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Title: On the structural-optical properties of Al-containing amorphous Si thin films and the metal-induced crystallization phenomenon

Abstract

Amorphous (a-)Si-based materials always attracted attention of the scientific community, especially after their use in commercial devices like solar cells and thin film transistors in the 1980s. In addition to their technological importance, the study of a-Si-based materials also present some interesting theoretical-practical challenges. Their crystallization as induced by metal species is one example, which is expected to influence the development of electronic-photovoltaic devices. In fact, the amorphous-to-crystalline transformation of the a-SiAl system has been successfully applied to produce solar cells suggesting that further improvements can be achieved. Stimulated by these facts, this work presents a comprehensive study of the a-SiAl system. The samples, with Al contents in the ∼0−15 at. % range, were made in the form of thin films and were characterized by different spectroscopic techniques. The experimental results indicated that: (a) increasing amounts of Al changed both the atomic structure and the optical properties of the samples; (b) thermal annealing induced the crystallization of the samples at temperatures that depend on the Al concentration; and (c) the crystallization process was also influenced by the annealing duration and the structural disorder of the samples. All of these aspects were addressed in view of the existing models of themore » a-Si crystallization, which were also discussed to some extent. Finally, the ensemble of experimental results suggest an alternative method to produce cost-effective crystalline Si films with tunable structural-optical properties.« less

Authors:
 [1];  [2]
  1. Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, SP 13566-590 (Brazil)
  2. Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701 (United States)
Publication Date:
OSTI Identifier:
22314294
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 116; Journal Issue: 7; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ANNEALING; CONCENTRATION RATIO; CRYSTALLIZATION; MATERIALS; METALS; OPTICAL PROPERTIES; PHOTOVOLTAIC EFFECT; SOLAR CELLS; THIN FILMS; TRANSISTORS

Citation Formats

Zanatta, A. R., and Kordesch, M. E. On the structural-optical properties of Al-containing amorphous Si thin films and the metal-induced crystallization phenomenon. United States: N. p., 2014. Web. doi:10.1063/1.4893654.
Zanatta, A. R., & Kordesch, M. E. On the structural-optical properties of Al-containing amorphous Si thin films and the metal-induced crystallization phenomenon. United States. doi:10.1063/1.4893654.
Zanatta, A. R., and Kordesch, M. E. Thu . "On the structural-optical properties of Al-containing amorphous Si thin films and the metal-induced crystallization phenomenon". United States. doi:10.1063/1.4893654.
@article{osti_22314294,
title = {On the structural-optical properties of Al-containing amorphous Si thin films and the metal-induced crystallization phenomenon},
author = {Zanatta, A. R. and Kordesch, M. E.},
abstractNote = {Amorphous (a-)Si-based materials always attracted attention of the scientific community, especially after their use in commercial devices like solar cells and thin film transistors in the 1980s. In addition to their technological importance, the study of a-Si-based materials also present some interesting theoretical-practical challenges. Their crystallization as induced by metal species is one example, which is expected to influence the development of electronic-photovoltaic devices. In fact, the amorphous-to-crystalline transformation of the a-SiAl system has been successfully applied to produce solar cells suggesting that further improvements can be achieved. Stimulated by these facts, this work presents a comprehensive study of the a-SiAl system. The samples, with Al contents in the ∼0−15 at. % range, were made in the form of thin films and were characterized by different spectroscopic techniques. The experimental results indicated that: (a) increasing amounts of Al changed both the atomic structure and the optical properties of the samples; (b) thermal annealing induced the crystallization of the samples at temperatures that depend on the Al concentration; and (c) the crystallization process was also influenced by the annealing duration and the structural disorder of the samples. All of these aspects were addressed in view of the existing models of the a-Si crystallization, which were also discussed to some extent. Finally, the ensemble of experimental results suggest an alternative method to produce cost-effective crystalline Si films with tunable structural-optical properties.},
doi = {10.1063/1.4893654},
journal = {Journal of Applied Physics},
number = 7,
volume = 116,
place = {United States},
year = {Thu Aug 21 00:00:00 EDT 2014},
month = {Thu Aug 21 00:00:00 EDT 2014}
}
  • We report a detailed study of the mechanisms and energetics of hydrogen (H) insertion into strained Si-Si bonds during H-induced crystallization of hydrogenated amorphous Si (a-Si:H) thin films. Our analysis is based on molecular-dynamics (MD) simulations of exposure of a-Si:H films to H atoms from a H{sub 2} plasma through repeated impingement of H atoms. Hydrogen atoms insert into Si-Si bonds as they diffuse through the a-Si:H film. Detailed analyses of the evolution of Si-Si and Si-H bond lengths from the MD trajectories show that diffusing H atoms bond to one of the Si atoms of the strained Si-Si bondmore » prior to insertion; upon insertion, a bridging configuration is formed with the H atom bonded to both Si atoms, which remain bonded to each other. After the H atom leaves the bridging configuration, the Si-Si bond is either further strained, or broken, or relaxed, restoring the Si-Si bond length closer to the equilibrium bond length in crystalline Si. In some cases, during its diffusion in the a-Si:H film, the H atom occupies a bond-center position between two Si atoms that are not bonded to each other; after the H diffuses away from this bond-center position, a Si-Si bond is formed between these previously nonbonded Si atoms. The activation energy barrier for the H insertion reaction depends linearly on both the initial strain in the corresponding Si-Si bond and a strain factor that takes into account the additional stretching of the Si-Si bond in the transition-state configuration. The role of the H insertion reactions in the structural relaxation of the a-Si:H network that results in disorder-to-order transitions is discussed.« less
  • The crystallization of amorphous silicon is studied by transmission electron microscopy. The effect of Ni on the crystallization is studied in a wide temperature range heating thinned samples in-situ inside the microscope. Two cases of limited Ni source and unlimited Ni source are studied and compared. NiSi{sub 2} phase started to form at a temperature as low as 250°C in the limited Ni source case. In-situ observation gives a clear view on the crystallization of silicon through small NiSi{sub 2} grain formation. The same phase is observed at the crystallization front in the unlimited Ni source case, where a secondmore » region is also observed with large grains of Ni{sub 3}Si{sub 2}. Low temperature experiments show, that long annealing of amorphous silicon at 410 °C already results in large crystallized Si regions due to the Ni induced crystallization.« less
  • The crystallization kinetics of ultrathin a-Si induced by Al under thermal annealing and pulsed laser irradiation has been studied. Under thermal annealing, the crystallization temperature and activation energy for crystallization of a-Si with a thin Al metal layer was reduced to around 340 degree sign C and 3.3 eV, respectively. The reaction exponent was determined to vary from 1.5 to 1.8, corresponding to a crystallization process in which grain growth occurs with nucleation, and the nucleation rate decreases with the progress of grain growth. Under high power pulsed laser irradiation, the crystallization and reamorphization of a-Si were found to takemore » place sequentially in a-Si/Al. The reamorphization of a-Si in contact with a thin Al metal layer can be attributed to the melting of a-Si/Al initiated at the interface, due to the low melting temperature of Si-Al alloy and the rapid solidification that followed. Considering only the crystallization process, the activation energy for crystallization of a-Si induced by Al, estimated to be about 0.22 eV, was nearly an order of magnitude lower than that under thermal annealing. This may be explained by the explosive crystallization of a-Si by mechanical impact with a high power pulsed laser. In the meantime, the reaction exponent, determined to range from 1.9 to 2.2, was slightly higher than that under thermal annealing, indicating that the decrease of nucleation rate with the progress of grain growth during crystallization was slower, and the crystallization process became more nucleation dominant.« less
  • Thin film polycrystalline Si (poly-Si) is of considerable interest today for microelectronics, flat panel displays, and photovoltaics. Low thermal budget solid-phase crystallization (SPC) of a-Si precursor films was achieved using surface treatments with metal-containing solutions. Two different treatment procedures were demonstrated. With these treatments, one based on a Pd solution and the other on a Ni solution, the SPC time at 600 C was reduced from 18 h to 10 min or less. This approach renders the usual vacuum deposition step used in metal-induced crystallization unnecessary. The authors find that the ultraviolet reflectance and Raman shift signals for the crystallizedmore » films are independent of whether the SPC-enhancing metal is applied by vacuum or solution. These characterization results do differ, however, with the metal applied.« less