Visible and infrared photoluminescence from Er-doped SiOx

The annealing behaviors of photoluminescence of SiO_x and Er-doped SiO_x grown by molecular beam epitaxy in the wavelength range of visible and infrared light are studied. For SiO_x, four PL bands located at 510, 600, 716 and 810 nm, respectively, are observed. For Er-doped SiO_x, the 716 nm band, which is believed to be originated from the electron–hole recombination at the interface between crystalline Si and amorphous SiO₂, disappears in the annealing temperature range of 500–900°C. It is suggested the enhancement of Er luminescence is partially due to the energy transfer from the recombination at the interface between crystalline Si and SiO₂ to Er ions.     

UHV-MOCVD growth of TiO2 on SiOx/Si(1 1 1): Interfacial properties reflected in the Si 2p photoemission spectra

Metal–organic chemical vapour deposition growth of titanium oxide on moderately pre-oxidised Si(1 1 1) using the titanium(IV) isopropoxide precursor has been studied for two different growth modes, reaction-limited growth at 300 °C and flux-limited growth at 500 °C. The interfacial properties have been characterized by monitoring synchrotron radiation excited Si 2p photoemission spectra. The cross-linking from oxidised Si to bulk Si after TTIP exposure has been found to be very similar to that of SiO_x/Si(1 1 1). However, the results show that the additional oxidation of Si most probably causes a corrugation of the SiO_x/Si interface. Those conclusions are valid for both growth modes. A model is introduced in which the amorphous interface region is described as (TiO₂)_x(SiO₂)_y where x and y changes linearly and continuously over the interface. The model quantifies how (TiO₂)_x(SiO₂)_y mixing changes the relative intensities of the signals from silicon oxide and silicon. The method can be generalised and used for the analyses of other metal-oxides on silicon.     

Substrate temperature dependence of the photoluminescence efficiency of co-sputtered Si/SiO2 layers

Si particles embedded in an SiO₂ matrix were obtained by co-sputtering of Si and SiO₂ at various deposition temperatures T_d (200–700°C) and annealing at different temperatures T_a (900–1100°C). The systems were characterized by X-ray photoelectron, Raman scattering, infrared absorption and photoluminescence spectroscopy techniques. The results show that the photoluminescence efficiency is strongly dependent on the degree of phase separation between the Si nanocrystals and the SiO₂ matrix. This is likely connected with the Si/SiO₂ interface characteristics, together with the features indicating the involvement of quantum confinement.     

Amorphous Si/insulator multilayers grown by vacuum deposition and electron cyclotron resonance plasma treatment

a-Si/insulator multilayers have been deposited on (0 0 1) Si by electron gun Si evaporation and periodic electron cyclotron resonance plasma oxidation or nitridation. Exposure to an O or N plasma resulted in the formation of a thin SiO₂ and SiN_x layer whose thickness was self-limited and controlled by process parameters. For thin-layer (2 nm) Si/SiO₂ and Si/SiN_x multilayers no visible photoluminescence (PL) was observed in most samples, although all exhibited weak “blue” PL. For the nitride multilayers, annealing at 750°C or 850°C induced visible PL that varied in peak energy with Si layer thickness. Depth profiling of a-Si caps on thin insulating layers revealed no detectable contamination for the SiN_x layers, but substantial O contamination for the SiO₂ films.     

Reversible H2 passivation of Si ≡ Si3 interface defects in (1 1 1)Si/SiO2

K-band electron spin resonance (ESR) at 4.3 K has revealed the dipole-dipole (DD) interaction effects between [1 1 1]P_b centers (^*Si ≡ Si₃ defects with unpaired sp³ hybrid [1 1 1]) at the 2 dimensional (1 1 1)Si/SiO₂ interface. This has been enabled by the perfectly reversible H₂ passivation of P_b, which affects the defect's spin state. Sequential hydrogenation at 253–353°C and degassing treatments in high vacuum at 743–835°C allowed to vary the P_b density in the range 5 × 10¹⁰ < [P_b] (1.14 ± 0.06) × 10¹³ cm^-2. With increasing [P_b] fine structure doublets are clearly resolved. It is found that (1 1 1)Si/SiO₂ interfaces, dry thermally grown at ≈920°C, naturally comprise a ^*Si ≡ Si₃ defect density — passivated or not — of 1.14 × 10¹³ cm^-2.     

Defect versus nanocrystal luminescence emitted from room temperature and hot-implanted SiO2 layers

Silicon nanocrystals have been synthesized in SiO₂ matrix using Si ion implantation. Si ions were implanted into 300-nm-thick SiO₂ films grown on crystalline Si at energies of 30–55 keV, and with doses of 5×10¹⁵, 3×10¹⁶, and 1×10¹⁷ cm⁻². Implanted samples were subsequently annealed in an N₂ ambient at 500–1100°C during various periods. Photoluminescence spectra for the sample implanted with 1×10¹⁷ cm⁻² at 55 keV show that red luminescence (750 nm) related to Si-nanocrystals clearly increases with annealing temperature and time in intensity, and that weak orange luminescence (600 nm) is observed after annealing at low temperatures of 500°C and 800°C. The luminescence around 600 nm becomes very intense when a thin SiO₂ sample is implanted at a substrate temperature of 400°C with an energy of 30 keV and a low dose of 5×10¹⁵ cm⁻². It vanishes after annealing at 800°C for 30 min. We conclude that this luminescence observed around 600 nm is caused by some radiative defects formed in Si-implanted SiO₂.     

Excess Si concentration dependence of the photoluminescence of Si nanoclusters in SiO2 fabricated by ion implantation

A method for the fabrication of luminescent Si nanoclusters in an amorphous SiO₂ matrix by ion implantation and annealing, and the detailed mechanisms for the photoluminescence are reported. We have measured the implanted ion dose, annealing time and excitation energy dependence of the photoluminescence from implanted layers. The samples were fabricated by Si ion implantation into SiO₂ and subsequent high-temperature annealing. After annealing, a photoluminescence band below 1.7 eV has been observed. The peak energy of the photoluminescence is found to be independent of annealing time and excitation energy, while the intensity of the luminescence increases as the annealing time and excitation energy increase. Moreover, we found that the peak energy of the luminescence is strongly affected by the dose of implanted Si ions especially in the high dose range. These results indicate that the photons are absorbed by Si nanoclusters, for which the band-gap energy is modified by the quantum confinement effects, and the emission is not simply due to direct electron–hole recombination inside Si nanoclusters, but is related to defects probably at the interface between Si nanoclusters and SiO₂, for which the energy state is affected by Si cluster–cluster interactions. It seems that Si nanoclusters react via a thin oxide interface and the local concentrations of Si nanoclusters play an important role in the peak energy of the photoluminescence.     

Characterization of high-k gate dielectric/silicon interfaces

We have shown that, for thermally evaporated Ta₂O₅ or ZrO₂ thin films on Si(1 0 0), O₂ annealing at 300–500 °C causes the formation of an interfacial silicon oxide layer as thin as 1–2 nm which can be interpreted in terms of their high permeability to oxygen. And we have demonstrated how useful the energy loss spectra of photoexcited electrons from core levels such as O 1s are to measure the energy bandgaps of very thin insulators. With the combination of measured bandgaps and valence band lineups determined for X-ray photoelectron spectroscopy valence band spectra, we have determined the energy band alignments of Ta₂O₅ and ZrO₂ with Si(1 0 0) before and after the O₂ annealing at 500 °C. In addition, we have demonstrated that total photoelectron yield spectroscopy provides us direct information to quantify the energy distributions of both the defect states in the high-k dielectrics and the dielectric/Si(1 0 0) interface states over nearly entire Si bandgap.     

Photoluminescence and paramagnetic defects in silicon-implanted silicon dioxide layers

Thermally grown SiO₂ layers on Si substrates implanted with Si⁺ ions with a dose of 6×10¹⁶ cm⁻² were studied by the techniques of photoluminescence, electron paramagnetic resonance (EPR), and low-frequency Raman scattering. Distinct oxygen-vacancy associated defects in SiO₂ and non-bridging oxygen hole centers were identified by EPR. The luminescence intensity in the 620 nm range was found to correlate with the number of these defects. The low-frequency Raman scattering technique was used to estimate the average size of the Si nanocrystallites formed after the implantation and thermal annealing at T>1100°C, which are responsible for the photoluminescence band with a maximum at 740 nm. The intensity of this band can be significantly enhanced by an additional treatment of the samples in a low-temperature RF plasma.     

Penetration of electronic states from silicon substrate into silicon oxide

The depth profiling of O 1s energy loss in silicon oxide near the SiO₂/Si interface was performed using extremely small probing depth. As a result, the energy loss of O 1s photoelectrons with threshold energy of 3.5 eV was found. This value of 3.5 eV is much smaller than the SiO₂ bandgap of 9.0 eV, but quite close to direct interband transition at Γ point in energy band structure of silicon. This can be explained by considering the penetration of electronic states from silicon substrate into silicon oxide up to 0.6 nm from the interface. In addition, the penetrating depth is larger than the thickness of the compositional transition layer.     

The effect of annealing under hydrostatic pressure on the visible photoluminescence from si-ion implanted SiO2 films

We have studied the influence of the hydrostatic pressure during annealing on the intensity of the visible photoluminescence (PL) from thermally grown SiO₂ films irradiated with Si⁺ ions. Post-implantation anneals have been carried out in an Ar ambient at temperatures T_a of 400°C and 450°C for 10 h and 1130°C for 5 h at hydrostatic pressures of 1 bar–15 kbar. It has been found that the intensity of the 360, 460 and 600 nm PL peaks increases with rising hydrostatic pressure during low-temperature annealing. The intensity of the short-wavelength PL under conditions of hydrostatic pressure continues to rise even at T_a=1130°C. Increasing T_a leads to a shift in the PL spectra towards the ultraviolet range. The results obtained have been interpreted in terms of enhanced, pressure-mediated formation of ≡Si–Si≡ centres and small Si clusters within metastable regions of the ion-implanted SiO₂.     

Structural and electrical properties of crystalline (1−x)Ta2O5−xTiO2 thin films fabricated by metalorganic decomposition

Polycrystalline (1−x)Ta₂O₅−xTiO₂ thin films were formed on Si by metalorganic decomposition (MOD) and annealed at various temperatures. As-deposited films were in the amorphous state and were completely transformed to crystalline after annealing above 600 °C. During crystallization, a thin interfacial SiO₂ layer was formed at the (1−x)Ta₂O₅−xTiO₂/Si interface. Thin films with 0.92Ta₂O₅–0.08TiO₂ composition exhibited superior insulating properties. The measured dielectric constant and dissipation factor at 1 MHz were 9 and 0.015, respectively, for films annealed at 900 °C. The interface trap density was 2.5×10¹¹ cm⁻² eV⁻¹, and flatband voltage was −0.38 V. A charge storage density of 22.8 fC/μm² was obtained at an applied electric field of 3 MV/cm. The leakage current density was lower than 4×10⁻⁹ A/cm² up to an applied electric field of 6 MV/cm.     

A comparative infrared study of H2O reactivity on Si(1 0 0)-(2 × 1), (2 × 1)-H, (1 × 1)-H and (3 × 1)-H surfaces

The water adsorption on the bare and H-terminated Si(1 0 0) surfaces has been studied by the BML-IRRAS technique. It is found that H-terminated surfaces are much less reactive compared to the bare silicon surfaces. The (1 × 1)-H and (3 × 1)-H surfaces show similar and less reactivity pattern compared to the (2 × 1)-H surface. At higher exposures, the water reaction with coupled monohydride species provides an effective channel for oxygen insertion into the back bonds of dihydride species. It is not attributed to the H–Si–Si–H + H₂O → H–S–Si–OH + H₂, which could give rise to the characteristic Si–H and Si–OH modes, respectively at 2081 and 921 cm⁻¹. A more suitable reaction mechanism involving a metastable species, H–Si–Si–H + H₂O → H₂Si  HO–Si–H (metastable) explains well the bending modes of oxygen inserted silicon dihydride species which are observed relatively strongly in the reaction of water with H-terminated Si(1 0 0) surfaces.     

Exchange coupling effects in NiO/Co and NiO/permalloy bilayers

NiO/Co and NiO/Ni₈₀Fe₂₀ bilayers were prepared at 293 onto SiO₂(1 0 1)/Si(1 1 1) and glass substrates using UHV (5×10⁻¹⁰ mbar) RF/DC magnetron sputtering. Results on magnetic measurements showed that the exchange biasing and coercive fields are inversely proportional to the Co and Ni₈₀Fe₂₀ (Py) layer thickness down to 2 nm. A maximal RT coupling energy for the NiO–Co and NiO–Py interface was estimated as 0.04 and 0.03 mJ/m² for the samples prepared onto SiO₂(1 0 1)/Si(1 1 1) substrates.     

Real-time monitoring of SiO2/Si(1 1 1) interlayer etching by Brewster-angle reflectometry

A transitory etching regime after SiO₂ dissolution and before bulk Si(1 1 1) etching in neutral NH₄F solutions was monitored by in situ Brewster-angle reflectometry (BAR). An observed intermediate increase of the BAR reflectance signal is attributed to a fast dissolution of a stressed/strained interlayer beneath the SiO₂/Si(1 1 1) interface. Similar effects were observed on thin thermal oxides (18.2 nm), grown on float zone silicon, as well as on ultra-thin native oxides (1.2 nm) on Czochralsky silicon. Native oxide covered samples showed an increased surface roughness in the course of interlayer dissolution while the surface is progressively covered with compounds of fluorinated silicon. The etch rate, determined by atomic force microscopy (AFM) and compared to the etch rate of bulk silicon, is increased by a factor of four. In the limit of extended etching, the known low etch rates for silicon in 40% NH₄F are observed. Structural and chemical properties of the interfacial layer were analyzed by synchrotron radiation photoelectron spectroscopy (SRPES) which confirmed the presence of Si^3+/4+ valence states throughout the interlayer and by near open-circuit potential (N-OCP) dark current measurements. As a result, oxide etch rates in NH₄F in the pH-range 7–8 as well as the silicon interlayer depth can be assessed by in situ BAR.     

Cathodoluminescence properties of silicon nanocrystallites embedded in silicon oxide thin films

Cathodoluminescence (CL) spectra for the Si nanocrystallites embedded in a matrix of silicon oxide films are measured at room temperature. The CL spectra consist of two principal bands whose peak energies are in a near-infrared (NIR) region (<1.6 eV) and in a blue region (2.6 eV), respectively. The spectral feature of the NIR CL band is similar to the corresponding PL spectra. The strong correlation between the presence of Si nanocrystallites and the formation of the NIR CL band are found as well as the PL spectrum. The peak energy of the blue CL band is slightly lower than that of the luminescence band originating from oxygen vacancies (≡Si–Si≡) in SiO₂. Therefore, the blue CL band is considered to come from Si_n clusters with n3 in the oxide matrix. Under irradiation of electron beams, degradation of the intensity is observed for both the CL bands but the decay characteristics are different.     

Fabrication of p-Si/β-FeSi2 balls/n-si structures by MBE and their electrical and optical properties

We report on the formation technique of single-crystalline β-FeSi₂ balls (<100 nm) embedded in a Si p–n junction region by Si molecular beam epitaxy (MBE). β-FeSi₂ films grown on Si (0 0 1) by reactive deposition epitaxy (RDE) aggregated into islands after annealing at 850°C in ultrahigh vacuum. The islands of β-FeSi₂ aggregated further into a ball shape by following the Si MBE overgrowth at 750°C. It was found from X-ray diffraction (XRD) patterns that the epitaxial relationship between the two materials, and single-crystalline nature were preserved even after the annealing and the Si overgrowth. Capacitance–voltage (C–V) characteristics and transmission electron microscope (TEM) images revealed that a lot of defects were introduced around the embedded β-FeSi₂ balls with an increase of embedded β-FeSi₂ quantity.     

Analysis of frequency-dependent series resistance and interface states of In/SiO2/p-Si (MIS) structures

In this work, the investigation of the interface state density and series resistance from capacitance–voltage (C–V) and conductance–voltage (G/ω−V) characteristics in In/SiO₂/p-Si metal–insulator–semiconductor (MIS) structures with thin interfacial insulator layer have been reported. The thickness of SiO₂ film obtained from the measurement of the oxide capacitance corrected for series resistance in the strong accumulation region is 220 Å. The forward and reverse bias C–V and G/ω−V characteristics of MIS structures have been studied at the frequency range 30 kHz–1 MHz at room temperature. The frequency dispersion in capacitance and conductance can be interpreted in terms of the series resistance (R_s) and interface state density (D_it) values. Both the series resistance R_s and density of interface states D_it are strongly frequency-dependent and decrease with increasing frequency. The distribution profile of R_s–V gives a peak at low frequencies in the depletion region and disappears with increasing frequency. Experimental results show that the interfacial polarization contributes to the improvement of the dielectric properties of In/SiO₂/p-Si MIS structures. The interface state density value of In/SiO₂/p-Si MIS diode calculated at strong accumulation region is 1.11×10¹² eV⁻¹ cm⁻² at 1 MHz. It is found that the calculated value of D_it (≈10¹² eV⁻¹ cm⁻²) is not high enough to pin the Fermi level of the Si substrate disrupting the device operation.     

Competing thermal relaxation processes in response to intrinsic defects produced by exposing SiO2 to synchrotron radiation

Irradiation of SiO₂ with soft X-ray photons (hν>100 eV) produces a variety of defects, of which E₁′ centers and neutral Si–Si bonds are mainly responsible for the dielectric response change. The thermal processes that modify the structures around the defect sites have been investigated by in situ spectroscopic ellipsometry. Annealing the irradiated SiO₂ film diminishes the number of defects which are assigned to E₁′ centers by about half. The competing channels for annihilation of E₁′ centers are the recovery of the Si–O–Si bonding configuration and, in the opposite direction, the decomposition of the material into volatile products until the network is completely restructured. The other half of the defects are converted to Si–Si bond units and precipitates as nanocrystalline particles of Si.     

Covalent binding of cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene on Si(1 1 1)-7 × 7

The adsorption reactions and binding configurations of cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene on Si(1 1 1)-7 × 7 were studied using high-resolution electron energy loss spectroscopy (HREELS), ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and DFT calculation. The covalent attachments of these unsaturated hydrocarbons to Si(1 1 1)-7 × 7 through the formation of Si–C linkages are clearly demonstrated by the observation of the Si–C stretching mode at 450–500 cm⁻¹ in their HREELS spectra. For chemisorbed cyclohexene, the involvement of π_C=C in binding is further supported by the absence of C=C stretching modes and the disappearance of the π_C=C photoemission. The chemisorption of both 1,3-cyclohexadiene and 1,4-cyclohexadiene leads to the formation of cyclohexene-like intermediates through di-σ bonding. The existence of one π_C=C bond in their chemisorbed states is confirmed by the observation of the C=C and (sp²)C---H stretching modes and the UPS and XPS results. DFT calculations show that [4 + 2]-like cycloaddition is thermodynamically preferred for 1,3-cyclohexadiene on Si(1 1 1)-7 × 7, but a [2 + 2]-like reaction mechanism is proposed for the covalent attachment of cyclohexene and 1,4-cyclohexadiene.