Si Oxide Films Research Articles

(Invited) Growing Si Nanocrystals within a-Si Nanoclusters Embedded in a-SiO2: Evolution of Photoluminescence

We combine X-ray absorption, electron spin resonance and Raman spectroscopies, X-ray diffraction and photoluminescence (PL) techniques to determine the structure and luminescence mechanisms in Si nanoclusters (Si-ncls) embedded within Si oxides at various intermediate formation stages. The Si-ncls/oxide systems are fabricated by thermally annealing plasma-enhanced chemical vapor deposited Si-rich Si-oxide films. The structural and chemical orders in the amorphous oxide matrix, the Si-ncls amorphous and crystalline volume fractions and sizes, the dangling bond density and PL spectra and decay rates are followed closely as a function of the annealing temperature. The results can be interpreted by a crystalline core/amorphous shell model for the Si-ncls. The important role of the shell, often ignored in the literature, is discussed. As the Si-ncls crystalline cores grow at the expense of thinning amorphous shells, the PL undergoes a transition from a regime dominated by disorder-induced effects to a situation where quantum confinement prevails.

Tunneling current modulation by Ge incorporation into Si oxide films for flash memory applications

Current-voltage characteristic for a Ge-incorporated Si oxide was investigated. Current enhancement was observed for the electric field larger than 10 MV/cm. Such a current enhancement only under high electric field is expected to improve programming performance without deteriorating reading performance. From secondary ion mass spectrometry and hard x-ray photoelectron spectroscopy analyses and current simulation, it is concluded that the Ge impurity in Ge4+ state around the tunnel oxide/substrate interface enhances the current by trap-assisted tunneling. The programming current enhancement induced by the Ge incorporation is expected to be one of the promising solutions for the next-generation flash memory.

From amorphous to crystalline silicon nanoclusters: structural effects on exciton properties

Synchrotron x-ray absorption spectroscopy (XAS) and electron spin resonance (ESR) experiments were performed to determine, in combination with Raman spectroscopy and x-ray diffraction (XRD) data from previous reports, the structure and paramagnetic defect status of Si-nanoclusters (ncls) at various intermediate formation stages in Si-rich Si oxide films having different Si concentrations (y = 0.36–0.42 in SiyO1−y), fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition and isochronally (2 h) annealed at various temperatures (Ta = 900–1100 °C) under either Ar or (Ar + 5%H2) atmospheres. The corresponding emission properties were studied by stationary and time dependent photoluminescence (PL) spectroscopy in correlation with the structural and defect properties. To explain the experimental data, we propose crystallization by nucleation within already existing amorphous Si-ncls as the mechanism for the formation of the Si nanocrystals in the oxide matrix. The cluster-size dependent partial crystallization of Si-ncls at intermediate Ta can be qualitatively understood in terms of a ‘crystalline core–amorphous shell’ Si-ncl model. The amorphous shell, which is invisible in most diffraction and electron microscopy experiments, is found to have an important impact on light emission. As the crystalline core grows at the expense of a thinning amorphous shell with increasing Ta, the PL undergoes a transition from a regime dominated by disorder-induced effects to a situation where quantum confinement of excitons prevails.

Excess silicon concentration dependence of the structural and optical properties of silicon nanocrystals embedded in silicon oxide matrix

Silicon (Si) nanocrystals embedded in Si oxide matrix have been formed by rapid thermal annealing of sub-stoichiometric Si oxide films (SiOx with x < 2). The SiOx films were deposited by co-sputtering of Si oxide and Si target using magnetron RF sputtering technique. The Si-to-SiO2 ratio was controlled by varying the number of Si chips being placed on the pure SiO2 target during sputtering. Rapid thermal anneal in nitrogen gas at 1100°C lead to the decomposition of SiOx into Si nanocrystals and SiO2. The structural (size of nanocrystals) and optical properties (absorption and luminescence) of Si nanocrystals embedded in oxide matrix, were found, strongly depend on the initial excess Si concentration in SiOx films.

Low-Temperature Deposition of Silicon Oxide Film from the Reaction of Silicone Oil Vapor and Ozone Gas

Using the chemical reaction between silicone oil vapor and ozone gas at atmospheric pressure and above 200 °C, we deposited a Si oxide film. Fourier transformer infrared (FT-IR) spectra of the films were very similar to those of thermal SiO2 films without peaks due to carbon for deposition temperatures Ts from 200 to 350 °C. Although a small peak due to the Si–OH bond appeared for Ts ≤300 °C, it was reduced with increasing Ts and almost disappeared around 350 °C. Secondary ion mass spectrometry measurements showed that the concentrations of metal and carbon impurities were at a negligible level for device performance even for Ts = 200 °C. The dielectric property of the film deposited at 350 °C was also shown to be comparable to the SiOx film produced by a conventional low-temperature deposition method, although the estimated density of interface trap was in the order of 1011/(cm2·eV).

Selective X‐ray analysis of electron localizing sites using capacitance or electrostatic force

AbstractFor selective analyses of nanostructures, an X‐ray analysis in which X‐ray‐induced photoionization of electron localizing sites is probed with capacitance (C) or electrostatic force (EF) is proposed. The photoionization of nanostructures with localized electrons provides a quasi‐stable state with a long lifetime of 1 ms or longer. The photoionization is rapidly relaxed to initial state within several femtoseconds at the other sites without localized electrons. Because of high sensitivity of C and EF to the quasi‐stable state with long lifetime, selective analyses of nanostructure can be achieved. We demonstrate this technique with samples of Cu metal and chemically deposited Si oxide film. We observe the modulation of C and EF with a parallel‐plate capacitor and electrostatic force micro‐scopy. The C modulation dependent on X‐ray photon energy indicated an electronic state of Cu/Cu2O interface, and the EF modulation realized stoichiometry mapping of thin SiOx film on Si wafer. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 92(3): 38–45, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10016

Photoluminescence from Er-doped Si-rich Si oxides deposited by magnetron sputtering in Ar or Ar+H2 plasmas

The authors have studied photoluminescence (PL) from Er-doped Si-rich Si oxide (SRSO) films deposited by magnetron sputtering of an Er+Si+SiO2 composite target in Ar or Ar+H2 ambients. When the samples were annealed in N2, for the film grown in an Ar ambient, the PL annealing behaviors reveal that the emissions from the film are defect-related and that the Er3+ PL at 1.54μm is possibly triggered by a defect-mediated energy transfer process; while for the films grown in an Ar+H2 ambient, the emissions from the SRSO matrix are suppressed and the Er PL intensities increase significantly but differently dependent on the Ar:H2 ratios during sputtering. After annealing the samples in an Ar+5%H2 (FG) ambient, however, almost no Er PL was observed from the film grown in the Ar ambient, while the Er PL intensities of the films grown in the Ar+H2 ambient increase further compared to those annealed in N2. Fourier transform infrared spectroscopy shows that the absorption of the samples after FG annealing is weaker th...

Electronic properties of the interface between Si and sputter deposited indium-tin oxide

We have investigated electronic properties of the interface between p-type Si and indium-tin oxide (ITO) films grown by magnetron sputtering. Current–voltage ( I– V) and capacitance–voltage ( C– V) measurements and deep level transient spectroscopy (DLTS) have been employed. The behaviour of the I– V and C– V characteristics for the ITO/p-Si junctions is correlated to the presence of interfacial states observed by DLTS. Thermal stability of the interfacial states is studied in the temperature range 100–400 °C. For the structures deposited at room temperature, the I– V characteristics show a diode-like behaviour with rectification of about three orders of magnitude. However, the I– V curve cannot be described within the thermionic emission–diffusion approximation. C– V measurements reveal non-linearity in the 1/ C 2 dependence on V in the near-interface region, indicating the presence of carrier traps. Hole traps have been indeed observed by DLTS and exhibit an activation energy of 0.3 eV with a physical location near the p-Si/ITO interface. Heat treatments in the temperature range 100–400 °C result in annealing of these hole traps and improvement of the I– V and C– V characteristics. The correlation between the electrical properties of the Si/ITO heterojunction and the presence of the electronic states is discussed.

Phase-separated Si nanoclusters from Si oxide matrix grown by laser-assisted chemicalvapor deposition

A CO2 laser was used in combination with a conventional plasma-enhancedchemical vapor deposition system to assist the decomposition ofSiH4 molecules and to grow Si nanoclusters embedded in Si oxide films. The compositional andstructural properties of the grown films were characterized by energy dispersivespectrometry (EDS) and Fourier transformation infrared (FTIR) spectrometry. A twophase mixture model of Si nanoclusters separated from the grown Si oxide matrix wasproposed to explain the formation mechanism of Si nanoclusters embedded in the Si oxidematrix. The dependences of the photoluminescence photon energy and intensity on theCO2 laser power were observed. According to the model, the refractive indices of the Si oxide phasein the grown film were deduced and are in agreement with the theoretically calculatedvalues. These results demonstrated the existence of laser-induced phase-separatedSi nanoclusters. In addition, the relation between the sizes of Si nanoclusters and theCO2 laser power was also elucidated.

Fabrication of multilayered Ge nanocrystals by magnetron sputtering and annealing

Multilayered Ge nanocrystals embedded in Si and Ge oxide films have been fabricated on Si substrateby a (SiO2+Ge)/(SiO2+GeO2) superlattice approach, using an rf magnetron sputtering technique with aGe+SiO2 composite target and subsequent thermal annealing inN2 ambientat 750 °C for 5 min. X-ray diffraction (XRD) measurements indicated the formation of Genanocrystals with an average size estimated to be 9.8 nm. Raman scatteringspectra showed a peak of the Ge–Ge vibrational mode shifted downwards to298.8 cm−1, which was caused by quantum confinement of phonons in the Ge nanocrystals. X-rayphotoemission spectroscopy (XPS) analysis demonstrated that the Ge chemical state is mainlyGe0 in the(SiO2+Ge) layerand Ge4+ in the (SiO2+GeO2) layer in the superlattice structure. Transmission electronmicroscopy (TEM) revealed that Ge nanocrystals were confined in(SiO2+Ge) layers, and had good crystallinity. This superlattice approach significantly improved boththe size uniformity of Ge nanocrystals and their uniformity of spacing on the‘Z’ growth direction compared with the conventional Ge-ncs fabrication method using a single and thickSiO2 matrix film.

Infrared transmittance spectra of photoluminescent oxide films with Si and Ge quantum dots formed by pulsed laser ablation

The composition of photoluminescent films containing low-dimensional silicon and germanium is studied. Si, Ge, and Al oxide films containing Si and Ge quantum dots are produced by pulsed laser ablation. The infrared transmittance spectra in the range of wave numbers from 650 to 1400 cm−1 and the time-resolved photoluminescence spectra in the energy range from 1.4 to 3.2 eV at room temperature are recorded. Correlations between the conditions of formation of the films, their photoluminescence properties, and the stretching vibrations of Si-O-Si, Ge-O-Ge, and Al-O bonds are established.

In situ monitoring of nucleation and evolution of Ge nanodots on faintly oxidized Si(1 1 1) surfaces

We have investigated the nucleation and evolution of germanium (Ge) nanodot (ND)s taking place while depositing Ge onto the silicon (Si) (1 1 1) surfaces with ultra-thin Si oxide films by using ultra-high vacuum in situ high-resolution transmission electron microscopy in the profile-imaging geometry. Various types of growth phenomena such as nucleation, growth and coalescence of Ge NDs have successfully been observed. The results show that the growth phenomena of the Ge NDs are dramatically rapid after their size reaches the size of the critical nucleus. The critical nucleus size estimated from a model using the cohesive energy of the Ge NDs has been consistent with observed one.

On the effects of double-step anneal treatments on light emission from Er-doped Si-rich silicon oxide

We have studied photoluminescence (PL) from an Er-doped Si-rich Si oxide (SRSO) film thermally annealed under different conditions. Compared to the case of annealing in N2 alone, double-step annealing the film at 875°C in N2 and then at ∼850°C in O2 or vice versa increases Er PL intensities by 10%–15%; while double-step annealing in N2+5%H2 (FG) and then in O2 or vice versa yields significant enhancements of the PL from the SRSO matrix and the Er PL intensity decreases differently by exchanging the processing order. Fourier transform infrared spectroscopy indicates that silicon oxynitride forms after annealing in FG ambient, and for the samples initially oxidized, the increase of Er PL intensity after secondary annealing in N2 (or FG) is due to more Si nanoclusters being formed. The PL spectra exhibit different annealing behavior with increasing the FG annealing temperature and the processing order. Weak oxygen bonds and silicon oxynitrides are believed to form upon O2 and FG annealing, respectively, and play important roles in the PL.

Optimization of the luminescence emission of Si nanocrystals synthesized from non-stoichiometric Si oxides using a Central Composite Design of the deposition process

Si oxide films with a controlled excess of Si were deposited on Si wafers by LPCVD using Si 2H 6 and O 2, thermally annealed to 1100 °C for 1 h to form Si nanocrystals embedded in SiO 2 and subsequently annealed at 450 °C in forming gas. The samples were characterized by Fourier transform infrared spectroscopy, spectroscopic ellipsometry and cathodoluminescence spectroscopy. The excess of Si in the as-deposited samples, ranging from 0 to 70% in volume, was obtained from the ellipsometry data analysis. After annealing at 1100 °C, the samples show a luminescence band (peaking at 665 nm) at 80 K and at room temperature which is associated to the presence of Si nanocrystals. The growth rate, the excess of Si incorporated to the films and the intensity of the luminescence band were modelled using a Face-Centered Central Composite Design as a function of the main deposition variables (pressure, 185–300 mTorr; temperature, 250–400 °C; Si 2H 6/O 2 flow ratio, 2–5) aiming to control the growth process and the incorporation of Si in excess as well as to determine the experimental conditions that yield the samples with the maximum intensity of the luminescence emission.

Chemical phase transitions of a Si oxide film on SiC by MeV electron beam irradiation

An ultrathin Si oxide film grown on a 6H-SiC(0001) wafer was irradiated with 1MeV electron beam to examine its effect on the chemical species of a Si oxide/SiC wafer, where the Si oxide film was composed of SiO2, Si suboxides (Si3+, Si2+, and Si1+), and Si oxycarbides (Si–C–O). Scanning photoelectron microscopy and Si 2p core-level spectroscopy show that e-beam irradiation induces chemical phase transitions from the Si suboxides and Si oxycarbides to SiO2. This suggests that e-beam irradiation is an efficient and simple method of producing a chemically uniform SiO2 film on SiC without thermal and chemical treatments.

Optical activation of Si nanowires by Er3+ doped binary Si–Al oxides films derived from sol–gel solutions

The optical activation of Si nanowires (SiNWs) by the coating of Er3+ doped binary silicon–aluminum oxides (Si–Al oxides) films derived from sol–gel solutions is reported. Continuous and crack-free Si–Al oxide film could be successfully coated onto an Si substrate where the SiNWs were grown. The strong Er3+ luminescence of 1.534μm from the SiNWs was observed at a high Er concentration (5at.%). These results suggest that the Al–Si oxide film makes it possible to realize a strong Er3+ luminescence by excluding concentration quenching while at the same time improving the quality of film.

Selective X-Ray Analyses of Electron Localizing Sites Using Capacitance or Electrostatic Force

For selective analyses of nano structures, an x-ray analysis in which x-ray induced photoionization of electron localizing sites is probed with capacitance (C) or electrostatic force (EF) is proposed. The photoionization of nano structures with localized electrons provides a quasi-stable state with a long lifetime of > ms. The photoionization is rapidly relaxed to initial state within ˜fs at the other sites without localized electrons. Because of high sensitivity of C and EF to the quasi-stable state with long lifetime, selective analyses of nano structure can be achieved. We demonstrate this technique with samples of Cu metal and chemically deposited Si oxide film. We observe the modulation of C and EF with a parallel plate capacitor and electrostatic force microscopy. The C modulation dependent on x-ray photon energy indicated an electronic state of Cu/Cu2O interface, and the EF modulation realized stoichiometry mapping of thin SiOx film on Si wafer.

Light Emission From Hydrogenated and Unhydrogenated Si-Nanocrystal/Si Dioxide Composites Based on PECVD-Grown Si-Rich Si Oxide Films

Hydrogenated and unhydrogenated Si-nanocrystal/Si dioxide (Si-nc/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) composites were obtained from Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> (y=0.36,0.42) thin films deposited by plasma-enhanced chemical vapor deposition. The unhydrogenated composites were fabricated by promoting the Si precipitation through the thermal annealing of the films in the flowing pure Ar at temperatures up to 1100degC. The hydrogenated composites were obtained from identical films by replacing the Ar with (Ar+5% H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) in the annealing step. The photoluminescence (PL) of the composites was studied as a function of the annealing temperature (T), annealing time, and pump laser power. The PL intensity increases with increasing annealing temperature and time; however, it increases faster and attains several hundreds percent larger values when the annealing is performed under (Ar+5% H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) as compared to the annealing under pure Ar. Fourier-transform infrared spectra show that H in these hydrogenated samples incorporates mainly as Si-H bonds. The dependence of the PL spectra on y,T, and laser power are consistent with the assumption that light emission in both the hydrogenated and unhydrogenated Si-nc/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composites originates from the bandgap transitions involving the electron quantum confinement in the Si-ncs. The PL spectra from the hydrogenated films are skewed to the red as compared to those from the unhydrogenated ones. The bulk of the data indicates that H passivates the nonradiative recombination centers, most probably Si dangling bonds in disordered Si-nc/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> regions, thus increasing the number of Si-ncs that contribute to the PL and modifying the distribution of the emission wavelengths

Correlation between optical properties and Si nanocrystal formation of Si-rich Si oxide films prepared by plasma-enhanced chemical vapor deposition

We have investigated the phase separation and silicon nanocrystal (Si NC) formation in correlation with the optical properties of Si suboxide (SiO x , 0 < x < 2) films by thermal annealing in high vacuum. The SiO x films were deposited by plasma-enhanced chemical vapor deposition at different nitrous oxide/silane (N 2O/SiH 4) flow ratios. The as-deposited films show increased Si concentration with decreasing N 2O/SiH 4 flow ratio, while the deposition rate and surface roughness have strong correlations with the flow ratio in the N 2O/SiH 4 reaction. After thermal annealing at temperatures above 1000 °C, Fourier transform infrared spectroscopy, Raman spectroscopy, and transmission electron microscopy manifest the progressive phase separation and continuous growth of crystalline-Si (c-Si) NCs in the SiO x films with increasing annealing temperature. We observe a transition from multiple-peak to single peak of the strong red-range photoluminescence (PL) with increasing Si concentration and annealing temperature. The appearance of the single peak in the PL is closely related to the c-Si NC formation. The PL also redshifts from ∼1.9 to 1.4 eV with increasing Si concentration and annealing temperature (i.e., increasing NC size). The good agreements of the PL evolution with NC formation and the PL peak energy with NC size distribution support the quantum confinement model.

Photoluminescence of Ge∕Si structures grown on oxidized Si surfaces

Ge ∕ Si structures grown on oxidized Si surfaces are studied by means of photoluminescence (PL). High-temperature annealing of the structures leads to the increase of their PL intensity by at least one order of magnitude at energies around 0.82eV. It is suggested that relaxation of strain, existing in our structures due to the Ge∕Si lattice mismatch, occurs through the formation of defects such as interstitial clusters in the Si layer capping the layer of Ge islands. These defects in conjunction with oxygen complexes of the rest of the Si oxide film provide the intense PL in the D1 range.