental legislation. The remains of these earthen pits comprise solids mixed with hydrocarbons, water, salts and metals. Current remediation techniques, such as biological or thermal treatments, may have limited applicability in highly contaminated sites (e.g., land-farming) or may be excessively expensive (e.g., pyrolysis). This paper describes a cost-effective method for treating contaminated soil "in situ" using low temperature oxidation (LTO) to convert the hydrocarbons to inert coke. The method was applied in a laboratory reactor to contaminated soil taken from a flare pit in Northwestern Alberta. The contaminated soil was oxidized with air at temperatures between 150 and 170? C for three weeks. Extractable hydrocarbon levels were reduced to less than 0.1% mass. Bioassays (14-day earthworm survival tests) showed that the toxicity associated with hydrocarbons was eliminated. Following LTO, remaining salts and metals were successfully removed by leaching with water. A conceptual design for application of LTO in the field is also presented in this paper. Introduction Contamination of soils by hydrocarbons, metals and salts is a hazard at many oilfield, well, and processing sites. Unlined pits used for holding drilling fluids or flare pits are examples of these sources of contamination. Although such practices are no longer acceptable in the oil and gas industry, many of these contaminated pits still exist, some dating back many years. Recently, the Government of Alberta, through the Alberta Energy Utilities Board (AEUB), issued new requirements for the remediation of such unlined earthen pits(1). Land-farming (i.e., the process of spreading contaminated waste on the surface of fields so that concentrations remain low and gradually working it into the soil to promote biodegradation) is often the preferred remediation method because of its simplicity. However, many pits have high hydrocarbon and salt concentrations that preclude the feasibility of land-farming. There is a need to develop cost-effective, alternative techniques for remediation of contaminated soil. One such possible technique is the application of a Low Temperature Oxidation (LTO) process. This involves the injection of air or an oxygen-containing gas at moderate temperatures (150 - 200 ° C) in the contaminated soil to convert the hydrocarbon contaminants into coke according to the following model: Equation (available in full paper) Coke, the dominant product, is an inert, immobile, and nontoxic substance consisting mostly of carbon. Low temperature oxidation of a wide variety of hydrocarbons (including aviation kerosene and bitumen) in the presence of core material and water has been investigated previously using plug flow reactors. For Athabasca bitumen, it was found that more than 80% of the initial oil was converted into coke after only a few hours of injecting oxygen/nitrogen mixtures with less than 10% oxygen content at temperatures ranging from 150 to 275 ° C(2). These results suggest that the conversion to coke can be further augmented, and the temperature requirements reduced, by injecting gas with a higher oxygen content or by increasing the reaction time.

Emission Spectra and Oxidation Products of Low-Temperature Flames in Flat-Flame Burner.

Low-temperature flames established on a flat-flame burner are investigated using rich diethyl ether/air mixtures to acquire a new knowledge of self-ignition processes. Temperature, chemical species and emission spectra are measured along with the burner axis. A series of cool flame, blue flame and a bright yellow column appears in a case of equivalence ratio 2.0. A dark zone demarcates between the blue flame and yellow column. Fuel enrichment causes a yellow column disappearance. Onset temperatures of cool and blue flame are 400 and 850 K respectively, which are independent to the equivalence ratios. Blue flame emission spectra contain an emission ranging from 720 to 810 nm in wave length, which can be estimated to be the H2O spectrum. Yellow column spectra are similar to that of black bodies; the emission probably from solid carbon particles therewith. The dark zone has similar near-infrared emission spectra and intensity to the blue flame though the visible emission is quite weak. The appearance of blue flame is closely related to an intensive decomposition of fuel, i.e., only when the decomposition reactions complete in the blue flame, the low-temperature oxidation process is raised to the final hot-flame stage.

Possible explanation of the contradictory results on the porous silicon photoluminescence evolution after low temperature treatments

The modification of the porous silicon photoluminescence (PL) after a low level thermal oxidation at 300°C is studied by analyzing the surface chemical composition using infra-red spectroscopy. The amount of oxide is determined quantitatively. Two different porosities of porous silicon are studied: 70% and 80%. In the case of the 70% porosity porous layer, this thermal process leads to an important PL enhancement in the early stages of oxygen incorporation followed by a degradation at higher oxidation levels whereas the photoluminescence of the 80% porosity porous layer only decreases. It is shown how this new analysis can account for the different and often contradictory results on the PL behavior under various low-temperature oxidation processes.

Characterization of Interface Electronic Properties of Low-Temperature Ultrathin Oxides and Oxynitrides Formed on Si(111) Surfaces by Contactless Capacitance-Voltage and Photoluminescence Methods

Using contactless capacitance-voltage (C-V), photoluminescence surface state spectroscopy ( PLS3) and X-ray photoelectron spectroscopy techniques, interface electronic properties of ultrathin-insulator films formed on Si (111) surfaces at low temperatures were characterized, paying particular attention to the effect of nitrogen-related plasmas. Hydrogen termination was used as the initial surface treatment. Low-temperature (400° C) thermal oxidation processes produced oxide/Si interfaces with a high-density of interface states that caused limited C-V variation and low PL efficiency. Treatment of low-temperature thermally grown oxides in electron cyclotron resonance (ECR) N2 plasma had no effect on the interface properties. On the other hand, marked enhancement of C-V variation and PL efficiency was achieved at the surface after ECR N2O-plasma oxynitridation at 400° C. The correlation between chemical and electronic properties of the interfaces is discussed.

Contactless capacitance–voltage and photoluminescence characterization of ultrathin oxide–silicon interfaces formed on hydrogen terminated (111) surfaces

Electronic properties of the interfaces between Si and ultrathin (≲10 Å) oxides formed by various low-temperature processes were characterized in contactless fashion, using contactless capacitance–voltage and photoluminescence surface state spectroscopy techniques together with x-ray photoelectron spectroscopy measurement. Hydrogen (H) terminated Si(111) surfaces were used as the initial surface. Ultrathin oxides were formed at low temperatures by chemical oxidation processes (hot HNO3, H2SO4+H2O2), long-time air exposure, and low-temperature oxidation processes below 350 °C. The initial H-terminated surfaces showed presence of Fermi-level pinning at E0=EV+0.65 eV due to high density of amphoteric discrete state probably originating from Si dangling bonds. On the other hand, all the ultrathin oxide–Si interfaces exhibited very limited capacitance variations with voltage at low capacitance levels similar to GaAs metal–insulator–semiconductor systems, and indicated that the Fermi level is pinned near the hybrid orbital charge neutrality level EHO due to presence of interface states with narrow U-shaped continuous distributions. Low-temperature oxidation at 350 °C slightly weakens such pinning. The present work indicates difficulty of realizing unpinned ultrathin oxide–silicon interface by low-temperature processes.

Oxidation of silicon (100): Experimental data versus a unified chemical model

It has been observed in many studies of oxidation kinetics that silicon dioxide growth in the thin regime (<30 nm) is faster than the predictions of the linear-parabolic Deal–Grove relationship. We have developed a conceptual and mathematical model for the thermal oxidation of silicon which is based on an initial dissociative chemisorption of molecular oxygen on the 2×1 silicon (100) surface followed by subsequent diffusion and dissociative reactions of molecular oxygen at the Si/SiO2 interface. This model accounts for current experimental observations on the structural modification of the reconstructed surface to a 1×1 superlattice on limited exposure to molecular oxygen and provides a mechanistic rationale for the self-limiting mechanism of the oxide film at ∼0.6 nm during low-temperature oxidation processes. The rate-equation model, consistent with the proposed reactions at the Si/SiO2 interface, has been refined to give an excellent fit to experimental data within all thickness regimes, but especially in the initial rapid growth regime where the growth rate is proportional to (PO2)1/2. The rate equation reduces to a linear dependence on oxygen partial pressure in the thicker regime, where it predicts classic Deal–Grove behavior. We present the development of the model for oxidations performed between 780–1100 °C consistent with observed oxide growth and film properties. The activation energies for the reaction-controlled regime and the diffusion-controlled regime are consistent with literature data. We observe a sharp transition in a characteristic length parameter at ∼950 °C which may be possibly due to the experimentally observed change in oxide film density. A detailed analysis and explanation is presented.

Low-temperature oxidation kinetics of high-volatile bituminous coal studied by dynamic in situ 9 GHz c.w. e.p.r. spectroscopy

This paper explores the potential of using the Arrhenius diagrammatic analysis of dynamic in situ 9 GHz c.w. e.p.r. data taken in constant time-constant temperature intervals to calculate ‘activation’ energies characterizing stable free radical reactions that contribute to the various stages of the low-temperature oxidation process. The measurements were made on an Alberta foothills (Luscar) h.v. bituminous coal. The most evident thermal feature of these diagrams is the existence of two and three linear regions characteristic of first-order reactions for samples of this coal when subjected to dry air and nitrogen gas flow, respectively. The two regions with positive Arrhenius slopes are similar for both gases. The third region, with a negative Arrhenius slope, appears in the temperature interval 293–323 K and is correlated with the decomposition of peroxygen radicals. The ‘activation’ energies in this region depend on the initial moisture content of the coal samples. The initial positive-slope region has a much larger Arrhenius slope than the second higher-temperature region when the samples are subjected to dry air and nitrogen gas flow. Hydroperoxygen decomposition reactions appear to dominate in the former region, which create other free radicals and produce primarily gaseous oxidation products, whereas the reactions in the latter region produce solid oxidation products with much lower ‘activation’ energy values. It shows that this analysis can be used to provide unambiguous identification of the role that moisture and the presence of oxygen play in determining the sequence of different reactions involving stable free radicals that contribute to the low-temperature oxidation of coal.

The Oxidation Behavior of a Si-Ti-C-O Fiber with a Low Oxygen Content

The Si-Ti-C-O fiber (STC (6)) with a low oxygen content of 6mass% was oxidized at temperatures in the range of 1273 to 1773K in order to measure its oxidation rate and thereby to elucidate its oxidation behavior. These results were compared to those of the commercial fibers having 18 and 13mass% O (STC (18) and STC (13)). The kinetic data of STC (6) were expressed by the contracting-cylinder formula. The activation energies were calculated to be 72 and 430kJ/mol at the low-temperature (T 1473K) regions, respectively. On the other hand, STC (18) and STC (13) had activation energies of oxidation of E=72kJ/mol and E=70kJ/mol within the whole temperature range, respectively. Oxidation of STC (6) with a low oxygen content is considered to be similar in kinetic behavior to that of pure SiC.The oxide film consisted of SiO2 and TiO2; and the former was amorphous during the low-temperature oxidation process tending to be crystallized into cristobalite at higher temperatures. The average size of crystallized β-SiC particles of the interface between the oxide film and unoxidized core fiber was larger than that of core of the fiber. This is because the progress of the crystallization in O2 stream is considered to have been suppressed when the fiber was completely covered with the oxide film. When the fiber was oxidized at 1773K, the interface between the oxide film and the unoxidized core fiber contained many microcracks; moreover, the core fiber lost 75% of its original strength at room temperature. It is quite possible that these defects were produced by a contraction due to transformation of cristobalite (α-β) during cooling from the oxidation temperatures.

The role of oxygen, nitrogen and argon in the low-temperature oxidation of a dried Alberta coal as studied by dynamic in situ 9 GHz CW-e.p.r.

The results are reported of a study of the changes in spin concentration and in the e.p.r. spectral parameters for the free-radical and the Mn 2 + HFS resonances observed in dried high-volatile bituminous Alberta coal samples due to low-temperature oxidation in dry air, nitrogen and argon gas flow in the interval 20–300 °C. The temperature-dependence of the free-radical resonance for air treatment differs significantly from that for nitrogen and argon treatment, but the Mn 2 + resonance is independent of the type of gas up to 190 °C. It is concluded that the low-temperature oxidation processes in coal depend critically on the presence of loosely bound water and oxygen.

The low temperature oxidation process in an Alberta hv bituminous coal as monitored using the 9GHz CW-EPR Mn 2+ hfs spectral parameter

The low temperature oxidation process in dried and moisture saturated samples of an Alberta hv bituminous coal containing ~237 ppm manganese impurity ions subjected to dry air flow has been studied using dynamic in situ 9 GHz continuous wave-electron paramagnetic resonance (CW-EPR) in the temperature interval from 293 to 523 K. It was found that there is a correlation between oxidation, as indicated by an increasing concentration of carbon free radicals, and the presence of moisture and inorganic matter as monitored by the Mn 2+ spin concentration. Arrhenius plots were used to characterize the changes in the spin concentration which were observed during the low temperature oxidation process. It was found that the lower values of the apparent activation energy can be attributed to solid (carbonyl and carboxyl) rather than gaseous oxidation products, and that these activation energy values depend on the sample pretreatment procedure. This kinetic analysis also has provided evidence for the existence of reactions involving changes in the oxidation state of the Mn ions which occur in characteristic temperature intervals.

Effects of predeposition HF/NH4F treatments on the electrical properties of SiO2/Si structures formed by low-temperature plasma-assisted oxidation and deposition processes

Si(100) and (111) wafers were prepared by a standard RCA cleaning process followed by a treatment in HF/NH4F solutions with different pH values. SiO2/Si structures were formed on these surfaces by a two-step low-temperature, 200–300 °C, plasma-assisted oxidation/deposition process. The SiO2/Si(111) interface, subjected to a predeposition rinse in a 40 wt % NH4F solution, displayed a midgap interface trap density Dit of approximately 5×1010 cm−2 eV−1, which is significantly lower than is generally observed on thermally oxidized Si(111). The Dit values increased systematically up to about 2×1011 cm−2 eV−1 as the pH of the HF/NH4F treatment was decreased. For Si(100) wafers, the absolute level, and the energy distribution of Dit within the Si band gap, were essentially independent of the pH of the HF/NH4F treatment. The micromorphology and perfection of the H passivation of the Si surfaces after the HF/NH4F treatment were characterized by Auger electron spectroscopy and low-energy electron diffraction, and were correlated with the electrical properties of the resulting SiO2/Si structure. It was observed for the first time that the Si(100) surface partially reconstructs into a 2×1 structure after a rinse in 40 wt % NH4F that follows a traditional RCA cleaning treatment.

Detection of residual impurities after the alkali metal enhanced oxidation of Si(100) surfaces by SIMS and Hall effect measurements

The ongoing miniaturisation of semiconductor devices with oxide thicknesses of less than 100 Å demands strongly low temperature oxidation processes. A significant reduction of the oxidation temperature to about 700 K has been accomplished by preadsorbed alkali metal layers on semiconductor surfaces, e.g. K on Si(100). The technical application of this process in the production of semiconductor devices such as MOS capacitors depends on the possibility of a complete removal of the alkali metal. So far it could be shown, applying surface sensitive methods such as XPS and AES, that a short anneal to 1000 K results in an alkali metal free surface region. However, depth profiles measured by SIMS clearly prove that these oxide layers contain alkali metal impurities. Upon thermal decomposition of this oxide at about 1273 K the K diffuses even into the Si bulk, based also on SIMS. Consistently an increased charge carrier concentration could be determined by Hall effect measurements. We conclude that the alkali metal induced oxidation of Si is not generally suitable for device applications.