Quantitative electron energy loss spectroscopy of Si nanoclusters embedded in SiOx

We have used a methodology, based on electron energy loss spectroscopy combined with energy filtered images, which allows us to quantify the clustered silicon concentration in annealed sub-stoichiometric silicon oxide layers (SiO_x). This information was coupled to the chemical silicon and oxygen concentrations determined from Rutherford backscattering analyses. The silicon agglomeration kinetics was investigated as a function of the gas flows used during the plasma enhanced chemical vapor deposition process. Although the clustered Si concentration and the average cluster radius do not follow a well defined dependence on Si, O, or N concentration, separately, the ratio between the former parameter and the silicon excess concentration decreases monotonically as a function of the nitrogen concentration approaching a saturation value equal to 0.2 for nitrogen concentrations above 10 at.%.     

X-ray-emission study of the structure of Si:H layers formed by low-energy hydrogen-ion implantation

Amorphous silicon layers formed by implantation of 24-keV hydrogen ions into SiO₂/Si and Si with doses of 2.7×10¹⁷ and 2.1×10¹⁷ cm^?2, respectively, were studied using ultrasoft X-ray emission spectroscopy with variations in the energy of excitation electrons. It is ascertained that the surface silicon layer with a thickness as large as 150–200 nm is amorphized as a result of implantation. Implantation of hydrogen ions into silicon coated with an oxide layer brings about the formation of a hydrogenated silicon layer, which is highly stable thermally.     

Silicon-on-insulator structures with a nitrogenated buried SiO<Subscript>2</Subscript> layer: Preparation and properties

Electrical properties of silicon-on-insulator (SOI) structures with buried SiO₂ layer implanted with nitrogen ions are studied in relation to the dose and energy of N⁺ ions. It is shown that implantation of nitrogen ions with doses >3 × 10¹⁵ cm⁻² and an energy of 40 keV brings about a decrease in the fixed positive charge in the oxide and a decrease in the density of surface stares by a factor of 2. An enhancement of the effect can be attained by lowering the energy of nitrogen ions. The obtained results are accounted for by interaction of nitrogen atoms with excess silicon atoms near the Si/SiO₂ interface; by removal of Si-Si bonds, which are traps of positive charges; and by saturation of dangling bonds at the bonding interface of the SOI structure.     

Remote plasma-enhanced CVD of silicon nitride films: effects of diluting nitrogen with argon. Part II: effect of nitrogen plasma parameters on layer characteristics

In Part I we reported the results of an emission spectroscopic study of the plasma obtained in an SiH₄–N₂–Ar mixture. It was shown that argon in metastable electronic excited states provides a high concentration of atomic nitrogen. In this part we report the results of a study of the influence of argon dilution on the growth rate, composition and properties of silicon nitride films. The exact influence of nitrogen dilution with argon depends on the process parameters and on the method of coupling of the RF power, but it is found in general that a high concentration of atomic nitrogen leads to changes in the relative amounts of Si–H_j and N–H_i bonds and in the Si/N ratio of deposited films. In particular, it is shown that hydrogen incorporation can be reduced and improved stoichiometry can be obtained. © 1998 John Wiley & Sons, Ltd.     

Effect of high-power nanosecond and femtosecond laser pulses on silicon nanostructures

The effect of high-power nanosecond (20 ns) and femtosecond (120 fs) laser pulses on silicon nanostructures produced by ion-beam-assisted synthesis in SiO₂ layers or by deposition onto glassy substrates is studied. Nanosecond annealing brings about a photoluminescence band at about 500 mn, with the intensity increasing with the energy and number of laser pulses. The source of the emission is thought to be the clusters of Si atoms segregated from the oxide. In addition, the nanosecond pulses allow crystallization of amorphous silicon nanoprecipitates in SiO₂. Heavy doping promotes crystallization. The duration of femtosecond pulses is too short for excess Si to be segregated from SiO₂. At the same time, such short pulses induce crystallization of Thin a-Si films on glassy substrates. The energy region in which crystallization is observed for both types of pulses allows short-term melting of the surface layer.     

Surface passivation of high‐efficiency silicon solar cells by atomic‐layer‐deposited Al2O3

Atomic‐layer‐deposited aluminium oxide (Al₂O₃) is applied as rear‐surface‐passivating dielectric layer to passivated emitter and rear cell (PERC)‐type crystalline silicon (c‐Si) solar cells. The excellent passivation of low‐resistivity p‐type silicon by the negative‐charge‐dielectric Al₂O₃ is confirmed on the device level by an independently confirmed energy conversion efficiency of 20·6%. The best results are obtained for a stack consisting of a 30 nm Al₂O₃ film covered by a 200 nm plasma‐enhanced‐chemical‐vapour‐deposited silicon oxide (SiO_x) layer, resulting in a rear surface recombination velocity (SRV) of 70 cm/s. Comparable results are obtained for a 130 nm single‐layer of Al₂O₃, resulting in a rear SRV of 90 cm/s. Copyright © 2008 John Wiley & Sons, Ltd.     

Epitaxial, high-K dielectrics on silicon: the example of praseodymium oxide

We show the first results for crystalline growth of praseodymium oxide on Si as a potential high-K dielectric with very promising electrical properties. All layer growth experiments were performed using solid source molecular beam epitaxy. The initial growth phase was studied using scanning tunneling microscopy. On Si(0 0 1) oriented surfaces, crystalline Pr₂O₃ grows as (1 1 0)-domains, with two orthogonal in-plane orientations. Epitaxial silicon overgrowth seems to be impossible. We obtain perfect epitaxial growth on Si(1 1 1). These layers can also be overgrown epitaxially with silicon. Finally, we show that the structural quality of epitaxial grown Pr₂O₃ on Si(0 0 1) degrades when the film is exposed to air due to silicon oxide formation at the interface based on oxygen indiffusion. However, it can be stabilized by capping with Si.     

Specific features of the segregation-related redistribution of phosphorus during thermal oxidation of heavily doped silicon layers

A model of the diffusion-segregation redistribution of phosphorus in an SiO₂/Si system during thermal oxidation of highly doped silicon layers is developed taking into account the formation of a peak of surface impurity concentration at the interface. The formation of this surface concentration peak is attributed to a change in the free energy of the impurity atoms near the silicon surface. This process is simulated by a diffusion-segregation equation. It is shown that the developed diffusion-segregation model is quite adequate for describing the phosphorus redistribution occurring during the oxidation of uniformly doped silicon layers. For the oxidation of implanted silicon layers, it was found that the segregation coefficient of the phosphorus at the SiO₂/Si interface is not constant but depends on time in the same way as the efficiency of transient enhanced diffusion in silicon. This phenomenon is explained by the reactivity of the impurity segregation during the thermal oxidation of silicon, when excess point defects in the implanted silicon layer affect both the oxidation process and the capture of impurity atoms by the growing silicon dioxide.     

Nitrogen bonding configurations near the oxynitride/silicon interface after oxynitridation in N2O ambient of a thin SiO2 gate

The identification of the bonding environments and their progressive modifications upon reaching the oxynitride/silicon interface, in a SiO₂/SiO_xN_y/Si structure, have been investigated by means of X-ray photoemission spectroscopy (XPS). The SiO₂ film was grown at 850 °C by means of a mixed dry-steam process, followed by a 60 min, 950 °C furnace oxynitridation in N₂O gas. A depth profile analysis was carried out by a progressive chemical etching procedure, reaching a residual oxide thickness of about 1.2 nm. XPS analysis of the Si 2p and N 1s photoelectron peaks pointed out that the chemistry of the oxynitride layer is a rather complex one. Four different nitrogen bonding environments were envisaged. Both the overall nitrogen content, which rises up to 2.5%, and its bonding configurations are progressively changing while moving towards the silicon interface.     

Electronic states on silicon surface after deposition and annealing of SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films

The spectrum of the photoconductivity induced by the polarization field of charges at surface states and traps in the film bulk has been analyzed to determine the energy band diagram at the c-Si-SiO _x interface and the changes in the electronic states after the film annealing. It is found that the energy bands are bent at the Si-SiO _x interface and the Si surface is enriched in electrons. In equilibrium the photocurrent peak at 1.1 eV is due to the band-to-band transitions in the silicon part of the interface. Annealing shifts the peak to higher energies; this shift increases with an increase in the annealing temperature from 650 to 1000°C. This effect is accompanied by a decrease in the photocurrent at ≤1.1 eV and weakening of the band-edge photoluminescence near the Si surface. The changes revealed are explained by the formation of an oxide layer with Si nanoclusters at the Si-SiO _x interface upon annealing. This process is caused by oxygen diffusion from the SiO _x film, which occurs mainly via defects on the Si wafer surface. The photoconductivity spectrum of the samples charged by short-term application of a negative potential to silicon exhibits electronic transitions in the SiO _x film, both from the matrix electronic states and from the states of the defects and Si nanoclusters in the film.     

Study of the layer-substrate interface in <Emphasis Type="Italic">nc</Emphasis>-Si-SiO<Subscript>2</Subscript>-<Emphasis Type="Italic">p</Emphasis>-Si structures with silicon quantum dots by the method of temperature dependences of photovoltage

Layers grown by magnetron deposition of Si and SiO₂ on a p-type silicon substrate and containing silicon nanocrystals in the oxide matrix have been studied by the method of temperature dependences of the capacitive photovoltage. The effect of the substrate orientation and natural oxidation preceding high-temperature annealing that results in the formation of Si nanocrystals in the SiO₂ matrix on the layer-substrate interface characteristics is studied. The density of fast interface states trapping majority carriers was estimated. It is found that structural changes occur at the layer-substrate interface in the case of a (111) substrate and are caused by stresses appearing upon cooling. It was shown that natural oxidation of the deposited layer, preceding high-temperature annealing, causes an increase in the charge trapped in the oxide.     

Growth and structural properties of crystalline LaAlO3 on Si (0 0 1)

Crystalline LaAlO₃ was grown by oxide molecular beam epitaxy (MBE) on Si (0 0 1) surfaces utilizing a 2 ML SrTiO₃ buffer layer. This SrTiO₃ buffer layer, also grown by oxide MBE, formed an abrupt interface with the silicon. No SiO₂ layer was detectable at the oxide-silicon interface when studied by cross-sectional transmission electron microscopy. The crystalline quality of the LaAlO₃ was assessed during and after growth by reflection high energy electron diffraction, indicating epitaxial growth with the LaAlO₃ unit cell rotated 45° relative to the silicon unit cell. X-ray diffraction indicates a (0 0 1) oriented single-crystalline LaAlO₃ film with a rocking curve of 0.15° and no secondary phases. The use of SrTiO₃ buffer layers on silicon allows perovskite oxides which otherwise would be incompatible with silicon to be integrated onto a silicon platform.     

Correlation between excess Si atoms near the ultrathin silicon oxide-Si(100) interface and oxidation temperature

The number densities (areal densities) of Si and O atoms for 3.5–8.0-nm-thick silicon oxide films on Si(100) oxidized at 800–950 C were determined by Rutherford backscattering spectrometry. It was confirmed that excess Si atoms relative to the stoichiometric SiO₂ composition exist near the oxide film-Si substrate interface and the oxidation temperature vs. their number density (N_Si(excess)) characteristic exhibits a minimum at 850 C. This oxidation temperature dependence contrasted with that for volumetric density; a maximum at 850 C. Moreover, a slight discrepancy of the oxide thickness (ΔT_ox), which probably corresponds to the slight microscopic structural change due to the N_Si(excess) difference, decreased with decreasing oxidation temperature without showing a minimum at 850 C. Therefore, although most of the fundamental characteristics for the ultrathin oxide faithfully follow the change of N_Si(excess), in the 800 C-oxidation films, it was suggested that N_Si(excess) is possibly increased by additional structural changes without increasing ΔT_ox.     

Study of nitrogen impact on VFB-EOT roll-off by varying interfacial SiO2 thickness

Flat band voltage (V_FB) roll-off in long channel devices at thin equivalent oxide thickness (EOT) is studied on SiO₂/nitrided-HfSiO stacks. V_FB increases when SiO₂ interfacial layer thickness decreases, and charges pumping (CP) frequency sweep analysis shows higher trap density near Si/SiO₂ interface. Based on this observation, an atomic diffusion model is introduced. Higher concentration of nitrogen atom in the HfSiO(N) layer diffuses to the Si/SiO₂ interface through the SiO₂ layer in thinner SiO₂ device, and accumulates near Si/SiO₂ interface which can introduce higher density of interfacial traps. Lifetime extracted from negative bias temperature instability (NBTI), and mobility are also degraded in thinner SiO₂ devices due to the higher interfacial trap density.The V_FB roll-off can be improved by lowering nitrogen concentration in the HfSiO(N) layer from optimizing plasma nitridation pressure, decreasing post deposition anneal temperature, or using defect absorbing layer on the high-k oxide.     

Silicon in functional epitaxial oxides: A new group of nanostructures

The ability to integrate low-dimensional crystalline silicon into crystalline insulators with high dielectric constant (high-k) can open the way for a variety of novel applications ranging from high-k replacement in future nonvolatile memory devices to insulator/Si/insulator structures for nanoelectronic applications. We will present an approach for nanostructure fabrication by incorporation of crystalline silicon into epitaxial oxide that is based on a solid-phase epitaxy of Si. In dependence on the preparation conditions we obtained nanostructures containing an either ultra-thin single-crystalline Si quantum-well buried in single-crystalline oxide matrix with sharp interfaces or Si-nanocrystals (ncs) embedded into single-crystalline oxide layer. As an example, we demonstrate the growth of Si buried in Gd₂O₃ and the incorporation of epitaxial Si clusters into single-crystalline Gd₂O₃ on silicon as well as silicon carbide substrates using molecular beam epitaxy. The leakage current of the obtained nanostructures exhibited negative differential resistance at lower temperatures. For structures containing Si-ncs a large hysteresis in capacitance–voltage measurements due to charging and discharging of the Si-ncs was obtained.     

Evolution of the Co–N/Ti/Si System in Magnetron Sputtering of Co onto a Heated Ti/Si(100) Substrate

The surface-diffusion interaction is studied experimentally between cobalt and a heated Ti/Si(100) substrate under reactive magnetron sputtering in an argon–nitrogen atmosphere. A model is proposed that accounts for the nature of silicide formation in the Co/Ti/Si system by volume and surface-diffusion reactions between cobalt and the substrate. It is shown that the diffusion of cobalt into the silicon is impeded by the TiSi _x layer to a far greater extent than by the Ti layer.     

Breakdown properties of metal/NIDOS SiO2/silicon structures

NIDOS/SiO₂/silicon structures have been annealed in a nitrogen (N₂) ambient and X-ray photoelectron spectroscopy (XPS) characterization has been performed in order to definitively demonstrate the nitrogen atoms out-diffusion from the nitrogen doped silicon (NIDOS) film towards the buried oxide layer. The nitridation of the SiO₂ layer is related to the competition between nitrogen atoms out-diffusion phenomena on one side into the underlying oxide layer and on the other side into an oxynitride layer grown during annealing. In order to analyse and optimize the corresponding MIS process, different structures such as metal/SiO₂/silicon, metal/(NH₃-‘nitrided’)SiO₂/silicon, metal/(N₂O-nitrided)SiO₂/silicon, metal/poly-Si*/SiO₂/silicon (* indicates deposited from disilane Si₂H₆) and metal/NIDOS/SiO₂/silicon have been realized and compared by capacitance–voltage, current–voltage and ageing under constant current injection experiments. The optimization of the NIDOS-nitridation process gives the highest charge-to-breakdown for the lowest nitridation level or even for no intentional nitridation. The dielectric breakdown improvement should therefore not be related to the nitridation phenomena alone but also to the intrinsic properties of the polysilicon layer itself.     

Structural properties and electrical behaviour of thin silicon oxynitride layers

Thin SiO₂ and SiO_xN_y layers were grown on silicon using Rapid Thermal Processing (RTP) in either O₂ or N₂O ambient. Subsequent annealing or nitridation was performed in order to improve the electrical stability. The composition of the films, in particular the incorporation of nitrogen and hydrogen, has been studied. We obtained the distribution of states at the Si/insulator interface through the evaluation of CV measurements and investigated the charge trapping in the layers analysing the voltage–time behaviour during Fowler–Nordheim constant current injection. Furthermore, assuming a trap assisted tunneling mechanism, the influence of near interface trap states on the current voltage characteristic was used to derive an effective insulator state distribution.     

Preparation and measurement of highly efficient a‐Si:H single junction solar cells and the advantages of μc‐SiOx:H n‐layers

Reducing the optical losses and increasing the reflection while maintaining the function of doped layers at the back contact in solar cells are important issues for many photovoltaic applications. One approach is to use doped microcrystalline silicon oxide (μc‐SiO_x:H) with lower optical absorption in the spectral range of interest (300 nm to 1100 nm). To investigate the advantages, we applied the μc‐SiO_x:H n‐layers to a‐Si:H single junction solar cells. We report on the comparison between amorphous silicon (a‐Si:H) single junction solar cells with either μc‐SiO_x:H n‐layers or non‐alloyed silicon n‐layers. The origin of the improved performance of a‐Si:H single junction solar cells with the μc‐SiO_x:H n‐layer is identified by distinguishing the contributions because of the increased transparency and the reduced refractive index of the μc‐SiO_x:H material. The solar cell parameters of a‐Si:H solar cells with both types of n‐layers were compared in the initial state and after 1000 h of light soaking in a series of solar cells with various absorber layer thicknesses. The measurement procedure for the determination of the solar cell performance is described in detail, and the measurement accuracy is evaluated and discussed. For an a‐Si:H single junction solar cell with a μc‐SiO_x:H n‐layer, a stabilized efficiency of 10.3% after 1000 h light soaking is demonstrated. Copyright © 2015 John Wiley & Sons, Ltd.     

A novel method for the deposition of Si–SiO2 superlattices

A novel procedure is proposed for the deposition of Si/SiO₂ multilayers by magnetron sputtering of an SiO₂ target under a hydrogen-rich plasma. The effect of increasing hydrogen pressure (P_H) was investigated for the case of a monolayer. This involved an increased enrichment in the Si of the deposited film, because of the incorporation deficit of oxygen that is efficiently “reduced” by hydrogen. This was found to favour the formation of Si crystallites, together with a significant improvement of the phase segregation between Si grains and silica. The increase of the deposition temperature (T_S) induced similar effects in such a way that the layer obtained with high P_H and T_S behaves as a pure silicon film with a refractive index close to 3.2. Thus, a 40-period Si-rich/silica multilayered system was fabricated by alternating the introduction and the switching off of the hydrogen flux. The related electron microscopy image showed evidence of good quality multilayers, which attests the reliability of the new method.