Dry Thermal Oxidation Research Articles

Valence-Band and Chemical-State Analyses of Zr and O in Thermally Grown Thin Zirconium-Oxide Films: An XPS Study

In situ X-ray photoelectron spectroscopy (XPS) was applied to investigate the valence-band spectrum and to analyze the local chemical states of Zr and O in thin (thickness <10 nm) oxide films, grown on bare single-crystalline Zr surfaces by dry thermal oxidation in the temperature range of T = 300–450 K. The oxide films grown at T ≤ 400 K are predominantly amorphous. The measured upper valence band (UVB) region of the grown oxide films shows pronounced changes in shape with increasing oxidation temperature, which can be attributed to the gradual formation of a tetragonal ZrO2-like phase at oxidation temperatures T > 400 K. The resolved Zr 3d5/2 and O 1s photoelectron lines and the Zr M45N1N23, Zr M45N23N23, Zr M45N23V, and O KL23L23 Auger transitions were combined to construct Wagner plots for Zr and O in the oxide films. The observed decreases of the Zr and O Auger-parameter values with increasing oxidation temperature evidence a lowering of the electronic polarizability around core-ionized Zr and O atoms. It was concluded that the amorphous-to-crystalline transition of the oxide films with increasing oxidation temperature is accompanied with an increase in the Zr–O bond ionicity and changes in the first coordination spheres of both Zr and O. The results obtained for the amorphous-to-crystalline transition of zirconium-oxide films were compared with those for aluminum-oxide films.

Polysilicon nanogap structure development using size expansion technique

PurposeNanobiosensors based on nanogap capacitor are widely used for measuring dielectric properties of DNA, protein and biomolecule. The purpose of this paper is to report on the fabrication and characterization polysilicon nanogap patterning using novelties technique.Design/methodology/approachOverall, the polysilicon nanogap pattern was fabricated based on conventional lithographic techniques. For size expansion technique, by employing simple dry thermal oxidation, the couple of nanogap pattern has been expanded to lowest nanogap value. The progress of nanogap pattern expansion was verified by using scanning electron microscopy (SEM). Conductivity, resistivity, and capacitance test were performed to characterize and to measure electrical behavior of full device fabrication.FindingsSEM characterization emphasis on the expansion of polysilicon nanogap pattern increasing with respect to oxidation time. Electrical characterization shows that nanogap enhanced the sensitivity of the device at the value of nano ampere of current.Originality/valueThese simple least‐cost method does not require complicated nanolithography method of fabrication but still possible to serve as biomolecular junction. This approach can be applied extensively to different design of nanogap structure down to several nanometer levels of dimensions. A method of preparing a nanogap electrode according to the present innovation has an advantage of providing active surface that can be easily modified for immobilizations of biomolecules.

Optical and electrical properties of thermally oxidized bismuth thin films

Bismuth trioxide (Bi 2O 3) thin films were prepared by dry thermal oxidation of metallic bismuth films deposited by vacuum evaporation. The oxidation process of Bi films consists of a heating from the room temperature to an oxidation temperature ( T o = 673 K), with a temperature rate of 8 K/min; an annealing for 1 h at oxidation temperature and, finally, a cooling to room temperature. The optical transmission and reflection spectra of the films were studied in spectral domains ranged between 300 nm and 1700 nm, for the transmission coefficient, and between 380 nm and 1050 nm for the reflection coefficient, respectively. The thin-film surface structures of the metal/oxide/metal type were used for the study of the static current–voltage ( I– U) characteristics. The temperature of the substrate during bismuth deposition strongly influences both the optical and the electrical properties of the oxidized films. For lower values of intensity of electric field ( ξ < 5 × 10 4V/cm), I– U characteristics are ohmic.

Nucleation properties of undercooled silicon at various substrates

When solidifying molten silicon in contact with a foreign substrate, the interfacial interaction between silicon and the substrate material can affect the nucleation behavior and therefore the final grain structure. This work illuminates the nucleation properties of different materials using differential scanning calorimetry to measure the undercooling below the melting temperature of molten silicon in contact with silicon oxides and nitrides. Both dry and wet thermal oxides show greater than 120 °C of undercooling before nucleation occurs, while silicon nitride consistently shows less than 17 °C. Variability in measured undercooling was minimized by improving coating uniformity but is not necessarily affected by overall sample deformation. A double-layer coating of silicon nitride over dry oxide reduced the dry oxide undercooling to &amp;lt;56 °C due to inter-layer thermal stresses, which can be minimized by reducing the outer layer of silicon nitride to &amp;lt;40 nm. The double-layer SiNx-SiO2 coating produced larger grains than the SiNx coating alone, when applied to a wafer recrystallization process.

Rear side passivation of PERC‐type solar cells by wet oxides grown from purified steam

AbstractTo considerably improve the conversion efficiency of industrial solar cells, the effective passivation of the rear surface is a prerequisite. Thermal grown silicon oxides provide an excellent level of surface passivation on lowly doped p‐type surfaces. However, dry thermal oxidation processes require relatively high temperatures (∼1000°C) and, due to the low growth rate, long process times. To decrease both oxidation temperature and process time, the dry oxidation process can be replaced by a wet oxidation. The most common way to introduce high purity water vapor into the oxidation tube is the pyrolytic generation from high purity gases (H2 and O2). A more easy and cost effective option for the supply of water vapor that was applied within this work is the direct purification of steam. The passivation quality of dry and wet oxides, the latter grown from pyrolytic generated water vapor and purified steam was compared within this work. The passivation quality obtained for the wet oxides grown from purified steam was found to be comparable to those oxides grown from pyrolytic generated water vapor. On laser fired contacts (LFC) solar cells conversion efficiencies well above 20% could be achieved independent of the oxide that was applied for the rear side passivation. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Fabrication and Characterization of Axially Doped Silicon Nanowire Tunnel Field-Effect Transistors

Tunnel field-effect transistors were fabricated from axially doped silicon nanowire p-n junctions grown via the vapor-liquid-solid method. Following dry thermal oxidation to form a gate dielectric shell, the nanowires have a p-n-n(+) doping profile with an abrupt n-n(+) junction, which was revealed by scanning capacitance microscopy. The lightly doped n-segment can be inverted to p(+) by modulating the top gate bias, thus forming an abrupt gated p(+)-n(+) junction. A band-to-band tunneling current flows through the electrostatically doped p(+)-n(+) junction when it is reverse biased. Current-voltage measurements performed from 375 down to 4.2 K show two different regimes of tunneling current at high and low temperatures, indicating that there are both direct band-to-band and trap-assisted tunneling paths.

Experimental Study on the Subthreshold Swing of Silicon Nanowire Transistors

SOI based wrap-gate silicon nanowire FETs are fabricated through electron beam lithography and wet etching. Dry thermal oxidation is used to further reduce the patterned fins cross section and transfer them into nanowires. Silicon nanowire FETs with different nanowire widths varying from 60 nm to 200 nm are fabricated and the number of the nanowires contained in a channel is also varied. The on-current (I(ON)) and off-current (I(OFF)) of the fabricated silicon nanowire FET are 0.59 microA and 0.19 nA respectively. The subthreshold swing (SS) and the drain induced barrier lowering are 580 mV/dec and 149 mV/V respectively due to the 30 nm thick gate oxide and 10(15) cm(-3) lightly doped silicon nanowire channel. The nanowire width dependence of SS is shown and attributed to the fact that the side-gate parts of a wrap gate play a more effectual role as the nanowires in a channel get narrower. It seems the nanowire number in a channel has no effect on SS because the side-gate parts fill in the space between two adjacent nanowires.

Defects in SiO2/Si Structures Formed by Dry Thermal Oxidation of RF Hydrogen Plasma Cleaned Si

The present paper reports characterization of the defects in ultrathin (∼10 nm) oxides grown by low-temperature (850°C) thermal oxidation of hydrogen plasma hydrogenated (100)Si and (111)Si substrates. Electrically active defects, studied by analyzing the frequency dispersion of the capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics are dominated by distinct defects, related to interface traps with different localized energy levels in the Si bandgap, border traps and bulk Si traps. Precursors of the trapping centers are defects in a thin, less dense Si surface region containing voids, which is formed during hydrogenation and is incorporated into the growing oxide layer. Oxidation-induced stress level, evaluated from ellipsometric and electroreflectance data analysis, is in the order of 108 N/m2.

Doping Dependence of Thermal Oxidation on n-Type 4H-SiC

The doping dependence of dry thermal oxidation rates in n-type 4H-SiC was investigated. The oxidation was performed in the temperature range 1000C to 1200C for samples with nitrogen doping in the range of 6.5e15/cm3 to 9.3e18/cm3, showing a clear doping dependence. Samples with higher doping concentrations displayed higher oxidation rates. The results were interpreted using a modified Deal-Grove model. Linear and parabolic rate constants and activation energies were extracted. Increasing nitrogen led to an increase in linear rate constant pre-exponential factor from 10-6m/s to 10-2m/s and the parabolic rate constant pre-exponential factor from 10e9m2/s to 10e6m2/s. The increase in linear rate constant was attributed to defects from doping-induced lattice mismatch, which tend to be more reactive than bulk crystal regions. The increase in the diffusion-limited parabolic rate constant was attributed to degradation in oxide quality originating from the doping-induced lattice mismatch. This degradation was confirmed by the observation of a decrease in optical density of the grown oxide films from 1.4 to 1.24. The linear activation energy varied from 1.6eV to 2.8eV, while the parabolic activation energy varied from 2.7eV to 3.3eV, increasing with doping concentration. These increased activation energies were attributed to higher nitrogen content, leading to an increase in effective bond energy stemming from the difference in C-Si (2.82eV) and Si-N (4.26eV) binding energies. This work provides crucial information in the engineering of SiO2 dielectrics for SiC MOS structures, which typically involve regions of very different doping concentrations, and suggests that thermal oxidation at high doping concentrations in SiC may be defect mediated.

Novel E-Mode GaN-on-Si MOSHEMT Using a Selective Thermal Oxidation

A novel normally-off AIN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMT) on 100-mm Si substrates for high-power applications is demonstrated for the first time by means of a selective thermal oxidation of AIN. The formation of a high-quality insulating AION layer resulting from the dry thermal oxidation of AIN at 900 °C in oxygen has been identified by transmission electron microscopy and X-ray photoelectron spectroscopy. The AIN thermal oxidation appears to be highly selective toward the SiN cap layer allowing the local depletion of the 2-D electron gas (self-aligned to the gate) and thus the achievement of normally-off operation. Threshold voltage (VT) of +0.8 V and drain leakage current at VGS = 0 V well below 1 μA/mm are obtained reproducibly over the wafer. The comparison of the fabricated MOSHEMTs with the control sample (identical but nonoxidized) reveals a drastic shift of VT toward positive values and three to four orders of magnitude drain leakage current reduction.

Electrical Characteristics of MOSFETs Using 3C-SiC with Buried Insulating Layer

Metal oxide semiconductor field effect transistors (MOSFETs) using SiC on insulator (SiC-OI) substrate with the structure of 3C-SiC (100)/SiO2/Si have been fabricated. SiC-OI substrates with SiC thicknesses of 100 nm and 600 nm are employed as starting materials and aluminum ions are implanted for p-regions or channel regions with a multi-implantation technique. Afterward, to form the source and drain regions, phosphorus ions are implanted. The gate oxide layer is grown in dry thermal oxidation, followed by post-oxidation annealing. Nickel is used as a contact material for the source and drain region, and aluminum is used for the gate material. From Id-Vd characteristics, 600 nm SiC-OI MOSFET is superior to 100 nm SiC-OI MOSFET. It is might that the crystalline quality of surface SiC layers affects the performance of MOSFET. SiC-OI MOSFET is operated successfully for the first time.

Low-temperature oxidation of SiC surfaces by supercritical water oxidation

The oxidation process on silicon carbide (SiC) surfaces is important for wide bandgap power semiconductor devices. We investigated SiC oxidation using supercritical water (SCW) at high pressure and temperature and found that a SiC surface can be easily oxidized at low temperature. The oxidation rate is 10 nm/min at 400 °C and 25 MPa, equal to that of conventional thermal dry oxidation at 1200 °C. Low-temperature oxidation should contribute to improved performance in future SiC devices. Moreover, we found that SCW oxidation at 400 °C forms a much smoother SiO 2/SiC interface than that obtained by conventional thermal dry oxidation. A higher oxidation rate and smaller microroughness are achieved at a lower oxidation temperature owing to the high density of oxidizers under SCW conditions.

Effect of Postoxidation Annealing on High Temperature Grown SiO2 / 4H-SiC Interfaces

was grown by dry thermal oxidation (at 1175, 1300, or ) on n-type 4H-SiC substrates. The samples were prepared by subsequently exposing the grown film on 4H-SiC to postoxidation annealing (POA) treatment using nitric oxide (NO) gas. The interfaces were characterized by high frequency capacitance–voltage measurements, X-ray photoelectron spectroscopy (XPS), and ellipsometry. The interface trap density of the dry oxide grown at was much lower than others. At a higher grown temperature , the electrical and physical properties of the oxide were not improved compared to those oxides grown at . The XPS measurements provided evidence for the presence of intermediate oxidation states of Si oxycarbide in all samples. The areal densities of the intermediate oxidation states affected the interface trap density. The NO POA treatment significantly improved the interface trap density, the near-interface trap density, and the effective oxide charge density of the oxides grown at 1175 and . But, this improvement was not observed for the oxide grown at . The electrical properties of the metal-oxide-semiconductor devices fabricated using these oxides have also been discussed in terms of the oxide chemical compositions, which were determined by XPS and an oxide etching test.

The effect of substrate material on nucleation behavior of molten silicon for photovoltaics

To manufacture crystalline silicon for photovoltaic applications, molten silicon is often solidified while in physical contact with a crucible or substrate, where nucleation of new grains should be minimized. This study aims to determine the propensity of silicon to nucleate in contact with different materials. Differential scanning calorimetry is used to measure the undercooling below the melting temperature of molten silicon in contact with these materials. Dry thermal oxide shows up to 141 °C of undercooling before nucleation occurs, while silicon nitride consistently shows less than 20 °C. Noticeable variation in the level of undercooling may be related to sample deformations or coating thickness. Greater undercooling indicates a lower driving force for the coating material to nucleate heterogeneously, thus promoting growth of larger grains, which will be useful in designing crystal growth processes for photovoltaics.

STRUCTURAL AND OPTICAL PROPERTIES OF LARGE-SCALE ZnO NANOWIRES AND NANOSHEETS PREPARED BY DRY THERMAL OXIDATION

In this work, we report the morphology and optical properties of zinc oxide ( ZnO ) layers prepared by dry thermal oxidation at different annealing conditions. Morphology studies using scanning electron microscope (SEM) show that the amount of nanowires and nanosheets increases with the introduction of a flow of O2 gas. High-resolution X-ray diffraction (HR-XRD) data show that typical polycrystalline ZnO nanostructure layers have been deposited. Near-perfect stoichiometry of Zn and O atom vacancies has been observed from energy dispersion spectroscopy (EDS) spectrum. Photoluminescence (PL) spectra show strong peaks at UV and green regions. An increase in the stoichiometry of ZnO has been achieved with the oxygen gas flow during annealing indicating that deep-level defects represented by interstitial oxygen and antisite oxygen are gas pressure dependent. A single exciton peak with binding energy 60 meV has been observed at room temperature.

Interfacial properties of AlN and oxidized AlN on Si

We report on the characteristics of metal–insulator-semiconductor (MIS) capacitors with aluminum nitride (AlN) as the insulator material. AlN has been grown on (1 1 1) Si by means of molecular beam epitaxy (MBE) and DC magnetron sputtering (SPU). AlN layers have been characterized before and after dry thermal oxidation in O 2. By analyzing changes in morphology and electrical properties, different oxidation mechanisms were identified, due to the crystalline quality difference of the AlN samples. In both cases, oxidation at 1000 °C was beneficial for the electrical characteristics of the MIS structures, presumably due to passivation of atom vacancies. Although AlN was only partially oxidized, the flat-band voltage was reduced and the density of interface traps improved. Dominant conduction mechanism was Poole–Frenkel for the SPU sample, and changed to hopping after oxidation.

Preferential oxidation of stacking faults in epitaxial off-axis (111) 3C-SiC films

The effect of thermal dry oxidation on an off-axis (111) 3C-SiC film have been studied in order to subsequently realize a metal-oxide-semiconductor structure. A morphological characterization of the SiO2 surface, grown at 1200 °C in an O2 flux, pointed out some defect-related effects as a consequence of the preferential oxidation of stacking faults over the (111) 3C-SiC surface. Scanning electron microscopy and atomic force microscopy confirmed such a hypothesis. Stacking faults are seen as promoters of a local polarity inversion in (111) 3C-SiC, from Si- to C-terminated surface, resulting in a higher oxidation rate as compared to defect-free zones.

Microstructural Evolution of Silica on Single‐Crystal Silicon Carbide. Part I: Devitrification and Oxidation Rates

Dry thermal oxidation of single‐crystal silicon carbide (6H‐SiC) at 1400°C in low‐water oxygen using an alumina tube furnace initially yields a predominantly vitreous oxide scale. After 28 h, approximately one quarter of the oxide scale's surface becomes crystalline, with disk‐like cristobalite aggregates (radialites) statistically distributed within the vitreous matrix. Crystalline areas were found to be thinner than vitreous regions using optical microscopy and atomic force microscopy/hydrofluoric acid (AFM/HF)‐etching analysis, providing evidence for different oxidation rates of SiC covered by vitreous silica and cristobalite. As the bulk oxide scale continues to grow during devitrification, the radialites assume their characteristic morphology with a deepened center. Line profiles of the oxide scale's thickness across radialites obtained from AFM/HF etching were used to determine the oxidation rate of SiC covered by crystalline silica and the crystallisation rate applying a two‐stage parabolic equation. As a result, it was found that the parabolic rate‐constant Bvitreous is ≈4.2 times larger than the corresponding rate‐constant in crystalline areas (Bcristobalite), suggesting similar differences in effective oxygen diffusion coefficients. For the crystallization rate ν, we determined a value of 1.5±0.1 μm/h.

Characterization of Plasma Damage in Low-k Films by TVS Measurements

AbstractWe evaluated Triangular Voltage Sweep (TVS) measurements as a technique to characterize plasma damage in low-k films. Blanket wafers with low-k films of different porosity and k value were prepared. Our samples included an SiOC:H material with 7% porosity and k value of 3.0, deposited on 200mm wafers, and two SiOC:H materials with 25% porosity and k value of 2.5, deposited on 300mm wafers. Before deposition, a thin layer of dry thermal oxide (2 – 5 nm) was grown on the n-type wafers to stabilize the silicon interface. After deposition, low-k films were exposed to N2/H2 plasma for different times in order to induce different degree of plasma damage. Untreated low-k films were always included as a reference. For electrical measurements, metal dots were deposited on pieces to fabricate Metal-Insulator-Semiconductor capacitors.TVS measurements were performed at 190°C on the different samples. On samples exposed to N2/H2 plasma, we detected a current peak in the TVS trace, whose magnitude increased with exposure time to plasma. No peaks were detected on untreated films. This indicates that TVS measurements are sensitive to plasma damage. Furthermore, TVS results correlated well with FTIR spectra that showed increasing damage and H2O uptake with increasing exposure time to plasma. We conclude that TVS measurements are suitable for characterizing the degree of plasma damage in low-k films and complement well materials analysis, because with the help of TVS a link to leakage properties can be made. As an application, we used TVS measurements for evaluating restoration of plasma damaged low-k films by long N2-bake at high temperature. Wafer pieces from each sample were baked at 350°C for 4h30min in N2 atmosphere. A few pieces were measured immediately after baking. The remaining pieces were either left exposed to ambient for a few days or dipped in deionized H2O for a few hours to evaluate recovery of hydrophobic properties. The different treatments (N2-bake, exposure to ambient, H2O dipping) were always performed on blanket wafer pieces. Metal dots for electrical measurements were only deposited after the treatment. CV and FTIR measurements were performed before and after treatments to evaluate change in k-value and material structure, respectively. Our data show that long N2-bake at high temperature can partially restore damaged low-k films. The magnitude of the damage-related TVS peak was significantly reduced after heat treatment and remained stable even after H2O dipping. CV measurements performed on baked pieces after 6 days of exposure to ambient showed a reduced k-value. Consistently, FTIR spectra showed a significant reduction of H2O content soon after baking. The materials remained stable over several days and only minor reincorporation of H2O occurred after exposure to ambient or H2O dipping. Therefore, long N2-bake at high temperature can partially restore leakage (TVS), k-value (CV) and hydrophobic properties (FTIR) of damaged low-k films.

Aqueous and Thermal Oxidation of Porous Silicon Microparticles: Implications on Molecular Interactions

Links between the mechanisms and kinetics of aqueous and dry thermal oxidation of porous silicon (pSi) microparticles have been investigated and the influence on molecular interaction established. zeta potential measurements have established the interplay between the dry oxidation state of pSi microparticles and their interfacial chemistry in aqueous solution, and Fourier transform infrared spectroscopy has demonstrated the effect of immersion time and oxidation temperature on surface chemistry. The influence of aqueous and thermal oxidation on molecular interactions and loading was investigated using methylene blue as a probe molecule. Aqueous immersion of pSi microparticles results in an initial increase in OySiH (y = 1-3) species with increasing immersion times, reducing O2SiH concentration, while O3SiH concentration remained constant. Thermal oxidation from 473 to 1073 K causes the gradual transition from SiySiHx to OySiH and finally OySiOH species. Both aqueous and thermal oxidations had an effect on the zeta potentials of pSi microparticles. Methylene blue discoloration occurred due to its reduction by the SiSiHx-terminated surface thereby demonstrating the reactivity of such species. Aqueous and thermal oxidations modify pSi microparticle surface chemistry, which has therefore shown to influence molecular interactions. Understanding the aqueous oxidation of pSi is crucial when loading pSi from aqueous solution due to its impact on molecular interactions. These molecular interactions play an important role in the loading of pSi since they dictate the attraction of the molecule toward the surface and therefore ultimately the loading level.