Interfacial Suboxide Research Articles

A model for oxidation kinetics in air at room temperature of hydrogen-terminated (1 ) Si

A quantitative model is proposed for the description of the oxidation kinetics in air at room temperature of single crystalline, hydrogen-terminated, (1 0 0) silicon. The theory separates the growth kinetics of the interfacial suboxide from those of the outer stoichiometric oxide. The theory proceeds assuming that the suboxide grows along the surface at the border of oxidized-silicon clusters, while the formation of the stoichiometric oxide takes place on the top of the suboxide at a rate decaying exponentially with the oxide thickness. In these hypotheses the kinetics of suboxide formation are found to depend on the initial concentration of (defective) oxo groups, while the growth of the stoichiometric oxide is described by the Elovich equation both in the short- and long-time limits.

Zirconia-germanium interface photoemission spectroscopy using synchrotron radiation

An ultrathin zirconia gate dielectric had been successfully incorporated into germanium metal-oxide-semiconductor (MOS) devices demonstrating very high-permittivity gate stacks with no apparent interfacial layer. In this study, synchrotron-radiation photoemission spectroscopy has been applied on the same gate stack to identify and quantify the presence of any interfacial germanium suboxide layer. By taking progressive core-level spectra during the layer-by-layer removal of the zirconia film, an oxidized germanium layer with submonolayer thickness was found, possibly arising from an interfacial Zr–O–Ge bonding configuration. In addition, the offsets in the valence-band spectra were also monitored and the energy-band diagram of the zirconia–germanium heterostructure was constructed. Compared to high-κ gate stacks on Si, the thinner interfacial layer and larger conduction-band offset in high-κ gate stacks on Ge suggest better scalability towards an ultimately higher MOS gate capacitance.

Theory of atomic-scale dielectric permittivity at insulator interfaces

We develop a theory for investigating atomic-scale dielectric permittivity profiles across interfaces between insulators. A local susceptibility $\ensuremath{\chi}(x;\ensuremath{\omega})$ is introduced to describe variations of the dielectric response over length scales of the order of interatomic distances. The nonlocality of the microscopic susceptibility tensor ${\ensuremath{\chi}}_{ij}(\mathbf{r},{\mathbf{r}}^{\ensuremath{'}};\ensuremath{\omega})$ occurs at smaller distances and therefore does not intervene in our formulation. The local permittivity is obtained from the microscopic charge density induced by an applied electric field. We show that the permittivity can conveniently be analyzed in terms of maximally localized Wannier functions. In this way, we can relate variations of the microscopic dielectric response to specific features of the local bonding arrangement. In addition to a continuous description in terms of the local permittivity, we introduce an alternative scheme based on discrete polarizabilities. In the latter case, electronic polarizabilities ${\ensuremath{\alpha}}_{\mathrm{elec}}^{(n)}$ are obtained in terms of the displacements of maximally localized Wannier functions, while ionic polarizabilities ${\ensuremath{\alpha}}_{\text{ion}}^{(I)}$ are determined from the induced ionic displacements and the corresponding dynamical charges. The potential of our scheme is illustrated through applications to two systems of technological interest. First, we consider the permittivity of Si slabs of finite thickness. Our approach indicates that the local permittivity in the slab interior approaches the corresponding value for bulk Si within a few atomic layers from the surface. Therefore, the decrease of the average slab permittivity with thickness originates from the lower permittivity of the outer planes and the increasing surface-to-volume ratio. Second, we address the dielectric permittivity across the $\mathrm{Si}(100)\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$ interface. Using two distinct interface models, we are able to show that the dielectric transition from the silicon to the oxide occurs within a width of only a few \AA{}. The polarizability associated to intermediate oxidation states of Si is found to be enhanced with respect to bulk $\mathrm{Si}{\mathrm{O}}_{2}$, resulting in a larger permittivity of the interfacial suboxide layer with respect to the stoichiometric oxide.

Influence of the oxygen concentration of atomic-layer-deposited HfO2 films on the dielectric property and interface trap density

The influence of the ozone concentration (160–370g∕m3) during atomic layer deposition of HfO2-gate dielectrics on the dielectric performance of the films grown on Si was studied. Although ozone was effective in reducing the impurity concentration in the film compared to H2O, the higher concentration slightly deteriorated the dielectric performance. More importantly, the degradation in the interface trap property with increasing post-annealing temperature became more serious as the ozone concentration increased. Investigation of the interface states using x-ray photoelectron spectroscopy revealed that the excessive oxygen incorporated during the film growth made the interfacial sub-oxide species (SiO, Si2O3, and silicate) and SiO2 coordinate more with oxygen. This increased the interface trap density and degraded the interface properties.

Suboxide characteristics in ultrathin oxides grown under novel oxidation processes

Interfacial suboxide (SiOx,0<x<2) between Si and its stable SiO2 is observed to be a key factor to affect the ultrathin-oxide quality. SiO2 grown by anodic oxidation (anodization) or by repeated-spike oxidation (RSO) presents a better dielectric characteristic than conventional rapid-thermal oxidation (RTO). This improvement can be attributed to two reasons: (1) A thinner transition region and/or (2) more stoichiometric S–O bonds under the same oxide thickness. Anodic SiO2 presents a thinner suboxide width (interface region) and RSO SiO2 demonstrates the existence of less suboxide existence at the near-surface region (oxide bulk). The reduction of the imperfect bonding structure in suboxide gives an abrupt Si–SiO2 interface and consequently, the reduction of electron tunneling probability through oxide.

Microscopic bonding and macroscopic strain relaxations at Si-SiO 2 interfaces

Optical second harmonic generation studies of the temperature dependence of the phase angle differences from contributions of terrace and edge bonding at silicon–silicon dioxide interfaces prepared on vicinal Si(111) wafers have revealed an interface relaxation process at an annealing temperature between 850 and 900 °C. Complementary studies by synchrotron soft X-ray photoelectron spectroscopy have established that this relaxation is associated with changes in the concentration and composition of local suboxide bonding environments in an ultrathin, interfacial transition region that is approximately one molecular layer thick. This relaxation occurs at a significantly lower temperature than an approximately 990±10 °C relaxation of macroscopic compressive strain in the bulk of the SiO2 film. This paper establishes an analogy between i) the Si–SiO2 interface in which there is a transition from a ‘rigid’ substrate, Si, to an ‘ideal continuous random covalent network’, SiO2, in which the average number of bonding constraints/atom matches the network dimensionality, and ii) a concentration dependent transition between ‘under-’ and ‘over-constrained’ local bonding in non-crystalline glass alloys such as GexSe1-x. The interfacial suboxide transition region is demonstrated to have properties in common with a regime of alloy compositions in which ‘self-organization’ reduces bond constraint induced strain, thereby stabilizing these compositions against ‘aging’ as for example in time-dependent changes in the glass transition temperature. These comparisons provide important new insights into defect formation at Si–SiO2 interfaces, as well as interfaces between Si and alternative high-k dielectrics being considered for advanced Si devices including Al2O3 and transition metal silicate alloys, e.g., (ZrO2)x(SiO2)1-x. This new perspective is also extended to interfaces between GaN and SiO2, where the interfacial transition region is a suboxide of Ga, GaOx, with x<1.5.

Molecular structure of interfaces formed with plasma-polymerized silica-like primer films: Part II. Characterization of the primer/metal interface using x-ray photoelectron spectroscopy in SITU

X-ray photoelectron spectroscopy (XPS) was performed in situ on plasma-polymerized silica-like films that were deposited onto metal substrates. Relatively thick films (∼8.0 nm) had spectra that were typical of bulk amorphous silicon dioxide (a-SiO 2 ). When thinner films were analyzed (∼2.4 nm), a Si(2p) peak emerged that was due to the formation of silicon suboxide at the interface. Changes in the metal and metal oxide peaks showed that oxidation of the substrates during plasma etching and deposition occurred. It was determined that during the initial stages of plasma deposition, metal atoms from the substrate migrated to the metal-oxide surface. This resulted in preferential oxidation of metal atoms with the formation of silicon suboxide at the film/metal interface. In addition, interfacial suboxide formation was shown to have a dependence upon the diffusivity of the metal substrate atoms through the surface oxide of the metal. As a result, more interfacial suboxide was observed to form for depositions on titanium substrates in comparison with depositions on aluminum substrates. A detailed analysis of the atomic species detected with in situ XPS enabled us to develop a model of the molecular structure at the a-SiO 2 /metal interface for plasma depositions on aluminum and titanium substrates. When the possible chemical reaction routes for film deposition were considered, the formation of primary Al-O-Si and Ti-O-Si bonds at the interface was proposed.

Molecular structure of interfaces formed with plasma-polymerized silica-like primer films: Part I. Characterization of the primer/metal interface using infrared spectroscopy in SITU

Reflection-absorption infrared (RAIR) spectroscopy was performed in situ on plasma polymerized silica-like films that were deposited onto metal substrates. Relatively thick films (∼8.0 nm) had infrared spectra that were typical of bulk amorphous silicon dioxide (a-SiO 2 ). When thinner films were analyzed (∼0.6 nm), other infrared bands emerged that were due to the formation of silicon suboxide at the interface. Infrared bands due to oxidation of the substrate during deposition were also observed. It was determined that during the initial stages of plasma deposition, metal atoms from the substrate migrated to the metal-oxide surface. This resulted in preferential oxidation of metal atoms with the formation of silicon suboxide at the film/metal interface. In addition, interfacial suboxide formation was shown to have a dependence upon the diffusivity of the metal substrate atoms through the surface oxide of the metal. As a result, more interfacial suboxide was observed to form for depositions on titanium substrates in comparison with depositions on aluminum substrates.

Monte Carlo study of interfacial silicon suboxide layers and oxidation kinetics

A simple simulation scheme that simultaneously describes the growth kinetics of SiO 2 films at the nanometer scale and the SiO x /Si interface dynamics (its extent, and spatial/temporal evolution) is presented. The simulation successfully applies to experimental data in the region above and below 10 nm, reproduces the Deal and Grove linear-parabolic law and the oxide growth rate enhancement in the very thin film regime (the so-called anomalous region). According to the simulation, the oxidation is governed mainly by two processes: (a) the formation of a transition suboxide layer and (b) its subsequent drift towards the silicon bulk. We found that it is the superposition of these two processes that produces the crossover from the anomalous oxidation region behavior to the linear-parabolic law.

Barrier layer model determined by XPS data for tunneling current reductions at monolayer nitrided Si–SiO 2 interfaces

This paper builds on previous work that has demonstrated that interfacial suboxide transition regions at Si–SiO 2 interfaces modify tunneling oscillations in the Fowler–Nordheim regime. This paper extends this approach to the direct tunneling regime, emphasizing differences in interfacial transition regions between Si–SiO 2 interfaces with and without monolayer level interface nitridation. Tunneling currents in devices with the same oxide-equivalent thickness are reduced by monolayer level interfacial nitrogen with respect to devices without interface nitridation for both substrate and gate injection in both the direct and Fowler–Nordheim tunneling regimes. These decreases have been combined with physically thicker stacked oxide/nitride dielectrics to yield significantly reduced tunneling compared to devices with oxides of the same equivalent oxide thickness, t ox-eq; e.g., tunneling currents ∼5×10 −3 A/cm 2 at 1 V for t ox-eq∼1.6 nm have been obtained.

The effects of interfacial sub-oxide transition regions and monolayer level nitridation on tunneling currents in silicon devices

Direct tunneling (D-T) in Si metal-oxide-semiconductor (MOS) devices having 1.8 to 3 nm thick gate oxides is reduced approximately tenfold by monolayer Si-dielectric interface nitridation with respect to devices with nonnitrided interfaces. The reduction is independent of gate oxide-equivalent thickness, and gate or substrate injection, and extends into the Fowler-Nordheim tunneling (F-N-T) regime for thicker oxides as well. A barrier layer model, including sub-oxide transition regions, has been developed for the interface electronic structure for tunneling calculations using X-ray photoelectron spectroscopy data. These calculations provide a quantitative explanation for the observed tunneling current reductions.

The effects of interfacial suboxide transition regions on direct tunneling in oxide and stacked oxide-nitride gate dielectrics

This paper builds on previous work that has demonstrated the effects of interfacial suboxide transition regions at SiSiO 2 interfaces on tunneling oscillations in the Fowler-Nordheim regime. This paper extends thes effects to the direct tunneling regime and focuses on differences in interfacial transition regions between SiSiO 2 interfaces with, and without monolayer level interface nitridation. Tunneling currents in devices with the same oxide-equivalent thickness are reduced by monolayer level interfacial nitrogen with respect to devices without interface nitridation for i) substrate and gate injection and ii) in both the direct and Fowler-Nordheim tunneling regimes.

X-ray spectroscopy of the oxidation of 6H-SiC(0001)

Chemically abrupt ${\mathrm{SiO}}_{2}/6\mathrm{H}\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{C}(0001)$ interfaces have been obtained after thin (\ensuremath{\cong}5 nm) oxide thermal growth of the Si-terminated face. The originality of the paper resides in a simultaneous use of standard technological conditions for this oxidation (dry ${\mathrm{O}}_{2}$ oxidation at 1 atmosphere and 1000 \ifmmode^\circ\else\textdegree\fi{}C) and in control and characterization of the starting substrates taken from in situ, ultrahigh vacuum-surface techniques. The oxidation of $3\ifmmode\times\else\texttimes\fi{}3$ Si-rich and $6\sqrt{3}\ifmmode\times\else\texttimes\fi{}6\sqrt{3}R30\ifmmode^\circ\else\textdegree\fi{}$ C-rich reconstructed surfaces is compared. In both cases no C-C bonds or C enrichment are observed at the ${\mathrm{SiO}}_{2}/\mathrm{S}\mathrm{i}\mathrm{C}$ interface by x-ray photoelectron spectroscopy $\mathrm{C}1s$ analysis, the C-C bonds of the initially graphitized C-rich surface being removed during the first nanometer oxide growth. The interfacial suboxide components with binding energy between the substrate and ${\mathrm{SiO}}_{2}\mathrm{}\mathrm{Si}2p$ features remain below our detection limit. X-ray photoelectron diffraction analyses indicate that, below the oxide layer, the ordering and polarity of the 6H-SiC(0001) are maintained. These results allow us to conclude that there is probably no fundamental contraindication to obtain nearly ideal ${\mathrm{SiO}}_{2}/\mathrm{S}\mathrm{i}\mathrm{C}$ interfaces on flat SiC terraces. Two growth regimes are observed in the oxide growth kinetics. They are discussed to determine the possible reasons of carbon exodiffusion or previously observed C enrichments at the ${\mathrm{SiO}}_{2}/\mathrm{S}\mathrm{i}\mathrm{C}$ interface.

The Effects of Interfacial Suboxide Transition Regions on Direct Tunneling in Oxide and Stacked Oxide-Nitride Gate Dielectrics

ABSTRACTMesarjian et al. were the first to recognize the effects of suboxide interfacial transition regions at Si-SiO2 interfaces on tunneling oscillations in the Fowler-Nordheim regime. This paper extends these ideas to the direct tunneling regime and focuses on differences in interfacial transition regions between Si-SO2 interfaces with, and without monolayer level interface nitridation. Tunneling currents in both the direct and Fowler-Nordheim tunneling regimes are reduced by monolayer level interface nitridation for PMOS and NMOS devices with the same oxide-equivalent thickness. This paper develops a modified barrier layer model based on analysis of XPS results that accounts for these reductions in current in the direct tunneling regime.

Investigation of postoxidation thermal treatments of Si/SiO2 interface in relationship to the kinetics of amorphous Si suboxide decomposition

Interfacial Si suboxides (SiOx, x&amp;lt;2) are detrimental to transistor performance and are typically minimized during postoxidation anneals. To study the kinetics of SiOx decomposition, thick films (∼2000 Å) of amorphous Si:O:H alloys (0.7&amp;lt;x&amp;lt;1.4) were deposited by remote plasma enhanced chemical vapor deposition and subjected to rapid thermal anneals. Films were characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, ellipsometry, photoluminescence (PL), and transmission electron microscopy. At temperatures &amp;gt;500 °C initially there is a rapid segregation into amorphous Si (a-Si) surrounded by a SiO2 shell which acts as a diffusion barrier decelerating the reaction. Phenomenological modeling of kinetics with a one-dimensional Avrami–Erofe’ve treatment gives an upper limit for a-Si lateral growth rates of 1.2 Å/s at 900 °C with an activation energy of 120 kJ/mol. PL, Raman, transmission electron microscopy and ellipsometry confirm this segregation model in the amorphous state. Due to the rapid initial decomposition and relatively large diffusion coefficients, a simple kinetic hindrance explanation for the 4–8 Å of SiOx at the SiO2/Si interface is unlikely.

Elimination of sub-oxide transition regions at SiSiO 2 interfaces by rapid thermal annealing at 900°C

In many spectroscopic studies, particularly by X-ray photoelectron spectroscopy, XPS, it has been generally assumed that the existence of sub-oxide transition regions at SiSiO 2 interfaces were not strongly dependent on synthesis chemistries and processing conditions. This paper presents experimental evidence for a kinetically-limited phase transition at SiSiO 2 interfaces at approximately 900°C that can minimize and to a large degree eliminate significant interfacial sub-oxide transition regions. The paper emphasizes a connection between (i) differences in photoluminescence from SiO x bulk films before and after 900°C annealing and (ii) differences in electrical performance of SiSiO 2 interfaces and optical second harmonic generation from SiSiO 2 interfaces, also before and after 900°C annealing

Minimization of sub-oxide transition regions at SiSiO 2 interfaces by 900°C rapid thermal annealing

Combining previously reported optical second harmonic generation (SHG) data with i) newly-reported X-ray photoelectron spectroscopy (XPS) data and ii) the Auger electron spectroscopy (AES) results presented in this paper demonstrates that interfacial sub-oxide bonding (SiO x, x<2) invariably results from rapid thermal or plasma-assisted oxidation at temperatures below 800°C, and is significantly reduced following a 30 s, 900°C RTA.

Size, shape, and composition of luminescent species in oxidized Si nanocrystals and H-passivated porous Si.

Near-edge and extended x-ray-absorption fine-structure measurements from a wide variety of oxidized Si nanocrystals and H-passivated porous Si samples, combined with electron microscopy, ir absorption, forward recoil scattering, and luminescence emission data, provide a consistent structural picture of the species responsible for the luminescence observed in these systems. For porous Si samples whose luminescence wavelengths peak in the visible region, i.e., at 700 nm, their mass-weighted-average structures are determined here to be particles (not wires) whose short-range character is crystalline and whose dimensions---typically 15 \AA{}---are significantly smaller than previously reported or proposed. Results are also presented which demonstrate that the observed visible luminescence is not related to either a photo-oxidized Si species in porous Si or an interfacial suboxide species in the Si nanocrystals. The structural and compositional findings reported here depend only on sample luminescence behavior, not on how the luminescent particles are produced, and thus have general implications in assigning quantum confinement as the mechanism responsible for the visible luminescence observed in both nanocrystalline and porous silicon.

Annealing effect on native-oxide/Si(111) interfaces studied by second-harmonic generation.

Structural changes in native-oxide/Si(111) interfaces caused by annealing in an ultrahigh vacuum were studied by using optical second-harmonic generation (SHG). The SHG intensity increased with annealing temperature. However, this increased SHG was then decreased by supplying atomic hydrogen to the interface at room temperature. This indicates that the increase in SHG was caused by the interface dangling bonds, which are easily terminated by hydrogen. Besides SHG, x-ray photoelectron spectroscopy indicated that annealing changed the stoichiometry of the interfacial suboxide layer. After annealing, the amount of stoichiometric ${\mathrm{SiO}}_{2}$ increased. Thus, annealing made the stoichiometry ${\mathrm{SiO}}_{2}$-like, while the accompanying stress increase was released by dangling-bond creation at the interface.

Size, Shape, and Crystallinity of Luminescent Structures in Oxidized Si Nanoclusters and H-Passivated Porous Si

ABSTRACTNear-edge and extended x-ray absorption fine structure measurements from a wide variety of H-passivated porous Si samples and oxidized Si nanocrystals, combined with electron microscopy, ir-absorption, α-recoil, and luminescence emission data, provide a consistent structural picture of the species responsible for the luminescence observed in these systems. For luminescent porous Si samples peaking in the visible region, i. e., ≤700nm, their mass-weighted-average structures are determined here to be particles–not wires, whose short-range character is crystalline – not amorphous, and whose dimensions – typically &lt;15 Å – are significantly smaller than previously reported or proposed. These results depend only on sample luminescence behavior, not on sample preparation details, and thus have general implications in describing the mechanism responsible for visible luminescence in porous silicon. New results are also presented which demonstrate that the observed luminescence is unrelated to either the photo-oxidized Si species in porous Si or the interfacial suboxide species in the Si nanocrystals.