Effusion Experiments Research Articles

The Microstructure of Underdense Hydrogenated Amorphous Silicon and its Application to Silicon Heterojunction Solar Cells

The application of thin underdense hydrogenated amorphous silicon (a‐Si:H) films for passivation of crystalline Si (c‐Si) by avoiding epitaxy in silicon heterojunction (SHJ) solar cell technology has recently been proposed and successfully applied. Herein, the microstructure of such underdense a‐Si:H films, as used in our silicon heterojunction solar cell baseline, is investigated mainly by Raman spectroscopy, effusion, and secondary ion mass spectrometry. In H effusion experiments, a low‐temperature (near 400 °C) effusion peak which has been attributed to the diffusion of molecular H2 through a void network is seen. The dependence of the H effusion peaks on film thickness is similar as observed previously for void rich, low substrate‐temperature a‐Si:H material. Solar cells using underdense a‐Si:H as i1‐layer with a maximum efficiency of 24.1% are produced. The passivation quality of the solar cells saturates with increasing i1‐layer thickness. The fact that with such underdense material combined with a following high‐quality i2‐layer, instead of only high‐quality a‐Si:H with a low defect density direct on the c‐Si substrate, good passivation of c‐Si solar cells is achieved, which demonstrates that in the passivation process, molecular hydrogen plays an important role.

Volatility Study of Amino Acids by Knudsen Effusion with QCM Mass Loss Detection.

This work presents a new Knudsen effusion apparatus employing continuous monitoring of sample deposition using a quartz-crystal microbalance sensor with internal calibration by gravimetric determination of the sample mass loss. The apparatus was tested with anthracene and 1,3,5-triphenylbenzene and subsequently used for the study of sublimation behavior of several proteinogenic amino acids. Their low volatility and thermal instability strongly limit possibilities of studying their sublimation behavior and available literature data. The results presented in this work are unique in their temperature range and low uncertainty required for benchmarking theoretical studies of sublimation behavior of molecular crystals. The possibility of dimerization in the gas phase that would invalidate the effusion experiments is addressed and disproved by theoretical calculations. The enthalpy of sublimation of each amino acid is analyzed based on the contributions in two hypothetical sublimation paths involving the proton transfer in the solid and in the gas phase.

Toward the Elucidation of the Competing Role of Evaporation and Thermal Decomposition in Ionic Liquids: A Multitechnique Study of the Vaporization Behavior of 1-Butyl-3-methylimidazolium Hexafluorophosphate under Effusion Conditions.

The evaporation/decomposition behavior of the imidazolium ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6) was investigated in the overall temperature range 425-551 K by means of the molecular-effusion-based techniques Knudsen effusion mass loss (KEML) and Knudsen effusion mass spectrometry (KEMS), using effusion orifices of different size (from 0.2 to 3 mm in diameter). Specific effusion fluxes measured by KEML were found to depend markedly on the orifice size, suggesting the occurrence of a kinetically delayed evaporation/decomposition process. KEMS experiments revealed that other species are present in the vapor phase besides the intact ion pair BMImPF6(g) produced by the simple evaporation BMImPF6(l) = BMImPF6(g), with relative abundances depending on the orifice size-the larger the orifice, the larger the contribution of the BMImPF6(g) species. By combining KEML and KEMS results, the conclusion is drawn that in the investigated temperature range, when small effusion orifices are used, a significant part of the mass loss/volatility of BMImPF6 is due to molecular products formed by decomposition/dissociation processes rather than to evaporated intact ion pairs. Additional experiments performed by nonisothermal thermogravimetry-differential thermal analysis (TG-DTA) further support the evidence of simultaneous evaporation/decomposition, although the conventional decomposition temperature derived from TG curves is much higher than the temperatures covered in effusion experiments. Partial pressures of the BMImPF6(g) species were derived from KEMS spectra and analyzed by second- and third-law methods giving a value of ΔevapH298K° = 145.3 ± 2.9 kJ·mol-1 for the standard evaporation enthalpy of BMImPF6. A comparison is done with the behavior of the 1-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide (BMImNTf2) ionic liquid.

Light-induced changes in silicon nanocrystal based solar cells: Modification of silicon–hydrogen bonding on silicon nanocrystal surface under illumination

Hydrogenated polymorphous silicon (pm-Si:H) is a material consisting of a small volume fraction of nanocrystals embedded in an amorphous matrix. pm-Si:H solar cells demonstrate interesting initial degradation behaviors such as rapid initial change in photovoltaic parameters and self-healing after degradation during light-soaking. The precise dynamics of the light-induced degradation was studied in a series of light-soaking experiments under various illumination conditions such as AM1.5G and filtered 570 nm yellow light. Hydrogen effusion experiment before and after light-soaking further revealed that the initial degradation of pm-Si:H solar cells originate from the modification of silicon–hydrogen bonding on the surface of silicon nanocrystals in pm-Si:H.

Features of surface layer of LiNbO3 as-received single crystals: Studied in situ on treatment samples modified by elevated temperature

The surface layer of LiNbO3 congruent crystals was studied at room temperature and the 300–1000°C temperature range. The XPS test discerned two temperature ranges with different electronic structures and chemical compositions of the crystal sample surface layer. We observed a deficiency of Li ions in surface of as-received epi-polished samples, at RT, and in range from 300 to 500°C. The concentration of Li ions in epi-polished surface did not exceed 4%, which was much lower than the Li concentration determined for cleaved crystal, at RT. We explained this Li shortage with the use of leaching procedure, which was used in the crystal sample preparation stage. The XPS spectra, which have been obtained in the temperature of 300–500°C, have shown three oxidation states of niobium Nb 3d0, 3d1, 3d3, broadening of O 1s, and occurrence of C 1s core lines. In higher temperature range (700–1000°C), we detected an increased concentration of lithium and changes in the oxidation state of niobium that indicated congruent composition in the surface of lithium niobate crystal. The conditions, which corresponded to migration of Li ions, have been checked with the use of thermogravimetric analysis, effusion test, and TOF-SIMS spectroscopy. Thermogravimetry has shown mass losses of bulk crystal and powdered samples. The powdered samples exhibited saturation of mass loss at reducing conditions. The effusion experiments confirmed that O and Li2O were the most volatile species in LiNbO3. Mass spectroscopy (effusion and TOF-SIMS) shown out-diffusion of lithium ions from the deeper part of the crystal to the surface layer. TOF-SIMS maps exhibited a non–homogeneous composition and granular structure of distribution of the Li and Nb elements on the surface of the reduced sample.

Resistivity of atomic layer deposition grown ZnO: The influence of deposition temperature and post-annealing

Conductive zinc oxide (ZnO) films deposited by atomic layer deposition were studied as function of post-annealing treatments. Effusion experiments were conducted on ZnO films deposited at different temperatures. The influence of different annealing atmospheres on the resistivity of the films was investigated and compared to reference samples. It was found that the influence of the deposition temperature on the resistivity is much higher than that of subsequent annealings. This leads to the conclusion that reduction of the resistivity by diffusion of different gases, such as oxygen and hydrogen, into annealed ZnO films is unlikely.

The advantage of using in-situ methods for studying hydrogen mass transport: Neutron radiography vs. carrier gas hot extraction

Neutron radiography (NR) is compared with the commonly used carrier gas hot extraction (CGHE) technique. We performed isothermal hydrogen effusion experiments at 623 K to study the mass transport kinetics. The investigated material was technical iron. The quantification of the hydrogen mass flow is done for NR by using concentration standards. The temporal hydrogen concentration evolution in the sample coincides well for both methods, i.e. NR and CGHE, and is in good agreement with literature. The advantages of the NR method are the non-destructive nature of measuring and the in-situ determination of hydrogen concentrations with high spatial and temporal resolution. Remaining hydrogen inside the sample can be identified directly by the NR method.

Diffusion of Saccharides and Sugar Alcohol Sorbitol in Chitosan Membranes and Beads

AbstractDiffusion coefficients of different sugars and the sugar alcohol sorbitol have been determined by two different experimental setups: mass transfer through a flat membrane in a diffusion cell and effusion experiments with beads in a batch stirred vessel. Diffusion experiments show highly consistent data with low uncertainties and high regression coefficients, while effusion experiments suffer from larger experimental uncertainties. For the diffusion through a chitosan membrane at 25 °C, typically one‐third of the free molecular diffusion coefficient is found, whereas at 40 °C only one‐quarter of it is reached. The influence of molecular size, structure, and temperature on the transport properties is discussed in detail.

Hydrogen related crystallization in intrinsic hydrogenated amorphous silicon films prepared by reactive radiofrequency magnetron sputtering at low temperature

We present an investigation on the transition from amorphous to nanocrystalline silicon and associated hydrogen changes during the first steps of hydrogenated nanocrystalline silicon growth for films elaborated by reactive radiofrequency magnetron sputtering at a substrate temperature as low as room temperature and for deposition times varying from 3 to 60min. Complementary experimental techniques have been used to characterize the films in their as-deposited state. They are completed by thermal hydrogen effusion experiments conducted in the temperature range, from room temperature to 800°C. The results show that, during the initial stages of growth, the presence of a hydrogen-rich layer is necessary to initiate the crystallization process.

Er3+ and Si luminescence of atomic layer deposited Er-doped Al2O3 thin films on Si(100)

Atomic layer deposition was used to deposit amorphous Er-doped Al2O3 films (0.9–6.2 at. % Er) on Si(100). The Er3+ photoluminescence (PL), Er3+ upconversion luminescence, as well as the Si PL and associated surface passivation properties of the films were studied and related to the structural change of the material during annealing. The PL signals from Er3+ and Si were strongly dependent on the annealing temperature (T = 450–1000 °C), but not directly influenced by the transition from an amorphous to a crystalline phase at T > 900 °C. For T > 650 °C, broad Er3+ PL centered at 1.54 μm (4I13/2) with a full width at half maximum of 55 nm was observed under excitation of 532 nm light. The PL signal reached a maximum for Er concentrations in the range of 2–3 at. %. Multiple photon upconversion luminescence was detected at 660 nm (4F9/2), 810 nm (4I9/2), and 980 nm (4I11/2), under excitation of 1480 nm light. The optical activation of Er3+ was related to the removal of quenching impurities, such as OH (3 at. % H present initially) as also indicated by thermal effusion experiments. In contrast to the Er3+ PL signal, the Si luminescence, and consequently the Si surface passivation, decreased for increasing annealing temperatures. This trade-off between surface passivation quality and Er3+ PL can be attributed to an opposite correlation with the decreasing hydrogen content in the films during thermal treatment.

Nature of doped a-Si:H/c-Si interface recombination

Doped hydrogenated amorphous silicon (a-Si:H) films of only a few nanometer thin find application in a-Si:H/crystalline silicon heterojunction solar cells. Although such films may yield a field effect at the interface, their electronic passivation properties are often found to be inferior, compared to those of their intrinsic counterparts. In this article, based on H2 effusion experiments, the authors argue that this phenomenon is caused by Fermi energy dependent Si–H bond rupture in the a-Si:H films, for either type of doping. This results in the creation of Si dangling bonds, counteracting intentional doping of the a-Si:H matrix, and lowering the passivation quality.

Post-deposition thermal annealing studies of hydrogenated microcrystalline silicon deposited at 40 °C

Post-deposition thermal annealing studies, including gas effusion measurements, measurements of infrared absorption versus annealing state, cross-sectional transmission electron microscopy (X-TEM) and atomic force microscopy (AFM), are used for structural characterization of hydrogenated amorphous and microcrystalline silicon films, prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) at low substrate temperature ( T S). Such films are of interest for application in thin semiconductor devices deposited on cheap plastics. For T S ∼ 40 °C, H-evolution shows rather complicated spectra for (near-) microcrystalline material, with hydrogen effusion maxima seen at ∼ 200–250 °C, 380 °C and ∼ 450–500 °C, while for the amorphous material typical spectra for good-quality dense material are found. Effusion experiments of implanted He demonstrate for the microcrystalline material the presence of a rather open (void-rich) structure. A similar tendency can be concluded from Ne effusion experiments. Fourier Transform infrared (FTIR) spectra of stepwise annealed samples show Si–H bond rupture already at annealing temperatures of 150 °C. Combined AFM/X-TEM studies reveal a columnar microstructure for all of these (near-) microcrystalline materials, of which the open structure is the most probable explanation of the shift of the H-effusion maximum in (near-) microcrystalline material to lower temperature.

Hydrogen migration in single crystal and polycrystalline zinc oxide

Hydrogen diffusion in single crystal and polycrystalline zinc oxide was investigated by deuterium diffusion and hydrogen effusion experiments. Deuterium concentration depth profiles were measured as a function of the passivation temperature, while in H effusion experiments the molecular hydrogen flux was measured as a function of the heating rate. The diffusion coefficient exhibits thermally activated behavior and varies between ${E}_{A}=0.17$ and $0.37\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The change of ${E}_{A}$ is accompanied by a change of the diffusion prefactor by eight orders of magnitude. This indicates that ${E}_{A}$ is not related to the energetic position of H transport sites or the barrier height between such sites. Using the microscopic diffusion prefactor, the position of the hydrogen chemical potential, ${\ensuremath{\mu}}_{H}$, was estimated. With increasing temperature, ${\ensuremath{\mu}}_{H}$ decreases with a rate of $\ensuremath{\approx}0.0013\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{K}$. At H concentrations of less than ${10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ ${\ensuremath{\mu}}_{H}$ is pinned. The hydrogen density of states was derived from H effusion data, which is consistent with a diffusion activation of about $1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ as was originally reported by Mollwo [Z. Phys. 138, 478 (1954)] and Thomas and Lander [J. Chem. Phys. 25, 1136 (1956)]. Clear evidence for hydrogen deep traps was found in single crystal and polycrystalline $\mathrm{ZnO}$.

Tritium locked in silica using 248nm KrF laser irradiation

In this Letter we report on selectively occluding tritium in a silica film on a silicon substrate using a combination of high-pressure tritium loading and 248nm KrF laser irradiation. Sixty percent of tritium dissolved in the silica film was bonded by laser irradiation. The concentration of the bonded tritium was proportional to the total laser fluence. Tritium effusion experiments indicated that the laser-locked tritium existed stably in the glass matrix up to 400°C. In this work we point a way to a safe and simple approach for the integration of on-chip radioisotope micropower sources for micromechanical and microelectronic applications.

Microstructure Characterization of Amorphous Silicon-Nitride Films by Effusion Measurements

AbstractThe microstructure of a-Si:N:H films, which are applied as antireflection coating and for defect passivation in multicrystalline silicon (mc-Si) solar cells, was studied by gas effusion experiments. The results show for as-deposited material of low substrate temperatures (TS = 200 – 300°C) a predominant diffusion of molecular hydrogen for temperatures up to 800°C in agreement with the presence of interconnected openings (voids). At higher substrate temperatures, the material has a more compact structure and atomic hydrogen is the dominant diffusing species in the accessible temperature range. Annealing effects were also studied. The results are consistent with the concept that atomic hydrogen released from the a-Si:N:H coating serves for defect passivation in μc-Si solar cells.

Energetics of the N−O Bonds in 2-Hydroxyphenazine-di-N-oxide

The standard enthalpy of formation and the enthalpy of sublimation of crystalline 2-hydroxyphenazine-di-N-oxide, at T = 298.15 K, were determined from isoperibol static bomb combustion calorimetry and from Knudsen effusion experiments, as -76.7 +/- 4.2 kJ.mol(-1) and 197 +/- 5 kJ.mol(-1), respectively. The sum of these two quantities gives the standard enthalpy of formation in the gas-phase for this compound, delta(f)H(m)degrees(g) = 120 +/- 6 kJ.mol(-1). This value was combined with the gas-phase standard enthalpy of formation for 2-hydroxyphenazine retrieved from a group estimative method yielding the mean (N-O) bond dissociation enthalpy, in the gas-phase, for 2-hydroxyphenazine-di-N-oxide. The result obtained with this strategy is (DH(m)degrees (N - O)) = 263 +/- 4 kJ.mol(-1), which is in excellent agreement with the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d) computed value, 265 kJ.mol(-1).

Hydrogen effusion from highly-ordered near-stoichiometric a-SiC:H

In this work, highly-ordered close-to-stoichiometry a-SiC:H alloys deposited by PECVD under the deposition conditions of a `silane-starving regime' and strong hydrogen dilution were characterized by gas effusion experiments. The results show that films deposited under these conditions are more stable than conventional materials showing a hydrogen effusion peak shifted to higher temperatures and almost no hydrocarbon effusion. This is an indication of preferential removal of less stable H and CH n radicals during growth resulting in a more compact material with reduced density of C–H and Si–H bonds as compared to conventionally deposited a-SiC:H films. The effects of varying deposition parameters, like substrate temperature, rf power density and hydrogen dilution ratio on the effusion spectra are analyzed.

Infrared studies combined with hydrogen effusion experiments on nanostructured porous silicon

Nanostructured porous silicon samples prepared by electrochemical anodization of p-type crystalline silicon were exposed to blue light irradiation ( λ=400 nm) in air atmosphere. We observed that both photoluminescence and IR spectra evolve during irradiation. We have monitored the evolution of the IR spectra and we have performed effusion experiments to study the photo-oxidation kinetics. Bands located at around 1100 and 900 cm −1 attributed to Si–O related modes are observed to grow with the illumination time, whereas a mode at 910 cm −1 assigned to SiH 2 scissoring vibration decreases. The existence of an isosbestic point and the results from factor analysis reveal the presence of only two species evolving in a correlated way. The hydrogen effusion experiments corroborate that the photo-oxidation takes place in a preferential manner, at the expense of hydrogen bonded as dihydride.

Helium Versus Hydrogen Dilution of Silane in the Deposition of Polymorphous Silicon Films: Effects on the Structure and the Transport Properties

AbstractWe compare the deposition rate, hydrogen incorporation and optoelectronic properties of hydrogenated polymorphous silicon films produced either by the decomposition of silane-hydrogen or of silane-helium mixtures. Our results clearly show that He dilution allows to drastically reduce the RF power needed to achieve the same deposition rate as in the case of H2 dilution. Infrared spectroscopy and hydrogen effusion experiments show clear differences in the hydrogen bonding and content in both series of films. Interestingly, both He and hydrogen dilution result in films with improved transport properties, in particular the hole diffusion length, with respect to standard amorphous silicon. These results indicate that He dilution is a good alternative to H2 dilution to prepare intrinsic layers for solar cells.

Thermodynamic Properties of 1-Butyl-3-methylimidazolium Hexafluorophosphate in the Ideal Gas State

Thermodynamic properties of an ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) in the ideal gas state were calculated from molecular and spectral data. Because quantum chemical calculations demonstrated that Kdis for [C4mim][PF6] did not exceed 10-11 at temperatures below 1000 K, the gas was assumed to consist of ion pairs. The product of the principal moments of inertia was found to be 16.49 × 10-132 kg3·m6. The frequencies of normal vibrations were obtained from the experimental and calculated spectra. Rotation of CH3N was assumed to be free. The parameters for all alkyl tops were taken to be close to those in alkanes. The parameters for Bu- and PF6 were calculated ab initio. The calculated thermodynamic functions of the ideal gas (S°, Cp, and −(G° − H°(0))/T) were (657.4, 297.0, and 480.3) J·K-1·mol-1, respectively, at 298 K and were (843.1, 424.4, and 252.8) J·K-1·mol-1, respectively, at 500 K. Experiments were performed to better characterize the thermal stability and vapor pressure Psat of this substance. DSC experiments were carried out in a temperature range from (303 to 523) K and suggest that the substance starts to decompose at temperatures greater than 473 K. Knudsen effusion experiments were attempted to measure Psat for [C4mim][PF6] in the temperature range (433 to 522) K, but no reproducible values of Psat were obtained. By combining a published value of the cohesive energy density, measured heat capacities, and thermodynamic properties in the ideal gas state, thermodynamic properties of vaporization ( Cp, ΔvapS, ΔvapH) and vapor pressure (Psat) were calculated. At room temperature, the calculated Psat was found to be 10-10 Pa, a value that is much smaller than the lower detection limit for effusion measurements.