Chemical and phase compositions of silicon oxide films with nanocrystals prepared by carbon ion implantation

The chemical and phase compositions of silicon oxide films with self-assembled nanoclusters prepared by ion implantation of carbon into SiO _x (x < 2) suboxide films with subsequent annealing in a nitrogen atmosphere have been investigated using X-ray photoelectron spectroscopy in combination with depth profiling by ion sputtering. It has been found that the relative concentration of oxygen in the maximum of the distribution of implanted carbon atoms is decreased, whereas the relative concentration of silicon remains almost identical over the depth in the layer containing the implanted carbon. The in-depth distributions of carbon and silicon in different chemical states have been determined. In the regions adjacent to the layer with a maximum carbon content, the annealing results in the formation of silicon oxide layers, which are close in composition to SiO₂ and contain silicon nanocrystals, whereas the implanted layer, in addition to the SiO₂ phase, contains silicon oxide species Si²⁺ and Si³⁺ with stoichiometric formulas SiO and Si₂O₃, respectively. The film contains carbon in the form of SiC and elemental carbon phases. The lower limit of the average size of silicon nanoclusters has been estimated as ∼2 nm. The photoluminescence spectra of the films have been interpreted using the obtained results.     

Influence of the ion synthesis and ion doping regimes on the effect of sensitization of erbium emission by silicon nanoclusters in silicon dioxide films

The photoluminescence spectra of erbium centers in SiO₂ films with ion-synthesized silicon nanoclusters under nonresonant excitation were investigated. Erbium was introduced into thermal SiO₂ films by ion implantation. The dependences of photoluminescence intensity on the dose, the order of ion implantation of Si and Er, the annealing temperature, and additional Ar⁺ and P⁺ ion irradiation regimes, i.e., factors determining the influence of radiation damage and doping on sensitization of erbium luminescence by silicon nanoclusters, were determined. It was found that the sensitization effect and its amplification due to doping with phosphorus are most pronounced under the conditions where nanoclusters are amorphous. The quenching of photoluminescence due to radiation damage in this case manifests itself to a lesser extent than for crystalline nanoclusters. The role of various factors in the observed regularities was discussed in the framework of the existing concepts of the mechanisms of light emission and energy exchange in the system of silicon nanoclusters and erbium centers.     

Local structure of porous silicon studied by means of X-ray emission spectroscopy

2,3 X-ray emission spectra of porous silicon (P-Si) and of spark-processed silicon (sp-Si). Both types of Si-structure display strong photoluminescence in the visible range of the spectrum. Porous samples were prepared by anodization of n^-- and p⁺-Si-wafers. Whereas for the P-Si processed from p⁺-Si the presence of some amorphous silicon is detected, the X-ray emission spectra of porous Si prepared from n^--Si display a higher content of SiO₂. For spark-processed Si the Si L_2,3 X-ray emission spectra reveal a much stronger degree of oxidation which extends to depths larger than 10000 Å. Furthermore, the chemical state of silicon atoms of sp-Si measured at the center of the processed area is close to that of silicon dioxide, and it has an influence on the photoluminescence energy. Specifically, green photoluminescent sp-Si shows a higher degree of oxidation than the blue luminescent specimen. However, the depth of oxidation consistently decreases in areas with weak or no PL. Possible origins of the observed photoluminescence are discussed. Accepted: 6 March 1997     

Large-scale potential fluctuations caused by SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> compositional inhomogeneity

The short-range order in amorphous SiO _x (0 ≤ x ≤ 2) films has been studied by high-resolution X-ray photoelectron spectroscopy. Both the random bonding and random mixture models do not describe experimental photoelectron spectra of SiO _x (x ≤ 2). An intermediate model of the SiO _x structure has been proposed. The measured photoelectron spectra of the SiO _x (x ≤ 2) valence band indicate the presence of the silicon phase and silicon oxide.     

Si-nanostructures formation in amorphous silicon nitride SiNx:H deposited by remote PECVD

The formation of silicon nanoclusters embedded in amorphous silicon nitride (SiN_x:H) can be of great interest for optoelectronic devices such as solar cells. Here amorphous SiN_x:H layers have been deposited by remote microwave-assisted chemical vapor deposition at 300 °C substrate temperature and with different ammonia [NH₃]/silane [SiH₄] gas flow ratios (R=0.5−5). Post-thermal annealing was carried out at 700 °C during 30 min to form the silicon nanoclusters. The composition of the layers was determined by Rutherford back scattering (RBS) and elastic recoil detection analysis (ERDA). Fourier transform infrared spectroscopy (FTIR) showed that the densities of SiH (2160 cm⁻¹) and NH (3330 cm⁻¹) molecules are reduced after thermal annealing for SiN:H films deposited at flow gas ratio R>1.5. Breaking the SiH bonding provide Si atoms in excess in the bulk of the layer, which can nucleate and form Si nanostructures. The analysis of the photoluminescence (PL) spectra for different stoichiometric layers showed a strong dependence of the peak characteristics (position, intensity, etc.) on the gas flow ratio. On the other hand, transmission electron microscopy (TEM) analysis proves the presence of silicon nanoclusters embedded in the films deposited at a gas flow ratio of R=2 and annealed at 700 °C (30 min).     

Group-IV nanocluster formation by ion-beam synthesis

A short review of our investigations devoted to the use of ion-beam-synthesized nanoclusters for silicon-based light emission and nonvolatile memory effects is presented. Blue-violet light emission is demonstrated based on Ge-implanted silicon dioxide layers thermally grown on silicon substrates. This version of silicon-based light emission relies on Ge-related defects in the amorphous ≡Si–O–Si≡ network. The photoluminescence and electroluminescence are excited by a singlet S₀–S₁ transition of a neutral oxygen vacancy and by electron injection from the silicon substrate into the silicon dioxide layer, respectively. Whereas the photoluminescence excitation is a well-known mechanism, for the case of electroluminescence an interpretation was performed for the first time in the course of our studies. It was found that the most probable way to excite luminescence centers is the impact excitation by hot electrons. Whereas the injection is explained by trap-assisted tunneling of electrons from the substrate into the oxide, the electrons will be transported via traps or in the SiO₂ conduction band. The application of the silicon-based light-emitting devices for an integrated optocoupler arrangement is described. Another application of nanoclusters is based on the investigation of thin Si-implanted silicon dioxide layers for nonvolatile memory devices. First promising results demonstrate that the observed programming window can reach several volts and the devices exhibit excellent retention behavior. A 256 K-nv-SRAM is demonstrated showing a programming window of >1 V for write pulses of 12 V/8 ms. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-351/260-3411, E-mail: w.skorupa@fz-rossendorf.de     

Thermally induced defect photoluminescence in hydrogenated amorphous silicon

The intrinsic defect photoluminescence of hydrogenated amorphous silicon (a-Si:H) films has been investigated at high intensities of optical pumping that lead to heating of the film. It has been revealed that, for short heating times, the intensity of the defect photoluminescence increases exponentially with an increase in the temperature with an activation energy of 0.85 eV, which is considerably higher than the activation energy (∼0.2 eV) determined from experiments on classical annealing. This and other experimental results on the temperature dependence of the intensity and kinetics of the defect photoluminescence have been explained in terms of the “hydrogen glass” model by thermally induced generation of intrinsic defects in amorphous silicon. The results of the calculations are in good agreement with the experimental data on the defect photoluminescence that reflects the formation and annihilation of defects for short heating times under optical excitation.     

Synchrotron investigations of electronic and atomic-structure peculiarities for silicon-oxide films’ surface layers containing silicon nanocrystals

Films obtained using molecular-beam deposition of SiO powder on c-Si (111) substrates for the purpose of SiO2 system formation with silicon nanocrystals were investigated before and after 900–1100°C annealing by photoluminescence, ultrasoft X-ray emission spectroscopy, X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy, and X-ray diffraction. The appearance of (111)-oriented luminescent silicon nanoclusters in considerable amounts upon annealing at T = 1000–1100°C is established in the investigated films. An anomalous phenomenon of X-ray absorption quantum yield intensity reversal for the L _2,3 elementary silicon edge is detected. Models for this phenomenon are suggested.     

Photoelectric properties of the GeSi/Si heterostructures with self-assembled nanoclusters grown by sublimation molecular beam epitaxy in a GeH4 medium

The photosensitivity (PS) spectra of the GeSi/Si(001) heterostructures with self-assembled nanoclusters grown by sublimation molecular beam epitaxy in a GeH₄ medium have been studied by photo-emf spectroscopy at the semiconductor/electrolyte junction (PSE) and by photo-emf and photocurrent spectroscopy of Schottky barriers including the temperature dependence of the PS spectra in the temperature range of 10–300 K. The bands related to the phonon assisted and phonon-less interband of spatially indirect optical transitions in the GeSi nanoclusters have been observed in the PS spectra even at 300 K. The scatter effect of the GeSi nanoclusters in size and/or in composition on the PS spectrum’s edge shape has been theoretically considered. The emission theory of the photoexcited carriers from the quantum wells of the GeSi/Si nanoclusters built-in to a Schottky barrier (p-tn junction, semiconductor/electrolyte junction) has been developed.     

Synthesis of hydrophobic gold nanoclusters: growth mechanism study,luminescence property and catalytic application

One-pot synthesis of well dispersed, size-controlled gold nanoparticles with the average size of 10–15 nm and luminescent gold nanoclusters with average size of 1.7–2.0 nm were successfully achieved by thermal decomposition of gold organometallic precursor CH₃AuPPh₃ in the presence of thiol surfactants in o-xylene. Only difference between the preparations of two types of Au nanoparticles is the amount of thiol surfactant employed. The mechanistic study of formation of gold nanoparticles was carried out by analyzing the samples at different reaction time intervals and revealed that two-staged growth process was involved. The nanoclusters showed strong red emission with the maximum intensity at about 600 nm. The maximum room temperature photoluminescence quantum yield was measured as 1.2%. The catalytic ability of the Au nanoclusters to promote Suzuki–Miyaura coupling involving the C–C bond formation was also investigated.     

Investigation of the dependence of the photoluminescence properties of silicon nanoclusters on their volume fraction in a silicon oxide matrix

The photoluminescence (PL) spectra and kinetics of amorphous and crystalline silicon nanoclusters are investigated. The given nanoclusters are formed by thermal annealing of thin suboxide silicon films with different volume fractions of silicon. It is demonstrated that the PL intensity and lifetime of the ensembles of silicon nanocrystals have a steplike dependence on the silicon volume fraction in the film. The influence of the percolation effect on the photoluminescence properties of the structures under study is discussed.     

Spatial versus quantum confinement in porous amorphous silicon nanostructures

Light-emitting porous amorphous silicon has been produced by anodization in HF of hydrogenated amorphous silicon films. The maximal thickness of the porous films is limited by the onset of an instability which results in the formation of large channels short-circuiting the amorphous layer. This is due to the high resistivity of the amorphous silicon films as compared to that of the electrolyte. Confinement effects on the electron wavefunction are analyzed in situ using photoluminescence measurements in hydrofluoric acid and compared to those observed in porous crystalline silicon. For crystalline silicon, a huge blue shift of the photoluminescence is observable upon reducing the size of the structures by photo-etch, showing clear evidence of quantum confinement effects in this material. No shift has been observed when carrying out the same experiment with amorphous silicon. This indicates that the extent of the wavefunction in the bandtail states involved in luminescence is too small to be sensitive to confinement down to the minimum sizes of our porous material ( 3 nm). Measurements of the width and the temperature dependence of the photoluminescence demonstrate that the Urbach energy does not change upon increasing the porosity, i.e., upon decreasing the size of the a-Si:H nanostructures, in contradiction with what has been reported in ultrathin a-Si:H multilayers. Received: 3 August 1998     

Specific features and mechanisms of photoluminescence of nanostructured silicon carbide films grown on silicon in vacuum

The light-emitting properties of cubic silicon carbide films grown by vacuum vapor phase epitaxy on Si(100) and Si(111) substrates under conditions of decreased growth temperatures (T _gr ∼ 900–700°C) have been discussed. Structural investigations have revealed a nanocrystalline structure and, simultaneously, a homogeneity of the phase composition of the grown 3C-SiC films. Photoluminescence spectra of these structures under excitation of the electronic subsystem by a helium-cadmium laser (λ_excit = 325 nm) are characterized by a rather intense luminescence band with the maximum shifted toward the ultraviolet (∼3 eV) region of the spectral range. It has been found that the integral curve of photoluminescence at low temperatures of measurements is split into a set of Lorentzian components. The correlation between these components and the specific features of the crystal structure of the grown silicon carbide layers has been analyzed.     

Spectral diffusion of quasi localized excitons in single silicon nanocrystals

Evolution in time of photoluminescence spectra of SiO_x capped single silicon nanocrystals has been investigated by means of confocal optical spectroscopy at room temperature. Large spectral jumps between subsequent spectra of up to 40 meV have been detected leading to noticeable line broadening and variation in the electron–phonon coupling. Further, a correlation between emission energy and emission intensity has been found and discussed in terms of an intrinsic Stark effect. Anti-correlated variations of the electron–phonon coupling to Si and SiO₂ phonons as a function of photoluminescence energy indicate that the nearly localized excition is to some extent coupled to phonons in the shell covering the silicon nanocrystal. However, coupling is reduced upon increasing Stark effect, while at the same time coupling to phonons of the Si core increases.     

Photoluminescence and Raman scattering in spatially inhomogeneous heteroepitaxial InGaN layers

Spatial inhomogeneities of the indium distribution in In _x Ga_1–x N epitaxial layers grown on sapphire substrate with a GaN buffer layer were investigated using photoluminescence (PL) in addition to confocal scanning Raman spectroscopy (RS) and PL. Broad emission bands from In-enriched InGaN nanoclusters (700–900 nm) and from the volume outside the clusters (about 460 nm) were observed in PL spectra of an epitaxial InGaN layer with an average In content of 25.7%. It was established that larger micro-PL intensities corresponded to energetically shallower clusters. The observed broadly asymmetric A₁(LO) RS band of InGaN confirmed that the In concentration in the layer was highly variable. Modeling the LO phonon band by two Lorentzian curves gave an average In concentration of 21% in the volume outside the clusters and 37% in the nanoclusters, which was considerably higher than the average concentration in the layer and agreed well with their PL band positions.     

The microstructure of SiO thin films: from nanoclusters to nanocrystals

The microstructure and electronic structure of silicon-rich oxide (SRO) films were investigated using transmission electron microscopy and electron energy loss spectroscopy as the main analytical techniques. The as-deposited SRO film was found to be a single phase SiO_1.0, as suggested by its electronic structure characteristics determined by the valence electron energy loss spectrum. This single phase undergoes a continuous but incomplete phase decomposition to Si and SiO₂ for films annealed between 300 and 1100°C. The resulting Si phase first appears as ～2?nm-diameter amorphous clusters which grow to larger sizes at higher annealing temperatures, but only crystallize at a critical temperature between 800 and 900°C. This cluster/matrix configuration of the SRO films is consistent with the appearance of the interface plasmon and its oscillator strength as a function of the nanoparticle size. Three separate stages were identified in the sequence of annealed films that were characterized by the presence of single-phase SiO, amorphous silicon nanoclusters, and silicon nanocrystals, respectively. The presence of amorphous silicon nanoclusters in the intermediate stage, the mean size of which can be controlled via annealing, may offer an alternative to silicon nanocrystal composites for optical applications.     

Porous silicon based β-FeSi2 and photoluminescence

Highly-pure iron powder was covered on porous silicon for fabricating semiconducting β-FeSi₂ structures. X-ray diffraction and Raman scattering results confirm the formation of pure-phase β-FeSi₂ after high-temperature annealing at 1100°C and then long-time persistence at 900°C. Scanning electron microscope observations reveal that large-size (>μm) β-FeSi₂ grains mainly form in the pores of porous silicon and some nanocrystals grow on local surfaces. The temperature-dependent photoluminescence spectra disclose that the observed ∼1.54 μm emission arises from free exciton recombination, which is confirmed via the activation energy (0.25 eV) measurement. Our method provides a way to synthesize single-phase β-FeSi₂ materials.     

Influence of natural aging on photoluminescence from porous silicon

The influence of natural aging on the photoluminescence intensity and the position of a photolu-minescence peak in n-type por-Si (por-Si) is studied. The variation of the phase composition and the relative content of the amorphous and oxide phases of silicon in por-Si during aging is determined by fitting simulated spectra to experimental ultrasoft Si L _2,3 X-ray emission spectra using reference spectra.     

a-Si:H/a-SiNx:H超晶格薄膜光致发光性质的研究

本文报道了a-Si:H/a-SiNx:H超晶格薄膜光致发光某些性质的研究。实验发现,这种超晶格薄膜光致发光的强度和峰值能量随交替层a-Si:H厚度,测量温度及光照时间等而变化。同时还发现,在阴、阳两极上,利用GD法沉积的样品,发光强度和峰值能量也有所不同。文中对这些实验结果作了初步解释。     

A study of photosensitive InAs/GaAs heterostructures with quantum dots grown by ion-beam deposition

The possibility of obtaining ion-beam-deposited InAs/GaAs heterostructures with quantum dots for photovoltaic converters is shown. The surface morphology of the grown heterostructures is analyzed by scanning probe microscopy. Quantum dots and InAs nanoclusters with planar dimensions from 20 to 100 nm and a height from 5 to 80 nm are detected. The average surface density of quantum-dimensional InAs objects with a size below 35 nm is 10⁵ mm⁻². In the photoluminescence spectra (T = 300 K), a peak is revealed with a maximum at the wavelength λ = 1150 nm (hν ≈ 1.1 eV), which shows that the grown heterostructures contain InAs quantum dots of various sizes.