The role of hydrogen in a-Si:H — results of evolution and annealing studies

Beyer, W.; Wagner, H.

doi:10.1016/0022-3093(83)90547-1

Cited by 182 publications

(73 citation statements)

References 15 publications

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“…18 The LT peak has been attributed to hydrogen located at the surfaces of internal voids or at the surface or near-surface regions in more compact films. 18,19 The significance of its kinetic parameters was clarified by Khait et al 1 According to their theory, hydrogen atoms bonded to neighboring Si atoms can evolve with activation enthalpies (⌬H) that can change depending on the contribution to the process of carriers trapped in deep levels. This change in ⌬H results in great displacements of the peak temperature from 650 K down to 490 K. Anyway, one can identify this particular evolution process because a linear relationship exists between the activation enthalpy ⌬H and activation entropy, ⌬S, which constitutes its signature:…”

Section: Analysis and Discussionmentioning

confidence: 99%

Calorimetry of hydrogen desorption froma-Si nanoparticles

et al. 2002

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The process of hydrogen desorption from amorphous silicon (a-Si͒ nanoparticles grown by plasmaenhanced chemical vapor deposition ͑PECVD͒ has been analyzed by differential scanning calorimetry ͑DSC͒, mass spectrometry, and infrared spectroscopy, with the aim of quantifying the energy exchanged. Two exothermic peaks centered at 330 and 410°C have been detected with energies per H atom of about 50 meV. This value has been compared with the results of theoretical calculations and is found to agree with the dissociation energy of Si-H groups of about 3.25 eV per H atom, provided that the formation energy per dangling bond in a-Si is about 1.15 eV. It is shown that this result is valid for a-Si:H films, too.

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Section: Analysis and Discussionmentioning

confidence: 99%

Calorimetry of hydrogen desorption froma-Si nanoparticles

et al. 2002

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“…As discussed before, 22 the appearance of interconnected voids with rising oxygen content is likely related to the increasing hydrogen content, in agreement with the concept of incorporation of hydrogen at positions interrupting bonds of the amorphous network so that the network loses its connectiveness. 31 Moreover, since oxygen in Si:O:H materials is not incorporated in Si-O-H but in Si-O-Si configurations, clustered oxygen even at low total oxygen content will result in the formation of silicon-oxide rings similar as in SiO 2 which are known to allow diffusion of molecular hydrogen at rather low temperatures. 32 …”

Section: Passivation Mechanismmentioning

confidence: 99%

Analysis of sub-stoichiometric hydrogenated silicon oxide films for surface passivation of crystalline silicon solar cells

Einsele¹,

Beyer²,

Rau³

2012

Journal of Applied Physics

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Thermal stability of passivating layers in amorphous/crystalline silicon (a-Si/c-Si) heterojunction solar cells is crucial for industrial processing and long-term device stability. Hydrogenated amorphous silicon (a-Si:H) yields outstanding surface passivation as atomic hydrogen saturates silicon dangling bonds at the a-Si/c-Si interface. Yet, a-Si surface passivation typically starts to degrade already at annealing temperatures in the range of 200 to 250 °C depending on annealing time, and optical absorption in front layers of a-Si reduces the short circuit current density. We show that oxygen incorporation into a-Si:H films enhances the thermal stability of the passivation and reduces parasitic absorption. We further show that for good passivation of the a-Si/c-Si interface, a compact material structure of the a-Si:O:H films is required where atomic hydrogen is the dominating type of diffusing hydrogen species. For plasma deposited a-Si:O:H films, oxygen incorporation of up to 10 at. % leads to an increase of the optical band gap while the hydrogen concentration is almost constant at approximately 10 at. %. For oxygen concentrations below 3%, the films yield surface recombination velocities as low as 10 cm/s on p-type wafers, and the temperature stability improves by about 50 K compared to pure a-Si:H. For films with relatively low oxygen content, hydrogen effusion spectra and Fourier transform infrared spectroscopy (FTIR) indicate a compact microstructure where only atomic H diffuses. For oxygen concentrations above 3%, the passivation quality reduces and H effusion and FTIR suggest the formation of an open, void-rich material where molecular H2 diffuses. In this case, annealing above 400 °C results in improved interface passivation, presumably due to a densification of the material. Likely, this densification results in an increased density of atomic H, which saturates Si dangling bonds near the c-Si interface.

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“…[9][10][11][12] When the material is heated at a constant rate, dehydrogenation peaks ͑proportional to the hydrogen desorption rate͒ appear at characteristic temperatures. From these experiments and provided that dehydrogenation is not diffusion controlled, the activation energy of Siu H bond breaking can be obtained.…”

Section: Introductionmentioning

confidence: 99%

Calorimetry of dehydrogenation and dangling-bond recombination in several hydrogenated amorphous silicon materials

et al. 2006

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Differential scanning calorimetry ͑DSC͒ was used to study the dehydrogenation processes that take place in three hydrogenated amorphous silicon materials: nanoparticles, polymorphous silicon, and conventional device-quality amorphous silicon. Comparison of DSC thermograms with evolved gas analysis ͑EGA͒ has led to the identification of four dehydrogenation processes arising from polymeric chains ͑A͒, SiH groups at the surfaces of internal voids ͑AЈ͒, SiH groups at interfaces ͑B͒, and in the bulk ͑C͒. All of them are slightly exothermic with enthalpies below 50 meV/͑H atoms͒, indicating that, after dissociation of any SiH group, most dangling bonds recombine. The kinetics of the three low-temperature processes ͓with DSC peak temperatures at around 320 ͑A͒, 360 ͑AЈ͒, and 430°C ͑B͔͒ exhibit a kinetic-compensation effect characterized by a linear relationship between the activation entropy and enthalpy, which constitutes their signature. Their Siu H bond-dissociation energies have been determined to be E͑Siu H͒ 0 = 3.14 ͑A͒, 3.19 ͑AЈ͒, and 3.28 eV ͑B͒. In these cases it was possible to extract the formation energy E͑DB͒ of the dangling bonds that recombine after Siu H bond breaking ͓0.97 ͑A͒, 1.05 ͑AЈ͒, and 1.12 ͑B͔͒. It is concluded that E͑DB͒ increases with the degree of confinement and that E͑DB͒ Ͼ 1.10 eV for the isolated dangling bond in the bulk. After Siu H dissociation and for the low-temperature processes, hydrogen is transported in molecular form and a low relaxation of the silicon network is promoted. This is in contrast to the high-temperature process for which the diffusion of H in atomic form induces a substantial lattice relaxation that, for the conventional amorphous sample, releases energy of around 600 meV per H atom. It is argued that the density of sites in the Si network for H trapping diminishes during atomic diffusion.

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The role of hydrogen in a-Si:H — results of evolution and annealing studies

Cited by 182 publications

References 15 publications

Calorimetry of hydrogen desorption froma-Si nanoparticles

Calorimetry of hydrogen desorption froma-Si nanoparticles

Analysis of sub-stoichiometric hydrogenated silicon oxide films for surface passivation of crystalline silicon solar cells

Calorimetry of dehydrogenation and dangling-bond recombination in several hydrogenated amorphous silicon materials

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