Si/SiO2 multilayers: synthesis by reactive magnetron sputtering and photoluminescence emission

This paper relates a complete study of Si/SiO₂ multilayer (ML) structures. First, we suggest an original way of synthesis based on reactive magnetron sputtering of a pure silica target. The photoluminescence spectra of these MLs consist of two Gaussian bands in the visible-near infrared spectral region. The stronger one (I band) is fixed at about 780 nm and probably due to interface states. The weaker one (Q band) is tuneable with the Si sublayer thickness and originates from a radiative recombination within the nanosized Si layers. For this latter band the peak position is a function of the Si sublayer thickness and shows a discontinuity at 30 Å. This corresponds to an Si phase change. For thicknesses above 30 Å, the sublayers are composed of nanocrystalline silicon whereas below 30 Å the sublayers are made of amorphous silicon. We develop a model based on a quantum well to which we have added an interfacial region between Si and SiO₂. It is characterised by an interfacial potential of 0.3 eV. This model depicts the simultaneous behaviour of Q and I bands for an Si sublayer thickness below 30 Å.     

Luminescent silicon nanocrystal dots fabricated by SiCl4/H2 RF plasma-enhanced chemical vapor deposition

Luminescent nanocrystalline Si dots were fabricated directly on thermally grown SiO₂ at 120°C by conventional RF plasma-enhanced chemical vapor deposition using tetrachlorosilane, SiCl₄ and H₂. As-deposited Si dot exhibits photoluminescence (PL) in the visible region, consisting of two broad bands corresponding to photon energies of 1.38 and 1.48 eV. Storage in air enhances PL and shifts the PL peak energy to higher wavelengths for dots of diameter less than 10 nm. Fourier transform attenuated total reflection absorption spectroscopy (FTIR-ATR) study reveals that the spontaneous oxidation proceeds until saturation after 70 h at dot sizes of 3–5 nm. The relationship between PL intensity, blueshift of PL peak energy, and surface termination species during oxidation indicates that these changes are attributed to the increased density of radiative centers at the Si nanocrystal dot/SiO₂ interface and enhancement of the quantum confinement effect.     

Electroluminescence from Si／SiO2 films deposited on p—Si substrates

The structure of Au/Si/SiO₂/p-Si has been fabricated using the magnetron sputtering technique. It has a very good rectifying behaviour. Visible electroluminescence (EL) has been observed from the Au/Si/SiO₂/p-Si structure at a forward bias of 5V or larger. A broad band with one peak around 650－660 nm appears in all the EL spectra of the structure. The effects of the thickness of the Si layer in the Si/SiO₂ films and of the input electrical power on EL spectra are studied systematically.     

Intrinsic ultraviolet luminescence from Lu2O3, Lu2SiO5 and Lu2SiO5:Ce

Radioluminescence and thermally stimulated luminescence measurements on Lu₂O₃, Lu₂SiO₅ (LSO) and Lu₂SiO₅:Ce³⁺ (LSO:Ce) reveal the presence of intrinsic ultraviolet luminescence bands. Characteristic emission with maximum at 256 nm occurs in each specimen and is attributed to radiative recombination of self-trapped excitons. Thermal quenching of this band obeys the Mott-Seitz relation yielding quenching energies 24, 38 and 13 meV for Lu₂O₃, LSO and LSO:Ce, respectively. A second intrinsic band appears at 315 nm in LSO and LSO:Ce, and at 368 nm in Lu₂O₃. Quenching curves for these bands show an initial increase in peak intensity followed by a decrease. Similarity in spectral peak position and quenching behavior indicate that this band has a common origin in each of the samples and is attributed to radiative recombination of self-trapped holes, in agreement with previous work on similar specimens. Comparison of glow curves and emission spectra show that the lowest temperature glow peaks in each specimen are associated with thermal decay of self-trapped excitons and self-trapped holes. Interplay between the intrinsic defects and extrinsic Ce³⁺ emission in LSO:Ce is strongly indicated.     

Au/(SiO2/Si/SiO2)纳米双势垒/n+-Si结构的电致发光研究

利用射频磁控溅射方法,在n⁺-Si衬底上淀积SiO₂/Si/SiO_{2纳米双势垒单势阱结构,其中Si层厚度为2至4nm,间隔为0.2nm,邻近n⁺-S i衬底的SiO2层厚度固定为1.5nm,另一SiO2层厚度固定为3nm.为了 对比研究,还制备了Si层厚度为零的结构,即SiO2(4.5nm)/n⁺-Si 结构.在经过600℃氮气下退火30min,正面蒸上半透明Au膜,背面也蒸Au作欧姆接触后,所 有样品都在反向偏置(n^＋-Si的电压高于Au电极的电压)下发光,而在正向偏压 下不发光.在一定的反向偏置下,电流和电致发光强度都随Si层厚度的增加而同步振荡,位 相相同.所有样品的电致发光谱都可分解为相对高度不等的中心位于2.26eV(550nm)和1.85eV (670nm)两个高斯型发光峰.分析指出该结构电致发光的机制是：反向偏压下的强电场使Au/( SiO2/Si/SiO2)纳米双势垒/n⁺-Si结构发生了雪崩击穿 ,产生大量的电子-空穴对,它们在纳米SiO2层中的发光中心(缺陷或杂质)上复 合而发光. 关键词： 电致发光 纳米双势垒 高斯型发光峰 雪崩击穿}     

Effect of laser annealing on crystallinity of the Si layers in Si/SiO2 multiple quantum wells

We report on continuous-wave laser induced crystallisation processes occurring in Si/SiO₂ multiple quantum wells (MQW), prepared by remote plasma enhanced chemical vapour deposition of amorphous Si and SiO₂ layers on quartz substrates. The size and the volume fraction of the Si nanocrystals in the layers were estimated employing micro-Raman spectroscopy. It was found that several processes occur in the Si/SiO₂ MQW system upon laser treatment, i.e. amorphous to nanocrystalline conversion, Si oxidation and dissolution of the nanocrystals. The speed of these processes depends on laser power density and the wavelength, as well as on the thickness of Si-rich layers. At optimal laser annealing conditions, it was possible to achieve ∼100% crystallinity for 3, 5 and 10 nm thickness of deposited amorphous Si layers. Crystallization induced variation of the light absorption in the layers can explain the complicated process of Si nanocrystals formation during the laser treatment.     

Properties of thick SiO2/Si structure formed at 120 °C by use of two-step nitric acid oxidation method

Thick (i.e., ∼10 nm) SiO₂/Si structure has been formed at 121 °C by immersion of Si in relatively low concentration HNO₃ followed by that in 68 wt.% HNO₃ (i.e., two-step nitric acid (HNO₃) oxidation method of Si, NAOS) and spectroscopic properties and electrical characteristics of the NAOS SiO₂ layers are investigated. The SiO₂ thickness strongly depends on the concentration of HNO₃ aqueous solutions employed in the initial oxidation, and it becomes the largest at the HNO₃ concentration of 40 wt.%. The MOS diodes with the ∼9 nm SiO₂ layer formed by the NAOS method possess a relatively low leakage current density (e.g., 10⁻⁸ A/cm² at the forward bias of 1 V) and it is further decreased by more than one order of magnitude by post-metallization annealing (PMA) in hydrogen at 250 °C. The good leakage characteristic is attributable to atomically flat SiO₂/Si interfaces and high atomic density of 2.30-2.32 × 10²² atoms/cm³ of the NAOS SiO₂ layers. High-density interface states are present in as-prepared SiO₂ layers and they are eliminated by PMA in hydrogen.     

Theoretical study of quantum-confined band-edge shifts and radiative lifetimes in oxidized Si(001) quantum films: comparison with experiment for Si/SiO2 quantum wells

The electronic structure and radiative lifetimes of Si(001) quantum films terminated by SiO₄ tetrahedra, which simulate Si/SiO₂ quantum wells (QWs), are calculated by the extended Hückel-type non-orthogonal tight-binding method. It is found that calculated band-gap widenings and radiative lifetimes account for band-edge shifts and photoluminescence (PL) peak shifts and lifetimes measured in amorphous-Si/SiO₂ QWs, suggesting that quantum confinement effects on the extended band-edge states in the amorphous-Si layer are responsible for the observed results. However, it is shown that band-edge shifts and PL energies and lifetimes observed in crystalline-Si/SiO₂ QWs cannot be reproduced properly by the interface model proposed in this study, implying that further studies are needed on the atomic structure of the crystalline-Si/SiO₂ interface.     

p型硅MOS结构Si/SiO2界面及其附近的深能级与界面态

用深能级瞬态谱(DLTS)技术系统研究了Si/SiO₂界面附近的深能级和界面态。结果表明,在热氧化形成的Si/SiO₂界面及其附近经常存在一个浓度很高的深能级,它具有若干有趣的特殊性质,例如它的DLTS峰高度强烈地依赖于温度,以及当栅偏压使费密能级与界面处硅价带顶的距离明显小于深能级与价带顶的距离时,仍然可以观测到一个很强的DLTS峰。另外,用最新方法测量的Si/SiO₂界面连续态的空穴俘获截面与温度有关,而与能量位置无明显关系,DLTS测 关键词：     

Electron density profile at the interface of SiO2/Si(0 0 1)

In this report we present grazing incidence X-ray reflectivity (GIXR) study of SiO₂/Si(0 0 1) system. We have analysed the X-ray reflectivity data using recursive formalism based on matrix method and distorted wave Born approximation (DWBA). From the analysis of the reflectivity data we could obtain the electron density profile (EDP) at the interface of the dielectric SiO₂ film and the Si(0 0 1) substrate. The EDP obtained from the matrix method follows the DWBA scheme only when two transition layers are considered at the interface of SiO₂/Si. The layer which is in proximity with the Si substrate has a higher electron density value than the Si and SiO₂ values and it appears as a maximum in the EDP. The layer which is in proximity with the dielectric SiO₂ layer has an electron density value lower than the SiO₂ value and it appears as a minimum in the EDP. When the thickness of the SiO₂ layer is increased the lower density layer diminishes and the higher density layer persists.     

Electroluminescence from Si quantum dots/SiO2 multilayers with ultrathin oxide layers due to bipolar injection

Si quantum dots/SiO₂ multilayers with ultrathin oxide layers (2.4 nm) were fabricated on a p-type Si substrate in order to enhance the hole injection. Besides the luminescence band at 900 nm which was also shown in photoluminescence spectra, another strong luminescence band near the infrared region (1200 nm) can be observed in electroluminescence spectra. It can be assigned to the band-edge emission from the quasi 2-dimensional potential well in the Si substrate. Moreover, it is interesting to find the reduction of photoluminescence intensity under biased conditions which can be attributed to the occurrence of non-radiative Auger recombination process in charged Si quantum dots.     

Photoluminescence study of disordering in the cation sublattice of Cu2ZnSnS4

In this study we investigated the influence of the degree of disordering in the cation sublattice on low temperature photoluminescence (PL) properties of Cu₂ZnSnS₄ (CZTS) polycrystals. The degree of disordering was changed by using different cooling rates after post-annealing at elevated temperatures. The results suggest that in the case of higher degree of cation sublattice disorder radiative recombination involving defect clusters dominates at T = 10 K. These defect clusters induce local band gap energy decrease in CZTS. The concentration of defect clusters can be reduced by giving more time for establishing ordering in the crystal lattice. As a result, radiative recombination mechanism changes and band-to-impurity recombination involving deep acceptor defect with ionization energy of about 200 meV starts to dominate in the low temperature PL spectra of CZTS polycrystals.     

Silicon nanoparticles formation in annealed SiO/SiO2 multilayers

We present a study on amorphous SiO/SiO₂ superlattice performed by grazing-incidence small-angle X-ray scattering (GISAXS). Amorphous SiO/SiO₂ superlattices were prepared by high-vacuum evaporation of 3 nm thin films of SiO and SiO₂ (10 layers each) onto Si(1 0 0) substrate. After the deposition, samples were annealed at 1100 °C for 1 h in vacuum, yielding to Si nanocrystals formation. Using a Guinier approximation, the shape and the size of the crystals were obtained. The size of the growing nanoparticles in the direction perpendicular to the film surface is well controlled by the bilayer thickness. However, their size varies more significantly in the direction parallel to the film surface.     

Structural and optical properties of Si/SiO2 multi-quantum wells

Structural and optical properties of Si/SiO₂ multi-quantum wells (MQW) were investigated by means of Raman scattering and photoluminescence (PL) spectroscopy. The MQW structures were fabricated on a quartz substrate by remote plasma enhanced chemical vapour deposition (RPECVD) of alternating amorphous Si and SiO₂ layers. After layer deposition the samples were subjected to heat treatments, i.e. rapid thermal annealing (RTA) and furnace annealing. Distinct PL signatures of confined carriers evidenced formation of Si-nanocrystals (nc-Si) in annealed samples. Analyses of Raman spectra also show presence of nc-Si phase along with amorphous-Si (a-Si) phase in the samples. The strong influence of the annealing parameters on the formation of nc-Si phase suggests broad possibilities in engineering MQW with various optical properties. Interestingly, conversion of the a-Si phase to the nc-Si phase saturates after certain time of furnace annealing. On the other hand, thinner Si layers showed a disproportionately lower crystalline volume fraction. From the obtained results we could assume that an interface strain prevents full crystallization of the Si layers and that the strain is larger for thinner Si layers. The anomalous dependence of nc-Si Raman scattering peak position on deposited layer thickness observed in our experiments also supports the above assumption.     

Spectroscopic and electrical properties of ultrathin SiO2 layers formed with nitric acid

Spectroscopic and electrical properties of ultrathin silicon dioxide (SiO₂) layers formed with nitric acid have been investigated. The leakage current density of the as-grown SiO₂ layers of 1.3 nm thickness is high. The leakage current density is greatly decreased by post-oxidation annealing (POA) treatment at 900 °C in nitrogen, and consequently it becomes lower than those for thermally grown SiO₂ layers with the same thickness. X-ray photoelectron spectroscopy measurements show that high density suboxide species are present before POA and they are markedly decreased by POA. Fourier transformed infrared absorption measurements show that water and silanol group are present in the SiO₂ layers before POA but they are removed almost completely by POA above 800 °C. The atomic density of the as-grown chemical SiO₂ layers is 4% lower than that of bulk SiO₂ layers, while it becomes 12% higher after POA. It is concluded that the high atomic density results from the desorption of water and OH species, and oxidation of the suboxide species, both resulting in the formation of SiO₂. The valence band discontinuity energy at the Si/SiO₂ interface increases from 4.1 to 4.6 eV by POA at 900 °C. The high atomic density enlarges the SiO₂ band-gap energy, resulting in the increase in the band discontinuity energy. The decrease in the leakage current density by POA is attributed to (i) a reduction in the tunneling probability of charge carriers through SiO₂ by the enlargement of the band discontinuity energy, (ii) elimination of trap states in SiO₂, and (iii) elimination of interface states.     

Photoemission studies of pulsed-RF plasma nitrided ultra-thin SiON dielectric layers

Results are presented of a photoemission study of the electronic structure of SiON layers formed by a pulsed-RF decoupled plasma nitration (DPN) of ultra-thin SiO₂ grown base layers approximately 1.0 nm thick. The optical thickness of these device grade nitrided dielectric layers was in the range 1.4-1.6 nm. X-ray photoelectron spectroscopy (XPS) studies indicate that the nitrogen is incorporated in a single chemical environment at concentration levels in the range 15-17%. Angle resolved XPS measurements show that the nitrogen is distributed through the layer, with the binding energy of the N 1s peak at 398.3 eV which is indicative of a Si₃N₄-like chemical species in an oxide environment. High resolution core level photoemission studies of the spin orbit stripped Si 2p⁴⁺ peak revealed full width half maximum values in the range 1.4-1.55 eV, which are significantly larger than the 1.15 eV value reported for SiO₂ layers. Synchrotron radiation photoemission studies of the valence band spectra enable the valence band off-set at the Si/SON interface to be evaluated as 2.3 eV and to infer a conduction band off-set of 2.1 eV.     

Ab initio calculation of optical absorption and reflectivity of Si(0 0 1)/SiO2 superlattices with varying interfaces

Using first-principles calculations we investigate the influence of interface modification and layer thicknesses on the optical properties of Si/SiO₂ superlattices. Four interface models with different dangling-bond passivation are considered. The results demonstrate confinement effects not only for the fundamental band gaps but also for the optical properties. While for a large Si layer thickness of the Si/SiO₂ superlattices the interface dependence is small, the calculations show a significant structure dependence for thin Si layers. © 2007 Elsevier Science. All rights reserved.     

Elucidating the role of non-radiative processes in charge transfer of core–shell Si–SiO2 nanoparticles

Crystalline silicon is the most commonly used material in photovoltaics but has limitations due to its high cost and non-tunable band gap. A new approach of using inexpensive, non-toxic materials with layers that have different band gaps which absorb a wide range of the solar spectrum has the potential to dramatically increase the efficiencies and lower the costs. Core–shell Si–SiO₂ nanoparticles are ideally suited for the photovoltaic application and have been synthesised by different groups in an array of sizes allowing for absorption in a wide spectral range. A theoretical investigation of fundamental charge transfer processes in these systems can potentially lead to improved devices. Calculations on a model core–shell interface with the formula Si₂₆₄O₁₆₀ which features a silicon layer sandwiched between two SiO₂ layers were performed using the Vienna ab initio software package. The Perdew–Burke–Ernzerhof functional in the basis of plane waves was used along with pseudopotentials to simulate electronic structure. The nuclear motion was considered using ab initio molecular dynamics. The density of states, absorption spectrum, partial charge densities, and radiative recombination lifetimes have been calculated. This interface shows quantum confinement behaviour similar to a particle in a box. The role of non-radiative recombination was also determined by relaxation dynamics.     

THE INTERACTION OF Au AND THE Si/SiO2 INTERFACE DEFECT Hit(0.494)

The defects at the Si/SiO₂ interface have been studied by the deep-level transient spectroscopy (DLTS) technique in p-type MOS structures with and without gold diffusion. The experimental results show that the interaction of gold and Si/SiO₂ interface defect,H_it(0.494), results in the formation of a new interface de-fect, Au-H_it(0.445). Just like the interface defect, H_it(0.494), the new interface defect possesses a few interesting properties, for example, when the gate voltage applied across the MOS structure reduces the energy interval between Fermi-level and Si valence band of the Si surface to values smaller than the hole ionization Gibbs free energy of the defect, a sharp DLTS peak is still observable; and the hole apparent activation energy increases with the decrease of the Si surface potential barrier height. These properties can be successfully explained with the transition energy band model of the Si/SiO₂ interface.     

Fabrication and characterization of Au/SiO2 nanocomposite films grown by radio-frequency cosputtering

Au/SiO₂ nanocomposite films were prepared on Si wafers by cosputtering of SiO₂ and gold wires. Au/Si atomic ratios in Au/SiO₂ nanocomposite films were varied from 0.53 to 0.92 by controlling the length of gold wire to study the evolution of the crystallization of gold, the size of Au/SiO₂ nanocomposite particles, and the optical properties of as-deposited Au/SiO₂ nanocomposite films. An X-ray photoelectron spectroscopy reveals that Au exists as a metallic phase in the bulk of SiO₂ matrix. Dome-shaped Au/SiO₂ nanocomposite particles and both Au (1 1 1) and (2 0 0) planes were observed in a field-emission scanning electron microscopy and X-ray diffraction studies respectively. With an ultraviolet-visible, absorption peaks of Au/SiO₂ nanocomposite films were observed at 525 nm.