Oxide Desorption Research Articles

(Invited) Atomic Layer Deposition for Epitaxial Oxides on Silicon

Thin films of crystalline oxides integrated with silicon are of interest for novel devices that utilize the rich variety of functional properties of oxide materials. Growth of crystalline oxides on silicon requires careful control of a nanometer thin transition region to avoid deleterious side reactions such as oxidation of the substrate. Submonolayer surface reconstructions of alkaline earth metals such as Sr are key to controlling the interface for successful growth. Previously, this has only been accomplished using molecular beam epitaxy growth methods, but there is interest to create an alternative process based on atomic layer deposition. In this work we demonstrate the successful deposition of the critical (2x1) Sr/Si reconstruction using SrO ALD deposition combined with oxide desorption. The results show that ALD based crystalline oxide heteroepitaxy is promising.

Thermal desorption of Ge native oxides and loss of Ge from the surface

The authors have identified oxidation and desorption processes of Ge native oxide by chemical bonding states measured by X-ray photoemission spectroscopy. Ge oxidation occurs at the temperatures of 450–500 °C in an oxidizing ambient. Ge desorption in nitrogen ambient is observed at the temperatures of 500–550 °C, which is higher than the oxidation temperature by 50 °C. Combined oxidation and desorption processes proceed subsequently and cause a loss of Ge from the surface when Ge is annealed in oxidizing ambient at a temperature higher than desorption temperature. The surface loss is avoided when Ge is annealed with SiO 2 cap layer in an identical annealing condition.

Site-controlled self-assembled InAs quantum dots grown on GaAs substrates

Atomically-flat surfaces are obtained after thin GaAsSb buffer layer growth on GaAssubstrates with regular-distributed nano-holes formed after oxide desorption of the localatomic-force-microscopy anode oxidation. Different from the samples with GaAsSbbuffer layers, increasing surface root-mean-square roughness is observed for theGaAs-buffered samples with increasing GaAs buffer layer thickness. The phenomenon isattributed to the enhanced adatom migration resulting from the incorporation ofSb atoms. By using the substrates with nano-holes after buffer layer growth,site-controlled self-assembled InAs quantum dots (QDs) are observed with thedeposition of a below-critical-thickness InAs coverage of 1.3 monolayer (ML).

Silicon thermal oxidation and its thermal desorption investigated by Si 2p core-level photoemission

Initial thermal oxidation of silicon and thermal desorption of silicon oxide have been investigated by Si 2p core-level photoemission with synchrotron radiation. The surface reaction processes are discussed from the difference in the chemically-shifted components, especially suboxides. On the thermal oxidation, time evolutions of suboxide intensities show a distinct temperature dependence, which is explained by two growth modes for the oxidation: first-order Langmuir-type adsorption mode and two-dimensional island growth mode at oxidation temperatures below and above 650 °C, respectively. On the thermal desorption, the spectrum for the thermal oxide followed by annealing at 1000 °C for 30 s in vacuum, whose thickness is nominally estimated at 3.7 Å, are compared with the 3.3 Å-thickness thermal oxide without the process of the annealing. The intensities of the suboxide components are considerably different between them, which can be explained by the formation and lateral growth of the voids in the annealed oxide layer.

Morphology and bonding states of chemical vapor deposition diamond films nucleation surface

The morphology and chemical bonding of the nucleation surface of polycrystalline diamond films were studied following the removal of the silicon substrate by acid etching and mild plasma cleaning. Atomic force microscopy characterization of this surface revealed a mosaic structure of cracks arranged in hexagonal patterns. These cracks are correlated with etched twin grain boundaries that surrounded hexagonal diamond crystals at the initial growth stages of the diamond films. High resolution electron energy loss spectroscopy studies indicate that mild annealing of the diamond film caused desorption of various oxides exposing a damaged diamond surface.

Ga assisted oxide desorption on GaAs(0 0 1) studied by scanning tunnelling microscopy

Native oxide removal on GaAs wafers under conventional thermal desorption causes severe surface degradation. Recently a new method of Ga assisted oxide removal has reported improved initial surface conditions. A precise dosing of Ga is required to optimise the oxide removal, however the effects of alternate temperatures on the desorption process effects the reaction kinetics. By using selected bias imaging, scanning tunnelling microscopy (STM) can be used to probe the underlying bulk whilst the native oxide is still present. Hence the effects of oxide removal on the surface can be identified during the native oxide desorption. By comparing Ga assisted oxide removal on both vicinal and off cut samples, the Ga adatom kinetics are shown to underpin the oxide removal process and a sample temperature in excess of 500 °C is necessary to optimise the procedure.

Micro-twin Defects in InSb/AlInSb layers grown on (001) GaAs ∼ Application of the [formula omitted]-directional TEM analysis ∼

Methods for the reduction of micro-twin defects in InSb/AlInSb epilayers were investigated by using 〈1̄16〉-directional TEM analysis. The use of a 2°off-axis GaAs (001) substrate which has been known as an effective way to reduce micro-twin densities was combined with another approach, the use of an As2 beam for oxide desorption from a GaAs substrate surface. The combined method yields a micro-twin density of 3.3×102/cm, which is 3.0 times smaller than for a similar previous method with an Sb2 flux. This study also demonstrates the usefulness of 〈1̄16〉-directional TEM analysis for investigating micro-twin defects in (001) thin films.

Optimization of MBE-grown GaSb buffer layers and surface effects of antimony stabilization flux

Growth optimization of unintentionally doped GaSb buffer layers on (1 0 0) GaSb substrates by molecular beam epitaxy is reported. Several growth parameters were varied to determine the optimal oxide desorption and GaSb growth conditions, as well as to investigate the conditions under which antimony condenses on the wafer surface. Variation of group-V stabilization flux level, timing of flux application during wafer heating and cooling, desorption anneal temperature, and GaSb growth temperature were investigated. Epilayer quality was gauged by optical microscopy, atomic force microscopy, 77 K photoluminescence, and X-ray photoelectron spectroscopy. Results indicate that growth of GaSb at 520 ∘ C , the highest temperature used here, combined with appropriate heating and cooling steps produces the highest quality bulk GaSb.

Role of nitrous acid decomposition in absorber and bleacher in nitric acid plant

The stringent statutory regulation for clean environment makes the absorber and the bleacher in the nitric acid plant two very important equipments. The NOx finally released to the atmosphere from the absorber, is based on the mathematical calculations of the nitric acid and nitrous acid concentration profiles along the length of the absorber. The present work focuses on the role of HNO 2 decomposition in the absorber and the bleacher in the nitric acid plant. The decomposition of HNO 2 is a heterogeneous reaction resulting into the formation of nitric acid and desorption of nitric oxide. At high nitric acid concentration (40–50 wt.%), the reversibility of HNO 2 decomposition due to NO oxidation with nitric acid is important. The model for bleacher column is presented, which takes into account the de-colorization of the product acid from the absorption column and makes it free from the nitrous acid. The model for the bleacher also takes into account the decomposition and oxidation of HNO 2 with dissolved oxygen. The models proposed for absorber and bleacher are validated from the data collected from a mono-pressure nitric acid plant as well as the data present in open literature.

NOx Reduction and Desorption Studies Under Electric Discharge Plasma Using a Simulated Gas Mixture: A Case Study on the Effect of Corona Electrodes

In this study, reduction and desorption of oxides of nitrogen (NOx) were conducted using an electrical discharge plasma technique. The study was carried out using a simulated gas mixture to explore the possibility of re-generation of used adsorbents by a nonthermal plasma desorption technique. Three different types of corona electrodes, namely, pipe, helical wire, and straight wire, were used for analyzing their effectiveness in NOx reduction/desorption. The pipe-type corona electrode exhibited a nitric oxide (NO) conversion of 50%, which is 1.5 times that of the straight-wire-type electrode at an energy density of 175 J/L. The helical-wire-type corona electrode exhibited a NOx desorption efficiency almost 4 times that of the pipe-type electrode, indicating the possibility that corona-generated species play a crucial role in desorption.

Topography of epitaxial GaAs surfaces for growth

The topography and surface roughness of (100) GaAs substrates and buffers after different preparation procedures were determined from atomic force microscopy (AFM) measurements. In order to characterize the topography over a wide range of length scales, multiple large 5×5 μm2 AFM scans were acquired for each sample. These scans were analyzed both by histogramming the distribution of pixel heights and by finding the rms roughness at length scales from 10 nm to 5 μm using a tiling analysis. The influence of substrate aging and chemical etching on buffers grown by molecular beam epitaxy was studied, as was the effect of different buffer growth procedures. Immediately after thermal desorption of the surface oxide, all wafers were extremely rough, as expected, with wafers etched in HCl:H20 (1:1) somewhat smoother than untreated epiready wafers, while wafers etched in H2O2:NH4OH:H2O (3:1:10) were rougher. After as little as 100 nm of buffer growth, however, there was no significant difference in roughness of etched or as-received substrates, and all samples were quite smooth with a rms roughness around 0.27 nm. The buffer growth conditions were found to significantly affect surface roughness. Interrupting the supply of Ga at the start of the growth enhanced mounding on the wafer surface, while postgrowth annealing at the growth temperature reduced the surface roughness and changed the characteristic topography of the surface.

Metalorganic Vapor-Phase Epitaxial Growth of (211)B CdTe on (211) Si Using Ge Interfacial Layer

We have achieved metalorganic vapor-phase epitaxial growth of (211)B CdTe on Si without the requirement of a pregrowth high-temperature oxide desorption step. This was achieved by growing a thin Ge film on the starting (211) Si substrates. To get (211)B CdTe orientation, the Ge surface was exposed to As prior to the start of CdTe growth. A thin ZnTe interlayer between Ge and CdTe has been shown to improve the CdTe surface morphology.

Roles of Mineral Matter in the Early Stages of Coal Combustion

In a recent study, we discovered that oxygen from the gas phase, organic portions of the coal, and minerals in the coal have profound influence on the formation and desorption of stable surface oxides in the early stages of coal combustion. In an attempt to isolate the effects of minerals, demineralized coals (DMC) are oxidized in O{sub 2} with a contact time less than 1 s, and the amount and strength of stable surface oxides are characterized by temperature-programmed desorption (TPD) up to 1650{sup o}C. Young chars derived from both demineralized lignite and bituminous coals show low and flat TPD profiles over a wide temperature range, signifying the minerals' catalytic activities in forming stable surface oxides for both coals. Indeed, the oxidation rates of chars from both bituminous coals and lignite, estimated based on the O{sub 2} concentrations entering and exiting the Al{sub 2}O{sub 3} reactor, were higher than their DMC counterparts. Moreover, graphite, containing no minerals and organically bound oxygen, has an even lower oxidation rate. Similar to those for the raw coals, the combined oxygen balance and elemental analysis of chars from DMC suggests that the oxygen in the organic portion of the lignite activates oxygen turnover andmore » carbon oxidation during its combustion; neither chars from raw nor demineralized bituminous coals possess these properties. X-ray photoelectron spectroscopy (XPS) of raw and demineralized bituminous coals and their char show peaks at around 532.0 eV in the O(1s) difference spectrum, suggesting the possible existence of intercalated stable surface oxides. 35 refs., 7 figs., 4 tabs.« less

New process results in smoother indium antimonide substrates

The high optical transparency of indium antimonide (InSb) substrates makes them attractive for IR focal-plane arrays (IRFPAs) and detectors, free-space communications, transistors, and integrated optoelectronics.1–3 A large-diameter crystalline InSb surface can accommodate new and bigger advanced IRFPA designs. However, the 150mm diameter of these antimonides poses unique surface-polishing challenges to substrate manufacturers. To improve the resolution and sensitivity of highperformance IRFPA imaging systems in the 1–5.4μm region (77◦K), the substrates’ surface must meet or surpass stringent demands. In particular, for detector-fabrication processes requiring epitaxy growth for the device layer, the starting InSb surface must be ultrasmooth, less than one atomic layer in roughness. It should also desorb the Sb and In surface oxides in an abrupt and fast outgassing process.4 A consistent and easily desorbable surface oxide helps improve advanced-device epilayer yield and performance in Sb-based IRFPA manufacturing. The substrate’s chemomechanical-polish (CMP) history is a primary factor in InSb surface smoothness and oxide desorption. We wanted to consistently produce surfaces suitable for recent epitaxy processes with stringent substrate-surface specifications. To do so, we analyzed the substrate-surface orientation, atomic roughness, oxide desorption, and crystallinity as a function of CMP process. Our resulting CMP process for largerdiameter InSb single-crystal surfaces has produced a smoother InSb starting surface and abrupt oxide-desorption characteristics suitable for device-layer molecular-beam-epitaxy (MBE) material growth.5 We applied a standard semiconductor-grade CMP to two groups of antimonide substrates separated from the same boule. We used a proprietary CMP process for group A. For group Figure 1. Thermal x-ray photoelectron spectroscopy analysis spectra in electron volts (eV) for the antimonide (Sb) 3d (electron orbital) binding-energy peaks of both Group A and B chemomechanical polish surfaces at 400◦C shows the Sb oxide removed for Group B polished surfaces. CMP: Chemomechanical polish.

Ethylene oxide removal by sorption on aluminium oxide

The dynamics of ethylene oxide sorption and desorption on Al2O3 sorbent were investigated. The investigations of ethylene oxide sorption on Al2O3 show that significant sorption appeared above 125?C. The removal of sorbed ethylene oxide from Al2O3 was achieved by continuous increasing of the temperature up to 450?C in air stream. The analysis of desorbed products show that 90% of adsorbed ethylene oxide is converted to CO2 and the rest consists of the three derivatives of ethylene oxide. The exact composition of desorbed organic products will be determined in further investigation. The desorption temperature profiles point out the presence of two exothermic picks, as was confirmed by detection of CO2 and derivates of ethylene oxide at these temperatures. Investigation of textural characteristics and thermal stability of Al2O3 sorbent show that there are no changes of any characteristics of Al2O3 in sorption/desorption operating temperatures regimes. Only at 700?C the specific surface area of Al2O3 decreases of about 10%. This indicates that the investigated Al2O3 is convenient material for removal of ethylene oxide by sorption.

Characterization of Early-stage Coal Oxidation by Temperature-programmed Desorption

To obtain representative temperature-programmed desorption (TPD) profiles of young oxidized chars up to 1650 °C with minimal reactor wall interferences, the chemistry and physics of four ceramic materials has been critically reviewed. A two-staged experimental apparatus is then uniquely designed to produce chars in an Al2O3 flow reactor with 1−21% O2 followed by in situ TPD with a SiC tube. Comparison of TPD profiles of oxidized chars with those from pyrolyzed chars and ashes suggests early-stage char oxidation is profoundly influenced by oxygen from three sources: organics oxygen, mineral matters, and gas phase O2. Young chars oxidized at 1000 °C with less than 0.3 s residence time shows CO desorption peaks during TPD at three distinct temperatures: 730, 1280, and 1560 °C. The peaks at 730 °C are mainly caused by incomplete devolatilization. The peaks at 1280 °C mainly represent desorption of stable surface oxides and incomplete devolatilization. Increasing the gas phase oxidants notably increases the amount of stable surface oxides. The broad peaks between 1400 and 1650 °C are attributed to the reactions of oxidants decomposed from minerals and carbon in the char or SiC tube. Gas-phase oxygen shifts these reactions to lower temperatures. Detailed oxygen balance based on the CO and CO2 yields and elemental compositions of both pyrolysis and oxidized chars reveals that oxygen uptakes are very high, +0.056 mg O per mg of carbon, in chars derived from bituminous coal, whereas lignite chars show negative oxygen uptake, −0.020 mg O per mg of carbon, in char. Indeed, lignite char seems to possess little amount of stable surface oxides other than those contributed by the minerals. The extensive emissions of CO from lignite chars during TPD seem to suggest that either O2 or minerals promotes the oxygen transfer on char surface and subsequent carbon oxidation.

Growth of ordered SrO layers on Si(100) using metal-organic surface reactions

The formation of ordered alkaline earth metal-oxide layers on Si(100) through metal-organic surface reactions has been investigated. Strontium oxide layers were deposited on a thin (1.0±0.2nm) SiO2 layer grown on Si(100) using a saturation coverage of the β-diketonate precursor bis(2,2,6,6-tetramethyl-3,5-heptanedionato)strontium [Sr(thd)2] followed by reaction with H2O. Oxide desorption at high temperatures (>800°C) leads to an ordered strontium layer on the silicon surface. Auger electron spectroscopy, x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and low-energy electron diffraction have been used to investigate the progress of the reaction. The data show a Sr∕Si(100) surface with 3× order and a Sr coverage of (0.9±0.1)×1014∕cm2. The observed metal coverage and ordering are explained by the saturation of the adsorbed metal-organic layer due to steric interactions that limit the adsorbed coverage. The results are promising for alternative methods to fabricate epitaxial oxides ...

Interface characteristics of ZrO 2 high-k gate dielectrics on epitaxial Ge capacitor layers after thermal desorption of Ge native oxide and Ge nitridation

The authors report on interface characteristics of ZrO 2 high-k gate dielectrics on Ge epitaxial layers integrated on Si substrates. For surface passivation, thermal desorption of Ge native oxide and Ge nitridation processes is applied to epitaxial Ge capacitor layers. Metal-oxide-semiconductor capacitors exhibit excellent electrical characteristics with a gate leakage current density of 6 × 10 − 7 –1 × 10 − 6 A/cm 2 with a capacitance equivalent thickness of 1.5–1.3 nm at a gate voltage of 1 V. Thermal behavior of Ge native oxide and nitrided Ge layers is correlated with electrical characteristics.

Hydrogen and thermal deoxidations of InSb and GaSb substrates for molecular beam epitaxial growth

Oxide removal processes for InSb(100) and GaSb(100) substrates utilizing exposure to molecular hydrogen at moderate temperatures were studied in an experimental UHV chamber and were compared to the common method of thermal oxide desorption (TOD). Molecular hydrogen cleaning (MHC) deoxidizes the substrate surfaces effectively at temperatures of ∼100 and ∼200°C lower than that in TOD of GaSb and InSb, respectively. Closely related hydrogen based procedures for the cleaning of these materials before molecular beam epitaxial growth were developed. In these procedures a very low flux of atomic hydrogen was used in order to achieve short, yet mild, deoxidation of the substrates both with and without an Sb flux. These mild atomic hydrogen cleaning (MAHC) methods give smoother, cleaner, and more perfect InSb and GaSb surfaces after a shorter cleaning cycle at temperatures of 100–200°C lower than that in TOD. An Sb flux is not necessary in MAHC, but if used it gives a smoother substrate surface, and a well ordered...

UV laser desorption of nitric oxide from semiconducting C60/Cu(111)

The desorption of NO from a well-characterized, epitaxially grown semiconducting C60 surface is reported. Two different channels are identified in the laser desorption. Both channels yield a comparably high desorption cross section of σ1=7.0×10-17 cm2 and σ2=5.5×10-17 cm2 for the first and second channel, respectively. The laser desorbed NO molecules are detected with rovibrational state selectivity by (1+1) REMPI in the \(A^2\Sigma^+ \to{X}^2 \Pi\,\gamma\)-bands. In the first channel the desorbing molecules are highly excited with an average kinetic energy of 〈Ekin〉=174 meV. The rotational population distribution can be fitted by a rotational temperature of Trot=800 K. A rotational–translational coupling is observed, with velocities ranging from 1000 m/s for low to 1300 m/s for high rotational states. The vibrationally excited population is estimated to be less than 1% of the ground state. The second channel yields less excited molecules and an almost Boltzmann distributed rotational population with a temperature of Trot=280 K. The apparent velocity distribution derived from the pump-probe delay yields molecules much too slow to be explained by even a thermal desorption. This desorption is probably caused by a long-lived electronic excitation in the substrate for which a lifetime of τ≈160 μs is estimated.