Hydrogen Effusion Measurements Research Articles

Hydrogen density-of-states distribution in β-Ga2O3

Hydrogen bonding in β-Ga2O2 is investigated using hydrogen effusion measurements. The samples were grown by plasma-assisted pulsed laser deposition. With increasing deposition temperature, the total H concentration decreases from 8.1 × 1019 to 9.2 × 1018 cm−3. The dependence of the hydrogen chemical potential, μH, on the H concentration is derived from the effusion spectra and subsequently used to determine the H density-of-states distribution. β-Ga2O2 deposited at Tdep ≤ 723 K exhibits a needle-like peak in the H density-of-states distribution at ≈1.8 eV below the H transport states. With increasing Tdep, the H density-of-states changes and two broad peaks emerge that are located at ≈1.7 and 2.4 eV below the H transport states for Tdep ≥ 873 K.

Monatomic hydrogen diffusion in β-Ga2O3

The nature of hydrogen migration in β-Ga2O3 was investigated with respect to molecular or monatomic diffusion. For this purpose, β-Ga2O3 samples were grown by plasma-assisted pulsed laser deposition in which H and D were spatially separated. The samples were characterized using secondary-ion-mass spectrometry and hydrogen effusion measurements. The combination of these methods provides clear evidence for monatomic H and D diffusion in β-Ga2O3.

Amorphous silicon passivation for 23.3% laser processed back contact solar cells

This paper presents amorphous silicon deposited at temperatures below 200 °C, leading to an excellent passivation layer for boron doped emitter and phosphorus doped back surface field areas in interdigitated back contact solar cells. A higher deposition temperature degrades the passivation of the boron emitter by an increased hydrogen effusion due to lower silicon hydrogen bond energy, proved by hydrogen effusion measurements. The high boron surface doping in crystalline silicon causes a band bending in the amorphous silicon. Under these conditions, at the interface, the intentionally undoped amorphous silicon becomes p-type conducting, with the consequence of an increased dangling bond defect density. For bulk amorphous silicon this effect is described by the defect pool model. We demonstrate, that the defect pool model is also applicable to the interface between amorphous and crystalline silicon. Our simulation shows the shift of the Fermi energy towards the valence band edge to be more pronounced for high temperature deposited amorphous silicon having a small bandgap. Application of optimized amorphous silicon as passivation layer for the boron doped emitter and phosphorus doped back surface field on the rear side of laser processed back contact solar cells, fabricated using four laser processing steps, yields an efficiency of 23.3%.

Using MDECR‐PECVD to study the impact of ion bombardment energy on microstructural properties of μc‐Si:H thin film grown from an SiF4/H2 chemistry

AbstractThe matrix‐distributed electron cyclotron resonance plasma‐enhanced chemical vapor deposition (MDECR‐PECVD) technique has been shown to achieve high deposition rates for hydrogenated microcrystalline silicon (μc‐Si:H) thin film. Due to the fact that plasma is sustained by a microwave discharge, by biasing the substrate holder with additional power supply, one can achieve independent control over the plasma density and the maximum ion bombardment energy (IBE). In this work, we present studies of the impact of IBE on the microstructural properties of the μc‐Si:H film deposited by MDECR‐PECVD. Insufficient ion bombardment is found to be responsible for the substantial presence of nano‐porous regions within the material, resulting in significant post‐deposition oxidation. Good agreement between transmission electron microscopy (TEM) Fresnel contrast analysis and the results of infrared absorption and hydrogen effusion measurements for the deposited films suggest that moderate IBE is of vital importance to achieve high quality μc‐Si:H. In doing so, denser films with significantly decreased nano‐porous regions and better stability are obtained, which is of great interest to optimize the process parameters for solar cell applications. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface passivation of heavily boron or phosphorus doped crystalline silicon utilizing amorphous silicon

Excellent surface passivation of heavily boron or phosphorus doped crystalline silicon is presented utilizing undoped hydrogenated amorphous silicon (a-Si:H). For passivating boron doped crystalline silicon surfaces, amorphous silicon needs to be deposited at low temperatures 150 °C≤Tdep≤200 °C, leading to a high bandgap. In contrast, low bandgap amorphous silicon causes an inferior surface passivation of highly boron doped crystalline silicon. Boron doping in crystalline silicon leads to a shift of the Fermi energy towards the valence band maximum in the undoped a-Si:H. A simulation, implementing dangling bond defects according to the defect pool model, shows this shift in the undoped a-Si:H passivation to be more pronounced if the a-Si:H has a lower bandgap. Hence, the inferior passivation of boron doped surfaces with low bandgap amorphous silicon stems from a lower silicon-hydrogen bond energy due to this shift of the Fermi energy. Hydrogen effusion and ellipsometry measurements support our interpretation.

Influence of annealing and Al2O3 properties on the hydrogen-induced passivation of the Si/SiO2 interface

Annealing at moderate temperatures is required to activate the silicon surface passivation by Al2O3 thin films while also the thermal stability at higher temperatures is important when Al2O3 is implemented in solar cells with screenprinted metallization. In this paper, the relationship between the microstructure of the Al2O3 film, hydrogen diffusion, and defect passivation is explored in detail for a wide range of annealing temperatures. The chemical passivation was studied using stacks of thermally-grown SiO2 and Al2O3 synthesized by atomic layer deposition. Thermal effusion measurements of hydrogen and implanted He and Ne atoms were used to elucidate the role of hydrogen during annealing. We show that the passivation properties were strongly dependent on the annealing temperature and time and were significantly influenced by the Al2O3 microstructure. The latter was tailored by variation of the deposition temperature (Tdep = 50 °C–400 °C) with hydrogen concentration [H] between 1 and 13 at.% and mass density ρmass between 2.7 and 3.2 g/cm3. In contrast to films with intermediate material properties, the passivation by low- and high density films showed a reduced thermal stability at relatively high annealing temperatures (∼600 °C). These observations proved to be in good agreement with thermal effusion results of hydrogen and inert gas atoms that were also strongly dependent on film microstructure. We demonstrate that the temperature of maximum effusion decreased for films with progressively lower density (i.e., with increasing [H]). Therefore, the reduced thermal stability of the passivation for low-density hydrogen-rich ([H] >∼5 at. %) films can be attributed to a loss of hydrogen at relatively low annealing temperatures. In contrast, the lower initial [H] for dense Al2O3 films can likely explain the lower thermal stability associated with these films. The effusion measurements also allowed us to discuss the role of molecular- and atomic hydrogen during annealing.

Voids in hydrogenated amorphous silicon materials

Effusion measurements of hydrogen and of implanted helium are used to characterize the presence of voids in hydrogenated amorphous silicon (a-Si:H) materials as a function of substrate temperature, hydrogen content, etc. For undoped plasma-grown a-Si:H, interconnected voids are found to prevail at hydrogen concentrations exceeding 15–20at.%, while isolated voids which act as helium traps appear at hydrogen concentrations≤15at.%. The concentration of such isolated voids is estimated to some 1018/cm3 for device-grade undoped a-Si:H deposited at a substrate temperature near 200°C. Higher values are found for, e.g., doped material, hot wire grown a-Si:H and hydrogen-implanted crystalline Si. The results do not support recent suggestions of predominant incorporation of hydrogen in a-Si:H in (crystalline silicon type) divacancies, since such models predict a concentration of voids (which act as helium traps) in the range of 1021/cm3 and a correlation between void and hydrogen concentrations which is not observed.

Firing stability of SiN y/SiN x stacks for the surface passivation of crystalline silicon solar cells

In the photovoltaic industry contacts to crystalline silicon are typically formed by firing of screen-printed metallization pastes. However, the stability of surface passivation layers during high temperature contact formation is a major challenge. Here, we investigate the thermal stability of the surface passivation by amorphous silicon nitride double layers (SiN y /SiN x ). The SiN y passivation layer is silicon rich with refractive index larger than 3. Whereas the SiN x capping layer has a refractive index of 2.05. Compared to pure hydrogenated amorphous silicon, the nitrogen in the SiN y passivation layer improves the firing stability. We achieve an effective surface recombination velocity after a conventional co-firing process of (5.2±2) cm/s on p-type (1.5 Ωcm) FZ-silicon wafers at an injection density of 10 15 cm −3. An analysis of the improved firing stability is presented based on FTIR and hydrogen effusion measurements. The incorporation of an SiN y /SiN x stack into the passivated rear of Cz silicon screen-printed solar cells results in an energy conversion efficiency of 18.3% compared to reference solar cells with conventional aluminum back surface field showing 17.9% efficiency. The short circuit current density increases by up to 0.8 mA/cm 2 compared to conventional solar cells due to the improved optical reflectance and rear side surface passivation.

Annealing Effects of Microstructure in Thin-film Silicon Solar Cell Materials Measured by Effusion of Implanted Rare Gas Atoms

ABSTRACTIn thin film silicon solar cell technology, annealing (heat treatment) effects are of interest since (i) annealing of underlying films often cannot be avoided during deposition and (ii) heat treatment (e.g. by laser) may be actively used for improvement of as-deposited material. Changes in the microstructure of several thin film silicon solar cell materials like hydrogenated amorphous silicon, microcrystalline silicon and zinc oxide by heat treatment were investigated by effusion measurements of hydrogen and implanted helium. Densification is observed for all materials studied, i.e. interconnected voids disappear or are transformed to isolated voids. We attribute the observed annealing effects primarily to an incomplete polymerization during growth. Important for solar cell processing is the result that the annealing effects involving structural changes set in at temperatures close to the temperature of deposition.

Microstructure Effects in Amorphous and Microcrystalline Ge:H Films

AbstractThe characterization of void-related microstructure in amorphous and microcrystalline Ge:H films is reported. Various methods are applied including effusion measurements of hydrogen and of implanted helium and neon, measurements of the infrared absorption of C-H bonds due to in-diffusion of contaminants and of the stretching modes of bonded hydrogen. Several microstructure effects like interconnected voids and isolated voids and a quite different material homogeneity are detected and are found to depend on the preparation conditions. Amorphous Ge:H can be prepared with a (largely) homogeneous structure while microcrystalline Ge:H tends to consist of compact grains surrounded by more or less open voids. Enhanced substrate temperatures (Ts ≈ 250°C) favour the growth of more compact material.

Hydrogen-Induced Passivation of Grain-Boundary Defects in Polycrystalline Silicon

The influence of the hydrogen content in the amorphous starting material on hydrogen bonding and defect passivation in laser annealed polycrystalline silicon is investigated. The samples are characterized using electron paramagnetic resonance and hydrogen effusion measurements. After laser dehydrogenation and crystallization the samples contain a residual H concentration of up to 8×1021 cm-3. During a vacuum anneal at least 1.5×1021 cm-3 are mobile of which only 3.7×1018 cm-3 H atoms passivate preexisting Si dangling bonds. It is shown that a vacuum anneal can cause the vast majority of H atoms to accumulate in platelet-like structures. Defect passivation and platelet nucleation and growth occur spatially separated requiring long range H diffusion.

Influence of laser crystallization on hydrogen bonding in poly‐Si

AbstractWe investigate the influence of the hydrogen content in the amorphous starting material on hydrogen bonding and defect passivation in laser crystallized poly‐Si using electron‐spin‐resonance and hydrogen effusion measurements. After laser dehydrogenation and crystallization the specimens contain a residual H concentration of 8×1021 cm–3 to 1.5×1022 cm–3. During a vacuum anneal at least 1.5×1021 cm–3 H atoms are mobile in the lattice, however, only about 3.7×1018 cm–3 H atoms passivate Si dangling‐bonds. Our results show that the annealing treatment can cause the vast majority of H atoms to accumulate in H stabilized platelets. Since defect passivation preferentially occurs at grain boundaries and platelet nucleation and growth is confined to the interior of single crystal grains, H equilibration is governed by two spatially separated processes. Moreover, our data demonstrate that the hydrogen density‐of‐states distribution derived from H effusion data is dynamic and changes in response to experimental parameters. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of Fermi energy on structural and electrical properties of laser crystallized P-doped amorphous silicon

A series of phosphorous-doped hydrogenated amorphous silicon films (a-Si:H) were crystallized using step-by-step laser crystallization process. The structural changes during the sequential crystallization process were detected by Raman measurements. The dehydrogenation was monitored by measuring the Si–H local vibrational modes using Raman spectroscopy and hydrogen effusion measurements. Interestingly, hydrogen bonding is affected by doping of the amorphous material. The influence of doping concentrations, thus the Fermi energy on electronic properties has been investigated employing secondary ion mass spectroscopy (SIMS), dark-conductivity- and Hall-effect measurements. The results from hydrogen effusion are consistent with the results obtained from Raman spectroscopy, Hall-effect- and dark-conductivity measurements.

Hydrogen transport properties in zinc oxide

Abstract Hydrogen diffusion in single crystal and polycrystalline zinc oxide (ZnO) was investigated by deuterium diffusion and hydrogen effusion measurements. The diffusion coefficient exhibits thermally activated behavior and varies between E A = 0.17 and 0.37 eV. Interestingly, the change in E A is accompanied by a variation of the diffusion prefactor by about eight orders of magnitude. This indicates that E A does not represent the diffusion barrier. On the other hand, the H density of states derived from effusion data is consistent with originally reported values of the diffusion activation energy of E A ≈ 1.0 eV .

Hydrogen equilibration in polycrystalline silicon

Hydrogen equilibration in polycrystalline silicon was investigated as a function of annealing time and temperature using electron spin resonance and hydrogen effusion measurements. During a vacuum anneal at least 1.5×1021cm−3 H atoms are mobile in the lattice, however, only about 3.7×1018cm−3 H atoms passivate Si dangling bonds. The results show that the annealing treatment can cause the vast majority of H atoms to accumulate in H stabilized platelets. Since defect passivation preferentially occurs at grain boundaries and platelet nucleation and growth are confined to the interior of single-crystal grains, H equilibration is governed by two spatially separated processes. Moreover, the data demonstrate that the hydrogen density-of-states distribution is dynamic and changes in response to experimental parameters.

Hydrogen induced voids in hydrogenated amorphous silicon carbon (a-SiC:H): Results of effusion and diffusion studies

The void formation in Si-rich a-SiC:H films deposited with dc magnetron sputtering is studied by effusion measurements of hydrogen and of implanted rare gases and secondary ion mass spectrometry (SIMS). Rare gas atoms were incorporated into the material by ion implantation. The results suggest a widening of the network openings with increasing alloy concentration. However, the void formation is mainly attributed not to an increase in carbon concentration but to an increase in hydrogen incorporation.

Boron Doping Effects in Microcrystalline Silicon

AbstractConditions leading to high conductivities (up to 300 S/cm) in chlorosilane-based boron-doped microcrystalline Si:Cl:H films are investigated. It is found that the high conductivity originates primarily from the growth of highly crystalline material with a high concentration of boron. Furthermore, these films grow with relatively low chlorine and hydrogen concentrations of a few percent and, according to effusion measurements of hydrogen and implanted helium, in a relatively compact structure. At a boron doping level of 1%, admixture of 10% silane to the tetrachlorosilane results in the growth of amorphous material of low conductivity while for admixture of up to 90% of silicontetrafluoride, microcrystalline Si films with high conductivities can be grown.

Effects of doping on structural change and hydrogen bonding in laser crystallized polycrystalline silicon films

Doped polycrystalline silicon films were produced by employing a step-by-step laser crystallization of doped hydrogenated amorphous silicon (a-Si:H). The influence of laser crystallization on structural properties and hydrogen bonding were investigated using Raman backscattering spectroscopy and hydrogen effusion measurements. Crystallization with low laser fluence, E L, results a stratified structure with polycrystalline silicon layer at the top of an amorphous layer. In fully crystallized polycrystalline silicon the Raman lines in both P- and B-doped specimens are asymmetric, which is indicative of the Fano effect. From the hydrogen effusion spectra, the hydrogen density-of-states distribution is derived. Laser crystallization results in an increase of the hydrogen binding energy by about 0.2–0.3 eV compared to the amorphous starting material.

The diffusion of hydrogen and inert gas in sputtered a-SiC:H alloys: Microstructure study

Abstract The microstructure of DC sputtered amorphous silicon carbon (a-SiC:H) is studied by effusion measurements of hydrogen and of implanted inert gases helium, neon, argon and secondary ion mass spectrometry. The results suggest that the motion of inert gas atoms is controlled by the diffusion, greatly depending on a broadening of network openings. Already at carbon concentrations of 25 at%, isolated voids disappeared presumably because interconnected voids are formed. A void formation is mainly attributed to an increase in hydrogen incorporation in the samples.

Structural properties of phosphorous-doped polycrystalline silicon

Abstract The structural properties and hydrogen bonding of undoped and phosphorous doped polycrystalline silicon produced by step-by-step laser dehydrogenation and crystallization technique were investigated using Raman spectroscopy and hydrogen effusion measurements. At low laser fluences, E L , a two-layer system is created. This is accompanied by the change in hydrogen bonding. The intensity of the Si–H vibration mode at 2000 decreases faster than the one at 2100 cm −1 . This is even more pronounced in phosphorous-doped specimens. The laser crystallization results in an increase of the hydrogen binding energy by approximately 0.2–0.3 eV compared to the amorphous starting materials.