Luminescence of amorphous silicon nanoclusters

Films of amorphous silicon suboxide α-SiO _x containing amorphous silicon nanoclusters have been grown by direct current magnetron sputtering. It has been found that two radiation bands are observed in the photoluminescence spectra of relatively large amorphous nanoclusters (a size of ∼2 nm) unlike one photoluminescence band of silicon nanocrystals of the same size. The form of the spectra upon the change in the nanocluster sizes agrees with that predicted in theoretical works, in which the energy spectrum of amorphous nanoclusters has been calculated taking into account quantum confinement of delocalized, weakly localized, and strongly localized states.     

Electronic structure and defect states of transition films from amorphous to microcrystalline silicon studied by surface photovoltage spectroscopy

In this paper, surface photovoltage spectroscopy (SPS) is used to determine the electronic structure of the hydrogenated transition Si films. All samples are prepared by using helicon wave plasma-enhanced chemical vapour deposition technique, the films exhibit a transition from the amorphous phase to the microcrystalline phase with increasing temperature. The film deposited at lower substrate temperature has the amorphous-like electronic structure with two types of dominant defect states corresponding to the occupied Si dangling bond states (D⁰/D^- and the empty Si dangling states (D⁺). At higher substrate temperature, the crystallinity of the deposited films increases, while their band gap energy decreases. Meanwhile, two types of additional defect states is incorporate into the films as compared with the amorphous counterpart, which is attributed to the interface defect states between the microcrystalline Si grains and the amorphous matrix. The relative SPS intensity of these two kinds of defect states in samples deposited above 300\du increases first and decreases afterwards, which may be interpreted as a result of the competition between hydrogen release and crystalline grain size increment with increasing substrate temperature.     

Emission of Si nanoclusters of different phases in amorphous hydrogenated silicon

This paper presents the results of PL spectrum studies for Si nano-clusters in an amorphous silicon matrix. The four amorphous Si layers were prepared by the hot-wire CVD method on glass substrates at a temperature of 250 ^°C and different filament temperatures in the range of 1650–1950 ^°C. The joint analysis of PL and X ray diffraction results dependant on technological conditions has been done. PL bands deal with Si nanocrystals and amorphous Si nanoclusters are discussed as well.     

Interaction between rare-earth ions and amorphous silicon nanoclusters produced at low processing temperatures

Temperatures of 1000 °C and higher are a significant problem for the incorporation of erbium-doped silicon nanocrystal devices into standard silicon technology, and make the fabrication of contacts and reflectors in light emitting devices difficult. In the present work, we use energy-filtered TEM imaging techniques to show the formation of size-controlled amorphous silicon nanoclusters in SiO films annealed between 400 and 500 °C. The PL properties of such films are characteristic of amorphous silicon, and the spectrum can be controlled via a statistical size effect—as opposed to quantum confinement—that has previously been proposed for porous amorphous silicon. Finally, we show that amorphous nanoclusters sensitize the luminescence from the rare-earth ions Er, Nd, Yb, and Tm with excitation cross-sections similar in magnitude to erbium-doped silicon nanocrystal composites, and with a similar nonresonant energy transfer mechanism.     

Auger electron spectroscopy of a hydrogenated amorphous silicon

Thin layers of a hydrogenated amorphous silicon were studied by means of the Auger electron spectroscopy (AES). It was found that the spectra of the a-Si : H samples exhibit a large peak at 34 eV which was ascribed to the L₁L₂₃V Coster-Kronig transition and that the intensity of the L₂₃VV transition was lowered, due to hydrogenation. The explanation of this feature is given on the basis of the electronic structure and the transition probabilities changes in silicon, due to hydrogenation. The results on the a-Si : H layer were compared with measurement of the a-Si layer and the influence of an electron and an ion bombardment, an elevated temperature and an exposure to oxygen on both layers was studied.The author would like to expres hiss thanks to Dr. J. Zemek for supplying the a-Si and a-Si : H layers, to Dr. J. Drahokoupil and Dr. J. imnek for stimulating discussions and to Dr. V. Cháb for helpful discussions and for his help with measurements.     

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 Si(y)O(1-y)), fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition and isochronally (2 h) annealed at various temperatures (T(a) = 900-1100 °C) under either Ar or (Ar + 5%H(2)) 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 T(a) 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 T(a), the PL undergoes a transition from a regime dominated by disorder-induced effects to a situation where quantum confinement of excitons prevails.     

Raman spectroscopy of amorphous silicon subjected to laser annealing

The effect of laser radiation power on the Raman spectra of amorphous silicon obtained by electron-beam evaporation has been revealed. The formation of nanocrystalline inclusions in the amorphous matrix under exposure to a laser with a power of more than 2.5 mW is established by Raman spectroscopy and photoluminescence. The influence of the fabrication conditions (substrate temperature and annealing in a vacuum) of source amorphous silicon films on the formation of nanocrystalline inclusions formed by subsequent laser treatment has been investigated. The features of silicon nanocrystal formation in cases when the original amorphous silicon film is obtained at a substrate temperature of ∼250°C have been revealed. These features may be associated with the presence of silicon-silicon multiple bonds.     

Surface chemistry of silicon nanoclusters

We employ density functional and quantum Monte Carlo calculations to show that significant changes occur in the gap of fully hydrogenated nanoclusters when the surface contains passivants other than hydrogen, in particular atomic oxygen. In the case of oxygen, the gap reduction computed as a function of the nanocluster size provides a consistent interpretation of several recent experiments. Furthermore, we predict that other double bonded groups also significantly affect the optical gap, while single bonded groups have a minimal influence.     

表面光电压谱及其检测技术实验

利用锁相放大器、单色仪、Xe灯光源等搭建了表面光电压谱检测仪.基于表面光电压谱技术的原理,并借助场诱导表面光电压谱测定了半导体材料的光伏响应、导电类型、能带隙和表面电荷分布.     

Quantitative analysis of hydrogen in amorphous silicon using Raman scattering spectroscopy

Hydrogenated amorphous silicon (a‐Si:H) films were studied using infrared and Raman spectroscopy. We have experimentally found that ratios of Raman scattering cross‐sections for Si–H to Si–Si bonds and for Si–H₂ to Si–Si bonds are equal to 0.65 ± 0.07 and 0.25 ± 0.03, respectively. It allows to measure the concentration of hydrogen in a‐Si:H films. The developed approach can be applied for in situ control of hydrogen in a‐Si:H films and also suitable for thin a‐Si:H films on substrates that are opaque in infrared spectral region. Copyright © 2013 John Wiley & Sons, Ltd.     

Photocurrent spectroscopy of a strained-layer superlattice

Strained-layer superlattices (SLS's) are a new kind of semiconductor material, grown from lattice-mismatched constituents, whose electronic properties may be tailored over a wide range. We report the growth by metal-organic chemical vapor deposition of a GaAs_0.4P_0.6/GaP SLS, and the characterization of its room temperature electronic properties by photocurrent spectroscopy. The results are consistent with predictions of an effective-mass (Kronig-Penney) model.     

Copper nanoclusters in amorphous hydrogenated carbon

A study of the geometric characteristics of copper nanoclusters incorporated in an amorphous hydrogenated carbon matrix is reported. It makes use of small-angle x-ray scattering and transmission electron microscopy (100 keV). The fractal dimension and nanocluster diameter have been determined from the x-ray scattering indicatrix for different copper concentrations. TEM images of copper nanoclusters have been analyzed, and a cluster distribution function in size constructed. The shape of the distribution function is discussed in terms of the theory of nucleation of a new phase. Fiz. Tverd. Tela (St. Petersburg) 40, 568–572 (March 1998)     

Photocurrent decay in silicon at low temperatures

We report measurements performed at low temperatures in silicon of the transient photovoltaic current following excitation by an intense laser light pulse. They show the occurrence of a delayed minor current peak, which we suggest is related to the free carriers obtained, when electron-hole droplets after drift in an exciton density gradient are destroyed by heating.     

Surface photovoltage spectroscopy study of CdS thin films

The method of surface photovoltage spectroscopy has been applied to the study of the surface properties of thin CdS films deposited by vacuum evaporation. The energy spectrum of the surface states is obtained and their kinetic parameters are calculated. A comparison is made of the results obtained in air and in vacuum for non-recrystallized and recrystallized samples. The results for thin films are compared with the data on monocrystal CdS in the literature. It is assumed that the surface states established in the films are of a donor-like type.     

Surface photovoltage spectroscopy and surface piezoelectric effect in GaAs

“Real” (111) surfaces of n-type GaAs were investigated employing surface photovoltage spectroscopy and the surface piezoelectric effect. Surface states at the energy position E_c ? E_t ? 0.72 eV were found on both the Ga and the As surfaces. Both types of surfaces exhibited a barrier of about 0.55 V. No variations in the surface barrier or the energy position of the surface states were observed in various ambients at atmospheric pressure (dry air, wet air, ammonia and ozone). However, the capture cross-section of the surface states for electrons, as determined from the surface piezoelectric effect transients (of the order of 10^?13 cm²), was found to be sensitive to the ambient. It decreased in wet air and increased in ozone. This effect was more pronounced on the As than on the Ga surfaces. Additional surface states were found to be present in the energy region of 0.9 to 1.0 eV, below the bottom of the conduction band. However, their exact energy positions could not be determined due to interference caused by the carrier trapping of the surface states at E_c ? E_t ? 0.72 eV.     

Laser crystallization of amorphous silicon films investigated by Raman spectroscopy and atomic force microscopy

The intrinsic and phosphorous (P)-doped hydrogenated amorphous silicon thin films were crystallized by laser annealing. The structural properties during crystallization process can be investigated. Observed redshifts of the Si Raman transverse optical phonon peak indicate tensile stress present in the films and become intense with the effect of doping, which can be relieved in P-doped films by introducing buffer layer structures. Based on experimental results, the established correlation between the stress and crystalline fraction (X_C) suggests that the relatively high stress can limit the increase in X_C and the highest crystalline fraction is obtained by a considerable stress release. At high laser energy density of 1250 mJ/cm², the poorer crystalline quality and disordered structure of the film originating from the irradiation damage and defects lead to the low electron mobility.