Kinetics of light-induced degradation in a-Si:H films investigated by computer simulation

In this work we investigated the stability of a-Si:H films under illumination and following recovery in darkness at different temperatures. The a-Si:H films were fabricated with 55 kHz PECVD and with conventional rf 13.56 MHz PECVD. We measured the steady-state photocurrent and the dark-current after switching off the light source as a function of time. We observed photocurrent degradation and the following recovery of the dark current. The kinetics of the photocurrent degradation as well as the dark-current recovery demonstrated stretched-exponential behavior. The results of these straightforward measurements in combination with computer simulation were used to determine the effect of light-induced degradation and thermal recovery on the density of states distribution in the band gap of a-Si:H. We have found that the photocurrent degradation and the corresponding increase in the total defect concentration have different kinetics. The different kinetics were also determined for the dark-current recovery and the corresponding decrease in the total defect concentration. The results point out that slow and fast types of defects in a-Si:H films control the kinetics of light-induced changes of the defect distribution in the band gap. A model is proposed that relates the origin of the fast and slow metastable defects with the distribution of Si-Si bond lengths.     

Laser-induced degradation of the photoluminescence intensity of porous silicon

Transmission Fourier-transform infrared (FTIR) spectroscopy has been used to monitor laser-induced degradation of the photoluminescence (PL) intensity of porous Si. It is observed that the release of hydrogen from silicon hydride surface species coincides with a decrease in the PL intensity and oxidation of the porous Si. The as-anodized PL characteristics can be recovered, with a slight blue shift, by a brief immersion in hydrofluoric acid.     

Complexing of nitrogen with carbon and oxygen in silicon: Photoluminescence studies

We study interactions of nitrogen with carbon and oxygen in crystalline silicon by photoluminescence spectroscopy. Such processes manifest themselves in five photoluminescence lines in the spectral region around 1.6 μm emerging after nitrogen and carbon implantations and furnace annealing with 550° C optimum temperature. Nitrogen and carbon isotope shifts of the lines confirm the incorporation of these atomic species in the optical defects. Nitrogen-oxygen interactions are demonstrated by differences in the lines’ appearance between oxygen-lean float-zone and oxygen-rich pulled silicon starting materials. The data suggests similar basic nitrogen-carbon units in all five defects.     

Origins of photoluminescence degradation in porous silicon under irradiation and the way of its elimination

Using of infrared (IR) and photoluminescence (PL) spectroscopy a comparative study of distinctions in composition and photoluminescence properties of porous silicon with different morphology was performed. Basing on the obtained experimental data and conventional theoretical models the main factors were found that have a negative effect on the intensity of PL in porous silicon and its degradation under the impact of directed irradiation in the visible range. With porous silicon as an example having the pores of 50–100 nm in size there was demonstrated a possibility for improving of these characteristics by its chemical treatment in polyacrylic acid.     

Investigation of polarization anisotropy in individual porous silicon nanoparticles

Polarization anisotropy is investigated in single porous silicon nanoparticles containing multiple chromophores. Two forms of nanoparticle samples are studied; low current density (LCD) and high current density (HCD). Photoluminescence measurements reveal that LCD samples exhibit red-shifted spectra and HCD particles display a blue-shifted spectrum. We utilize single molecule spectroscopy to detect the polarization effects of spatially isolated individual nanoparticles, and show that LCD nanoparticles demonstrate strong polarization anisotropy, whereas a dynamic polarization response is collected from HCD nanoparticles.     

Growth and visible photoluminescence of highly oriented (1 0 0) zinc oxide film synthesized on silicon by plasma immersion ion implantation

High-quality (1 0 0) ZnO films with smooth surface topography have been synthesized on Si substrate by plasma immersion ion implantation. The materials exhibit compressive stress because of room temperature growth. After annealing at different temperatures, various visible photoluminescence bands are observed. The optical phenomenon as well as the transition mechanism which may involve defects such as [Zn_I], [V_Zn], and [O_i⁻] induced by the high substrate bias are discussed in this paper.     

Effect of stacking fault in silicon induced by nanoindentation with MD simulation

A silicon model with the vacancy type stacking fault is built and used for MD nano-indentation simulation to study the different nano-processing characteristics of silicon, compared with the ideal silicon model. During the research, the load–displacement curve, the nano-hardness curve and the strain distribution figure are drawn to study the nano-mechanics properties. The coordination analysis method is introduced to visualize the motion of the silicon and study the structural phase transformations. The results show that the hardness of the model with stacking fault (8.9–9.9 GPa) is lower than the ideal model (9.6–10.4 GPa). The model with stacking fault has a large amount of plastic deformation, which eventually leads to a smaller elastic recovery. During the nano-indentation, there is a new structure β-Si forming in the perfect model. But in the stacking fault model, a large number of amorphous structures are formed. The material property of amorphous structure is unstable, which is not suitable for ultra-precision machining. Therefore, the stacking fault of interstitial type has an adverse impact on the nano-machining performance of the monocrystalline silicon.     

Kinetic Monte Carlo simulation of low-pressure chemical vapor deposition of silicon nitride: Impact of gas flow rate and temperature on silicon cluster size and density

In the present study, the deposition process of SiN_x thin films obtained by a low-pressure chemical vapor deposition technique with a mixture of disilane (Si₂H₆) and ammonia (NH₃) was simulated by using the kinetic Monte Carlo method. A new pattern describing the distribution of ammonia molecules in the simulation matrix was proposed. The influences of the NH₃/Si₂H₆ gas flow ratio and the deposition temperature on the obtained films structure in terms of silicon cluster size and density were analyzed. The simulation results indicate that an increase in the gas flow ratio leads to the deposition of amorphous silicon clusters characterized by small sizes. Nevertheless, an increase in the temperature values of the process provokes an enhancement in the silicon cluster size along with a decrease in their density.     

Microstructural and photoluminescence properties of Co-doped ZnO films fabricated using a simple solution growth method

Nanocrystalline 2% cobalt doped ZnO films were successfully prepared using a simple chemical solution method on glass substrates and subsequently annealed in air at 300 and 500 °C. Structural, morphology, chemical composition and photoluminescence properties of the films were characterized using X-ray diffractometry (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and Fourier transform infra-red spectroscopy (FTIR) and photoluminescence (PL) spectroscopy. X-ray diffraction studies of the annealed films reveal the formation of polycrystalline hexagonal wurtzite structure of ZnO crystals without any co-related secondary phases. SEM micrographs of the films show the formation of spherical nanoparticles. Photoluminescence of the films showed a weak UV and defect related visible emissions like blue, blue–green, yellow and relatively intense orange–red emissions and their mechanism was discussed in detail.     

On the simulation of the formation and dissolution of silicon self-interstitial clusters and the corresponding inverse modeling problem

The formation and dissolution of silicon self-interstitial clusters is linked to the phenomenon of transient-enhanced diffusion (TED) which in turn has gained importance in the manufacturing of semiconductor devices. Based on theoretical considerations and measurements of the number of self-interstitial clusters during a thermal step, a model for the formation and dissolution of self-interstitial clusters is presented including the adjusted model parameters for two different technologies (i.e. material parameter sets). In order to automate the inverse modeling part, a general optimization framework was used. In addition to solving this problem, the same setup can solve a wide range of inverse modeling problems occurring in the domain of process simulation. Finally, the results are discussed and compared with a previous model.     

Investigation of GaAs]-x_x P crystals using the pseudo-kossel X-ray and photoluminescence techniques

Two indirect methods of determining composition in GaAs_{1-x x}P films grown epitaxially on GaAs substrates are discussed in this paper. In the first method precision lattice parameters, obtained using the pseudo-Kossel technique in back reflection, are compared to compositions determined with the electron microprobe. A plot of corrected lattice parameter vs. %GaP (microprobe) over the range of composition investigated (0.10 < x < 0.55) shows that Vegard’s law holds very closely in this system. The slope of the line is -0.002042 Å/%GaP while the intercept is within 0.028% of the established lattice parameter for GaAs. No point lies farther than 0.003 Å from the line describing Vegard’s law.     

Estimation of drift mobility in InGaAsP semiconducting alloys from photoluminescence at 77 and 300 K

In this work, we propose a method of calculation for estimating the drift mobility from photoluminescence (PL). The method is based on the difference between the temperature of the scattered carriers after thermalization and the lattice temperature. The effective carrier temperature T_E was determined experimentally by fitting the high-energy region of PL spectra. The total mobility is obtained from the mobility calculated for different scattering mechanisms and the application of the Matthiessen rule. The method was used on InGaAsP semiconducting alloys grown by liquid phase epitaxy. The calculated mobility values for different InGaAsP samples agree with those reported for these alloys.     

Modeling of silicon oxynitride etch microtrenching using genetic algorithm and neural network

A prediction model of etch microtrenching was constructed by using a neural network. The etching of silicon oxynitride films was conducted in C₂F₆ inductively coupled plasma. The process parameters that were varied in a statistical experimental design include radio frequency source power, bias power, pressure, and C₂F₆ flow rate. The etch microtrenching was quantified from scanning electron microscope images. The prediction accuracy of optimized neural network model with genetic algorithm had a root mean-squared error of 0.03 nm/min. Compared to conventional model, this demonstrates an improvement of about 32%. The constructed model was used to infer etch mechanisms particularly as a function of pressure. Roles of profile sidewall variations were investigated by relating them to the microtrenchings. The pressure effect was conspicuous at lower source power, lower bias power, or higher C₂F₆ flow rate. Microtrenching variations could be reasonably explained by the expected ion reflection from the profile sidewall. The pressure effect seemed to be strongly affected by the relative dominance of fluorine-driven etching over polymer deposition initially maintained in the chamber.     

Characterization of the silicon nitride–thermal oxide interface in oxide–nitride–oxide structures by ELS, XPS, ellipsometry, and numerical simulation

This work studies the properties of the SiO₂-Si₃N₄ interface in oxide-nitride-oxide (ONO) structures by using energy loss spectroscopy, X-ray photoelectron spectroscopy, ellipsometry measurements and numerical simulation. By oxidation the as-deposited Si₃N₄, silicon-silicon bonds at Si₃N₄-thermal SiO₂ interface are found. These excess Si-Si bonds are produced by replacing nitrogen with oxygen during the oxidation of Si₃N₄. We further propose that the Si-Si bonds are the major trap center at the Si₃N₄-SiO₂ interface. With MINDO/3 numerical simulation, we have found that the Si-Si bonds can capture both electrons and holes at the top Si₃N₄-SiO₂ interface. These bonds are proposed to be the responsible candidate for the positive charge accumulation in re-oxidized nitrided oxide.     

The temperature mobility degradation influence on the zero temperature coefficient of partially and fully depleted SOI MOSFETs

The zero temperature coefficient (ZTC) is investigated experimentally in partially (PD) and fully depleted (FD) SOI MOSFET fabricated in a 0.13 μm SOI CMOS technology. A simple model to study the behavior of the gate voltage at ZTC (V_ZTC) is proposed in the linear and the saturation region. The influence of the temperature mobility degradation on V_ZTC is analyzed for PD and FD devices. Experimental results show that the temperature mobility degradation is larger in FD than in PD devices, which is responsible for the V_ZTC decrement observed in FD instead of the increment observed in PD devices when the temperature increases. The analysis takes into account temperature dependence model parameters such as threshold voltage and mobility. The analytical predictions are in very close agreement with experimental results in spite of the simplification used for the V_ZTC model as a function of temperature in the linear and the saturation region.     

Study of SiC and Si3N4 inclusions in industrial multicrystalline silicon ingots grown by directional solidification method

In directional solidification of multicrystalline silicon ingots for solar cells, the concentration of C and N impurities in the silicon melts increases with progression of solidification due to their relatively low segregation coefficients. In the case of supersaturation of C and N in silicon melts, SiC and Si₃N₄ inclusions are formed. In this work, a piece of multicrystalline silicon was selected from a central block, which was cut out from an industrial multicrystalline silicon ingot grown by directional solidification method. The distribution of SiC and Si₃N₄ inclusions from the top to bottom regions was systematically studied. It was found that majority of SiC and Si₃N₄ inclusions are present in the top region and the amount of inclusions decreases exponentially from the top surface down to the bulk of the ingot. Morphologies and characteristics of the SiC and Si₃N₄ inclusions were investigated. The presence of SiC and Si₃N₄ inclusions generates high density of dislocations in multicrystalline silicon, and sometimes can also introduce pores into multicrystalline silicon. The results of this work will be of practical interest to the photovoltaic industry.     

Growth,photoluminescence and thermal conductance of graphene-like nanoflakes grown on copper foils in methane environment

In this paper, we reported a simple and effective synthesis method of graphene-like nanoflakes (GNFs) on the copper foils by hot filament chemical vapor deposition in methane environment. The structure and composition of GNFs were studied by field emission scanning electron microscope, micro-Raman spectroscope, and Fourier transform infrared spectroscope, respectively. According to the characterization results and the growth process, the formation mechanism of GNFs was investigated, which was based on the formation of carbon particles and the diffusion and assembly of carbon atoms. The photoluminescence (PL) of GNFs was measured in a Ramalog system and the PL spectra show a weak and a strong PL bands centered at about 411 and 515 nm, respectively. The measurement results of thermal conductance of GNFs indicate that the thermal conductivity of GNFs is up to 480 W/mK. Our results can enrich the knowledge on the synthesis, optical and thermal properties of graphene-based nanomaterials and contribute to the development of graphene-based devices.     

Analysis of degradation mechanisms of crystalline silicon PV modules after 12 years of operation in Southern Europe

The long‐term reliability of photovoltaic modules is crucial to ensure the technical and economic viability of PV as a successful energy source. The analysis of degradation mechanisms of PV modules is key to ensure current lifetimes exceeding 25 years. This paper presents the results of the investigations carried out on the degradation mechanisms of a crystalline silicon PV installation of 2 kWp after 12 years of exposure in Málaga, Spain. The analysis was conducted by visual inspection, infrared thermography and electrical performance evaluation. By visual inspection, the most relevant defects in the modules were identified and ranked according to their frequency. The electrical performance was assessed by comparing the characteristic parameters of the individual modules, obtained by outdoor measurements at the start and end of the exposure period. The correlation of the visual defects and the shifts in the electrical parameters was analysed. The results presented show that glass weathering, delamination at the cell‐EVA interface and oxidation of the antireflective coating and the cell metallization grid were the most frequently occurring defects found. The total peak power loss, including the initial light induced degradation, was 11.5%, which corresponded almost totally to a loss in short‐circuit current. Copyright © 2011 John Wiley & Sons, Ltd.     

Role of a Si0.95Ge0.05 epilayer cap on boron diffusion in silicon under inert and dry oxidizing ambient annealing

Silicon (Si) and Si with a 60 nm Si_0.95Ge_0.05 epilayer cap (Si_0.95Ge_0.05/Si) were implanted with 60 keV, 1×10¹³ cm⁻² boron (B) followed by annealing in nitrogen (N₂) or dry oxygen (O₂) in two different anneal conditions. B⁺implantation energy and dose were set such that the B peak is placed inside Si in Si_0.95Ge_0.05/Si samples and concentration independent B diffusion is achieved upon annealing. For samples annealed above 1075 °C, Ge diffusing from the Si_0.95Ge_0.05 epilayer cap in Si_0.95Ge_0.05/Si samples reached the B layer inside Si and resulted in retarded B diffusion compared to the Si samples. For annealing done at lower temperatures, diffusion of Ge from Si_0.95Ge_0.05 epilayer cap does not reach the B layer inside Si. Thus B diffusion profiles in the Si and Si_0.95Ge_0.05/Si samples appear to be similar. B diffusion in dry oxidizing ambient annealing of Si_0.95Ge_0.05/Si samples further depends on the nature of Si_0.95Ge_0.05 oxidation which is set by the duration and the thermal budget of the oxidizing anneal.     

Surface integrity and removal mechanism of silicon wafers in chemo-mechanical grinding using a newly developed soft abrasive grinding wheel

A new soft abrasive grinding wheel (SAGW) used in chemo-mechanical grinding (CMG) was developed for machining silicon wafers. The wheel consisted of magnesia (MgO) soft abrasives, calcium carbonate (CaCO₃) additives and magnesium oxychloride bond. Surface topography, roughness and subsurface damage of the silicon wafers ground using the new SAGW were comprehensively investigated. The results showed that the grinding with the new SAGW produced a surface roughness of about 0.5 nm in R_a and a subsurface damage layer of about 10 nm in thickness, which is comparable to that produced by chemo-mechanical polishing. This study also revealed that the chemical reactions between MgO abrasive, CaCO₃ additives and silicon material did occur during grinding, thereby generating a soft reactant layer on the ground surface. The reactant layer was easily removed during the grinding process.