Both experimental and theoretical studies of fast and microscale physical phenomena occurring during the growth of vapour bubbles in nucleate pool boiling are reported. The focus is on the liquid film of micrometric thickness (a ‘microlayer’) that can form between the heater and the liquid–vapour interface of a bubble. The microlayer strongly affects the macroscale heat transfer and is thus important to be understood. The microlayer appears as a result of the inertial forces that cause the hemispherical bubble shape. It is shown that the microlayer can be seen as the Landau–Levich film deposited by the bubble foot edge during its receding. Paradoxically, the deposition is controlled by viscosity and surface tension. The microlayer profile measured with white-light interferometry, the temperature distribution over the heater, and the bubble shape are observed with synchronised high-speed cameras. According to the numerical simulations, the microlayer consists of two regions: a dewetting ridge near the contact line, and a longer and flatter bumped part. It is shown that the ridge cannot be measured by interferometry because of its intrinsic limitation on the interface slope. The ridge growth is linked to the contact line receding. The simulated dynamics of both the bumped part and the contact line agrees with the experiment. The physical origin of the bump in the flatter part of microlayer is explained.

Pm-Si:H PIN and NIP solar cells structures grown using plasma enhanced chemical vapor deposition (PECVD) technique were analyzed during 400 hrs of light-soaking exposition. The evolution of the structural and optical properties was observed and characterized by Raman spectroscopy, spectroscopic ellipsometry. The effect observed is related to defects creation due to induced hydrogen diffusion, break of Si-H bonds and the generation of dangling bonds that causes less passivated films. The film microstructure, and therefore the optical properties varied with the exposition time. The crystalline fraction of these structures presents a slight decrease and it is observed to be between 15 to 24% for the PIN and 5 to 10% for the NIP. The optical gap increases from 1.66 to 1.68 eV for the PIN structure while for the NIP no significant change is observed during light-soaking. Hydrogen diffusion during lights soaking generates a decrease on the absorption properties of the films which in turn is expected to reduce the device efficiency during operation. In this work we show that long range motion of hydrogen during light-soaking causes a hydrogen rearrangement on the film and microstructure changes. We determined that there is not an pronounced change on the film structure during prolonged light exposition related to the stability of the pm-Si:H films. The PIN structure properties are more affected during light soaking in comparison to the NIP structure which is expected to cause less degradation of its optoelectronic properties under illumination, and a more stable device during operation.

High-quality epitaxial silicon thin films have been deposited by plasma-enhanced chemical vapor deposition (PECVD) at 200 °C in a standard radiofrequency (RF) PECVD reactor. We optimized a silicon tetrafluoride (SiF4) plasma to clean the surface of <100> crystalline silicon wafers and without breaking vacuum, an epitaxial silicon film was grown from SiF4, hydrogen (H2), and argon (Ar) gas mixtures. We demonstrate that the H2/SiF4 flow rate ratio is a key parameter to grow high-quality epitaxial silicon films. Moreover, by changing this ratio, we can fine-tune the composition of the interface between the crystalline silicon (c-Si) wafer and the epitaxial film. In this way, at low values of the H2/SiF4 flow rate ratio, an abrupt interface is achieved. On the contrary, by increasing this ratio we can obtain a porous and fragile interface layer, composed of hydrogen-rich microcavities, which allows the transfer of the epitaxial film to foreign substrates.

The use of a laser annealing and chemical texturing process (dubbed the LaText process) on room-temperature sputtered ZnO:Al has been shown to generate unusually high haze properties, favorable for thin film silicon solar cells.This is due to the melting of the ZnO:Al layer by the XeCl laser, and the formation of crystalline domains onthe surface, for which the grains and grain boundaries are subsequently etched at different rates. The unusual surface morphology produced through this process can strongly impact the nature of the amorphous or microcrystalline silicon material deposited thereupon. In this paper, we report on results for amorphous silicon devices, for which the surface texture is seen to slightly impact thelight absorption in the material, but more interestingly, also the light-induced degradation of the cells.For co-deposited cells, devices deposited on surfaces with the characteristic "LaText" morphologyundergo a much lesser degradation. Furthermore, the decreased degree of degradation coincides with a notable shift in the Raman scattering peak. This provides a rapid diagnostic for testing multiple textures and deposition parameters.

We propose an implementation of the PCD technique to minority carrier effective lifetime assessment in crystalline silicon at 77K. We focus here on (n)-type, FZ, polished wafers passivated by a-Si:H deposited by PECVD at 200°C. The samples were immersed into liquid N2 contained in a beaker placed on a Sinton lifetime tester. Prior to be converted into lifetimes, data were corrected for the height shift induced by the beaker. One issue lied in obtaining the sum of carrier mobilities at 77K. From dark conductance measurements performed on the lifetime tester, we extracted an electron mobility of 1.1x104 cm².V-1.s-1 at 77K, the doping density being independently calculated in order to account for the freezing effect of dopants. This way, we could obtain lifetime curves with respect to the carrier density. Effective lifetimes obtained at 77K proved to be significantly lower than at RT and not to depend upon the doping of the a-Si:H layers. We were also able to experimentally verify the expected rise in the implied Voc, which, on symmetrically passivated wafers, went up from 0.72V at RT to 1.04V at 77K under 1 sun equivalent illumination.

Hydrogenated polymorphous silicon (pm-Si:H) is one of the most promising candidates for a stable top cell material in multi-junction thin film solar cells. Solar cells using pm-Si:H as their absorbing layer show very interesting degradation kinetics when compared to hydrogenated amorphous silicon (a-Si:H), summarized by macroscopic structural changes and irreversible changes in solar cell characteristics, while nevertheless preserving a higher stabilized efficiency. Notably, pm-Si:H solar cells, once degraded, respond to neither annealing nor further light-soaking. Such results suggest a device degradation mechanism including structural changes, active hydrogen motion, and interface delamination mediated by fast hydrogen diffusion and accumulation at the interface. Interestingly, a similar behavior was reported for a-Si:H solar cells under severe light soaking conditions (at 350 °C or under 50 suns) while pm-Si:H solar cells show such behavior under 1 sun at 40 °C.

Aluminum-doped zinc oxide (ZnO:Al) thin films were prepared on glass substrates by radio frequency (RF) magnetron sputtering from a ceramic mixed target ZnO:Al2O3 (1 wt.%) with a power of 250 W. Two series of samples were deposited at room temperature, the first one in pure Ar atmosphere, the second one in Ar/O2 gas mixture. Effects of post-deposition annealing treatments carried out from 400 °C to 500 °C under vacuum and in N2/H2 (5%) atmosphere have been investigated. The influence of these parameters was studied by a detailed microstructural analysis using X-Ray diffraction and Raman spectroscopy. For N2/H2 annealing process, the increase of charge carrier concentration limits the increase of the mobility while after vacuum annealing, an improvement of both electrical and optical properties was observed. The increase of the crystallinity and grain size for ZnO:Al films deposited in Ar/O2 gas mixture could explain their improvements. Resistivity was reduced down to 3.5×10-4 Ω.cm, for a mobility of 49 cm2/V.s with a vacuum annealing at 450 °C for ZnO:Al deposited in Ar/O2 gas mixture.

In the optimization of high-growth rate hydrogenated microcrystalline silicon (μc-Si:H) for photovoltaics, the observation of the infrared (IR) absorption peaks around 2000 cm-1 - which originate from silicon hydrogen (Si-HX) bonds in the film bulk - can provide useful information about the properties of the film. If the films are deposited on IR transparent substrates, Fourier Transform infrared (FTIR) spectroscopy is the most commonly used technique to measure the absorption in this regime. The rich spectrum of Si-HX peaks that can be observed in this wavenumber region describes the distribution of the bonded hydrogen amongst the various possible SiHX configurations, and as well, the presence of a pair of narrow, twin peaks at 2085 and 2100 cm-1 is indicative of material that is porous and prone to oxidation upon exposure to air. Many of these high wavenumber peaks are also observable using Raman scattering spectroscopy, allowing one to observe films on substrates that are not IR transparent. As well, the different absorption/scattering cross sections of the two techniques provide a useful tool to understand the origins of the various peaks, and in particular, those of the twin narrow peaks. In this work, we examine the dynamics upon exposure to atmosphere of these twin peaks in μc-Si:H grown by Matrix Distributed Electron Cyclotron Resonance (MD-ECR) PECVD. This deposition technique is an extremely promising one for growing μc-Si:H thin films at high deposition rates, and has demonstrated μc-Si:H film deposition at up to 28Å/s. By using both FTIR and Raman spectroscopy to characterize these films, we examine the differences in the dynamics of each of the twin peaks according to the technique used. In addition, secondary ion mass spectrometry (SIMS) measurements on the films show the physical distribution of oxygen in the films after five months of air exposure, and XRD measurements correlate the presence of these twin peaks with a [111] preferential surface orientation. Using these observations, we assign these absorption and scattering features to specific configurations of Si-HX within the film, and explain both the dynamics of the peaks over time and the similarity of the peaks for a wide range of samples.

Silicon nanowires (Si NWs) were grown directly on transparent conductive oxide layers using a single pump down process in a plasma enhanced chemical vapour deposition (PECVD) system. Layers of ITO and SnO2 on glass substrates were exposed to a hydrogen plasma leading to the reduction of the oxide and to the agglomeration of the metal into catalyst droplets of a few tens of nanometers diameter. The diameter and the density of the nanowires depend on the catalysts droplets size and density, we studied step by step the evolution of the surface prior to and at the initial stage of the nanowire growth. The catalyst droplets size and distribution were essentially investigated through Scanning Electron Microscopy (SEM).

Here we report a new in-plane solid-liquid-solid (IPSLS) mode for obtaining in-plane silicon nanowires (SiNW), which can be controlled and directly guided into various desired patterns for circuit architecture. Indium catalyst drops are firstly formed by a H2 plasma reduction of a thin layer of ITO on Corning glass substrate and then covered by an a-Si:H layer deposited at low temperature (100oC-200oC). The growth of SiNWs is activated in a reacting-gas-free thermal annealing process and led by the indium catalyst drops, that absorb and transform the a-Si:H matrix into crystalline SiNWs behind. At least two guided modes, that is, the a-Si:H channel guided mode and the step edge guided mode, can be applied to effectively control the growth routes for the lateral SiNWs. This guided growth of the IPSLS SiNWs lays an important basis for realizing various SiNWs-based device applications directly on top of low-cost substrates.

One of the primary challenges in the application of hydrogenated microcrystalline silicon (μc-Si:H) to photovoltaic cells is achieving high growth rates while maintaining good material quality over a wide process window. The rapid characterization of the material without generating a complete cell is thus a useful tool to determine said process window. Infrared absorption due to the various vibrational modes of the material has been used as a coarse tool towards this purpose, but the use of FTIR to perform this diagnosis limits the substrates upon which the analysis can be performed. We report on the use of high wave-number (1800-2200 cm-1) Raman scattering to perform a similar role of telltale peak detection directly on solar cells and on substrates suitable for thin-film photovoltaics. We evaluate material grown from SiF4 by RF-PECVD and from SiH4 by Matrix Distributed Electron Cyclotron Resonance (MDECR-) PECVD.

The efficiencies of hydrogenated polymorphous silicon (pm-Si:H) solar cells have been previously demonstrated to show superior stability under light-soaking. This stability arises due to the fact that the decrease they show in fill factor (FF) is partially offset by an accompanying increase in open circuit voltage (VOC). Recently, high-deposition rate (9Å/s) pm-Si:H material deposited by standard RF-PECVD at 13.56MHz has been investigated as the intrinsic layer in photovoltaic modules as it has shown excellent electronic properties. The degradation behaviour of these high-deposition rate cells, however, differs significantly from that of lower deposition rate material. In particular, no beneficial increase in Voc is observed during light soaking. We investigate the degradation dynamics of solar cells made from this high growth rate material using a Variable Illumination Method (VIM) during light soaking to quantify the changes to these high-rate cells during light-soaking and directly contrast them with those of low-rate (1.5Å/s) cells. In particular, we discuss the importance of bulk recombination effects vs interface quality changes, as well as the dynamics of changes in VOC.

The grain size of thermally crystallized a-Si films is controlled by the nucleation, rN, and growth, rG, rates according to the standard Avrami's theory. Despite this evidence, most papers devoted to improve the crystallized grain size analyze their results with a qualitative reference to this theory. In this paper, we will show that one can identify the standard set of rN and rG values for a-Si and that experiments show that deviations from this standard values always result in a smaller grain size. It is also shown that one cannot expect any substantial improvement with non-conventional heat treatments. Finally, it is argued that a larger grain size is expected from a-Si films containing, in their as-grown state, a controlled density of embedded nanocrystals.

We present a material study on RF PECVD-grown a-Ge:H showing that thin films of such material can be produced without using the conventional techniques of high power density or powered-electrode substrate placement. We demonstrate the production of material with PDS signatures superior to material produced at ten times higher power density. This is achieved through the use of Ar and H2 dilution and by growing the films at high pressures under conditions where nanocrystals formed in the gas phase contribute significantly to the growth as confirmed by HRTEM. The conditions described result in material which demonstrates activated conduction down to room temperature. Additionally, the quality of the material has been demonstrated through its application in n-i-p diodes. A spectral response at 0.9um of 0.38 and an AM1.5 efficiency of 2.1% have been demonstrated utilizing an absorber layer thickness of only 60nm.

We have developed a model to account for the effects of hydrogen and voids on the structural stability of silicon thin films. The model is based on both experiments and theory. First, hydrogenated amorphous silicon films (a-Si:H) with various hydrogen contents were obtained by Plasma Enhanced Chemical Vapor Deposition. A linear correlation between hydrogen content and void fraction was observed. By tuning the deposition conditions, polymorphous silicon films with hydrogen contents up to 15%, very small void fractions (0.5%) and excellent electronic properties were also obtained. Density Functional Theory (DFT) calculations were performed to determine the formation energy for four types of silicon tetrahedra of the form Si-SinH4−n (n=1, 2, 3, 4). In our model, these tetrahedral units are considered as the building blocks of the silicon thin films. Considering a homogeneous distribution of hydrogen in the solid, the proportion of the different SiSinH4−n tetrahedra as a function of the hydrogen concentration was calculated. Then, the formation energy of hydrogenated amorphous silicon (a-Si:H) was calculated as a function of the hydrogen content and for various porosities. The model predicts that hydrogen incorporation does render the a-Si:H structure unstable for different hydrogen contents depending on the void fraction. Our results show that polymorphous silicon films with hydrogen concentrations up to 15% can be as stable as standard amorphous silicon with 2% hydrogen content, provided that the presence of hydrogen is not associated with the incorporation of porosity in the film.