Si Oxide Films Research Articles

Percolation threshold in annealed ultrathin SiOx films by 2D Monte Carlo simulations

In this work, the kinetics of phase separation in ultrathin non-stoichiometric Si oxide (SiOx, x < 2) films during high-temperature annealing is studied by Monte Carlo simulations on the two-dimensional...

Study on residual OH content in low-temperature Si oxide films after in situ post-deposition heating (PDH)

We investigated the post-deposition heating (PDH) effect on OH content in SiOx films deposited by atmospheric-pressure CVD using a deposition source of silicone oil (SO) with O3 and TCE vapor at a temperature T d of 180 °C–250 °C. The PDH is performed in situ for 5 min in the deposition chamber just after film deposition without any supply of SO, where the heating temperature is the same as T d. The OH content in the films deposited normally decreases with increasing T d. In contrast, those with PDH decrease with deceasing T d from 220 °C, and, at T d = 190 °C, a minimum OH content can be obtained. This means that lower OH content remains at a lower deposition temperature. The PDH effect on OH reduction can be explained by easily reconstructible structure of SiOx films deposited at low temperature. Furthermore, we discuss the mechanism of the PDH effect from other points of view.

(Invited) Potential of Silicon Oxide Films for Low-Cost and High-Performance Resistive Switching Devices

Although resistance switching in transition-metal oxide (TMO) films has been widely studied [1], the physics and chemistry of resistance switching in non-transition-metal oxide (nTMO) films remains under-studied [2]. This must be corrected because inexpensive and chemically stable device configurations are required for the future Internet-of-Things society. It is easily anticipated that the switching electric field of silicon oxide films is apt to be higher than that of TMO films because the dielectric constant of silicon oxide films is smaller than those of the TMO films. However, the use of two-layer stacks like SiOx/hi-k oxide reduces the switching electric field [3]. Therefore, the study of silicon oxide films is still meaningful.Many scientists have recently investigated resistance switching in sputter-deposited silicon oxide films in detail [4,5] because this structure dispenses with the TMO (Fig. 1). Many papers addressed the role of silicon sub-oxide (SiOx) [6] because it is anticipated that the unstable bonds of non-stoichiometric silicon oxide can create degraded, but reversible, conductive paths inside the film. However, it is not yet clear how the SiOx region can trigger resistance switching, how important the SiOx region is, and whether the SiOx region is the only determiner of resistance switching [7] (Figs. 2, 3). The author demonstrated that hot-electron injection from the Si substrate had great potential in triggering resistance switching and lowering the switching voltage of sputter-deposited Si oxide films [5,8]; it was also mentioned that Si precipitates played an important role in realizing repeatable bipolar switching [9] (Fig. 2). Relating to this study, the author also proposed the physical and chemical structure of conductive filaments and their switching behavior based on an analysis of a possibly equivalent circuit model [10] (Fig. 4). However, it was not definitely elucidated why unipolar switching is not easily observed in sputter-deposited silicon oxide films, even though it is not a TMO.Recently, the author performed various Monte Carlo simulations to elucidate the physical and chemical parameters that rule the unipolar switching process in sputter-deposited silicon oxide films. Generations of simple bond breaking, oxygen vacancies, metallic Si sites, and E’’ centers were implemented in the simulation algorithm [8]. All-positive voltage stress mode for both the electroforming process and the reset process will not yield devices with stable, repeatable switching [11]. On the other hand, the all-negative stress mode results in stable, repeatable switching because the recovery of the internal degradation of the Si oxide film is not completed [11] (Fig. 5). This difference stems from the physical asymmetry of the electrode materials (Fig. 1).Though some may consider that silicon oxide films are not preferable to ReRAM devices from the chemical points of view, the theoretical analysis provided by the author in this paper suggests that silicon oxide films can be applied to the ReRAM device.[1] Y. Tokura, Physics Today, vol. 56, pp. 50-55, 2003.[2] T. Yanagida, et al., Sci. Rep., vol. 3, No.1657, pp.1-6, 2013.[3] P. Broqvist and A. Pasquarello, Appl. Phys. Lett., vol. 91, 192905, 2007.[4] J. Yao, et al., Appl. Phys. Lett, vol. 93, pp. 253101-1-253101-3, 2008.[5] R. Yamaguchi, S. Sato, and Y. Omura, Jpn. J. Appl. Phys., vol. 56, pp. 041301-1-041301-6, 2017.[6] A. Mehonic, et al., J. Appl. Phys., vol. 111, pp. 074507-1-074507-9, 2012.[7] Y. Wang, et al., Appl. Phys. Lett., vol.102, pp. 042103-1-042103-5, 2013.[8] Y. Omura, Ind. J. Electrical Eng. & Comput. Sci., vol. 24, pp. 1367-1378, 2021.[9] Y. Omura, R. Yamaguchi, and S. Sato, IEEE Trans. Device Reliab. and Mat. Vol. 17, pp. 561-567 (2017).[10] Y. Omura, ECS J. Solid State Sci. and Technol., vol. 10, pp. 124006-1-124006-10, 2021.[11] Y. Omura, Materials Today Proc., vol. 20, pp. 273-282, 2020; the advanced study will be published in the ECS J. Solid State Sci. and Technol. Figure 1

Thermodynamic Theory of Phase Separation in Nonstoichiometric Si Oxide Films Induced by High-Temperature Anneals

High-temperature anneals of nonstoichiometric Si oxide (SiOx, x < 2) films induce phase separation in them, with the formation of composite structures containing amorphous or crystalline Si nanoinclusions embedded in the Si oxide matrix. In this paper, a thermodynamic theory of the phase separation process in SiOx films is proposed. The theory is based on the thermodynamic models addressing various aspects of this process which we previously developed. A review of these models is provided, including: (i) the derivation of the expressions for the Gibbs free energy of Si oxides and Si/Si oxide systems, (ii) the identification of the phase separation driving forces and counteracting mechanisms, and (iii) the crystallization behavior of amorphous Si nanoinclusions in the Si oxide matrix. A general description of the phase separation process is presented. A number of characteristic features of the nano-Si/Si oxide composites formed by SiOx decomposition, such as the local separation of Si nanoinclusions surrounded by the Si oxide matrix; the dependence of the amount of separated Si and the equilibrium matrix composition on the initial Si oxide stoichiometry and annealing temperature; and the correlation of the presence of amorphous and crystalline Si nanoinclusions with the presence of SiOx (x < 2) and SiO2 phase, respectively, in the Si oxide matrix, are explained.

Simulation study on physical parameters ruling unipolar resistance switching of sputter-deposited silicon oxide film on Si substrate

This paper performs Monte Carlo simulations to elucidate the physical parameters that rule the unipolar switching process in sputter-deposited Si oxide films, where the metal/sputter-deposited Si oxide film/Si substrate stack structure is assumed. Simulation parameters are based on the results of experiments performed in parallel. Generations of simple bond breaking, oxygen vacancies, metallic Si sites, and E″ centers are implemented in the simulation algorithm, where various chemical parameters are used to control bonding conditions. In this study, the impact of hole injection from the Si substrate on the switching operation is also discussed. All-positive voltage stress mode for both the electroforming process (or set process) and the reset process and all-negative voltage stress mode for both the electroforming process (or set process) and the reset process are compared in detail as to their effect on the film’s degradation process.The comparison of the two different stress modes reveals that the positive stress mode does not yield devices with stable, repeatable switching. On the other hand, the negative stress mode results in stable, repeatable switching. It is strongly suggested that the difference stems from the physical asymmetry of the electrode materials; that is, the energy spectra of electrons injected from the Si substrate has a wide range in the all-positive stress mode and the energy transfer efficiency from electrons injected from Si substrate to the atomic bonding sites is small due to the large tunnel probability, which does not result in the successful switching of the Si oxide film. The role of hole injection from the n-Si substrate on the negative-stress-mode switching process is also discussed. These new findings should contribute to the design methodology of such resistance switching devices.

(Invited) Potential of Silicon Oxide Films for Low-Cost and High-Performance Resistive Switching Devices

This paper reviews our experimental studies on the resistive switching behaviors of sputter-deposited Si oxide films. Theoretical simulations are also demonstrated to elucidate the physical mechanisms of resistive switching behaviors. These suggest the various potential applications for low-energy sensor network systems, neuromorphic computing systems and low-cost commercial electronic systems.

Enhancing capacitance of dielectric Si-oxide film by inserting indium-tin-oxide interlayer

Two parallel-plate capacitors, Cu/Si-oxide/Cu (MIM) and Cu/Si-oxide/indium-tin-oxide/Cu (MIM-ITO), were fabricated. The capacitance of MIM-ITO structure (1365.5 pF) was measured to be much larger than MIM structure (442 pF) by two folds. The ITO interlayer enhances the ‘edge effect’ and results in non-stoichiometric Si2O3 phase formation in Si-oxide film. Si2O3 tetrahedrons present strong spontaneous dipoles, which result in an additional net polarization in the Si-oxide film under an applied electric field. With TEM images, (222)-preferred ITO crystalline phase was observed at the Si-oxide/ITO interface and served as the growth seed layer for Si2O3-contained Si-oxide film.

Quantitative Evaluation of Low-temperature Si Oxide Films by Derivative FT-IR Spectra

Quantitative Evaluation of Low-temperature Si Oxide Films by Derivative FT-IR Spectra

New SIMS method to characterize hydrogen in polysilicon films

A new analysis protocol for profiling the hydrogen (H) concentration depth distributions in polysilicon (poly-Si) thin films on Si oxide was developed by using secondary ion mass spectrometry (SIMS). Traditional SIMS determination of H concentrations in poly-Si films is to monitor and collect the atomic H−, but this analysis method presents some challenges for providing stable and repeatable H depth profiles, especially, for shallow profiling in thinner poly-Si films on thicker Si oxide films. In this paper, we present a thorough study of the SIMS characterization of H levels in poly-Si thin films on Si oxide. We describe an approach for the use of cluster SiH− instead of atomic H− to determine the H concentrations. The utilization of the SiH− cluster ion minimizes both H adsorption effects on the film surface regions and sample surface charging effects arising from the Si oxide layer underneath, thus enhancing the secondary ion signal stability in poly-Si films, as discussed with secondary ion energy distribution on the samples. This method differs from the conventional SIMS analysis of H in thin films and significantly improves the data quality and accuracy. Fourier transform infrared spectroscopy analysis was used to study the Si–H chemical bonding information for investigating the nature of the SIMS collected SiH− cluster ions. Both the H implant sample with no significant Si–H bonding and the poly-Si samples with high levels of H and the weak Si–H bonding exhibited strong SiH− secondary cluster ion intensities in our studies. This indicates the cluster ion signals arise from either the combination of Si and H ions through the physics formation process or weak chemically bonded Si and H.

First-principles study of strain effect on oxygen vacancy in silicon oxide

The strain dependence of stability and diffusion barrier height of the O vacancy in Si oxide is examined using the first-principles calculation. It is found that the stability and the diffusion barrier height increase as the oxide is compressed. The analysis shows that the diffusion barrier height is determined by the Si–Si distance of the O vacancy. Based on these results, the dielectric breakdown of the gate Si oxide film in the three-dimensional structure metal-oxide-semiconductor field-effect transistor is also considered. It is suggested that the interfacial O-vacancy density increases because the interfacial oxide is subjected to compressive strain. On the other hand, the surface O-vacancy density decreases and the surface O-vacancy diffusion is promoted because the surface oxide is subjected to tensile strain. The reliability of the oxide film might be thus affected by the strain.

Effect of different substrates on Si and Ta co-doped Ga2O3 films prepared by pulsed laser deposition

In this study, the structure, morphology, electrical and optical properties of Si and Ta co-doped gallium oxide (Ga2O3) films deposited on (1 0 0) Ga2O3, (0 0 0 1) sapphire and quartz substrates by pulsed laser deposition are analyzed. The Si and Ta co-doped Ga2O3 film on Ga2O3 substrate possessed the best crystal quality with a high carrier concentration of 1.12 × 1020 cm−3 and a conductivity of 2.13 S·cm−1 compared to the sapphire and quartz substrates. The Si and Ta co-doped Ga2O3 films on different substrates exhibited high transmittance (>73%) above 300 nm and smooth film surface. The optical gap of Si and Ta co-doped Ga2O3 films on Ga2O3, sapphire and quartz substrates decreases from 4.89 to 4.85 eV, and the corresponding Urbach energy increases from 0.396 to 0.704 eV, so the homoepitaxial Si and Ta co-doped Ga2O3 film possess the best quality.

Density Functional Theory Study on Stability of Fe, Cu, and Ni Atoms Near (001) Surface of Si Wafer

“Metal gettering” technology is becoming increasingly important to keep the active region of state-of-the-art electronic devices extremely clean. Metal atoms entering the silicon (Si) wafer will be out-diffused at the wafer surface if the internal gettering (IG) does not work well. Therefore, an understanding the stability of metal atoms near the Si (001) wafer surface is important in addition to understanding the IG mechanism. The calculations with density functional theory are preformed to investigate the stability of Fe, Cu, and Ni atoms near the (001) surface with and without ultra-thin oxide film. The following main results are as obtained. (1) Compared to the Si bulk, Fe, Cu, and Ni atoms are more stable up to the fifth atomic layer from the Si (001) surface. (2) Fe0, Fe+, Ni0, and Cu0 atoms at the wafer surface are energetically very stable compared to those internally gettered by B atom, with the order of Fe0 > Fe+ > Ni0 > Cu0, while Cu+ is not so stable at the wafer surface. (3) Just below the ultra-thin Si oxide film, the formation energy of Fe, Cu, and Ni atoms is increased. Furthermore, the segregation coefficient of these metals between the surface region and Si bulk was evaluated.

Spinodal decomposition versus nucleation and growth mechanism of phase separation in nonstoichiometric silicon oxide films during high temperature annealing

In this work, kinetic mechanisms of phase separation in nonstoichiometric Si oxide (SiOx) films during high temperature annealing are studied theoretically. Analysis of the composition dependence of the Gibbs free energy of SiOx phase is carried out to derive its binodal and spinodal characteristics. The ranges of the values of x corresponding to stability, metastability, and instability of nonstoichiometric Si oxide with respect to phase separation are determined as the functions of annealing temperature and SiOx elastic properties. The composition-temperature regions of the spinodal decomposition and the nucleation and growth mechanisms of phase separation are presented. Obtained results open a way to a development of the kinetic theory of phase separation in SiOx films and formation of Si quantum dots/SiO2 matrix composite structures.

Effect of trichloroethylene enhancement on deposition rate of low-temperature silicon oxide films by silicone oil and ozone

A low-temperature silcon oxide film was deposited at 160 to 220 °C using an atmospheric pressure CVD system with silicone oil vapor and ozone gases. It was found that the deposition rate is markedly increased by adding trichloroethylene (TCE) vapor, which is generated by bubbling TCE solution with N2 gas flow. The increase is more than 3 times that observed without TCE, and any contamination due to TCE is hardly observed in the deposited Si oxide films from Fourier transform infrared spectra.

Optical Property of Porous Silicon Produced by Metal-Assisted Etching

A number of ideas have been applied to improve the conversion efficiency of crystalline silicon (Si) solar cells. Antireflection of Si surface is an important technology to increase the photocurrent density of solar cells. Both random pyramid structures produced by anisotropic etching of crystalline Si wafers and antireflection coatings which have lower refractive index than Si are generally used for commercial solar cells. Common antireflection coatings using Si nitride have such problems as high manufacturing cost and limited antireflection effect only in a specific range of wavelength. Previously, antireflection coatings of porous Si formed by stain etching using a mixture solution of hydrofluoric acid and nitric acid or by anodic etching were reported1, 2. This method can control the refractive index by changing the structure of the porous Si, so it can decrease the reflection of Si surface in a wide wavelength range. We reported that macro porous Si formed by a metal assisted etching, which is immersing metal catalyst modified Si in a hydrofluoric acid (HF) aqueous solution, can be used as a texture layer for antireflection of solar cells3. The macro porous layers consisting of pores in µm-size are expected to damage p-n junction and reduce the conversion efficiency of solar cells. In this study, we investigated the optical properties of porous Si thin films, which were thinner than p-n junction. Single-crystalline n-Si wafers (CZ, (100), ca. 10 Ω cm) were used as substrates. The Si wafers were immersed in 1 mM (M = mol dm-3) silver nitrate (AgNO3) solution including 0.15 M HF at 278 K for electroless displacement metal deposition. Metal deposited Si wafers were immersed in a 6.6 M HF solution including hydrogen peroxide (H2O2) at 298 K for 1 ~ 60 s. The samples after etching were immersed in a 13 M nitric acid and then 7.3 M HF to remove silver nanoparticles remaining in the bottom of the pores and the Si oxide film, respectively. A multi-photometric system (Otsuka Electronics, MCPD-7000KA) with an integrating sphere was used to measure reflectance of Si surface. A spectroscopic ellipsometer (Otsuka Electronics, FE-5000) was used to analyze the optical constants of the porous Si thin film. Fig. 1 shows that an uniform porous Si thin film consisting of straight pores, few tens nm in diameter and 250 nm in length, was formed by the etching for 15 s. The structure of porous Si was changed with HF and H2O2 concentrations of the etching solution and etching time. The pore size was consistent with the size of Ag nanoparticles as shown in Fig. 1. Thus, the porosity of porous Si can be evaluate as the coverage of Ag nanoparticles on Si before etching. The coverage, that is porosity, can be controlled by deposition time of Ag. The reflection of porous Si surface was much lower than mirror polished Si wafer before metal assisted etching (Fig. 2). As the porosity increased, interference waveform in the reflectance spectra appeared and the peak interval of reflectance spectra decreased. On the other hand, it was possible to control the thickness of the porous Si thin film by changing the etching time. As porous layer thickness increased, the reflectance decreased and the amplitude of the interference waveform decreased. The reflectance reached lower than 5.5%, which is much lower than 10% of a pyramidal structure, in the wavelength range of 400-1000 nm. From these results, we assumed that the porous Si functions as an optical thin film of a smooth single-layer. The refractive index of porous Si thin film at the porosity of 33 ~ 56% was observed as 1.1 ~ 2.2. The attenuation factor (factor that affects light absorption) of porous Si thin film was ~100 times higher than that of crystalline Si. The thin porous Si layers produced by the metal assisted etching are expected to work as suitable antireflection films for crystalline Si solar cells. ACKNOWLEDGEMENT The present work was partly supported by JSPS KAKENHI (26289276). The authers wish to express their thanks to Otsuka Elecronics Co., LTD for ellipsometric analysis. REFERENCES 1. R. R. Bilyalov, R. Lüdemann, W. Wettling, L. Stalmans, J. Poortmans, J. Nijs, L. Schirone, G. Sotgiu, S. Strehlke, C. Lévy-Clément, Sol. Energy Mater. Sol. Cells 60, 391-420 (2000). 2. L. Remachea, E. Fourmonda, A. Mahdjoubb, J. Dupuisa, M. Lemitia, Mater. Sci. Eng. B 176, 45–48 (2011). 3. S. Yae, Y. Kawamoto, H. Tanaka, N. Fukumuro, H. Matsuda, Electrochem.Commun. 5, 632 (2003). Figure 1

The role of operations after the deposition on the performance of SiOx films in optoelectronics devices

In this study, we have investigated phase separation, silicon Nano crystal (Si-NC) formation and optoelectronics properties of Si oxide (SiOx, 0.7<x<1.3) films in high-vacuum annealing and ion bombardment conditions. The SiOx films were deposited by physical vapor deposition (PVD). The internal structure properties of these films were the main factor in applications of optoelectronic. Possible changes in the structure, composition and electro physical properties were investigated by FTIR and TEM spectroscopy. The measurements show that SiOx film is the dominant phase in the ultra-thin layer. Also, high-temperature annealing ion bombardment results in further increase of the phase separation of the whole layer.

Observation of the enhancement of electric fields normal to the surface using mid-infrared slot antennas and an atomic layer deposition technique

Optical electric field enhancement in the normal direction was experimentally investigated using mid-infrared slot antennas that were formed on a thin Al2O3 layer/Si substrate. The Al2O3 layer thicknesses could be controlled to an accuracy of a given atomic layer through the use of atomic layer deposition, and varied from 0nm to 60nm. An in-depth probe of the electric field was performed by observing the change in the reflection signal arising from the Restrahlen band of the natural oxide of Si formed on the surface of a Si substrate. In contrast to dipole nanoantennas, we could clearly observe Restrahlen bands of Al2O3 as well as the native Si oxide film. This was because the direction of the enhanced electric field was primarily parallel to the substrate surface in the slot antennas, which was different from the dipole nanoantenna having strong normal electric fields at the antenna ends. The atomic layer deposition technique provides versatile information on the electric field distribution within the depth direction, being considered complementary to the electromagnetic simulation of nanoantennas.

The formation of Yb silicates and its luminescence in Yb heavily doped silicon oxides after high temperature annealing

We report on the formation of ytterbium (Yb) silicates and its photoluminescence (PL) properties for heavily Yb doped Si oxide films after various annealings. X-ray diffraction patterns and transmission electron microscopy indicate that different Yb silicates have formed in the oxides upon 1100 and 1200°C annealing. The Yb PL intensities after the high temperature annealings are much stronger than those after lower temperatures, which indicates that the Yb silicates have higher emission efficiency than the Yb configurations found for lower temperature annealing. The PL intensities of the films can be altered considerably by secondary oxidizing or annealing in forming gas (N2+7% H2) ambience.

Photoemission study of the identification of Mn silicate barrier formation on carbon containing low-κ dielectrics

In this study X-ray photoelectron spectroscopy (XPS) has been used to characterize manganese silicate Cu diffusion barrier layer formation on a range of ultralow-κ (ULK) carbon doped oxide (CDO) layers. Ultra-thin Si and Mn oxide films were deposited in order to provide accurate binding energy (BE) references for the Si 2p and O 1s core levels of the dielectric materials. The results indicate that there is a strong correlation between carbon content in the CDO films and the BE position of both the Si 2p and O 1s core level peaks. Furthermore, it has been shown that the full width half maximum (FWHM) of these peaks are significantly larger than those observed in SiO2 leading to complications in the analysis and identification of barrier layer formation on these low-κ substrates. In a separate experiment, the deposition and high temperature annealing of thin fully oxidized Mn layers (MnOy, where y⩾1) on these ULK CDO substrates suggests the formation of an interface layer consistent with MnSiO3 based on analysis of both the O 1s and Mn 2p core level spectra. It is also shown that the use of oxidized Mn films tend to minimize the formation of Mn carbide within the barrier layer region in agreement with previous studies on other CDO substrates.

Mixed Ti–O–Si oxide films formation by oxidation of titanium–silicon interfaces

The reaction of oxygen with titanium deposited on Si (100) surfaces has been studied at room temperature and low oxygen pressures, using XPS and ARXPS. The experimental results for Ti growth on Si can be explained using a model involving a two stage mechanism. The first stage is characterized by the formation of a uniform TiSix layer ∼4ML thick and the second one by the formation of metallic titanium that grows following a Stranki–Krastanov mechanism, that is, the formation of a Ti monolayer followed by the growth of Ti islands (7ML thick) over the TiSix layer previously formed. The oxidation of Ti/Si interfaces strongly depends on the interface that is oxidized. For an interface corresponding to the first stage of deposition a Ti–O–Si mixed oxide layer is formed on the near surface. This layer is on top of a multilayer structure which is composed of TiO2 (Ti4+), titanium suboxides along with TiSi (TiSi+Ti1++Ti2++Ti3+), and substrate when going from the outer surface to the substrate whereas for an interface corresponding to the second stage no Ti–O–Si mixed oxide is detected and a Ti0 rich layer is observed between the titanium suboxides and the Si substrate.